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We report on pairs of converging-diverging spin vortices in Co/Rh/NiFe trilayer disks. The lateral magnetization distribution of these effective spin merons is directly imaged by means of element-selective x-ray microscopy. By this method, both the divergence and circulation states of the individual layers are identified to be antisymmetric. Reversal measurements on corresponding continuous films reveal that biquadratic interlayer exchange coupling is the cause for the formation of the effective meron pairs observed. Furthermore their three-dimensional magnetization structure is determined by micromagnetic simulations. It is shown that in order to reduce magnetic surface and volume charges the magnetic induction aligns along a flux-closing torus. This toroidal topology enforces a symmetry break, which links the core polarities to the divergence configuration.

We report on pairs of diverging/converging spin vortices in Co/Rh/Ni81Fe19 disks. The lateral magnetization distribution of these effective spin merons [1] is directly imaged by means of element-selective x-ray microscopy. By this method, both the circulation and divergence states of the individual layers are identified as antisymmetric [2] (see Figure 1). Magnetization reversal measurements of corresponding continuous films reveal that biquadratic interlayer exchange coupling is the origin for the formation of such effective meron pairs. Furthermore, the effective meron pair’s three-dimensional magnetic structure is determined via micromagnetic simulations. We find that the perpendicular magnetization component is distributed nonhomogeneously, namely M is partially circulating on a flux-closing torus. This toroidal topology enforces a symmetry break, which ties the core polarities to the divergence configuration. Upon excitation, such topology-induced core stabilization could lead to enhanced maximum amplitudes of steady gyration compared to single layer vortices, as the meron pair’s core switching onset velocity is expected to be higher than that of a single vortex.

We report on pairs of diverging/converging spin vortices in Co/Rh/Ni81Fe19 trilayer disks. The lateral magnetization distribution of these effective spin merons is imaged directly by means of element-selective x-ray microscopy. By this method, both the divergence and circulation states of the individual layers are identified as antisymmetric. Reversal measurements on corresponding continuous films reveal that biquadratic interlayer exchange coupling is the origin for the formation of effective meron pairs. Furthermore, their three-dimensional magnetization structure is determined by micromagnetic simulations. Interestingly, the magnetic induction follows a kind of flux-closing torus. This toroidal topology enforces a symmetry break, which ties the core polarities to the divergence configuration.

We report on the response of multilayer spin textures to static magnetic fields. Coupled magnetic vortex pairs in trilayer elements (ferromagnetic/nonmagnetic/ferromagnetic) are imaged directly by means of layer selective magnetic x-ray microscopy. We observe two different circulation configurations with parallel and opposing senses of magnetization rotation at remanence. Upon application of a field, all of the vortex pairs investigated react with a displacement of their cores. For purely dipolar coupled pairs, the individual core displacements are similar to those of an isolated single layer vortex but also a noticeable effect of the mutual stray fields is detected. Vortex pairs that are linked by an additional interlayer exchange coupling (IEC) mainly exhibit a layer-congruent response. We find that –apart from a possible decoupling at higher fields– these strict IEC vortex pairs can be described by a single layer model with effective material parameters. This result implies the possibility to design multilayer spin structures with arbitrary effective magnetization.

Ion implantation of ferromagnetic films has proven to be a promising tool for the fabrication of fully planar samples with a microscopic magnetic substructure. A waveguide-like propagation of spin waves in a Ni81Fe19 film which was locally patterned by ion implantation could be observed. The investigations have been performed using Brillouin light scattering microscopy on samples patterned with varying ion fluences. Further investigations on the coupling behavior of two parallel stripes in this fully planar structures have been performed. The presented fabrication technique of spin-wave waveguides provides much lower stray fields and better heat conduction. Especially the latter is a matter of interest when the objects are exposed to intense microwave fields (excitation of spin waves) or investigated by laser spectroscopy like Brillouin light scattering. Financial support by the DFG (GRK 792) is gratefully acknowledged.

Spin structures have been a relevant topic of magnetism research for many years. In particular, magnetic vortices have attracted much at- tention, due to their non-trivial topology and the various dynamic modes they exhibit. A magnetic vortex consists of a planar, flux- closing magnetization curl that turns out of the plane in the central nanoscopic core. For a single layer structure, the curl’s radial components typically cancel each other out. Recent investigations show that this holds also true for multilayer vortex systems comprising bilinear interlayer exchange coupling (IEC). In this contribution we report on pairs of diverging-converging spin vortices occurring in biquadratically coupled systems. Using magnetic x-ray microscopy we directly observe that the individual vortices of such pairs possess a residual radial magnetization component. From this ∇M𝑥𝑦 ̸= 0, an additional perpendicular magnetization divergence ∇M𝑧 is analytically deduced. We compare our continuous model with discrete micromagnetic simulations.

The magneto-dynamics of lense shaped thin lm elements where studied using vector network analyzer ferromagnetic resonance (VNA-FMR). An unexpected strong increase of the resonance frequency was found when approaching the switching eld. Using Magnetic Force Microscopy this resonance increase could be ascribed to the formation and evolution of a buckling domain state. The experimental data have been qualitatively reproduced by micromagnetic simulations of a model element. Thereby the role of the external magnetic eld and the buckling wavelength was studied separately. Domain modes with dynamic magnetization modulations parallel and perpendicular to the static magnetization were identied. We derive qualitative arguments based on mageto-static energy considerations that allow for an interpretation of the dynamic response in such low-symmetric magnetization distributions.

