arXiv subjects

S. Majumdar

Publications and source records attributed to S. Majumdar.

At least 55 records · Page 3Linked to original sources

arXiv2009

Exchange bias effect involved with tunneling magnetoresistance in polycrystalline La_{0.88}Sr_{0.12}CoO_3

We report the exchange bias (EB) effect along with tunneling magnetoresistance (MR) in polycrystalline La_{0.88}Sr_{0.12}CoO_3. Analogous to the shift in the magnetic hysteresis loop along the field (H)-axis a shift is clearly observed in the MR-H curve when the sample is cooled in a static magnetic field. Training effect (TE) is a significant manifestation of EB effect which describes the decrease of EB effect when sample is successively field-cycled at a particular temperature. We observe TE in the shift of the MR-H curve which could be interpreted by the spin configurational relaxation model. A strong field-cooled (FC) effect in the temperature as well as time dependence of resistivity is observed below spin freezing temperature. The unusual MR results measured in FC mode are interpreted in terms of intragranular interface effect between short range ferromagnetic clusters and spin-glass matrix giving rise to the EB effect. EB effect in MR has been observed in bilayer or multilayer films which has not yet seen in a polycrystalline compound. EB effect involved with tunneling MR and semiconducting transport property attributed to the intragranular intrinsic nanostructure is promising for the spintronic applications.

cond-mat.str-el↗

arXiv2009

Possible magnetoelectric coupling in the half doped charge ordered manganite, Pr_0.5Ca_0.5MnO_3-d

Magnetization, magnetoresistance, and magnetodielectric measurements have been carried out on the half doped charge ordered manganite, Pr_0.5Ca_0.5MnO_3. The low temperature state is found to be strongly dependent on the oxygen stoichiometry whereas the high temperature state remains almost unaltered. A disorder driven phase separation in the low temperature state is noticed in the magnetic, magnetoresistance, and dielectric measurements which is attributed to the oxygen deficiency in the compound. A considerable magnetodielectric (MD) effect is noticed close to room temperature at 280 K which is fascinating for technological applications. The strongest MD effect observed in between 180 K and 200 K is found to be uncorrelated with magnetoresistance but it is suggested to be due to a number of intricate processes occuring in this temperature range which includes paramagnetic to antiferromagnetic transition, incommensurate to commensurate charge ordering and orbital ordering. The strongest MD effect seems to emerge from the high sensitivity of the incommensurate state to the external perturbation such as external magnetic field. The results propose the possible magnetoelectric coupling in the charge ordered compound.

cond-mat.str-el↗

arXiv2009

Intragrain electrical inhomogenities and compositional variation of static dielectric constant in LaMn_{1-x}Fe_xO_3

Bulk dc resistivity and dielectric constant measurements with temperature and frequency have been performed in LaMn_{1-x}Fe_xO_3 (0 <= x <= 1.0) as a result of Fe substitution both for the as-synthesized and oxygen annealed samples. Temperature dependence of real part of dielectric constant at different frequencies show a frequency and temperature independent value (epsilon_s) at low temperature for x >= 0.15 where epsilon_0 rises with x reaching a maximum at x = 0.5 and then it decreases sharply at x = 0.70 showing a further increasing trend with the further increase in x. The sharp drop of epsilon_s at x = 0.70 is correlated with the structural change from rhombohedral to orthorhombic structure. A considerable increase of epsilon_s ~ 43% is observed for an increase of x from 0.15 to 0.50. Furthermore, epsilon_s is increased considerably (up to ~ 17% at x = 0.5) due to the oxygen annealing for x <= 0.50. The low temperature resistivities satisfying Variable Range Hopping model are found to be related with the increase of epsilon_s with x for x <= 0.50. The analysis of the complex impedance and modulus planes at low temperature indicates the electrical inhomogeneities in the grain interior of the compounds.

