Magnetic and thermal properties of low-dimensional single-crystalline transition-metal antimonates and tantalates
Christian, Aaron Brandon
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This work contributes to the study of magnetic interactions in the low-dimensional antiferromagnets M(Sb,Ta) 2O 6, where M is a transition metal. By virtue of the trirutile structure, M-O-O-M chains propagate along  at z = 0 and [11overline0] at z = 1=2 of the unit cell. These chains are separated along  by sheets of weakly-interacting diamagnetic ions. The spin-exchange coupling perpendicular to the chains is weak, permitting the low-dimensional classification. Single crystals have been grown using chemical vapor deposition and the floating zone method. Magnetization, in-field heat capacity, and high-resolution thermal expansion measurements have been performed along various axes, revealing significant anisotropy due to the peculiar magnetic structures and low dimensionality. The Neel temperature, TN, at which long-range order occurs is found to be unstable against the application of magnetic field above 2 T. Large fields tend to lower TN of the set of moments with projections along the applied field. Moments which are aligned perpendicular to the field are significantly less affected. This can lead to the formation of a secondary peak in heat capacity when magnetic field is along either  or [11overline0]. The change in heat capacity at the location of the newly formed peak means there is a change in entropy, which depends upon the direction of applied field with respect to the magnetic moments. Consequently, an anisotropic magnetocaloric effect arises due to the unique magnetic structure. The anisotropic nature of this effect has potential applications in magnetic refrigeration.