Ion implantation is a standard process in semiconductor tech¬nology. However, since a doping of metals typically does not have such a tremendous effect as compared to semiconductors ion implantation in metals to achieve a desired electrical, optical or magnetic property is much less explored. A combination of litho-graphy and ion implantation to achieve a lateral ion implantation pattern is a novel route to design and construct artificial materials like photonic or magnonic crystal which rely on the local band gap or spin wave dispersion engineering [1]. The contribution will provide an overview over the different physical mechanisms which allow modifying and tailoring the dynamical magnetic properties, i. e., magnetic damping even in an anisotropic fashion, aiming at the creation of new functional materials. In one example [2] it is demonstrated that the mag¬netic damping parameter in a ferromagnetic/antiferromag¬ne¬tic/ferro¬magnetic trilayer stack can be continuously and spatially varied between two extremal values (see Fig. 1). In a second example [3] the creation of a lateral magnetization pattern by ion implantation gives rise to additional and anisotropic relaxation channels. Potential areas of application are magnonic crystals and band-stop filters in the GHz range. Acknowledgement: This work was supported by Deutsche Forschungsgemeinschaft SFB491, FA 314/3, FA314/6-1 and MC 9/7. [1] J. Fassbender, J. McCord, J. Magn. Magn. Mater. 320, 579 (2008). [2] J. McCord, T. Strache, I. Mönch, R. Mattheis, J. Fassbender, Phys. Rev. B 83, 224407 (2011). [3] I. Barsukov, F. M. Römer, R. Meckenstock, K. Lenz, J. Lindner, S. Hemken to Krax, A. Banholzer, M. Körner, J. Grebing, J. Fassbender, M. Farle, Phys. Rev. B 84, 140410(R) (2011).

Current-induced domain wall motion for systematic studies of spin momentum transfer requires the reliable preparation of domain walls. Since high current densities can change or destroy the structures investigated, weak pinning potentials allowing the controlled and reliable depinning of domain walls at low current densities are desirable. A prerequisite for the preparation of a domain wall at such pinning sites is, that the domain wall nucleates at fields smaller than the field required to depin the domain wall. We suggest methods to tune the nucleation field of lithographically patterned Co/Pt multilayer wires. An up to fourfold reduction of the nucleation field could be achieved through altering the lateral shape of the wires or by depositing iron stripes on top. Furthermore we explored the applicability of geometric constrictions and ion implantation for the creation of pinning sites. The magnetization reversal of the structures was imaged by transmission X-ray microscopy at the Advanced Light Source in Berkeley, CA, USA. The authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft via SFB 668 and GrK 1286 as well as by the DOE.

Nanostructuring by means of ion erosion has proven its versatility with respect to the surface morphology control. By varying different ion irradiation parameters, e.g. ion energy, fluence, incident angle, and sample temperature, the assembly of self-organized periodically ordered arrays of nano-dots [1] and ripples [2] is possible. This has been proven for semiconductors as well as for metals [3,4]. Especially, nanopatterning of magnetic materials is intriguing due to the fact that not only the surface morphology is affected, but the overall magnetic properties are accordingly modified. For example, ion bombardment of single crystal Fe films enables the manipulation of the in-plane uniaxial magnetic anisotropy (UMA) that is associated with the ripple morphology [5]. Here we present a novel bottom-up method of magnetic film patterning, where a highly ordered periodic MgO ripple surface with a wavelength on the nanometer scale is coated by a magnetic Fe layer. The modulations can be induced along any arbitrary in-plane orientation and outstandingly, the surface stays crystalline upon ion irradiation. Thus, due to the low lattice mismatch single crystalline Fe can be grown onto such templates. Despite the intrinsic magnetic cubic anisotropy (CA) of bcc Fe additional UMA is introduced. As a reference a model cubic system, i.e. Fe on flat MgO(100), has been grown. 15 nm of Fe (tFe) was deposited onto the MgO(100) substrate at room temperature by means of molecular beam epitaxy (MBE). In-plane magneto-optic Kerr effect (MOKE) and ferromagnetic resonance (FMR) angular measurements were performed revealing clear evidence of a four-fold symmetry (CA; K4||/M=186.3 Oe) superimposed with a very small two-fold symmetry contribution (UMA; K2||/M=2 Oe). The magnetic reversal curves (MRCs) shown in Fig. 1a (top panel) correspond to three distinguished sample orientations with respect to the angle φH of the external magnetic field. The curves represent the hard (HA), intermediate, and easy (EA) magnetization orientations of the cubic system. The MRC at φH=18° exhibits a two-jump magnetization process that is a consequence of nucleation and propagation of 90° domain walls (DWs), which is confirmed by magnetic domain observations performed by magneto-optical Kerr microscopy. In a second step, ripple MgO substrates of a wavelength λrip≈20 nm were prepared (for the AFM micrograph refer to Fig. 1a, top panel) and coated with Fe. Several Fe thicknesses were evaporated in the range of 5-30 nm onto templates with a few distinct in-plane ripple orientations with respect to the MgO [100] direction (0°≤φrip≤60°). Thus, due to the combination of the periodic morphology and the intrinsic magnetic properties of Fe, an ensemble of two-fold and four-fold magnetic symmetry is created. This is confirmed by FMR and MOKE measurements. In general, the UMA originates from dipolar and step-edge effects. The orientation and strength of the UMA depends on the angle of ripple ridges elongation φrip with respect to the MgO [100] direction and Fe film thickness, respectively. First evidence of strong UMA emerges already from MRCs analysis as shown for a sample with φrip≈25° (Fig. 1a, bottom panel), where for in-plane orientations between HA and EA directions a three-jump magnetization process is found. From Kerr microscopy investigations, the magnetization reversal process can be described as a subsequent nucleation and propagation of 90°, 180°, and then again 90° DWs. FMR analysis, i.e. fit of the in-plane angular resonance frequency fres dependence, reveals anisotropy fields and orientations of both anisotropy contributions (Fig. 1b). E.g., for the ripple sample with tFe=15 nm the CA and UMA fields are K4||/M=306 Oe and K2||/M=20 Oe, respectively. Both values are strongly increased relative to flat Fe films. Moreover, the orientation of the UMA coincides with the elongation direction of the ripple ridges for all the samples. In addition, we find that the UMA decreases as a function of Fe thickness, whereas the CA behaves the opposite way. The origin and interaction of the magnetic anisotropy contributions will be discussed in detail. References: [1] S. Facsko et al., Science 285, 1551 (1999) [2] E. Chason et al., Phys. Rev. Lett. 72, 3040 (1994) [3] R. Moroni et al., Phys. Rev. Lett. 91, 167207 (2003) [4] F. Bisio et al., Phys. Rev. Lett. 96, 057204 (2006) [5] Q. F. Zhan et al., Phys. Rev. B 80, 094416 (2009)

