arXiv subjects

Yong-Chang Lau

Publications and source records attributed to Yong-Chang Lau.

At least 37 records · Page 2Linked to original sources

arXiv2020

Giant inverse Faraday effect in Dirac semimetals

We have studied helicity dependent photocurrent (HDP) in Bi-based Dirac semimetal thin films. HDP increases with film thickness before it saturates, changes its sign when the majority carrier type is changed from electrons to holes and takes a sharp peak when the Fermi level lies near the charge neutrality point. These results suggest that irradiation of circularly polarized light to Dirac semimetals induces an effective magnetic field that aligns the carrier spin along the light spin angular momentum and generates a spin current along the film normal. The effective magnetic field is estimated to be orders of magnitude larger than that caused by the inverse Faraday effect (IFE) in typical transition metals. We consider the small effective mass and the large $g$-factor, characteristics of Dirac semimetals with strong spin orbit coupling, are responsible for the giant IFE, opening pathways to develop systems with strong light-spin coupling.

cond-mat.mes-hall↗

arXiv2020

Interlayer exchange coupling through Ir-doped Cu spin Hall material

Metallic superlattices where the magnetization vectors in the adjacent ferromagnetic layers are antiferromagnetically coupled by the interlayer exchange coupling through nonmagnetic spacer layers are systems available for the systematic study on antiferromagnetic (AF) spintronics. As a candidate of nonmagnetic spacer layer material exhibiting remarkable spin Hall effect, which is essential to achieve spin-orbit torque switching, we selected the Ir-doped Cu in this study. The AF-coupling for the Co / Cu$_{95}$Ir$_{5}$ / Co was investigated, and was compared with those for the Co / Cu / Co and Co / Ir / Co. The maximum magnitude of AF-coupling strength was obtained to be 0.39 mJ/m$^{2}$ at the Cu$_{95}$Ir$_{5}$ thickness of about 0.75 nm. Furthermore, we found a large spin Hall angle of Cu$_{95}$Ir$_{5}$ in Co / Cu$_{95}$Ir$_{5}$ bilayers by carrying out spin Hall magnetoresistance and harmonic Hall voltage measurements, which are estimated to be 3 ~ 4 %. Our experimental results clearly indicate that Cu$_{95}$Ir$_{5}$ is a nonmagnetic spacer layer allowing us to achieve moderately strong AF-coupling and to generate appreciable spin-orbit torque via the spin Hall effect.

cond-mat.mtrl-sci↗

arXiv2019

The spin Hall effect of Bi-Sb alloys driven by thermally excited Dirac-like electrons

We have studied the charge to spin conversion in Bi$_{1-x}$Sb$_x$/CoFeB heterostructures. The spin Hall conductivity (SHC) of the sputter deposited heterostructures exhibits a high plateau at Bi-rich compositions, corresponding to the topological insulator phase, followed by a decrease of SHC for Sb-richer alloys, in agreement with the calculated intrinsic spin Hall effect of Bi$_{1-x}$Sb$_x$ alloy. The SHC increases with increasing thickness of the Bi$_{1-x}$Sb$_x$ alloy before it saturates, indicating that it is the bulk of the alloy that predominantly contributes to the generation of spin current; the topological surface states, if present in the films, play little role. Surprisingly, the SHC is found to increase with increasing temperature, following the trend of carrier density. These results suggest that the large SHC at room temperature, with a spin Hall efficiency exceeding 1 and an extremely large spin current mobility, is due to increased number of Dirac-like, thermally-excited electrons in the $L$ valley of the narrow gap Bi$_{1-x}$Sb$_x$ alloy.

