α”-Fe16N2 is a promising environmentally-friendly rare-earth-free permanent magnet material with ultra-high saturation magnetization. Recent research has demonstrated experimentally through a thermally quenching treatment using γ’ phase Fe4N as a precursor to synthesize α”-Fe16N2 in bulk format. In this research, we investigated γ’-Fe4N thin film thermal decomposition process and potential localized phase transition from fcc phase to bct phase using Molecular Dynamics (MD) simulation method. As nitrogen concentration is much higher in γ’-Fe4N (5.9 wt.%) than that in α’-Fe8N or α’’-Fe16N2 (3 wt.%), Nitrogen “depletion” process must have to occur during the thermal decomposition to form possible lower-Nitrogen content bct Fe-N solid solution. A localized “Nitrogen-rich” grain boundary lattice defect model is designed to induce Nitrogen bond forming through the recombination of nitrogen pair atoms, and therefore significantly improves local reduction of Nitrogen content in the α’-Fe4N lattice sample during thermal treatment. Modified Embedded Atom Method (MEAM) interatomic potential of Fe-N system is applied. Bond forming/breaking and local Nitrogen displacement analysis (MSD) are also performed in the thermostat-controlled heating/quenching simulation process. We use virtual XRD method and formation energy calculation to assess and detect the new material phases on the thermally-treated and energy minimized lattice sample. 
**References:** [1] Jian-Ping Wang, “Environment-friendly bulk Fe16N2 permanent magnet: Review and Prospective”, Journal of Magnetism and Magnetic Materials, 497(2020) 165962 
[2] J. Zhu, G, Guo and J.-P. Wang, "Study of γ'-F4N Annealing Process Through Molecular Dynamics Modeling", TMS 2022 151st Annual Meeting, The Minerals, Metals & Materials Series, Page 109-117, https://doi.org/10.1007/978-3-030-92381-5_11 
[3] M.H. Wetzel, M.R. Schwarz, A. Leineweber, High-pressure high-temperature study of the pressure induced decomposition of the iron nitride γ′-Fe4N, Journal of Alloys and Compounds, Volume 801, 2019, Pages 438-448

Simulation of Thermal Decomposition of γ’ Fe4N using Molecular Dynamics Method

Patterned FePt media solutions, such as Bit-Patterned Media (BPM) and Embedded Hardmask Patterning (EMP), have been proposed as alternative methods to improve Heat-Assisted Magnetic Recording areal density. Understanding etching mechanism of FePt material is important to warrent the low damage to the magnetic properties caused by micro-fabrication process. In this research, Reactive Molecular Dynamics (MD) modeling is carried out to study methanol (MeOH) plasma etch on L10-FePt media. The atomistic-level etch simulation process is divided into two sequential MD simulation steps: the radical-based reactive treatment step, followed by the ion-based physical etch step. Carbon-monoxide (CO) and H2 molecules are identified as the main etch precursors in methanol plasma interacting with substrate. Reactive interaction between CO/H2 radicals and Fe/Pt atoms is studied using bond-order ReaxFF force field with atomic charge calculation. Bond energy calculation shows the positively charged Fe and Pt and the negatively charged C atoms in CO molecules can form ionic carbonyl bonds that are shown more stable than Fe-Pt metallic bonds. As the result, it is found that the number of Fe(CO) and Pt(CO) carbonyl bonds are dominated among the new bonds produced after the reactive-treatment process. In the following physical etch step simulated with Ar or He ion bombardment, in comparison with FePt surface without CO/H2 treatment, the sputter yield of FePt can significantly improve by 2-4X on the samples with the CO/H2-treatment step. 
**References** 
[1] H. Wang, H. Zhao, P. Quarterman, and J.-P. Wang, Appl. Phys. Lett., vol. 102, no. 5, p. 052406, 2013 
[2] Patent: US10,347,467B2, J.-P. Wang, P. Quarterman and J. Zhu 
[3] LAMMPS - A flexible simulation tool for particle-based materials modeling at the atomic, meso, and continuum scales, Comp. Phys. Comm. (accepted 09/2021),
DOI:10.1016/j.cpc.2021.108171 
[4] J. Zhu, P. Quarterman, J.-P. Wang, AIP Advances 2017, 7 (5) , 056507. https://doi.org/10.1063/1.4977223 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/a01666868c95347f3bf6c30ec19d4fec.png) 
Figure 1. Number and types of carbonyl bonds created on FePt surface after CO/H2 exposure

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/535be17232038eb114b6084ff7a87b38.png) 
Figure 2. FePt sputter yield ( with Ar ions) comparison on CO/H2-treated and non-treated surface

