Field induced lattice deformation in a quasi-one-dimensional antiferromagnet BaCo₂V₂O₈

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 M², where M is the magnetization. A good agreement between theory and experiment is obtained for the a-axis up to the saturation field, B_s. Above B_s, an M⁴ term is necessary, in addition to the M² one, to obtain a better agreement. For the c-axis, a sum of the M² and M ⁴ terms gives a reasonable fit to the experimental data.