Anhysteretic and biased first magnetization curves for Finemet-type toroidal samples

The anhysteretic and a set of biased first magnetization (BFMC) curves together with a set of first-order reversal curves (FORC) were measured and modeled by the hyperbolic T(x) model for a Finemet-type nanocrystalline toroidal sample with a round hysteresis loop. Similar to the FORC diagram, a “fingerprint”-like distribution has been obtained from a set of BFMC curves using the mixed second-derivate method of Pike. It is concluded that while the FORC diagram gives the distribution of coercive fields (or Preisach distribution), the BFMC diagram gives the distribution of the critical field where the domain wall magnetization become unstable and split up.     

Reversible and irreversible magnetization behavior in SmCo films

Recent studies of demagnetization processes in permanent magnets have used two different methods for determining the reversible magnetization, often arriving at conflicting results. In the present work, enhanced-remanence SmCo films have been prepared by DC magnetron sputtering. The evolution of the reversible (M_rev) and irreversible (M_irr) magnetization along the initial magnetization curve and the demagnetization curve of the films was determined using the two methods. In one case, the DCD definition of the reversible magnetization was used to measure the quantity from a set of recoil curves. Alternatively, the moving Preisach model was used to separate the magnetization into its components M_rev and M_irr. We discuss the different behaviors encountered for the reversible magnetization and the inter-relation function η=(∂M_rev/∂M_irr)_Hi and interpret the differences.     

The effect of evolving grain shape and alignment on the coercivity in thermomechanically deformed Nd13.9(Fe0.92Co0.08)80.3B5.3Ga0.5 permanent magnets

A series of samples of nominally identical composition have been melt-quenched and thermomechanically deformed to varying degrees, resulting in a variation in crystallographic alignment. The effect of the alignment on the coercivity has been studied using measurements of the temperature dependence of coercivity. The analysis was performed using a well-known phenomenological model which relates the coercivity to the anisotropy field, through a parameter α, and the saturation magnetization, through an effective demagnetization coefficient N_eff. It was discovered that the trends in α and N_eff as a function of crystallographic grain alignment are the opposite to what is expected and measured previously in analogous sintered samples of varying crystallographic alignment. An explanation for this behavior is proposed in which the evolving grain shape and grain boundary topography caused by deformation becomes the controlling factor in the coercivity rather than the crystallographic alignment of the grains.     

On the product Preisach model of hysteresis

The product model is an output-dependent modification of the traditional Preisach model in order to remove its non-real congruency property. The differential susceptibility is proposed to be a product of a magnetization-dependent factor and an expression containing two terms: one for the reversible process and the other an integral of a Preisach type distribution for the irreversible part. Thus the magnetization is an indirect function, in which the saturation is a natural property of the hysteresis model, and the reversible and irreversible parts of the magnetization are added up indirectly. The envelope function is related to the paramagnetic process and in the specific case of uni-axial anisotropy it is a hyperbolic tangent function. The measurement of the anhysteretic curve may provide the direct link to the evaluation methods of experimental data applied for traditional Preisach modeling.     

Theory of thermal remagnetization of permanent magnets

A self-consistent mean-field theory explaining the thermal remagnetization (TR) of polycrystalline permanent magnets is given. The influence of the environment of a grain is treated by an inclusion approximation, relating the field inside the grain to the local field outside by means of an internal demagnetization factor n. For the switching fields and the fluctuations of the local fields around the mean field, Gaussian distributions of widths σ_s, and σ_f, respectively, are assumed. The isothermal hysteresis curve, the recoil curves, and the TR dependent on the model parameters n, σ_s, and σ_f are calculated. Furthermore, the influence of the initial temperature and the strong dependence of the TR on the demagnetization factor of the sample are studied, and it is shown that for reasonable parameter sets TR effects up to 100% are possible. The theoretical results correspond well with the experimental situation.     

