Ferromagnetic resonance study of Fe/FePt coupled films with perpendicular anisotropy

Exchange spring magnets with perpendicular magnetic anisotropy represent new magnetic properties with respect to their constituent components. These systems typically consist of a hard magnetic layer and a soft magnetic layer which are strongly coupled. The modification of their bulk magnetic properties arises from this strong ferromagnetic exchange coupling, interfacial effects and competing magnetic anisotropies of the two magnetic layers. We have studied the magnetic bilayer system which consists of an Fe (soft) film exchange coupled to an FePt (hard) layer which has an easy axis aligned along the direction perpendicular to the film plane. The entire structure has the form: MgO/FePt (10 nm)/Fe (2nm or 3.5nm)/ Ag (2nm), where the Ag overlayer acts as protection against oxidation. The epitaxial FePt layers were deposit- ed on MgO (100) substrates using the RF sputtering technique at a substrate temperature of about 390 C. The epitaxy of this layer was studied using x-ray and electron diffraction techniques. Layer morphologies were further studied using atomic force microscopy (AFM), these studies reveal a granular morphology with grain sizes of the order of 40 - 50 nm. We have made detailed angular measurements using the ferromagnetic resonance (FMR) at room temperature. This angular FMR study, which includes the orientations of in-plane and out-of-plane, was performed in order to study the magnetic anisotropies as well as the exchange coupling between the magnetic layers and interfacial effects. In particular, we have chosen to study two samples with 2 nm and 3.5 nm of Fe, which effectively constitute the rigid magnet (RM) and exchange spring (ES) regimes, respectively. The RM and ES regimes depend implicitly on the magnetic anositropies and properties of the two coupled layers [1]. In figure 1 (a) we show an example of an FMR spectrum for the RM (2 nm Fe) sample. Of the various resonances observed, only the three low field lines are due to the Fe layer. It will be noted that the FePt does not have any FMR signature in the field range studied due to it very high magnetocrystalline anisotropy. The other resonance features evident in the spectrum arise from the MgO substrate and show no significant angular variations. As such the only FMR signals observed in our samples will arise from the Fe layer. In figure 1 (b) we show the angular variation of the resonance field of the three Fe resonance lines. Of these, two resonances display a uniaxial anisotropy with the easy axes aligned along the direction perpendicular to the film plane and will be directly related to the exchange coupling with the hard (FePt) layer. The third resonance, while also manifesting a uniaxial anisotropy, displays an easy axis direction which is canted by about 50 degrees from the film normal. While the origin of this resonance is not entirely clear, we suspect it may arise from the interfacial region between the FePt and Fe layers. In figure 2 we show the corresponding FMR results for the ES (3.5 nm Fe) sample. It will be noted that in addition to the resonances observed in the RM sample, there are a further two resonance, whose angular dependences are illustrated in figure 2 (b). These also display a uniaxial like behaviour with easy axes close to the film normal. In all spectra lines were fit using a home made programme which allows multiple peak fitting of Lorentzian and Gaussian lines. We develop a model of FMR based on the magnetic free energy of the coupled layers which is required to interpret the angular dependences of the resonance fields [1]. Existing models fall short of a full explanation of all the resonance lines and we are working to bridge this gap by considering the effects of boundary conditions and spin wave modes. [1] G. Asti et al., Phys. Rev. B, 73, 094406 (2006

Measuring magnetic properties in exchange spring systems using ferromagnetic resonance

We have studied the magnetic bilayer system which consists of Fe (soft ferromagnet) layer exchange coupled with FePt (hard ferromagnet) which have been deposited on an MgO substrate. Angular ferromagnetic resonance (FMR) measurements were made (0-360 degrees), in the plane which includes the in-plane and out of film plane directions, to study the magnetic anisotropies and the exchange coupling mechanism

Control of structural and magnetic properties of polycrystalline Co2FeGe films via deposition and annealing temperatures

This article belongs to the Special Issue Magnetic and Magnetoelectric Nanomaterials: Synthesis, Characterization and Applications.Thin polycrystalline Co2FeGe films with composition close to stoichiometry have been fabricated using magnetron co-sputtering technique. Effects of substrate temperature (TS) and post-deposition annealing (Ta) on structure, static and dynamic magnetic properties were systematically studied. It is shown that elevated TS (Ta) promote formation of ordered L21 crystal structure. Variation of TS (Ta) allow modification of magnetic properties in a broad range. Saturation magnetization ~920 emu/cm3 and low magnetization damping parameter α ~ 0.004 were achieved for TS = 573 K. This in combination with soft ferromagnetic properties (coercivity below 6 Oe) makes the films attractive candidates for spin-transfer torque and magnonic devices.Portuguese team acknowledges Network of Extreme Conditions Laboratories-NECL and Portuguese Foundation of Science and Technology (FCT) support through the projects PTDC/FIS-MAC/31302/2017, NORTE-01-0145-FEDER-022096, EXPL/IF/00541/2015 (S.A.B.), SFRH/BPD/84948/2012 (A.V.) and SFRH/BPD/87430/2012 (A.A.). This work was partially supported by Spanish Ministerio de Economía y Competitividad through project MAT2017-82970-C2 and from regional Gobierno de Aragón through project E28 20R including FEDER funding.Peer reviewe