Micromagnetic modelling of nanocrystalline magnets and structures

Abstract

Numerical micromagnetic 3D simulation of nanocrystalline magnetic materials and structures determine the microstructural factors controlling the magnetisation reversal process. Predictions for size and shape of grains and elements in order to obtain optimum magnetic properties are made. The micromagnetic predictions of coercivity and remanence for composite hard magnets cover the range of (H c , J r )=(0.34, 1.4 T) to (0.61 MA/m, 1.1 T), assuming a grain size of 20 nm and an Nd 2Fe 14B content of 40%. The calculations reveal a linear trade off of remanence and coercivity as a function of the α-Fe to Fe 3B ratio. Increasing Fe 3B content improves the coercive field, but reduces the remanence and deteriorates the squareness of the hysteresis loop. The coercive field shows a maximum at a grain size of about D=15–20 nm. Intergrain exchange interactions override the magnetocrystalline anisotropy of the ND 2Fe 14B grains for smaller grains, whereas exchange hardening of the soft phases becomes less effective for larger grains. The magnetisation distribution shows a vortex-like structure within soft magnetic regions with an extension greater than 80 nm. The vortices have zero net magnetisation and act as nucleation sites, reducing remanence and coercivity. The shape of the grains considerably influences the squareness of the demagnetisation curve. Our simulation shows that the magnetisation reversal starts with reversible rotations of the magnetisation within the soft magnetic phase followed by irreversible switching of the hard magnetic grains. The minimum size and the domain wall jaggedness of nanocrystalline Co/Pt multilayered films strongly depend on the average size and misorientation of grains. Switching fields in patterned NiFe and Co elements are also influenced and the formation of a multidomain structure is suppressed on the type of anisotropy and the size and shape of the elements containing nanocrystalline grains.