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Spin Torque Oscillators (STOs) are promising solutions in a wide variety of next generation technologies from read-head sensors in high-density magnetic recording arrays [1] to neural oscillator units for neuromorphic computing [2-3]. There are several metrics that can be used to quantify the performance of an STO such as power, quality factor, frequency tunability, etc., most of which are dependent on the design of the STO device itself. Furthermore, determining the most important metric will be contingent on its desired application, meaning that it is crucial to understand how the STOs design parameters influence all aspects of its performance so that its design can be optimized to perform the desired function. Our previous studies have investigated the influence of STO size [4] and shape anisotropy [5] on precession frequency and linewidth. In this work, we expand on our previous research and study the influence of device size and shape on several STO performance metrics and use correlation coefficients to quantify relative magnitude of these effects. This was done by analyzing spin torque oscillations generated from 20 magnetic tunnel junctions with in-plane anisotropy and patterned into elliptical nano-pillars with a wide range of sizes and aspect ratios. For each device, we acquired 20 to 50 data sets at various bias fields and currents and obtained power spectral density (PSD) plots for each set, examples of which are shown in Fig. 1a. Each PSD plot was used to measure frequency and linewidth (as illustrated in Fig. 1b) as well as output power. From these measurements, we then calculated quality factor and power-to-linewidth ratio for each set. We also analyzed each STOs performance in terms of the bias fields and bias currents required to maximize output power and signal quality as well as the frequency tunability with both field and current. All of these performance metrics were compared between all 20 STOs tested.References: [1] H. Suto et al., “Real-Time Measurement of Temporal Response of a Spin-Torque Oscillator to Magnetic Pulses”, Appl. Phys. Express, vol. 4, p. 013003, Dec. 2010, doi: 10.1143/APEX.4.013003. [2] D. E. Nikonov et al., “Coupled-Oscillator Associative Memory Array Operation for Pattern Recognition”, IEEE J. Explor. Solid-State Comput. Devices Circuits, vol. 1, pp. 85–93, Nov. 2015, doi: 10.1109/JXCDC.2015.2504049. [3] M. R. Pufall et al., “Physical Implementatin of Coherently Coupled Oscillator Networks”, IEEE J. Explor. Solid-State Comput. Devices Circuits, vol. 1, pp. 76-84, Aug. 2015, doi: 10.1109/JXCDC.2015.2468070. [4] X. Chao, M. Jamali, and J.-P. Wang, “Scaling effect of spin-torque nano-oscillators”, AIP Advances, vol. 7, issue 5, p. 056624 Jan. 2017, doi: 10.1063/1.4974014. [5] X. Chao, M. Jamali, and J.-P. Wang, “Shape anisotropy effects on spin-torque oscillators”, AIP Advances, vol. 10, issue 4, p. 045101, Mar. 2020, doi: 10.1063/1.5130480.

MMM 2020

Influence of size and shape on key performance metrics in spin-torque oscillators

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Magnetic core has been widely used in integrated inductor as it can provide a high inductance and reduce the device size due to its high permeability [1]. At the same time, the relatively high loss tangent and saturation current will diminish the performance enhancement in the high frequency and high current applications, respectively [2]. The dilemma has resulted in the investigation of magnetic core materials, among which CoZrTaB demonstrated a low Hk, high resistivity and low loss tangent below 500 MHz, which is compatible with most of the integrated voltage regulators’ working frequency [3]. Here we fabricated CoZrTaB thin films with sputtering on different substrates and characterized them comprehensively. The CoZrTaB thin films have been fabricated in single layer and multi-layer structures (Fig. 1(a)), of which the VSM results, shown in Fig. 2, suggest that the Hk change of the multi-layer structure can be neglected while it can suppress the eddy current at high working frequency. Compared with the continuously sputtered 400nm CoZrTaB thin film, the multi-layer thin film shows a more desirable hysteresis loop with a single domain. The surface roughness of the Si and two kinds polyimide was extracted with the AFM, followed by the VSM characterization to verify the performance degradation. In addition, the material characterization such as XRD and RBS have been conducted to acquire the information of phase and element ratio of the CoZrTaB thin film, as shown in Fig.1 (b)(c). A comparison is made with the previous reported results from Gardner [4] and Koh [5] to see the successful implantation of the CoZrTaB thin film on different substrates.References: [1] Xu, Wei, et al. "Performance enhancement of on-chip inductors with permalloy magnetic rings." IEEE Electron Device Letters 32.1 (2010): 69-71. [2] Bechir, M. H., et al. "Magnetic inductor model which take into account permeability and loss tangent of the magnetic material." 2015 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP). IEEE, 2015. [3] Burton, Edward A., et al. "FIVR—Fully integrated voltage regulators on 4th generation Intel® Core™ SoCs." 2014 IEEE Applied Power Electronics Conference and Exposition-APEC 2014. IEEE, 2014. [4] Gardner, Donald S., et al. "Integrated on-chip inductors with magnetic films." 2006 International Electron Devices Meeting. IEEE, 2006. [5] Koh, Kisik, et al. "High frequency microwave on-chip inductors using increased ferromagnetic resonance frequency of magnetic films." 2015 28th IEEE International Conference on Micro Electro Mechanical Systems (MEMS). IEEE, 2015.

