arXiv subjects

R. Mahendiran

Publications and source records attributed to R. Mahendiran.

At least 19 recordsLinked to original sources

arXiv2026

High-frequency magnetotransport in LaMnO3 samples synthesized by microwave irradiation versus conventional heating

Magnetoresistance of hole-doped LaMnO3 at frequencies above a few MHz has been seldom reported compared to numerous studies on magnetoresistance measured at direct current. Here, we contrast the high-frequency magnetoresistance in the frequency range f = 0.9-3 GHz in polycrystalline LaMnO3 (LMO) synthesized by microwave irradiation of oxide precursors in a microwave furnace (MW-LMO) and by conventional heating (CH-LMO) in an electrical furnace. The structure at room temperature changed from orthorhombic in CH-LMO to rhombohedral in MW-LMO. A combination of magnetization and resistivity studies suggest that the CH-LMO sample is a canted antiferromagnetic insulator below 140 K but the MW-LMO is a ferromagnetic metal with T_C = 240 K. While the high-frequency resistance of the CH-LMO sample at 300 K decreases gradually with increasing strength of the applied dc magnetic field for all f, a peak appears at H = H_r larger than 0 gauss when f equal or greater than 1.4 GHz in the MW-LMO sample and H_r increases linearly with f. We attribute the observed features in the high-frequency magnetoresistance of MW-LMO to current-driven resonant excitation of spins in the paramagnetic state. Its absence in the CH-LMO sample highlights the need for an optimum hole density to observe this effect.

physics.app-ph↗

arXiv2026

Magnetovolume effect in SrRu1-xCoxO3 (x = 0.0, 0.05)

Polycrystalline SrRu1-xCoxO3 (x = 0.0, 0.05) and CaRuO3 were rapidly synthesized (< 1 hour) by microwave irradiation of oxide powders, and their magnetic, magnetoresistance, thermal expansion, and magnetostriction properties were investigated. The microwave-synthesised SrRuO3 exhibits a ferromagnetic transition at TC = 160 K, metallic-type resistivity, and negative magnetoresistance, with magnitudes comparable to those of a sample synthesized by conventional heating over 24 hours. Upon lowering the temperature from 300 K, the linear thermal expansion shows a transition from the usual contraction in the paramagnetic state to spontaneous expansion in the ferromagnetic state (invar-like effect). The application of an external magnetic field at a fixed temperature results in isotropic expansion of the length, implying a positive magnetovolume effect. The volume magnetostriction is 40 ppm at 10 K in a magnetic field of 50 kOe, and it reaches a maximum value of 60 ppm close to TC. The spontaneous thermal expansion is diminished in SrRu0.95Co0.05O3. While the magnetostriction is anisotropic at 10 K, the isotropic behaviour is recovered above 80 K, and the maximum value of the positive magnetovolume is comparable to that of the parent compound. Our results suggest that the magnetovolume effect in SrRu1-xCoxO3 is related to competition between robust tilting/rotation of RuO6 octahedra and spin-orbit interaction of the doped Co2+ ions

cond-mat.str-el↗

arXiv2026

Evolution of anisotropic magnetostriction in LaMn1-xCoxO3 (x= 0.1-0.9)

Polycrystalline samples of LaMn1-xCoxO3 series over a wide compositional range (x = 0.1 - 0.9) were synthesized by microwave irradiation of oxide precursors and their magnetic and magnetostrictive properties were investigated. Magnetostrictions parallel (l_par) and perpendicular (l_per) to the applied magnetic field were measured to estimate anisotropic(l_anis) and volume magnetostrictions(l_vol). In all the compositions, l-par (l_per) is negative (positive) and (l_anis) >> (l_vol), suggesting dominance of anisotropic lattice distortion under a magnetic field. The value of l_anis at 10 K for H = 50 kOe initially increases with x from 178 ppm for x = 0.1 to a maximum of 1221 ppm at x = 0.5 before decreasing for higher x. The composition dependence of magnetostriction is asymmetric about x = 0.5, the decrease for x > 0.5 is more steeper than for x < 0.5 whereas saturation magnetization decreases monotonically with increasing x except for an abrupt change at x = 0.6. The largest anisotropic magnetostriction observed for x = 0.5 is attributed to the presence of high-spin Co2+ ions with a non-zero orbital moment in the maximum fraction whereas Mn3+/4+ or Co3+ ions play minor roles. The composition dependence of magnetostriction is suggested to arise from changes in the structure, valence states of Mn and Co ions and magnetic interactions among them.

