Study of microstructure and magnetotransport properties of CrO2 prepared under HTHP

Abstract The microstructure of the CrO 2 particles prepared by high temperature and high pressure (HTHP) method was studied by HRTEM. It is found that the CrO 2 particles synthesized at 500 and 550 °C are covered by Cr 2 O 3 surface layers of about 6 nm thick. However, the CrO 2 particles synthesized at 400 and 450 °C do not have Cr 2 O 3 surface layers. The saturation magnetization of the CrO 2 particles synthesized at different temperatures is all very close to the theoretical value. The magnetoresistance (MR) of the CrO 2 particles synthesized at 500 and 550 °C is much larger than that of the CrO 2 particles synthesized at 400 and 450 °C, which should be due to the enhancement of tunneling magnetoresistance by insulating Cr 2 O 3 surface layers. The tunneling MR of the CrO 2 particles can be fitted well by expression of C 1 ( M / M s ) 2  +  C 2 ( M / M s ) 4  +  C 3 ( M / M s ) 6 . The proportion of the higher-order terms of ( M / M s ) 2 in the expression is tightly related to the existence of the Cr 2 O 3 surface layer.