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Magnetic nanofibers are of great interest in basic research, as well as for possible applications in spintronics and neuromorphic computing. Here we report on the preparation of magnetic nanofiber mats by electrospinning polyacrylonitrile (PAN)/nanoparticle solutions, creating a network of arbitrarily oriented nanofibers with a high aspect ratio. Since PAN is a typical precursor for carbon, the magnetic nanofiber mats were stabilized and carbonized after electrospinning. The magnetic properties of nanofiber mats containing magnetite or nickel ferrite nanoparticles were found to depend on the nanoparticle diameters and the potential after-treatment, as compared with raw nanofiber mats. Micromagnetic simulations underlined the different properties of both magnetic materials. Atomic force microscopy and scanning electron microscopy images revealed nearly unchanged morphologies after stabilization without mechanical fixation, which is in strong contrast to pure PAN nanofiber mats. While carbonization at 500 &deg, C left the morphology unaltered, as compared with the stabilized samples, stronger connections between adjacent fibers were formed during carbonization at 800 &deg, C, which may be supportive of magnetic data transmission.

International audience; Magnetoelectric composites (CoFe2O4) x-(BaTiO3)1-x with x = 0.1, 0.2, 0.3, 0.4 and 0.5 were prepared by a polyol mediated synthesis route. The densification step was performed in a reducing forming gas atmosphere at 1250 1C. Under these conditions Co 2+ and Fe 3+ are reduced to the corresponding metals and thus a reaction of these elements with the BaTiO 3 matrix is prevented. Reoxidation in air to CoFe 2 O 4 at a rather low temperature of 800 °C leads to samples free of secondary phases. While the course of the synthesis was monitored by TGA, XRD and photometric analysis, the final composites were characterized via SEM, EDX, impedance spectroscopy and magnetic and magnetoelectric (ME) measurements. All samples show pronounced ME hystereses and a ME maxima dependent on the H dc field sweep direction. The sample with x = 0.4 exhibits the highest maximum a ME of 420 mV Oe-1 cm-1 .

It was investigated NiMg 0.1 M x Fe 2 O 4 ferrite where M stands for Mn, Co or simultaneously Mn and Co dopants. The concentration of M is 0.1 and it was divided by two in the sample with addition of Mn and Co. It was used the method of citrate precursors with 1100 °C calcination. The materials were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM) and reflectivity measures by waveguide method. The X-ray diffraction measures, with Rietveld refinement, present average crystallite sizes between 0.576 and 0.626 µm. The SEM analysis shows clustered particles smaller than 1 µm at 1100 °C, in agreement with Rietveld refinement. The compositions with Mn reach magnetization between 42.09 and 53.20 Am 2 /kg, which does not generate high microwave absorption. The 0.1 Co addition reached greater coercivity (2.96×10 −2 T), with up to 84% reflectivity at 10.17 GHz frequency. The Co material has high magnetocrystalline anisotropy, which is associated with the increase of coercive field, H c . The higher coercivity optimizes the reflectivity results.

Electronic and magnetic properties of the charge ordered phase of LuFe2 O4 are investigated by means of x-ray spectroscopic and theoretical electronic structure approaches. LuFe2 O4 is a compound showing fascinating magnetoelectric coupling via charge ordering. Here, we identify the spin ground state of LuFe2 O4 in the charge ordered phase to be a 2:1 ferrimagnetic configuration, ruling out a frustrated magnetic state. An enhanced orbital moment may enhance the magnetoelectric coupling. Furthermore, we determine the densities of states and the corresponding correlation potentials by means of x-ray photoelectron and emission spectroscopies, as well as electronic structure calculations. © 2009 The American Physical Society.