cond-mat.str-el↗

arXiv2025

Enhanced magnetic, electrical, and magnetostrictive properties of La-doped SrCoO3 synthesized by microwave heating

We report microwave-assisted synthesis and physical properties of La-doped SrCoO3-δ. The Sr0.8La0.2CoO3-δ synthesized via microwave heating (MWH) exhibits superior physical properties compared to a nominally identical composition obtained by conventional heating (CH) in an electrical furnace. The MWH sample exhibits an enhanced ferromagnetic Curie temperature (158 K in CH and 176 K in MWH samples) and higher saturation magnetization but smaller coercive field and magnetoresistance at 10 K compared to the CH sample. While the dc resistivity increases substantially below 120 K in the CH sample, it is metallic from 350 to 10 K in the MWH sample. Further, the longitudinal magnetostriction of the MWH sample (λpar = 247 ppm for H = 50 kOe) at 10 K is also higher than that of the CH sample (λpar = 128 ppm). The observed enhanced properties of the MWH sample can not be attributed to grain size and grain boundaries but are likely to arise from lesser oxygen defects and partial oxidation of Co3+ into Co4+, however, the exact mechanism is not fully understood at present.

cond-mat.mtrl-sci↗

arXiv2023

Giant magnetostriction in La2CoMnO6 synthesized by microwave irradiation

Polycrystalline insulating ferromagnetic double perovskite La2CoMnO6 possessing monoclinic structure and a high ferromagnetic Curie temperature (TC = 222 K) was rapidly synthesized ( 30 min) by irradiating stoichiometric mixture of oxides with the microwave. The sample exhibits negative magnetostriction, i.e., contraction of length along the magnetic field direction in the ferromagnetic state. At 10 K, the parallel magnetostriction does not show saturation up to a magnetic field of 50 kOe where it reaches 610 ppm which is one of the highest values of magnetostriction found so far among perovskite oxides with 3d ions. The magnitude of magnetostriction decreases monotonically as the temperature increases and becomes negligible above TC. The giant magnetostriction in this double perovskite is suggested to originate from large spin-orbit coupling associated with Co2+ (d7) cation. The obtained magnetostriction value is comparable to 630 ppm in an identical composition obtained through solid-state reaction over several days in a conventional furnace which indicates the advantages of microwave-assisted synthesis in saving reaction time and electric power without deteriorating physical properties.

cond-mat.mtrl-sci↗

arXiv2022

Magnetostriction in microwave synthesized La0.5Ba0.5CoO3

A single-phase polycrystalline La0.5Ba0.5CoO3-d sample was synthesized by microwave irradiation within 20 minutes of processing time and its structural, magnetic, electrical, and magnetostrictive properties were investigated. While the temperature dependence of field-cooled magnetization (M) in a field of H = 0.5 kOe indicates the onset of ferromagnetic transition at TC = 177 K, irreversibility between the zero field-cooled and field cooled M(T) persists even at H = 3 kOe. M(H) at 10 K does not saturate at the maximum available field and has a much smaller value (0.87 μB/Co in a field of 50 kOe) than 1.9 μB/Co expected for spin-only contribution from intermediate Co3+ and Co4+ spins. The resistivity shows insulating behavior down to 10 K and only a small magnetoresistance (~ 2% for H = 70 kOe) occurs around TC. All these results suggest a magnetically heterogeneous ground state with weakly interacting ferromagnetic clusters coexisting with a non-ferromagnetic phase. The length of the sample expands in the direction of the applied magnetic field (positive magnetostriction) and does not show saturation even at 50 kOe. The magnetostriction has a maximum value (= 252 ppm) at 10 K and it decreases with increasing temperature. The smaller value of magnetostriction compared to the available data on La0.5Sr0.5CoO3 suggests that non-ferromagnetic matrix is most likely antiferromagnetic and it restrains the field-induced expansion of ferromagnetic clusters in the microwave synthesized La0.5Ba0.5CoO3-d sample.

