Apparent critical behaviour of sputter-deposited magnetoelectric antiferromagnetic Cr₂O₃ films near Néel temperature

Chromium(III) oxide is a collinear antiferromagnet with a linear magnetoelectric effect. We are presenting the measurements of the magnetoelectric susceptibility α of a sputter-deposited 500 nm film and a bulk single-crystal substrate of Cr₂O₃. We investigated the magnetic phase-transition and the critical exponent β of the sublattice magnetization near Néel temperature. For the film, an exponent of 0.49(1) was found below 293 K, and changed to 1.06(4) near the Néel temperature of 298 K. For the bulk substrate, the exponent was constant at 0.324(4). We investigated the reversal probability of antiferromagnetic domains during magnetoelectric field cooling. For the sputtered films, reversal probability was zero above 298 K and stabilized only below 293 K. We attribute this behaviour to formation of grains during film growth, which gives different intergrain and intragrain exchange-coupling energies. The reversal probability dependence on the magnitude of cooling magnetic field could be explained by a phenomenological model. For the bulk substrate, reversal probability was stabilized immediately at the Néel temperature of 307.6 K.