[en] The magnetic properties of substituted hard-magnetic M-type ferrites are surveyed. A combined La-Co substitution leads to magnets of compositions Ba_1-xLa_xFe_12-xCo_xO₁₉ and Sr_1-xLa_xFe_12-xCo_xO₁₉ (x= 0, 0.1, 0.2, 0.3, 0.4). Due to this substitution, the anisotropy as well as the coercivity increases. The effect of the ion size was studied on the system (Sr_1-yCa_y)_1-xLa_xFe_12-x Co_0.75xFe²⁺_0.25xO₁₉ (x= 0.2, 0.4; y= 0, 0.33, 0.66, 1). Additionally, Sm substitution which leads to a strong increase of the coercivity was studied, but the effect on the anisotropy is small (1%)

[en] We investigated the charge distribution in rare-earth (R=La, Nd, Sm) doped hexaferrites R_xSr_1-xFe₁₂O₁₉ at x≤0.33, T=4.2 K and in magnetic fields up to 7 T with the local probe of ⁵⁷Fe- nuclear magnetic resonance (NMR). All five crystallographic Fe-sites in the doped material are subject to an additional internal field ΔB_hf, independent of R. ΔB_hf increases to 0.4 T with x≤0.06, then decreases again. For x>0.15, the Fe-spectrum is nearly indistinguishable from the one observed in undoped samples. The homogeneous shift of all Fe-lines is ascribed to the formation of large polarons with a magnetic polarization increasing at small x, and decreasing again when additional states are involved in their formation

[en] The magnetic viscosity coefficient in Y_xSm_1-xCo₃Cu₂ alloys, with x=0, 0.25, 0.5, 0.75, and 1, was determined from measurements of the magnetization reversal relaxation close to the coercive field using a vibrating sample magnetometer and from the coercive field relaxation by the sweep-rate method using a pulse-field magnetometer, respectively. The coercive field and the magnetic viscosity coefficient, as determined from both methods, decrease with increasing yttrium concentration. However, the magnetic viscosity coefficient obtained from magnetization relaxation is always smaller (about six times) than that obtained by means of pulsed field measurements. These results are explained on the basis of different time windows of the measurements, proving different flat distributions of energy barriers due to a complex distribution of activation energies

[en] Full text: One problem of pulsed field magnetometry is an error in magnetization, which appears in measurements of conducting samples. This error is due to eddy currents induced by a time varying field. To allow predictions how eddy currents exert influence on the hysteresis loop, systematic experimental and theoretical studies of pulsed field measurements of metallic samples were performed. The theoretical studies include analytical calculations as well as numerical ones using a 2D finite element software. In the measurements three physical parameters have been varied: i) the conductivity of the sample by using two different materials, in this case technical Cu and Al ii) size and shape of the sample by using cylinders, spheres and cuboids iii) the pulse duration of the external field by changing the capacitor battery from 8mF ( =9.1ms) to 24mF ( =15.7ms). The time dependence of the external field corresponds with a pulsed damped harmonic oscillation with a maximum value of 5.2T. The samples were studied in the as cast state (after machining) as well as after heat treatment. Theoretical calculations showed not only good agreement with the absolute values of the measured eddy current ^magnetization^, they also gave an explanation of the shape of the eddy current hysteresis and the dependence of the eddy current 'magnetization' on parameters as pulse duration of the external field and conductivity of the sample. (author)

[en] In the group of M-type hexaferrites LaFe₁₂O₁₉ shows distinct magnetic behavior at low temperatures. The magnetocrystalline anisotropy at 4.2 K increases more than 200 % compared with is value at room temperature. It was reported that the increase in anisotropy is related to a change from Fe³⁺ state to Fe²⁺ at the 2a site in order to compensate charge excess due to the La valency. Due to difficulties in preparing this material no detailed investigations took place. Therefore an appropriate method for preparation of La hexaferrite has been found using co-precipitation and mechanical alloying. Annealing temperature and quenching where found as significant factors whereas the annealing duration is arbitrary. The samples have been investigated using XRD and LaFeO₃ and Fe₂O₃ were found as secondary phases. Although varying the weight ratio of the precursors according to the method and varying the annealing temperature, it was impossible to avoid formation of secondary phases. The sample with the highest amount of LaFe₁₂O₁₉ phase was obtained for the method of coprecipitation with 50 % La excess, annealed at 1365 ^oC. The saturation magnetization was estimated as 66.8 emu/g using a M over 1/H² plot of a hystersis loop measured in a pulsed field magnetometer up to 4.9 T. Comparing this value with the saturation magnetization of pure La hexaferrite gives an estimation of the hexaferrite phase concentration of around 95 %. The increase in anisotropy has been verified by measurement using the SPD technique. The anisotropy field was found to be μ₀H_Α = 4.01 T at 10 K compared with 1.64 T at room temperature. (author)

