Magnetovolume and magnetoelastic effects in dilute Cr-Ga alloys

Abstract

The thermal expansion and elastic constants of a series of dilute Cr-Ga alloys are reported between 77 and 500K. In addition to magnetic anomalies at the Neel temperature, TN, well defined anomalies in both properties have been observed for the first time at the incommensurate-commensurate transition temperatures of this system which has a triple point near 0.5 at.% Ga and 280K where the paramagnetic, incommensurate (I) and commensurate (C) spin density wave (SDW) states coexist. The temperature dependence of the magnetovolume, Delta omega , of alloys showing I-C transitions is analysed to yield the ratio (m2(0))CSDW/(m2(0))ISDW=1.5+or-0.2, where (m2(0)) is the mean squared value of the effective local moment at 0K. Using this result the magnetic contribution to the bulk modulus at 0K in a Cr+1.00 at.% Ga alloy is found to be mod Delta B(0) mod CSDW=(0.20+0.05)*1011 N m-2. Also for this alloy, for example, delta ln TN/ delta omega ISDW=20+or-4, in good agreement with independent measurements. For some alloys the calculated pressure dependence of the Neel temperatures agrees fairly well with direct measurements. Above TN the magnetovolume due to antiferromagnetic spin fluctuations decays roughly exponentially with temperature.