cond-mat.mtrl-sci↗

arXiv2022

Anomalous Nernst effect in La0.5Ca0.5Coo3

We report the occurrence of the anomalous Nernst effect (ANE) in polycrystalline perovskite La0.5Ca0.5CoO3. The sample is ferromagnetic below TC = 147 K and resistivity shows non-metallic behavior above and below the TC with only a small negative magnetoresistance (~2%) around TC. Field dependence of magnetization at 10 K shows large hysteresis with a coercive field of 6 kOe but a small magnetization ~ 0.64 Bohr magneton/Co even in a field of 50 kOe, which indicates the presence of magnetically heterogeneous ground state consisting of ferromagnetic and non-ferromagnetic phases. The field dependence of the Nernst thermopower (Sxy) at low temperatures shows complete saturation but the magnetization does not. This indicates that the ANE in La0.5Ca0.5CoO3 depends only on the transport properties of the ferromagnetic phase, while it is not affected by the non-ferromagnetic phase. Due to the higher value of remnant Sxy, the magnetized polycrystalline sample exhibits ANE in absence of an external magnetic field.

cond-mat.mtrl-sci↗

arXiv2021

Effect of phonon anharmonicity on ferroelectricity in EuxBa1-xTiO3

Investigating the competition between ferroelectric ordering and quantum fluctuations is essential to tailor the desired functionalities of mixed ferroelectric and incipient ferroelectric systems, like, (Ba,Sr)TiO3 and (Eu,Ba)TiO3. Recently, it has been shown that suppression of quantum fluctuations increases ferroelectric ordering in (Eu,Ba)TiO3 and since these phenomena are coupled to crystallographic phase transitions it is essential to understand the role of phonons. Here, we observe that the unusual temperature dependence of phonons in BaTiO3 gets suppressed when Ba2+ is replaced by Eu2+. This manifests in a decrease in the cubic-to-tetragonal (i.e., para-to-ferroelectric) phase transition temperature (by 150 K) and a complete suppression of tetragonality of the lattice (at room temperature by 40% replacement of Ba2+ by Eu2+). We have quantified the anharmonicity of the phonons and observed that the replacement of Ba2+ by Eu2+ suppresses it (by 93%) with a resultant lowering of the ferroelectric ordering temperature in the EuxBa1-xTiO3. This suggests that tuning phonon anharmonicity can be an important route to novel ferroelectric materials.

cond-mat.mtrl-sci↗

arXiv2021

Anomalous Nernst thermopower and giant magnetostriction in microwave synthesized La0.5Sr0.5CoO3

Ferromagnetic metallic oxides have potential applications in spincaloric devices which utilize the spin property of charge carriers for interconversion of heat and electricity through the spin Seebeck or the anomalous Nernst effect or both. In this work, we synthesized polycrystalline La0.5S0.5CoO3 by microwave irradiation method and studied its transverse thermoelectric voltage (Nernst thermopower) and change in the linear dimension of the sample (Joule magnetostriction) in response to external magnetic fields. In addition, magnetization, temperature dependences of electrical resistivity, and longitudinal Seebeck coefficient (Sxx) in absence of an external magnetic field were also measured. The sample is ferromagnetic with a Curie temperature of TC = 247 K and shows a metal-like resistivity above and below TC with a negative sign of Sxx suggesting charge transport due to electrons. Magnetic field dependence of the Nernst thermopower (Sxy) at a fixed temperature shows a rapid increase at low fields and a tendency to saturate at high fields as like the magnetization. Anomalous contribution to Sxy was extracted from total Sxy measured and it exhibits a maximum value of ~ 0.21 microV/K at 180 K for H = 50 kOe, which is comparable to the value found in a single crystal for a lower Sr content. The Joule magnetostriction is positive, i.e., the length of the sample expands along the direction of the magnetic field and it does not saturate even at 50 kOe. The magnetostriction increases with decreasing temperature below TC and reaches a maximum value of 500 ppm at T = 40 K and below. Coexistence of the anomalous Nernst thermopower and giant magnetostriction in a single compound has potential applications for thermal energy harvesting and low-temperature actuators, respectively.

