arXiv subjects

N. P. Lalla

Publications and source records attributed to N. P. Lalla.

At least 19 recordsLinked to original sources

arXiv2025

Magnetic structure evolution and magnetoelastic coupling across the spin reorientation transition in TmCrO3

We present a comprehensive study of the magnetic structure evolution across the spin reorientation transition in orthorhombic (Pnma) TmCrO3. Magnetic susceptibility reveals canted antiferromagnetic (CAFM) ordering at T_N = 125 K, two compensation points (T_comp1 and T_comp2), followed by magnetization reversal with a magnetic susceptibility minimum between T_comp1 and T_comp2. Heat capacity shows a sharp lambda-type transition at T_N, associated with the long-range antiferromagnetic ordering of Cr, followed by a broad feature near 9 K. Neutron powder diffraction (NPD) establishes the Pn'm'a (Gamma2) magnetic structure below T_N. A gradual change in magnetic structure occurs during the spin-reorientation (SRO) transition below 30 K, where the magnetic symmetry transforms from Pn'm'a (Gamma2) to Pn'ma' (Gamma4) phase. However, below the SRO, neither Gamma2 nor Gamma4 alone adequately fit the intensity of magnetic reflections. A satisfactory refinement is achieved using the monoclinic subgroup P21'/c', derived from a combination of Gamma2 and Gamma4. The gradual SRO of Tm and Cr moments across the compensation regime is consistent with the magnetic symmetry P21'/c'. Furthermore, the ordered moments of Cr and Tm in TmCrO3 exhibit a complex, non-monotonic temperature dependence, with the Tm sublattice driving the spin-reorientation transition near the compensation point. Anomalies in the lattice parameters reveal strong magnetoelastic coupling, linking structural distortions to the SRO.

cond-mat.str-el↗

arXiv2025

Structural Inhomogeneities and Suppressed Magneto-Structural Coupling in Mn-Substituted GeCo2O4

A comprehensive study of Ge1-xMnxCo2O4 (GMCO) system was conducted using neutron powder diffraction (NPD), x-ray diffraction (XRD), Scanning electron microscopy, magnetometry, and heat capacity measurements. Comparative analysis with GeCo2O4 (GCO) highlights the influence of Mn substitution on the crystal and magnetic structure at low temperature. Surprisingly, phase separation is observed in GMCO with a targeted nominal composition of Ge0.5Mn0.5Co2O4. SEM/EDX analysis reveals that the sample predominantly consists of a Mn-rich primary phase with approximate stoichiometry Mn0.74Ge0.18Co2O4, along with a minor Ge-rich secondary phase of composition Ge0.91Mn0.19Co2O4. Although both GCO and GMCO crystallize in cubic symmetry at room temperature, a substantial difference in low-temperature structural properties has been observed. Magnetic and heat capacity data indicate ferrimagnetic ordering in the Mn-rich phase near TC = 108 K, while the Ge-rich phase exhibits antiferromagnetic order at TN = 22 K in GMCO. Analysis of heat capacity data reveals that the estimated magnetic entropy amounts to only 63% of the theoretical value expected in GMCO. A collinear ferrimagnetic arrangement is observed in the Mn rich phase below the magnetic ordering temperature, characterized by antiparallel spins of the Mn at A site and Co at B site along the c-direction. At 5 K, the refined magnetic moments are 2.31(3) for MnA and 1.82(3) uB for CoB in the Mn rich ferrimagnetic phase. The magnetic structure at 5 K in the Ge rich secondary phase is identical to the antiferromagnetic structure of the parent compound GeCo2O4. The refined value of the CoB moment in this phase at 5 K is 2.53(3) uB.

