Name: sputter growth and characterization of metamagnetic b2-ordered ferh epilayers
Uploaded: 2013-10-05T23:30:00
Duration: 12 min 20 s
Description: Watch this Scientific Journal Video about Sputter Growth and Characterization of Metamagnetic B2-ordered FeRh Epilayers at JoVE.com

This work was supported by the UK Engineering and Physical Sciences Research Council under grant number EP/G065640/1 and by the US National Science Foundation under grant number DMR-0908767 [M.L. and L.H.L.] and grant number DMR-0907007 [D.H.].

In this protocol, thin films of the ordered FeRh alloy are made with dc-magnetron sputtering on MgO (001) substrates. The samples are grown in a magnetic field of about 200 Oe provided by a permanent magnet array, which is used to set an in-plane magnetic anisotropy. The target diameter is 50 mm and the distance between the target and substrates is about 10 cm. To grow FeRh, a dc-magnetic Torus magnetron gun suitable for magnetic materials is used. The heaters are bulbs positioned 2 cm above the substrates and surrounded by a metal cylinder to keep the heated volume small. The maximum temperature possible in this system is ~1,050 K. This system is capable of holding 24 different substrates; however, we typically grow fewer than 10 when making epitaxial samples due to time constraints. The details presented here for this sample preparation protocol are known to work well in our vacuum system. As there are many equivalent vacuum systems that differ in their details, the requirements for the quantitative parameters such as temperature, time, etc. may well take different optimal values in other systems. Nevertheless, the description below will prove a useful guide to the reader.
In the detailed protocol below, it is assumed that the reader is familiar with the basics of good vacuum practice, such as the use of gloves to handle all components that will enter the vacuum chamber [see, for instance, reference 26].
1. Substrate and Target Preparation
This section describes the preparation of the sputter deposition chamber and the single crystal MgO substrates.
<ol>
	<li>Rinse the (001) MgO substrates in isopropanol and mount them in the substrate holders. Load these into the vacuum chamber.</li>
	<li>Mount the FeRh target in the magnetron gun and reassemble the gun. For a sample with an equiatomic composition, we have found that a target with Fe47Rh53 is most suitable, yielding the clearest magnetostructural phase transition. Test that there is no short circuit between the magnetron and the surrounding shield. Similarly, prepare any target(s) to be used for capping layers.</li>
	<li>Close the vacuum chamber and pump it down.</li>
	<li>Once the vacuum is better than 1 x 10-6 Torr, heat the substrates to 870 K. Monitor the vacuum level and heating rate to ensure that the pressure does not rise above this level. Hold at this temperature overnight.</li>
	<li>One hour before commencing growth, begin to flow liquid nitrogen through the Meissner trap. The vacuum should improve to better than 4 x 10-7 Torr.</li>
	<li>Set the mass flow controller to 65 sccm of working gas flow. The sputter gas is Ar with 4% hydrogen to avoid sample oxidation during growth. Open the gas flow valve. The pressure in the chamber should rise to the low mTorr range.</li>
	<li>Before the growth, pre-sputter the FeRh target for 1,200 sec at 30 W.</li>
</ol>
2. Epilayer Deposition
This section describes the deposition of the FeRh layer by dc-magnetron sputtering.
<ol>
	<li>Adjust the set point of the mass flow controller to give a chamber pressure of 4 x 10-3 Torr. Allow the pressure to settle to a stable value.</li>
	<li>Check that the substrate temperature remains at 870 K and is stable.</li>
	<li>Apply power to the magnetron to yield an overall deposition rate of 0.4 &#197;/sec. In chambers equipped with a quartz crystal monitor, this can be done using that monitor, if suitably calibrated. If the chamber does not have a quartz crystal monitor, post-growth thickness measurements can be helpful, as well as a high level of reproducibility between runs.</li>
	<li>Open the shutter and deposit FeRh on the heated substrate for a length of time suitable to give the desired thickness. For instance, a 500 sec deposition will yield a sample 20 nm thick.</li>
	<li>Close the shutter.</li>
	<li>Shut off power to the magnetron.</li>
	<li>Close the gas valve.</li>
	<li>Increase the sample temperature to 970 K. Hold the samples at this temperature for one hour. The pressure should remain better than 1 x 10-6 Torr throughout. The effects of varying this anneal temperature can be found in de Vries et al.20</li>
	<li>Shut off heater power and cool the samples to room temperature. In this system, this takes at least three hours.</li>
	<li>Deposit any capping layer required, using steps similar to steps 2.1-2.7. Depositing the capping layer at a temperature below ~370 K is essential to prevent interdiffusion into the FeRh layer27.</li>
	<li>Vent the chamber with dry nitrogen, open it, and remove the samples. They should appear bright and shiny.</li>
</ol>
3. Routine Post-growth Characterization
This section provides an overview of the basic characterization steps carried out on the majority of our FeRh samples. As there are many possible equivalent methods for making these measurements, the nature of the descriptions here are less detailed and prescriptive, and rather concentrate on the essential features of any such measurements.
<ol>
	<li>Perform a low angle &#935;-ray reflectivity scan to determine the sample thickness. Mount the sample in the diffractometer and align it in &#969; with the detector angle 2&#952; &#8776; 1&#176;. If available, &#967; should also be aligned. Run a standard &#952;-2&#952; scan with &#952; running from 0&#176; until the noise floor of the instrument is reached, typically once &#952;&#8805;6&#176; for a good quality sample. Clear Kiessig (thin-film interference) fringes should be visible, from which the epilayer thickness can be determined.</li>
	<li>Perform a high-angle &#935;-ray diffraction scan to determine the degree of chemical order. This can be carried out with the sample still mounted in the diffractometer from step 3.1. The MgO substrate peak should be found (at 2&#952; = 42.9&#176; if Cu K&#945; radiation is used) and the sample aligned again in &#969;. Again, run a &#952;-2&#952;, covering at least the range 12.5&#176;&#60;&#952;&#60;62.5&#176; (again assuming Cu K&#945; radiation) so that both the FeRh (001) and (002) peaks, as well as the substrate peak, are captured.</li>
	<li>Perform a measurement of the temperature dependence of the sample resistivity to determine the transition temperature. Make electrical contacts to the sample such that a standard 4-point measurement can be made to avoid contact resistance problems. If a dc method is being used, make measurements for forward and reverse current directions and the resistances averaged in order to null off any thermal e.m.f. generated at elevated temperatures. Then place the sample on a temperature controlled hot stage (in this set-up the stage is in a small turbo-pumped high vacuum chamber to be sure of avoiding any oxidation), and measure the resistance as a function of temperature on both heating and cooling sweeps so that any hysteresis in the first order magnetostructural phase transition can be determined.</li>
</ol>

