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The X/Co 3nm/Y (where X, Y=Au, Pt) trilayers with as deposited in-plane magnetization alignment were irradiated with 30 keV Ga+ ions in the wide range of ion fluence. The samples were investigated by means of complementary techniques: magneto-optical magnetometry and spectroscopy (in the photon energy range from 1.2 eV to 4.5 eV), magnetic force microscopy, positron annihilation spectroscopy, X-ray diffraction and reflectivity. Difference in miscibility of interface atoms is clearly manifested in various intermixing extent at Co/Pt and Co/Au interfaces and consequently in magnetic properties of the irradiated trilayers. Low irradiation fluence (~1014 ions/cm2) leads to ~1nm interfaces broadening without visible surface etching for all samples, which is related with a distinct drop of magnetic anisotropy. However, the high irradiation fluence (~5·1015 ions/cm2) results in enhanced interface broadening and significant surface etching (~5 nm) partially removing also Co atoms. Tensile strains (up to 0.5%) were developed in the cover layers. The tensile strain, layers intermixing and the creation of Co-Pt(Au) alloys with different composition formed by irradiation are correlated with the increase of magnetic anisotropy. Moreover it was observed that substitution of Au instead of Pt (as a cap or buffer layer) results in substantial increase of perpendicular magnetic anisotropy. Maximal increase of magnetooptical parameters was observed for Pt/Co/Pt layer. Irradiation induced changes of concentration profiles are revealed using magnetooptical spectra, X-ray reflectivity spectra and simulations with use of binary collision approximation.

Modifications of magnetic anisotropy of 30 keV Ga + ion irradiated ultrathin Co films sandwiched between Au or Pt buffer and capping layers are investigated as a function of magnetic layer thickness, d Co , and the ion fluence, F. Maps (d Co , F) of saturation fields have been derived from local magnetooptical polar Kerr effect (PMOKE) measurements. The areas with increased remanent magnetization and/or saturation fields, which are directly related to the uniaxial anisotropy, adopt linear shapes for the two branches in the maps. They are very distinct, especially for the Pt/Co/Pt system irradiated at lower and higher fluence. Replacement of Pt with Au in the buffer layer results in minor influence on the magnetization properties of the irradiated trilayers. Au as a capping layer significantly decreases the anisotropy in the branch appearing at lower fluence. In the Au/Pt/Au sandwich, a severe reduction of induced anisotropy is observed in both branches. The proposed phenomenological model describing experimentally investigated magnetic anisotropies enables separation of surface and volume contributions to both branches of enhanced anisotropy.

Modifications of the magnetic properties of Co=Mo=Co films activated by irradiation with 30-keV Ar and 17-keV Ne ion beams are investigated and compared with the influence of 35-keV Ga ions. This system is magnetized in the sample plane and exhibits a twofold anisotropy. The interlayer coupling of magnetization in as-deposited structures is parallel except for the Mo spacer thickness range between 0.5 and 1.0 nm, where the magnetization of the Co layers is antiparallel oriented. The coupling changes and gradually reduced strength of the ferromagnetic properties are compared for all ion types and discussed as a function of the Mo spacer thickness and the ion fluence. The structural evolution of the studied films with increasing fluence determined from TRIDYN simulations is discussed in relation to the observed magnetic changes.We also propose various types of magnonic crystals that can be fabricated by exploiting the results presented in this work.

Molecular beam epitaxy-grown layered structures Co/Mo/Co exhibit an antiparallel coupling of Co films magnetization in the Mo spacer thickness range between 0.5 nm and 1.0 nm and parallel beyond this range. Magnetic properties are substantially modified by beam irradiation of 35 keV Gaþ ions. With the increase in ion fluence, antiparallel coupling switches to the parallel one. Further increase in fluence results in gradual suppression of ferromagnetic behavior of the system. Experimental results are correlated with numerical simulations of layered structure evolution driven by irradiation.

