arXiv subjects

E. V. Sampathkumaran

Publications and source records attributed to E. V. Sampathkumaran.

At least 145 records · Page 8Linked to original sources

arXiv2003

Magnetic frustration in a stoichiometric spin-chain compound, Ca$_3$CoIrO$_6$

The temperature dependent ac and dc magnetization and heat capacity data of Ca$_3$CoIrO$_6$, a spin-chain compound crystallizing in a K$_4$CdCl$_6$-derived rhombohedral structure, show the features due to magnetic ordering of a frustrated-type below about 30 K, however without exhibiting the signatures of the so-called "partially disordered antiferromagnetic structure" encountered in the isostructural compounds, Ca$_3$Co$_2$O$_6$ and Ca$_3$CoRhO$_6$. This class of compounds thus provides a variety for probing the consequences of magnetic frustration due to topological reasons in stoichiometric spin-chain materials, presumably arising from subtle differences in the interchain and intrachain magnetic coupling strengths. This compound presents additional interesting situations in the sense that, ac susceptibility exhibits a large frequency dependence in the vicinity of 30 K uncharacteristic of conventional spin-glasses, with this frustrated magnetic state being robust to the application of external magnetic fields.

cond-mat.str-el↗

arXiv2003

Anisotropic spin-glass-like and quasi-one-dimensional magnetic behaviour in an intermetallic compound, Tb2PdSi3

We report temperature dependent ac susceptibility ($χ$) measurements on a high quality single crystal of Tb$_2$PdSi$_3$, crystallizing in a AlB$_2$-derived hexagonal structure. This compound is found to exhibit features attributable to quasi-low-dimensional magnetism at high temperatures and {\it anisotropic} spin-glass-like behavior at low temperatures ($<$ 10 K) with an unusually large frequency dependence of peak temperature in ac $χ$(T). This compound thus presents a novel situation in {\it metallic magnetism}, considering that the former phenomenon is normally encountered only among insulators, whereas the anisotropic spin-glass-like behavior, to our knowledge, has not been known in stoichiometric intermetallic compounds.

cond-mat.str-el↗

arXiv2002

Do we understand electron correlation effects in Gadolinium based intermetallic compounds?

Recognising the difficulties in systematic understanding of the physical characteristics of strongly correlated f-electron systems, we considered it worthwhile to subject the so-called "normal" f-electron systems like those of Gd to careful investigations. We find that the spin-disorder contribution to electrical resistivity ($ρ$) in the paramagnetic state, instead of remaining contant, surprisingly increases with decreasing temperature (T) in the paramagnetic state in some of the Gd alloys. In some cases, this "excess resistance" is so large that a distinct minimum in the plot of $ρ$ versus T can be seen, mimicking the behaviour of Kondo lattices. This excess resistance can be suppressed by the application of a magnetic field, naturally resulting in large magnetoresistance. In addition, these alloys are found to exhibit heavy-fermion-like heat-capacity behavior. These unusual findings imply hither-to-unexplored electron correlation effects even in Gd-based alloys.

cond-mat.str-el↗

arXiv2002

ESR investigation of the spin dynamics in (Gd_{1-x}Y_{x})_2PdSi_{3}

We report electron-spin resonance measurements in polycrystalline samples of (Gd_{1-x}Y_{x})_2PdSi_{3}. We observe the onset of a broadening of the linewidth and of a decrease of the resonance field at approximately twice the Neel temperature in the paramagnetic state, this characteristic temperature coincides with the electrical resistivity minimum. The high-temperature behaviour of the linewidth is governed by a strong bottleneck effect.

cond-mat.str-el↗

arXiv2002

Ac susceptibility behaviour of Ce_2Pd_{1-x}Co_{x}Si_3

We report the results of ac susceptibility chi measurements for various compositions of the series, Ce_2Pd_{1-x}Co_xSi_3, in the polycrystalline form, which has been previously reported to undergo a transformation from magnetic ordering to non-magnetic Kondo lattice behavior as x is varied from 0 to 1. A finding of emphasis here is that the features in ac chi data for the compositions, x = 0.6 and 0.7, in the close vicinity of quantum critical point (QCP), are typical of long range magnetic ordering systems, whereas the compositions far away from QCP at the Pd-end including Ce_2PdSi_3 exhibit spin-glass characteristics. This trend is different from hitherto known behavior among Kondo alloys.

