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H Ohno - One of the best experts on this subject based on the ideXlab platform.

B. L. Gallagher - One of the best experts on this subject based on the ideXlab platform.

  • molecular beam epitaxy grown ga mn as p with perpendicular to plane magnetic Easy Axis
    2008
    Co-Authors: A W Rushforth, M Wang, N R S Farley, R P Campion, K W Edmonds, C R Staddon, C T Foxon, B. L. Gallagher
    Abstract:

    We present an experimental investigation of the magnetic, electrical, and structural properties of Ga0.94Mn0.06As1−yPy layers grown by molecular beam epitaxy on GaAs substrates for y≤0.3. X-ray diffraction measurements reveal that the layers are under tensile strain, which gives rise to a magnetic Easy Axis perpendicular to the plane of the layers. The strength of the magnetic anisotropy and the coercive field increases as the phosphorous concentration is increased. The resistivity of all samples shows metallic behavior with the resistivity increasing as y increases. These materials will be useful for studies of micromagnetic phenomena requiring metallic ferromagnetic material with perpendicular magnetic anisotropy.

  • Molecular Beam Epitaxy grown (Ga,Mn)(As,P) with perpendicular to plane magnetic Easy Axis
    2008
    Co-Authors: B. L. Gallagher
    Abstract:

    We present an experimental investigation of the magnetic, electrical and structural properties of Ga0.94Mn0.06As1-yPy layers grown by molecular beam epitaxy on GaAs substrates for y less than or equal to 0.3. X-ray diffraction measurements reveal that the layers are under tensile strain which gives rise to a magnetic Easy Axis perpendicular to the plane of the layers. The strength of the magnetic anisotropy and the coercive field increase as the phosphorous concentration is increased. The resistivity of all samples shows metallic behaviour with the resistivity increasing as y increases. These materials will be useful for studies of micromagnetic phenomena requiring metallic ferromagnetic material with perpendicular magnetic anisotropy.

Hideo Ohno - One of the best experts on this subject based on the ideXlab platform.

  • non linear variation of domain period under electric field in demagnetized cofeb mgo stacks with perpendicular Easy Axis
    2018
    Co-Authors: N. Ichikawa, Shunsuke Fukami, Takaaki Dohi, Atsushi Okada, Hideo Sato, Hideo Ohno
    Abstract:

    We investigate a variation of magnetic domain pattern in demagnetized perpendicular-Easy-Axis CoFeB/MgO stacks with a wide-range electric field and discuss the electric-field effect on the exchange stiffness constant AS. We observe a non-linear variation of domain period DP with the applied electric field, in contrast to the effective perpendicular anisotropy field HKeff that is evaluated from a ferromagnetic resonance and shows a linear response. An analysis based on a model describing the relation of DP to AS and HKeff suggests that AS varies non-linearly with the electric field, implying an involved characteristic mechanism of the exchange stiffness in the studied systems. A possible scenario for explaining the non-linear behavior of AS is discussed.We investigate a variation of magnetic domain pattern in demagnetized perpendicular-Easy-Axis CoFeB/MgO stacks with a wide-range electric field and discuss the electric-field effect on the exchange stiffness constant AS. We observe a non-linear variation of domain period DP with the applied electric field, in contrast to the effective perpendicular anisotropy field HKeff that is evaluated from a ferromagnetic resonance and shows a linear response. An analysis based on a model describing the relation of DP to AS and HKeff suggests that AS varies non-linearly with the electric field, implying an involved characteristic mechanism of the exchange stiffness in the studied systems. A possible scenario for explaining the non-linear behavior of AS is discussed.

  • non linear variation of domain period under electric field in demagnetized cofeb mgo stacks with perpendicular Easy Axis
    2018
    Co-Authors: N. Ichikawa, Shunsuke Fukami, Takaaki Dohi, Atsushi Okada, Hideo Sato, Hideo Ohno
    Abstract:

    We investigate a variation of magnetic domain pattern in demagnetized perpendicular-Easy-Axis CoFeB/MgO stacks with a wide-range electric field and discuss the electric-field effect on the exchange stiffness constant AS. We observe a non-linear variation of domain period DP with the applied electric field, in contrast to the effective perpendicular anisotropy field HKeff that is evaluated from a ferromagnetic resonance and shows a linear response. An analysis based on a model describing the relation of DP to AS and HKeff suggests that AS varies non-linearly with the electric field, implying an involved characteristic mechanism of the exchange stiffness in the studied systems. A possible scenario for explaining the non-linear behavior of AS is discussed.

