H. Micklitz

Pressure-induced antiferromagnetism in fcc Fe-Ni Invar alloys

Abstract We report the experimental observation of a pressure-induced magnetic phase transition in the ferromagnetic fcc Fe 68.5 Ni 31.5 Invar alloy. It is shown that above a critical pressure of 5.8 GPa the system displays antiferromagnetic order at low temperatures. The Neel temperature decreases on further increasing the pressure. The experimental results are compared with several related theoretical models.

Pressure induced instability of the Fe magnetic moment in an fcc-like environment: La(Fe0.88Al0.12)13

Abstract La(Fe0.88Al0.12)13 is a cubic compound (NaZn13 structure) in which the Fe lattice sites have a nearest neighbor (nm) configuration very similar to that in fcc (γ)-Fe. 57 Fe high-pressure Mossbauer experiments on La(Fe0.88Al0.12)13 show an abrupt decrease of the magnetic ordering temperature and the average magnetic hyperfine field Beff at a critical pressure pc≈4.5 GPA, corresponding to an average Fe-Fe nn distance d c =(2.53±0.05) A. This value of d is compared with the value measured in fcc FexNi1-x Invar and the theoretical values of dc deduced from spin polarised band structure calculations.

Mössbauer studies of single and polycrystalline Bi2Sr2CaCu2O8 doped with 57Fe

Abstract 57Fe Mossbauer studies of oriented single crystals of Bi2Sr2CaCu2O8 doped with ≈ 3.5 at % Fe (with respect to Cu) essentially show one quadrupole doublet (ΔEQ = 1.8 mms−1) with the direction of Vzz parallel to the c-axis and a positive sign of Vzz. This doublet can be attributed to Fe3+ (3d5) on a regular Cu-site with a pyramidal oxygen coordination. Polycrystalline samples with the same composition show an additional quadrupole doublet (ΔEQ = 0.68 mms−1) which probably is due to Fe either at grain boundaries or surfaces.

The stability of the ferromagnetic state in La(Fe0.86Al0.14)13 under high pressure

Abstract The ferromagnetic (FM) La(Fe0.86Al0.14)13 Invar alloy displays a pressure-induced FM to antiferromagnetic (AF) phase transition at a pressure as low as p⪅0.1 GPa. A quantitative analysis of the results shows that the FM → AF magnetic phase transition recently observed in La(FexAl1−x)13 at ambient pressure is governed by the decrease of the unit cell volume rather than the increase of the number of Fe nearest neighbor atoms.

Metal-nonmetal transition in solid argon-lanthanum mixtures☆

Abstract A continuous metal-nonmetal transition is observed in solid ArLa mixtures. The d.c. conductivity σ starts at a La atomic fraction cLa ≅ 0.15 with σ = 2 × 10 -2 [Ω-cm] -1 and increases exponentially with cLa until σ reaches the minimum metallic conductivity σ min = 300 ± 100[Ω-cm] -1 at cLa ≅ 0.4. The temperature dependence of σ shows variable range hopping between localized states for c>La ⪅ 0.4 and metallic behaviour for c La ⪆ 0.4.

Test of scaling theories for metal-insulator transition in amorphous, superconducting BixKr1−x mixtures

Abstract Measurements of the d.c. electrical conductivity σ and the superconducting transition temperature T c in amorphous Bi x Kr 1−x mixtures as a function of the Bi concentration x give clear evidence for the occurence of an Anderson or Anderson-Mott type metal-insulator transition at x c = 0.55 ± 0.02. From our σ and T c data we estimate two characteristic exponents which we compare with those derived from existing scaling theories for the metal-insulator transition in disordered systems.

Metal-insulator transition and superconductivity in SnAr films

Abstract We studied the conductivity and superconducting transition temperature T c of SnAr films. The films were prepared by condensing the SnAr mixture on a sapphire substrate held at 5 K. A plot of the conductivity as a function of Sn concentration shows a metal-insulator transition which agrees with a percolation model consisting of Sn clusters embedded in solid Ar. A drop of T c is observed below the percolation threshold.

On the valence stability of Eu in the laves phase compounds Eu1-xPtxPt2 under high-pressure

Abstract High-pressure Mossbauer studies of the 151Eu resonance in Eu1-xPtxPt2 (x = 0;0.143;0.25) have been performed between 4.2 and 300 K. A linear decrease in the magnitude of the interconfiguration excitation energy Eexc on the Eu2+-site with decreasing lattice constant was found (∂|Eexc|/∂ ln V = 1.6 x 104 K). The isomer shift for EuPt3 at 70 kbar and 4.2 K shows that the Eu atoms on the Eu2+-site are no longer pure divalent but have ≅ 5% admixture of the Eu3+ cofiguration.