cond-mat.str-el↗

arXiv2002

Interesting thermomagnetic history effects in the antiferromagnetic state of SmMn_2Ge_2

We present results of magnetization measurements showing that the magnetic response of the antiferromagnetic state of SmMn_2Ge_2 depends on the path used in the field(H)-temperature(T) phase space to reach this state. Distinct signature of metastablity is observed in this antiferromagnetic state when obtained via field-cooling/field-warming paths. The isothermal M-H loops show lack of end-point memory, reminiscent of that seen in metastable vortex states near the field-induced first order phase transition in various type-II superconductors.

cond-mat.str-el↗

arXiv2002

Magnetic behavior of single crystalline Ho$_2$PdSi$_3$

The magnetic behavior of single-crystal Ho$_2$PdSi$_3$, crystallizing in an AlB$_2$-derived hexagonal structure, is investigated by magnetic susceptibility ($χ$) and electrical resistivity ($ρ$) measurements along two directions. There is no dramatic anisotropy in the high temperature Curie-Weiss parameter or in the $ρ$ and isothermal magnetization data, though there is a noticeable anisotropy in the magnitude of $ρ$ between two perpendicular orientations. The degree of anisotropy is overall less prominent than in the Gd (which is an S-state ion!) and Tb analogues. A point of emphasis is that this compound undergoes long range magnetic ordering below 8 K as in the case of analogous Gd and Dy compounds. Considering this fact for these compounds with well-localised f-orbital, the spin glass freezing noted for isomorphous U compounds in the recent literature could be attributed to the role of the f-ligand hybridization, rather than just Pd-Si disorder.

cond-mat.str-el↗

arXiv2002

Thermomagnetic history effects in SmMn$_2$Ge$_2$

The intermetallic compound SmMn$_2$Ge$_2$, displaying multiple magnetic phase transitions, is being investigated in detail for its magnetization behavior near the 145 K first order ferromagnetic to antiferromagnetic transition occuring on cooling, in particular for thermomagnetic history effects in the magnetization data. The most unusual finding is that the thermomagnetic irreversibility, [= M$^{FCW}$(T)-M$^{ZFC}$(T)] at 135 K is higher in intermediate magnetic field strengths. By studying the response of the sample (i.e., thermomagnetic irreversibility and thermal hysteresis) to different histories of application of magnetic field and temperature, we demonstrate how the supercooling and superheating of the metastable magnetic phases across the first order transition at 145 K contribute to overall thermomagnetic irreversibility.

cond-mat.str-el↗

arXiv2002

An unusual interplay among disorder, Kondo-effect and spin-glass behavior in the Kondo lattices, Ce$_2$Au$_{1-x}$Co$_x$Si$_3$

We report the results of magnetic measurements for the solid solution Ce$_2$Au$_{1-x}$Co$_x$Si$_3$. The results reveal that this solid solution is characterized by a magnetic phase diagram (plot of magnetic transition temperature versus $x$) unusual for Kondo lattices. In particular, the spin-glass freezing induced by disorder is observed only for the compositions at the weak coupling limit; as one approaches the quantum critical point by a gradual replacement of Au by Co, this disorder effect is surprisingly suppressed in favor of long range antiferro-magnetic ordering in contrast to expectations. This unusual interplay between disorder, spin-glass freezing and the Kondo-effect calls for further refinement of theories on competition between magnetism and the Kondo effect.

cond-mat.str-el↗

arXiv2002

Superparamagnetic-like ac susceptibility behavior in a "partially disordered antiferromagnetic" compound, Ca$_3$CoRhO$_6$

We report the results of dc and ac magnetization measurements as a function of temperature (1.8 - 300 K) for the spin chain compound, Ca$_3$CoRhO$_6$, which has been recently reported to exhibit a partially disordered antiferromagnetic (PDAF) structure in the range 30 - 90 K and spin-glass freezing below 30 K. We observe an unexpectedly large frequency dependence of ac susceptibility in the T range 30 - 90 K, typical of superparamagnets. In addition, we find that there is no difference in the isothermal remanent magnetization behavior for the two regimes below 90 K. These findings call for more investigations to understand the magnetism of this compound.

