Amorphous Ferromagnetic Films Research Articles

The Effect of Size and Strain on Micro Stripe Magnetic Domain Structure of CoFeB Thin Films

The prerequisite for flexible magnetic electronic devices is the knowledge of the preparation technology of flexible magnetic films and the evolution of the film properties under strain. In this work, CoFeB amorphous ferromagnetic films with stripe domains were prepared on flexible polyimide (PI) substrates by oblique sputtering. The results show that oblique sputtering induces the formation of columnar crystal structure in CoFeB films, which increases the perpendicular magnetic anisotropy of the films, thus leading to the appearance of stripe magnetic domain structures. On this basis, the CoFeB films with stripe domains were processed on a microscopic scale to investigate the size effect and strain regulation on the microscopic domain structure of the magnetic films. The characterization of the magnetic domain structure shows that the stripe domain contrast is reduced by the striped structure prepared by lithography. The triangular, circular and ring patterns deflect the alignment of the stripe domain to different degrees. The experimental results show that the deflection of the stripe domains is caused by the anisotropy of the shapes produced by the different patterns and that the size of the microstructure needs to be close to the period of the stripe domains for the size effect to be significant. In addition, the strain-induced magnetoelastic anisotropy effectively rotates the orientation of the stripe domains, and the variation in domain contrast demonstrates that tensile/compressive strains vary the magnitude of the out-of-plane stray field of the film. Our results provide some insight into the modulation of the physical properties of flexible magnetic films.

Playing with universality classes of Barkhausen avalanches

Many systems crackle, from earthquakes and financial markets to Barkhausen effect in ferromagnetic materials. Despite the diversity in essence, the noise emitted in these dynamical systems consists of avalanche-like events with broad range of sizes and durations, characterized by power-law avalanche distributions and typical average avalanche shape that are fingerprints describing the universality class of the underlying avalanche dynamics. Here we focus on the crackling noise in ferromagnets and scrutinize the traditional statistics of Barkhausen avalanches in polycrystalline and amorphous ferromagnetic films having different thicknesses. We show how scaling exponents and average shape of the avalanches evolve with the structural character of the materials and film thickness. We find quantitative agreement between experiment and theoretical predictions of models for the magnetic domain wall dynamics, and then elucidate the universality classes of Barkhausen avalanches in ferromagnetic films. Thereby, we observe for the first time the dimensional crossover in the domain wall dynamics and the outcomes of the interplay between system dimensionality and range of interactions governing the domain wall dynamics on Barkhausen avalanches.

Universal temporal characteristics and vanishing of multifractality in Barkhausen avalanches.

Barkhausen effect in ferromagnetic materials provides an excellent area for investigating scaling phenomena found in disordered systems exhibiting crackling noise. The critical dynamics is characterized by random pulses or avalanches with scale-invariant properties, power-law distributions, and universal features. However, the traditional Barkhausen avalanches statistics may not be sufficient to fully characterize the complex temporal correlation of the magnetic domain walls dynamics. Here we focus on the multifractal scenario to quantify the temporal scaling characteristics of Barkhausen avalanches in polycrystalline and amorphous ferromagnetic films with thicknesses from 50 to 1000 nm. We show that the multifractal properties are dependent on film thickness, although they seem to be insensitive to the structural character of the materials. Moreover, we observe for the first time the vanishing of the multifractality in the domain walls dynamics. As the thickness is reduced, the multifractal behavior gives place to a monofractal one over the entire range of time scales. This reorganization in the temporal scaling characteristics of Barkhausen avalanches is understood as a universal restructuring associated to the dimensional crossover, from three- to two-dimensional magnetization dynamics.

