Lakhan Bainsla | Tohoku University (original) (raw)

Papers by Lakhan Bainsla

Spin-Hall nano oscillators (SHNOs) [1-4] have recently emerged as an alternative for microwave si... more Spin-Hall nano oscillators (SHNOs) [1-4] have recently emerged as an alternative for microwave signal generation and bio-inspired oscillatory computing [3,4]. However, the ferromagnetic layer of the SHNO comes with drawbacks: (a) need for strong external magnetic fields, and (b) limited operational frequencies. As a solution, compensated ferrimagnets provide an alternative as they combine the ultrafast magnetization dynamics of antiferromagnets with a ferromagnet-like spin-orbit-torque (SOT) behavior. For use in SHNOs, it is however important to ensure that such films retain their ferrimagnetic behavior also in ultrathin films. In this work, ferrimagnetic Gdx(FeCo)1-x thin films were grown using co-sputtering of Gd and Fe87.5Co12.5 on high resistance (HR) Si (100) substrates, and their magneto-dynamics was studied using ferromagnetic resonance m...

Topological semimetals (TSs) are promising candidates for low-power spin-orbit torque (SOT) devic... more Topological semimetals (TSs) are promising candidates for low-power spin-orbit torque (SOT) devices due to their large charge-to-spin conversion efficiency. Here, we investigated the charge-to-spin conversion efficiency of amorphous PtSn4 (5 nm)/CoFeB (2.5-12.5 nm) layered structures prepared by a magnetron sputtering method at room temperature. The charge-to-spin ratio of PtSn4/CoFeB bilayers was 0.08, characterized by a spin torque ferromagnetic resonance (ST-FMR) technique. This ratio can further increase to 0.14 by inducing dopants, like Al and CoSi, into PtSn4. The dopants can also decrease (Al doping) or increase (CoSi doping) the resistivity of PtSn4. The work proposed a way to enhance the spin-orbit coupling (SOC) in amorphous TSs with dopants.

Journal of Physics D: Applied Physics, 2018

Magnetic materials with low Gilbert damping and low magnetization are necessary for the realizati... more Magnetic materials with low Gilbert damping and low magnetization are necessary for the realization of faster or more energy-efficient spintronic devices based on spin-transfer-torque. Here, we report Gilbert damping in epitaxially grown equiatomic quaternary CoFeMnSi Heusler alloy films. The 10 nm-thick films show a saturation magnetization of M S = 630 emu cm−3 and a Gilbert damping constant of , which are relatively small values among transition metal ferromagnets, in addition to its soft magnetic properties. The physical origin of the relatively low damping and the possibility of a further reduction of α to the ultra-low damping regime ~10−4 are discussed in terms of the spin-gapless-like electronic structure and the effect of the chemical order computed from first principles.

Journal of Alloys and Compounds, 2015

Abstract In this paper, CoRuFeX (X = Si, Ge) alloys were investigated by means of x-ray diffracti... more Abstract In this paper, CoRuFeX (X = Si, Ge) alloys were investigated by means of x-ray diffraction, Mossbauer spectroscopy, magnetic and magneto-transport measurements. Alloys are found to be exist in Y-type structure with presence of some B2 disorder. High values of Curie temperature (TC) of 867 K and 833 K are obtained for CoRuFeSi and CoRuFeGe alloys, respectively. The obtained TC values are found to be highest among all the equiatomic quaternary Heusler alloys. The saturation magnetization (MS) values are found to be in good agreement with the values obtained from Slater-Pauling estimation, which is a prerequisite for half-metallic ferromagnets. Electrical resistivity in the temperature range of 5–300 K gives an indirect evidence of half-metallic behavior in these alloys at low temperatures. Considering high TC and the transport behavior, both these materials seem to be interesting for spintronic applications.

Journal of Applied Physics, 2014

We present here the detailed structural, magnetic, and magnetodielectric properties of polycrysta... more We present here the detailed structural, magnetic, and magnetodielectric properties of polycrystalline DyFe0.5Cr0.5O3. Rietveld refinement reveals that DyFe0.5Cr0.5O3 crystallizes in an orthorhombic structure, with Pnma space group, having a disordered arrangement of Fe and Cr ions. An anomaly observed around 250 K in the temperature dependence of magnetization indicates an onset of magnetic ordering. The field dependent magnetization of DyFe0.5Cr0.5O3 at 13 K, 120 K, and 240 K has a small loop indicating weak ferromagnetic nature and the corresponding curve at 5 K indicates a metamagnetic behavior. The small loop and the unsaturated magnetization even for a high applied magnetic field of 7 T indicate a canted antiferromagnetic structure, the canting can be explained due to Dzyaloshinsky–Moriya antisymmetric exchange interaction. The temperature dependence of the dielectric permittivity shows a transition around 500 K. An anomaly in the temperature dependence of magnetization (supported by differential sc...

