Neutron—Diffraction Study of Cr and Cr Alloys

Abstract

The principle quantities which parameterize the antiferromagnetism of Cr are studied by neutron diffraction as a function of temperature, pressure, magnetic field, and concentration of solute atoms. In order to account for the observed intensities of the magnetic reflections, their dependence on magnetic field and temperature, the torque measurements of Montalvo and Marcus, and the creation of a ``single‐Q'' state in field‐cooling, a model is proposed based on the presumed existence of thermally active polarization domains within a ``single‐Q'' region of the crystal. The variation of the polarization axis from place to place and with time lowers the free energy by an increase in entropy. The pressure dependence of the first‐order phase change at 38.5°C is given as dTN∕dP=−5.4 deg∕kbar. The temperature dependence of the length of the wave vector below TN is given as (1∕Q) (dQ∕dT)=6.3×10−5 deg−1. Alloys with 0.5 and 0.78 wt% Fe and with 0.9 wt% Co show a decrease in TN of ∼20°K per at.% of solute. The amplitudes of the magnetization waves increase and the wave vector Q approaches commensurateness with the lattice periodicity with increasing solute concentration in contrast to results for other solutes. Some unusual effects were observed for 2.3 wt% Fe samples