Noncollinear magnetic hyperfine fields in the Ag spacers of Fe/Ag multilayers

Nearly perpendicular magnetic hyperfine fields have been observed for the first time in the Ag "spacers" of Fe/Ag multilayers using low temperature nuclear orientation of (110)Ag(m) at 6 mK. At the same time, vibrating sample magnetometry measurements at temperatures down to 4 K have shown the magnetic anisotropy of the Fe to be in plane. The direction of the Ag hyperfine field is thus noncollinear (nearly orthogonal) to the Fe anisotropy. These results are compared with full potential linearized augmented plane wave calculations using the wien97 code.     

Mössbauer studies of spin wave excitations in Fe/Ag multilayers

The magnetic properties of molecular beam epitaxially (MBE) grown FE(110)/Ag(111) heterostructures were investigated with Mössbauer spectroscopy. The Fe bilayers were fixed at 3 ML (monolayer) thickness and the Ag bilayer thickness varied from 4 ML to 20 ML. We found that as the Ag layer became thick enough (>17 ML) to magnetically isolate the Fe layers, a quasi-linear temperature dependence of the hyperfine field results due to the 2-D spin wave excitations. As the Ag layer is reduced, a dimensional crossover in the excitations is induced by the magnetic interaction between Fe layers which makesM(T) change from a two-dimensionalT relation to a three-dimensionalT ^3/2 dependence. We constructed a simple theoretical model to motivate the explanation for the experimental results and obtained approximate values for the interlayer coupling strength for various Ag bilayer thicknesses.     

Magnetic anisotropy in multilayers

Magnetic anisotropy between in-plane and out of plane magnetic alignments is studied in a variety of multilayer systems using Mössbauer spectrosopy to observe the (Fe) magnetic orientation. The surface anisotropy in Fe/Au (1 1 1) multilayers is measured as K _s = 0.9 × 10^?3 Jm^?2. In Fe/Ni multilayers the dependence of magnetic orientation on external field applied normal to the layers enables volume and interface anisotropies K _v = (?5 ± 1) × 10⁴ Jm^?3 and K _s = (?0.6 ± 0.4)× 10^?3 Jm^?2 to be evaluated. In similar applied field experiments coherent rotation of the magnetic Fe and NiFe layers in Fe/Cu/NiFe/Cu multilayers was observed for intervening Cu layer thickness x = 5 Å but independent rotation for x = 50 Å. Out of plane magnetic components are observed for DyFe₂, YFe₂ thin films and DyFe₂/YFe₂ multilayers. In fields of up to 0.25 T applied inplane only the moments of the YFe₂ film showed significant rotation.     

CEMS-study of the interface in Fe/Cr multilayer systems

The magnetic hyperfine (hf.) fields at the Fe/Cr interface were analyzed in epitaxial Fe/Cr thin film structures of (100)- and (110)-orientation with monolayer resolution by means of in-situ⁵⁷Fe Conversion Electron Mössbauer Spectroscopy (CEMS). The hf. field (300 K) in the 1st Fe-monolayer (ML) at the interface has been found to be strongly reduced to 22.0/20.9 T for (110)-/(100)-orientation, whereas the 2nd and 3rd ML reveal a slightly increased hf. field of 33.7 T as compared with the Fe-bulk value of 33.4 T. The temperature dependence of the hf. field at the interface shows aT ^3/2 spin wave law. The spin wave parameters are enlarged with respect to the bulk value indicating a reduced exchange interaction. A discontinuity in theT ^3/2-dependence is interpreted by the onset of magnetic order (Néel-temperature) of the Cr layers adjoining the⁵⁷Fe probe layer.     

