E. V. Sampathkumaran's research while affiliated with Tata Institute of Fundamental Research and other places

The spin-chain compound Sr3NiPtO6 is known to have a nonmagnetic ground state. We investigate the nature of the ground state of Sr3NiPtO6 using the magnetic susceptibility χ(T), heat capacity Cp(T), muon spin relaxation (μSR), and inelastic neutron scattering (INS) measurements. χ(T) and Cp(T) do not exhibit any pronounced anomaly that can be associated with a phase transition to a magnetically ordered state. Our μSR data confirm the absence of long-range magnetic ordering down to 0.04 K. Furthermore, the muon spin relaxation rate increases below 20 K and exhibits temperature-independent behavior at low temperature, very similar to that observed in a quantum spin-liquid system. The INS data show a large excitation near 8 meV, and the analysis of the INS data reveals a singlet crystal-electric-field (CEF) ground state with a first excited CEF doublet state at ΔCEF=7.7 meV. The estimated CEF parameters reveal strong planar anisotropy in the calculated χ(T), consistent with the reported behavior of χ(T) of single-crystal Sr3NiPtO6. We propose that the nonmagnetic singlet ground state and a large ΔCEF (much larger than the exchange interaction Jex) are responsible for the absence of long-range magnetic ordering and can mimic a classical spin-liquid behavior in this quasi-one-dimensional spin-chain system Sr3NiPtO6. The classical spin-liquid ground state observed in Sr3NiPtO6 is due to the single-ion property, which is different from the quantum spin-liquid ground state observed in geometrically frustrated systems, in which two-ion exchanges play an important role.

Partially disordered antiferromagnetism (PDA) (in which one of the three magnetic ions in a triangular network remains magnetically disordered) has been known commonly among geometrically frustrated insulating materials. The 1/3 plateau in isothermal magnetization M of such materials has been of great theoretical interest. Here we report these properties in an AlB2-structure-derived metallic material, Er2RhSi3, in which the Er sublattice has triangular networks. The presence of a well-defined λ anomaly in the temperature T dependence of heat capacity and its magnetic-field H dependence and the loss of spin-disorder contribution to electrical resistivity ρ confirm antiferromagnetic order below TN=5K. On the other hand, the separation of zero-field-cooled and field-cooled dc magnetic susceptibility χ curves, the decay of isothermal remnant magnetization, and the frequency dependence of real and imaginary components of ac χ suggest the onset of spin-glass freezing concomitant with the antiferromagnetic order. In addition, interestingly, we observe the 1/3 plateau in M(H) below 20 kOe for T<TN. The change in ρ as a function of H at a given temperature well below TN is also revealing, with this compound exhibiting a plateau below 20 kOe, with complexities at higher fields. Therefore, this compound serves as a prototype for theoretical understanding of transport behavior across the 1/3 plateau due to PDA magnetism in a metal without any interference from the 4f delocalization phenomenon.

There have been constant efforts to find 'exotic' quantum spin-liquid (QSL) materials. Some of the transition metal insulators dominated by the direction-dependent anisotropic exchange interaction ("Kitaev model" for honeycomb network of magnetic ions) are considered to be promising cases for the same. In such Kitaev insulators, QSL is achieved from the zero-field antiferromagnetic state by the application of magnetic field, suppressing other exchange interactions responsible for magnetic order. Here, we show that the features attributable to long-range magnetic ordering of the intermetallic compound, Tb5Si3, (TN = 69 K), containing honey-comb network of Tb ions, are completely suppressed by a critical applied field, Hcr, in heat-capacity and magnetization data, mimicking the behavior of Kitaev physics candidates. The neutron diffraction patterns as a function of H reveal that it is an incommensurate magnetic structure that gets suppressed, showing peaks arising from multiple wave vectors beyond Hcr. Increasing magnetic entropy as a function of H with a peak in the magnetically ordered state is in support of some kind of magnetic disorder in a narrow field range after Hcr. Such a high-field behavior for a metallic heavy rare-earth system to our knowledge has not been reported in the past and therefore is intriguing.

