J. Dolinšek

Experimental study of the electronic density of states in aluminium-based intermetallics

We report on an experimental investigation of the electronic density of states (DOS) in aluminium-based intermetallics by three experimental techniques: electrical resistivity, NMR spin–lattice relaxation and soft x-ray emission spectroscopy (SXES). The investigated samples were alloys of Al with transition elements Cu, Fe, Cr, Pd and Mn and sometimes also a small amount of B. The samples were structurally very different and included (1) regular periodic intermetallic compounds, (2) large-unit-cell intermetallics that are translationally periodic on the scale of several nanometres and exhibit local polytetrahedral order (like quasicrystals) on the scale of interatomic distances and (3) quasicrystals. Correlation analysis between the DOS parameters determined independently by the three techniques employed showed that the order of samples with decreasing metallic character was the same in all experiments. Quantitative evaluation of the DOS at EF has shown that in regular alloys the DOS is reduced to about 50% of the free-electron-like value in fcc Al metal, whereas the reduction becomes increasingly larger on going to large-unit-cell periodic solids and quasicrystals. Our results also demonstrate that low resistivities are accompanied by positive temperature coefficient (PTC) variation, whereas samples with large resistivity exhibit negative temperature coefficient (NTC). Samples with a resistivity of about 200 µΩ cm appear to be at a crossover from PTC to NTC resistivity, resulting in a temperature-compensated resistivity with essentially zero temperature coefficient. Magnetic properties of the samples are also presented.

Magnetic memory effect in multiferroic K3Fe5F15 and K3Cr2Fe3F15

The fluorides K3Fe5F15 and K3Cr2Fe3F15 are known as multiferroic materials. Here we report the detection of a magnetic memory effect in these materials and its dependence on temperature and aging time. We succeeded in writing, reading, and deleting 3-bits digital information in these systems. These results show that in addition to their already known magneto-electric multiferroic properties, K3Fe5F15 and K3Cr2Fe3F15 also possess a new functionality: they can be used as materials for a thermal memory cell.

The hydrogen dynamics of CsH5(PO4)2 studied by means of nuclear magnetic resonance.

We have investigated the hydrogen dynamics of cesium pentahydrogen diphosphate, CsH(5)(PO(4))(2), by means of nuclear magnetic resonance (NMR) spectroscopy, in order to address the question of why there is no superprotonic phase transition in this compound, in contrast to other structurally similar hydrogen-bonded ionic salts, where a superprotonic transition is frequently found to be present. The analysis of the NMR spectrum and the spin-lattice relaxation rate revealed that the temperature-dependent hydrogen dynamics of CsH(5)(PO(4))(2) involves motional processes (the intra-H-bond jumps and the inter-H-bond jumps at elevated temperatures, as a mechanism of the ionic conductivity) identical to those for the other H-bonded superprotonic salts. The considerably stronger H-bond network in CsH(5)(PO(4))(2) prompts the search for a higher superprotonic transition temperature. However, due to the relatively weak bonding between the {[H(2)PO(4)]}∞ planes in the [100] direction of the CsH(5)(PO(4))(2) structure by means of the ionic bonding via the cesium atoms and the small number of H bonds in that direction (where out of five H bonds in the unit cell, four are directed within the {[H(2)PO(4)]}∞ planes and only one is between the planes), the bonds between the planes become thermally broken and the crystal melts before the H-bond network rearranges via water release into an open structure typical of the superprotonic phase. Were the coupling between the {[H(2)PO(4)]}∞ planes in the CsH(5)(PO(4))(2) somewhat stronger, the superprotonic transition would occur in the same manner as it does in other structurally related hydrogen-bonded ionic salts.

Antiferromagnetic order competing with topological state in CexBi2−xTe3

The topological surface states in three-dimensional topological insulators are easily tuned by chemical doping, especially by magnetic impurities. We prepared single crystals of CexBi2−xTe3 with various x (=0.04, 0.06, 0.08, 0.10, and 0.12). The obtained crystals were characterized by X-ray diffraction and scanning electron microscopy. The magnetic susceptibility data revealed that the Ce atoms are well substituted for Bi into Bi2Te3. From the Curie-Weiss fits, we observed that the effective magnetic moments μeff are close to 2.54 μB for free Ce ion, and the paramagnetic Curie-Weiss temperatures θp are negatively increased from 2.87 K to −59.3 K with increasing x. The magnetization data clearly showed antiferromagnetic orders around TN = 4.1 K for x ≥ 0.08, where θp suddenly increases, and the electrical resistivity is simply metallic and the magnetoresistance is parabolic. Only for x = 0.06, exotic physical properties arising from the topological states were observed such as non-metallic behavior in the ...