Application of M?ssbauer spectroscopy in magnetism

An overview is provided on our recent work that applies ⁵⁷Fe M?ssbauer spectroscopy to specific problems in nanomagnetism. ⁵⁷Fe conversion electron M?ssbauer spectroscopy (CEMS) in conjunction with the ⁵⁷Fe probe layer technique as well as ⁵⁷Fe nuclear resonant scattering (NRS) were employed for the study of various nanoscale layered systems: (i) metastable fct-Fe; a strongly enhanced hyperfine magnetic field B_hf of ～39?T at 25?K was observed in ultrahigh vacuum (UHV) on uncoated three-monolayers thick epitaxial face-centered tetragonal (fct) ⁵⁷Fe(110) ultrathin films grown by molecular-beam epitaxy (MBE) on vicinal Pd(110) substrates; this indicates the presence of enhanced Fe local moments, μ_Fe, as predicted theoretically; (ii) Fe spin structure; by applying magnetic fields, the Fe spin structure during magnetization reversal in layered (Sm–Co)/Fe exchange spring magnets and in exchange-biased Fe/MnF₂ bilayers was proven to be non-collinear and depth-dependent; (iii) ferromagnet/semiconductor interfaces for electrical spin injection; CEMS was used as a diagnostic tool for the investigation of magnetism at the buried interface of Fe electrical contacts on the clean surface of GaAs(001) and GaAs(001)-based spin light-emitting diodes (spin LED) with in-plane or out-of-plane Fe spin orientation; the measured rather large average hyperfine field of ～27?T at 295?K and the distribution of hyperfine magnetic fields, P(B_hf), provide evidence for the absence of magnetically “dead” layers and the existence of relatively large Fe moments (μ_Fe ～ 1.8?μ_B) at the ferromagnet/semiconductor interface. - Finally, a short outlook is given for potential applications of M?ssbauer spectroscopy on topical subjects of nanomagnetism/spintronics.     

Modeling of the angular dependences of nuclear resonant reflectivity for different types of noncollinear magnetic ordering

The peculiarities of nuclear-resonance reflectometry (NRR) for different types of magnetic ordering in multilayers are considered. The theory of NRR for noncollinear magnetic structures is briefly presented. Model calculations are carried out for the [⁵⁷Fe(8Å)/Cr(20 Å)]₃₀ structure, in which the magnetization varies with depth in different ways. The results show that the “magnetic” maximum on the NRR curve occurs only when the directions of the hyperfine fields in adjacent iron layers have different projections on the synchronous beam direction. Spiral alignment of the multilayer magnetization causes the emergence of satellites near the Bragg maximum, while the “magnetic” maximum does not occur. With more complicated magnetic ordering profiles the characteristic features of available experimental curves can be explained.     

Hyperfine interaction experienced by181Ta nuclei at the Hf site in HfFe2−xSix compounds through a decoupling experiment

The time-differential perturbed angular correlation (TDPAC) experiments involving the 133–482 keV γ-γ cascade in¹⁸¹Ta have been performed in the presence of an external magnetic field along the quantization axis to measure the hyperfine magnetic field experienced by¹⁸¹Ta nuclei at the Hf site in the pseudobinary compounds HfFe_2?xSi_x with x=0.1 and x=0.3. The hyperfine magnetic fields measured at 298 K are H_hf=133.1±12.0 kG in the cubic (C15) Laves phase compound HfFe_1.9Si_0.1 and H_hf=76.8±7.0 kG in the hexagonal (C14) Laves phase compound HfFe_1.7Si_0.3. The measured hyperfine fields are discussed within the framework of the Campbell-Blandin model.     

Mössbauer and TDPAC Studies on Fe/Mo Multilayers

Hyperfine fields at Fe and Mo layers in polyimide/Fe(10 nm)/[Mo(1.1 nm)/Fe(2.0 nm)]₁₂₀ and [Mo(1.3 nm) /Fe(2.0 nm)]₁₂₀ multilayers prepared by the electron-beam evaporation technique were measured at room-temperature by Mössbauer spectroscopy and perturbed-angular-correlation spectroscopy. The hyperfine fields in the Fe layers do not show a clear dependence on the Mo layer thickness. On the other hand, the hyperfine fields in the Mo layers show different magnetic structures in these samples. The difference suggests a variation of electron spin polarization in the Mo layers.     

Layer magnetization canting in 57Fe/FeSi multilayer observed by synchrotron Mössbauer reflectometry