Quantifying magnetic anisotropy dispersion: Theoretical and experimental study of the magnetic properties of anisotropic FeCuNbSiB ferromagnetic films

We performed a theoretical and experimental investigation of the quasi-static magnetic properties of anisotropic systems. We considered a modified Stoner-Wohlfarth model to describe anisotropic systems, and a distribution function to express the magnetic anisotropy dispersion. We also proposed a procedure to calculate the magnetic properties from experimental results of the quadrature of magnetization curves, thus quantifying the magnetic anisotropy dispersion. To test the robustness of the approach, we applied the theoretical model to describe the quasi-static magnetic properties of amorphous FeCuNbSiB ferromagnetic films, and directly compared the theoretical results with longitudinal and transverse magnetization curves measured for the films. Our films are characterized by anisotropy fields between 7 and 10.5 Oe, values compatible with that obtained for several amorphous magnetic materials, as well as by anisotropy dispersions expressed by exponents n between 14 and 30. Thus, the excellent agreement between numerical calculation and experimental results provides support to confirm the validity of our theoretical approach to describe the magnetic properties of anisotropic amorphous ferromagnetic films.

Statistical properties of Barkhausen noise in amorphous ferromagnetic films.

We investigate the statistical properties of the Barkhausen noise in amorphous ferromagnetic films with thicknesses in the range between 100 and 1000 nm. From Barkhausen noise time series measured with the traditional inductive technique, we perform a wide statistical analysis and establish the scaling exponents τ,α,1/σνz, and ϑ. We also focus on the average shape of the avalanches, which gives further indications on the domain-wall dynamics. Based on experimental results, we group the amorphous films in a single universality class, characterized by scaling exponents τ=1.28±0.02,α=1.52±0.3, and 1/σνz=ϑ=1.83±0.03, values compatible with that obtained for several bulk amorphous magnetic materials. Besides, we verify that the avalanche shape depends on the universality class. By considering the theoretical models for the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium found in literature, we interpret the results and identify an experimental evidence that these amorphous films, within this thickness range, present a typical three-dimensional magnetic behavior with predominant short-range elastic interactions governing the domain-wall dynamics. Moreover, we provide experimental support for the validity of a general scaling form for the average avalanche shape for non-mean-field systems.

Coercivity of Amorphous Со-Р Single-Layer and Trilayer Films Fabricated by Chemical Deposition

Coercivity of amorphous Co-P single-layer and trilayer films fabricated by chemical deposition has been investigated. It has been established that the thickness dependences of the coercivity for isotropic and anisotropic films are different. Data on the coercivity variation with a nonmagnetic spacer thickness and a number of magnetic layers in the trilayer films are reported. The results obtained are analyzed with regard to the features of the induced anisotropy in amorphous ferromagnetic films and the magnetic coupling between magnetic layers in sandwich structures.

Nonlinear magnetoelectric interactions in composite multiferroic structures

A theory of magnetoelectric interactions in ferromagnetic-ferroelectric composite structures is presented. Resonant frequency doubling and magnetic field mixing effects in the planar structure of a lead zirconate-titanate amorphous ferromagnetic film are investigated experimentally.

Resonance mixing of alternating current magnetic fields in a multiferroic composite

Theory for nonlinear mixing of harmonic magnetic fields in a ferromagnetic-ferroelectric composite structure has been developed and compared with data. In the voltage response of the composite, the model predicts a dc voltage proportional to the magnetostriction λ and its second derivative p with respect to the bias field H, an ac voltage due to linear magnetoelectric effect that is proportional to the piezomagnetic coefficient q, and a third term due to nonlinear mixing of the ac magnetic fields that is proportional to p. Doubling of the frequency and generation of voltages with sum and difference frequencies are expected due to nonlinearity of λ (H). The theoretically predicted effects are investigated in a sample of amorphous ferromagnetic film FeBSiC and a bimorph of lead zirconate titanate. Both the efficiency of frequency doubling and nonlinear mixing of the ac magnetic fields are found to be proportional to p. The effects discussed here are of interest for magnetic field sensors and signal processing devices such as mixers, frequency doublers, frequency dividers, and modulators.

Origin of in-plane uniaxial magnetic anisotropy in CoFeB amorphous ferromagnetic thin films

Describing the origin of uniaxial magnetic anisotropy (UMA) is generally problematic in systems other than single crystals. We demonstrate an in-plane UMA in amorphous CoFeB films on GaAs(001) which has the expected symmetry of the interface anisotropy in ferromagnetic films on GaAs(001), but strength which is independent of, rather than in inverse proportion to, the film thickness. We show that this volume UMA is consistent with a bond-orientational anisotropy, which propagates the interface-induced UMA through the thickness of the amorphous film. It is explained how, in general, this mechanism may describe the origin of in-plane UMAs in amorphous ferromagnetic films.

