111 research outputs found
Random local strain effects in homovalent-substituted relaxor ferroelectrics: a first-principles study of BaTi0.74Zr0.26O3
We present first-principles supercell calculations on BaTi0.74Zr0.26O3, a
prototype material for relaxors with a homovalent substitution. From a
statistical analysis of relaxed structures, we give evidence for four types of
Ti-atom polar displacements: along the , , or
directions of the cubic unit cell, or almost cancelled. The type of a Ti
displacement is entirely determined by the Ti/Zr distribution in the adjacent
unit cells. The underlying mechanism involves local strain effects that ensue
from the difference in size between the Ti4+ and Zr4+ cations. These results
shed light on the structural mechanisms that lead to disordered Ti
displacements in BaTi(1-x)Zr(x)O3 relaxors, and probably in other BaTiO3-based
relaxors with homovalent substitution.Comment: 5 pages, 4 figure
Colossal Spin-Phonon Anomalies and the Ferroelectric Phase Transition in the Model Multiferroic Bifeo3
We report a temperature-dependent Raman and neutron scattering investigation
of the multiferroic material bismuth ferrite BiFeO3 (BFO).Comment: submitted to PRL (July 2005
Experimental study of the competition between Kondo and RKKY interactions for Mn spins in a model alloy system
The quasicrystal Al-Pd-Mn is a model system for an experimental study of the
competition between Ruderman-Kittel-Kasuya-Yoshida (RKKY) and Kondo
interactions. First, specific of such alloys, only a few Mn atoms carry an
effective spin and their concentration x is tunable over several orders of
magnitude, even though the Mn amount is almost constant. Second, the
characteristic energy scales for the interactions lie in the Kelvin range.
Hence we could study the magnetization on both side of these energy scales,
covering a range of temperatures [0.1-100 K] and magnetic fields (mu_B H/k_B= 0
to 5 K) for 22 samples and x varying over 2 decades. Using very general Kondo
physics arguments, and thus carrying out the data analysis with no preconceived
model, we found a very robust and simple result: The magnetization is a sum of
a pure Kondo (T_K=3.35K) and a pure RKKY contributions, whatever the moment
concentration is and this surprisingly up to the concentration where the RKKY
couplings dominate fully and thus cannot be considered as a perturbation.Comment: 18 pages, 18 figure
Crystallization of Ge2Sb2Te5 nanometric phase change material clusters made by gas-phase condensation
International audienceThe crystallization behavior of Ge2Sb2Te5 nanometric clusters was studied using X-ray diffraction with in situannealing. Clusters were made using a sputtering gas-phase condensation source, which allowed for the growth of well-defined, contaminant-free, and isolated clusters. The average size for the clusters is 5.7 ± 1 nm. As-deposited amorphous clusters crystallize in the fcc cubic phase at 180 °C, while for thin films, the phase change temperature is 155 °C. This observation illustrates the scalability of the Ge2Sb2Te5phase change from the amorphous to the cubic state in three-dimensionally confined systems in this size range
Non-collinear magnetism in Al-Mn topologically disordered systems
We have performed the first ab-initio calculations of a possible complex
non-collinear magnetic structure in aluminium-rich Al-Mn liquids within the
real-space tight-binding LMTO method. In our previous work we predicted the
existence of large magnetic moments in Al-Mn liquids [A.M. Bratkovsky, A.V.
Smirnov, D. N. Manh, and A. Pasturel, \prb {\bf 52}, 3056 (1995)] which has
been very recently confirmed experimentally. Our present calculations show that
there is a strong tendency for the moments on Mn to have a non-collinear
(random) order retaining their large value of about 3~. The d-electrons
on Mn demonstrate a pronounced non-rigid band behaviour which cannot be
reproduced within a simple Stoner picture. The origin of the magnetism in these
systems is a topological disorder which drives the moments formation and
frustrates their directions in the liquid phase.Comment: 10 pages, RevTex 3.0, 24kb. 3 PS figures available on request from
[email protected] The work has been presented at ERC
``Electronic Structire of Solids'' (Lunteren, The Netherlands, 9-14 September
1995
EXAFS study of lead-free relaxor ferroelectric BaTi(1-x)Zr(x)O3 at the Zr K-edge
Extended X-ray absorption fine structure (EXAFS) experiments at the Zr K-edge
were carried out on perovskite relaxor ferroelectrics BaTi(1-x)Zr(x)O3 (BTZ) (x
= 0.25, 0.30, 0.35), and on BaZrO3 for comparison. Structural information up to
4.5 A around the Zr atoms is obtained, revealing that the local structure
differs notably from the average Pm-3m cubic structure deduced from X-ray
diffraction. In particular, our results show that the distance between Zr atoms
and their first oxygen neighbors is independent of the Zr substitution rate x
and equal to that measured in BaZrO3, while the X-ray cubic cell parameter
increases linearly with x. Furthermore, we show that the Zr atoms tend to
segregate in Zr-rich regions. We propose that the relaxor behavior in BTZ is
linked to random elastic fields generated by this particular chemical
arrangement, rather than to random electric fields as is the case in most
relaxors.Comment: 13 pages, 12 figures, 4 tables. Submitted to Phys. Rev.
