Pressure-induced anomalies in the magnetic transitions of the exotic multiferroic material ${\mathrm{Tb}}_2{\mathrm{BaNiO}}_5$

We have studied the influence of external pressure up to 1 GPa on the magnetic transitions of the orthorhombic Haldane-spin chain compound ${\mathrm{Tb}}_2{\mathrm{BaNiO}}_5$, an exotic multiferroic material. This parent compound is known to undergo Néel ordering at $T_{N1}=63\mathrm{K}$ and another magnetic transition at $T_{N2}=25\mathrm{K}$ at which ferroelectricity sets in, however, without any change in the magnetic symmetry, but with only a sharp change in the canting angle of Tb $4f$ and Ni $3d$ magnetic moments. There is a subtle difference in the antiferromagnetic state above and below $T_{N2}$, which is supported by the fact that there is a metamagnetic transition below $T_{N2}$ only (for 5 K, at ∼60 kOe). We report here that, with the application of external pressure, there is an upward shift of $T_{N1}$, while $T_{N2}$ shifts towards lower temperatures. It is interesting that the two magnetic transitions in the same compound behave differently under pressure and the opposite behavior at $T_{N2}$ is attributed to local distortion leading to ferroelectricity. The results are augmented by temperature dependent x-ray diffraction and positive chemical pressure studies. The chemical pressure caused by the isoelectronic doping at Ba site by Sr reduces both the transition temperatures. Clearly, the external pressure favors antiferromagnetic coupling (that is, leading to $T_{N1}$ enhancement), whereas the chemical pressure reduces $T_{N1}$, suggesting an important role of the changes in local hybridization induced by doping on magnetism in this material.

Figure 1

Magnetic susceptibility (χ) curves obtained as a function of temperature for ${\mathrm{Tb}}_2{\mathrm{BaNiO}}_5$ under various external pressures in the range 2–100 K measured in 5 kOe are shown in (a). The high pressure χ(T) curves are shifted for the sake of clarity, as otherwise the curves are essentially indistinguishable. Derivative curves, also for the data measured in 100 Oe, are shown in (b) and (c) in an expanded form in the vicinity of magnetic transitions ($T_{N1}$ and $T_{N2}$) described in the text. Vertical arrows mark the $T_{N1}$ and $T_{N2}$ under ambient pressure conditions. Inclined arrows are shown as guide to the eye for the reader to show the direction in which the curves move with pressure around the transition. The plot of inverse susceptibility above 150 K is shown in the inset of (a) in the absence of external pressure (points) and for 1 GPa (continuous line), to highlight that the paramagnetic Curie temperatures and the effective magnetic moment do not change with pressure.

Figure 2

(a) Magnetic susceptibility (χ) as a function of temperature for Sr-doped specimens, ${\mathrm{Tb}}_2{\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_5$ in the range 2–100 K, measured with 5 kOe. In (b), the derivative curves are shown in an expanded form in the vicinity of $T_{N1}$ and $T_{N2}$. Vertical arrows mark $T_{N1}$ and $T_{N2}$. Inclined arrows are guides to the eyes to show how the transitions shift with $x$ ($=0.0,0.025,0.05,0.075$, and 0.1).

Figure 3

(a) Isothermal magnetization (virgin) at 5 K for ${\mathrm{Tb}}_2{\mathrm{BaNiO}}_5$ under pressure in the metamagnetic transition region. The profile of the curve in the range 0–160 kOe under ambient pressure is shown in the inset. (b) Isothermal magnetization at 5 K for the Sr-doped specimens. Inclined arrows are showing the direction in which the curves shift with increasing pressure/composition.

Figure 4

Synchrotron based powder x-ray diffraction (S-PXRD) patterns of selected temperatures for ${\mathrm{Tb}}_2{\mathrm{BaNiO}}_5$ and ${\mathrm{Tb}}_2{\mathrm{Ba}}_{0.9}{\mathrm{Sr}}_{0.1}{\mathrm{NiO}}_5$ are shown to highlight that there is no change in structural symmetry across the magnetic transitions. Vertical bars are shown to mark the expected positions of Bragg peaks.