United States

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.&lt;br&gt;
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/300448a262a8a122d2d995834592eccc.png)&lt;br&gt;
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&gt; = (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/m&lt;sup&gt;2&lt;/sup&gt;. (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/m&lt;sup&gt;2&lt;/sup&gt;, MFCs are placed at 70 nm to the STNO.

MMM 2022

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

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.

poster

#### 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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Recent experimental and theoretical work has focused new interest on how magnons can influence the transport properties of metallic ferromagnets (FM). Here we demonstrate the influence the Gilbert damping parameter, αGD, has on both the electron and magnon populations available for transport in two distinctly different CoxFe1-x alloys. The absolute Seebeck coefficient, αabs, is measured for each of the films [1], and we compare to our calculated total thermopower, αtot. To calculate the electronic and magnonic components,
we employ a semi-classical approach; the classical free-electron model estimates the diffusive thermopower component, αMott [2], while we compute the relativistic magnon-drag component, αmd, based on the spin transfer mechanism [3,4]. From these results, we obtain quantitatively comparable measured Seebeck coefficients for both alloys, subsequently showing αmd dominates the low-damping sample, while the alloy with normal damping follows αMott well over our temperature regime. We further present anisotropic magnetoresistance (AMR) [5] and magnetothermopower (MTP) [6] measurements over the temperature range of interest from 125 to 250 K, where we previously reported a significant non-electronic thermal conductivity [7]. AMR measurements align with expected FM metal behavior, while MTP shows a field-dependence as H ∥ ▽T in the low-damping FM metal. We estimate the magnitude of this behavior using the relationship between α(H,⊥) and the inverse resistance, R -1, and calculated the expected α(H,∥) [6]. This argues for an additional field-direction dependent
contribution to thermopower, which could be a novel form of magnon-drag. 
**References** 1. S. J. Mason, A. Hojem, and D. J. Wesenberg, Journ. of Appl. Phys. 127 (2020). 
2. M. Jonson and D. Mahan, Phys. Rev. B 21 (1980). 
3. M. E. Lucassen, C. H. Wong, and R. A. Duine, Appl. Phys. Let. 99 (2011). 
4. B. Flebus, R. A. Duine, and Y. Tserkovnyak, EPL 115 (2016). 
5. J. Shi, S. S. Parkin, and L. Xing, Journ. Of Mag. and Mag. Mater. 125, (1993). 
6. A. D. Avery, M. R. Pufall, and B. L. Zink, Phys. Rev. B – Cond. Mat. and Mater. Phys. 86,(2012). 
7. M. R. Natale, D. J. Wesenberg, and E. R. Edwards, Phys. Rev. Mater. 5, (2021). 
8. M. R. Natale, D. J. Wesenberg, and E. R. Edwards, in preparation 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/cd43a414a645b7b8620c807946139b73.png) 
Fig. 1: Temperature dependence of absolute thermopower for Co25Fe75 alloy. Calculated diffusive, αMott, (green), magnon-drag, αmd, (orange), and total thermopower, αtot, (marron)overlayed with experimental measurements, αmeas
, from 125-250K.

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/769106818bac00aeeb9ad072f65dd768.png) 
Fig. 2: MTP measurement for Co25Fe75 at 200K. αabs(H) is measured in-plane for Hext ⊥ ▽T and Hext ∥ ▽T. Curved arrows indicate the field sweep direction and are color coordinated to distinguish between field-orientation. Dotted lines are expected values.