Reactive Molecular Dynamics Modeling of Plasma Etching of L10 FePt Magnetic Media

In this talk, we first report the roadmap and development of advanced perpendicular magnetic tunnel junctions based on low-damping magnetic materials with crystalline anisotropy that can be scaled down to sub-3-nm diameter with 10 years’ retention time [1]. Then, we will present the perpendicular magnetic tunnel junctions (p-MTJs) switched utilizing bipolar electric fields. Traditional voltage-controlled magnetic anisotropy only linearly lowers the energy barrier of ferromagnetic layer via electric field effect and efficiently switches p-MTJs only with a unipolar behavior. Here we propose and demonstrate a bipolar electric field effect switching of 100-nm p-MTJs through voltage-controlled exchange coupling (VCEC) [2]. The switching current density, ~1.1×105 A/cm2, is one order of magnitude lower than that of the best-reported spin-transfer torque devices. Theoretical results suggest that electric field induces a ferromagnetic-antiferromagnetic exchange coupling transition and generates a field-like interlayer exchange coupling torque, which cause the bidirectional magnetization switching. We will further report our recent results to switch the p-MTJs using the interplay of SOT and VCEC, which lowers the VCEC switching current density to ~5×103 A/cm2[3]. These results could eliminate the major obstacle in the development of spin memory devices beyond their embedded applications. This technology can be used directly for the future spin-orbit-torque (SOT) MRAM [3] and spin logic devices [4]. 
**References** [1] D. Zhang, et al, Phys. Rev. Applied, (2018) vol. 9, 044028 
[2] D. Zhang, et al, Nano Letters (2022), doi.org/10.1021/acs.nanolett.1c03395 
[3] B. Zink, et al, Advanced Electronic Materials, (2022), DOI: 10.1002/aelm.202200382 
[4] M. Mankalale, et al, IEEE Journal on Exploratory Solid-State Computational Devices and Circuits, 3 (2017) 27-36; 10.1109/JXCDC.2017.2690629 

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Perpendicular Magnetic Tunnel Junctions with Crystalline Anisotropy and Energy efficient Switching of Synthetic Antiferromagnet pMTJs by voltage

The unidirectional spin Hall magnetoresistance (USMR) is initially discovered in ferromagnetic (FM) and normal metal (NM) structures[1] and magnetic semiconductor structures[2]. It is a phenomenon observed in bilayer structures and is related to interfacial spin generation and scattering processes at the interface. The similar unidirectional spin Hall and Rashba-Edelstein magnetoresistance (USRMR) is observed in FM-topological insulator (TI) bilayers being much larger than that in FM-NM bilayers in bilayer structures[3]. They are of particular interest due to its potential of 180-degree sensitive electrical readout of magnetization in bilayer structures for spin-orbit torque switching devices.
Here, we report large unidirectional spin Hall and Rashba−Edelstein magnetoresistance in a new material family - magnetic insulator (MI)/TI Y3Fe5O12 (YIG)/Bi2Se3 bilayers. Such heterostructures exhibit a USRMR that is about an order of magnitude larger than the highest values reported in all-metal Ta/Co bilayers[1]. The polarized neutron reflectometry (PNR) reveals a unique temperature-dependent magnetic intermediary layer at the MI-substrate interface and a proximity layer at the MI-TI interface. These PNR findings echo the magnetoresistance results in a comprehensive physics picture. Finally, we demonstrate a prototype memory device based on a MI/TI bilayer, using USRMR for electrical read out of current-induced magnetization switching aided by a small Oersted field[4].
 
**References** [1] C. O. Avci, K. Garello, A. Ghosh et al., Nat. Phys., vol. 11, no. 7, p. 570–575 (2015)
[2] K. Olejník, V. Novák, J. Wunderlich et al., Phys. Rev. B - Condens. Matter Mater. Phys., vol. 91, no. 18, p. 180402 (2015)
[3] Y. Lv et al., Nat. Commun., vol. 9, no. 1, p. 111 (2018)
[4] Y. Lv et al., Appl. Phys. Rev., vol. 9, no. 1, p. 011406, (2022) 
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Large unidirectional spin Hall and Rashba−Edelstein magnetoresistance in topological insulator/magnetic insulator heterostructures

