The influence of Re layer insertion either at the bottom or top interface of epitaxially grown Pt/Co/Pt heterostructures on their static and dynamic magnetic properties is discussed in terms of their crystalline structure. Magnetic properties were studied as a function of both Re and Co layer thicknesses (d(Re) and d(Co), respectively) in the matrix-like samples with a double-wedge structure, in which the layer thickness gradients were oriented orthogonally. The comprehensive investigations of the changes in coercivity, perpendicular magnetic anisotropy, spin reorientation transition, interfacial Dzyaloshinskii-Moriya interaction (iDMI) and spin wave damping are reported. Two different magnetic phases depending on the Co layer thickness with volume anisotropies (determined without demagnetization term) of K-V similar to 0.25 (low) and K-V similar to 0.75 MJ/m(3) (high) were observed. The relation between these phases depends on the stack sequence and thickness of Re inserted layer. For d(Re) = 0.2 divided by 0.7 nm deposited as the bottom interface, d(Co) induced transition at d(tr) similar to 2 nm from low to high volume anisotropy phases was observed. Low and high volume anisotropy phases are associated to Co fcc and hcp structural phases. The insertion of half atomic layer of Re had no influence on surface anisotropy but significantly enhanced iDMI above 1 pJ/m and reduced spin wave damping.

The results of extensive combined experimental and theoretical investigations of static and dynamic properties of an Ir/Co/Pt multilayer with low uniaxial anisotropy and asymmetric Ir/Co and Co/Pt interfaces responsible for large interfacial Dzyaloshinskii-Moriya interaction (IDMI) are presented. Within longitudinal magnetooptical Kerr effect-based microscopy and magnetic force microscopy studies a complex magnetic configuration was detected: Large in-plane magnetized domains several dozen micrometers in size were modulated by a weak stripe domain pattern with periods of about 100 nm. Using Brillouin light scattering spectrometry, the hysteresis of the Stokes and anti-Stokes peaks frequencies was observed as a function of the magnetic field. This hysteretic behavior associated with IDMI-induced asymmetry of spin wave dispersion is correlated with the switching of the large macrodomains. Using micromagnetic simulations, we determine field-dependent magnetization distributions and dispersion relations, proposing an explanation of the observed behavior. The investigated nanostructure can be used as a nonvolatile spin wave velocity switcher.

The results of magnetic investigations of epitaxial trilayers Pt/Co/Pt asymmetrically modified by inserting W layer at the bottom or top Co interfaces in the wide ranges of Co and W layer thicknesses are reported. The samples were epitaxially grown on Pt buffer in double wedge geometry: the d(Co) thickness gradient of the continuous Co wedge was orthogonal to the d(W) thickness gradient of non-magnetic W underlayer (overlayer) deposited either as step-like or continuous wedge. Resulting samples have Pt/W/Co/Pt (Pt/Co/W/Pt) stacking sequences. The influence of (d(Co), d(W)) on static magnetization reversal processes were studied using magnetooptical Kerr effect; Brillouin light scattering (BLS) method was applied for dynamical characterization. The magnetic dead layer thickness d(0) abruptly increases until d(W) reaches similar to 0.5 nm for both orderings. Then its value saturates for Pt/Co/W/Pt, while for Pt/W/Co/Pt samples a slow growth is observed. For d(Co) corresponding to out-of-plane magnetization the inserting of W layer leads to the strong reduction in coercivity (two orders of magnitude) and transition to in-plane magnetization in the case of Pt/W/Co/Pt ordering, while the Pt/Co/W/Pt samples demonstrates weak changes of coercivity with small change in magnetic anisotropy. The strength of interfacial Dzyaloshinskii-Moriya interaction (iDMI) and spin wave damping for selected d(Co) thicknesses as a function of d(W) were derived from BLS measurements. Inserting W with thickness d(W) similar to 0.1 nm induces significant iDMI changes, iDMI saturates for d(W) > 0.4 nm. Our findings demonstrate the efficiency of thin W interlayer on modification of magnetic parameters in Pt/Co/Pt trilayer.

