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A. Biswas

Publications and source records attributed to A. Biswas.

32 records · Page 2Linked to original sources
arXiv2017

Cryogenic magnetic coil and superconducting magnetic shield for neutron electric dipole moment searches

A magnetic coil operated at cryogenic temperatures is used to produce spatial, relative field gradients below 6 ppm/cm, stable for several hours. The apparatus is a prototype of the magnetic components for a neutron electric dipole moment (nEDM) search, which will take place at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory using ultra-cold neutrons (UCN). That search requires a uniform magnetic field to mitigate systematic effects and obtain long polarization lifetimes for neutron spin precession measurements. This paper details upgrades to a previously described apparatus, particularly the introduction of super-conducting magnetic shielding and the associated cryogenic apparatus. The magnetic gradients observed are sufficiently low for the nEDM search at SNS.

physics.ins-det↗
arXiv2015

Composition dependence of electronic, magnetic, transport and morphological properties of mixed valence manganite thin films

We present a comparison of the in-plane length scale over which charge and magnetism are correlated in (La0.4Pr0.6)1-xCaxMnO3 films with x = 0.33 and 0.375, across the metal to insulator transition (MIT) temperature. We combine electrical transport (resistance) measurements, x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and specular/off-specular x-ray resonant magnetic scattering (XRMS) measurements as a function of temperature to elucidate relationships between electronic, magnetic and morphological structure of the thin films. Using off-specular XRMS we obtained the charge-charge and charge-magnetic correlation length of these LPCMO films near the MIT. The charge-magnetic correlation length (~ 12000 Å) for x = 0.33 was much larger (~4 times) than the charge-charge correlation length (~ 3200 Å) at 20 K. Whereas for x = 0.375 the charge-magnetic correlation length (~ 7500 Å) was smaller than the charge-charge correlation length (~ 9000 Å).

cond-mat.str-el↗
arXiv2014

Manipulation of the magnetic order parameter and the metal-insulator-transition of a manganite thin film with applied elastic stress

We measured the temperature dependence of the saturation magnetization (Ms) of a (La1-xPrx)1-yCayMnO3 (x ~ 0.60, y ~ 0.33) film as a function of applied bending stress. Stress producing a compressive strain of -0.01% along the magnetic easy axis increased the Curie temperature by ~6 K and the metal-insulator-transition by ~4 K. Regardless of whether or not stress is applied to the film, magnetic ordering occurs at temperatures significantly higher than the metal-insulator-transition temperature. The magnetization of the sample at the temperature of the metal-insulator-transition is approximately the site percolation threshold for a two-dimensional spin lattice.

cond-mat.str-el↗
arXiv2012

Images of metallic and insulating domains on the surface of a (La0.4Pr0.6)0.67Ca0.33MnO3 film

Using conductive atomic force microscopy, we observed non-uniform distributions of metallic and insulating domains on the surface of a single crystalline (La0.4Pr0.6)0.67Ca0.33MnO3 film grown on a (110) NdGaO3 substrate. The electronic properties of the surface exhibit thermal hysteresis. The hysteresis is similar to that of the transport and magnetism of the film bulk.

cond-mat.mtrl-sci↗
arXiv2012

Measurement of the coupling between applied stress and magnetism in a manganite thin film

We measured the magnetization depth profile of a (La1-xPrx)1-yCayMnO3 (x = 0.60\pm0.04, y = 0.20\pm0.03) film as a function of applied bending stress using polarized neutron reflectometry. From these measurements we obtained a coupling coefficient relating strain to the depth dependent magnetization. We found application of compressive (tensile) bending stress along the magnetic easy axis increases (decreases) the magnetization of the film.

cond-mat.mtrl-sci↗
arXiv2011

Magnetic non-uniformity and thermal hysteresis of magnetism in a manganite thin film

We measured the chemical and magnetic depth profiles of a single crystalline (La$_{1-x}$Pr$_x$)$_{1-y}$Ca$_y$MnO$_{3-δ}$ (x = 0.52\pm0.05, y = 0.23\pm0.04, δ = 0.14\pm0.10) film grown on a NdGaO3 substrate using x-ray reflectometry, electron microscopy, electron energy-loss spectroscopy and polarized neutron reflectometry. Our data indicate that the film exhibits coexistence of different magnetic phases as a function of depth. The magnetic depth profile is correlated with a variation of chemical composition with depth. The thermal hysteresis of ferromagnetic order in the film suggests a first order ferromagnetic transition at low temperatures.

cond-mat.mtrl-sci↗
arXiv2010

Hardy-Sobolev Type Equations for p-Laplacian, 1 < p < 2, in Bounded Domain

We study quasilinear degenerate singular elliptic equation of type -Delta_p u = \frac{u^{p^*(s)-1}}{|y|^t}$ in a smooth bounded domain Ωin R^n=R^k \times R^{N-k}$, x=(y,z) in R^k \times R^{N-k}, 2 \leq k<N and N \geq 3, 1<p<2, 0\leq s\leq p, 0\leq t\leq s, p^*(s)=\frac{p(n-s)}{n-p}. We study existence of solution for t<s, non-existence in a star-shaped domain for t=s and s<k(\frac{p-1}{p}). We also show that solution is in C^{1,\al}(Ω) for some 0<\al<1 provided t<\frac{k}{N}(\frac{p-1}{p}). The regularity of solution can be improved to the class W^{2,p}(Ω) when t<k(\frac{p-1}{p}). We also study some properties of the singular sets in a cylindrically symmetric domain using the method of symmetry.

