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Application of Bismuth Ferrite, BiFeO3, in Heterostructures

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Inge Leer

on 6 November 2014

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Transcript of Application of Bismuth Ferrite, BiFeO3, in Heterostructures

Application of Bismuth Ferrite, BiFeO3, in Heterostructures
for HEMTs and High-Tc Materials

A. N. Kalinkin, V. M. Skorikov, and A. Ya. Vasil’ev
2014
http://www.christophschuette.com/physics/skyrmions.php
nice website where explaning stuf
about skyrmions
random introduction:

Microwave electonics, used in microwave transistors of transmitter modules in cellular phones, active phased array radars ed.
goes from GaAs-based heterostructures
GaN: Cree Inc. (USA); 0.5kW power with working frequency of 1.4GHz
Oxides (field-effect high electron mobility transistor) can give 0.3-10THz frequency to be used in Real-time subcloud monitoring of Earth' surface from planes and satellites.
2013: first HEMTs and chips based on LAO/STO and STO/GTO, non superconducting \7,8.
Oxide review \9
afkortingen:
HS: heterostructures
APAR: active phased array radar
HEMT: high electron mobility transistor
CM: combined modules
MIC: monolithic integrated circuits
LCS: liquid cooling system
PLD: pulsed laser deposition
MBE: molecular beam epitaxy
FE: ferroelectric
DW: domain wall

at the moment problem with monolithic integrated circuits in airborne APARs is necessity of active cooling: now there is a liquid cooling system which is most of the weight of the radar.

Detecting range: D~E^0.25. (E is power emmitted by the radar)
increas in energy release: Oxide electronics and chalcogenide heterojuntions with a superconducting channel, operate at liquid-nitrogen or higher temp. could make HEMTs.
without the cooling problem, the dissipation-free channel can get an increase electron mobility and considerable raise the working frequency.
oxide HS: 2D ES:
n_2D~ 10^15 or 10^16 cm-2
--> meaning power level of order of 1kW (in superconducting mode)
In combined modules this can be
1MW integrated APAR power
Nonsuperconducting Heterostructures
BiFeO3 (BFO, bismunth ferrite)
BFO, single crystals first prepared by melt growth in 1965 \11

in 2003 films were found to possess giant polarization: P~150uC/cm2\12
now subject or electronic and spintronic applications.
multiferroic: Neel (643K) and Curie (1103K) temperature high
and small ferromagnetic magnetic moment
STO/BFO was predicted to form a 2DEG (1nm thick) between BO and TO \15.
Electrons pass from TO to BO to prevent polar catastrophe. In the para-electric phase, there are symmetric peaks on both junctions=2DEG formation.
An electric field (
gate
) can destroy these symmetric peaks : a ferroelectric phase kept a peak at the right side (2DEG in BFO) --> this gate/polarisation ensures additional electron injection into the 2DEG.
Extra electron injection due to oxygen vacancies in BFO was not taken into account, so the n was maybe overestimated (n_2D=10^14cm-2 \15)
Experiment with PLD did not show 2DEG \16
Other experiment with MBEdid show 2DEG, by transition to tetragonal BFO, with reduced lattice parameter (3.9A) and increased polarization \17
table1 for promising bismuth ferrite-based HS.
by bandstructure calculations in the local-density-functional approach
film grown by PLD,
conduction band offset (photoelectron spectroscopy): dE=0.5 - 0.6 eV (estimated from the bandgap difference) \16
film (4nm) grown with MBE
another example: Perovskite-wurtzite:
PbTiO3(111)/GaN(0001) and BaTiO3(111)/GaN(0001)
lots of info about the latice parameters, energy gaps etc.
bismuth ferrite, antiferromagnetic semiconductor multiferroic with bandgap Eg-2.7eV has rather high intrinsic conductivity and exhibits a diode effect due to oxygen vacancies (electron donors), --> used as ferroelectric resistive memory (FeRRAM) \22
BFO shows diode effect, as does Gd-dopeded BFO (BFGO) or Nb-doped
STO
(STON) \23

all the components of table 1 are told about.
expect high 2DEG for antiparallel orientation of polarization vectors
better electrical prameters
deep well at interface --> high 2DEG density, high HEMT power
TiO2/BiFeO3: good for photovoltaic cells, solar energy converters.\28
at domain walls of BTO2 a DEG found
2DEG found at domain walls (reasonalble potential well and high 2DEG due to small domain wall width) means we can have heterostructure devices of one compound:
"nanodomain" electronics to have chips in the bulk of one single-crystal film without multilayer deposition.