Magnetic random access memory (MRAM) is a promising candidate for beyond-CMOS memory and logic technologies and has been developed for commercial purposes in recent years (1). The key building block of commercial MRAM is magnetic tunnel junction (MTJ), which is operated through either spin transfer torque (STT) or spin-orbit torque (SOT). STT-based MRAM benefits from higher cell density but suffers from reliability and energy-efficiency due to the current flow through the MTJ whereas SOT-based MRAM has superior reliability, endurance, and faster switching times (2). For realizing the ultralow switching current density and ultrafast switching MRAM cell, two or more driving forces are normally employed to switch MTJs. One acts as the principal switching mechanism and the other serves to assist in switching. For example, ultrafast switching speed of ~ 0.27 ns has been reported in SOT switching of perpendicular MTJs (p-MTJs) assisted with STT and voltage-controlled magnetic anisotropy (VCMA), however, the key challenge is still large switching current densities (Jc) ~ 2.0×108 A/cm2 for SOT and ~ 2.3×106 A/cm2 for STT (3).
Voltage controlled exchange coupling (VCEC) occurs in perpendicular MTJs (p-MTJs) with synthetic antiferromagnetic (SAF) free layers and can realize bidirectional switching at switching current densities as low as 1 x 105 A/cm2 (4). In this work, we designed and fabricated the p-MTJ stacks with SAF free layers on bi-layered spin Hall channels (Ta/Pd). The p-MTJ stacks were patterned into 150-nm pillars, and MTJ devices are switched through combination of SOT and VCEC effects, as illustrated in Fig. 1(a). Bidirectional switching of p-MTJs were obtained with current densities as low as 3×103 A/cm2 for VCEC and 6.8×107 A/cm2 for SOT, as shown in Fig. 1(b), where the VCEC switching current density is two orders of magnitude lower than the reported value for VCEC-only switching [4]. Furthermore, by studying the contribution of SOT and VCEC for MTJ switching, it is found that SOT plays a crucial role for ultralow-energy VCEC bidirectional magnetization switching. 

**References:** 
(1) J.-P. Wang et al, DAC '17: Proceedings of the 54th Annual Design Automation Conference 16, pp. 1-6 (2017). 
(2) Y. C. Liao et al, IEEE J. Explor. Solid-State Computat. 6, pp. 9-17 (2020). 
(3) E. Grimaldi et al, Nat. Nanotechnol. 15, pp. 111-117 (2020). 
(4) D. Zhang et al, Nano Lett., 22, pp. 622-629 (2022). 

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Ultra low current density switching via voltage

Unidirectional spin Hall magnetoresistance (USMR) is a magnetoresistance effect with potential applications to read two-terminal spin-orbit-torque (SOT) devices directly. In this work, we observed a large USMR value (up to 0.710-11 per A/cm2, 50% larger than reported values from heavy metals) in sputtered amorphous PtSn4/CoFeB bilayers. Ta/CoFeB bilayers with interfacial MgO insertion layers are deposited as control samples. The control experiments show that increasing the interfacial resistance can increase the USMR value, which is the case in PtSn4/CoFeB bilayers. The observation of a large USMR value in an amorphous spin-orbit-torque material has provided an alternative pathway for USMR application in two-terminal SOT devices. 

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Observation of unidirectional spin Hall magnetoresistance in amorphous PtSn4/CoFeB bilayers

In this work, we report the design of a spin torque nano-oscillator (STNO) combined with magnetic flux concentrators (MFCs) to improve the sensitivity of detecting magnetic fields, where the STNO consists of two out-of-plane (OP) ferromagnetic layers separated by a MgO insulating layer (see Fig. 1(a)) and the MFCs placed next to STNO are permalloy. We carried out micromagnetic simulations on the spin-transfer torque (STT) induced magnetic dynamics in the free layer of STNO. Based on the Landau-Lifshitz-Gilbert (LLG)
equation with STT term, the time evolved magnetization precession of the free layer is plotted in Fig. 1(b). First, we show that the ferromagnetic resonance frequency (FMR) of the free
layer magnetization precession can be tuned by the charge current density and external magnetic fields. In addition, the FMR as a function of external magnetic field strength is also studied
on STNO with and without MFCs, see Fig. 1(c). We observed a general suppression of the resonance frequency due to the damping effect of the magnetization induced by MFCs placed at
different distances to the STNO (see Fig. 1(d)). The FMR shift of the STNO free layer magnetization as function of external magnetic field is summarized in Fig 1. (e). By placing MFCs
next to STNO, the lowest detectable magnetic field strength is enhanced from 10 μT to 10 nT. It is concluded that MFCs improve the sensitivity of STNO to external fields due to the
distortion caused by the magnetization damping. The results presented in this work could inspire the optimal design of STNO and MFC-based ultra-low magnetic field sensors. 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/300448a262a8a122d2d995834592eccc.png) 
Fig. 1. (a) Schematic view of STNO with a diameter of 80 nm. Charge current is defined as +J and applied in the positive direction. (b) Evolution of the free layer magnetization resonating
in-plane from the initial condition x,y,z> = (0, 0, 1). (c) Schematic view of MFCs placed next to STNO. (d) The distance between MFCs and STNO affects the resonance frequency. J=5e10 A/m2. (e) FMR shift of the magnetization induced by an external magnetic field for an STNO with and without MFCs, where LOD stands for limit of detection. J=5e10 A/m2, MFCs are placed at 70 nm to the STNO.