Hexagonal ferrites, or hexaferrites, are hugely important materials commercially and technologically, with common applications as permanent magnets, magnetic recording and data storage media, components in electrical devices operating at wireless frequencies, and as GHz electromagnetic wave absorbers for EMC, RAM and stealth technologies. Hexaferrites are all ferrimagnetic materials, and their magnetic properties are intrinsically linked to their crystalline structures, all having a strong magnetocrystalline anisotropy; that is the induced magnetisation has a preferred orientation within the crystal structure. They can be divided into two main groups: those with an easy axis of magnetisation (known as uniaxial), the hard hexaferrites, and those with an easy plane (or cone) of magnetisation (known as ferroxplana or hexaplana), soft ferrites. The common hexaferrite members are:M-type ferrites, such as BaFe12O19and SrFe12O19Z-type ferrites (Ba3Me2Fe24O41)Y-type ferrites (Ba2Me2Fe12O22)W-type ferrites (BaMe2Fe16O27)X-type ferrites (Ba2Me2Fe28O46)U-type ferrites (Ba4Me2Fe36O60)where Me = a small 2+ion such as cobalt, nickel or zinc, and Ba can be fully substituted by Sr. Generally, the M ferrites are hard, the Y, Z and U ferrites are soft, and the W and X ferrites can very between these two extremes, but all have large magnetisation (M) values.There is currently increasing interest in composite materials containing hexaferrite fibres. It had been predicted that properties such as thermal and electrical conductivity, and magnetic, electrical and optical behaviour will be enhanced in material in fibrous form. This is because a continuous fine fibre can be considered as effectively one-dimensional, and it does not behave as a homogeneously distributed solid. Although the intrinsic magnetisation of the material is unaffected, the effective magnetisation of an aligned fibre sample should be greater when a field is applied parallel with fibre alignment compared to when applied perpendicularly to fibre alignment. This feature was first demonstrated by the author for aligned hexaferrite fibres in 2006. This chapter will deal with progress in the manufacture and properties of hexaferrite fibres, from the first syntheses of BaM, SrM,Co2Y,Co2Z, Co2W, Co2X and Co2U micron-scale fibres by the author 12-15 years ago, to recent developments in M ferrite hollow fibres and nanofibres, and hexaferrite-coated CNTs (carbon nanotubes).The relative properties of all reported hexaferrite fibres are compared and summarised at the end of this chapter.

