The effect of an oblique magnetic field on the superparamagnetic relaxation time

The effect of a constant magnetic field, applied at an angle ψ to the easy axis of magnetization, on the Néel relaxation time τ of a single domain ferromagnetic particle with uniaxial anisotropy is investigated by calculating the smallest non-vanishing eigenvalue of the appropriate Fokker-Planck equation as a function of ψ. The curve of λ₁ versus ψ is symmetric about ψ = 90°. Thus the maximum decrease in τ occurs at ψ = 45° with maximum increase to a value exceeding that at ψ = 0, at ψ = 90°, the ψ = 0 value being again attained at ψ = 180°. The results are shown to be consistent with the behaviour predicted by the Kramers theory of the rate of escape of particles over potential barriers. This theory when applied to the potential barriers for the ψ = 90° orientation for rotation in space yields a simple approximate formula for the escape rate which is in reasonable agreement with the exact λ₁ calculated from the Fokker-Planck equation.     

Anomalous slowing down of the longitudinal spin relaxation near the antiferromagnetic phase transition in LaMnO<Subscript>3</Subscript> manganite

It has been found that, instead of the expected critical acceleration of the longitudinal spin relaxation near the Néel temperature in stoichiometric LaMnO₃ samples, the relaxation is sharply retarded. This slowing down is similar to that observed earlier in doped manganites with ferromagnetic ordering.     

Anomalously strong relaxation of the polarization of muons in the magnetically ordered and paramagnetic states of the TbMnO<Subscript>3</Subscript> multiferroic

An anomalously strong relaxation of the muon polarization in a magnetically ordered state in the TbMnO₃ multiferroic has been revealed by the method below the μSR Néel temperature (42 K). Such a relaxation is due to the muon channel of relaxation of the polarization and the interaction of the magnetic moment of the muon with inhomogeneities of the internal magnetic field of an ordered state in the form of a cycloid. Above the Néel temperature, beginning with temperatures depending on the applied magnetic field, a two-phase state has been revealed where one phase has an anomalously strong relaxation of the muon polarization for a paramagnetic state. These features of the paramagnetic state are due to short-range magnetic order domains that appear in strongly frustrated TbMnO₃. A true paramagnetic state has been observed only at T ≥ 150 K.     

The squared modulated magnetic structure of HoRu2Ge2

The holmium ions order antiferromagnetically in the ternary tetragonal compound HoRu₂Ge₂, at the Néel temperature T_N = 20 K. This compound exhibits a squared modulated structure with a propagation vector k = [0.2216, 0.0111, 0.0] with a holmium magnetic moment of 6.6μ_B, parallel to the c-axis of the crystal. Crystal field calculation show that the tetragonal axis of HoRu₂Ge₂ is the easy axis.     

The temperature dependence of the angular variation of,and the critical point exponent for the EPR linewidth in the two-dimensional canted antiferromagnetic (C2H5NH3)2MnBr4: Evidence for a structural phase transition

The angular variation of the EPR linewidths in single crystals of (C₂H₅NH₃)₂MnBr₄ has been measured as a function of temperature. The angular dependence is well characterized by δH(θ) = a + b(3 cos²θ ? 1) + c(3 cos²θ ? 1)². The temperature dependence of the expansion coefficients is reported, and the effect of critical point fluctuations near the Néel temperature as well as a linear temperature dependence at high temperature are observed. A sharp decrease in linewidths at 160°K is attributed to a structural phase transition. The Néel temperature is determined to be 46°K (± 1°) from linewidth measurements of a powder sample. The linewidths diverge exponentially near the Néel temperature with a critical point exponent of 1.5.     

Magnetostriction and the two-spin correlation function in MnF2

The longitudinal magnetostriction along the [001] axis of MnF₂ was measured at temperatures 64 < T < 300 K in magnetic fields, H, up to 130 kOe. This magnetostriction is proportional to H² at low H, exhibits a λ anomaly near the Néel temperature T_N, and shows the field-induced transition from the antiferromagnetic phase to the paramagnetic phase for T just below T_N. The results are well described by a model which relates the magnetostriction to the two-spin correlation function.     

