66 research outputs found
Ultrafast terahertz field control of the emergent magnetic and electronic interactions at oxide interfaces
Ultrafast electric-field control of emergent electronic and magnetic states
at oxide interfaces offers exciting prospects for the development of new
generations of energy-efficient devices. Here, we demonstrate that the
electronic structure and emergent ferromagnetic interfacial state in epitaxial
LaNiO3/CaMnO3 superlattices can be effectively controlled using intense
single-cycle THz electric-field pulses. We employ a combination of
polarization-dependent X-ray absorption spectroscopy with magnetic circular
dichroism and X-ray resonant magnetic reflectivity to measure a detailed
magneto-optical profile and thickness of the ferromagnetic interfacial layer.
Then, we use time-resolved and temperature-dependent magneto-optical Kerr
effect, along with transient optical reflectivity and transmissivity
measurements, to disentangle multiple correlated electronic and magnetic
processes driven by ultrafast high-field (~1 MV/cm) THz pulses. These processes
include an initial sub-picosecond electronic response, consistent with
non-equilibrium Joule heating; a rapid (~270 fs) demagnetization of the
ferromagnetic interfacial layer, driven by THz-field-induced nonequilibrium
spin-polarized currents; and subsequent multi-picosecond dynamics, possibly
indicative of a change in the magnetic state of the superlattice due to the
transfer of spin angular momentum to the lattice. Our findings shed light on
the intricate interplay of electronic and magnetic phenomena in this strongly
correlated material system, suggesting a promising avenue for efficient control
of two-dimensional ferromagnetic states at oxide interfaces using ultrafast
electric-field pulses
Observation of Coherently Coupled Cation Spin Dynamics in an Insulating Ferrimagnetic Oxide
Many technologically useful magnetic oxides are ferrimagnetic insulators,
which consist of chemically distinct cations. Here, we examine the spin
dynamics of different magnetic cations in ferrimagnetic NiZnAl-ferrite
(NiZnAlFeO) under continuous microwave
excitation. Specifically, we employ time-resolved x-ray ferromagnetic resonance
to separately probe Fe and Ni cations on different sublattice
sites. Our results show that the precessing cation moments retain a rigid,
collinear configuration to within 2. Moreover, the effective
spin relaxation is identical to within 10% for all magnetic cations in the
ferrite. We thus validate the oft-assumed ``ferromagnetic-like'' dynamics in
resonantly driven ferrimagnetic oxides, where the magnetic moments from
different cations precess as a coherent, collective magnetization
Direct experimental evidence of tunable charge transfer at the ferromagnetic interface
Interfacial charge transfer in oxide heterostructures gives rise to a rich
variety of electronic and magnetic phenomena. Designing heterostructures where
one of the thin-film components exhibits a metal-insulator transition opens a
promising avenue for controlling such phenomena both statically and
dynamically. In this letter, we utilize a combination of depth-resolved soft
X-ray standing-wave and hard X-ray photoelectron spectroscopies in conjunction
with polarization-dependent X-ray absorption spectroscopy to investigate the
effects of the metal-insulator transition in on the electronic and
magnetic states at the interface. We report on a direct
observation of the reduced effective valence state of the interfacial Mn
cations in the metallic superlattice with an above-critical
thickness (6 u.c.) due to the leakage of itinerant Ni 3d electrons into
the interfacial layer. Conversely, in an insulating superlattice
with a below-critical thickness of 2 u.c., a homogeneous effective
valence state of Mn is observed throughout the layers due to the
blockage of charge transfer across the interface. The ability to switch and
tune interfacial charge transfer enables precise control of the emergent
ferromagnetic state at the interface and, thus, has
far-reaching consequences on the future strategies for the design of
next-generation spintronic devices
Coherent transfer of spin angular momentum by evanescent spin waves within antiferromagnetic NiO
This is the final version. Available from the publisher via the DOI in this record.Insulating antiferromagnets have recently emerged as efficient and robust conductors of spin current. Element-specific and phase-resolved x-ray ferromagnetic resonance has been used to probe the injection and transmission of ac spin current through thin epitaxial NiO(001) layers. The spin current is found to be mediated by coherent evanescent spin waves of GHz frequency, rather than propagating magnons of THz frequency, paving the way towards coherent control of the phase and amplitude of spin currents within an antiferromagnetic insulator at room temperature.Engineering and Physical Science Research Council (EPSRC
Dependence of spin pumping and spin transfer torque upon Ni81Fe19 thickness in Ta/Ag/Ni81Fe19/Ag/Co2MnGe/Ag/Ta spin-valve structures
This is the final version of the article. Available from American Physical Society via the DOI in this record.Spin pumping has been studied within Ta / Ag /
Ni
81
Fe
19
(0–5 nm) / Ag (6 nm) /
Co
2
MnGe
(5 nm) / Ag / Ta large-area spin-valve structures, and the transverse spin current absorption of
Ni
81
Fe
19
sink layers of different thicknesses has been explored. In some circumstances, the spin current absorption can be inferred from the modification of the
Co
2
MnGe
source layer damping in vector network analyzer ferromagnetic resonance (VNA-FMR) experiments. However, the spin current absorption is more accurately determined from element-specific phase-resolved x-ray ferromagnetic resonance (XFMR) measurements that directly probe the spin transfer torque (STT) acting on the sink layer at the source layer resonance. Comparison with a macrospin model allows the real part of the effective spin mixing conductance to be extracted. We find that spin current absorption in the outer Ta layers has a significant impact, while sink layers with thicknesses of less than 0.6 nm are found to be discontinuous and superparamagnetic at room temperature, and lead to a noticeable increase of the source layer damping. For the thickest 5-nm sink layer, increased spin current absorption is found to coincide with a reduction of the zero frequency FMR linewidth that we attribute to improved interface quality. This study shows that the transverse spin current absorption does not follow a universal dependence upon sink layer thickness but instead the structural quality of the sink layer plays a crucial role.The authors gratefully acknowledge the support of EPSRC Grant No. EP/J018767/1, and the award of the Exeter-Brown Scholarship in High Frequency Spintronics to C.J.D