cond-mat.str-el↗

arXiv2002

Inhomogeneous magnetism in single crystalline Sr$_3$CuIrO$_{6+δ}$: Implications to phase-separation concepts

The single crystalline form of an insulator, Sr$_3$CuIrO$_{6+δ}$, is shown to exhibit unexpectedly more than one magnetic transition (at 5 and 19 K) with spin-glass-like magnetic susceptibility behaviour. On the basis of this finding, viz., inhomogeneous magnetism in a chemically homogeneous material, we propose that the idea of "phase- separation" described for manganites [1] is more widespread in different ways. The observed experimental features enable us to make a comparison with the predictions of a recent toy model [2] on {\it magnetic} phase separation in an insulating environment.

cond-mat.str-el↗

arXiv2002

Crossover in the nature of the metallic phases in the perovskite-type RNiO_3

We have measured the photoemission spectra of Nd$_{1-x}$Sm$_{x}$NiO$_{3}$, where the metal-insulator transition and the Néel ordering occur at the same temperature for $x \lesssim 0.4$ and the metal-insulator transition temperature ($T_{MI}$) is higher than the Néel temperature for $x \gtrsim 0.4$. For $x \le 0.4$, the spectral intensity at the Fermi level is high in the metallic phase above $T_{MI}$ and gradually decreases with cooling in the insulating phase below $T_{MI}$ while for $x > 0.4$ it shows a pseudogap-like behavior above $T_{MI}$ and further diminishes below $T_{MI}$. The results clearly establish that there is a sharp change in the nature of the electronic correlations in the middle ($x \sim 0.4$) of the metallic phase of the $R$NiO$_3$ system.

cond-mat.str-el↗

arXiv2001

Magnetic anomalies in the spin chain system, Sr$_3$Cu$_{1-x}$Zn$_x$IrO$_6$

We report the results of ac and dc magnetization (M) and heat-capacity (C) measurements on the solid solution, Sr$_3$Cu$_{1-x}$Zn$_x$IrO$_6$. While the Zn end member is known to form in a rhombohedral pseudo one-dimensional K$_4$CdCl$_6$ structure with an antiferromagnetic ordering temperature of (T$_N$ =) 19 K, the Cu end member has been reported to form in a monoclinically distorted form with a Curie temperature of (T$_C$ =) 19 K. The magnetism of the Zn compound is found to be robust to synthetic conditions and is broadly consistent with the behavior known in the literature. However, we find a lower magnetic ordering temperature (T$_o$) for our Cu compound (~ 13 K), thereby suggesting that T$_o$ is sensitive to synthetic conditions. The Cu sample appears to be in a spin-glass-like state at low temperatures, judged by a frequency dependence of ac magnetic susceptibility and a broadening of the C anomaly at the onset of magnetic ordering, in sharp contrast to earlier proposals. Small applications of magnetic field, however, drive this system to ferromagnetism as inferred from the M data. Small substitutions for Cu/Zn (x = 0.75 or 0.25) significantly depress magnetic ordering; in other words, T$_o$ varies non-monotonically with x (T$_o$ ~ 6, 3 and 4 K for x = 0.25, 0.5, and 0.67 respectively). The plot of inverse susceptibility versus temperature is non-linear in the paramagnetic state as if correlations within (or among) the magnetic chains continuously vary with temperature. The results establish

cond-mat.str-el↗

arXiv2001

Sr$_3$CuIrO$_6$, a spin-chain compound with random ferromagnetic-antiferromagnetic interactions

Ac and dc magnetization and heat-capacity (C) measurements performed on the pseudo-one-dimensional compound Sr$_3$CuIrO$_6$ reveal a competition between antiferromagnetic (AF) and ferromagnetic (F) exchange couplings, as evidenced by frequency dependence of ac susceptibility and by the absence of a C anomaly at the magnetic transition. The value of the saturation moment (about 0.35 $μ_B$/formula unit) is much smaller than expected for ferromagnetism from the two S=1/2 ions (Cu and Ir). Thus, this compound is not a ferromagnet in zero magnetic field, in contrast to earlier beliefs. Of particular importance is the finding that the value of the magnetic ordering temperature is sample dependent, sensitive to synthetic conditions resulting from deviations in oxygen/Cu content. We propose that this compound serves as a unique model system to test theories on random AF-F interaction in a chain system, considering that this competition can be tuned without any chemical substitution.