  • magnetic field angle dependence of coercivity in cofeb mgo magnetic tunnel junctions with perpendicular Easy Axis
    2017
    Co-Authors: Junta Igarashi, F Matsukura, Hideo Sato, J Llandro, Hideo Ohno
    Abstract:

    We investigate the magnetization-reversal mode in CoFeB/MgO magnetic tunnel junctions with perpendicular Easy Axis from the magnetic-field-angle dependence of coercivity. The reversal in a free layer with diameter of ∼20 nm is found to be in a good agreement with coherent reversal mode for a device with a reference layer much larger than the free layer. However, the reversal mode is quite different in a junction in which the two layers are almost the same size. From micromagnetic simulation, the difference is attributed to the reduction of the magnetic anisotropy in the vicinity of the device edge.

  • a spin orbit torque switching scheme with collinear magnetic Easy Axis and current configuration
    2016
    Co-Authors: Shunsuke Fukami, Tetsuro Anekawa, Chaoliang Zhang, Hideo Ohno
    Abstract:

    Spin-orbit torque, a torque brought about by in-plane current via the spin-orbit interactions in heavy-metal/ferromagnet nanostructures, provides a new pathway to switch the magnetization direction. Although there are many recent studies, they all build on one of two structures that have the Easy Axis of a nanomagnet lying orthogonal to the current, that is, along the z or y axes. Here, we present a new structure with the third geometry, that is, with the Easy Axis collinear with the current (along the x Axis). We fabricate a three-terminal device with a Ta/CoFeB/MgO-based stack and demonstrate the switching operation driven by the spin-orbit torque due to Ta with a negative spin Hall angle. Comparisons with different geometries highlight the previously unknown mechanisms of spin-orbit torque switching. Our work offers a new avenue for exploring the physics of spin-orbit torque switching and its application to spintronics devices.

  • magnetization reversal by field and current pulses in elliptic cofeb mgo tunnel junctions with perpendicular Easy Axis
    2016
    Co-Authors: Eriko Hirayama, F Matsukura, Hideo Sato, Shun Kanai, Hideo Ohno
    Abstract:

    We fabricate elliptic CoFeB/MgO magnetic tunnel junctions (MTJs) with perpendicular magnetic Easy Axis with different in-plane aspect ratios (ARs) from 1 to 4 with the same area as that of a 100 nm diameter circular MTJ. We evaluate their thermal stability factors from magnetization switching probability measurements by using either applied magnetic field pulses or current pulses. The thermal stability factors determined from field-pulse measurement appear to decrease with increasing AR, whereas those from current-pulse measurement depend much less on AR. These results imply that the different drives induce switching through different magnetization reversal modes.

Kedar Damle - One of the best experts on this subject based on the ideXlab platform.

  • two step melting of three sublattice order in s 1 Easy Axis triangular lattice antiferromagnets
    2018
    Co-Authors: Dariush Heidarian, Kedar Damle
    Abstract:

    We consider S = 1 triangular lattice Heisenberg antiferromagnets with a strong single-ion anisotropy D that dominates over the nearest-neighbour antiferromagnetic exchange J. In this limit of small J∕D, we study low temperature (T ~ J ≪ D) properties of such magnets by employing a low-energy description in terms of hard-core bosons with nearest neighbour repulsion V ≈ 4J + J2∕D and nearest neighbour unfrustrated hopping t ≈ J2∕2D. Using a cluster Stochastic Series Expansion (SSE) algorithm to perform sign-problem-free quantum Monte Carlo (QMC) simulations of this effective model, we establish that the ground-state three-sublattice order of the Easy-Axis spin-density Sz(r) melts in zero field (B = 0) in a two-step manner via an intermediate temperature phase characterized by power-law three-sublattice order with a temperature dependent exponent η(T)∈[1/9,1/4]. For η(T)<2/9 in this phase, we find that the uniform Easy-Axis susceptibility of an L × L sample diverges as χL ~ L2−9η at B = 0, consistent with recent predictions that the thermodynamic susceptibility to a uniform field B along the Easy Axis diverges at small B as χEasy-Axis(B)~B−4−18η/4−9η in this regime.

