Multiferroicity and magneto-electric effect in Gd2BaNiO5

Abstract

We report the observation of electric polarization in the magnetically ordered state of the Haldane chain compound, Gd 2 BaNiO 5 , with strongly correlated magnetic and dielectric properties. The results of dc magnetic susceptibility and heat capacity measurements indicate two magnetic transitions, one corresponding to the anti-ferromagnetic order at T N ~55 K and the other to spin-reorientation transition at T SR ~24 K. The dielectric permittivity and loss (tanδ) also exhibit anomalies in the vicinity of T SR and T N respectively. Below the spin-reorientation transition, concurrently magnetic-field-induced spin-flop and the meta-electric transitions are observed at a critical magnetic field in isothermal magnetization and magneto-dielectric results respectively. Another interesting finding is that Δ (= changes its sign at the critical magnetic field. The origin of the observed magneto-electric effect is discussed on the basis of spin-phonon coupling.

Extraordinarily large intrinsic magnetodielectric coupling of the Tb member within the Haldane spin-chain family R2BaNiO5

“…The fact that the magnetic ordering is of an antiferromagnetic-type is confirmed by the observation that the peak gets gradually suppressed, shifting towards low-temperature range with increasing dc magnetic-field (figure 1b, inset). It is worth noting that the observed value of TN is the highest in this series; for the Gd, Dy, Ho and Er cases, respective TN value is equal to ~55, 58, 53 and 32 K [22,19,18,7]. It was theoretically established long ago [26][27][28] that the anisotropy of the 4f orbital of the crystal-field split ground state (that is, the sign of some of the crystal-field terms to Hamiltonian) in general plays a role in such an enhancement of TN for a heavy member in a given rare-earth family.…”

“…The homogeneity of the sample was further confirmed by scanning electron microscope and the composition (within 2%) was confirmed by energydispersive x-ray analysis. Instruments used for measurements of magnetization, heat-capacity (C), and dielectric permittivity as a function of T down to 1.8 K and in magnetic-fields have been described in our earlier publications [7,[19][20][21][22][23]. Bias electric-field (E) measurements were done with the help of a Keithley 6517B electrometer.…”

“…These compounds crystallizing in Immm-type orthorhombic structure has been of special interest in the field of magnetism for over a quarter century for anomalies associated with Haldane spin-chain gap and magnetic ordering of Ni and (magnetic-moment containing) R ions at the same temperature (see, for instance, Refs. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Though this family has been considered to be prototype for Haldane spin-chain behavior, there was very little focus to understand magnetoelectric behavior of these insulators till recently.…”

Magnetic and magnetodielectric coupling anomalies in the Haldane spin-chain system Nd2BaNiO5

“…The reason is that similar inferences have made by us in some other families as well in recent years, [15][16][17] where the magnetoelectric coupling is strong. We have tried to look for ferroelectricity observed for heavy rare-earth members; 4,6,8 however, we could not detect the same (that is, remnant polarization and its reversibility of sign with opposite poling by electric field by measuring pyroelectric current) in this compound. We wonder whether there is any additional factor playing a role, that is, the radial extension/contraction of 4f orbital, to create the local distortion of Ni-O responsible for the ferroelectricity in this series.…”

“…This situation is completely different from that observed for heavy rare-earth members. 5,7,8 In case of heavy rare-earth members in this series [see Fig. 3 in Refs.…”