[en] 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 SmCo₅, Sm₂Co₁₇ 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 SmCo₅ 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 SmCo₅ demonstrate clearly that the TR is mainly due to the contribution of the 'weak' grains, remagnetizing in the interaction fields of the 'hard' grains

[en] 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

[en] Torque curves of SmCo₅ sintered magnets with regularily lowered coercivity up to a maximum field H = 28 x 10⁵ Am^-1 have been registered automatically. From the observed rotational hysteresis the values of the rotational integral could be estimated in dependence on the degree of lowering of the coercivity. By comparison with theoretically calculated values for a simple nucleation model for a single crystal it follows that the mechanism of the magnetization reversal for SmCo₅ magnets with lowered coercivity should be pinning of Bloch walls in the multidomain state. The absolute value of the maximum rotational hysteresis work is comparable with the maximum linear hysteresis work. (author)

[en] The known process of the thermal remagnetization of SmCo₅ is explained half quantitatively as a typical effect in a polycrystal by assuming a square spectrum of its coercivities and parallel orientation of its easy axes. Together with large (H/sub R/ - H/sub c/)-differences it is caused by a fluctuating interaction field H' combined with the irreversible reduction of the low coercivity side of the H/sub c/-spectrum. From the measured field dependence on SmCo₅ as well as on BaFe₁₂O₁₉ magnets the field H' can be estimated experimentally and found to be H' = 0.3 and 0.5 x 10⁵ Am^-1 for a temperature T₀ = 180 and 270 ⁰C, respectively. (author)

[en] The torque T and the rotational hysteresis (rh) of permanent magnets with the composition Sm(Co/sub 0.65/Fe/sub 0.27/Cu/sub 0.06/Zr/sub 0.02/)/sub 7.44/ have been investigated at room temperature. T and rh depend sensitivity on the magnetization state of the magnet. A dependence of the rh work on the coercivity and the magnetic state of the material could be shown. Results give evidence that in the annealed samples the domain walls are easily movable where after the thermal treatment the hard magnetic characteristics of the sample are nearly recovered

[en] 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 T₀ followed by heating. The TR is especially large for well-aligned sintered SmCo₅ 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 T₀ on the TR of SmCo₅ sintered magnets experimentally and theoretically. We find that the TR-maximum increases with decreasing T₀, whereas its position T_max shifts slightly to lower temperatures with decreasing T₀. This together with the dependence on the sample demagnetization factor N, is in agreement with the results of our model calculation

[en] A simplified method for preparing sintered samarium-cobalt magnets without applying high pressures is presented. Depending on the samarium content and the course of sintering, relative densities up to 99 per cent could be reached. The limits of the method are given and possibilities for improvements are shown. (author)

[en] The effect of 'thermal remagnetization' as a function of the positive applied field has been studied in two different technical 2:17 magnets: Sm₂(Co_0.65Fe_0.27Cu_0.06Zr_0.02)_14.9 and Sm₂(Co_0.709Fe_0.205Cu_0.06Zr_0.02)_15.5. After a thermal demagnetization of the samples the field has been increased to H and the magnetization M₁ has been measured. Then the field has been decreased to -H_c and -H_R and the coercivities H_c and H_R have been measured. In a zero field the temperature was then increased to T₁ and decreased to T₀ and the magnetization increase ΔM has been registered. Two different types of virgin and remanence curves have been found. The values of H_c, H_R, (H_R - H_c), and normalized ΔM as functions of external saturation field are plotted and discussed

[en] The optical generation of coherent surface acoustic waves (SAW) is shown up to frequencies as high as 2 GHz on semiconductor surfaces. The frequency response of the acoustic coherence is directly related to thermal relaxation processes within the surface layer. Propagation of the optically generated SAW is shown to be sensitive to phase restructuring that occurs at solid-liquid interfaces

[en] In order to test the model assumptions on hard magnetic properties of sintered NdFeB magnets, a comparison of the measured and calculated values of the magnetization and remanence coercivities H_C and H_R, respectively, as a function of the angle between texture axis and external field θ in Nd₁₆Fe₇₆B₈ and (Nd_0.9Tb_0.1)₁₆Fe₇₆B₈ is given and explained qualitatively