United States

Spin-orbit torque (SOT) arising from spin-orbit coupling promises efficient magnetization switching in spintronic devices. It is important for device applications that the SOT switches perpendicular magnetizations without an external magnetic field. In addition to field-free switching, reducing SOT switching current is another essential requirement for low energy consumption.
In this talk, we first demonstrate spin currents and associated SOT in ferromagnet/non-magnet/ferromagnet trilayers, in which the torque on the top magnetic layer can be controlled by changing the magnetization of the bottom ferromagnetic layer. In this structure, by aligning the magnetization of the bottom magnetization parallel to the current direction, spin current can carry out-of-plane spin polarization, enabling field-free SOT switching of the top perpendicular magnetization [1]. The out-of-plane SOT is generated by spin current precession at the interface due to the interfacial spin-orbit field. Second, we show that the spin current caused by the spin anomalous Hall effect can be additionally exploited in epitaxial Co-based magnetic trilayers (2). The large in-plane magnetic anisotropy of the epitaxial Co makes it possible to control the magnetic easy axis of the bottom magnetic layer away from the current direction. It is found that the critical current for field-free switching is reduced when the magnetization is at an angle between 30 and 45 degrees from the current direction where the spin anomalous Hall effect is non-negligible.
References (1) S-h. C. Baek, et al, Nat. Mater. 17, 509 (2018)
(2) J. Ryu, et al, Nat. Electron. 5, 217 (2022)

MMM 2022

Efficient field free spin

technical paper

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Valley, as a new degree of freedom of electrons besides charge and spin, can be used to encode, transmit, store, and manipulate information [1-3]. By using various external methods, such as optical pumping, magnetic field, magnetic doping, and magnetic proximity effect, the valley polarization can be induced in materials for realizing practical valleytronic devices. Recently, ferrovalley materials, a system with intrinsic magnetic and valley characteristics, was proposed for 2H-VSe2 [4]. Compared to traditional valleytronic materials, the ferrovalley materials have potential to eliminate the limitations associated with external fields in inducing valley polarization. However, to date, most of the predicted ferrovalley materials possess in-plane magnetization, which impede the appearance of valley polarization in nature. The crucial challenge is to induce large perpendicular magnetic anisotropy to achieve spontaneous valley polarization in two-dimensional ferrovalley materials. The van der Waals heterostructure, constructed by isolated atomic layer by layer in a chosen sequence, can reveal unusual properties and phenomena. Here, using first-principles calculations, we find that A-type antiferromagnetic VSe2/CrI3 heterostructure can realize perpendicular magnetic anisotropy and spontaneous valley polarization for realizing anomalous valley Hall effect, while the magnetic anisotropy of VSe2 is in-plane. More interestingly, the electric-field-controlled valley states can be realized by capping a ferroelectric layer (In2Se3) on VSe2/CrI3. The half metal-to-semiconductor transition and turning off and on valley are achieved by switching the ferroelectric polarization. Our results of constructing ferrovalley/ferromagnetic and ferroelectric/ferrovalley/ferromagnetic two-dimensional van der Waals heterostrucutre to engineer electronic and valley states can be useful for developing low-dimensional spintronic and valleytronic devices.

Anomalous Valley Hall Effect in A type Antiferromagnetic Van der Waals Heterostructures