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The authors declare that they have no competing financial interests.

Here we have demonstrated that this method can be used to prepare epilayer samples of FeRh of good crystallographic quality and a high degree of B2 chemical ordering. The method is suited to the preparation of a wide variety of epitaxial metallic layers, including ordered alloys. Whilst we have used the B2-ordered FeRh alloy as an example here, as it shows a dramatic phase transition when the stoichiometry is correct and chemical ordering is present, this method can also be used for other materials. For instance, both FePd and FePt have L10 phases, which leads to a very strong uniaxial magnetocrystalline anisotropy. We have successfully grown this material in the past, showing domain wall resistance in FePt8, and large anomalous Hall effects in both FePd and FePt10. With an appropriate adjustment of growth temperatures and rates and a suitable choice of substrate, this method ought to be useful for preparing a wide variety of different magnetic and non-magnetic metal epilayers displaying chemical order.
Nevertheless, a limitation of this approach is the need for a single crystal substrate to achieve epitaxy. This means difficulties will be encountered in performing experiments such as plan-view transmission electron or X-ray microscopy or integration into a technology built on another substrate wafer such as the near-ubiquitous Si. A possible means to get around this problem is to grow a thin MgO layer on which the FeRh can then be deposited. This can yield out-of-plane texture that nucleates local epitaxial growth on top of each MgO grain37. Remarkably, it is possible to grow a thin MgO layer that has both (001) texture and in-plane crystallographic alignment on an amorphous surface using a method with an ion-beam assist gun that is oriented at 45&#176; to the substrate normal 38. This could permit growth of B2-ordered FeRh on e.g. electron or X-ray transparent Si3N4 membranes, which are capable of surviving the high growth temperatures required in our protocol, or on the native oxide layer of a Si wafer.
Further refinements of the method include the use of B2-ordered underlayers, such as NiAl39, to promote B2-ordering in the FeRh epilayer when it is ultrathin, or its use to build heterostructures involving multiple chemically-ordered layers37. Since FeRh can be doped on the Rh site to adjust the transition temperature TT up (for example using Ir40, 41 or Pt40, 42) or down (e.g. using Au40, 27 or Pd40, 43), the creation of doping profiles in FeRh layers can lead to designed-in magnetic profiles as the sample is heated and cooled. This opens up a route to generating purely magnetic stratification of an epilayer in a controllable way44.

