arXiv subjects

D. D. Sarma

Publications and source records attributed to D. D. Sarma.

At least 55 records · Page 3Linked to original sources

arXiv2013

Effect of "dipolar-biasing" on the tunability of tunneling magnetoresistance in transition metal oxide systems

We observe an unusual tunneling magnetoresistance (TMR) phenomenon in a composite of La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ with CoFe$_{2}$O$_{4}$ where the TMR versus applied magnetic field loop suggests a "negative coercive field". Tracing its origin back to a "dipolar-biasing" of La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ by CoFe$_{2}$O$_{4}$, we show that the TMR of even a single composite can be tuned continuously so that the resistance peak or the highest sensitivity of the TMR can be positioned anywhere on the magnetic field axis with a suitable magnetic history of the sample. This phenomenon of an unprecedented tunability of the TMR should be present in general in all such composites.

cond-mat.mtrl-sci↗

arXiv2013

Engineered spin-valve type magnetoresistance in Fe$_3$O$_4$-CoFe$_2$O$_4$ core-shell nanoparticles

Naturally occurring spin-valve-type magnetoresistance (SVMR), recently observed in Sr2FeMoO6 samples, suggests the possibility of decoupling the maximal resistance from the coercivity of the sample. Here we present the evidence that SVMR can be engineered in specifically designed and fabricated core-shell nanoparticle systems, realized here in terms of soft magnetic Fe3O4 as the core and hard magnetic insulator CoFe2O4 as the shell materials. We show that this provides a magnetically switchable tunnel barrier that controls the magnetoresistance of the system, instead of the magnetic properties of the magnetic grain material, Fe3O4, and thus establishing the feasibility of engineered SVMR structures.

cond-mat.mtrl-sci↗

arXiv2013

Observation of magnetically hard grain boundaries in double-perovskite Sr$_{2}$FeMoO$_{6}$

Unusual low temperature magneto-resistance (MR) of ferromagnetic Sr$_{2}$FeMoO$_{6}$ polycrystals has been attributed to magnetically hard grain boundaries which act as spin valves. We detected the different magnetic hysteresis curves for the grains and the grain boundaries of polycrystalline Sr$_{2}$FeMoO$_{6}$ by utilizing the different probing depths of the different detection modes of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), namely, the total electron yield (TEY) mode (probing depth $\sim$5 nm) and the total fluorescence yield (TFY) mode (probing depth $\sim$100 nm). At 20 K, the magnetic coercivity detected in the TEY mode ($H_{\rm c,TEY}$) was several times larger than that in the TFY mode ($H_{\rm c,TFY}$), indicating harder ferromagnetism of the grain boundaries than that of the grains. At room temperature, the grain boundary magnetism became soft and $H_{\rm c,TEY}$ and $H_{\rm c,TFY}$ were nearly the same. From line-shape analysis of the XAS and XMCD spectra, we found that in the grain boundary region the ferromagnetic component is dominated by Fe$^{2+}$ or well-screened signals while the non-magnetic component is dominated by Fe$^{3+}$ or poorly-screened signals.

cond-mat.str-el↗

arXiv2013

Interface characterization of Co2MnGe/Rh2CuSn Heusler multilayers

All-Heusler multilayer structures have been investigated by means of high kinetic x-ray photoelectron spectroscopy and x-ray magnetic circular dichroism, aiming to address the amount of disorder and interface diffusion induced by annealing of the multilayer structure. The studied multilayers consist of ferromagnetic Co$_2$MnGe and non-magnetic Rh$_2$CuSn layers with varying thicknesses. We find that diffusion begins already at comparably low temperatures between 200 $^{\circ}$C and 250 $^{\circ}$C, where Mn appears to be most prone to diffusion. We also find evidence for a 4 Å thick magnetically dead layer that, together with the identified interlayer diffusion, are likely reasons for the small magnetoresistance found for current-perpendicular-to-plane giant magneto-resistance devices based on this all-Heusler system.

