917 research outputs found
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 N\'eel-like domain walls are found. These
results show the difficulty of studying the domain wall type from resistance
measurements
Spin-orbit torque switching without external field with a ferromagnetic exchange-biased coupling layer
Magnetization reversal of a perpendicular ferromagnetic free layer by
spin-orbit torque (SOT) is an attractive alternative to spin-transfer torque
(STT) switching in magnetic random-access memory (MRAM) where the write process
involves passing a high current across an ultrathin tunnel barrier. A small
symmetry-breaking bias field is usually needed for deterministic SOT switching
but it is impractical to generate the field externally for spintronic
applications. Here, we demonstrate robust zero-field SOT switching of a
perpendicular Co90Fe10 (CoFe) free layer where the symmetry is broken by
magnetic coupling to a second in-plane exchange-biased CoFe layer via a
nonmagnetic Ru spacer. The preferred magnetic state of the free layer is
determined by the current polarity and the nature of the interlayer exchange
coupling (IEC). Our strategy offers a scalable solution to realize
bias-field-free SOT switching that can lead to a generation of SOT-based
devices, that combine high storage density and endurance with potentially low
power consumption.Comment: 6 pages,3 figures, first submitted on 31st July 201
The spin Hall effect of Bi-Sb alloys driven by thermally excited Dirac-like electrons
We have studied the charge to spin conversion in BiSb/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
BiSb alloy. The SHC increases with increasing thickness of the
BiSb 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 valley of the narrow gap
BiSb alloy
Spin Hall effect from hybridized 3-4 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 or 6 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 +0.050.01, 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 - Ga 4 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 -orbital element and by Fermi level tuning.Comment: 17 pages, 4 figures, Supplementary information provide
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
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
Structure, site-specific magnetism and magneto-transport properties of epitaxial D0 MnFeGa thin films
Ferrimagnetic MnFeGa thin films have been
characterised by X-ray diffraction, SQUID magnetometry, X-ray absorption
spectroscopy, X-ray magnetic circular dichroism and M\"{o}ssbauer spectroscopy
with the aim of determining the structure and site-specific magnetism of this
tetragonal, D0-structure Heusler compound. High-quality epitaxial films
with low RMS surface roughness ( nm) are grown by magnetron
co-sputtering. The tetragonal distortion induces strong perpendicular magnetic
anisotropy along the -axis with a typical coercive field T
and an anisotropy field ranging from to T. Upon increasing the Fe
content , substantial uniaxial anisotropy, MJ/m
can be maintained over the full range, while the magnetisation of the
compound is reduced from to 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 exhibit a high anomalous Hall angle of % and a
high Fermi-level spin polarisation, above %, as measured by point contact
Andreev reflection. The Fe-substituted MnGa 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 nm.Comment: 11 pages, 11 figure
Magnetization switching induced by spin-orbit torque from Co2MnGa magnetic Weyl semimetal thin films
This study reports the magnetization switching induced by spin-orbit torque
(SOT) from the spin current generated in Co2MnGa magnetic Weyl semimetal (WSM)
thin films. We deposited epitaxial Co2MnGa thin films with highly B2-ordered
structure on MgO(001) substrates. The SOT was characterized by harmonic Hall
measurements in a Co2MnGa/Ti/CoFeB heterostructure and a relatively large spin
Hall efficiency of -7.8% was obtained.The SOT-induced magnetization switching
of the perpendicularly magnetized CoFeB layer was further demonstrated using
the structure. The symmetry of second harmonic signals, thickness dependence of
spin Hall efficiency, and shift of anomalous Hall loops under applied currents
were also investigated. This study not only contributes to the understanding of
the mechanisms of spin-current generation from magnetic-WSM-based
heterostructures, but also paves a way for the applications of magnetic WSMs in
spintronic devices.Comment: 15 pages, 4 figure
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
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
() in W/CoFeB based heterostructures can be modulated with external current
passed along the film plane. At higher current, decreases with increasing
current, which we infer is partly due to the adiabatic spin transfer torque. At
lower current, increases linearly with current regardless of the polarity
of current flow. The rate of increase in 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
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