Successive multiferroic orderings in Na₄Ni₇(PO₄)₆polar magnetic oxide

The blending of multiple ferroic orders offers a compelling avenue to exploit intricate correlations among spin, charge and lattice degrees of freedom, potentially paving the way for innovations in spin-based electronics. In this study, we investigate the magnetic, dielectric, and ferroelectric properties of the polar, non-centrosymmetric Na₄Ni₇(PO₄)₆ magnetic oxide. Magnetic and neutron scattering measurements reveal multiple long-range magnetic orderings, including a commensurate magnetic phase at T_N1 (~17 K), T_N2 (~9 K), and an incommensurate phase at T_N3 (~4.6 K). Dielectric measurements exhibit distinct frequency-independent anomalies, at T_N1 followed by frequency-independent peaks at T_N2 and T_N3, indicating a strong coupling between magnetic and dielectric properties. Furthermore, field-dependent magnetization and dielectric measurements reveal a progressive broadening of the ferroelectric-like peak at T_N2 under increasing magnetic field up to 7 T. Interestingly, the dielectric anomaly at T_N1 remains unaffected by the external magnetic field. Pyroelectric current measurements under different protocols confirm switchable electric polarization at T_N1, while non-switchable polarization emerges at T_N2, suggesting an unusual ferroelectric-to-pyroelectric transition as the temperature decreases. These results indicate that the Na₄Ni₇(PO₄)₆ system presents a fertile ground for exploring successive multiferroic ordering, identifying it as a promising type-II multiferroic material.