The magnetic properties of NiO/Co/Pt as a function of Co layer thickness were investigated by polar magneto-optical Kerr effect (PMOKE) (magnetometry and microscopy) and Brillouin Light Scattering (BLS) spectroscopy. PMOKE measurements revealed strong surface anisotropy (1.8 mJ/m 2 ) favoring perpendicular magnetic anisotropy and asymmetric domain wall propagation explained by anticlockwise chirality. BLS measurements show that this chirality is induced by strong interfacial Dzyaloshinskii–Moriya interaction (+ 2.0 pJ/m). This is one of the highest values reported so far for Co layers surrounded by different layers. The observed chirality is opposite to what has been found in Co/oxide interfaces. These results and data published earlier, indicate that the strength of interfacial Dzyaloshinskii–Moriya interaction increases with the amount of stoichiometric NiO. Therefore, this work shows that NiO is the source of the interfacial Dzyaloshinskii–Moriya interaction.

Magnetic multilayers of (Ir/Co/Pt)6 with interfacial Dzyaloshinskii-Moriya interaction (IDMI) were deposited by magnetron sputtering with Co thickness d=1.8 nm. Exploiting magneto-optical Kerr effect in longitudinal mode microscopy, magnetic force microscopy, and vibrating sample magnetometry, the magnetic field-driven evolution of domain structures and magnetization hysteresis loops have been studied. The existence of weak stripe domains structure was deduced – tens micrometers size domains with in-plane “core” magnetization modulated by hundred of nanometers domains with out-of-plane magnetization. Micromagnetic simulations interpreted such magnetization distribution. Quantitative evaluation of IDMI was carried out using Brillouin light scattering (BLS) spectroscopy as the difference between Stokes and anti-Stokes peak frequencies Δf. Due to the additive nature of IDMI, the asymmetric combination of Ir and Pt covers led to large values of effective IDMI energy density Deff. It was found that Stokes and anti-Stokes frequencies as well as Δf, measured as a function of in-plane applied magnetic field, show hysteresis. These results are explained under the consideration of the influence of IDMI on the dynamics of the in-plane magnetized “core” with weak stripe domains.

We discuss the magnetic properties of epitaxial Pt/W(d(w))/Co(d(Co))/Pt layered films as a function of W(d(w)) and Co (d(Co)) layer thicknesses. The samples were investigated by means of: (i) polar magnetooptical Kerr effect based magnetometry and microscopy as well as (ii) Brillouin light scattering (BLS) spectrometry. The bottom W layer exhibits a strong influence on the magnetic anisotropy, d(Co)-dependent spin reorientation transition and magnetic polarization of atoms forming the interfaces. The areas with (i) in-plane, (ii) out-of-plane, (iii) super-paramagnetic with out-of-plane anisotropy, and (iv) nonmagnetic states appear with d(Co) decrease. The d(w) driven transition from hard to soft magnetic material with out-of-plane magnetization was found. The observed domain structures (micrometer range bubbles, stripes and dendritic-like) at out-of-plane state are very sensitive to d(Co), d(w) thicknesses and external magnetic field. The presence of Dzyaloshinskii-Moriya interaction was deduced from BLS measurements.

