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MMM 2020

Novel Approaches to the Excitation and Control of Nano-Scale Propagating Spin Waves 

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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Manganites have shown potential in spintronics due to their low damping and insulating characteristics. Here, we studied the damping properties of La0.67Sr0.33MnO3/Pt bilayer samples which are prepared using oxide molecular beam epitaxy. We have observed decrease in damping coefficient (α) with increase in Pt thickness. Further, we investigated inverse spin Hall effect (ISHE) and observed that ISHE signal gets enhanced with Pt thickness. We observed maximum spin pumping voltage of 20.05 µV for the sample with Pt thickness of 3 nm. We have evaluated spin Hall angle for these samples. Introduction: Pure spin current based devices are potential for faster and low power consumption [1]. Generation of pure spin current has been demonstrated by ferromagnetic resonance (FMR) through spin pumping mechanism [2]. This pure spin current can loss their spin angular momentum in the presence of high spin orbit coupling (SOC) material e.g. Pt, W, Ta. The loss of spin angular momentum can develop voltage by asymmetric scattering of spins, which is known as inverse spin Hall effect (ISHE) [3]. So far studies have been concentrated mostly on Pt and FM metals [4]. However, magnetic oxides are less explored. La0.67Sr0.33MnO3 (LSMO) is well known FM oxide for exhibiting high Curie temperature (TC=350 K) and nearly 100% spin polarization (in bulk) [5]. In this work, in order to get high spin Hall angle, high resistive Pt film is grown on LSMO thin film. Experimental details: LSMO (20 nm)/Pt (tPt = 0, 3 and 10 nm) bilayer samples have been prepared on SrTiO3(001) substrate using an oxygen plasma assisted molecular beam epitaxy system. ISHE measurements are performed using home modified coplanar wave-guide (CPW) based ferromagnetic resonance (FMR) spectroscopy [6]. Results: We have studied the static and dynamic properties of the LSMO/Pt systems. A decrease in α has been observed with increase in Pt thickness. We performed angle dependent ISHE at frequency of 7 GHz. From angle dependent ISHE measurement spin Hall angle were calculated 0.033 and 0.014 for samples with 3 and 10 nm of Pt, respectively.References: [1]. C. Chappert, A. Fert, and F. N. Van Dau, Nature Materials 6, 813 (2007). [2]. O. Mosendz, J. Pearson, F. Fradin, Phys. Rev. Lett. 104, 046601 (2010). [3]. J. Hirsch, Phys. Rev. Lett. 83, 1834 (1999). [4]. J.-C. Rojas-Sanchez, N. Reyren, P. Laczkowski, Phys. Rev. Lett. 112, 106602 (2014). [5]. J.-H. Park, E. Vescovo, H.-J. Kim, Nature 392, 794 (1998). [6]. B. B. Singh, S. K. Jena, M. Samanta, Phys. Status Solidi Rapid Res. Lett. 13, 1800492 (2019).

Simultaneous observation of anti-damping and inverse spin Hall effect in La0.67Sr0.33MnO3/Pt bilayers

Ferrimagnetic insulator garnet films play an important role in magnon spintronics where magnon-mediated spin currents are used to carry, transport, and process information. This wave-based computing technologies lead to energetically more efficient devices and allows a decrease in the footprint of computing elements [1]. When a magnetic insulator (MI) film is combined with a non-magnetic (NM) spin current detector material like platinum, it is possible to electrically control the MI’s magnetization via the spin-orbit torque (SOT) effect. If the MI presents a perpendicular magnetic anisotropy (PMA), the electrical current density required to switch the magnetization is lower, and the process is faster. This makes the magnetoresistive random access memories based on SOT (SOT-MRAM) challenge the MRAMs based on the spin-transfer torque effect (STT-MRAM) [2]. Moreover, ferrimagnetic garnets with PMA enables high-speed domain-wall (DW) motion that also can be driven by electrical currents via SOT and are ideal materials for developing efficient DW racetrack memories [3]. Thulium iron garnet (TIG – Tm3Fe5O12) is a good candidate due to its high critical temperature of 549 K, a Gilbert damping of 10-2, and negative magnetostriction constant that favors the control of the PMA [4]. When combined with Pt, TIG/Pt is a reach system for the study of spin waves and the observation of spin-orbit effects. We investigate the generation and detection of magnon-based spin currents driven by microwaves and heat currents via the spin pumping (SPE) and spin Seebeck (SSE) effects in TIG(60 nm)/Pt(4 nm), at room temperature. The TIG film was deposited over GGG (Gd3Ga5O12) substrate using the sputtering technique and presented a flat surface with an RMS roughness of 0.1 nm. The magnetization saturation was ~ 100 emu/cm3, and the Gilbert damping parameter was 0.02. The results showed an effective spin-to-charge conversion via the inverse spin Hall effect (ISHE). It made it possible to detect a micro-voltage at the edges of the Pt film and confirmed high spin transparency of the interface TIG/Pt. The spin Seebeck coefficient was 0.54 μV/K, whereas the ratio between the microwave-driven voltage and the microwave power was 4μV/W.References: [1] A. V. Chumak et al., Nature Physics, Vol. 11, p. 453 (2015). [2] M. Cubukcu et al., IEEE Transactions on Magnetics, Vol. 54, p. 1 (2018). [3] S. Vélez et al., Nature Communications, Vol. 10, p. 4750 (2019). [4] G. Vilela et. al., Journal of Applied Physics, Vol. 127, p. 115302 (2020).