The Improvement of the CoZrTaB Thin Films on Different Substrates for Flexible Device Applications

It is well known that both the anisotropy K and the exchange stiffness A in magnetic materials decrease as the temperature increases [1]. The temperature dependence of these parameters is often expressed as a power law in the reduced magnetization (m), e.g. calculations for Co indicate that the Callen-Callen law K(m)∼ m3 is a very good approximation for the anisotropy, while for the exchange stiffness, A(m) ∼ m1.8 [1]. Since the domain wall width is proportional to the square root of A/K, it increases with temperature, as does the domain wall energy. However, in thin films with an interfacial Dzyaloshinskii-Moriya interaction (DMI), the DMI constant (D) also enters in the expression for the domain wall energy. As the DMI leads to chiral domain walls and skyrmions, determining the temperature dependence of D and hence the domain wall energy is important from both fundamental and applied points of view. We report on our study of multilayers of Pt/ Co68Fe22B10/Ir and Pt/Co68B32/Ir multilayers, deposited by dc magnetron sputtering onto a silicon oxide substrate. Wide-field Kerr microscopy was used to image the domains in the demagnetised state. We measured the domain period in the temperature range from -264 to 17 °C [Fig.1], enabling a calculation of D following the approach of Woo et al [2] as shown in [Fig.2]. In a second study, we deposited the same multilayers onto BaTiO3 (BTO) (100), which undergoes structural phase transitions close to 0 and -100 °C and thus exerts a strain on the multilayers. Images of domains in a [Pt/CoB/Ir]x10 superlattice were obtained using X-ray magnetic circular dichroism - photoemission electron microscopy (XMCD-PEEM). It was found that the domain period increases as the temperature increases as shown in [Fig.1]. However the DMI for all the samples decreases when the temperature increases as shown in [Fig.1].References: [1] Moreno et al., Phys. Rev. B 94, 104433 (2016) [2] Woo et al., Nat. Mater. 15, 501 (2016) [3]Bennett, et al., Scientific reports, 6, p.22708 (2016).

Temperature dependence of the Dzyaloshinskii-Moriya constant in ultrathin Pt/Co(Fe)B/Ir films

A- and B-site ordered quadruple perovskites with the chemical formula AA'3B2B'2O12 can form with a 1:3 ratio at the A site. A unique feature for this specially ordered perovskite is that three different atomic sites (A', B, and B' sites) can all accommodate magnetic transition metals. As a consequence, multiple magnetic interactions can occur at A', B and/or B' sites, giving rise to a series of intriguing physical phenomena. CaCu3Fe2Re2O12 is a good example, which shows the half-metallic electronic structure, large magnetization, and very high Curie temperature [1]. Here we have investigated a series of ferrimagnetic compounds ACu3Fe2Re2O12(A=Ca, Sr, Ba, Pb, Sc, Y, La) by using density functional theory and Monte Carlo simulations. We found that all compounds with A2+ ions exhibit high Curie temperatures (above 405 K), and the compounds with A3+substitution yield even higher Tc (above 502 K). By examining interatomic exchange parameters, we found that the antiferromagnetic exchange couplings between Re and Cu as well as Re and Fe are responsible for this very high Curie temperature. For the compounds with A3+ substitution, an electron doping at the band around the Fermi level dominated by the Re ions strengthen the Re-Cu, Re-Fe and Re-Re exchange interactions, which cause an increase in the critical temperatures. In summary, this work demonstrates a design strategy of enhancing spin ordering temperatures by replacing A-site non-magnetic ions.References: 1. Chen, Wei-tin, Masaichiro Mizumaki, Hayato Seki, Mark S. Senn, Takashi Saito, Daisuke Kan, J. Paul Attfield, and Yuichi Shimakawa. "A half-metallic A-and B-site-ordered quadruple perovskite oxide CaCu3Fe2Re2O12 with large magnetization and a high transition temperature." Nature Communications 5, no. 1 (2014): 1-7.