cond-mat.mtrl-sci↗

arXiv2012

Ferrimagnetism, Antiferromagnetism and Magnetic Frustration in La$_{2-x}$Sr$_{x}$CuRuO$_{6}$ (0 <= x <= 1)

We studied structural and magnetic properties of a series of insulating double perovskite compounds, La$_{2-x}$Sr$_{x}$CuRuO$_{6}$ (0 $\le$ x $\le$ 1), representing doping via \textit{A}-site substitution. The end members La$_{2}$CuRuO$_{6}$ and LaSrCuRuO$_{6}$ form in monoclinic structure while the intermediate Sr doped compounds stabilise in triclinic structure. The Cu and Ru ions sit on alternate B-sites of the perovskite lattice with $\sim$15% anti-site defects in the undoped sample while the Sr doped samples show a tendency to higher ordering at B-sites. The undoped (x = 0) compound shows a ferrimagnetic-like behaviour at low temperatures. In surprising contrast to the usual expectation of an enhancement of ferromagnetic interaction on doping, an antiferromagnetic-like ground state is realized for all doped samples (x $>$ 0). Heat capacity measurements indicate the absence of any long range magnetic order in any of these compounds. The magnetic relaxation and memory effects observed in all compounds suggest glassy dynamical properties associated with magnetic disorder and frustration. We show that the observed magnetic properties are dominated by the competition between the nearest neighbour Ru -- O -- Cu 180$^\circ$ superexchange interaction and the next nearest neighbour Ru -- O -- O -- Ru 90$^\circ$ superexchange interaction as well as by the formation of anti-site defects with interchanged Cu and Ru positions. Our calculated exchange interaction parameters from first principles calculations for $x$ = 0 and $x$ = 1 support this interpretation.

cond-mat.str-el↗

arXiv2012

Near room-temperature colossal magnetodielectricity and multiglass properties in partially-disordered La2NiMnO6

We report magnetic, dielectric and magnetodielectric responses of pure monoclinic bulk phase of partially-disordered La2NiMnO6, exhibiting a spectrum of unusual properties and establish that this system intrinsically is a true multiglass with a large magnetodielectric coupling (8-20%) over a wide range of temperatures (150 - 300 K). Specifically, our results establish a unique way to obtain colossal magnetodielectricity, independent of any striction effects, by engineering the asymmetric hopping contribution to the dielectric constant via the tuning of the relative spin orientations between neighboring magnetic ions in a transition metal oxide system. We discuss the role of anti-site (Ni-Mn) disorder in emergence of these unusual properties.

cond-mat.mtrl-sci↗

arXiv2012

XPS evidence for molecular charge-transfer doping of graphene

By employing x-ray photoelectron spectroscopy (XPS), we have been able to establish the occurrence of charge-transfer doping in few-layer graphene covered with electron acceptor (TCNE) and donor (TTF) molecules. We have performed quantitative estimates of the extent of charge transfer in these complexes and elucidated the origin of unusual shifts of their Raman G bands and explained the differences in the dependence of conductivity on n- and p-doping. The study unravels the cause of the apparent difference between the charge-transfer doping and electrochemical doping.

cond-mat.mtrl-sci↗

arXiv2011

Mechanism for the high Neel temperature in SrTcO_3

The microscopic origin of the high Neel temperature (T_N) observed experimentally in SrTcO_3 has been examined using a combination of ab-initio electronic structure calculations and mean-field solutions of a multiband Hubbard model. The G-type antiferromagnetic state is found to be robust for a large region of parameter space, with large stabilization energies found, surprisingly, for small values of intraatomic exchange interaction strength as well as large bandwidths. The microscopic origin of this is traced to specific aspects associated with the d3 configuration at the transition-metal site. Considering values of interaction strengths appropriate for SrTcO3 and the corresponding 3d oxide SrMnO_3, we find a ratio of 4:1 for the TN as well as magnitudes consistent with experiment.

cond-mat.str-el↗

arXiv2011

Tuning of dielectric properties and magnetism of SrTiO3 by site-specific doping of Mn

