A Mössbauer study of Pb4Fe3O8Cl,a three-layer magnet

The study of Mössbauer spectra from Pb₄Fe₃O₈Cl allows the observation of crystallographic sites associated with two kinds of magnetic layers present in the structure. Magnetic order appears simultaneously in the two sites below T_N at 602 K. The following features are observed (i) magnetic moments in a plane of easy magnetization perpendicular to the fourfold axis, (ii) antiferromagnetic behavior, as evidenced by the rotation of the magnetic moments under the effect of an external field, (iii) slightly different critical exponents β (0.311 ± 0.003 and 0.295 ± 0.003) for the two magnetic layers.     

Field dependence of uniform magnon energies in lamellar CoCl2 and CoBr2 by AFMR experiments

AFMR experiments in the frequency range 77<v<600 GHz and in the magnetic field range 0<H<75 kG on antiferromagnetic CoCl₂ and CoBr₂ single crystals at 4.2 K are reported. The waves propagate along the c axis of the crystals and the steady magnetic field is in the plane of the layers, which is also the easy plane of the spins.The influence of a rotation of the field in the easy magnetization plane is investigated, showing the existence of antiferromagnetic domains.The results of the field dependence of uniform magnon energies are reported in detail for the two compounds CoCl₂ and CoBr₂. In CoBr₂, the two magnon modes at zero field are non degenerate, their frequencies are v₁(0) = 60 GHzandv₂(0) = 675 GHz, and the field required for saturation is H_s⊥ = 74.2 kG.     

Magneto-optical studies on ferromagnetic stripe domains in K2CuF4

Ferromagnetic stripe domains are observed in K₂CuF₄ below T_c=6.19 K by use of Faraday rotation (FR). They are about 4 μm wide and lie parallel to the c-planes. They are due to a very small intraplanar anisotropy field as a result of the minute orthorhombic lattice distortion observed by Hidaka and Walker. A stripe domain model, including closure domains for complete demagnetization, is in accordance with the experimental observations if the anisotropy fields H^out_A=1.4kOe and Hⁱⁿ_A=0.1 Oe are introduced. Long-range order, presumably induced by interplanar exchange, is a accompanied by 3d Ising critical behavior as verified by critical exponents obtained from different magneto-optical measurements (α=0.07, β=0.33 and δ=4.6). Changes of the domain structure, of the ac-plane FR, and of the ac-plane linear birefringence due to a transverse magnetic field H 6 a are discussed. A non-vanishing internal field is preceded by a domain instability. The compatibility of the domain structure with neutron scattering results of Hirakawa and Yoshizawa is discussed.     

Magnetic superconductors of HoMo6S8 type: The effect of magnetic field and supercurrent

We study the magnetic superconductors for which the second order phase transition to the ferro-magnetic state would take place at the temperature θ_c ? T_c1 if the superconductivity was absent. In this case the inhomogeneous magnetic structure of domain-like type (DS phase) appears in superconductors below T_M ≈ θ_c. The effect of the magnetic field H on DS phase is analysed and the region of DS phade existence in the plane (H, T) is found. The new peaks 2nQ (n is integer) should appear in the neutron scattering in the presence of the magnetic field. The wavevector Q of magnetic structure decreases with the growth of magnetic field or supercurrent.     

Triangular spin configuration and weak ferromagnetism of Mn3Sn and Mn3Ge

Polarized-neutron diffraction by Mn₃Sn and Mn₃Ge in magnetic field reveals that the spin triangle coincides with the c-plane, is the inversion of the atomic triangle, and rotates opposite to the rotation of the c-plane component of magnetic field. Theory predicts that the inverted spin triangle is free to rotate and the associated weak ferromagnetic moment rotates oppositely.     

Glassy-ferromagnetic behavior in Eu0.5Sr0.5CoO3

Polycrystalline perovskite cobalt oxide Eu_0.5Sr_0.5CoO₃ was prepared by the conventional solid-state reaction method. X-ray powder patterns indicated the prepared samples are pure, cubic perovskite structure (Pm3?m), and with no evidence of any secondary phases. The dc magnetization and ac susceptibility measurements were carried out to investigate the magnetic properties of the sample, and which indicated that cluster-glasses properties are suppressed with the increasing of the coercive field. We denied the possibility of spin-glasses and the existence of the Hopkinson effect in Eu_0.5Sr_0.5CoO₃ through the temperature-dependent ac susceptibility measurements, and explained the magnetic behavior of Eu_0.5Sr_0.5CoO₃ with the competition between magnetic anisotropy and the external magnetic field.     

