arXiv subjects

E. V. Sampathkumaran

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

At least 19 recordsLinked to original sources

arXiv2024

Magnetic ground state and excitations in mixed 3$d$-4$d$ quasi-1D spin-chain oxide Sr$_3$NiRhO$_6$

Entanglement of spin and orbital degrees of freedom, via relativistic spin-orbit coupling, in 4$d$ transition metal oxides can give rise to a variety of novel quantum phases. A previous study of mixed 3$d$-4$d$ quasi-1D spin-chain oxide Sr$_3$NiRhO$_6$ using the magnetization measurements by Mohapatra et al. [Phys. Rev. B 75, 214422 (2007)] revealed a partially disordered antiferromagnetic (PDA) structure below 50 K [Mohapatra et al, Phys. Rev. B 75, 214422 (2007)]. We here report the magnetic ground state and spin-wave excitations in Sr$_3$NiRhO$_6$, using muon spin rotation and relaxation ($μ$SR), and neutron (elastic and inelastic) scattering techniques. Our neutron diffraction study reveals that in the magnetic structure of Sr$_3$NiRhO$_6$, Rh$^{4+}$ and Ni$^{2+}$ spins are aligned ferromagnetically in a spin-chain, with moments along the crystallographic $c$-axis. However, spin-chains are coupled antiferromanetically in the $ab$-plane. $μ$SR reveals the presence of oscillations in the asymmetry-time spectra below 50 K, supporting the long-range magnetically ordered ground state. Our inelastic neutron scattering study reveals gapped quasi-1D magnetic excitations with a large ratio of gap to exchange interaction. The observed spin-wave spectrum could be well fitted with a ferromagnetic isotropic exchange model (with $J = 3.7 $ meV) and single ion anisotropy ($D=10$ meV) on the Ni$^{2+}$ site. The magnetic excitations survive up to 85 K, well above the magnetic ordering temperature of $\sim 50$ K, also indicating a quasi-1D nature of the magnetic interactions in Sr$_3$NiRhO$_6$.

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arXiv2024

Emergent Griffiths-phase-like behavior in the ball-milled nanocrystalline Dy4RhAl and its implication

We report the results of dc susceptibility and heat capacity measurements on the (ball-milled) nanocrystalline rare-earth (R) ternary compound, crystallizing in Gd4RhIn type, cubic Dy4RhAl compound. The bulk form of this compound has been known to undergo antiferromagnetic ordering at (TN=) 18 K with concomitant cluster spin glass anomalies. The present studies on the nano-form obtained by ball milling reveal that this antiferromagnetic ordering is suppressed with the reduction of particle size with no feature attributable to a well defined long-range magnetic ordering down to 1.8 K, but showing an inhomogeneous magnetism below 10 K. The point being stressed is that the results show the dominance of a feature around 30 K in the magnetic susceptibility data (well above TN of the bulk form) mimicking Griffiths phase. We infer that surface magnetism dominates before long range magnetic ordering occurs in this material.

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arXiv2023

Role of crystal field ground state in the classical spin-liquid behavior of a quasi-one dimensional spin-chain system Sr3NiPtO6

The spin-chain compound Sr3NiPtO6 is known to have a nonmagnetic ground state. We have investigated the nature of ground state of Sr3NiPtO6 using magnetic susceptibility $χ(T)$, heat capacity $C_{\rm p}(T)$, muon spin relaxation ($μ$SR) and inelastic neutron scattering (INS) measurements. The $χ(T)$ and $C_{\rm p}(T)$ do not exhibit any pronounced anomaly that can be associated with a phase transition to a magnetically ordered state. Our $μ$SR 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 CEF ground state with a first excited CEF doublet state at $Δ_{\rm CEF}$ = 7.7 meV. The estimated CEF parameters reveal a strong planar anisotropy in the calculated $χ(T)$, consistent with the reported behavior of the $χ(T)$ of single crystal Sr3NiPtO6. We propose that the nonmagnetic singlet ground state and a large $Δ_{\rm CEF}$ (much larger than the exchange interaction $\mathcal{J}_{\rm ex}$) are responsible for the absence of long-range magnetic ordering and can mimic a classical spin-liquid behavior in this quasi-1D 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, where two-ion exchanges play an important role.