Ultrathin cobalt layers sandwiched between noble metals exhibit magnetic properties attractive for different applications. In particular in these systems relatively strong perpendicular magnetic anisotropy (PMA) can be obtained in Co thickness range of several monolayers. Moreover, for Pt/Co/Ir and Ir/Co/Pt systems strong Dzyaloshinskii-Moriya interaction (responsible for creation of skyrmions and domain walls with a given chirality) can be achieved [1]. Therefore, magnetic systems with structure X/Co/Y (X,Y = Au, Ir, Pt) are very important for applications in information technologies. We have investigated X/Co-wedge/Y (X, Y = Au, Ir, Pt) layered system deposited by magnetron sputtering on naturally oxidized Si substrate covered with Ti-4nm/Au-30nm buffer layer. The thickness of wedge shaped Co layer (tCo) was varied from 0 to 3.6 nm and the thickness of X and Y layers were 2 nm. The magnetic properties of nine systems (all combination of X/Co/Y) were characterized by measurements of magneto-optical hysteresis loops measured in polar configuration (P-MOKE) along the Co thickness gradient. On this basis we have defined tCo changes of: (i) coercive field HC(tCo); (ii) and squareness of the hysteresis loops (ratio of P-MOKE signal in remanence and saturation φR/φS(tCo)); (iii) magnetic uniaxial anisotropy field. The Co thickness corresponding to spin reorientation transition, and volume as well as surface contributions to effective anisotropy constant were determined for each system. For selected systems the magnetic characterization was supplemented by determination of Dzyaloshinskii-Moriya interaction (using Brillouin light spectroscopy) and observation of magnetic domain structure evolution with magnetic field (using P-MOKE-microscopy). References: [1] A. Fert et al., Nature Materials 2 (2017), 17031.

Magnetization processes and magnetic domain structures in Ta/CoFeB/MgO stacks were studied in a series of samples with various CoFeB thicknesses d ranging from 1.24 to 1.60 nm with a step of 0.04 nm, using polar magneto-optical Kerr effect (PMOKE) magnetometry and microscopy. Thickness dependence of the magnetic anisotropy was evaluated and the first and second order anisotropy constants were quantified for each thickness. Accordingly, this dependence was deduced to result in magnetization reorientation from out-of-plane to in-plane through an easy-cone magnetization region (1.39 nm <= d <= 1.41 nm) as d was increased. PMOKE imaging of the magnetization reversal processes for stacks with out-of-plane easy axis indicated both a significant increase of the density of nucleation centers and a change in domain morphology with increasing d up to the magnetization reorientation thickness. Magnetization reversal dynamics was described by a thermal activation model consistent with a Barkhausen length of about 120 nm. The thinnest films with d = 1.24 and 1.28 nm exhibited straightened narrow stripe domains resulting from magnetic dipolar repulsion. A thorough study of narrow stripe domains was performed via direct and indirect magnetization reversal processes. The application of such structures as spin wave nano-channels could be promising.

We have studied the solid state dewetting of ten monolayers thick Ag film deposited on periodically patterned Si(557) surface. The annealing of the system in the ultra-high vacuum at the temperatures between 300 and 400 °C resulted in full agglomeration of the film and formation of faceted single crystalline Ag nanoparticles exhibiting bimodal size distribution. We demonstrated that some particles contain screw dislocations producing a step on the upper particle facet. We related the bimodality in particles distribution with the ability of dislocation-containing Ag particles to evolve by Ostwald ripening mechanism.

The X /Co 3 nm/ 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 (∼10 14 ions cm −2 ) leads to ∼1 nm 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 × 10 15 ions cm −2 ) 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.

The influence of 30 keV Ga+ ion irradiation in the fluence range 0 divided by 1.10(16) ions/cm(2) on magnetic properties of Pt (bottom)/Co(3 nm)/Au(top) trilayers grown by the molecular beam epitaxy was studied. Polar magnetooptical Kerr effect magnetometry and Brillouin light scattering spectrometry were used to measure the magnetic anisotropy and the strength of Dzyaloshinskii-Moriya interaction (DMI). The as-deposited sample exhibits inplane magnetization with an effective anisotropy field of -0.35 +/- 0.02 T. Magnetic anisotropy gradually grows as the ion fluence is increased and out-of-plane magnetic anisotropy appears for the fluence above similar to 3.10(15) ions/cm(2). The frequency asymmetry in Stokes and anti-Stokes lines Delta f, being a measure of DMI strength, was determined from Brillouin light scattering investigations as a function of applied fluence. The effective DMI constant is equal to -0.48 +/- 0.05 mJ/m(2) in the as-deposited sample. While increasing fluence the parameter Delta f: (i) varies non-monotonically (with two maxima) for F smaller than F approximate to 10(15) ions/cm(2) and (ii) gradually decreases to zero for higher fluences. Our results can be used for the adjustment of DMI interaction in the manufacturing of magnonic nano-devices.

