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Antiferromagnetic thin films attract significant interest for future low-power spintronic devices. They are largely insensitive to external magnetic fields, exhibit domains with negligible cross-talk and can be switched at THz frequencies [1]. Furthermore, in multiferroics where antiferromagnetism can coexist with ferroelectricity, it is possible to control the magnetic state electrically - a particularly appealing feature for future device applications. In this talk, we show how the Qnami ProteusQTM -- a commercial room temperature scanning nitrogen-vacancy magnetometer (SNVM) -- can be used to quantitatively image antiferromagnetic spin textures with proven DC field sensitivity ≤ 2 μT/√Hz. We image the spin cycloid of a multiferroic bismuth ferrite (BiFeO3) thin film and extract a period of about 103 nm, indicative of a type II spin cycloid. [2] In addition, we take advantage of the quantitative nature of SNVM to extract an average magnetic moment density of 0.08 ±0.02 μB /Fe in the BiFeO3 film. These findings agree well with reported vibrating sample magnetometer results (MS = 0.06 μB /Fe) on similar, strained BiFeO3 thin films. [3] Our results shed light on future design requirements for reconfigurable nanoscale spin textures in such multiferroic systems.References: [1] V. Baltz, et al., Rev. Mod. Phys. 90, 015005 (2018) [2] A. Haykal, et al., Nat. Commun. 11, 1704 (2020) [3] W Eerenstein, et al., Science 307, 1203a (2005)

MMM 2020

Antiferromagnetic Spin Cycloidal Orders Imaged with a Commercial Scanning Nitrogen-Vacancy Magnetometer

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INTERMAG 2021

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The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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We present successful fabrication and demonstration of magnetic vortex core tip (VC) for magnetic force microscopy (MFM). The design of VC probe is based on permalloy disk with diameter of 330nm and thickness of 55nm fabricated on intentionaly blunted tip (see the Fig.1). The ground state of this permalloy disk is the magnetic vortex state with out-of-plane magnetic moment determined by well spatially localized vortex core. The novelty of this design is creation of spatially well localized magnetic out-of-plane field in mechanically blunt tip. This design offers two advantages: First, due to large area of the disk, VC probe has higher wear resistance and longer lifetime than sharp commercial MFM probes. Second, it has very good stability of the out-of-plane magnetic moment, because vortex core is almost independent on the edge imperfections. However, VC probe has also some limitations: the sample should be flat and it should not produce quasi-uniform field in large volume nearby the tip, because the polarization affects vortex core position. We analyze the permalloy tip polarization effect by micromagnetic calculations of MFM images of periodic magnetic strips. By this approach we identify the limits on samples for this new kind of MFM probe.References: J. Soltys, J. Feilhauer, I. Vetrova, J. Tobik, K. Bublikov, T. Scepka, J. Fedor, J. Derer, and V. Cambel, Appl. Phys. Lett. 116, 242406 (2020).

New Vortex Core Probe for Magnetic Force Microscopy

Rich variation of magnetic texture attracts attention in connection with topology of electronic structures. The temperature range slightly below Curie temperature (Tc) is interesting, because the contributing micromagnetic energies (demagtization, exchange, Dzyaloshinsky-moriya and anisotropy) change rapidly with temperature. Recently we reported bubble domain formation immediately below Tc within a kagome ferromagnet Co3Sn2S2 (Tc=176 K) in which anomalous high-temperature, low-field magnetic phase occurred [1]. Self-reorganization of domain walls (DWs) occurred even at zero field, and the domain evolution was dependent not only on temperature and field but also on temperature variation rate [2]. To examine such domain evolution associated with slow spin dynamics, we developed a method to record Lorentz images continuously at a fixed time interval when a specimen is heated/cooled at a constant rate, not like an usual manner to wait until thermal drift settles down. We applied it to examine domain evolution within another uniaxial kagome magnet, Fe3Sn2 (Tc= 657 K), in the temperature range at 303-673 K under no external field. A psuedo-three-dimensional (x-y-time (temperature)) image was constructed from a series of drift-corrected Lorentz images. Fig. 1 shows a cross-section image including time-axis, exhibiting time(temperature) evolution of the magnetic domains. The stripe domain stricture was very stable between room temperature (RT) and 620 K at which DW contrast disappeared due to reduced magnetization both on heating and cooling across Tc, although interesting temperature-dependent texture such as skyrmionic bubble was observed for the alloy below RT. It was also found that the domain structure was nearly independent of cooling/heating rate. The differece in domain evolution between Fe3Sn2 and Co3Sn2S2 is explained in terms of DW creep associated with thermal depinning in Co3Sn2S2.References: [1] M. A. Kassem et al., Phys. Rev. B 96, 014429 (2017). [2] Akira Sugawara et al., Phys. Rev. Mater. 3, 104421 (2019)

