Magnetic structures of cubic FeGe studied by small-angle neutron scattering

Abstract

The magnetic ordering of a single crystal of the cubic polymorph of FeGe has been studied by small-angle neutron scattering. The compound orders magnetically at TN=278.7 K into a long-range spiral (period approximately 683-700 AA) propagating along equivalent <100> directions at high temperatures and along equivalent <111> directions at low temperatures. The length of the spiral wavevector is nearly independent of temperature. The transition at TN is first order with very little hysteresis. The transition at which the direction of the spiral turns is rather sluggish. It takes place in a temperature interval of approximately 40 K and shows pronounced temperature hysteresis (T2 down arrow =211 K, T2 up arrow =245 K). Applied magnetic fields of 20-40 mT, depending on the temperature and the field direction, cause the spiral axis to turn into the direction of the applied field. As the field is further increased, the amplitude of the antiferromagnetic spiral decreases and the ferromagnetic component increases until at fields above approximately 200-300 mT cubic FeGe becomes magnetically saturated. The magnetic ordering in cubic FeGe is a Dzyaloshinskii spiral similar to the structure observed in the isostructural compound MnSi. However, in MnSi the spiral propagates along equivalent <111> directions at all temperatures below TN=29.5 K.