Ion erosion as a tool for nanostructuring has proven its versatility with respect to surface morphology modifications. Ion irradiation parameters, e.g. ion energy, fluence, incident angle, and sample temperature, can be varied in order to assemble self-organized periodically ordered arrays of nano-dots and ripples. Particularly, nanopatterning of magnetic materials is meaningful because not only the surface morphology is affected, but the overall magnetic properties are accordingly modified. Here we present a novel bottom-up method of magnetic film patterning, where ordered periodic MgO ripple surfaces with a wavelength on the nanometer scale, ion sculptured along a few arbitrary in-plane orientations and outstandingly fully crystalline upon ion irradiation, are coated by a magnetic Fe layer. Due to a cubic symmetry of Fe an in-plane fourfold magnetic anisotropy is induced and in addition, an uniaxial magnetic anisotropy arises due to the surface morphology. The uniaxial magnetic anisotropy orientation and strength is controlled by an arbitrarily chosen irradiation direction with respect to the sample plane and the ripple wavelength is set by the ion energy, respectively. Thus an ensemble of twofold and fourfold anisotropy is created and analyzed by ferromagnetic resonance, magnetooptic Kerr effect, and X-ray diffraction techniques. Theoretical analysis reveals both the anisotropy fields and their directions that are in agreement with the experiment. This work is supported by DFG FA314/6-1.

Recently, there is a rise of interest in fabrication and investigation of nanometre sized magnetic objects. Magnetic properties can easily be manipulated by ion beam implantation. Focused-ion beam (FIB) techniques are one way to combine both nanopatterning and implantation. The main difference between standard ion implantation and FIB implantation is the beam current density, which could lead to differences in the structural and magnetic properties. The aim of this work is to compare both implantation techniques in terms of structural changes and to relate them to magnetic property changes. For our investigation we have used 50 nm thick non-ordered nano-crystalline permalloy (Ni81Fe19) films modified by a 30 keV Ga+ ion beam. The magnetic properties were characterised via magneto-optic Kerr effect measurements at room temperature. Both types of implantation demonstrate a degradation of saturation magnetisation with increasing ion fluence. For structural investigations we have applied several techniques. We have used the advantage of non-destructive X-ray techniques to study the structural changes. Besides X-ray diffraction, providing the long-range order information, EXAFS measurements to probe the local structure were performed. Both methods are statistical ones, whereas TEM imaging provides information on a local scale. Implantation leads to a crystallite growth from ~12 nm up to ~25 nm and further texturing of the material towards (111) direction at almost constant lattice parameter. In the case of FIB implanted samples the TEM images show crystallites growing through the entire film at high implantation fluences. The EXAFS analysis shows an almost perfect near-order coordination, corresponding to an fcc cell; only in the vicinity of the Ga atoms a small local deviation could be observed. In general FIB and standard implantation demonstrate similar behaviour with a shift in the fluence value. These results let us conclude that the simple presence of the Ga atoms is the dominating effect leading the degradation of saturation magnetisation.

A magnetic vortex structure consists of a planar magnetization curl with a perpendicularly magnetized nanoscopic core in its center. As a consequence of the different combinations possible for the curl’s rotation sense (circulation: c ∈ {+1,-1}) and the orientation the core (polarity: p ∈ {+1,-1}), magnetic vortices occur with opposite handednesses. When excited by magnetic fields or spin polarized currents, magnetic vortices exhibit different kinds of eigen modes of which the gyrotropic core mode is most prominent. It corresponds to an orbiting of the core around its equilibrium position, where the sense of motion is determined by p as a topological charge only[1,2]. The static and dynamic properties of single layer vortices have been the subject of numerous fundamental investigations during the past decades[e.g.1,2,3], which also led to the proposal of their application for memory devices[4] and spin-torque oscillators[5]. On the technological context as well as from a basic perspective, the coupling between spatially confined vortices is a key issue. Here we report on the magnetization dynamics of coupled vortex pairs, separated by a nonmagnetic spacer in a stacked geometry. Besides magnetodipolar interaction, interlayer exchange coupling (IEC) can be present in such a system, depending on the spacer material and thickness. We have experimentally identified two nongenerate configurations for both, IEC and purely dipolar coupled vortex pairs regarding their relative circulation orientations by means of scanning transmission x-ray microscopy (STXM)[6]. Time-resolved STXM furthermore allows to probe the layer-specific response of coupled vortex pairs to magnetic excitations. By this, we have observed a principally different core gyration behavior for the different circulation configurations. Also, a collective reorientation of the core polarities has been detected for sufficiently strong excitations.

Spin vortices have attracted much attention due to their chiral nature and the variety of dynamic phenomena associated with them. In this contribution we present experimental findings on vortex coupling in trilayer elements, where two ferromagnetic layers are separated by a nonmagnetic spacer. For such systems the relative configurations of the in-plane flux senses (circulations) as well as the core orientations (polarities) of layered vortices are identified by means of scanning transmission x-ray microscopy (STXM). The dominant coupling mechanisms here are the magneto-dipolar interaction and interlayer exchange coupling (IEC). Remarkably, a modification of the IEC, which can be induced by noble gas ion irradiation, allows to specifically set the circulation configuration of a layered vortex pair to be either an- tiferromagnetic or ferromagnetic. In addition, time-resolved measurements of the response of interlayer coupled vortices to an excitation by sinusoidal magnetic fields will be shown.

The controlled motion of magnetic domain walls in nanowires is of scientific and technological relevance since it is necessary for new concepts of ultrafast, high-density, and nonvolatile data storage devices. Locally well-defined confining potentials, e.g., created by notches, act as pinning sites for individual domain walls. Magnetic configurations can be manipulated by external magnetic fields and by spin-polarized currents via transfer of spin-angular momentum. The local modification of magnetic properties by ion irradiation opens the possibility to create pinning sites without geometric constrictions. Implantation of chromium ions into permalloy nanowires causes alloying and structural defects which leads to a reduction of the Curie temperature and the saturation magnetization Ms as well as small changes of the magnetocrystalline anisotropy. The energy associated with the domain wall is locally reduced and a position at the so-called magnetic soft spot is energetically favorable in comparison to the environment, Field- driven pinning and depinning of a domain wall at the soft spots has been directly observed using magnetic transmission soft X-ray microscopy (MTXM) at beamline 6.1.2. of the Advanced Light Source in Berkeley, CA, USA. Magnetic contrast is provided via X-ray magnetic circular dichroism (XMCD) at the Ni L3-absorption edge. We have shown that the strength of the pinning potential and thus the depinning field can be tuned by the chromium ion fluence applied to induce the soft spots.

