Composition induced lowering of metamagnetic critical field and static magnetic properties of Co1-xNixCl2⋅2H2O

Abstract

The magnetic behavior of ${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{Cl}}_{2}\ensuremath{\cdot}{2\mathrm{H}}_{2}\mathrm{O}$ has been studied by dc magnetization and susceptibility measurements on mixtures spanning the entire composition range. The pure components are three-dimensional Ising (Co) and Heisenberg (Ni) antiferromagnets, ordering at 17.2 K and at 7.25 K (with a 6.31 K spin reorientation transition), respectively. Each contains ${\mathrm{MCl}}_{2}{\mathrm{MCl}}_{2}\mathrm{M}\dots{}$ chemical (and structural) chains, with ferromagnetic intrachain exchange and weaker but significant antiferromagnetic interchain exchange. Curie-Weiss fits, ${\ensuremath{\chi}}_{M}=C/(T\ensuremath{-}\ensuremath{\theta}),$ to high-temperature powder susceptibilities of mixtures yield Curie and Weiss constants that vary regularly with composition. C vs x is close to linear. The $\ensuremath{\theta}(x)$ dependence is less conventional, exhibiting only weak variation for small x, a marked increase between $x>~0.2$ and $x<~0.4,$ and a steady increase from $x>0.4$ to 1. In the susceptibilities of all mixtures below 20 K antiferromagnetic maxima appear. For Ni-rich mixtures only one maximum occurs, with ${T}_{\mathrm{max}}$ increasing weakly but uniformly from $x=1$ ${(7.4}_{7}\mathrm{K})$ to ${x=0.56}_{4}{(8.8}_{3}\mathrm{K}).$ A maximum in this region persists even down to ${x=0.27}_{2}$ ${(9.1}_{5}\mathrm{K});$ for ${x=0.21}_{0}$ only a shoulder in $\ensuremath{\chi}(T)$ near 9 K appears. For Co-rich mixtures a maximum near 18 K occurs, becoming merely incipient for $x\ensuremath{\sim}0.4;$ its location varies weakly between ${17.9}_{5}$ and ${18.2}_{5}\mathrm{K}.$ Transition temperatures, slightly lower than the associated ${T}_{\mathrm{max}},$ can often be estimated; these display a weak but regular composition dependence as well. Magnetization vs field isotherms evolve with composition. For high x the appearance of $M(H)$ is similar to that of ${\mathrm{NiCl}}_{2}\ensuremath{\cdot}{2\mathrm{H}}_{2}\mathrm{O}.$ For low x somewhat more curvature in $M(H)$ occurs than for ${\mathrm{CoCl}}_{2}\ensuremath{\cdot}{2\mathrm{H}}_{2}\mathrm{O},$ and there is a suggestion of qualitatively different behavior, though hysteresis effects are very small. Dramatically different properties are observed for ${x=0.27}_{2}$ and ${0.40}_{9},$ where field induced transitions in the 6 to 10 kG range occur for temperatures below 7 K. Major hysteretic effects, especially below 2 K, also appear. The overall behavior is reminiscent of metamagnets, and since the inferred transition fields are two or more times smaller than those previously determined for the pure components, this is a composition induced effect. A mean-field theory analysis of the temperature dependence of the hysteresis is made, suggesting that the ratio of interchain to intrachain exchange is somewhat larger in the ${x=0.27}_{2}$ and ${0.40}_{9}$ mixtures than in the pure components. The temperature dependence of the hysteresis loop area suggests the presence of an activation process with a characteristic temperature of the order of 1 K.