Magnetic Field Detection Using Spin Torque Nano Oscillator Combined with Magnetic Flux Concentrators

Future embedded and standalone magnetic random access memory (MRAM) solutions require materials possessing bulk magnetocrystalline anisotropy at sub-10 nm feature sizes. FePd in its L10-ordered phase delivers MJ/m3 bulk anisotropy energy density (Ku) [1], large interlayer exchange coupling through Ru [2], Gilbert damping (α) below 0.008 [3] and robust compatibility across a range of Pt-group buffer layers [4]. Transitioning FePd films into mature spintronic applications requires a higher level of process control realize highly uniform microstructural, magnetic and magnetotransport properties. 
We demonstrate control of the orientation distribution of the c-axis in highly (001)-oriented L10 FePd. Using direct current magnetron sputtering on (001)-cut, polished MgO substrates, we explore an N2 substrate anneal, variation in the individual thicknesses of the Cr/Pt underlayer and post-anneal modules between layers. Each sample had an 8-nm-thick FePd layer grown at 350 °C and post-annealed at 500 °C, leading to a high degree of L10 ordering, with Ku ranging between 0.8 MJ/m3 and 1.1 MJ/m3 and α ranging from 0.0067 to 0.0133. Despite the relatively narrow spread in these key parameters, we observed a wide range of inhomogeneous linewidth broadening (µ0ΔH0) from ferromagnetic resonance measurements, ranging from below 0.01 T to nearly 0.2 T (Fig. 1(a)). Variation in µ0ΔH0 correlates with the FePd grain orientation distribution, as measured by x-ray diffraction rocking curve scans of the (002) Bragg peak (Fig. 1(b)). Our process controls enable varying the full-width-half-maximum (FWHM) of the (002) peak from 14 degrees down to nearly 2 degrees, leading to narrowing of the local anisotropy distribution and reduction in the µ0ΔH0 (Fig. 1(b) inset). These process improvements will deliver the large-scale uniformity needed to implement FePd-based MRAM solutions. 
**References:** [1] M. Futamoto et al., AIP Advances 6, 085302 (2016) 
[2] D.-L. Zhang et al., Phys. Rev. Appl. 9, 044028 (2018) 
[3] D.-L. Zhang et al., Appl. Phys. Lett. 117, 082405 (2020) 
[4] X. Wang et al., AIP Advances 11, 025106 (2021) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/aa4b6704bacd0147dba693b652175112.png) 
Figure 1: (a) FMR Linewidth versus frequency for three samples with inhomogeneous line broadening of 0.178 T (blue), 0.075 T (green) and 0.015 T (red). (b) Inhomogeneous line broadening versus FWHM of FePd(002) (Δω), with (inset) exemplary FePd(002) rocking curve scans of the red, blue and green star samples highlighted with colored markers in (a)

Control of grain orientation spread and inhomogeneous broadening in L10 phase FePd thin films with low damping and high perpendicular magnetic anisotropy

In this in vivo study, we have used Magnetic Pen or MagPen (see Fig. 1(a)) to stimulate the rat sciatic nerve which led to the report of the dosage response curve for
micromagnetic stimulation (μMS). As reported earlier [1], the MagPen prototype consists of sub-mm size microcoil (μcoil) at its tip (see Fig. 1(b)). When driven by alternating current, this
μcoil generates a time-varying magnetic field, which induces an electric field on any conductor (here, the sciatic nerve) according to Faraday’s Law of electromagnetic induction. This
induced electric field activates the nerve. This work reports how the directionality of the magnetic μcoil plays a direct role in activating nerves directly observed in hind limb muscle
twitches. These limb twitches have been quantitatively characterized using electromyography (EMG) recordings. Furthermore, we have studied how on varying the amplitude and
frequency of the micromagnetic stimuli, alters hind limb muscle movements. Thereby, a dosage response curve for μMS was generated. The highlight of this dosage response curve is that,
at higher frequencies, significantly smaller amplitudes of MagPen stimuli can trigger limb twitches. This frequency-dependent nerve activation can be explained directly from Faraday’s
Law as the magnitude of the induced electric field used in neuromodulation is proportional to frequency. In our previous work of in vitro μMS of rat hippocampal slices [1], we explained
the frequency-dependent, low-power activation of μMS through numerical simulations. This work further corroborates the same concept through experiments. Implantable μMS is a novel
neuromodulation technique which is still in its infancy and aims to address the potential drawbacks of electrical implants in terms of MRI compatibility and biofouling. Since the induced
electric fields from these magnetic μcoils are not in galvanic contact with the nerve, μMS offers flexibility in applying waveforms to drive these magnetic implants (see Fig. 1(c)).
Fig 1. (a) Different directionalities of the MagPen prototype. Only Type V can activate the sciatic nerve. (b) Size of the sub-mm size μcoil implant compared to a rice grain. (c) The
waveform that activated the nerve. 

