L. Jahn

Theory of thermal remagnetization of permanent magnets

Abstract 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.

Investigation of the thermal remagnetization in sintered hard magnets

The maximum thermal remagnetization (TR) after dc-demagnetization was studied at sintered RE/T hard magnets and at barium ferrite. We found very large TR-effects for SmCo/sub 5/, large effects for hard ferrites, medium for NdFeB and a very low effect for Sm/sub 2/Co/sub 17/ magnets. The experimental results are interpreted on the basis of a theory taking into account the local fluctuations of the internal magnetic field as well as the internal demagnetization effects. The relevant model parameters were estimated for SmCo/sub 5/ and barium ferrite.

Influence of the initial temperature on the thermal remagnetization of SmCo5 sintered magnets

Abstract Modern permanent magnets show an irreversible increase of the remanent magnetization for increasing temperature. This thermal remagnetization (TR) occurs after DC-demagnetization at the initial temperature T 0 followed by heating. The TR is especially large for well-aligned sintered SmCo 5 permanent magnets and is strongly correlated with the grain-interaction and the temperature coefficient of the coercivity. We present a systematical study on the influence of the initial temperature T 0 on the TR of SmCo 5 sintered magnets experimentally and theoretically. We find that the TR-maximum increases with decreasing T 0 , whereas its position T max shits slightly to lower temperatures with decreasing T 0 . This together with the dependence on the sample demagnetization factor N , is in agreement with the results of our model calculation.

Influence of texture on the magnetic viscosity

Abstract The influence of texture on the time and field dependence of the magnetization, the magnetic viscosity, the irreversible susceptibility and the fluctuation field is calculated for two models of non-interacting uniaxial particles differing in the angular dependence of the switching field. The first model consists of single-domain particles reversing their magnetization by coherent rotation. In the second model the grains exhibit a 1/cos ϑ dependence of the switching field on the misalign angle. A detailed comparison of our results with the existing theory of the magnetic viscosity is given. For both cases it was shown, that texture alone cannot give a barrier distribution smooth enough to get the mostly measured ln t behaviour of the magnetization. The fluctuation field S v is found to be independent on the texture, in good agreement with the experimental situation, but in contrast to the theoretical considerations reported in the literature.

Field dependence of the thermal remagnetization in sintered hard magnets

Abstract The dependence of the thermal remagnetization (TR) in sintered hard magnets on external magnetic fields has been investigated in order to determine more precisely the grain-interaction fields in the DC-demagnetized state. That negative external field at which the TR or its initial slope vanishes or changes its sign, corresponds well to the theoretical parameter σ f , i.e. the distribution width of internal field fluctuations. For the “normal” TR of the metallic magnets SmCo 5 , Sm 2 Co 17 and NdFeB these fields are of the order of −0.5, 0.25 and −0.04 T , respectively. For the “inverse” TR at the hard ferrites this external field is about −0.02 T . The systematic variation of the TR-curves with a small external field is well explained by our theoretical model and leads to a refinement of the parameters for SmCo 5 and barium ferrite. Susceptibility measurements with small alternating fields, carried out at different points of the TR curve, as well as repeating TR-experiments at SmCo 5 demonstrate clearly that the TR is mainly due to the contribution of the “weak” grains, remagnetizing in the interaction fields of the “hard” grains.

Magnetic viscosity of modified neodymium iron boron magnets with high coercivities

Abstract In order to examine the long-term stability of novel NdDyFeCoMoAlB magnets with enhanced temperature stability, after-effect measurements have been carried out in an open circuit in the temperature range 20–230°C. In this temperature range the ln( t ) law for the irreversible polarization losses is confirmed and the after-effect constant S and the fluctuation field S v decrease monotonically with temperature for the sample annealed at an optimum. It can be proved that the open circuit measurements of S ′ and the irreversible susceptibility, χ ′ irr = χ ′ tot − χ ′ rev , should be corrected by the same the demagnetization coefficient N depending factor and thereafter that the S v values can be derived directly by S v = S ′ v = S ′ χ ′ irr . The determined room-temperature values of S v between 8 and 11 kA m −1 are related to the coercivity approximately within the limits of a Barbier plot (log( S v ) = a · log( H cJ ) with a ≈ 0.5−1) and do not diminish the advantages of the low-temperature coefficient of the coercivity of the NdDyFeCoMoAlB magnets. The experimentally determined ratio S v / T yields an activation volume V c for the magnetization reversal corresponding to a critical diameter d c ≈ 7–14 nm, which is 1.5–2.5 times the Bloch wall thickness δ.