278 research outputs found
Metastabiliteit in Nikkel Silicide en Nikkel Germanide Dunne Film Reacties
No full textThin films have been subject of intensive research, spurring many scientific discoveries and technological innovations. A long-standing, open question in this field has been how a system evolves from a metastable to a stable configuration. More specifically, when a film is deposited onto a substrate and the system is annealed, which phases form before thermal equilibrium is reached and why? This is a deep and complex problem, as the path of such thin film reactions is determined by many thermodynamic and kinetic factors. To address this question, the reactions of metal thin films with silicon or germanium have been historically studied as model systems for thin film reactions. Moreover, understanding the formation of metal silicides and germanides is relevant for the ongoing improvement of computer chips, due to their role as contact material in complementary metal-oxide-semiconductor (CMOS) technology. Two prominent reactions often studied, for both fundamental and technological reasons, are those of the Ni-Si and Ni-Ge systems.
In this research, we aim to gain more insights into the Ni-Si and Ni-Ge solid-phase reactions. We do so by bringing the initial configuration (a Ni film on a Si or Ge single-crystalline substrate) further away from equilibrium and studying how it evolves towards stability, i.e. increasing the metastability of the system prior to the reaction, effectively changing the starting point of the reaction path. We achieve this in three different ways: reducing film thickness to only a few nm, creating substrate disorder or substrate amorphization by ion implantation or evaporation, and introducing nitrogen impurities.
These modifications are found to have strong effects on a number of metastable phases appearing in the thin film reaction and on the morphological stability of the films. The hexagonal, metastable phase appearing in both reactions is destabilized by the amorphization of the substrate, whereas the morphology of the films is stabilized by removing the substrate template. Nitrogen impurities stabilize the hexagonal phase and delay the morphological degradation of the film. Moreover, the amorphous Ni-Si alloy preceding the crystalline silicides during the reaction is stabilized by nitrogen as well. If a sufficient amount of nitrogen is implanted, the entire metal supply is consumed by the amorphous phase and the system takes a fundamentally different reaction path (an impurity-enhanced solid-state amorphization reaction).
Our results highlight the interwoven nature of diffusion, nucleation, film grain orientation (texture), agglomeration... during thin film solid-phase reactions. More specifically, these insights stress the importance of the exact nature of the interface periodicity (or lack thereof), and of the kinetic side of nucleation, i.e. how atoms arrange themselves in a crystal lattice via short-range diffusion.status: Publishe
Large positive magnetoresistance at low temperatures in a ferromagnetic natural multilayer, LaMn<SUB>2</SUB>Ge<SUB>2</SUB>
No full textThe results of magnetoresistance measurements on a naturally occurring multilayer LaMn2Ge2, which is ferromagnetic below 326 K, are reported. The magnitude of magnetoresistance is found to be positive below 70 K gradually increasing to an unusually large value (nearly 100%) at 4.2 K in the presence of a field of 70 kOe as the temperature is lowered, similar to the recent observations by Verbanck, Temst, Mae, Schad, Van Bael, Moshchalkov, and Bruynseraede [Appl. Phys. Lett. 70, 1477 (1997)] in Cr/Ag/Cr trilayers. The positive sign of magnetoresistance for a ferromagnet is unexpected and possible explanations are offered
Composiet multiferroïsche materialen gesynthetiseerd door ionenimplantatie in dunne films
No full textGrowth and characterization of BFO Ferro-electric thin films to be usedin spin transfer torque devices.status: Publishe
Grensvlakeffecten in Magnetische Nanostructuren
No full textThe quest to follow Moore's law [1] brings solid state physics research down to
smaller and smaller length scales. In this regime surface and interface effects
become more and more important due to the increase in surface to volume
ratio. It is therefore paramount to study the physics at the interface between
two materials in detail. An unexpected and prominent interface effect was
discovered in 1957 by Meiklejohn and Bean when they discovered an exchange
interaction between a ferromagnet and an antiferromagnet in close proximity [2].
Even though this effect, called exchange bias, was discovered more than half a
century ago, it is still widely studied today, since no universal theory exists yet
to describe all the phenomena associated with exchange bias, such as a shift
in the hysteresis loop, increased coercivity and training effects. Additionally,
exchange bias systems are attractive candidates for applications in magnetic
random access memory and magnetoresistive readheads based on spin valves or
tunnel junctions [3; 4]. Here, we study exchange bias in Co/CoO nanostructures,
grown by oblique angle deposition, a technique to create unique nanostructures
that can be modified by varying the angle used during sample growth. We
investigate how a patterned substrate influences the magnetic properties of an
obliquely grown, exchange biased Co/CoO film using SQUID magnetometry
and polarized neutron reflectometry.
Reaching smaller and smaller length scales as well as the hunt for more energy
efficient and localized ways to switch mangetic bits have not just led to
the discovery of new phenomena but also new classes of materials, such as
artificial multiferroics. Multiferroics are materials with two or more ferroic order
parameters, such as ferromagnetism, ferroelectricity, ferroelasticity... Very few
naturally occurring materials are multiferroics, therefore artificial multiferroics
are fabricated by combining materials with different ferroic order parameters.
