D. Spenato

Asymmetric magnetization reversal behavior in exchange-biased NiFe∕MnPt bilayers in two different anisotropy regimes: Close and far from critical thickness

The effect of the ratio between the uniaxial and unidirectional anisotropy on magnetization reversal in NiFe∕MnPt bilayers has been systematically studied using vectorial vibrating magnetometer. Depending on the balance between these two anisotropies the magnetization reverses either in the opposite or the same semicircles during the ascending and descending branches of the hysteresis loop. A simple modified coherent rotation model provides a good description of the magnetization reversal in these bilayers.

Study of dynamic properties and magnetic anisotropies of NiFe/MnPt in the critical thickness range

The interfacial ferromagnetic/antiferromagnetic exchange coupling and ferromagnetic and antiferromagnetic anisotropies of polycrystalline ferromagnetic/antiferromagnetic exchange biased bilayers of NiFe/MnPt grown by RF sputtering have been investigated with quasi-static and dynamic measurements for different MnPt and NiFe thicknesses. The dynamic properties have been investigated through complex permeability frequency spectra measurements in the 100 MHz–3 GHz range. An effective field is deduced from dynamic measurements. A sharp enhancement of this effective field appears above a critical MnPt thickness (4.7 nm) lower than the critical thickness for the appearance of the easy axis hysteresis loop shift (7.8 nm). We observe also a strong enhancement of the damping parameter for the former thickness. This may be attributed to the superparamagnetic to antiferromagnetic transition in the antiferromagnetic layer. The anisotropy constant of the antiferromagnetic layer determined with dynamic measurements is in agreement with the one deduced from static measurements. The interface exchange energy JE deduced from dynamic measurement is twice higher than the one obtained with hysteresis loop measurements. The dispersion relation of the resonance frequency of imaginary part of the permeability shows hysteretic behaviour and discontinuities when measured in presence of a static magnetic field applied along the easy axis of the ferromagnetic layer. This is attributed to partial magnetization switching by rotation mechanism in the ferromagnetic layer.

Experimental evidence for exchange bias in polycrystalline BiFeO3/Ni81Fe19 thin films

We report on experimental evidence of exchange bias between a polycrystalline antiferromagnetic BiFeO3 and a ferromagnetic Ni81Fe19 film at room temperature. The measured 17 Oe hysteresis loop shift corresponds to an exchange energy of 11×10-3 erg/cm2 per unit area of the interface coupling the two films, which is comparable with those observed for similar epitaxially-grown systems. The azimuthal behavior of the longitudinal and transverse magnetization components revealed the presence of induced unidirectional and biquadratic anisotropies. A misalignment between unidirectional and biquadratic anisotropy axes was also observed.

Large exchange bias in interdiffused NiFe/Mn bilayers

The effect of the annealing temperature a well as the antiferromagnetic (AF) and ferromagnetic (F) thickness on exchange coupling in Ni81Fe19/Mn single bilayers was systematically studied. We then show that a large exchange coupling is observed when the bilayers are annealed at 300 °C: it corresponds to an exchange field of about 290 Oe. This large exchange field is induced with a rather thick F layer of 46 nm. Both the exchange field Hex and coercive field Hc increase with Mn thickness reaching saturation around 100 nm of Mn. The exchange coupling is associated with interfacial diffusion at the NiFe/Mn interface. This result is confirmed by a large change in magnetization and resistivity before and after annealing. We show that the existence of the exchange field is associated with the clear evidence of a new FeMnNi phase in the x-ray spectrum.

Asymmetric magnetization reversal in exchange-biased polycrystalline F/AF bilayers

This paper describes a model for magnetization reversal in polycrystalline ferromagnetic (F)/antiferromagnetic (AF) exchange-biased bilayers. We assume that the exchange energy can be expanded into cosine power series. We show that it is possible to fit experimental asymmetric shape of hysteresis loops in exchange-biased bilayer for any direction of the applied field. The hysteresis asymmetry is discussed in terms of energy considerations. An angle β is introduced to quantify the easy axis dispersion of AF grains.

Probing misalignment in exchange biased systems: A dynamic approach

An experimental method based on ferromagnetic resonance is proposed to quantify the misalignment between ferromagnetic and antiferromagnetic easy axis in exchange biased systems. Our experimental study deals with Ni81Fe19/Al2O3 and exchange biased Ni81Fe19/NiO bilayers. Performing ferromagnetic resonance studies, we demonstrate that the misalignment is present in the exchange biased system. It is revealed by an asymmetry of the resonance field angular dependence. Using a simple model, the angle of misalignment is determined.

Effect of magnetization ripple at high frequencies in amorphous ferromagnetic thin films

An analytical calculation of the permeability frequency spectra using Landau–Lifshitz theory is presented and compared to experimental results obtained on soft amorphous ferromagnetic thin films with an in-plane uniaxial magnetic anisotropy. The disagreement between theory and experiment is attributed to a small magnetic anisotropy dispersion in the film (magnetization ripple). We have measured the real part of the transverse biased permeability μt′ at different frequencies of the exciting field in order to estimate the ripple effects in our films and to improve our model. We have plotted the value of the bias field Hm, at which μt′ versus exciting field frequency reaches a maximum. Hm is found to be frequency independent at low frequencies (up to 280 MHz). Above a critical frequency it increases rapidly, suggesting that ripple effects are strongly frequency dependent.

Domain walls dynamics and separation of magnetization processes in in-plane anisotropic amorphous ferromagnetic thin films

Permeability due to domain wall motion and spin rotation is studied on CoZr and CoZrRe thin films presenting different distributions of anisotropy. The influence of the amplitude and frequency of the exciting field on the two magnetization processes, domain wall motion and spin rotation, is studied, showing the existence of a critical field for domain wall depinning and a domain wall relaxation. We show that it is possible to separate and evaluate the contribution of the two magnetization processes to the permeability and analyse the influence of the anisotropy dispersion on the frequency spectra of the permeability.

Driving mechanism of exchange bias and magnetic anisotropy in multiferroic polycrystalline BiFeO3/permalloy bilayers

Magnetization reversal processes of Permalloy exchange coupled with thin polycrystalline antiferromagnetic multiferroic BiFeO3 films were investigated. The BiFeO3 critical thickness and the interfacial exchange energy are comparable with similar epitaxially grown systems. However, the coercive field dependence on multiferroic thickness presents an atypical behaviour. This system shows induced unidirectional, uniaxial, and biquadratic-like behaviour with misalignment of magnetic easy axis with respect to pinning field direction.

A broad-band method for measuring the complex permeability of thin soft magnetic films

Abstract An automated method for measuring the complex permeability of anisotropic thin magnetic films over a broad frequency band is described. The method is based on the measurement, by a network analyser, of scattering matrix elements of a rectangular coaxial transmission line loaded by the sample. The sample is deposited onto a nonmagnetic substrate, so that the measured S parameters are characteristic of the total transverse section of the line. To relate those measured values to thin-film permeability, a numerical resolution of Laplaces equation in a quasi-TEM approximation is used. Results showing the complex permeability of CoZr thin films versus frequency for different orientations of the magnetic exciting field, obtained by this method, are presented.