math.AP↗
arXiv2009

Tunneling magnetoresistance in (La,Pr,Ca)MnO3 nanobridges

The manganite (La,Pr,Ca)MnO3 is well known for its micrometer scale phase separation into coexisting ferromagnetic metallic and antiferromagnetic insulating (AFI) regions. Fabricating bridges with widths smaller than the phase separation length scale has allowed us to probe the magnetic properties of individual phase separated regions. We observe tunneling magnetoresistance across naturally occurring AFI tunnel barriers separating adjacent ferromagnetic regions spanning the width of the bridges. Further, near the Curie temperature, a magnetic field induced metal-to-insulator transition among a discrete number of regions within the narrow bridges gives rise to abrupt and colossal low-field magnetoresistance steps at well defined switching fields.

cond-mat.mtrl-sci↗
arXiv2009

Intrinsic tunneling in phase separated manganites

We present evidence of direct electron tunneling across intrinsic insulating regions in sub-micrometer wide bridges of the phase separated ferromagnet (La,Pr,Ca)MnO$_3$. Upon cooling below the Curie temperature, a predominantly ferromagnetic supercooled state persists where tunneling across the intrinsic tunnel barriers (ITBs) results in metastable, temperature-independent, high-resistance plateaus over a large range of temperatures. Upon application of a magnetic field, our data reveal that the ITBs are extinguished resulting in sharp, colossal, low-field resistance drops. Our results compare well to theoretical predictions of magnetic domain walls coinciding with the intrinsic insulating phase.

cond-mat.str-el↗
arXiv2004

Origin of the anomalous low temperature upturn in resistivity in the electron-doped cuprates

The temperature, doping and field dependences of the magnetoresistance (MR) in Pr_(2-x)Ce_(x)CuO_(4-delta) films are reported. We distinguish between orbital MR, found when the magnetic field is applied perpendicular to the ab planes, and the nearly isotropic spin MR. The latter, the major MR effect in the superconducting samples, appears in the region of the doping-temperature phase diagram where d(rho)/dT<0, or an upturn in the resistivity appears. We conclude that the upturn originates from spin scattering processes.

cond-mat.supr-con↗
arXiv2003

First-order nature of the ferromagnetic phase transition in (La-Ca)MnO_3 near optimal doping

Neutron scattering has been used to study the nature of the ferromagnetic transition in single crystals of La_0.7Ca_0.3MnO_3 and La_0.8Ca_0.2MnO_3, and polycrystalline samples of La_0.67Ca_0.33MnO_3 and La_5/8Ca_3/8MnO_3 where the naturally occurring O-16 can be replaced with the O-18 isotope. Small angle neutron scattering on the x=0.3 single crystal reveals a discontinuous change in the scattering at the Curie temperature for wave vectors below ~0.065 A^-1. Strong relaxation effects are observed for this domain scattering, for the magnetic order parameter, and for the quasielastic scattering, demonstrating that the transition is not continuous in nature. There is a large oxygen isotope effect observed for the T_C in the polycrystalline samples. For the optimally doped x=3/8 sample we observed T_C(O-16)=266.5 K and T_C(O-18)=261.5 K at 90% O-18 substitution. The temperature dependence of the spin-wave stiffness is found to be identical for the two samples despite changes in T_C. Hence, T_C is not solely determined by the magnetic subsystem, but instead the ferromagnetic phase is truncated by the formation of polarons which cause an abrupt transition to the paramagnetic, insulating state. Application of uniaxial stress in the x=0.3 single crystal sharply enhances the polaron scattering at room temperature. Measurements of the phonon density-of-states show only modest differences above and below T_C and between the two different isotopic samples.

cond-mat.str-el↗
arXiv2003

Substrate-induced strain effects on Pr_{0.6}Ca_{0.4}MnO_{3} films

We report the characterization of the crystal structure, low-temperature charge and orbital ordering, transport, and magnetization of Pr_{0.6}Ca_{0.4}MnO_{3} films grown on LaAlO_{3}, NdGaO_{3}, and SrTiO_{3} substrates, which provide compressive (LaAlO_{3}) and tensile (NdGaO_{3} and SrTiO_{3}) strain. The films are observed to exhibit different crystallographic symmetries than the bulk material, and the low-temperature ordering is found to be more robust under compressive-- as opposed to tensile-- strain. In fact, bulk-like charge and orbital ordering is not observed in the film grown on NdGaO_{3}, which is the substrate that provides the least amount of nominal and measured, but tensile, strain. This result suggests the importance of the role played by the Mn--O--Mn bond angles in the formation of charge and orbital ordering at low temperatures. Finally, in the film grown on LaAlO_{3}, a connection between the lattice distortion associated with orbital ordering and the onset of antiferromagnetism is reported.

cond-mat.str-el↗
arXiv2001

Observation of large anisotropy in c-axis oriented thin films of MgB$_{2}$ prepared by an in situ pulsed laser deposition process

A large anisotropy in the upper critical field (H$_{C2}$) is observed for MgB$_{2}$ films grown in situ by a pulsed laser deposition process involving growth and annealing of Mg and B multilayers. Measurements of resistivity as a function of temperature and magnetic field yield the estimated zero temperature values of H$_{C2}$ in the range 100 to 130 T and about 10 T, for the field in ab plane and along c axis, respectively, depending on the criterion chosen for the transition temperature. The corresponding anisotropy parameter is thus in the range 9 to 13. The estimated coherence length in ab plane is about 50 Å, whereas that in the c direction is much smaller (\approx 5 Å). No significant magnetoresistance is observed in the normal state.

cond-mat.supr-con↗