Magnetocaloric cooling observed in polycrystalline ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$

We report the observation of magnetocaloric cooling in polycrystalline ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ at T≪${\mathit{T}}_{\mathit{c}}$. By using a temperature measurement, the field-induced heating or cooling in an adiabatic environment has been measured for magnetic fields up to 300 Oe applied at constant ramp rates in the absence of a transport current. In hysteretic runs at 6.5 and 7.1 K cooling of the order ∼0.35 mK has been observed at applied fields up to 230 and 150 Oe, respectively. For fields larger than these heating occurs and takes over as the dominant effect. The cooling is proportional to the square of the applied field and independent of the ramp rate of the magnetic field. The volume average magnetic-flux density B has been simultaneously measured and found to increase as the square of the applied field up to 150 Oe and then to increase nearly linearly with applied field up to 300 Oe. Using classical thermodynamic arguments, we find that 0.001–0.0045 % of the material has returned to the normal state in an applied field of 100 Oe at 7.1 K.