Enhanced spin correlations in the Bose-Einstein condensate compound Sr3Cr2O8

TL;DR

This study combines experimental and modeling approaches to investigate the spin correlations and phase diagram of Sr3Cr2O8, revealing significant magnetic entropy effects and lattice-magnetism coupling near the Bose-Einstein condensate of magnons.

Contribution

It provides new experimental data and a phenomenological model elucidating the spin-lattice interactions in Sr3Cr2O8 near the field-induced magnetic order.

Findings

Magnetic entropy causes significant temperature drops under high magnetic fields.

The phase diagram features a dome with critical fields at 30.4 T and 62 T.

Lattice coupling influences the exchange interactions in the material.

Abstract

Combined experimental and modeling studies of the magnetocaloric effect, ultrasound, and magnetostriction were performed on single-crystal samples of the spin-dimer system Sr$_3$Cr$_2$O$_8$ 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 T, $H_{c2}$ = 62 T, and a single maximum ordering temperature $T_{{\rm max}}(45~$T$)\simeq$8 K. The sample temperature was observed to drop significantly as the magnetic field is increased, even for initial temperatures above $T_{{\rm max}}$, indicating a significant magnetic entropy associated to 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$_3$Cr$_2$O$_8$. Our experimental results are qualitatively reproduced by a minimalistic phenomenological model of the exchange-striction by which sound waves renormalize the effective exchange couplings.