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Multilayer ultrathin-films with perpendicular magnetic anisotropy (PMA) show a perspective as candidates for spintronic devices. The effective magnetic anisotropy strongly depends on the magnetic layer thickness – spin reorientation transition (SRT) at a critical thickness takes place from the inplane to out-of-plane magnetization direction. Moreover, SRT can be shifted to higher thicknesses by means of ion [1] and fs-laser irradiation [2]. Here, we employ variable energy positron annihilation spectroscopy (PAS) that is sensitive to open volume defects and their atomic surrounding to compare these two methods of the SRT inducement. Molecular beam epitaxy deposited films with different bottom Pt layer thicknesses and growth temperatures have been investigated. In case of Ar+ ion irradiated trilayers magnetization can be driven to the out-of-plane state twice as a function of ion fluence [1], whereas our preliminary studies of UV laser irradiation suggest only one SRT. PAS analysis (Fig. 1) of the as-grown samples reveal slight differences in crystal quality and defect concentration that is likely due to a thicker bottom Pt layer used for the ion irradiation experiment. Films utilized for the laser irradiation exhibit larger initial positron diffusion length, L+ that is close to L+ of the substrate (~60nm). In case of lower ion and laser fluences only minor and no interface intermixing has been found, respectively. The sample irradiated with lower ion fluence shows a slight increase of S (for E~1.5keV) being likely due to Co-vacancy increase, which could be responsible for first SRT. The sample irradiated with lower laser fluence shows no difference compare to the asgrown stacks, where only increase of magnetic anisotropy was found. The larger ion and laser fluences reorients magnetization to the out-of-plane state that is evidenced by both magneto-optical and PAS studies – S(E) curves nicely overlap. Moreover, large intermixing is expected, thus creation of the CoPt alloy with PMA. To elucidate stoichiometry and atomic surrounding of open volume defect coincidence Doppler broadening investigations will be presented. *Corresponding author. E-mail address: m.liedke@hzdr.de [1] A. Maziewski et al., Phys. Rev. B. 85, 054427 (2012) [2] J. Kisielewski et al., J. Appl. Phys. 115, 053906 (2014)

The origin of magnetic phase transition from in-plane to perpendicular magnetic anisotropy (PMA) of Pt/Co/Pt thin film by Ga+ ion irradiation at fluences of 1-5 x 10(15) ions/cm(2) is investigated by means of x-ray magnetic circular dichroism (XMCD) and extended x-ray absorption fine structure (EXAFS) analyses. We find that Pt and Co atoms are mixed with each other and that Co is oxidized near the surface due to removal of the Pt overlayer. Furthermore, polarization-dependent EXAFS analysis shows that Co is firstly dispersed as separated single-atom-thick sheets in a Pt matrix at 1 x 10(15) ions/cm(2), then the Co sheets are divided into a few angstrom clusters at 5 x 10(15) ions/cm(2), which are regarded as nanodiscs parallel to the film plane. This process is accompanied by the appearance of an out-of-plane magnetization component and a remanence peak is observed. Because we do not observe an enhancement in anisotropy of Co orbital moment which leads to change in magnetic anisotropy through the transition at about 5 x 10(15) ions/cm(2), it might be possible that such nanodisc formation induces increase of magnetic anisotropy via a shape effect. By comparing with the phase transition observed at lower fluence [Phys. Rev. B 86, 024418 (2012)], we find that the mechanism of two transitions is different, i.e., the transition at lower fluence is caused by anisotropy of orbital moment due to structural strain, while the present transition is possibly by shape effect due to nanodisc formation.

Magnetic and magneto-optical properties of Pt/Co/Au and Pt/Co/Pt trilayers subjected to 30 keV Ga+ ion irradiation are compared. In two-dimensional maps of these properties as a function of cobalt thickness and ion fluence, two branches with perpendicular magnetic anisotropy (PMA) for Pt/Co/Pt trilayers are well distinguished. The replacement of the Pt capping layer with Au results in the two branches still being visible but the in-plane anisotropy for the low-fluence branch is suppressed whereas the high-fluence branch displays PMA. The X-ray absorption spectra and X-ray magnetic circular dichroism (XMCD) spectra are discussed and compared with non-irradiated reference samples. The changes of their shapes and peak amplitude, particularly for the high-fluence branch, are related to the modifications of the local environment of Co(Pt) atoms and the etching effects induced by ion irradiation. Additionally, in irradiated trilayers the XMCD measurements at the Pt L-2,L-3-edge reveal an increase of the magnetic moment induced in Pt atoms.

