Terbium orthovanadate TbVO4 is a textbook example of a material exhibiting the cooperative JT effect driven by phonon-mediated interactions between the terbium quadrupole moments . At high temperatures, it crystallizes in the tetragonal zircon structure whereas on lowering the temperature to less than 33 K it undergoes a cooperative JT transition and the crystal spontaneously distorts along the  direction, lowering the symmetry to orthorhombic. The balance between TbVO4’s magnetic and quadrupolar effects can be tuned by varying the strength of an applied field. The effect of large external magnetic field on TbVO4 has only recently been studied . It was predicted that the JT distortion would be suppressed when fields more than 29 T were applied along the c-axis of the sample.
In this experiment the sample was placed in an environment that permits the application of high magnetic fields while monitoring changes using X-ray diffraction . The sample consisted of finely powderedd crystal of TbVO4 embedded in polyvinylpyrolidone to prevent movement of the powder grains due to magnetic forces.The data shown on the figures were collected on the DUBBLE CRG beamline (BM26B) by accumulating about 45 magnetic field pulses per powder diffraction spectrum. For each field pulse a mechanical shutter exposed the image plate detector for 4.9 ms centered around the maximum field.
The JT transition manifests itself as a splitting of some of the powder lines in the spectrum due to the distortion of the crystal lattice. Within the photon energy range studied, the (311)/(131) and (202)/(022) pairs of reflections are sensitive to the JT distortion (figures above). Spectra taken at different fields strengths (15 and 30 T) showed a reduction of the splitting below the JT transition (figure above, left) and the appearance of splitting above the transition (figure above, right), thus providing evidence for the modification of the JT distortions of TbVO4 by magnetic fields. Although the observed splitting followed the predicted spectra, the degree of splitting was lower than expected. Various theories to explain the quantitative difference have been proposed , one being sample heating due to the magnetocaloric effect.
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Members of the laboratory implied in this activity
Former members of the lab implied in this activity
C. Detlefs, T. Roth (ESRF, Grenoble)
J. Vanacken (INPAC, Leuven)
J.E. Lorenzo (Institut Néel, Grenoble)
W. Bras (DUBBLE CRG, ESRF)
Z.A. Kazeĭ (Moscow State University, Russia)
P.C. Canfield (Ames Laboratory, USA)