The quasi-magnetic-hysteresis behavior of polydisperse ferrofluids with small coupling constant

The magnetization behaviors of ferrofluids based on γ-Fe₂O₃/Ni₂O₃ composite nanoparticles of size about 11 nm have been investigated. The dipole coupling constant λ of these particles is so small (0.43) that they cannot form aggregates through magnetic interaction alone. Experimental results have shown that for a polydisperse ferrofluid with a particle volume fraction of ?_V=2.4%, the magnetization curve exhibits quasi-magnetic-hysteresis behavior, i.e., the demagnetization curve lies above the magnetization curve in a high field. However, for a more dilute γ-Fe₂O₃/Ni₂O₃ ferrofluid with ?_V=0.94%, the magnetization curve does not show such behavior. According to the bidisperse model for polydisperse ferrofluids, these magnetization behaviors may be attributed to field-induced effects of self-assembled pre-existing chain-like aggregates. For such pre-existing chain-like aggregates, the orientation of the moments inside the particles is not co-linear, so that during the magnetization and demagnetization processes, their apparent magnetizations at the high-field limit are different. As a consequence, the magnetization curve of the ferrofluid with ?_V=2.4% displays quasi-magnetic-hysteresis.     

First order spin flop transition in yttrium iron garnet (YIG)

We have studied the field dependence of the sublattice magnetization of ferrimagnetic yttrium iron garnet (YIG) using neutron scattering. In contrast to the macroscopic spontaneous magnetization that shows the normal field dependence of a soft ferromagnet (sudden saturation at the demagnetization field and no hysteresis) in neutron scattering a field induced first order spin flop transition with considerable hysteresis is observed at a critical field of H_c∼580 G (external field). Considering that with neutron scattering the antiferromagnetic component of ∼4/5 of the total moment is detected preferentially while in the macroscopic magnetization samples the ferromagnetic component of ∼1/5 exclusively it becomes clear that ferromagnetic and antiferromagnetic component have a completely independent field (and temperature) dependence. This indicates that the two magnetic structures have to be viewed as two weakly coupled order parameters. In the zero field ground state the moment orientations of the two ordering structures are orthogonal. Only for fields H₀>H_c a nearly collinear ferrimagnetic order is established by the field.     

Eine neue methode der konstruktion des verallgemeinerten preisach-diagramms

The author proposes the construction of a generalized Preisach diagram which is valid in the whole interval of fields coming into consideration in processes of technical magnetization. Fundamentally, the paper uses the method of variable probability density used in the papers by Feldtkeller-Wilde [2], Wilde-Girke [3] and later by Biorci-Pescetti [4]. A contribution of this paper is the method permitting the determination of the probability density distribution in the Preisach diagram by means of the experimentally obtained anhysteretic and primary magnetization curve. The advantage of this method compared with that of [4] is that the original Preisach diagram is used as the basis, permitting a clearer physical interpretation of the two separated components of the probability density than the method based on a modification of the Preisach diagram given in [4]. Some more complicated curves of magnetization reversal, calculated by means of the P-diagram proposed here, are compared in the figures with the corresponding measured curves. The values obtained by the method after Biorci-Pescetti are given for comparison.     

Temperature dependence of the hyperfine field distribution in an amorphous ferromagnet

The rapid temperature decrease in magnetization characteristic of amorphous alloys is often attributed to short-range exchange interactions. The temperature dependence of the hyperfine field distribution in an amorphous Fe₈₀B₂₀ (METGLAS 2605) measured by Mössbauer technique disagrees with such an explanation. It is shown that for this alloy each magnetic moment follows the same magnetization curve, i.e. the temperature dependence of the magnetization is determined by long-range correlations.     

Delayed demagnetization jumps in (NdDy)(FeCo)B magnets in a steady-state magnetic field

Spontaneous demagnetization jumps are observed in sintered magnets (Nd_0.6Dy_0.4)₁₆(Fe_0.77Co_0.23)₇₈B₆ in a constant magnetic field after a sharp decrease in an external magnetic field from the value corresponding to the saturation to a value close to the coercive force. It is shown that the number of the magnetization jumps is proportional to their amplitudes. A low value of the autocorrelation coefficient between the jump amplitude and the time of its appearance (R < 0.1) demonstrate the stochasticity of the jumps. It is found that the spectral jump density is independent of the frequency, i.e., a white magnetic noise is observed. The distribution of the magnetic field gradient has been obtained near the sample surface that makes it possible to distinguish domains and the grain magnetization in the dependence on the direction of the texturing of the sintered magnet.