The central paradigm of the microelectronics industry is the method of planar processing: the sequential deposition and patterning of thin films on the surface of a wafer of substrate material. Very often, the substrate is a single crystal, and the films need to be epitaxial, that is to say in crystal register with the underlying substrate. With semiconductor materials, this is typically achieved either using molecular beam epitaxy (MBE) in a laboratory setting1 or metalorganic vapor phase epitaxy (MOVPE) in manufacturing2.
Whilst the epitaxial growth of metals by MBE is possible, they are easily deposited by sputtering, and this is the most common method for the deposition of thin magnetic films in both research and industrial settings. Whilst this method is commonly associated with the growth of polycrystalline films, epitaxial growth on a single crystal substrate is possible under certain conditions3. These generally include a raised substrate temperature (at least for the initial layers), a slow deposition rate, and a low vacuum chamber base pressure. This approach has been used to prepare giant magnetoresistance multilayer materials4, 5, for instance.
In our own laboratory, we have used epitaxial sputtering to prepare a variety of magnetic materials on single crystal substrates. It has been possible to grow CoFe alloy epilayers on GaAs(001), for instance, by selecting the lattice-matched Co70Fe30 composition6. This material is a solid solution, where the Co and Fe atoms randomly populate the bcc lattice sites. We have also grown chemically ordered magnetic alloys, where the different atomic species are required to take up particular lattice sites. The growth protocol we shall describe here was initially developed for the growth of L10-ordered FePd and FePt alloys, which are of interest since they possess a very high magnetocrystalline anisotropy7. We have studied the relationship between ballistic and diffusive spin-polarized transport8, 9 and the anomalous Hall effect10 in these materials, which are of comparable quality to layers grown by MBE11.
Here we will illustrate our epitaxial growth method using the example of B2-ordered FeRh epilayers. Fe and Rh will form alloys at any composition, however a B2-ordered compound is the equilibrium state for stoichiometries in the near-equiatomic range 49-53% atomic Fe12. This so-called &#945;&rdquo;-phase is an antiferromagnet (AF) that exhibits a first-order phase transition on heating, becoming an &#945;&#39;-phase ferromagnet (FM) around TT = 350&#8594;400K13, 14, 15. This metamagnetic transition between the two different but both fully ordered magnetic states (type II AF 16 and FM) is accompanied by an isotropic 1% volume expansion in the B2 lattice17, 18, a large entropy release19, a large drop in the resistivity14, and a large increase in the carrier concentration20. Neutron diffraction21, 16 and more recently XMCD measurements22 indicate that part of the 3.3 &#956;B magnetic moment centered on the Fe in the AF phase is transferred to the Rh in the FM phase, with &#956;Fe ~2.2 &#956;B and &#956;Rh ~0.6 &#956;B. The Curie temperature for the FM &#945;&#39; phase is ~670 K14, comparable to the Curie temperature of alloys with x&#62;0.5323. The metamagnetic transition temperature TT is highly sensitive to the composition &#215; in Fe&#215;Rh1-&#215;23, 24, and is suppressed by ~8 K/T of applied magnetic field25, 15. This rich array of physical behavior depends critically on achieving the proper B2-ordered structure and so permits a wide variety of measurement techniques to be deployed to detect proper chemical ordering in a specimen, making it a convenient example to demonstrate a method of growing high-quality ordered alloy epilayers.

<table border="1">
	<tbody>
		<tr>
			<td>Name of Reagent/Material</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Sputter Deposition System</td>
			<td>Kurt J. Lesker Company</td>
			<td>Bespoke</td>
			<td></td>
		</tr>
		<tr>
			<td>MgO Single Crystal Substrate</td>
			<td>Pi-Kem</td>
			<td>Single-sided epi-polished</td>
			<td>(001) orientation</td>
		</tr>
		<tr>
			<td>FeRh sputtering target</td>
			<td>Pi-Kem</td>
			<td>Bespoke</td>
			<td>50 mm diameter</td>
		</tr>
		<tr>
			<td>Transmission Electron Microscope</td>
			<td>FEI</td>
			<td>Tecnai TF20</td>
			<td></td>
		</tr>
		<tr>
			<td>X-ray Diffractometer</td>
			<td>Br&#252;ker</td>
			<td>D8 Discover</td>
			<td></td>
		</tr>
		<tr>
			<td>SQUID Magnetometer</td>
			<td>Quantum Design</td>
			<td>MPMS-XL 5</td>
			<td></td>
		</tr>
	</tbody>
</table>

sputter growth and characterization of metamagnetic b2-ordered ferh epilayers

Chemically ordered alloys are useful in a variety of magnetic nanotechnologies. They are most conveniently prepared at an industrial scale using sputtering techniques. Here we describe a method for preparing epitaxial thin films of B2-ordered FeRh by sputter deposition onto single crystal MgO substrates. Deposition at a slow rate onto a heated substrate allows time for the adatoms to both settle into a lattice with a well-defined epitaxial relationship with the substrate and also to find their proper places in the Fe and Rh sublattices of the B2 structure. The structure is conveniently characterized with X-ray reflectometry and diffraction and can be visualised directly using transmission electron micrograph cross-sections. B2-ordered FeRh exhibits an unusual metamagnetic phase transition: the ground state is antiferromagnetic but the alloy transforms into a ferromagnet on heating with a typical transition temperature of about 380 K. This is accompanied by a 1% volume expansion of the unit cell: isotropic in bulk, but laterally clamped in an epilayer. The presence of the antiferromagnetic ground state and the associated first order phase transition is very sensitive to the correct equiatomic stoichiometry and proper B2 ordering, and so is a convenient means to demonstrate the quality of the layers that can be deposited with this approach. We also give some examples of the various techniques by which the change in phase can be detected.

