Bulk Amorphous Ferromagnets Research Articles

The peak in anomalous magnetic viscosity

Anomalous magnetic viscosity, where the magnetization as a function of time exhibits non-monotonic behaviour, being seen to increase, reach a peak, and then decrease, is observed on recoil lines in bulk amorphous ferromagnets, for certain magnetic prehistories. A simple geometrical approach based on the motion of the state line on the Preisach plane gives a theoretical framework for interpreting non-monotonic behaviour and explains the origin of the peak. This approach gives an expression for the time taken to reach the peak as a function of the applied (or holding) field. The theory is applied to experimental data for bulk amorphous ferromagnet alloys of composition Nd60−xFe30Al10Dyx, x = 0, 1, 2, 3 and 4, and it gives a reasonable description of the observed behaviour. The role played by other key magnetic parameters, such as the intrinsic coercivity and fluctuation field, is also discussed. When the non-monotonic behaviour of the magnetization of a number of alloys is viewed in the context of the model, features of universal behaviour emerge, that are independent of alloy composition.

Structural analysis of nanocrystals and their role in the coercivity mechanism of Nd–Fe–Al–Dy bulk amorphous ferromagnets

The magnetic property and microstructure of Nd60−xFe30Al10Dyx (x=0, 2, 4) bulk ferromagnets have been investigated. At room temperature, each alloy sample shows hard magnetic behavior, and the intrinsic coercivity increases significantly with increasing Dy content. Transmission electron microscopy (TEM) and Atom probe tomography (APT) results show nanocrystals of width 5–20nm embedded in the Fe-rich amorphous matrix. The coercivity mechanism of the Nd60−xFe30Al10Dyx (x=0, 2, 4) amorphous alloys is discussed in the context of Gaunt's strong pinning model of domain walls. The crystallographic phases of nanocrystals identified by high-resolution TEM can act as pinning centers. Our findings give further insight into the coercivity mechanism of Nd-based bulk amorphous alloys through a better understanding of the nanostructural environment.

Application of the Stoner–Wohlfarth model with interaction for the determination of the saturation magnetisation, anisotropy field, and mean field interaction in bulk amorphous ferromagnets

Magnetic hysteresis curves of bulk amorphous ferromagnet alloys of composition Nd 60Fe 30Al 10, Nd 60Fe 20Co 10Al 10 and Pr 58Fe 24Al 18 have been measured in applied magnetic fields up to 9 T at temperatures in the range 10–350 K. The behaviour of the demagnetisation curve in the first quadrant is interpreted using a mean field interaction model as proposed by Callen et al. [Phys. Rev. B 16 (1977) 263], which extends the Stoner–Wohlfarth model [Philos. Trans. Roy. Soc. A 240 (1948) 599] for a random distribution of non-interacting uniaxial grains. Application of the mean field interaction model enables the determination of the saturation magnetisation M s, anisotropy field H a, and interaction parameter d, and from these other magnetic parameters, such as the anisotropy constant, K, are deduced. For the three alloys, the temperature dependent behaviour of M s, H a, d and K over the range 20–350 K are found to be qualitatively similar, though there are quantitative differences. In all cases M s increases with decreasing temperature, both H a and K increase with decreasing temperature, reaching a peak in the range 75–120 K, and then decreasing, and d decreases approximately linearly as the temperature decreases. The physical mechanisms responsible for coercivity in these materials are discussed in the context of random anisotropy and a strong pinning model of domain walls.

Temperature dependence of the spontaneous remagnetization in Nd60Fe30Al10 and Nd60Fe20Co10Al10 bulk amorphous ferromagnets

Time dependent behavior of the magnetization, i.e., magnetic viscosity, in ferromagnetic materials is well known. Less well known is the phenomenon of spontaneous remagnetization, where following a magnetic history that results in the dc demagnetized state, a ferromagnetic material spontaneously develops a magnetic moment as a function of time. Spontaneous remagnetization behavior of the bulk amorphous ferromagnets Nd60Fe30Al10 and Nd60Fe20Co10Al10 is investigated as a function of temperature from 50 to 400 K. At all temperatures the spontaneous remagnetization, Mspon, follows the relationship Mspon=Sspon ln(t+t0), where Sspon is a measure of the spontaneous remagnetization processes, t is the time, and t0 is a reference time. Sspon is strongly temperature dependent, increasing approximately linearly with temperature from 50 to ≈300 K, where it reaches a peak, before decreasing rapidly. This behavior is similar to that observed for the magnetic viscosity coefficient, S, in ferromagnetic materials.

Sweep rate dependence of the coercivity and fluctuation field in some RE-Fe(Co)–Al, RE=Nd or Pr, bulk amorphous ferromagnets

Bulk amorphous alloys of composition Nd60Fe30Al10, Nd60Fe20Co10Al10, and Pr58Fe24Al18 have been prepared, and all alloys exhibit hard magnetic properties at room temperature. Magnetic field sweep rate measurements show the intrinsic coercivity to be proportional to the logarithm of the field sweep rate. The fluctuation field, Hf, for each alloy, is determined from both sweep rate and magnetic viscosity measurements. In the case of the latter it is determined by using both the waiting time method and S/χirr, where S is the magnetic viscosity parameter and χirr the irreversible susceptibility. Differences in the value of Hf determined from the sweep rate and magnetic viscosity experiments are discussed, and the value of the activation volume for each alloy determined.