Micromagnetic structures have been a focus of both fundamental and technological research for several decades. As a part of this effort, spin vortices (which are characterized by an in-plane magnetization curl and a small out-of-plane core region) have attracted much attention due to their chiral nature and the variety of dynamic phenomena associated with them. Although the static and dynamic properties of individual vortices are now reasonably well understood, there have been relatively few studies addressing their mutual interactions, particularly for the case of a vertically stacked arrangement. Here we present experimental findings on vortex coupling in trilayer elements, where two ferromagnetic layers are separated by a nonmagnetic spacer. For these systems the relative configurations of the in-plane flux senses (circulations) as well as the core orientations (polarities) of layered vortices have been identified by means of scanning transmission x-ray microscopy (STXM). The dominant coupling mechanisms here are the magneto-dipolar interaction and interlayer exchange coupling (IEC).

Focused ion beam irradiation is a versatile tool that can be used for magnetic nanostructuring. In this work we compare both FIB irradiation and a standard implantation taking into account their distinctive irradiation features. A 50nm thick permalloy layer (Ni80Fe20) irradiated with different Ga+ ion fluences was used for the investigations. The structure was studied via XRD and EXAFS measurements carried out on the ESRF ROBL and ID01 facilities. Additionally TEM and magneto-optic Kerr effect magnetometry were performed. Both types of irradiation demonstrate a similar behaviour: increasing the ion fluence causes a further material crystallization and a decrease of the magnetic moment. However FIB irradiation leads to a stronger crystallite growth due to the high current densities used.

Ion erosion of MgO substrates produces highly ordered surface patterns. The so-called ripples are not only induced along any arbitrary in-plane orientation but outstandingly, they stay crystalline upon ion irradiation. Due to the low lattice mismatch single crystalline Fe films can be grown onto these periodically modulated MgO(100) templates. Despite the intrinsic magnetic property of bcc Fe, i.e. cubic anisotropy, an additional ripple morphology driven uniaxial magnetic anisotropy is introduced. Thus an ensemble of twofold and fourfold symmetry is created, which is confirmed by ferromagnetic resonance and magneto-optic Kerr effect measurements. The orientation and strength of the uniaxial anisotropy, which mainly originates from shape and step-edge contributions, depends on the angle of the ripple ridges elongation with regard to the [100] direction of MgO and on the Fe film thickness, respectively. Theoretical analysis reveals anisotropy fields and orientations of both anisotropy contributions that are in agreement with the experiment.

The controlled motion of magnetic domain walls (DWs) in nanowires is one of the fundamental issues for the realization of new concepts of high-density and ultrafast non-volatile data-storage devices. Locally welldefined confining potentials, e.g., created by notches, act as pinning sites for individual DWs . A spin polarized current, driven through the nanowire, yields the possibility to manipulate the magnetic configuration due to a transfer of spin-angular momentum. For a reliable spin-torque induced domain-wall depinning, low threshold currents are required in order to reduce stochastic temperature effects, caused by Joule heating as well as to avoid damage to the nanowire. Here we present a concept to create pinning sites for DWs in magnetic nanowires without geometric constrictions: The local modification of magnetic properties by means of ion irradiation (magnetic soft spots). Implantation of chromium ions into Ni80Fe20 causes a reduction of the saturation magnetization (Ms) and changes the crystalline magnetic anisotropy. The application of electron beam lithography (EBL) or focused ion beam (FIB) on the other hand enables a spatial resolution below 50 nm for this kind of magnetic patterning process. Pinning of DWs in magnetic soft spots is expected to be preferred compared to the environment, as the locally reduced Ms causes a decrease of the exchange energy associated with the DW. In order to verify the suitability of our concept, we fabricate magnetic soft spots into Ni80Fe20 nanowires by means of 15 kV Cr ion irradiation through an EBL shadow mask and examine the pinning characteristics for DWs. In conclusion, magnetic soft spots realized via implantation of chromium are suitable as pinning sites for domain walls, avoiding a local increase of the current density in the nanowire due to supplementary spatial constriction.

On a continuous 20 nm thick Permalloy film, magnetically softer and harder stripes, which are in direct lateral contact by means of exchange coupling, were fabricated by focused Ga+ irradiation. The irradiation dose was 6.00 x 1015 Ga/ions and the width of the alternative exposed and unexposed stripes were varied from 1000nm to 200nm. Low angle electron diffraction experiment confirmed that due to this amount of ion dose, saturation magnetic induction of the irradiated stripe is reduced to 70 % of the unirradiated stripe, assuming a thickness reduction of 2.5 nm determined by AFM. Magnetisation reversal experiments were carried out using high resolution Lorentz microscopy. For a stripe width of 1000 nm a pronounced 2 step reversal with an intermediate antiparallel orientation of magnetisation in neighboring stripes is found. Differential phase contrast (DPC) images demonstrate a transition from the discrete switching of the wider nanostripes to the collective switching of the narrower stripes of width 200 nm. This can be attributed to the local modification of the magnetic thin film parameters below the intrinsic magnetic length scales associated with the domain wall width. By using DPC mode of Lorentz microscopy, domain wall width was calculated and compared with theory for 20 nm thick continuous Permalloy film.

We report on an approach to tailor the magnetic exchange in a conventional granular CoCrPt:SiO2 recording medium by irradiation with Co+ ions. Irradiation at low fluences enhances the intergranular exchange resulting in a narrowing of the switching field distribution (SFD). The modification of magnetic exchange coupling was evidenced by measuring the angular dependence of the switching field and is supported by an increase in the magnetic domain size. Further, micromagnetic simulations of a granular magnetic medium confirm the correlation between intergranular exchange and SFD. At high fluences, however, the irradiation damages lead to the degradation of the magnetic layer as magnetic anisotropy and saturation magnetization decrease. Ion irradiation simulations suggest that this is caused by strong intermixing at the grain and layer interfaces.

