Name: optimizing magnetic force microscopy resolution and sensitivity to visualize nanoscale magnetic domains
Uploaded: 2022-07-20T14:40:00
Description: Watch this Scientific Journal Video about Optimizing Magnetic Force Microscopy Resolution and Sensitivity to Visualize Nanoscale Magnetic Domains at JoVE.com

All AFM/MFM imaging was performed in the Boise State University Surface Science Laboratory (SSL). The glovebox AFM system used in this work was purchased under National Science Foundation Major Research Instrumentation (NSF MRI) Grant Number 1727026, which also provided partial support for PHD, ACP, and OOM. Partial support for OOM was further provided by NSF CAREER Grant Number 1945650. Research at the University of Delaware, including fabrication and electron microscopy characterization of artificial spin-ice structures, was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0020308. The authors thank Drs. Medha Veligatla and Peter M&#252;llner for helpful discussions and preparation of the Ni-Mn-Ga samples shown here, as well as Dr. Corey Efaw and Lance Patten for their contributions to the MFM standard operating procedure including in the Supplementary File 1.

NOTE: In addition to the protocol below, a detailed step-by-step MFM standard operating procedure (SOP) specific to the instrument used here and geared towards general MFM imaging is included as Supplementary File 1. To supplement the video portion of this manuscript, the SOP includes images of the probe holder, tip magnetizer and magnetization procedure, software settings, etc.
1. MFM probe preparation and installation
<ol>
	<li>Open the AFM control softwar...

High-resolution MFM imaging requires that a corresponding high-resolution, high-fidelity topography scan first be acquired for each line. This topography scan is typically obtained through intermittent contact or tapping mode AFM, which employs an amplitude modulation feedback system to image sample topography47. The fidelity of the topography scan can be optimized by adjusting the amplitude set point of the cantilever and feedback gains as described in the Protocol. The amplitude setpoint is crit...