Combining experiments with first principles calculations, we show that site-specific doping of Mn into SrTiO3 has a decisive influence on the dielectric properties of these doped systems. We find that phonon contributions to the dielectric constant invariably decrease sharply on doping at any site. However, a sizable, random dipolar contribution only for Mn at the Sr site arises from a strong off-centric displacement of Mn in spite of Mn being in a non-d0 state; this leads to a large dielectric constant at higher temperatures and gives rise to a relaxor ferroelectric behavior at lower temperatures. We also investigate magnetic properties in detail and critically reevaluate the possibility of a true multi-glass state in such systems.

cond-mat.mtrl-sci↗

arXiv2011

Glass-like ordering and spatial inhomogeneity of magnetic structure in Ba3FeRu2O9 : The role of Fe/Ru-site disorder

Several doped 6H hexagonal ruthenates, having the general formula Ba3MRu2O9, have been studied over a significant period of time in order to understand the unusual magnetism of ruthenium metal. However, among them, the M=Fe compound appears different since it is observed that unlike others, the 3d Fe ions and 4d Ru ions can easily exchange their crystallographic positions and as a result many possible magnetic interactions become realizable. The present study involving several experimental methods on this compound establish that the magnetic structure of Ba3FeRu2O9 is indeed very different from all other 6H ruthenates. Local structural study reveals that the possible Fe/Ru-site disorder further extends to create local chemical inhomogeneity, affecting the high temperature magnetism of this material. There is a gradual decrease of 57Fe Mössbauer spectral intensity with decreasing temperature (below 100 K), which reveals that there is a large spread in the magnetic ordering temperatures, corresponding to many spatially inhomogeneous regions. However, finally at about 25 K, the whole compound is found to take up a global glass-like magnetic ordering.

cond-mat.mtrl-sci↗

arXiv2011

Origin of the unconventional magnetoresistance in Sr2FeMoO6

The unusual magnetoresistance (MR) behavior in Sr2FeMoO6, recently termed as spin-valve type MR (SVMR), presents several anomalies that are little understood so far. The difficulty in probing the origin of this phenomenon, arising from the magnetic property of only a small volume fraction of the ferromagnetic bulk, is circumvented in the present study by the use of ac susceptibility measurements that are sensitive to the slope rather than the magnitude of the magnetization. The present study unravels a spin-glass (SG) like surface layer around each soft ferromagnetic (FM) grain of Sr2FeMoO6. It is also observed that there is a very strong exchange coupling between the two, generating `exchange bias' effect, which consequently creates the `valve', responsible for the unusual MR effects.

cond-mat.mtrl-sci↗

arXiv2011

Re-entrant ordering of solute in a colloidal suspension during solvent evaporation

We study the phenomenon of self-assembly of silica micro-spheres on a glass plate during evaporation of the solvent from a colloidal suspension. Our experiments unveil an interesting competition between ordering and compaction in a strongly driven, out of equilibrium system arising from a slowing down of dynamics due to an impending glass transition. A suitable choice of experimental conditions minimizing the influence of many other competing phenomena that usually complicate probing of this underlying physics is crucial for our study. A re-entrant behavior in the order-disorder phase diagram as a function of particle density and drying time is established and the results are explained with the help of simulations and phenomenological theory.

cond-mat.soft↗

arXiv2011

Relative stability of zincblende and wurtzite structure in CdX(X = S, Se, Te) series - A NMTO study

Using NMTO-downfolding technique we have revisited the issue of relative stability between zincblende (ZB) and wurtzite (WZ) symmetries in case of CdS, CdSe and CdTe semiconductors. Our computed ionicity factors using accurate NMTO-downfolding successfully brings out the right trend within the CdX series - CdS being most ionic stabilizes in WZ symmetry while CdSe and CdTe being more covalent stabilizes in ZB symmetry. Our NMTO constructed Wannier functions corresponding to only valence bands provide a nice demonstration of this fact. The tendency towards ZB stability is governed by the covalency which prefers isotropic nature of the tetrahedral bonds.