cond-mat.mtrl-sci↗

arXiv2020

Paramagnetic resonance in La2NiMnO6 probed by impedance and lock-in detection techniques

We report the detection of paramagnetic resonance in the double perovskite La2NiMnO6 at room temperature for microwave magnetic fields with frequencies, f = 1 GHz to 5 GHz, using two cavity-less methods. We use an indirect impedance method which makes use of a radio frequency impedance analyzer and a folded copper strip coil for the frequency range f = 1 to 2.2 GHz. In this method, when an applied dc magnetic field is swept, high-frequency resistance of the strip coil exhibits a sharp peak and the reactance curve crosses zero exhibiting resonance. A lock-in based broadband setup using a coplanar waveguide for microwave excitation was used for f = 2 to 5 GHz The resonance fields (Hr) obtained from both the techniques increase linearly with frequency and a large spectroscopic g-factor, equal to 2.1284, which supports the presence of Ni2+ cation with strong spin-orbit coupling. Line shape analysis and analytical fitting were performed to characterize the material in terms of its initial susceptibility and damping parameters.

cond-mat.mtrl-sci↗

arXiv2020

Sharp steps in magnetization, magnetoresistance and magnetostriction in Pr0.6Sr0.4Co1-yGayO3

We report the effect of Ga substitution on magnetization, magnetoresistance, and magnetostriction in polycrystalline Pr0.6Sr0.4Co1-yGayO3 (y=0.0-0.3) samples. Upon substitution of the non-magnetic Ga3+ cation for magnetic Co3+, the low-temperature ground state transforms from ferromagnetic metallic for y = 0 to cluster glass semiconductor for y = 0.2. Magnetoresistance at 7T is negative and its magnitude increases from 2 percentage for y = 0 to 30 percentage for y = 0.3 at 10 K. On the other hand, magnetostriction at 10 K is positive and its value decreases with increasing y. Interestingly, the field-dependent magnetization, magnetoresistance, and magnetostriction for y greater than or equal to 0.2 and at T less than or equal to 3 K show reversible abrupt steps for both positive and negative magnetic field whereas all these quantities vary smoothly with the magnetic field above 4 K. Such steps in all three distinct physical quantities were never reported earlier in perovskite cobaltites and they differ from observations made in manganites and intermetallic alloys. It is suggested that field-induced avalanche flipping of ferromagnetic clusters could be the origin of observed steps in all these three quantities.

cond-mat.str-el↗

arXiv2020

Broadband Magnetoresistance in Ferromagnetic and Paramagnetic Samples of La0.7Ca0.3-xSrxMnO3

We have studied the room-temperature magnetoimpedance of paramagnetic (x = 0.06) and ferromagnetic (x = 0.1) samples in La0.7Ca0.3-xSrxMnO3 series using a radio-frequency impedance analyzer and also microwave power absorption using a network analyzer. In both measurements, samples were enclosed tightly inside a copper stripcoil and impedance or reflection coefficient of this copper stripcoil was measured as a function of the applied magnetic field for different frequencies of current (f = 0.1 to 2.5 GHz). The direction of the applied magnetic field was perpendicular to the alternating magnetic field produced by the coil. In the ferromagnetic sample (x = 0.1), magnetoresistance shows a peak around zero field for lower frequencies but a peak appears at H away from the origin at higher frequencies. The position of the peak shifts towards higher fields with increasing frequency. A similar trend is also found for the paramagnetic sample (x = 0.06) but the peak occurs at a higher field compared to the ferromagnetic sample for the same frequency. Microwave power absorption also shows features similar to magnetoresistance. Line shape analysis of the data was performed by fitting the data to a Lorentzian function. It is concluded that the observed features are imprints of ferromagnetic resonance in x = 0.1 and paramagnetic resonance in x = 0.06 samples.

