Soumik Mukhopadhyay

Inversion of Magnetoresistance in Magnetic Tunnel Junctions: Effect of Pinhole Nanocontacts

Inverse magnetoresistance has been observed in magnetic tunnel junctions with pinhole nanocontacts over a broad temperature range. The tunnel magnetoresistance undergoes a change of sign at higher bias and temperature. This phenomenon is attributed to the competition between the spin conserved ballistic transport through the pinhole contact where the transmission probability is close to unity and spin polarized tunneling across the insulating spacer with weak transmittivity.

Tracking random walk of individual domain walls in cylindrical nanomagnets with resistance noise.

The stochasticity of domain-wall (DW) motion in magnetic nanowires has been probed by measuring slow fluctuations, or noise, in electrical resistance at small magnetic fields. By controlled injection of DWs into isolated cylindrical nanowires of nickel, we have been able to track the motion of the DWs between the electrical leads by discrete steps in the resistance. Closer inspection of the time dependence of noise reveals a diffusive random walk of the DWs with a universal kinetic exponent. Our experiments outline a method with which electrical resistance is able to detect the kinetic state of the DWs inside the nanowires, which can be useful in DW-based memory designs.

Giant enhancement of room-temperature magnetoresistance in La0.67Sr0.33MnO3∕Nd0.67Sr0.33MnO3 multilayers

The metal-insulator transition temperature in colossal magnetoresistance manganites has been altered and brought close to the room temperature by preparing La0.67Sr0.33MnO3 (LSMO)∕Nd0.67Sr0.33MnO3 (NSMO) multilayers with ultrathin individual layers of LSMO and NSMO. The LSMO/NSMO multilayers with ultrathin individual layers of thickness of about 10 A exhibits 150% magnetoresistance at 270 K whereas LSMO/NSMO multilayers with moderate individual layer thickness of about 40 A each exhibits a mere 15% magnetoresistance at the same temperature. We have shown that the reduction in thickness of the individual layers leads to increased spin fluctuation which results in the enhancement of magnetoresistance.

Low-temperature magnetotransport properties in granular ferromagnetic manganites

The magnetotransport properties at low temperature in La0.67Sr0.33MnO3 nanoparticles, La0.67Sr0.33MnO3/Al2O3 nanocomposites and ferromagnetic-manganite thick films have been studied. All the samples exhibit low-temperature upturn in resistivity and enhanced low-field magnetoresistance of varying magnitude below the resistivity minima. The voltage dependence of conductance at low temperature for all the samples can be described by a universal scaling function G=G0 exp (V/V0)1/2, where the scaling parameter V0 is sensitive to the magnetic field. The observed magnetotransport properties are explained in a unified treatment incorporating both inter-grain tunneling conduction and thermal activation over the inter-grain potential barrier lowered by the electric field.

Colossal enhancement of magnetoresistance in La0.67Sr0.33MnO3/Pr0.67Ca0.33MnO3 multilayers : Reproducing the phase separation scenario

Colossal enhancement of magnetoresistance has been achieved over a broad temperature range which extends up to the room temperature, in ferromagnetic metal-charge–ordered insulator manganite multilayers. The artificially created phase coexistence in the multlayers reproduces the characteristic signatures of metastability in the magnetotransport properties commonly observed in electronically phase-separated manganites.

Low temperature conductivity in ferromagnetic manganite thin films : quantum corrections and inter-granular transport

The interplay between inter-granular transport and quantum corrections to low temperature transport properties of La(0.67)Sr(0.33)MnO(3) (LSMO) and Nd(0.67)Sr(0.33)MnO(3) (NSMO) thin films has been discussed. All the samples exhibit characteristics of renormalized electron-electron interaction in two dimensions. The contrasting response of the low temperature transport to magnetic field in the LSMO and NSMO films is attributed to the strikingly different magnetic field sensitivity of the inter-granular transport.

Anomalous bias dependence of tunnel magnetoresistance in a magnetic tunnel junction

We have fabricated a spin-polarized tunneling device based on half-metallic manganites incorporating Ba2LaNbO6 as an insulating barrier. An anomalous bias dependence of tunnel magnetoresistance (TMR) has been observed, the first of its kind in a symmetric electrode tunnel junction with a single insulating barrier. The bias dependence of TMR shows an extremely sharp zero-bias anomaly, which can be considered as a demonstration of the drastic density of states variation around the Fermi level of the half-metal. This serves as strong evidence for the existence of minority-spin tunneling states at the half-metal–insulator interface.

Colossal enhancement of magnetoresistance in La0.67Sr0.33MnO3 thin films: possible evidence of electronic phase separation

A gigantic three orders of magnitude change of resistivity in La(0.67)Sr(0.33)MnO(3) (LSMO) thin film, on application of magnetic field, has been observed. The transport and magnetic properties are characteristic of electronic phase separation between ferromagnetic metallic and antiferromagnetic charge-ordered insulating regions, unusual for a canonical double exchange system such as LSMO.

The role of triplet states in the emission mechanism of polymer light-emitting diodes

The blue emission of polyfluorene (PF)-based light-emitting diodes (LEDs) is known to degrade due to a low-energy green emission, which hitherto has been attributed to oxidative defects. By studying the electroluminescence (EL) from ethyl-hexyl substituted PF LEDs in the presence of oxygen and in an inert atmosphere, and by using trace quantities of paramagnetic impurities (PM) in the polymer, we show that the triplet states play a major role in the low-energy emission mechanism. Our time-dependent many-body studies show a large cross-section for the triplet formation in the EL process in the presence of PM, primarily due to electron-hole recombination processes.

Field-tunable stochasticity in the magnetization reversal of a cylindrical nanomagnet

The nature of magnetization reversal in an isolated cylindrical nanomagnet has been studied employing time-resolved magnetoresistance measurement. We find that the reversal mode is highly stochastic, occurring either by multimode or single-step switching. Intriguingly, the stochasticity was found to depend on the alignment of the driving magnetic field to the long axis of the nanowires, where predominantly multimode switching gives way to single-step switching behavior as the field direction is rotated from parallel to transverse with respect to the nanowire axis.