Magnetic force microscopy (MFM), a scanning probe microscopy (SPM) derivative of atomic force microscopy (AFM), enables imaging of the relatively weak but long-range magnetic forces experienced by a magnetized probe tip as it travels above a sample surface1,2,3,4,5. AFM is a non-destructive characterization technique that employs a nanometer-scale tip at the end of a pliable cantilever to map surface topography6 as well as measure material (e.g., mechanical, electrical, and magnetic) properties7,8,9 with nanoscale resolution. Deflection of the cantilever due to tip-sample interactions of interest is measured via reflection of a laser off the back of the cantilever and into a position-sensitive photodiode10. High-resolution imaging of a material&#39;s local magnetic properties via MFM provides the unique opportunity to characterize the magnetic field strength and orientation in novel materials, structures, and devices at the nanoscale4,5,11,12,13,14,15,16,17. To perform MFM, an AFM probe whose tip has been magnetized vertically (i.e., perpendicular to the probe cantilever and sample surface) is mechanically oscillated at its natural resonance frequency at a fixed height above the sample surface. Resultant changes in oscillation amplitude (less sensitive, and hence less common), frequency, or phase (described here) are then monitored to measure magnetic field strength qualitatively. More specifically, frequency modulation MFM produces a map of shifts in the oscillation frequency or phase, proportional to the magnitude and sign of the magnetic force gradient experienced by the probe. In order to maintain a constant height above the sample during MFM measurements, a dual-pass mode of operation is typically employed. The sample topography is first mapped via standard AFM techniques, followed by interleaved MFM imaging of each sequential scan line at a user-determined lift height (tens to hundreds of nm) off the sample surface. Employing such an interleaved dual-pass acquisition mode enables separation of the short-range tip-sample van der Waals interactions used to map the topography from the relatively longer-range magnetic forces experienced during the interleaved lift mode pass. However, MFM spatial resolution increases with decreasing lift height18, so there is an inherent tension between increasing MFM resolution and avoiding topographical artifacts due to van der Waals forces. Likewise, MFM sensitivity is proportional to the oscillation amplitude during the lift mode pass, but the maximum allowable oscillation amplitude is limited by the lift height and rapid changes in sample topography (i.e., high aspect ratio features).
Recent studies have highlighted the wealth of opportunities associated with the application of nanomagnetism and nanomagnonics, developed via artificial spin-ice (ASI) structures and magnonic crystals, as functioning devices for logic, computation, encryption, and data storage19,20,21,22. Composed of nanomagnets arranged in distinct extended lattice formations, artificial spin ices exhibit emergent magnetic dipoles or monopoles that can be controlled via an external stimulus19,20,23,24,25. In general, ASIs favor a moment configuration that minimizes the energy (e.g., in a two-dimensional (2D) square ASI, two moments point in and two point out of every vertex), with the low energy microstates following rules analogous to crystalline spin-ice materials21,26,27,28. Similarly, a recent MFM-enabled study demonstrated a three-dimensional (3D) ASI lattice system constructed from rare-earth spins situated on corner-sharing tetrahedra, where two spins point toward the center of the tetrahedra and two spins point out, resulting in two equal and opposite magnetic dipoles and hence a net zero magnetic charge at the tetrahedra centers23. Depending upon the alignment of an applied magnetic field relative to the sample surface, significant differences in the magnetic ordering and correlation length were observed. The alignment and control of ASI dipoles thus warrant further investigation. Methods for measuring ASI magnetic field distributions have included using a magneto-optical noise spectrometer29 or X-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM)25; however to achieve spatial resolutions equal to or greater than that of MFM with XMCD-PEEM, extremely short wavelengths (i.e., high energy X-rays) are required. MFM offers a much simpler characterization technique that does not require exposure of samples to potentially damaging high energy X-rays. Additionally, MFM has been used to not only characterize ASI microstates21,23,27, but also for topological defect driven magnetic writing using high magnetic moment tips30. Accordingly, MFM can play a vital role in furthering ASI research and development, specifically through its ability to correlate sample topography with magnetic field strength and orientation, thereby revealing the magnetic dipoles associated with specific topographic features (i.e., ASI lattice elements).
High-resolution MFM likewise provides significant insight into the relationship between the structure of ferromagnetic shape memory alloys and their nanoscale magnetomechanical properties14,17,31,32,33. Ferromagnetic shape memory alloys, commonly referred to as magnetic shape memory alloys (MSMAs), exhibit large (up to 12%) magnetic field induced strains, carried through twin boundary motion29,33,34,35. MFM techniques have been used to investigate the complex relationships between twinning during deformation and martensitic transformation, indentation, micro-pillar deformation, and nanoscale magnetic responses of MSMAs15,16,17,36. Of particular note, MFM has been combined with nanoindentation to create and read a four-state nanoscale magnetomechanical memory17. Similarly, next-generation magnetic recording technologies are being pursued via heat-assisted magnetic recording (HAMR), achieving linear densities of 1975 kBPI and track densities of 510 kTPI37. The increased areal density required to enable greater, more compact data storage has resulted in a significant reduction in the defined track pitch of HAMR technologies, accentuating the need for high-resolution MFM imaging.
