Magnetic field-induced ferroelectricity in S = 1/2 kagome staircase compound PbCu3TeO7

Ni 3 V 2 O 8 is an archetypical multiferroic material with a kagome staircase structure of S = 1 (Ni 2+ ) spins whose complex interplay between spin ordering and ferroelectricity has been studied for more than a decade. Here, we report a new kagome staircase compound PbCu 3 TeO 7 with Cu 2+ ( S = 1/2) spins that exhibits two Néel temperatures at T N1 = 36 K and T N2 = 24 K, and a magnetic field ( H )-induced electric polarization ( P ) below T N2 . Pyroelectric and magnetoelectric current measurements in magnetic fields up to 60 T reveal that for H||c of ~ 8.3 T, a spin-flop transition induces a transverse P||a with a magnitude of 15 µC/m 2 below T N2 . Furthermore, for a parallel configuration with P//H // a , two spin-flop transitions occur, the first at ~ 16 T with P//a of 14 µC/m 2 , and the second at ~ 38 T, where P disappears. Monte Carlo simulations based on 12 major exchange interactions uncover that a sinusoidal amplitude modulation of the spins occurs along the b -axis below T N1 and an incommensurate, proper screw-type spin order occurs in the ac -plane below T N2 . The simulation results show that P//a under H // c stems from a spin-flop transition facilitating an ab -plane-type spiral order, while the two successive spin-flop transitions for H // a result in spiral spin orders in the ab - and bc -planes. Based on the experimental and theoretical results, we establish field-induced magnetic and electric phase diagrams for the two H directions, demonstrating that the distorted kagome staircase structure with competing intra–interlayer interactions and lifted frustration creates a plethora of different noncollinear spin textures of S = 1/2 spins that in turn induce electric polarization. Multiferroics: Inducing ferroelectricity magnetically In multiferroic materials, the ferroelectric polarization can be induced by the spin order; now a Cu-based material with a geometrically frustrated (kagome) lattice joins the list of oxides in which this effect has been observed. In an ideal material the frustration would give rise to degenerate ground states such as the quantum-spin liquid, but the presence of lattice defects lifts the spin frustration, allowing the formation of a long-range spin order. Kee Hoon Kim from Seoul National University and colleagues observed magnetic-field-induced ferroelectricity in a material containing Cu2+ ions with spin 1/2 that has a kagome spin structure. They established the phase diagram of the material for different