Synchrotron Mössbauer reflectometry and CEMS results on a [⁵⁷Fe(2.55 nm)/FeSi\break(1.57 nm)]₁₀ multilayer (ML) on a Zerodur substrate are reported. CEMS spectra are satisfactorily fitted by α‐Fe and an interface layer of random α‐(Fe, Si) alloy of 20% of the ⁵⁷Fe layer thickness on both sides of the individual Fe layers. Kerr loops show a fully compensated AF magnetic layer structure. Prompt X‐ray reflectivity curves show the structural ML Bragg peak and Kiessig oscillations corresponding to a bilayer period and total film thickness of 4.12 and 41.2 nm, respectively. Grazing incidence nuclear resonant Θ–2Θ scans and time spectra (E = 14.413 keV, λ = 0.0860 nm) were recorded in different external magnetic fields (0 < B^ext < 0.95 T) perpendicular to the scattering plane. The time integral delayed nuclear Θ–2Θ scans reveal the magnetic ML period doubling. With increasing transversal external magnetic field, the antiferromagnetic ML Bragg peak disappears due to Fe layer magnetization canting, the extent of which is calculated from the fit of the time spectra and the Θ–2Θ scans using an optical approach. In a weak external field the Fe layer magnetization directions are neither parallel with nor perpendicular to the external field. We suggest that the interlayer coupling in [Fe/FeSi]₁₀ varies with the distance from the substrate and the ML consists of two magnetically distinct regions, being of ferromagnetic character near substrate and antiferromagnetic closer to the surface.     

Ferromagnetic Semiconductors Studied by Hyperfine Interaction of Nuclear Probes

The ferromagnetic spinels CdCr₂Se₄ and CuCr₂Se₄ were investigated by PAC (perturbed angular correlations) after implantation of the probes ¹¹¹In(¹¹¹Cd), ^111mCd, ¹¹¹Ag(¹¹¹Cd) and ⁷⁷Br(⁷⁷Se). The site occupation of different probes was determined by magnetic and electric hyperfine interactions. Theoretical calculations of hyperfine parameters by the WIEN97 code gave satisfactory efg (electric field gradients) but the magnetic hyperfine fields are inconsistent with the experiments. This revised version was published online in September 2006 with corrections to the Cover Date.     

Apparent giant hyperfine anomaly of nuclear levels in the indium isotopes — A puzzle

The magnetic hyperfine fields of In in Fe, Co and Ni were measured by PAC using¹¹⁷In, and by NMR-ON using^114mIn and¹¹⁴In. There is a disparity of about 8% (4%) between the fields derived from PAC on¹¹⁷In (NMR-ON on¹¹⁴In) and the values that have been measured using isomeric 1g_9/2 proton states in^109,111,115In. This is tentatively attributed to a giant hyperfine anomaly.     

Magnetic ordering in a [Fe/Co]<Subscript>35</Subscript> BCC film studied by nuclear resonant reflectometry

An analysis of the angular dependences of nuclear resonant reflectivity time spectra for different models of magnetic ordering in films reveals an ambiguity in the magnetization direction determined from spectra measured at one orientation of the sample. This analysis explains features in the spectra of the nuclear resonant reflectivity from a MgO/[Fe(6 ML)/Co(3 ML)]₃₅/V (1 nm) film measured before and after sample rotation by 90° about the normal to the surface. It is shown that the spectrum measured only at one orientation of the sample determines only the effective azimuth angle of magnetization γ^eff. This does not exclude the occurrence of a domain structure, while the angle γ^eff does not correspond to the true direction of the preferred orientation of magnetization. The results of measurements at two orientations of the sample can be satisfactorily matched using a model that considers a coherent mixture of states with magnetization directed along the 〈110〉 axis (77%) and with a chaotic orientation of the magnetic hyperfine field B _hf in the film plane for the other nuclei.     

Hyperfine fields and Mössbauer isomer shifts of 57Fe in PdAgFe alloys

The hyperfine fields B_hf and Mössbauer isomer shifts S of ⁵⁷Fe in two PdAgFe alloy series containing 3 and 7 at% have been measured at 4.2 K and 295 K. With increasing Ag content, a change in the magnetic ordering from ferromagnetism towards a spin-glass behaviour is indicated in the 3 at% Fe series.     

Magnetism of ultrathin Cr layers and Fe/Cr multilayers studied by 119Sn probes

Cr/Sn and Fe/Cr/Sn/Cr multilayers, where monatomic Sn layers are embedded in Cr layers and Fe/Cr multilayers respectively, were prepared by means of ultrahigh-vacuum deposition technique, and the magnetic hyperfine field induced at the ¹¹⁹Sn nuclear sites was examined using conversion electron Mössbauer spectroscopy. The magnetic structures of the Cr layers are inferred from the size and direction of the magnetic hyperfine field transferred at the Sn sites.     

