Imaging the magnetic spin structure of exchange-coupled thin films

Magnetic Friedel Oscillation at the Fe(001) Surface: Direct Observation by Atomic-Layer-Resolved Synchrotron Radiation ^{57}Fe Mössbauer Spectroscopy

The surface magnetism of Fe(001) was studied in an atomic layer-by-layer fashion by using the in situ iron-57 probe layer method with a synchrotron Mössbauer source. The observed internal hyperfine field H_{int} exhibits a marked decrease at the surface and an oscillatory behavior with increasing depth in the individual upper four layers below the surface. The calculated layer-depth dependencies of the effective hyperfine field |H_{eff}|, isomer shift δ, and quadrupole shift 2ϵ agree well with the observed experimental parameters. These results provide the first experimental evidence for the magnetic Friedel oscillations, which penetrate several layers from the Fe(001) surface.

Incoherent Nuclear Resonant Scattering from a Standing Spin Wave

We introduce a method to study the spatial profiles of standing spin waves in ferromagnetic microstructures. The method relies on Nuclear Resonant Scattering of ⁵⁷Fe using a microfocused beam of synchrotron radiation, the transverse coherence length of which is smaller than the length scale of lateral variations in the magnetization dynamics. Using this experimental method, the nuclear resonant scattering signal due to a confined spin wave is determined on the basis of an incoherent superposition model. From the fits of the Nuclear Resonant Scattering time spectra, the precessional amplitude profile across the stripe predicted by an analytical model is reconstructed. Our results pave the way for studying non-homogeneous dynamic spin configurations in microstructured magnetic systems using nuclear resonant scattering of synchrotron light.

Nuclear resonance reflectivity from a [57Fe/Cr]30 multilayer with the Synchrotron Mössbauer Source

Mössbauer reflectivity spectra and nuclear resonance reflectivity (NRR) curves have been measured using the Synchrotron Mössbauer Source (SMS) for a [⁵⁷Fe/Cr]₃₀ periodic multilayer, characterized by the antiferromagnetic interlayer coupling between adjacent ⁵⁷Fe layers. Specific features of the Mössbauer reflectivity spectra measured with π-polarized radiation of the SMS near the critical angle and at the `magnetic' maximum on the NRR curve are analyzed. The variation of the ratio of lines in the Mössbauer reflectivity spectra and the change of the intensity of the `magnetic' maximum under an applied external field has been used to reveal the transformation of the magnetic alignment in the investigated multilayer.

Stabilization of magnetic helix in exchange-coupled thin films

Based on micromagnetic simulations, we report on a novel magnetic helix in a soft magnetic film that is sandwiched between and exchange-coupled to two hard magnetic layers with different anisotropies. We show that such a confined helix stays stable without the presence of an external magnetic field. The magnetic stability is determined by the energy minimization and is a result of an internal magnetic field created by the exchange interaction. We show that this internal field stores a magnetic energy density of a few kJ/m³. We also find that it dramatically modifies ferromagnetic resonances, such that the helix can be used as a ferromagnetic resonance filter and a fast acting attenuator.

Spin precession mapping at ferromagnetic resonance via nuclear resonant scattering of synchrotron radiation

We probe the spin dynamics in a thin magnetic film at ferromagnetic resonance by nuclear resonant scattering of synchrotron radiation at the 14.4 keV resonance of ^{57}Fe. The precession of the magnetization leads to an apparent reduction of the magnetic hyperfine field acting at the ^{57}Fe nuclei. The spin dynamics is described in a stochastic relaxation model adapted to the ferromagnetic resonance theory by Smit and Beljers to model the decay of the excited nuclear state. From the fits of the measured data, the shape of the precession cone of the spins is determined. Our results open a new perspective to determine magnetization dynamics in layered structures with very high depth resolution by employing ultrathin isotopic probe layers.

Magnetization reversal process at atomic scale in systems with itinerant electrons

The magnetic response of itinerant electrons systems to an external magnetic field is investigated on the basis of a microscopic Hamiltonian from which the spin-polarized electronic structure is determined. The magnetic moment and grand thermodynamic potential of the d-electronic subsystem on a particular atomic site in the presence of the external field are calculated as a function of the moment's orientation for fixed electron configuration of its local environment. Self-consistent magnetic solutions strongly depend on the d-electron number, determined by the position of the d level relative to the Fermi energy. For parameters corresponding to α-Fe, two branches of self-consistent solutions with high and low magnetic moments are found. For parameters corresponding to bulk Cr, a Fe impurity in the Cr matrix and a Cr impurity in the Fe matrix, there are only low-spin solutions. The theory is also applied for describing magnetization reversal processes in exchange spring magnets. A slab of Fe was considered as a soft magnetic layer. The influence of the hard magnet is modeled by the inclusion of an external magnetic field applied to the interface Fe layers. The dependence of the hysteresis loop on the thickness of the Fe slab and on the value of the interface field is investigated.