We demonstrate spin orbit torque (SOT) magnetization switching of (Pt/Co) multilayers with large perpendicular magnetic anisotropy (PMA) by a topological insulator Bi0.85Sb0.15 layer in both thermal activation and non-thermal regime with current pulse width τ down to 1 ns. For this purpose, we deposited a stack of Bi0.85Sb0.15(10 nm)/[Pt(0.4 nm)/Co(0.4 nm)]2/insulating buffer on a Si/SiOx substrate by magnetron sputtering, as shown in Fig. 1(a). The (Pt/Co) multilayers have a large PMA with anisotropy magnetic field of 6 kOe. We then fabricated 1000 nm × 800 nm Hall bar devices for ultrafast SOT switching measurement. Figure 1 (b) shows the threshold switching current density JBiSbth as a function of τ at Hx = 1 kOe. The red dashed line is fitting by the thermal activation model (τ >> 10 ns), which yields a zero-Kelvin JBiSbth0 = 2.5×106 A/cm2. Meanwhile, the green dashed line is fitting for the non-thermal regime (τ < 10 ns), which yields a larger JBiSbth0 = 4.1×106 A/cm2 but still smaller than those observed in heavy metals by nearly two orders of magnitude. Figure 1(c) shows SOT switching loops at various t from 4 ns to 1 ns. From time-resolved measurement using 1 ns pulses, we observed a fast domain wall velocity of 470 m/s at a modest JBiSb =1.6×107 A/cm2. We then successfully demonstrated deterministic SOT switching in our device by multi pulses (JBiSb =1.3×107 A/cm2, τ = 3 ns) at Hx = -1 kOe and +1 kOe as shown in Fig. 2. These results demonstrate the potential of BiSb topological insulator for ultralow power and ultrafast operation of SOT-based spintronic devices [1,2]. 
**References** [1] N. H. D. Khang, et al, Nature Mater. 17, 808 (2018).
[2] N. H. D. Khang, et al, Appl. Phys. Lett. 120, 152401 (2022). 
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Ultralow power nanosecond spin orbit torque magnetization switching induced by BiSb topological insulator

Weyl semimetals can generate large current-induced spin-orbit torques (SOT) that can manipulate the magnetization dynamics in the ferromagnetic materials, and thus play an important role in spintronic devices. Due to their concomitant reduced symmetries, Weyl semimetals are also promising for generating SOTs with novel symmetries that may be able to switch perpendicular magnetic anisotropy thin films. Recently, the Td phase MoTe2, which is Weyl semimetal, has attracted a lot of attentions in the field of spintronics, due to its theoretically predicted SOT efficiencies that are even larger than those of WTe2 [1]. Here, we studied the effects of processing conditions on the stoichiometry and crystal structure of the magnetron-sputtered MoTe2 films by using Rutherford backscattering and X-ray diffraction. Furthermore, we identified different phases of the sputtered MoTe2 thin films using Raman spectroscopy, and we have observed a stable Td semimetal phase even at room temperature. The Td phase is generally considered to be a low-temperature phase and only appears at temperatures below 250 K, and at room temperatures, MoTe2 will be in the 1Tâ€™ monoclinic phase or the 2H hexagonal phase [2]. We have also observed dimensionality-dependent phase transitions from the Td to the 1Tâ€™ phase at room temperature for sputtered MoTe2 thin films as the characteristic peak for the Td phase MoTe2 gradually shifts from 264 cm-1 to 259 cm-1(the Raman peak position for the 1Tâ€™ phase), as the thickness of the sputtered films increases (shown in Figure 1). Finally, SOT efficiencies of the MoTe2 thin films were measured via spin-torque ferromagnetic resonance (ST-FMR) on MoTe2/Ni80Fe20 heterostructures and we observed a large novel SOT due to the spins with out-of-plane polarization directions in both the symmetric and anti-symmetric components of the homodyne mixing voltage signals (shown in Figure 2). The efficiencies of the novel damping-like and field-like SOT are estimated to be 16% and 27%, respectively.

