Amorphous Ferromagnetic Layer Research Articles

Fourfold magnetic anisotropy induced in CoFeB/IrMn bilayers by interfacial exchange coupling

Exchange bias (EB) occurring in ferromagnetic (FM)/antiferromagnetic (AFM) bilayers conventionally can lead to a unidirectional magnetic anisotropy () as well as an accompanied uniaxial magnetic anisotropy (). We observed an additional fourfold magnetic anisotropy () induced by interfacial exchange coupling in amorphous CoFeB/epitaxial IrMn bilayers with an EB. Because of the combined effect of the three kinds of magnetic anisotropies, one- and two-step magnetic switching processes were observed at different magnetic field orientations, which usually appear in single-crystal FM layer with an intrinsic magnetocrystalline anisotropy but not in amorphous FM layer. The angular dependent magnetic switching fields can be nicely fitted by a phenomenological model based on domain wall nucleation and propagation with the in-plane along <100>. The ferromagnetic resonance measurements indicate that the specific strength of for EB along [100] is larger than that for EB along [110]. The induced can be understood by considering two types of AFM domains caused by both monatomic steps and defects and their induced net uncompensated spins along the in-plane <100> axes. The different dependence of on the EB direction are because of the different effects of growth magnetic field on the presence of AFM domains.

Anisotropy of magnetoelectric effects in an amorphous ferromagnet-piezoelectric heterostructure

In-plane anisotropy of linear and nonlinear magnetoelectric (ME) effects in a heterostructure composed of mechanically coupled layers of amorphous iron-based ferromagnet and piezoelectric lead zirconate-titanate ceramics has been investigated. The measurements were performed on a disk-shaped sample subjected to a low-frequency excitation magnetic field h oriented either parallel or perpendicular to a bias magnetic field H0. The first harmonic of the voltage response was recorded in the case of the linear ME effect, which was excited at frequencies of acoustic resonances of the sample, and the second harmonic in the case of the nonlinear ME effect excited in the non-resonance regime. Strong uniaxial or biaxial anisotropy for different directions of the bias field within the sample plane was observed. The uniaxial anisotropy of the ME effect occurs due to the uniaxial magnetic and magnetostrictive anisotropy of the amorphous ferromagnetic layer that is a result of its fabrication by the ultrafast cooling method. The biaxial anisotropy at resonant frequencies is the manifestation of the spatial structure of the excited acoustic modes. The anisotropy of ME effects should be taken into account when developing magnetoelectric devices based on composite heterostructures.

Magnetocrystalline anisotropy imprinting of an antiferromagnet on an amorphous ferromagnet in FeRh/CoFeB heterostructures

Magnetic anisotropy is a fundamental key parameter of magnetic materials that determines their applications. For ferromagnetic materials, the magnetic anisotropy can be easily detected by using conventional magnetic characterization techniques. However, due to the magnetic compensated structure in antiferromagnetic materials, synchrotron measurements, such as X-ray magnetic linear dichroism, are often needed to probe their magnetic properties. In this work, we observed an imprinted fourfold magnetic anisotropy in the amorphous ferromagnetic layer of FeRh/CoFeB heterostructures. The MOKE and ferromagnetic resonance measurements show that the easy magnetization axes of the CoFeB layer are along the FeRh〈110〉 and FeRh〈100〉 directions for the epitaxially grown FeRh layer in the antiferromagnetic and ferromagnetic states, respectively. The combined Monte Carlo simulation and first-principles calculation indicate that the fourfold magnetic anisotropy of the amorphous CoFeB layer is imprinted due to the interfacial exchange coupling between the CoFeB and FeRh moments from the magnetocrystalline anisotropy of the epitaxial FeRh layer. This observation of imprinting the magnetocrystalline anisotropy of antiferromagnetic materials on easily detected ferromagnetic materials may be applied to probe the magnetic structures of antiferromagnetic materials without using synchrotron methods.

Amorphous FeCoSiB for exchange bias coupled and decoupled magnetoelectric multilayer systems: Real-structure and magnetic properties

The effect of field annealing for exchanged biased multilayer films is studied with respect to the resultant structural and magnetic film properties. The presented multilayer stacks comprise repeating sequences of Ta/Cu/{1 1 1} textured antiferromagnetic Mn70Ir30/amorphous ferromagnetic Fe70.2Co7.8Si12B10. Within the ferromagnetic layers crystalline filaments are observed. An additional Ta layer between the antiferromagnet and ferromagnet is used in order to investigate and separate the influence of the common Mn70Ir30/Fe70.2Co7.8Si12B10 interface on the occurring filaments and structural changes. In situ and ex situ transmission electron microscopy is used for a comprehensive structure characterization of multilayer stacks for selected temperature stages. Up to 250 °C, the multilayers are structurally unaltered and preserve the as-deposited condition. A deliberate increase to 350 °C exhibits different crystallization processes for the films, depending on the presence of crystal nuclei within the amorphous ferromagnetic layer. The influence of volume-to-surface ratio of the multilayer stacks to the crystallization process is emphasized by the comparison of in situ and ex situ investigations as the respective specimen thickness is changed. Complementary magnetic studies reveal a defined exchange bias obtained at the first annealing step and a decrease of total anisotropy field with partial crystallization after the subsequent annealing at 350 °C.

