CRYSTAL GROWTH AND MAGNETO·STRUCTURAL CHARACTERISATION OF Zn1-xMnxO4 SINGLE CRYSTALS

Magnetic oxide spinels with the general formula M'l-x M"xCr204 (M' and M" are divalent metals) are being extensively studied because of their interesting properties. The system Znl_xMnxCr204 seems particularly attractive because of the coexistence of competitive Cr-Cr, Cr-Mn and Mn-Mn antiferromagnetic interactions. The pure compounds ZnCr204 (antiferromagnetic, T~ 16 K, 9= -424 K) and MnCr204 (noncollinear ferromagnet, T c= 43 K) are normal spinels with the chromium ions occupying the octahedral sites; the substitution of Zn by Mn ions, by changing the spin structure and the resulting magnetic interactions, leads to a magnetic moment on the tetrahedral sites. In this work single crystals of Znl-xMnxCrz04' up to 1x1x1 mm, in the range of compositions x = 0 to I, have been grown by the chemical vapour transport (CVT) technique using chlorine as the transport agent A thermodynamic study of the closed tube system has been done in order to determine the most favourable conditions for growth. The gaseous species taken into consideration were Cl2, O2, MnCl2, CrCI:z, CrCl3 CrC4, C~12 and CrzC4. In addition the calculations also take into account the presence of MnCl2 liquid, a spurious species which may modify the crystal perfection and stoichiometry. Single crystals of different compositions were analyzed by X-ray difIractomettic methods using MoKa radiation. All the crystals examined showed a spinel structure (space group Fd3m), with zinc which partially substitutes manganese in tetrahedral sites. The variation of the parameters of the cells does not appear to be linear. Susceptibility measurements as a function of temperature from 4 to 300 K for the different compositions are reported. The magnetic phase diagram has been determined as a function of x and T: it was observed that the antiferromagnetic and ferromagnetic states are progressively perturbed and then destroyed, leading to