Spin orbit Torques in Magnetron

Nowadays, magnetic nanoparticles may be arranged into highly ordered, crystal-like structures, so-called mesocrystals. Compared to their bulk counterpart, the
characteristics of mesocrystals are not only determined by the specific material employed, but also by the shape of its nanoscale constituents, their ordering on the meso- and on the
macroscale and the resulting mutual interactions between them. We use ferromagnetic resonance (FMR) experiments and Brillouin light scattering (BLS) microscopy in combination with micromagnetic simulations to investigate and characterize
dipolar interactions between magnetic nanoparticles (MNPs) within such mesocrystals. The MNPs investigated in this work consist of iron oxide (magnetite - Fe3O4
) and are coated with
non-magnetic polymers, forming highly ordered hexagonal monolayer crystals as shown in fig. 1 a.
The magnetic response of the regularly arranged hexagonal mesocrystals can be tuned in a controlled way by varying the thickness of the non-magnetic polymer coating of the MNPs and
thus the lattice constant of the mesocrystal. The spectral features reveal that the dipolar coupling strength between the MNPs decreases as the spacing between MNPs increases, as shown in
fig. 2 a.
Structuring the monolayer by employing electron beam lithography, well-defined, circularly shaped MNP assemblies can be fabricated as shown in fig. 1 b. Performing BLS experiments a
main signal accompanied by a satellite signal can be observed, showing distinct dependencies on the externally applied field. 2D-BLS-mapping of the circular assembly reveals the
associated active areas within the assembly for the two resonances. The experimental findings of the FMR and BLS experiments are fully corroborated by micromagnetic simulations. 
**References** 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/5a5627a37b171538d2fba65619fefed4.png)
 
(a) Self-assembled, hexagonally arranged MNP monolayer. (b) Using electron beam lithography, well-defined MNP assemblies can be fabricated from monolayers as shown in (a). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/838fcbb80ef8fe957517300f67728c54.png)
 
(a) In FMR experiments on MNP monolayers (see fig. 1 (a)) of different spacing between the MNPs, but constant MNP size. (b) Investigations of structured MNP assemblies as shown in
fig. 1 (b) by BLS reveal two signals, both showing distinct dependencies on the externally applied field.

Dynamic properties of magnetic nanoparticles in highly ordered arrangements: distance and frequency dependent properties

Soft magnetic material with high magnetic flux density (B), high permeability (µ), and high resistivity (ρ) is the key for miniaturized high frequency (>10 MHz) passive devices. The popularity of ferrites wane beyond 10 MHz owing to their prohibitively large magnetic losses (Pm), similarly, the spherical ferromagnetic alloys are too lossy due to eddy current losses (Pe). Of late, composites of electrically insulated magnetic particles attracted much attention. However, the trade-offs between µ, ρ, Pm and Pe, etc. demand judicious design of materials to stem eddy-current losses and skin effect by optimizing the thickness and properties of the insulation layer surrounding the magnetic body [1,2]. 
Here we demonstrate magnetically-aligned FeSiB-based soft magnetic flakes (SMFs; 63-106 µm, t:2-5 µm) coated with optimally thick (~10 nm) SiO2 as the potential magnetic core for power inductors operating beyond 10 MHz. The SMFs with such a high aspect ratio was fabricated by combination of mechanical ball milling of ribbons and then electrically insulted by an optimized sol-gel procedure in presence of silane coupling agent (APTES). Milling helped to reduce ribbon thickness below its skin depth to eliminated Pe at high frequencies. The effect of milling (duration, RPM, BPR) and coating conditions on the properties of the SMFs were thoroughly investigated. An optimally-tiny fraction of the milled flakes is allowed to become nanocrystalline (Fig. 1c) that augments B and µ without incurring losses. XPS and magnetization measurement results indicate that the thin but robust insulation layer is preserved through the post-annealing process which is necessary for removal of stress. B-H loop shows post-processing, coercivity is reduced significantly (from ~57 to ~6 Oe), but magnetic flux remains same. 
**References:** [1] T. Suzukia, P. Sharmaa, A. Makino, J. Magn. Magn. Mater. 491 (2019) 165641 
[2] F. Luo, X. Fana, Z. Luoa, W. Hua, J. Wangd, Z. Wu, G. Li, Y. Li and X. Liu, J. Magn. Magn. Mater. 498 (2020) 166084 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/080b991caad833ef058948eeb3dc2146.png) 
Fig 1: (a) Schematic representation of the SMFs core. (b) SEM image of the FeSiB-based as-milled flakes. (c) XRD patterns and (d) hysteresis loops of the as-milled and of SiO2 coated flakes (~63-106 µm) before and after annealing at 425oC in an inert atmosphere.

