Domain wall pinning and hard magnetic phase in Co-doped bulk single crystalline ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$

We report the effects of cobalt doping on the magnetic properties of two-dimensional van der Waals ferromagnet ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$. Single crystals of ${\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_3{\mathrm{GeTe}}_2$ with $x=0–0.78$ were successfully synthesized and characterized with x-ray diffraction, energy dispersive x-ray spectroscopy, and magnetization measurements. Both the Curie-Weiss temperature and ferromagnetic (FM) ordered moment of ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ are gradually suppressed upon Co doping. A kink in the zero-field-cooling low field $M\left(T\right)$ curve which is previously explained as an antiferromagnetic transition is observed for samples with $x=0–0.58$. Our detailed magnetization measurements and theoretical calculations strongly suggest that this kink is originated from the pinning of magnetic domain walls. The domain pinning effects are suddenly enhanced when the doping concentration of cobalt is around 50%, both the coercive field $H_c$ and the magnetic remanence to saturated magnetization ratio $M_R/M_S$ are largely improved and a hard magnetic phase emerges in bulk single crystal samples. The strong doping dependent magnetic properties suggest more spintronic applications of ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$.

Figure 1

(a) The x-ray diffraction patterns measured on single crystals of ${\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_3{\mathrm{GeTe}}_2$ showing ($00L$) diffraction peaks. (b) Doping dependence of $c$-axis lattice parameters. Inset shows one single crystal of $x=0.58$ imaged by a scanning electron microscope. (c) Temperature dependence of the magnetization $M$ measured with $H=0.5\mathrm{T}$ applied either parallel to the $c$-axis ($H\parallel c$, solid symbols) or parallel to the $ab$ plane ($H\parallel ab$, open symbols) for ${\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_3{\mathrm{GeTe}}_2$. (d) Isothermal magnetization curves for different samples measured with $H\parallel c$ and $H\parallel ab$ up to 5 T at $T=1.8\mathrm{K}$. The curve with $x=0.78$ is fitted by Eq. (1). (e) Doping dependence of saturated magnetic moment per Fe calculated from isothermal magnetization curves.

Figure 2

(a)–(e) Temperature dependence of magnetization for $x=0–0.78$ with 100 Oe magnetic field applied in directions of $H\parallel c$ and $H\parallel ab$. (f) Temperature vs doping phase diagram for ${\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_3{\mathrm{GeTe}}_2$. The FM transition temperature $T_c$ is defined by the minimum of the $dM/dT$ curve. The $T^{*}$ is defined as the ZFC kink temperature at $H=100$ Oe, which indicates a crossover from freely moved magnetic domains to pinned ones under this magnetic field.

Figure 3

(a) ZFC magnetization curves for $x=0.58$ under different magnetic fields. Inset shows the corresponding FC magnetization curves. (b) Isothermal magnetization curves for $x=0.58$ with $H\parallel c$ at different temperatures. Inset shows the enlarged view of the low field data. (c) Crossover points in ZFC $M\left(T\right)$ curves and $M\left(H\right)$ curves as marked by black arrows in (a) and (b) can be scaled together in a temperature vs field plot. (d) Hysteresis loops for $x=0.58$ with different maximum magnetic fields applied parallel to $c$ axis at $T=2\mathrm{K}$.

Figure 4

(a) Hysteresis loops for samples with different doping concentrations at $T=2\mathrm{K}$ and $H\parallel c$. The inset shows the enlarged view of the dashed box area. The data were measured with $H_{\text{max}}=\pm 50$ kOe. Doping dependent of coercive fields (b) and $M_R/M_S$ values (c) at $T=2\mathrm{K}$ for ${\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_3{\mathrm{GeTe}}_2$. The data are calculated from hysteresis loops.