Magnetic phases of erbium in a c-axis field
Introduction
The discovery in the mid-eighties of long-period commensurate magnetic structures in holmium and erbium rekindled intense interest in the magnetic structures and excitations of these and other heavy rare earth metals. The existence of these long-period phases is due to competition between the exchange interaction which favours simple incommensurate ordering and the crystal field interaction which favours formation of commensurable structures.
Application of a magnetic field alters the balance between these interactions producing modifications of the zero-field structures, and even completely new types of magnetic order, such as the helifan observed in holmium when the field is applied along the easy b-axis.
In the case of erbium even the zero-field structures were not clearly understood until a fairly recent comprehensive study by Cowley and Jensen showed that the cycloidal phase purported to exist between 54K and 18K possesses an oscillatory component perpendicular to the plane of the cycloid, and that its existence requires a magnetic interaction which distinguishes between the two lattice sites in the chemical unit cell.
Their model has now pointed to detailed solutions to the structures of erbium in a c-axis field, from measurements performed comparatively recently on D10. These have allowed further detail to be added to the phase diagram (shown below).
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Magnetic phase diagram of erbium in a c-axis field. Solid lines separate the main phases; dashed lines separate similar commensurate and incommensurate phases.
Results
New results were firstly that in addition to the longitudinal modulation along c, the high temperature phase of erbium in a c-axis field has a small basal plane moment, probably helically ordered, with a different modulation vector to the longitudinal component. Secondly the most noticeable effect of applying a field is that the temperature intervals over which the wavevector of the cycloidal phase is commensurate are enhanced at the expence of those over which it is incommensurate.
The oscillatory component perpendicular to the a-c cycloidal plane is still present but application of a field does reduce its moment, as well as the basal-plane component of the cycloid, relative to their values in zero field. Finally applying a field enhances the stability of the commensurate and incommensurate cone phases at the expence of the longitudinal and cycloidal phases, although details of the phase diagram at high field and low temperature are still uncertain.
Last updated by Andrew Crowe on 15/02/1996
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