cond-mat.str-el↗

arXiv2001

Observation of superconductivity in Y$_2$PdGe$_3$, structurally same as MgB$_2$

The results of electrical resistance (1.4 - 300 K), magnetization (2-300 K) and heat-capacity (2 - 50 K) measurements in Y$_2$PdGe$_3$, found to crystallize in a AlB$_2$-derived hexagonalstructure, are reported. The results establish that this compound is superconducting below 3 K. This obervation is interesting considering that this compound is the first superconductor among the ternary members derived from the hexagonal AlB$_2$ structure. With superconductivity being uncommon even among binary alloys derived from AlB$_2$ structure and with recent excitement on the observation of high temperature superconductivity in Mg$B_2$, this finding gains importance.

cond-mat.supr-con↗

arXiv2000

Magnetic, thermal, and transport properties on single crystals of antiferromagnetic Kondo-lattice Ce2PdSi3

Magnetization, heat capacity, electrical resistivity, thermoelectric power, and Hall effect have been investigated on single-crystalline Ce_2PdSi_3. This compound is shown to order antiferromagnetically below N'eel temperature (T_N) \~3 K. The Sommerfeld coefficient far below T_N is found to be about 110 mJ/K^2 mol Ce, which indicates the heavy-fermion character of this compound. The transport and magnetic properties exhibit large anisotropy with an interplay between crystalline-electric-field (CEF) and Kondo effects. The sign of thermoelectric power is opposite for different directions at high temperatures and the ordinary Hall coefficient is anisotropic with opposite sign for different geometries, indicating the anisotropic Fermi surface. The CEF analysis from the temperature dependence of magnetic susceptibility suggests that the ground state is |+/-1/2>. The first and the second excited CEF doublet levels are found to be located at about 30 and 130 K, respectively. The Kondo temperature is estimated to be the same order as T_N, indicating the presence of a delicate competition between the Kondo effect and magnetic order.

cond-mat.str-el↗

arXiv2000

Magnetoresistance in ordered and disordered double perovskite oxide, Sr$_2$FeMoO$_6$

We have prepared crystallographically ordered and disorder specimens of the double perovskite, Sr$_2$FeMoO$_6$ and investigated their magnetoresistance behaviour. The extent of ordering between the Fe and Mo sites in the two samples is determined by Rietveld analysis of powder x-ray diffraction patterns and reconfirmed by Mössbauer studies. While the ordered sample exhibits the sharp low-field response, followed by moderate changes in the magnetoresistance at higher fields, the disordered sample is characterised by the absence of the spectacular low-field response. We argue that the low field response depends crucially on the half-metallic ferromagnetism, while the high-field response follows from the overall magnetic nature of the sample, even in absence of the half-metallic state.

cond-mat.str-el↗

arXiv2000

Magnetic and electrical resistance behaviour of the oxides, Ca$_{3-x}$Y$_x$LiRuO$_6$ (x= 0.0, 0.5 and 1.0)

We have investigated the magnetic and electrical resistance behaviour of Ca$_{3-x}$Y$_x$LiRuO$_6$. The parent compound exhibits magnetic ordering from Ru sublattice at a rather high temperature, 113 K. Though the paramagnetic Curie temperature ($θ$$_p$) is negative indicative of antiferromagnetic ordering, the large magnitude (-250 K) of $θ$$_p$ reveals complex nature of the magnetism in this compound. Ru ions appear to be in the pentavalent state. We note that the Néel temperature undergoes only a marginal reduction by Y substitution. All these compositions are found to be insulators and thus the electron doping does not result in metallicity. Thus the overall magnetic and transport behaviour are found to be essentially insensitive to Y substitution for Ca, a finding which may favour the idea of quasi-one-dimensional magnetism in these compounds.

cond-mat.mtrl-sci↗