U. Köbler

High field Mössbauer effect study of LaFe12O19

Abstract Mossbauer effect measurements on 57 Fe in LaFe 12 O 19 at room temperature and at 4.2 K in an external magnetic field of 10 T confirm the suggestion of Lotgering[2] that the replacement of Ba 2+ in BaFe 12 O 19 by La 3+ is accompanied by a valence change of Fe 3+ to Fe 2+ in the 2a-sublattice. The corresponding Fe 2+ -subspectrum has been detected. The increased hyperfine fields of the 12 k - and 2 b -sites in LaFe 12 O 19 are discussed in terms of the different exchange paths between the iron sublattices.

Field induced valence shift in mixed valent EuCu2Si2

Abstract The mixed valent intermetallic compound EuCu 2 Si 2 has been studied at temperatures 4.2 K ⩽ T ⩽ 320 K and in external magnetic fields up to B o = 12.8 T using the 151-Eu Mossbauer effect. In a limited temperature range around T = 180 K the application of B o = 12.8 T induces a valence shift towards Eu 2+ indicated by a change of the isomer shift S by Δ S = 0.2 mm/s. Magnetization measurements up to B o = 8 T at different temperatures 1.9 ⩽T⩽ 240 K and low field susceptibility data are shown to be consistent with the Mossbauer effect results.

Magnetic properties of amorphous Fe in Fe/Y layered structures

Abstract In Y/Fe/Y layered structures the Fe interlayer grows in an amorphous structure up to a critical thickness of d Fe = 2.3 nm. Above this thickness the Fe film transforms to a nanocrystalline structure. The magnetic properties of the amorphous Fe film (a-Fe) were determined by means of 57 Fe conversion electron Mossbauer spectroscopy and magnetization measurements. The average magnetic moment of a-Fe amounts to if μ = (1.2±0.2)μ B and the ferromagnetic Curie temperature to T c = (80±3) K. There seems to be a broad distribution of the magnitude of the magnetic moments. No indication of spin glass properties was found.

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.

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.

Transferred hyperfine fields in the EuS-SrS system studied by NMR

Abstract The NMR spectra of (Eu x Sr 1− x )S for x = 0.99, 0.5 and 0.05 have been studied at 1.1 K. The transferred hyperfine field of each nearest Eu neighbour is -0.335 T. No contribution of the other neighbour shells is found.

Effect of Sn substitution for Cu on the magnetic and superconducting properties of La2−xSrx(Cu1−ySny)O4+δ (0≤x≤0.15)

Abstract We have investigated the effect of Sn doping on the magnetic and superconducting properties of La 2− x Sr x (Cu 1− y Sn y ) O 4+δ with 0≤ x ≤0.15 and 0≤ y ≤0.01. Measurements of the 119 Sn Mossbauer isomer shift of samples doped with y =0.005 ( 119 Sn) show that Sn is tetravalent (Sn 4+ ) in the whole concentration range 0≤ x ≤0.15. On the other hand, the electrical resistivity, the Neel temperature ( T N ), and the superconducting temperature ( T c ) are found to be affected by increasing the Sn content. In the antiferromagnetic state ( x =0,0.008), we find that T c is slightly affected by increasing Sn content. In the superconducting state ( x =0.065, 0.10, and 0.15), T c is strongly suppressed with increasing Sn content. It is shown that the variation of T N upon Sn doping depends on the competition between two mechanisms: (1) magnetic disorder due to the nonmagnetic impurity which induces an uncompensated local magnetic moment on the CuO 2 lattice and thereby tends to decrease T N ; (2) reduction (compensation) of the number of the free O − holes by excess electrons introduced by Sn 4+ which leads to stabilizing the antiferromagnetic order and increasing T N . We further show that the suppression of T c with increasing Sn content can be explained by the same magnetic mechanism (1) responsible for decreasing T N .