19 research outputs found
Unraveling the nature of carrier mediated ferromagnetism in diluted magnetic semiconductors
After more than a decade of intensive research in the field of diluted
magnetic semiconductors (DMS), the nature and origin of ferromagnetism,
especially in III-V compounds is still controversial. Many questions and open
issues are under intensive debates. Why after so many years of investigations
Mn doped GaAs remains the candidate with the highest Curie temperature among
the broad family of III-V materials doped with transition metal (TM) impurities
? How can one understand that these temperatures are almost two orders of
magnitude larger than that of hole doped (Zn,Mn)Te or (Cd,Mn)Se? Is there any
intrinsic limitation or is there any hope to reach in the dilute regime room
temperature ferromagnetism? How can one explain the proximity of (Ga,Mn)As to
the metal-insulator transition and the change from
Ruderman-Kittel-Kasuya-Yosida (RKKY) couplings in II-VI compounds to double
exchange type in (Ga,Mn)N? In spite of the great success of density functional
theory based studies to provide accurately the critical temperatures in various
compounds, till very lately a theory that provides a coherent picture and
understanding of the underlying physics was still missing. Recently, within a
minimal model it has been possible to show that among the physical parameters,
the key one is the position of the TM acceptor level. By tuning the value of
that parameter, one is able to explain quantitatively both magnetic and
transport properties in a broad family of DMS. We will see that this minimal
model explains in particular the RKKY nature of the exchange in
(Zn,Mn)Te/(Cd,Mn)Te and the double exchange type in (Ga,Mn)N and simultaneously
the reason why (Ga,Mn)As exhibits the highest critical temperature among both
II-VI and III-V DMS.Comment: 6 figures. To appear in Comptes Rendus de l'Acad\'emie des Sciences
(2015
Why RKKY exchange integrals are inappropriate to describe ferromagnetism in diluted magnetic semiconductors
We calculate Curie temperatures and study the stability of ferromagnetism in
diluted magnetic materials, taking as a model for the exchange between magnetic
impurities a damped Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and a shor
t range term representing the effects of superexchange. To properly include
effects of spin and thermal fluctuations as well as geometric disorder, we
solve the effective Heisenberg Hamiltonian by means of a recently developed
semi-analytical approach. This approach, ``self-consistent local Random Phase
Approximation (SC-L RPA)'', is explained. We show that previous mean-field
treatments, which have been widely used in the literature, largely overestimate
both the Curie temperatures and the stability of ferromagnetism as a function
of carrier density. The discr epancy when compared to the current approach was
that effects of frustration in RKKY oscillations had been strongly
underestimated by such simple mea n-field theories. We argue that the use, as
is frequent, of a weakly-disordered RKKY exchange to model ferromagnetism in
diluted III-V systems is inconsistent with the observation of ferromagnetism
over a wide region of itinerant carrier densities. This may be puzzling when
compared to the apparent success of calculations based on {\it ab-initio}
estimates of the coupling; we propose a resolution to this issue by taking
RKKY-like interactions between resonant states close to the Fermi level.Comment: Accepted for publication in Physical Review B. 22 pages, 7 figure
Optical conductivity of Mn doped GaAs
We study the optical conductivity in the III-V diluted magnetic semiconductor
GaMnAs and compare our calculations to available experimental data. Our model
study is able to reproduce both qualitatively and quantitatively the observed
measurements. We show that compensation (low carrier density) leads, in
agreement to the observed measurements to a red shift of the broad peak located
at approximately 200 meV for the optimally annealed sample. The non
perturbative treatment appears to be essential, otherwise a blueshift and an
incorrect amplitude would be obtained. By calculating the Drude weight (order
parameter) we establish the metal-insulator phase diagram. We indeed find that
Mn doped GaAs is close to the metal-insulator transition and that for 5 and
7 doped samples, 20 of the carriers only are delocalized. We have found
that the optical mass is approximately 2 m. We have also interesting
results for overdoped samples which could be experimentally realized by Zn
codoping.Comment: the manuscript has been extended, new figures are include
Spontaneous magnetization in presence of nanoscale inhomogeneities in diluted magnetic systems
The presence of nanoscale inhomogeneities has been experimentally evidenced
in several diluted magnetic systems, which in turn often leads to interesting
physical phenomena. However, a proper theoretical understanding of the
underlying physics is lacking in most of the cases. Here we present a detailed
and comprehensive theoretical study of the effects of nanoscale inhomogeneities
