Anisotropic coercivity and the effects of interlayer exchange coupling in CoFeB/FeRh bilayers

In an amorphous CoFeB layer, coercivity becomes anisotropic with fourfold symmetry when the CoFeB layer exchange couples to an FeRh layer. The angular dependence of coercivity of the CoFeB layer coincides with the in-plane easy-axis direction of the FeRh layer and experiences a ${45}^{\circ }$ shift with the occurrence of a metamagnetic phase transition of the FeRh layer from antiferromagnetism at room temperature to ferromagnetism at 400 K. The intriguing phenomena are well reproduced by our unbiased Monte Carlo simulation. The interfacial exchange and anisotropy energies, as well as the interfacial magnetization in the CoFeB/FeRh bilayer, are disentangled to demonstrate the strong dependence of the imprinting of anisotropy in the CoFeB layer on the interfacial exchange coupling. The evolution of the easy-axis direction of the induced anisotropy arises from the reconstruction of the interfacial exchange energy profile accompanied with the change of the magnetic state of FeRh, which governs the magnetization reversal of the CoFeB layer at both branches. Moreover, the imprinting is further applicable for the uniaxial magnetocrystalline anisotropy. This work not only presents the possibility of directly duplicating anisotropy between dissimilar materials, but it also provides a powerful tool to probe the hidden magnetic structures and/or the properties of materials that have weak magnetism, such as antiferromagnetic materials.

Figure 1

(a) Sample structure of the CoFeB/FeRh bilayer grown on the MgO(001) substrate. The weak uniaxial anisotropy of the CoFeB layer and the strong in-plane fourfold anisotropy of the FeRh layer are shown in the bottom panel. (b) Typical MOKE loops of the CoFeB(10 nm)/FeRh(30 nm) bilayer in selected magnetizing directions at room temperature, where $\phi$ is designated as an angle between the in-plane measuring field and MgO[100] directions, $H_{\text{cI}}$ and $H_{\text{cII}}$ are weak- and strong-field coercivities at ascending branches, and blue arrows indicate the moment orientations of the CoFeB layer. (c) $H_{\text{cI}}$ and $H_{\text{cII}}$ as a function of $\phi$.

Figure 2

(a) Typical FMR spectra measured in different field orientations $\left(\phi \right)$ for the CoFeB(10 nm)/FeRh(30 nm) bilayer at room temperature, where black dots indicate the resonance fields. (b) Typical angular dependence of resonance field $H_r$ at selected temperatures.

Figure 3

(a) Calculated magnetization hysteresis loops at selected magnetizing directional angles with respect to the positive $x$-direction ($\phi$) in CoFeB single layer (open symbol) and CoFeB/(FM) FeRh bilayer (solid symbol). (b) Calculated coercivity as a function of $\phi$. The inset shows the model of CoFeB(2)/FeRh(20) bilayer, where numbers are in units of monolayers.

Figure 4

(a) Calculated angular $\left(\phi \right)$ dependence of coercivity in the CoFeB layer, which is coupled to a ferromagnetic (FM) or an antiferromagnetic (AFM) FeRh layer. (b) Calculated magnetization hysteresis loops in the CoFeB/(AFM) FeRh bilayer at selected $\phi$. (c) Schematic illustrations of virgin isotropy in the CoFeB single layer and the induced cubic anisotropy in the CoFeB/(FM) FeRh bilayer, with the easy-axis direction identical to that of the FeRh layer, and in the CoFeB/(AFM) FeRh bilayer, with a ${45}^{\circ }$ deviation of the easy-axis directions from that of the FeRh layer.

Figure 5

(a) Magnetization of each monolayer in the CoFeB/FeRh bilayer for the FeRh layer with a ferromagnetic (FM) or an antiferromagnetic (AFM) state at low temperature after zero-field cooling, and the insets show the magnetization vs temperature ($M-T$) behaviors of layers 17–22 (L17–L22) in the CoFeB/FeRh bilayer for FM or AFM FeRh. (b) Microscopic spin configurations of L17–L22 in CoFeB/(FM) FeRh (upper) and CoFeB/(AFM) FeRh (bottom) bilayers, where (1)–(6) give the zoomed-in views of configurations in the selected layers. (c) Schematic illustrations of the role of the FeRh layer played on the CoFeB layer during magnetization reversal, where dashed arrows represent the dragging directions via interfacial exchange coupling, and two-headed arrows stand for two directions that are possible with equal probability.

Figure 6

(a) Calculated coercivity as a function of directional angle $\left(\phi \right)$ after cooling under selected fields $\left(H{}_{\mathrm{FC}}\right)$ in the CoFeB/(FM) FeRh bilayer. (b) Calculated coercivity as a function of temperature under zero $H{}_{\mathrm{FC}}$ at selected $\phi$. Energy densities of (c) interfacial exchange coupling $\left({\varepsilon }_{j12}\right)$ and (d) cubic anisotropy $\left({\varepsilon }_{k2}\right)$ and their temperature derivative as a function of temperature calculated at selected $\phi$. The inset shows the coercivity as a function of $\phi$ at selected temperatures.

Figure 7

(a) Unit vector of the easy axis of uniaxial anisotropy (open symbols) and angular $\left(\phi \right)$ dependence of the normalized coercivity (solid symbols) in the CoFeB layer, which is coupled to a hard-magnet FM layer with strong uniaxial anisotropy along selected easy-axis directions. (b) Calculated magnetization hysteresis loops of the CoFeB layer, which is coupled to the hard-magnet FM layer with uniaxial anisotropy along the ${45}^{\circ }$ and ${90}^{\circ }$ easy-axis directions with respect to the positive $x$-direction, at selected $\phi$.