Characterization of the magnetic properties of NdFeB thick films exposed to elevated temperatures

Hard magnetic films used in magnetic micro-systems may be exposed to elevated temperatures during film and system fabrication and also during use of the micro-system. In this work, we studied the influence of temperature on the magnetic properties of 10 μm thick out-of-plane textured NdFeB films fabricated by high rate triode sputtering. Out-of-plane hysteresis loops were measured in the range 300K – 650K to establish the temperature dependence of coercivity, magnetization at 7 T and remanent magnetization. Thermal demagnetization was measured and magnetization losses were recorded from 350K in films heated under zero or low (-0.1 T) external field and from 325 K for films heated under an external field of -0.5 T. The effect of thermal cycling under zero field on the remanent magnetization was also studied and it was found that cycling between room temperature and 323 K did not lead to any significant loss in remanence at room temperature, while a 4% drop is recorded when the sample is cycled between RT and 343K. Measurement of hysteresis loops at room temperature following exposure to elevated temperatures reveals that while remanent magnetisation is practically recovered in all cases, irreversible losses in coercivity occur (6.7 % following heating to 650K, and 1.3 % following heating to 343K). The relevance of these results is discussed in terms of system fabrication and use.