First-principles studies of the magnetic hyperfine field in Fe multilayers

A detailed theoretical study of the magnetic moments and magnetic hyperfine fields in several Fe multilayers (Fe fcc(001)/5X fcc(001), X=Cu and Ag, and Fe bcc(001)/5X fcc(001), X=Ag and Au) as well as in bulk Fe is presented. The calculations have been performed using the spin-polarized, relativistic linear muffin-tin orbital (SPR-LMTO) method of band structure calculation. Therefore, not only the contribution to the hyperfine fields due to the conventional Fermi contact interaction but also due to the spin dipolar and orbital contributions induced by the crystal field and by spin-orbit coupling are accounted for. To decompose the hyperfine field of non-s-electrons into these contributions it has been assumed that they are proportional to the corresponding so-called magnetic dipole moment and the orbital magnetic moment, respectively. In contrast to previous results for pure metals and alloys not only the orbital but also the spin dipolar hyperfine field was found to be non-negligible. The anisotropy of the hyperfine field determined by calculations for in-plane and perpendicular orientation of the magnetisation was found to be very pronounced and closely connected with the corresponding anisotropy of the magnetic dipole moment and the orbital moment.     

Fe–Ag granular multilayers and heterostructures studied in applied magnetic field

An (0.2 nm ⁵⁷Fe / 2.6 nm Ag)₇₅ granular multilayer sample and heterostructures with additional continuous Fe layers in different sequences were studied in magnetic field applied at different temperatures. The broadening of the superparamagnetic lines was found to be very similar for the three samples in applied fields both parallel and perpendicular to the sample plane. While the layer sequence has no significant effect on the superparamagnetic properties, the continuous magnetic layers follow a different approach to saturation in perpendicular magnetic fields.     

Hyperfine magnetic fields in Fe−Co alloys and their tempera ture dependences

We obtained⁵⁷Fe hyperfine field parameters from Fe₁−x-Co _x alloys (0≤x≤0.6) from 77 K to 900 K. We first discuss the origin of the low temperature hyperfine fields in terms of the 3d and 4s electrons at⁵⁷Fe atoms. The⁵⁷Fe hyperfine magnetic field (hmf) of Fe-Co alloys depends more weakly on temperature than the hmf of pure Fe. This temperature dependence occurs because the alignment of the magnetic moments at both the Fe atoms and at the Co atoms depend on temperature in the same way as the bulk magnetization of Fe-Co alloys.     

151Eu and57Fe Mössbauer effect studies of magnetic interactions in europium transition element oxides solution

The solid state solutions of europium transition element oxides Eu (Fe0.8M0.2)O₃ (M=Sc,Cr,Mn,Co) are synthesized. The X-ray diffraction of the compound shows that all the compounds possess the perovskite structures. Both the¹⁵¹Eu Mössbauer spectra and the⁵⁷Fe Mössbauer spectra are measured. The hyperfine magnetic field and non-axisymmetric electric field gradient are observed in the¹⁵¹Eu Mössbauer spectrum. The⁵⁷Fe Mössbauer spectrum shows that there are four components of hyperfine fields corresponding to four kinds of different neighbours of the Fe ion.     

Magnetic hyperfine fields at Fe,Cd and Sn sites in an FeCo alloy

The magnetic hyperfine field at an Fe site in the ferromagnetic alloy Fe_0.475Co_0.525 was measured using the Mössbauer effect. The value obtained at room temperature was 343 kOe. The hyperfine field at a substituted Cd impurity was measured by the method of time differential perturbed angular correlations. A single frequency was observed at room temperature, corresponding to a field of -177 kOe. Using the Mössbauer effect, the Sn site hyperfine field was measured in a sample in which 0.3 atomic percent of ¹¹⁹Sn had been substituted. The room temperature spectrum consisted of the superposition of a single line, together with a six-line hyperfine spectrum, corresponding to a field of 231 kOe. A phenomenological interpretation is proposed for Fe, Cd and Sn fields in the binary alloys of iron.     

Electronic structure and magnetism of Cr/Sn multilayer systems

The electronic structure and magnetism of Cr/Sn and Fe/Cr/Sn/Cr multilayer systems with monolayer Sn are studied by means of a first-principles method. The calculated hyperfine field at Sn site is significantly large (∼20 T) in Cr/Sn multilayers, while the value is remarkably diminished (∼4 T) in the case of Fe/Cr/Sn/Cr multilayers. This trend of the hyperfine field is consistent with recently reported experimental results. It is found that the hyperfine field at Sn site is determined by the spin magnetic moment at the interface Cr site. The most important feature in the electronic structure of the multilayer systems is the existence of an interface state at the Cr interface layer near the Fermi energy region.