Non-Langevin high-temperature magnetization of nanoparticles in a weak magnetic field

Experimental evidence and theoretical substantiation are presented for the asymptotic behavior of high-temperature magnetization of an ensemble of nanoparticles in a weak magnetic field, which was predicted earlier and which differs qualitatively from the “Langevin” limit for ideal superparamagnetic particles. It is shown that the physical reason for the new asymptotic behavior is the temperature-independent “positive” tilt of the uniform magnetization vector at local energy minima in the direction of the field; this asymptotic behavior is associated with the nonstandard thermodynamics of single-domain particles, which depends on the ratio of characteristic frequencies of regular precession and random diffusion of this vector. An alternative approach is proposed for describing the magnetic dynamics of an ensemble of nanoparticles in a magnetic field, and the precession orbits of the magnetization vector are considered as stochastic states of each particle, whereas each state is characterized by the trajectory-averaged value of magnetization.     

Effect of an AC magnetic field on the steady-state magnetization distribution in a weak ferromagnet

Possible steady-state magnetization distributions in a domain wall are found in a weak ferromagnet subjected to an ac magnetic field. The character of the rotation of the magnetization vector in the domain wall is determined. It is predicted that domain structures can be rearranged and reoriented under an ac magnetic field.     

Zn-doping effect on the energy barrier to magnetization reversal in superparamagnetic nickel ferrite nanoparticles

Measurements of ac-susceptibility and dc-magnetization were carried out on samples of Ni_1-xZn_xFe₂O₄ nanoparticles (x=0, 0.25, 0.5, 0.75) with average diameters 〈D〉≈7 nm. Values of the superparamagnetic blocking temperature T̄_B were obtained from the characteristic temperature behavior of the imaginary susceptibility χ_imag. An Arrhenius-type law, which accurately describes the relationship between the observation time τ_obs and the blocking temperature, was used to determine the effective energy barrier to magnetization reversal U_eff. A Zn-content dependence of the energy barrier is observed, where U_eff changes little for 0≤x≤0.25, it peaks at x=0.5, and decreases back upon further Zn-doping. The large increase of U_eff at x=0.5 is attributed to an enhanced magnetic anisotropy induced by the crossover between two spatial arrangements of spins in the A and B sub-lattices of the ferrimagnetic inverse spinel. PACS 75.50.Bb; 75.50.Gg; 75.30.Et     

Temperature dependence of the field induced magnetization reorientation in dzialoshinsky-moriya type weak ferromagnets

Using a Hamiltonian consisting of isotropic, anisotropic, and antisymmetric exchange terms plus a Zeeman term, the temperature dependence of the weak ferromagnetic-spin-flop phase transition is calculated in the molecuar field approximation. For all temperatures less than the tricritical one, the phase transition is of first order. The stability limits of the two phases and the thermodynamic phase boundary are calculated for this temperature region. For temperatures greater than the tricritical one, the second order transition boundary between the phases is calculated. A set of equations precisely defining the tricritical point in the field-temperature plane is derived and results are presented for the location of this point as a function of the parameters of the system. The methods developed in this work are general and can be applied directly to similar problems.     

Time-dependent magnetization in co-precipitated cobalt ferrite

A study of the magnetic aftereffect in co-precipitated cobalt ferrite is presented. Measurements of the magnetic viscosity S were performed at room temperature along the demagnetization curve for different applied fields H_ap over a wide range of fields (0 kOe<H_ap<−7 kOe). The interrelation function η=(∂M_rev/∂M_irr)_Hi between the DCD reversible M_rev and irreversible M_irr magnetization components was determined as well. The experimental results for S_η(H_i), where H_i is the internal field, showed a broad distribution with a maximum at H_i=2.7 kOe. However, the irreversible susceptibility χ_irr displays a maximum at H_c=0.75 kOe, the coercivity of the material. The experimental behavior of η and the non-proportionality between S_η and χⁱ_irr suggest that the magnetic viscosity in this material is principally supplied by events of nucleation of inverse domains and the depinning of domain walls. When the main mechanism of reversal magnetization changes to rotation of magnetic moments for all the grains, the magnetic viscosity decreases.     

