Abstract
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.
Highlights
Microelectromechanical systems (MEMS) based fabrication technologies are suitable for fabricating inexpensive and complex micro-devices thanks to batch-fabrication and the use of photolithography based processes derived from integrated circuit (IC) fabrication technologies
It is known that applying a demagnetizing field factor of N = 1 to hard magnetic films can result in over-skewing of corrected loops, due to the discrete nature of magnetization reversal in these materials, and we previously reported an experimental method based on minor loop measurements to establish the effective demagnetization factor (Neff) to be applied.[9]
It can be seen that cycling between RT 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
Summary
Microelectromechanical systems (MEMS) based fabrication technologies are suitable for fabricating inexpensive and complex micro-devices thanks to batch-fabrication and the use of photolithography based processes derived from integrated circuit (IC) fabrication technologies. MEMS have recently replaced machined sensors, such as inertia sensors, pressure sensors and micro-phones, which are used in mobile devices such as smartphones, smart watches, tablets and laptop computers. The use of MEMS technology is a promising way to reduce the size and the cost of such devices and allows for the smart integration of micro-elements. Sintered bulk NdFeB-based magnets which show excellent magnetic properties are not compatible with MEMS technologies, and there have been many reports on the development of hard magnetic NdFeB films.[1,2,3,4,5,6,7] Triode sputtering is well adapted to MEMS development, as very high sputtering rates can be achieved over large surface areas and thick textured films can be produced having magnetic properties comparable to those of sintered magnets.[2,7] Hard magnetic films may be
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