Tuning the Dzyaloshinskii-Moriya interaction (DMI) using electric (E)-fields in magnetic devices has opened up new perspectives for controlling the stabilization of chiral spin structures. Recent efforts have used voltage-induced charge redistribution at magnetic/oxides interfaces to modulate the DMI. This approach is attractive for active devices but tends to be volatile, making it energy-demanding. Here we demonstrate nonvolatile E-field manipulation of the DMI by ionic-liquid gating of Pt/Co/HfO2 ultra thin films. The E-field effect on the DMI is linked to the migration of oxygen species from the HfO2 layer into the Co and Pt layers and subsequent anchoring. This effect permanently changes the properties of the material, showing that E-fields can be used not only for local gating in devices but also as a material design tool for post growth tuning of the DMI.
In this paper, we present the functionality and model the performance of a new spin-based logic device called the voltage-controlled topological-spin switch (vTOPSS). This device stores information in the magnetization of a thin magnetic insulator (MI) layer, which has ultra-fast dynamics and low-energy dissipation due to its small damping factor. To control the magnetization of the MI, a voltage signal is applied to a proximal topological insulator (TI) layer, which has a high charge-to-spin conversion efficiency at room temperature. The information in the MI layer is read using a magnetic tunnel junction (MTJ) voltage divider with sub-100 mV read voltages. Since its input/output state variables are in the voltage domain, the vTOPSS device does not require any transduction circuitry to be integrated with the CMOS technology. Device optimization shows that the vTOPSS device can operate with sub-25 aJ energy dissipation and < 30 nW power in on-state, these values are much lower than those reported in contemporary spin-based devices. Results confirm that the dominant component of energy dissipation is due to the TI leakage, which can be reduced by suppressing the surface and bulk charge conduction in the TI. Unlike CMOS devices, energy dissipation of the VTOPSS device is proportional to its switching delay. To simultaneously achieve low latency and energy dissipation in vTOPSS, a TI material with a large spin Hall conductivity and negligible charge conductivity is preferred. Interconnect burden on the performance of the vTOPSS device is minimal, which opens up the possibility of using highly resistive nanowires as potential interconnects for this technology.
In this work we report on time resolved magnetization reversal driven by spin transfer torque in an orthogonal
spin transfer (OST) magnetic tunnel junction device. We focus on the transitions from parallel (P) to antiparallel
(AP) states and the reverse transitions (AP to P) under the influence of 10 ns voltage pulses. The electrical
response is monitored with a fast real-time oscilloscope and thus timing information of the reversal process
is obtained. The P to AP transition switching time decreases with increasing current and shows only direct
switching behavior. The AP to P transition shows similar behavior, but has a broader distribution of switching
times at high currents. The rare AP to P switching events that occur at later times are due to the occurrence of a
pre-oscillation, which could be identified in time resolve voltage traces. A possible origin of these pre-oscillations
is seen in micromagnetic simulations, where they are associated with the breakdown of the uniform precession
of the magnetization, and lead to reversal of the magnetization at later times.
Random, spatially uncorrelated nuclear-hyperfine fields in organic materials dramatically affect electronic transport properties such as electrical conductivity, photoconductivity, and electroluminescence. Competition between spin-dynamics due to these spatially uncorrelated fields and an applied magnetic field leads to large magnetoresistance, even at room temperature where the thermodynamic influences of the resulting nuclear and electronic Zeeman splittings are negligible. Here, we discuss a new method of controlling the electrical conductivity of an organic film at room temperature, using the spatially varying magnetic fringe fields of a magnetically unsaturated ferromagnet. Fringe-field magnetoresistance has a magnitude of several percent, and is hysteretic and anisotropic. This new method of control is sensitive to even remanent magnetic states, leading to different conductivity values in the absence of an applied field. The fringe field effects are insensitive to the ferromagnetic film’s thickness (and therefore the fringe field magnitude) but sensitive to the magnetic domain’s correlation length. This points at fringe-field gradients as an important ingredient of this mechanism. We develop a model based on fringe-field induced polaron-pair spin-dynamics that successfully describes several key features of the experimental fringe-field magnetoresistance.
Spin-transfer devices that incorporate a polarizer with its magnetization orthogonal to a switchable (free) layer
offer the potential for ultra-fast switching, low power consumption and reliable operation. The non-collinear
magnetizations lead to large initial spin-transfer torques, eliminating the incubation delay seen in devices with
collinear magnetization. Here we present the basic electrical and magnetic characteristics of spin-valve nanopillars
that incorporate a perpendicularly magnetized polarizer and demonstrate current-induced switching with
short current pulses, down to 100 ps in duration. We have fabricated devices that have a CoNi polarizer with
perpendicular magnetization and an in-plane magnetized 3 nm thick Co free layer and a 12 nm thick Co reference
layer, each separated by thin (~ 10 nm) Cu layers. The magnetization of the reference layer is collinear with that
of free layer to read out the device state. The reference layer also contributes to the spin-accumulation acting on
the free layer and leads to a spin-torque that favors the parallel (P) or antiparallel (AP) state depending on the
current pulse polarity, reducing the requirement of precise pulse timing in precessional reversal. The anisotropy
field of the perpendicular polarizer is 1.3 T, i.e. it is high enough so that in-plane fields (< 0.3 T) applied to
switch the magnetizations of the reference and free layers do not reorient the polarizer. Our typical nanopillar
device lateral dimensions are between 60 nm and 300 nm and nanopillars are positioned on coplanar waveguides
to allow for broadband electrical connections and studies with fast rise time pulses, generated by an arbitrary
waveform generator. The switching probability has been determined for variable pulse amplitude and duration,
from 0.1 to 10 ns at room temperature.
We consider magnetization reversal due to thermal fluctuations in thin, submicron-scale rings. These mesoscopic ferromagnetic particles are of particular interest as potential information storage components in magnetoelectronic devices, because their lack of sharp ends result in a magnetization density that is significantly more stable against reversal than in thin needles and other geometries. Their two-dimensional nature and rotational symmetry allow us to incorporate long-range magnetostatic forces in a fully analytic treatment, which is not possible in most geometries. We uncover a type of 'phase transition' between different activation regimes as magnetic field is varied at fixed ring size. Previous studies of such transitions in classical activation behavior have found that they occur as length is varied, which cannot be realized easily or continuously for most systems. However, the different activation regimes in a single mesoscopic ferromagnet should be experimentally observable by changing the externally applied magnetic field, and by tuning this field the transition region itself can be studied in detail.
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