Magnetic ordering in ${}^{57}\mathrm{Fe}$-doped $\mathrm{Eu}\mathrm{Ni}{\mathrm{O}}_3$ and $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ studied by Mössbauer spectroscopy

${}^{57}\mathrm{Fe}$ Mössbauer spectra of $\mathrm{Eu}\mathrm{Ni}{\mathrm{O}}_3$ and $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ doped with $1\mathrm{at.}\%$ ${}^{57}\mathrm{Fe}$ taken in the temperature range $1.5⩽T⩽300\mathrm{K}$ reveal two magnetically inequivalent Fe sites. About half of the Fe atoms show a magnetic hyperfine interaction that follows the macroscopic magnetization: a second-order magnetic phase transition is observed for $\mathrm{Eu}\mathrm{Ni}{\mathrm{O}}_3$ at $T_N=190\left(10\right)\mathrm{K}$, while a first-order magnetic transition is seen for $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ at $T_N=150\left(10\right)\mathrm{K}$. The other half of the Fe atoms reveals a slowing down of fluctuating hyperfine fields on decreasing temperature, indicating that these sites are magnetically frustrated. This can be well explained by a charge disproportionation occurring at the Ni sites.

Figure 1

${}^{57}\mathrm{Fe}$ ME spectra of $\mathrm{Eu}{\mathrm{Ni}}_{0.99}$${}^{57}\mathrm{Fe}_{0.01}$ ${\mathrm{O}}_3$ at temperatures above $T_N$.

Figure 2

${}^{57}\mathrm{Fe}$ ME spectra of $\mathrm{Eu}{\mathrm{Ni}}_{0.99}{}^{57}\mathrm{Fe}_{0.01}{\mathrm{O}}_3$ at temperatures below $T_N$. Drawn lines are fits with the model as described in text.

Figure 3

${}^{57}\mathrm{Fe}$ ME spectra of $\mathrm{Nd}{\mathrm{Ni}}_{0.99}{}^{57}\mathrm{Fe}_{0.01}{\mathrm{O}}_3$ at temperatures above $T_N$.

Figure 4

${}^{57}\mathrm{Fe}$ ME spectra of $\mathrm{Nd}{\mathrm{Ni}}_{0.99}{}^{57}\mathrm{Fe}_{0.01}{\mathrm{O}}_3$ at various temperatures below $T_N$. Drawn lines are fits with the model as described in the text. Data recorded on cooling.

Figure 5

Temperature dependence of magnetic hyperfine field ${\mathrm{B}}_{{h}{f}}$ for (a) $\mathrm{Eu}{\mathrm{Ni}}_{0.99}{}^{57}\mathrm{Fe}_{0.01}{\mathrm{O}}_3$ and (b) $\mathrm{Nd}{\mathrm{Ni}}_{0.99}{}^{57}\mathrm{Fe}_{0.01}{\mathrm{O}}_3$ (as observed when cooling down and warming up, respectively). The lines through the data points are least-squares fits with a common magnetization curve for $S=5∕2$. $T_{N^{*}}$ is the extrapolated “real” Néel temperature of the insulating $\mathrm{Nd}{\mathrm{Ni}}_{0.99}{}^{57}\mathrm{Fe}_{0.01}{\mathrm{O}}_3$.

Figure 6

Fit functions for spectra of $\mathrm{Nd}{\mathrm{Ni}}_{0.99}{}^{57}\mathrm{Fe}_{0.01}{\mathrm{O}}_3$ at 4.2, 20, and $60\mathrm{K}$, showing the static and dynamic (spherical fluctuations) subspectra.

Figure 7

Dynamical subspectra for (a) spherical and (b) axial fluctuations for several fluctuation rates $\gamma$ (Mc/s).

Figure 8

Temperature dependence of fluctuation rates for (a) $\mathrm{Eu}{\mathrm{Ni}}_{0.99}{}^{57}\mathrm{Fe}_{0.01}{\mathrm{O}}_3$ and (b) $\mathrm{Nd}{\mathrm{Ni}}_{0.99}{}^{57}\mathrm{Fe}_{0.01}{\mathrm{O}}_3$ as obtained from the fits to the ${}^{57}\mathrm{Fe}$ ME spectra as described in the text.

Figure 9

${}^{57}\mathrm{Fe}$ ME spectra of $\mathrm{Eu}{\mathrm{Ni}}_{0.99}{}^{57}\mathrm{Fe}_{0.01}{\mathrm{O}}_3$ in external magnetic field at $4.2\mathrm{K}$. Magnetic field direction parallel to $\gamma$-ray direction. Drawn lines are fits with the model as described in the text.