Observation of ferroelectricity among non-d0 systems, which was believed for a long time an unrealistic concept, led to various proposals for the mechanisms to explain the same (i.e. magnetically induced ferroelectricity) during last decade. Here, we provide support for ferroelectricity of a displacive-type possibly involving magnetic ions due to short-range magnetic correlations within a spin-chain, through the demonstration of magnetoelectric coupling in a Haldane spin-chain compound Er2BaNiO5 well above its Néel temperature of (TN = ) 32 K. There is a distinct evidence for electric polarization setting in near 60 K around which there is an evidence for short-range magnetic correlations from other experimental methods. Raman studies also establish a softening of phonon modes in the same temperature (T) range and T-dependent x-ray diffraction (XRD) patterns also reveal lattice parameters anomalies. Density-functional theory based calculations establish a displacive component (similar to d0-ness) as the root-cause of ferroelectricity from (magnetic) NiO6 chain, thereby offering a new route to search for similar materials near room temperature to enable applications.

Abstract:We report the results of various measurements, namely magnetization, complex dielectric permittivity and electric polarization (P) on Dy 2 BaNiO 5 as a function of temperature (T) and magnetic-field (H), apart from heat-capacity (C), with the primary motivation of exploring the existence of magnetoelectric (ME) coupling among Haldane spin-chain systems. The M(T) and C(T) data establish long range magnetic ordering at 58K. The most noteworthy observations are:

We bring out novel dielectric behavior of a spin-chain compound, Ca 3 Co 2 O 6 , undergoing Néel order at (T N 5) 24 K. It is found that the virgin curve in the plot of isothermal dielectric constant (e') versus magnetic-field lies outside the 'butterfly-shaped' envelope curve well below T N (e.g., 2.6 K), with a signature of a partial arrest of the high-field magnetoelectric (ME) phase in zero-field after travelling through magnetic-field-induced magnetic transitions. This behavior is in contrast to that observed in the isothermal magnetization data. Thus, this work brings out a novel case for 'phase-coexistence phenomenon' due to ME coupling. Another strange finding is that there is a weak, but a broad, peak in e' around 85-115 K well above T N , attributable to incipient spin-chain magnetic ordering. This finding should inspire further work to study ME coupling on artificial assemblies of magnetic chains, also keeping in mind miniaturization required for possible applications.T he search for materials with multiferroicity (for instance, those in which magnetic ordering and ferrolectricity are coupled) and with strong magnetolectric (ME) coupling has picked up momentum in recent years 1,2 due to application potential of such materials as well as from the fundamental science angle. As a result of such a coupling between seemingly different phenomena, these two phases can intriguingly coexist under favorable circumstances 3 , bearing relevance to the currently active topic of 'phase-coexistence phenomenon' in general in solids 4 . It is therefore of great interest to search for novel aspects of such a phase-coexistence situations arising out of this cross-coupling -an area which is yet to pick-up momentum. Another area remaining largely unexplored theoretically as well as experimentally is the investigation of ME coupling in magnetically isolated spin-chains, though this question was previously addressed in an organic one-dimensional quantum magnet and spin-Peierls instability was proposed to play a role on ME coupling 5 . Therefore, in order to enable magnetically controllable electric effects, for miniaturization of devices for applications commonly in particular, it is important to investigate whether the ME coupling within spin-chains (in the paramagnetic state) is in general more widespread without any restriction on spin values. Here, we demonstrate experimentally that the naturally occurring spin-chain oxide, Ca 3 Co 2 O 6 6 , with spin 5 2 on Co, turns out to be a novel material with 'exotic' dielectric properties, providing an ideal playground to probe both these aspects. We carried out complex dielectric measurements for this compound as a function of temperature (T) and magnetic-field (H). We find distinct intriguing features attributable to (i) a novel magnetoelectric phase-coexistence phenomenon following a magnetic-field-induced transition in the magnetically ordered state, and (ii) magnetoelectric coupling effect from spin-chains well above long range magnetic ordering.The Co spin-chains are place...

The compound, Fe 2 TiO 5 (FTO) is a well-known uniaxial anisotropic spin-glass insulator with two successive glassy freezing temperatures i.e. transverse (T TF = 9K) and longitudinal (T LF = 55 K). In this article, we present the results of measurements of complex dielectric behavior, electric polarization as a function of temperature (T), in addition to characterization by magnetic susceptibility and heat-capacity, primarily to explore magnetoelectric (ME) coupling and multiglass properties in uniaxial anisotropic spin cluster-glass FTO. The existence of two magnetic transitions is reflected in the isothermal magnetodielectric (MD) behavior in the sense that the sign of MD is different in the T regime TT TF . The data in addition provide evidence for the glassy dynamics of electric-dipoles; interestingly, this occurs at much higher temperature (~100-150 K) than T LF with high remnant polarization at 10 K ( 4000C/m 2 ) attributable to short-range magnetic correlations, thereby offering a route to attain ME coupling above 77 K.

We report that the spin-chain compound Dy2BaNiO5, recently proven by us to exhibit magnetoelectric coupling below its Néel temperature (TN) of 58 K, exhibits strong frequency-dependent behavior in ac magnetic susceptibility and complex dielectric properties at low temperatures (<10 K), mimicking the 'reentrant' multiglass phenomenon. Such a behavior is not known among undoped compounds. A new finding in the field of multiferroics is that the characteristic magnetic feature at low temperatures moves towards higher temperatures in the presence of a magnetic field (H), whereas the corresponding dielectric feature shifts towards lower temperatures with H, unlike the situation near TN. This observation indicates that the alignment of spins by external magnetic fields tends to inhibit glassy-like slow electric-dipole dynamics, at least in this system, possibly arising from peculiarities in the magnetic structure.