cond-mat.mes-hall↗

arXiv2019

Giant perpendicular magnetic anisotropy in Ir/Co/Pt multilayers

We have studied the magnetic properties of multilayers composed of ferromagnetic metal Co and heavy metals with strong spin orbit coupling (Pt and Ir). Multilayers with symmetric (ABA stacking) and asymmetric (ABC stacking) structures are grown to study the effect of broken structural inversion symmetry. We compare the perpendicular magnetic anisotropy (PMA) energy of symmetric Pt/Co/Pt, Ir/Co/Ir multilayers and asymmetric Pt/Co/Ir, Ir/Co/Pt multilayers. First, the interface contribution to the PMA is studied using the Co layer thickness dependence of the effective PMA energy. Comparison of the interfacial PMA between the Ir/Co/Pt, Pt/Co/Ir asymmetric structures and Pt/Co/Pt, Ir/Co/Ir symmetric structures indicate that the broken structural inversion symmetry induced PMA is small compared to the overall interfacial PMA. Second, we find the magnetic anisotropy field is significantly increased in multilayers when the ferromagnetic layers are antiferromagnetically coupled via interlayer exchange coupling (IEC). Macrospin model calculations can qualitatively account for the relation between the anisotropy field and the IEC. Among the structures studied, IEC is the largest for the asymmetric Ir/Co/Pt multilayers: the exchange coupling field exceeds 3 T and consequently, the anisotropy field approaches 10 T. Third, comparing the asymmetric Ir/Co/Pt and Pt/Co/Ir structures, we find the IEC and, to some extent, the interface PMA are stronger for the former than the latter. X-ray magnetic circular dichroism studies suggest that the proximity induced magnetization in Pt is larger for the Ir/Co/Pt multilayers than the inverted structure, which may partly account for the difference in the magnetic properties. These results show the intricate relation between PMA, IEC and the proximity induced magnetization that can be exploited to design artificial structures with unique magnetic characteristics.

cond-mat.mtrl-sci↗

arXiv2019

Current-induced modulation of interfacial Dzyaloshinskii-Moriya interaction

The Dzyaloshinskii-Moriya (DM) interaction is an antisymmetric exchange interaction that is responsible for the emergence of chiral magnetism. The origin of the DM interaction, however, remains to be identified albeit the large number of studies reported on related effects. It has been recently suggested that the DM interaction is equivalent to an equilibrium spin current density originating from spin-orbit coupling, an effect referred to as the spin Doppler effect. The model predicts that the DM interaction can be controlled by spin current injected externally. Here we show that the DM exchange constant ($D$) in W/CoFeB based heterostructures can be modulated with external current passed along the film plane. At higher current, $D$ decreases with increasing current, which we infer is partly due to the adiabatic spin transfer torque. At lower current, $D$ increases linearly with current regardless of the polarity of current flow. The rate of increase in $D$ with the current density agrees with that predicted by the model based on the spin Doppler effect. These results imply that the DM interaction at the HM/FM interface partly originates from an equilibrium interface spin (polarized) current which can be modulated externally.

cond-mat.mes-hall↗

arXiv2019

Giant spin-orbit torque in a single ferrimagnetic metal layer

Antiferromagnets and compensated ferrimagnets offer opportunities to investigate spin dynamics in the 'terahertz gap' because their resonance modes lie in the 0.3 THz to 3 THz range. Despite some inherent advantages when compared to ferromagnets, these materials have not been extensively studied due to difficulties in exciting and detecting the high-frequency spin dynamics, especially in thin films. Here we show that spin-obit torque in a single layer of the highly spin-polarized compensated ferrimagnet Mn2RuxGa is remarkably efficient at generating spin-orbit fields μ_0H_eff, which approach 0.1x10-10 T m2/A in the low-current density limit -- almost a thousand times the Oersted field, and one to two orders of magnitude greater than the effective fields in heavy metal/ferromagnet bilayers. From an analysis of the harmonic Hall effect which takes account of the thermal contributions from the anomalous Nernst effect, we show that the antidamping component of the spin-orbit torque is sufficient to sustain self-oscillation. Our study demonstrates that spin electronics has the potential to underpin energy-frugal, chip-based solutions to the problem of ultra high-speed information transfer.

