171 research outputs found
Exchange-torque-induced excitation of perpendicular standing spin waves in nanometer-thick YIG films
Spin waves in ferrimagnetic yttrium iron garnet (YIG) films with ultralow
magnetic damping are relevant for magnon-based spintronics and low-power
wave-like computing. The excitation frequency of spin waves in YIG is rather
low in weak external magnetic fields because of its small saturation
magnetization, which limits the potential of YIG films for high-frequency
applications. Here, we demonstrate how exchange-coupling to a CoFeB film
enables efficient excitation of high-frequency perpendicular standing spin
waves (PSSWs) in nanometer-thick (80 nm and 295 nm) YIG films using uniform
microwave magnetic fields. In the 295-nm-thick YIG film, we measure intense
PSSW modes up to 10th order. Strong hybridization between the PSSW modes and
the ferromagnetic resonance mode of CoFeB leads to characteristic anti-crossing
behavior in broadband spin-wave spectra. A dynamic exchange torque at the
YIG/CoFeB interface explains the excitation of PSSWs. The localized torque
originates from exchange coupling between two dissimilar magnetization
precessions in the YIG and CoFeB layers. As a consequence, spin waves are
emitted from the YIG/CoFeB interface and PSSWs form when their wave vector
matches the perpendicular confinement condition. PSSWs are not excited when the
exchange coupling between YIG and CoFeB is suppressed by a Ta spacer layer.
Micromagnetic simulations confirm the exchange-torque mechanism.Comment: 9 pages, 6 figure
Pulsed laser deposition of La1-xSrxMnO3 : thin-film properties and spintronic applications
Materials engineering on the nanoscale by precise control of growth parameters can lead to many unusual and fascinating physical properties. The development of pulsed laser deposition (PLD) 25 years ago has enabled atomistic control of thin films and interfaces and as such it has contributed significantly to advances in fundamental material science. One application area is the research field of spintronics, which requires optimized nanomaterials for the generation and transport of spin-polarized carriers. The mixed-valence manganite La1−xSrxMnO3 (LSMO) is an interesting material for spintronics due to its intrinsic magnetoresistance properties, electric-field tunable metal–insulator transitions, and half-metallic band structure. Studies on LSMO thin-film growth by PLD show that the deposition temperature, oxygen pressure, laser fluence, strain due to substrate–film lattice mismatch and post-deposition annealing conditions significantly influence the magnetic and electrical transport properties of LSMO. For spintronic structures, robust ferromagnetic exchange interactions and metallic conductivity are desirable properties. In this paper, we review the physics of LSMO thin films and the important role that PLD played in advancing the field of LSMO-based spintronics. Some specific application areas including magnetic tunnel junctions, multiferroic tunnel junctions and organic spintronic devices are highlighted, and the advantages, drawbacks and opportunities of PLD-grown LSMO for next-generation spintronic devices are discussed.Peer reviewe
Temperature control of local magnetic anisotropy in multiferroic CoFe/BaTiO3
This paper reports on the temperature evolution of local elastic interactions between ferromagnetic CoFe films and ferroelectric BaTiO3 substrates. Polarization microscopy measurements indicate that growth-induced stripe domains in the CoFe films are preserved and strengthened during cooling and heating through the structural phase transitions of BaTiO3. Moreover, rotation of the magnetic easy axes at the tertragonal-to-orthorhombic transition (T = 278 K) and at T  ≈  380 K simultaneously switches the local magnetization of both uniaxial domains by 90° . Irreversible changes in the ferromagnetic domain pattern are induced when the room-temperature ferroelectric domain structure is altered after temperature cycling.Peer reviewe
Spin transfer torque oscillator based on asymmetric magnetic tunnel junctions
We present a study of the spin transfer torque oscillator based on
CoFeB/MgO/CoFeB asymmetric magnetic tunnel junctions. We observe microwave
precession in junctions with different thickness of the free magnetization
layer. Taking advantage of the ferromagnetic interlayer exchange coupling
between the free and reference layer in the MTJ and perpendicular interface
anisotropy in thin CoFeB electrode we demonstrate the nanometer scale device
that can generate high frequency signal without external magnetic field
applied. The amplitude of the oscillation exceeds 10 nV/Hz^0.5 at 1.5 GHz.Comment: 4 pages, 4 figures, to be submitted to Applied Physics Letter
Coherent piezoelectric strain transfer to thick epitaxial ferromagnetic films with large lattice mismatch
Strain control of epitaxial films using piezoelectric substrates has recently attracted significant scientific interest. Despite its potential as a powerful test bed for strain-related physical phenomena and strain-driven electronic, magnetic, and optical technologies, detailed studies on the efficiency and uniformity of piezoelectric strain transfer are scarce. Here, we demonstrate that full and uniform piezoelectric strain transfer to epitaxial films is not limited to systems with small lattice mismatch or limited film thickness. Detailed transmission electron microscopy (TEM) and x-ray diffraction (XRD) measurements of 100 nm thick CoFe2O4 and La2/3Sr1/3MnO3 epitaxial films on piezoelectric 0.72Pb(Mg1/3Nb2/3)O3–0.28PbTiO3 substrates (+4.3% and -3.8% lattice mismatch) indicate that misfit dislocations near the interface do not hamper the transfer of piezoelectric strain. Instead, the epitaxial magnetic oxide films and PMN-PT substrates are strained coherently and their lattice parameters change linearly as a function of applied electric field when their remnant growth-induced strain state is negligible. As a result, ferromagnetic properties such as the coercive field, saturation magnetization, and Curie temperature can be reversibly tuned by electrical means. The observation of efficient piezoelectric strain transfer in large-mismatch heteroepitaxial structures opens up new possibilities for the engineering of strain-controlled physical properties in a broad class of hybrid material systems.Peer reviewe
Magnetic field sensor with voltage-tunable sensing properties
We report on a magnetic field sensor based on CoFeB/MgO/CoFeB magnetic tunnel
junctions. By taking advantage of the perpendicular magnetic anisotropy of the
CoFeB/MgO interface, the magnetization of the sensing layer is tilted
out-of-plane which results in a linear response to in-plane magnetic fields.
The application of a bias voltage across the MgO tunnel barrier of the field
sensor affects the magnetic anisotropy and thereby its sensing properties. An
increase of the maximum sensitivity and simultaneous decrease of the magnetic
field operating range by a factor of two is measured. Based on these results,
we propose a voltage-tunable sensor design that allows for active control of
the sensitivity and the operating filed range with the strength and polarity of
the applied bias voltage.Comment: 4 pages, 4 figures, lette
Influence of magnetic field and ferromagnetic film thickness on domain pattern transfer in multiferroic heterostructures
Domains in BaTiO induces a regular modulation of uniaxial magnetic
anisotropy in CoFeB via an inverse magnetostriction effect. As a result, the
domain structures of the CoFeB wedge film and BaTiO substrate correlate
fully and straight ferroelectric domain boundaries in BaTiO pin magnetic
domain walls in CoFeB. We use x-ray photoemission electron microscopy and
magneto-optical Kerr effect microscopy to characterize the spin structure of
the pinned domain walls. In a rotating magnetic field, abrupt and reversible
transitions between two domain wall types occur, namely, narrow walls where the
magnetization vectors align head-to-tail and much broader walls with
alternating head-to-head and tail-to-tail magnetization configurations. We
characterize variations of the domain wall spin structure as a function of
magnetic field strength and CoFeB film thickness and compare the experimental
results with micromagnetic simulations.Comment: 5 pages, 5 figure
- …