United States

To improve magnetic memories, significant efforts are made to reduce the size and spacing of magnetic bits. This implies that failures and defects can only be discovered with a non-invasive technique that can resolve small magnetic fields with high spatial resolution. Scanning NV magnetometry (SNVM) is the emerging quantum sensing technique that offers the required sensitivity.&lt;br/&gt;We will demonstrate magnetic images of a few hot candidate materials for future magnetic memory devices. We will look at memory bits in antiferromagnetic chromia, antiferromagnetic cycloids in BiFeO3 [1], cobalt nanomagnets and ultra-scaled CoFeB nanowires [2]. We will reveal magnetic textures that are undetectable with standard characterization techniques. In this context, we will more broadly discuss the potential of SNVM as a powerful magnetic characterization tool.

MMM 2022

Investigation of Ferro and Antiferromagnetic Memory Structures using Scanning NV Magnetometry

The Institute of Physics and IOP Publishing, in partnership with the Chinese Physical Society (CPS) and the University of Science and Technology of China (USTC) are proud to have brought together the international quantum science community online for this inaugural conference.


Early career and leading researchers from universities, industry and government worldwide came together to share, learn and collaborate on the latest research and emerging areas of high interest across quantum science and technology. The broad themes covered by Quantum 2020 included:

Quantum communication;
Quantum computing and quantum simulation;
Quantum sensing, metrology and imaging;
Quantum information theory, software and algorithms;
Quantum foundations;
Quantum technology in industry.
The four-day event featured talks from world-renowned scientists, interactive poster sessions and two exciting panel discussions on the themes of Quantum Technology in Industry and Quantum Technology Programmes Around the World.


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Quantum 2020

The Institute of Physics and IOP Publishing, in partnership with the Chinese Physical Society (CPS) and the University of Science and Technology of China (USTC) are proud to have brought together the international quantum science community online for this inaugural conference.

Early career and leading researchers from universities, industry and government worldwide came together to share, learn and collaborate on the latest research and emerging areas of high interest across quantum science and technology. 

To improve magnetic memories, significant efforts are made to reduce the size and spacing of magnetic bits. This implies that failures and defects can only be discovered with a non-invasive technique that can resolve small magnetic fields with high spatial resolution. Scanning NV magnetometry (SNVM) is the emerging quantum sensing technique that offers the required sensitivity. We will demonstrate magnetic images of a few hot candidate materials for future magnetic memory devices. We will look at memory bits in antiferromagnetic chromia, antiferromagnetic cycloids in BiFeO3 [1], cobalt nanomagnets and ultra-scaled CoFeB nanowires [2]. We will reveal magnetic textures that are undetectable with standard characterization techniques. In this context, we will more broadly discuss the potential of SNVM as a powerful magnetic characterization tool.

technical paper

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

The Conference was held from October 31 to November 4 and offers both in-person and on-demand content.

MMM 2022 is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a tutorial, and several special sessions. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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Ultra-thin silicon steel with thickness less than 0.1 mm can be used to make the iron core of high-frequency transformers and high-speed motor. The design of these electrical equipment often relies on the data of alternating magnetic characteristics measured under standard excitation, ignoring the influence of working conditions such as stress and temperature, etc.. Under stress conditions, the magnetic properties of the magnetic material are quite different from those under standard sinusoidal excitation. Therefore, it is very important to accurately measure the magnetic properties of ultra-thin silicon steel under stress conditions.
Previously, many scholars have studied the influence of mechanical stress on the magnetic properties of conventional electrical steels [1-5]. However, there are few reports on the properties of high frequency materials under stress. When testing the properties of high frequency materials under stress, we can refer to the traditional testing methods of silicon steel. Compared with the other methods of stress application, such as motor or oil pressure, piezoelectric material has good flexibility and control accuracy, and it will not produce stray magnetic field to interference the measurement. The most important thing is that piezoelectric materials have good applicability to the magnetizer. In other words, it is not necessary to reconstruct the magnetic circuit structure and stress actuator, and the relevant experiments can be carried out directly by pasting piezoelectric materials in the sample.
This paper uses the magnetizer proposed in paper [6], which has good test accuracy. The sample is ultra-thin silicon steel with 0.1 mm thickness (GT100). The size of the sample is 50 mm×50 mm. Two Macro Fiber Composite (MFC) [7] are symmetrically pasted on the upper and lower surfaces of the sample center. A maximum tensile or compressive stress of about 150 N or 107 MPa can be obtained in the sample along one direction. The distribution of stress is calculated by the FEM and shown in Fig. 1. It can be seen that the stress is uniform in the measuring region. Fig. 2 gives the hysteresis loops under different stress along rolling direction. 