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arXiv2023

Emergence of partially disordered antiferromagnetism and isothermal magnetization plateau due to geometrical frustration in a metallic compound, Er2RhSi3

Partially disordered antiferro (PDA) magnetism (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 one-third plateau in isothermal magnetization (M) of such materials has been of great theoretical interest. Here, we report these properties in a AlB2-structure derived metallic material, Er2RhSi3 in which Er sublattice has triangular networks. The presence of a well-defined lamda anomaly in the temperature (T) dependence of heat capacity and its magnetic-field (H) dependence, and the loss of spin-disorder contribution in electrical resistivity (rho) confirm antiferromagnetic order below (TN=) 5 K. On the other hand, the separation of zero-field-cooled and field-cooled dc magnetic susceptibility (chi) curves, decay of isothermal remnant magnetization and the frequency dependence of real and imaginary components of ac chi suggest the onset of spin-glass freezing concomitant with the antiferromagnetic order. In addition, interestingly, we observe one-third plateau in M(H) below 20 kOe for T less than TN. The change in rho 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 one-third plateau due to PDA magnetism in a metal without any interference from the 4f delocalization phenomena.

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arXiv2023

Magnetic behavior of cubic Dy4RhAl with respect to isostructural Dy4PtAl, revealing a novel 4f d-band interaction

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.

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arXiv2023

Canting angle behavior of magnetic moments in Y- substituted Tb2BaNiO5 and its relevance for magnetoelectric coupling

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.

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arXiv2023

Magnetic-field induced melting of long-range magnetic order akin to Kitaev insulators in the metallic compound Tb5Si3

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 (T_N= 69 K), containing honey-comb network of Tb ions, are completely suppressed by a critical applied field, H_cr, 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 H_cr. 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.

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arXiv2021

Pressure-induced anomalies in the magnetic transitions of the exotic multiferroic material, Tb2BaNiO5

We have studied the influence of external pressure up to 1 GPa on the magnetic transitions of the orthorhombic Haldane-spin chain compound Tb2BaNiO5 an exotic multiferroic material. This parent compound is known to undergo Néel ordering at TN1= 63 K and another magnetic transition at TN2= 25K at which ferroelectricity sets in, however, without any change in the magnetic symmetry, but with only a sharp change in the canting angle of Tb 4f and Ni 3d magnetic moments. There is a subtle difference in the antiferromagnetic state above and below TN2, which is supported by the fact that there is a metamagnetic transition below TN2only (for 5 K, at about 60 kOe). We report here that, with the application of external pressure, there is an upward shift of TN1, while TN2 shifts towards lower temperatures. It is interesting that the two magnetic transitions in the same compound behave differently under pressure and the opposite behavior at TN2 is attributed to local distortion leading to ferroelectricity. The results are augmented by temperature dependent x-ray diffraction and positive chemical pressure studies. The chemical pressure caused by the isoelectronic doping at Ba site by Sr reduces both the transition temperatures. Clearly, the external pressure favors antiferromagnetic coupling (that is, leading to TN1 enhancement), whereas the chemical pressure reduces TN1, suggesting important role of the changes in local hybridization induced by doping on magnetism in this material.

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arXiv2021

Magnetic frustration and paramagnetic state transport anomalies in Ho4RhAl and Er4RhAl: Possible test cases for newly identified roles of itinerant electrons

We report the results of magnetic, heat-capacity, electrical and magnetoresistance measurements on Ho4RhAl and Er4RhAl, characterized by 3 sites for rare-earths (R). Antiferromagnetic ordering sets in at (T_N =) about 8.8 and 4.0 K respectively. While Ho compound appears to enter into a complex spin-glass phase at T less thanT_N (at nearly 5 K), spin-glass component appears to set in essentially almost at T_N for the Er case. The loss of the spin-disorder contribution in the magnetically ordered state is not pronounced, mimicking that in Gd2PdSi3, a compound which now attracts interest in the area of topological Hall effect and magnetic skyrmions, indicating complex Fermi surface. There is a minimum in the temperature dependence of electrical resistivity in the case of only Ho above T_N, but significant negative magnetoresistance is observed over a wide temperature range in the paramagnetic state increasing with decreasing temperature for both the cases. This finding establishes that these compounds belong to a select group of intermetallics in which spin-disorder contribution apparently increases gradually as one approaches respective TN with decreasing temperature. This could be an experimental signature for the effect due to classical spin-liquid above T_N. In view of these properties analogous to those of Gd2PdSi3, it is of interest to investigate these 4:1:1 compounds further to understand possible unconventional roles of itinerant electrons, not only in the magnetically ordered state but also in the paramagnetic state, predicted by some theories in recent years for which this Gd compound is considered to be a classic example.Besides, magnetoresistance and isothermal entropy change (magnetocaloric effect) track each other.

