arXiv subjects

P. N. Vishwakarma

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

5 recordsLinked to original sources

arXiv2021

Investigation of spin-phonon coupling and local magnetic properties in magnetoelectric Fe2TeO6

Spin-phonon coupling originated from spin-lattice correlation depends upon different exchange interactions in transition metal oxides containing 3d magnetic ions. Spin-lattice coupling can influence the coupling mechanism in magnetoelectric material. To understand the spin-lattice correlation in inverse trirutile Fe2TeO6 (FTO), magnetic properties and phonon spectra are studied. Signature of short-range magnetic correlation induced by 5/2-5/2 dimeric interaction and magnetic anomaly at 150 K is perceived apart from the familiar sharp transition (TN~210K) corresponding to long-range order by magnetization and heat capacity measurement. The magnetic transitions and the spin dynamics are further locally probed by muon spin resonance (μSR) measurement in both zero fields (ZF) and longitudinal field (LF) mode. Three dynamically distinct temperature regimes; (i) T >TN, (ii) TN>T>150 K, and (iii) T<150 K, are observed. A swift change in spin dynamics is realized at 150K by μSR, though previous studies suggest long-range antiferromagnetic order. The observation of renormalization of different Raman modes below 210K suggests the existence of spin-phonon coupling in the material. The coupling strength is quantified as in the range 0.1-1.2 cm-1 following the two-spin cluster approximation. We propose that the spin-phonon coupling is mediated by the Fe-O2-Fe interbilayer exchange play a significant role in ME coupling observed in the material.

cond-mat.str-el↗

arXiv2021

d5-off-centering induced ferroelectric and magnetoelectric correlations in trirutile-Fe2TeO6

We present the rare existence of d5 off-centering, weak ferroelectric polarization and demonstrate its correlation with observed magnetoelectric (ME) properties in the G type (TN~210 K) antiferromagnet Fe2TeO6 (FTO) compound. The origin of ferroelectricity (FE) is associated with both lattice and asymmetric electron density distribution around the ion cores. ME coupling is observed in magnetic field-dependent polarization, ME voltage, and magnetostrain measurements. Short-range magnetic ordering due to intrabilayer dimeric exchange coupling via the double oxygen bridged Fe-O1-Fe pathway is proposed to play a dominating role to exhibit the negative nonlinear magnetic field dependent ME behavior at 300 K. Interbilayer exchange via Fe-O2-Fe pathways dominantly determines the hysteretic nonlinear magnetic field dependent ME response below TN. The observed nonlinear ME coupling signifies magnetoelasticity as manifested in the temperature and magnetic field-dependent strain measurement. Hence the rare existence of ferroelectricity and magnetoelectric coupling by d5 ion is presented in FTO.

cond-mat.mtrl-sci↗

arXiv2017

Cobalt substitution induced magnetodielectric enhancement in multiferroic Bi2Fe4O9

Antiferromagnetic Bi_{2}Fe_{4}O_{9} (BFO), lightly substituted by cobalt is studied for magnetodielectricity. The substitution causes a substantial decrease in the Neel temperature (T_N) from 250 K (in parent sample, BFO) to 152 K (in 2% Co substituted sample). At the same time, the substituted samples display a pronounced irreversibility in the ZFC-FC magnetization data for T $<$ 370 K and opening of hysteresis in the M-H plot, thus signifying the onset of weak ferromagnetism (FM) and magnetic glassiness. The induced magnetic glassiness is found to slow down the dynamics such that the magnetization decay follows. The dielectric measurement in the same temperature window shows unusual oppression in, for T$\sim$T_N and contrasting nature of tan loss for temperatures above and below T_N, thus hinting a plausible coupling between the magnetic and electric order parameters. A confirmation to this coupling is seen in the magnetodielectric (MD) results, in which it is found that the substitution induces an additional component in the MD, apart from the usual components in BFO. This additional component of MD is found to obey behaviour, with the n values being comparable to 1-p of magnetization. The temperature variation of MD also shows a contrasting behaviour for the parent and 2% Co substituted sample with an enhancement of two times in MD value. In summary, our study shows ME coupling introduced by the magnetic glassiness and its behaviour is very much different from the intrinsic one.

cond-mat.mtrl-sci↗

arXiv2005

M-I Transition in a-Conducting Carbon Films Induced by Boron Doping

The amorphous conducting carbon films have been prepared at three different preparation temperatures with different boron-doping levels. The structural and transport properties of the same have been studied. X-ray diffraction measurements show that the 'd' value of the carbon depends both on atomic percentage of B in the carbon network and also on the preparation temperature. Doping of boron increases the structural graphitic ordering of the films prepared at lower temperatures. On the contrary for the films prepared at higher temperatures the ordering deteriorates as the boron content increases. The d.c electrical transport measurements on these amorphous conducting carbon films show, doping induced metal-insulator transition via critical regime, in the temperature interval of 1.3 K to 300K. Also the films in the insulating regime show a crossover from Mott to ES VRH for T < 55K. Additional support to this transition is evident from negative magnetoresistance in VRH regime when the sample is deep inside the insulating side of MI transition. The calculated value of density of states at Fermi level shows a gradual change with corresponding variation in boron doping level, indicating a change in the number of conducting pi electrons due to substitutional doping of boron in the carbon network. However for the films exhibiting critical behaviour, the magnetic field dependence of magnetoresistance was not as predicted by the available theoretical models. Various calculated parameters like localization length, density of states at the Fermi level and coulomb gap for insulating samples were calculated from the experimental data.

cond-mat.dis-nn↗

arXiv2005

AC Conductivity in Boron Doped Amorphous Conducting Carbon Films on the Insulating Side of MI Transition

Boron doped amorphous conducting carbon films show MI transition induced by doping. In this paper we discuss the ac conductivity of the films, which lie on the insulating side of MI transition. The ac conductivity data is analyzed as a function of frequency as well as temperature for both real as well as imaginary part of the conductivity data. The ac conductivity of these samples shows enhanced interaction effect at low temperature. The conduction mechanism at high frequency and at low temperature is due to the tunneling mechanism. At intermediate temperatures and at moderate frequencies, the conductivity data is in good agreement with extended pair approximation modified for interaction effect. At high temperature and at low frequencies, the mechanism of ac conductivity is similar to that of dc conduction. The conductivity data for the insulating samples near the boundary of MI transition were analyzed using an RC element model. While fitting the impedance data, a new type of constant phase element (inductive instead of capacitive) was found to be appropriate. At present the origin of this inductive CPE is not is clear but we believe its origin is in the electron-electron interaction, which makes any attempt of applied frequency a slower response. Our ac conductivity results are well explained by tunneling models rather than hopping models.

cond-mat.dis-nn↗