We have prepared many FeRh samples according to this protocol. In this section we show typical results obtained using the most common characterization procedures for a selection of representative samples. Results such as these are expected for samples in the thickness range 20-50 nm. Other methods we have used to characterize our material in more depth include X-ray magnetic circular dichroism28, grazing incidence X-ray scattering29, and polarized neutron reflectometry30. We have also studied the effects of doping the alloy with Au27. Further data on the properties that can be expected from this material can be found in those reports, and the references contained therein.
The structure of one of our epilayers is shown in detail in the transmission electron micrographs shown in Figure 1. The sample cross-section was prepared by the conventional dimpling and ion polishing technique- a standard specimen preparation method (see, for instance Williams and Carter31)- and observed using a 200 kV electron beam. The overall layer structure can be seen in Figure 1(a). In this case, a 30 nm FeRh film was epitaxially grown onto an MgO substrate, followed by a ~4 nm Cr layer and a ~1 nm thick Al layer. (The Cr layer was included here for a particular experiment and is not needed in general). The roughness of the FeRh/MgO and FeRh/Cr interfaces are 0.6 nm and 2.8 nm, respectively, as measured from the image. In Figure 1(b) a high resolution micrograph of the MgO/FeRh interface is shown. The epitaxial relationship as confirmed from selected area diffraction is FeRh[100](001)||MgO[110](001). The lattice matching across the interface demonstrates the high quality of the epitaxial growth. We do not show the data here but have used energy dispersive X-ray spectroscopy in the TEM to check the composition on a selection of samples: it has always been equiatomic to within the uncertainty of the measurement.
&#935;-ray reflectometry data are shown in Figure 2 for a nominally 25 nm thick FeRh epilayer capped with a thin, polycrystalline layer of Al. The measurement was performed in a standard two-circle diffractometer in the Bragg-Brentano geometry, using Cu K&#945; radiation (&#955; = 0.1541 nm), with a Ni filter to attenuate the K&#945; radiation. The pronounced Kiessig fringes, which arise from the interference of X-ray beams that reflect from the various interfaces in the layer stack, indicate that those interfaces are smooth and well-correlated. The solid red line shows a fit to the data that has been performed using the GenX software32. The best fit parameters for the multilayer structure are shown in Table 1. The fact that a portion of the Al layer will have oxidized and self-passivated once the sample is exposed to the air is accounted for in the model.
&#935;-ray diffraction data for the same sample are shown in Figure 3, collected on the same instrument. The (002) reflection of the MgO substrate is strong and sharp enough to just resolve the Cu K&#945;1and K&#945;2 lines. The FeRh layers shows both (001) and (002) reflections. There is some broadening due to the finite thickness of the epilayer and strain gradients. The (002) FeRh B2 peak is centered at 2&#952; = 61.3&#176;&#177;0.02, yielding an average out-of-plane lattice constant of 3.02&#177;0.05 &#197;. It is possible to determine the chemical order parameter S of the FeRh B2 structure from the relative integrated intensities of these two peaks. This quantity is defined as S = rFe+rRh -1, where is the fraction of Fe(Rh) sites occupied by Fe(Rh) atoms33. A brief inspection of the formula shows that when rFe = rRh = 1 and the structure is perfect, S = 1, whereas when rFe = rRh = 0.5, so that all the lattice sites are randomly occupied, S = 0. The reason is that when S = 0 the site-averaged structure is bcc, for which the structure factor forbids the (001) reflection, whereas when S = 1 the structure is primitive cubic, for which the (001) reflection is allowed. This means that in practical terms, <img alt="Equation 1" fo:content-width="2.5in" src="/files/ftp_upload/50603/50603eq1.jpg" width="230px" />, where <img alt="Equation 2" fo:content-width=".2in" src="/files/ftp_upload/50603/50603eq2.jpg" width="20px" /> and <img alt="Equation 3" fo:content-width=".2in" src="/files/ftp_upload/50603/50603eq3.jpg" width="20px" /> are the experimental and theoretical intensities of the (00I) Bragg reflection, respectively33. For the calculation of the theoretical intensities the Debye-Waller factors from EXAFS measurements on FeRh were used34. In this case, S = 0.855&#177;0.001, typical for a sputtered thin film of this material.