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arXiv2021

Competing magnetic interactions and magnetoresistance anomalies in cubic intermetallic compounds, Gd4RhAl and Tb4RhAl, and enhanced magnetocaloric effect for the Tb case

We report complex magnetic, magnetoresistance (MR) and magnetocaloric properties of Gd4RhAl and Tb4RhAl forming in the Gd4RhIn type cubic structure. Though the synthesis of the compounds was reported long ago, to our knowledge, no attempt was made to investigate the properties of these compounds. The present results of ac and dc magnetization, electrical resistivity and heat-capacity measurements down to 1.8 K establish that these compounds undergo antiferromagnetic order initially, followed by complex spin-glass features with decreasing temperature. These characteristic temperatures are: For Gd case, TN is about 46K and TG is about 21 K, and for Tb, about 32 and 28 K respectively. Additionally, there are field induced magnetic effects, interestingly leading to non-monotonic variations in MR. There is a significant MR over a wide temperature range above TN, similar to the behavior of magnetocaloric effect (MCE) as measured by isothermal entropy change (DeltaS). An intriguing finding we made is that DeltaS at the onset of magnetic order is significantly larger for the Tb compound than that observed for the Gd analogue near its TN. On the basis of this observation in a cubic material, we raise a question whether aspherical nature of the 4f orbital can play a role to enhance MCE under favorable circumstances, a clue that could be useful to find materials for magnetocaloric applications.

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arXiv2020

Origin of destruction of multiferroicity in Tb2BaNiO5 by Sr doping and its implications

The orthorhombic Haldane spin chain compound Tb2BaNiO5 (Neel order, TN1= 63 K) has been shown to be an exotic multiferroic system below (TN2) 25 K due to various fascinating features, pointing to a strong potential for the advancement of concepts in this field. In particular, the rare-earth ions play a direct decisive role unlike in many other well known multiferroic materials and there appears to be a critical canting angle, developing below TN2, subtended by Tb 4f and Ni 3d moments to trigger this cross coupling phenomenon. However, for a small replacement of Sr for Ba, viz. in Tb2Ba0.9Sr0.1NiO5, ferroelectricity was reported to get destroyed, but retaining magnetic features at (TN1) 55 K and (TN2) 14 K. In this article, we address the origin of suppression of multiferrocity in this Sr doped system through neutron diffraction studies and density functional theory calculations. We find that, unlike in Tb2BaNiO5, there is no pronounced change in the relative canting angle of the magnetic moments around TN2 and that the absolute value of this parameter down to 2 K fails to exceed the critical value noted for the parent, thereby explaining the origin of destruction of magnetoelectric coupling in the Sr doped material. This finding renders strong support to the proposal of possible existence of critical canting angle, at least in some cases, to induce multiferroicity, apart from serving as a route to engineer multiferroic materials for applications.

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arXiv2020

Non-collinear Order and Spin-Orbit Coupling in Sr$_3$ZnIrO$_6$

Sr$_{3}$ZnIrO$_{6}$ is an effective spin one-half Mott insulating iridate belonging to a family of magnets which includes a number of quasi-one dimensional systems as well as materials exhibiting three dimensional order. Here we present the results of an extensive investigation into the magnetism including heat capacity, a.c. susceptibility, muon spin rotation ($μ$SR), neutron diffraction and inelastic neutron scattering on the same sample. It is established that the material exhibits a transition at about $17$ K into a three-dimensional antiferromagnetic structure with propagation vector $\boldsymbol{k}=(0,\frac{1}{2},1)$ in the hexagonal setting of R$\bar{3}$c and non-collinear moments of $0.87$$μ_B$ on Ir$^{4+}$ ions. Further we have observed a well defined powder averaged spin wave spectrum with zone boundary energy of $\sim 5$ meV at $5$ K. We stress that a theoretical analysis shows that the observed non-collinear magnetic structure arises from anisotropic inter- and intra- chain exchange which has its origin in significant spin-orbit coupling. The model can satisfactorily explain the observed spin wave excitations.

