Magnetic behavior of bulk and fine particles of RCr2Si2C (R = La, Ce)compounds: possible magnetic ordering from Cr

The magnetic behavior of the quaternary compounds, RCr2Si2C (R = La, Ce), has been investigated by magnetization (M) and heat-capacity (C) measurements (1.8-300 K) in the bulk polycrystals and nano forms (<1 {\mu}m) obtained by high-energy balling. Our finding is that Cr appears to exhibit magnetic ordering of an itinerant type at low temperatures (<20 K) in the bulk form, as inferred from a combined look at all the data. The magnetic ordering gets gradually suppressed with increasing milling time. Evidence for a mixed-valence state of Ce for the bulk form is obtained from the tendency of magnetic susceptibility to exhibit a maximum above 300 K. However, this feature vanishes in the nano form, which exhibits a Curie-Weiss behavior above 200 K as though Ce tends towards trivalency in these fine particles; in addition, there is a weak upturn in C/T below 10 K in the bulk, which becomes very prominent in the milled Ce-based specimens at lower temperatures, as though heavy-fermion behavior gets stronger in smaller particles

Magnetic behavior of new compounds, Gd3RuSn6 and Tb3RuSn6

We report temperature (T) dependence of dc magnetization, electrical resistivity (rho(T)), and heat-capacity of rare-earth (R) compounds, Gd3RuSn6 and Tb3RuSn6, which are found to crystallize in the Yb3CoSn6-type orthorhombic structure (space group: Cmcm). The results establish that there is an onset of antiferromagnetic order near (T_N) 19 and 25 K respectively. In addition, we find that there is another magnetic transition for both the cases around 14 and 17 K respectively. In the case of the Gd compound, the spin-scattering contribution to rho is found to increase below 75 K as the material is cooled towards T_N, thereby resulting in a minimum in the plot of rho(T) unexpected for Gd based systems. Isothermal magnetization at 1.8 K reveals an upward curvature around 50 kOe. Isothermal magnetoresistance plots show interesting anomalies in the magnetically ordered state. There are sign reversals in the plot of isothermal entropy change versus T in the magnetically ordered state, indicating subtle changes in the spin reorientation with T. The results reveal that these compounds exhibit interesting magnetic properties.Comment: 7 figure

Magnetic and magnetotransport behavior of RFe5Al7 (R= Gd and Dy): Observation of re-entrant inverse-magnetocaloric phenomenon and asymmetric magnetoresistance behavior

We have compared and contrasted magnetic, magnetocaloric and magnetoresistive properties of Gd and Dy members of the rare-earth (R) series RFe5Al7, crystallizing in ThMn12 structure, known to order antiferromagnetically. Among other observations, we would like to emphasize on the following novel findings: (i) There are multiple sign-crossovers in the temperature (T) dependence of isothermal entropy change (DeltaS) in the case of Dy compound; in addition to nil DeltaS at the magnetic compensation point known for two-magnetic-sublattice systems, there is an additional sign-crossover at low temperatures, as though there is a re-entrant inverse magnetocaloric phenomenon. Corresponding sign reversals could also be observed in the magnetoresistance data. (ii) The plots of magnetoresistance versus magnetic field are found to be highly asymmetric with the reversal of the direction of magnetic-field (H) well below TN for both compounds, similar to that known for an antiferromagnetic tunnel junctions. We attribute these to subtle changes in spin orientations of R and Fe moments induced by T and H.Comment: 8 figures Physical Review B, in pres

Magnetism of a rhombohedral-type pyrochlore-derived Kagome series, Mn2R3Sb3O14 (R= Rare-earths)

The results of magnetic investigations on a new series of compounds, Mn2R3Sb3O14, containing 2-dimensional Kagome lattice of R ions and belonging to pyrochlore series, are presented. Crystallographic features of light R members (R= La, Pr and Nd) of this family, as established in the recent literature, have been reported to be novel in many respects, in particular, the rhomohedral nature of the structure which is rare among pyrochlores. It was also reported that, as the R becomes heavier, beyond R= Sm, the fraction of well-known cubic pyrochlore phase tends to gradually dominate. Here, we report that we are able to form the Gd member in the rhombohedral form without noticeable admixture from the cubic phase. With respect to magnetic behavior, our magnetization measurements on the La member reveal that Mn exists in divalent state without any evidence for long range magnetic ordering down to 2 K (that is, suppressed magnetism), which is not so common for Mn based oxides, though antiferromagnerism below 2 K is not ruled out. Nd and Gd members, are however, found to show distinct features above 2 K in magnetic susceptibility and heat-capacity, attributable to long-range magnetic ordering from respective rare-earth sublattice. The experimental results with respect to magnetism are found to be consistent with the results from ab initio band structure calculations performed for the La case. The calculations imply that electron correlation is important to describe insulating behavior.Comment: 7 figure