cond-mat.mes-hall↗

arXiv2019

Dzyaloshinskii-Moriya interaction and spin-orbit torque at the Ir/Co interface

We studied the spin torque efficiency and the Dzyaloshinskii-Moriya interaction (DMI) of heterostructures that contain interface(s) of Ir and Co. The current-induced shifts of the anomalous Hall loops were used to determine the spin torque efficiency and DMI of [Pt/Co/X] multilayers (X=Ir, Cu) as well as Ir/Co and Pt/Ir/Co reference films. We find the effective spin Hall angle and the spin diffusion length of Ir to be ~0.01 and less than ~1 nm, respectively. The short spin diffusion length and the high conductivity make Ir an efficient spin sink layer. Such spin sink layer can be used to control the flow of spin current in heterostructures and to induce sufficient spin-orbit torque on the magnetic layer. The DMI of Ir and Co interface is found to be in the range of ~1.4 to ~2.2 mJ/m2, similar in magnitude to that of the Pt and Co interface. The Ir/Co and Pt/Co interfaces possess the same sign of DMI, resulting in a reduced DMI for the [Pt/Co/Ir] multilayers compared to that of the [Pt/Co/Cu] multilayers. These results show the unique role the Ir layer plays in defining spin-orbit torque and chiral magnetism in thin film heterostructures.

cond-mat.mes-hall↗

arXiv2018

Circular photogalvanic effect in Cu/Bi bilayers

We have studied the circular photogalvanic effect (CPGE) in Cu/Bi bilayers. When a circularly polarized light in the visible range is irradiated to the bilayer from an oblique incidence, we find a photocurrent that depends on the helicity of light. Such photocurrent appears in a direction perpendicular to the light plane of incidence but is absent in the parallel configuration. The helicity dependent photocurrent is significantly reduced for a Bi single layer film and the effect is nearly absent for a Cu single layer film. Conventional interpretation of the CPGE suggests the existence of spin-momentum locked band(s) of a Rashba type in the Cu/Bi bilayer. In contrast to previous reports on the CPGE studied in other systems, however, the light energy used here to excite the carriers is much larger than the band gap of Bi. Moreover, the CPGE of the Cu/Bi bilayer is larger when the energy of the light is larger: the helicity dependent photocurrent excited with a blue light is nearly two times larger than that of a red light. We therefore consider the CPGE of the Cu/Bi bilayer may have a different origin compared to conventional systems.

cond-mat.mes-hall↗

arXiv2018

Magnetocrystalline anisotropy and exchange probed by high-field anomalous Hall effect in fully-compensated half-metallic Mn2RuxGa thin films

Magnetotransport is investigated in thin films of the half-metallic ferrimagnet Mn$_2$Ru$_x$Ga in pulsed magnetic fields of up to 58 T. A non-vanishing Hall signal is observed over a broad temperature range, spanning the compensation temperature 155 K, where the net magnetic moment is strictly zero, the anomalous Hall conductivity is 6673 $Ω^{-1}.m^{-1}$ and the coercivity exceeds 9 T. Molecular field modelling is used to determine the intra- and inter-sublattice exchange constants and from the spin-flop transition we infer the anisotropy of the electrically active sublattice to be 216 kJ/m$^3$ and predict the magnetic resonances frequencies. Exchange and anisotropy are comparable and hard-axis applied magnetic fields result in a tilting of the magnetic moments from their collinear ground state. Our analysis is applicable to collinear ferrimagnetic half-metal systems.

cond-mat.mtrl-sci↗

arXiv2018

Magneto-optic Kerr effect in a spin-polarized zero-moment ferrimagnet

The magneto-optical Kerr effect (MOKE) is often assumed to be proportional to the magnetisation of a magnetically ordered metallic sample; in metallic ferrimagnets with chemically distinct sublattices, such as rare-earth transition-metal alloys, it depends on the difference between the sublattice contributions. Here we show that in a highly spin polarized, fully compensated ferrimagnet, where the sublattices are chemically similar, MOKE is observed even when the net moment is strictly zero. We analyse the spectral ellipsometry and MOKE of Mn 2 Ru x Ga, and show that this behaviour is due to a highly spin-polarized conduction band dominated by one of the two manganese sublattices which creates helicity-dependent reflectivity determined by a broad Drude tail. Our findings open new prospects for studying spin dynamics in the infra-red.

cond-mat.mtrl-sci↗

arXiv2018

Domain wall resistance in CoFeB-based heterostructures with interface Dzyaloshinskii-Moriya interaction

