Enhanced spin correlations in the Bose-Einstein condensate compound Sr₃Cr₂O8

Combined experimental and modeling studies of the magnetocaloric effect, ultrasound, and magnetostriction were performed on single-crystal samples of the spin-dimer system Sr3Cr2O8 in large magnetic fields to probe the spin-correlated regime in the proximity of the field-induced XY-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) similar or equal to 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 Sr3Cr2O8. Our experimental results are qualitatively reproduced by a minimalistic phenomenological model of the exchange striction by which sound waves renormalize the effective exchange couplings