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Soumen Mandal

Publications and source records attributed to Soumen Mandal.

41 records · Page 3Linked to original sources
arXiv2009

Magnetic interactions and electron transport in hole-doped manganite-superconducting cuprate heterostructures

This thesis presents a systematic study of magnetotransport and magnetic ordering in manganite-high T$_c$ cuprate spin valve structures. YBa$_{2}$Cu$_{3}$O$_{7}$ - La$_{2 / 3}$Sr$_{1 / 3}$MnO$_{3}$ heterostructures of (110) orientation are grown to allow direct injection of spin polarized holes from the La$_{2 / 3}$Sr$_{1 / 3}$MnO$_{3}$ (LSMO) into the CuO$_2$ superconducting planes of the YBa$_{2}$Cu$_{3}$O$_{7}$ (YBCO). Galvanomagnetic studies on the LSMO-YBCO-LSMO trilayers reveal unusually high AMR ($\sim$72000%) on rotating the field in the plane of the heterostructure whose magnetic ground state is antiferromagnetic (AF). The coupling energy J$_1$ of the AF state in these trilayers is much higher as compared to energy of (001) oriented hybrids. First the preparation and measurement of magnetic and galvanomagnetic properties of (110) and (001) oriented La$_{2 / 3}$Sr$_{1 / 3}$MnO$_{3}$ films are described. The magnetization vector ($\vec{M}$) of the (001) and (110) type films is pinned along the (110) and (001) directions respectively at low fields. A magnetization orientation phase transition (MRPT) which manifests itself as a discontinuity and hysteresis in $R(ψ)$ where $ψ$ is the angle between $\vec{H}$ and the easy axis for the $\vec{H}$ below a critical value $\vec{H}^*$ has been established. Further, the relevance of pair-breaking by exchange and dipolar fields, and by injected spins in a low carrier density cuprate Y$_{1-x}$Pr$_x$Ba$_2$Cu$_3$O$_7$ sandwiched between two ferromagnetic LSMO layers is examined. At low external field ($H_{ext}$), the system shows a giant magnetoresistance(MR), which diverges deep in the superconducting state.

cond-mat.supr-con↗
arXiv2008

Magnetization depinning transition, anisotropic magnetoresistance and inplane anisotropy in two polytypes of La$_{2 / 3}$Sr$_{1 / 3}$MnO$_{3}$ epitaxial films

The isothermal magnetoresistance [R($θ$)] of [001] and [110] epitaxial films of La$_{2 / 3}$Sr$_{1 / 3}$MnO$_{3}$ measured as a function of the angle $θ$ between current ($\vec{I}$) and magnetic field ($\vec{H}$), both in the plane of the film, is measured at several temperatures between 10 and 300K. The magnetic easy axis of these polytypes is intimately related to the orientation of Mn - O - Mn bonds with respect to the crystallographic axis on the plane of the substrate and energy equivalence of some of these axes. The magnetization vector ($\vec{M}$) of the [001] and [110] type films is pinned along the [110] and [001] directions respectively at low fields. A magnetization orientation phase transition (MRPT) which manifests itself as a discontinuity and hysteresis in $R(ψ)$ where $ψ$ is the angle between $\vec{H}$ and the easy axis for the $\vec{H}$ below a critical value $\vec{H}^*$ has been established. The boundary of the pinned and depinned phase on the H-T plane has been established. The highly robust pinning of magnetization seen in [110] films is related to their uniquely defined easy axis. The isothermal resistance R$_\bot$ and R$_\|$ for $\vec{I} \bot \vec{H}$ and $\vec{I} \| \vec{H}$, respectively for both polytypes follows the inequality R$_\bot >$ R$_\|$ for all ranges of fields ($0 \leq H \leq 3500Oe$) and temperatures (10K - 300K). A full fledged analysis of the rotational magnetoresistance is carried out in the framework of Döring theory for MR in single crystal samples. Strong deviations from the predicted angular dependence are seen in the irreversible regime of magnetization.

cond-mat.mtrl-sci↗
arXiv2008

Diverging Giant Magnetoresistance in the Limit of Infinitely Conducting Spacer

The relevance of pair-breaking by exchange and dipolar fields, and by injected spins in a low carrier density cuprate Y$_{1-x}$Pr$_x$Ba$_2$Cu$_3$O$_7$ sandwiched between two ferromagnetic La$_{2 / 3}$Sr$_{1 / 3}$MnO$_{3}$ layers is examined. At low external field ($H_{ext}$), the system shows a giant magnetoresistance(MR), which diverges deep in the superconducting state. We establish a distinct dipolar contribution to MR near the switching field(H$_c$) of the magnetic layers. At H$_{ext} \gg$ H$_c$, a large positive MR, resulting primarily from the motion of Josephson vortices and pair breaking by the in-plane field, is seen.

cond-mat.supr-con↗
arXiv2007

Correlation between incoherent phase fluctuations and disorder in Y$_{1-x}$Pr$_x$Ba$_2$Cu$_3$O$_{7-δ}$ epitaxial films from Nernst effect measurements

Measurements of Nernst effect, resistivity and Hall angle on epitaxial films of Y$_{1-x}$Pr$_x$Ba$_2$Cu$_3$O$_{7-δ}$(Pr-YBCO, 0$\leq x\leq$0.4) are reported over a broad range of temperature and magnetic field. While the Hall and resistivity data suggest a broad pseudogap regime in accordance with earlier results, these first measurements of the Nernst effect on Pr-YBCO show a large signal above the superconducting transition temperature(T$_c$). This effect is attributed to vortex-like excitations in the phase incoherent condensate existing above T$_c$. A correlation between disorder and the width of the phase fluctuation regime has been established for the YBCO family of cuprates, which suggests a T$_c\approx$110K for disorder-free YBa$_2$Cu$_3$O$_{7-δ}$.

cond-mat.supr-con↗