Anomalous magnetic viscosity in the bulk amorphous ferromagnets Nd60Fe30Al10 and Nd60Fe20Co10Al10

The bulk amorphous ferromagnets Nd60Fe30Al10 and Nd60Fe20Co10Al10 exhibit hard magnetic properties, and time dependent magnetisation measurements have been performed to explore similarities and differences between them. Magnetic viscosity measurements on the major hysteresis loop of Nd60Fe30Al10 are reported, along with data for both alloys showing the behaviour of the irreversible magnetic susceptibility. The fluctuation field, Hf, for each alloy is determined. A focus of the study is investigation of the anomalous magnetic viscosity i.e. non-monotonic behaviour of the time dependent magnetisation (where a peak is observed in the magnetisation), on the recoil curve that leads to the dc demagnetised state. Data showing non-monotonic behaviour in both Nd60Fe30Al10 and Nd60Fe20Co10Al10 is presented. The non-monotonic behaviour is interpreted using the Preisach model, as it predicts a simple functional relationship between the time taken to reach a peak and the applied magnetic field. The experimental data is in good agreement with this simple relationship.

The fluctuation field and anomalous magnetic viscosity in commercial NdFeB alloys, AlNiCo and the bulk amorphous ferromagnets [formula omitted] and [formula omitted

The fluctuation field, H f , is a useful parameter for characterising any ferromagnetic material that displays hysteresis, as it is a measure of the thermally activated rate processes that govern magnetisation reversals. Anomalous magnetic viscosity, i.e. nonmonotonic behaviour of the time dependent magnetisation, where the magnetisation is seen to increase, reach a peak, and then decrease, has been observed on both the upper and lower branches of minor loops or recoil curves in some ferromagnetic materials. Parameters relevant to the Preisach model are discussed as to their usefulness in predicting anomalous magnetic viscosity in ferromagnetic materials. This is done with reference to measurements of H f and the time dependent magnetisation in commercial NdFeB alloys, AlNiCo and the bulk amorphous ferromagnets Nd 60 Fe 30 Al 10 and Nd 60 Fe 20 Co 10 Al 10 .

Thermal and spontaneous remagnetization in the bulk amorphous ferromagnets Nd60Fe30Al10 and Nd60Fe20Co10Al10

Bulk amorphous Nd60Fe30Al10 and Nd60Fe20Co10Al10 alloys, which display hard magnetic properties, have been prepared by arc melting and quenching into a copper mould. Following magnetic saturation and dc demagnetization, at room temperature, both materials are seen to exhibit the phenomena of thermal and spontaneous remagnetizations. The behavior of the observed thermal and spontaneous remagnetizations is similar to that exhibited in nucleation controlled sintered permanent magnets. The thermal remagnetization data are fitted using a model, developed from the theory of Schumann and Jahn [J. Magn. Magn. Mater. 232, 231 (2001)] for explaining thermal remagnetization in polycrystalline magnets, to gauge its usefulness when applied to amorphous ferromagnets. The spontaneous remagnetization behavior is interpreted in the context of the Preisach model.

Co-based soft magnetic bulk amorphous ferromagnets prepared by powder consolidation

Co70.3Fe4.7Si10B15 and Co56Fe16Zr8B20 amorphous ribbons have been short time ball-milled in order to prepare powder samples. The powders were consolidated by hot pressing into disc-shaped bulk specimens. The influence of the consolidation temperature on the magnetic properties of bulk samples was investigated. Optimized consolidation parameters allowed to obtain bulk glassy samples without significant crystallization. In contrast, the gradual devitrification of the Co-based bulk metallic glasses during consolidation leads to a giant increase in coercivity. The observed magnetic hardening is assumed to be due to pinning of domain walls by crystallites. The lowest values of coercivity of 23 A/m and 13 A/m were achieved after consolidation at 540 °C and 600 °C for Co70.3Fe4.7Si10B15 and Co56Fe16Zr8B20 bulk samples, respectively. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Abstract: Soft magnetic properties and magnetostriction of sputter-deposited bulk amorphous alloys

Bulk amorphous ferromagnets, interesting in soft magnets or magnetostrictive materials, were produced by a high rate sputtering technique based on dc-triode method. The deposition was done onto a water cooled copper substrate. About 0.5−1 mm thick disks or rings (10–40 mm in diameter) in the amorphous state were obtained for Fe or Co rich alloys including metalloids and rare earth elements. All the alloy deposits were removed from the substrate, and then measurements were made of the B–H hysteresis loop and magnetostriction at room temperature and the saturation magnetization in the wide temperature range. (1) In the CO rich Co–Fe–Si–B alloys, the B–H loops show low remanence and large Barkhausen effect, representing coercivity ranging from 0.1 to 0.3 Oe. By annealing above the Curie point, the Barkhausen effect disappears and the coercivity decreases markedly. Particularly in the nearly zero magnetostrictive alloys very low values near 0.002 Oe can be obtained, Co67Fe5Si10B18 is typical. (2) Fe rich Fe–C and Fe–B alloys near 20 at.% of C and B exhibit large saturation magnetostriction of about 32×10−6. Distinguishably, the magnetostriction can be changed by adding only a few percent of rare earth elements; the magnetostriction constant of (Fe1–XRX)83B17, where R is Sm, Tb and Dy, increases up to about 40×10−6 for x=0.05 of Tb or Dy, while it decreases down to about 20×10−6 for x=0.05 of Sm. In the Tb case, longitudinal magnetostriction is as large as 60×10−6 but transverse value is vanishingly small. This suggests that spontaneous magnetization is declined from the in-plane of the disk deposits. Mean magnetic moment per Fe+R in these alloys can be explained by assuming the antiparallel coupling of the atomic moments of Fe and R.