Microwave and Thermally Driven Spin Currents in Tm3Fe5O12/Pt with Perpendicular Magnetic Anisotropy

Spintronics has traditionally relied on ferromagnetic materials to provide non-volatility and to generate and detect spin currents. However, magnetization reversal by spin transfer and spin orbit torques is a power-consuming process. This is driving research on multiferroics to achieve low-power electric-field control of the magnetization, but practical materials are scarce and magnetoelectric switching remains difficult to control. In this presentation we will focus on an alternative strategy: use ferroelectricity to obtain the remanent control of the spin-charge interconversion. At the interface between a ferroelectric and an ultrathin spin orbit coupling (SOC) systems -such as 2DEG- electrons are accumulated or depleted depending on the polarization direction. This modifies the electric field in the interface region, and in the ideal case changes its sign. If a Rashba state is present in the SOC system at the interface with the ferroelectric, reversing the sign of the local electric field reverses the chirality of the spin textures in both split Fermi contours as depicted in figure 1.This mechanism offers the possibility to design a wealth of devices such as bipolar memories and can also be the basis of logic devices akin to the magnetoelectric spin–orbit device proposed by Intel A highly efficient conversion was evidenced in the two-dimensional electron gas (2DEG) obtained at the interface between SrTiO3 (STO) and LaAlO3 (LAO). Moreover, thanks to the high dielectric constant of STO at cryogenic temperature the spin current generation and detection can be tuned using gate voltage [1,2]. A similar 2DEG can be obtained by depositing a thin layer of a reducing metal as Al on top of STO alleviating the need for the high-temperature growth of crystalline LAO and allowing the modulation of the carrier density through the adjustment of the Al thickness. In this system the gate tunability of the spin to charge conversion and its link to the bandstructure of STO was evidenced [3]. But despite the possibility to tune the conversion using gate voltage in STO based Rashba system, the remanent control of the 2DEG was still not achieved. While STO is not a ferroelectric it is possible to induce ferroelectricity in STO based structure either by doping it with Calcium [4] or by applying high electric field. In this presentation we will evidence that it is possible to control the 2DEG carrier density and the sign of the spin to charge conversion in a remanent way in ferroelectric-like STO based structures [5]. After applying a high enough electric field to induce a ferroelectric state in STO It is possible to modulate the sign of the conversation with positive and negative voltage pulses as depicted in figure 2.References: [1] E. Lesne, Y. Fu, S. Oyarzun et al., Nature Materials 15,1261–1266(2016) [2] F. Trier, D.C. Vaz, P. Noël et al., Nano Lett. 20, 1, 395–401 (2020) [3] D. C. Vaz,P. Noël, A. Johansson et al., Nature Materials 18, 1187–1193(2019) [4] J. Bréhin, F. Trier, P. Noël et al., Phys. Rev. Materials 4, 041)002(R) (2020) [5] P. Noël, F. Trier, L. M. Vicente Arche et al., Nature 580, 483–486(2020)

Non-volatile electric control of spin–charge conversion in a SrTiO3 Rashba system INVITED