Handbook of Magnetic Materials, 2016

Ferromagnetic materials dominate as the magnetically active element in spintronic devices, but co... more Ferromagnetic materials dominate as the magnetically active element in spintronic devices, but come with drawbacks such as large stray fields, and low operational frequencies. Compensated ferrimagnets provide an alternative as they combine the ultrafast magnetization dynamics of antiferromagnets with a ferromagnet-like spin-orbit-torque (SOT) behavior. However to use ferrimagnets in spintronic devices their advantageous properties must be retained also in ultrathin films (t < 10 nm). In this study, ferrimagnetic Gdx(Fe87.5Co12.5)1−x thin films in the thickness range t = 2–20 nm were grown on high resistance Si(100) substrates and studied using broadband ferromagnetic resonance measurements at room temperature. By tuning their stoichiometry, a nearly compensated behavior is observed in 2 nm Gdx(Fe87.5Co12.5)1−x ultrathin films for the first time, with an effective magnetization of Meff = 0.02 T and a low effective Gilbert damping constant of α = 0.0078, comparable to the lowest va...

$Research paper thumbnail of Energy-efficient W$_{\text{100-x}}$Ta$_{\text{x}}$/CoFeB/MgO spin Hall nano-oscillators$

We investigate aW–Ta alloying route to reduce the auto-oscillation current densities and the powe... more We investigate aW–Ta alloying route to reduce the auto-oscillation current densities and the power consumption of nano-constriction based spin Hall nano-oscillators. Using spin-torque ferromagnetic resonance (ST-FMR) measurements on microbars of W100-xTax(5 nm)/CoFeB(t)/MgO stacks with t = 1.4, 1.8, and 2.0 nm, we measure a substantial improvement in both the spin-orbit torque efficiency and the spin Hall conductivity. We demonstrate a 34% reduction in threshold autooscillation current density, which translates into a 64% reduction in power consumption as compared to pure W based SHNOs. Our work demonstrates the promising aspects of W–Ta alloying for the energy-efficient operation of emerging spintronic devices.

Advanced Functional Materials, 2022

In this report, structural, electronic, magnetic and transport properties of quaternary Heusler a... more In this report, structural, electronic, magnetic and transport properties of quaternary Heusler alloys CoRuMnGe and CoRuVZ (Z = Al, Ga) are investigated. All the three alloys are found to crystallize in cubic structure. CoRuMnGe exhibits L2_1 structure whereas, the other two alloys have B2-type disorder. For CoRuMnGe and CoRuVGa, the experimental magnetic moments are in close agreement with the theory as well as those predicted by the Slater-Pauling rule, while for CoRuVAl, a relatively large deviation is seen. Among these alloys, CoRuMnGe has the highest T_C of 560 K. Resistivity variation with temperature reflects the half-metallic nature in CoRuMnGe alloy. CoRuVAl shows metallic character in both paramagnetic and ferromagnetic states, whereas the temperature dependence of resistivity for CoRuVGa is quite unusual. In the last system, ρ vs. T curve shows an anomaly in the form of a maximum and a region of negative temperature coefficient of resistivity (TCR) in the magnetically ord...

The Japan Society of Applied Physics, 2017

K. Z. Suzuki, R. Ranjbar, Lakhan Bainsla, 1 Atsuo Ono, 1 Yuta Sasaki, 1,4 J. Okabayashi, Y. Miura... more K. Z. Suzuki, R. Ranjbar, Lakhan Bainsla, 1 Atsuo Ono, 1 Yuta Sasaki, 1,4 J. Okabayashi, Y. Miura, and S. Mizukami 1)WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577,Japan 2)Research Center for Spectrochemistry, University of Tokyo, Tokyo 113-0033,Japan 3)Department of Electronics, Kyoto Institute of Technology, Kyoto 606-8585,Japan 4)Department of Applied Physics, Tohoku University, Sendai 980-8579, Japan E-mail: kazuya.suzuki.d8@tohoku.ac.jp Mn-Ga alloy films have a large perpendicular magnetic anisotropy (PMA), small saturation magnetization (Ms), and small damping constant, thus it is potentially attractive for spin-transfer-torque (STT) applications, such as magnetoresistive random access memory (STT-MRAM) with high recording density and STT-oscillator/diode devices with the operation frequency of THz range. Recently, we discovered B2-ordered CoGa as an excellent growth seed layer for ultra-thin L10-orderedMnGa films and demonstrated that the pe...

