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Thin film NdFeB permanent magnets are important for future advances in microsensors, micromotors and microelectromechanical systems (MEMS). The miniaturisation of these microdevices requires magnetic thin films with high coercivity and remanence in order to provide strong forces [1]. The addition of some suitable elements to NdFeB films is an effective way to improve magnetic properties and their thermal stability. An uniform and fine structure at nanometric scale is a key factor for obtaining of film NdFeB permanent magnets with optimal magnetic performances (high coercivity, remanence and maximum energy product). This can be achieved by composition adjustment and a good control of the processing conditions. For all the samples, the Nb-Cu or Ta films are used as buffer layers (BL). This work investigates the influence of the M additional elements (with M = Nb - Cu, Co) on the hard magnetic properties and thermal stability of the multilayer BL/[NdFeB/M]xn/BL thin films obtained in certain processing conditions. The influence of the Nb-Cu and Ta buffer layers on the hard magnetic properties of [NdFeB/M]xn films is also presented. The metallic elements such as Nb - Cu combination are added with the aim of promoting the formation of nucleation sites and inhibiting the grain growth [2], whilst Co acts in the sense of increasing the Curie temperature and improving the thermal stability of the hard magnetic properties [3]. The samples were deposited by R.F sputtering, at room temperature, on Si substrates. The as–deposited samples were subsequently annealed, in vacuum, for different periods of time, at temperatures between 550°C and 750°C. For annealing temperatures around 650°C, the grains of Nd2Fe14B reach an optimum size of about 30 nm and some secondary phases appear. The coercive field Hc increases to about 1800 kA/m due to the domain wall pinning effect at grain boundaries. The enhancement of the magnetic properties and their thermal stability of multilayer NdFeB/M films is attributed to the refinement of the structure by simultaneous addition of Nb-Cu and Co.
[1] H. Jiang et al. J.M.M.M. 224 (2001) 233.
[2] H.Chiriac, M.Grigoras and M. Urse, J.M.M.M., 224 (2006), in press.
[3] B.Z.Cui, M.J.O’Shea, J.M.M.M., 279, (2004) 27.
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