Asymmetric Ir/Co/Pt and Pt/Co/Ir trilayers with a wedged Co layer (Co thickness d = 0 divided by 3.6 nm) were deposited by magnetron sputtering on naturally oxidized Si substrates with a Ta/Au buffer. Their magnetic properties have been investigated using magneto-optical polar Kerr effect and Brillouin light scattering (BLS) technique in the Damon-Eshbach geometry. The BLS measurements shows a frequency asymmetry between Stokes and anti-Stokes peaks position, exploited to determine interfacial Dzyaloshinskii-Moriya interaction energy density DS. DS was nearly two times larger for the Ir/Co/Pt trilayer than for the Pt/Co/Ir with opposite chirality. The effective uniaxial magnetic anisotropy and Ds depend non-monotonically on d with a maximum at d approximate to 1.2 nm. The asymmetry in linewidth for Stokes and anti-Stokes peaks is observed for both Ir/Co/Pt and Pt/ Co/Ir trilayers for d < 1.6 nm, and it is increasing with decrease of Co thickness.

temperature-induced changes of magnetic anisotropy in ultrathin a Co/NiO bilayer P. Mazalski,1, 2 B. Anastaziak,3, 4 P. Kuświk,3 I. Sveklo,1 and A. Maziewski1 1Faculty of Physics, University of Bialystok, Bialystok, Poland 2Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow 3Institute of Molecular Physics Polish Academy of Sciences, Poznan, Poland 4NanoBioMedical Centre, Adam Mickiewicz University in Poznan, Poland

We have investigated the magnetization in a ferromagnetic/antiferromagnetic Co/NiO bilayer from room temperature (RT) to 200 degrees C. Using polar magneto-optical Kerr effect (PMOKE) based magnetometry, we observe the temperature-induced spin reorientation transition of the Co layer at 150 degrees C (from out-of-plane to in-plane state). Zero field cooling (ZFC) down to RT leads to the appearance of a micrometer size domain structure and demagnetization of the sample. The remanence magnetization after the ZFC process strongly depends on the temperature from which the cooling process starts (onset temperature). The domain structure and its evolution under the external out-of-plane magnetic field were studied with magnetic force and PMOKE microscopies.

Modifications of magnetic and magneto-optical properties of Pt/Co(dCo)/Pt upon Ar+ irradiation (with energy 1.2, 5 and 30 keV) and fluence, F at the range from 2 · 1013–2 · 1016 Ar+ cm−2) were studied. Two ‘branches’ of increased perpendicular magnetic anisotropy (PMA) and enhanced magneto-optical response are found on 2D (dCo, F) diagrams. The difference in F between ‘branches’ is driven by ion energy. Structural features correlated with magnetic properties have been analysed thoroughly by x-ray diffraction, Rutherford backscattering spectrometry and positron annihilation spectroscopy. Experimental results are in agreement with TRIDYN numerical calculations of irradiation-induced layers intermixing. Our work discusses particularly structural factors related to crystal lattice defects and strain, created and modified by irradiation, co-responsible for the increase in the PMA.

The interlayer exchange coupling and magnetotransport properties have been investigated in trilayer structures where two coupled symmetric Co layers were separated by a spacer. The trilayers have the following general structure: Co(dc(Co))/X(d(spacer))/Co(dc(Co)), where X stands for used spacer (pure Mo or codeposition Mo and Fe with chemical composition Mo0.8Fe0.2), dc(Co) and d(spacer) are the thicknesses of the cobalt and spacer layers. Two samples sets were fabricated by molecular beam epitaxy: (i) wedge-like: the both Co wedges are similar (0-3 nm) and directed along one the side of the substrate while the spacer wedge (also 0-3 nm) was grown along perpendicular direction; (ii) uniform in thickness Co(3 nm)/X(0.7 nm)/Co(3 nm), exhibiting in-plane magnetization configuration and the strongest antiferromagnetic coupling. Magnetization processes were investigated using polar and longitudinal magneto-optical Kerr effect (MOKE) for studies of out-of-plane and in-plane magnetization configurations, respectively. With increase of d(Mo), the interlayer exchange coupling alternates between ferromagnetic and antiferromagnetic exhibiting as high as 0.3 T coupling field H-IEC of antiparallel alignment for d(Mo) = 0.7 nm. Iron doping results in about 3.5 times decrease of the H-IEC, measured at first antiferromagnetic coupling range for both perpendicular and in-plane magnetization configurations. The oscillating dependence of H-IEC on d(spacer) is analyzed in the frame of a Ruderman-Kittel-Kasuya-Yosida model. The giant magnetoresistance (GMR) effect has been investigated in the current in plane geometry. Relation between MOKE and GMR results is discussed.

