Ch. Sauer

Investigation of magnetite thin films produced by pulsed laser deposition

Abstract Thin films of magnetite (Fe 3 O 4 ) with thicknesses in the range 10–1000 nm have been produced by the XeCl excimer laser ( λ = 308 nm) ablation of both Fe 3 O 4 and metallic 57 Fe targets. Deposition conditions have been investigated in an attempt to fabricate films reproducibly at low laser fluences so as to minimize ablation bouldering. The ablation of metallic Fe has been shown to produce a smaller concentration of boulders than the ablation of bulk magnetite. It is believed that this is mainly due to the difference in primary ablation mechanisms between the two target materials. The use of single-crystal, lattice-matching substrates was found to be necessary at lower fluences (1–3 J cm −2 ) for the ablation of metallic Fe, whereas polycrystalline, non-lattice-matching substrates were adequate for deposition with moderate laser fluences (5–6 J cm −2 ). Film quality has been assessed by a range of techniques including XRD, SEM, AFM/MFM and CEMS. Using CEMS it has been established that the films produced from a metallic target contained a significant amount of metallic Fe, together with secondary oxide phases. The influence of film thickness on the Verwey transition has been investigated via electrical conductivity and SQUID magnetometry. It is found that there is a systematic reduction of Verwey temperature with decreasing film thickness; this is attributed to the effect of strain.

Exchange biasing in MBE-grown Ni80Fe20/Fe50Mn50 bilayers

The exchange biasing field (Heb) and coercive field (Hc) of molecular-beam-epitaxy (MBE) grown Cu/Ni80Fe20/Fe50Mn50 samples in [111], [001] and [110] orientations were investigated by Kerr effect measurements. A strong dependence of Heb and Hc on the growth orientation is observed. A strong uniaxial in-plane anisotropy introduced by the Fe50Mn50 layer was found for the [110]-oriented sample. Conversion electron Mossbauer studies revealed a roughness of 2–3 A and no significant moment reduction at the Ni80Fe20/Fe50Mn50 interface.

Magnetic hyperfine fields near the Fe / Cr (100, 110) interface studied by CEMS

Abstract Magnetic hyperfine (hf) fields near the Fe/Cr-interface in epitaxial thin film structures with (110)- and (100)-orientation were analyzed by means of in situ conversion electron Mossbauer spectroscopy (CEMS). The hf field at room temperature in the first Fe-monolayer (ML) at the interface is strongly reduced to 22.0 T (110) and 20.9 T (100) compared with the Fe-bulk value of 33.4 T. In contrast, in the second and third Fe-monolayer with respect to the interface the hf field was enhanced by 0.3 T to 33.7 for both film orientations.

Influence of stoichiometry on hexaferrite structure

Abstract Barium hexaferrite particles prepared by several techniques yield best magnetic characteristics if a certain barium surplus is used. In the case of unsubstituted sol-gel-derived barium hexaferrites, highest values of the specific saturation magnetization occur in powders with n = Fe 2 O 3 /BaO = 5.25 in the starting composition. Best magnetic properties are in coincidence with an ideal occupancy of the 12k sites in the magnetoplumbite structure.

The sign of the EFG in semiconducting and grain-oriented high-Tc YBa2(Cu1−χ57Feχ)3Oy by Mössbauer spectroscopy

Abstract The structure of both the orthorhombic (y→7, high Tc) and tetragonal (y→6, semiconducting) YBa2Cu3Oy phases includ es two Cu sites, denoted Cu1 and Cu2. From NQR/NMR measurements [1] on the high-Tc sample the ratio of 63Cu quadrupole frequencies ν(Cu2) /ν(Cu1) ⋍ 1.43 was determined. The results were consistent with the structural hypothesis that the asymmetry parameter η and the orientation of Vzz obey the following: Cu2 (η⋍0, Vzz|c axis); Cu1 (η⋍1). These conclusions have been checked, and the sign of the EFG determined, using Mossbauer spectroscopy from 57Fe-doped semiconducting and grain-oriented high-Tc samples. We have shown that for y=6.9: (1) the 57Fe splitting ΔE Q (2)/ ΔE Q (1)⋍1.39 , consistent with the above NQR/ NMR results; (2) that for Fe on Cu2 sites, the large line intensity asymmetry shows that the EFG is negative, and implies that η is small (η⋍0 and Vzz|c); (3) that for Fe on site Cu1, the symmetrical doublet implies η⋍1 (and so no EFG sign). For y=6.0: (1) there is a large decrease in ΔEQ(2), and (2) Vzz(2) is now positive. Thus the sign of the EFG for 57Fe on Cu2 sites different for the superconducting and the semiconducting states.

Interface roughness in Fe(100)/Cr film structures studied by CEMS

Abstract Various epitaxial Fe(100)/Cr film structures were MBE-grown on MgO(100) and GaAs(100) substrates with the aim to modify the roughness of the Fe/Cr interfaces. By introducing a 2 monolayer thick 57Fe probe layer at the interface the distribution of the magnetic hyperfine (hf) fields could be measured locally by means of 57Fe conversion electron Mossbauer spectroscopy (CEMS). A simple model is applied which allows the determination of a pattern of the average interface roughness from this hf field distribution. It was observed that even samples of high epitaxial quality according to LEED and RHEED reveal a micro-roughness on a lateral scale of 1–2 nm due to intermixing of Fe and Cr within 1–2 monolayers.

On the Mössbauer site assignment in YBa2(Cu1−xFex)3Oy

The assignment of the four57Fe Mössbauer sites which are observed in Fe-substituted YBa2Cu3Oy is presented. We present a systematic study of highly-controlled samples with low Fe-concentration over the whole oxygen concentration range. Two sites are found to dominate in the limit of high oxidation (y→7), indentified as Fe4+ on the two crystallographic Cu sites: Cu(1) and Cu(2). These are replaced at lower oxidation (y→6) by two others, which we identify with Fe3+ on these same two sites. Consistency is found with the sign of the electric field gradient (EFG), the asymmetry parameter η and the isomer shiftIS for this model. The magnetic structure in the limit y→6 shows changes in the antiferromagnetic (AF) structure dependent on iron concentration. Mössbauer and spin-echo NMR results show a change in the Cu(2) AF layer sequence at low temperatures, without any magnetic moments on the Cu(1) chain sites.

An electron spectrometer for depth selective conversion electron Mössbauer spectroscopy

Abstract A spectrometer has been constructed for depth selective conversion electron Mossbauer spectroscopy using an electrostatic cylindrical mirror analyzer. The spectrometer allows depth selective investigations even of samples with 57Fe in natural abundance. This was demonstrated on an Fe78B11Si9C2 amorphous metglas where the magnetic hyperfine field and the orientation of the magnetization was determined in dependence of the distance from the sample surface.

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.