W. Zinn

Short- and long period oscillations in the exchange coupling of Fe across epitaxially grown Al- and Au-interlayers

Abstract We have investigated the exchange coupling of Fe films across Al- and Au-interlayers with good epitaxial growth. For the first time coupling across Al was found to be antiferromagnetic (a.f.). In addition to the long period oscillations of the coupling now well known from many other systems there are short period oscillations which are clearly visible for Au and faintly visible also for Al. This observation can be correlated with the much better growth of Au interlayers. In both cases we find also a strong contribution of biquadratic coupling, previously identified for the Fe/Cr system. The results strongly support an RKKY-type mechanism as the microscopic origin of these couplings.

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.

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.

Interlayer coupling of Fe films across Cr interlayers and its relation to growth and structure

In order to investigate the exchange coupling of Fe films across Cr interlayers we have grown Fe/Cr/Fe structures with good epitaxy and crystallinity, which is documented by in situ SPALEED and ex situ RBS investigations. For samples deposited at substrate temperature T s = 293 K, the coupling oscillates as a function of the interlayer thickness with a period of approximately 1.8 nm. For samples made at T s = 523 K, there is, in addition, a short period oscillation with a period of two ML and an appreciable contribution of 90° type coupling. The temperature dependence of the oscillations has been investigated.

Interlayer coupling across noble metal spacers

Abstract We have investigated the exchange coupling of Fe layers across interlayers of Au, Ag, Cu and Au-Cu alloys. In order to evaluate the dependence of the coupling on the interlayer thickness the functional form expected from RKKY-like theory is used. In this way two oscillation periods are evaluated which can be compared with predictions based on the known Fermi surfaces of the interlayer materials. The data from the alloy interlayers suggest that the coupling strength depends strongly on the structural order of the interlayer material. Also in the case of the alloys the changes in the Fermi surfaces with composition are evaluated.

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.

Higher order interaction terms in coupled Fe/Cr/Fe sandwich structures

Abstract From experimental studies of magnetization curves of Fe/Cr/Fe sandwich structures grown epitaxially on GaAs (100) substrates there is evidence for a cosine-square term in addition to the linear Heisenberg-like interlayer exchange interaction. The different dependences of both exchange mechanisms with the interlayer thickness are shown to result in thickness ranges where the biquadratic term even dominates the linear one which results in characteristic anomalies in the remagnetization behaviour.

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.

Investigation of 90° coupling in Fe/Ag/Fe structures: ‘‘Loose spins’’ and fluctuation mechanism

Results are outlined of experimental work concerning Slonczewski’s theories explaining 90° coupling: ‘‘loose spin’’ theory and thickness‐fluctuation mechanism. The loose spin theory suggests that 90° coupling originates from paramagnetic impurities in the interlayer of an exchange‐coupled layered structure. The influence of these paramagnetic impurities on the magnetic exchange coupling was studied using the wedge technique. High‐quality single‐crystalline Fe/Ag/Fe samples were prepared in UHV and each sample consisted of two wedges: one wedge with additional Fe deliberately inserted during growth into the Ag interlayer and one wedge with a pure Fe/Ag/Fe structure. The detailed analysis of magneto‐optic hysteresis loops revealed quantitatively the bilinearly and the 90° coupling strength. As predicted by loose spin theory additional Fe in the Ag spacer lead to a strong temperature dependence of the 90° coupling. According to the theory an increase of 90° coupling strength proportional to impurity concentr...