In addition to ASIs and MSMAs, MFM has been successfully used to characterize various magnetic nanoparticles, nanoarrays, and other types of magnetic samples3,38,39. However, ultimate MFM resolution and sensitivity are limited both by things beyond the user&#39;s control (e.g., AFM detection electronics, MFM probe technology, underlying physics, etc.) and by choice of imaging parameters and environment. Meanwhile, feature sizes in magnetic devices continue to decrease40,41, creating smaller magnetic domains, thus making MFM imaging increasingly more challenging. Additionally, the magnetic dipoles of interest are not always oriented out-of-plane, parallel to the magnetization vector of the probe. High-resolution imaging of the stray fields emanating from the ends of in-plane or nearly in-plane oriented dipoles, as is the case in the ASI structures shown here, requires greater sensitivity. Achieving high-resolution MFM images, especially of such in-plane magnetized samples composed of nanoscale magnetic domains, thus depends on appropriate choice of MFM probe (e.g., thickness, coercivity, and moment of the magnetic coating, which can at times be at odds with improving sensitivity or lateral resolution18 or preservation of the sample&#39;s magnetic alignment30), imaging parameters (e.g., lift height and oscillation amplitude, as mentioned above, as well as minimizing tip coating wear during topography line imaging), and sample quality (e.g., surface roughness and contamination, including polishing debris or surface water due to ambient humidity). In particular, the presence of water adsorbed on the sample surface due to ambient humidity can introduce strong tip-sample van der Waals forces that can significantly interfere with measuring magnetic forces and limit the minimum achievable lift height for MFM measurements. MFM operation within an inert atmosphere glovebox eliminates nearly all surface contaminants, allowing for lower lift heights and higher resolution coupled with greater sensitivity. Accordingly, in the sample examples shown here, an AFM system housed in a custom inert atmosphere glovebox filled with argon (Ar) containing &#60;0.1 ppm oxygen (O2) and water (H2O) has been employed to enable extremely low lift heights (down to 10 nm). This subsequently enables exquisitely high-resolution MFM imaging capable of resolving alternating magnetic domains &#60;200 nm wide within a larger crystallographic twin and magnetic dipoles (nanoscale bar magnets) &#60;100 nm wide and ~250 nm long.
This article explains how to acquire high-resolution, high-sensitivity MFM images by combining the use of an inert atmosphere glovebox with careful sample preparation and optimal choice of imaging parameters. The described methods are especially valuable for imaging in-plane oriented dipoles, which are traditionally difficult to observe, and therefore exemplary high-resolution MFM images are presented of both Ni-Mn-Ga MSMA crystals exhibiting distinct nanoscale magnetic domains within crystallographic twins and across twin boundaries, as well as nanomagnetic ASI arrays fabricated with an in-plane magnetic dipole orientation. Researchers in a wide variety of fields desiring high-resolution MFM imaging can significantly benefit from employing the protocol outlined here, as well as the discussion of potential challenges such as topographical artifacts.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Atomic force microscope</td>
			<td>Bruker</td>
			<td>Dimension Icon</td>
			<td>Uses Nanoscope control software</td>
		</tr>
		<tr>
			<td>Glovebox, inert atmosphere</td>
			<td>MBraun</td>
			<td>LabMaster Pro MB200B + MB20G gas purification unit</td>
			<td>Custom design (leaktight electrical feedthroughs, vibration isolation, acoustical noise and air current minimization, etc.) and depth for use with Bruker Dimension Icon AFM, 3 gloves, argon atmosphere</td>
		</tr>
		<tr>
			<td>MFM probe</td>
			<td>Bruker</td>
			<td>MESP</td>
			<td>k = 3 N/m, f0 = 75 kHz, r = 35 nm, 400 Oe coercivity, 1 x 10-13 EMU moment. An improved version with tighter specifications, the MESP-V2, is now available. We have also used Bruker&#39;s MESP-RC (2x higher resonance frequency than the standard MESP, f0 = 150 kHz, with a marginally stiffer nominal spring constant of 5 N/m) and other MESP variants designed for low (0.3 x 10-13 EMU) or high (3 x 10-13 EMU) moment (i.e., MESP-LM or MESP-HM, respectively) or coercivity. A variety pack of 10 probes containing 4x regular MESP, 3x MESP-LM, and 3x MESP-HM variants is available from Bruker as MESPSP. Other vendors also manufacture MFM probes with specifications similar to the MESP (e.g., PPP-MFMR from Nanosensors, also available in a variety of variants, including -LC for low coercivity, -LM for low moment, and SSS for &#34;super sharp&#34; decreased tip radius; MAGT from AppNano, available in low moment [-LM] and high moment [-HM] variants). Similarly, Team Nanotec offers a line of high resolution MFM probes (HR-MFM) with several options in terms of cantilever spring constant and magnetic coating thickness.</td>
		</tr>
		<tr>
			<td>MFM test sample</td>
			<td>Bruker</td>
			<td>MFMSAMPLE</td>
			<td>Section of magnetic recording tape mounted on a 12 mm diameter steel puck; useful for troubleshooting and ensuring the MFM probe is magnetized and functioning properly</td>
		</tr>
		<tr>
			<td>Nanscope Analysis</td>
			<td>Bruker</td>
			<td>Version 2.0</td>
			<td>Free AFM image processing and analysis software package, but proprietary, designed for, and limited to Bruker AFMs; similar functionality is available from free, platform-independent AFM image processing and analysis software packages such as Gwyddion, WSxM, and others</td>
		</tr>
		<tr>
			<td>Probe holder</td>
			<td>Bruker</td>
			<td>DAFMCH or DCHNM</td>
			<td>Specific to the particular AFM used; DAFMCH is the standard contact and tapping mode probe holder, suitable for most MFM applications, while DCHNM is a special nonmagnet version for particularly sensitive MFM imaging</td>
		</tr>
		<tr>
			<td>Probe magnetizer</td>
			<td>Bruker</td>
			<td>DMFM-START</td>
			<td>MFM &#34;starter kit&#34; designed specifically for the Dimension Icon AFM; includes 1 box of 10 MESP probes (see above), a probe magnetizer (vertically aligned, ~2,000 Oe magnet in a mount designed to accommodate the DAFMCH or DCHNM probe holder, above), and a magnetic tape sample (MFMSAMPLE, above)</td>
		</tr>
		<tr>
			<td>Sample Puck</td>
			<td>Ted Pella</td>
			<td>16218</td>
			<td>Product number is for 15 mm diameter stainless steel sample puck. Also available in 6 mm, 10 mm, 12 mm, and 20 mm diameters at https://www.tedpella.com/AFM_html/AFM.aspx#anchor842459</td>
		</tr>
		<tr>
			<td>Scanning electron microscope (SEM)</td>
			<td>Zeiss Merlin</td>
			<td>Gemini II</td>
			<td>SEM parameters: 5 keV accelaration voltage, 30 pA electron current, 5 mm working distance. Due to nm scale ASI lattice features, the aperture and stigmation alignment were adjusted before acquisition to produce high quality images.</td>
		</tr>
	</tbody>
</table>

