In this paper, we review some of the key concepts in ultrathin film magnetism which underpin nanomagnetism. We survey the results of recent experimental and theoretical studies of well characterized epitaxial structures based on Fe, Co and Ni to illustrate how intrinsic fundamental properties such as the magnetic exchange interactions, magnetic moment and magnetic anisotropies change markedly in ultrathin films as compared with their bulk counterparts, and to emphasize the role of atomic scale structure, strain and crystallinity in determining the magnetic properties. After introducing the key length scales in magnetism, we describe the 2D magnetic phase transition and survey studies of the thickness dependent Curie temperature and the critical exponents which characterize the paramagnetic–ferromagnetic phase transition. We next discuss recent experimental and theoretical results on the determination of the exchange constant, followed by an overview of measurements of the magnetic moment in the elemental 3d transition metal thin films in the various crystal phases that have been successfully stabilized, thereby illustrating the sensitivity of the magnetic moment to the local symmetry and to the atomic environment. Finally, we discuss briefly the magnetic anisotropies of Fe, Co and Ni in the fcc crystalline phase, to emphasize the role of structure and the details of the interface in influencing the magnetic properties. The dramatic effect that adsorbates can have on the magnetic anisotropies of thin magnetic films is also discussed. Our survey demonstrates that the fundamental properties, namely, the magnetic moment and magnetic anisotropies of ultrathin films have dramatically different behaviour compared with those of the bulk while the comparable size of the structural and magnetic contributions to the total energy of ultrathin structures results in an exquisitely sensitive dependence of the magnetic properties on the film structure.

https://iopscience.iop.org/article/10.1088/0034-4885/71/5/056501/pdf

Magnetism in ultrathin film structures

A theoretical description of magneto-optical effects in metallic transition metal systems on the basis of an itinerant description for the underlying electronic structure is presented and applications to the magneto-optical Kerr effect and the circular dichroism in x-ray absorption are discussed Simple arguments based on symmetry considerations as well as electronic excitation schemes are given to show that magneto-optical effects are caused by the interplay of magnetic ordering and spin - orbit coupling The various band structure techniques developed to deal with this situation are reviewed with an emphasize on fully relativistic methods based on the Dirac equation The theoretical framework to calculate magneto-optical spectra on the basis of the underlying band structure is outlined in some detail In addition, a number of sum rules derived by various authors are presented that allow one to estimate spin and orbital magnetic moments of an absorbing atom from the magnetic x-ray dichroism spectra For the magneto-optical Kerr effect, as well as the magnetic x-ray dichroism, application of the theoretical formalism is presented for a great variety of systems, ie pure elements, compounds, alloys and multilayer systems For both kinds of spectroscopies it is demonstrated that model calculations which allow one to manipulate the exchange splitting and the spin - orbit coupling strength give valuable information for a better understanding of the rather complex spectra Concerning the sum rules, this technique together with the direct calculation of the dichroism spectra allow for a stringent test of the various assumptions on which these rules are based

https://iopscience.iop.org/article/10.1088/0034-4885/59/12/003/pdf

Magneto-optical effects in transition metal systems

Using x-ray absorption spectroscopy recent progress is achieved all over in solid state physics. This review focuses on these advances, with particular emphasis on applications to surface physics and to magnetism of ultrathin 3d and 5d films that are made possible by the use of undulators in third generation synchrotron radiation sources: the unambiguous appearance of an atomic extended x-ray absorption fine structure for atomic adsorbates and of σ* resonances in near-edge x-ray absorption fine structure spectra of oriented molecules is demonstrated. The induced magnetism at the interfaces of 3d and 5d layers is studied by x-ray magnetic circular dichroism. Fundamental aspects of the spectroscopy are clarified for rare earth crystals. The determination of the ground state properties and the detailed understanding of the underlying mechanisms was obtained by comparison of the experimental data to state-of-the-art ab initio calculations.

https://qmlab.ubc.ca/ARPES/PEOPLE/Andrea/Teaching/PHYS555_2007/XASreview.pdf

Recent advances in x-ray absorption spectroscopy

An implementation of the multiple-scattering approach to x-ray magnetic circular dichroism (XMCD) in K edge x-ray absorption spectroscopy is presented. The convergence problems due to the cluster size and the relativistic corrections are solved using an expansion of the Dirac Green function for complex energies up to second order in 1/c. The Fermi energy is dealt with via a complex plane integration. Numerical methods used to obtain the semirelativistic Green function in the whole complex plane are explained. We present a calculation of the magnetic circular dichroism at the K edge of bcc iron including the core hole effect. A good agreement is found at high energy. The physical origins of the XMCD spectrum near the edge and far from the edge are analyzed. The influence of the core hole, the possibility of a multiple-scattering expansion, and the relation of XMCD with the spin polarized density of states are discussed. A simple interpretation of XMCD at the K edge is presented in terms of a rigid-band model. \textcopyright{} 1996 The American Physical Society.

Multiple-scattering theory of x-ray magnetic circular dichroism: Implementation and results for the iron K edge.