We have studied the domain wall resistance in W/Ta/CoFeB/MgO heterostructures. The Ta layer thickness is varied to control the type of domain walls via changes in the interfacial Dzyaloshinskii Moriya interaction. We find a nearly constant domain wall resistance against the Ta layer thickness. Adding contributions from the anisotropic magnetoresistance, spin Hall magnetoresistance and anomalous Hall effect describe well the domain wall resistance of the thick Ta layer films. However, a discrepancy remains for the thin Ta layer films wherein chiral Néel-like domain walls are found. These results show the difficulty of studying the domain wall type from resistance measurements.

cond-mat.mes-hall↗

arXiv2018

Anomalous spin Hall magnetoresistance in Pt/Co bilayers

We have studied the spin Hall magnetoresistance (SMR), the magnetoresistance within the plane transverse to the current flow, of Pt/Co bilayers. We find that the SMR increases with increasing Co thickness: the effective spin Hall angle for bilayers with thick Co exceeds the reported values of Pt when a conventional drift-diffusion model is used. An extended model including spin transport within the Co layer cannot account for the large SMR. To identify its origin, contributions from other sources are studied. For most bilayers, the SMR increases with decreasing temperature and increasing magnetic field, indicating that magnon-related effects in the Co layer play little role. Without the Pt layer, we do not observe the large SMR found for the Pt/Co bilayers with thick Co. Implementing the effect of the so-called interface magnetoresistance and the textured induced anisotropic scattering cannot account for the Co thickness dependent SMR. Since the large SMR is present for W/Co but its magnitude reduces in W/CoFeB, we infer its origin is associated with a particular property of Co.

cond-mat.mes-hall↗

arXiv2017

The Spin Nernst effect in Tungsten

The spin Hall effect allows generation of spin current when charge current is passed along materials with large spin orbit coupling. It has been recently predicted that heat current in a non-magnetic metal can be converted into spin current via a process referred to as the spin Nernst effect. Here we report the observation of the spin Nernst effect in W. In W/CoFeB/MgO heterostructures, we find changes in the longitudinal and transverse voltages with magnetic field when temperature gradient is applied across the film. The field-dependence of the voltage resembles that of the spin Hall magnetoresistance. A comparison of the temperature gradient induced voltage and the spin Hall magnetoresistance allows direct estimation of the spin Nernst angle. We find the spin Nernst angle of W to be similar in magnitude but opposite in sign with its spin Hall angle. Interestingly, under an open circuit condition, such sign difference results in spin current generation larger than otherwise. These results highlight the distinct characteristics of the spin Nernst and spin Hall effects, providing pathways to explore materials with unique band structures that may generate large spin current with high efficiency.

cond-mat.mes-hall↗

arXiv2017

Spin Hall effect from hybridized 3$d$-4$p$ orbitals

Electrical manipulation of magnetization by spin-orbit torque (SOT) has shown promise for realizing reliable magnetic memories and oscillators. To date, the generation of transverse spin current and SOT, whether it is of spin Hall effect (SHE), Rashba-Edelstein effect or spin-momentum locking origin, relies primarily on materials or heterostructures containing 5$d$ or 6$p$ heavy elements with strong spin-orbit coupling. Here we show that a paramagnetic CoGa compound possesses large enough spin Hall angle to allow robust SOT switching of perpendicularly-magnetized ferrimagnetic MnGa films in CoGa/MnGa/Oxide heterostructures. The spin Hall efficiency estimated via spin Hall magnetoresistance and harmonic Hall measurements is +0.05$\pm$0.01, which is surprisingly large for a system that does not contain any heavy metal element. First-principles calculations corroborate our experimental observations and suggest that the hybridized Co 3$d$ - Ga 4$p$ orbitals along R-X in the Brillouin zone is responsible for the intrinsic SHE. Our results suggest that efficient spin current generation can be realized in intermetallic by alloying a transition metal with a $p$-orbital element and by Fermi level tuning.

cond-mat.mtrl-sci↗

arXiv2017

Exchange coupling of a perpendicular ferromagnet to a half-metallic compensated ferrimagnet via a thin hafnium interlayer

