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 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.

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.

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We report the bulk magnetic characterization of a dimeric chain material, BiMnVO, by means of magnetic susceptibility, magnetization and heat capacity measurements. Our results provide compelling evidence of an antiferromagnetic (AFM) transition at (T ) ~ 11.5 K. Moreover, the magnetic entropy change in zero field saturates to 14.6 J mol K which is close to the total spin entropy of Mn. The development of long-range magnetic order in this chain material demonstrates the interplay of strong intra-chain and inter-chain interactions between the dimers, in addition to the intra-dimer interaction. Low-temperature (T < T ) heat capacity data indicate the presence of a gap (Δ/k ≈  5 K) in the spin excitations. Furthermore, the isothermal magnetization below T shows an anomaly in the slope between 30 and 40 kOe which is suggestive of a spin-flop transition. Such a low-field spin-flop transition and gapped spin wave excitations may be attributed to the presence of (weak) magnetic anisotropy in this material. We attempt to construct a phase diagram in the magnetic field-temperature plane by extracting data from in-field heat capacity and isothermal magnetization measurements.