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Register a new userDielectric and multiferroic behavior in a Haldane spin-chain compound Sm2BaNiO5 due to an interplay between crystal-field effect and exchange interaction
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E. V. Sampathkumaran Professor
Publication date
2017
fields
Physics
and research's language is
English
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The Haldane spin-chain (S=1) insulating compound, Sm2BaNiO5, which has been proposed to order antiferromagnetically around (T_N=) 55 K, was investigated for its complex dielectric permittivity, magnetodielectric and pyrocurrent behavior as a function of temperature (T). In order to enable meaningful discussions, the results of ac and dc magnetizatioin and heat-capacity studies are also reported. We emphasize on the following findings: (i) There is a pyrocurrent peak near T_N, but it is shown not to arise from ferroelectricity, but possibly due to thermally stimulated depolarization current, unlike in many other members of this rare-earth series, in which case ferroelectric features were reported at or above T_N; (ii) however, the pyrocurrent measured in the presence of a bias electric field (after cooling in zero electric field) as well as dielectric constant reveal a weak peak with increasing T around 22 K - the temperature around which population of the exchange-split excited state of Kramers doublet has been known to occur. This finding suggests that this compound presents a novel situation in which multiferroicity is induced by an interplay between crystal-field effects and exchange interaction. No multiglass features could be observed down to 2 K unlike in many members of this family.
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In the multiferroic hexagonal manganite HoMnO3, inelastic neutron scattering and synchrotron based THz spectroscopy have been used to investigate the spin waves associated to the Mn order together with Ho crystal field excitations. While the Mn order sets in first below 80 K, a spin reorientation occurs below 37 K, a rare feature in the rare earth manganites. We show that severalHo crystal field excitations are present in the same energy range as the magnons, and that they are all affected by the spin reorientation. Moreover, several anomalous features are observed in the excitations at low temperature. Our analysis and calculations for the Mn spin waves and Ho crystal field excitations support Mn-Ho coupling mechanisms as well as coupling to the lattice affecting the dynamics.
We have recently reported that the Haldane spin-chain system, Er2BaNiO5, undergoing antiferromagnetic order below 32 K, is characterized by the onset of ferroelectricity near 60K due to magnetoelectric coupling induced by short-range magnetic-order within spin-chains. We have carried out additional magnetic and dielectric studies to understand the properties well below antiferromagnetic ordering temperature. We emphasize here on the following: (i) A strong frequency dependent behaviors of ac magnetic susceptibility and complex dielectric properties have been observed at much lower temperatures (below 8 K), that is, reentrant multiglass-like phenomenon, naturally suggesting the existence of an additional transition well below Neel temperature; ii) Magnetoelectric phase coexistence is observed at very low temperature (e.g., T =2K), where the high-field magnetoelectric phase is partially arrested on returning to zero magnetic field after a cycling through metamagnetic transition.
The bulk single crystals of $S = 1$ chain compound Ni(C$_3$H$_{10}$N$_2$)$_2$NO$_2$ClO$_4$ are grown by using a slow evaporation method at a constant temperature and a slow cooling method. It is found that the optimum condition of growing large crystals is via slow evaporation at 25 $^circ$C using 0.015 mol Ni(ClO$_4$)$_2$$cdot$6H$_2$O, 0.015 mol NaNO$_2$, and 0.03 mol 1,3-propanediamine liquid dissolved into 30 ml aqueous solvent. High-quality crystals with size up to $18 times 7.5 times 5$ mm$^3$ are obtained. The single crystals are characterized by measurements of x-ray diffraction, magnetic susceptibility, specific heat and thermal conductivity. The susceptibilities along three crystallographic axes are found to exhibit broad peaks at $sim 55$ K, and then decrease abruptly to zero at lower temperatures, which is characteristic of a Haldane chain system. The specific heat and the thermal conductivity along the $c$ axis can be attributed to the simple phononic contribution and are analyzed using the Debye approximation.
We present sharp magnetization jumps and field induced irreversibility in magnetization in multiferroic Y2CoMnO6. Appearance of magnetic relaxation and field sweep rate dependence of magnetization jumps resemble the martensite like scenario and suggests the coexistence of E*-type antiferromagnetic and ferromagnetic phases at low temperatures. In Y2CoMnO6, the critical field required for the sharp jump can be increased or decreased depening on the magnitude and direction of the cooling field; this is remarkably different from manganites or other metamagnetic materials where the critical field increases irrespective of the direction of the field cooling. The cooling field dependence on the sharp magnetization jumps has been described by considering exchange pinning mechanism at the interface, like in exchange bias model.
SrTm$_2$O$_4$ has been investigated using heat capacity, magnetic susceptibility, magnetization in pulsed fields, and inelastic neutron scattering measurements. These results show that the system is highly anisotropic, has gapped low-energy dispersing magnetic excitations, and remains in a paramagnetic state down to 2K. Two theoretical crystal field models were used to describe the single-ion properties of SrTm$_2$O$_4$without any optimization procedures; a standard point-charge model and a Density Functional Theory (DFT) based model that uses Wannier functions. The DFT model was found to better describe the system at low energy by predicting a singlet ground state for one Tm site and a doublet for the second Tm site and anisotropy of second site Tm dominating the anisotropy of the system. Additionally, muon spin rotation/relaxation ($mu^+$psr) spectra reveal oscillations, typically a sign of long-range magnetic order. We attribute these observations to lattice distortion induced by muon implantation, causing renormalization of the gap size.
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