Observation of an atomic exchange bias effect in DyCo<inf>4</inf> film

Abstract

The fundamental important and technologically widely employed exchange bias effect occurs in general in bilayers of magnetic thin films consisting of antiferromagnetic and ferromagnetic layers where the hard magnetization behavior of an antiferromagnetic thin film causes a shift in the magnetization curve of a soft ferromagnetic film. The minimization of the single magnetic grain size to increase the storage density and the subsequent demand for magnetic materials with very high magnetic anisotropy requires a system with high H EB . Here we report an extremely high H EB of 4 Tesla observed in a single amorphous DyCo 4 film close to room temperature. The origin of the exchange bias can be associated with the variation of the magnetic behavior from the surface towards the bulk part of the film revealed by X-ray absorption spectroscopy and X-ray magnetic circular dichroism techniques utilizing the bulk sensitive transmission and the surface sensitive total electron yield modes. The competition between the atomic exchange coupling in the single film and the Zeeman interaction lead to an intrinsic exchanged coupled system and the so far highest exchange bias effect H EB = 4 Tesla reported in a single film, which is accommodated by a partial domain wall formation.Exchange bias effect (EB) was discovered in 1956 by Meiklejohn and Bean when studying Co particles embedded in their native antiferromagnetic oxide 1 . It is generally considered to form from an uncompensated spin configuration at the ferromagnetic/antiferromagnetic (FM/AF) interface 2,3 , as it is the case in small particles, inhomogeneous materials, FM films on AF single crystals and FM on AF thin films 4 with frozen and rotatable spins at their interfaces [5][6][7][8][9] . Phenomenologically, the EB effect in FM/AF systems displays a shift of the hysteresis by an EB field H EB that is achieved by a magnetic field cooling procedure down to the Néel temperature T N of the AF. The experimentally observed value of the H EB in FM/AF systems, however, is in general several orders of magnitude below the theoretical prediction for a perfect EB system 10 . This discrepancy resulted in a heavy debate and the development of sophisticated models for the explanation of the origin of the EB effect and its drastic reduction of the EB effect in real EB systems 4,11,12 . Besides the classical system of AF/FM interfaces, EB and related effects have been observed also in other types of samples, e.g. involving ferrimagnets (FI): AF/FI 13 , FI/FM 14-16 and lately also in FI/FI 17 with a compensated spin structures at the interface. Transition Metal-Rare Earth (TM-RE) alloys, in particular, are nowadays suggested to used as FI materials in magnetic hybrid structures exhibiting strong EB effects 18 . Besides an interfacial exchange between two chemically and magnetically different compositions, a physically induced magnetic phase deviation from the bulk to the surface may also results into an exchange bias effect although there is no obvious chemical interface in the sample. The...

Maximizing Exchange Bias in Co/CoO Core/Shell Nanoparticles by Lattice Matching between the Shell and the Embedding Matrix

“…1,2,4,5 In this respect, although archetypical exchange bias systems, e.g., AFM/FM bilayers or FM-AFM core/shell nanoparticles, typically exhibit HE values in the range of tens to hundreds of Oe; 1,2,4,5 HE values in excess of 10 kOe have also been occasionally reported. [14][15][16][17][18][19] However, apart from some possible minor-loop issues, 20,21 many of these systems are based on phase-separation. 14,16,17 This leads to illdefined FM and AFM counterparts often with FM phases of very small dimensions.…”