**References:** 

[1] Permiakov, V. , et al. "Loss separation and parameters for hysteresis modelling under compressive and tensile stresses." Journal of Magnetism & Magnetic Materials 272.supp-S(2004):E553-E554. 
[2] Singh, D. , et al. "Effect of stress on excess loss of electrical steel sheets." IEEE (2015):1-1. 
[3]Hameyer, et al. "Effect of mechanical stress on different iron loss components up to high frequencies and magnetic flux densities." COMPEL: The international journal for computation and mathematics in electrical and electronic engineering 36.3(2017):580-592. 
[4]Ali, K. , Atallah, K. and Howe, D. . "Prediction of mechanical stress effects on the iron loss in electrical machines." Journal of Applied Physics 81.8(1997). 
[5]Naumoski, H. , Maucher , A. and Herr, U. . "Investigation of the influence of global stresses and strains on the magnetic properties of electrical steels with varying alloying content and grain size." Electric Drives Production Conference IEEE, 2015. 
[6]Yue, S. , et al. "Comprehensive Investigation of Magnetic Properties for Fe–Si Steel Under Alternating and Rotational Magnetizations Up to Kilohertz Range." IEEE Transactions on Magnetics 55.7(2019):1-5. 
[7]Wilkie, W. K. , et al. "Low-cost piezocomposite actuator for structural control applications." Proceedings of Spie the International Society for Optical Engineering (2000). 

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Influence of stress on the magnetic properties of ultra thin silicon steel in high frequency range

Abstract—The Jiles-Atherton (J-A) model [1] predicts that an inductor has different inductances at the current ramp-up time (ton) and ramp-down time (toff). For the first time, this phenomenon is observed at the boost converter (Fig. 1) during the voltage converter (Fig. 2) and a simple equation to depict it is derived. Moreover, the methodology to calculate the inductor power loss from the voltage conversion data directly for a boost converter is presented.
I. Model and Experiment Result
From Fraday law, the inductance is
L=NdBA/(dt(dI/dt))=uNA d(H+M)/dI= Lo(1+dM/dH) (1)
From the J-A model, the differential of total magnetization M is
dM/dH=(Man-M)/((dk/m)-a(Man-M)) (2)
where d=1 if dH/dt>0 and d=-1 if dH/dt < 0.
From the experiment result in Figs. 3-5, Man-M can be approximated to a constant MD since the voltage almost keeps constant at each ton or toff based on L=V/(dI/dt). So, the inductance at ton is given by
L= Lo(1+mMD/(k-aMD)=L1 (3)
The inductance at toff is given by
L= Lo(1-mMD/(k+aMD)=L2 (4)
Figs. 3-5 show the voltage and current waveforms of the boost converter during the voltage conversion for the output currents (IOUT in Fig. 1) 0A, 0.2A and 0.4A. However, the three different IOUT waveforms have different ton and toff due to different slops by aligning their base current level with the current waveform for IOUT=0A as shown in Fig. 6. It proves that an inductor has the different inductances at ton and toff and L1 at ton is larger than Lo and L2 at toff is smaller than Lo. This is caused by the hysteresis effect as shown in Fig. 7 since it follows the initial hysteresis loop, minor hysteresis curves i and ii for Lo, L1 and L2. Integrating the powers at ton and toff and divided by the period T, the inductor power loss for boost converter during the voltage conversion is Ploss=(L1-L2)dIL(dIL+2IMIN)/2T (5)
where dIL is the peak-to-peak inductor current, IMIN is the minimum inductor current in Fig. 2.
Table-I shows the input power, measured inductor power loss based on Figs. 3-5 and calculated inductor power loss based on Eq. (5). The calculated power loss is very close to the measured power loss, verifying Eq. (5) valid to calculate the inductor power loss from the electrical voltage conversion data directly. 


