Simultaneous enhancements of magnetization and remanence in sufficiently exchange-coupled Co_(0.8)Al_(0.2)Nd_xFe_(2-x)O_4/Co_7Fe_3(Co) composites

摘要

Hard/soft magnetic Co_(0.8)Al_(0.2)Nd_xFe_(2-x)O_4/Co_7Fe_3(Co) composites are synthesized by the ball-milling-assisted ceramic process, followed by 5% H_2-Ar reduction. The effects of the reduction time on their structure and magnetic properties are studied at room temperature. XRD patterns indicate that Co_(0.8)Al_(0.2)Nd_xFe_(2-x)O_4 sample, calcined at 900 ℃ for 3 h, contains the main spinel CoFe_2O_4 phase in combination of a small amount of foreign Fe_2O_3 and/or FeNdO_3 phases. After being reduced at 550 ℃ for 30 min, surface magnetic particles of spinel Co_(0.8)Al_(0.2)Nd_xFe_(2-x)O_4 ferrite samples are transformed from the hard magnetic Co_(0.8)Al_(0.2)Nd_xFe_(2-x)O_4 to the soft magnetic Co_7Fe_3(Co). The magnetic measurement suggests that the specific saturation magnetization and remanence of spinel Co_(0.8)Al_(0.2)Nd_xFe_(2-x)O_4 decrease with increasing Nd~(3+) content. After being reduced at 550 ℃, specific saturation magnetization and remanence of the composites, Co_(0.8)Al_(0.2)Nd_xFe_(2-x)O_4/Co_7Fe_3(Co), increase markedly. The hysteresis loop of the composites shows a single-phase magnetization behavior, implying that the magnetic hard (Co_(0.8)Al_(0.2)Nd_xFe_(2-x)O_4) phase and soft [Co_7Fe_3(Co) alloy] phase in Co_(0.8)Al_(0.2)Nd_xFe_(2-x)O_4/ Co_7Fe_3(Co) composites are well exchange-coupled, which is confirmed by the switching field distribution curves further.