Amorphous exchange-spring magnets with crossed perpendicular and in-plane anisotropies

We study the magnetic coupling in a thin film bilayer exchange-spring magnet composed of an amorphous ${\mathrm{Tb}}_{10}{\mathrm{Co}}_{90}$ layer with a large perpendicular anisotropy and an amorphous ${\mathrm{Co}}_{85}{\left({\mathrm{Al}}_{70}{\mathrm{Zr}}_{30}\right)}_{15}$ layer with uniaxial in-plane anisotropy. The CoAlZr is directly exchange coupled to the TbCo with an interface region of at least 7.5 nm within which the CoAlZr magnetization is perpendicular to the plane and switches with the underlying TbCo. The influence of the coupling extends up to 30 nm into the CoAlZr resulting in an effective tilted anisotropy. The coercivity of the CoAlZr is greatly enhanced due to the coupling and is tuneable over a large range by varying its thickness.

Figure 1

(a) A representative XRR measurement including a fit, showing the low interface roughness of the bilayers. The inset shows a schematic of the sample structure corresponding to the measurement, where $d_{\text{A}}=30$ nm and $d_{\text{B}}=15$ nm. (b) A GIXRD measurement of the same sample, showing a single broad peak at approximately $2\theta ={45}^{\circ }$. The measurement is representative of the entire sample series.

Figure 2

Magnetic hysteresis loops measured by VSM in the out-of-plane and in-plane directions on (a) a single layer of CoAlZr and (b) a single layer of TbCo. The TbCo has a well-defined PMA whereas the CoAlZr has a clear uniaxial IMA. (c) The magnetic response of a TbCo(30 nm)/AlZr(2.5 nm)/CoAlZr(15 nm) sample, measured both in-plane and out-of-plane by VSM. The AlZr spacer layer decouples the two magnetic layers such that they switch independently. Data are shown between $-1.5$ T to 1.5 T for clarity, with the measurement extending to $\pm 5$ T.

Figure 3

The magnetic response of TbCo/CoAlZr bilayers with crossed anisotropies measured by VSM. (a) Out-of-plane hysteresis loops. (c) In-plane hysteresis loops, parallel to the growth field direction (in-plane easy axis). The data shown are between $\pm 1.5$ T for clarity, with the measurements extending to $\pm 5$ T.

Figure 4

(a) The hysteresis loop of a bilayer with $d_{\text{B}}=33$ nm (red) perpendicular to the plane. The data is fitted using Eq. (1) resulting in the solid black line. The fit is composed of two loops, shown in (b), a square loop and an S-shaped loop, corresponding to different components of the bilayer. (c) The saturation moment of the different components extracted from fitting the VSM data, as depicted in (a) and (b). The (blue) squares and (green) circles correspond to the square and S-loop, respectively. Up until $d_{\text{B}}=7.5$ nm there is no contribution from the hard axis loop, indicating that at least 7.5 nm of the soft CoAlZr switches with the TbCo. The dashed lines are a guide to the eye and extend to $d_{\text{B}}=80$ nm (not shown). The insets show the different magnetic components schematically.

Figure 5

(Left) Coercive field of hysteresis loops measured in-plane [Fig. 3] as a function of thickness. For thin $d_{\mathrm{B}}$ the coercivity is enhanced by two orders of magnitude compared to bulk CoAlZr. (Right) The saturation field of the S-shape (hard axis) component loop from the out-of-plane measurement [Fig. 3] plotted as a function of $d_{\text{B}}$. The field required to saturate the CoAlZr out-of-plane is significantly reduced for thicknesses below $d_{\mathrm{B}}=33$ nm. Dashed lines are a guide to the eye.