**References:** 

[1] Saha R., Faramarji S et al. (2022) Strength-frequency curve for micromagnetic neurostimulation (μMS) through EPSPs and numerical modeling. J. Neural Eng. 19 016018. 

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In vivo Micromagnetic Stimulation of the Rat Sciatic Nerve

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

The generation of phonons by ferromagnetic resonance in magnetic thin films and subsequent propagation of phonons into the substrate is an effect known as phonon pumping [1-2]. This effect can be utilized in spintronic devices that rely on magnetoelastic coupling for information transfer. The phonon pumping effect is most efficient in the perpendicular configuration, and can therefore be suitably integrated in devices composed of PMA materials. Here we demonstrate the pumping of phonons by ferromagnetic resonance in a series of [Co(0.8 nm)/Pd(1.5 nm)]n multilayers (n = 6, 11, 15, and 20) with large magnetostriction and perpendicular anisotropy. The effect is shown using broadband ferromagnetic resonance over a range of temperatures (10 to 300 K), where a resonant damping enhancement is observed at frequencies corresponding to standing wave phonons across the multilayer. The strength of this effect is strongly dependent on temperature—approximately a factor of 4 larger at 10 K compared to room temperature (see figure). Lastly, we demonstrate that the damping enhancement coincides with a shift in the ferromagnetic resonance field, and further that it is a complementary effect that can be predicted from the resonant damping enhancements. We emphasize that [Co/Pd]n multilayers are an ideal platform for studying this effect due to their large magnetostriction and perpendicular anisotropy. Importantly, the PMA does not show significant dependence on multilayer thickness, so devices can be engineered that have phonon pumping resonances at zero applied field. This work was supported by SMART, a center funded by nCORE, a Semiconductor Research Corporation program sponsored by NIST.
References [1] S. Streib, H. Keshtgar, and G. E. W. Bauer, Damping of Magnetization Dynamics by Phonon Pumping, Phys. Rev. Lett. 121, p.027202 (2018).
[2] K. An, A. N. Litvinenko, R. Kohno, A. A. Fuad, V. V. Naletov, L. Vila, U. Ebels, G. de Loubens, H. Hurdequint, N. Beaulieu, J. Ben Youssef, N. Vukadinovic, G. E. W. Bauer, A. N. Slavin, V. S. Tiberkevich, and O. Klein, Coherent long-range transfer of angular momentum between magnon Kittel modes by phonons, Phys. Rev. B 101, p.060407(R) (2020).

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Ferromagnetic resonance linewidth enhancements due to phonon pumping at different temperatures for the [Co/Pd]11 multilayer. Linewidth contributions from Gilbert damping and inhomogeneous broadening are excluded. Vertical dashed lines at 23 and 46 GHz show positions of phonon pumping resonances.

Anomalous Temperature Dependence of Phonon Pumping by Ferromagnetic Resonance in [Co/Pd]n Multilayers with Perpendicular Anisotropy

Embedded Mask Patterning (EMP) has been demonstrated as a potential HAMR media solution with formation of ultra-fine and highly-ordered L10-phase FePt grains[1]. We used a hybrid model including both reactive and pair-wise interatomic potentials in the classic Molecular Dynamic (MD) simulation to study the methanol-based reactive plasma etching mechanism of the FePt lattice. The major reactive redical etchants in the methanol plasma are identified as Carbon monoxide and Hydrogen spieces[2]. Adsorbed Carbon monoxide molecules reacts with the Fe and Pt atoms and forms caronyl bonds. With assisted ion beams such as Ar+ or He+, we show the chemical etch yield is increased. The ordering change of FePt crystal surface during the etch are also simulated.