cond-mat.mtrl-sci↗

arXiv2024

Structural and Magnetic properties of Ge0.5Mn0.5Co2O4 using neutron diffraction

The structural and magnetic properties of Ge0.5Mn0.5Co2O4 (GMCO) have been investigated in detail utilizing neutron powder diffraction (NPD), x-ray diffraction (XRD), DC magnetometry, and heat capacity analysis and compared with GeCo2O4. Despite both compounds exhibiting a cubic structure at room temperature, a substantial difference on low temperature structural properties have been observed for GMCO, indicating the effect of Mn substitution on crystal structure. The magnetic and heat capacity data reveal a ferrimagnetic ordering around 108 K in GMCO. A minor secondary phase is confirmed which undergoes long range AFM ordering at further lower temperatures. This secondary phase remains undetected in XRD due to identical lattice parameters. Furthermore, the analysis of heat capacity data indicates a broadening of the high-temperature transition, attributing to the short-range correlation persisting up to higher temperatures. The estimated magnetic entropy amounts is 63% of the value expected for GMCO. The missing entropy is likely linked with the short-range magnetic correlations persisting well above the transition temperature. Cation distribution at the A and B sites has been estimated in GMCO using NPD. Magnetic structures are also confirmed in the main phase as well as in the secondary phase using NPD analysis. The high-temperature transition corresponds to the ferrimagnetic ordering of A and B site cations in the main phase. A collinear ferrimagnetic arrangement of A and B site spins aligned parallel to c axis is observed. The average values of A and B site moments in the ferrimagnetic phase at 5 K are 2.31(3) and 1.82(3)mB, respectively, with the temperature dependence of moments following the expected power law behavior. The low-temperature ordering is attributed to the antiferromagnetic ordering of B site ions associated with the secondary phase, something similar to GeCo2O4.

cond-mat.str-el↗

arXiv2023

Charge density wave and superconductivity in 6R-TaS2

The layered transition metal dichalcogenide compounds 1T-TaS2 and 4H-TaS2 are well known for their exotic properties, which include charge density wave, superconductivity, Mott transition, etc., and lately quantum spin liquid. Here, we report the magnetic, transport and transmission electron microscopy study of the charge density wave and superconductivity in 6R-TaS2 which is a relatively less studied polymorph of this dichalcogenide TaS2. Our high temperature electron microscopy reveals multiple charge density wave transitions between room temperature and 650K. Magnetization, and the electrical resistivity measurements in the temperature range of 2-400 K reveal that 6R-TaS2 undergoes a charge density wave transition around 305 K and is followed by a transition to a superconducting state around 3.5 K. The low temperature specific heat measurement exhibits anomaly associated with the superconducting transition around 2.4 K. The estimated Ginzburg Landau parameter suggests that this compound lies at the extreme limit of type-II superconductivity.

cond-mat.supr-con↗

arXiv2020

Strain Induced Relaxor-type Ferroelectricity Near Room Temperature in Delafossite CuCrO2

Polycrystalline samples of CuCrO2 were synthesized by solid state reaction method. Temperature dependent dielectric measurements, synchrotron x-ray diffraction (SXRD), pyroelectric current and Raman measurements have been performed on these samples. Evidences of the presence of relaxor type ferroelectricity, which otherwise have gone unnoticed in CuCrO2 system (a member of delafossite family) near room temperature, have been presented. Presence of broad maximum in dielectric permittivity and its frequency dispersion indicates relaxor-type ferroelectricity in CuCrO2 near room temperature. Careful analysis of temperature dependent SXRD data and Raman spectroscopic data indicates that the distorted CrO6 octahdera, is giving rise to strain in the sample. Due to this strain, polar regions are forming in an otherwise non-polar matrix, which is giving rise to relaxor type ferroelectricity in the sample. Regularization of CrO6 octahedra and disappearance of disorder induced peak in Raman spectra at high temperatures could be the reason behind observed dielectric anomaly in this sample. Present investigations propose that relaxor type ferroelectricity near room temperature is an inherent property of the CuCrO2 system, making it a fascinating material to be explored further.

cond-mat.mtrl-sci↗

arXiv2019

Evidence of a cluster spin-glass state in the B-site disordered perovskite SrTi0.5Mn0.5O3

SrTi0.5Mn0.5O3 (STMO) is a chemically disordered perovskite having random distribution of Ti and Mn over 1b site. Striking discrepancies about the structural and magnetic properties of STMO demands detailed analysis which is addressed. To explore the magnetic ground state of STMO, static and dynamic magnetic properties were studied over a broad temperature range (2-300 K). The dc, ac magnetization show a cusp like peak at Tf ~ 14 K, which exhibits field and frequency dependence. The thermoremanent magnetization is characterized by using stretched exponential function and characteristic time suggests the existence of spin clusters. Also the other features observed in magnetic memory effect, muon spin resonance/rotation and neutron powder diffraction confirm the existence of cluster spin glass state in STMO, rather than the long range ordered ground state. Intriguingly, the observed spin relaxation can be attributed to the dilute magnetism due to non-magnetic doping at Mn-site and competing antiferromagnetic and ferromagnetic interactions resulting from the site disorder.

cond-mat.mtrl-sci↗

arXiv2018

Huge magnetoresistance and ultra-sharp metamagnetic transition in polycrystalline ${Sm_{0.5}Ca_{0.25}Sr_{0.25}MnO_3}$

