A Mössbauer study of FeC2O4·2D2O below the néel temperature

From low-temperature Mössbauer measurement on FeC₂O₄·2D₂O the reported difference in quadrupole splitting from the simple dihydrate is inferred to be due to lattice effects. The Zeeman split spectrum has been analyzed taking into account the ambiguity problem and the hyperfine parameters were determined to be I.S.=1.22 mm/sec; Q.S.=1.93 mm/sec; =0.65 to 0.72; =90 to 83.1^o and =0 to 11.8^o. The principal electric field gradient axis lies along the crystal a-axis with V_YY and lying along the crystal b-axis. The crystal field parameters 10Dq, Ds and Dt have been determined to be 10500, 185 and 211 cm^–1, respectively.     

The influence of internal strain on the Néel temperature of rare earth metals

Measurements of the thermal expansion and elastic constant C₃₃ of single crystal 50% TbHo near the Néel temperature (T_N) indicates that T_N increases with the increase in internal strain in the sample. After repeated thermal cycling or direct straining of the sample this effect on T_N is irreversible.     

Néel temperature of quasi-low-dimensional Heisenberg antiferromagnets

The Néel temperature T(N) of quasi-one- and quasi-two-dimensional antiferromagnetic Heisenberg models on a cubic lattice is calculated by Monte Carlo simulations as a function of interchain (interlayer) to intrachain (intralayer) coupling J(')/J down to J(')/J approximately = 10(-3). We find that T(N) obeys a modified random-phase approximationlike relation for small J(')/J with an effective universal renormalized coordination number, independent of the size of the spin. Empirical formulas describing T(N) for a wide range of J(') and useful for the analysis of experimental measurements are presented.     

On the mechanism of the temperature evolution of the symmetric magnetic hyperfine structure of Mössbauer spectra of magnetic nanoparticles to the quadrupole doublet of lines

The multilayer relaxation model of Mössbauer spectra of an ensemble of single-domain particles has been generalized to the case of the presence of the electric field gradient on nuclei with a chaotic orientation of its principal axes. The generalized model makes it possible to take into account the physical mechanisms of the formation of the hyperfine structure of the spectra in the real situation and to numerically describe the qualitative features of the temperature evolution of the spectra from symmetric magnetic sextet to the quadrupole doublet of lines, which is observed in most experimental spectra of ⁵⁷Fe nuclei in magnetic nanoparticles.     

Evolution of magnetic domain structure of martensite in Ni–Mn–Ga films under the interplay of the temperature and magnetic field

Ferromagnetic shape memory Ni-Mn-Ga films with 7M modulated structure were prepared on MgO （001） substrates by magnetron sputtering. Magnetization process with a typical two-hysteresis loop indicates the occurrence of the reversible magnetic field-induced reorientation. Magnetic domain structure and twin structure of the film were controlled by the in- terplay of the magnetic and temperature field. With cooling under an out-of-plane magnetic field, the evolution of magnetic domain structure reveals that martensitic transformation could be divided into two periods： nucleation and growth. With an in-plane magnetic field applied to a thermomagnetic-treated film, the evolution of magnetic domain structure gives evidence of a reorientation of twin variants of martensite. A microstructural model is described to define the twin structure and to produce the magnetic domain structure at the beginning of martensitic transformation; based on this model, the relationship between the twin structure and the magnetic domain structure for the treated film under an in-plane field is also described.     

Monte Carlo simulation of the néel relaxation of a single-domain particle

The magnetization reversal kinetics in a single-domain particle with uniaxial magnetic anisotropy was simulated using the Monte Carlo method. The algorithm developed here yields good agreement with the analytical estimates for superparamagnetic relaxation and qualitatively describes some features of magnetic behavior of fine-particle systems.     

Dynamic-susceptibility studies of the interplay between the Néel and Brown magnetic relaxation mechanisms

We have used temperature- and frequency-resolved ac-susceptibility measurements to investigate the magnetic relaxation of a Co_0.2Fe_2.8O₄ magnetic fluid above the freezing point of the liquid carrier. Our data show that both the Néel and the Brown relaxation mechanisms are operative at temperatures in the vicinity of the out-of-phase (imaginary) susceptibility peak. We separate the contributions of the two mechanisms to the overall relaxation time, and demonstrate that Brownian relaxation plays a dominant role at all temperatures within this high-dissipation regime.     

A review of: “Comparative crystal chemistry: temperature,pressure, composition and the variation of crystal structure”

By R. M. Hazen and L. W. Finger. Wiley (1982). Price: £19.50 (U.K.)     

Complex magnetic differentiation of cobalts in Na x CoO2 with 22 K Néel temperature

Single crystals of sodium cobaltates Na _x CoO₂ with x ≈ 0.8 were grown by the floating zone technique. Using electrochemical Na de-intercalation method we reduced the sodium content in the as-grown crystals down to pure phase with 22 K Néel temperature and x ≈ 0.77. The ⁵⁹Co NMR study in the paramagnetic state of the T _N = 22 K phase permitted us to evidence that at least 6 Co sites are differentiated. They could be separated by their magnetic behavior into three types: a single site with cobalt close to non-magnetic Co³⁺, two sites with the most magnetic cobalts in the system, and the remaining three sites displaying an intermediate behavior. This unusual magnetic differentiation calls for more detailed NMR experiments on our well characterized samples.     

