Magnetic and magnetodielectric behavior of the Haldane spin-chain system, HO2BaNiO5

The Haldane spin-chain compound, HO2BaNiO5, crystallizing in an orthorhombic space group (Immm) and known to order antiferromagnetically below (T-N=) 53 K, is investigated by magnetization, heat-capacity, dielectric, magnetodielectric, and pyrocurrent studies as well as neutron diffraction experiments down to 2 K. We report here another magnetic feature in the close vicinity of 12 K (as a consequence of the saturation of the magnetic moment of Ho) as revealed by qualitative changes in the behavior of isothermal magnetization when the temperature is varied across 12-15 K. Heat-capacity and magnetic susceptibility data also are consistent with this conclusion. The compound can be classified as 'metamagnetodielectric', when comparing the plots of dielectric constant as a function of temperature and magnetic field with the corresponding plots of magnetization. We are not able to find any evidence for ferroelectricity. This is notable considering that all the heavy rare-earth members of this family with the same crystal structure were known to be multiferroics. Pyrocurrent data however reveal a feature around 55 K, which is attributable to `thermally stimulated depolarization current'.