Physics
http://hdl.handle.net/2262/86
PhysicsThu, 05 Mar 2015 20:18:52 GMT2015-03-05T20:18:52ZMagnetic and transport properties of epitaxial stepped Fe3O4(100) thin films
http://hdl.handle.net/2262/73491
Magnetic and transport properties of epitaxial stepped Fe3O4(100) thin films
SHVETS, IGOR
We investigate the magnetic and transport properties of epitaxial stepped Fe3O4 thin films grown with different thicknesses. Magnetization measurements suggest that the steps induce additional anisotropy, which has an easy axis perpendicular to steps and the hard axis along the steps. Separate local transport measurements, with nano-gap contacts along a single step and perpendicular to a single step, suggest the formation of a high density of anti-phase boundaries (APBs) at the step edges are responsible for the step induced anisotropy. Our local transport measurements also indicate that APBs distort the long range charge-ordering of magnetite
PUBLISHED
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/2262/734912014-01-01T00:00:00ZEvidence for spin glass state of NdCo1-xNixO 3 (x = 0.3- 0.5)
http://hdl.handle.net/2262/73490
Evidence for spin glass state of NdCo1-xNixO 3 (x = 0.3- 0.5)
SHVETS, IGOR
Low-temperature magnetic properties of single phase NdCo1−x Ni xO3(x = 0.3−0.5) have been studied using ac and dc magnetic susceptibility measurements. Nickel substituted samples have been found to exhibit a different magnetic state at low temperature as compared to pristine NdCoO3. The temperature dependent dc magnetization M (T) revealed the presence of a sharp cusp occurring at characteristic temperatures TP , for x = 0.3, 0.4, 0.5. Below TP , clear effect of magnetic field can be seen in M (T) curves and TP decreases with increasing magnetic field as well as Ni substitution content. The isothermal magnetization measurements at low temperatures shows small unsaturated hysteresis loop at lowest temperature (10 K). The ac susceptibility results show a clear frequency dependent feature. These results are analyzed to distinguish superparamagnetic and spin glass behavior by using Néel-Arrhennius, Vogel-Fulcher law, and power law fitting. This analysis ruled out the superparamagnet like state and suggests the presence of significant inter-cluster interactions, giving rise to spin-glass like cooperative freezing
PUBLISHED
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/2262/734902014-01-01T00:00:00ZI-V curves of Fe/MgO (001) single- and double-barrier tunnel junctions.
http://hdl.handle.net/2262/73408
I-V curves of Fe/MgO (001) single- and double-barrier tunnel junctions.
RUNGGER, IVAN; SANVITO, STEFANO
In this work, we calculate with
ab initio
methods the current-voltage characteristics for ideal single- and
double-barrier Fe/MgO
001
magnetic tunnel junctions. The current is calculated in the phase-coherent limit
by using the recently developed
SMEAGOL
code, combining the nonequilibrium Green function formalism with
density-functional theory. In general we find that double-barrier junctions display a larger magnetoresistance,
which decays with bias at a slower pace than their single-barrier counterparts. This is explained in terms of
enhanced spin filtering from the middle Fe layer sandwiched in between the two MgO barriers. In addition, for
double-barrier tunnel junctions, we find a well defined peak in the magnetoresistance at a voltage of
V
=0.1 V. This is the signature of resonant tunneling across a majority quantum well state. Our findings are
discussed in relation to recent experiment
PUBLISHED
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/2262/734082008-01-01T00:00:00ZAlgorithm for the construction of self-energies for electronic transport calculations based on singularity elimination and singular value decomposition
http://hdl.handle.net/2262/73407
Algorithm for the construction of self-energies for electronic transport calculations based on singularity elimination and singular value decomposition
RUNGGER, IVAN; SANVITO, STEFANO
We present a complete prescription for the numerical calculation of surface Green’s functions and self-
energies of semi-infinite quasi-one-dimensional systems. Our work extends previous results generating a robust
algorithm to be used in conjunction with
ab initio
electronic structure methods. We perform a detailed error
analysis of the scheme and find that the highest accuracy is found if no inversion of the usually ill conditioned
hopping matrix is involved. Even in this case however a transformation of the hopping matrix that decreases
its condition number is needed in order to limit the size of the imaginary part of the wave vectors. This is done
in two different ways: either by applying a singular value decomposition and setting a lowest bound for the
smallest singular value or by adding a random matrix of small amplitude. By using the first scheme the size of
the Hamiltonian matrix is reduced, making the computation considerably faster for large systems. For most
energies the method gives high accuracy, however in the presence of surface states the error diverges due to the
singularity in the self-energy. A surface state is found at a particular energy if the set of solution eigenvectors
of the infinite system is linearly dependent. This is then used as a criterion to detect surface states, and the error
is limited by adding a small imaginary part to the energy
PUBLISHED
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/2262/734072008-01-01T00:00:00Z