Spin transfer torque (STT) and tunnelling magnetoresistance (TMR) in magnetic tunnel junctions with ferromagnetic (F) leads are two essential underlying phenomena of modern spintronics. We present here a theoretical study of STT in antiferromagnet (AF) based tunnel junctions, where two AF metal electrodes are separated by a thin nonmagnetic insulating barrier. In particular, the behaviour of STT and TMR in epitaxial AF-based tunnel junctions is investigated using tight binding calculations in the framework of the Keldysh formalism. The spatial distribution of the STT out-of-plane component is found to be staggered, similar to the in-plane component. This behaviour is specific to the use of a tunnel barrier and significantly differs from the out-of-plane torques reported in previous works using a metallic spacer. Additionally, we show that unlike conventional ferromagnetic-based tunnel junctions, the TMR can increase with applied bias and reach values comparable to typical magnetoresistances found for usual spin valves.Next, the analysis carried out for AFs is extended to ferrimagnets (FI), for which AFs constitute simpler limiting cases. The additional magnetic complexity inherent to FI materials yields to a richer physics concerning the STT spatial behaviour in FI based tunnel junctions.Electronic structure parameters such as band widths and exchange splittings of the FI are shown to have a strong influence on STT. In particular, the STT spatial distribution within the leads exhibits a striking spin-modulated wave-like behaviour resulting from the interplay between the exchange splittings of the two FI sublattices. This wave-like behaviour can also be tuned via the applied voltage across the junction. Furthermore, the fundamental intrinsic parameter for quantifying STT characteristic lengths in FI metals is identified. This fundamental parameter can be considered as an effective exchange field in FIs, similar to the homogeneous exchange field in the F case.Finally, the STT characteristic lengths in AF materials are investigated experimentally. Here, room temperature critical depths and absorption mechanisms of spin currents in Ir20Mn80 and Fe50Mn50 are determined by F-resonance and spin pumping. In particular, room temperature critical depths are observed to be originated from different absorption mechanisms: dephasing for Ir20Mn80 and spin flipping for Fe50Mn50., En électronique de spin, le couple de transfert de spin (STT) et la magnétorésistance tunnel (TMR) dans les jonctions tunnel magnétiques à électrodes ferromagnétiques (F) sont deux phénomènes physiques essentiels. Dans cette thèse, nous présentons une étude théorique du STT dans des jonctions tunnel antiferromagnétiques (AF), où deux électrodes non-plus F mais AF sont séparées par une barrière isolante non-magnétique. Plus concrètement, les comportements du STT et de la TMR sont étudiés dans des jonctions tunnel AF cristallines, et ce, à l´aide de calculs de liaisons fortes dans le cadre du formalisme de Keldysh. Nous avons observé une distribution spatiale de la composante perpendiculaire du STT régulière et de signe alternatif, ce qui est similaire au comportement de la composante parallèle. Ces variations spatiales de la composante perpendiculaire sont cependant spécifiques à l'utilisation d'une barrière tunnel et contrastent avec les effets observés par le passé pour le cas de couches séparatrices métalliques. De plus, contrairement aux jonctions tunnel F conventionnelles, nous avons montré que la TMR peut augmenter avec la tension appliquée et atteindre des valeurs du même ordre de grandeur que pour des vannes de spin usuelles : tout-métallique et à électrodes F.L´analyse effectuée pour des AF est ensuite étendue aux matériaux ferrimagnétiques (FI), pour lesquels les AF constituent, somme toute, des cas limites. La complexité magnétique additionnelle inhérente aux FI se traduit par un comportement spatial du STT beaucoup plus riche dans les jonctions tunnel FI. Nous observons notamment que les paramètres électroniques tels que les largeurs et les décalages de bandes ont une très forte influence sur le STT. Plus particulièrement, la différence entre les couplages d'échange inter-spin locaux des deux sous-réseaux du FI donne lieu à une distribution spatiale du STT ondulatoire qui est modulée par la densité locale de spin. Il est possible d'ajuster cet effet en jouant sur la tension appliquée aux bornes de la jonction tunnel FI. Nous trouvons de plus que la différence entre les couplages d'échange inter-spin locaux constitue un paramètre fondamental pour la quantification des longueurs caractéristiques du STT dans les FIs. Ce paramètre peut être considéré comme un champ d´échange effectif, par similitude avec le cas usuel des Fs qui présentent un champ d´échange homogène.Pour finir, nous avons sondé expérimentalement les longueurs caractéristiques du STT dans des AFs. Pour de l'Ir20Mn80 et du Fe50Mn50, nous avons déterminé les longueurs de pénétration de spin et les mécanismes d'absorption de courants de spin à température ambiante en utilisant la résonance F et le pompage de spin. Plus précisément, nous avons associé les profondeurs de pénétration critiques à deux mécanismes d'absorption distincts: du déphasage pour l´Ir20Mn80 et du retournement de spin pour le Fe50Mn50.

Ba–Fe–Ti oxides are nowadays attracting considerable interest for the production of permanent magnets and microwave devices, due to their high dielectric constant. Among these materials, recently the quaternary ferrite Ba12Fe28Ti15O84 (BFT) was discovered to possess ferrimagnetic properties at room temperature with a main magnetic transition at about 420 K and complete disappearance of magnetisation above 700 K. In this study, we report for the first time on the Raman spectrum of BFT samples prepared with different methods. Raman spectra were recorded in dependence of temperature and a preliminary assignment of modes was attempted. Coupling the Raman results with previous magnetic studies allowed gaining more insight on the structural mechanism at play in correspondence of the main magnetic transition.

