41 research outputs found
Spin relaxation mechanism in Silver nanowires covered with MgO protection layer
Spin-flip mechanism in Ag nanowires with MgO surface protection layers has
been investigated by means of nonlocal spin valve measurements using
Permalloy/Ag lateral spin valves. The spin flip events mediated by surface
scattering are effectively suppressed by the MgO capping layer. The spin
relaxation process was found to be well described in the framework of
Elliott-Yafet mechanism and then the probabilities of spin-filp scattering for
phonon or impurity mediated momentum scattering is precisely determined in the
nanowires. The temperature dependent spin-lattice relaxation follows the
Bloch-Gr\"uneisen theory and falls on to a universal curve for the monovalent
metals as in the Monod and Beuneu scaling determined from the conduction
electron spin resonance data for bulk.Comment: 18 pages, 3figure
Spin relaxation characteristics in Ag nanowire covered with various oxides
We have studied spin relaxation characteristics in a Ag nanowire covered with
various oxide layers of Bi2O3, Al2O3, HfO2, MgO or AgOx by using non-local spin
valve structures. The spin-flip probability, a ratio of momentum relaxation
time to spin relaxation time at 10 K, exhibits a gradual increase with an
atomic number of the oxide constituent elements, Mg, Al, Ag and Hf.
Surprisingly the Bi2O3 capping was found to increase the probability by an
order of magnitude compared with other oxide layers. This finding suggests the
presence of an additional spin relaxation mechanism such as Rashba effect at
the Ag/Bi2O3 interface, which cannot be explained by the simple Elliott-Yafet
mechanism via phonon, impurity and surface scatterings. The Ag/Bi2O3 interface
may provide functionality as a spin to charge interconversion layer.Comment: 11pages, 1table, and 3figure
Effect of anisotropic spin absorption on the Hanle effect in lateral spin valves
We have succeeded in fully describing dynamic properties of spin current
including the different spin absorption mechanism for longitudinal and
transverse spins in lateral spin valves, which enables to elucidate intrinsic
spin transport and relaxation mechanism in the nonmagnet. The deduced spin
lifetimes are found independent of the contact type. From the transit-time
distribution of spin current extracted from the Fourier transform in Hanle
measurement data, the velocity of the spin current in Ag with Py/Ag Ohmic
contact turns out much faster than that expected from the widely used model.Comment: 21 pages, 3 figures, 9 pages (supplementary information
Experimental Verification of Comparability between Spin-Orbit and Spin-Diffusion Lengths
We experimentally confirmed that the spin-orbit lengths of noble metals
obtained from weak anti-localization measurements are comparable to the spin
diffusion lengths determined from lateral spin valve ones. Even for metals with
strong spin-orbit interactions such as Pt, we verified that the two methods
gave comparable values which were much larger than those obtained from recent
spin torque ferromagnetic resonance measurements. To give a further evidence
for the comparability between the two length scales, we measured the disorder
dependence of the spin-orbit length of copper by changing the thickness of the
wire. The obtained spin-orbit length nicely follows a linear law as a function
of the diffusion coefficient, clearly indicating that the Elliott-Yafet
mechanism is dominant as in the case of the spin diffusion length.Comment: 5 pages, 3 figure
Revisiting the measurement of the spin relaxation time in graphene-based devices
A long spin relaxation time (tausf) is the key for the applications of
graphene to spintronics but the experimental values of tausf have been
generally much shorter than expected. We show that the usual determination by
the Hanle method underestimates tausf if proper account of the spin absorption
by contacts is lacking. By revisiting series of experimental results, we find
that the corrected tausf are longer and less dispersed, which leads to a more
unified picture of tausf derived from experiments. We also discuss how the
correction depends on the parameters of the graphene and contacts
5d transition metal oxide IrO2 as a material for spin current detection
Devices based on a pure spin current (a flow of spin angular momentum) have
been attracting increasing attention as key ingredients for low-dissipation
electronics. To integrate such spintronics devices into charge-based
technologies, an electric detection of spin current is essential. Inverse spin
Hall effect converts a spin current into an electric voltage through spin-orbit
coupling. Noble metals such as Pt and Pd, and also Cu-based alloys, owing to
the large direct spin Hall effect, have been regarded as potential materials
for a spin-current injector. Those materials, however, are not promising as a
spin-current detector based on inverse spin Hall effect. Their spin Hall
resistivity rho_SH, representing the performance as a detector, is not large
enough mainly due to their low charge resistivity. Here we demonstrate that
heavy transition metal oxides can overcome such limitations inherent to