cond-mat.mes-hall↗

arXiv2011

Origin of Magnetism and trend in $T_{c}$ in Cr-based double perovskites: Interplay of two driving mechanisms

Employing first principles density functional calculations, together with solution of the low-energy, model Hamiltonian constructed in a first principles manner, we explored the origin of magnetism and $T_c$ trend in Cr-based double perovskite series, Sr$_2$CrB$'$O$_6$ (B$'$=W/Re/Os). Our study shows that the apparently puzzling $T_c$ trend in Sr$_2$CrB$'$O$_6$ (B$'$=W/Re/Os) series can be understood in terms of the interplay of the hybridization driven mechanism and the super-exchange mechanism.

cond-mat.mtrl-sci↗

arXiv2010

Electric and magnetic polarizabilities of hexagonal Ln2CuTiO6 (Ln=Y, Dy, Ho, Er and Yb)

We investigated the rare-earth transition metal oxide series, Ln2CuTiO6 (Ln=Y, Dy, Ho, Er and Yb), crystallizing in the hexagonal structure with non-centrosymmetric P63cm space group for possible occurrences of multiferroic properties. Our results show that while these compounds, except Ln=Y, exhibit a low temperature antiferromagnetic transition due to the ordering of the rare-earth moments, the expected ferroelectric transition is frustrated by the large size difference between Cu and Ti at the B-site. Interestingly, this leads these compounds to attain a rare and unique combination of desirable paraelectric properties with high dielectric constants, low losses and weak temperature and frequency dependencies. First-principles calculations establish these exceptional properties result from a combination of two effects. A significant difference in the MO5 polyhedral sizes for M = Cu and M = Ti suppress the expected co-operative tilt pattern of these polyhedra, required for the ferroelectric transition, leading to relatively large values of the dielectric constant for every compound investigated in this series. Additionally, it is shown that the majority contribution to the dielectric constant arises from intermediate-frequency polar vibrational modes, making it relatively stable against any temperature variation. Changes in the temperature stability of the dielectric constant amongst different members of this series are shown to arise from changes in relative contributions from soft polar modes.

cond-mat.mtrl-sci↗

arXiv2009

Pressure and alloying effects on the metal to insulator transition in NiS{2-x}Se{x} studied by infrared spectroscopy

The metal to insulator transition in the charge transfer NiS{2-x}Se{x} compound has been investigated through infrared reflectivity. Measurements performed by applying pressure to pure NiS2 (lattice contraction) and by Se-alloying (lattice expansion) reveal that in both cases an anomalous metallic state is obtained. We find that optical results are not compatible with the linear Se-alloying vs Pressure scaling relation previously established through transport, thus pointing out the substantially different microscopic origin of the two transitions.

cond-mat.str-el↗

arXiv2008

First principles study of structural stability and electronic structure of CdS nanoclusters

Using first-principles density functional calculations, we have studied the structural stability of stoichiometric as well as non-stoichiometric CdS nanoclusters at ambient pressure with diameters ranging up to about 2.5 nm. Our study reveals that the relative stability of the two available structures for CdS, namely zinc blende and wurtzite, depends sensitively on the details like surface geometry and/or surface chemistry. The associated band gap also exhibits non-monotonic behavior as a function of cluster size. Our findings may shed light on reports of experimentally observed structures and associated electronic structures of CdS nanoclusters found in the literature.

physics.atm-clus↗

arXiv2008

Electronic Structure, Phonons and Dielectric Anomaly in Ferromagnetic Insulating Double Perovskite La2NiMnO6

Using first-principles density functional calculations, we study the electronic and magnetic properties of ferromagnetic insulating double-perovskite compound La2NiMnO6, which has been reported to exhibit interesting magnetic field sensitive dielectric anomaly as a function of temperature. Our study reveals existence of very soft infra-red active phonons that couple strongly with spins at the Ni and Mn sites through modification of the super-exchange interaction. We suggest that these modes are the origin for observed dielectric anomaly in La2NiMnO6.

cond-mat.mtrl-sci↗