Non-ideal exchange springs in DyFe2/YFe2 superlattices

Abstract

Magnetic exchange spring systems have potential applications in (i) the next generation of permanent magnets and (ii) storage elements in information systems. However any practical realization of this potential will involve a study of non-ideal exchange spring behaviour. Within the context of molecular-beam-epitaxy (MBE)-grown DyFe2/YFe2 superlattices, two non-ideal processes are examined in this work. The first involves the anisotropy of the so-called hard pinning DyFe2 layer. At room temperature, the anisotropy of the Dy pinning ions is reduced substantially, and approaches that of the so-called soft YFe2 layers, which is usually neglected. In this regime, the magnetic switching process becomes more complicated. The latter has been studied experimentally using the magneto-optic Kerr effect (MOKE), at room temperature. This work was also complimented by 1D computer simulations, involving numerical solution of the Landau-Lifshitz-Gilbert equations. In contrast to the well accepted low-temperature behaviour, both the MOKE and computer simulations show that the anisotropy of the soft YFe2 layers now plays a significant role in determining the coercive/switching field, even though exchange-springs are formed in the YFe2 layers. Thus the well established bending field relationship BB cx 1/t2s found at low temperatures, is not obeyed.

The second non-ideal exchange process studied involved the deliberate insertion of a few hard DyFe2 layers, directly into the middle of the soft YFe2 layers. This inevitably injects anisotropy into the 'soft' YFe2 layers, which should give rise to irreversible processes as the exchange springs wind-up and unwind. At low temperatures, irreversible exchange spring processes were observed for fields applied along both the [00¹1] and [¹110] axes of MBE-grown (110) DyFe2/YFe2 multilayer samples. For fields applied along the [00¹1], vibrating sample measurements reveal that there are two irreversible exchange processes. The experimental results are complemented by 1D Object Oriented Micromagnetic Framework (OOMMF) simulations. It is shown that deliberate doping of the magnetically soft layers, with one or two hard mono-layers, can modify exchange spring behaviour, substantially. Similar behaviour was observed in a uperlattice sample with soft YFe2 layers doped with DyFe2.

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