D. T. Dekadjevi

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.

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.

Driving mechanism for damping and g-factor in non-amorphous ferromagnetic CoFeZr ultrathin films

– We demonstrate that an in-plane uniaxial anisotropy may be induced in non-amorphous soft CoFeZr films. We used broadband ferromagnetic resonance spectroscopy and complex permeability spectra to investigate the spin dynamics in CoFeZr films. We report a systematic study of the FM thickness on the fundamental dynamic parameters such as the effective magnetisation, the g-factor and relaxation mechanisms. Our study reveals that the decrease of the effective magnetisation mesured with FMR with thickness is not due to perpendicular anisotropy but to low dimentionality. Moreover, we observed a decrease of the g-factor with thickness and a modification of the ratio of the orbital to the spin magnetic moment. These films exhibit good high-frequency performance red (i.e. high permeability in a broad frequency range and a low damping) at low thickness of about a few nanometers. Copyright c EPLA, 2016 Nowadays, spintronics devices and magnetic media have to operate at the gigahertz regime (i.e. with ns reversal times). Magnetic thin films are widely used and studied because the ability of their magnetization to precess/reverse in a high frequency-short time range (several GHz/ns) [1,2]. For high-frequency applications, these magnetic materials need to have a large permeabil-ity in a broad frequency range [3]. The key parameters that govern spin dynamics are the saturation magnetiza-tion, the effective field and the (Lande) factor [4]. FeCo alloy should be, at first glance, one of the most competitive candidates because of its high saturation mag-netization M S (2.45 T). However, as-deposited FeCo films exhibit high coercive field ranging from 100 to 200 Oe, in-plane isotropic magnetic anisotropy and a large magne-tocrystalline anisotropy which hinders the high-frequency applications ([5] and references therein). A solution is to produce amorphous CoFe-based alloys by alloying met-alloid into the FM matrix: the addition of a metal-loid in the ferromagnet (FM) destroys the cristallinity, (a)

FMR studies of exchange-coupled multiferroic polycrystalline Pt/BiFeO3/Ni81Fe19/Pt heterostructures

An experimental study of the in-plane azimuthal behaviour and frequency dependence of the ferromagnetic resonance field and the resonance linewidth as a function of BiFeO3 thickness is carried out in a polycrystalline exchange-biased BiFeO3/Ni81Fe19 system. The magnetization decrease of the Pt/BiFeO3/Ni81Fe19/Pt heterostructure with BiFeO3 thickness deduced from static measurements has been confirmed by dynamic investigations. Ferromagnetic resonance measurements have shown lower gyromagnetic ratio in a perpendicular geometry compared with that of parallel geometry. The monotonous decrease of gyromagnetic ratio in perpendicular geometry as a function of the BiFeO3 film thickness seems to be related to the spin–orbit interactions due to the neighbouring Pt film at its interface with Ni81Fe19 film. The enhancement of gyromagnetic ratio in Pt/Ni81Fe19/Pt is attributed to the Pt. The in-plane azimuthal shape of the total linewidth of the uniform mode shows isotropic behaviour that increases with BiFeO3 thickness. The study of the frequency dependence of the resonance linewidth in a broad band of 3–35 GHz has allowed the determination of intrinsic and extrinsic contributions to the relaxation as a function of BiFeO3 thickness in perpendicular geometry. In our system the magnetic relaxation is dominated by the spin-pumping mechanism due to the presence of Pt. The insertion of BiFeO3 between Pt and Ni81Fe19 attenuates the spin-pumping damping at one interface.

Thermal simulation of magnetization reversals for size-distributed assemblies of core-shell exchange biased nanoparticles

A temperature dependent coherent magnetization reversal model is proposed for size-distributed assemblies of ferromagnetic nanoparticles and ferromagnetic-antiferromagnetic (AF) core-shell nanoparticles. The nanoparticles are assumed to be of uniaxial anisotropy and all aligned along their easy axis. The thermal dependence is included by considering thermal fluctuations, implemented via the Neel-Arrhenius theory. Thermal and angular dependence of magnetization reversal loops, coercive field, and exchange-bias field are obtained, showing that ferromagnetic-antiferromagnetic size-distributed exchange-coupled nanoparticles exhibit temperature-dependent asymmetric magnetization reversal. Also, non-monotonic evolutions of exchange-bias and coercive fields with temperature are demonstrated. The angular dependence of coercive field with temperature exhibits a complex behavior, with the presence of an apex, whose position and amplitude are strongly temperature-dependent. The angular dependence of exchange bias wi...

Temperature dependence of the exchange bias properties in polycrystalline BiFeO 3 /Ni 80 Fe 20

In this study, we report a temperature dependent analysis of the exchange bias properties in BFO/Py bilayers as a function of the BFO thickness. The temperature dependence of the exchange bias field (H_e) and the coercive field (H_c) are first presented. To understand exchange bias magnetization reversal and the magnetic anisotropies, we will then present an azimuthal study of H_e and H_c at 300 K and 77 K. Finally, results of a controlled field cooling protocol applied on all samples will be discussed to understand anisotropy energy distribution. The bilayers were grown by radio-frequency sputter deposition, with the following structure: Si/Pt(14 nm)/BiFeO₃(t_BFO)/Ni₈₀Fe₂₀(10 nm)/ Pt(10 nm), with t_BFO among 0 nm, 29 nm and 177 nm . To induce uniaxial anisotropy, a 300 Oe field H_dep was applied during the growth . The XRD analysis confirmed a single polycrystalline structure for the BFO layer.

Driving the magnetization reversal below the blocking temperature in exchange biased NiFe/NiO

The temperature dependence of the exchange bias field and coercive field was studied in a polycrystalline NiFe layer coupled with a diluted NiO layer. The temperature behavior of both fields is modified by cooling the bilayer below the Curie, Neel, and/or blocking temperatures. Below these temperatures, the presence of double hysteresis loops demonstrates the key role of the NiFe multidomain state during the cooling procedure.