Fabrication and soft magnetic properties of FeSiB based flakes with insulating surface layer

Achieving Boolean logic via shape anisotropy is not a foreign concept and the usage of fringe field-based interactions between single-domain nanomagnets has been explored [1-2]. However, bi-material ASI systems to fulfil two-input Boolean logic has not been reported. We are motivated to explore field-orientation-dependent logic-based reversal on bi-material structures. Co and Py nano-trapezoids orientated at 90Â° were fabricated via self-aligned stepwise nanosphere lithography [3] forming L-shape structures. Field-orientation dependent magnetization reversal studies were conducted via MFM by applying magnetic field at different angles and values. Fig.1(a) shows the AFM image while Fig.1(b-e) show the remnant magnetic domain images after applying a saturation field of 3 kOe at various field angles. Different configurations of Co (red) and Py (blue) can be defined as logic 0 or 1, as shown in Fig.2(a). The configurations can be divided into four stable and repeatable states â€˜00â€™, â€˜10â€™, â€˜11â€™, '01â€™ which agree well with MFM in Fig.1(b-c) respectively. All the conceivable states vs various orientations are shown in Fig.2(b). The one-in-one-out configuration, which can be attained when the structures are magnetized along the geometrical easy axis for each component, was found to be the most stable [4]. By setting this as logic 0 and others as logic 1, each L-shape can be considered as a two-input XOR gate. Under various external field values and orientation, the magnetic switching behavior of bi-material L-shape structures will be explored by MFM and compared to OOMMF [5] simulation results. Our preliminary simulations show that the bi-material system exhibits three stable states which can be set as logic 0 and 1 whereas the pure material system only produced two stable states which can only be set as logic 0. We will explore how the bi-material system can be exploited to fulfil other logic functions.

Study of field orientation dependent logic for bi

La(Fe,Si)13-based (1:13) compounds exhibit giant magnetocaloric effect and are promising candidates for room temperature as well as cryogenic magnetic refrigeration
[1,2]. The magnetic refrigeration materials require a comprehensive evaluation of their properties such as transition temperature (Ttr), entropy change (△Sm), adiabatic temperature change
(△Tad), thermal hysteresis (△Thys), relative cooling power (RCP), thermal conductivity and mechanical stability [3]. Extensive studies on tuning these properties have been carried out for
the 1:13 system by varying the chemical compositions and processing conditions. However, an excellent combination of properties has not been achieved yet, hindering any practical
application [4]. In this work, we employed machine learning (ML) to address the optimization problem in the 1:13 system. A dataset containing over 1000 samples in total was collected
using data mining across 200 articles. Several machine learning models were trained to predict Ttr, △Sm, RCP and Thys as targets and compared using the root mean squared error (RMSE)
metric. We found that random forest (RF) and gradient boosting (GB) regressors outperformed other models such as decision tree and k-NN regressors. The achieved performance of the
former model is demonstrated in Fig. 1 for the case of predicting Ttr. The relative feature importance extracted from the trained RF model in Fig. 2 shows the important parameters for
tuning the Ttr, while the corresponding Pearson correlation co-efficients (PCC) shows how these parameters influence the Ttr. Similarly, the critical parameters were found out for the other
target properties and used for further optimization study. The compositional optimization of the 1:13 system was performed by means of the differential evolution technique with a multiobjective
target toward the best combination of the magnetocaloric properties. 

**References:** 

1. A. Fujita et al., Phys. Rev. B, 67 <a href="tel:(2003) 104416">(2003) 104416</a>. 
2. S. Fujieda et al., Appl. Phys. Lett., 89 <a href="tel:(2006) 062504">(2006) 062504</a>. 
3. V. Franco et al., Annual Review of Materials Research, 42 <a href="tel:(2012) 305-342">(2012) 305-342</a>. 
4. V. Paul-Boncour et al., Magnetochemistry, 7 (2021) 13. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/69bb2304646da6758f791f3edd915c36.png)