Hidden magnetic frustration by quantum relaxation in anisotropic Nd-langasite
The static and dynamic magnetic properties of the NdGaSiO
compound, which appears as the first materialization of a rare-earth
kagome-type lattice, were re-examined, owing to contradictory results in the
previous studies. Neutron scattering, magnetization and specific heat
measurements were performed and analyzed, in particular by fully taking account
of the crystal electric field effects on the Nd ions. One of the novel
findings is that the peculiar temperature independent spin dynamics observed
below 10 K expresses single-ion quantum processes. This would short-circuit the
frustration induced cooperative dynamics, which would emerge only at very low
temperature
Spin-orbit readout using thin films of topological insulator Sb2Te3 deposited by industrial magnetron sputtering
Driving a spin-logic circuit requires the production of a large output signal
by spin-charge interconversion in spin-orbit readout devices. This should be
possible by using topological insulators, which are known for their high
spin-charge interconversion efficiency. However, high-quality topological
insulators have so far only been obtained on a small scale, or with large scale
deposition techniques which are not compatible with conventional industrial
deposition processes. The nanopatterning and electrical spin injection into
these materials has also proven difficult due to their fragile structure and
low spin conductance. We present the fabrication of a spin-orbit readout device
from the topological insulator Sb2Te3 deposited by large-scale industrial
magnetron sputtering on SiO2. Despite a modification of the Sb2Te3 layer
structural properties during the device nanofabrication, we measured a sizeable
output voltage that can be unambiguously ascribed to a spin-charge
interconversion process
The role of C and N dopants incorporation in phase change materials
Phase change memory (PCM) technology is considered to be among the most promising alternatives to conventional technologies in embedded memories [1]. To allow operation at relatively high temperatures in embedded applications, it is crucial to improve the stability of the amorphous phase. Carbon and nitrogen doping have been shown to significantly increase the crystallization temperature [1-3]. Moreover, the high RESET current requirement [2], which is a limit to the scalability of GeTe and GST, can be reduced by the incorporation of a dopant element [4].
In this presentation we focus on correlating experimental results and ab initio simulations to understand the effect of C and N incorporation in GeTe and GST PCM devices. Understanding the effect of dopants on the change of electronic properties and the mechanisms of the phase transformation requires analysis of the local order and structure of the amorphous to crystalline phases.
In this context, we demonstrate that carbon and nitrogen deeply affects the structure and the dynamical properties of the amorphous phase of GeTe. In particular, the inclusion of N and C dopant elements in GeTe has a drastic effect on the vibrational modes of GeTe therefore improving the stability of the glass. This effect goes with an increased mechanical rigidity explaining why these doped GeTe compounds have a higher crystallization temperature than the undoped ones.
Finally we will explore, mainly by FTIR and XRD measurements, the effect of C and N dopants during the annealing of amorphous PCMaterials towards their crystalline phases. These results will be discussed in order to understand the origin of the differences of the doped PCMaterials amorphous phase stability (data retention) observed between full sheet materials and the materials integrated in PCM devices.
[1] A. Fantini et al., 2010 IEEE International Electron Devices Meeting (IEDM), 2010, pp. 29.21.21-29.21.24.
[2] G. Betti Beneventi et al., Solid-State Electronics, 65-66 (2011) 197-204.
[3] V. Sousa et al., EPCOS 2011.
[4] Q. Hubert et al., IMW 2012.A.R.C. Themoter
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