![](https://assets.underline.io/uploads/markdown_image/1/image/83abf9422e8c50f158b238db1a099582.jpg)

MD Simulation of Plasma Etch of FePt Media

Reactive Molecular Dynamic Simulation of Plasma Etching Process of L10-FePt HAMR Media in Embedded Mask Patterning

Spin-orbit torque (SOT) switching of perpendicular magnetic tunnel junctions (p-MTJs) has been attracted considerable interest recently for spin memory applications due to the fast magnetization switching speed (1~4). Recently, the synthetic antiferromagnetic (SAF) free layer provides lots of benefits for MTJ performance, such as very small stray field, ultrafast switching speed, and large thermal stability (5~6). In this presentation, we will show our experimental results about the SOT switching of the SAF free layer via the bilayer spin Hall channel. The SAF stack of the [Co/Pd]/Co/Ru/Ta/CFB with perpendicular magnetic anisotropy (PMA) was developed on Ta/Pd seed layers which also serve as the spin Hall channel for spin-orbit torque (SOT) switching. First of all, the antiferromagnetic coupling behavior of the SAF layers was verified under a sweeping magnetic field with VSM and AHE measurements. Then, the Second Harmonic Hall measurement was conducted to characterize the SOTs generated by the Ta/Pd bilayers. The results are plotted in Fig. 1(a). We can find that the spin-torque efficiency of the Ta/Pd bilayer increases as increasing of the thickness of the Pd layer. With the thinner Pd layer, the spin torque efficiency cancels each other because of the opposite spin Hall angle. When the thickness of the Pd layer increases, the spin torque efficacy is dominant by the Pd layer. Furthermore, the current switching of the SAF layers with SOT was then demonstrated as shown in Fig. 1(b). In addition, magnetic field-free SOT switching of the SAF layer with PMA was also realized at a certain configuration of the Ta/Pd bilayer structure.

**References:**

 1. I. M. Miron, K. Garello, G. Gaudin, P.-J. Zermatten, M. V. Costache, S. Auffret, S. Bandiera, B. Rodmacq, A. Schuhl and P. Gambardella, Nature 476, 189–193 (2011) 2. L. Liu, C. F. Pai, Y. Li, H. W. Tseng, D. C. Ralph, R. A. Buhrman, Science 336, 555-558 (2012) 3. A. Manchon, J. Zelezny, I. M. Miron, T. Jungwirth, J. Sinova, A. Thiaville, et al., Rev. Mod. Phys. 91, 035004 (2019). 4. E. Grimaldi, V. Krizakova, G. Sala, F. Yasin, S. Couet, G. S. Kar, K. Garello, and P. Gambardella, Nat. Nanotech. 15, 111–117(2020) 5. C. Yoshida, T. Takenaga, Y. Iba, Y. Yamazaki, H. Uehara, H. Noshiro, K. Tsunoda, A. Hatada, M. Nakabayashi, A. Takahashi, M. Aoki, and T. Sugii, IEDM13-68, 3.4.1 6. A. Bergman, B. Skubic, J. Hellsvik, L. Nordström, A. Delin, and O. Eriksson, Phys. Rev. B 83, 224429 (2011)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/92188869b59cbf4de9d50735a1fa44b6.jpg)
FIG. 1. (a) the damping-like spin Hall efficiency (τDL) as a function of the thickness of the Pd layer (tPd). (b) SOT switching of SAF layer through the bilayer spin Hall channel.

 Spin-orbit torque switching of synthetic antiferromagnetic layer by metallic bilayers with opposite spin Hall angles