Modifications of magnetic and magnetooptical properties of Pt/Co/Pt tri-layers upon irradiation with ion beam are investigated in this work. They are studied in detail as a function of the Co layer thickness, dCo (up to 5 nm), ion species (Ne+, Ar+, Ga+) the energy of ions, E (up to 30 keV) and ion fluence, F (range from 1013 to 3∙1016 ions/cm2). Modified magnetic properties are correlated with structural features of irradiated samples. Numerical simulations of in-depth atomic concentration profile and surface etching carried out by TRIDYN software complements explanation of the observed effects. MBE grown Pt/Co/Pt tri-layers display a spin reorientation transition (SRT) at a well-defined Co thickness, dSRT from out-of-plane to in-plane magnetization alignment as dCo increases. Upon ion irradiation various processes undergo: degradation of chemically sharp interfaces, formation of Co-Pt alloys and lattice strain development [1, 2]. Spread interfaces reduce perpendicular anisotropy whereas formation of the ordered alloy may strength this property. Therefore a resulting magnetic state of the sample is a product of interplay of these two opposing trends in anisotropy evolution and the sample surface etching, particularly effective for highest fluences. It is very sensitive to dCo, ion species, their energy and the ion fluence. Discussion is focused on results obtained with Ar+ ions irradiation, which is commonly used for technological processes especially for nanostructures sputtering. Set of (dCo, log F) diagrams of remanence, mr (Figure. 1), (normalized magnetization perpendicular component) measured by means of magnetooptical Kerr effect magnetometry in polar configuration (PMOKE) upon irradiation with Ar+ ions with different energies. Irradiation of the samples modifies magnetic properties in various manners depending on dCo, F and ion energy. The suppression of the dSRT with F in the range from 1013 to 1014 ions/cm2 and formation of branch(es) corresponding to strong perpendicular anisotropy (higher value of mr) are the most distinguished differences depending on the ion energy. As the ion energy increases a single branch with high mr (for 1.2 keV) begins to split into two parts for 5.0 keV, which are clearly separated for 30.0 keV. Qualitatively this evolution resembles that one already reported for Ga ions [1]. However, the mr modifications strength are weaker due to the lower mass of Ar atoms. Nevertheless, up to 5 transitions between various magnetic states are observed with F increase, e. g. for dCo = 1.6 nm and E = 30 keV. It is worth to notice that even the lowest applied energy of Ar+ ions induces visible modifications of magnetic properties. In the case of the lightest studied Ne ions modification of magnetic anisotropy is weaker than for Ar+ ions and the branches in a similar diagram are non-continuous. TRIDYN software determined in-depth chemical profile (Figure 2) is helpful in identification of possible alloy composition being responsible for enhanced perpendicular magnetic anisotropy. Also the surface etching extent allows to estimate an amount of sputtered (also magnetic) material that may result in superparamagnetic state observed for the highest ion doses. Structural properties of the irradiated samples are investigated by Rutherford backscattering (RBS). Evolution of the acquired chemical profiles is correlated with that simulated with TRIDYN software. Moreover, the etching efficiency is estimated from topography step profile between irradiated and non-irradiated areas of the sample investigated by means of profilometer and atomic force microscopy. Discussed above abundance of magnetic investigated states in combination with controlled structural features developing upon ion irradiation enable a precise tuning of desired magnetic properties. In particular, such approach, exploiting a focused ion beam, might be of great importance in fabrication of magnonic crystals – a new type of metamaterial with reprogrammable band structure.

