A.-M. Lamarche

Low temperature magnetic behaviour of CuFeS2 from neutron diffraction data

Abstract Measurements of neutron diffraction spectra were made at temperatures 4.2, 25, 45, 65, 85, 150 and 300 K on powdered polycrystalline synthetic CuFeS2. Standard Rietveld profile analysis using GSAS showed that at all temperatures, a good fit to the data could be obtained with both the chemical and magnetic space groups taken as I 4 2d, and an antiferromagnetic configuration of the Fe spins, as reported previously by Donnay et al. However, graphs of lattice parameters a and c against temperature showed a distinct change in slope close to 50 K, indicating the presence of a transition. Graphs of integrated intensity of diffraction lines against temperature showed corresponding discontinuities in the case of magnetic and mixed nuclear-magnetic lines but not for nuclear lines, indicating that the transition was magnetic. Detailed calculations of predicted magnetic intensities showed that the intensity variations could be explained by Cu spins, having a paramagnetic arrangement down to 50 K and then ordering to an antiferromagnetic form at lower temperatures. The analysis gave a value of ∼ 0.05 μB for the magnetic moment of the Cu ions.

Crystal structures of I2 · Mn · IV · VI4 compounds

Polycrystalline samples of 18 quaternary compounds of the form I{sub 2} {center dot} Mn {center dot} IV {center dot} VI{sub 4} with I = Cu,Ag, IV = Si,Ge,Sn, and VI = S,Se,Te were made from the elements using the melt and anneal technique. By using various annealing temperatures, it was shown that while the compounds involving I = Cu, IV = Si and Ge, and VI = S and Se showed only one structure which was stable down to room temperature, for most of the other compounds, two different structures could be found, depending upon the annealing temperatures used. Also for all of the Te compounds, annealing or slow cooling below 400C produced a two- or multiphase condition. However, single-phase conditions produced by rapid quenching from higher temperatures were quasistable at room temperature. Lattice parameter values were determined for all of the single-phase forms and values were calculated for the effective lattice parameter a{sub e} (=(V/N){sup 1/3}). It was found that with a{sub e} plotted against the molecular weight W, five straight lines were obtained, each characteristic of a particular structure and magnetic behavior.

Magnetic behaviour of some Mn.III2.VI4 compounds and their alloys

Measurement of the magnetic susceptibility χ as a function of temperature were made on polycrystalline samples from the alloy systems Cd1−zMnzGa2Se4, Zn1−zMnzGa2Se4 and Cd1−zMnzIn2Te4 which had been subjected to various heat treatments. The 1/χ versus T curves indicated that for the ZnSe alloys, for all values of z, samples slowly cooled to room temperature were antiferromagnetic showing ideal Curie-Weiss behaviour, but for samples quenched from 700°C the behaviour was a mixture of antiferromagnetic and spin-glass. For the CdSe alloys, samples from the I42 m range (0.6 < z < 1.0) showed very similar behaviour, but in the I4 range (0 < z < 0.6) even the very slowly cooled samples showed a mixture of antiferromagnetic and spin-glass behaviour. For the CdTe alloys, all samples, however heat-treated, showed spin-glass form. Values of the Curie-Weiss constant Ф were determined from all of the 1/χ versus T curves, and by comparison with the T(z) phase diagrams for the different alloy systems, the values of Ф were correlated with the ordering of the Mn atoms on the cation lattice. It is shown that the experimental values of Ф can give a very convenient way of determining the type of ordering and the degree of order in such alloys.

Magnetic susceptibility and exchange interaction parameters for some Mn.III2.VI4 compounds

Abstract Measurements of magnetic susceptibility χ as a function of temperature were made on polycrystalline samples of the compounds MnGa 2 Se 4 , MnIn 2 Te 4 , MnGa 2 Te 4 and MnIn 2 Se 4 . From the 1 / χ versu that MnGa 2 Se 4 was antiferromagnetic but the other three compounds showed spin-glass behaviour. This is consistent with the known crystal structures, since in MnGa 2 Se 4 the Mn atoms are ordered while in MnIn 2 Te 4 they are disordered on the cation sublattice. It is concluded that similar disorder occurs in MnGa 2 Te 4 and MnIn 2 Se 4 . Values of the critical temperature T N and the Curie-Weiss θ were determined for each compound. In the cases of MnGa 2 Se 4 and MnIn 2 Te 4 , analysis was carried out in terms of a simple mean-field theory and using the virtual transition model of Geertsma et al. for exchange interaction, and values of exchange interaction parameters determined from the measured T N and θ data.

Neutron diffraction study of magnetic phases in polycrystalline Mn2SiS4

Abstract A narrow region of spontaneous magnetization was found to occur in the synthetic olivine Mn 2 SiS 4 between 83 and 86 K. Neutron diffraction analysis shows that for the manganese ions there is antiferromagnetic spin alignment along the crystallographic a -axis, with opposing ferromagnetic c -components on two sites of different symmetries, inversion and mirror. Below 83 K the analysis shows canted antiferromagnetism progressing to collinear alignment on the b -axis at 4.2 K.

