Impact of incomplete martensitic phase transition on the magnetic behavior of Co-doped ${\mathrm{MnNiGe}}_{1.05}$ alloys

Co-doped ${\mathrm{MnNiGe}}_{1.05}$ alloys of nominal compositions ${\mathrm{MnNi}}_{1-x}{\mathrm{Co}}_x{\mathrm{Ge}}_{1.05}$ (for $x=0.075$ and 0.125), with a significant amount of vacancies at the transition-metal sites, have been explored through temperature-dependent structural and magnetic investigations. Such studies confirm the presence of martensitic phase transition (MPT) in both the alloys. However, the observed MPT is found to be incomplete in nature. The presence of a significant amount of high-temperature austenite phase, well below the MPT, results in several interesting features, including field-induced metamagnetic transition and nonmonotonic change in the inverse magnetocaloric behavior, etc. In addition, a possible spin-reorientation transition has also been noticed in the magnetically ordered austenite phase for both the alloys.

Figure 1

X-ray powder diffraction data, recorded using P21.1 beamline of PETRA-III electron storage ring, at 300 K [(a) and (b)] and 25 K [(d) and (e)] for both the alloys are plotted. Here, black circles, red lines, and blue lines represent the observed, calculated, and difference patterns, respectively. The olive and pink vertical bars indicate the Bragg's peak position for hexagonal ($P{6}_3/mmc$) and orthorhombic ($Pnma$) phases, respectively. (c) and (f) depict the hexagonal (austenite) and orthorhombic (martensite) structures, respectively, of a representative alloy of composition ${\mathrm{MnNi}}_{0.925}{\mathrm{Co}}_{0.075}{\mathrm{Ge}}_{1.05}$.

Figure 2

Contour plots of a restricted region of the temperature evolution of the hexagonal and orthorhombic reflections are depicted in (a) and (b) for $x=0.075$ and 0.125 alloys, respectively. The white dashed line indicates the martensitic phase transition temperature of the studied alloys. The color bar presented in the right-hand side specifies the normalized intensities. The temperature variation of the orthorhombic and hexagonal phase fractions for $x=0.075$ and 0.125 alloys are plotted in (c) and (d), respectively.

Figure 3

Temperature variation of dc magnetization data, recorded in the presence of 1 kOe of external magnetic fields, under field cooling (FC) and field-cooled heating (FCH) protocols are depicted in the main panels of (a) and (b) for $x=0.075$ and 0.125 alloys, respectively. Insets show the enlarged view of the anomaly observed in the magnetically ordered austenite phase.

Figure 4

(a) and (b) depict the dc magnetization $M$ data as a function of temperature $T$ in the presence of 10, 50, 100, and 140 kOe of external magnetic field ($H$) under field cooling (FC) and field-cooled heating (FCH) protocols for $x=0.075$ and 0.125 alloys, respectively. Isothermal variation of dc $M$ as a function of $H$ are plotted in (c) and (d) for $x=0.075$ and 0.125 alloys, respectively. Insets of (c) and (d) depict the enlarged view of the $M\left(H\right)$ isotherm, recorded just below the MPT.

Figure 5

The main panels of (a) and (b) show the variation of entropy change $\mathrm{\Delta }S$ as a function of temperature $T$ for different values of changing magnetic fields ($H$) for $x=0.075$ and 0.125 alloys, respectively. Insets depict the isothermal variation of $\mathrm{\Delta }S$ as a function of $H$ at different constant $T$.