  • two step melting of three sublattice order in s 1 Easy Axis triangular lattice antiferromagnets
    2015
    Co-Authors: Dariush Heidarian, Kedar Damle
    Abstract:

    We consider $S=1$ triangular lattice Heisenberg antiferromagnets with a strong single-ion anisotropy $D$ that dominates over the nearest-neighbour antiferromagnetic exchange $J$. In this limit of small $J/D$, we study low temperature ($T \sim J \ll D$) properties of such magnets by employing a low-energy description in terms of hard-core bosons with nearest neighbour repulsion $V \approx 4J + J^2/D$ and nearest neighbour unfrustrated hopping $t \approx J^2/2D$. Using a cluster Stochastic Series Expansion (SSE) algorithm to perform sign-problem-free quantum Monte Carlo (QMC) simulations of this effective model, we establish that the ground-state three-sublattice order of the Easy-Axis spin-density $S^z(\vec{r})$ melts in zero field ($B=0$) in a {\em two-step} manner via an intermediate temperature phase characterized by power-law three-sublattice order with a temperature dependent exponent $\eta(T) \in [\frac{1}{9}, \frac{1}{4}]$. For $\eta(T) < \frac{2}{9}$ in this phase, we find that the uniform Easy-Axis susceptibility of an $L \times L$ sample diverges as $\chi_L \sim L^{2-9 \eta}$ at $B=0$, consistent with a recent prediction that the thermodynamic susceptibility to a uniform field $B$ along the Easy Axis diverges at small $B$ as $\chi_{\rm Easy-Axis}(B) \sim B^{-\frac{4-18\eta}{4-9\eta}}$ in this regime.

  • melting of three sublattice order in Easy Axis antiferromagnets on triangular and kagome lattices
    2015
    Co-Authors: Kedar Damle
    Abstract:

    When the constituent spins have an energetic preference to lie along an Easy Axis, triangular and kagome lattice antiferromagnets often develop long-range order that distinguishes the three sublattices of the underlying triangular Bravais lattice. In zero magnetic field, this three-sublattice order melts either in a two-step manner, i.e., via an intermediate phase with power-law three-sublattice order controlled by a temperature-dependent exponent η(T)∈(1/9,1/4), or via a transition in the three-state Potts universality class. Here, I predict that the uniform susceptibility to a small Easy-Axis field B diverges as χ(B)∼|B|^{-[(4-18η)/(4-9η)]} in a large part of the intermediate power-law ordered phase [corresponding to η(T)∈(1/9,2/9)], providing an Easy-to-measure thermodynamic signature of two-step melting. I also show that these two melting scenarios can be generically connected via an intervening multicritical point and obtain numerical estimates of multicritical exponents.

  • triangular and kagome antiferromagnets with a strong Easy Axis anisotropy
    2009
    Co-Authors: Arnab Sen, Kedar Damle, Fa Wang, Roderich Moessner
    Abstract:

    We consider S>3/2 kagome and triangular lattice magnets with strong Easy-Axis single-ion anisotropy D and antiferromagnetic exchange J. When D>>J, the low energy states selected by the anisotropy map onto configurations of the corresponding classical Ising antiferromagnet. Subleading O(J;{3}S/D;{2}) multispin interaction arising from the transverse quantum dynamics makes the low-temperature behavior very different from the well-known classical case: The kagome magnet goes into a semiclassical spin-liquid state with distinctive and unusual short-range correlations below a crossover temperature T;{*} approximately 0.08J;{3}S/D;{2}, while the triangular magnet undergoes a first-order transition at T_{c} approximately 0.1J;{3}S/D;{2} to an orientationally ordered collinear state that gives rise to a novel zero-magnetization plateau for small magnetic fields along the Easy Axis. Possible experimental implications are also discussed.

  • magnetization plateaus and sublattice ordering in Easy Axis kagome lattice antiferromagnets
    2008
    Co-Authors: Arnab Sen, Kedar Damle, Ashvin Vishwanath
    Abstract:

    We study kagome lattice antiferromagnets where the effects of Easy-Axis single-ion anisotropy ($D$) dominates over the Heisenberg exchange $J$. For $S\ensuremath{\ge}3/2$, virtual quantum fluctuations help lift the extensive classical degeneracy. We demonstrate the presence of a one-third magnetization plateau for a broad range of magnetic fields ${J}^{3}/{D}^{2}\ensuremath{\lesssim}B\ensuremath{\lesssim}JS$ along the Easy Axis. The fully equilibrated system at low temperature on this plateau develops an unusual nematic order that breaks sublattice rotation symmetry but not translation symmetry; however, extremely slow dynamics associated with this ordering is expected to lead to glassy freezing of the system on intermediate time scales.

D L Atherton - One of the best experts on this subject based on the ideXlab platform.