Effect of inhomogeneity on magnetic, magnetocaloric, and magnetotransport properties of La0.6Pr0.1Ca0.3MnO3 single crystal

The effect of magnetic inhomogeneity on magnetic, magnetocaloric, and transport properties of the colossal magnetoresistance manganites with first order ferromagnetic-to-paramagnetic phase transition is studied. The experiments were performed on the single-crystalline samples of La_0.6Pr_0.1Ca_0.3MnO₃. The inhomogeneity is described by the Curie temperature distribution function, which is found from the magnetization data. The temperature dependence of the magnetic field induced change in the entropy is shown to be determined by the distribution function and the shift of the transition temperature in a magnetic field. Similarly, magnetoresistance in the transition region is determined by the resistivity at H=0 and the shift of the transition temperature. The maximum entropy change as well as maximum magnetoresistance can be achieved in the magnetic field of order δT_C/B_M where δT_C is the transition width and B_M is the rate of change of the Curie temperature with magnetic field.Our approach to analysis of the effects of inhomogeneity is general and therefore can be used for all compounds with the first order magnetic phase transition.     

Magneto-optical study of the 5ƒ2 → 5ƒ16d1 transition in UO2

High field Faraday rotation and magnetic circular dichroism measurements in the energy range between 1.4 and 5.4eV are reported for UO₂. The measurements are compared with two modified atomic models for the 5?² → 5?¹6d¹ transition using LS- and jj-coupling, respectively. Excellent agreement between theory and experiment is found only for a jj-coupling treatment of the final state.The Faraday rotation for magnetic saturation is estimated to be 3 × 10⁶ deg cm^?1 at 4.5 eV.     

Exchange bias effect in multiferroic Eu0.75Y0.25MnO3

The exchange bias phenomenon has been investigated in multiferroic Eu_0.75Y_0.25MnO₃. The material shows a weak ferromagnetism with cone spin configuration induced by external magnetic field below 30 K. Consequently, the electric polarization coming from the cycloid spin order below 30 K can be suppressed by external magnetic fields. The magnetic hysteresis loops after cooling in a magnetic field exhibit characteristics of exchange bias below the spin glassy freezing temperature (T_g)∼16 K. The exchange bias field, coercivity field, and remanent magnetization increase with increasing cooling magnetic field. The exchange bias effect is ascribed to the frozen uncompensated spins at the antiferromagnetism/weak ferromagnetism interfaces in the spin-glass like phase.     

Rotation des moments magnetiques dans BaLa2 Fe2 O7

In the compound BaLa₂Fe₂O₇, below 235 K, spins are oriented along the diagonal of the quadratic cell in an antiferromagnetic configuration. Above this temperature we observe by neutron diffraction a continuous rotation of the magnetic moments in the base plane xyO, the configuration remaining antiferromagnetic; the magnetic symmetry goes from I_pm′mm′ to P2′/m Shubnikov's group. At the transition temperature we observe a discontinuity of the thermal expansion coefficient. This phenomenon can be interpreted in agreement with Landau's theory of second order transitions.     

Field induced phase transitions on NdRhIn5 and Nd2RhIn8 antiferromagnetic compounds

In this work, we have investigated the low temperature magnetic phase diagram of the tetragonal NdRhIn₅ and Nd₂RhIn₈ single crystals by means of temperature and field dependent heat capacity and magnetic susceptibility measurements. These compounds order antiferromagnetically with a Néel temperature (T_N) of 11 and 10.7 K for NdRhIn₅ and Nd₂RhIn₈, respectively. The constructed magnetic phase of both compounds are anisotropic and show, as expected, a decrease of T_N as a function of the magnetic field for c crystallographic direction. However when the magnetic field is applied along of the c-axis, which is the magnetic easy axis, first-order-like field induced transitions are observed within the antiferromagnetic state. We compare the phase diagrams obtained for NdRhIn₅ and Nd₂RhIn₈ with those for their cubic relative NdIn₃.     

Contact and non-contact magnetoimpedance in amorphous and nanocrystalline Fe73.5Si13.5B8CuV3Al ribbons

The giant magnetoimpedance (GMI) effect in amorphous and nanocrystalline Fe_73.5Si_13.5B₈CuV₃Al ribbons has been studied as a function of a dc magnetic field, by contact and non-contact methods. In the contact method, an MI of 19% was obtained in the nanocrystalline ribbon while it is of the order of 10⁴ in the non-contact method. The huge sensitivity of the non-contact method is promising with regard to the increase of the sensitivity of the MI sensors. Field dependence of MI has shown double peak behavior in contact method and single peak profile in non-contact method. The domains were found to lie normal to the plane of the ribbon, through MFM studies. All the experimental results were discussed using the Polivanov model.     

Magnetic field effect on the susceptibility of a Heisenberg ferromagnet CuK2Cl4.2H2O near the critical point

The results on the investigation of the susceptibility for the Heisenberg ferromagnet CuK₂Cl₄.2H₂O in the magnetic field are reported. The susceptibility divergence at the critical temperature in the magnetic field is shown to transform into susceptibility anomalies of two types, their shifts being approximated by the power functions with indices ω = 2.6 and ? = 0.58. The experimental data support the assumption about the complex critical temperature. The regions for the existence of phases with uniform and non-uniform magnetizations are determined.     