Electronic structure and magnetism of half-metallic quadruple perovskite ACu3Fe2Re2O12(A=Ca, Sr, Ba, Pb, Sc, Y, La) by first principles theory

Rare-earth orthochromites (RCrO3) with orthorhombically distorted perovskite (ABO3) structure exhibit a wealth of magnetic phenomena such as temperature-induced magnetization reversal (TMR), spin-reorientation (SR), spin-flipping (SF), and exchange-bias (EB). These occur as a result of magnetic interactions among the cations such as Cr3+–Cr3+, Cr3+–R3+ and R3+–R3+, where R is the rare earth element, such as Gd, Sm, Tm [1]. Rare-earth orthochromites also form a class of magnetoelectric multiferroics which exhibit the coexistence of ferroelectric and magnetic orders [1]. Single crystalline GdCrO3, exhibit a giant magnetic entropy change (ΔSm) at a reasonable magnetic field, making it a potential candidate for the application in magnetic refrigeration [2]. TMR and EB have been reported in several RCrO3 compounds due to the competition between the R3+ moment and the canted weak ferromagnetic (FM) component of Cr3+ ions [3]. In the present work, GdCrO4 samples were obtained using sol-gel technique [4]. X-ray diffraction and Raman techniques were used to identify the phase of the samples. Calcination of the samples at 600 °C led to GdCrO4 phase formation [5]. Further calcination of the samples at 1000 °C led to the decomposition of GdCrO4 into GdCrO3. Upon heat treatment, the Cr5+ oxidation state in the GdCrO4-zircon phase reduces to the relatively stable Cr3+ together with oxygen loss to stabilize the GdCrO3 perovskite-structure [5]. The role of thermal decomposition of GdCrO4 to GdCrO3 on crystal structure and magnetic transitions, studied using XRD and vibrating sample magnetometer (VSM), will be discussed. Magnetization measurements as a function of temperature (M-T) with different probing magnetic fields were carried out to locate the various magnetic transitions in the samples under different measurement protocols such as zero field cooled (ZFC), field cool cooling (FCC) and field cool warming (FCW). Anomalies in the M-T curves observed at T ≈ 10 K and T ≈ 170 K correspond to spin - flip and Néel transitions, respectively. This is in agreement with previously reported values [1]. The hysteresis loops measured across the transition temperatures validate the magnetic transitions as observed in the M-T curves.References: [1] S. Mahana, U. Manju and D. Topwal, J. Phys. D: Appl. Phys. 51 (2018) 305002. [2] L. Yin, J. Yang, X. Kan, W. Song, J. Dai and Y. Sun, J. Appl. Phys. 117 (2015)133901. [3] B.B. Dash and S. Ravi, Solid State Sciences 83 (2018) 192. [4] P. Mohanty, A.R.E. Prinsloo, B.P. Doyle, E. Carleschi, C.J. Sheppard, AIP Advances 8 (2018) 056424. [5] A.J. Dossantos-Garcia, E. Climent-Pascual, J.M. Gallardo-Amores, M.G. Rabie, Y. Doi, J. Romero de Paz, B. Beuneu, R. Saez-Puche, Journal of Solid State Chemistry 194 (2012) 119.

Thermal Decomposition of GdCrO4 to GdCrO3: Structure and Magnetism

We present the results of density functional calculations of the proposed low temperature charge quadrupolar ordering in Ba2MgReO6. Ba2MgReO6 is a spin-orbit-driven Mott insulator with a symmetry-lowering structural phase transition at 33 K and a canted antiferromagnetic ordering of the small (around 0.25 μB) Re magnetic moments at 18 K. The goal of our calculations is to test the existence of the quadrupolar order, and to understand its relationship to the structural and magnetic orders, which we achieve by separately isolating the ordering of the charge, spins, orbital moments, and structure. We find that the charge quadrupole ordering exists, and can be realized in the absence of magnetic ordering. In addition to shedding light on the detailed behavior of Ba2MgReO6, our work provides insight into the intriguing physics of 5d1 double perovskites in general, and illustrates a method for the analysis of magnetic ordering interactions combined with higher orders such as charge quadrupoles.

