Specific features of the structural and magnetic states of a Zn$_{1-x}$Ni$_x$Se crystal ($x$ = 0.0025) at low temperatures

The magnetic state and the structure of a Zn$_{1-x}$Ni$_x$Se ($x$ = 0.0025) bulk crystal were studied at low temperatures. It is revealed that the magnetic and crystal structures below $T\cong$ 15 K are dependent on the cooling rate of this dilute semiconductor. For example, on fast cooling to 4.2 K, about 10% hexagonal ferromagnetic phase is formed in the crystal. During heating, the phase disappears at $T\cong$ 15 K. The results obtained are discussed with allowance for the specific features of the Jahn-Teller distortions in this compound.