S. P. Pogossian

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.

Neutron and x-ray propagation laws in thin film waveguides

Abstract The guide propagation properties of neutron waves and x-rays are described. In spite of apparent resemblance to optical dielectric waveguides, these new neutron/x-ray waveguides have several interesting features. First, the modal dispersion function has the same form as those of the media composing the guide. Second, there is no cutoff wavelength. Third, the neutron waves/x-rays with different wavelengths may be guided with the same mode number and effective coherent nuclear scattering length densities. Fourth, the energy distribution on the cross-section of the guide is independent from the neutron waves/x-rays wavelength, which leads to an energy transfer by channels through the thin film neutron/x-ray guide.

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.

Direct and inverse measurement of thin films magnetostriction

Abstract Two techniques of measurements of thin film magnetostriction are compared: direct, when changes of the substrate curvature caused by the film magnetization are controlled, and inverse (“indirect”), when the modification of the magnetic anisotropy induced by the substrate deformation (usually bending) is measured. We demonstrate how both the elastic strength of the substrate and the effective magneto-mechanical coupling between the substrate deformation and magnetic anisotropy of the film depend on different conditions of bending. Equations to be used for magnetostriction value determination in typical cases are given and critical parameters for the corresponding approximations are identified.

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.

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.

Observation of magnetic thin films neutron waveguides

Abstract Magnetic Co 95 Zr 05 /Al/Co 95 Zr 05 tri-layers structure have been deposited on float glass and on Gd 3 Ga 5 O 12 . Polarised guided neutron waves in the Al layer are expected in such structures. Two different type of air prism have been used for coupling the neutronwave to the waveguide. In both structures the measured transmitted resonance are presented and compared with the predicted one.

Determination of the Parameters of Rectangular Dielectric Waveguides by New Effective Index Methods

Abstract New methods based on effective index considerations are proposed to determine the propagation characteristics of rectangular dielectric waveguides. One of these methods, being based on symmetry considerations, does not have the ambiguity inherent to the effective index method. The ambiguous character of effective index method is thoroughly analysed.