Domain walls, magnetization, and magnetoelectric effect in bismuth ferrite films

The specific features of the antiferromagnetic domain structure, magnetization, and polarization induced by an inhomogeneous micromagnetic distribution in films of bismuth ferrite multiferroics have been investigated. It has been shown that the magnetic domain structure correlates with the ferroelectric domain structure, and the character of the rotation of the antiferromagnetic vector depends on the type of ferroelectric domain walls. An asymmetry in the distribution of the antiferromagnetic vector has been observed for the cases of 109° and 71° ferroelectric domain walls. It has been demonstrated that there are differences in the distributions of the polarization and magnetization in bismuth ferrite films with ferroelectric domains separated by 109° and 71° walls. The basic mechanisms responsible for the magnetization in domain walls in multiferroics have been considered.     

Strong electric field effect on weak localization

The influence of an electric field on weak localization is studied by a recently proposed generalized quantum Langevin equation approach to the conductivity problem. A general formula for the memory function of the non-interacting electron gas, in the presence of high order impurity scattering and an arbitrary electric field, is derived. In the low field case, a scale and frequency dependent conductivity is obtained, which reduces to the well known scale dependent conductivity in the static limit. In the high field case, the conductivity is field dependent through the drift velocity. It is shown that the presence of strong electric fields tends to delocalize the one and two dimensional electron systems if one adopts the electron heating model. However, if the electron gas does not heat up, the conductivity will be field independent.     

Some comments on magnetization reversal in uniaxial ferrite magnets

The interpretation of Hadjipanayis et al. of their experimental results on magnetization reversal in ferrite magnets is critically analyzed on the basis of the present knowledge of the mechanisms governing magnetization, magnetization reversal and coercivity, both in single and polydomain particles.     

Influence of frustration on magnetization processes in the ferrite CoFeCrO4

The behavior of isotherms of the magnetization σ(H) and the longitudinal λ_‖ and transverse λ_⊥ magnetostriction of the polycrystalline ferrite CoFeCrO₄ with frustrated magnetic structure has been investigated for the first time in magnetic fields up to H=50 kOe at T=4.2 K. It is found that the magnetization grows with increasing field due to two different paraprocesses. Fiz. Tverd. Tela (St. Petersburg) 41, 2042–2043 (November 1999)     

The effect of hydrogenation on the magnetization reversal process in amorphous ferromagnets

The influence of hydrogenation and dehydrogenation on the power spectra of Barkhausen noise in amorphous alloys has been investigated. It has been shown that after hydrogenation local stress centers occur in amorphous structure. Presence of the centers leads to long-wave fluctuations of domain wall energy which cause the decrease of the number of irreversible domain wall displacements.     

The effect of strain on the magnetization process in CeAg1-xInx

Measurements of magnetization and strain were performed on CeAg, and CeAg_0.99In_0.01 polycrystals as a function of the external magnetic field. We found that anomalous stairs-like magnetization curves observed below the temperature (T_Q) of Jahn-Teller transformation are strongly correlated with the strain induced by the external magnetic field.     

The effect of weak disorder on random walks in a magnetic field

Renormalization group analysis has shown a weak disorder in d = 2 dimensions to change considerably the transport properties (magnetoresistance, diffusion coefficient, etc.) of random walks in a magnetic field.     

The effect of a weak magnetic field on the mobility of dislocations in silicon

The magnetoplastic effect in dislocation silicon is discovered. It is shown that in the presence of tensile stresses (up to 20 MPa), the mechanically activated path of surface dislocation half-loops is limited mainly by the dynamics of defects in various slip systems relative to the applied load. The activation barriers for the motion of dislocations controlled by various conditions in the temperature range T=850–950 K are E _aF=2.1±0.1 eV and E _aS=1.8±0.1 eV. An increase in the path of surface dislocation half-loops and a change in the activation barriers are detected (E _aF=1.4±0.1 eV and E _aS=1.6±0.1 eV) after subjecting silicon to a magnetic field (B=0.7 T) for 30 min. Possible reasons behind the observed effects are discussed.     