Magnetic behavior of metallic kagome lattices, Tb3Ru4Al12 and Er3Ru4Al12

We report magnetic behavior of two intermetallics-based kagome lattices, Tb3Ru4Al12 and Er3Ru4Al12, crystallizing in the Gd3Ru4Al12-type hexagonal crystal structure, by measurements in the range 1.8-300 K with bulk experimental techniques (ac and dc magnetization, heat-capacity and magnetoresistance). The main finding is that the Tb compound, known to order antiferromagnetically below (T_N=) 22 K, shows glassy characteristics at lower temperatures (<15K), thus characterizing this compound as a re-entrant spin-glass. The data reveal that glassy phase is quite complex and is of a cluster type. Since the glassy behavior was not seen for the Gd analogue in the past literature, this finding for the Tb compound emphasizes that this kagome family could provide an opportunity to explore the role of higher order (such as quadrupole) in bringing out magnetic frustration. Additional findings reported here for this compound are: (i) The temperature dependence of magnetic susceptibility and electrical resistivity in the range 12 - 20 K are found to be hysteretic leading to a magnetic phase in this temperature range, mimicking disorder-induced first-order magnetic phase-transition. (ii) Features attributable to an interesting magnetic phase co-existence phenomenon in the magnetoresistance in zero field, after cycling across metamagnetic transition fields, are observed. With respect to the Er compound, we do not find any evidence for long-range magnetic ordering down to 2 K, but this appears to be on the verge of magnetic order at 2 K.Comment: 11 figures. JPCM (in press

Refined similarity hypothesis using 3D local averages

The refined similarity hypotheses of Kolmogorov, regarded as an important ingredient of intermittent turbulence, has been tested in the past using one-dimensional data and plausible surrogates of energy dissipation. We employ data from direct numerical simulations, at the microscale Reynolds number $R_\lambda \sim 650$, on a periodic box of $4096^3$ grid points to test the hypotheses using 3D averages. In particular, we study the small-scale properties of the stochastic variable $V = \Delta u(r)/(r \epsilon_r)^{1/3}$, where $\Delta u(r)$ is the longitudinal velocity increment and $\epsilon_r$ is the dissipation rate averaged over a three-dimensional volume of linear size $r$. We show that $V$ is universal in the inertial subrange. In the dissipation range, the statistics of $V$ are shown to depend solely on a local Reynolds number

Dielectric and multiferroic behavior in a Haldane spin-chain compound Sm2BaNiO5 due to an interplay between crystal-field effect and exchange interaction

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.Comment: 6 figure

Scaling exponents saturate in three-dimensional isotropic turbulence

From a database of direct numerical simulations of homogeneous and isotropic turbulence, generated in periodic boxes of various sizes, we extract the spherically symmetric part of moments of velocity increments and first verify the following (somewhat contested) results: the $4/5$-ths law holds in an intermediate range of scales and that the second order exponent over the same range of scales is {\it{anomalous}}, departing from the self-similar value of $2/3$ and approaching a constant of $0.72$ at high Reynolds numbers. We compare with some typical theories the dependence of longitudinal exponents as well as their derivatives with respect to the moment order $n$, and estimate the most probable value of the H\"older exponent. We demonstrate that the transverse scaling exponents saturate for large $n$, and trace this trend to the presence of large localized jumps in the signal. The saturation value of about $2$ at the highest Reynolds number suggests, when interpreted in the spirit of fractals, the presence of vortex sheets rather than more complex singularities. In general, the scaling concept in hydrodynamic turbulence appears to be more complex than even the multifractal description

Magnetic anomalies in a spin-chain compound, Sr3CuRhO6: Griffiths-phase-like behavior of magnetic susceptibility

We report the results of ac and dc magnetic susceptibility (chi), heat-capacity (C), isothermal magnetization and isothermal remanent magnetization measurements on the compound, Sr3CuRhO6, crystallizing in a K4CdCl6-derived monoclinic structure. The magnetization data reveal distinct magnetic anomalies near 6 and 12 K with decreasing temperature (T). While the transition below (To=) 6K appears to be of a spin-glass-type as inferred from all the data, the one at 12 K is not typical of bulk ferromagnetism in contrast to an earlier proposal. In the range 6 to 12 K, the dc chi obeys the theoretically expected form for very low dc fields and the values of chi decrease gradually with the application of higher magnetic fields, mimicking the behavior Griffiths-phases.Comment: Phys. Rev. B, in pres

Novel dielectric anomalies due to spin-chains above and below Neel temperature in Ca3Co2O6

We bring out novel dielectric behavior of a spin-chain compound, Ca3Co2O6, undergoing Neel order at (T_N=) 24 K. It is found that the virgin curve in the plot of isothermal dielectric constant 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 dielectric constant 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.Comment: Scientific Reports, 31st October 201