Machine learning assisted optimization towards high performance La(Fe,Si)13

Magnetic Particle Imaging (MPI) is a novel medical imaging modality that is in a preclinical stage [1]. MPI scanners use rf magnetic fields to excite and measure oscillations in superparamagnetic iron oxide nanoparticles (SPIO) so that the signal is proportional to SPIO's concentration. Applying a gradient magnetic field allows an MPI scanner to spatially localize the signal. Our group has designed a single sided field-free line (FFL) scanner, which has all hardware on a single side of the device providing an open imaging volume, useful for imaging larger subjects such as humans. Compared to a field-free point scanner [2], our device promises higher sensitivity and robust image reconstruction. In the past, we demonstrated 1D imaging capabilities by implementing electronic scanning of field-free line (FFL) across a simple rod phantom [3] and simulated 2D image reconstruction with backprojection technique [4]. In this work we demonstrate the 2D imaging capability of our prototype scanner by imaging phantoms, which include a single rod and two rods in orthogonal orientation, i.e. "elbow". Each rod has a diameter of 1.2 mm and a length of 10 mm and filled with undiluted SynomagD SPIO. To excite the SPIO in our samples we applied an rf magnetic field created by a low noise sinusoidal current source at 25 kHz producing a magnetic field of 1.5 mT at the surface of the scanner. For image encoding the FFL is created and dynamically scanned across the sample by two programmed DC power supplies providing a gradient of 0.58 T/m. The signal is recorded at 0.5 mm step over 4 cm interval of a rotated sample at diffrent discrete angles. The resulted backprojection reconstruction images are shown in the Fig.1. The final images were also deconvoluted with the experimentally obtained point spread function providing a spatial resolution of 4 mm. At this relatively low field gradient this is a reasonable result and in the future we will incorporate higher field gradient and dynamic trajectory correction to improve the uniformity of the image. This work is a significant first step towards human MPI imaging that can be potentially used for diagnostics and biopsy of cancer.

Two Dimensional Imaging Using a Single Sided Field Free Line Magnetic Particle Imaging Scanner

The search for efficient approaches to realize local switching of magnetic moments in spintronic devices has attracted intense interest. In particular, spin-orbit torque (SOT) that arises from materials with large spin-orbit coupling (SOC) offers a new pathway for energy-efficient and fast magnetic information storage. Recent studies on SOT-induced magnetization switching and dynamics in magnetic heterostructures have shown promise for SOT-MRAM [1-2]. We have demonstrated electrical manipulation of magnetization and exchange bias (EB) in an antiferromagnet/ferromagnet (AFM/FM) based device via SOT [3]. Perpendicular EB reversal across AFM IrMn and FM [Co/Pt]2 multilayer has been shown, both in extended and confined geometries. In three-terminal perpendicular magnetic tunnel junction (MTJ) devices (Figs. 1a and 1b), we have achieved the switching of the magnetization and EB using the SOT (Fig. 1c). Both high- and low- resistances have been observed at zero magnetic field during EB switching (Fig. 1d). These findings provide a new direction in exploring the electrical control of EB in SOT magnetic random access memory (SOT-MRAM). In another study, we have demonstrated that the magnon current excited in an insulating NiO AFM layer by an electronic spin current in an all-oxide epitaxial SrRuO3/NiO/SrIrO3 heterostructure is effective for manipulating the perpendicular magnetization in the SrRuO3 layer [4]. Fig. 2 shows the schematic of the heterostructure and TEM image of the interfaces and the current driven magnetization switching. Interestingly, the threshold current density to generate a sufficient magnon current to manipulate the magnetization was one order of magnitude smaller than that in conventional metallic systems. These results suggest a route for developing highly efficient magnon based all-oxide spintronic devices. 

This work has been supported by KAUST, SRC/NIST SMART center, and US-NSF. 
**References** [1] I.M. Miron, et al, Nature, 476 (2011) 189. 
[2] L.Q. Liu, et al, Science, 336 (2012) 555. 
[3]. B. Fang, et al, Advanced Functional Materials,2022, 32, 2112406. 
[4]. D.X. Zheng, et al, Advanced Materials, revised and resubmitted. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/921ef79571b61a3518c8be48d735b57c.png)

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