The metamagnetic phase transition can be detected in several ways. Its presence indicates the correct equiatomic stoichiometry and B2-ordering of the lattice. The lattice expansion that accompanies the metamagnetic transition may be detected by the shift in the position of the Bragg peaks27; however, this requires a diffractometer with a heater stage.
Perhaps the most obvious method is to detect the appearance of the ferromagnetic moment as the sample is heated through TT. This can be done using any temperature dependent magnetometer with sufficient sensitivity, for instance using the magneto-optical Kerr effect or a vibrating sample magnetometer. In Figure 4 we show the temperature dependence of the magnetization M, measured using a superconducting quantum interference device (SQUID) magnetometer. Measurements were made in the temperature range of 275-400 K with a temperature sweep rate of 2 K/min. The curve shown displays the anticipated AF &#8594; FM transition (heating) and FM &#8594; AF transition (cooling) with a 15 K thermal hysteresis. This measurement was made at high field (50 kOe) and yielded a transition temperature TT &#8776; 365 K. The transition temperature is field-dependent, as a higher magnetic field reduces the free energy of the FM phase with respect to the AF phase. Typically dTT/dH &#8776; 0.8 mK/Oe14,15, 27. Note that the magnetic moment in the AF phase is not quite zero, but is a few tens of emu/cm3 when averaged over the volume of the entire sample. This moment resides in the near-interface regions of the FeRh epilayer, which remain ferromagnetic (albeit with a reduced magnetization) when the bulk of the sample transforms into the AF phase28, 30.
A way to detect the transition that uses simpler equipment and is often used in our laboratory is to make an electron transport measurement. The simplest measurement is of the resistivity &#961; of the film, since &#961; in the FM phase is much less than in the AF phase35, 36, 20. The temperature dependence of &#961; for the same 25 nm FeRh epilayer for which X-ray data were shown is plotted in Figure 5, measured using a standard four-point probe method: spring-loaded, gold-plated pins were pressed on to the sample surface to make contact to the sample, which was mounted on a heater stage in a small custom vacuum chamber to prevent any sample oxidation when hot. A linear, metallic &#961;(T) dependence is seen in both the AF and FM phases, but there is a marked drop in resistivity between the two. The hysteresis seen in Figure 5 is a clear fingerprint of the magnetostructural phase transition taking place and is a convenient method to measure the transition temperature, which is given by the minimum point in d&#961;/dT (shown in the inset of Figure 5). Another easily measured transport property, the Hall effect, can also be used to confirm the presence of the transition, as there is a large difference in the Hall coefficient between the two phases20.
<img alt="Figure 1" fo:content-width="5in" src="/files/ftp_upload/50603/50603fig1highres.jpg" width="600" /> 
Figure 1. Transmission electron micrographs of an FeRh epilayer on an MgO substrate. (a) Image demonstrating the structure of the layer. The FeRh is 30 nm thick with a further ~4 nm Cr layer and ~1 nm Al cap deposited on top. The amorphous region at the top of the image is an epoxy resin used during cross-section sample preparation. (b) A high resolution image of the MgO FeRh interface. The epitaxial matching across the interface is seen here, and the associated relationship, as confirmed from selected area diffraction, is FeRh[100](001)||MgO[110](001). <a href="https://www.jove.com/files/ftp_upload/50603/50603fig1highres.jpg" target="_blank">Click here to view larger figure</a>
<img alt="Figure 2" fo:content-width="5in" src="/files/ftp_upload/50603/50603fig2.jpg" width="600" /> 
Figure 2. X-ray reflectometry spectrum from a 25 nm thick FeRh epilayer capped with polycrystalline Al. The solid line is a fit as described in the text, using the parameters given in Table 1. The inset shows the scattering length density profile associated with that set of fitting parameters. <a href="https://www.jove.com/files/ftp_upload/50603/50603fig2highres.jpg" target="_blank">Click here to view larger figure</a>
 