Magnetic texture evolution within uniaxial kagome ferromagnets at constant heating/cooling rates

In wide-range magnetometry using optically detected magnetic resonance of NV- centers, we demonstrate more than order-of-magnitude speedup with sequential Bayesian experimental design as compared with conventional frequency-swept measurements. The NV- center is a quantum defect with spin 1 and coherence time up to several milliseconds at room temperature. Zeeman splitting of the NV- energy levels allows detection of the magnetic field via photoluminescence. NV-center is an excellent platform for magnetometry with potential spatial resolution down to few nanometers and demonstrated sensitivity down to nT/Hz1/2. We used a commercially available type IIa single crystal diamond grown by chemical vapor deposition (CVD). Green laser with 520 nm wavelength was used to optically excite NV- center, and a microstrip line was used to supply microwaves to manipulate spin state of the NV- center. For each data point, photons were counted for 50 ms. Measurement of each data point was followed by 50 ms measurement of a reference data point with microwaves switched off. The combination of laser power, microwave power and counting time produced measurements with a signal-to-noise ratio on the order of 1. Moreover, the magnetic field was slightly misaligned from the NV- center axis to produce a three-dip signal. Such experimental setup showcased ability of sequential Bayesian experiment design to locate and measure complex multiple-peak signal even in extremely noisy data and showed its broad dynamical range for sensitivity. Using the measurement described above, we compare conventional magnetic field measurements of fluorescence that use pre-determined sweeps of the microwave frequency with the measurement using a Bayesian inference methodology. In sequential Bayesian experiment design, the frequency of each measurement is determined in real-time from utility predictions based on the accumulated experimental data. We report more than order-of-magnitude speedup in magnetic field and hyperfine splitting determination with sequential Bayesian experimental design, compared with the conventional NV- magnetometry measurement.

Center Magnetometry Using Sequential Bayesian Experiment Design

Since it was first demonstrated in 1987, magnetic force microscopy (MFM) has become a truly widespread and commonly used characterization technique that has been applied to a variety of research and industrial applications. Some of the main advantages of the method includes its high spatial resolution (typically ∼50 nm), ability to work in variable temperature and applied magnetic fields, versatility, and simplicity in operation, all without almost any need for sample preparation. However, the technique has historically provided only qualitative information, and the number of available modes was typically limited, thus not reflecting the experimental demands. We present the recent progress and development of MFM as well as a summary of the current state-of-the-art techniques and objects for study. Aspects including quantitative MFM, the accurate interpretation of the MFM images, new instrumentation, probe-engineering alternatives, and applications of MFM to new (often interdisciplinary) areas of the materials science, physics, and biology are discussed. We outline the importance of the technique in emerging fields including skyrmions, 2D-materials, and topological insulators.References: O. Kazakova, et al., ‘Frontiers of Magnetic Force Microscopy’ J Appl. Phys. 125, 060901 (2019) H. Corte-León, et al., ‘Magnetic imaging using geometrically constrained nano-domain walls’ Nanoscale 11, 4478 (2019) V. Panchal, et al., ‘Calibration of multilayered magnetic force microscopy probes’ Sci Reports 7, 7224 (2017) F. Moro, et al., ‘Room temperature uniaxial magnetic anisotropy induced by Fe-islands in the InSe semiconductor van der Waals crystal’, Advanced Science, 5, 1800257 (2018) H. Corte-León, et al., ‘Comparison and validation of different magnetic force microscopy calibration schemes’ Small 20, 1906144 (2020) X. Hu, et al., ‘Round Robin Comparison on Quantitative Nanometer Scale Magnetic Field Measurements by Magnetic Force Microscopy’ J. Magn. Magn Mater. 511, 166947 (2020)