Enhanced initial susceptibility in small amorphous disks

A highly enhanced initial susceptibility is predicted theoretically for thin, amorphous ferromagnetic films when they are patterned as separate disks of appropriate dimensions. The relation between the required disk radius and the film thickness is given as a function of the exchange constant and the saturation magnetization of the amorphous material.

An ideally soft ferromagnet: is it feasible?

The infinite initial susceptibility encountered in the micromagnetics computations of sufficiently small, amorphous ferromagnetic spheres is argued to be a real physical phenomenon, which may actually be realized in practice as a very large enhancement of the initial susceptibility of thin, amorphous ferromagnetic films, when they are subdivided into separate discs. The only difficulty is to make the films highly homogeneous, and to avoid mechanical strains as much as possible.

Preparation and characterisation of a new amorphous tip coating for application in magnetic force microscopy

Using sputter deposition, we have coated standard silicon pyramidal force microscopy tips with an amorphous ferromagnetic film. The film composition is close to that of the commercial METGLAS ® 2605SC amorphous ribbon. Using a NIST magnetic imaging reference sample, and a Digital Instruments Dimension 3000 microscope platform, we demonstrate the good spatial resolution of the coated tip, and that it behaves in a manner consistent with a magnetic moment intermediate between currently available commercial tips and those possessing a low coercivity. Using Lorentz electron tomography techniques, the stray field pattern of the tip in the remanent state was shown to be consistent with a magnetic moment aligned along the tip axis.

Conditional replaceability of magnetic surface anisotropies by effective volume anisotropies in the ferromagnetic resonance of ultrathin films

Specific conditions are proposed for the replaceability of magnetic surface anisotropies by effective volume anisotropies and for the concomitant replaceability of the actual ferromagnetic resonance (FMR) surface mode or spin wave mode by a uniform mode. These conditions are then applied to the parallel and perpendicular FMR configurations in monocrystalline and amorphous ferromagnetic films. The significance and limitations of the results are discussed.

Investigation of the approach to ferromagnetic saturation in sputtered amorphous Fe 80−xNi xB 20 films

The magnetization of sputtered amorphous ferromagnetic films was investigated between 10 and 240 K in magnetic fields up to 4 T. The law of approach to ferromagnetic saturation is found to be described by a 1/H 1 2 term which is attributed to small elastic stress sources of diameter <4 nm.

Magnetoresistance of amorphous ferromagnetic metals

Amorphous ferromagnetic films of NiAu and and Co are prepared by quenched condensation onto a substrate at He temperature. Amorphous ferromagnetic metals are particularly well suited to investigate the influence of an external magnetic field in the ferromagnetic state on the transport properties. The small mean free path of the conduction electrons suppresses the complications of the band structure and its consequences on the transport properties. The magnetoresistance is measured in an external field up to 80 kG. $\frac{d\ensuremath{\rho}}{dB}$ is negative and takes values up to 2 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}9}$ ${\mathrm{m}}^{3}$/Asec. The results are discussed within the band model of ferromagnetism. They contradict the rigid-band model for NiAu alloys. Within the Heisenberg model the influence of anisotropy fields on the resistivity again yields values for $\frac{d\ensuremath{\rho}}{dB}$ which are too small.

Annealing effects in amorphous CoP alloys

The in-plane magnetic saturation field and the intensity of small-angle X-ray scattering have been measured for amorphous, ferromagnetic CoP alloy films (22.5 at.% P), 31 μm thick, produced by electrodeposition. The observed intensity of small-angle X-ray scattering depends on the angle φ between the diffraction vector K and the plane of the film. These observations have been interpreted in terms of oriented, ellipsoidal scattering regions. They may correspond to regions of high cobalt concentration and therefore of high magnetization. After annealing at a temperature (200 °C for 6 h) which is lower than the crystallization temperature (340 °C), the in-plane magnetic saturation field decreases from 70 to 40 Oe and the intensity of small-angle X-ray scattering decreases by 40%. These observations suggest that internal demagnetizing effects associated with anisotropic microstructure contribute to perpendicular easy-axis magnetic anisotropy in these amorphous ferromagnetic films.