A surface acoustic wave (SAW) actuated rotary motor is reported here, consisting of a millimeter-sized spherical metal rotor placed on the surface of a lead zirconate titanate piezoelectric substrate upon which the SAW is made to propagate. At the design frequency of 3.2 MHz and with a fixed preload of 41.1 N, the maximum rotational speed and torque achieved were approximately 1900 rpm and 5.37 N-mm, respectively, producing a maximum output power of 1.19 W. The surface vibrations were visualized using laser Doppler vibrometry and indicate that the rotational motion arises due to retrograde elliptical motions of the piezoelectric surface elements. Rotation about orthogonal axes in the plane of the substrate has been obtained by using orthogonally placed interdigital electrodes on the substrate to generate SAW impinging on the rotor, offering a means to generate rotation about an arbitrary axis in the plane of the substrate. © 2011 American Institute of Physics.

In this paper we report results of structural, spectroscopic, and magnetic investigations of MgFe2O4 nanoparticles prepared by soft mechanochemical synthesis. MgFe2O4 nanoparticles crystallize in Fd3m space group with mixed cation distribution and reduced percentage of Fe3+ at tetrahedral (8a) sites. Discrepancy in the cation distribution compared to that in the bulk Mg-ferrite is one of the highest known. X-ray line broadening analysis reveals crystallite size and strain anisotropy. The saturation magnetization, M-sat=62 emu/g measured at 5 K is twice higher than that found in the bulk counterparts. Such high value of M-sat is attributed to the low value of cation inversion parameter (delta=0.69), to the core/shell structure of the nanoparticles and to the surface/volume ratio. Moumlssbauer spectrum collected at room temperature reveals ferrimagnetic ordering between Fe3+ ions in 8a and 16d sites, while zero-field-cooled (ZFC) and field-cooled (FC) M(T) measurements were shown superparamagnetic state above 350 K.

Electronic and magnetic properties of the charge ordered phase of LuFe2O4 are investigated by means of x-ray spectroscopic and theoretical electronic structure approaches. LuFe2O4 is a compound showing fascinating magnetoelectric coupling via charge ordering. Here, we identify the spin ground state of LuFe2O4 in the charge ordered phase to be a 2:1 ferrimagnetic configuration, ruling out a frustrated magnetic state. An enhanced orbital moment may enhance the magnetoelectric coupling. Furthermore, we determine the densities of states and the corresponding correlation potentials by means of x-ray photoelectron and emission spectroscopies, as well as electronic structure calculations.

Strain relaxation studies in epitaxial magnetite, Fe3O4, thin films grown on MgAl2O4(100) substrates are reported. The study shows that the films were relaxed in line with the theoretical model prediction with a critical thickness, t(c)=5 nm. Antiphase boundaries (APBs) are not expected to form in Fe3O4 films grown on MgAl2O4 substrates because both film and substrate have the same crystal symmetry. In contrast, our study reveals the formation of APBs within the Fe3O4 films. Our analysis shows that the APBs in a Fe3O4/MgAl2O4 heteroepitaxial system are formed by partial dislocations, which accommodate the misfit. This formation mechanism of APBs is fundamentally different from the one found in the Fe3O4/MgO system, where APBs are formed as a consequence of equivalent nucleation sites on the MgO substrate separated by nontranslational vectors of the Fe3O4 lattice. The mechanism for the formation of antiphase boundaries through partial dislocations should be applicable to a wide range of epitaxial systems having identical symmetries of the film and the substrate and significant lattice mismatch.

