Amorphous Ferromagnets Research Articles

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.

Temperature dependence of the coercivity in Nd 60Fe 30− xCo xAl 10, x=0 or 10, and Pr 58Fe 24Al 18 bulk amorphous ferromagnets

Bulk amorphous ferromagnet alloys of composition Nd 60Fe 30Al 10, Nd 60Fe 20Co 10Al 10 and Pr 58Fe 24Al 18 have been prepared by argon arc melting and quenching into a copper mould. General insight into the magnetic behaviour of the alloys was gained from measurement of the major hysteresis loop at room temperature, and from zero-field cooled and field-cooled magnetisation measurements in the range 10–400 K. Measurements of the coercivity were made from 10 to 400 K, and for all alloys, the coercivity is seen to increase steeply with decreasing temperature to a peak at a temperature in the range 25–50 K, before decreasing. For all alloys, the temperature dependence of the coercivity between 50 and 400 K is well explained by the strong pinning model of domain walls of Gaunt [Philos. Mag. B 48 (1983) 261]. Quantities deduced from the Gaunt model, along with other relevant magnetic parameters, are used to estimate values for the exchange and anisotropy constants.

Perpendicular surface magnetic anisotropy in an amorphous ferromagnet induced by adsorption occurring through the mechanism of the formation of hydrogen bonds

It has been found that the domain structures of amorphous magnetically soft iron-based ferromagnetic ribbons in an atmosphere of water vapor and methyl alcohol are significantly different. In the former case, several domains separated by domain walls oriented at an angle of 30°–40° to the long side of the sample are observed. In the latter case, two domains separated by one wall located in the middle of the sample in parallel to its long side are observed. In the former case, the normal component of the magnetization on the sample surface has been detected using the polar magnetooptical Kerr effect. In the latter case, the normal component of the magnetization is almost absent. The observed effects are reversible. The normal component of the magnetization is induced by the desorption of water and methyl alcohol molecules, which are absorbed through the mechanism of the formation of hydrogen bonds, from the sample surface. According to the performed estimate, the effective field of the perpendicular magnetic anisotropy reaches a value of 1.6 kA/m.

Anomalously high surface deceleration of the domain wall and magnetic stray field micro-distribution over a surface of amorphous ferromagnet

It was founded using the magneto-optical technique that the domain wall in the iron-based amorphous ferromagnet in its movement experienced enhanced deceleration on the surface compared to the volume. The surface deceleration of the wall intensifies under water molecules adsorption on the sample surface. This effect is explained by the domain wall interaction with magnetic micro-defects observed on the sample surface using a magnetic force microscope.

Magnetic and anomalous magnetic viscosity in the bulk amorphous ferromagnet [formula omitted] and partially amorphous ferromagnet [formula omitted

A number of ferromagnetic Pr–Fe–Al alloys have been prepared by argon arc melting and quenching into a copper mould. The alloy of composition Pr 58Fe 24Al 18 is identified as being amorphous (bulk metallic glass or bulk amorphous ferromagnet), and a range of magnetic measurements have been performed to explore differences and similarities between it and a partially amorphous alloy, containing a significant crystalline fraction, Pr 60Fe 24Al 16. For both alloys, measurements of the irreversible susceptibility, and magnetic viscosity on the major hysteresis loop are reported. From the magnetic viscosity data, the fluctuation field is determined. The behaviour of the anomalous magnetic viscosity (non-monotonic behaviour of the magnetic viscosity, where the magnetisation as a function of time is seen to increase, reach a peak, and then decrease), on the recoil curve that leads to the dc demagnetised state is investigated. Both alloys display non-monotonic behaviour. After dc demagnetisation, spontaneous remagnetisation is observed in both alloys, and some comments are made on the thermal remagnetisation behaviour of the amorphous alloy. The anomalous magnetic viscosity is interpreted in the context of 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 for both alloys is in good agreement with this prediction.

Metamaterials from Amorphous Ferromagnetic Microwires: Interaction between Microwires

For inhomogeneous mediums the оptical Magnus effect has been derived. The metamaterials fabricated from amorphous ferromagnet Co-Fe-Cr-B-Si microwires are shown to exhibit a negative refractive index for electromagnetic waves over wide scale of GHz frequencies. Optical properties and optical Magnus effect of such metamaterials are tunable by an external magnetic field.

