59 research outputs found

### Measurement of orbital asymmetry and strain in Co90Fe10/Ni multilayers and alloys: Origins of perpendicular anisotropy

We use broadband ferromagnetic resonance spectroscopy and x-ray diffraction
to investigate the fundamental origin of perpendicular anisotropy in
Co90Fe10/Ni multilayers. By careful evaluation of the spectroscopic g-factor,
we determine the orbital moment along the out-of-plane and in-plane directions.
For the multilayers, we find a direct relationship between the orbital moment
asymmetry and the perpendicular anisotropy, consistent with the theory of Bruno
[P.Bruno, Phys. Rev. B, 39, 865 (1989)]. A systematic x-ray diffraction study
revealed the presence of a trigonal strain as high as 0.7 % in some samples.
However, we found no direct correlation between the strain and the anisotropy,
indicating that the anisotropy is not dominated by magnetoelastic effects. In
order to further study the interface structure on the anisotropy, we prepared a
set of equivalent alloy samples. The strain in the alloy samples was comparable
to that of the multilayer samples; however the orbital moment asymmetry in the
alloy samples showed a very different trend allowing us to isolate the effect
of the interfaces in the multilayers.Comment: In press at Physical Review

### Detection of the dc inverse spin Hall effect due to spin pumping in a novel meander-stripline geometry

The dc voltage obtained from the inverse spin Hall effect (iSHE) due to spin
pumping in ferromagnet/normal-metal (NM) bilayers can be unintentionally
superimposed with magnetoresistive rectification of ac charge currents in the
ferromagnetic layer. We introduce a geometry in which these spurious
rectification voltages vanish while the iSHE voltage is maximized. In this
geometry, a quantitative study of the dc iSHE is performed in a broad frequency
range for Permalloy/NM multilayers with NM={Pt, Ta, Cu/Au, Cu/Pt}. The
experimentally recorded voltages can be fully ascribed to the iSHE due to spin
pumping. Furthermore we measure a small iSHE voltage in single CoFe thin films

### Comment on "Detection of Microwave Spin Pumping Using the Inverse Spin Hall Effect"

A Comment on Phys. Rev. Lett. 111, 217204 (2013), "Detection of Microwave
Spin Pumping Using the Inverse Spin Hall Effect

### Phase-sensitive detection of spin pumping via the ac inverse spin Hall effect

An intriguing feature of spintronics is the use of pure spin-currents to
manipulate magnetization, e.g., spin-currents can switch magnetization in
spin-torque MRAM, a next-generation DRAM alternative. Giant spin-currents via
the spin Hall effect greatly expand the technological opportunities.
Conversely, a ferromagnet/normal metal junction emits spin-currents under
microwave excitation, i.e. spin-pumping. While such spin-currents are modulated
at the excitation frequency, there is also a non-linear, rectified component
that is commonly detected using the corresponding inverse spin Hall effect
(iSHE) dc voltage. However, the ac component should be more conducive for
quantitative analysis, as it is up to two orders of magnitude larger and
linear. But any device that uses the ac iSHE is also sensitive to inductive
signals via Faraday's Law and discrimination of the ac iSHE signal must rely on
phase-sensitive measurements. We use the inductive signal as a reference for a
quantitative measurement of the magnitude and phase of the ac iSHE

### Mode- and size-dependent Landau-Lifshitz damping in magnetic nanostructures: Evidence for non-local damping

We demonstrate a strong dependence of the effective damping on the nanomagnet
size and the particular spin-wave mode that can be explained by the theory of
intralayer transverse-spin-pumping. The effective Landau-Lifshitz damping is
measured optically in individual, isolated nanomagnets as small as 100 nm. The
measurements are accomplished by use of a novel heterodyne magneto-optical
microwave microscope with unprecedented sensitivity. Experimental data reveal
multiple standing spin-wave modes that we identify by use of micromagnetic
modeling as having either localized or delocalized character, described
generically as end- and center-modes. The damping parameter of the two modes
depends on both the size of the nanomagnet as well as the particular spin-wave
mode that is excited, with values that are enhanced by as much as 40% relative
to that measured for an extended film. Contrary to expectations based on the ad
hoc consideration of lithography-induced edge damage, the damping for the
end-mode decreases as the size of the nanomagnet decreases. The data agree with
the theory for damping caused by the flow of intralayer transverse
spin-currents driven by the magnetization curvature. These results have serious
implications for the performance of nanoscale spintronic devices such as
spin-torque-transfer magnetic random access memory.Comment: The manuscript is published in Physical Review Letters. We revised
the manuscript to meet the length requiremen

### Suppression of Spin Pumping Between Ni$_{80}$Fe$_{20}$ and Cu by a Graphene Interlayer

We compare ferromagnetic resonance measurements of Permalloy
Ni$_{80}$Fe$_{20}$ (Py) films sputtered onto Cu(111) films with and without a
graphene (Gr) interlayer grown by chemical vapor deposition before Py
deposition. A two-angle sputtering method ensured that neither Gr nor Py was
degraded by the sample preparation process. We find the expected damping
enhancement from spin pumping for the Py/Cu case and no detectable enhancement
for the Py/Gr/Cu case. Since damping is sensitive to effects other than spin
pumping, we used magnetometry to verify that differences in Py magnetostatic
properties are not responsible for the difference in damping. We attribute the
suppression of spin pumping in Py/Gr/Cu to the large contact resistance of the
Gr/Cu interface