When decreasing ultrathin film thickness d, an increase of the perpendicular magnetic anisotropy (PMA) occurs, resulting in the magnetization reorientation phase transition (RPT) from the in-plane to out-of-plane magnetization state below the critical thickness dRPT. The results of combined experimental, analytical, and micromagnetic simulations studies on the evolution of magnetization states and processes in ultrathin films and multilayered systems [1] will be presented. Possibilities of tuning the magnetic domain sizes in a broad range (of several orders of magnitude) by changing the geometrical parameters of the nanostructure, as well as by an applied external magnetic field will be discussed. Transitions between two- and three-dimensional magnetization distributions will be shown. As a result of the ion irradiation of the metallic ultrathin films, a decrease of PMA was usually reported [2]. Contrariwise, Ga+ ion irradiation driven creation of the out-of-plane magnetization states, dependent on ion fluence, have been found [3]. Similarly, femtosecond light pulses of high energy density may also induce irreversible enhancement of PMA [4], while low energy density pulses can be used to trigger the magnetization oscillations, via reversible thermal changes of the magnetic anisotropy [5]. 1. A. Maziewski, J. Fassbender, J. Kisielewski, M. Kisielewski, Z. Kurant, P. Mazalski, F. Stobiecki, A. Stupakiewicz, I.Sveklo, M.Tekielak, A.Wawro, and V. Zablotskii, Magnetization states and magnetization processes in nanostructures: From a single layer to multilayers, Phys. Status Solidi A, 211, 1005 (2014). 2. H. Bernas (Ed.), Materials Science with Ion Beams, Vol. 116 (Springer-Verlag, Berlin, Berlin, 2010). 3. A. Maziewski, P. Mazalski, Z. Kurant, M. O. Liedke, J. McCord, J. Fassbender, J. Ferré, A. Mougin, A. Wawro, L. T. Baczewski, A. Rogalev, F. Wilhelm, and T. Gemming, Tailoring of magnetism in Pt/Co/Pt ultrathin films by ion irradiation, Phys. Rev. B 85, 054427 (2012). 4. J. Kisielewski, W. Dobrogowski, Z. Kurant, A. Stupakiewicz, M. Tekielak, A. Kirilyuk, A. Kimel, Th. Rasing, L. T.Baczewski, A. Wawro, K. Balin, J. Szade, and A. Maziewski, Irreversible modification of magnetic properties of Pt/Co/Pt ultrathin films by femtosecond laser pulses, Journal of Applied Physics 115, 053906 (2014), 5. J. Kisielewski, A. Kirilyuk, A. Stupakiewicz, A. Maziewski, A. Kimel, Th. Rasing, L. T. Baczewski, and A. Wawro, Laser-induced manipulation of magnetic anisotropy and magnetization precession in an ultrathin cobalt wedge, Phys. Rev. B 85 (2012) 184429.

The results of combined (experimental, analytical, and micromagnetic simulations) studies on the evolution of magnetization states and processes in ultrathin films and multilayered systems are presented. We show ways to manipulate magnetization distributions in ultrathin magnetic single or multilayers by tuning: the thickness of the magnetic layer, the thickness of either the non-magnetic cap or spacer layer, the magnetic anisotropy, and the geometrical constrictions of the system. In ultrathin magnetic films, both the magnetization distribution and the critical thickness of the magnetization reorientation phase transition (RPT) between perpendicular and in-plane states can be also controlled by post-growth treatments, e.g., by either ion or light irradiation. By changing the geometrical parameters of the nanostructure, as well as by an applied external magnetic field, one can tune magnetic domain sizes in a giant range (of a few orders of magnitude) and induce the RPT. Transitions between two- and three-dimensional magnetization distributions are discussed. The authors present possibilities of the engineering of magnetic properties (e.g. magnetic anisotropy and coercivity field) of nanostructures during deposition processes and post growth treatments, e.g. by ion irradiation and laser annealing. Magnetization distributions in single ultrathin layer and multilayers have been studied both experimentally (using Co films sandwiched between Au, Pt or Mo layers) and theoretically. These huge distribution changes, driven by nanostructure geometry or magnetic field, are shown.

Ga+, He+ irradiated sapphire/Pt/Co(dCo)/Pt(dPt) ultrathin films were studied using polar Kerr effect in wide ranges of both cobalt dCo and platinum dPt thicknesses as well as ions fluences F. Ga+, He+ irradiation induce: creation of two branches with increased magnetic anisotropy and enhancement of Kerr rotation angle. Ga+ irradiation induces creation of two branches of out-of-plane magnetization state.

Magnetization orientation in Ga+-irradiated Pt/Co(d(Co))/Pt ultrathin films can be changed in a controlled way by adjusting the ion fluence, F. Two-dimensional (d(Co), F) diagrams of magnetic and magneto-optical properties have been derived. Distinct out-of-plane magnetic anisotropy states with enhanced magneto-optical effects were evidenced for specific (d(Co), F) values. This rich behavior originates from two competing mechanisms: intermixing of Co and Pt atoms at the interfaces and the formation of ordered CoPt alloy phases with high magnetic anisotropy. The irradiation-induced effects open novel routes for both tailoring thin-film magnetic and magneto-optical properties and patterning of magnetic nanostructures.