Focused ion beam (FIB) irradiation was applied to an ultrathin Pt/Co/Pt film to create micrometer-sized, square-shaped regions with perpendicular orientation of magnetization embedded in an in-plane magnetized environment. The FIB parameters like ion fluence and scanning direction are used to control the magnetic properties of the irradiated areas. We have studied the magnetic and magnetooptical properties in the irradiated areas as a function of ion fluence by means of component selective magnetooptical Kerr microscopy that measures separately pure polar and pure longitudinal components of magnetization. Two fluence ranges that induce perpendicular magnetization components were observed. In the higher fluence range, four regions with independently inclined out-of-plane magnetization, depending on FIB fast scan direction, and an unexpected magnetic domain structure were distinguished inside the squares irradiated by FIB.

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.

The origin of the focused single-pulse laser irradiation-induced changes in magnetic anisotropy of a Pt/Co/Pt film is investigated by the x-ray absorption near-edge structure and extended x-ray absorption fine structure techniques combined with the photoelectron emission microscope. A significant increase of the Co–Co bond length in both in-plane and out-of-plane directions is observed on the periphery of the laser spot, at which perpendicular magnetization appears. With increasing laser power density towards the center of the laser spot, anisotropic structural changes are observed accompanied by the reappearance of in-plane magnetization. The enhancement of perpendicular magnetization is attributed to the lattice expansion-induced magnetoelastic effect, while the in-plane compressive strain in the Co film is suggested to be the origin of the reappearance of in-plane magnetization at higher laser power densities.

We have studied the structural mechanisms responsible for the magnetic reorientation between in-plane and out-of-plane magnetization in the (25 nm Pt)/(3 and 10 nm Co)/(3 nm Pt) trilayer systems irradiated with nanosecond XUV pulses generated with laser-driven gas-puff target plasma source of a narrow continuous spectrum peaked at wavelength of 11 nm. The thickness of individual layers, their density, chemical composition and irradiation-induced lateral strain were deduced from symmetric and asymmetric X-ray diffraction (XRD) patterns, grazing-incidence X-ray reflectometry (GIXR), grazing incidence X-ray fluorescence (GIXRF), extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) measurements. In the as grown samples we found, that the Pt buffer layers are relaxed and that the layer interfaces are sharp. As a result of a quasi-uniform irradiation of the samples, the XRD, EXAFS, GIXR and GIXRF data reveal the formation of two distinct layers composed of Pt1-xCox alloys with different Co concentrations, dependent on the thickness of the as grown magnetic Co film but with similar similar to 1% lateral tensile residual strain. For smaller exposure dose (lower number of accumulated pulses) only partial interdiffusion at the interfaces takes place with the formation of a tri-layer composed of Co-Pt alloy sandwiched between thinned Pt layers, as revealed by TEM. The structural modifications are accompanied by magnetization changes, evidenced by means of magneto-optical microscopy. The difference in magnetic properties of the irradiated samples can be related to their modification in Pt1xCox alloy composition, as the other parameters (lateral strain and alloy thickness) remain almost unchanged. The out-of-plane magnetization observed for the sample with initially 3 nm Co layer can be due to a significant reduction of demagnetization factor resulting from a lower Co concentration. (C) 2018 Elsevier B.V. All rights reserved.