Coherent ac spin current transmission across an antiferromagnetic CoO insulator
This is the final version. Available on open access from Nature Research via the DOI in this recordData availability:
Data are available from the corresponding author upon reasonable request.The recent discovery of spin current transmission through antiferromagnetic insulating materials opens up vast opportunities for fundamental physics and spintronics applications. The question currently surrounding this topic is: whether and how could THz antiferromagnetic magnons mediate a GHz spin current? This mismatch of frequencies becomes particularly critical for the case of coherent ac spin current, raising the fundamental question of whether a GHz ac spin current can ever keep its coherence inside an antiferromagnetic insulator and so drive the spin precession of another ferromagnet layer coherently? Utilizing element- and time-resolved x-ray pump-probe measurements on Py/Ag/CoO/Ag/Fe75Co25/MgO(001) heterostructures, here we demonstrate that a coherent GHz ac spin current pumped by the Py ferromagnetic resonance can transmit coherently across an antiferromagnetic CoO insulating layer to drive a coherent spin precession of the Fe75Co25 layer. Further measurement results favor thermal magnons rather than evanescent spin waves as the mediator of the coherent ac spin current in CoO.US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering DivisionNational Science Foundation (NSF)National Research Foundation of KoreaEngineering and Physical Sciences Research Council (EPSRC)National Key Research and Development Program of CHIN
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Element- and Time-Resolved Measurements of Spin Dynamics Using X-ray Detected Ferromagnetic Resonance
This is the author accepted manuscript. The final version is available from Taylor & Francis via the DOI in this recordThe technique of x-ray detected ferromagnetic resonance (XFMR) represents an indispensable new tool in the
investigation of spin current effects in complex heterostructures, as it enables the observation of magnetization and
spin dynamics with element-, site-, and valence state-specificity. Here we give an overview of the development of XFMR
and characterize different approaches to measure spin dynamics using synchrotron radiation. We provide a detailed
description of the working principle of the technique and give an overview of recent work carried out at beamline 4.0.2
of the Advanced Light Source and beamline I10 of the Diamond Light Source using XFMR. Results from our latest
publications demonstrate the capabilities and sensitivity of the technique. Element- and phase-resolution provide
intriguing insights into the mechanisms of spin current propagation in multilayers, while the high sensitivity of XFMR
allows for detection of even miniscule signals. Most recently, the utilization of linearly polarized x-rays for XFMR and
the detection of XFMR by means of x-ray diffraction rather than x-ray absorption demonstrate two new capabilities in
the investigation of spin dynamics.Engineering and Physical Sciences Research Council (EPSRC)US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering DivisionNational Research Foundation of Kore
Observation of room-temperature polar skyrmions
peer reviewe
Optically and microwave induced magnetization precession in [Co/Pt]/NiFe exchange springs
This is the final version. Available from the American Chemical Society via the DOI in this recordMicrowave and heat-assisted magnetic recordings are two competing technologies that have greatly increased the capacity of hard disk drives. The efficiency of the magnetic recording process can be further improved by employing non-collinear spin structures that combine perpendicular and in-plane magnetic anisotropy. Here, we investigate both microwave and optically excited magnetization dynamics in [Co/Pt]/NiFe exchange spring samples. The resulting canted magnetization within the nanoscale [Co/Pt]/NiFe interfacial region allows for optically stimulated magnetization precession to be observed for an extended magnetic field and frequency range. The results can be explained by formation of an imprinted domain structure, which locks the magnetization orientation and makes the structures more robust against external perturbations. Tuning the canted interfacial domain structure may provide greater control of optically excited magnetization reversal and optically generated spin currents, which are of paramount importance for future ultrafast magnetic recording and spintronic applications.Engineering and Physical Sciences Research Council (EPSRC
Manipulating chiral-spin transport with ferroelectric polarization
A collective excitation of the spin structure in a magnetic insulator can
transmit spin-angular momentum with negligible dissipation. This quantum of a
spin wave, introduced more than nine decades ago, has always been manipulated
through magnetic dipoles, (i.e., timereversal symmetry). Here, we report the
experimental observation of chiral-spin transport in multiferroic BiFeO3, where
the spin transport is controlled by reversing the ferroelectric polarization
(i.e., spatial inversion symmetry). The ferroelectrically controlled magnons
produce an unprecedented ratio of up to 18% rectification at room temperature.
The spin torque that the magnons in BiFeO3 carry can be used to efficiently
switch the magnetization of adja-cent magnets, with a spin-torque efficiency
being comparable to the spin Hall effect in heavy metals. Utilizing such a
controllable magnon generation and transmission in BiFeO3, an alloxide,
energy-scalable logic is demonstrated composed of spin-orbit injection,
detection, and magnetoelectric control. This observation opens a new chapter of
multiferroic magnons and paves an alternative pathway towards low-dissipation
nanoelectronics
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