Magnon drag observed via field

Skyrmions are of great interest in spintronic devices because their small size and the high-speed motion when driven by a current or a spin-wave (SW). Skyrmion motion
along a narrow track is important for racetrack-type spintronic devices, since a skyrmion moving along the propagation direction of an injected electrical or SW current (forward) is the
means by which data are moving[1]. To design the devices, knowing how to adjust the skyrmion motion becomes important.
We carried out simulations of SW driven skyrmion motion using OOMMF [2], with fixed parameters D = 1.5 mJ/m2, MS = 1004 kA/m, A = 3.6 pJ/m, and Ku = 1 MJ/m3. We found that for tracks narrower than 50 nm the skyrmion always moves forwards. For wider tracks, the skyrmion will initially move both backward and close to the track edge, before moving forward at longer times once the track edge is reached, as shown in Fig 1. We attribute this to an additional repulsive force on the skyrmion from the track edge. For samples with Gilbert damping
constant α > 0.06, the skyrmion will move forward without retrograde motion.
The Thiele equation is a widely-used way to analyse skyrmion motion. The SW driving force on a skyrmion is usually the SW scattering force [3], but the strength of the scattering and the
energy change associated with changes in skyrmion size owing to absorbed SWs are hard to extract from magnetization data. Thus, we use the SW transmission T and reflection
coefficients R to make an estimate of the forces involved. We can obtain T from the ratio of the SW amplitude downstream of the skyrmion and without skyrmion at same position.
Similarly, we obtain R from the difference between magnetization data upstream of the skyrmion and without skyrmion.
As an example, we consider the data from a simulation with track width w = 100 nm and α = 0.03. By Analysing T and R, the decrease in T after ~30 ns, shown in Fig.2, will reduce the
effort that leads to the skyrmion moving backward. Meanwhile the increasing force from the sample edge also helps the skyrmion overcome the Spin-Ttansfer Torque between transmitted
SW and skyrmion[4], allowing it to move forwards. 
**References** 
1. Sampaio, J., Cros, V. et al. Nature Nanotech 8, 839–844 (2013) 
2. Donahue, M., Porter D. et al, National Institute of Standards and Technology(1999) 
3. Zhang, X. et al New J. Phys. 19, 065001 (2017) 
4. Yan, X., S. Wang et al. Phys. Rev. Lett. 107, 177207 (2011) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/d19058f8a75b91bfae124e6b897fb99c.png) 
The snapshots of w = 40 nm, 90 nm tracks 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/1486ee39a892911ac9e318a9a335f631.png) 
Time-dependent SW T, R, A and skyrmion acceleration data

Transient retrograde motion of spin wave driven skyrmions in multilayer wave guides

Two-dimensional (2D) intrinsic half-metallic ferromagnets (HMFs) are potential candidates for ultrathin spintronic device applications [1]. The understanding of
magnetocrystalline anisotropy energy (EMCA) and Gilbert damping (α) is required to determine their use in practical applications. In particular, the thermal stability and critical current of
magnetization switching of spin-transfer-torque magnetic random-access memories highly depend on EMCA and α [2]. Therefore, using density functional theory, we study the monolayer
Rhodium dihalides family, RhX2, where X= I, Br, Cl. First, we find that RhX2 prefer 1T phase (octahedral symmetry) than the other possible 1H phase (trigonal prismatic symmetry). The
ferromagnetic ordering is energetically favourable in RhX2 monolayers. The Curie temperature of RhI2, RhBr2 and RhCl2 crystals calculated using spin-wave theory (& Ising model) [3,4]
is 182 (304) K, 92 (189) K and 17 (45) K respectively. We then calculate the spin-polarized electronic band structure which reveals wide intrinsic half-metallic gap (> 1.1 eV) in the down
spin bands of RhX2 monolayers. It results in 100% spin-polarization about the Fermi level and should prevent the spin-current leakage in nanoscale devices. The inclusion of spin-orbit
coupling (SOC) does not affect the band’s profile much and the half-metallic character remains intact. Moreover, the metallic nature arises mainly from the hybridization of Rh 4d-orbitals
and X p-orbitals which are present on the Fermi level. The EMCA and α which originate from SOC control the spin dynamics. We use force theorem [5] for EMCA calculation and the
results show substantial in-plane magnetic anisotropy in RhI2(-2.3 mJ/m2) and RhBr2(-0.57 mJ/m2) whereas small perpendicular anisotropy in RhCl2(0.04 mJ/m2) [6]. To calculate α we employ Kambersky's torque-torque correlation model [7] and it comes out relatively low (2.1×10-2to 4.0×10-3). This work highlights the importance of 2D RhX2 HMFs in fabrication of future spintronic devices. 
**References** 
[1] T. Song, X. Cai and M. W. Y. Tu, Science 360, 1214 (2018). 
[2] S. Ikeda, K. Miura and H. Yamamoto, Nat. Mater. 9, 721 (2010). 
[3] J. L. Lado and J. Fernández-Rossier, 2D Mater. 4, 035002 (2017). 
[4] J. M. Dixon, J. A. Tuszynski and E. J. Carpenter, Physica A 349, 487 (2005). 
[5] D. Li, C. Barreteau, M. R. Castell and F. Silly, Phys. Rev. B 90, 205409 (2014). 
[6] Ram Krishna Ghosh, Ashna Jose and Geetu Kumari, Phys. Rev. B 103, 054409 (2021). 
[7] K. Gilmore, Y. U. Idzerda and M. D. Stiles, Phys. Rev. Lett. 99, 027204 (2007). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/38c5680407b04da4dbeea66d451fd8d3.png) 
Fig.1 a) Side view of 1T-RhX2 monolayer and difference between spin charge densities about Rh and I atoms, b) Electronic band structure of RhI2 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/a967f0757a86ddc2bd90cdd09d0e489b.png) 
Fig.2 a) EMCA vs. θ, b) Gilbert damping tensors vs. life-time broadening.