Frontiers of magnetic force microscopy INVITED

Antiferromagnets attract a great interest for spintronics owing to the robustness of their magnetic textures and their fast dynamics. However, since they exhibit no net magnetization, antiferromagnets are challenging to work with. NV-center magnetometry, which provides a sensitivity of a few µT/Hz-1/2 combined with a nanoscale spatial resolution, has emerged in the last years as a powerful technique to investigate them [1]. Here we introduce a new imaging mode of the NV-center magnetometer which does not rely on the measurement of the static magnetic stray field but on the detection of magnetic noise originating from spin waves inside the non-collinear antiferromagnetic textures of interest. The presence of magnetic noise accelerates the NV spin relaxation. As a consequence, the emitted photoluminescence is reduced, allowing a simple detection of the noise sources [2]. We demonstrate this new technique on synthetic antiferromagnets (SAF) [3] consisting of two ferromagnetic Co layers antiferromagnetically coupled through a Ru/Pt spacer. We first image domain walls and prove that we perform noise-based imaging by measuring a shorter NV spin relaxation time above an antiferromagnetic domain than above a domain wall. Calculations of the spin waves dispersion both in the antiferromagnetic domains and in the domains walls as well as maps of simulated magnetic noise intensity enable us to conclude that the noise which we probe arises from spin waves channelled in the domain walls. Going further, we tune the composition of the SAF stacks in order to stabilize spin spirals or antiferromagnetic skyrmions. In both cases, our relaxometry-based technique is able to image the non-collinear structures, demonstrating its efficiency and opening new avenues of exploration in the characterization of complex structures in magnetically-compensated materials. This work was performed in collaboration with the UMR CNRS/Thalès and the Center for Nanoscience and Nanotechnology (C2N) in Palaiseau, France. It has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 846597 and from the DARPA TEE Program.References: [1] I. Gross et al, Nature, Vol 549, p 252 (2017) [2] A. Finco et al, arXiv:2006.13130 (2020) [3] W. Legrand et al, Nature Materials, Vol 19, p 34 (2020)

Imaging non-collinear antiferromagnetic textures via single spin relaxometry INVITED

Efficient and versatile spin-to-charge current conversion is crucial for the development of spintronic applications, which strongly rely on the ability to electrically generate and detect spin currents. In this context, the spin Hall effect has been widely studied in heavy metals with strong spin-orbit coupling. While the high crystal symmetry in these materials limits the conversion to the orthogonal configuration [1], unusual configurations are expected in low symmetry transition metal dichalcogenide semimetals [2], which could add flexibility to the electrical injection and detection of pure spin currents. Here, we report the observation of spin-to-charge conversion in MoTe2 flakes, which are stacked in graphene lateral spin valves [3]. We detect two distinct contributions arising from the conversion of two different spin orientations. In addition to the conventional conversion where the spin polarization is orthogonal to the charge current, we also detect a conversion where the spin polarization and the charge current are parallel. Both contributions, which could arise either from bulk spin Hall effect or surface Edelstein effect, show large efficiencies comparable to the best spin Hall metals and topological insulators. Our finding enables the simultaneous conversion of spin currents with any in-plane spin polarization in one single experimental configuration [3].References: [1] Sinova, J.; Valenzuela, S. O.; Wunderlich, et al., Rev. Mod. Phys., Vol. 87, p.1213-1260 (2015) [2] MacNeill, D.; Stiehl, G. M.; Guimaraes, et al., Nat. Phys. Vol. 13, p.300-305 (2017) [3] Safeer, C. K.; Ontoso, N.; Ingla-Aynés, et al., Nano Lett., Vol.19, p8758-8766 (2019)

