Enhanced spin correlations in the Bose-Einstein condensate compound Sr_3Cr_2O_8

摘要

Combined experimental and modeling studies of the magnetocaloric effect, ultrasound, and magnetostriction were performed on single-crystal samples of the spin-dimer system Sr_3Cr_2O_8 in large magnetic fields to probe the spin-correlated regime in the proximity of the field-induced XT-type antiferromagnetic order also referred to as a Bose-Einstein condensate of magnons. The magnetocaloric effect, measured under adiabatic conditions, reveals details of the field-temperature (H, T) phase diagram, a dome characterized by critical magnetic-fields H_(c1) = 30.4. H_(c2) = 62 T, and a single maximum ordering temperature T_(max)(45 T) ≃ 8 K. The sample temperature was observed to drop significantly as the magnetic field is increased, even for initial temperatures above T_(max), indicating a significant magnetic entropy associated with the field-induced closure of the spin gap. The ultrasound and magnetostriction experiments probe the coupling between the lattice degrees of freedom and the magnetism in Sr_3Cr_2O_8. Our experimental results are qualitatively reproduced by a minimalistic phenomenological model of the exchange striction by which sound waves renormalize the effective exchange couplings.