Localization: The effect of a weak magnetic field

The effect of a weak magnetic field on the diffusion of noninteracting electrons in a disordered system is studied in a nonlinear-model context. The effective Lagrangian describing the soft modes of the system in the weak field limit is derived. The result does not have the simple form that has been suggested by several authors. Therefore the crossover of the system under a weak perturbing magnetic field is not analogous to that found in spin systems.     

Fractal model of magnetization reversal in a strained garnet ferrite film

The domain structure in strained garnet ferrite films and its behavior in an external magnetic field are studied using the Faraday effect. Based on the experimental results, a model of magnetization reversal in thin polycrystalline layers is proposed that describes the process of remagnetization as the development of fractal clusters. The model proposed is verified using a computer simulation of magnetization reversal.     

Low field microwave absorption and magnetization process in CoFeNi electroplated wires

Ferromagnetic resonance （FMR）, Ferromagnetic antirresonance （FMAR） and low field magnetoimpedance （MI） are the characteristic features of high frequency losses in applied fields. While some results on FMR and FMAR in CoFeNi electroplated wires were reported earlier, here we present microwave absorption in CuBe wires electroplated by 1 μm FeCoNi magnetic layer at very low fields. These data are comparatively analysed together with longitudinal hysteresis loops in order to reveal the correlation between power absorption and magnetization processes. Microwave studies are made by using the cavity perturbation method at 9.65 GHz for a DC field parallel to the sample axis, and with microwave magnetic field hrf parallel or perpendicular to the wire axis. Two peaks have been observed in all samples, one is due to FMR, and the other is, at very low fields, related to MI. The MI peaks represent minima in power absorption. By comparing with the hysteresis loop we remark the close correspondence between the MI phenomena in the axial mode and the concomitant magnetization process.     

Selective effect of a weak DC magnetic field on triglycine sulfate crystals

The effect of the selective influence of a dc magnetic field on the characteristics of nominally pure triglycine sulfate crystals was detected for the first time. A short (minutes) exposure to a weak magnetic field B₀=0.08±0.01 T caused long-term (hundreds of hours) changes in the spontaneous polarization, coercive field, Curie temperature, and permittivity of the crystal at the Curie point. The effect is selective in nature presumably because of the participation of hydrogen bond protons in the spin-dependent processes of the transformation of defect complexes in real crystals.     

The effect of Gd3+ and Cd2+ substitution on magnetization of copper ferrite

Polycrystalline compositions of soft ferrite system, Cd_xCu_1−xFe_2−yGd_yO₄ (X=0.00, 0.20, 0.40, 0.60, 0.80 and 1.00; y=0.00, 0.10 and 0.30) were prepared by standard ceramic method. X-ray diffraction study show formation of single phase cubic spinel ferrite for the compositions X⩾0.20 and tetragonal nature for compositions X=0.00, for all values of Gd³⁺ (y=0.00, 0.10 and 0.30) concentration. Saturation magnetization and magnetic moments were found to be increasing with cadmium concentration up to X=0.40, for all values of Gd³⁺ content, obeying Neel's two sublattice model and decreases thereafter, showing existence of non-collinear spin interaction. The Gd³⁺ substitution results into reaction in the magnetic moments. This is due to occupancy of Gd³⁺ ion on octahedral (B) site, resulting into dilution in the magnetization of B sublattices. The Curie temperatures for all compositions are found to be decreasing with substitution of Cd²⁺ concentration. This is attributed to the occupancy of cadmium on tetrahedral (A) site, causing dilution in the inter site magnetic interaction. The temperature dependence of AC susceptibility is also studied and its behaviour is explained on the basis of domain structure.     

An order-by-disorder process in the cyclic phase of spin-2 condensate with a weak magnetic field

We present in this paper a model study on the “order-by-disorder” process in the cyclic phase of spin-2 condensate, which forms a family of incommensurable, spiral degenerate ground states. On the basis of the ordering mechanism of entropic splitting, it is demonstrated that the energy corrections resulting from quantum fluctuations of disorder lift the accidental degeneracy of the cyclic configurations and thus lead to an eventual spiral order called the cyclic order. The order-by-disorder phenomenon is then realized even if the magnetic field exists. Finally, we show that our theoretic observations can be verified experimentally by direct detection of the cyclic order in the ⁸⁷Rb condensate of a spin-2 manifold with a weak magnetic field.