Uncompensated antiferromagnetism and phase boundary with variable curvature in the mixed magnet ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$

The magnetic properties of ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$ are examined by dc magnetization and susceptibility measurements on a wide range of compositions. The pure components are isomorphous three-dimensional Heisenberg antiferromagnets, ordering at $1.62\mathrm{K}\left({\mathrm{MnCl}}_2·4{\mathrm{H}}_2\mathrm{O}\right)$ and $2.99\mathrm{K}\left({\mathrm{NiCl}}_2·4{\mathrm{H}}_2\mathrm{O}\right)$. So far as is known, exchange interactions in either material are antiferromagnetic only. The Curie and Weiss constants, in ${\chi }_M=C∕\left(T-\theta \right)$ fits to high temperature data, vary with composition in somewhat unexpected ways. $C\left(x\right)$ is not as linear as often seen in mixed magnets; $\theta \left(x\right)$ exhibits a high degree of scatter. Remarkably, for compositions from $x=0.10$ through $x=0.95$, the susceptibility shows no standard antiferromagnetic maximum; rather, sharp upturns appear, at temperatures which vary systematically with composition. Only very near the composition extremes does the susceptibility exhibit somewhat oddly shaped maxima. Ordering temperatures are estimated, and a $T_c\left(x\right)$ phase boundary deduced. It is monotonic but displays several curvature changes, more than anticipated from certain theoretical treatments. Low temperature magnetization isotherms display inflections, the location of which varies with composition. There is also an evolution with composition of the extrapolated zero-field magnetization at sufficiently low temperatures. It is likely that incomplete cancellation of antiferromagnetic sublattice moments occurs in mixtures.

Figure 1

Inverse molar magnetic susceptibility vs temperature for various compositions of ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$. Solid lines are Curie-Weiss fits; dashed lines extend these to temperatures higher than fitted. From bottom to top, and with successive shifts of $10,20,\dots ,140\mathrm{mol}∕\mathrm{emu}$ (for clarity), compositions are $x=0$, 0.10, 0.20, 0.30, 0.40, 0.50, 0.525, 0.55, 0.60, 0.70, 0.80, 0.90, 0.95, 0.975, and 1.

Figure 2

Curie constant and Weiss $\theta$ vs composition for ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$. A straight line connects $C$ values for $x=0$ and 1.

Figure 3

Molar magnetic susceptibility vs temperatures for an $x=0.10$ composition of ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$. The arrow shows the estimated transition temperature.

Figure 4

Molar magnetic susceptibility vs temperature for several compositions of ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$ not richer in nickel. For clarity shifts are applied to $x=0.20$, 0.30, 0.40, and 0.50; 1.25, 1.00, 1.65, and $0.75\mathrm{emu}∕\mathrm{mol}$, respectively. Arrows indicate transition temperatures.

Figure 5

Molar magnetic susceptibility vs temperature for three slightly $\mathrm{Ni}$-rich compositions of ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$. For clarity shifts are applied to $x=0.525$, 0.55, and 0.60; 0.55, 0.40, and $0.30\mathrm{emu}∕\mathrm{mol}$, respectively. Arrows indicate estimated transition temperatures.

Figure 6

Molar magnetic susceptibility vs temperatures for three rather $\mathrm{Ni}$-rich compositions of ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$. For clarity shifts are applied to $x=0.70$ and 0.80; $-0.01$ and $-0.04\mathrm{emu}∕\mathrm{mol}$, respectively. Arrows indicate estimated transition temperatures.

Figure 7

Molar magnetic susceptibility vs temperature for two very $\mathrm{Ni}$-rich compositions of ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$. Arrow indicates transition for $x=0.95$.

Figure 8

Transition temperature vs composition for ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$. Curve through results is a guide to the eye only.

Figure 9

Molar magnetization vs field at various temperatures for $x=0.20$ ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$.

Figure 10

Molar magnetization vs field at various temperatures for $x=0.40$ ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$.

Figure 11

Molar magnetization vs field at various temperatures for $x=0.60$ ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$. For clarity the $1.840\mathrm{K}$ isotherm is shifted up $500\mathrm{emu}∕\mathrm{mol}$.

Figure 12

Molar magnetization vs field at various temperatures for $x=0.80$ ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$. For clarity the $1.844\mathrm{K}$ isotherm is shifted up $500\mathrm{emu}∕\mathrm{mol}$.

Figure 13

Isotherms taken near $1.85\mathrm{K}$ for compositions of ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$ for which an inflection appears in $M$ vs $H$, indicated by arrows. From bottom to top, and with successive shifts of $500,100,\dots ,4500\mathrm{emu}∕\mathrm{mol}$ (for clarity), compositions are $x=0.90$, 0.80, 0.70, 0.60, 0.55, 0.525, 0.50, 0.40, 0.30, and 0.20.

Figure 14

Characteristic field vs composition for ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$, from $1.85\mathrm{K}$ isotherm inflection points in previous figure.

Figure 15

Low field regions of lowest temperature isotherms, including hysteresis, for $x=0.10$ and 0.40 compositions of ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$. Open symbols are field-up data; closed symbols are field-down data. Lines or curves through data are extrapolations to zero field. For clarity the $x=0.40$ data are shifted up $20\mathrm{emu}∕\mathrm{mol}$.

Figure 16

Low field regions of lowest temperature isotherms, including hysteresis, for $x=0.60$ and 0.80 compositions of ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$. Open symbols are field-up data; closed symbols are field-down data. Lines through data are extrapolations to zero field. For clarity the $x=0.60$ data are shifted up $20\mathrm{emu}∕\mathrm{mol}$.

Figure 17

Magnetization vs temperature for $x=0.050$ and 0.025 compositions of ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{Cl}}_2·4{\mathrm{H}}_2\mathrm{O}$. Samples masses are 267.2 and $229.4\mathrm{mg}$, respectively, with an applied field of $200\mathrm{G}$. For clarity the $x=0.050$ data are shifted up $0.002\mathrm{emu}$.