A study of the magnetic critical behavior and Hall effect, and magnetic interactions is presented for Fe73.5-xCrxCu1Nb3Si13.5B9 metallic glasses on the road to percolation (xp ≈ 24 at.% Cr) [1]. The samples under study were prepared as amorphous ribbons from Fe73.5-xCrxCu1Nb3Si13.5B9 with 10 ≤ x ≤ 20 at. % ingot alloys by means of the melt-spinning technique. The amorphous structural state of each sample was confirmed using X-ray diffraction with CuKa radiation. The critical behavior is studied by determining the critical exponents for the zero-field magnetization (β), zero-field isothermal susceptibility (γ), critical isotherm (δ), and critical temperature TC, from magnetization measurements using the Kouvel-Fisher method [2]. The obtained (β, γ, δ) values indicate that the universality class of these metallic glasses is consistent with the mean-field model with long-range interactions. The critical temperature, TC, quasi-linearly decreases with the increase of Cr content. This behavior indicates that the exchange interaction weakens with the increase of Cr content. The Hall effect measurements were carried out in the field range – 10 < H < 10 kOe for several DC electrical currents applied to the samples at room temperature. These measurements reveal an ordinary and extraordinary contribution to the Hall effect and a positive Hall constant. From the ordinary contribution to the Hall effect, we obtained the following physical parameters: ordinary Hall coefficient, resistivity, concentration and drift velocity of charge carriers, besides mean free path of charge carriers and relaxation time. An analysis of the physical parameters previously mentioned and of their dependence on Cr concentration is presented. Special attention is given to exchange interaction and spin-orbit interaction that might be active in these materials, and their possible effects on the (β, γ, δ) values are analyzed.References: 1. M.S. Mahmud, “Magnetic behavior of Fe73.5-xCrxCu1Nb3Si13.5B9 amorphous alloys at low temperature”, J. Mat. Sci. & Eng., A1, 58-63 (2011). 2. J.S. Kouvel, M.E. Fisher, Phys. Rev. 136 (1964) 1626, https://doi.org/10.1103/ PhysRev. 136.A1626.

Magnetic Critical Behavior, Hall Effect, and Magnetic Interactions in Fe73.5-xCrxCu1Nb3Si13.5B9 Metallic Glasses on the Road to Percolation.

Magnetic sensors based on the giant magnetic impedance effect (GMI) have become a hotspot in the development of high-performance magnetic sensors due to their miniaturization, high sensitivity, and fast response. However, low frequency 1/f noise and other issues have seriously troubled the development of GMI magnetic sensors. Due to the fact that its noise sources have not yet been unified, it is increasingly difficult to optimize 1/f noise by means such as improving circuit structure and improving material quality. The magnetic field modulation technology based on the idea of dynamic shunt has achieved good results in improving the effect of 1/f noise on MTJ magnetic sensors[1]. This technology is mainly realized through the function of the magnetic flux concentrator(MFC) to modulate the magnetic field. Therefore, this paper studies the relationship between MFC parameters and the function of modulating the magnetic field to be measured. The effect of MFC on the amplitude of the measured magnetic field is studied with COMSOL software in this paper[2]. As can be seen from Figure 1, when a trapezoidal MFC with larger bottom-top ratio and longer height is close to the point to be measured, it has a better amplification effect on the magnetic field of the point to be measured. Figure 2 shows a structure with a frequency modulation of the magnetic field to be measured. The cantilever beam is driven by piezoelectric material to achieve periodic MFC vibration. The curves of Modulated magnetic field versus drive signal under different frequencies are obtained by simulation. Through the magnetic field modulation technology, the frequency of the signal to be measured is modulated to a high frequency for detection, so as to avoid the influence of 1/f noise and achieve the purpose of optimizing the signal to be measured.References: [1] J. Hu, et al, Sensors, Vol.19, No.20, p.4475 (2019) [2] Predrag M., et al, Sensors and Actuators A: Physical, Vol.97-98, p.10-14 (2002)