metal-based spintronics materials. A binary 5d transition metal oxide IrO2,
owing to its large resistivity as well as a large spin-orbit coupling
associated with 5d character of conduction electrons, was found to show a
gigantic rho_SH ~ 38 microohm cm at room temperature, one order of magnitude
larger than those of noble metals and Cu-based alloys and even comparable to
those of atomic layer thin film of W and Ta
Tunneling Devices with Perpendicular Magnetic Anisotropy Electrodes on Atomically Thin van der Waals Heterostructures
We report the fabrication of perpendicular ferromagnetic electrodes for
tunneling devices consist of van der Waals heterostructure. We found MgO/Co/Pt
films on Hexagonal BN shows perpendicular magnetic anisotropy (PMA) with the
easy axis perpendicular to the substrate. Vacuum annealing enhances the
perpendicular anisotropy. The easy axis along the perpendicular direction
persists up to room temperature with the Pt layer thickness ranges from 1.5 to
5 nm. We employed the PMA electrodes to construct tunneling devices on graphene
and monolayer WSe2, where spin injection characteristics and field effect
transistor behavior were demonstrated without strong Schottky barrier
formation
Spin injection characteristics of Py/graphene/Pt by gigahertz and terahertz magnetization dynamics driven by femtosecond laser pulse
Spin transport characteristics of graphene has been extensively studied so
far. The spin transport along c-axis is however reported by rather limited
number of papers. We have studied spin transport characteristics through
graphene along c-axis with permalloy(Py)/graphene(Gr)/Pt by gigahertz (GHz) and
terahertz (THz) magnetization dynamics driven by femtosecond laser pulses. The
relatively simple sample structure does not require electrodes on the sample.
The graphene layer was prepared by chemical vapor deposition and transferred on
Pt film. The quality of graphene layer was characterized by Raman microscopy.
Time resolved magneto-optical Kerr effect is used to characterize gigahertz
magnetization dynamics. Magnetization precession is clearly observed both for
Pt/Py and Pt/Gr/Py. The Gilbert damping constant of Pt/Py was 0.015, indicates
spin pumping effect from Py to Pt. The Gilbert damping constant of Pt/Gr/Py is
found to be 0.011, indicates spin injection is blocked by graphene layer. We
have also performed the measurement of THz emission for Pt/Py and Pt/Gr/Py.
While the THz emission is clearly observed for Pt/Py, a strong reduction of THz
emission is observed for Pt/Gr/Py. With these two different experiments, and
highly anisotropic resistivity of graphite, we conclude that the vertical spin
transport is strongly suppressed by the graphene layer.Comment: Submitted to AIP adv (MMM
Increased Curie temperature and enhanced perpendicular magneto anisotropy of Cr2Ge2Te6/NiO heterostructure
Magnetism in two-dimensional van der Waals materials has received significant
attention recently. The Curie temperature reported for those materials,
however, has been so far remained relatively low. Here, we measure
magneto-optical Kerr effects (MOKE) under perpendicular magnetic field for van
der Waals ferromagnet Cr2Ge2Te6 as well as its heterostructure with
antiferromagnetic insulator NiO. We observe a notable increase in both Curie
temperature and magnetic perpendicular anisotropy in Cr2Ge2Te6/NiO
heterostructures compared to those in Cr2Ge2Te6. Measurements on the same
exfoliated Cr2Ge2Te6 flake (on a SiO2/Si substrate) before and after depositing
NiO show that the hysteresis loop can change into a square shape with larger
coercive field for Cr2Ge2Te6/NiO. The maximum Curie temperature (TC) observed
for Cr2Ge2Te6/NiO reaches ~120 K, is nearly twice the maximum TC ~ 60 K
reported for Cr2Ge2Te6 alone. Both enhanced perpendicular anisotropy and
increased Curie temperature are observed for Cr2Ge2Te6 flakes with a variety of
thicknesses ranging from ~5 nm to ~200 nm. The results indicate that magnetic
properties of two-dimensional van der Waals magnets can be engineered and
controlled by using the heterostructure interface with other materials
Van der Waals Heterostructure Magnetic Josephson Junction
When two superconductors are connected across a ferromagnet, the spin
configuration of the transferred Cooper pairs can be modulated due to magnetic
exchange interaction. The resulting supercurrent can reverse its sign across
the Josephson junction (JJ) [1-4]. Here we demonstrate Josephson phase
modulation in van der Waals heterostructures when Cooper pairs from
superconducting NbSe tunnel through atomically thin magnetic insulator (MI)
CrGeTe. Employing a superconducting quantum interference device
based on MI JJs, we probe a doubly degenerate non-trivial JJ phase ()
originating from the magnetic barrier. This -phase JJ is formed by
momentum conserving tunneling of Ising Cooper pairs [5] across magnetic domains
in the CrGeTe barrier. The doubly degenerate ground states in MI
JJs provide a two-level quantum system that can be utilized as a new
disipationless component for superconducting quantum devices, including phase
batteries [6], memories [7,8], and quantum Ratchets [9,10]