Abstract
Exchange bias properties of MnFe_2O_4@gamma–Fe_2O_3 core–shell nanoparticles are investigated. The measured field and temperature dependencies of the magnetization point out a well-ordered ferrimagnetic core surrounded by a layer with spin glass-like arrangement. Quasi-static SQUID magnetization measurements are presented along with high-amplitude pulse ones and are cross-analyzed by comparison against ferromagnetic resonance experiments at 9 GHz. These measurements allow one to discern three types of magnetic anisotropies affecting the dynamics of the magnetic moment of the well-ordered ferrimagnetic NP’s core viz. the easy-axis (uniaxial) anisotropy, the unidirectional exchange-bias anisotropy and the rotatable anisotropy. The uniaxial anisotropy originates from the structural core–shell interface. The unidirectional exchange-bias anisotropy is associated with the spin-coupling at the ferrimagnetic/spin glass-like interface; it is observable only at low temperatures after a field-cooling process. The rotatable anisotropy is caused by partially-pinned spins at the core/shell interface; it manifests itself as an intrinsic field always parallel to the external applied magnetic field. The whole set of experimental results is interpreted in the framework of superparamagnetic theory, i.e., essentially taking into account the effect of thermal fluctuations on the magnetic moment of the particle core. In particular, it is found that the rotatable anisotropy of our system is of a uniaxial type.
Highlights
Exchange bias properties of MnFe2O4@γ–Fe2O3 core–shell nanoparticles are investigated
Comparative analysis of the quasistatic and magnetodynamic measurements points out that in CS particles, as in multilayer films, the exchange coupling, along with customary exchange bias (EB) anisotropy, causes another type of a nisotropy[22,23]. The latter is known as rotatable anisotropy (RA)[24], and is unambiguously detected in ferromagnetic resonance (FMR) experiments where it manifests itself as an additional internal field HRA that readily follows the direction of the imposed magnetizing field H
In this paper, taking as test objects the samples consisting of MnFe2O4@γ–Fe2O3 CS nanoparticles dispersed in a carrier, we show that the temperature dependence of anisotropy fields ( HEB and HRA ) is a key issue for distinguishing the interfacial magnetic anisotropies of CS nanoparticles
Summary
Exchange bias properties of MnFe2O4@γ–Fe2O3 core–shell nanoparticles are investigated. The unidirectional exchange-bias anisotropy is associated with the spin-coupling at the ferrimagnetic/spin glass-like interface; it is observable only at low temperatures after a field-cooling process. Comparative analysis of the quasistatic and magnetodynamic (ferromagnetic resonance) measurements points out that in CS particles, as in multilayer films, the exchange coupling, along with customary EB anisotropy, causes another type of a nisotropy[22,23] The latter is known as rotatable anisotropy (RA)[24], and is unambiguously detected in ferromagnetic resonance (FMR) experiments where it manifests itself as an additional internal field HRA that readily follows the direction of the imposed magnetizing field H. The existence of the RA as such complies in full with the general concept that the interface spin clusters together with the spin layers adjacent to the surfaces are responsible for a plethora of strong and observable magnetic e ffects[2]
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