EFFECT OF AN OBLIQUE MAGNETIC FIELD ON THE SUPERPARAMAGNETIC RELAXATION TIME. II. INFLUENCE OF THE GYROMAGNETIC TERM
EFFECT OF AN OBLIQUE MAGNETIC FIELD ON THE SUPERPARAMAGNETIC RELAXATION TIME. II. INFLUENCE OF THE GYROMAGNETIC TERM
复制标题
倾斜磁场对超顺磁性弛豫时间的影响。
DOI：
10.1103/physrevb.58.3249
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发表时间：
1998
影响因子：
3.7
通讯作者：
E. Kennedy
中科院分区：
文献类型：
--
作者：
W. Coffey;D. Crothers;J. Dormann;L. Geoghegan;E. Kennedy
以与易磁化轴倾斜的角度施加的均匀磁场对超顺磁性〜纵向或尼尔！通过数值求解 Fokker-Planck 方程以获得最小非零特征值来研究弛豫时间。结果表明，当应用于当前问题时，一般非轴对称势的中到高阻尼极限下克莱默逃逸率的渐近公式的倒数对于中到高阻尼值产生了可接受的尼尔时间渐近近似。或者相应的 Kramers 低耗散公式~能量控制扩散！为非常小的阻尼值提供可接受的近似值。引起纵向和横向运动模式之间的耦合的旋磁项的效果通常对应于最小非零本征值的增加，并因此对应于尼尔弛豫时间的减少。还评估磁化强度衰减曲线斜率下的积分弛豫时间或面积。事实证明，对于足够高的均匀场值〜然而，比破坏势的双稳态性质所需的值要少得多！最低非零特征值的倒数〜与尼尔时间或磁化反转时间成正比！并且积分弛豫时间可能彼此呈指数差异，这表明除了与过度势垒相关的模式之外的模式的贡献~Neel！弛豫过程到整体弛豫过程。除了方位角依赖性之外，整体行为在质量上是相似的！轴对称情况是由于均匀场耗尽两个势阱中较浅的部分而产生的，因此，在足够高的均匀场值下，两个势阱较深处的快速过程可能会主导弛豫过程。@S0163-1829~98！04229-5#
The effect of a uniform magnetic field applied at an oblique angle to the easy axis of magnetization on the superparamagnetic ~longitudinal or Neel! relaxation time is investigated by numerically solving the Fokker- Planck equation for the smallest nonvanishing eigenvalue. It is demonstrated that the reciprocal of the asymptotic formula for the Kramers escape rate in the intermediate to high damping limit for general nonaxi- ally symmetric potentials when applied to the present problem, yields an acceptable asymptotic approximation to the Neel time for moderate to high values of the damping. Alternatively the corresponding Kramers low dissipation formula ~energy controlled diffusion! provides an acceptable approximation for very small values of the damping. The effect of the gyromagnetic term which gives rise to coupling between the longitudinal and transverse modes of motion generally corresponds to an increase of the smallest nonvanishing eigenvalue and so to a decrease of the Neel relaxation time. The integral relaxation time or area under the slope of the curve of the decay of the magnetization is also evaluated. It is demonstrated that for sufficiently high values of the uniform field ~much less, however, than that required to destroy the bistable nature of the potential ! the reciprocal of the lowest nonvanishing eigenvalue ~proportional to the Neel time, or the time of reversal of the magnetization! and the integral relaxation time may differ exponentially from one another signifying the contributions of modes other than that associated with the overbarrier ~Neel! relaxation process to the overall relaxation process. The overall behavior is qualitatively similar ~apart from the azimuthal dependence! to that of the axially symmetric case which arises due to the depletion of the shallower of the two potential wells by the uniform field, so that the fast processes in the deeper of the two wells may come to dominate the relaxation process at sufficiently high values of the uniform field.@S0163-1829~98!04229-5#