**References:** 

[1] D. C. Jiles and D. L. Atheron, “Theory of Ferromagnetic Hysteresis,” J. of Magnetism and Magnetic Materials, pp. 48-60, 1986. 

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A New Inductor Power Loss Model for Boost Converter during the Voltage Conversion Based on the Jiles Atherton Model

Different contributions and mechanisms of the unidirectional magnetoresistance (UMR) have become a recent research hotspot in spintronics. Field sweeping DC measurements are used to measure the value of UMR instead of second harmonic measurements. [1,2] In this paper, potential measurement errors in conventional DC measurements are studied. Oersted field and field-like torque usually do not influence the measurement, but a large field-like torque can result in anisotropic magnetoresistance (AMR) difference when the sample is not perfectly aligned. The existence of ordinary magnetoresistance (OMR) can also contribute to a large background. An alternative measurement method is demonstrated to address this issue. This work broadens the understanding of the error sources and provides a standard measurement method for UMR DC measurements. 

**References:** 

[1] Duy Khang, Nguyen Huynh, and Pham Nam Hai. "Giant unidirectional spin Hall magnetoresistance in topological insulator–ferromagnetic semiconductor heterostructures." Journal of Applied Physics 126.23 (2019): 233903. 
[2] Chang, Ting-Yu, et al. "Large unidirectional magnetoresistance in metallic heterostructures in the spin transfer torque regime." Physical Review B 104.2 (2021): 024432.

Origins of potential observational errors in field sweeping DC measurements for unidirectional magnetoresistance

Amniotic fluid is a clear and pale yellowish liquid that surrounds and protects the fetus (unborn baby) throughout pregnancy [1]. The amniotic fluid analysis can diagnose certain health disorders in the unborn baby such as Down syndrome, Cystic fibrosis, Sickle cell disease, Tay-Sachs disease, and Neural tube defects [2]. Fig. 1 shows a 10-week-old human fetus surrounded by amniotic fluid within the amniotic sac [3]. 

This abstract presents a deep learning/machine learning pipeline that can be used for the prediction of health problems in an unborn baby by using deep learning or machine learning algorithm. The work is divided into two parts; (1) data collection, and (2) data processing and analysis. Fig. 2(a) shows data collection workflow. In the data collection, 20 samples of amniotic fluid are collected from a healthy and an unhealthy woman during 15~20 weeks of pregnancy [4]. After collecting the samples, the nuclear magnetic resonance (NMR) is performed for data acquisition [5]. Fig. 2(b) depicts workflow of data processing and analysis. In this part, metabolites data is normalized and a data reduction method like PCA is applied. After getting the preprocessed data, feature selection algorithms are performed to remove the irrelevant data. Three variable selection algorithms such as (i) Correlation-based feature selection (CFS), (ii) Partial least squares regression (PLS), and (iii) Learning Vector Quantization (LVQ) [4] are used. For each selected set of metabolites, support vector machine (SVM) models are generated using 10-fold cross-validation. To evaluate the performance and significance of the optimized model, we perform a permutation test. In the 10-fold cross-validation, the permutation test shuffles the class labels while using the same set of metabolites. For data analysis, we calculate true/false positives and true/false negatives for each round of our 10-fold cross-validation to report average sensitivity (TP/P) and specificity (TN/N) values along with the average of the area under the curve (AUC) [4]. 