Combining density-functional theory calculations with many-body Green's-function technique, we reveal that the macroscopic magnetization in half-metallic antiferromagnets does not vanish at finite temperature as for the T=0 limit. This anomalous behavior stems from the inequivalent magnetic sublattices which lead to different intrasublattice exchange interactions. As a consequence, the spin fluctuations suppress the magnetic order of the sublattices in a different way leading to a ferrimagnetic state at finite temperatures. Computational results are presented for the half-metallic antiferromagnetic CrMnZ (Z=P,As,Sb) semi-Heusler compounds., Comment: 4 pages, 2 figures

MgO-based magnetic tunnel junctions with up to 230% tunnel magnetoresistance ratio at room temperature and up to 345% at 13 K are prepared. The lower electrode is either exchange-biased or free, while the top electrode is free or an exchanged-biased artificial ferrimagnet, respectively. Additionally, a pseudo-spin-valve (hard-soft switching) design with two unpinned electrodes is used. Inelastic electron-tunneling spectra for each of these systems show a strong variation in the zero-bias anomaly with a reduced peak for some of the junctions. At voltages around 200 mV additional structures are found, which are not known from junctions with lower magneto resistance, such as alumina-based junctions. We discuss the spectra for the different electrode types and compare our findings with respect to barrier material and magnetoresistance ratio.

Fe1.5Ti0.5O3± delta epitaxial thin films have been grown on alpha-Al2O3 (0001) substrates by pulsed laser deposition technique. The samples are both ferrimagnetic and semiconducting beyond room temperature. Oxygen pressure (PO2) during the deposition appears to be a critical parameter in promoting high temperature long range magnetic order and semiconducting properties. For all oxygen pressures, Fe1.5Ti0.5O3± delta thin films are single phase with twin epitaxy. High dc conductivity and low magnetization are obtained at low PO2, whereas high saturation magnetization and low conductivity stand for high PO2. Oxygen vacancies and/or change of iron valence state are pointed out to be responsible of these properties. Superexchange mechanism via oxygen bonds seems rather to dominate the magnetic properties especially for high PO2, whereas for low PO2 a double exchange mechanism might occur. Fe1.5Ti0.5O3± delta appears thus to be an interesting material for high temperature spintronics applications.

6 pages, 9 figures, 2 tables.-- PACS number(s): 75.30.Cr, 61.66.Dk, 75.50.Gg, 61.10.Nz, The compounds RNi2Mn (R=Tb, Dy, Ho, and Er) with a MgCu2-type structure have been synthesized. The R to transition metal atom ratio is confirmed to be 1:3 using the energy dispersive spectroscopy. The structural and magnetic properties have been investigated by various experimental methods. The x-ray diffraction patterns (XRD) can be well indexed with a cubic Laves cell and space group Fd3m. The refinement results of the XRD patterns show the presence of vacancies in the crystallographic structure. The ordering temperatures TC have been derived to be 131, 94, 75, and 50 K for R=Tb, Dy, Ho and Er, respectively, which are much higher than those of their corresponding RNi2 and RMn2 compounds. A large difference of M-T curves between zero-field-cooling and field-cooling magnetization for all samples at a certain temperature range is observed in a low field, which can be understood in the terms of narrow-domain-wall pinning and a sensitive temperature dependence of coercivity.

Single-phase polycrystalline powder samples of the double perovskite oxide LaSrMnCoO6 were synthesized by the Pechini (citrate-gel) technique. The structural, magnetic and electrical properties of the obtained powders were investigated by X-ray diffraction, electron microscopy, dc magnetization, ac susceptibility and dc resistivity measurements. The crystal structure of the new compound was found to be cubic of space group Fm3̄m at room temperature. Below 225K, the samples exhibit ferrimagnetic behavior with a spin-glass-like character. Resistivity measurements indicate semiconducting behavior with two different conductivity mechanisms: thermally activated behavior below 190K and variable range hopping above 190K. © 2003 Elsevier Science (USA). All rights reserved. 173 350 354 350-354

Under the terms of the Creative Commons Attribution (CC BY) license to their work., We report on the magnetic properties of the ordered bimetallic compound CoCo(NiEDTA)2⋅4H2O (abbreviated as [CoCoNi]). The structure consists of ordered bimetallic layers formed by alternating octahedral sites of Co and Ni(II); tetrahedral Co sites connect different Co‐Ni layers. We discuss the low‐dimensional ferrimagnetic behavior of this compound in terms of a model that assumes three spin sublattices exchange coupled by an Ising interaction., This work was supported by the Comision Interministerial en Ciencia y Tecnologia (Grant No. PB 85-OI06-C02-02), the Iustitucio Valenciana d'Estudis i Investigació, and the National Science Foundation (Grant No. DMR-8515224). F. S. andP. G. R. thank the Spanish Ministerio de Educacion y Ciencia for a predoctoral and postdoctoral fellowship, respectively.