Simulation of Magnetic Field Modulation of Magnetic Flux Concentrator

Axial-flux permanent magnet synchronous motors (AFPMSMs) have higher torque densities than the conventional radial-flux permanent magnet synchronous motors (RFPMSMs). However, RFPMSMs have the advantage of lower eddy current losses in the core, owing to the use of electrical steel plates that can be stacked. Therefore, most studies on reducing the eddy current losses of the existing AFPMSMs cores have focused on reducing the eddy current loss by using a soft magnetic composite (SMC) material or rolling an amorphous electrical steel plate [1]-[3]. The method proposed for reducing the eddy current loss of AFPMSMs in this paper uses 3D printing technology to insert a 3D-printed SMC core into the stator shape of a motor. A 3D-printed shape design that reduced eddy current loss among core loss components using 3D-Printed SMC material, which is currently under development, was analyzed. In the case of the AFPMSM proposed in the paper, the eddy current loss was reduced by inserting a slit structure into the stator core of the proposed motor as air insulation, as in the principle of laminating the core in order to reduce the eddy current loss in RFPMSM. Figure 1 shows the theoretical conventional AFPMSM and proposed AFPMSM eddy current loss loop. As a result, it was confirmed through finite element analysis (FEA) analysis that the actual eddy current loss was reduced by up to 37.6%, when compared to that of the previous models. Figure 2 shows eddy current loss loops through FEA. Moreover, while reducing the eddy current loss, seven models that satisfied the target specifications were derived. Finally, when comparing the core loss of the proposed model and the model designed with AFPMSM as a high performance electrical steel plate with low core loss, the feasibility was verified through the FEA that there is no difference in core loss.References: [1] B. Scheerlinck, H. De Gersem, P. Sergeant, “3-D Eddy current and fringing-flux distribution in an axial-flux permanent-magnet synchronous machine with stator in laminated iron or SMC”, IEEE Transactions on Magnetics, vol. 51, Nov 2015. [2] Xiaoguang Wang, Sheng Zhou, Lei Wu, Meng Zhao, Cangxian Hu, "Iron Loss and Thermal Analysis of High Speed PM motor Using Soft Magnetic Composite Material", Electrical Machines and Systems (ICEMS) 2019 22nd International Conference on, pp. 1-4, 2019. [3] X. Sun, Y. Shen, S. Wang, G. Lei, Z. Yang, S. Han, “Core Losses Analysis of a Novel 16/10 Segmented Rotor Switched Reluctance BSG Motor for HEVs Using Nonlinear Lumped Parameter Equivalent Circuit Model,” IEEE/ASME Transactions on Mechatronics, vol. 23, no. 2, pp.747-757, April. 2018

Eddy Current Loss Reduction in Axial-Flux Motors using 3D Printing

Temperature dependence of electrical resistivity as well as neutron diffraction measurements of (Cr84Re16)100−xVx alloys with x = 0, 4.2, 5.5 and 6.2 are reported here. Previous studies probing the quantum critical behaviour (QCB) in the (Cr84Re16)100−xVx alloy system revealed prominent anomalies in physical properties close to the previously defined critical concentration of xc ≈ 10.5 [1]. One of the key indicators of critical behaviour is the Sommerfeld coefficient, γ [2,3]. γ is obtained by fitting the low temperature specific heat (Cp) data to the equation Cp = γT + βT3 [3], where the first and second terms are the electronic and lattice contributions to specific heat respectively [4]. For the (Cr84Re16)100−xVx alloy system, γ(x) showed a broad deep minimum at x ≈ 4.4 and a continuous decrease through x = 10.5 [1]. The latter anomaly was associated with the onset of QCB, but the anomaly at x ≈ 4.4 had to be probed further. It was proposed that the minimum corresponds to the transition from the commensurate spin-density-wave (CSDW) phase to the incommensurate (I) SDW phase upon increasing x. There is a larger gap in the electronic spectrum in the CSDW phase than in the ISDW phase [5]. In the CSDW phase, there is perfect nesting of the electron and hole Fermi sheets which leads to stronger reduction of density of states at the Fermi level. This hypothesis has now been further investigated with neutron powder diffraction (NPD) that provides information directly from the reciprocal lattice to facilitate identification of the magnetic order. Results indicate that the base alloy with x = 0 is in the CSDW state as expected and the x = 4.2, 5.5 and 6.2 alloys display ISDW trends. An additional magnetic peak associated with the Cr2O3 phase was also identified in the (Cr84Re16)100−xVx alloys using NPD. Quantification analyses revealed that the Cr2O3 content was less than 2 wt.% in all of the samples.References: 1. B.S. Jacobs, A.R.E. Prinsloo, C.J. Sheppard and A.M. Strydom, J. Appl. Phys., Vol. 113, p.17E126 (2013) 2. L. Reddy, H.L. Alberts, A.M. Strydom, A.R.E. Prinsloo and A.M. Venter, J. Appl. Phys., Vol. 103, p.07C903 (2008) 3. J. Takeuchi, H. Sasakura and Y. Masuda, J. Phys. Soc. Jpn., Vol. 49, p.508 (1980) 4. T. Mamiya and Y. Masuda, J. Phys. Soc. Jpn., Vol. 40, p.390 (1976) 5. E. Fawcett, H.L. Alberts, V.Y. Galkin, D.R. Noakes and J.V. Yakhmi, Rev. Mod. Phys., Vol. 66, p.25 (1994)

Neutron Diffraction Studies of the (Cr84Re16)100−xVx Alloy System

Modern spintronic based devices, such as magnetic random-access memories and magnetic read head sensors have been designed on the principles of exchange bias effect [1]. A unidirectional interfacial exchange coupling between a ferromagnet (FM) and an antiferromagnet (AFM) is the primary reason for the shift of field-cooled hysteresis loop leading to the exchange bias effect [2]. Other prominent signatures of exchange bias are enhancement of coercivity and observation of training effect. This phenomenon is generally interfacial in origin, however sometimes spins from the ‘bulk’ part of the AFM can also contribute to exchange bias [3]. Besides, the interface can be ferromagnetic, antiferromagnetic or spin-glass in nature. Interestingly, all the above features are system dependent and can vary with the thickness of the films.
 