**References:** 

[1]. C. M. Broek, J. Bots, I. V. Lasheras, M. Bugiani, F. Galis, S. V. Dongen, “Amniotic fluid deficiency and congenital abnormalities both influence fluctuating asymmetry in developing limbs of human deceased fetuses,” PLoS One, vol. 8, no. 11, 2013. 
[2]. MedlinePlus [Internet], National Library of Medicine (US), “Amniocentesis (amniotic fluid test),” Available from: https://medlineplus.gov/lab-tests/amniocentesis-amniotic-fluid-test/. 
[3]. https://en.wikipedia.org/wiki/Amniotic_fluid. 
[4]. R. O. B. Singh, A. Yilmaz, H. Bisgin, O. Turkoglu, P. Kumar, E. Sherman, A. Mrazik, A. Odibo, and S. F. Graham, “Artificial intelligence and the analysis of multi-platform metabolomics data for the detection of intrauterine growth restriction,” PLoS One vol. 14, no. 4, 2019. 
[5]. https://www.jeol.co.jp/en/products/nmr/basics.html. 

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Analysis of Nuclear Magnetic Resonance Metabolomics data for the detection of unborn baby health problems using deep learning

Superparamagnetic nanoparticles (SNPs) are widely used in biomedical applications like imaging, magnetic hyperthermia, drug delivery, etc. [1-2]. We present a continuation of our study of the scattering of dilute aqueous suspensions of single-domain magnetite nanoparticles using an AC Faraday rotation setup [3-5]. The setup employs a stabilized He-Ne laser (633 nm) along with an AC magnetic field (130-1120 Hz) that enables lock-in detection. We have measured the scattering response of magnetite nanoparticles that vary in diameter from 15 to 25 nm. Our previous results analysed the scattering of light by SNPs subjected to an AC magnetic field, utilizing the even harmonics of the intensity signal. In this study, we continue to investigate the leading even harmonic (2f) signal to analyse scattering, but with added emphasis on the initial polarization angle of the incident light (linearly polarized) and its relationship to the orientation of the applied magnetic field. This approach allows us to investigate how the direction of the applied magnetic field results in a shape anisotropy for the SNP aggregates, and the effect that this anisotropy has on the scattering of light with varying angles of incident polarization. Differences in scattering are then related to the structural details of the aggregates as well as the time dynamics of the formation of these structures. The two orthogonal cases (transverse and longitudinal) that form the basis of this analysis are where incident polarization is perpendicular and parallel to the applied field (also the presumed long axis of aggregates), respectively. The scattering is also measured for various other angles between a parallel and perpendicular orientation. The model allows us to predict an angle for which the scattering is not affected by the application of the magnetic field. We show that the presence of this angle is confirmed by our data.
Our results provide important insight into the role of nanoparticle size (core and hydrodynamic), particle concentration, and magnetic field profile (intensity and frequency) in the formation dynamics of clusters. 

**References:** 

1. R. Hergt, S. Dutz, and M. Zeisberger, Nanotechnology, 21, (2010). 
2. Y. Xiao and J. Du, J. Mater. Chem. B, 8, 3, 354–367, (2020). 
3. S. Vandendriessche, W. Brullot, D. Slavov, V. Valev, and T. Verbiest, Appl. Phys. Lett., 102, (2013). 
4. C. Patterson, M. Syed, Y. Takemaura, Journal of Magnetism and Magnetic Materials, 451, 248-253 (2018).<br
5. M. Syed, W. Li, N. Fried, and C. Patterson, AIP Advances, 11, 015328 (2021).