Optical evidence for symmetry changes above the Néel temperature of KCuF3

We report on optical measurements of the 1D Heisenberg antiferromagnet KCuF3. The crystal-field excitations of the Cu2+ ions have been observed and their temperature dependence can be understood in terms of magnetic and exchange-induced dipole mechanisms and vibronic interactions. Above TN we observe a new temperature scale TS characterized by the emergence of narrow absorption features that correlate with changes of the orbital ordering as observed by Paolasini et al. [Phys. Rev. Lett. 88, 106403 (2002)]. The appearance of these optical transitions provides evidence for a symmetry change above the Néel temperature that affects the orbital ordering and paves the way for the antiferromagnetic ordering.     

Field-induced Néel vector bi-reorientation of a ferrimagnetic insulator in the vicinity of compensation temperature

The spin Hall magnetoresistance(SMR)effect in Pt/Gd₃Fe₅O₁₂(Gd IG)bilayers was systematically investigated.The sign of SMR changes twice with increasing magnetic field in the vicinity of the magnetization compensation point(TM)of Gd IG.However,conventional SMR theory predicts the invariant SMR sign in the heterostructure composed of a heavy metal film in contact with a ferromagnetic or antiferromagnetic film.We conclude that this is because of the significant enhancement of the magnetic moment of the Gd sub-lattice and the unchanged moment of the Fe sub-lattice with a relatively large field,meaning that a small net magnetic moment is induced at TM.As a result,the Néel vector aligns with the field after the spin-flop transition,meaning that a bi-reorientation of the Néel vector is produced.Theoretical calculations based on the Néel’s theory and SMR theory also support our conclusions.Our findings indicate that the Néel-vector direction of a ferrimagnet can be tuned across a wide range by a relatively low external field around TM.     

Influence of growth morphology on the Néel temperature of CrRu thin films and heterostructures

Dimensionality effects on epitaxial and polycrystalline Cr_1?xRu_x alloy thin films and in Cr/Cr–Ru heterostructures are reported. X-ray analysis on Cr_0.9965Ru_0.0035 epitaxial films indicates an increase in the coherence length in growth directions (1 0 0) and (1 1 0) with increasing thickness (d), in the range 20≤d≤300 nm. Atomic force microscopy studies on these films shows pronounced vertical growth for d>50 nm, resulting in the formation of columnar structures. The Néel temperatures (T_N) of the Cr_0.9965Ru_0.0035 films show anomalous behaviour as a function of d at thickness d≈50 nm. It is interesting to note that this thickness corresponds to that for which a change in film morphology occurs. Experiments on epitaxial Cr_1?xRu_x thin films, with 0≤x≤0.013 and d=50 nm, give T_N–x curves that correspond well with that of bulk Cr_1?xRu_x alloys. Studies on Cr/Cr_0.9965Ru_0.0035 superlattices prepared on MgO(1 0 0), with the Cr layer thickness varied between 10 and 50 nm, keeping the Cr_0.9965Ru_0.0035 thickness constant at 10 nm, indicate a sharp decrease in T_N as the Cr separation layers reaches a thickness of 30 nm; ascribed to spin density wave pinning in the Cr layers for d<30 nm by the adjacent CrRu layers.     

Radial structure of Alfvén eigenmodes in the DIII-D tokamak through electron-cyclotron-emission measurements

The spatial structure of toroidal Alfvén eigenmodes and reversed shear Alfvén eigenmodes in DIII-D is obtained from electron-cyclotron-emission measurements. Peak measured temperature perturbations are of similar magnitude for both toroidal Alfvén eigenmodes and reversed shear Alfvén eigenmodes and found to be deltaT(e)/T(e) approximately equal to 0.5%. Simultaneous measurements of density fluctuations using beam-emission spectroscopy indicate deltan(e)/n(e) approximately equal to 0.25%. Predictions of the measured temperature and density perturbation profiles as well as deltaT(e)/deltan(e) from the ideal magnetohydrodynamic code NOVA are in close agreement with experiment.     

Dynamics of domain walls in magnetically multiaxial films in the thickness range corresponding to the Bloch-Néel transformations of the structure

The numerical minimization of the total energy functional and the solution of the nonlinear Landau-Lifshitz equation have been performed exactly taking into account the fundamental (including dipole-dipole) interactions in terms of the two-dimensional magnetization distribution. The equilibrium structure, energy, mobility, and scenario of the dynamic transformation of the domain walls (in their non- steady-state motion) have been determined as a function of the film thickness b and external magnetic field H for two different ((010) and (110)) orientations of the surfaces of magnetically triaxial films. The range of film thicknesses, including the thickness b = b _N, for which the Néel domain walls can be transformed into the Bloch domain walls, has been investigated. The phenomena of anisotropy of the domain-wall energy, the domain-wall mobility, and the period of dynamic transformations of the domain walls have been analyzed as a function of the film thickness b and external magnetic field H. The range of film thicknesses has been determined, in which the non-steady-state motion of the Néel domain walls is accompanied by the creation and annihilation of vortex-like structures despite the one-dimensional character of the magnetization distribution in these walls.     

Influence of magnetic ordering on the resistivity anisotropy of α-MnS single crystal

The resistivity and the optical absorbtion spectra of single crystal α-MnS are studied in the temperature range 80-300 K along two directions [100] and [111]. Strong anisotropy of the resistivity, and the shift of absorbtion spectra band edge below are explained in terms of model involving delocalized holes in 3d-band manganese ions interacting with localized spins by using the sd-model.