Large magnetoresistive materials are of immense interest for a number of spintronic applications by developing high density magnetic memory devices, magnetic sensors and magnetic switches. Colossal magnetoresistance, for which resistivity changes several order of magnitude (${\sim10^4 \%}$) in an external magnetic field, occurs mainly in phase separated oxide materials, namely manganites, due to the phase competition between the ferromagnetic metallic and the antiferromagnetic insulating regions. Can one further enhance the magnetoresistance by tuning the volume fraction of the two phases? In this work, we report a huge colossal magnetoresistance along with the ultra-sharp metamagnetic transition in half doped ${Sm_{0.5}Ca_{0.25}Sr_{0.25}MnO_3}$ manganite compound by suitably tuning the volume fraction of the competing phases. The obtained magnetoresistance value at 10 K is as large as $\sim10^{13}\%$ in a 30 kOe external magnetic field and $\sim10^{15}\%$ in 90 kOe external magnetic field and is several orders of magnitude higher than any other observed magnetoresistance value reported so far. Using model Hamiltonian calculations we have shown that the inhomogeneous disorder, deduced from tunneling electron microscopy, suppresses the CE-type phase and seeds the ferromagnetic metal in an external magnetic field.

cond-mat.mtrl-sci↗

arXiv2017

Room temperature ferromagnetism in transparent and conducting Mn-doped $SnO_{2}$ thin films

The magnetization as a function of magnetic field showed hysteretic behavior at room temperature. According to the temperature dependence of the magnetization, the Curie temperature $(T_{C})$ is higher than 350 K. Ferromagnetic Mn-doped tin oxide thin films exhibited low electrical resistivity and high optical transmittance in the visible region (400-800 nm). The coexistence of ferromagnetism, high visible transparency and high electrical conductivity in the Mn-doped $SnO_{2}$ films is expected to be a desirable trait for spintronics devices.

cond-mat.mtrl-sci↗

arXiv2016

Structural and magnetic properties of S = 3/2 chemically disordered perovskite Sr2TiMnO6

Single phase polycrystalline sample of perovskite Sr2TiMnO6 (STMO) has been successfully synthesized via solid state reaction route. Its detailed structural and physical properties have been studied using x-ray diffraction, transmission electron microscopy, magnetization, specific heat, and dielectric measurements. Rietveld analysis of the powder XRD data and transmission electron microscopy studies confirm that STMO is not a double perovskite structure with Fm-3m space group. But it is a disordered perovskite structure with Pm-3m space group where Ti4+ and Mn4+ ions are randomly distributed at the 1b site. The dc magnetization (\c{hi}dc-vs-T) measurements in the temperature range 2-300K reveals a single anomaly at ~13K with predominant long range canted antiferromagnetic (AFM) ordering. The frequency invariant maxima at 13K in the ac susceptibility (\c{hi}ac-vs-T) discards the spin-glass behaviour in STMO. Despite the presence of sharp feature in M-T data, the specific heat data (Cp-vs-T) shows only a broad hump around 13K, which has been attributed to the magneto-structural ordering giving rise to canted AFM state in chemically disorder STMO. Matching with the magnetic and specific heat anomaly, the dielectric permittivity also shows a broad anomaly around 14K, indicating the possible magneto-dielectric coupling in STMO.

cond-mat.mtrl-sci↗

arXiv2015

Complex dielectric and impedance behavior of magnetoelectric Fe2TiO5

We have investigated the complex dielectric and impedance properties of magnetoelectric compound Fe2TiO5 (FTO) as a function of temperature (T) and frequency (f) to understand the grain (G) and grain boundary (Gb) contributions to its dielectric response. The temperature and frequency dependent dielectric permittivity data shows a sharp increase in permittivity above 200K accompanied with a frequency dependent peak in loss. At T less than 175K, only G contribution dominates even at lower frequency (nearly 100Hz), but for T greater than 175K, the Gb contribution starts appearing at low frequency. The value of critical frequency distinguishing these two contributions increases with increasing temperature. The observed non-Debye dielectric relaxation follows thermally activated process and is attributed to polaron hopping. Further the frequency dependence of ac conductivity follows the Jonscher power-law. The temperature dependency of critical exponent s shows that the correlated barrier hopping model is appropriate to define the conductivity mechanism of FTO in the studied temperature regime.