     

Thermodynamic properties of two-dimensional few-electrons quantum dot using the static fluctuation approximation (SFA)

The two-dimensional GaAs parabolic quantum dot (QD) is investigated in the static fluctuation approximation (SFA) where both the magnetic field and the electron-electron interaction are fully taken into account. The thermodynamic properties of the system are computed as functions of the temperature and magnetic field for different numbers of electrons (N). It is noted that the mean internal energy increases with increasing number of electrons and magnetic field. This increase is not smooth, and a sudden change in slope is observed at a certain value of magnetic field for N≥4. It is found that the magnetization has sharp jumps for N≥4; for N≤3 a smooth monotonic diamagnetism is exhibited. A sharp peak is observed in the specific heat when the magnetic field is increased. It is found that the lowest-state transitions occur at a critical value of magnetic field. These transitions appear in the mean internal energy and magnetization.     

The role of demagnetization factor in determining the ‘true’ value of the Curie temperature

The Curie temperature, T_C, is the temperature above which a material loses its long-range ferromagnetic order. Considering the equation of state of a ferromagnet in the mean-field approximation it has been shown theoretically that the value of the demagnetization factor N has a significant influence on the perceived location of T_C on the temperature scale. A series of precise measurements of magnetization using two differently shaped single crystals of high-purity gadolinium was carried out to prove this result experimentally and develop a procedure leading to the ‘true’ value of T_C.     

Evolution of domain walls and reversal mechanism in exchange-coupled nanolayers

The demagnetization process for an exchange-coupled double-nanolayer system with perpendicular easy axes has been investigated within a micromagnetic model. The nucleation field, coercivity and angular distribution of the magnetization, have been obtained as functions of the thickness L of the misaligned layer, the layer with the easy axis perpendicular to the applied field. It is found that the coercivity is identical to the nucleation field only for very small L. For larger L (larger than a quarter of its Bloch wall width), the nucleation field is negative while the coercivity saturates at 0.414H_K, where H_K is the anisotropy field. Thus for larger L, the coercivity mechanism is self-pinning rather than pure nucleation. This self-pinning has both attributes of traditional nucleation and pinning.     

Manipulating the magnetoimpedance by dc bias current in amorphous microwire

We have studied the effect of the internal circumferential magnetic field H_B created by the dc bias current I_B on longitudinal and off-diagonal magnetoimpedance (MI) in amorphous microwire with helical anisotropy and experimentally demonstrated that by changing the dc current I_B it is possible to considerably change the MI dependencies. The bias current applied to such microwire transforms the symmetric and hysteretic MI curve to asymmetrical and anhysteretic. The minimum of longitudinal MI curve shifts from the zero-field point. Reversing the bias current causes reversal of the bias field direction and results in a mirroring of the MI curves. It is proposed to apply a cross-checking of two MI curves with I_B of different polarity for magnetic field sensing. In particular, this method allows to overcome the drawbacks usually associated with longitudinal MI—namely the impossibility to determine the direction of an external axial magnetic field H_E and the low sensitivity near the zero-field point. Moreover, the operating range of the longitudinal MI sensor, in contrast to the off-diagonal one, can be much extended as it exhibits a quite high sensitivity in the field range up to one order of magnitude higher than the anisotropy field.     

Gap-related trapped magnetic flux dependence between single and combined bulk superconductors

Aiming at examining the trapped-flux dependence between single and combined bulk superconductors for field-pole applications, three rectangular Y_1.65Ba₂Cu₃O_7?x (YBCO) bulks with a possibly compact combination were employed to investigate the trapped-flux characteristics of single and combined bulks with a field-cooling magnetization (FCM) method. A gap-related dependence was found between them. At lower gaps of 1 mm and 5 mm, the peak trapped fields and total magnetic flux of combined bulks are both smaller than the additive values of each single bulk, which can be ascribed to the demagnetization influences of the field around the bulk generated by the adjacent ones. While, at larger gaps like 10 mm, the situation becomes reversed. The combined bulks can attain bigger peak trapped fields as well as total magnetic flux, which indicates that the magnetic field by the bulk combination can reach higher gaps, thanks to the bigger magnetic energy compared with the single bulk. The presented results show that, on one hand, it is possible to estimate the total trapped magnetic flux of combined bulks by an approximate additive method of each single bulk while considering a demagnetization factor; on the other hand, it also means that the performance of combined bulks will be superior to the addition of each single bulk at larger gaps, thus preferable for large-scaled magnet applications.     

