Observation of domains first order transitions in normal spinel ZnCr2Se4

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Abstract
Specific heat, neutron diffraction, magnetization and susceptibility measurements have been performed in an extended range of temperatures on powder ZnCr2Se4. They indicate that in this compound the usually admitted Néel temperature TN=21 K is actually a first order transition temperature at which helimagnetic macrodomains spontaneously break into metamagnetic microdomains. Most of these domains split again at T=45.5 K. The high temperature magnetic structure is centred tetragonal and a true TN=105 K is determined. Entropy due to Block walls flexibility explains the stability of the magnetic phase. A(T,H) phase diagram is obtained which indicates that in the presence of a magnetic field, the micromagnetic phase may be stable at T<21 K; this also explains the observed kink of the magnetization curves. From this phase diagram, an entropy value is obtained which is in good agreement with the one obtained from direct calorimetric measurements.

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