In recent years, magnon-phonon coupling has attracted renewed interest[1] both from the physics and technological perspectives owing to its efficient conversion of energy between magnon and phonon. In particular, the magneto-acoustic resonance, has become a focal point, partly owing to the possibility for rapid switching, which is significant for applications in cloud storage, advanced spin memory, logic, and other spintronic devices. An ultrafast optical approach, specifically femtosecond laser pulses, is a powerful tool to initiate the coupling and its successive detection. The laser pulse simultaneously generates the phonon and magnon, through thermal expansion and fast demagnetization. The thermal expansion produces a local strain or phonon, whose frequency is tunable by both the material composition and geometry. At the same time, the fast demagnetization excites the magnetization out of equilibrium and creates magnons. The femtosecond time scale of the laser pulse benefits the detection of both magnon and phonon, as their dynamics endure longer than the pulse time scale. However, research reported in the literature mostly launch strain electrically in piezoelectric materials, which leads to a different phonon propagation direction. In addition, the literature study of the coupling is confined to materials unsuitable for spintronic applications. These materials, limit the resonance frequency or require low temperature for the magnetoacoustic resonance. To date, direct demonstration of magnon-phonon coupling remains elusive in materials with high perpendicular magnetic anisotropy, which are capable of achieving ultrahigh frequency resonance at room temperature. Here, we report an extremely high frequency magnetoacoustic resonance up to 60 GHz, originating from magnon-phonon coupling in [Co/Pd]n multilayer with perpendicular magnetic anisotropy. Our theory uncovers the physics of resonance in a coupled system, and is based on the equation of motion for both magnon and phonon, with coupling introduced in the Hamiltonian through the magnetostrictive energy. Owing to the magnon and phonon possessing the same symmetry, the physics at resonance presents an enhanced wave envelope in the time domain, an anticrossing in the frequency domain, and significant hybridization of both magnons and phonons. When the frequencies of magnon and phonon approach each other in the resonance regime, these two modes hybridize into a quasiparticle contributed by both magnon and phonon. This hybridization generates an anticrossing in the dispersion with a frequency gap Δf in Fig. (a), which is quantitatively determined by the coupling coefficient in our model and consistent with our experiment. The amplitudes of both modes increase in the hybridization regime in Fig. (b): the original phonon mode is now highly visible owing to the admixed magnon, the original magnon mode is enhanced owing to the pumping from phonons. In addition, the resonance in the time domain (an enhanced wave envelope) indicates the strain substantially influences the spin dynamics at an ultrafast picosecond time scale. All these physical features are demonstrated in both ultrafast optical measurements and micromagnetic simulations. The ultrafast optical measurements [3] of the time-resolved magneto-optical Kerr effect (TR-MOKE) and time-domain thermoreflectance (TDTR) directly observe the dynamics of magnon and phonon. The frequency and the amplitude of TR-MOKE and TDTR signals are consistent with the modeling, which indicates the transition of the system from hybridization to de-hybridization, controlled by the external applied field. The simulations apply the properties of the [Co/Pd]n multilayers, whose structures are optimized to achieve a maximum resonance frequency allowable with current experimental settings. All the [Co/Pd]n multilayers samples were prepared in an ultra-high vacuum magnetron sputtering system. With an appropriate strain, the simulation predicts that the system is able to undergo a large-angle magnetization precession and achieve a rapid switching. In summary, our work demonstrates an efficient energy transfer among magnon and phonon at an ultrafast picosecond scale, which paves a potential pathway for enabling an ultrahigh speed strain-assisted magnetization switching in a technologically relevant magnetic system.

**References:**

[1] A. Kirilyuk, A. V. Kimel, T. Rasing, Ultrafast optical manipulation of magnetic order. Rev. Mod. Phys. 82, 2731–2784 (2010). [2] D.-L. Zhang, J. Zhu, T. Qu, D. M. Lattery, R. H. Victora, X. Wang, J.-P. Wang, High frequency magnetoacoustic resonance through strain-spin coupling in perpendicular magnetic multilayers. Sci. Adv. 6, eabb4607 (2020) [3] D. B. Huang, D. Lattery, X. Wang, Materials Engineering Enabled by Time-Resolved Magneto-Optical Kerr Effect for Spintronic Applications, ACS Appl. Electron. Mater., 2020 (appears online).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/da1d7a3d7e4d08d14d19d9c77f1710f1.jpg)
The frequency of the [Co(0.8 nm)/Pd(1.5 nm)]11 multilayers as a function of Hext. Two frequencies of spin precession (mode 1, open black circles; mode 2, open red diamonds) are derived by fitting the experimental data of TR-MOKE. The figure also includes the frequency of acoustic waves measured from TDTR (blue stars). The anticrossing point of mode 1 and mode 2 occur at the resonance field (Hext ≈ 21 kOe), where the frequencies of modes 1 and 2 split and open a gap Δf. We assign the strain mode as the one with field-independent frequencies that are nearly identical to the acoustic wave frequencies from TDTR. The frequency of the FMR mode increases linearly with Hext.

![](https://assets.underline.io/uploads/markdown_image/1/image/33d75b34edaa03dce17f237aff203cd9.jpg)
The individual Mz amplitudes of modes 1 and 2 as a function of Hext for the [Co(0.8nm)/Pd(1.5nm)]11 multilayers. There exists an apparent amplification of both modes due to the coupling between these two modes near the anticrossing point. (Figure is taken from Ref. 2.)