physics.app-ph↗

arXiv2020

Magnetoimpedance based detection of L-band electron paramagnetic resonance in 2,2-diphenyl-1-picrylhydrazyl (DPPH) molecule

Detection of electron paramagnetic resonance (EPR) using a microwave cavity resonating at a fixed frequency (between 9 and 10 GHz) remains the most popular method until now. Here, we report a cavity-less technique which makes use of only an impedance analyzer and a copper strip coil to detect L-band EPR (f = 1-3 GHz) in the standard EPR marker 2,2-diphenyl-1-picrylhydrazyl (DPPH). Our method relies on measuring the magnetoimpedance (MI) response of DPPH through a copper strip coil that encloses DPPH. In contrast to commercial EPR which measures only the field derivative of power absorption, our method enables us to deduce both absorption and dispersion. Changes in resistance (R) and reactance (X) of the copper strip while sweeping an external dc magnetic field, were measured for different frequencies (f = 0.9 to 2.5 GHz) of radio-frequency current in the coil. R exhibits a sharp peak at a critical value of the dc magnetic field, which is identified as the resonance field and X shows a dispersion at the same frequency. The data were analyzed to obtain line width and resonance field parameters. The resonance field increased linearly with frequency and the obtained Landé g factor of 1.999 is close to the accepted value of 2.0036, measured in X-band. The simplicity of this technique can be exploited to study paramagnetic centers in catalysis and other materials.

physics.app-ph↗

arXiv2020

Microwave magnetoabsorption in R0.6Sr0.4MnO3 (R = Pr and Nd)

We report microwave magnetoabsorption (P) at room temperature in R0.6Sr0.4MnO3 (R = Pr and Nd) samples. P as a function of dc magnetic field (-2.5 kOe to +2.5 kOe) was measured for a broad frequency range (f = 0.1 - 4 GHz) of microwave magnetic field using a vector network analyzer and a copper strip coil which encloses one of the above samples. As the external dc magnetic field decreased from the maximum field, P initially increases and shows a maximum for a critical field and then decreases as the field approaches zero. The critical field increases with increasing frequency of the microwave signal. Line shape analysis of the obtained spectra suggests that the observed features in P are caused by ferromagnetic resonance (FMR) in R = Pr and electron paramagnetic resonance (ESR) in R = Nd samples, which were confirmed using a coplanar waveguide-based broadband magnetic resonance spectrometer.

cond-mat.mtrl-sci↗

arXiv2018

Effects of external magnetic field and hydrostatic pressure on magnetic and structural phase transitions in Pr0.6Sr0.4MnO3

We investigate the effect of hydrostatic pressure on temperature dependence of magnetization and also the influence of magnetic field on linear thermal expansion in polycrystalline Pr0.6Sr0.4MnO3, which is ferromagnetic at room temperature (TC = 305 K) but its magnetization undergoes an abrupt decrease at TS = 89 K within the ferromagnetic state. Normal and inverse magnetocaloric effects around TC and TS, respectively, were reported earlier in this single phase compound [D. V. M. Repaka et al., J. Appl. Phys. 112, 123915 (2012)]. The thermal expansion shows an abrupt decrease at TS in zero magnetic field but it transforms into an abrupt increase at the same temperature under 7 T, which we interpret as the consequence of magnetic field-induced structural transition from a low-volume monoclinic (I2/a symmetry) to a high volume orthorhombic (Pnma symmetry) phase in corroboration with a published neutron diffraction study in zero magnetic field. While the external magnetic field does not change TS, application of a hydrostatic pressure of P = 1.16 GPa shifts the magnetic anomaly at TS towards high temperature. The pressure induced shift of the low-temperature anomaly (deltaTS = 27 K) is nine-times more than that of the ferromagnetic Curie temperature (deltaTC = 3K). Our results suggest that while hydrostatic pressure stabilizes the low temperature monoclinic phase at the expense of orthorhombic phase, magnetic field has an opposite effect.