cond-mat.str-el↗

arXiv2008

Reentrant spin glass state in Mn doped Ni2MnSn shape memory alloy

The ground state properties of the ferromagnetic shape memory alloy of nominal composition Ni2Mn1.36Sn0.64 have been studied by dc magnetization and ac susceptibility measurements. Like few other Ni-Mn based alloys, this sample exhibits exchange bias phenomenon. The observed exchange bias pinning was found to originate right from the temperature where a step-like anomaly is present in the zero-field-cooled magnetization data. The ac susceptibility study indicates the onset of spin glass freezing near this step-like anomaly with clear frequency shift. The sample can be identified as a reentrant spin glass with both ferromagnetic and glassy phases coexisting together at low temperature at least in the field-cooled state. The result provides us an comprehensive view to identify the magnetic character of various Ni-Mn-based shape memory alloys with competing magnetic interactions.

cond-mat.str-el↗

arXiv2008

Exchange bias effect in the phase separated Nd_{1-x}Sr_{x}CoO_3 at the spontaneous ferromagnetic/ferrimagnetic interface

We report the new results of exchange bias effect in Nd_{1-x}Sr_{x}CoO_3 for x = 0.20 and 0.40, where the exchange bias phenomenon is involved with the ferrimagnetic (FI) state in a spontaneously phase separated system. The zero-field cooled magnetization exhibits the FI (T_{FI}) and ferromagnetic (T_C) transitions at ~ 23 and \sim 70 K, respectively for x = 0.20. The negative horizontal and positive vertical shifts of the magnetic hysteresis loops are observed when the system is cooled through T_{FI} in presence of a positive static magnetic field. Training effect is observed for x = 0.20, which could be interpreted by a spin configurational relaxation model. The unidirectional shifts of the hysteresis loops as a function of temperature exhibit the absence of exchange bias above T_{FI} for x = 0.20. The analysis of the cooling field dependence of exchange bias field and magnetization indicates that the ferromagnetic (FM) clusters consist of single magnetic domain with average size around \sim 20 and ~ 40 Å~ for x = 0.20 and 0.40, respectively. The sizes of the FM clusters are close to the percolation threshold for x = 0.20, which grow and coalesce to form the bigger size for x = 0.40 resulting in a weak exchange bias effect.

cond-mat.str-el↗

arXiv2008

Polaron relaxation and hopping conductivity in LaMn$_{1-x}$Fe$_{x}$O$_3$

Dc and ac transport properties as well as electric modulus spectra have been investigated for the samples LaMn$_{1-x}$Fe$_{x}$O$_3$ with compositions 0 $\leq x \leq$ 1.0. The bulk dc resistivity shows a temperature variation consistent with the variable range hopping mechanism at low temperature and Arrhenius mechanism at high temperatures. The ac conductivity has been found to follow a power law behavior at a limited temperature and frequency region where Anderson-localization plays a significant role in the transport mechanism for all the compositions. At low temperatures large dc resistivities and dielectric relaxation behavior for all the compositions are consistent with the polaronic nature of the charge carriers. Scaling of the modulus spectra shows that the charge transport dynamics is independent of temperature for a particular composition but depends strongly on different compositions possibly due to different charge carrier concentrations and structural properties.

cond-mat.str-el↗

arXiv2008

Metastability and magnetic memory effect in Ni-Mn-Sn alloy

Magneto-structural instability in the ferromagnetic shape memory alloy of composition Ni$_2$Mn$_{1.4}$Sn$_{0.6}$ is investigated by transport and magnetic measurements. Large negative magnetoresistance is observed around the martensitic transition temperature (90-210 K). Both magnetization and magnetoresistance data indicate that upon the application of an external magnetic field at a constant temperature, the sample attains a field-induced arrested state which persists even when the field is withdrawn. We observe an intriguing behavior of the arrested state that it can remember the last highest field it has experienced. The field-induced structural transition plays the key role for the observed anomaly and the observed irreversibility can be accounted by the Landau-type free energy model for the first order phase transition.

cond-mat.str-el↗

arXiv2007

Studies of self-organized Nanostructures on InP(111) surfaces after low energy Ar+ ion irradiation

