Exchange Bias Demonstrated in Bulk Nanocomposites Processed by High-Pressure Torsion

Abstract

Ferromagnetic (Fe or Fe20Ni80) and antiferromagnetic (NiO) phases were deformed by high-pressure torsion, a severe plastic deformation technique, to manufacture bulk-sized nanocomposites and demonstrate an exchange bias, which has been reported predominantly for bilayer thin films. High-pressure torsion deformation at elevated temperatures proved to be the key to obtaining homogeneous bulk nanocomposites. X-ray diffraction investigations detected nanocrystallinity of the ferromagnetic and antiferromagnetic phases. Furthermore, an additional phase was identified by X-ray diffraction, which formed during deformation at elevated temperatures through the reduction of NiO by Fe. Depending on the initial powder composition of Fe50NiO50 or Fe10Ni40NiO50 the new phase was magnetite or maghemite, respectively. Magnetometry measurements demonstrated an exchange bias in high-pressure torsion-processed bulk nanocomposites. Additionally, the tailoring of magnetic parameters was demonstrated by the application of different strains or post-process annealing. A correlation between the amount of applied strain and exchange bias was found. The increase of exchange bias through applied strain was related to the microstructural refinement of the nanocomposite. The nanocrystalline maghemite was considered to have a crucial impact on the observed changes of exchange bias through applied strain.

Magnetic Materials via High-Pressure Torsion of Powders

“…Reprinted from. [130] The coercivity and the squareness of the hysteresis loops can be also enhanced at room temperature in nanocomposites consisting of ferromagnetic and antiferromagnetic components with a Neel temperature, TN, above room temperature using the exchange bias effect. The enhancement is maximum after field annealing above TN.…”

“…In a first study, the possibility to synthesize nanocomposites by HPT-deformation consisting of Fe (ferromagnetic) and NiO (antiferromagnetic) phases with a phase ratio of 1:1 was investigated. In [130] , the synthesis of ferromagnetic-antiferromagnetic nanocomposites by HPT was evaluated to a much greater extent by studying also different ratios of ferromagnetic-antiferromagnetic phases (Fe or Fe20Ni80 and NiO) and by further investigating the influence on magnetic properties. With rising strain, an increase in exchange bias (Heb) was found which could be related to the microstructural refinement of the nanocomposites.…”