Electronic topological transitions, magnetic ordering and the Fermi surfaces of Y and the heavy rare earths

The magnetic order of the heavy rare earths has long been associated with the presence of a certain feature in the Fermi surface (FS) topology (being common to non-magnetic Y also), known as the “webbing”. Furthermore, there has been speculation that changes in the observed ordering, brought about by temperature-induced changes in the crystalline c / a ratio, are due to the presence or absence of that webbing feature. Such a simplistic model is appealingly simple in systems where physics of the exchange interaction as a function of composition and temperature is complicated. The Gd 1 − x Y x alloy system exhibits ferromagnetism or helical antiferromagnetism at different compositions and temperatures. We present our experimental measurement of the FS topology of such an alloy at a composition close to the Y concentration where helical antiferromagnetism occurs. The data show that the evolution of the FS in Gd 1 − x Y x is different from that predicted by the model based on the c / a ratio, suggesting a less simplistic interpretation.