We report the electronic properties of R4PtAl (R = Ho, and Er), which contains 3 sites for R, by the measurements of magnetization (ac and dc), heat-capacity, transport, and magnetoresistance (MR). Dc magnetization data reveal antiferromagnetic order below 19 K and 12 K in Ho and Er compounds, respectively. Additional features observed at lower temperatures (12 K for Ho4PtAl and 5 K for Er4PtAl) are akin to cluster spin-glass phase. Resistivity data exhibit a weak minimum at a temperature marginally higher than their respective N\'eel temperature (T_N) which is unusual for such rare-earths with well localized 4f states. Isothermal magnetization and magnetoresistance data well below T_N exhibit signatures of a subtle field-induced magnetic transition for a small magnetic field (less than 10 kOe). Notably, the isothermal entropy change at T_N has the largest peak value within this rare-earth family; for a field change from zero to 50 kOe, the entropy change is about 14.5 J/kg K (Ho4PtAl) and 21.5 J/kg K (Er4PtAl) suggesting a role of anisotropy of 4f orbital in determining this large value. The results provide some clues for the advancement of the field of magnetocaloric effect. The magnetocaloric property of Er4PtAl is nonhysteretic meeting a challenge to find materials with reversible magnetocaloric effect.

The Haldane-spin chain compound, Tb2BaNiO5, has been known to be an exotic multiferroic system, exhibiting antiferromagnetic anomalies at T_N1= 63 K and T_N2= 25 K, with ferroelectricity appearing below T_N2 only. Previous reports in addition established that, interestingly, Tb ions play a direct and decisive role to lead to multiferroic properties with a critical canting angle of magnetic moments, unlike other well-known multiferroics. Here, we report the results of temperature dependent neutron powder diffraction studies on Tb_2-x Y_x BaNiO_5, to get an insight into the critical canting angle for multiferroic behavior. While multiferroic transition temperature decreases linearly with Y concentration, there is an abrupt drop of relative canting angle (of Tb and Ni magnetic moments) with respect to that in parent compound for an initial substitution ofx = 0.5 in the multiferroic region, without any notable change thereafter. We therefore infer that this critical canting angle is made up of two components - cooperative (long-range) and local (short-range) contributions.

We have investigated for the first time the magnetic behaviour of an intermetallic compound, Dy4RhAl, crystallizing in Gd4RhIn type cubic structure containing 3 sites for rare-earth (R), by several bulk measurements down to 1.8 K. This work is motivated by the fact that the isostructural Dy compound in the R4PtAl family surprisingly orders ferromagnetically unlike other members of this series, which order antiferromagnetically. The results reveal that the title compound undergoes antiferromagnetic order at about 18 K, similar to other heavy R members of R4RhAl family, unlike its Pt counterpart, indicating a subtle difference in the role of conduction electrons to decide magnetism of these compounds. Besides, spin-glass features coexisting with antiferromagnetic order could be observed, which could mean cluster antiferromagnetism. The electrical resistivity and magnetoresistance behaviours in the magnetically ordered state are typical of magnetic materials exhibiting antiferromagnetic gap. Features attributable to spin-reorientation as a function of temperature and magnetic field can be seen in the magnetization data.

We report the electronic properties of R4PtAl (R = Ho, and Er), which contains three sites for R, by the measurements of magnetization (ac and dc), heat-capacity, transport, and magnetoresistance (MR). Dc magnetization data reveal antiferromagnetic order below 19 K and 12 K in Ho and Er compounds, respectively. Additional features observed at lower temperatures (12 K for Ho4PtAl and 5 K for Er4PtAl) are akin to the cluster spin-glass phase. Resistivity data exhibit a weak minimum at a temperature marginally higher than their respective Néel temperature (TN), which is unusual for such rare-earths with well-localized 4f states. Isothermal magnetization and magnetoresistance data well below TN exhibit signatures of a subtle field-induced magnetic transition for a small magnetic field (<10 kOe). Notably, the isothermal entropy change at TN has the largest peak value within this rare-earth family; for a field change from zero to 50 kOe, the entropy change is ~14.5 J/kg K (Ho4PtAl) and ~21.5 J/kg K (Er4PtAl) suggesting a role of anisotropy of 4f orbital in determining this large value. The results provide some clues for the advancement of the field of magnetocaloric effect. The magnetocaloric property of Er4PtAl is nonhysteretic, meeting a challenge to find materials with reversible magnetocaloric effect.