Influence of thermal energy on exchange-bias studied by finite-element simulations

In this article we describe the thermal relaxation in anti-ferromagnetic/ferromagnetic bilayers using a hybrid method that combines a kinetic Monte Carlo technique with magnetization dynamics following the Landau Lifshitz Gilbert equation. A granular anti-ferromagnetic layer is exchange coupled to an amorphous ferromagnetic layer and discretized using a finite element method. Calculations are made to help clarify how the underlying magnetic structure is related to the measured exchange bias fields as a function of temperature for the case of amorphous Co65.5Fe14.5B20/granular Ir22Mn78 bilayers. Our calculations are in excellent agreement with experimentally measured macro-magnetic properties of these bilayers.

Exchange-bias in amorphous ferromagnetic and polycrystalline antiferromagnetic bilayers: Structural study and micromagnetic modeling

We study the role of the structure of antiferromagnetic polycrystalline metallic films in determining the magnetic properties of an exchange-coupled amorphous ferromagnetic layer. The bilayers are sputter-deposited, highly textured {111} Ir22Mn78 and Co65.5Fe14.5B20 thin films. We focus on structural characterization of Ir22Mn78 as a function of layer thickness in the range having the strongest influence over the exchange-bias field and training effect. We have used transmission electron microscopy to characterize defects in the form of interface steps and roughness, interdiffusion, twin- and grain-boundaries. Such defects can result in uncompensated magnetic spins in the antiferromagnet, which then contribute to exchange-bias. These experimental results form the basis of a general model, which uses finite element micromagnetic simulations. The model incorporates the experimental structural parameters of the bilayer by implementing a surface integral technique that allows numerical calculations to solve the transition from an amorphous to a granular structure. As a result, a detailed calculation of the underlying magnetic structure within the antiferromagnetic material is achieved. These calculations are in good agreement with micromagnetic imaging using Lorentz transmission electron microscopy and the macro-magnetic properties of these bilayers.

Exchange bias interactions in polycrystalline/amorphous bilayers

For technologically relevant systems of polycrystalline antiferromagnetic layer coupled to an amorphous ferromagnetic layer, quantitative models and micromagnetic simulations are challenging due to inherent structural differences. We present a numerical study, performed using a surface integral technique with finite element micromagnetic simulations, that can incorporate structural and magnetic parameters such as grain crystallography, mixed spin-interface coupling and granular stability, arising from grain volume and anisotropy. We show that this model is in good agreement with experimental results for exchange bias and coercive fields as well as the training effect.

Interface stability between amorphous ferromagnetic layer and Al oxide barrier in tunneling magnetoresistive films at elevated temperatures

The interface stability and microstructure between amorphous and crystalline ferromagnetic (FM) layers Fe56Co24B20 and Co70Fe30 (at. %) and oxide barrier layers (AlO) deposited by physical vapor deposition, in both as-deposited and annealed states, have been studied using hysteresis loops for magnetic measurement, x-ray photoelectron spectroscopy for elemental depth profiling, and transmission electron microscopy for atomic-level microstructure. AlO was found to be amorphous on both amorphous Fe56Co24B20 and polycrystalline Fe30Co70 FM layers. Substantial Fe diffusion towards the AlO layer and Al diffusion towards the FM layer are clearly observed for the Fe56Co24B20∕AlO system when annealed above 360°C and will likely cause magnetic tunneling junction devices made from this system to fail.

Theoretical and experimental investigation on giant magnetoresistive materials with amorphous ferromagnetic layer

Pseudo-spin-valve (PSV) sandwiches using amorphous CoNbZr alloy as soft magnetic layer were fabricated by magnetron sputtering. The giant magnetoresistance (GMR) and its dependence on the thickness of magnetic layer were investigated. Anti-parallel magnetization alignments were observed in the samples with very thin CoNbZr thickness (2–4 nm) and a maximum GMR ratio of 6.5% was obtained. The Camley-Barnas semiclassical model was extended for amorphous layer based magnetic sandwiches by considering that the mixed layers exist between the ferromagnetic and nonmagnetic layer. The calculated results agree with the experimental results very well, indicating that the new model gives a more realistic picture of the physical processes that take place in the magnetic sandwiches. Moreover, the calculated results for amorphous sandwiches also clarify that the occurrence of maximum GMR at very small thickness of amorphous layer is ascribed to the short mean-free-path in amorphous materials.

Theoretical model of giant magnetoresistive sandwiches with polycrystalline and amorphous ferromagnetic layer

The Camley–Barnas semiclassical model has been extended for magnetic sandwiches by considering the existing of inter-diffused region between the ferromagnetic and nonmagnetic layer. The giant magnetoresistance (GMR) curves of magnetic sandwiches with polycrystalline and amorphous magnetic layer were calculated respectively with this model. The calculations reproduce the experimental results well for both kinds of sandwiches, indicating that this new model is more effective than the original one.

Observation of thickness dependence of magnetic surface anisotropy in ultrathin amorphous films.

Ferromagnetic resonance (FMR) and SQUID magnetometry measurements have been made on multilayers of amorphous ${\mathrm{Fe}}_{70}$${\mathrm{B}}_{30}$/Ag. The dependence of the magnetic surface anisotropy constant ${\mathit{K}}_{\mathit{s}}$ on the magnetic layer thickness 2L has been determined in the range 1.6 \AA{}2L90 \AA{} using more than twenty samples. It is found that ${\mathit{K}}_{\mathit{s}}$ is constant for 2L&gt;16.5 \AA{}, but decreases monotonically towards zero as 2L decreases from 16.5 \AA{} towards zero. The FMR results can be well described by a theory developed for ultrathin amorphous ferromagnetic layers.