Grazing-incidence synchrotron-radiation ⁵⁷Fe-Mössbauer spectroscopy using a nuclear Bragg monochromator and its application to the study of magnetic thin films

Energy-domain grazing-incidence (57)Fe-Mössbauer spectroscopy (E-GIMS) with synchrotron radiation (SR) has been developed to study surface and interface structures of thin films. Highly brilliant (57)Fe-Mössbauer radiation, filtered from SR by a (57)FeBO(3) single-crystal nuclear Bragg monochromator, allows conventional Mössbauer spectroscopy to be performed for dilute (57)Fe in a mirror-like film in any bunch-mode operation of SR. A theoretical and experimental study of the specular reflections from isotope-enriched ((57)Fe: 95%) and natural-abundance ((57)Fe: ∼2%) iron thin films has been carried out to clarify the basic features of the coherent interference between electronic and nuclear resonant scattering of (57)Fe-Mössbauer radiation in thin films. Moreover, a new surface- and interface-sensitive method has been developed by the combination of SR-based E-GIMS and the (57)Fe-probe layer technique, which enables us to probe interfacial complex magnetic structures in thin films with atomic-scale depth resolution.

Electron proportional gas counter for linear and elliptical Mössbauer polarimetry

Design, characterization, and selected applications of a novel electron detector dedicated to conventional perpendicular- and low-angle-incidence conversion electron Mössbauer spectroscopy are presented. The setup is suitable for varying the incident angle and external magnetic fields on Mössbauer source and absorber. Test experiments were performed on alpha-(57)Fe films using a conventional single-line (57)Co(Rh) and magnetically split, (57)Co(alpha-Fe) Mössbauer sources. The integral "blackness effect" in conversion-electron Mössbauer spectra of (57)Fe isotope-enriched absorbers is demonstrated and shown to be pronounced at shallow angles of incidence. In order to determine the alignment and sign of the hyperfine field in an isotope-enriched absorber, the blackness effect is accounted for in a semiempirical way by using single-line source/absorber experimental relative intensities determined independently. This method works with high accuracy for linear polarimetry; however it is only a rough approximation in the case of nearly circular polarimetry.

Artificial neural networks applied to the analysis of synchrotron nuclear resonant scattering data

The capabilities of artificial neural networks (ANNs) have been investigated for the analysis of nuclear resonant scattering (NRS) data obtained at a synchrotron source. The major advantage of ANNs over conventional analysis methods is that, after an initial training phase, the analysis is fully automatic and practically instantaneous, which allows for a direct intervention of the experimentalist on-site. This is particularly interesting for NRS experiments, where large amounts of data are obtained in very short time intervals and where the conventional analysis method may become quite time-consuming and complicated. To test the capability of ANNs for the automation of the NRS data analysis, a neural network was trained and applied to the specific case of an Fe/Cr multilayer. It was shown how the hyperfine field parameters of the system could be extracted from the experimental NRS spectra. The reliability and accuracy of the ANN was verified by comparing the output of the network with the results obtained by conventional data analysis.

A setup combining magneto-optical Kerr effect and conversion electron Mössbauer spectrometry for analysis of the near-surface magnetic properties of thin films

We propose a system allowing the characterization of thin magnetic multilayer structures that combine conversion electron Mossbauer spectrometry (CEMS) under applied magnetic field with the magneto-optical Kerr effect (MOKE) technique. Measured hysteresis loops obtained from the MOKE part are used for investigation of sample surface magnetic properties. The CEMS part of such a system is suitable for studying the spatial spin distribution during magnetization reversal under applied magnetic field, whose values are established from the measured MOKE loop. The combined technique is demonstrated on the results obtained at 300 K on an exchange-coupled ferrimagnetic amorphous GdFe/TbFe bilayer, where the center of the GdFe layer is enriched in (57)Fe. Both techniques confirm in-plane uniaxial anisotropy. The spin structure at the position of the probe layer is analyzed for several values of the external magnetic field applied in the hard magnetization axis direction.

