Coercivity and magnetization reversal mechanism in ferromagnet/antiferromagnet bilayers: Correlation with microstructure of ferromagnetic layers

Abstract

Exchange biasing in Co 20 Ni 80 / FeMn and Co 80 Cr 20 / FeMn bilayers has been investigated, where the CoCr layers are of granular structure and the CoNi layers are in the form of a single phase. In the above two series of bilayers, the exchange field is proportional to 1 / t FM ͑t FM denotes ferromagnetic layer thickness͒. For CoNi/ FeMn bilayers, the coercivity and the uniaxial anisotropic field decrease with increasing t FM with a linear scale of 1 / t FM. Since they are equal to each other, the magnetization reversal process can be described by magnetization coherent rotation and the coercivity enhancement can be explained in terms of a uniaxial anisotropy model. For CoCr/ FeMn bilayers, however, the coercivity displays unusual behaviors. First, in comparison with that of single CoCr layer films, the coercivity is reduced instead of enhanced. Secondly, it increases with increasing t FM. Finally, the coercivity of the bilayers is not equal to the uniaxial anisotropic field. A noncoherent rotation process is proposed to occur during the magnetization reversal process. The different characteristics of the coercivity and magnetization reversal mechanisms in the two series of bilayers result from the different microstructures in the CoNi and CoCr layers. The present work might be helpful to clarify the mechanism for the coercivity enhancement in ferromagnet/antiferromagnet bilayers.

Influence of spacer layer in exchange coupled NiFe/Cu/IrMn trilayer structure

“…2. It means that the uniaxial anisotropy of exchange coupled NiFe film could be associated with the exchange coupling [7,8].…”

Exchange bias in ferromagnet/antiferromagnet bilayers

“…Although H C of an FM/AFM bilayer is often enhanced in comparison with that of a single FM layer, it may be reduced for a particulate FM layer as discussed below. For Si substrate/Cu(30 nm)/Co 80 Cr 20 (= CoCr)(0-30 nm)/FeMn (20 nm) multilayer, [47] a broad diffraction peak of CoCr (1011) was observed in an X-ray diffraction spectra. Moreover, α-FeMn (422) and γ-FeMn (111) diffraction peaks were also observed.…”

“…Measurements were performed at room temperature. Solid lines each serve as a guide to the eye [47,48].…”

Exchange bias in Cr/Gd multilayers with TC<TN

“…First, the value of H C of the former is about three times higher than that of the latter (e.g., at T = 100 K, H C is 158 Oe and 53 Oe, respectively), corroborating that H C decreases with increasing FM layer thickness [25]. Secondly, the most fascinating result is the linear temperature dependence of H C and H C M FM for the [Cr(3.4 nm)/Gd(3 nm)] 30 multilayer, as shown in Fig.…”