Magnetic Structure of CrAs and Mn-Substituted CrAs

A neutron diffraction study has been made of powdered samples of CrAs and Mn-substituted CrAs, namely, Cr0.7Mn0.3As, Cr0.4Mn0.6As, and Cr0.3Mn0.7As. Although CrAs shows a kink in susceptibility at 796°K, the anomaly was found to be due to a crystallographic transformation, from MnP-type structure (low-temperature side) to NiAs type. The liquid-nitrogen temperature diffraction pattern contains a number of magnetic peaks. An analysis of the data gives a double-layered helical structure, with the propagation vector along the c axis (the space group Pnma), like that found in MnP. The Néel temperature as determined from the temperature dependence of (101)− intensity is about 280°K. The magnetic structure of Cr0.7Mn0.3As is much the same as that of CrAs, but with the Néel temperature at 263°K. Cr0.4Mn0.6As is ferromagnetic with a Curie temperature at 160°K, and has a magnetic transition in the paramagnetic range, which separates the low-temperature low-spin state from the high-temperature intermediate-spin state. Cr0.3Mn0.7As is metamagnetic at low temperatures.