[en] Three different methods (mechanical alloying, coprecipitation and sol-gel) were used for preparing LaFe₁₂O₁₉ hexaferrite and all seem to be suitable for preparation of this phase. A heat treatment in the small temperature range between 1360 and 1390 deg. C followed by a rapid quenching is necessary for all preparation methods, since without quenching it is not possible to preserve the La-hexaferrite phase. Detailed investigations were performed for the method of coprecipitation varying the La excess, the heat treatment temperature and the quenching method

[en] We discuss Barkhausen noise experiments performed in interstitial-free low-carbon steel laminations, subjected to increasing deformation either by tensile straining or cold rolling. The spectral density of the noise, measured under a constant magnetization rate J^.(t) using both sensing coils and a soft ferrite probe, exhibits a defined dependence on the deformation ε_p. In particular, the low- and high-frequency portions of the spectra display opposite trends with ε_p, reflecting the behavior of remanent induction (permeability) and coercivity, respectively. Different roles are played by surface and bulk magnetization processes in noise buildup. Fast domain wall unpinning events in the outer lamination layer determine the spectra at high frequencies, while long-range propagation of the Barkhausen reversals in the bulk affects the low frequencies. By conventional measurements and noise analysis, one can thus achieve a consistent picture of the effect of plastic deformation on the magnetization process and provide a rationale for nondestructive steel testing by magnetic methods

[en] Numerous applications of hexagonal ferrites are related to their easy axis or easy plane magnetocrystalline anisotropy configurations. Certain W-type ferrites undergo spin reorientation transitions (SRT) between different anisotropy states on magnetic field or temperature variation. The transition point can be tuned by modifying the chemical composition, which suggests a potential application of hexaferrites in room temperature magnetic refrigeration. Here we present the results of structural and magnetic characterization of BaCo_xZn_2-xFe₁₆O₂₇ (0.7 ≤ x ≤ 2) doped barium ferrites. Fine powders were prepared using a sol-gel citrate precursor method. Crystal structures and particle size distributions were examined by X-ray diffraction and transmission electron microscopy. The optimal synthesis temperature ensuring complete formation of single W-phase with limited grain growth has been determined. Spin reorientation transitions were revealed by thermomagnetic analysis and AC susceptibility measurements.

[en] We investigate the temperature induced ferromagnetic to paramagnetic phase transition in Co substituted La(Fe_xCo_ySi_1−x−y)₁₃ with x = 0.9 and low Co content of y = 0.015 (T_c≃200 K) by means of magneto-optical imaging with indicator film and by calorimetry at very low temperature rates. We were able to visualize the motion of the ferromagnetic (FM)/paramagnetic (PM) front which is forming reproducible patterns independently of the temperature rate. The average velocity of the FM/PM front was calculated to be 10⁻⁴ m/s during the continuous propagation and 4×10⁻³ m/s during an avalanche. The heat flux was measured at low temperature rates by a differential scanning calorimeter and shows a reproducible sequence of individual and separated avalanches which occurs independently of the rate. We interpret the observed effects as the result of the athermal character of the phase transition

[en] Interstitial-free (IF) low carbon steel laminations have been subjected to plastic deformation either by tensile straining or cold rolling and their magnetic hysteresis behavior has been investigated for permanent elongation up to approximately 6%. Weaker magnetic hardening is observed, for a given strain level, in the cold-rolled (CR) materials, an effect chiefly ascribed to a more favorable profile of the residual stresses. Cold rolling leads to more homogeneous distribution of the internal compressive and tensile stresses compared to deformation by tension, and this results in a more uniform domain structure and softer magnetic behavior