We report formation of self organized InP nano dots using 3 keV Ar+ ion sputtering, at $15^\circ$ incidence from surface normal, on InP(111) surface. Morphology and optical properties of the sputtered surface, as a function of sputtering time, have been investigated by Scanning Probe Microscopy and Raman Scattering techniques. Uniform patterns of nano dots are observed for different durations of sputtering. The sizes and the heights of these nano dots vary between 10 to 100 nm and 20 to 40 nm, respectively. With increasing of sputtering time, t, the size and height of these nano dots increases up to a certain sputtering time $t_c$. However beyond $t_c$, the dots break down into smaller nanostructures, and as a result, the size and height of these nanostructures decrease. The uniformity and regularity of these structures are also lost for sputtering beyond $t_c$. The crossover behavior is also observed in the rms surface roughness. Raman investigations of InP nano dots reveal optical phonon softening due to phonon confinement in the surface nano dots.

cond-mat.mtrl-sci↗

arXiv2007

Exchange bias with Fe substitution in LaMnO_3

The exchange bias (EB) in LaMn_{0.7}Fe_{0.3}O_3 is observed by the negative shift and training effect of the hysteresis loops, while the sample was cooled in external magnetic field. The analysis of cooling field dependence of EB gives the size of the ferromagnetic (FM) cluster ~ 25 Angstrom, where the magnetic anisotropy of FM cluster is found two order of magnitude higher than the FM bulk manganites. We propose that the nanoscale FM clusters are embedded in the glassy magnetic host with EB at the FM/glassy magnetic interface.

cond-mat.str-el↗

arXiv2007

Realization of biferroic properties in La0.6Sr0.4MnO3 0.7Pb(Mg0.33Nb0.67)O3 0.3(PbTiO3) epitaxial superlattices

A set of symmetric and asymmetric superlattices with ferromagnetic La0.6Sr0.4MnO3 and ferroelectric 0.7Pb(Mg0.33Nb0.67)O3 0.3(PbTiO3) as the constituting layers were fabricated on LaNiO3 coated (100) oriented LaAlO3 substrates using pulsed laser ablation. The crystallinity, magnetic and ferroelectric properties were studied for all the superlattices. All the superlattice structures exhibited a ferromagnetic behavior over a wide range of temperatures between 10K and 300K, whereas only the asymmetric superlattices exhibited a reasonably good ferroelectric behaviour. Strong influence of an applied magnetic field was observed on the ferroelectric properties of the asymmetric superlattices. Studies were conducted towards understanding the influence of conducting LSMO layers on the electrical responses of the heterostructures. The absence of ferroelectricity in the symmetric superlattice structures has been attributed to their high leakage characteristics. The effect of an applied magnetic field on the ferroelectric properties of the asymmetric superlattices indicated strong influence of the interfaces on the properties. The dominance of the interface on the dielectric response was confirmed by the observed Maxwell Wagner type dielectric relaxation in these heterostructures.

cond-mat.mtrl-sci↗

arXiv2007

The Red-Sequence Cluster Surveys

The Red-Sequence Cluster Surveys (RCS-1 and RCS-2) are large optical imaging surveys optimized to create well-characterized catalogs of clusters of galaxies up to a redshift of ~1. We describe our first cosmological analysis, using the self-calibration technique, of a cluster sample of ~1000 from the 90 sq. deg RCS-1, using optical richness as a mass proxy. We obtain values for the cosmological parameters $Ω_m$ and $σ_8$ that are in excellent agreement with the year-three WMAP results. Furthermore, the derived cluster richness-mass relationship is entirely consistent with those measured directly using dynamical mass measurements.

astro-ph↗

arXiv2006

Inhomogeneous phase separation and domain wall dynamics in the orthorhombically distorted La_{0.87}MnO_x

We observe the interesting magnetization and resistivity results in the orthorhombically distorted self doped manganite, La_{0.87}MnO_x. A significant irreversibility between zero-field-cooled and field-cooled resistivities as well as magnetization data is clearly observed up to a temperature which is well above the both paramagnetic to ferromagnetic (T_c) and metal to insulator transition (T_{MI}) temperatures. In the ferromagnetically ordered and metallic states the magnetotransport results suggest the influence of domain wall dynamics. Interesting scenario of memory effects through the domain wall dynamics is strikingly observed in the relaxation of resistivity in a limited temperature region. The characteristic features of magnetotransport behaviour are suggested duo to the co-existence of structurally driven inhomogeneous phase separation and domain wall dynamics.