In this research, a low temperature in-field 57Fe Mössbauer spectroscopy is used for investigation of anomalous magnetic properties of MnZn nanoferrite. Based on Mössbauer spectroscopy results, the reduced saturation magnetization of MnZn nanoferrite in respect to that of bulk sample is due to nonequilibrium cation distribution and spin disorder. The enhanced Curie temperature of MnZn nanoferrite in respect to that of bulk sample can be attributed to the strengthening of the (A)-O-[B] superexchange interactions due to an increase of the magnetic ion concentration in the (A) site.

Electronic and magnetic properties of the charge ordered phase of LuFe2O4 are investigated by means of x-ray spectroscopic and theoretical electronic structure approaches. LuFe2O4 is a compound showing fascinating magnetoelectric coupling via charge ordering. Here, we identify the spin ground state of LuFe2O4 in the charge ordered phase to be a 2:1 ferrimagnetic configuration, ruling out a frustrated magnetic state. An enhanced orbital moment may enhance the magnetoelectric coupling. Furthermore, we determine the densities of states and the corresponding correlation potentials by means of x-ray photoelectron and emission spectroscopies, as well as electronic structure calculations.

Magnetic torque (3≤T≤46 K), dc magnetic susceptibility (2≤T≤300 K), and ac magnetic susceptibility (2≤T≤30 K) are reported for PTMA0.5[Fe(Pc)(CN)2]･CH3CN. The torque curves exhibit a sinusoidal line shape for temperatures above 6 K. This result as well as the temperature dependence of the dc magnetic susceptibility above 13 K was analyzed by employing an anisotropic Heisenberg model in one dimension. The compound exhibits spontaneous magnetization below 6 K. The torque measurements at 3 K revealed that the direction of this spontaneous magnetization is parallel to the easy axis of the mother antiferromagnetism. The ac susceptibility measurements demonstrated the fluctuation of the spontaneous magnetization up to 13 K. A model of charge-ordered ferrimagnetism is proposed in order to explain these results. This model explains the spontaneous magnetization which commonly appears in conducting salts of [Fe(Pc)(CN)2] and their derivatives.

International audience; The Nd49Al13Ni38 amorphous alloy has been prepared by melt-spinning in the form of ribbons. Its magnetic properties have been investigated via superconducting quantum interference device magnetometry and its magnetic phase diagram was established. Hysteresis and temperature-dependent magnetization measurements show the occurrence of a reentrant spin-glass behavior on cooling. With increasing applied field, the spin-freezing temperature decreases and disappears at very high field, and the Curie temperature increases, broadening the temperature range of the ferromagnetic state. The resulting magnetocaloric effect was evaluated and compared to other interesting magnetic refrigerant materials.

Cr 2p core excited x-ray absorption and x-ray magnetic circular dichroism (XMCD) spectra of ferromagnetic Cr1-δTe with several concentrations of δ=0.11-0.33 and ferrimagnetic Cr5S 6 have been measured. The observed XMCD line shapes are found to very weakly depend on δ for Cr1-δTe, The experimental results are analyzed by means of a configuration-interaction cluster model calculation with consideration of hybridization and electron correlation effects. The obtained values of the spin magnetic moment by the cluster model analyses are in agreement with the results of the band structure calculation. The calculated result shows that the doped holes created by the Cr deficiency exist mainly in the Te 5p orbital of Cr1-δTe, whereas the holes are likely to be in the Cr 3d state for Cr5S6.

