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Yang J.,Institute Sistemas e Computadores Microsistemas e Nanotecnologias | Cardoso S.,Institute Sistemas e Computadores Microsistemas e Nanotecnologias | Freitas P.P.,Institute Sistemas e Computadores Microsistemas e Nanotecnologias | Devolder T.,University Paris - Sud | Ruehrig M.,Siemens AG
Applied Physics Letters | Year: 2010

The stability of the exchange bias field Heb has been studied for Mn80Ir20/Co60Fe20B20 thin films by means of network analyzer ferromagnetic resonance. The results demonstrated that Heb decreases with increasing temperature. The observed variation in the magnitude of Heb and even a reversal in the sign of Heb as a function of time above room temperature can be interpreted by a thermally activated reversal of antiferromagnetic domains as evidenced by the linear dependence of Heb on ln t according to the Ńel-Arrhenius law. Moreover, the correlation between the reversal of antiferromagnetic domains and the antiferromagnetic grain volume is obtained. © 2010 American Institute of Physics.


Yang J.,Institute Sistemas e Computadores Microsistemas e Nanotecnologias | Cardoso S.,Institute Sistemas e Computadores Microsistemas e Nanotecnologias | Cardoso S.,University of Lisbon | Freitas P.P.,Institute Sistemas e Computadores Microsistemas e Nanotecnologias | And 3 more authors.
Journal of Applied Physics | Year: 2011

We investigated the dynamics of exchange bias field HEB of Mn80Ir20 (80 A°)/Co60Fe20B 20 (110 ) thin films at temperatures of 300, 375, and 450 K by means of vector network analyzer ferromagnetic resonance. The results show that H EB deduced from the resonance frequencies along the hysteresis loop varies significantly and that this large variation is due to thermally activated reversal of the antiferromagnetic domains during measurements. Furthermore, the results of HEB as a function of angle show that HEB is mainly unidirectional with fewer contributions from the uniaxial anisotropy as evidenced by the fits to the data based on the Fourier cosine series. It is also found that the symmetries of HEB at temperatures higher than 300 K decreases compared with that at 300 K. © 2011 American Institute of Physics.


Bellini E.,University of Glasgow | Bellini E.,University of Salento | McVitie S.,University of Glasgow | Maclaren D.A.,University of Glasgow | And 5 more authors.
Journal of Physics D: Applied Physics | Year: 2013

We investigated the physical microstructure and the domain structure of a CoFeB layer 30 and 14 thick, embedded in a multilayer composed of MgO (50)/CoFeB/MgO (15), by transmission electron microscopy and Lorentz microscopy. Structurally both films appeared similar and polycrystalline in character. However, the magnetization reversal behaviour observed during Lorentz microscopy experiments was found to vary considerably with the CoFeB thickness. Indeed, in the 14 CoFeB layer we found an unusual asymmetric orientation of domain walls when varying the direction of the applied field. To assist in the interpretation of the experimental results, we carried out calculations using a modified Stoner-Wohlfarth model, including unidirectional anisotropy. A good agreement was found between measured and calculated domain wall orientations. To explain the presence of the unidirectional anisotropy, we propose the possibility of formation of an interfacial structure that can give rise to an exchange bias effect, which has been previously seen from other studies. © 2013 IOP Publishing Ltd.

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