The control of the effective magnetic anisotropy, saturation magnetization as well as the dynamic magnetic properties in ferromagnetic thin films is of significant importance for most applications in spin electronics. The two materials’ parameters which control the dynamic response of soft magnetic thin films are the precessional frequency f_res and the effective damping parameter alpha. Whereas anisotropy and saturation magnetization together determine the precessional frequency of the films, the magnetic damping parameter cannot be varied easily. However it was shown that exchange coupled ferromagnetic/antiferromagnetic (F/AF) systems exhibit strong changes in f_res and alpha independent of the occurrence of exchange bias [1, 2]. On the other hand, ion irradiation of magnetic thin films [3] has been shown to provide an alternative method to alter soft magnetic properties of magnetic multilayer films due to ion induced changes in the interfacial structure. Here we use Ni-ion irradiation to control the dynamic magnetic properties in Ni81Fe19/IrMn/Ni81Fe19 structures in a controlled way. We show that for very thin AF layers alpha can be increased significantly [4]. Selecting an IrMn thickness below the onset of exchange bias, it is possible to solely alter alpha. This effect is reversible by low fluence Ni ion irradiation without changing the composition of the ferromagnetic alloy and thus can be used to adjust alpha over a wide range. As a consequence, the method can be used to laterally pattern samples only by the damping parameter. As shown in Fig. 1 the overall integral damping parameter then is determined by the relative fraction of materials with high and low damping parameter. alpha is thereby varied linearly without any significant change in f_res. The demonstrated paths of film alteration by ion irradiation provide an additional degree of freedom for the tailoring of dynamic magnetic properties of softmagnetic thin films. The method not only allows for general adjustment of magnetic properties, but also for a local setting of dynamic magnetic properties. This work was supported by the German Science Foundation DFG under grant no. MC9-8/2. [1] J. McCord et al., Phys. Rev. B 70, 094420 (2004) [2] J. McCord et al., Phys. Rev. B 75, 134418 (2007). [3] J. Fassbender and J. McCord, J. Magn. Magn. Mater. 320, 579 (2008). [4] J. McCord, et al., Appl. Phys. Lett. 92 162506 (2008).

Ion beam irradiation and ion implantation of ferromagnetic films is a smart technique to tailor their magnetic properties and structural composition of multilayers or nanostructured samples [1,2]. Metals like Ta are commonly used as seed and cap layers in spintronic devices like Giant Magneto-Resistance sensors as Ta is chemically stable. However, it is known that 12% of Ta intermixing in Py leads to magnetically dead layers of 0.6-1.2 nm in thickness [3]. These dead layers make it impossible to determine the correct magnetic volume, which is needed to obtain the saturation magnetization from the magnetic moment measured e.g. by SQUID. This is especially true for multilayer samples which typically have a large number of interfaces. Here we present a method to determine the saturation magnetization of Py/Ta multilayers from VNA-FMR (Vector Network Analyzer Ferromagnetic Resonance) and MOKE (Magneto-optical Kerr Effect) measurements even in the case of interfacial mixing due to ion irradiation, where SQUID magnetometry fails due to the unknown magnetic volume. Three sets of Py/Ta thin film multilayer systems were sputter-deposited on a Si/SiO2 substrate: (1xPy) is a single 20 nm thick Py layer, (5×Py) a multilayer of the structure 31 nm Ta/[4 nm Py/1 nm Ta]5/2 nm Ta and (10xPy) a multilayer of 30.5 nm Ta/[2 nm Py/0.5 nm Ta]10/2.5 nm Ta. The overall Py amount was always 20 nm and the total Ta thickness including seed and cap layer corresponds to 38 nm. Finally, the films have been irradiated with Ne ions at 40 keV with ion fluences in the range of 5×1013 to 5×1016 Ne/cm2. FMR shows that the FMR frequency vs. field dependence is significantly influenced by the amount of irradiation and number of interfaces (see Fig 1). At fluences above 2.5×1015 Ne/cm2 a significant decrease of the resonance frequency can be observed for the 1xPy samples. For the three unirradiated samples the FMR frequency decreases with an increasing number of Py/Ta repetitions, i.e. increasing number of interfaces. This decrease will be even more pronounced if larger numbers of interfaces are used. This can be explained by the higher number of neighboring Ta atoms in those cases. The deleterious effect of Ta on the ferromagnetic properties is becoming much stronger reducing the effective ferromagnetic film thickness by creating magnetically dead layers close to the interface [1,3-6]. This reduction of the saturation magnetization is directly linked to the resonance frequency. This allow to determine not only the uniaxial in-plane anisotropy field K2|| but also the saturation magnetization μ0Ms from the FMR frequency vs. field dependence. However, this can only be done, if there is only shape anisotropy but no uniaxial out-of-plane anisotropy as it is the case for our Py/Ta multilayers. From polar MOKE loops μ0Ms can be obtained by determining the perpendicular anisotropy field of the samples as well. This feature proves to be very useful here since SQUID magnetometry suffers from a major drawback: It requires the exact effective Py volume to calculate μ0Ms from the magnetic moment, which becomes more and more difficult to determine due to the increasing interfacial mixing induced by the ion irradiation and thinner Py layers. In our case, polar MOKE and FMR represent suitable alternatives since they allow μ0Ms to be determined without the knowledge of any film thickness. The good agreement with the SQUID data of the non-irradiated samples supports this (see Fig 2). ...