Domain Structure Reversible Reconstruction of Amorphous Ferromagnetic by Water Molecules Weak Adsorption

Influence of water molecules reversible adsorption on domain structure and domain wall structure in amorphous Fe-rich ferromagnet samples is investigated by magnetooptical method. Before investigation the samples were kept in the air. Domain structure of the samples significantly changes in wacuum: instead one 180-degree domain wall several domain walls appear which are oriented with angle 45o to the long side of the sample. This effect is completely reversible. Another new effect is found: domain wall width on the sample surface in vacuum is 35% bigger (13.5 µm) than in atmosphere of water vapor (10 µm).

Metamaterials with Tunable Negative Refractive Index Fabricated from Amorphous Ferromagnetic Microwires: Magnetostatic Interaction between Microwires

For inhomogeneous mediums the optical Magnus efiect has been derived. The metamaterials fabricated from amorphous ferromagnet Co-Fe-Cr-B-Si microwires are shown to exhibit a negative refractive index for electromagnetic waves over wide scale of GHz frequencies. Optical properties and optical Magnus efiect of such metamaterials are tunable by an external magnetic fleld, magnetic fleld from neighbourhood microwires and mechanical stress.

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.

Electrical resistivity in Co-rich pseudobinary Ni–Cr ferromagnetic metallic glasses from 1.5to300K

Electrical resistivity ρ(T) has been measured in two series of melt-spun amorphous ferromagnets Fe5Co50Ni17−xCrxB16Si12 (x=0,5,10,15) and Fe7.8Co31.2Ni39−xCrxB14Si8 (x=5,10,15) and Fe7.8Co31.2Ni24Mn15B14Si8 between 1.5 and 25K. ρ(T) of all these high-resistive, soft ferromagnetic samples with wide device applications show pronounced minima at Tmin. In the region well below Tmin, it shows very conclusively a (−√T) dependence coming from electron-electron interaction effects in the presence of weak localization due to their high structural disorder. ρ(T), measured over a wider temperature range of 1.5–300K in the second series, shows in the region well above Tmin a T2 dependence coming from either the dynamical structure factor or magnetic scattering or from both. At still higher temperatures, ρ(T)∼T through the structural term only. The above interpretation is severely restricted by various characteristic temperatures such as Debye, Curie, and Tmin in the relevant theories used for the analysis. Reasonable isolations of different contributions were only possible because of the very high resolution of the ρ(T) data.

Ferroelectric solid solutions with morphotropic boundaries: Vanishing polarization anisotropy, adaptive, polar glass, and two-phase states

The generic case of a ferroelectric solid solution is considered wherein different symmetry phases located at opposing ends of the diffusionless phase diagram are separated by a morphotropic boundary (MB). It is shown that the Landau theory of weak first-order phase transformations automatically predicts vanishing of the anisotropy of polarization, continuity of thermodynamic properties, and a drastic decrease in domain wall energy near the MB line that results in the formation of adaptive ferroelectric nanodomain states. Low-resolution diffraction from these adaptive states acquired at the coherence lengths of elastic x-ray or neutron scattering probes will produce the same diffraction pattern as attributed to monoclinic (MA,MB,MC) phases. It is further shown that the electric- or stress-field-induced reconfiguration of these adaptive nanodomain states results in a softening of the piezoelectric, elastic, and dielectric properties near the MB line. In addition, the spherical degeneration of the polarization direction, reflecting the decoupling of the polarization from the crystal lattice at the MB, also predicts the formation of a polar glass state whose properties should be similar to the special properties of amorphous ferromagnets. In particular, the vanishing of the polarization anisotropy at the MB should result in ferroelectric domains with irregular shapes exhibiting high configurational sensitivity to external forces. The theory further predicts that two tricritical points will occur on the line of paraelectric→ferroelectric transitions and it is shown that all two-phase equilibrium lines of the diffusionless phase diagram—including the MB line—must be replaced by two-phase fields. Within these two-phase fields, the adjacent ferroelectric-ferroelectric and paraelectric-ferroelectric phases coexist. Possible topologies of the equilibrium MB phase diagram illustrating these two-phase equilibrium fields are computed and discussed.