cond-mat.str-el↗

arXiv2006

Magnetic field induced metastability in the self-doped manganites

We observe an interesting scenario of field induced metastabilities in the resistivity measurements. The resistivity exhibits a bifurcation of zero-field cooled and field-cooled (FC) temperature dependence below the paramagnetic to ferromagnetic transition ($T_c$). The clear evidence of {\it inverse} thermal hysteresis of resistivity is observed under FC condition in between 85 and 200 K, indicating the first order phase transition at $T_c$. The additional observation of minor hysteresis loops confirms the phase coexistence. The interesting features in the dynamics of resistivity is observed at 40 K, which is suggested due to the effect of domain wall dynamics.

cond-mat.str-el↗

arXiv2005

Surface and interface study of pulsed-laser-deposited off-stoichiometric NiMnSb thin films on Si(100) substrate

We report a detailed study of surface and interface properties of pulsed-laser deposited NiMnSb films on Si (100) substrate as a function of film thickness. As the thickness of films is reduced below 35 nm formation of a porous layer is observed. Porosity in this layer increases with decrease in NiMnSb film thickness. These morphological changes of the ultra thin films are reflected in the interesting transport and magnetic properties of these films. On the other hand, there are no influences of compositional in-homogeneity and surface/interface roughness on the magnetic and transport properties of the films.

cond-mat.mtrl-sci↗

arXiv2005

An X-ray Galaxy Cluster Survey for Investigations of Dark Energy

The amount and nature of dark energy (DE) can be tightly constrained by measuring the spatial correlation features and evolution of a sample of ~ 100,000 galaxy clusters over the redshift range 0<z < 1.5. Such an X-ray survey will discover all collapsed structures with mass above 3.5e14 solar masss at redshifts z<2 (i.e. the full range where such objects are expected) in the high Galactic latitude sky. Above this mass threshold the tight correlations between X-ray observables and mass allow direct interpretation of the data. We describe the constraints on Dark Energy that can be inferred from such a survey, using powerful self-calibration techniques to relate the X-ray observables (luminosity and temperature) to the underlying mass.

astro-ph↗

arXiv2005

Unusual transport properties of ferromagnetic Heusler alloy Co$_2$TiSn

We report results of magnetization, zero field resistivity and magnetoresistance measurements in ferromagnetic Heusler alloy Co$_2$TiSn. There is a striking change in the character of electron transport as the system undergoes the paramagnetic to ferromagnetic transition. In the paramagnetic state the nature of the electron transport is like that of a semiconductor and this changes abruptly to metallic behaviour at the onset of ferromagnetic ordering. Application of external magnetic field tends to suppress this semiconducting like transport leading to a negative magnetoresistance which reaches a peak in the vicinity of Curie temperature. Comparison is made with the similar unusual behaviour observed in other systems including UNiSn and manganites.

cond-mat.mtrl-sci↗

arXiv2005

Neutron inelastic scattering investigation of the magnetic excitations in Cu_2Te_2O_5X_2 (X=Br, Cl)

Neutron inelastic scattering investigations have been performed on the spin tetrahedral system Cu_2Te_2O_5X_2 (X = Cl, Br). We report the observation of magnetic excitations with a dispersive component in both compounds, associated with the 3D incommensurate magnetic order that develops below $T^{Cl}_{N}$=18.2 K and $T^{Br}_{N}$=11.4 K. The excitation in Cu_2Te_2O_5Cl_2 softens as the temperature approaches $T^{Cl}_{N}$, leaving diffuse quasi-elastic scattering above the transition temperature. In the bromide, the excitations are present well above $T^{Br}_{N}$, which might be attributed to the presence of a degree of low dimensional correlations above $T^{Br}_{N}$ in this compound.

cond-mat.other↗

arXiv2004

Investigation of the field-treated magnetic state in Gd5Ge4

At low temperatures, the intermetallic compound Gd$_5$Ge$_4$ shows a sharp field-induced transition into a ferromagnetic state around 25 kOe of applied field. The material remains in the field treated ferromagnetic state even when the magnetic field is removed. We have investigated the character of this field-treated state by magnetization and heat capacity measurements. The nature of the magnetization and the heat capacity are found to be different above and below a charactersitic temperature Tirr = 25 K.

cond-mat.str-el↗