Magnetocrystalline anisotropy energy and Gilbert damping of two dimensional half

Electrically driven magnetization switch utilizing voltage-controlled magnetic anisotropy (VCMA) have attracted much attention in the new generation memory, especially
for spin-orbit torque (SOT)-based magnetic random access memory (MRAM) [1,2]. For the traditional SOT switching, an additional magnetic field is needed to break the inversion
symmetry, which makes the circuit difficult to implement [3]. Combining the SOT with the VCMA effect can realize the certain switching in SOT-MRAM with lower critical switching
current [4]. With the development of the SOT-MRAM, device model is urgently needed for the device characterization. However, the published models are facing limitations as the size shrinks down
to nanometer. Also, the effect of the elevated temperature caused by repeated operation on the device is non-negligible [5]. Therefore, an effective model is needed to represent the
switching dynamic of the device concerning the influence of the nanoscale and the thermal effect.
In the paper, a compact model of three-terminal VCMA assisted SOT switching is established. The influence of the VCMA effect on the field-free SOT switching is considered by
numerically solving the LLG equations. Furthermore, a 3D model of the VCMA+SOT device is established by finite element method to demonstrate the thermoelectric behavior.
Considering the influence of the thermal effect generated in the working state on the switching characteristics, the electrothermal behavior is integrated into the compact model to show the
influence of the temperature on the switching behavior, highlighting the importance of the thermal effect for the realistic modelling of SOT switching. Additionally, considering the
influence of thermal effect, the switching error rate of the device under different working voltage conditions is evaluated. Finally, a novel voltage pulse scheme is proposed, which can
effectively shorten the inversion time and improve the reliability of the device. The established model provides strategies and guidelines for next-generation memory design and
application. 
**References** [1] W. Kang, Y. Ran and Y. G. Zhang et al., IEEE Trans. Nanotechnol., Vol. 16, p. 387 (2017). 
[2] S. Sharmin, A. Jaiswal, and K. Roy, IEEE Trans. Electron Devices, vol. 63, p. 3493 (2016). 
[3] A. Manchon, I. M. Miron and T. Jungwirth et al., Rev. Mod. Phys., vol. 91, p. 035004 (2019). 
[4] S. Shreya and B. K. Kaushik, IEEE Trans. Electron Devices, vol. 67, p. 90 (2019). 
[5] J. J. Kan, C. Park and C. Ching et al., IEEE Trans. Electron Devices, vol. 64, p. 3639 (2017). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/3ae5dbed55d72a570c2440ec178e1e30.png) 
Fig.1 Schematics of the three-terminal device allowing for field-free SOT switching with an illustration of the influences of VCMA and thermal effects.