on the temperature dependent spontaneous magnetization in diluted magnetic
systems, which is found to exhibit an unusual and unconventional behavior. The
effects of impurity clustering on the magnetization response have hardly been
studied until now. We show that nanosized clusters of magnetic impurities can
lead to drastic effects on the magnetization compared to that of homogeneously
diluted compounds. The anomalous nature of the magnetization curves strongly
depends on the relative concentration of the inhomogeneities as well as the
effective range of the exchange interactions. In addition we also provide a
systematic discussion of the nature of the distributions of the local
magnetization.Comment: 18 pages, 9 figures, 4 new references added and Text modified to
match the published versio
Nanoscale inhomogeneities: A new path toward high Curie temperature ferromagnetism in diluted materials
Room temperature ferromagnetism has been one of the most sought after topics
in today's emerging field of spintronics. It is strongly believed that defect-
and inhomogeneity- free sample growth should be the optimal route for achieving
room-temperature ferromagnetism and huge efforts are made in order to grow
samples as "clean" as possible. However, until now, in the dilute regime it has
been difficult to obtain Curie temperatures larger than that measured in well
annealed samples of (Ga,Mn)As (190 K for 12% doping). In the present
work, we propose an innovative path to room-temperature ferromagnetism in
diluted magnetic semiconductors. We theoretically show that even a very small
concentration of nanoscale inhomogeneities can lead to a tremendous boost of
the critical temperatures: up to a 1600% increase compared to the homogeneous
case. In addition to a very detailed analysis, we also give a plausible
explanation for the wide variation of the critical temperatures observed in
(Ga,Mn)N and provide a better understanding of the likely origin of very high
Curie temperatures measured occasionally in some cases. The colossal increase
of the ordering temperatures by nanoscale cluster inclusions should open up a
new direction toward the synthesis of materials relevant for spintronic
functionalities.Comment: 16 pages, 4 figures, New references added and Text revised to match
the accepted versio
Unified picture for diluted magnetic semiconductors
For already a decade the field of diluted magnetic semiconductors (DMS) is
one of the hottest. In spite of the great success of material specific Density
Functional Theory (DFT) to provide accurately critical Curie temperatures
() in various III-V based materials, the ultimate search for a unifying
model/theory was still an open issue. Many crucial questions were still without
answer, as for example: Why, after one decade, does GaMnAs still exhibit the
highest ? Is there any intrinsic limitations or any hope to reach room
temperature? How to explain in a unique theory the proximity of GaMnAs to the
metal-insulator transition, and the change from RKKY couplings in II-VI
materials to the double exchange regime in GaMnN? The aim of the present work
is to provide this missing theory. We will show that the key parameter is the
position of the Mn level acceptor and that GaMnAs has the highest among
III-V DMS. Our theory (i) provides an overall understanding, (ii) is
quantitatively consistent with existing DFT based studies, (iii) able to
explain both transport and magnetic properties in a broad variety of DMS and
(iv) reproduces the obtained from first principle studies for many
materials including both GaMnN and GaMnAs. The model also reproduces accurately
recent experimental data of the optical conductivity of GaMnAs and predicts
those of other materials.Comment: 5 figures includes, accepted for publication in Eur. Phys. Let
Carrier induced ferromagnetism in the insulating Mn doped III-V semiconductor InP
Although InP and GaAs have very similar band-structure their magnetic
properties appear to drastically differ. Critical temperatures in (In,Mn)P are
much smaller than that of (Ga,Mn)As and scale linearly with Mn concentration.
This is in contrast to the square root behaviour found in (Ga,Mn)As. Moreover
the magnetization curve exhibits an unconventional shape in (In,Mn)P
contrasting with the conventional one of well annealed (Ga,Mn)As. By combining
several theoretical approaches, the nature of ferromagnetism in Mn doped InP is
investigated. It appears that the magnetic properties are essentially
controlled by the position of the Mn acceptor level. Our calculations are in
excellent agreement with recent measurements for both critical temperatures and
magnetizations. The results are only consistent with a Fermi level lying in an
impurity band, ruling out the possibility to understand the physical properties
of Mn doped InP within the valence band scenario. The quantitative success
found here reveals a predictive tool of choice that should open interesting
pathways to address magnetic properties in other compoundsComment: 5 pages and 5 figures, accepted for publication in Phys. Rev.