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Register a new userMagnetic and dielectric behavior of the spin-chain compound Er2BaNiO5 well below its Neel temperature
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E. V. Sampathkumaran Professor
Publication date
2014
fields
Physics
and research's language is
English
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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.
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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.
Static magnetic susceptibility chi, ac susceptibility chi_{ac} and specific heat C versus temperature T measurements on polycrystalline samples of In2VO5 and chi and C versus T measurements on the isostructural, nonmagnetic compound In2TiO5 are reported. A Curie-Wiess fit to the chi(T) data above 175 K for In2VO5 indicates ferromagnetic exchange between V^{4+} (S = 1/2) moments. Below 150 K the chi(T) data deviate from the Curie-Weiss behavior but there is no signature of any long range magnetic order down to 1.8 K. There is a cusp at 2.8 K in the zero field cooled (ZFC) chi(T) data measured in a magnetic field of 100 Oe and the ZFC and field cooled (FC) data show a bifurcation below this temperature. The frequency dependence of the chi_{ac}(T) data indicate that below 3 K the system is in a spin-glass state. The difference Delta C between the heat capacity of In2VO5 and In2TiO5 shows a broad anomaly peaked at 130 K. The entropy upto 300 K is more than what is expected for S = 1/2 moments. The anomaly in Delta C and the extra entropy suggests that there may be a structural change below 130 K in In2VO5.
We have investigated the magnetic behavior of the nano crystals, synthesized by high-energy ball-milling, for a well-known geometrically frustrated spin-chain system, Ca3CoRhO6, and compared its magnetic characteristics with those of the bulk form by measuring ac and dc magnetization. The features attributable to the onset of partially disordered antiferromagnetism (characterizing the bulk form) are not seen in the magnetization data of the nano particles; the magnetic moment at high fields in the very low temperature range in the magnetically ordered state gets relatively enhanced in the nano particles. It appears that the ferromagnetic intrachain interaction, judged by the sign of the paramagnetic Curie temperature, is preserved in the nano particles. These trends are opposite to those seen in Ca3Co2O6. However, the complex spin-dynamics as evidenced by large frequency dependence of ac susceptibility is retained in the nano particles as well. Thus, there are some similarities and dissimilarities between the properties of the nano particles and those of the bulk. We believe that these findings would be useful to understand correlation lengths deciding various properties of geometrical frustration and/or spin-chain phenomena.
We report that the major features in the temperature dependence of dc and ac magnetization of a well-known spin-chain compound, Ca3Co2O6, which has been known to exhibit two complex magnetic transitions due to geometrical frustration (one near 24 K and the other near 10 K), are found to be qualitatively unaffected in its nano form synthesized by high-energy ball-milling. However, the multiple steps in isothermal magnetization - a topic of current interest in low-dimensional systems - known for the bulk form well below 10 K is absent in the nano particles. We believe that this finding will be useful to the understanding of the step magnetization behavior of such spin-chain systems.
We have measured the specific heat of the S = 1/2 alternating Heisenberg antiferromagnetic chain compound pentafluorophenyl nitronyl nitroxide in magnetic fields using a single crystal and powder. A sharp peak due to field-induced magnetic ordering (FIMO) is observed in both samples. The H-T phase boundary of the FIMO of the single crystal is symmetric with respect to the central field of the gapless field region HC1 < H < HC2, whereas it is distorted for the powder whose ordering temperatures are lower. An analysis employing calculations based on the finite temperature density matrix renormalization group indicates the possibility of novel incommensurate ordering due to frustration in the powder around the central field.
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