Anisotropic transport properties of the Al13TM4 and T-Al–Mn–Fe complex metallic alloys

Anisotropy of the transport properties (electrical resistivity, ρ(T), thermoelectric power, S(T), and thermal conductivity, κ(T)) of the Al76Co22Ni2 (Y-Al–Ni–Co), o-Al13Co4 and T-Al72.5Mn21.5Fe6.0 complex metallic alloys was investigated experimentally. These compounds belong to the class of approximants in decagonal quasicrystals phases with stacked-layer crystallographic structure and enabled us to study the evolution of transport properties with increasing structural complexity and the unit cell size. For Y-Al–Ni–Co and o-Al13Co4, the anisotropic electronic transport coefficients were analyzed theoretically by Boltzmann transport theory and ab initio calculated anisotropic Fermi surface. The non-metallic anisotropic electrical resistivity of the T-Al72.5Mn21.5Fe6.0 may be analyzed in a semi-quantitative way by the theory of quantum transport of slow charge carriers.

Geometric origin of magnetic frustration in the μ-Al4Mn giant-unit-cell complex intermetallic

The structurally ordered μ-Al(4)Mn complex intermetallic phase with 563 atoms in the giant unit cell shows the typical broken-ergodicity phenomena of a magnetically frustrated spin system. The low-field zero-field-cooled and field-cooled magnetic susceptibilities show splitting below the spin freezing temperature T(f) = 2.7 K. The ac susceptibility exhibits a frequency-dependent cusp, associated with a frequency-dependent freezing temperature T(f)(ν). The decay of the thermoremnant magnetization is logarithmically slow in time and shows a dependence on the aging time t(w) and the cooling field H(fc) typical of an ultraslow out-of-equilibrium dynamics of a nonergodic spin system that approaches thermal equilibrium, but can never reach it on the experimentally accessible time scale. The above features classify the μ-Al(4)Mn complex intermettalic among spin glasses. The origin of frustration of magnetic interactions was found to be geometrical due to the distribution of a significant fraction of Mn spins on triangles with antiferromagnetic coupling. The μ-Al(4)Mn phase is a geometrically frustrated spin glass.

Deuterium dynamics in the icosahedral and amorphous phases of the Ti40Zr40Ni20 hydrogen-absorbing alloy studied by 2H NMR

The Ti40Zr40Ni20 hydrogen-absorbing alloy was prepared in the icosahedral and amorphous phases by controlling the rotation speed of the melt–spinning method of sample preparation, and the deuterium dynamics was investigated by 2H NMR dynamic lineshape and spin–lattice relaxation. The results were analysed by the lineshape and relaxation models that assume deuterium thermally activated hopping within a manifold of different chemical environments. The observed 8% larger activation energy for the deuterium hopping over the interstitial sites and the 10% larger static spectrum width of the amorphous phase, as compared to the icosahedral phase, can be accounted for by the larger deuterium content of the investigated amorphous sample. From the deuterium dynamics point of view, the icosahedral phase is not special with respect to the amorphous modification of the same material.

Electrical resistivity of the μ-Al4Mn giant-unit-cell complex metallic alloy

The μ-Al4Mn complex intermetallic phase with 563 atoms in its giant unit cell exhibits a complicated temperature dependence of electrical resistivity that has a broad maximum at about 175 K and a minimum at 13 K. The temperature dependence of the resistivity was reproduced by employing the theory of quantum transport of slow charge carriers, which predicts a crossover from the metallic (Boltzmann-type) positive-temperature-coefficient electrical resistivity at low temperatures to the insulator-like (non-Boltzmann) negative-temperature-coefficient resistivity at elevated temperatures. The low-temperature resistivity minimum was reproduced by considering it as a magnetic effect due to increased scattering of the conduction electrons by the Mn spins on approaching the spin glass phase that develops below the spin freezing temperature Tf = 2.7 K.

A Study of Phase Transformations in Complex Metallic Alloys Al73Mn23Pd4 and Al73Mn21Pd6

The microstructure characterization of Al73Mn23Pd4 and Al73Mn21Pd6 alloys was done after annealing at 900°C for 312 h and subsequent water quenching, as well as after thermal cycling. DTA and EDX/WDX/SEM techniques were used in the investigation. It was found out that the alloys consist of the single ternary T-phase after annealing and water quenching. The DTA experiment confirmed the stability of this phase also at lower temperatures. After DTA, the alloys exhibited double-phase microstructure consisting of the ternary T-phase and probably the icosahedral I-phase. It was proved an incongruent transformation of the ternary T-phase into the liquid and vice versa.