Role of competing interactions on dynamic relaxation and exchange bias in spin-glass/ferromagnet bilayer

Abstract When a ferromagnet (FM) couples to a spin glass (SG), the hidden nature of SG may imprint on the FM via their interfacial coupling, probably resulting in elongated magnetic relaxation and enhanced unidirectional and uniaxial anisotropies. In parallel with the study of interfacial roles, e.g. interfacial coupling and matching, we focused on the influence of the competing interactions in SG (JSG) on the magnetic relaxation properties of SG/FM bilayer and the FM magnetization reversal involving exchange bias field (HE) and coercivity (HC). A fastest decay of relaxation in SG/FM bilayer is found at an intermediate JSG∼5.2 erg/cm2 and the relaxation parameters related to JSG is nonmonotonic. On the other hand, a pronounced HE of ∼-0.26 kOe is observed at small JSG, and as JSG increases larger than 2.275 erg/cm2, HE decreases by an order of magnitude and levels off, i.e., a net EB remains. HC is sensitive to JSG only at JSG above 2.275 erg/cm2 and exhibits a minimum value at JSG = 4.55 erg/cm2, just below 5.2 erg/cm2 where the relaxation decay is the fastest. Microscopic analysis unravels that the net magnetization associated with unidirectional magnetic stabilization and the large-area FM-like domains in SG are favored at small JSG, while with increasing JSG, the SG spins spontaneously reverse and thus the energy barriers are smeared out. A high susceptibility and a strong interfacial coupling lead to FM magnetization reversal under weaker fields and thus a decrease in coercivity. For too large JSG (exceeding 5.2 erg/cm2), magnetic frustration extends over the domains in SG meanwhile fierce energy competition occurs frequently, resembling enhanced quantum fluctuations. As a result, dynamic relaxation and statistic coercivity are both recovered. This work tells us that the independent modulations of unidirectional and uniaxial anisotropies are possible in SG-based systems through precisely controlling JSG, which is desired for application in the read/write head in future data storage media.