<img alt="Figure 3" fo:content-width="5in" src="/files/ftp_upload/50603/50603fig3.jpg" width="500" /> 
Figure 3. X-ray diffraction spectrum from a 25 nm thick FeRh epilayer capped with polycrystalline Al. The presence of the (001) FeRh peak indicates that B2 ordering has taken place. The chemical order parameter is S = 0.855&#177;0.001, as determined using the method described in the text. <a href="https://www.jove.com/files/ftp_upload/50603/50603fig3highres.jpg" target="_blank">Click here to view larger figure</a>
 
<img alt="Figure 4" fo:content-width="5in" src="/files/ftp_upload/50603/50603fig4.jpg" width="500" /> 
Figure 4. Temperature dependence of magnetization M of a 50 nm thick FeRh epilayer capped with polycrystalline Al. These data were taken with a 50 kOe field applied in the film plane. The transition temperature TT is seen to be ~365 K with a hysteresis width of about 15 K. <a href="https://www.jove.com/files/ftp_upload/50603/50603fig4highres.jpg" target="_blank">Click here to view larger figure</a>
 
<img alt="Figure 5" fo:content-width="5in" src="/files/ftp_upload/50603/50603fig5.jpg" width="500" /> 
Figure 5. Temperature dependence of the resistivity &#961; of a 25 nm thick FeRh layer capped with Al. Inset is the derivative of &#961; with respect to temperature T. The transition temperature TT is seen to be 447 K on warming into the FM phase and 375 K on cooling into the AF phase. <a href="https://www.jove.com/files/ftp_upload/50603/50603fig5highres.jpg" target="_blank">Click here to view larger figure</a>
 
<table border="1">
	<tbody>
		<tr>
			<td>Layer </td>
			<td>Density (atoms/nm3) </td>
			<td>Thickness (nm) </td>
			<td>Roughness (nm) </td>
		</tr>
		<tr>
			<td>Al2O3 passivation layer</td>
			<td>25.5&#177;0.9</td>
			<td>2.18&#177;0.08</td>
			<td>1.0&#177;0.1</td>
		</tr>
		<tr>
			<td>Al cap</td>
			<td>60.6&#177;0.6</td>
			<td>0.91&#177;0.02</td>
			<td>0.6&#177;0.2</td>
		</tr>
		<tr>
			<td>FeRh epilayer</td>
			<td>38.7&#177;0.3</td>
			<td>25.09&#177;0.06</td>
			<td>0.400&#177;0.002</td>
		</tr>
		<tr>
			<td>MgO substrate</td>
			<td>53.4&#177;1.3</td>
			<td>&#8734;</td>
			<td>0.1761&#177;0.0003</td>
		</tr>
	</tbody>
</table>
Table 1. Fitting parameters for the X-ray reflectivity spectrum shown in Figure 2, leading to the scattering length density profile shown in the inset of that Figure.

Watch this Scientific Journal Video about Sputter Growth and Characterization of Metamagnetic B2-ordered FeRh Epilayers at JoVE.com

Sputter Growth and Characterization of Metamagnetic B2-ordered FeRh Epilayers

A method to prepare epitaxial layers of ordered alloys by sputtering is described. The B2-ordered FeRh compound is used as an example, as it displays a metamagnetic transition that depends sensitively on the degree of chemical order and the exact composition of the alloy.

Chemically ordered alloys are useful in a variety of magnetic nanotechnologies. They are most conveniently prepared at an industrial scale using ...

sputter-growth-characterization-metamagnetic-b2-ordered-ferh

Research

JoVE Journal

Engineering

Simulation, Fabrication and Characterization of THz Metamaterial Absorbers

Metamaterials (MM), artificial materials engineered to have properties that may not be found in nature, have been widely explored since the first theoretical1 and experimental demonstration2 of their unique properties. MMs can provide a highly controllable electromagnetic response, and to date have been demonstrated in every technologically relevant spectral range including the optical3, near IR4, mid IR5 , THz6 , mm-wave7 , microwave8 and radio9 bands. Applications include perfect lenses10, sensors11, telecommunications12, invisibility cloaks13 and filters14,15. We have recently developed single band16, dual band17 and broadband18 THz metamaterial absorber devices capable of greater than 80% absorption at the resonance peak. The concept of a MM absorber is especially important at THz frequencies where it is difficult to find strong frequency selective THz absorbers19. In our MM absorber the THz radiation is absorbed in a thickness of ~ &lambda;/20, overcoming the thickness limitation of traditional quarter wavelength absorbers. MM absorbers naturally lend themselves to THz detection applications, such as thermal sensors, and if integrated with suitable THz sources (e.g. QCLs), could lead to compact, highly sensitive, low cost, real time THz imaging systems.

This protocol outlines the simulation, fabrication and characterization of THz metamaterial absorbers. Such absorbers, when coupled with an appropriate sensor, have applications in THz imaging and spectroscopy.

Metamaterials (MM),  artificial materials engineered to have properties that may not be found in  nature, have been widely explored since the first ...

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals1-4 and the enhancement of both linear5-7 and nonlinear devices.8-11
Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically12 and electrically13 pumped ultra-low threshold lasers and the enhancement of nonlinear effects.14-16 Furthermore, passive filters17 and modulators18-19 have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption.
To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues.
The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum20-21 and interferometric techniques.22-25 Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.26 Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans.
When characterising photonic crystal cavities, techniques involving internal sources21 or external waveguides directly coupled to the cavity27 impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.28 and further developed by Galli et al.29

Use of photonic crystal slow light waveguides and cavities has been widely adopted by the photonics community in many differing applications. Therefore fabrication and characterization of these devices are of great interest. This paper outlines our fabrication technique and two optical characterization methods, namely: interferometric (waveguides) and resonant scattering (cavities).