Growth and structure of cobalt oxide on (001) bct cobalt film

In this paper we discuss the specific features of K-edge magnetic circular x-ray-dichroism (MCXD) spectra of Fe and Co in bcc Fe-Co alloys and Fe/Co multilayers. In the alloys, the systematic variations of shape and amplitude of the near-edge MCXD signals as a function of concentration can be correlated with the changes in local atomic environment. The difference between the Co K-edge and Fe K-edge spectra of the same alloy shows that the signal is central atom dependent. Well above the absorption edge (E-${\mathit{E}}_{0}$g80 eV) the extended x-ray-absorption fine structure (EXAFS)-like oscillations in the MCXD spectra clearly reflect the local bcc structure and appear not to depend on the absorbing atom. In both alloys and multilayers, the magnetic EXAFS intensity can be correlated to the average magnetization in the neighborhood of the absorbing atom.

Magnetic circular x-ray dichroism measurements of Fe-Co alloys and Fe/Co multilayers

Multilayers alternating cobalt with different spacers such as iron and chromium at nanometric scale have been deposited by rf diode sputtering. The structures have been characterized in situ by kinetic ellipsometry and ex situ by grazing x‐ray reflection, x‐ray diffraction, Auger profile analysis, and transmission electron microscopy. Nuclear magnetic resonance (NMR) and high‐field SQUID magnetometry have been used to determine the magnetic properties. In Co/Fe multilayers the structure strongly depends on the cobalt thickness tCo. For tCo below 2 nm, the cobalt layers exhibit a bcc crystalline structure, and the crystalline coherence extends over many periods. For tCo above 2 nm, for Co a mixed fcc‐hcp phase has been observed. In this case the crystalline coherence is destroyed from one iron layer to the other. The NMR frequency (198 MHz), and the magnetic moment (1.58 μB/atom) of bcc cobalt have been evaluated. A similar behavior is observed in Co/Cr multilayers, the bcc phase disappearing around tCo=1.5 nm. Moreover, interdiffusion is observed at the Cr‐on‐Co interface: The samples have zero magnetization up to tCo=0.7 nm. Preliminary experiments show some antiferromagnetic coupling between Co layers.

Magnetic and structural properties of rf‐sputtered Co/Fe and Co/Cr multilayers

Structural properties of diode radio‐frequency (rf)‐sputtered Cr/Co multilayers have been investigated using in situ kinetic ellipsometry, grazing x‐ray reflection, x‐ray diffraction, and nuclear magnetic resonance. Results have been correlated with magnetic characterizations obtained by SQUID susceptometry. Interdiffusion along ≂10 A occurs at the ‘‘Cr on Co’’ interface. The Co on Cr interface appears sharper. X‐ray diffraction and nuclear magnetic resonance show that Co layers grow with bcc structure when the Co layer thickness dCo is lower than ≂15 A. In this case, the films are strongly textured with the Cr bcc (100) direction perpendicular to the plane of the substrate. A better structural coherence is observed for the thinner layers. When dCo is thicker than ≂15 A, a mixture of hcp and fcc Co phases appears by x‐ray diffraction and nuclear magnetic resonance. The good structural quality of the films is confirmed by the occurrence of satellite peaks in the x‐ray diffraction patterns. The Co magnetic ...

Structural and magnetic properties of diode radio‐frequency sputtered Cr/Co multilayers

Radio‐frequency‐sputtered Co/Fe multilayers with layer thicknesses varying in the 8–100 A range were studied by extended x‐ray absorption fine structure (EXAFS), x‐ray absorption near edge spectroscopy (XANES), and nuclear magnetic resonance (NMR) techniques. While the Fe environment is body‐centered‐cubic (bcc) in all the samples, the Co environment depends drastically on the Co layer thickness. Under ≂20 A, investigation at the Co K edge by XANES provides a bcc fingerprint in both polarization directions. The XANES profile becomes typical of a full hcp stacking only for Co layer thicknesses larger than 98 A. This behavior is confirmed by EXAFS spectra at the Co K edge and NMR analysis. For Co layer thickness between 6 and 18 A, eight first neighbors at 2.48 A and six second neighbors at 2.83 A are detected which match the bcc phase. Resonance peak at a very low frequency is also observed (198 MHz). The second shell radius appears at 2.78 A only for larger Co layer thicknesses. This effect goes along wit...

Direct experimental evidence of a body‐centered‐cubic Co phase in radio‐frequency‐sputtered Co/Fe multilayers

We report on structural and magnetic properties of diode rf‐sputtered fcc Cu/Co(100) multilayers. All the samples exhibit a pseudo‐epitaxial structure with interface roughness of the order of 6 A. As the Cu layer thickness is varied, we observe oscillations of magnetoresistance corresponding to oscillations of the interlayer coupling. The maximum of magnetoresistance is found for tCu=18.5 A (up to ΔR/R≊6%). We furthermore measure a very high differential magnetoresistance (ΔR/RΔH=1.4 kOe−1). A combination of antiferromagnetic coupling and quadratic in‐plane anisotropy is responsible for this large value of magnetoresistivity.

F. Giron

Papers

Magnetic circular x-ray dichroism measurements of Fe-Co alloys and Fe/Co multilayers

Magnetic and structural properties of rf‐sputtered Co/Fe and Co/Cr multilayers

Structural and magnetic properties of diode radio‐frequency sputtered Cr/Co multilayers

Direct experimental evidence of a body‐centered‐cubic Co phase in radio‐frequency‐sputtered Co/Fe multilayers

Anisotropic differential magnetoresistance of Cu/Co(100) multilayers