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Richard Bouzerar

Publications and source records attributed to Richard Bouzerar.

16 recordsLinked to original sources
arXiv2016

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 compounds

cond-mat.dis-nn↗
arXiv2015

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.

cond-mat.dis-nn↗
arXiv2014

Spin-wave excitations in presence of nanoclusters of magnetic impurities

Nanoscale inhomogeneities and impurity clustering are often found to drastically affect the magnetic and transport properties in disordered/diluted systems, giving rise to rich and complex phenomena. However, the physics of these systems still remains to be explored in more details as can be seen from the scarce literature available. We present a detailed theoretical analysis of the effects of nanoscale inhomogeneities on the spin excitation spectrum in diluted magnetic systems. The calculations are performed on relatively large systems (up to $N$=$66^3$). It is found that even low concentrations of inhomogeneities have drastic effects on both the magnon density of states and magnon excitations. These effects become even more pronounced in the case of short ranged magnetic interactions between the impurities. In contrast to the increase of critical temperatures $T_C$, reported in previous studies, the spin-stiffness $D$ is systematically suppressed in the presence of nanoscale inhomogeneities. Moreover $D$ is found to strongly depend on the inhomogeneities' concentration, the cluster size, as well as the range of the magnetic interactions. The findings are discussed in the prospect of potential spintronics applications. We believe that this detailed numerical work could initiate future experimental studies to probe this rich physics with the most appropriate tool, Inelastic Neutron Scattering (INS).

cond-mat.dis-nn↗
arXiv2012

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.

cond-mat.dis-nn↗
arXiv2012

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 ($\sim$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.

cond-mat.mtrl-sci↗
arXiv2011

Magnetic spin excitations in Mn doped GaAs : A model study

We provide a quantitative theoretical model study of the dynamical magnetic properties of optimally annealed Ga$_{1-x}$Mn$_x$As. This model has already been shown to reproduce accurately the Curie temperatures for Ga$_{1-x}$Mn$_x$As. Here we show that the calculated spin stiffness are in excellent agreement with those which were obtained from ab-initio based studies. In addition, an overall good agreement is also found with available experimental data. We have also evaluated the magnon density of states and the typical density of states from which the "mobility edge", separating the extended from localized magnon states, was determined. The power of the model lies in its ability to be generalized for a broad class of diluted magnetic semiconductor materials, thus it bridges the gap between first principle calculations and model based studies.

cond-mat.mtrl-sci↗
arXiv2010

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$_{e}$. We have also interesting results for overdoped samples which could be experimentally realized by Zn codoping.

cond-mat.dis-nn↗
arXiv2010

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 ($T_{C}$) 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 $T_{C}$? 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 $T_{C}$ 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 $T_{C}$ 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.

cond-mat.dis-nn↗
arXiv2009

On the reliability of recent Monte Carlo studies of dilute systems of localized spins interacting with itinerant carriers

In this paper, we discuss magnetic properties of dilute systems of localized spins interacting with itinerant carriers. More precisely, we compare recent available Monte Carlo results with our two step approach (TSA) calculations. The TSA consists first on the determination of the magnetic couplings and then on a proper treatment of the resulting effective dilute Heisenberg Hamiltonian. We show important disagreement between the Monte Carlo results (in principle exact) and our TSA calculations. We analyze the origins and shed light on the reasons of those dissensions. In contrast to one could expect, we demonstrate that the available MC calculations suffer from severe numerical shortcomings. More precisely, (i) the finite size effects appear to be huge in dilute systems, (ii) the statistical sampling (disorder configurations) was far too small, and (iii) the determination of the Curie temperature was too rough. In addition, we provide new arguments to explain a recent disagreement between the Monte Carlo simulations and TSA for the model study of the well known III-V diluted magnetic semiconductor Ga$_{1-x}$Mn$_{x}$As. We hope that this work will motivate new systematic large scale Monte Carlo calculations.