 High-frequency magnetoacoustic resonance through strain-spin coupling in perpendicular magnetic multilayers INVITED

Spin-torque oscillators (STOs) have been extensively studied due to their potential applications in hardware neural networks [1]. For future practical STO devices, multiple magnetic tunnel junctions in series are demanded. Because it will involve a complicated stack structure, there are very limited experimental attempts so far. In this work, the spin-torque oscillation modes of a composite synthetic antiferromagnetic free layer in dual magnetic tunnel junctions with MgO barriers are studied experimentally by analyzing its field- and current-dependent power spectra. Two oscillation modes are observed under different magnetic fields and bias currents due to the unique layer structure as follows: seed layer/PtMn/Co70Fe30/Ru/Co60Fe20B20/MgO/Co20Fe60B20/Ru/Co20Fe60B20/MgO/Co60Fe20B20/Ru/Co70Fe30/PtMn/capping layer. With increasing field, the first mode shows a frequency reduction at low magnetic fields followed by an increasing frequency at high magnetic fields, while the second mode reveals a frequency increase at both low and high magnetic fields (Fig. 1(b)). The current-dependent power spectra of these two modes are also different. With increasing current, the frequency of the first mode drops at low magnetic fields (Fig. 2(a)) and rises at high magnetic fields (Fig. 2(c)), while the other decreases at low and high magnetic fields. These two modes could be tentatively understood by considering the coupling, the bias asymmetry of the spin torque, and the different thicknesses of Co20Fe60B20 in the composite synthetic antiferromagnetic free layer.

**References:**

[1] M. Romera, P. Talatchian, S. Tsunegi et al., Nature, Vol. 563, p. 230, (2018).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/5e2f10db377ec008ba72f706161f7df6.jpg)
Fig. 1 (a) The power spectral density of the free layer measured at the current of 0.3 mA and the bias fields of 400, 800, and 1000 Oe. The hollow signs are experimental results. The solid curves are Lorentzian fittings. (b) The power spectral density of the free layer as a function of magnetic field from 0 to 1100 Oe. The triangles and the crosses correspond to the frequencies of the two modes.

![](https://assets.underline.io/uploads/markdown_image/1/image/ec7159002dc17d22c71088883f41636f.jpg)
Fig. 2 The current-dependent power spectral density of the two modes measured at the magnetic fields of (a) 400 Oe, (b) 800 Oe, and (c) 1000 Oe. The triangles are the frequencies of the first mode, and the crosses are the frequencies of the second mode.

 Spin-Torque Oscillation Modes of a Composite Synthetic Antiferromagnetic Free Layer in Dual Magnetic Tunnel Junctions

Enhancement of switching perpendicular magnetic tunnel junctions (p-MTJs) with an electric-field has been a milestone for realizing ultra-low energy memory and computing devices. To integrate with current STT-MTJ and SOT-MTJ devices the typical linear fJ/Vm range voltage controlled magnetic anisotropy (VCMA) needs to be enhanced through stack engineering. We have investigated possible non-linear voltage controlled magnetic anisotropy (VCMA) in heavily electron depleted devices, where the magnetic anisotropy could be reduced with positive and negative voltages, differing with the typical linear VCMA voltage behavior. A possible bidirectional and 1.1 pJ/Vm VCMA effect can be realized based on theoretical predictions with heavily electron depleted Fe/MgO interfaces. To improve upon existing VCMA technology we have proposed inserting high work function materials under the magnetic layer which will deplete electrons from the magnetic layer biasing the gating window into the electron-depleted regime where the pJ/Vm and bidirectional VCMA effect was predicted. We have demonstrated tunable control of the under-layer’s work function by utilizing Ta/Pd(x)/Ta trilayers. By modulating the Pd thickness from 0-20 nm we have observed a tunable change in the Ta layer’s work function from 4.3eV to 4.8eV. To investigate the extent of the electron depletion as a function of the Pd thickness in the trilayer we have performed DFT calculations on supercells of Ta/Pd (X)/Ta /CoFe /MgO. We are working to improve the quality of our gate dielectric materials to avoid hysteretic ionic effects allowing for a reliable VCMA measurement of the Ta/Pd/Ta samples.

![](https://assets.underline.io/uploads/markdown_image/1/image/a8360ad2cb5f5ae617004083260b5005.jpg)

Enhancement of Voltage Controlled Magnetic Anisotropy (VCMA) Through Electron Depletion

Enhancement of Voltage Controlled Magnetic Anisotropy (VCMA) Through Electron Depletion

KOB: Hard Magnetic Materials IV: Rare-Earth Free Hard Magnetic Materials

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