Micromagnetic simulations of magnetic exchange spring systems

Magnetic exchange spring systems are multi-layers or composites of magnetically hard and soft materials that are exchange-coupled across their interfaces. In recent years, research into exchange spring systems has flourished, with potential for application in high-performance permanent magnets, GMR spin devices, magnetic MEMS technology, and in magnetic data storage. We investigate the magnetic properties of MBE grown superlattices with alternating layers of magnetically hard rare earth-iron (DyFe2, ErFe2) and soft yttriumiron (YFe2) compounds. They are ideal model systems to study exchange spring phenomena. We develop numerical models of the investigated systems and apply micromagnetic simulations. The simulation code OOMMF is extended and used to solve Landau-Lifshitz-Gilbert and Brown’s equations. This allows us to determine the microscopic configuration of the magnetisation that is not directly accessible by experiment. Magnetic field-sweep measurements of a multilayered DyFe2/YFe2 system show an unexpected triple switching of the magnetically hard DyFe2 layers. The magnetisation of the hard magnetic layers reverse before the soft magnetic layers. We reproduce the experimental hysteresis loops of the net and compound-specific magnetisation by means of simulations and explain the switching behaviour. Using similar numerical methods, we interpret experimental data on ErFe2/YFe2 multilayers. At sufficiently high fields, applied perpendicular to the multilayer film plane, the energy is minimised by a multilayer spin flop. This is a particular spin configuration where the magnetisation aligns with a direction perpendicular to the applied field. Taking the preceding findings further, we investigate multilayers of ErFe2/YFe2/ DyFe2/YFe2. We gain insight in the complex spin configurations in systems of different magnetically hard materials, with a pre-strung domain wall in the soft YFe2 layers. Varying the thickness of the YFe2 layers, we study the changing mutual interference of the switching patterns in the ErFe2 and DyFe2 layers.