Magnetization and neutron-diffraction measurements were performed on a single crystal of Cu 2 MnSnS 4. This quartenary magnetic semiconductor has the stannite structure ͑derived from the zinc-blende structure which is common to many II-VI dilute magnetic semiconductors͒, and it orders antiferromagnetically at low temperature. The neutron data for the nuclear structure confirm that the space group is I42m. Both the neutron and magnetization data give T N ϭ8.8 K for the Néel temperature. The neutron data show a collinear antiferromagnetic ͑AF͒ structure with a propagation vector kϭ͓1/2,0,1/2͔, in agreement with earlier neutron data on a powder. However, the deduced angle between the spin axis and the crystallographic c direction is between 6°and 16°, in contrast to the earlier value of 40°. The magnetization curve at TӶT N shows the presence of a spin rotation ͑analogous to a spin flop͒, which indicates that the spin axis is indeed close to the c direction. The deduced magnetic anisotropy gives an anisotropy field H A Х2 kOe. At high magnetic fields the magnetization curve at TӶT N shows the transition between the canted ͑spin-flop͒ phase and the paramagnetic phase. The transition field, Hϭ245.5 kOe, yields an intersublattice exchange field H E ϭ124 kOe. The exchange constants deduced from H E and the Curie-Weiss temperature ⌰ϭϪ25 K show that the antiferromagnetic interactions are an order of magnitude smaller than in II-VI dilute magnetic semiconductors ͑DMS's͒. The much weaker antiferromagnetic interactions are expected from the difference in the crystal structures ͑stannite versus zinc-blende͒. A more surprising result is that the exchange constant which controls the AF order below T N is not between Mn ions with the smallest separation. This result contrasts with a prediction made for the related II-VI DMS, according to which the exchange constants decrease rapidly with distance. ͓S0163-1829͑97͒04234-3͔
The critical temperature is calculated as a function of the ratio for an S = 5/2 Heisenberg spin lattice with antiferromagnetic ordering of types I and II on a face-centred cubic lattice. and represent, respectively, the nearest- and next-nearest-neighbour exchange constants. Both possibilities for ordering of type II, antiferromagnetic and ferromagnetic, are considered. The critical region is studied by applying the Padé approximant method to the corresponding high-temperature series expansion of the staggered susceptibility. The results presented here provide a useful tool for a straightforward interpretation and understanding of experimental data. The approach is applied to various experimental systems and the values obtained compared with those provided by other approximations.
The controlled substitution of Mn by Zn within the diluted magnetic semiconductor Zn 1Ϫx Mn x Ga 2 Se 4 (0 рxр1) series induces a pumping of magnetic ions from a defined site of the crystal lattice to another one driving the system to an order-disorder phase transition with x c ϭ0.50Ϯ0.01. A long-range order parameter, , is defined and directly determined as a function of x within the whole range of composition from neutrondiffraction experiments. An augmentation of the magnetic exchange field by increasing the magnetic dilution occurs as a consequence of that order-disorder process.
The crystal and magnetic structures of RbMnF4 and KMnF4 have been determined by neutron powder diffraction. The crystal symmetry of both compounds belongs to the layered perovskite structure and exhibits a pseudo-tetragonal unit cell, space group P21/a. The (MnF2F42/)- octahedra show a distortion induced by both steric and Jahn-Teller effects. They are also tilted by an angle which depends on the size of the alkali ion. KMnF4 orders as a noncollinear antiferromagnet below 5.2 +or- 0.1 K exhibiting four magnetic sublattices with an angle between the two spin directions of 17 degrees . RbMnF4 is a collinear antiferromagnet below 3.7 +or- 0.1 K. Interestingly enough and contrary to what is found for the K compound, there are two active irreducible representations in the magnetic structure of the Rb derivative. Moreover, the relationship between crystal structure and magnetic behaviour has been investigated in the AMnF4 (A=Na, K, Rb, Cs) series. The sign of the isotropic magnetic interaction is studied as a function of the superexchange angle Mn-F-Mn and the degree of distortion of the octahedra.
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