TOPICAL REVIEW: Nanomagnetics

The contribution that the technique of ferromagnetic resonance (FMR) has made to the understanding of the magnetic behaviour of ultrathin single films is reviewed. Experimental methods to measure FMR in situ in ultrahigh vacuum are presented. The temperature dependence of the magnetization, of the magnetic relaxation rate in the vicinity of the Curie temperature, and of the second- and fourth-order magnetic anisotropy energy (MAE) constants can be measured by FMR in situ for magnetic monolayers. Using the cases of Ni/Cu(001) and Gd/W(110) as examples, the role of the MAE for the quantitative description of temperature- and thickness-dependent reorientation transitions of the magnetization is discussed. Initial results for the anisotropy of the g-factor which is related to the anisotropy of the orbital moment (and the MAE) are presented.

https://iopscience.iop.org/article/10.1088/0034-4885/61/7/001/pdf

Ferromagnetic resonance of ultrathin metallic layers

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

The impact of stress-driven structural transitions and of film strain on the magnetic properties of nm ferromagnetic films is discussed. The stress-induced bending of film-substrate composites is analysed to derive information on film stress due to lattice mismatch or due to surface-stress effects. The magneto-elastic coupling in epitaxial films is determined directly from the magnetostrictive bending of the substrate. The combination of stress measurements with magnetic investigations by the magneto-optical Kerr effect (MOKE) reveals the modification of the magnetic anisotropy by film stress. Stress-strain relations are derived for various epitaxial orientations to facilitate the analysis of the substrate curvature. Biaxial film stress and magneto-elastic coupling coefficients are measured in epitaxial Fe films in situ on W single-crystal substrates. Tremendous film stress of more than 10 GPa is measured in pseudomorphic Fe layers, and the important role of film stress as a driving force for the formation of misfit distortions and for inducing changes of the growth mode in monolayer thin films is presented. The direct measurement of the magneto-elastic coupling in epitaxial films proves that the magnitude and sign of the magneto-elastic coupling deviate from the respective bulk value. Even a small film strain of order 0.1% is found to induce a significant change of the effective magneto-elastic coupling coefficient. This peculiar behaviour is ascribed to a second-order strain dependence of the magneto-elastic energy density, in contrast to the linear strain dependence that is valid for bulk samples.

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The correlation between mechanical stress and magnetic anisotropy in ultrathin films

There are two main thrusts towards new permanent-magnet materials: improving extrinsic properties by nanostructuring and intrinsic properties by atomic structuring. Theory-both numerical and analytical-plays an important role in this ambitious research. Our analysis of aligned hard-soft nanostructures shows that soft-in-hard geometries are better than hard-in-soft geometries and that embedded soft spheres are better than sandwiched soft layers. Concerning the choice of the hard phase, both a high magnetization and a high anisotropy are necessary. As an example of first-principle research, we consider interatomic Mn exchange in MnAl and find strongly ferromagnetic intralayer exchange, in spite of the small Mn-Mn distances.

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Predicting the Future of Permanent-Magnet Materials

The heteroepitaxy of Fe on W(110) is governed mainly by the large lattice mismatch f › 9.4% derived from the elemental lattice constants aW › 3.165 A and aFe › 2.866 A [1]. As a consequence of this mismatch only the first monolayer (ML) [2] iron grows pseudomorphically on W(110) at 300 K [3]. Misfit dislocations caused by the reduction of the strain energy of the Fe film are already created in islands of the second monolayer at a Fe coverage of about 1.5 pseudomorphic monolayers, as shown in a recent scanning tunneling microscopy (STM) study [4]. A central-force model [5] involving bulk iron elastic constants yields an elastic energy per surface atom of order 0.3 eV, which is a formidable contribution to any energy consideration regarding growth, structure, and magnetism of the iron film. Thus, stress induced effects are likely to affect the behavior of the film and will be discussed in this Letter. Not only the morphology of the film undergoes a transition from 1 to 2 ML thickness [4], but also four different magnetic regimes are of interest: (i) a submonolayer region, paramagnetic due to the absence of magnetic percolation [6], (ii) a ferromagnetic one monolayer region, characterized by a pronounced twofold in-plane anisotropy [7], (iii) an intermediate, “ sesquilayer” region, consisting of second layer islands on top of a one monolayer sea exhibiting, reportedly [8], antiferromagnetic order, and (iv) a 2 ML region without striking magnetic properties. The subject of this work is the investigation of the sesquilayer region at a coverage of 1.5 ML. We present for the first time stress measurements with submonolayer sensitivity taken during the growth of ultrathin iron films on W(110). The magnetism of the sesquilayer films is investigated by in situ Kerr effect measurements; the observed coercivity is explained by a novel domain wall pinning mechanism which is estimated to be 10 times stronger than the stress induced coercivity increase. The iron films were grown under ultrahigh vacuum (UHV) conditions on clean W(110) substrates at 300 K. Film and sample cleanliness were checked by Auger electron spectroscopy, the contamination level due to the only contaminants oxygen and carbon was found to be less than

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Film Stress and Domain Wall Pinning in Sesquilayer Iron Films on W(110).

The cantilever bending beam technique was applied to measure film stress, film magnetization and magneto-elastic coupling in nanometre Fe films grown epitaxially on W substrates. A simple optical deflection technique yielded sub-monolayer sensitivity for stress measurements and was used to determine magnetization and magnetostrictive properties of nanometre Fe films in situ. The combination of an electromagnet inside an ultra-high-vacuum chamber with a rotatable external magnet was employed to perform magneto-optical Kerr-effect measurements in the transversal, longitudinal and polar geometry in fields of up to 0.4 T. Examples for stress-driven structural changes in monolayer Fe films are discussed with respect to the unusual high coercivity found for sesquilayer Fe films and the re-orientation of the easy axis of magnetization in Stranski-Krastanov Fe films. The direct correlation between strain and magnetism was exploited to measure the magnetostrictive bending of the film-substrate composite. The magnitude and sign of the magneto-elastic coupling coefficient were found to depend on the film thickness, in contrast to the respective bulk values.

http://iopscience.iop.org/article/10.1088/0022-3727/31/6/014/pdf

The correlation between mechanical stress and magnetic properties of ultrathin films

Scanning tunneling microscopy and low energy electron diffraction show that high quality fcc Ni(111) films can be prepared on W(110). The subsequent coverage of this Ni template by monolayers of Fe leads to a Fe/Ni bilayer with striking magnetic properties. The Fe cap layer induces a spin reorientation of the easy axis of magnetization from in-plane to perpendicular to the film, as checked with the magneto-optic Kerr effect. At higher Fe coverages, an in-plane magnetization of the bilayer is found, which is proposed to be caused by the fcc to bcc transition in the Fe layer.

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C. Schmidthals

Papers

Film Stress and Domain Wall Pinning in Sesquilayer Iron Films on W(110).

The correlation between mechanical stress and magnetic properties of ultrathin films

Structure and perpendicular magnetization of Fe/Ni(111) bilayers on W(110)

Annealing effect on morphology and magnetism of ultrathin films of Fe and Ni on Cu(100)

Stranski-Krastanov layers of Fe on W(1 1 0): A combined MOKE, LEED, SAM and stress measurement investigation