Large multi-directional spin-to-charge conversion in low symmetry semimetal MoTe2 at room temperature

Metallic-based nanoparticles have been investigated extensively for their robustness, effectiveness and the potential functionalization of their surface. Especially, magnetic nanoparticles with their distinctive characteristics are widely used in bioassays and magnetic drug delivery systems in biomedical applications as an anti-cancer agent. In this study, Fe/Fe3O4 core-shell nanoparticles (13-20nm) (Figure 1), coated with PEG, have been successfully synthesized by Y-junction flow tube technique. The room temperature coercivity was found to be 350 Oe. A549 non-small cell lung cancer and NIH3T3 mouse embryonic fibroblast cell lines were used to monitor cellular toxicity (IC50) and proliferation effects of PEG-ylated Fe/Fe3O4 nanoparticles. The inhibitory potential on the cell motility of A549 cancer cells were higher than NIH3T3 cells. The results showed that the toxic effect of particles was higher on cancer cells. Upon examination, IC50 values were found to be 27.5 μg/ml and 1 μg/ml for NIH3T3 cell and A549 cell, respectively. (Figure 1 a and b) The nanoparticles were found to be 100 times more toxic on cells when compared to the previous studies. According to the results, PEG-ylated Fe/Fe3O4 nanoparticles not only have significantly lower IC50 values compared to similar research but also they could be used in diagnosis, magnetic hyperthermia and various drug systems as a treatment method against cancer. This work was supported by BAP.2018-2.05References: Domac, B. H., AlKhatib, S., Zirhli, O., Akdogan, N. G., Öcal Dirican, S. C., Bulut, G., & Akdogan, O., Effects of PEGylated Fe–Fe3O4 core-shell nanoparticles on NIH3T3 and A549 cell lines. Heliyon, 6(1), (2020).

Effects of PEGylated Fe/Fe3O4 Core Shell Nanoparticles on Cancer Cells

Combining two materials having different functional properties has become a current research area for biomedical applications. The progress of nanoplatforms brings a new non-invasive imaging and therapeutic tools for cancer. Here, biocompatible and multifunctional magnetic Fe3O4-ZnO core–shell nanostructure has been developed by using a soft chemical approach. Fe3O4-ZnO core–shell is well characterized by various sophisticated characterization techniques such as X-ray diffraction (XRD), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-visible absorption, photo-luminescent spectroscopy and physical properties measurement system (PPMS). X-ray diffraction (XRD) confirms the presence of both Fe3O4 and ZnO phases. XPS analysis confirms the magnetite phase of Fe3O4 nanodots in the core-shell structures. TEM micrograph clearly reveals that 5 nm Fe3O4 nanodots are well decorated with ZnO nanoparticles shell. This multi-functional approach will facilitate magnetic targeting and generation of reactive species under an external AC magnetic field, providing therapy a smart therapeutic agent for cancer. Fe3O4-ZnO core–shell conserves the intrinsic superparamagnetic behaviour of its constituent Fe3O4 with a magnetization value of ~ 36.5 emu/g. The aqueous colloidal stability, optimal magnetization, and good specific absorption rate (under external AC magnetic field) of Fe3O4-ZnO core–shell particles perform as an effective heating source for thermal therapy. Cytotoxicity studies exhibit the reduction of cellular viability for human cervical cancer cells (HeLa). Further, A combined effect of thermo-therapy and generation of reactive oxygen species under AC magnetic field results in a synergistic effect on the cancer cell death. Therefore, these results indicate that core–shell is sufficient materials for magnetic hyperthermia and bio-imaging.References: 1. J. Wan, H. Li, and K. Chen, Materials Chemistry and Physics, 114, 2009, 30-32. 2. H. Zhang, L. Guo, S. Ding, J. Xiong, and B. Chen, Oncotarget. 7, 2016, 36602–36613. 3. K. Patel, B. S. Raj, Y. Chen, and X.Lou,Colloids and Surfaces B: Biointerfaces, 150, 2017, 317–325.

