Irreversible Susceptibility Research Articles

Magnetic After-Effect in Amorphous Tb-Fe Thin Films

Magnetic after-effect in sputter deposited TbxFe1-x (x×0.17∼0.24) films is studied. The after-effect is directly observed using polar Kerr magneto-optic effect as a slow propagation of domain walls. The velocity of the domain growth (10-4∼10-1 cm/s) changes exponentially with the applied field. The viscosity coefficient Sv≡dM/d ln t·χ-1 (χ: irreversible susceptibility), which was used in describing Jordan type after-effect, is applied to characterize the after-effect. The value of Sv (22∼110 Oe) is inversely proportional to the saturation magnetization at room temperature and decreases with rising temperature. The origin of the slow wall motion is inferred to be the random anisotropy of Tb ion rather than the oxidation of the film surface.

Amorphous FeF3: a non-crystalline magnet with antiferromagnetic interactions

Susceptibility and magnetisation curves for amorphous FeF3 indicate the presence of strong antiferromagnetic exchange coupling which would normally lead to a Neel temperature of the order of 300K in a crystalline lattice. In fact the appearance of remanence and an irreversible susceptibility indicate that spin freezing sets in only below Tf=29K, but there is no sharp phase transition. Mossbauer measurements indicate that the magnetic order is speromagnetic. A low value of the molar Curie constant measured in the range 50-300K (3.54) may be explained by assuming a spread in magnitude of the antiferromagnetic interactions. The results are discussed in terms of magnetic frustration.

Thermally activated motion of a 180° Bloch wall in a Fe-Si single crystal

The activated motion of a 180° Bloch wall is investigated in a frame-shaped Fe-Si single crystal. Two kinds of behaviour can be observed. In one of them, the irreversible motion of the wall takes place in a small scale by localized, independent jumps. When a constant magnetic field is applied, the jumps can be thermally activated and the induction increases. The temperature dependence of the change of induction is consistent with a spectrum of energy barriers and can be interpreted by the activation model of Street and Woolley. The order of magnitude of the energy barriers is 1000 K. The magnetic after-effect recorded on a small-amplitude hysteresis loop is proportional to the irreversible susceptibility. The wall motion can also take place by large, collective jumps. The after-effect then exhibits an exponential time dependence which reflects a single energy barrier (∼ 7000 K). When the sample is cycled, the losses per cycle are frequency dependent ; a logarithmic deviation from the linear law is observed at low frequencies, showing that thermal fluctuation assists wall motion. A good agreement is found with experiments previously reported by Sasaki on commercial Fe-Si.

Magnetic viscosity in ferromagnets

Abstract The thermal activation theory of magnetic viscosity is reviewed. Equations are derived relating the maximum slope of the magnetization decrement in (time) curve to absolute temperature, irreversible susceptibility, and the rate of change of activation energy with field at constant temperature, (∂∊/∂H)T The activation energy ∊ is, in general, a function of the applied field H and an intrinsic function of the temperature T. If, however, (∂∊/∂T)H=0 then (∂∊/∂H)T≃28·5k/(dH/dT) where H is the field at which the viscosity curve slope is a maximum. Deviations from this relationship indicate an intrinsic temperature variation of the activation energy ∊ and thus enable thermal activation and intrinsic temperature effects to be separated. Experimental data on magnetic viscosity are re-examined from this point of view.

Barkhausen noise of 3% Si-Fe strips after plastic deformation

Grain-oriented 3% Si-Fe strips were plastically strained up to \epsilon = 3 %. The frequency spectrum E(f) of the Barkhausen noise was measured. From E(O) a mean Barkhausen volume v eff is calculated. The noise and v eff are diminished after plastic deformation mainly already in the region \epsilon %, where the coercive field remains still constant, v eff is related to a mean irreversible susceptibility, which is mainly determined by the hysteresis loop. By measurements of the correlation of the flux changes along the specimen a mean length of the Barkhausen volumes was obtained. Its diminution mainly causes the diminution of v eff . The connection of the results with changes of the domain and dislocation structure is discussed.

The Age of Viscous Remanent Magnetization of Hadrian's Wall (Northern England)

Summary Following Neel's considerations on magnetic viscosity, an experimental approach is made to determine the age of viscous remanent magnetization (VRM) of certain rocks. The time-dependent critical internal fluctuation field is measured directly using AC demagnetization techniques. There are still some unsolved theoretical problems such as the ambiguous significance of the logarithmic time constant Q and the irreversible susceptibility χirr in the formulas developed by Neel. To prove the proposed method we have used blocks of quartz dolerite (Whin Sill) which were built into the Roman Hadrian's Wall in northern England. The historical age of the wall–constructed during the first centuries after Christ–and the age of VRM calculated from the magnetic measurements coincide fairly well. The method may therefore be applicable to certain dating problems in geology, e.g. to correlate Quaternary rock deposits, and in archaeology, if well-calibrated apparatus and statistical methods are used. Clearly the method cannot be used for ages greater than that of the last main reversal of the geomagnetic field.

Interdependence of thermal aftereffect and natural aging in permanent magnet materials

The thermal aftereffect is proportional to the irreversible susceptibility \chi irr and thus has a peak value in the zone of coercive force. It follows a logarithmic time law of the type \Delta I= S \cdot \log t + const whose constant S shows a linear increase with the absolute temperature T . The quotient S/\chi irr is a material constant and proportional to the coercive force. Natural aging also follows a law of the form \Delta B = S \cdot \log t + const , but is not proportional to \chi irr and therefore does not have a peak value in the zone of coercive force. Natural aging disappears at the remanence and at the coercive force. The constant S of natural aging is proportional to the product I \cdot H \cdot \chi irr and inversely proportional to the coercive force. The quotient S/\chi irr is, in this case, not a material constant and not proportional to the coercive force.

The power spectrum of the barkhausen noise of grain-oriented three-percent silicon iron

The Barkhausen effect in specimens of 3-percent silicon iron has been investigated by measuring the Barkhausen noise. The cross power spectrum of the noise voltages, picked up by two coils at different points of one specimen, has also been analyzed ; it yields information about the correlation of the Barkhausen discontinuities in time and space. Because of this correlation, Barkhausen jumps occur in groups, thus causing the normal power spectrum to rise remarkably at low frequencies. Comparing the experimental results with a formula developed by Mazzetti and Montalenti, one gets quantitative data about the groups. We have found that the mean number \bar{\rho} of pulses per group and the mean time interval τ between subsequent pulses in a group vary with the irreversible susceptibility like \rho \sim \chi_{irr}^{2}, \tau \sim \chi_{irr}^{-1} , no matter how \chi_{irr} is changed.

Long Term Magnetic Stability of Alnico and Barium Ferrite Magnets

Alnico and barium ferrite materials do not age at room temperature. The decreases in remanence, occurring with time, are adjustments by the magnet to its environment. Remanence adjustment proceeds with the logarithm of time and amounts to 2% or less one year after magnetization. Remanences in magnets with Hci > 2200 oe show no changes. Also, the smaller the irreversible susceptibility at the operating point, the more stable is remance. Remanences can be stabilized completely by preadjustment. Alnico V magnets are more stable than similar Alnico III magnets, even when the Alnico V magnets operate at a higher susceptibility. Néel's theory suggests that the oriented material is more stable than the isotropic material because larger volumes are involved in each magnetization reversal. To explain relatively large domain-like regions of reversal, we have to assume that magnetic regions cooperate during the magnetization reversal.