Exchange-coupling in thermal annealed bimagnetic core/shell nanoparticles

Abstract

In this study we demonstrate that the effective coupling of the magnetic phases in core/shell nanoparticles can be promoted by an appropriate thermal annealing. In this way, the magnetization thermal stability of the hard ferrimagnetic CoFe 2 O 4 oxide can be increased up to room temperature when coupled to a CoO antiferromagnetic core in an inverse core/shell structure. In addition, the results show that, being encapsulated in a ~2 nm thick CoFe 2 O 4 shell, the CoO core is successfully protected against oxidation which is crucial for the effectiveness of the magnetic coupling at the interface.

Tunnel Magnetoresistance in Self-Assemblies of Exchange-Coupled Core/Shell Nanoparticles

“…[28] The coercive field in these systems can be finely modified through the interface magnetic coupling [29][30][31][32][33][34], the core size and shell thickness, [35][36][37] or the magnetic anisotropy of the components. [23,[38][39][40] Devices of this type should provide a way to manipulate at will the characteristic switching field of TMR by controlling the magnetic coupling across the core/shell interface.…”

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

“…[30][31][32][33][34][35][36][37][38][39][40][41][42] Interestingly, in this system H E values ranging from a few to thousands of Oe have been reported for relatively similar nanoparticles. Several effects have been reported to influence HE in Co/CoO: core diameter, 34,39,40 shell thickness, 34,37,39,40 crystallinity of the shell, 32,43,44 exchange interactions with neighboring particles, 32,45 strains, 34 orbital moments, 45 uncompensated spins 33,41 and the matrix they are embedded in (through lattice matching effects or antiferromagnetic proximity effects). 31,33,36,46 Concerning the role of the matrix in Co/CoO, HE values extending from 10 Oe (in non-AFMnon-lattice matched matrices, e.g., Al2O3 ) 31 to 7500 Oe (in AFMlattice matched matrices, e.g., CoO) 31,36 have been reported for rather similar nanoparticles.…”

Enhanced Magnetic Properties in Antiferromagnetic-Core/Ferrimagnetic-Shell Nanoparticles

“…Interestingly, currently, there is an increasing interest in, so-called, inverted structures (see Fig. 1), where the shell is FM or ferrimagnetic (FiM) and the core is AFM, containing for example Mn oxides12131415161718, Fe oxides1920212223242526272829, Co oxides30313233343536, Cr oxides373839, metallic FePt40 or even multiferroic BiFeO 3 (Refs. 41,42).…”

“…It has been demonstrated, experimentally and theoretically, that the poor crystallinity of the AFM counterpart can result in considerably inferior exchange bias properties4344. In fact, inverted structures have already demonstrated very large coercivities and loop shifts, tunable blocking temperatures, enhanced Néel temperatures or proximity effects12131415161718192021222324252627282930313233343536373839404142 and have been proposed as potential magnetoelectric random access memories41. However, despite their potential, systematic studies of size effects (i.e., core diameter or shell thickness) are still rather scarce12162225333435.…”