21 research outputs found
MAGNETISM AND MAGNETOTRANSPORT STUDIES OF IV-VI FERROMAGNETIC SEMICONDUCTOR
Ph.DDOCTOR OF PHILOSOPH
Field-free spin-orbit torque switching of a perpendicular ferromagnet with Dzyaloshinskii-Moriya interaction
Leveraging on interfacial Dzyaloshinskii-Moriya interaction (DMI) induced intrinsic magnetization tilting in nanostructures, a parametric window enabling field-free spin-orbit torque (SOT) magnetization switching in a perpendicular ferromagnet is established. The critical current density (Jc) bounds for SOT switching are highly dependent on the DMI, producing a distorted diamond-shaped region bounded by the Jc-DMI curves. The widest Jc interval is found for DMI values between 0.5 mJ/m2 and 0.8 mJ/m2. Geometrical modulation, of the ferromagnetic layer, reveals that the circular structure is optimum for minimizing the switching energy while maximizing the parametric window. For all the structures investigated, the SOT induced reversal process is via domain wall nucleation and propagation, and the switching is practical at room temperature
Magnetization dynamics and its scattering mechanism in thin CoFeB films with interfacial anisotropy
Studies of magnetization dynamics have incessantly facilitated the discovery
of fundamentally novel physical phenomena, making steady headway in the
development of magnetic and spintronics devices. The dynamics can be induced
and detected electrically, offering new functionalities in advanced electronics
at the nanoscale. However, its scattering mechanism is still disputed.
Understanding the mechanism in thin films is especially important, because most
spintronics devices are made from stacks of multilayers with nanometer
thickness. The stacks are known to possess interfacial magnetic anisotropy, a
central property for applications, whose influence on the dynamics remains
unknown. Here, we investigate the impact of interfacial anisotropy by adopting
CoFeB/MgO as a model system. Through systematic and complementary measurements
of ferromagnetic resonance (FMR), on a series of thin films, we identify
narrower FMR linewidths at higher temperatures. We explicitly rule out the
temperature dependence of intrinsic damping as a possible cause, and it is also
not expected from existing extrinsic scattering mechanisms for ferromagnets. We
ascribe this observation to motional narrowing, an old concept so far neglected
in the analyses of FMR spectra. The effect is confirmed to originate from
interfacial anisotropy, impacting the practical technology of spin-based
nanodevices up to room temperature.Comment: 23 pages,3 figure
Diode Like Attributes in Magnetic Domain Wall Devices via Geometrical Pinning for Neuromorphic Computing
Neuromorphic computing (NC) is considered as a potential vehicle for
implementing energy-efficient artificial intelligence (AI). To realize NC,
several materials systems are being investigated. Among them, the spin-orbit
torque (SOT) -driven domain wall (DW) devices are one of the potential
candidates. To implement these devices as neurons and synapses, the building
blocks of NC, researchers have proposed different device designs. However, the
experimental realization of DW device-based NC is only at the primeval stage.
In this study, we have proposed and investigated pine-tree-shaped DW devices,
based on the Laplace force on the elastic DWs, for achieving the synaptic
functionalities. We have successfully observed multiple magnetization states
when the DW was driven by the SOT current. The key observation is the
asymmetric pinning strength of the device when DW moves in two opposite
directions (defined as, xhard and xeasy). This shows the potential of these DW
devices as DW diodes. We have used micromagnetic simulations to understand the
experimental findings and to estimate the Laplace pressure for various design
parameters. The study leads to the path of device fabrication, where synaptic
properties are achieved with asymmetric pinning potential
All-Electrical Skyrmionic Bits in a Chiral Magnetic Tunnel Junction
Topological spin textures such as magnetic skyrmions hold considerable
promise as robust, nanometre-scale, mobile bits for sustainable computing. A
longstanding roadblock to unleashing their potential is the absence of a device
enabling deterministic electrical readout of individual spin textures. Here we
present the wafer-scale realization of a nanoscale chiral magnetic tunnel
junction (MTJ) hosting a single, ambient skyrmion. Using a suite of electrical
and multi-modal imaging techniques, we show that the MTJ nucleates skyrmions of
fixed polarity, whose large readout signal - 20-70% relative to uniform states
- corresponds directly to skyrmion size. Further, the MTJ exploits
complementary mechanisms to stabilize distinctly sized skyrmions at zero field,
thereby realizing three nonvolatile electrical states. Crucially, it can write
and delete skyrmions using current densities 1,000 times lower than
state-of-the-art. These results provide a platform to incorporate readout and
manipulation of skyrmionic bits across myriad device architectures, and a
springboard to harness chiral spin textures for multi-bit memory and
unconventional computing.Comment: 8 pages, 5 figure
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Confined spin-wave characteristics in magnetic nanowire ensembles approaching the ultrathin regime
We present a comprehensive study on the high-frequency magnetization dynamics of homogeneously patterned ultrathin magnetic nanowire arrays. The backward volume magnetostatic spin-wave (BVMSW) and Damon-Eshbach (DE) configurations are studied along with the intermediate transition states to understand the edge mode’s evolution in depth. We find at the sub-10-nm ultrathin regime the dynamics are heavily influenced by geometrical parameters such as magnetic layer thickness (tFM), demagnetization factors, and interfaces. Critical entities such as field separation δH between uniform to edge mode increase linearly with 1/tFM, while the edge saturation field Hedgesat increase monotonically with increasing tFM, revealing excellent agreement between findings from experimental and micromagnetic simulations. The dynamics are less sensitive to the width of the nanowire but very sensitive to the adjacent material or the interface of the ferromagnet, especially at the ultrathin limits
Role of CoFeB thickness in electric field controlled sub-100 nm sized magnetic tunnel junctions
We report a comprehensive study on the role of the free layer thickness (tF) in electric-field controlled nanoscale perpendicular magnetic tunnel junctions (MTJs), comprising of free layer structure Ta/Co40Fe40B20/MgO, by using dc magnetoresistance and ultra-short magnetization switching measurements. Focusing on MTJs that exhibits positive effective device anisotropy (Keff), we observe that both the voltage-controlled magnetic anisotropy (ξ) and voltage modulation of coercivity show strong dependence on tF. We found that ξ varies dramatically and unexpectedly from ∼−3 fJ/V-m to ∼−41 fJ/V-m with increasing tF. We discuss the possibilities of electric-field tuning of the effective surface anisotropy term, KS as well as an additional interfacial magnetoelastic anisotropy term, K3 that scales with 1/tF2. Voltage pulse induced 180° magnetization reversal is also demonstrated in our MTJs. Unipolar switching and oscillatory function of switching probability vs. pulse duration can be observed at higher tF, and agrees well with the two key device parameters — Keff and ξ