Field free switching model of Spin

Spin torque nano oscillator(STNO) and spin Hall nano oscillator(SHNO) are the promising devices to generate spin wave with thermal stability and low power
consumption.
[1,2] But the spin wave has in nature the low power density for device applications. Thus, various spin wave excitation methods have been studied, such as synchronization of
auto-oscillation via multiple SHNO array
[4-7] and utilizing high spin Hall angle in a heavy metal.
[8] Here, we designed a permalloy (Py) ferromagnetic layer in a rectangular bar type for
spin wave excitation by auto oscillation and investigated the effects of a transition metal (Co) alloying on the excitation efficiency. The magnetic fluctuation in bar type layers of Py1-xCox
(x = 0, 0.11, 0.21, 0.29, and 0.40), fabricated by Py-Co cosputtering, is estimated from micro-Brillouin light scattering (μ-BLS) spectroscopy data, as shown in Fig. 1. And the threshold
current for auto oscillation in the bar layers is estimated from the current dependence of integral intensity of the spectra. It is found that Py-Co cosputtering reduces the threshold current for
auto oscillation, e.g., 27.3% for Py1-xCox
(x = 0.21), compared with that of Py layer. It is conjectured that the cosputtering enhances the spin Hall transparency while it causes almost no
change in the effective Gilbert damping constant in the layer. Details will be discussed in the poster presentation and conference proceeding paper. 
**References** 
[1] J. Wunderlich, B. Kaestner, and T. Jungwirth, Phys Rev Lett 94, 047204 (2005). 
[2] J. Torrejon et al., Nature 547, 428 (2017). 
[3] A. Slavin and V. Tiberkevich, IEEE Transactions on Magnetics 45, 1875 (2009). 
[4] Y. Ou, D. C. Ralph, and R. A. Buhrman, Phys Rev Lett 120, 097203 (2018). 
[5] Y. Yin et al., Physical Review B 92 (2015). 
[6] M. A. W. Schoen, D. Thonig, and J. M. Shaw, Nature Physics 12, 839 (2016). 
[7] Y. Fu et al., IEEE Transactions on Magnetics 45, 4004 (2009). 
[8] H. Mazraati et al., Applied Physics Letters 109 (2016). 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/b078253a80d9187c1b90409a52a46000.png) 
Figure 1. Brillouin light scattering (BLS) intensity spectra of Py1-xCox
(x = 0.21) (Py = permalloy) nanowires with various applied DC currents of 3.5, 4.0, 4.5, 5.0 and 5.5 mA

Effects of Permalloy Cobalt Cosputtering in a Nanowire Structure on Magnetic Fluctuation and Auto

Non-reciprocal microwave components such as circulators, isolators, and phase shifters are indispensable tools in both today’s communication systems and future quantum
computers [1-2]. However, these components rely almost entirely on field-based bulky ferrimagnet, which tend to be relatively large, off-chip, and costly to assemble. In this context,
different fields of research are investigating solutions to miniaturize these non-reciprocal devices compatible with integrated circuit technology. Among them, the field of magnonic plays a
key role in this search benefiting from a wide range of non-reciprocal properties in the propagation of spin waves [3]. Recently, unidirectional transmission of spin waves was achieved by
taking advantage of the chiral coupling between the uniform resonance of Co nanowires and exchange spin waves in a thin YIG film [4, 5]. Here, we further miniaturized this method and
demonstrate the possibility of shaping non-reciprocal spin wave beams in a continuous thin YIG film. We performed spin wave spectroscopy on a series of devices made of arrays of Co
bars of dimensions: L=10µm in length, w=200 nm in width, 40nm in thickness, and laterally spaced by a=400nm (see Fig. 1). The transmission spectra done with the external field applied
along the length of the Co bars show perfect non-reciprocity for several peaks located around the resonance frequency of the Co bars, e.g. 8.4 to 10.4 GHz range as shown in Fig. 2. These
peaks correspond to integer values of the lateral spacing between Co bars, e.g. kn=nπ/a ranging up to 80rad/µm. Additional non-reciprocal peaks occurring at lower frequencies ranging
between 2 and 6 GHz correspond to the satellite peaks of the antenna directly coupled to the YIG film. Surprisingly, some of the peaks display a reversed non-reciprocity. Finally, we make
use of this multitude of transmission peaks to characterize the k-dependence of the relaxation time and the group velocity of dipole-exchange spin waves. 
**References** 87.png)[1] W. Palmer, et al., IEEE Micr. Magazine 20, 36 (2019) 
[2] M. Devoret, et al., Superconducting circuits for quantum information,Science 339, 1169 (2013) 
[3] J. Chen, et al., J. Phys. D. Appl. Phys. 55, 123001 (2022) 
[4] H. Wang, et al., Nano Res. 14, 2133–2138 (2021) 
[5] J. Chen, et al., Phys. Rev. B 100, 104427 (2019) 
 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/94e4cd227f20528c8226450f05b99d7c.png) 

SEM image of Co nanowires with length L=10μm, width w=200nm, and period a=400nm lithographed on a 50nm YIG film with thickness, and located under 2 µm wavelength Au coplanar waveguides (CPW)

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/0d51030b4d6ea35e8b9bbc51296dd09e.png) 
Transmission spectrum ΔL21
(blue) and ΔL12
(red) at 24mT applied field.

