Doping site induced alteration of incommensurate antiferromagnetic ordering in Fe-doped MnNiGe alloys

The magnetic structure of Fe-doped MnNiGe alloys of nominal compositions ${\mathrm{MnNi}}_{0.75}{\mathrm{Fe}}_{0.25}\mathrm{Ge}$ and ${\mathrm{Mn}}_{0.85}{\mathrm{Fe}}_{0.15}\mathrm{NiGe}$ has been explored through detailed neutron powder diffraction (NPD) study in ambient and high-pressure (6 kbar) conditions. Both these alloys crystallize with ${\mathrm{Ni}}_2\mathrm{In}$-type hexagonal structure (with $P{6}_3/mmc$ space group) at room temperature and undergo a martensitic transition to low-temperature TiNiSi-type orthorhombic structure (with space group $Pnma$). Both the alloys show incommensurate antiferromagnetic ordering at the low-temperature martensitic phase. However, the modulation of the magnetic structures depends strongly on the doping site of Fe. The incommensurate propagation vector $\mathbf{k}$ for ${\mathrm{MnNi}}_{0.75}{\mathrm{Fe}}_{0.25}\mathrm{Ge}$ and ${\mathrm{Mn}}_{0.85}{\mathrm{Fe}}_{0.15}\mathrm{NiGe}$ alloys are found to be (0.1790(1),0,0) and (0.1543(3),0,0), respectively at 1.5 K, and these remain almost unchanged with increasing sample temperature under ambient conditions. The application of the external pressure results in a significant effect on both martensitic transition temperature and low-temperature magnetic structure. The propagation vectors $\mathbf{k}$ for both the alloys show a monotonic decrease with increasing sample temperature in the presence of external pressure. Interestingly, no sign of the magnetically ordered hexagonal austenite phase was observed in any alloys down to the lowest measurement temperature.

Figure 1

Main panels of (a) and (b) show NPD data from 5–6 detector bank of WISH, recorded at 320 K, for ${\mathrm{MnNi}}_{0.75}{\mathrm{Fe}}_{0.25}\mathrm{Ge}$ (MNFG) and ${\mathrm{Mn}}_{0.85}{\mathrm{Fe}}_{0.15}\mathrm{NiGe}$ (MFNG) alloys, respectively. Here the observed, calculated, and difference patterns are represented by black circles, red lines, and blue lines (lowermost lines in the main panels), respectively. The olive (labeled i) and magenta (marked ii) vertical bars represent the peak positions for the ${\mathrm{Ni}}_2\mathrm{In}$-type (for MNFG and MFNG) and ${\mathrm{MgZn}}_2$-type (for ${\mathrm{MnNi}}_{1.3}{\mathrm{Ge}}_{0.7}$) hexagonal nuclear phases, respectively. Inset of (b) shows the enlarged view of the nuclear reflections originating from the ${\mathrm{MnNi}}_{1.3}{\mathrm{Ge}}_{0.7}$ composition.

Figure 2

NPD patterns for (a), (b) MNFG and (c), (d) MFNG alloys recorded at 1.5 K at ambient pressure (samples mounted in 6-mm vanadium cell) from 3–8 and 1–10 detector banks of WISH are plotted for two different lattice spacing ($d$) ranges. The enlarged view of the (001)${}_{\mathrm{o}}^{\pm }$ magnetic reflection is shown in the insets of (a) and (c). Here the black circles, red lines, and blue lines represent observed, calculated, and difference patterns, respectively. The vertical bars represent the peak positions for the ${\mathrm{Ni}}_2\mathrm{In}$-type hexagonal nuclear phase (olive, labeled i), TiNiSi-type orthorhombic nuclear phase (orange, labeled ii), orthorhombic magnetic phase (purple, marked iii), and ${\mathrm{MgZn}}_2$-type hexagonal nuclear phase (magenta, marked iv).

Figure 3

(a), (b) and (d), (e) show the illustration of the helical and cycloidal magnetic structure at 1.5 K in ambient conditions for MNFG and MFNG alloys, respectively. The schematic representation of helical and cycloidal modulation of magnetic moments are depicted in (c) and (f), respectively.

Figure 4

NPD patterns recorded (black circles) from 3–8 and 1–10 detector banks of WISH in the presence of 6 kbar of external pressure ($P$) at 10 K for (a), (b) MNFG and (c), (d) MFNG along with the calculated (red lines) and difference patterns (blue lines) are plotted for two different $d$ ranges. Insets depict the enlarged view of the magnetic reflections recorded in ambient and high $P$ conditions at 10 K. The vertical bars, olive (labeled i), orange (labeled ii), purple (labeled iii), and magenta (labeled iv) represent the position of reflections from ${\mathrm{Ni}}_2\mathrm{In}$-type hexagonal nuclear phase, TiNiSi-type orthorhombic nuclear phase, orthorhombic magnetic phase, and ${\mathrm{MnZn}}_2$-type hexagonal nuclear phase, respectively.

Figure 5

Contour plots of the temperature ($T$) evolution of ${\left(101\right)}^{+}, {(-101)}^{-}, {\left(101\right)}^{-}, {(-101)}^{+}$, and ${\left(000\right)}^{\pm }$ magnetic satellite reflections at ambient pressure are plotted in (a), (b) for MNFG and (c), (d) for MFNG alloys. Contour plots in the presence of 6 kbar of external pressure for the $T$ evolution of the same set of magnetic satellite reflections are depicted in (e), (f) for MNFG and (g), (h) for MFNG alloys. Normalized intensity is indicated by the right-side scale bar. White dotted horizontal lines indicate the martensitic phase transition temperature.

Figure 6

(a), (b) show the variation of orthorhombic lattice parameters as a function of temperature ($T$) for MNFG and MFNG alloys, respectively. The $T$ variation of hexagonal lattice parameters are plotted in (c), (d) for MNFG and MFNG alloys, respectively.

Figure 7

Temperature variation of phase fraction both in ambient as well as in the presence of 6 kbar of external pressure ($P$) are depicted in (a), (b) for MNFG and MFNG alloys, respectively. The main panels of (c), (d) indicate integrated intensities of ${\left(000\right)}^{\pm }$ magnetic satellite reflection (both in ambient and high $P$ situation) for MNFG and MFNG alloys, respectively. Insets of (c), (d) show the $T$ variation of the $x$ component of propagation vector $k_a$ in ambient and high $P$ situations for MNFG and MFNG alloys, respectively.