We report field-induced switchable polarization (P ~ 0.2 -0.8 µC/cm 2 ) below the Néel temperature of chromium (T N Cr ) in weakly ferromagnetic rare-earth orthochromites, RCrO 3 (R=rare-earth) but only when the rare-earth ion is magnetic.Intriguingly, the polarization in ErCrO 3 (T C = 133 K) disappears at a spinreorientation (Morin) transition (T SR ~ 22 K) below which the weak ferromagnetism associated with the Cr-sublattice also disappears, demonstrating the crucial role of weak ferromagnetism in inducing the polar order. Further, the polarization (P) is strongly influenced by applied magnetic field, indicating a strong magnetoelectric effect. We suggest that the polar order occurs in RCrO 3 , due to the combined effect of poling field that breaks the symmetry and the exchange field on R-ion from Crsublattice stabilizes the polar state. We propose that a similar mechanism could work in the isostructural rare-earth orthoferrites, RFeO 3 as well.

-Raman study on a select few orthochromites, RCrO 3 (R = Y, Lu, Gd, Eu and Sm) shows that the phonon behavior at T N in compounds with magnetic R-ion (Gd and Sm) is remarkably different from that of non-magnetic R-ion (Y, Lu and Eu). While anomalies in most of the observed phonon frequencies in all these compounds may result from the distortion of CrO 6 octahedra due to size effect and magnetostriction arising from Cr-ordering, the anomalous behavior of their linewidths observed at T N for the compounds with only magnetic R-ion suggests spin-phonon coupling. The presence of spin-phonon coupling and the anomalies in the low frequency modes related to R-ion motion in orthochromites (R = Gd and Sm) support the suggestion that the coupling between 4f-3d moments play important role in inducing switchable electric polarization.Introduction.-Magnetoelectric multiferroic materials with their coupled ferroelectric and ferromagnetic order parameters are promising for developing a new generation of both electrically and magnetically controlled multifunctional devices [1][2][3][4][5]. Multiferroic materials are broadly classified into two types; in type I multiferroics, the ferroelectricity and magnetism occur at high temperatures but with different temperature scale. However, the coupling between the two order parameters are rather weak. Type II multiferroics are generally centrosymmetric and magnetic where the ferroelectricity is caused by certain type of magnetic ordering. In the well-known example of TbMnO 3 , the manganese moments order antiferromagnetically at T N = 41 K and at 25 K it undergoes another magnetic transition below which a cycloidal spin structure breaks the inversion symmetry and thus induces ferroelectricity [6]. In case of HoMnO 3 [7], a collinear magnetic ordering with E-type magnetic structure gives rise to ferroelectricity. It has been shown recently that canted antiferromagnetic ordering with two non-equivalent spin pairs in the orthoferrite, SmFeO 3 [8] induces ferroelectric polarization at the magnetic ordering temperature of iron. Rajeswaranet al. [9] have reported electric polar order at the magnetic ordering temperature of chromium in the isostructural orthochromites, RCrO 3 with magnetic R ion, where the interactions between R 3+ and Cr 3+ ions have been

Rare-earth orthochromites, RCrO3 (R = rare-earth and Y) with magnetic R-ions are reported to exhibit ferroelectricity at the magnetic ordering temperature of Cr3+-ions (Rajeswaran, B.; Khomskii, D. I.; Zvezdin, A. K.; Sundaresan, A.; Rao, C. N. R. arXiv 2012, 1201, 0826v1 ). Interestingly, we observe ferroelectricity at the Néel temperature of orthochromites containing a nonmagnetic R-ion (Y) upon introducing cation-disorder at the Cr-site by incorporation of another magnetic ion of nonequivalent spin. For example, the weakly ferromagnetic perovskite, YCr0.5Fe0.5O3, where the Cr3+ (S = 3/2) and Fe3+ (S = 5/2) ions at the B-site are disordered, exhibits ferroelectricity at T N = 260 K. Ferroelectricity is observed not only when the relative proportions of Fe3+ and Cr3+ ions as in YCr1–x Fe x O3 are varied but also when Y3+ ions are replaced by other nonmagnetic R-ion and the transition metal ions changed as in YCr1–x Mn x O3. While the local polarization cancels out in the weakly ferromagnetic YMO3 (M = Cr and Fe), a nonzero polarization in these materials seem to arise due to disordered transition metal ions of nonequivalent spins.

We report synthesis, characterization, and properties of a multifunctional oxalate framework, {KDy(C2O4)2(H2O)4} n (1) (C2O4 2– = oxalate dianion) composed of two absolutely different metal ions in terms of their size, charge, and electronic configuration. Dehydrated framework (1′) exhibits permanent porosity and interesting solvent (H2O, MeOH, CH3CN, and EtOH) vapor sorption characteristics based on specific interactions with unsaturated alkali metal sites on the pore surface. Compound 1 shows solvent responsive bimodal magnetic and luminescence properties related to the DyIII center. Compound 1 exhibits reversible ferromagnetic to antiferromagnetric phase transition upon dehydration and rehydration, hitherto unknown for any lanthanide based coordination polymer or metal–organic frameworks. Both the compounds 1 and 1′ exhibit slow magnetic relaxation with very high anisotropic barrier (417 ± 9 K for 1 and 418 ± 7 K for 1′) which has been ascribed to the single ion magnetic anisotropy of the DyIII centers. Nevertheless, compound 1 shows a metal based luminescence property in the visible region and H2O molecules exhibit the strongest quenching effect compared to other solvents MeOH, MeCN, and EtOH, evoking 1′ as a potential H2O sensor.

DyFeO 3 exhibits a weak ferromagnetism (T N Fe ~ 645 K) that disappears below a spin-reorientation (Morin) transition at T SR Fe ~ 50 K. It is also known that applied magnetic field induces ferroelectricity at the magnetic ordering temperature of Dy-ions (T N Dy ~ 4.5 K). Here, we show that the ferroelectricity exists in the weak ferromagnetic state (T SR Fe < T < T N,C ) without applying magnetic field, indicating the crucial role of weak ferromagnetism in inducing ferroelectricity. 57 Fe Mössbauer studies show that hyperfine field (B hf ) deviates from mean field-like behaviour that is observed in the weak ferromagnetic state and decreases below the onset of spin-reorientation transition (80 K), implying that the B hf above T SR had additional contribution from Dy-ions due to induced magnetization by the weak ferromagnetic moment of Fe-sublattice and below T SR , this contribution decreases due to collinear ordering of Fe-sublattice. These results clearly demonstrate the presence of magnetic interactions between Dy(4f) and Fe(3d) and their correlation with ferroelectricity in the weak ferromagnetic state of DyFeO 3 .