Despite a plethora of materials suggested for spintronic applications, a new class of materials h... more Despite a plethora of materials suggested for spintronic applications, a new class of materials has emerged, namely spin gapless semiconductors (SGS), that offers potentially more advantageous properties than existing ones. These magnetic semiconductors exhibit a finite band gap for one spin channel and a closed gap for the other. Here, supported by the first-principles, electronic-structure calculations, we report the first experimental evidence of SGS behavior in equiatomic quaternary CoFeCrGa, having a cubic Heusler (L21) structure but exhibiting chemical disorder (DO3 structure). CoFeCrGa is found to transform from SGS to half-metallic phase under pressure, which is attributed to unique electronic-structure features. The saturation magnetization (MS) obtained at 8 K agrees with the Slater-Pauling rule and the Curie temperature (TC) is found to exceed 400 K. Carrier concentration (up to 250 K) and electrical conductivity are observed to be nearly temperature independent, prerequi...

Journal of Applied Physics, 2020

Spin-gapless semiconductors (SGSs) are new states of quantum matter, which are characterized by a... more Spin-gapless semiconductors (SGSs) are new states of quantum matter, which are characterized by a unique spin-polarized band structure. Unlike conventional semiconductors or half-metallic ferromagnets, they carry a finite bandgap for one spin channel and a close (zero) gap for the other and thus are useful for tunable spin transport applications. It is one of the latest classes of materials considered for spintronic devices. A few of the several advantages of SGS include (i) a high Curie temperature, (ii) a minimal amount of energy required to excite electrons from the valence to conduction band due to zero gap, and (iii) the availability of both charge carriers, i.e., electrons as well as holes, which can be 100% spin-polarized simultaneously. In this perspective article, the theoretical foundation of SGS is first reviewed followed by experimental advancements on various realistic materials. The first band structure of SGS was reported in bulk Co-doped PbPdO 2 , using first-principles calculations. This was followed by a large number of ab initio simulation reports predicting SGS nature in different Heusler alloy systems. The first experimental realization of SGS was made in 2013 in a bulk inverse Heusler alloy, Mn 2 CoAl. In terms of material properties, SGS shows a few unique features such as nearly temperature-independent conductivity (σ) and carrier concentration, a very low temperature coefficient of resistivity, a vanishingly small Seebeck coefficient, quantum linear magnetoresistance in a low temperature range, etc. Later, several other systems, including 2-dimensional materials, were reported to show the signature of SGS. There are some variants of SGSs that can show a quantum anomalous Hall effect. These SGSs are classic examples of topological (Chern) insulators. In the later part of this article, we have touched upon some of these aspects of SGS or the so-called Dirac SGS systems as well. In general, SGSs can be categorized into four different types depending on how various bands corresponding to two different spin channels touch the Fermi level. The hunt for these different types of SGS materials is growing very fast. Some of the recent progress along this direction is also discussed.

Extended Abstracts of the 2018 International Conference on Solid State Devices and Materials, 2018

