Magnetic properties of cobalt nickel/gadolinium multilayers

CoNi/Gd multilayers were grown on Corning glass substrate at room temperature using DC magnetron sputtering. 10.0 nm Ag films were used as buffer and cap layers. A large increase in the coercivity of CoNi/Gd bilayer, compared to the coerciv ity of CoNi film, was observed. The increase in coercivity was not significant when the order of film growth was reversed for the same thicknesses and identical deposition conditions. The large increase in coercivity is attributed to the pinning of the CoNi layer by the magnetically compensated ferrimagnetic alloy forming at the CoNi (top)/Gd (bottom) interface due to the diffusion of Co and Ni into Gd. The exponentially decaying interlayer coupling in CoNi/Gd/CoNi trilayers was measured by minor hysteresis loop shifts and calculated from major hysteresis loops. The strong ferromagnetic coupling up to 3 nm Gd film thickness can be explained by the development of ferrimagnetic alloy in the Gd layer caused by the diffusion of Co and Ni into the Gd film or by pinholes. The coupling strength decreases rapidly. It can not be explained by Néel's “orange-peel” coupling and domain wall coupling. The exchange interaction among d electrons may be the reason for the exponentially decaying coupling. Strong positive and negative exchange bias in some CoNi/Gd/CoNi trilayers and CoNi/Gd bilayers were observed at low temperature. Gd or CoNi layer domination in total magnetization determines exchange bias direction due to negative exchange coupling between Gd and Co, and between Gd and Ni. However, the exchange bias field becomes zero when hysteresis loop measurements are done in large magnetic field intervals. Changing the remanent state of the trilayer altered the exchange bias field strength of the trilayer. This suggests that the magnetic state of top CoNi/Gd bilayer, which is the hard bilayer, determines the exchange bias field strength. If the sample was cooled at remanent state corresponding to the hard layer coercive field, the exchange bias becomes zero. The angular dependency of coercive field indicates that the average exchange bias became zero due to altered exchange coupling at the interface of hard CoNi/Gd bilayer and CoNi layer by domains and domain walls in the hard layer.