Effects of Co on the Néel temperature and thermal expansion in dilute CrFe alloys

The Néel temperature and thermal expansion characteristics of dilute CrFe alloys containing Co have been investigated. It has been found that the Néel temperature of Cr_99-xFe₁Co_x alloys (0.5?x?1) increases with increasing Co content and that these ternary alloys exhibit a large spontaneous volume magnetostriction, indicating Invar characteristics around room temperature. These behaviors are extremely different from those of CrFe and CrCo binary alloys.     

Nonlinear reconfiguration of vortexlike domain walls in magnetically uniaxial films under the action of an external field normal to the easy magnetic axis

The effect of external magnetic field H normal to the anisotropy axis on the energy and configuration of vortexlike asymmetric magnetic walls in a magnetically uniaxial film with an easy magnetic axis parallel to its surface is studied. The investigation is based on minimizing the energy functional of the film with due regard to exchange energy, magnetic anisotropy energy, magnetostatic energy, and Zeeman energy. The range of H below the anisotropy field is found where the asymmetric Néel wall is stable, unlike the case H = 0, when the asymmetric Bloch wall is stable. It is shown that an asymmetric Bloch wall becomes absolutely unstable and reconfigures into an asymmetric Néel wall at some critical values of H = H _⊥. The dependences of critical field H _⊥ on the film thickness and saturation induction at different values of the anisotropy field are determined: field H _⊥ depends on the thickness nonlinearly and on the saturation induction nonmonotonically.     

Magnetostriction studies of magnetic phase transitions in the copper metaborate CuB2O4

The field dependences of the longitudinal and transversal magnetostriction of the copper metaborate CuB₂O₄ were measured at various temperatures below the Néel point in magnetic fields directed along the tetragonal axis or in the basal plane. Magnetostriction was found to exhibit jumps at magnetic-field-induced phase transitions to a commensurate weak ferromagnetic state, as well as to grow smoothly in fields above and below the critical level. The magnetostriction observed in a magnetic field directed along the tetragonal axis is shown to be primarily caused by volume dilatation of the crystal. The experimental data obtained were used to construct the magnetic phase diagram of copper metaborate magnetized along the tetragonal axis.     

Solitons of TMMC in a strong magnetic field — Switching of hard anisotropy axis —

A switching of effective hard anisotropy axis is expected to occur in TMMC in the experimentally accessible field range, when the magnetic field is applied parallel to the easy plane. This effect explains well the observed deviation at high fields of nuclear spin-lattice relaxation time and Néel temperature from the previously proposed sine-Gordon soliton theory.     

Néel lines in ferrimagnetic garnet epilayers with orthorhombic anisotropy and canted magnetization

Néel lines in (TbY)₃(FeAl)₅O₁₂ epitaxial layers with orthorhombic anisotropy and canted magnetization are optically observed by the presence of a kink of the wall plane. The intrinsically asymmetrical character of Bloch walls is demonstrated by wall widening experiments. It follows that wall kinking may either be a consequence of the anisotropy specific to the samples, and/or be the result of magnetostatic interactions. A relaxation type numerical computation of the Néel line configuration indicates that anisotropy is the main source of wall kinking, demagnetizing field effects contributing to an amplification of the kink. The structure of those Néel lines is shown to bear two characters, that of a half-circular line characteristic of samples with in-plane anisotropy and that of a splay line, well known in bubble materials. Further, the charge distribution appears basically dipolar and lines contract when the saturation magnetization increases. They are therefore magnetostatistically analogous to vertical Néel (most generally called Bloch) lines in garnet epilayers supporting bubbles.     

A study of the conduction mechanism of single crystal Li0.5Fe2.5O4 through the transition points

The differential thermal analysis, electrical resistivity and thermopower are studied as a function of temperature in the range 300–1100 K. The measurements show two transition points at 905 and 1030 K. The first point is linked with the magnetic transition at the Néel temperature (T_N), and the second with the order-disorder phase. The conduction mechanism through the Néel point is discussed using the spin polaron model.     

Ultrasonic attenuation anomalies at the two transitions in antiferromagnetic FeGe2

The attenuation of longitudinal and shear sound waves is measured through the Néel temperature, T_N≈286 K, and the lower transition temperature, T_K≈265 K, of FeGe₂. Longitudinal sound with wavevector q along the [100] axis of this tetragonal antiferromagnetic metal shows an attenuation peak at T_N, which is reversibly suppressed by compressive uniaxial stress σ along [010]. The estimated pressure dependence of T_N is dT_N/dp=(?2.8±0.3) mK bar^?1. The peak at T_N shows thermal and stress hysteresis, which suggests that it is associated with domain wall motion and that this transition is first order.     

