Magnetostriction, magnetoelastic coupling, and the magnetic Grüneisen constant in the antiferromagnet RbMnF3

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Abstract
The magnetostriction in RbMnF3 (a cubic low-anisotropy Heisenberg antiferromagnet) was measured at 4.2 K in magnetic fields H up to 137 kOe. Capacitance dilatometers were used. The change Δl in the length l along the [001] direction was measured with H→ parallel to the [001], [100], or [110] directions. Two distinct phenomena were observed. At low fields, of order 1 kOe, l underwent a fractional change of order 1×10−6. The sign and magnitude of this change depended on the direction of H→. This low-field magnetostriction is related to the H-induced reorientation of the sublattice magnetizations (analogous to the spin-flop transition), and is interpreted quantitatively in terms of a model for the magnetoelastic coupling in RbMnF3 which was originally proposed by Eastman et al. The low-field magnetostriction data for H→∥[001] and H→∥[100] lead to the values b1=(1.88±0.15)×106 and (1.77±0.14)×106 erg/cm3, respectively, for the dominant magnetoelastic coupling constant. These values are in agreement with other determinations of b1. In fields above several kilo-oersted, l increased linearly with H2, with a slope d(Δll)d(H2)=4×10−16 Oe−2 which was the same (within the experimental error) for H→∥[001] and H→∥[100]. This high-field magnetostriction is related to the H-induced canting of the sublattice magnetizations and the concomitant change in the exchange energy. The high-field magnetostriction arises from the dependence of the dominant exchange "constant" J on the volume V. Analysis of the data yields a magnetic Grüneisen constant γmag=−(∂