M. M. Abd-Elmeguid

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.

Valence state and magnetic interactions in magnetically ordered Eu(Pd1-xAux)2Si2

X-ray diffraction,151Eu Mössbauer effect (ME) and magnetic susceptibility measurements have been performed to investigate the valence behavior and the magnetic interactions of the Eu ions in the intermetallic series Eu(Pd1−xAux)2Si2 (0≦x≦1) as a Forx>0.4 all experimental techniques are in agreement with each other and suggest a divalent 4f7 ground state of the Eu ion. Belowx=0.4 the different methods lead to different results: while the lattice parameters and the ME isomer shift suggest an instable behavior of the valence the magnetic susceptibility proves a pure divalent 4f7 ground state. These differences can be explained by assuming a partial extension of the 4f-shell radius such that the localized character of the 4f-electrons is preserved.

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.

Valence instability in magnetically ordered Eu(Pd1−xAux)2Si2

Abstract 151 Eu Mossbauer effect (300, 77, 4.2 K), X-ray diffraction (300 K), L III -X-ray absorption (300 K) and magnetic susceptibility measurements have been performed on the series Eu(Pd 1−x Au x ) 2 Si 2 (0⩽ x ⩽ 1). The variation of both the isomer shift and the magnetic hyperfine field with volume is found to be unusually large. Although mixed valence behaviour is deduced from L III -X-ray absorption, the magnetic susceptibility suggests divalent 4f 7 states.

57Fe and119Sn Mössbauer studies on La1.25Nd0.6Sr0.15CuO4

Abstract57Fe- and119Sn Mössbauer effect studies on 0.5 at %57Fe (or119Sn) dopednon-superconducting La1.25Nd0.6Sr0.15CuO4 in the temperature region 300 K ≧T≧4.2 K reveal an onset of local magnetic ordering occurring atTm≅32 K for both57Fe—as well as119Sn-doped samples. The local magnetic ordering shows up in the presence of a very large transferred hyperfine field ofBthf≅11.0(5)T at the119Sn nuclei. Since such a large field is neither present in antiferromagnetic La2CuO4, nor in superconducting La1.85Sr0.15CuO4 or overdoped La2−xSrxCuO4 (x=0.4), the occurrence of such local spin correlations seems to be a signature of the non-superconducting low temperature tetragonal phase of La1.25Nd0.6Sr0.15CuO4.

High-pressure studies on Invar alloys: a key for understanding itinerant electron magnetism

Abstract One of the favourable approaches to understand itinerant electron magnetism in 3d transition metals and their alloys is the study of the pressure effect on their magnetic properties. In this respect, the fcc Fe-based Invar alloys are the best candidates for such studies due to their known strong mutual relationship between magnetism and volume. In this paper we demonstrate how high-pressure 57 Fe Mossbauer studies on selected Fe-based Invar alloys provide new findings on the stability of the magnetic moments and their interactions in these systems. The experimental results are in a good agreement with recent spin-polarized band calculations of the volume dependence of the Fe magnetic moment in ordered Fe 3 Ni and Fe 3 Pt. It is shown that such high pressure studies not only shed light on the origin of the Invar anomalies in those alloys but also provide the basis for a better understanding of itinerant 3d magnetism.

On the nature of the valence transition in Eu(Pd1−xAux)2Si2

The intermediate valent compound series Eu(Pd1−xAux)2Si2 has been studied by Mossbauer effect measurements on 151Eu(T = 4.2−300K) and 197Au (4.2K) and by X-ray diffraction (10K, 300K). The temperature induced valence transition Eu2+ → Eu3+ for x < 0.175 is not of first order type, as suggested in a previous phase diagram [1]. The valence change of the Eu-ion is reflected also in the isomer shift of the 197Au Mossbauer-resonance.

Tilting of the CuO{sub 6} octahedra in La{sub 1.83{minus}{ital x}}Eu{sub 0.17}Sr{sub {ital x}}CuO{sub 4} as seen by {sup 151}Eu M{umlt o}ssbauer spectroscopy

{sup 151}Eu M{umlt o}ssbauer studies of La{sub 1.83{minus}{ital x}}Eu{sub 0.17}Sr{sub {ital x}}CuO{sub 4} for 0{le}{ital x}{le}0.26 exhibit a quadrupole interaction at the Eu site whose magnitude {ital eQV}{sub {ital zz}} strongly depends on both the Sr content {ital x} and the temperature {ital T}. For a fixed temperature, a linear relationship is obtained between the quadrupole interaction and the averaged CuO{sub 6} octahedra tilting angle determined from diffraction studies. The unusually large temperature dependence of {ital eQV}{sub {ital zz}} strongly indicates a temperature dependence of the local structure. In contrast to recent studies of the local structure La{sub 2}CuO{sub 4}-type superconductors by means of x-ray-absorption and neutron-scattering techniques, our data give strong evidence that the local octahedra tilting in La{sub 1.83{minus}{ital x}}Eu{sub 0.17}Sr{sub {ital x}}CuO{sub 4} corresponds well to the average tilting obtained by diffraction techniques. {copyright}{ital 1996 The American Physical Society.}

Magnetic order versus intermediate valence in Eu(Pd1−xAux)2Si2 under high pressure

Abstract The magnetically ordering compound Eu(Pd 0.7 Au 0.3 ) 2 Si 2 has been studied by 151 Eu Mossbauer effect at 300 and 4.2 K under pressure up to 3.9 GPa. Above ≈ 2.7 GPa the Eu-ion changes its valence from divalent to intermediate valent (IV) character. The results indicate an increase of the magnetic hyperfine field B eff in the valence region of 2.0 2.1 B eff vanished abruptly displaying a collapse of the magnetic order (MO). It is shown, that MO and IV can coexist in the valence region 2.0

57Fe and 119Sn Mössbauer studies on La1.25Nd0.6Sr0.15CuO4: Evidence for local magnetic ordering below ≂32 K (abstract)

57Fe and 119Sn Mossbauer effect studies on 0.5 at. % 57Fe or 119Sn doped nonsuperconducting La1.25Nd0.6Sr0.15CuO4 in the temperature region 300 K≥T≥4.2 K reveal an onset of local magnetic ordering occurring at TM≂32 K for both 57Fe as well as 119Sn‐doped samples. The local magnetic ordering shows up in the presence of a very large transferred hyperfine field of Bthf≂11.0(5) T at the 119Sn nuclei. Since such a large field is present in neither antiferromagnetic La2CuO4 superconducting La1.85Sr0.15CuO4 nor overdoped La2−xSrxCuO4 (x=0.4), the occurrence of such local spin correlations seems to be a signature of the nonsuperconducting low temperature tetragonal phase of La1.25Nd0.6Br0.15CuO4.