Figure 1

(a) The room temperature XRD pattern (Cu $K_{\alpha 1}\lambda =1.54059\AA$) for ${\mathrm{Ba}}_3{\mathrm{NiNb}}_2{\mathrm{O}}_9$ (plus marks). The solid curve is the best fit from the Rietveld refinement using FullProf. Inset: the (112) and (103) peaks from the XRD pattern. (b) The neutron diffraction pattern measured using the neutron powder diffractometer HB2A at ORNL showing several magnetic Bragg reflections at 2K. (c) Atomic resolution STEM high angle annular dark field $Z$-contrast image looking down [010]. Ba, Nb, and Ni atoms show brightest, brighter, and weak contrast, respectively. (d) Line profile of the atomic columns intensity indicated by the box in (c) showing the ordering sequence of -Ni-Nb-Nb-. (e) Schematic crystal structures for ${\mathrm{Ba}}_3{\mathrm{NiNb}}_2{\mathrm{O}}_9$: the green octahedra represent Ni sites and the blue octahedra represent Nb sites. (f) Zero field magnetic structure below $T_{\mathrm{N}1}$ composed of ${\mathrm{Ni}}^{2+}$ ions and the arrows representing their spins.Reuse & Permissions

Figure 2

(a) Temperature dependence of $\chi$ for ${\mathrm{Ba}}_3{\mathrm{NiNb}}_2{\mathrm{O}}_9$ at different fields. Inset: $d\chi /dT$ vs $T$ for $H=7$ and 9 T. (b) Temperature dependence of $C_{\mathrm{P}}/T$ at 0, 3, 5, 7, 8, and 9 T. Inset: Expansion of the 9 T data. (c) Temperature dependence of ${\epsilon }^{\prime }$ at 0, 4, 8, 10, 12, 14, 16, and 18 T, and (d) magnetic field dependence of ${\epsilon }^{\prime }$ at 0.5, 0.9, 1.6, 2, 2.6, and 3.1 K. Inset: Expansion around $H=8\mathrm{T}$ for 0.5 and 1.6 K (data is offset for clarity). The vertical lines in (a) and (b) indicate $T_{\mathrm{N}1}$ (higher $T$) and $T_{\mathrm{N}2}$ (lower $T$).Reuse & Permissions

Figure 3

Pyroelectric current ($I_p$) (a) and polarization (b) as a function of temperature at different magnetic fields under positive electric poling field and FC conditions. The inset of (b) shows polarization vs magnetic field at 2.3 and 3.8 K under ZFC conditions. $I_p$ (c) and polarization (d) at $H=0$ and 10 T under positive (solid symbols) and negative (open symbols) poling fields and ZFC and FC conditions.Reuse & Permissions

Figure 4

(a) dc magnetization ($M$) and its derivative ($dM/dH$) for $T=1.4\mathrm{K}$. (b) ac susceptibility as a function of magnetic field $H$ for four temperatures. The asterisks and the triangles indicate $A\mathrm{\text{−}}C$ and $C\mathrm{\text{−}}D$ phase boundary respectively. (c) $H\mathrm{\text{−}}T$ phase diagram for ${\mathrm{Ba}}_3{\mathrm{NiNb}}_2{\mathrm{O}}_9$. Different symbols denote phase boundaries obtained from different techniques. $\square$:${\epsilon }^{\prime }$ $H$-sweep, $•$:${\epsilon }^{\prime }$ $T$-sweep, $\diamond$: polarization, $◃$: $C_{\mathrm{P}}$, $▴$: ac susceptibility $H$-sweep, $△$: ac susceptibility $T$-sweep, $\oplus$: dc susceptibility $T_{\mathrm{N}1}$, $+$: dc susceptibility $T_{\mathrm{N}2}$, $*$: dc magnetization. The arrows indicate spin structures. The dashed (solid) vertical lines in (a) and (b) indicate the transition fields to $uud$ (oblique) phases. The dashed lines in (c) are guides to the eyes.Reuse & Permissions