Magnetovolume and magnetoelastic effects in dilute Cr-Ga alloys

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, T N , 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 (m 2 (0)) CSDW /(m 2 (0)) ISDW =1.5+or-0.2, where (m 2 (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)*10 11 N m -2 . Also for this alloy, for example, delta ln T N / 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 T N the magnetovolume due to antiferromagnetic spin fluctuations decays roughly exponentially with temperature.