32 research outputs found
Emergent tri-criticality in magnetic metamaterials
Metallic discs engineered on the 100 nm scale have an internal magnetic
texture which varies from a fully magnetized state to a vortex state with zero
moment. The interplay between this internal structure and the inter-disc
interactions is studied in magnetic metamaterials made of square arrays of the
magnetic discs. The texture is modeled by a mesospin of varying length with
O(2) symmetry and the inter-disc interaction by a nearest neighbour coupling
between mesospins. The thermodynamic properties of the model are studied
numerically and an ordering transition is found which varies from
Kosterlitz-Thouless to first order via an apparent tri-critical point. The
effective critical exponent characterising the finite size magnetization
evolves from the value for the 2D-XY model to less than half this value at the
tri-critical point. The consequences for future experiments both in and out of
equilibrium are discussed.Comment: 10 pages, 9 figure
Hydrogen site occupancy and strength of forces in nano-sized metal hydrides
The dipole force components in nano-sized metal hydrides are quantitatively
determined with curvature and x-ray diffraction measurements. Ab-initio density
functional theory is used to calculate the dipole components and the symmetry
of the strain field. The hydrogen occupancy in a 100 nm thick V film is shown
to be tetrahedral with a slight asymmetry at low concentration and a transition
to octahedral occupancy is shown to take place at around 0.07 [H/V] at 360 K.
When the thickness of the V layer is reduced to 3 nm and biaxially strained, in
a Fe_0.5V_0.5/V superlattice, the hydrogen unequivocally occupies octahedral
z-like sites, even at and below concentrations of 0.02 [H/V]
Influence of misfit strain on the physical properties of Fe thin films
We investigate the growth of thin Fe layers on MgAlO (001) and MgO
(001) substrates using dc magnetron sputtering. The crystal quality of Fe
layers deposited on MgAlO is found to be substantially higher as
compared to Fe grown on MgO substrates. The effects of the crystal quality on
the magnetic and electric transport properties are discussed.Comment: 8 pages, 6 figure
Acute COVID-19 severity and mental health morbidity trajectories in patient populations of six nations:An observational study
Accurate measurement of hydrogen concentration in transition metal hydrides utilizing electronic excitations by MeV ions
This study focuses on enhancing the accuracy of hydrogen content verification in hydrogen-rich ultrathin materials relevant for sustainable energy applications. Ion beams are used for distinctive real-space detection leveraging elastic and inelastic interactions with hydrogen atoms. However, the lack of experimental reference data on electronic interactions poses a challenge to the accuracy of analytical techniques. We investigate the effect of absorbed hydrogen on the electronic energy deposition of 15N-ions in amorphous transition metal compounds, specifically V and Zr, covering concentrations >1H/M. Employing resonant nuclear reactions and Rutherford backscattering, the energy loss is found to increase considerably with hydrogen content, in line with Bragg's additivity. The electronic energy loss cross section for 15N-ions at 6.5 MeV measured (64.55±3.38) eV cm2/1015 atoms. Results are compared to semi-empirical and theoretical models. The findings improve hydrogen profiling accuracy using 15N-nuclear reaction analysis and enable unprecedented methods for hydrogen quantification by other, commonly available ion beams
Hydrogen-induced enhancement of thermal stability in VZr(H) metallic glasses
Prediction of crystallization temperatures in metallic glasses is still an open question. Investigations of multi component alloys are common in the literature, however, binary and ternary alloys are more suitable for funda-mental studies due to their simplicity. Here, we show that a low thermodynamic driving force for crystallization can be associated with a high crystallization temperature. The driving force is determined by calculating - for the first time in metallic glasses - the temperature dependent Gibbs free energies of the alloys using ab initio density functional theory, in combination with the stochastic quenching method. The crystallization tempera-tures of VxZr100-x and VxZr67-xH33 have been determined using simultaneous in-situ x-ray scattering techniques and resistivity measurements. The onset of crystallization is found to exhibit a parabolic dependence throughout the composition range, whereas alloying with hydrogen increases the thermal stability up to 150 K close to the amorphous-crystalline boundaries. These findings suggest that hydrogen acts as an alloying element with the ability to dynamically tune the intrinsic properties of the material. Lastly, temperature-dependent descriptions of the Gibbs free energy and kinetic considerations of a metallic glass are necessary for a complete characterization of the crystallization process
Influence of misfit strain on the physical properties of Fe thin films
We investigate the growth of thin Fe layers on MgAl2O4 (001) and MgO (001) substrates using dc magnetron sputtering. The crystal quality of Fe layers deposited on MgAl2O4 is found to be substantially higher as compared to Fe grown on MgO substrates. The effects of the crystal quality on the magnetic and electric transport properties are discussed.
Hydrogen-induced volume changes, dipole tensor, and elastic hydrogen-hydrogen interaction in a metallic glass
Hydrogen and its isotopes, absorbed in metals, induce local stress on the atomic structure, which generates a global expansion in proportion to the concentration of hydrogen. The dipole force tensor and its interaction with the stress fields give rise to an effective attractive nonlocal potential between hydrogen atoms-the elastic hydrogen-hydrogen interaction-which is a key quantity governing the phase transitions of hydrogen in metals. While the dipole tensor and the elastic interaction have been researched in crystalline materials, they remain experimentally unexplored in metallic glasses and it is unclear how these quantities are affected by the lack of point group symmetries. Here, we investigate both experimentally and theoretically the volume changes, the components of the force dipole tensor, and ultimately the elastic hydrogen-hydrogen interaction in the metallic glass V80Zr20. In situ neutron reflectometry was used to determine the deuterium-induced volume changes as a function of deuterium concentration. The one-dimensional volume expansion is found to change by more than 14% without any structural degradation, up to concentrations of one deuterium atom per metal atom. From the expansion, we determine that the out-of-plane component of the elastic dipole tensor is remarkably similar to a composition weighted sum of the ones found in crystalline vanadium and zirconium. Via ab initio calculations of both free and biaxially constrained expanded metallic structures, we determine that the trace of the dipole tensor varies with hydrogen concentration and is essentially invariant of global elastic boundary conditions. As a consequence, the elastic hydrogen-hydrogen interaction energy is found to be concentration-dependent as well, illustrating that the disordered nature of a metallic glass does not impede the mediation of the elastic attraction, but rather allows it to vary with hydrogen content
Phase formation and thermal stability of amorphous ZrNbCrMo thin films
We demonstrate that the atomic size mismatch, δ, can be used to predict the boundary between amorphous and crystalline phase formation ranges in ZrNbCrMo thin films. The concentrations of all alloying elements are varied systematically, which induces a change in δ. The formation of an amorphous phase is observed by x-ray diffraction for δ ≥ 6.9%, whereas crystallization is detected for δ ≤ 6.9%. The crystallization temperature increases from 575 to 625 ∘C by increasing Nb + Mo content from 10 to 34 at.%, and decreases to 475 ∘C upon further additions of Nb + Mo. Calphad calculated liquidus temperatures, Tl, increases monotonously and δ decreases monotonously with Nb and Mo additions. Consequently, δ and Tl do not serve as predictors for thermal stability of ZrNbCrMo thin films
Similar molecular alterations occur in related leukemias with and without the Philadelphia chromosome.
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