Antiferromagnetic order with atomic layer resolution in EuTe(111) films

The temperature dependences of the magnetizations of individual atomic layers across an epitaxial antiferromagnetic EuTe film were derived from virtually background-free magnetic Bragg peaks with pronounced Laue oscillations recorded with soft x rays at the Eu-M5 resonance. The magnetizations of the outermost layers decrease significantly differently from those of bulk layers, in agreement with Heisenberg-Monte Carlo calculations. The results demonstrate the applicability of the method to complex ordering phenomena at surfaces and interfaces of thin films.

A compact UHV deposition system for in situ study of ultrathin films via hard x-ray scattering and spectroscopy

We report on a compact ultrahigh vacuum deposition system developed for in situ experiments using hard x rays. The chamber can be mounted on various synchrotron beamlines for spectroscopic as well as scattering experiments in grazing incidence geometry. The deposition process is completely remotely controlled and an ellipsometer is available for online monitoring of the layer growth process. The unique sample position in the chamber allows one to perform deposition, grazing incidence x-ray experiments, and ellipsometry measurements at the same time, enabling to correlate the x-ray analysis with parameters of the growth process. Additionally, the setup can be used to study in situ chemical and structural changes in an element specific manner by x-ray absorption spectroscopy. The flexibility and versatility of the system brings new possibilities to study the chemistry and structure of surfaces and interfaces in thin films systems during their formation.

The magnetization reversal process in spin spring magnets

The behavior of soft magnetic films in spring magnets, as a function of the intensity and orientation of an external magnetic field, is described in the framework of spin-polarized non-collinear electronic structure calculations. As experimentally observed, the critical intensity of the external field required for the onset of the non-collinear spiral formation depends on both the thickness of the soft magnetic phase and the orientation of the field. The spin spiral structure undergoes a change of chirality in rotating fields. Our theoretical approach opens new prospects for investigating the response of other nanostructures to external magnetic fields beyond usual phenomenological models.

MAGNETISM AND LATTICE DYNAMICS OF NANOSCALE STRUCTURES STUDIED BY NUCLEAR RESONANT SCATTERING OF SYNCHROTRON RADIATION

Nuclear resonant scattering of synchrotron radiation is applied to investigate the magnetic structure and the lattice dynamics of nanoscale systems. The outstanding brilliance of modern synchrotron radiation sources allows for sensitivities to smallest amounts of material. Due to the isotopic sensitivity of the scattering process, ultrathin probe layers of Mössbauer isotopes can be used to map out the magnetic and vibrational structure of thin films with sub-nm spatial resolution. Elastic nuclear resonant scattering is applied to determine the magnetic spin structure of an exchange-coupled bilayer system. Inelastic nuclear resonant scattering was used to determine the vibrational density of states in Fe islands on the W(110) surface.

Accelerating the spontaneous emission of x rays from atoms in a cavity

We have investigated the spontaneous radiative decay of resonant nuclei in a planar x-ray waveguide after excitation by synchrotron radiation pulses. The waveguide acts as a cavity and modifies the mode structure of the electromagnetic field. As a result, the rate of spontaneous emission is enhanced by a factor proportional to the density of photon states in the cavity. In this experiment, we have observed a sixfold acceleration of the coherent radiative decay of 57Fe nuclei located in the center of the first-order guided mode. This is in very good agreement with theoretical predictions.

Nuclear resonant magnetometry and its application to Fe/Cr multilayers

We introduce nuclear resonant magnetometry as a means to record the magnetization curve of isotopically enhanced regions of a sample. It is based on nuclear resonant scattering with circularly polarized synchrotron radiation and the use of a nuclear resonant reference sample. We apply this approach to study the interlayer coupling in Fe/Cr(100) multilayers and to obtain a layer-specific magnetization curve. Our measurements provide experimental evidence for the existence of a nontrivial interlayer-coupling angle in Fe/Cr/Fe.

Site-specific phonon density of states discerned using electronic states

We have performed the measurement of the site-specific phonon densities of states (PDOS) discerned using electronic states. As far as we know, no general method could give the site-specific PDOS, although oscillating properties of the individual atoms in nonequivalent positions are not necessarily equivalent. However, the combination of the energy and time domain measurements of the nuclear resonant scattering of synchrotron radiation allows the identification of site-specific PDOS. We measured the site-specific PDOS of iron atoms in magnetite, which is a mixed valent compound, and the difference between partial phonon densities of the iron sites was clearly observed.