Second Order Anisotropy in Exchange Spring Systems

Abstract

XCHANGE spring systems have been proposed recently as a possible solution for increasing write-ability of recording media. The structure allows for the reduction of the switching field, significantly more substantial than the reduction expected by averaging the anisotropy fields of the hard and soft layers. The reversal process in case an external field is applied parallel to the easy axis of the hard layer has been described in [2]: a domain wall develops in the soft phase that subsequently propagates through the interface and assists switching the hard phase. Separating the magnetic properties of the soft and hard layers in the structure is essential in the process of developing the media. Films containing only the hard phase can be used to measure the saturation moment and anisotropy field and , for example by employing the torque method. The torque curve—showing how the component of magnetization perpendicular to the field varies with the angle when the sample is rotated in a constant field—has a sinusoid shape and is easily expressed mathematically if we assume that the hard layer behaves as a single spin. By fitting the experimental data an effective anisotropy is determined, to which the demagnetization term equal to must be added to obtain the crystalline Hk. The difficulty of measuring the properties of the soft phase-M and come from the fact its microstructure changes significantly if the soft layer is grown by itself, compared to the case when it is grown on top of the hard layer. Thus we cannot use films composed of the soft layer only, we must measure directly the exchange spring structure. In analyzing the torque curve of the exchange spring, we must take into account the nonuniformity of magnetization orientation. Partial domain walls develop at the boundary of the two phases, due to the difference in anisotropy [3]. The magnetization of the hard/soft layer combination can be modeled theoretically as a one-dimensional chain of spins, extending from the bottom of the hard layer to the top of the soft layer. We deduced analytical formulas for the dependence of spin orientation on the position in the chain, when the sample is rotated in constant