cond-mat.mtrl-sci↗

arXiv2018

Large thermoelectric response in a diluted ferroelectric system: Ba0.7Eu0.3Ti1-xNbxO3

We investigated the electrical conductivity, thermal conductivity and thermopower as a function of Nb content (x) in Ba0.7Eu0.3Ti1-xNbxO3 (x = 0.001- 0.10) in the temperature range T = 400-2 K. The substitution of Nb destabilizes the ferroelectric insulating ground state of Ba0.7Eu0.3TiO3 and transforms into a paramagnetic metal for x = 0.1. Thermopower is negative in the entire composition range (S = -613 microVolt/K at 400 K for x = 0.001) and its magnitude decreases with increasing Nb content which suggests doping of electrons into empty Ti-3d(t2g) conduction band. In this series, the dimensionless figure of merit (ZT) increases with temperature for all the compositions and the x = 0.03 composition exhibits the maximum ZT (= 0.12 at 400 K). The enhanced value of ZT is primarily due to the low thermal conductivity of samples in this series (~ 0.7 to 1 W/(m.K) at 400 K) compared to other potential high temperature n-type thermoelectric oxides such as carrier doped SrTiO3 and CaMnO3. The low thermal conductivity in our compounds most likely arises from heavy Eu2+ ion and lattice disorder introduced by Nb5+ which scatter phonons effectively.

cond-mat.mtrl-sci↗

arXiv2017

Strong Electron-Phonon Interaction and Colossal Magnetoresistance in EuTiO$_3$

At low temperatures, EuTiO$_3$ system has very large resistivities and exhibits colossal magnetoresistance. Based on a first principle calculation and the dynamical mean-field theory for small polaron we have calculated the transport properties of EuTiO$_3$. It is found that due to electron-phonon interaction the conduction band may form a tiny subband which is close to the Fermi level. The tiny subband is responsible for the large resistivity. Besides, EuTiO$_3$ is a weak antiferromagnetic material and its magnetization would slightly shift the subband via exchange interaction between conduction electrons and magnetic atoms. Since the subband is close to the Fermi level, a slight shift of its position gives colossal magnetoresistance.

cond-mat.str-el↗

arXiv2016

Colossal magnetoresistance over a wide temperature range in Eu0.99La0.01TiO3

We report occurrence of large magnetoresistance in lightly doped antiferromagnetic paraelectric EuTiO3. Reports of magnetoresistance in rare-earth titanates (RTiO3 oxides) are very scarce because they are highly insulating at low temperatures. EuTiO3 is an insulator at 2.5 K and 1% La3+ substitution for Eu2+ lowers the resistivity over five orders of magnitude at 2.5 K. It is shown that Eu0.99La0.01TiO3 which is antiferromagnetic below T= 5.43 K shows large magnetoresistance over a wide temperature. At 2.5 K, magnetoresistance is -42 % for H = 6 kOe and it increases to -75 % for H = 7 T. Even at 50 K which is ten times higher than Neel temperature, magnetoresistance is very significant (-20 % for H = 7 T). It is suggested that 4f spins on Eu2+ ion is strongly exchange coupled to doped electron in Ti-3d band via f-d interation and field-induced suppression of 4f spin fluctuations decrease spin-disorder scattering experienced by 3d electrons. In view of our results, it will be interesting to investigate magnetoresistance of other rare earth titanates.

cond-mat.str-el↗