  • origin of a magnetic Easy Axis in pipeline steel
    1999
    Co-Authors: L Clapham, C Heald, T W Krause, D L Atherton, P Clark
    Abstract:

    Oil and gas pipelines are generally magnetically anisotropic, with a magnetic Easy Axis in the pipe axial direction. This is of interest because magnetic flux leakage tools are commonly used for the detection and sizing of defects. In the present study we investigate the origin of this magnetic Easy Axis, using an angular magnetic Barkhausen noise technique to characterize the magnetic anisotropy. The texture, microstructure, and residual stress are examined as possible causes of the Easy Axis, using x-ray pole figure analysis and microstructural examination along with high and low temperature annealing treatments. Our results indicate that plastic deformation and residual stress are responsible for the magnetic Easy Axis, since an elimination of the residual stresses through low temperature “stress relief” heat treatment produces a magnetically isotropic structure without altering the texture or microstructure. X-ray pole figure analysis supports the conclusion that magnetic anisotropy is not related to texture in these materials. We conclude that the axial magnetic Easy Axis is due to a compressive residual hoop stress resulting from the cold bending and cold expansion of the pipe during processing.

  • investigation of the stress dependent magnetic Easy Axis in steel using magnetic barkhausen noise
    1996
    Co-Authors: T W Krause, L Clapham, Andras Pattantyus, D L Atherton
    Abstract:

    Angular‐dependent magnetic Barkhausen noise (MBN) measurements were performed on a pipeline steel sample for various values of applied uniaxial stress at three angles with respect to the sample’s zero stress magnetic Easy Axis direction. It was observed that the response of the MBN signal to stress was dependent upon the direction of the stress with respect to the zero stress Easy Axis. The stress response of the MBN signal was greatest for (i) tensile stresses oriented perpendicular to the zero stress Easy Axis direction and (ii) compressive stresses applied parallel to the Easy Axis direction. The modification of the MBN signal under an applied stress was attributed primarily to a change in the 180° domain wall population in the material investigated. Results were described by a model that considered regions of locally correlated domain behavior, termed ‘‘interaction regions,’’ that were typically the size of grains within the steel material. A basic result of the model was the stress required to modify the number of 180° domain walls within an interaction region. Theoretical calculations of these threshold stresses for a typical grain size were found to be in agreement with the range of applied stresses that was observed to modify the angular‐dependent MBN signal obtained from the sample.

  • characterization of the magnetic Easy Axis in pipeline steel using magnetic barkhausen noise
    1994
    Co-Authors: T W Krause, L Clapham, D L Atherton
    Abstract:

    The angular dependence of magnetic Barkhausen noise (MBN) on eight surfaces through the thickness of a 2% Mn steel pipeline sample was investigated. The MBN signal was analyzed by integrating the square of the MBN voltage signal with respect to the time Axis. The resulting value, referred to as the MBN energy signal, was modeled by considering the irreversible motion of 180° domain walls, under the influence of an oriented magnetic field. An expression for the angular dependence of the MBN energy signal was derived and was given by energy=α cos2 θ+β, where α and β are the fitting parameters and θ is the angle between the maximum MBN signal and the applied sweep field. The α parameter was associated with the irreversible motion of 180° walls that contributed to the net macroscopic Easy Axis near the surface of the sample, while the β parameter was associated with the isotropic background MBN signal. The energy equation could be used to fit the data for all sweep field amplitudes in which the MBN spectrum w...

  • characterization of the magnetic Easy Axis in pipeline steel using magnetic barkhausen noise
    1994
    Co-Authors: T W Krause, L Clapham, D L Atherton
    Abstract:

    The angular dependence of magnetic Barkhausen noise (MBN) on eight surfaces through the thickness of a 2% Mn steel pipeline sample was investigated. The MBN signal was analyzed by integrating the square of the MBN voltage signal with respect to the time Axis. The resulting value, referred to as the MBN energy signal, was modeled by considering the irreversible motion of 180° domain walls, under the influence of an oriented magnetic field. An expression for the angular dependence of the MBN energy signal was derived and was given by energy=α cos2 θ+β, where α and β are the fitting parameters and θ is the angle between the maximum MBN signal and the applied sweep field. The α parameter was associated with the irreversible motion of 180° walls that contributed to the net macroscopic Easy Axis near the surface of the sample, while the β parameter was associated with the isotropic background MBN signal. The energy equation could be used to fit the data for all sweep field amplitudes in which the MBN spectrum was observed. A dependence of the α and β parameters on the applied sweep field amplitude was observed.