Structural modulation of spinel ZnCr₂Se₄ in the vicinity of antifferromagnetic phase transition induced by an applied magnetic field

Magnetic and structural phase transitions of normal-type spinel ZnCr₂Se₄ have been studied as functions of applied magnetic field and temperature by means of neutron diffraction. It is found that the spiral spin order of Cr³⁺ ions is largely affected by the magnetic field, and the satellite-like magnetic reflections are largely decreased in the magnetic field of 0.6 T < H < 1.2 T and disappear at H ≥ 1.2 T, when the magnetic field was applied vertically to an orthorhombic axis [001]. This means that the spiral configuration of the ferromagnetic CrSe₄ chains is perfectly disordered around two-fold screw axes in an orthorhombic symmetry phase (Fddd structure) by the magnetic field H ≥ 1.2 T, though the structural phase transition (T_c) between cubic and orthorhombic symmetry phases simultaneously occurs at the antiferromagnetic phase transition temperature (T_N ≈ 21 K) in no magnetic field. It is also found that the main reflections show a magnetic field dependence similar to that of the magnetic reflections. Pseudo-tetragonal displacements of Se ions below T_N are modulated by the applied magnetic field. The 3 T magnetic field induces a metastable structural transition at about 15 K, after initially cooling down to about 2 K in no applied magnetic field. The transition is related to the modulation from pseudo-tetragonal to pseudo-cubic displacements of Se ions.