Field Induced Lattice Deformation in a Quasi-One-Dimensional Antiferromagnet BaCo2V2O8

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Synchrotron X-ray diffraction is used to study the correlation between crystal structure and magnetism in a quasi-one-dimensional uniaxial antiferromagnet BaCo2V2O8. When an external magnetic field, $B$, is applied parallel to the $c$-axis in the short-range-ordered phase, the lattice constant, $a$ elongates, while $c$ shrinks with increasing $B$. We calculate the exchange energy under applied magnetic fields based on the proposed magnetic structure. We are successful in explaining the contrasting behavior of the lattice deformations. Our calculation shows that the change in the lattice constants is proportional to $M^{2}$, where $M$ is the magnetization. A good agreement between theory and experiment is obtained for the $a$-axis up to the saturation field, $B_{\text{s}}$. Above $B_{\text{s}}$, an $M^{4}$ term is necessary, in addition to the $M^{2}$ one, to obtain a better agreement. For the $c$-axis, a sum of the $M^{2}$ and $M^{4}$ terms gives a reasonable fit to the experimental data.
2010-04-15