Exchange Coupling and Remanence Enhancement in Nanocomposite Multilayer Magnets

 

Z.D. Zhang

Shenyang National Laboratory for Materials Science, Institute of Metal Research, and International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China

 

After experimental evidence of intergrain exchange coupling was reported, nanocomposite magnets with high remanence and large energy products were predicted. However, the experimental values of the maximum magnetic energy product of nanocomposite bulk magnets have been much less than the theoretically predicted ones. We report the exchange coupling and remanence enhancement in nanocomposite Sm-Co/Co (or FeCo),  (Nd,Dy)(Fe,Co,Nb,B)5.5/a-Fe thin films prepared by sputtering and heat treatments. The effects of the thickness of magnetic layers, coercivity mechanism, and magnetic viscosity have been systematically studied by using high resolution transmission electron microscope (HRTEM), Auger electron spectra, vibrating sample magnetometer (VSM), physical property measurement system (PPMS) and alternating gradient magnetometer (AGM). Additionally, the effect of the magnetic heat-treatment on nanocomposite Sm-Co/Co (or FeCo) has also been studied. The coercivity of a single Ti-buffered (Nd,Dy)(Fe,Co,Nb,B)5.5 layer is as large as 1.85 T, while a high remanence of Jr = 1.31 T and a high maximum energy product of (BH)max = 203 kJ/m3 are achieved in the nanocomposite multilayer films. Well-designed multilayer films consist of magnetically hard Nd2Fe14B-type phase with the grain size of 40 nm and magnetically soft a-Fe phase existing in the form of the continuous layers.  Our results suggest that nanocomposite multilayer films with well-distributed fine grains of the hard and soft magnetic phases could constitute a new generation of permanent- magnet materials

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