Exchange Springs and Hysteresis Loop - An Analytical Approach

Abstract:
Exchange springs (soft FerroMagnet/hard FM bilayer) are nowadays implemented as basic elements in magnetic recording heads and magnetic random access memories (MRAM). However, it remains a challenge to describe accurately their physics. I will present analytical expressions for the magnetization profile of an exchange spring with arbitrary layer thicknesses and material parameters (exchange coupling and anisotropy). This opens up the possibility to analyze in detail the mechanisms governing magnetization reversal under an external field. In particular, I will show how the interface coupling induces a twist of the hard layer well below its intrinsic reversal field, in agreement with recent experimental observations [Phys.Rev.Lett. 88, 067201 (2002)]. The method also allows to discuss the reversible and irreversible parts of the hysteresis loop and to identify the barrier between different magnetization states. I will shortly address the effect of thermal fluctuations on the magnetization reversal process. Finally, the crossover between power-law and exponential behaviour of the coercivity as a function of layer thickness and material parameters will be described.

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