Magnetic behaviour of some I2·Mn·IV·VI4 compounds

Abstract Polycrystalline samples of quaternary compounds I 2 · Mn · IV · VI 4 , with I = Cu , Ag , IV = Si , Ge , Sn and Vi = Se , Te , were prepared from the elements using the melt and anneal technique. The majority of these compounds can take more than one structure, depending upon heat treatment, with subcell symmetry of orthorhombic, tetragonal or cubic form, so that a total of 19 phases could be investigated. Because of this behaviour, in some cases preparation involved quenching of a high-temperature phase to a quasistble state at room temperature. For each sample, measurements of magnetic susceptibility χ were made as a function of temperature in the range 4–250 using SQUID magnetometer. The resulting data indicated that seven of these phases were antiferromagnetic while the other 12 showed ferrimagnetic behaviour. A value for the Neel temperature T N was obtained in every case, but because of the presence of small amounts of other phases in some of the quenched samples, values of other parameters such as the Curie-Weiss Θ a , etc. could be obtained in only 11 cases. It was found that when T N and Θ a were plotted against the effective lattice parameter a e , the phases were divided into five sets, each set being characterized by a particular crystal structure and magnetic behaviour and showing a linear variation of these magnetic parameters with a e .

Low temperature magnetic behaviour of CuFeSe2 from neutron diffraction data

Abstract Measurements of neutron diffraction spectra were made at temperatures 4.2, 60, 75, 90, 150 and 300 K on powdered synthetic CuFeSe2. The data showed that a magnetic transition occurs in CuFeSe2 at approximately 80 K, graphs of integrated intensity for the magnetic lines extrapolating to zero at this temperature. Rietveld analysis using GSAS showed that at the temperatures above 80 K, a good fit to the data could be obtained with a tetragonal unit cell and the space group taken as P 4 2c, as reported previously by Delgado et al. For temperatures below 80 K, the positions of the magnetic lines indicated that the magnetic unit cell was larger, with the a lattice parameter twice that of the chemical unit cell. GSAS analysis including magnetic contributions showed that the Fe magnetic moments had components along all three reference axes, giving the overall space group as triclinic, P1. The resultant magnetic moment has a magnitude of ∼ 1.75 μB and a direction at an angle of ∼ 15° to the y—z plane. The Fe atoms on the 2e and 2a sites (of the tetragonal cell) have slightly different magnetic moments, resulting in a weak ferrimagnetic behaviour, as observed from magnetic susceptibility measurements. Analysis of the nuclear lines below 80 K indicated that the Debye-Waller factor Uiso fell rapidly below the magnetic transition. Conversion to a Debye θD showed that θD increased by ∼25% below the transition, and this is attributed to a large value of ∂J/∂a in this material.

Magnetic properties of MnGa2Se4 in the temperature range of 2–300K

Measurements of low field static magnetic susceptibility and of magnetization with pulsed magnetic fields up to 32T have been made as a function of temperature on polycrystalline samples of the compound MnGa2Se4 which has a tetragonal structure. The resulting data have been used to give information on the magnetic spin-flop and magnetic saturation transitions. It has been found that MnGa2Se4 has a zero-field Neel temperature TN of 8.1K and shows a triple point at (7.8K, 2.2T). Details of the magnetic B-T phase diagrams were determined for the phases and the results compared with the predictions of theoretical uniaxial models. The susceptibility χ(T) curves for B=3 and 5T show magnetothermal effects below 4.5K.

Magnetic spin-flop and magnetic saturation in Ag2FeGeSe4, Ag2FeSiSe4 and Cu2MnGeSe4 semiconductor compounds

Measurements of magnetization (M) at helium temperatures and with pulsed magnetic fields up to 32 T, have been made on polycrystalline samples of the three compounds Ag 2 FeGeSe 4 , Ag 2 FeSiSe 4 and Cu 2 MnGeSe 4 . All three compounds have orthorhombic symmetry, with the wurtz-stannite structure, and have semiconductor properties. The resulting curves of M versus B have been used to give information on the magnetic spin-flop and magnetic saturation transitions. The observed values of the spin-flop field B f differ appreciably for the three cases, with mean values of 0.35 T for Ag 2 FeSiSe 4 , 4.0 T for Cu 2 MnGeSe 4 and 13.5 T for Ag 2 FeGeSe 4 . The saturation behavior is also very different from one compound to another. In the case of Ag 2 FeSiSe 4 , the magnetization curve at low fields ( < 3 T) is quite different for the cases of pulsed field and steady field measurements. This is attributed to domain effects, with Ag 2 FeSiSe 4 showing weak ferromagnetic behavior because of crystallographic spin-canting. This effect can also be the cause for the very low value of B f in this compound.

Crystallographic, Electrical and Magnetic Properties of the Cu2FeGeSe4 Compound

Measurements of X-ray powder diffraction at room temperature, of electrical transport in the temperature range from 100 to 300 K, of magnetic susceptibility χin the range from 2 to 300 K and of magnetization M as a function of applied magnetic field H at a number of fixed temperatures were made on polycrystalline samples of Cu2FeGeSe4. The analysis of the X-ray diffraction lines show that Cu2FeGeSe4 has the tetragonal stannita structure with lattice parameter values of a = 5.60078 A and c = 11.05609 A. From the analysis of the electrical data, the values of the hole binding energy EA and the density of states effective mass of the holes mp are estimated. The resulting χvs. T data show that Cu2FeGeSe4is antiferromagnetic with a N�el temperature TN = 20 K and that a second transition occurs at ≈8 K indicating a transition to weak ferromagnetic state. The magnetization and susceptibility results obtained on Cu2FeGeSe4 show that at all temperatures below ≈70 K bound magnetic polarons (BMPs) occur, in the paramagnetic, antiferromagnetic and weak ferromagnetic ranges. The number of BMPs remained practically constant with temperature having a mean value of 6.55 × 1018 cm—3. Using a simple spherical model, this gives the radius of a BMP as 12.0 A.