Artificial antiferromagnets made from magnetically coupled trilayer structures are the basis for all types of spintronic devices like MRAM, GMR sensors etc. For years major effort lay on adjusting the coupling strength by changing the spacer thickness or material. Today, nanostructures offer a different approach as they add additional coupling mechanisms like proximity effects or Néel orange-peel coupling to the common interlayer exchange coupling (IEC). By means of ion beam erosion techniques it is possible to create well ordered substrate ripples with nanometer periodicity. They are transferred into the films grown on these rippled substrates. Hence, such ripples are a convenient way to induce Néel orange-peel coupling [1] and thus allow for tailoring the magnetic properties [2] as well as the coupling strength by varying the ripple periodicity without adjusting the spacer thickness. We have investigated the influence of rippled vs. flat Si substrates on the interlayer exchange coupling contributions in polycrystalline Fe (4nm)/Cr (x nm)/Fe (4nm) thin film trilayers (x=0–5 nm). The substrate surface was periodically modulated (periods of 23 nm and 37 nm) by Ar+ ion beam erosion. The influence of the resulting surface and interface structure on the magnetic properties has been investigated by longitudinal magneto-optical Kerr effect (MOKE) applying a Stoner-Wohlfarth model on the magnetization reversal loops. Using 23 nm period ripples, we find an orange peel type coupling, predicted by Néel's theory superimposed on the IEC. In addition due to the morphology of the magnetic layers, a strong uniaxial magnetic anisotropy is induced. This work is supported by DFG grant FA 314/6-1. REFERENCES 1. Körner M. et al., Physical Review B 80, 214401, 2009. 2. Fassbender J. et al., New Journal of Physics 11, 125002, 2009

Lateral patterning of thin magnetic films allows structuring on dimensions below certain intrinsic length scales like the domain wall width. By means of magnetic patterning using local ion irradiation periodic patterns of stripes with alternating saturation magnetization value were created in a ferromagnetic Ni80Fe20 matrix. The domain configuration during magnetization reversal was investigated using Kerr microscopy, scanning transmission x-ray microscopy, as well as Lorentz microscopy. The reversal mechanisms in the stripe panels are influenced by the domain configuration of the surrounding film. Starting from 1 um stripe width, changes in the micromagnetic behavior with respect to decreasing width are investigated. Special emphasis is put on the formation of 180∘ walls between adjacent stripes with different saturation magnetization, on the orientation of the magnetization inside the stripes with respect to the stripe axis orientation, as well as on the transition of the patterned material to an effective medium.

We present a method to determine the saturation magnetization of samples for which the magnetic volume is unknown and thus cannot be calculated from the magnetic moment. This can happen, e.g., in multilayers, where the spacer material is likely to cause intermixing or whenever ion irradiation is used to modify the magnetic properties of samples on purpose. In both cases the active magnetic volume is altered from its nominal value in unknown manner. Therefore magnetometry like supraconducting quantum interference devices (SQUID) or vibrating sample magnetometry (VSM) fail, because they detect the magnetic moment but do not provide information on the respective magnetic volume. In this article we have used thin films of Permalloy (Py) and Py/Ta multilayers. Some of the Py/Ta samples were irradiated with Ne ions in order to modify the interfacial mixing. The saturation magnetization is determined by ferromagnetic resonance (FMR).

The control of dynamic magnetic properties in ferromagnetic thin films is of significant importance for applications in spin electronics. We demonstrate the use of low fluence Ni-ion irradiation to locally and solely tailor the magnetic damping parameter e in ferromagnetic-antiferromagnetic Ni81Fe19/IrMn/Ni81Fe19 sandwich structures. The overall relaxation time then depends on the relative fraction of the phases with different e , which thereby can be controlled without changing the precessional frequency. The superposition of alpha is confirmed by modeling of the dynamic response. The demonstrated path of film engineering by local ion irradiation [J. Fassbender, J. McCord, JMMM 320 (2008) 579] provides an additional degree of freedom for the local tailoring of dynamic magnetic properties of soft-magnetic thin films.

Lateral patterning of thin ferromagnetic films allows the modification of the magnetic parameters below certain intrinsic magnetic length scales like the domain wall width. In contrast to the creation of isolated micro- and nanostructures, magnetic patterning by means of local ion irradiation results in direct exchanged coupled regions in the thin film, which have different magnetic properties. At the lateral interfaces of these regions magnetic domain walls can be trapped and thus easily investigated. The modification of the material properties, e.g. saturation magnetization and magnetic anisotropy, also directly affects the intrinsic length scales. As a model system periodic patterns of Ni80Fe20 stripes with alternating saturation magnetization value located either inside a Ni80Fe20 matrix or isolated are used. During the magnetic reversal with the external field parallel to the stripes, magnetic domain walls are created between adjacent stripes due to a noneparallel alignment of the stripe’s magnetization (Fig. 1). Fig. 1: Magnetization distribution in periodic pattern of 10 μm wide stripes with alternating saturation magnetization value. The magnetization directions are indicated by arrows. (Non-) irradiated regions with (unchanged) decreased saturation magnetization are indicated with (gray) white shaded areas at the bottom. The location of a 180° domain wall is indicated by the green rectangle. As the wall profile for stripe widths in the sub-μm regime is not easily accessible in experiment, changes of the possible orientations of the magnetization inside the stripes with respect to the stripe orientation are used to obtain informations about the domain walls. For special pattern configurations these domain walls are able to mediate an exchange spring behavior through the storage of energy [1]. By scaling the stripes width down, a crossing between the patterning size and the intrinsic length scales associated with the domain walls is expected. This is combined with a transformation to an effective magnetic medium, showing neither the properties of the fully irradiated nor the unirradiated material (so-called hybrid material). Local ion irradiation is accomplished by irradiation in combination with a lithographically defined mask or the by the use of a focused ion beam. For comparision the ion energies for both methods are choosen to be 30 keV. For a Ni80Fe20 film thickness of 20 nm the resulting penetration depths of the ions cover the complete film as well as the interface to the seed layer. In order to understand the ion irradiation induced changes in the magnetic parameters of the film, ions of different species (Ar+, Ga+, Cr+, Ni+, Co+, Si+, O+) as well as different seed layers are used. Changes of the magnetic and structural properties are studied as function of ion species, fluence and seed layer material. The uniaxial magnetic anisotropy of the Ni80Fe20 film is not affected by the implanted ion species, but by the species of the recoils originating from the seed layer. Using SiO2 as seed layer material, the anisotropy can be successively annihilated with increasing fluence. Irradiation using a Ga+ focused ion beam with its high current density induces a grain growth to the material [2], limiting therefore also the minimum patterning size (Fig. 2). Fig. 2: Bright field plane view TEM image of 200 nm wide stripes. The focused ion beam irradiated areas are observable by the increased grain size. References: [1] McCord J. et al. Advanced Materials 20 (2008) 2090. [2] Ozkaya D.L. et al. Journal of Applied Physics 91 (2002) 9937.