or BTO as ferroelectric material in HS's for HEMTs\25.
superconducting Heterostructures
important factor in fabrication of HS (for reliability of transistors): the compatibility (coherence) between the lattices of their components.
For targeted synthesis of high-Tc materials a mechanism has been proposed in terms of spontaneous skyrmion lattice in 2DEG layers.
Skyrmions
Spin skyrmion lattices first predicted for magtic materials \55. but recently also local states found

eg in MnSi (at T=26K in B=0.164T) \56
Fe_{1-x}Co_{x}Si (at x=0.5, T=25) \57
FeGe
monatomic Fe film on Ir(111) (T=11K) \58 (spontainious lattice of skyrmion size 1nm)
Skyrmions in LAO/STO
TO-LO is the interface with electron doping and 2DEG.

charge transfer is needed to prevent polar catastrophe \3.
(this 2DEG was found to have superconducting and ferromagnetic properties, 10uc)
We assume that the
conduction band offset
en the
charge transfer
between a (TiO2)0 layer and the neighboring (LaO)+ layer lead to the formation of a
2DEG region containing a spontaneous skyrmion lattice
. This lattice is
formed in the ferromagnetic 2DEG by antiferromagnetically ordered moments of the Ti
that are situated several nanometers away in the neighboring TiO2 layer.
effective magnetic field of Ti ions resembles a checkerboard: its direction varies in checkerboard order.
therefore
neighboring skyrmions in 2DEG differ in chirality and attract each other, see fig6.
contrasting normal skyrmions (formed in uniform Bfield):
in contrast to vortices in hydrodynamics (where higher velocity gradients leads to attraction) spin vortices identical in chirality repulse each other \59 (in uniform B field)
absence of inversion symmetry and presence of spin-orbit interaction:
there is Dzyaloshinskii-Moriya and Rashba interactions, which stabilize the skyrmion lattice \55

The Coulombic barrier leads to a stable state of a skyrmion pair and then to the formation of an entire skyrmion lattice.

--> in some sense resembles the formation of an Abrikosov vortex lattice in a magnetic field, but on nanometer scale
what are:
- Dzyaloshinskii-Moriya interactions
- what is a skyrmion pair
- what is Abrikosov vortex

Rashba interactions are momentum-dependent splitting of spin bands in two-dimensional condensed matter systems
The antisymmetric exchange is a contribution to the total magnetic exchange interaction between two neigboring magnetic spins, \mathbf{S}_i and \mathbf{S}_j.
This term is also called Dzyaloshinskii-Moriya interaction. In magnetically ordered systems, it favors a spin canting of otherwise (anti)parallel aligned magnetic moments and thus, e.g., is a source of weak ferromagnetic behavior in an antiferromagnet.
In superconductivity, an Abrikosov vortex is a vortex of supercurrent in a type-II superconductor.
The supercurrent circulates around the normal (i.e. non-superconducting) core of the vortex.

also called fluxon
Collective vibrations of the skyrmion lattice (quasi-magnons) are initiated by phonons (thermal vibrations of the Ti ions on lattice sites)

"Excess" electrons, which do not take part in the formation of skyrmion lattice, fom "Cooper" pairs by virtue of electron-magnon interaction.
This Quasi-magnon modes are characteristi frequencies of the skyrmion lattice, and the mechanism of electron attraction is similar to the Fr"ohlich mechanism for phonons.

Even thought the two (crystal and skyrmion) lattices have identical vibrational frequencies, the magnon energy exceeds the phonon energy owing to the larger exchange interaction constatn J. Also possible is additional electron attraction due to electron localization at skyr,mions of opposite chiralities.
Electoron localizationthen has a toplogical nature and is similar to hole localization at a skyrmion in a nonlinear sigma model \60.


Lowering T:
- reduction in thermal fluctuations leads to Berezinskii-Kosterlitz-Thouless transition.
- then phase fluctuations of the order parameter sharply decrease at Tc
- skyrmion lattice begins to ensure cohrerence of states of all Cooper pairs: a boson condenstate is formed and superconductivity develops.
Berezinskii-Kosterlitz-Thouless transition:

is a phase transition in the 2D XY model. It is a transition from bound vortex-antivortex pairs at low temperatures to unpaired vortices and anti-vortices at some critical temperature.
STOPED at P. 1262 top right. -5/11/2014

vragen: waarom is het nuttig te weten of er een skyrmion lattice is?
Hoe relateerd zo'n skyrmion aan SC, of magnetism?

hi
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