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Piezomagnetism and magnetoelastic memory in uranium dioxide

M. Jaime, A. Saul, M. Salamon, V.S. Zapf, N. Harrison, T. Durakiewicz, J.C. Lashley, D.A. Andersson, C.R. Stanek, J.L. Smith, and K. Gofryk. Nature Communications 24th July (2017).

The thermal and magnetic properties of uranium dioxide, a prime nuclear fuel and thoroughly studied actinide material, remain a long standing puzzle, a result of strong coupling between magnetism and lattice vibrations. At high temperatures, the magnetic moments of the U atoms of this cubic material are disordered and the system is paramagnetic. At temperatures below 30.5 K, a magnetic order characterized by a 3-k non-collinear antiferromagnetic structure is observed. The magnetic order is associated with a Jahn-Teller distortion of the cubic cage formed by the oxygen atoms. In this work, we show that single crystals of uranium dioxide subjected to strong magnetic fields exhibit the abrupt appearance of a positive linear magnetostriction when the magnetic order is established at temperatures below 30.5 K. Upon reversal of the field the linear term also reverses sign, a hallmark of piezomagnetism. A switching phenomenon occurs at ±18 T, which persists during subsequent field reversals, demonstrating a robust magneto-elastic memory that makes uranium dioxide the hardest piezomagnet known. A model including a strong magnetic anisotropy, elastic, Zeeman, Heisenberg exchange, and magnetoelastic contributions to the total energy is proposed. This simple model successfully reproduces the intriguing experimental observations. The energy minimization of the model Hamiltonian with respect to the components of the strain tensor allows a clear understanding of the link between the magnetic domains connected by time reversal and the sign of the piezomagnetic tensor.

The experimental measurements have been performed at Los Alamos National Laboratory and Idaho National Laboratory, USA. Contact : A. Saúl.

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Top Left : The low-temperature antiferromagnetic phase of UO2 displaying the transverse 3-k TA order of the magnetic moments (violet arrows) of the U atoms (grey spheres) and the distortion of the cubic oxygen cage (red spheres) due to the Jahn-Teller distortion (not in scale). Top Right : experimental deformation versus magentic field. Bottom : Theoretical deformation versus magnetic field