Thermally activated self-alignment of exchange coupling in NiO/NiFe bilayers

Abstract

Long-term and temperature stability of the exchange coupling are important conditions for the practical application of spin-valve devices. Here we present magnetic measurements on the properties of sputtered NiO/NiFe bilayers. After deposition a temperature dependent magnetic self-alignment process was observed where the exchange bias field H eb increased according to a ln(t) law. The temperature dependence of this aftereffect indicates that a thermally activated process takes place which can be explained by a two energy-level relaxation model. In addition the long-term stability of the exchange coupling during a forced antiparallel alignment of the NiFe layer was investigated. A significant improvement of the coupling stability can be reached by cooling the samples through the Neél temperature in a magnetic field. Finally, the temperature dependence of H eb was determined for differently treated samples.

Fundamentals for magnetic patterning by ion bombardment of exchange bias layer systems

“…Since for defect creation within the AF volume the exchange coupling constant and the magnetically effective contact area is not changed the energy barrier ∆ i between the two minima in the free energy function is lowered and therefore some AF domains may thermally change their anisotropy axes even at room temperature. This leads to a slow increase of H eb with time after the IB, similar to the thermal after effect of the EB [2].…”

“…Some AF/FM bilayers showed a thermal after effect [2] of H eb , i.e. after the H eb has been initially set H eb slowly increases with time to a certain saturation value H eb,0 .…”

“…after the H eb has been initially set H eb slowly increases with time to a certain saturation value H eb,0 . By increasing the temperature the time to reach H eb,0 decreases [2] and, therefore, an annealing step after one of the above initialization procedures is required to achieve a stable exchange bias field.…”

In-plane magnetic pattern separation in NiFe/NiO and Co/NiO exchange biased bilayers investigated by magnetic force microscopy

“…To establish the initial unidirectional anisotropy (defined here as the ''x-direction'') throughout the whole sample, a magnetic field H gro ¼ 40 Oe was applied in x-direction parallel to the substrate surface plane during film growth. After deposition the exchange coupling was stabilized by annealing the samples at 540 K for 20 min and subsequent cooling to room temperature in a magnetic field of 1 kOe parallel to the easy axis of magnetization [15]. A hysteresis loop of sample no.…”

“…The bilayer was deposited in a Leybold Z400 rf diode sputtering system on a 0:2 nm thin glass substrate. NiFe oxidation was prevented by a Ta capping layer of 2 nm: The deposition process of this sample has been described previously [9,15], so only the most important parameters should be briefly mentioned here. The antiferromagnetic NiO layers were deposited in a reactive sputtering process from a pure Ni target in an Ar=O 2 gas mixture of 100 sccm Ar and 0:6 sccm O 2 : The deposition rates were determined by calibration films and amounted to 4:3 nm=min; 7:8 nm=min; and 5:7 nm=min for NiO, NiFe, and Ta, respectively.…”