Field Induced Lattice Deformation in the Quantum Antiferromagnet SrCu₂(BO₃)₂

The results of synchrotron X-ray diffraction by the coupled dimer compound SrCu 2 (BO 3 ) 2 subjected to a pulsed high magnetic field are reported. We find the lattice constant, a , contracts with increasing magnetic field. This change in the lattice constant scales with the magnetization change. We argue that the nearest neighbor exchange interaction between Cu 2+ spins is an antiferromagnetic one through an oxygen and its strength depends on the bond angle Cu 2+ –O 2- –Cu 2+ . The bond length between the Cu ions in a triplet becomes shorter to make the bond angle smaller and to diminish the antiferromagnetic exchange interaction. We propose this mechanism as a main source of the lattice contraction.