Focused ion beam irradiation can be used as a tool for creation of magnetic nanos-tructures. Previous studies have shown that FIB irradiation of thin metallic films could induce significant grain growth and therefore modify magnetic properties [1-2]. In this work we compare X-ray diffraction studies with magnetic properties that were characterized by magneto-optic Kerr effect sensing both the in-plane and the out-of-plane components of the magnetization. A 50nm thick permalloy layer (Ni80Fe20) irradiated with different fluences of Ga+ ions was chosen for the investigations of grain size and microstrain. Due to the small irradiated areas produced by FIB irradiation XRD measurements were carried out on an optimized X-ray laboratory setup with a focused X-ray beam of 200µm as well as on the ESRF ID01 facility using a 1µm focused beam. Figure 1a shows rocking curves of the (111) permalloy reflection and demonstrates the material texturing with increase of ion fluence as there is a narrowing in FWHM and a rise of intensity. Low irradiation fluences up to 6.24*10E15 ions/cm2 modify the material and induce crystal-lite growth, whereas larger fluences completely destroy the crystalline structure. A further material crystallization should improve the magnetic properties. Due to the incorporation of Ga+ ions into the lattice additional softening of the permalloy film occurs and leads to the degradation of magnetic properties. In figure 1b a corre-sponding saturation Kerr rotation measurement is shown. It demonstrates a decrease of the magnetic moment with increasing ion fluence. [1] C.M. Park and J. A. Bain., J. of Appl. Phys. 91, 6830 (2002). [2] W.M. Kaminsky et al., Appl. Phys. Lett. 78, 1589 (2001).

Conventional exchange spring systems consist of directly exchange coupled hard and soft magnetic layers, which characteristic behaviour is related to an interlayer domain wall that is formed during reversal of magnetization. Recently, a similar exchange spring effect was achieved in lateral hard/soft magnetic stripe structures, which were fabricated by means of ion implantation of single layer films [1]. This technique offers the possibility to directly investigate the changes of magnetization in and between the hard/soft phases. In the presented work, lateral exchange spring structures were prepared on patterned samples to investigate the interplay between structuring and additional dipolar fields. In contrast to extended films, the lateral exchange spring effect only occurs for a higher difference in saturation magnetization Ms between the two magnetic phases. The measured collective magnetization reversal of hard and soft phase is attributed to strong dipolar fields at the element edges that cause a hysteresis behaviour which is comparable to that of a magnetic homogeneous square element. The exchange spring behaviour, occurring with increasing difference in Ms between the two phases is related to an increase in effective shape anisotropy in the high Ms stripes. The magnetization reversal approaches an individual switching of the stripes, with the low Ms stripes switching first. This results in an antiparallel alignment of magnetization of the individual stripes with lateral domain walls in between, which configuration is stabilized by the inter-stripe flux closure. The resulting two-step reversal is modelled, taking the demagnetization and domain wall energy terms into account.

Due to its low coercivity and negligible magnetostriction, permalloy (Ni80Fe20) is one of the most used materials in thin film and micro/nano magnetism. By means of ion irradiation the magnetic properties can be modified [1], and in combination with a lithographically defined mask or the use of a focused ion beam magnetic patterning can be achieved [2]. The changes of the magnetic properties due to ion-solid- interaction must be related to different origins, e.g. direct implantation, surface sputtering and interfacial mixing. Their respective influence depends strongly on the chosen multilayer system as well as on the implantation conditions. Here we present a systematic study of irradiation of permalloy with common ion species. Special emphasis is put on the separation of the effect of direct implantation from mixing and sputtering. By transferring this knowledge to laterally resolved irradiation, direct exchange coupled magnetic stripes of submicron width are created. The magnetization reversal process of this lateral exchange spring structures depends on the interaction between adjacent soft and hard magnetic stripes. By scaling the stripe sizes down, fundamental questions regarding the maximum domain wall density and the domain–wall interaction may be addressed. References [1] J. Fassbender et al., Physical Review B 73, 184410 (2006). [2] J. McCord et al., Advanced Materials 20, 2090 (2008).

rtificial antiferromagnets made from magnetically coupled trilayer structures are the basis for all types of spintronic devices like MRAM, GMR sensors etc. For years major effort lay on adjusting the coupling strength by changing the spacer thickness or material. Today, nanostructures offer a different approach as they add additional coupling mechanisms like proximity effects or N\'eel orange-peel coupling to the common interlayer exchange coupling (IEC). By means of ion beam erosion techniques it is possible to create well ordered substrate ripples with nanometer periodicity. They are transferred into the films grown on these rippled substrates. Hence, such ripples are a convenient way to induce N\'eel orange-peel coupling [1] and thus allow for tailoring the magnetic properties [2] as well as the coupling strength by varying the ripple periodicity without adjusting the spacer thickness. We have investigated the influence of rippled vs. flat Si substrates on the interlayer exchange coupling contributions in polycrystalline Fe (4nm)/Cr ($x$ nm)/Fe (4nm) thin film trilayers ($x$=0--5 nm). The substrate surface was periodically modulated (periods of 23 nm and 37 nm) by Ar$^+$ ion beam erosion. The influence of the resulting surface and interface structure on the magnetic properties has been investigated by longitudinal magneto-optical Kerr effect (MOKE) applying a Stoner-Wohlfarth model on the magnetization reversal loops. Using 23 nm period ripples, we find an orange peel type coupling, predicted by N\'eel's theory superimposed on the IEC. In addition due to the morphology of the magnetic layers, a strong uniaxial magnetic anisotropy is induced. [1] M. Körner, K. Lenz, M.O. Liedke, T. Strache, S. Dzenisevich, A. Keller, S. Facsko, and J. Fassbender, submitted to Phys. Rev. B. [2] J. Fassbender, T. Strache, M.O. Liedke, D. Mark\'o, S. Wintz, K. Lenz, A. Keller, S. Facsko, I. M\"onch, and J. McCord, submitted to New. J. Phys. This work is supported by DFG grant FA 314/6-1.

