Magnetization Process of a Helical Spin Configuration

Abstract

The behavior of a helical spin configuration of a hexagonal crystal in an applied field is discussed for the case where the axis of the helix is identical with the hexagonal c axis and the magnetization vector in each layer is parallel to the basal plane. In a small field applied parallel to the basal plane, a slight deformation of the helix occurs resulting in a small increase in over-all magnetization. If the field surpasses a certain critical value, the helix changes abruptly into a state with high resulting magnetization. In this state the spin directions are oscillating about the direction of the applied field, the magnetization being about 85% of the saturation value. In still higher fields, saturation is reached completely. A helical spin configuration has been found in MnAu2 by Herpin,Mériel, and Villain. We have investigated the properties of the hexagonal oxide (Ba,Sr)2Zn2Fe12O22 which is of the Y-type. The Ba-rich composition is ferrimagnetic and has a preferential plane for the magnetization at all temperatures. The Sr-rich composition has no spontaneous magnetization, but a magnetization equal to the one in the Ba case can easily be induced by an applied field. This behavior can be explained by the assumption of a helical spin configuration. As an other example, we show that the magnetic properties of dysprosium as investigated by Behrendt,Legvold, and Spedding can be explained by assuming a helical spin configuration between 85°K and 178°K. The angle between two neighboring layers is equal to zero at 85°K, the ferromagneticCurie temperature, and increases at higher temperatures. The magnetization process mentioned above is modified slightly in dysprosium by the presence of a strong magnetic anisotropy in the plane. In the presence of this anisotropy the transition between the ferromagnetic state and the helical state turns out to be a first-order transition, which explains the observed specific heat.