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We have studied various spontaneous and magnetic-field-induced phase transitions in single crystals of multiferroic Mn 0.9 Co 0.1 WO 4 using magnetic and magnetoelectric measurements and neutron diffraction. Compared to pure MnWO 4 , our data consistently confirm that the anisotropic Co 2+ ions induce reorientation of the spin cycloid structure to the ac plane and reveal P a and P c components of spontaneous electric polarization. Field-induced phase transitions accompanied by anomalies of magnetic susceptibility and suppression of both P a and P c polarizations have been observed for H c (∼3 T) and H a (∼8.5 T). Neutron diffraction has revealed that in both cases the spin cycloid plane flops in direction almost perpendicular to H , i.e., close to the ab and bc planes, respectively. Parameters describing the magnetic structures including wave vectors, orientations of the main elliptical axes, etc., have been determined in all spontaneous and field-induced states.

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Evolution of competing commensurate collinear (AF4) and incommensurate cycloidal (AF2) magnetic structures in Mn 0.8 Co 0.2 WO 4 multiferroic was studied by neutron diffraction, magnetic, and pyroelectric characterization measurements. In contrast to pure and slightly Co doped MnWO 4 , the antiferromagnetic AF4 collinear phase [k 1 = ( Ferroelectric polarization along b axis was revealed below T FE in the low temperature conical phase resulting from the superposition of the AF4 and AF2 spin structures. The arrangement of the spins after the two successive magnetic transitions are thoroughly described. In particular, we found that spins in the AF4 phase are aligned along the easy direction in the ac plane (∼142• with respect to the c * axis), while the cycloidal AF2 spin order is developed in the magnetically hard directions, perpendicular to the easy one, and consequently the T FE decreases compared to the pure MnWO 4 .

A powdered La2CoMnO6 double perovskite was synthesized by the solid-state reaction method, and its crystal structure was investigated by (mode-crystallography) Rietveld analysis using X-ray and neutron powder diffraction data. La2CoMnO6 material is a monoclinic perovskite at room temperature, adopting the space group P21/n (a(-)a(-)b(+)), , c ≈ 2ap and Z = 2. The P21/n phase can be described effectively by three distortion modes (GM4(+), X3(+), X5(+)) of the Fm3[combining macron]m (a(0)a(0)a(0)) parent phase. The comparative study of the material and those in the Ln2CoMnO6 and Ln2NiMnO6 families has shown a general trend in nearly all the materials, has served to select a common direction in the sub-space spanned by X5(+), tri-linearly coupled to the order parameters of the cubic to monoclinic first order phase transition. This direction has been used to parametrize the refinements and to perform reliable refinements in the high-temperature intermediate distorted trigonal phase, R3[combining macron] (a(-)a(-)a(-)), for which only one effectively acting irrep has been deduced: GM5(+), physically a tilt of the oxygen sharing octahedra of Co and Mn. Its temperature evolution up to the prototype cubic phase has been fitted in the framework of the Landau Theory of Phase Transitions, showing a behavior typical of a tricritical point. The low-temperature neutron powder diffraction data have served to solve the magnetic structure: three indistinguishable ferromagnetic models with the space groups P21/n and P2/n' are proposed.

The structural and magnetic properties of a series of ordered double perovskites with the formula Sr(2)Co(1-x)Mg(x)TeO(6) (x = 0.1, 0.2 and 0.5) are investigated by X-ray diffraction, low temperature neutron diffraction, electron paramagnetic resonance and magnetic susceptibility. The progressive substitution of the paramagnetic Co(2+) high spin ion by the diamagnetic Mg(2+), of about the same size, induces changes in the room temperature crystal structure, from a distorted P2(1)/n phase for the undoped Sr(2)CoTeO(6) oxide to the I4/m of the end member (Sr(2)MgTeO(6)). These perovskites experience structural transitions on heating, the temperature at which the transitions occur being smaller as x increases. The novel approach of mode-crystallography is used for the analysis. All oxides show antiferromagnetic exchange interactions between Co(2+) ions but the long range antiferromagnetic order is not achieved for the phase with x = 0.5. The low temperature neutron diffraction data have been evaluated using a full symmetry analysis. Results are consistent with an unquenched orbital contribution of a high spin Co(2+) ion.

We investigate the magnetic-field-induced (H b) transition in the reference improper multiferroic MnWO 4 using superspace formalism. The distinct superspace group symmetries are determined from single-crystal neutron diffraction. The study of the full magnetic symmetry of the incommensurate magnetic phases under field provides the keys for understanding its magnetic and ferroelectric behavior up to 12 T. At moderate fields, below the transition, the magnetic modulations of the two Mn sites in the simple cell are independent. At the field-induced transition between multiferroic phases, the electric polarization undergoes a flip from the monoclinic b axis to the ac plane. An additional component in the polarization is predicted along the c axis, due to a point symmetry change from 2 y 1 to m y 1 . In contrast with the low-field phase, above the transition the influence of the magnetic field results in the coupling between the magnetic modulations of the two Mn sites of the original monoclinic cell.

In this report, the structural, magnetic and spectroscopic properties of the freeze-drying synthesized Sr2Ni1-xMgxTeO6 (x = 0.0, 0.1, 0.2, 0.3 and 0.5) oxides are analyzed by means of X-ray powder diffraction (XRPD) and neutron powder diffraction (NPD), electron paramagnetic resonance, diffuse reflectance and magnetic susceptibility. The XRPD and NPD data analysis using the mode-crystallography approach have revealed that at room temperature (RT), all the compositions are monoclinically distorted with the space group I2/m. The high and low temperature analyses have shown that these materials suffer a series of three structural phase transitions. The EPR results have shown that the spectra of all the compositions are centred at g≈ 2.28, indicating a slightly distorted octahedral environment of Ni(2+), which is in agreement with the crystal structure analysis. The increase of the Mg(2+) content in Sr2Ni1-xMgxTeO6, provokes a decrease of the dipolar interaction effects and thus, the resonance becomes narrower. This resonance does not completely disappear which leads to the idea that the long-range magnetic order is not completely established when x≥ 0.3. The substitution of the Ni(2+) (S = 1) ions by Mg(2+) (S = 0) ions, also induces a weakening of the antiferromagnetic interactions, which is reflected in the diminishing of the absolute value of θ and the Néel temperature TN. The magnetic structure determination revealed the existence of an antiferromagnetic coupling for x- and z-spin components of the nickel atoms.