Perpendicularly magnetized Tb/Co multilayers featuring tilted uniaxial anisotropy: Experiments and modeling

Abstract

Rare earth/transition metal (RE/TM) multilayers with perpendicular magnetic anisotropy are key ingredients for the development of spintronic applications. Their compensation temperature depends on the ratio of the thicknesses of rare earth and transition metal, allowing their magnetic properties to be tuned with temperature while maintaining their anisotropy even in nanometer-scale devices. In this work, we performed a thorough structural characterization and systematically investigate the magnetic properties of a whole family of ferrimagnetic [Tb/Co]$_{\times 5}$ multilayers varying the Tb thickness in the range of 0.4 nm - 1.25 nm. A linear dependence of the compensation temperature on the Tb layer thickness was observed. Moreover, a uniaxial anisotropy constant of 330$\pm$30 kJ/m$^3$, which is close to the values reported by other authors, was estimated. Additionally, we proposed a model to gain a better understanding of the angular dependence of the magnetization loops and the linear dependence of the compensation temperature. We present strong evidence demonstrating that the perpendicular anisotropy must be tilted away from the perpendicular axis in order to explain the observed features, particularly the hysteresis in the in-plane loops. Our work advances the understanding of DC magnetic properties in thin RE/TM ferrimagnetic films, which has the potential to impact different fields where these materials are involved.

Figures (7)

• Figure 1: Sample structure and composition profile. (a) Schematic of ferrimagnetic multilayers. (b) HAADF-STEM cross-sectional image for an TbCo(1.08) multilayer (c) EDX map plotted with depth of 20 thick specimen indicated by the red dashed line in panel (b).
• Figure 2: The experimental PIXE spectrum for 3$H^{+}$ of the TbCo(0.49) multilayer (black squares and solid line) using Mylar as outgoing X-rays filter. The magenta solid line represents the best fit provided by the GUPIX code. The inset shows the experimental PIXE spectra of TbCo($t_\mathrm{Tb}$) multilayer highlighting the Tb and Co peaks between 6.0 and 8 for $t_\mathrm{Tb}$= 0.49, 0.83 and 1.17, with black, blue and red lines, respectively.
• Figure 3: Magnetization behavior corresponding to the TbCo(0.66) multilayer. (a) Hysteresis loops for the out-of-plane (OOP) and in plane (IP) field sweeps at 300). (b) Temperature dependence of the saturation magnetization (blue open circles) and coercive field (black open squares) for the same sample.
• Figure 4: Absolute remanent OOP magnetization as a function of temperature for TbCo($t_\mathrm{Tb}$) multilayers with different Tb thicknesses $t_\mathrm{Tb}$, as indicated in the labels.
• Figure 5: a) Slope $b(t_\mathrm{Tb})$ and intercept $a(t_\mathrm{Tb})$ as a function of $t_\mathrm{Tb}$ extracted from the different measurements shown in Fig. \ref{['fig:MdeT']} in the range $T \in[\qty{80}{\kelvin}, \qty{280}{\kelvin}]$. The dashed lines correspond to linear fits, as explained in the text. b) Magnetic compensation temperature TM as a function of the tTb. The blue solid line was calculated using the Eq. \ref{['eq:ModelTm']}, while the magenta solid stars represent the $\mathrm{T_{M}}$ values extracted from the measurements shown in Fig. \ref{['fig:MdeT']}. The uncertainty in each $\mathrm{T_{M}}$ determination is $\qty{+-2}{\kelvin}$, which is much smaller than the symbol size and has therefore been omitted for clarity. The uncertainties associated with the TM computed by Eq. \ref{['eq:ModelTm']} is represented by the blue shaded region.