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).     

Errata

The Mössbauer effect has been used to study the behavior of the sublattice magnetization of the antiferromagnet RbFeF₄ near the Néel temperature T_N = 133.6 K. In the asymptotic critical region (1 ? T/T_N < 10^-2), a critical exponent β = 0.316 ± 0.005 was found, indicating a three-dimensional critical behavior.     

Optical investigation of metamagnetic DyAlO3

We have investigated the optical absorption spectra of metamagnetic DyAlO₃ (T _N =3.4 °K) above and below the Néel temperature. We find that the magnetic interactions are predominantly between the ions along the crystallographicc-axis; the interaction energy between two ions isW _C =? (2.9±0.6) cm^?1. Additional line-shifts below 3.4 °K yield the Néel temperature. Zeeman effect studies give the magnetic moment of the groundstate as μ=(9.2 ± 0.9)μ _B . The direction of the magnetic moments is in thea-b plane, with angles of ± 33.5° and ± 146.5° from theb-axis. The Zeeman effect studies reveal two metamagnetic transitions (forH _ext∥μ) at 7.0 and 7.3 kOe respectively. Additional susceptibility and Mössbauer measurements confirm the Néel temperature and the magnetic moment.     

Determination of the Néel temperature from measurements of the thermal conductivity of the Co3O4 antiferromagnet nanostructured in porous glass channels

The Néel temperature T _N(n) of the Co₃O₄ antiferromagnet nanostructured in channels of porous borosilicate glass with channel cross sections of ??7 nm has been determined from thermal conductivity measurements. It has been shown that the Néel temperature T _N(n) of this nanomaterial is approximately equal to 20 K, which is considerably lower than T _N = (30?C40) K for the bulk Co₃O₄ sample.     

Spin excitations and thermodynamics of the antiferromagnetic Heisenberg model on the layered honeycomb lattice

We present a spin-rotation-invariant Green-function theory for the dynamic spin susceptibility in the spin-1/2 antiferromagnetic Heisenberg model on a stacked honeycomb lattice. Employing a generalized mean-field approximation for arbitrary temperatures, the thermodynamic quantities (two-spin correlation functions, internal energy, magnetic susceptibility, staggered magnetization, Néel temperature, correlation length) and the spin-excitation spectrum are calculated by solving a coupled system of self-consistency equations for the correlation functions. The temperature dependence of the magnetic (uniform static) susceptibility is ascribed to antiferromagnetic short-range order. The Néel temperature is calculated for arbitrary interlayer couplings. Our results are in a good agreement with numerical computations for finite clusters and with available experimental data on the β-Cu₂V₂O₂ compound.     

A phenomenological theory for polarization flop in spiral multiferroic TbMnO<Subscript>3</Subscript>

A phenomenological Landau theory has been used to explain magnetic field-driven polarization flop in TbMnO ₃. The Néel wall-like magnetic structure in spiral multiferroics induces a space-dependent internal magnetic field which exerts a torque on spins to rotate bc-spiral to ab-spiral. The external magnetic field is argued to be competing with easy axis anisotropy and the system stabilizes when anisotropy is minimum. With the help of Landau free energy with DM magnetoelectric coupling and a general ansatz for magnetization, the phenomenon of polarization flop has been explained. Relation between T_flop and critical magnetic field has been established and found to be in good agreement with the experiment. This could be an indication that anisotropy of the system is temperature- and magnetic field-dependent.     

Magnetic order in an MnF2 epitaxial layer with the orthorhombic structure

The magnetic structure of MnF₂ is determined by the neutron diffraction method in the metastable orthorhombic phase grown in the form of thin (～1 μm) film on a CaF₂ buffer layer by the molecular beam epitaxy method. The magnetic moments of Mn⁺⁺ form a simple two-sublattice antiferromagnetic structure and are directed along the c crystallographic axis parallel to the film plane. Using the temperature dependence of magnetic reflections, a Néel temperature of 67.19(7) K and a critical index of 0.50(2), which corresponds to the mean field approximation, are determined.