### Radiative damping in wave guide based FMR measured via analysis of perpendicular standing spin waves in sputtered Permalloy films

The damping $\alpha$ of the spinwave resonances in 75 nm, 120 nm, and 200nm
-thick Permalloy films is measured via vector-network-analyzer
ferromagnetic-resonance (VNA-FMR) in the out-of-plane geometry. Inductive
coupling between the sample and the waveguide leads to an additional radiative
damping term. The radiative contribution to the over-all damping is determined
by measuring perpendicular standing spin waves (PSSWs) in the Permalloy films,
and the results are compared to a simple analytical model. The damping of the
PSSWs can be fully explained by three contributions to the damping: The
intrinsic damping, the eddy-current damping, and the radiative damping. No
other contributions were observed. Furthermore, a method to determine the
radiative damping in FMR measurements with a single resonance is suggested

### Determination of spin Hall effect and spin diffusion length of Pt from self-consistent fitting of damping enhancement and inverse spin-orbit torque measurements

Understanding the evolution of spin-orbit torque (SOT) with increasing
heavy-metal thickness in ferromagnet/normal metal (FM/NM) bilayers is critical
for the development of magnetic memory based on SOT. However, several
experiments have revealed an apparent discrepancy between damping enhancement
and damping-like SOT regarding their dependence on NM thickness. Here, using
linewidth and phase-resolved amplitude analysis of vector network analyzer
ferromagnetic resonance (VNA-FMR) measurements, we simultaneously extract
damping enhancement and both field-like and damping-like inverse SOT in
Ni$_{80}$Fe$_{20}$/Pt bilayers as a function of Pt thickness. By enforcing an
interpretation of the data which satisfies Onsager reciprocity, we find that
both the damping enhancement and damping-like inverse SOT can be described by a
single spin diffusion length ($\approx$ 4 nm), and that we can separate the
spin pumping and spin memory loss (SML) contributions to the total damping.
This analysis indicates that less than 40% of the angular momentum pumped by
FMR through the Ni$_{80}$Fe$_{20}$/Pt interface is transported as spin current
into the Pt. On account of the SML and corresponding reduction in total spin
current available for spin-charge transduction in the Pt, we determine the Pt
spin Hall conductivity ($\sigma_\mathrm{SH} = (2.36 \pm 0.04)\times10^6
\Omega^{-1} \mathrm{m}^{-1}$) and bulk spin Hall angle
($\theta_\mathrm{SH}=0.387 \pm0.008$) to be larger than commonly-cited values.
These results suggest that Pt can be an extremely useful source of SOT if the
FM/NM interface can be engineered to minimize SML. Lastly, we find that
self-consistent fitting of the damping and SOT data is best achieved by a model
with Elliott-Yafet spin relaxation and extrinsic inverse spin Hall effect, such
that both the spin diffusion length and spin Hall conductivity are proportional
to the Pt charge conductivity

### $\text{Co}_{25}\text{Fe}_{75}$ Thin Films with Ultralow Total Damping

We measure the dynamic properties of $\text{Co}_{25}\text{Fe}_{75}$ thin
films grown by dc magnetron sputtering. Using ferromagnetic resonance
spectroscopy, we demonstrate an ultralow total damping parameter in the
out-of-plane configuration of < 0.0013, whereas for the in-plane configuration
we find a minimum total damping of < 0.0020. In both cases, we observe low
inhomogeneous linewidth broadening in macroscopic films. We observe a minimum
full-width half-maximum linewidth of 1 mT at 10 GHz resonance frequency for a
12 nm thick film. We characterize the morphology and structure of these films
as a function of seed layer combinations and find large variation of the
qualitative behavior of the in-plane linewidth vs. resonance frequency.
Finally, we use wavevector-dependent Brillouin light scattering spectroscopy to
characterize the spin-wave dispersion at wave vectors up to 23 $\mu
\text{m}^{-1}$.Comment: v

### Precise determination of the spectroscopic g-factor using broadband ferromagnetic resonance spectroscopy

We demonstrate that the spectroscopic g-factor can be determined with high
precision and accuracy by broadband ferromagnetic resonance measurements and
applying an asymptotic analysis to the data. Spectroscopic data used to
determine the g-factor is always obtained over a finite range of frequencies,
which can result in significant errors in the fitted values of the
spectroscopic g-factor. We show that by applying an asymptotic analysis to
broadband datasets, precise values of the intrinsic g-factor can be determined
with errors well below 1 %, even when the exact form of the Kittel equation
(which describes the relationship between the frequency and resonance field) is
unknown. We demonstrate this methodology with measured data obtained for
sputtered Ni80Fe20 ("Permalloy") thin films of varied thicknesses, where we
determine the bulk g-factor value to be 2.109 +/- 0.003. Such an approach is
further validated by application to simulated data that includes both noise and
an anisotropy that is not included in the Kittel equation that was used in the
analysis. Finally, we show a correlation of thickness and interface structure
to the magnitude of the asymptotic behavior, which provide insight into
additional mechanisms that may induce deviations from the Kittel equation.Comment: Submitted to Journal of Applied Physic

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