cond-mat.mtrl-sci↗

arXiv2015

Low Temperature Structural and Transport Studies of La0.175Pr0.45Ca0.375MnO3

The temperature (T) dependent x-ray diffraction (XRD) and resistivity measurements of La0.175Pr0.45Ca0.375MnO3 (LPCMO) have been performed down to 2K to understand the structural and transport properties. From room temperature down to 220K, LPCMO exists in orthorhombic phase with Pnma structure and at 220K, it transforms to charge ordered (CO) monoclinic phase with P21/m structure and remains as it is down to 2K. The CO phase is evident from the occurrence of weak but well defined superlattice peaks in the XRD pattern. This structural transformation is of first order in nature as evident from the phase coexistence across the transition region. These results thus clearly illustrate that LPCMO undergoes a first order structural phase transition from charge disordered orthorhombic phase to CO monoclinic phase at 220K, consistent with temperature dependent resistivity results. Our structural analysis of T dependent XRD data using Rietveld refinement infers that below 220K, LPCMO forms commensurate CO monoclinic P21/m structure with four times structural modulation.

cond-mat.mtrl-sci↗

arXiv2015

Structural evidence of magnetic field induced devitrification of kinetically arrested antiferromagnetic phase in La0.175Pr0.45Ca0.375MnO3: A Low-temperature high-magnetic field x-ray diffraction study

The low-temperature and high-magnetic field (2K, 8T) powder x-ray diffraction (LTHM-XRD) measurements have been carried out at different temperatures (T) and magnetic fields (H) to investigate the structural phase diagram for phase separated La0.175Pr0.45Ca0.375MnO3 (LPCMO) manganite. The antiferromagnetic (AFM) P21/m insulating phase undergoes field induced transformation to ferromagnetic (FM) Pnma metallic ground state below its AFM ordering temperature (220K) in zero-field cooling (ZFC) from room temperature. At temperature greater than 25K, the field induced FM Pnma phase remained irreversible even after complete removal of field. However, for T ( 39-65K), the field induced transformation is partially reversible. This behaviour has been attributed to magnetic field induced devitrification of the glass-like arrested AFM P21/m phase to FM Pnma equilibrium phase. The devitrified FM Pnma phase starts transforming back to AFM P21/m phase around ~39K on heating the sample under zero field. Our results corroborate the evidence of strong magneto-structural coupling in this system. An H-T phase-diagram has been constructed based on LTHM-XTD data, which resembles with the one made from magnetic measurements. These results have been explained on the basis of kinetic arrest of first order phase transition and field induced devitrification of the arrested phase.

cond-mat.mtrl-sci↗

arXiv2015

Development of Low Temperature and High Magnetic Field X-Ray Diffraction Facility

The current progress of materials science regarding multifunctional materials (MFM) has put forward the challenges to understand the microscopic origin of their properties. Most of such MFMs have magneto-elastic correlations. To investigate the underlying mechanism, it is therefore essential to investigate the structural properties in the presence of magnetic field. Keeping this in view low temperature and high magnetic field (LTHM) powder x-ray diffraction (XRD), a unique state-of-art facility in India has been developed at CSR Indore. This setup works on symmetric Bragg Brentano geometry using a parallel incident x-ray beam from a rotating anode source working at 17 kW. Using this one can do structural studies at non-ambient conditions i.e. at low- temperatures (2-300 K) and high magnetic field (+8 to -8 T). The available scattering angle ranges from 5° to 115° 2θ with a resolution better than 0.1°. The proper functioning of the setup has been checked using Si sample. The effect of magnetic field on the structural properties has been demonstrated on Pr0.5Sr0.5MnO3 sample. Clear effect of field induced phase transition has been observed. Moreover, the effect of zero field cooled and field cooled conditions is also observed.

cond-mat.mtrl-sci↗

arXiv2015

Strong charge ordering above room temperature in B-site disordered electron-doped manganite SrMn0.875Mo0.125O3-δ

Low as well as high-temperature electron and x-ray diffraction studies have been carried out on a rare-earth free B-site disordered electron-doped manganite SrMn0.875.Mo0.125O3-δ in the temperature range of 83K to 637K. These studies reveal the occurrence of strong charge ordering (CO) at room temperature in a pseudo tetragonally distorted perovskite phase with space-group Pmmm. Non integral modulation vector of 8.95 times along [-110] indicates a charge density wave type modulation. The CO phase with basic perovskite structure Pmmm transforms to a charge disorder cubic phase through a first order phase transition at 355K. Supporting temperature dependent measurements of resistance and magnetization show a metal-insulator and antiferromagnetic transitions across 355K with a wide hysterisis ranging from 150K to 365K. The occurrence of pseudo tetragonality of the basic perovskite lattice with c/a < 1 together with charge-ordered regions with 2-dimensional modulation have been analyzed as the coexistence of two CO phases with 3dx2/3dy2 type and 3dx2-y2 type orbital ordering.