Hidden Multipolar Orders in 5d1 Ba2MgReO6

Theoretical prediction and rare experimental observation of room temperature ferromagnetism in transition metal doped semiconductors/insulators such as ZnO, TiO2, SrTiO3 and BaTiO3 has been of key fundamental interest in understanding materials [1-3]. In this work, 5 mol% of Mn doped SrTiO3 was synthesized by chemical coprecipitation [4]. This synthesis technique has been adopted specifically to ensure the most homogeneous doping of Mn in SrTiO3 lattice thus largely preventing the formation of any other impurity phases. Analysis of high resolution XRD data collected for the sample sintered at 1150°C revealed that the phase was of high purity. Magnetization measurements were carried out on the same sample where the variation of Magnetization (M) with temperature under both zero field cooled (ZFC) and field cooled conditions (FC) were measured under an applied magnetic field (H) of 100 Oe. The trend in ZFC is quite contrary to the expected paramagnetic nature in this system. The increase in ZFC magnetization values when the temperature was increased beyond 150 K has to be taken into consideration [Fig. 1]. Also the dispersion in the ZFC and FC curves is quite significant near room temperature and there is a difference of an order in the ZFC and the FC magnetization values around 300 K unlike conventional paramagnetic systems. Therefore the temperature dependent magnetization data (>150 K) clearly exhibits the sign for an impending ferromagnetic behavior around RT. The M versus H behavior was studied in the -1.5 T to 1.5 T range at three different temperatures - 10 K, 150 K and 300 K. Saturated hysteresis loop observed at RT corroborates our observation of the bifurcation observed in ZFC/FC behavior around 300 K [Fig.2]. At 10 K, hysteresis loop turns to look more paramagnetic-like indicating the presence of only a weak ferromagnetic ordering with a strong paramagnetic matrix. A suitable mechanism based on defects induced by Mn doping in SrTiO3 will be discussed to explain this observation of RT Ferromagnetic behavior [5].References: K. Ueda, H. Tabata and T. Kawai, Appl. Phys. Lett. 79, 988 (2001) J.-Y. Kim, J.-H. Park, H.-J. Noh, Phys. Rev.Lett. 90, 017401 (2003) M. Valant, T. Kolodiazhnyi, I. Arčon, Adv. Funct. Mater. 22, 2114 (2012) Preethi Meher K.R.S, Christine B, Pierre-Eymeric Janolin and K.B.R. Varma, Journal of Solid state Chemistry, 192, 296 (2012) R. Ishikawa, Y. Shimbo, I. Sugiyama, Phys Rev B, 96, 024440 (2017)

Clinching experimental evidence of "Room Temperature Ferromagnetic Behavior" in 5 mol% Mn doped SrTiO3

Rare-earth-lean permanent magnetic materials play a key role in many technology sectors like spintronics, energy transport, aerospace, and biomedicine. Structure and composition are likely to control material properties such as magnetic moment and coercivity. Here, we study thin-film libraries of Sm-Co-Ti, fabricated on Si wafer by combinatorial DC magnetron sputtering, with a focus on magnetic and structural properties over wide ranges of composition and thickness. The samples had a thickness ranging from 370 to 450 Å and SmxCoyTi100-x-y alloy compositions with x = 10-25 at% and y = 65-86 at%, as estimated by MATLAB theoretical calculations and subsequently confirmed by Rutherford backscattering spectroscopy. Structural properties were investigated by GIXRD, confirming all samples to be amorphous. MOKE, VSM, and SQUID magnetometry were used to investigate the magnetic properties. In-plane magnetic hysteresis loops show a coercive field of 10-250 mT and a saturation magnetization of approximately 800 kA/m in the full thin libraries. RT-MOKE measurements confirmed that the addition of Ti to the SmCo alloy makes it likely to obtain greater coercivity (~180 mT) as compared to SmCo (~100 mT) [1] while reducing the necessity for a high Sm content. Fig 1 shows representative in-plane and out-of-plane hysteresis loop measured with VSM and In-plane coercivity measurement along the center stipe of the libraries. The Sm-Co gradient is along this stripe, and the Ti content is roughly constant (~7 at%).References: [1] Magnus, F., et al (2013). Tunable giant magnetic anisotropy in amorphous SmCo thin films. Applied Physics Letters, 102(16), 162 402.

