Tunable high-field magnetization in strongly exchange-coupled freestanding Co/CoO core/shell coaxial nanowires

The exchange bias properties of Co/CoO coaxial core/shell nanowires were investigated with cooling and applied fields perpendicular to the wire axis. This configuration leads to unexpected exchange-bias effects. First, the magnetization value at high fields is found to depend on the field-cooling conditions. This effect arises from the competition between the magnetic anisotropy and the Zeeman energies for cooling fields perpendicular to the wire axis. This allows imprinting predefined magnetization states to the antiferromagnetic (AFM) shell, as corroborated by micromagnetic simulations. Second, the system exhibits a high-field magnetic irreversibility, leading to open hysteresis loops attributed to the AFM easy axis reorientation during the reversal (effect similar to athermal training). A distinct way to manipulate the high-field magnetization in exchange-biased systems, beyond the archetypical effects, was thus experimentally and theoretically demonstrated.The work was supported by the 2014-SGR-1015 and 2014-SGR-150 projects of the Generalitat de Catalunya, the Spanish MAT2012-35370 project, European Community’s Seventh Framework Programme (FET-Open/FP7/2007-2013) under grant agreement 296679 (MANAQA), and the Brazilian agency CAPES (BEX 0298/2015-08). G.S.A. thanks the Knut and Alice Wallenberg Foundation (Project 3DEM-NATUR) for the partial financial support. A.S. acknowledges funding from an ICREA Academia award. The authors thank Maria Esther Mata-Zamora and Raul Valenzuela for providing the nanowires. ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295).Peer Reviewe