Annealing driven positive and negative exchange bias in Fe–Cu–Pt heterostructures at room temperature

Abstract

We report annealing induced exchange bias in Fe-Cu-Pt based heterostructures with Cu as an intermediate layer (Fe/Cu/Pt heterostructure) and capping layer (Fe/Pt/Cu heterostructure).Exchange bias observed at room temperature (300 K) is found to be dependent on the annealing temperature. We obtained positive exchange bias of ~ +120 Oe on annealing both the heterostructures at 400 o C, while on annealing these heterostructures at 500 and 600 o C a negative exchange bias of ~ -100 Oe was found. X-ray reflectivity and polarized neutron reflectivity measurements provided evolution of depth dependent structure and magnetic properties of the heterostructures on annealing at different temperatures and revealed coexistence of soft and hard (alloy) magnetic phases across the thickness of the films. Rapid and long range interdiffusion at interfaces on annealing the systems at a temperature ˃ 400 o C resulted into formation of a ternary alloy phase. These results can be understood within the context of a very unusual interface exchange interaction at the interface of hard/soft magnetic phases, which are dependent on the annealing temperature.Recently, efforts have been made to reduce the ordering temperature by introducing (a) a buffer layer [16][17][18][19][20], (b) a cap layer [21,22], and (c) addition of third element to form a ternary alloy compound [23][24][25][26]. One of the effective ways to reduce the ordering (FCC to FCT) temperature of FePt film is by adding Cu to FePt and realizing the formation of (FePt) 1-x Cu x ternary alloys (x = 0-30%) [27][28][29][30][31][32]. These ternary alloys show drastically different magnetic properties [27][28][29][30][31][32]. Earlier, we studied the formation of ternary alloy FePtCu on annealing FePt/Cu multilayer, where Cu thickness was larger than the FePt alloy and it resulted in formation of (FePt) 0.42 Cu 0.58 [26]. The ternary alloy of FCT phase was obtained on annealing the multilayer at 400 o C, due to high diffusivity of the elements in this system, which exhibited a superparamagnetic behavior.The exchange bias effect at room temperature [33,34] in a coupled FM layer system without any field cooling procedure also generates a lot of interest. Usually, exchange bias is a consequence of the interfacial exchange interaction between a FM and an antiferromagnetic (AFM) material [33,34] and is manifested by a shift of the FM hysteresis loop along the magnetic field axis after field cooling through the Neel temperature of the AFM. An induced