Martensitic transitions and the nature of ferromagnetism in the austenitic and martensitic states of $\mathrm{Ni}-\mathrm{Mn}-\mathrm{Sn}$ alloys

Structural and magnetic transformations in the Heusler-based system ${\mathrm{Ni}}_{0.50}{\mathrm{Mn}}_{0.50-x}{\mathrm{Sn}}_x$ are studied by x-ray diffraction, optical microscopy, differential scanning calorimetry, and magnetization. The structural transformations are of austenitic-martensitic character. The austenite state has an $L{2}_1$ structure, whereas the structures of the martensite can be $10M$, $14M$, or $L{1}_0$ depending on the Sn composition. For samples that undergo martensitic transformations below and around room temperature, it is observed that the magnetic exchange in both parent and product phases is ferromagnetic, but the ferromagnetic exchange, characteristic of each phase, is found to be of different strength. This gives rise to different Curie temperatures for the austenitic and martensitic states.

Figure 1

(a) ZFC, FC, and FH $M\left(T\right)$ of the samples with $x=0.25$ and 0.20 in $H=50\mathrm{Oe}$. The inset, with a higher resolution in the vertical scale, highlights the splitting of the ZFC and FC curves for $x=0.20$. $M$ in the inset is in units of $\mathrm{emu}{\mathrm{g}}^1$ (b) $M\left(H\right)$ of the sample with $x=0.25$. Prior to each $M\left(H\right)$ measurement, the samples were prepared in the ZFC state by bringing them above $380\mathrm{K}$.

Figure 2

(a) ZFC, FC, and FH $M\left(T\right)$ of the sample $x=0.18$ in $H=50\mathrm{Oe}$. The inset, with a higher resolution in the vertical scale, highlights the splitting of the ZFC and FC curves. (b) $M\left(H\right)$ at $5\mathrm{K}$ of the samples $x=0.25$, 0.20, and 0.18. Prior to each $M\left(H\right)$ measurement the samples were prepared in the ZFC state by bringing them above $380\mathrm{K}$.

Figure 3

(a) ZFC, FC, and FH $M\left(T\right)$ of the sample with $x=0.15$ in $H=50\mathrm{Oe}$. (b) $M\left(T\right)$ plotted in a temperature range around the martensitic transition. (c) $M\left(H\right)$ at 5, 160, 195, 220, and $350\mathrm{K}$. Prior to each $M\left(H\right)$ measurement, the samples were prepared in the ZFC state by bringing them above $380\mathrm{K}$.

Figure 4

(a) ZFC, FC, and FH $M\left(T\right)$ of the sample with $x=0.13$ in $H=50\mathrm{Oe}$. (b) $M\left(T\right)$ plotted in a temperature range around the martensitic transition. (c) $M\left(H\right)$ at 5, 150, 275, 300, 306, and $330\mathrm{K}$. Prior to each $M\left(H\right)$ measurement, the samples were prepared in the ZFC state by bringing them above $380\mathrm{K}$.

Figure 5

(a) ZFC, FC, and FH $M\left(T\right)$ for $x=0.10$ and (b) $M\left(H\right)$ at 100 and $350\mathrm{K}$. (c) ZFC, FC, and FH $M\left(T\right)$ for $x=0.05$ and (d) $M\left(H\right)$ at 5 and $100\mathrm{K}$.

Figure 6

$dQ∕dT$ vs temperature for the alloys undergoing martensitic transformations (a) $x=0.15$, (b) $x=0.13$, (c) $x=0.10$, and (d) $x=0.05$.

Figure 7

X-ray diffraction pattern at room temperature for $x=0.25$ (lower) and $x=0.15$ (upper). The crystal structure is in both cases $L{2}_1$.

Figure 8

X-ray diffraction pattern at room temperature for $x=0.13$. The crystal structure is $10M$ with an orthorhombic unit cell.

Figure 9

X-ray diffraction pattern at room temperature for $x=0.10$. The inset shows details in the range $40°⩽2\theta ⩽46°$. The crystal structure is $14M$ with a monoclinic unit cell.

Figure 10

X-ray diffraction pattern at room temperature for $x=0.10$. The crystal structure is $L{1}_0$.

Figure 11

Optical microscopy images of the microstructure for $x=0.25$ $\left(L{2}_1\right)$, $x=0.13$ $\left(10M\right)$, $x=0.10$ $\left(14M\right)$, and $x=0.05$ $\left(L{1}_0\right)$.

Figure 12

$M\left(T\right)$ measured in $50\mathrm{kOe}$. The dashed line is a guide showing the expected behavior of $M\left(T\right)$ of the martensitic state beyond the stability range. The position of $M_s$ indicated in the figure is taken from the results of DSC measurements. The inset shows the linear behavior of $M∕M_0$ vs $T^{3∕2}$.

Figure 13

The structural and magnetic transition temperatures as a function of the valence electron concentration for $\mathrm{Ni}-\mathrm{Mn}-\mathrm{Sn}$.

Figure 14

$\Delta S$ and $\Delta H$ as a function of $e∕a$.