Physical Review Materials, 2019

The equiatomic quaternary Heusler alloy CoFeCrAl is a candidate material for spin-gapless semicon... more The equiatomic quaternary Heusler alloy CoFeCrAl is a candidate material for spin-gapless semiconductors (SGSs). However, to date, there have been no experimental attempts at fabricating 3 a junction device. This paper reports a fully epitaxial (001)-oriented MgO barrier magnetic tunnel 4 junction (MTJ) with CoFeCrAl electrodes grown on a Cr buffer. X-ray and electron diffraction 5 measurements show that the (001) CoFeCrAl electrode films with atomically flat surfaces have a 6 B2-ordered phase. The saturation magnetization is 380 emu/cm 3 , almost the same as the value 7 given by the Slater-Pauling-like rule, and the maximum tunnel magnetoresistance ratios at 300 8 K and 10 K are 87% and 165%, respectively. Cross-sectional electron diffraction analysis shows 9 that the MTJs have MgO interfaces with fewer dislocations. The temperature-and bias-voltagedependence of the transport measurements indicates magnon-induced inelastic electron tunneling overlapping with the coherent electron tunneling. X-ray magnetic circular dichroism (XMCD) measurements show a ferromagnetic arrangement of the Co and Fe magnetic moments of B2ordered CoFeCrAl, in contrast to the ferrimagnetic arrangement predicted for the Y-ordered state possessing SGS characteristics. Ab-initio calculations taking account of the Cr-Fe swap disorder qualitatively explain the XMCD results. Finally, the effect of the Cr-Fe swap disorder on the ability for electronic states to allow coherent electron tunneling is discussed. I. INTRODUCTION A spin-gapless semiconductor (SGS) is a material in which the Fermi level is located at a zero-energy gap state for a majority spin band and at an energy gap for a minority spin band.[1-3] SGSs belong to the class of half-metals that have fully spin-polarized carriers at the Fermi level, so they exhibit a huge magnetoresistance (MR) and low spin relaxation (the so-called Gilbert damping). These physical properties are ideally suited to solid-state spintronic devices, and are commonly observed in half-metals.[4-10] In addition to such physical properties, SGSs could be used to realize devices with new functionalities, such as reconfigurable magnetic tunnel diodes and transistors,[11] which use their gapless electronic characteristics. Therefore, it is of fundamental and technological importance to investigate

Extended Abstracts of the 2017 International Conference on Solid State Devices and Materials, 2017

Extended Abstracts of the 2016 International Conference on Solid State Devices and Materials, 2016

Physical Review B, 2019

Spin gapless semiconductors (SGS) are interesting class of materials which bridge the gap between... more Spin gapless semiconductors (SGS) are interesting class of materials which bridge the gap between semiconductors and half-metallic ferromagnets. This class of materials shows band gap in one of the spin channels and a zero band gap in the other, and thus promote tunable spin transport. Here, we present structural, electronic, magnetic and transport properties of Co-rich spin gapless semiconductor CoFeCrGa using both theoretical and experimental techniques. The key advantage of Co-rich samples Co1+xFe1−xCrGa is the high Curie temperature (TC) and magnetization, without compromising the SGS nature (up to x = 0.4), and hence our choice. The quaternary Heusler alloys Co1+xFe1−xCrGa (x = 0.1 to 0.5) are found to crystallize in LiMgPdSn-type structure having space group F43m (# 216). The measured Curie temperature increases from 690 K (x = 0) to 870 K (x = 0.5). The obtained TC for x = 0.3 (790 K) is found to be the highest among all the previously reported SGS materials. Observed magnetization values follow the Slater-Pauling rule. Measured electrical resistivity, in the temperature range of 5-350 K, suggests that the alloys retain the SGS behavior up to x = 0.4, beyond which it reflects metallic character. Unlike conventional semiconductors, the conductivity value (σxx) at 300 K lies in the range of 2289 S cm −1 to 3294 S cm −1 , which is close to that of other reported SGS materials. The anomalous Hall effect is comparatively low. The intrinsic contribution to the anomalous Hall conductivity increase with x, which can be correlated with the enhancement in chemical order. The anomalous Hall coefficient is found to increase from 38 S/cm for x = 0.1 to 43 S/cm for 0.3. Seebeck coefficients turn out to be vanishingly small below 300 K, another signature for being SGS. All the alloys (for different x) are found to be both chemically and thermally stable. Simulated magnetization agrees fairly with the experiment. As such Co-rich CoFeCrGa is a promising candidate for room temperature spintronic applications, with enhanced TC, magnetic properties and SGS nature.

Applied Physics Letters, 2018

Tunnel magnetoresistance (TMR) in MgO-based magnetic tunnel junctions (MTJs) with equiatomic quat... more Tunnel magnetoresistance (TMR) in MgO-based magnetic tunnel junctions (MTJs) with equiatomic quaternary CoFeMnSi Heusler and CoFe alloy electrodes is studied. The epitaxial MTJ stacking structures were prepared using ultrahigh-vacuum magnetron sputtering, where the CoFeMnSi electrode has a full B2 and partial L2 1 ordering crystal structure. Maximum TMR ratios of 101% and 521% were observed at room temperature and 10 K, respectively, for the MTJs. The large bias voltage dependence of the TMR ratio was also observed at low temperature (LT), as similarly observed in Co 2 MnSi Heusler alloy-based MTJs in the past. The physical origins of this relatively large TMR ratio at LT were discussed in terms of the half-metallicity of CoFeMnSi.