Combinatorial study of Sm-Co-Ti amorphous hard magnet thin films

The rare-earth (RE) supply crisis around 2010/2011 led to a drastic increase in prices and revealed the dependency on critical RE elements [1] for the production of high-performance permanent magnets. Significant efforts have been made since in the search for new material systems which present magnetic properties similar or superior to those of benchmark Nd2Fe14B, but which contain less critical elements [2]. One of such alternatives are Nd- and Sm-based intermetallic compounds with a tetragonal ThMn12-type crystal structure, exhibiting intrinsic magnetic properties comparable to Nd2Fe14B [3-6]. In this study, we carried out advanced characterization of grain boundaries of polycrystalline bulk SmFe11Ti samples. Detailed microstructural analysis was carried out using HR-SEM coupled with TEM and APT. Additionally, for the determination of the magnetic properties room-temperature isothermal magnetization measurements were carried out. To get a better understanding of the demagnetization behavior of the samples, in-situ demagnetization measurements were done using MOKE microscopy. The EDX analysis results indicate that the chemical composition of the suction casted and annealed sample is slightly off-stoichiometric SmFe11.1Ti0.9. The APT measurements revealed a distinct composition of grain boundaries compared to the adjoining 1:12 matrix grains where a grain boundary composition with more than 75 at. % of iron was measured. Additionally, three different phases, namely, monoclinic Sm3(Fe,Ti)29, cubic SmFe2-, and Fe2Ti-Laves phases, all form preferably at the grain boundaries and at triple junctions. The MOKE measurements reveal a strong ferromagnetic coupling of the 1:12 grains across the grain boundaries. Based on these results, we assume that the grain boundaries, with more than 75 at% of iron, behave like ferromagnetic regions and therefore no magnetic isolation is observed at the grain boundaries. Financial support of the future pioneering program Development of Magnetic Material Technology for High-Efficiency Motors (MagHEM) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan and the DFG – Project-ID 405553726 – TRR 270 are gratefully acknowledged.References: [1] O. Gutfleisch et al., Adv. Mat. 23 (2011) 821. [2] K.P. Skokov and O. Gutfleisch, Scr. Mat. 154 (2018) 289. [3] K.H.J. Buschow, Journal of Appl. Phys. 63 (1988) 3130. [4] D.B. De Mooij and K.H.J. Buschow, J. Less Common Met. 136 (1988) 207. [5] F. Maccari et al., Acta Mat. 180 (2019) 15-23. [6] D. Palanisamy et al., Phys. Rev. Mat. 4 (2020) 054404.

Grain boundary segregation, phase formation, and their influence on the coercivity of rapidly solidified SmFe11Ti hard magnetic alloys

Itinerant Contribution to the Anisotropy of Rare-Earth Transition-Metal Permanent Magnets

One of the most intriguing outcomes of symmetry breaking and spin-obit interactions in magnetic systems is the possibility to create non-collinear and chiral spin textures. The Dzyaloshinskii-Moriya interaction (DMI) results from strong spin-orbit coupling and broken inversion symmetry to generate magnetization rotations with fixed chirality. The discovery of magnetic skyrmions originating from strong DMI in metal thin films has led to an explosion of efforts to manipulate magnetic phases originating from interfaces. We have studied interface-induced magnetism in epitaxial 3d/5d iridate/manganite superlattices. In LaMnO3/SrIrO3 superlattices, we find a large additional topological Hall effect arising from the interaction of charge carriers with a noncoplanar chiral spin texture induced by interfacial DMI [1]. I will describe how the interfacial atomic layer stacking and symmetry enabled by the nonmagnetic A-sites determine the competition between collinear and chiral magnetic interactions originating from the oxide interface. Our findings [2] provide insight to the manipulation of chiral magnetism from atomic-scale control of exchange interactions at oxide interfaces.References: [1] P. Bruno et al., Phys. Rev. Lett. 93, 096806 (2004). [2] E. Skoropata et al., Sci. Adv., 6, eeaz3902 (2020).

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