Anomalous magnetization processes and non-symmetrical domain wall displacements in L10 FePt particulate films

Anomalous magnetization processes and non-symmetrical domain wall displacements in the minor loop of L1₀ FePt particulate films were investigated by magnetization measurements and in situ magnetic force microscopy. Magnetization (M) decreases dramatically on increasing the magnetic field to ∼3 kOe after which M becomes small and constant in the range of 5–20 kOe as observed in the successive measurement of minor loops. The domain wall displacement is non-symmetrical with respect to the field direction. The anomalous magnetization behavior was attributed to the non-symmetrical domain wall displacement and large magnetic field required for domain wall nucleation. Energy calculations from modeling suggest that non-symmetrical domain wall displacement is caused by the existence of metastable domains in which the domain edges are stuck to the particle boundaries.     

Giant strain associated with microstructure control by magnetic field in Fe-31.2Pd, CoO and Nd0.5Sr0.5MnO3

We have made in situ optical microscope observation for the microstructure control driven by magnetic field in Fe-31.2Pd (at%), CoO and Nd_0.5Sr_0.5MnO₃. These materials exhibit structural transitions, and their low-temperature phases are composed of several crystallographic domains (variants), which are separated by twinning planes. In the case of ferromagnetic Fe-31.2Pd and antiferromagnetic CoO, the magnetic field promotes the twinning plane movement. This movement gives a large strain of several percent and is essentially explained by the fact that the magnetic shear stress, which corresponds to the magnetic anisotropy energy divided by the twinning shear, is larger than the twinning stress. In the case of Nd_0.5Sr_0.5MnO₃, the twinned microstructure of the charge-ordered phase disappears under a magnetic field in association with the melting of the charge-ordered phase.     

Magnetic and transport properties of Pr2Pd3Si5

We have investigated the magnetic and transport properties of a new ternary intermetallic compound Pr₂Pd₃Si₅ which forms in U₂Co₃Si₅-type orthorhombic structure (space group Ibam). At low field (0.01 T) magnetic susceptibility exhibits an abrupt increase below 7 K and peaks at 5 K, revealing a magnetic phase transition. The onset of magnetic order is also confirmed by well defined anomalies in the specific heat and electrical resistivity data. Apart from the sharp λ-type anomaly, magnetic part of specific heat also shows a broad Schottky-type hump due to crystal field effect. Magnetoresistance data as a function of temperature exhibits a pronounced peak in paramagnetic state which could be interpreted in terms of crystal field effect and short-range ferromagnetic correlations.     

Etude par effet Mössbauer de la briartite (Cu2FeGeS4)

Mössbauer experiments on Cu₂FeGeS₄ have shown that the ground orbital level of the Fe²⁺ ions is |L_z=0>, which corresponds to the existence of an easy plane of magnetization perpendicular to the tetragonal axis 0z; in the antiferromagnetic phase the magnetic moments actually lie inside this plane. The tetragonal splitting of the ground orbital doublet ₃ is found to be about 1430 cm^?1, and the Néel temperature is 12,3±0,3 K. A. self consistent calculation of molecular field is used to explain the order of magnitude of the hyperfine field observed at low temperature (H_hf=167±2 kOe at 4,2 K).     

Fine structure of the field-induced magnetic transition in Nd0.1La0.9Fe11.5Al1.5

For the Nd_0.1La_0.9Fe_11.5Al_1.5 compound, the fine structure of the magnetic transition from the ferromagnetic (FM) to the antiferromagnetic (AFM) states has been studied carefully by means of magnetization (M) and heat capacity (C_p) measurements. Although a single phase with the cubic NaZn₁₃-type structure (Fm3c) has been proved by the room temperature X-ray diffraction pattern, the phase transition has been clearly found to be a stepwise process in M(T) and C_p(T) curves under proper fields. Due to the strong competition between the FM order and AFM order, the characteristic is especially evident under low fields, weakens gradually with the increasing applied field and finally vanishes when the field is higher than 2 T. This multi-step magnetic transition results from the inhomogeneity of the sample, probably due to the inhomogeneous distribution of Nd atoms.     

Raman study of NH4NO3 and ND4NO3—250–420K

Raman spectra of NH₄NO₃, and ND₄NO₃, were studied from 250 to 420K. The results show that there are four phases separated by first order transitions. No evidence of the previously reported phase II' was observed.The present results combined with the results of other experiments present the following picture of the state of order of the molecules.In phase I, the highest temperature phase, the NH₄⁺ groups are in a free rotation and the nitrate groups are likely in random reorientation among 12-equivalent positions. In phase II, the NH₄⁺ groups are likely in rapid random reorientation under the local force field of S₄ symmetry. The nitrate groups are in hindered rotation but are disordered with one of the O-N bonds directed in one sense or the other along the c-axis. In phase III, the absence of the librational mode indicates that the NH₄⁺ groups are in nearly free rotation but the rotational motion is restricted by the local force field of C₃ symmetry. The nitrate groups are probably ordered as suggested by the well polarized character of the modes associated with the nitrate groups. In phase IV, the nitrate groups are ordered with their molecular planes perpendicular to the b-axis. The NH₄⁺ groups are in orientational disorder but may undergo bindered rotations. An optical mode was observed to couple to an anomalous mode which is believed to be a zone edge acoustical mode.