Phase diagram of the antiferromagnet CsMnCl3·2H2O near the bicritical point. Effect of impurities

Abstract

The phase diagram of the quasi-one-dimensional antiferromagnet CsMn${\mathrm{Cl}}_3$·2${\mathrm{H}}_2$O (CMC) was determined with great accuracy near the bicritical point $T_b=4.3541\mathrm{}$ K, ${\mathrm{H}}_b=2.0144\mathrm{}$ T, and near the Néel point $T_N=4.880\mathrm{}$ K. The experimental procedure included susceptibility measurements and ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ and ${}_{}{}^{133}{}_{}{}^{}\mathrm{Cs}$ nuclear magnetic resonance. In the neighborhood of the bicritical point, the two second-order transitions are described by the theoretical laws provided by the extended scaling theory. The anisotropic crossover exponent $\phi =1.225\mathrm{}\pm 0.005$ is intermediate between the theoretical values for the uniaxial symmetry ($n=3\mathrm{}$) and for the orthorhombic symmetry ($n=2\mathrm{}$). The ratio of the bicritical amplitudes $\frac{{\omega }_{\perp }}{{\omega }_{\parallel }}$ is 1, in accordance with a theoretical calculation of the crossover temperature to the orthorhombic behavior in a quasi-one-dimensional system. We have done similar measurements in CMC doped with Co (4 at.%) and Cu (0.4, 0.7, and 2.6 at.%) impurities. The experimental reduction of the bicritical field in CMC:Cu agrees with a theoretical calculation for a quasi-one-dimensional system with diamagnetic impurities. We have determined the phase diagram of CMC:0.7 at.% Cu near the bicritical point. A theoretical calculation shows that the random-field behavior must be observable. In the neighborhood of the bicritical point, the two second-order transition lines are described each by a different shift exponent. The upper shift exponent is $1.7<\mathrm{}{\phi }_{\perp }<3.7\mathrm{}$ qualitatively in accordance with the theoretical random-field value obtained by $6-d$ expansions but the lower shift exponent $1.2<\mathrm{}{\phi }_{\parallel }<1.5\mathrm{}$ does not agree with the expected value close to 1. An explanation can be the destruction of the genuine antiferromagnetic-paramagnetic transition in the presence of random fields at the lower critical dimensionality $d=3\mathrm{}$.