cond-mat.dis-nn↗
arXiv2007

Ordering effects in diluted magnetic semiconductors

We review recently developed two-step approach for description of electronic and magnetic properties of a new class of materials, the diluted magnetic semiconductors. In the first step we construct, on the basis of the state-of-the-art first-principles electronic structure calculations, the effective Ising and Heisenberg Hamiltonians which describe, respectively, the alloy phase stability and the magnetic excitations in the system. In the second step, we analyze properties of these effective Hamiltonians by various methods of statistical physics. As a case study, the prototypical diluted magnetic semiconductor Ga_{1-x}Mn_{x}As is studied in detail. We determine, among others, a possibility for clustering in this system, formation energies of various compensating defects, and estimate short-range order parameters describing ordering tendencies in a system. On the other hand, by using recently developed local random-phase approximation approach, we evaluate the system Curie temperature and demonstrate its strong dependence on the sample preparation. We also emphasize the relevance of proper inclusion of the randomness in positions of magnetic impurities for a reliable estimate of the system critical temperature. Finally, we compare calculated Curie temperatures with available experimental data and briefly mention relation to other theoretical approaches.

cond-mat.mtrl-sci↗
arXiv2007

Superexchange induced canted ferromagnetism in dilute magnets

We argue, in contrast to recent studies, that the antiferromagnetic superexchange coupling between nearest neighbour spins does not fully destroy the ferromagnetism in dilute magnets with long-ranged ferromagnetic couplings. Above a critical coupling, we find a \textit{canted} ferromagnetic phase with unsaturated moment. We have calculated the transition temperature using a simplified local Random Phase Approximation procedure which accounts for the canting. For the dilute magnetic semiconductors, such as GaMnAs, using \textit{ab-initio} couplings allows us to predict the existence of a canted phase and provide an explanation to the apparent contradictions observed in experimental measurements. Finally, we have compared with previous studies that used RKKY couplings and reported non-ferromagnetic state when the superexchange is too strong. Even in this case the ferromagnetism should remain essentially stable in the form of a canted phase.

cond-mat.dis-nn↗
arXiv2006

Non-perturbative $J_{pd}$ model and ferromagnetism in dilute magnets

We calculate magnetic couplings in the $J_{pd}$ model for dilute magnets, in order both to identify the relevant parameters which control ferromagnetism and also to bridge the gap between first principle calculations and model approaches. The magnetic exchange interactions are calculated non-perturbatively and disorder in the configuration of impurities is treated exacly, allowing us to test the validity of effective medium theories. Results differ qualitatively from those of weak coupling. In contrast to mean field theory, increasing $J_{pd}$ may not favor high Curie temperatures: $T_C$ scales primarily with the bandwidth. High temperature ferromagnetism at small dilutions is associated with resonant structure in the p-band. Comparison to diluted magnetic semiconductors indicate that Ga(Mn)As has such a resonant structure and thus this material is already close to optimality.

cond-mat.str-el↗
arXiv2006

Comparison between ab-initio and phenomenological modeling of the exchange couplings in diluted magnetic semiconductors: the case of $Zn_{1-x}Cr_{x}Te$

Using a recently developed semi-analytical method (Self-Consistent Local RPA or SC-LRPA) we study the stability of the ferromagnetic phase in diluted magnetic systems where the exchange coupling between magnetic impurities are of RKKY form. A short discussion of the relevance of these calculations with respect to the ferromagnetism observed in diluted ferromagnetic materials is provided. Then, within a two step approach, we study ferromagnetism in $Zn_{1-x}Cr_{x}Te$. In the first step of our study, we calculate the magnetic couplings between Mn impurities within the LDA. In the second step, we diagonalize the resulting effective Heisenberg Hamiltonian using the SC-LRPA. We also compare, when available, our calculations with Monte Carlo simulations and experimental measurements.

cond-mat.dis-nn↗
arXiv2005

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.

cond-mat.str-el↗