A thin Hafnium film is shown to act both as an effective diffusion barrier for manganese at a thickness of 0.7 nm, and as an effective exchange coupling layer in a sandwich structure with perpendicular magnetic anisotropy. The magnetic layers are Co$_{20}$Fe$_{60}$B$_{20}$ and the low moment ferrimagnet Mn$_2$Ru$_x$Ga (MRG). The coupling changes sign at the compensation temperature of MRG and the exchange energy reaches 0.11 mJm$^{-2}$ for the thinnest Hf interlayers. Ruthenium, the usual metal of choice for coupling ferromagnetic layers in thin film heterostructures, cannot be used with the zero-moment half metal MRG because of Ru interdiffusion. Due to its large coercivity near compensation, the MRG can act as an effective source of exchange pinning.

cond-mat.mtrl-sci↗

arXiv2017

Structure, site-specific magnetism and magneto-transport properties of epitaxial D0$_{22}$ Mn$_2$Fe$_x$Ga thin films

Ferrimagnetic Mn$_2$Fe$_x$Ga $(0.26 \leq x \leq 1.12)$ thin films have been characterised by X-ray diffraction, SQUID magnetometry, X-ray absorption spectroscopy, X-ray magnetic circular dichroism and Mössbauer spectroscopy with the aim of determining the structure and site-specific magnetism of this tetragonal, D0$_{22}$-structure Heusler compound. High-quality epitaxial films with low RMS surface roughness ($\sim 0.6$ nm) are grown by magnetron co-sputtering. The tetragonal distortion induces strong perpendicular magnetic anisotropy along the $c$-axis with a typical coercive field $μ_0 H\sim 0.8$ T and an anisotropy field ranging from $6$ to $8$ T. Upon increasing the Fe content $x$, substantial uniaxial anisotropy, $K_\mathrm{u} \geq 1.0$ MJ/m$^3$ can be maintained over the full $x$ range, while the magnetisation of the compound is reduced from $400$ to $280$ kA/m. The total magnetisation is almost entirely given by the sum of the spin moments originating from the ferrimagnetic Mn and Fe sublattices, with the latter being coupled ferromagnetically to one of the former. The orbital magnetic moments are practically quenched, and have negligible contributions to the magnetisation. The films with $x=0.73$ exhibit a high anomalous Hall angle of $2.5$ % and a high Fermi-level spin polarisation, above $51$ %, as measured by point contact Andreev reflection. The Fe-substituted Mn$_2$Ga films are highly tunable with a unique combination of high anisotropy, low magnetisation, appreciable spin polarisation and low surface roughness, making them very strong candidates for thermally-stable spin-transfer-torque switching nanomagnets with lateral dimensions down to $10$ nm.

cond-mat.mtrl-sci↗

arXiv2017

Electric field modulation of the non-linear areal magnetic anisotropy energy

We study the ferromagnetic layer thickness dependence of the voltage-controlled magnetic anisotropy (VCMA) in gated CoFeB/MgO heterostructures with heavy metal underlayers. When the effective CoFeB thickness is below ~1 nm, the VCMA efficiency of Ta/CoFeB/MgO heterostructures considerably decreases with decreasing CoFeB thickness. We find that a high order phenomenological term used to describe the thickness dependence of the areal magnetic anisotropy energy can also account for the change in the areal VCMA efficiency. In this structure, the higher order term competes against the common interfacial VCMA, thereby reducing the efficiency at lower CoFeB thickness. The areal VCMA efficiency does not saturate even when the effective CoFeB thickness exceeds ~1 nm. We consider the higher order term is related to the strain that develops at the CoFeB/MgO interface: as the average strain of the CoFeB layer changes with its thickness, the electronic structure of the CoFeB/MgO interface varies leading to changes in areal magnetic anisotropy energy and VCMA efficiency.

cond-mat.mtrl-sci↗

arXiv2015

Designing a fully-compensated half-metallic ferrimagnet

Recent experimental work on Mn2RuxGa demonstrates its potential as a compensated ferrimagnetic half-metal (CFHM).Here we present a set of high-throughput ab initio density functional theory calculations and detailed experimental characterisation, that enable us to correctly describe the nominal Mn2RuxGa thin films, in particular with regard to site-disorder and defects. We then construct models that accurately capture all the key features of the Mn-Ru-Ga system, including magnetic compensation and the spin gap at the Fermi level. We find that electronic doping is neccessary, which is achieved with a Mn/Ga ratio smaller than two. Our study shows how composition and substrate-induced biaxial strain can be combined to design the first room-temperature CFHM.

cond-mat.mtrl-sci↗