High Wave Vector Non reciprocal spin Wave Beams

Understanding magnons in confined geometry that relates to experimental devices is rather challenging, since device heterostructures, e.g. Spin Seebeck devices, are a
combination of both ferromagnetic (FM) and antiferromagnetic (AFM) layers [1]. The size of the layers can range from hundreds to several atomic layers, hence discontinuities on a short
length scale where a change of magnetic properties is abrupt, in presence of magnetic anisotropies, bring an extra challenge in determining the magnon dynamics, dispersion as well as
prorogation in such structures.
In this work, we extend the method, used in [2] where the spin-scattering function was calculated for 1D finite chains, to 3D systems. In particular, we show how to modify this approach to
calculate the dynamics of magnon modes in finite systems using second quantisation of the Heisenberg Hamiltonian for thin film and interfaces, as well as hetero-structure between FM and
AFM layers, taking into account surface and interface anisotropies and exchange parameters. The effect of such changes in the parameters of the system leads to the appearance of confined
states at the surfaces and interfaces of the heterostructures, Fig. 1. We study the effect that such confined states have on the spin-scattering function, which can be regarded as a proxy for
neutron-scattering. 
**References** 
[1] Lin, W., et. al. Physical Review Letters, Vol.116, p.186601 (2016) 
[2] Bearisto, B., Phys. Rev. B, Vol.104, p.134415 (2021) 
[3] Pajda, M., Phys. Rev. B, Vol.64, p.174402 (2001) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/6f4fdee497bbb1a8a4599d57036da149.png) 
Spin scattering function for Fe BCC, exchange parameters are taken from [3] and experimental points (solid black dots) were reproduced from the same paper. We can see a clear resonance matching the bulk dispersion, as well as a line that corresponds to localized surface states.

Theory of confined magnons in 3D heterostructures

Magnetic modelling for ultrafast simulations at the atomistic and micromagnetic scale make common use of Langevin Dynamics to model the effect of thermal fluctuations on the procession of the magnetic moment. Such a model is simple to add to any finite-temperature simulation: a Gaussian distribution scaled against a collection of material dependent parameters are included in the effective field term in the LLG spin Hamiltonian, and at pico-second timescales is uncorrelated in space or time [1]. In general, however, the thermal fluctuations in a material are correlated through collective occupation of electron, phonon, and magnon modes in the system. Analytical solutions for these relationships under conditions of non-equilibrium require simple materials or drastic approximations, conditions ill-suited to ultrafast ferri and antiferromagnetic materials. We present an improved thermostat for ultrafast atomistic scale magnetic simulations using a computationally determined conduction band environment based on semi-classical electron-electron and electron-phonon scattering events [2][3]. Currently, our environment simulates charge and heat transport in metals on ultra-fast time scales during severe non-equilibrium caused from applied electric fields or laser-heating pulses. The electron-electron and electron-phonon relationship is parametrized into the relaxation time approximation using constants derived from experimental data [3][4]. Our environment successfully reproduces the popular two-temperature model (TTM) used to simulate laser-heating of experimental samples (fig. 1), and currently reproduces the phenomena of effective Joule heating from applied external fields and super-diffusive electron transport. The current work lays the foundation for the inclusion of electron spin into the conduction band environment, offering the ability to include correlated spin effects not possible in Langevin Dynamics. Figure 1. Simulated electron and lattice temperatures (solid) following laser exposure compared to the standard TTM (dash). The varying colours and dynamics are a result of varying the electron-electron scattering constant.