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arXiv2020

Magnetic and transport anomalies in R2RhSi3 (R= Gd, Tb, and Dy), resembling those of an exotic magnetic skyrmion Gd2PdSi3

We have carried out magnetization, heat capacity, electrical and magnetoresistance measurements (2-300 K) for the polycrystalline form of intermetallic compounds, R2RhSi3 (R= Gd, Tb, and Dy), forming in a AlB2 derived hexagonal structure with a triangular R network. This work was primarily motivated by a revival of interest on Gd2PdSi3 after about two decades in the field of Toplogical Hall Effect due to magnetic skyrmions. We report here that these compounds are characterized by double antiferromagnetic transitions (T_N= 13.5 and 12 K for Gd, 13.5 and 6.5 K for Tb; 6.5 and 2.5 for Dy), but antiferromagnerism seems to be complex. The most notable observations common to all these compounds are: (i) There are many features in the data mimicking those seen for Gd2PdSi3, including the two field-induced changes in isothermal magnetization as though there are two metamagnetic transitions well below T_N. In view of such a resemblance of the properties, we speculate that these Rh-based materials offer a good playground to study toplogical Hall effect in a centrosymmetric structure, with its origin lying in triangular lattice of magnetic R ions; (ii) There is an increasing contribution of electronic scattering with decreasing temperature towards T_N in all cases, similar to Gd2PdSi3, thereby serving as examples for a theoretical prediction for a classical spin-liquid phase in metallic systems due to geometrical frustration.

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arXiv2019

A report of (topological) Hall anomaly two decades ago in Gd2PdSi3, and its relevance to the history of the field of Topological Hall Effect due to magnetic skyrmions

The area of magnetic skyrmions, with potential revolution in certain applications, in condensed matter physics is considered about a decade old. In this article, we draw the attention of the community to the recent work of Kurumaji et al [Science 365, 914 (2019)] establishing magnetic skyrmion lattice behavior in Gd2PdSi3. We consider it important to bring out the two decades old history, in particular Hall transport anomalies characterizing magnetic skyrmions. The key experimental data in support of the skyrmion lattice that Kurumaji et al present is the observation of topological Hall resistivity in an intervening field range, following two metamagnetic transitions. In addition, the values are giant, compared to those known for other magnetic skyrmions. We point out here that such features in Hall resistivity data, arising from Gd 4f magnetism, can be found in our publication in 1999 (along with the metamagnetic transitions) and we expressed difficulties in explaining the results at that time. In view of above, we propose to the scientific community that the compound Gd2PdSi3, ex post facto, can be considered as the first experimental demonstration for giant topological Hall effect in this field, discovered about two decades ago. We take this opportunity to share some thoughts, offering clues for further investigations, in particular to identify novel magnetic skyrmions and to find the same to enable applications.

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arXiv2019

Magnetic order in Nd$_2$PdSi$_3$ investigated using neutron scattering and muon spin relaxation

The rare-earth based ternary intermetallic compounds $R_2TX_3$ ($R$ = rare-earth, $T$ = transition-metal, $X$ = Si, Ge, Ga, In) have attracted considerable interest due to a wide range of interesting low temperature properties. Here we investigate the magnetic state of Nd$_{2}$PdSi$_{3}$ using neutron diffraction, muon spin relaxation ($μ$SR) and inelastic neutron scattering (INS). This compound appears anomalous among the $R_{2}$PdSi$_{3}$ series, since it was proposed to order ferromagnetically, whereas others in this series are antiferromagnets. Although some members of the $R_2TX_3$ series have been reported to form ordered superstructures, our data are well described by Nd$_{2}$PdSi$_{3}$ adopting the AlB$_2$-type structure with a single Nd site, and we do not find evidence for superlattice peaks in neutron diffraction. Our results confirm the onset of long range magnetic order below $T_0=17$~K, where the whole sample enters the ordered state. Neutron diffraction measurements establish the presence of a ferromagnetic component in this compound, as well as an antiferromagnetic one which has a propagation vector $\mathbf{k_2}=(1/2,1/2,1/4-δ)$ with a temperature dependent $δ\approx0.02-0.04$, and moments orientated exclusively along the $c$-axis. $μ$SR measurements suggest that these components coexist on a microscopic level, and therefore the magnetic structure of Nd$_{2}$PdSi$_{3}$ is predominantly ferromagnetic, with a sinusoidally modulated antiferromagnetic contribution which reaches a maximum amplitude at 11~K, and becomes smaller upon further decreasing the temperature. INS results show the presence of crystalline-electric field (CEF) excitations above $T_0$, and from our analysis we propose a CEF level scheme.