Role of Aggregation Dynamics in Magneto Optical Scattering by Superparamagnetic Nanoparticles

In recent years there is a growing interest in synthesizes of novel iron-based nanoparticles and nanocomposites with high efficiency of thermal energy transfer suitable for use in magnetic fluid hyperthermia. The development of novel biocompatible magnetic nanoparticles (MNPs) for biomedical applications has been the subject of extensive exploration over the past two decades. Here we study the characteristics of carbon-coated ferromagnetic (Fe-Fe3O4)@C “core-shell” nanoparticles in samples with different concentration of iron synthesized by a solid-phase pyrolysis (SPP) of iron phthalocyanine (FeC32H16N8) molecules. The sample with higher iron concentration additionally annealed at 250°C under the oxygen media produces (Fe3O4) shell on Fe nanoparticles. The structural and magnetic properties of these nanomaterials were conducted using Scanning Electron Microscope (SEM), High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images with elemental mapping data, Raman spectroscopy, X-ray diffraction (XRD), magnetometry, electron paramagnetic and ferromagnetic resonances (EPR, FMR). The STEM image in Fig. 1 demonstrates a presence of Fe-Fe3O4 “core-shell” nanoparticles of order ~ 50 nm with oxide shells in carbon matrix. The ferromagnetic hysteresis loops with residual magnetizations and coercive forces is shown in Fig. 2 at two different temperatures. The characteristics of magnetic heating of water-based solution with different concentrations of synthesized nanocomposites under the influence of an external magnetic field have been studied. The magnetic characteristics such as saturation magnetization and coercivity as well as the specific absorption rate (SAR) make these materials attractive for magnetic hyperthermia applications.This work was supported by European Union’s Horizon 2020 research and innovation programme under grant agreement No 857502 (MaNaCa). The work at CSULA supported by the NSF CREST Grant #HRD-1547723. 

**References**

Fig. 1 High-resolution transmission electron microscopy images of iron/iron-oxide nanoparticles with core-shell architecture embedded in carbon matrix 


Fig. 2 Magnetization versus magnetic field shown for Fe-Fe3O4 nanoparticles at 3K and 300K

Syntheses and Characterization of Core Shell Iron

Magnetic force microscope (MFM) is a type of scanning probe microscope (SPM) that detects and images the gradient of the stray magnetic field of the observed sample. Therefore, the magnetic domain image is greatly affected by the surface morphology of the probe and the magnetic properties of the coated magnetic thin film. If the strong stray magnetic field for the permanent magnet material disturbs the magnetization of the magnetic probe, accurate observation of the magnetic domain structure is difficult. L10 ordered FePt alloy with high uniaxial magnetocrystalline anisotropyis thought to be an candidate probe material because of its high corrosion resistance and high coercivity[1]. It has also reported that the orientation of the FePt layer can be controlled by introducing MgO layer[2]. However, there are only a few reports of magnetic domain images observed by MFM probes using FePt film. In this study, the orientation of the FePt layer was controlled by using a MgO under layer with varying film thickness and element additions in order to observe high-resolution magnetic domain images. The FePt coated probes were prepared using an ultra-high vacuum magnetron sputtering system. A MgO buffer layer was deposited on Si cantilever and Si substrate at room temperature (R.T.). Then, FePt was deposited at a substrate temperature Ts of 873 K and they were annealed at 973 K. The surface morphology of the probes was observed by scanning electron microscope (SEM). The crystal structure was evaluated by X-ray diffraction (XRD), and the magnetic properties weremeasured usinga superconducting quantum interference device (SQUID) magnetometer. From XRD patterns, the fundamental (002) and the superlattice (001) peaks from L10 ordered FePt phase have been clearly observed atthe FePt probe by the introduction of MgO buffer layer, and a clear magnetic domain image was observed. However, when a Cu-doped MgO layer was used, the peak intensity from the L10ordered FePt phase increased and a magnetic domain image with even higher resolution was obtained. High-resolution mechanism will be discussed at the conference.