In order to increase the storage density of modern computer disk drives and to push the superparamagnetic limit to the smallest achievable bit sizes further, smaller grains with even larger magnetic anisotropies are required, which are accompanied by large coercive fields obstructing the writing process. One route to overcome this problem is to independently reduce the coercive field without altering anisotropy and remanence by tailoring the intergranular exchange in granular CoCrPt-SiO2 films. Here we demonstrate that by means of ion implantation of Co and Ne a continuous reduction of the coercive field can be achieved without significant modification of the remaining magnetic parameters. In addition to the magnetization reversal behavior of the entire film investigated by magneto-optic Kerr effect and SQUID magnetometry, also the magnetic domain configuration in the demagnetized state is imaged by magnetic force microscopy.

We have investigated the interlayer exchange coupling in Fe (4nm)/Cr (x nm)/Fe (4nm) thin film trilayers (x=0–5 nm) deposited on rippled armorphous silicon substrates. The substrate surface was periodically modulated (periods of 20 nm, 35 nm, and 50 nm) by Ar+ ion erosion. The influence of the resulting surface and interface structure on the magnetic properties has been investigated by magneto-optical Kerr effect (MOKE). We found an orange peel type coupling, predicted by Néel’s theory and, due to the morphology of the magnetic layers, a strong uniaxial magnetic anisotropy in the system. This work is supported by DFG grant FA 314/6-1.

Over the last years the modification of magnetic parameters of thin films and multilayers by ion irradiation and implantation has become fashionable. However, especially in multilayer structures ion irradiation can lead to interfacial mixing and thus to a significant reduction of the magnetically active volume, i.e. 'dead' layers. Thus, it can be quite difficult---if not at all impossible---to determine the correct effective magnetic volume, e.g. for measurements of the saturation magnetization by SQUID. Here we show how ferromagnetic resonance (FMR) and magneto-optical Kerr effect (MOKE) measurements can be utilized to determine the saturation magnetization of irradiated Py/Ta multilayers instead and how the magnetic properties change upon irradiation. We prepared thin films and multilayers of Py/Ta with an overall Py thickness of 20 nm and varying number of Py/Ta stacks and irradiated them with Ne ions at various fluences. FMR, MOKE, and SQUID magnetometry were used for investigation. With both, increasing ion fluences and increasing number of Py/Ta interfaces, a decrease of saturation magnetization and an increase of precessional damping can be observed. The uniaxial anisotropy of the samples is only of small magnitude and remains almost unaffected. There is, depending on the number of interfaces, a critical ion fluence at which ferromagnetic order in the multilayers vanishes. This work is supported by DFG grant FA 314/3-1.

We have investigated Py-Ta thin film systems with varying numbers of interfaces, but a constant overall thickness of 20 nm Py and 38 nm Ta. The samples have been irradiated with Ne+ ions at various fluences in order to modify their magnetic properties, that have been probed using ferromagnetic resonance (FMR), magneto-optical Kerr effect (MOKE), and SQUID magnetometry. The saturation magnetization Ms decreases both with increasing number of Py-Ta inferfaces and with increasing ion fluences. The uniaxial anisotropy of the samples is only of small magnitude and remains almost unaffected. There is a critical fluence depending on the number of interfaces for which ferromagnetism vanishes.

The magnetic anisotropy direction and strength of amorphous FeCoSiB thin films was modified locally by masked ion implantation without alteration of the magnetic material’s structure and the intrinsic magnetic properties of the ferromagnetic film. The changes were introduced by local ion implantation in a SiO2 covering and protection layer, inducing additional stress-induced magnetic anisotropy to the magnetostrictive ferromagnetic layer. Hybrid hysteresis curves combining switching and rotational processes were measured and the underlying local variation of magnetic anisotropy was confirmed by magnetic domain observations. A good agreement between the calculated stress distribution and the experimentally obtained magnetic data was found. The described indirect method, relying purely on magneto-elastics, introduces a new path to the creation or alteration of magnetic properties subsequent to magnetic film preparation in structured magnetic samples without introducing any structural changes to the ferromagnetic layers.

Ion irradiation of Fe60Al40 alloys results in the phase transformation from the paramagnetic, chemically ordered B2-phase to the ferromagnetic, chemically disordered A2-phase. The magnetic phase transformation is related to the number of displacements per atom (dpa) during the irradiation. For heavy ions (Ar+, Kr+, Xe+) a universal curve is observed with a steep increase in the fraction of ferromagnetic phase reaching saturation, i. e., a complete phase transformation, at about 0.5 dpa. This proves the purely ballistic nature of the disordering process. If light ions are used (He+, Ne+) a pronounced deviation from the universal curve is observed. This is attributed to bulk vacancy diffusion from dilute collision cascades, which leads to a partial recovery of the thermodynamically favored B2-phase. Comparing different noble gas ion irradiation experiments allows to asses the corresponding counteracting contributions. In addition, the potential to create local ferromagnetic areas embedded in a paramagnetic matrix is demonstrated.

Ion irradiation of Ni80Fe20/Ta multilayers induces an interfacial mixing of these two materials, which subsequently leads to a suppression of the ferromagnetic properties. In order to quantify the structural change grazing incidence x-ray reflectometry has been used. The ion irradiation induced intermixing has been studied by means of TRIDYN simulations. The broadening of the interfaces due to intermixing causes an enrichment of Ta in Ni80Fe20 and finally an amorphization of Ni80Fe20 above a Ta concentration of 10 to 20 percent [1]. Consequently the ferromagnetic properties are altered or even suppressed completely for high enough ion fluences. It is observed that the critical ion fluence for a complete suppression of the ferromagnetism depends sensitively on the number of interfaces. It is demonstrated that pure magnetic microstructures can be easily fabricated by local ion irradiation without changing the substrate topography. [1] J. Fassbender et al., Nucl. Instr. and Meth. in Phys. Res. B 248, 343 (2006).