cond-mat.str-el↗

arXiv2015

Evidence of ferromagnetic short-range correlations in cubic La1-xSrxMnO3-δ (x=0.80, 0.85) above antiferromagnetic ordering

We report the existence of ferromagnetic correlations (FMC) in paramagnetic (PM) matrix of cubic La1-xSrxMnO3-δ (x = 0.80, 0.85) well above its coupled structural, magnetic and electronic phase transitions. The dc-magnetization vs temperature [M(T)] behaviour under different magnetic fields (from 100 Oe to 70 kOe) shows the presence of short range magnetic correlations up to (TFMC ~) 365 K, far above the antiferromagnetic ordering temperatures (TN =) 260 K and 238 K for x=0.80 and 0.85, respectively. More importantly the observed short-range FMC survive even up to 70 kOe, which indicates their robust nature. The temperature region between TN to TFMC is dominated by the presence of correlated ferromagnetic (FM) entities within the PM matrix and stabilized due to A-site chemical disorder. Our results further illustrate that for the studied compositions, the oxygen off-stoichiometry does not have any significant effect on the nature and strength of these FM entities; however, FM interactions increase in the oxygen deficient samples. These compositions are the unique examples, where the presence of FMC is observed in an undistorted basic cubic perovskite lattice well above TN and therefore are novel to understand the physics behind the colossal magneto-resistance effect.

cond-mat.str-el↗

arXiv2015

Comment on Quantum paraelectric glass state in SrCu3Ti4O12 [Appl. Phys. Lett.104, 262905 (2014)]

Recently, Kumar et al. [APL 104, 262905 (2014)] have reported quantum paraelectric glass state in SrCu3Ti4O12. Their analysis is based on the misconception about the effect of quantum fluctuations on paraelectric properties. There are very serious issues with their analysis of permittivity vs T data where authors tried to prove the presence of quantum paraelectricity in SrCu3Ti4O12. In the comment we have clearly affirm that the analysis presented by the authors do not prove the presence of quantum paraelectric behavior and there by the quantum paraelectric glass state. The quantum fluctuations and the frequency dependence of permittivity, showing relaxor like behavior cannot exist simultaneously.

cond-mat.mtrl-sci↗

arXiv2014

Multiglass properties and magnetoelectric coupling in uniaxial anisotropic spin-cluster-glass Fe2TiO5

The compound, Fe2TiO5 (FTO)is a well-known uniaxial anisotropic spin-glass insulator with two successive glassy freezing temperatures i.e. transverse (TTF= 9K) and longitudinal (TLF= 55 K). In this article, we present the results of measurements of complex dielectric behavior, electric polarization as a function of temperature (T), in addition to characterization by magnetic susceptibility and heat-capacity, primarily to explore magnetoelectric (ME) coupling and multiglass properties in uniaxial anisotropic spin cluster-glass FTO. The existence of two magnetic transitions is reflected in the isothermal magnetodielectric (MD) behavior in the sense that the sign of MD is different in the T regime T TTF.The data in addition provide evidence for the glassy dynamics of electric-dipoles; interestingly, this occurs at much higher temperature (~100-150 K) than TLF with high remnant polarization at 10 K( 4000 micro C/m2) attributable to short-range magnetic correlations, thereby offering a route to attain ME coupling above 77 K.

cond-mat.mtrl-sci↗

arXiv2014

Displacive-type ferroelectricity from magnetic correlations within spin-chain

Observation of ferroelectricity among non-d0 systems, which was believed for a long time an unrealistic concept, led to various proposals for the mechanisms to explain the same (i.e. magnetically induced ferroelectricity) during last decade. Here, we provide support for ferroelectricity of a displacive-type possibly involving magnetic ions due to short-range magnetic correlations within a spin-chain, through the demonstration of magnetoelectric coupling in a Haldane spin-chain compound Er2BaNiO5 well above its Néel temperature of (TN=) 32K. There is a distinct evidence for electric polarization setting in near 60 K around which there is an evidence for short-range magnetic correlations from other experimental methods. Raman studies also establish a softening of phonon modes in the same temperature (T) range and T-dependent x-ray diffraction (XRD) patterns also reveal lattice parameters anomalies. Density-functional theory based calculations establish a displacive component (similar to d0-ness) as the root-cause of ferroelectricity from (magnetic) NiO6 chain, thereby offering a new route to search for similar materials near room temperature to enable applications.

cond-mat.str-el↗