Investigation of 'magnetic hardness' of the grain boundary in Sr2FeMoO6 ceramics

Intrinsic spin-valve-type magnetoresistance SVMR) behavior recently observed in Sr2FeMoO6 SFMO) is proposed to originate from the 'magnetic hardness' of the insulating grain boundary. It suggests the potential for tuning the SVMR by manipulating the magnetoresistance peak position of the grain boundary with a suitable magnetic material. In this paper,CoFe2O4/Sr2FeMoO6 (CFO(x)/SFMO(100%): x = 0%-30%) composites were specifically designed by introducing the hard magnetic insulator CFO into the grain boundary of SFMO. Insulating CFO can increase the grain boundary strength (macroscopic resistivity), resulting in an enhanced low-field magnetoresistance (LFMR) of similar to 1.6 times with respect to the maternal SFMO. The magnetic coercivity (H-c) of the CFO/SFMO grain material shows a complete enhancement which is dependent on the CFO content (x) and is due to the high magnetic anisotropy of the CFO. Vastly different from the H-c response is the coercive field variance of the magnetoresistance (H-c (MR)), which exhibits a complex correlation with the CFO content (x). Regardless of how much the H-c (MR) changes, the SVMR behavior is still maintained because of a larger H-c (MR) than that of H-c when x <= 18%. However, almost comparable values of H-c (MR) and H-c are observed when x increases up to 30%, where the SVMR property is greatly destroyed. In addition, the ferromagnetic transition temperature T-c values of all the samples show no obvious dependence on the CFO content (x). These interesting observations indicate that the CFO content (x) not only significantly optimizes the LFMR but it also effectively tunes the SVMR behavior. Our work may encourage further investigation of SVMR behavior in SFMO and other similar systems.