New magnetic phase in the nickelate perovskite ${\mathrm{TlNiO}}_3$

The ${\mathrm{RNiO}}_3$ perovskites are known to order antiferromagnetically below a material-dependent Néel temperature $T_{\text{N}}$. We report experimental evidence indicating the existence of a second magnetically ordered phase in ${\mathrm{TlNiO}}_3$ above $T_{\text{N}}=104\mathrm{K}$, obtained using nuclear magnetic resonance and muon spin rotation spectroscopy. The new phase, which persists up to a temperature $T_{\text{N}}^{*}=202\mathrm{K}$, is suppressed by the application of an external magnetic field of approximately 1 T. It is not yet known if such a phase also exists in other perovskite nickelates.

Figure 1

Phase diagram of the ${\mathrm{RNiO}}_3$ perovskites with respect to temperature and Ni-O-Ni superexchange angle. Data for compounds other than ${\mathrm{TlNiO}}_3$ shown in this plot are taken from Refs. [2, 19, 20, 21, 22, 23, 24]. The red triangle indicates the new magnetic phase transition described in this work.

Figure 2

Magnetic susceptibility of ${\mathrm{TlNiO}}_3$ powder, measured using a commercial DC magnetometer at different magnetic fields. Vertical dashed lines at $T_{\text{N}}=104\text{K}$ and $T_{\text{N}}^{*}=202\text{K}$ indicate the magnetic transition temperatures in zero field, as determined by $\mu \mathrm{SR}$. The magnetic response below 25 K (not shown) is dominated by a ferromagnetic impurity contribution from unreacted Ni(OH)${}_2$ (see text for details).

Figure 3

${}^{203}\mathrm{Tl}$ and ${}^{205}\mathrm{Tl}$ NMR spectra at 295 K in the PM phase (left) and at 50 K in the AFM phase (right). Note the different frequency scales in the two panels. The broadened upper edges of two overlapping AFM powder patterns are emphasized by vertical dashed lines.

Figure 4

Zero-field Tl NMR frequencies plotted against temperature, demonstrating the presence of static internal magnetic fields up to $\sim 200$ K. The solid lines are guides to the eye. Inset: Zero-field NMR spectra acquired at 5 K. Two different sets of probe-head configurations and NMR pulse-sequence parameter sets were used to acquire the two parts of the spectrum, shown in blue and red. The intensities measured in two different setups are not directly comparable; hence signals are normalized to their maximal amplitude.

Figure 5

Transverse NMR relaxation rate $T_2^{-1}$ in an applied magnetic field of several teslas, plotted against temperature. Note the large hysteresis between 125 and 200 K. Below 100 K, the relaxation rate follows an Arrhenius law with an activation energy $\mathrm{\Delta }/k_{\text{B}}=82\pm 5\text{K}$.

Figure 6

Comparison of experimental zero-field $\mu \mathrm{SR}$ asymmetries to the fits (solid line) described in the text body. Note that the oscillations mostly dephase within $0.2\mu \mathrm{s}$, corresponding to a strong broadening of the frequency spectrum. Data acquired at different temperatures are offset vertically for clarity.

Figure 7

Zero-field $\mu \mathrm{SR}$ frequencies ${\nu }^{\left(i\right)}$ in ${\mathrm{TlNiO}}_3$ powder, as obtained from Eqs. (3) and (4). Parameters were fitted with a maximum-likelihood method using Minuit [30]. Different markers are assigned to different components of the $\mu \mathrm{SR}$ asymmetry. Error bars are omitted because they are smaller than the symbol size.