Preparation of probes for high resolution magnetic force microscopy by orientation control of FePt layer

Transcranial magnetic stimulation (TMS) [1] is a non-invasive pain-free medical technology clinically approved for treatment of drug-resistant depression [2, 3]. TMS uses
pulsed magnetic fields to induce eddy currents [4] in the brain. The biological mechanism of rTMS effects is tought to be mostly relying on the long-lasting changes of the neuronal
excitability and the brain functional plasticity, respoectively [5]. However, the effects of TMS-like fields on non-neuronal cells and artificial systems are almost unknown. The aim of this
study was to explore the potential of repetitive magnetic stimulation (RMS) performed by TMS devices in oncology, controllable drug release and regenerative medicine applications.
Using a standard TMS device “Magstim Rapid2”, we examined effects of >30 experimental regimes of RMS on viability and phenotype of various tumour (glioblastoma, pancreatic, liver,
colorectal [6] and breast cancers) and immune cells (microglia and macrophages). We also tested the effect of RMS on drug release from polymer nanoparticles and reactive oxygen species
generation (ROSG) in cancer and immune cells. Finally, we examined the combined effects of RMS and drugs targeting tissue growth.
RMS selectively modulated viability and functional polarisation of microglia and macrophages in a frequency/intensity-dependent manner and affected the proliferation/viability of cancer
cells (cancer type, frequency- and pulse number-dependent up- and downregulation). RMS induced triggering of drug release from nanoparticles, enhancement of ROSG and additive drug
effects.
Our pioneering findings demonstrate the potential of TMS technology repurposing for immunomodulation, cancer treatment, and drugs and nanomedicines treatment.
We thank Sydney Vital (for seed grant) and Medilink Australia (for providing the “Magstim”). The authors gratefully acknowledge the support of the Macquarie University FMHHS
Laboratory Operations Team. A.G. is thankful for support of her work by the Macquarie University Research Fellowship. 

**References:** 

1. A. T. Barker, R. Jalinous, and I. L. Freeston, "Non-invasive magnetic stimulation of human motor cortex," Lancet, vol. 1, no. 8437, pp. 1106-7, May 11 1985, doi:
10.1016/s0140-6736(85)92413-4. 
2. A. T. Barker, I. L. Freeston, R. Jalinous, and J. A. Jarratt, "Magnetic stimulation of the human brain and peripheral nervous system: an introduction and the results of an initial clinical
evaluation," Neurosurgery, vol. 20, no. 1, pp. 100-9, Jan 1987, doi: 10.1097/00006123-198701000-00024. 
3. P. M. Rossini et al., "Non-invasive electrical and magnetic stimulation of the brain, spinal cord, roots and peripheral nerves: Basic principles and procedures for routine clinical and
research application. An updated report from an I.F.C.N. Committee," Clinical neurophysiology : official journal of the International Federation of Clinical Neurophysiology, vol. 126, no.
6, pp. 1071-1107, Jun 2015, doi: 10.1016/j.clinph.2015.02.001. 
4. M. Hallett, "Transcranial magnetic stimulation and the human brain," Nature, vol. 406, no. 6792, pp. 147-50, Jul 13 2000, doi: 10.1038/35018000. 
5. J. P. Lefaucheur et al., "Evidence-based guidelines on the therapeutic use of repetitive transcranial magnetic stimulation (rTMS)," (in English), Clinical neurophysiology : official journal
of the International Federation of Clinical Neurophysiology, vol. 125, no. 11, pp. 2150-2206, Nov 2014, doi: 10.1016/j.clinph.2014.05.021. 
6. B. Heng, S. B. Ahn and A. Guller, "TMS-like Magnetic Fields Modulate Metabolic Activity of Hepatic and Colorectal Cancer Cells." IEEE Transactions on Magnetics: 1-1, doi:
10.1109/tmag.2022.3147219.