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view ...

Design, Fabrication, and Experimental Characterization of Plasmonic Photoconductive Terahertz Emitters

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on photoconduction, which has been one of the most commonly used techniques for terahertz generation 1-8. Terahertz generation in a photoconductive emitter is achieved by pumping an ultrafast photoconductor with a pulsed or heterodyned laser illumination. The induced photocurrent, which follows the envelope of the pump laser, is routed to a terahertz radiating antenna connected to the photoconductor contact electrodes to generate terahertz radiation. Although the quantum efficiency of a photoconductive emitter can theoretically reach 100%, the relatively long transport path lengths of photo-generated carriers to the contact electrodes of conventional photoconductors have severely limited their quantum efficiency. Additionally, the carrier screening effect and thermal breakdown strictly limit the maximum output power of conventional photoconductive terahertz sources. To address the quantum efficiency limitations of conventional photoconductive terahertz emitters, we have developed a new photoconductive emitter concept which incorporates a plasmonic contact electrode configuration to offer high quantum-efficiency and ultrafast operation simultaneously. By using nano-scale plasmonic contact electrodes, we significantly reduce the average photo-generated carrier transport path to photoconductor contact electrodes compared to conventional photoconductors 9. Our method also allows increasing photoconductor active area without a considerable increase in the capacitive loading to the antenna, boosting the maximum terahertz radiation power by preventing the carrier screening effect and thermal breakdown at high optical pump powers. By incorporating plasmonic contact electrodes, we demonstrate enhancing the optical-to-terahertz power conversion efficiency of a conventional photoconductive terahertz emitter by a factor of 50 10.

We describe methods for the design, fabrication, and experimental characterization of plasmonic photoconductive emitters, which offer two orders of magnitude higher terahertz power levels compared to conventional photoconductive emitters.

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on ...

Writing and Low-Temperature Characterization of Oxide Nanostructures

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions. Oxide interfaces exhibit a wide range of properties that can be controlled include conduction, piezoelectric behavior, ferromagnetism, superconductivity and nonlinear optical properties. Recently, methods for controlling these properties at extreme nanoscale dimensions have been discovered and developed. Here are described explicit step-by-step procedures for creating LaAlO3/SrTiO3 nanostructures using a reversible conductive atomic force microscopy technique. The processing steps for creating electrical contacts to the LaAlO3/SrTiO3 interface are first described. Conductive nanostructures are created by applying voltages to a conductive atomic force microscope tip and locally switching the LaAlO3/SrTiO3 interface to a conductive state. A versatile nanolithography toolkit has been developed expressly for the purpose of controlling the atomic force microscope (AFM) tip path and voltage. Then, these nanostructures are placed in a cryostat and transport measurements are performed. The procedures described here should be useful to others wishing to conduct research in oxide nanoelectronics.

Oxide nanostructures provide new opportunities for science and technology. The interfacial conductivity between LaAlO3 and SrTiO3 can be controlled with near-atomic precision using a conductive atomic force microscopy technique. The protocol for creating and measuring conductive nanostructures at LaAlO3/SrTiO3 interfaces is demonstrated.

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions.  Oxide ...

Patterning via Optical Saturable Transitions - Fabrication and Characterization

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical Saturable Transitions (POST). In this technique the chemical properties of organic photochromic molecules that undergo single-photon reactions are exploited, enabling rapid top-down nanopatterning over large areas at low light intensities, thereby, allowing for the circumvention of the far-field diffraction barrier.4 Simple, cost-effective, high throughput and resolution alternatives to nanopatterning are being explored, such as, two-photon polymerization5,6, beam pen lithography (BPL)7, scanning electron beam lithography (SEBL), and focused ion beam (FIB) patterning. However, multi-photon approaches require high light intensities, which limit their potential for high throughput and offer low image contrast. Although, electron and ion beam lithographic processes offer increased resolution, the serial nature of the process is limited to slow writing speeds, which also prevents patterning of features over large areas. Beam-pen lithography is an approach towards parallel near-field optical lithography. However, the gap between the source of the beam and the surface of the photoresist needs to be controlled extremely precisely for good pattern uniformity and this is very challenging to accomplish for large arrays of beams. Patterning via Optical Saturable Transitions (POST) is an alternative optical nanopatterning technique for patterning sub-wavelength features1-3. Since this technique uses single photons instead of electrons, it is extremely fast and does not require high light intensities1-3, opening the door to massive parallelization.

We report that the diffraction limit of conventional optical lithography can be overcome by exploiting the transitions of organic photochromic derivatives induced by their photoisomerization at low light intensities.1-3 This paper outlines our fabrication technique and two locking mechanisms, namely: dissolution of one photoisomer and electrochemical oxidation.