We have investigated the role of ‘bulk’ spins of the AFM and also explored the magnetic nature of interface in exchange coupled Fe/IrMn bilayers by varying the thickness of the IrMn layer. In contrast with the conventional FM/AFM exchange bias systems [4], the exchange bias field (HEB) was found to decrease with increasing cooling field (HFC) due to random interface effect [2, 5]. The analysis of the training effect data indicates the presence of frozen and rotatable interfacial spins that contributed to the observed loop shift and coercivity enhancement, respectively.

 
References: [1] K. O’Grady, S. J. Greaves, S. M. Thompson, J. Magn. Magn. Mater., 322, 883 (2010)

[2] K. D. Usadel, U. Nowak, Phys. Rev. B, 80, 014418 (2009)

[3] A. C. Basaran, T. Saerbeck, J. de la Venta, H. Huckfeldt, A. Ehresmann, Ivan K. Schuller Appl. Phys. Lett., 105, 072403 (2014)

[4] L. D. Bianco, F.Spizzo, M.Tamisari, S.Laureti, Solid State Commun., 151, 351 (2011)

[5] S. Nayak, P. K. Manna, V. Thiruvengadam, B. B. Singh, S. Bedanta, J. Magn. Magn. Mater., 499, 166267 (2020)

Exchange bias in Fe/Ir20Mn80 bilayers: role of spin-glass like interface and ‘bulk’ antiferromagnet spins

Topological spin textures, such as skyrmions, have attracted intense interest in recent years. Magnetic skyrmions have been studied almost exclusively in bulk ferromagnets (FM) or FM multilayers. Antiferromagnets (AF) have also been predicted to harbor topological spin textures such as AF skyrmions. Topological Hall effect has been a primary indicator of spin textures in magnetic materials. We observe unambiguous evidence of topological Hall effect in Pt/Cr2O3 bilayers grown on Al2O3(0001) and Al2O3(11-20), where Cr2O3 epitaxial film is an antiferromagnetic insulator[1]. Although Cr2O3 is insulating, the topological magnetic textures can be detected through a novel phenomenon: “spin-Hall topological Hall effect” (SH-THE), similar to what recently observed topological Hall effect in Pt/Tm3Fe5O12 bilayers[2]. The Pt/Cr2O3 bilayers exhibit clear topological Hall resistivity for Cr2O3 thicknesses below 6 nm near and above room temperature, which is above the Néel temperature of Cr2O3, revealing the key role of thermal fluctuations in the formation of spin textures[3]. The similarity of topological Hall signals in (0001) and (11-20)-oriented Cr2O3 films indicates that the emergence of spin textures is insensitive to crystalline orientation. This first observational evidence of topological Hall effect in HM/AFI bilayers significantly expands our materials base to include the large family of AF insulators for the exploration of AF-based skyrmion technology.References: [1]. Yang Cheng, Sisheng Yu, Menglin Zhu, Jinwoo Hwang and Fengyuan Yang, “Evidence of Topological Hall Effect in Pt/Antiferromagnetic-Insulator Bilayers,” Phys. Rev. Lett. 123, 237206. [2]. A. S. Ahmed, A. J. Lee, N. Bagués, B. A. McCullian, A. M. A. Thabt, A. Perrine, P.-K. Wu, J. R. Rowland, M. Randeria, P. C. Hammel, D. W. McComb, and F.. Y. Yang, “Spin-Hall topological Hall effect in highly tunable Pt/ferrimagnetic-insulator bilayers,” Nano Lett., 19, 5683 (2019). [3]. Wenbo Wang, Matthew W Daniels, Zhaoliang Liao, Yifan Zhao, Jun Wang, Gertjan Koster, Guus Rijnders, Cui-Zu Chang, Di Xiao and Weida Wu, "Spin chirality fluctuation in two-dimensional ferromagnets with perpendicular magnetic anisotropy" Nat. Mater. 2019, 18, 1054.

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