Beyond conventional neurostimulation: effects of TMS like magnetic fields on cells and nanomaterials and their potential applications in oncology and regenerative medicine

Due to their high spin polarization and easy manipulation, ferromagnetic materials dominate as the magnetically active element in spintronic devices but come with drawbacks such as large stray fields, limited scalability, and limited operational frequencies [1-3]. These shortcomings have recently inspired researchers to consider antiferromagnetic spintronics [4], as antiferromagnetic materials have no stray fields and possess ultrafast spin dynamics. However, antiferromagnets have no net magnetic moment, which leads to weak read-out signals and more difficult operation. Compensated ferrimagnets (FiMs) provide an alternative as they combine the properties of ferromagnets and antiferromagnets. In particular, FiMs can have near THz resonances as in antiferromagnets, while still having a net magnetic moment which can be used for strong read-out signals and frequency control, such as efficient tunable microwave signal output from FiM based nano-oscillators. Due to these unique properties, research in FiMs for spintronic applications is intensifying [5-7]. However, to use ferrimagnets in spintronic devices such as in spin Hall nano-oscillators (SHNOs), it is important to ensure that such films retain their advantageous properties in ultrathin films and in nano devices.
In the present study, ferrimagnetic GdFeCo thin films were grown using co-sputtering of Gd and Fe87.5Co12.5 on high resistance Si (100) substrates, and their magnetodynamic properties were studied using broadband ferromagnetic resonance measurements at room temperature. By tuning the stoichiometry of the GdFeCo films, a nearly compensated behavior is for the first time demonstrated in 2 nm thin film, as shown in Fig. 1. Values for the effective magnetization and effective Gilbert damping constant (α) of 0.02 Tesla and 0.0078±0.0002 are obtained for a 2 nm Gd24.4(FeCo)75.6 film [8], where α is comparable to the lowest value obtained in thick films [9]. The vertical dotted lines in Fig. 1 show literature values [6, 9-10] for the spin angular momentum (xa) and magnetic momentum (xm) compensation points in thick films. The composition of the films for the magnetic moment and angular momentum compensation in this work is very close to those reported in the literature.
In search of high-frequency auto-oscillations in spin Hall nano-oscillators (SHNOs), constrictions with 50-300 nm widths were also prepared using GdFeCo(2-10nm)/Pt(5nm) based stacks. Brillouing Light Scattering microscopy measurements on SHNOs show auto-oscillations in the 8 GHz range, as shown in Fig. 2 [11]. Very interestingly, different Gd compositions result in different signs of the threshold current, which indicates different signs of the effective spin-orbit torque as a function of the composition. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/0ec1329d0bf0522e3ffef8763deade00.png) 
Fig. 2. (a,b) Brillouin Light Scattering Spectroscopy (BLS) vs. current of two GdFeCo based SHNOs with different Gd content, believed to operate at opposite sides of the angular momentum conpensation point (SHNO temperature > RT due to Joule heating). While thermal SWs [ferromagnetic resonace (FMR)] are seen at all currents, the Gd rich SHNO only shows auto-oscillations (AO) at negative current, and the Gd poor SHNO only shows AO at positive current. (c,d) Log plots of the BLS counts from (a,b) for the thermal spin waves SWs (green) and the AO SWs (red). While the FMR show a slight current dependence due to heating, the AO intensity shows an initially exponential increase, as expected around threshold. Insets shows the BLS spectrum profile at the highest current.

**References:** 

[1] M. Romera et al., Nature 2018, 563, 230. 
[2] M. Zahedinejad et al., Nat. Nanotechnol. 15, 47 (2020). 
[3] B. Dieny et al., Nat. Elec. 3, 446 (2020). 
[4] T. Jungwirth et al., Nat. Nanotechnol. 11, 231 (2016). 
[5] S. K. Kim et al., Nat. Mat. 21, 24 (2022). 
[6] C. Stanciu et al., Phys. Rev. B 73, 220402 (2006). 
[7] D. Cespedes-Berrocal et al., Adv. Mat. 33, 2007047 (2021). 
[8] Lakhan Bainsla et al., Adv. Funct. Mater. 32, 2111693 (2022). 
[9] D.H. Kim et al., Phys. Rev. L 122, 127203 (2019). 
[10] Y. Hirata et al., Phys. Rev. B 97, 220403 (R) (2018). 
[11] L. Bainsla, A. A. Awad, J. Åkerman et al., unpublished.