For these materials the coupling between the different ferroic orders, such as for
example the magnetoelectric coupling between a ferromagnet and a ferroelectric,
is mediated via the interface between the two materials. We are studying this
magnetoelectric coupling across the interface, which can be described using the
magnetoelectric voltage coefficient, in multiferroic multilayers. The multilayers
described here consist of BiFeO3-BaTiO3 double layers. While we keep the
thickness of the BaTiO3 constant, we vary the BiFeO3 thickness as well as
the double layer repetition to optimize the magnetoelectric coupling and gain
a deeper understanding of the rich interface physics as well as the emergent
physical properties of multiferroic heterostructures.
[1] G. E. Moore, "Cramming more components onto integrated circuits,
reprinted from electronics, volume 38, number 8, april 19, 1965, pp.114 ff.,"
IEEE Solid-State Circuits Society Newsletter, vol. 11, no. 3, pp. 33-35,
2006.[2] W. Meiklejohn and C. Bean, "New magnetic anisotropy," Physical Review,
vol. 105, no. 3, pp. 904-913, 1957.
[3] V. Baltz, J. Sort, S. Landis, B. Rodmacq, and B. Dieny, "Tailoring
size effects on the exchange bias in ferromagnetic-antiferromagneticnanostructures," Physical Review Letters, vol. 94, p. 117201, 2005.
[4] B. Dieny, V. S. Speriosu, S. S. P. Parkin, B. A. Gurney, D. R. Wilhoit,
and D. Mauri, "Giant magnetoresistive in soft ferromagnetic multilayers,"
Physical Review B - Condensed Matter and Materials Physics, vol. 43,
pp. 1297-1300, 1991.status: Publishe
Het samenspel tussen fononen en supergeleiding in Sn-supergeleiders op nanoschaal
No full textSuperconductivity is one of the most intriguing phenomena in solid state physics. It is characterized by the condensation of electrons into Cooper pairs, i.e. below a particular critical temperature there is an effective attractive interaction between electrons, and in bulk materials this manifests itself in (i) the disappearance of electrical resistance and (ii) the expulsion of an external magnetic field. This attractive interaction between electrons is mediated via the lattice, and therefore the electron-phonon coupling is essential for superconductivity. The current challenge in the field, with the rising interest in nanoscale phenomena, is if and how superconductivity can be established when the system size becomes smaller than the typical size of a Cooper pair, a situation occurring in thin films and nanoparticles.
We aim to get a deeper understanding of the intimate link between the overall superconducting properties and the lattice dynamics of nanoscale systems. We will use synchrotron-based techniques to measure phonons in (ultra) thin films, nanowires and nanoparticles as well as the superconducting gap energy for selected systems. Focusing on low-dimensional systems (films, nanowires and nanoparticles) will allow to gauge the influence of surfaces and interfaces and of finite dimensions on the superconducting properties. As model systems we will use Nb3Sn films, Sn nanowires and Sn and Pb nanoparticles.status: Publishe
A polarized neutron reflectometry study of the spin glass freezing in a 29?nm thick AuFe film
No full textWe performed polarized neutron reflectometry (PNR) experiments on a 29?nm thick Au93Fe7 film in a temperature range from 295?K down to 2?K in a vertical magnetic field up to 6?T. These high-field experiments were performed on the C5 spectrometer in Chalk River, Canada, using a split-pair cryomagnet. The magnetization as determined by PNR can be described with a Brillouin function from 295?K down to 50?K assuming the magnetic moment of isolated Fe atoms, i.e. 4[mu]B per Fe atom. Below 50?K the onset of the spin-glass freezing is observed as a strong deviation from this Brillouin type behavior of isolated atoms.NRC publication: Ye
Morphology-induced spin frustration in granular BiFeO3 thin films: Origin of the magnetic vertical shift
No full text© 2018 Author(s). Pronounced room temperature vertical shifts in the magnetic hysteresis loops of granular, highly polycrystalline and ferromagnetic-like BiFeO3 thin films are observed upon field-cooling from a temperature above the Néel temperature of bulk BiFeO3. This is ascribed to the interplay between the preferential alignment, established by the field-cooling process, of the net magnetic moment, which arises from uncompensated antiferromagnetic spins, and the pinning of a fraction of these spins at the particle boundaries. Conversely, field-cooling of an epitaxially grown BiFeO3 film results in no vertical shift, confirming the effective role played by the particle boundaries (i.e., morphology) of the granular-like BiFeO3 films in the process of spin frustration.sponsorship: This work was financed by the Research Foundation-Flanders (FWO) and the KU Leuven Concerted Action (GOA/09/006 and GOA/14/007) programs. E.M. acknowledges the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement No. 665919. M.L. and M.G. thank the Deutsche Forschungsgemeinschaft within SFB 762 "Functionality of Oxide Interfaces" for financial support. The authors thank the ESRF, Grenoble, France (Proposal No. HC-1586, BM20 beamline) for the allocation of SR beam time and C. Bahtz for the assistance during the experiments. The authors also extend their gratitude to Kerstin Brachwitz and Peter Schwinkendorf for the atomic force microscopy measurements. (Research Foundation-Flanders (FWO), KU Leuven Concerted Action|GOA/09/006, KU Leuven Concerted Action|GOA/14/007, European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant|665919, Deutsche Forschungsgemeinschaft within "Functionality of Oxide Interfaces"|SFB 762)status: Publishe
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