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical ...

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices have used devices prepared by spin coating, a process that is not amenable to large-scale fabrication. This mismatch in device fabrication makes it difficult to translate quantitative results obtained in the laboratory to the commercial level, making optimization difficult. Using a mini-slot die coater, this mismatch can be resolved by translating the commercial process to the laboratory and characterizing the structure formation in the active layer of the device in real time and in situ as films are coated onto a substrate. The evolution of the morphology was characterized under different conditions, allowing us to propose a mechanism by which the structures form and grow. This mini-slot die coater offers a simple, convenient, material efficient route by which the morphology in the active layer can be optimized under industrially relevant conditions. The goal of this protocol is to show experimental details of how a solar cell device is fabricated using a mini-slot die coater and technical details of running in situ structure characterization using the mini-slot die coater.

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Here, we present a protocol to fabricate organic thin film solar cells using a mini-slot die coater and related in-line structure characterizations using synchrotron scattering techniques.

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices ...

Protocol of Electrochemical Test and Characterization of Aprotic Li-O2 Battery

We demonstrate a method for electrochemical testing of an aprotic Li-O2 battery. An aprotic Li-O2 battery is made of a Li-metal anode, an aprotic electrolyte, and an O2-breathing cathode. The aprotic electrolyte is a solution of lithium salt with aprotic solvent; and porous carbon is commonly used as the cathode substrate. To improve the performance, an electrocatalyst is deposited onto the porous carbon substrate by certain deposition methods, such as atomic layer deposition (ALD) and wet-chemistry reaction. The as-prepared cathode materials are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray absorption near edge structure (XANES). A Swagelok-type cell, sealed in a glass chamber filled with pure O2, is used for the electrochemical test on a battery test system. The cells are tested under either capacity-controlled mode or voltage controlled mode. The reaction products are investigated by electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and Raman spectroscopy to study the possible pathway of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). This protocol demonstrates a systematic and efficient arrangement of routine tests of the aprotic Li-O2 battery, including the electrochemical test and characterization of battery materials.

Protocol of Electrochemical Test and Characterization of Aprotic Li-O2 Battery

A protocol for the electrochemical testing of an aprotic Li-O2 battery with the preparation of electrodes and electrolytes and an introduction of the frequently used methods of characterization is presented here.

We demonstrate a method for electrochemical testing of an aprotic Li-O2 battery. An aprotic Li-O2 battery is made of a Li-metal anode, an aprotic ...

Fabrication and Characterization of Superconducting Resonators

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.

Superconducting microwave resonators are of interest for detection of light, quantum computing applications and materials characterization. This work presents a detailed procedure for fabrication and characterization of superconducting microwave resonator scattering parameters.

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors ...

Production and Characterization of Vacuum Deposited Organic Light Emitting Diodes

A method for producing simple and efficient thermally-activated delayed fluorescence organic light-emitting diodes (OLEDs) based on guest-host or exciplex donor-acceptor emitters is presented. With a step-by-step procedure, readers will be able to repeat and produce OLED devices based on simple organic emitters. A patterning procedure allowing the creation of personalized indium tin oxide (ITO) shape is shown. This is followed by the evaporation of all layers, encapsulation and characterization of each individual device. The end goal is to present a procedure that will give the opportunity to repeat the information presented in cited publication but also using different compounds and structures in order to prepare efficient OLEDs.

A protocol for the production of simple structured organic light-emitting diodes (OLEDs) is presented.

A method for producing simple and efficient thermally-activated delayed fluorescence organic light-emitting diodes (OLEDs) based on guest-host or exciplex ...

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LAO thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. Although electrostatic mechanisms have been proposed in the past to describe the existence of this critical thickness, the importance of chemical defects has been recently accentuated. Here, we describe the growth of metal/LAO/STO heterostructures in an ultra-high vacuum (UHV) cluster system combining pulsed laser deposition (to grow the LAO), magnetron sputtering (to grow the metal) and X-ray photoelectron spectroscopy (XPS). We study step by step the formation and evolution of the q2DES and the chemical interactions that occur between the metal and the LAO/STO. Additionally, magnetotransport experiments elucidate on the transport and electronic properties of the q2DES. This systematic work not only demonstrates a way to study the electrostatic and chemical interplay between the q2DES and its environment, but also unlocks the possibility to couple multifunctional capping layers with the rich physics observed in two-dimensional electron systems, allowing the fabrication of new types of devices.

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures

We fabricate metal/LaAlO3/SrTiO3 heterostructures using a combination of pulsed laser deposition and in situ magnetron sputtering. Through magnetotransport and in situ X-ray photoelectron spectroscopy experiments, we investigate the interplay between electrostatic and chemical phenomena of the quasi two-dimensional electron gas formed in this system.