Exchange bias and spin-reorientation transition in α-Fe/PrDyCoFeB core/shell microwires

Highlights: • The α-Fe/PrDyCoFeB core/shell microwires possess exchange biased hysteresis loop. • Transition from ferrimagnet to Ising spin-glass state in PrDyCoFeB shell. • Core/shell exchange coupling provides hysteresis with four magnetization levels. • Amorphous shell contributes to magnetization in high magnetic fields ~10 kOe. • Polycrystalline single domain core manifests rectangular hysteresis loop. Magnetic anisotropy and interface exchange interaction were analyzed in α-Fe core microwire covered with PrDyCoFeB amorphous shell. The α-Fe/PrDyCoFeB core/shell microwires were grown by pendant drop melt extraction technique providing separation of the α-Fe and PrDyCoFeB phases during the ultrafast cooling. Low saturation field of the core ~100 Oe and high saturation field of amorphous shell ~10 kOe with zero coercivity were distinguished from angular dependences of the magnetic moment, recorded in low and high magnetic fields. Sharp decrease of the longitudinal magnetization, effective anisotropy field and magnetic susceptibility have been observed below the critical temperature, T_crit = 240 K, in zero field. The Almeida-Thouless transition from ferrimagnetic state to Ising spin glass state has been revealed in PrDyCoFeB amorphous shell. Such spin reorientation transition is very attractive for magnetocaloric applications. We found exchange bias effect controlled by exchange coupling between ferromagnetic core and ferrimagnetic shell. The significance of the obtained data lies in possible applications of the core/shell microwires for tweezers with magnetic moment, stepwise changing in external field. Exchange bias in core-shell interface provides determined initial state of the microwire used as working media of field sensor. The sharp decrease of the magnetization associating with spin-reorientation transition looks very promising for magnetocaloric applications close to room temperature.