畴壁切换：优化能源格局
Domain wall switching: optimizing the energy landscape
摘要：
最近有人提出，交换弹簧介质提供了一种增加介质密度而不引起热不稳定性（超顺磁性）的方法，通过使用通过交换耦合的硬层和软层。Victora提出了一个品质因数xi = 2 E_b/mu_0 m_s H_sw，即与具有相同开关场的Stoner-Wohlfarth系统的能垒之比。对于Stoner-Wohlfarth（相干开关）粒子，该品质因数为1;对于最佳双层复合介质，该品质因数为2。许多理论方法已经用于该问题（例如，各种数量的耦合Stoner-Wohlfarth层和连续微磁性）。在本文中，我们表明，许多这些方法可以被视为特殊情况下或近似的变分制定的问题，其中的能量最小化固定磁化。结果可以很容易地直观地显示在能量作为磁矩m_z的函数的曲线图中，其中开关场[E（m_z）的最大斜率]和稳定性（由能垒E_b决定）都是几何可见的。在这个公式中，我们可以证明品质因数xi的严格限制，它不能高于4。我们还表明，Suess等人提出的二次各向异性非常接近这个极限。
英文摘要：
It has recently been suggested that exchange spring media offer a way to increase media density without causing thermal instability (superparamagnetism), by using a hard and a soft layer coupled by exchange. Victora has suggested a figure of merit xi = 2 E_b/mu_0 m_s H_sw, the ratio of the energy barrier to that of a Stoner-Wohlfarth system with the same switching field, which is 1 for a Stoner-Wohlfarth (coherently switching) particle and 2 for an optimal two-layer composite medium. A number of theoretical approaches have been used for this problem (e.g., various numbers of coupled Stoner-Wohlfarth layers and continuum micromagnetics). In this paper we show that many of these approaches can be regarded as special cases or approximations to a variational formulation of the problem, in which the energy is minimized for fixed magnetization. The results can be easily visualized in terms of a plot of the energy as a function of magnetic moment m_z, in which both the switching field [the maximum slope of E(m_z)] and the stability (determined by the energy barrier E_b) are geometrically visible. In this formulation we can prove a rigorous limit on the figure of merit xi, which can be no higher than 4. We also show that a quadratic anistropy suggested by Suess et al comes very close to this limit.
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