Stripe domains in Fe based thin films: magnetic texture, magneto- transport and magnetic coupling

S. BUSTINGORRY; M. EDDRIEF; J. GÓMEZ; L. LOUNIS; B. PIANCIOLA; M. VÁSQUEZ MANSILLA; M. BARTUREN; E. DE BIASI; D. GOIJMAN; C. HEPBURN; F. OTT; M. SACCHI; N. ÁLVAREZ; A. BUTERA; S. FLEWETT; M. GRANADA; M. MARANGOLO; G. RAMÍREZ; J. MILANO

Ollantaytambo

Workshop; International workshop Spintronics 2019 (Spin Peru); 2019

In magnetic systems, domain and pattern formation have attracted much interest since its understanding can be crucial for many technological applications [1]. The competition between short range exchange interaction, which promotes homogeneous magnetic configurations at small length scales, and the unavoidable long-range dipolar interaction, which favors inhomogeneous configurations at large length scales, is primarily responsible for pattern formation. One of these particular patterns is formed by weak magnetic stripe domains. This particular behavior occurs due to the presence of a perpendicular-to-the- film magnetic anisotropy (PMA) that induces to magnetic moments to not keep the usual in-plane configuration imposed by magnetostatics. The parameter Q defined as Q = 2KPMA/u_0M^2, where KPMA is the strength of the PMA and u_0M2/2 is the demagnetizing energy (Edem) for a magnetic thin film, helps us to measure how far the system is from a fully in-plane magnetic configuration (Q < 0). If Q > 1, PMA overcomes Edem, so the magnetization points perpendicular to the thin films. However, if 0 < Q < 1, there exists an intermediate stage where PMA competes with Edem. In this case, the film presents self organized stripe-shaped magnetic domains with a very complex magnetic structure along the sample. This periodically modulated arrangement across the sample present domains pointing in the three spatial directions: Some of those domains remain pointing along the direction of the saturating field (MH), other one perpendicular to the film plane (Mperp) and the last one forming closures domains (MCD) in order to reduce the stray magnetic field.In this work, we present results concerning to several aspects to stripe domains: i) the determination of the magnetic domain structure; ii) magnetoresistant effects and iii) magnetic coupling. The systems involved in this study are thin films of FePt, permalloy and Fe0.8Ga0.2. The first two kind of films were grown by sputtering at INN, Argentina and the last one by molecular beam epitaxy, MBE, at INSP, France. In order to study the stripe magnetic texture, we have performed circular dichroism in x-ray resonant magnetic scattering, CDXRMS, (at Circular Polarization beamline in synchrotron ELETTRA) in Fe0.8Ga0.2 thin films, and neutron polarized reflectometry experiments, PNR, (at PRISM beamline in Leon Brillouin Laboratory), in FePt thin films. The asymmetry of the first order magnetic bragg peaks observed in the rocking curve [Figure 1(a)] of CXMRS experiments, when both circular polarizations are used, shows explicitly the existence of closure domains, which is a fingerprint of the presence of the weak stripe pattern. On the other hand, PNR experiments allow us to determine the M// behavior as a function of the thin film depth when the stripes are set. PNR indicates that M// is reduced respect to the saturated state, reaching the lower value at the middle of the film [Figure 1(b)].Magneto-transport properties of Fe0.8Ga0.2 films are studied. The anisotropic magnetoresistance, AMR, dominates the low field behavior, which is extremely dependent on the magnetic domain configuration [2]. At room temperature, the AMR shows two different behaviors depending on the measurement configuration, i.e., when the stripes are oriented along the electric current (parallel geometry) the resistivity increases with the applied field (positive AMR) [Figure 1(c)], while for the electronic current flowing perpendicular to the stripes (perpendicular geometry) AMR is negative [Figure 1(c)]. Moreover, when the temperature is decreased, the AMR changes in sign for the parallel geometry while AMR is nearly temperature independent in the perpendicular geometry. A simple model considering parallel and series conduction, plus the temperature dependence of anisotropic magnetoresistance and domains configuration, gives an insight of the phenomenology of these experimental results.In order to study the effects of stripe domains on the magnetic coupling, we have grown FePt/permalloy bilayers [3], where both alloys present stripe domains. Magnetic force microscopy, MFM, images show that the bilayer stray field is a convolution arising from the stripe pattern of both layers. However, as it is shown in Figure 1(d), the stripe domains are not aligned along the same direction. We can observe wide stripes that correspond to the permalloy, and the thin ones that are canted with respect to the permalloy stripes. This effect can be attributed to the influence of the dipolar stray field of the permalloy layer on the stripe structure of the FePt film.[1] Current-controlled propagation of spin waves in antiparallel, coupled domains. Chaunpu Liu et al., Nature Nanotech (2019). https://doi.org/10.1038/s41565-019-0429-7.[2] Magnetotransport properties of Fe0.8Ga0.2 films with stripe domains. M. Granada et al., Phys. Rev. B 94, 184435 (2016).[3] Magnetic coupling of stripe domains in FePt/Ni80Fe20 bilayers. N.R. Álvarez et al., J. Phys. D: Appl. Phys. 50 115001 (2017).