Emergence of noncollinear anisotropies from interfacial magnetic frustration in exchange-bias systems

Abstract

Exchange bias, referred to the interaction between a ferromagnet ͑FM͒ and an antiferromagnet ͑AFM͒, is a fundamental interfacial magnetic phenomenon, which is key to current and future applications. The effect was discovered half a century ago, and it is well established that the spin structures at the FM/AFM interface play an essential role. However, currently, ad hoc phenomenological anisotropies are often postulated without microscopic justification or sufficient experimental evidence to address magnetization-reversal behavior in exchange-bias systems. We advance toward a detailed microscopic understanding of the magnetic anisotropies in exchange-bias FM/AFM systems by showing that symmetry-breaking anisotropies leave a distinct fingerprint in the asymmetry of the magnetization reversal and we demonstrate how these emerging anisotropies are correlated with the intrinsic anisotropy. Angular and vectorial resolved Kerr hysteresis loops from FM/AFM bilayers with varying degree of ferromagnetic anisotropy reveal a noncollinear anisotropy, which becomes important for ferromagnets with vanishing intrinsic anisotropy. Numerical simulations show that this anisotropy naturally arises from the inevitable spin frustration at an atomically rough FM/AFM interface. As a consequence, we show in detail how the differences observed for different materials during magnetization reversal can be understood in general terms as originating from the interplay between interfacial frustration and intrinsic anisotropies. This understanding will certainly open additional avenues to tailor future advanced magnetic materials.

Highly asymmetric magnetic behavior in exchange biased systems induced by noncollinear field cooling

“…In general, the interfacial exchange coupling effects depend on the strength of the anisotropies 5 as well as their relative orientation, 6 exhibiting a complex phase diagram of different reversal modes. 5,6,[11][12][13][14][15] In fact, the relative orientation between the intrinsic FM anisotropy and the induced interfacial unidirectional anisotropy can be controlled by different FC procedures, varying both strength, 7,8 FC angle, 6,9,10 and/or interfacial magnetic frustration. 14,15 In this letter we present a detailed study on the magnetization reversal of FM/AFM systems with a noncollinear uniaxial, K U , and unidirectional, K E , anisotropy configuration.…”

“…5,6,[11][12][13][14][15] In fact, the relative orientation between the intrinsic FM anisotropy and the induced interfacial unidirectional anisotropy can be controlled by different FC procedures, varying both strength, 7,8 FC angle, 6,9,10 and/or interfacial magnetic frustration. 14,15 In this letter we present a detailed study on the magnetization reversal of FM/AFM systems with a noncollinear uniaxial, K U , and unidirectional, K E , anisotropy configuration. Our work reveals the importance of taking into account the misalignment between the K U direction and the direction of the applied field during the FC procedure in order to properly account for the asymmetry of the magnetization reversal and the angular dependences of 0 H C and 0 H E .…”

“…Similar noncollinear configurations have already been used ad hoc to model recent experimental observations in exchange bias systems. 6,[12][13][14][15] In our case, the values K U and K E were extracted from the experimental data of the reference FM film and the FM/AFM bilayers. The same anisotropy values were used to simulate the behavior of bilayer with collinear anisotropies, i.e., ␤ FC = 0°, shown in Figs.…”

Role of anisotropy configuration in exchange-biased systems

“…4,11,12 In particular, the angular dependence of the exchange bias (l 0 H E ), coercivity (l 0 H C ), and magnetization reversal, including its asymmetric behavior, depends on the ratio of the involved anisotropies 4 as well as on their relative orientation. 11,12 The latter can be promoted either intrinsically by interfacial frustration 10,11 or extrinsically via patterning 5,6 and/or special field cooling (FC) procedures.…”

“…4,11,12 In particular, the angular dependence of the exchange bias (l 0 H E ), coercivity (l 0 H C ), and magnetization reversal, including its asymmetric behavior, depends on the ratio of the involved anisotropies 4 as well as on their relative orientation. 11,12 The latter can be promoted either intrinsically by interfacial frustration 10,11 or extrinsically via patterning 5,6 and/or special field cooling (FC) procedures. [12][13][14][15][16] Here we compare the angular dependence of the magnetic properties of a 18 nm Co/5 nm IrMn bilayer with three tailored anisotropy configurations, including collinear and two opposite noncollinear configurations.…”