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arXiv2019

Electronic structure studies on single crystalline Nd2PdSi3, an exotic Nd-based intermetallic: Evidence for Nd 4f hybridization

In the series R2PdSi3, Nd2PdSi3 is an anomalous compound in the sense that it exhibits ferromagnetic order unlike other members in this family. The magnetic ordering temperature is also unusually high compared to the expected value for a Nd-based system, assuming 4f localization. Here, we have studied the electronic structure of single crystalline Nd2PdSi3 employing high resolution photoemission spectroscopy and ab initio band structure calculations. Theoretical results obtained for the effective electron correlation strength of 6 eV corroborate well with the experimental valence band spectra. While there is significant Pd 4d-Nd 4f hybridization, the states near the Fermi level are found to be dominated by hybridized Nd 4f-Si 3p states. Nd 3d core level spectrum exhibits multiple features manifesting strong final state effects due to electron correlation, charge transfer and collective excitations. These results serve as one of the rare demonstrations of hybridization of Nd 4$f$ states with the conduction electrons possibly responsible for the exoticity of this compound.

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arXiv2019

Reentrant spin-glass and transport behavior of Gd4PtAl, a compound with three sites for Gd

We report temperature (T) dependence (2-330 K) of DC and AC magnetization (M), isothermal remnant magnetization (M_IRM), heat capacity (C), electrical resistivity (rho), and magnetoresistance (MR) of a ternary intermetallic compound, Gd4PtAl, crystallizing in a cubic (space group F-43m) structure. In this structure, there are three sites for the rare-earth. The magnetization data reveal that, in addition to a magnetic transition at 64 K, there is another magnetic feature below 20 K. The C(T) data reveal an upturn below 64 K, shifting to a lower temperature with increasing field, which establishes that the onset of magnetic order is of an antiferromagnetic type. However, there is no worthwhile feature near 20 K in the C(T) curve. AC susceptibility peak undergoes an observable change with frequency and, in particular, the peak around 20 K gets suppressed with the application of a dc magnetic field; in addition, M_IRM undergoes a slow decay with time and isothermal M exhibits low-field hysteresis below 20 K only, which is typical of spin-glasses. The results overall suggest that this compound is a reentrant spin-glass in zero-field. There are experimental signatures pointing to the existence of both antiferromagnetic and ferromagnetic components, competing with the variation of temperature and magnetic field, as a result of which electrical and magnetoresistance behaviors are peculiar. The results overall suggest this compound exhibits interesting magnetic and transport properties.

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arXiv2019

Destruction of multiferroicity in Tb2BaNiO5 by Sr doping and its implication to magnetoelectric coupling

The Haldane spin-chain compound, Tb2BaNiO5, with two antiferromagnetic transitions, one at T1=63K, and the other at T2=25K, has been recently shown by us to be an exotic multiferroic below T2. Here, we report the results of our investigations of Sr doping at the Ba site by magnetization, heat-capacity, magnetoelectric (MDE), and pyrocurrent measurements. An intriguing finding, which we stress, is that the ferroelectricity is lost even for a doping level of 10 atomic percent, though magnetic ordering prevails. The doped specimens however retain significant magnetodielectric behaviour, but with reduced magnitudes and qualitative changes with respect to the behaviour of the parent compound. This implies that ferroelectric order is also crucial for the anomalously large MDE in the parent compound, in addition to the role of 4f single-ion anisotropy.

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