Core-Shell ZnO@Fe3O4 Nanostructure for Magnetic Hyperthermia and Bio-Imaging Applications

Transcranial Magnetic Stimulation (TMS) is a non-invasive technique of modulating the neurons of the brain that functions on the principle of electromagnetic induction to treat disorders.1 TMS, which has been employed in research and clinically, has proven to be a useful tool for the treatment for certain neurological and psychiatric disorders.2 The modulation of the neurons of the brain is dependent on the magnitude and direction of the electric field (E-Field) induced from the time-varying magnetic field generated from TMS coils. The coil geometry and orientation are known to define the E-Field magnitude and direction and to achieve high effectiveness, and these parameters are required to be optimized.3 In recent years, researchers have proposed several coil designs for TMS with the aim of achieving focality and deep penetration.4 Amongst this design is the commercially available Figure-of-Eight Coil and the novel Quadruple Butterfly Coil (QBC).5 The QBC consists of two sets of coils that are inclined at an angle of 450 to the vertical axis.6 In this research, the QBC orientation in steps of 150 within a range of ± 600 over the vertex location was modeled using a finite element tool, Sim4life, the maximum electric field intensity (E-Max) and stimulated volume of the brain (V-Half) were computed and compared to determine the optimal coil orientation. Fig. 1 shows the different QBC orientation and the E-Max distribution in the grey matter of the brain.References: 1Zhong, X., Rastogi, P., Wang, Y., Lee, E. G., & Jiles, D. C. (2019). IEEE Transactions on Magnetics, 55(7), 1–5. https://doi.org/10.1109/TMAG.2018.2890069 2Chail, A., Saini, R. K., Bhat, P. S., Srivastava, K., & Chauhan, V. (2018). Industrial Psychiatry Journal, 27(2), 172–180. https://doi.org/10.4103/ipj.ipj_88_18 3Reijonen, J., Säisänen, L., Könönen, M., Mohammadi, A., & Julkunen, P. (2020). Journal of Neuroscience Methods, 331, 108521. https://doi.org/10.1016/j.jneumeth.2019.108521 4Deng, Z.-D., Lisanby, S. H., & Peterchev, A. V. (2013). Brain Stimulation, 6(1), 1–13. https://doi.org/10.1016/j.brs.2012.02.005 5Rastogi, P., Lee, E. G., Hadimani, R. L., & Jiles, D. C. (2017). AIP Advances, 7(5), 056705. https://doi.org/10.1063/1.4973604 6Rastogi, Priyam (2019), Iowa State University, Ames, IA, USA, https://lib.dr.iastate.edu/etd/17547

Effect of Coil Positioning and Orientation of the Quadruple Butterfly Coil during Transcranial Magnetic Stimulation

Co1-xMgxFe2O4（x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0）nanoparticles surrounded by amorphous SiO2 with particle sizes around 5 nm were prepared by an original wet chemical method. Furthermore, in order to develop the particles for conjugation with proteins, thiol group was modified. These particles were further confirmed as an agent for hyperthermia treatment. The heat dissipation characteristics of the particles were investigated by measuring AC magnetic susceptibility and the relationship between the imaginary part of AC magnetic susceptibility χ” and the increase in temperature in the AC field was estimated. We have carried out spin echo MRI measurements for Co1-xMgxFe2O4 nanoparticles using a 0.3-T MRI system. The particles showed drastic T2 shortening effect comparing to agarose as background depending on the composition. All the particles showed effective relaxivity, R2 values and significant contrast was observed in the phantom image. These particles are expected for future agents for theranostics.References: [1] T. Kondo, K. Mori, M. Hachisu, T. Yamazaki, D. Okamoto, M. Watanabe, K. Gonda, H. Tada, Y. Hamada, M. Takano, N. Ohuchi, and Y. Ichiyanagi, J. Appl. Phys. 11717D157 (2015). [2] D. Shigeoka, T. Yamazaki, T. Ishikawa, K. Miike, K. Fujiwara, T. Ide, A. Usui, Y. Hosokai, H. Saito and Y. Ichiyanagi, IEEE Trans. Magn., 54 6100707 (2018).


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