Les cristaux cubiques ferro ou ferrimagnétiques, présentant une anisotropie uniaxe, induite par traitement thermomagnétique, peuvent, à l'état vierge, être divisés en régions magnétiquement uniaxes. L'ignorance de cette division entraîne des erreurs sur la détermination de K1 qui peuvent aller jusqu'au changement apparent du signe de K1.

We report on the magnetic behavior of ‘‘rigid’’ ferrimagnetic chains isolated in bimetallic complexes of the EDTA and ‘‘flexible’’ ones obtained in the amorphous variety. As shown by LAXS, the only noteworthy difference in the amorphous state is the random distribution of bond angles between nearest neighbors within chains. The ‘‘rigid’’ bimetallic chains in CoNi(EDTA)6H2O are described in terms of Heisenberg model with an exchange coupling J=−7.5 K. The behavior of the amorphous variety somewhat differs, following the law X=AT−0.8 typical of REHAC. A classical spin chain model involving a J distribution and alternating g factors allows to explain successfully the temperature dependence of the susceptibility. Daniel.Beltran@uv.es

On decrit un ensemble de mesure de l'effet magneto-electrique, induit par un champ electrique, a des temperatures comprises entre 1,8 °K et 330 °K. On commente les courbes indiquant le comportement de la susceptibilite magneto-electrique de Cr2O3 et Fe1,15Ga0,85O 3 en fonction de la temperature.

Pronounced anisotropy of magnetic properties and complex magnetic order of a new oxi-halide compound Co7(TeO3)4Br6 has been investigated by powder and single crystal neutron diffraction, magnetization and ac susceptibility techniques. Anisotropy of susceptibility extends far into the paramagnetic temperature range. A principal source of anisotropy are anisotropic properties of the involved octahedrally coordinated single Co(2+) ions, as confirmed by angular-overlap-model calculations presented in this work. Incommensurate antiferromagnetic order sets in at TN=34 K. Propagation vector is strongly temperature dependent reaching k1=(0.9458(6), 0, 0.6026(5)) at 30 K. A transition to a ferrimagnetic structure with k2=0 takes place at TC=27 K. Magnetically ordered phase is characterized by very unusual anisotropy as well: while M-H scans along b-axis reveals spectacularly rectangular but otherwise standard ferromagnetic hysteresis loops, M-H studies along other two principal axes are perfectly reversible, revealing very sharp spin flop (or spin flip) transitions, like in a standard antiferromagnet (or metamagnet). Altogether, the observed magnetic phenomenology is interpreted as an evidence of competing magnetic interactions permeating the system, first of all of the single ion anisotropy energy and the exchange interactions. Different coordinations of the Co(2+)-ions involved in the low-symmetry C2/c structure of Co7(TeO3)4Br6 render the exchange-interaction network very complex by itself. Temperature dependent changes in the magnetic structure, together with an abrupt emergence of a ferromagnetic component, are ascribed to continual spin reorientations described by a multi-component, but yet unknown, spin Hamiltonian., Comment: 12 pages, 13 figures; submitted to PRB

The resonance properties of a new Cu2OSeO3 ferrimagnet have been investigated in a wide range of frequencies (17-142 GHz) at liquid helium temperature. The resonance data were used to plot the frequency-field curve of the ferromagnetic spectrum described in the model of an anisotropic two-sublattice ferrimagnet. The effective magnetic anisotropy corresponding to the gap in the spin wave spectrum was estimated (3 GHz). It was found that the spectrum has a multicomponent structure due to the diversity of the types of magnetization precession. As the amplitude of the high-frequency magnetic field increased, additional absorption was observed in an external magnetic field below the main resonance field. The addition absorption detected corresponds to a nonuniform nonlinear parametric resonance due to the nonuniformity of the magnetic structure in the ferrimagnetic crystal Cu2OSeO3.