Magnetic properties and magnetic structures of Sr3−xCaxRu2O7

We have measured magnetization curves and powder neutron diffraction of double-layered Ruddlesden-Popper type ruthenate Sr_3−xCa_xRu₂O₇ (x=1.5, 2.0 and 3.0). The field dependence of the magnetization revealed that the transition field of metamagnetic transition along the b-axis shifted to lower fields and that the transition became broad with increasing Sr content. The slope of the magnetization curve also increased with increasing Sr content below the metamagnetic transition. These results indicate that an itinerant component is partly introduced by the Sr substitution. From the magnetic reflection, on cooling below T_N, an additional reflection was observed at (0 0 1) for each x, and the amplitude increased with decreasing temperature. The observed diffraction patterns are very similar to those of Ca₃Ru₂O₇. We conclude that the magnetic structure of the antiferromagnetic ordered phase is basically the same structure with that of Ca₃Ru₂O₇.     

Nonlinear dynamics of domain walls with a vortex internal structure in magnetouniaxial films with planar anisotropy

The nonlinear and generally unsteady dynamics of domain walls with a vortex internal structure in a constant magnetic field H is investigated on the basis of the numerical solution of the Landau-Lifshitz equation for a 2D distribution of magnetization M in magnetic films with planar anisotropy taking into account exactly the main interaction, including the dipole-dipole interaction. It is shown that in addition to field H _c (bifurcation field) above which the motion of a wall becomes unsteady and its internal structure experiences global dynamic changes, there exists a field H₀ separating two steady motions of the wall with different structures. The data clarifying the physical origin of the nonlinear dynamic rearrangement of the wall structure are presented. New rearrangement mechanisms associated with the generation and attenuation of vortices as well as their tunneling through the central surface of the wall are established. The existence of subperiod oscillations of the wall velocity in a static field in addition to the oscillations associated with the precession of M around the easy magnetization axis is predicted. The period T of dynamic variations of the wall structure is studied, and an empirical formula is proposed for describing the singular behavior of the T(H) dependence near H=H _Hc with the critical index depending on the film parameters. The bifurcation process is studied, and a nonlinear dependence of the critical field H _c on the film thickness and the saturation magnetization is established. The possibility of direct experimental investigation of the dynamic rearrangement of the internal structure of the wall is indicated.     

Specimen geometry effects on the irreversible magnetization in the low field regime for specimens of bulk Nb3Sn

Irreversible magnetization curves in particular in self field conditions were investigated for specimens of bulk Nb₃Sn as an isotropic and non-granular alternative to high T _c materials. The specimens were cut into squares with side length to thickness ratios of about 0.8 to almost 20, leading to diamagnetic slopes |χ _d | ranging from slightly more than 1 (slab in parallel field) to 13 (plate in perpendicular field). The peak in the low field magnetization (caused by the field dependence of j _c ) moves from negative field for specimens with small |χ _d | (where it should be according to the standard Bean model) to zero or even positive fields for specimens with large |χ _d | . Finite element self consistent calculations with field dependent and thus spacially varying j _c were used to fit the measured hysteresis curves. A strong current peak in the center plane of the specimens — caused by the demagnetization field — probably causes the shift of the magnetization peak to positive field. Best agreement between the experimental and calculated magnetizations have been obtained for the extreme geometries: either thin disk in perpendicular (χ _d < ?4) or thin slab in parallel magnetic field (χ _d > ?1.2).     

The physical theory of rock magnetism

Magnetite and hematite are representative of the ferrimagnetic and weakly ferromagnetic minerals which are responsible for the magnetic properties of rocks reviewed in this paper. Magnetite grains are multi-domains in the size range of interest (0.1 μ–1000 μ), whereas hematite grains in the same size range are almost certainly single domains. Properties discussed are coercivity, susceptibility, magnetic viscosity, thermo- and isothermal remanence, alternating field demagnetization, anhysteretic, chemical and detrital magnetization, anisotropy, piezomagnetic effects and self-reversals. Problems requiring more experimental data are emphasized, especially the basal plane anisotropy of hematite, the Barkhausen discreteness of domains in magnetite and the possibility that grains of cubic minerals may have some uniaxial character arising from grain shape or internal strains.