Composition dependent threshold current polarity in ferrimagnetic spin Hall nano oscillators based on GdFeCo

The combination of inversion symmetry breaking and spin-orbit coupling (SOC) give rise to spin-orbit torque (SOT), charge pumping, and chiral magnetism. In conventional SOT studies, the inversion symmetry breaking usually comes from the structure or the geometry, e.g. crystal inversion assymmety in the bulk of diluted ferromagnet with zinc-blende structure (Fig. 1a) and space inversion assymmetry in a heavy metal/ferromagnet bilayer (Fig. 1b). It was known that the SOT strength can be controlled by the magnitude of the SOC, therefore, heavy metals with large SOC has been widely used for SOT-based magnetic random-access memory (MRAM).
In our study, we demonstrated that the vertical composition gradient in a single FePt layer acts as a new type of inversion symmetry breaking (Fig. 1c). The SOT efficiencies scale almost linearly with the composition gradient (G), which is defined to be the change of the Co/Pt ratio per 1 nm. We found that by changing G from 0.31 %/nm to 0.71 %, the SOT efficiency increases by 300 %. In experiments, the composition gradient can be easily controlled during sample growth. Therefore, our work provides a powerful tool to tune the SOT strength.
Then we demonstrated the current-induced magnetization switching in a FePt single layer (Fig. 1e). However, this switching requires an in-plane external magnetic field to break the symmetry. For real application, it is pursued to realize magnetic field-free switching. To achieve this goal, we move our attention from L10 FePt (P4/mmm) to fcc CoPt3. Both of them possess good perpendicular magnetic anisotropy (PMA) and hold promise in magnetic storage (media and memory). We found that the CoPt3 has two structural properties, as illustrated in Fig. 2(a). One is the composition gradient along the thickness direction which can break the inversion symmetry and allows for the generation of the in-plane damping-like torque. The other is the formation of the Co platelets in the Pt-rich matrix near the substrate during growth. We can make a symmetry analysis of the Co platelet/Pt structure, and we can easily find that the mirror symmetry is broken relative to the (11-2) plane and preserved relative to the (1-10) plane. Therefore, when the current is applied along the [1-10] direction (low-symmetry axis), the lateral mirror symmetry is broken so that the out-of-plane SOT can be allowed. Then we can achieve the field-free magnetization switching (0 deg in Fig. 2c). In contrast, when the current is applied along the [11-2] direction (high-symmetry axis), the lateral mirror symmetry is preserved and there is no field-free switching (90 deg in Fig. 2c). We also found that the current-induced switching exhibit a three-fold angular dependence on the current angle (θI), which is in perfect agreement with the three-fold rotational symmetry of the crystal structure in Fig. 2b.
To summarize, we observed the field-assisted magnetization switching in FePt single layer by introducing a new type of inversion symmetry breaking: composition gradient. Furthermore, we demonstrated the symmetry-dependent field-free magnetization switching in the CoPt3 single layer. The composition gradient along the film's normal direction gives rise to the in-plane damping-like torque while the low symmetry (3m1) property at the interface of Co platelet/Pt gives rise to the 3m torque in CoPt3. The cooperation of these two effects leads to a three-fold field-free switching in the CoPt3 single layer. Our result of the self-switching in CoPt3 has provided one of the most simplified structures for field-free switching of perpendicular magnetization. The good endurance and high thermal stability make it a good candidate for magnetic memory devices and other spintronic applications.

**References:** 

[1] L. Liu, et.al., Electrical switching of perpendicular magnetization in a single ferromagnetic layer Physical Review B 101 (22), 220402 (2020). 

[2] L.Liu, et.al., Current-induced self-switching of perpendicular magnetization in CoPt single layer Nature Communication 13, 3539 (2022).

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