optimizing magnetic force microscopy resolution and sensitivity to visualize nanoscale magnetic domains

Magnetic force microscopy (MFM) enables mapping local magnetic fields across a sample surface with nanoscale resolution. To perform MFM, an atomic force microscopy (AFM) probe whose tip has been magnetized vertically (i.e., perpendicular to the probe cantilever) is oscillated at a fixed height above the sample surface. The resultant shifts in the oscillation phase or frequency, which are proportional to the magnitude and sign of the vertical magnetic force gradient at each pixel location, are then tracked and mapped. Although the spatial resolution and sensitivity of the technique increases with decreasing lift height above the surface, this seemingly straightforward path to improved MFM images is complicated by considerations such as minimizing topographical artifacts due to shorter range van der Waals forces, increasing the oscillation amplitude to further improve sensitivity, and the presence of surface contaminants (in particular water due to humidity under ambient conditions). In addition, due to the orientation of the probe's magnetic dipole moment, MFM is intrinsically more sensitive to samples with an out-of-plane magnetization vector. Here, high-resolution topographical and magnetic phase images of single and bicomponent nanomagnet artificial spin-ice (ASI) arrays obtained in an inert (argon) atmosphere glovebox with &lt;0.1 ppm O2 and H2O are reported. Optimization of lift height and drive amplitude for high resolution and sensitivity while simultaneously avoiding the introduction of topographical artifacts is discussed, and detection of the stray magnetic fields emanating from either end of the nanoscale bar magnets (~250 nm long and &lt;100 nm wide) aligned in the plane of the ASI sample surface is shown. Likewise, using the example of a Ni-Mn-Ga magnetic shape memory alloy (MSMA), MFM is demonstrated in an inert atmosphere with magnetic phase sensitivity capable of resolving a series of adjacent magnetic domains each ~200 nm wide.

Artificial spin-ice (ASI) lattices 
Artificial spin ices are lithographically defined two-dimensional networks of interacting nanomagnets. They exhibit frustration by design (i.e., the existence of many local minima in the energy landscape)21,42,43. High-resolution MFM imaging to elucidate the magnetic configurations and interactions between the array components offers the unique opportunity to better under...

Watch this Scientific Journal Video about Optimizing Magnetic Force Microscopy Resolution and Sensitivity to Visualize Nanoscale Magnetic Domains at JoVE.com

Optimizing Magnetic Force Microscopy Resolution and Sensitivity to Visualize Nanoscale Magnetic Domains

Magnetic force microscopy (MFM) employs a vertically magnetized atomic force microscopy probe to measure sample topography and local magnetic field strength with nanoscale resolution. Optimizing MFM spatial resolution and sensitivity requires balancing decreasing lift height against increasing drive (oscillation) amplitude, and benefits from operating in an inert atmosphere glovebox.

Magnetic force microscopy (MFM) enables mapping local magnetic fields across a sample surface with nanoscale resolution. To perform MFM, an atomic force ...

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