Electronic Heat Bath Simulations for Ultra fast Spin Dynamics

Ferrimagnets (FiMs) persist in ultra-fast dynamics similar to antiferromagnets but are as easy to be electrically manipulated as ferromagnets, leading to a promising approach for achieving ultrafast devices with feasible tunability. Furthermore, spin-chain is of fundamental importance in condensed-matter physics, revealing unique properties such as quantum phase transition, edge effect, etc.[1] Although the dynamics of FiM have been widely described by a two sublattices macro-spin model[2], it hardly captures the sophisticated physical properties in multi-spin systems such as spin chains. Hence, we developed an atomistic spin model to investigate the spin dynamics in the FiM spin chain deployed by current-induced spin-transfer torque(STT). Our simulation results showed that in the finite FiM spin chain, first, the oscillation started from opposite-spin spatial region, defined as "oscillation core (OC)". Next, the oscillation spatial region would expand and finally only some specific spins formed stable oscillation (Fig.1). To further understand this phenomenon, we first introduce one OC into the spin chain. When excited by STT, the exchange mode can be further classified into non-flipped exchange mode (region I), critical exchange mode (region II), and flipped exchange mode (region III). In region I and III, the frequency increased linearly with different slops as the current density increased, yet in region II, the frequency slightly decreased, indicating the competition among exchange field (Hex), uniaxial anisotropy, and STT (Fig.2a). Next, when two OCs interacted, we find that smaller distance gives lager frequency, which is attributed to different Hex. Hence, proper choice of distance allows us to tune the Hex, achieving large-angle precession with THz frequency(Fig.2b). In summary, our work offers a novel understanding of FiM oscillation and proposes a strategy to construct energy-efficient spintronic oscillators with THz frequency. (This work at the National University of Singapore is supported by MOE-2019-T2-2-215 and FRC-A-8000194-01-00) 

**References** 
[1] H. Brune, Science., Vol. 312, p.1005-1006 (2006)
[2] M. Guo, H. Zhang and R. Cheng, Phys. Rev. B., Vol. 105, p.064410 (2022) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/6a6c6abaafc634f3233c0686bd548bad.png) 
Fig.1 Schematic of FiM spin chain oscillation behavior evolved with time. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/9635db2888915fc5843feae8047b4e3c.png) 
Fig.2 Numerical results of the oscillation in the FiM spin chain with (a)one OC and (b)two OCs.

Theoretical Investigation of Tunable Ferrimagnetic Spin Chain Oscillation Based on Atomistic Spin Model Simulation

Dzyaloshinskii-Moriya interaction (DMI) that occurs in structural inversion asymmetric systems stabilizes chiral domain-walls (DWs) which is a key issue to achieve high performance spintronic applications such as memory and data storage devices with high speed and high durability [1-4]. It is therefore important to analyze the strength of DMI and thus, there have been numerous efforts devoted to quantifying the DMI both theoretically and experimentally [4-6]. Unfortunately, the inconsistency between theory and experiment inevitably occurs because the theory predicts the strength of DMI based on single interface, however, the experiments must be implemented based on at least, double interface because of Ex-Situ nature [2-6]. Here, we first, measure the strength of DMI at Pt/Co single interface in In-Situ nature. To measure the strength of DMI at single interface, we set up In-Situ Magneto-Optical-Kerr-Effect (MOKE) microscopy with UHV magnetron sputtering chamber. Then, we quantified the strength of DMI with respect to Co layer thickness. Fig.1 clearly shows the plot of HDMI with respect to tCo based on Je's method [5]. 
**References** [1] S. S. P. Parkin, M. Hayashi, and L. Thomas, Science 320 (5873), 190-194. (2008). 
[2] I. E. Dzialoshinskii, Sov. Phys. JETP 5, 1259 (1957). 
[3] T. Moriya, Phys. Rev. 120, 91 (1960). 
[4] A. Fert, V. Cros, and J. Sampaio, Nat. Nanotechnol. 8, 152 (2013). 
[5] S.-G. Je, K.-J. Lee, and S.-B. Choe, Phys. Rev. B 88, 214401 (2013). 
[6] J. Cho, B. Koopmans, and C.-Y. You, Nat. Commun. 6, 7635 (2015). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/ca39e5c56db70732287c6e9d8334f74e.png) 

Fig. 1 The plot of HDMI with respect to tCo.

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