School of Physics
http://hdl.handle.net/2262/14
School of Physics2015-03-04T16:46:57ZI-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
2008-01-01T00:00:00ZAlgorithm 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
2008-01-01T00:00:00ZAb initio study on the magnetostructural properties of MnAs
http://hdl.handle.net/2262/73406
Ab initio study on the magnetostructural properties of MnAs
RUNGGER, IVAN; SANVITO, STEFANO
The magnetic and structural properties of MnAs are studied with
ab initio
methods and by mapping total
energies onto a Heisenberg model. The stability of the different phases is found to depend mainly on the
volume and on the amount of magnetic order, confirming previous experimental findings and phenomenologi-
cal models. It is generally found that for large lattice constants the ferromagnetic state is favored, whereas for
small lattice constants different antiferromagnetic states can be stabilized. In the ferromagnetic state the
structure with minimal energy is always hexagonal, whereas it becomes orthorhombically distorted if there is
an antiferromagnetic alignment of the magnetic moments in the hexagonal plane. For the paramagnetic state
the stable cell is found to be orthorhombic up to a critical lattice constant of about 3.7 Å, above which it
remains hexagonal. This leads to the second-order structural phase transition between paramagnetic states at
about 400 K, where the lattice parameter increases above this critical value with rising temperature due to the
thermal expansion. We also evaluate the magnetic susceptibility as a function of temperature, from which a
semiquantitative description of the MnAs phase diagram emerges
2006-01-01T00:00:00ZLattice distortion effects on the magneto-structural phase transition of MnAs.
http://hdl.handle.net/2262/73405
Lattice distortion effects on the magneto-structural phase transition of MnAs.
RUNGGER, IVAN; SANVITO, STEFANO
We present a systematic experimental and theoretical study of the first-order phase transition of epitaxially grown MnAs thin films under biaxial tensile stress. Our results give direct information on the dependence of the phase-transition temperature of MnAs films on the lattice parameters. We demonstrate that an increase of the lattice constant in the hexagonal plane raises the phase-transition temperature (Tp), while an increase of the perpendicular lattice constant lowers Tp. The results of calculations based on density functional theory are in good agreement with the experimental ones. Our findings open exciting prospects for magneto-mechanical devices, where the critical temperature for ferromagnetism can be engineered by external stress.
PUBLISHED; PMID: 16196819
2005-01-01T00:00:00Z