From ferromagnetic to helical order with a discussion of the low-temperature antiferromagnetism in composite ${\mathrm{Cd}}_{1–x}{\mathrm{Mn}}_x\mathrm{Ge}{\mathrm{P}}_2+\mathrm{MnP}$ semiconductors

Mn-doped chalcopyrites exhibit rich and unique physical properties due to the presence of magnetic clusters, both at ambient and under high external pressures. Despite recent efforts and rich literature on this topic, it is still necessary to explain some of the phenomena occurring in these systems, such as the origin of magnetic interactions at ambient pressure. In this context, we report the investigations of physical mechanisms responsible for the magnetic order of a ${\mathrm{Cd}}_{1–x}{\mathrm{Mn}}_x\mathrm{Ge}{\mathrm{P}}_2+\mathrm{MnP}$ composite system with changeable Mn contents $x$, ranging from 0.008 to 0.037. The presence of MnP clusters, confirmed by energy dispersive x-ray spectrometry and scanning electron microscopy methods, is responsible for the ferromagnetic phase transition with the Curie temperature $T_{\mathrm{C}}$ around 295 K. Moreover, the transition from ferromagnetic to helical order is observed, with the critical temperature $T_{\mathrm{S}}$ changing from 78 K for the sample with $x=0.008$ down to 54 K for $x=0.037$. Besides the common explanation based on the structural disorder at the cluster surface and finite-size effects, we associate the observed $T_{\mathrm{S}}\left(x\right)$ dependence with the local pressure effect due to Mn doping, where the influence of the external pressure $P\sim 0.25\mathrm{GPa}$ corresponds to the influence of the Mn content $x=0.035$.

Figure 1

Results of the HRXRD measurements obtained at room temperature for the exemplary ${\mathrm{Cd}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{Ge}{\mathrm{P}}_2+\mathrm{MnP}$ samples with $x=0$ and $x=0.023$ showing three detected phases in these alloys including (a) chalcopyrite $\mathrm{CdGe}{\mathrm{P}}_2$, (b) cubic Ge, and (c) hexagonal Cd phases.

Figure 2

Results of the SEM/EDS measurements obtained at room temperature for the exemplary ${\mathrm{Cd}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{Ge}{\mathrm{P}}_2+\mathrm{MnP}$ sample with $x=0.012$ including (a) SEM image and [(b)–(f)] EDX microprobe results including (b)–(e) maps of the distribution of the alloying elements and (f) the detailed chemical content measurement executed at different spots [marked as numbers in Fig. 2] of the sample surface.

Figure 3

Temperature dependence of the AC magnetic susceptibility (top figure) and its derivative (bottom figure) obtained for the ${\mathrm{Cd}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{Ge}{\mathrm{P}}_2+\mathrm{MnP}$ samples with different average Mn content $x$.

Figure 4

Temperature dependencies of the inverse DC magnetic susceptibility collected at $B=500\mathrm{mT}$ for the ${\mathrm{Cd}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{Ge}{\mathrm{P}}_2+\mathrm{MnP}$ samples with different average Mn content $x$ (markers) and fitted Curie-Weiss law to the high-temperature part of the experimental data (lines).

Figure 5

Selected results of the magnetization measurements (points) obtained for ${\mathrm{Cd}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{Ge}{\mathrm{P}}_2+\mathrm{MnP}$ samples with different average Mn content $x$, including magnetization curves (a) measured up to 9 T at $T=4.5\mathrm{K}$ and lines representing fitting the experimental data to different models, and [(b),(c)] in the field range $\pm 0.2\mathrm{T}$ at $T=4.5\mathrm{K}$ [top (b)] and $T=295\mathrm{K}$ [bottom (c)].

Figure 6

Comparison between Mn doping and pressure effect, plotted against the $\mathrm{CdGe}{\mathrm{P}}_2$ volume cell. Solid lines indicate the linear fitting of both dependences. The red dashed lines show extrapolation from the point of Mn doping to linear dependence with $P$. The obtained extrapolated pressure data are plotted against the average Mn content, as shown in the inset. Green dashed lines correspond to hypothetical continuation of the $x\left(P\right)$ dependence.

Figure 7

T-P magnetic phase diagram of ${\mathrm{Cd}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{Ge}{\mathrm{P}}_2+\mathrm{MnP}$, including magnetic transitions $T_{\mathrm{C}}$ and $T_{\mathrm{S}}$ from Table 2, compared with the same transitions in bulk MnP from Refs. [25, 39, 40]. Dashed lines correspond to hypothetical continuation of the $T_{\mathrm{C}}\left(P\right)$ and $T_{\mathrm{S}}\left(P\right)$ dependencies. FM denotes the ferromagnetic, SCR denotes the helical (screw), and PM denotes the paramagnetic state.