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.

Mean-field description of the magnetic structure of ultradispersed magnets

A modification of a mean-field method for the description of a magnetic structure of an ultradisperse or an amorphous ferromagnet is performed. A block size and an induced anisotropy in one-and three-dimensional models of a polycrystal are calculated.

Exchange coupling between an amorphous ferromagnet and a crystalline antiferromagnet

We have investigated the exchange bias effect in bilayers of an amorphous ferromagnet (CoFeB) and a crystalline antiferromagnet (IrMn) in a top-pinned configuration. When the crystalline IrMn layer was deposited on top of the amorphous CoFeB layer, no exchange bias was observed. On insertion of a thin crystalline ferromagnetic layer of NiFe between the amorphous CoFeB and the crystalline IrMn, exchange coupling appeared and it was dependent on the thickness of the NiFe layer. An enhancement in the blocking temperature of the CoFeB/NiFe/IrMn layers was observed on increasing the thickness of the NiFe layer. These effects were directly correlated with the (111) texture in the antiferromagnetic phase of the IrMn layer, which developed progressively with increasing thickness of the NiFe layer. The blocking temperature was found to vary linearly with the intensity under the (111) IrMn X-ray diffraction peak. A NiFe interlayer can be used to introduce an additional source of anisotropy in a giant magnetoresistance sensor, by exchange coupling the free ferromagnetic (FM) layer of CoFeB in an orthogonal direction to the anisotropy direction of the pinned FM layer.

Kapitza pendulum effect in an amorphous magnet with a weak disorder

The mechanism of magnetization orientation in an amorphous ferromagnet under the effect of a weak anisotropic disorder of the random anisotropy type is considered. The disorder orients the magnetization so as to minimize the transverse magnetization fluctuations. The effect of the disorder is found to be analogous to the effect of vibrations of the suspension point on the orientation of a mechanical pendulum (the Kapitza pendulum). The disorder types ensuring the presence of the orienting effect are specified; simultaneously with the effect, the continuous degeneracy is lifted and the Larkin-Imry-Ma mechanism of long-range order collapse is suppressed.

Magnetic properties of amorphous Fe 73.5Cu 1Mo 3Si 12.5Al 1B 9 alloy

The FINEMET type Fe 73.5Cu 1Mo 3Si 12.5Al 1B 9 (numbers indicate at.%) alloy has been synthesized using single wheel melt spinning technique. The samples have been thermally annealed at about 40 K less than the crystallization temperature for various durations. The samples show increase in Curie temperature of the amorphous phase with annealing. To get a better insight into their magnetic behaviour we have done spin wave analysis on the as-spun and thermally annealed samples and extracted the spin wave stiffness constant and mean square range of exchange interaction. Low temperature thermomagnetic measurements from 10 to 300 K were performed with an applied field of 0.5 T. The data were fitted using the Bloch equation. The value of the C/ B ratio and the mean square range of exchange interaction were found to be characteristic of amorphous ferromagnets. The variation of spin wave stiffness constant is correlated to the changes in Curie temperature and the nature of the exchange interaction existing in this alloy is determined.

Critical behavior in a random field classical XY model for amorphous systems

By using the differential operator technique and the effective field theory (EFT) scheme, the critical behavior of amorphous classical XY ferromagnet in a random field is studied. The phase diagram in the T – H and T – α planes on a simple cubic lattice for a cluster with two spins is obtained. Calculation is carried out within the EFT approximation. Tricritical points, reentrant phenomena and influence of the random field and amorphization on the transition temperature are discussed.

Magnetic tunnel junctions comprising amorphous NiFeSiB and CoFeSiB free layers

Magnetic and magneto-transport behaviors of magnetic tunnel junctions (MTJs) incorporating newly developed amorphous NiFeSiB and CoFeSiB as free layers were investigated. Both experimental and theoretical approaches were carried out to understand the details of switching characteristics. The MTJs with traditional free layer materials such as NiFe and CoFe were also fabricated and compared. The studied amorphous ferromagnets appeared beneficial for reducing switching fields without loosing tunnelling magnetoresistive outputs. Further layer thickness, composition, and MTJ structure optimization are expected.