Perpendicular Surface Anisotropy Research Articles

Theoretical estimation of surface magnetic anisotropy on [formula omitted]-FePt thin films: Case of perfect and defect Surfaces.

We carried out first-principle calculations of magnetocrystalline anisotropy of L10-FePt thin films terminated by either perfect Fe (Pt) layers or defect surfaces, including vacancies or substituted surface atoms. Inspired by the experimental strategy for determining magnetic anisotropy, we present here a theoretical model for analyzing volume and surface magnetic anisotropies for L10-FePt thin films with different surface terminations. We demonstrate that surface anisotropy does not depend solely on the film thickness, but also on the atomic environment of the surface layer. Perpendicular surface anisotropy of L10-FePt thin films reaches its maximum with full Pt termination, which decreases with increasing Pt vacancies and becomes parallel to the surface for a full Fe surface. In the case of mixed surfaces, surface anisotropy of (3Pt,1Fe) is enhanced compared to platinum-rich surface with one vacancy, and tends to be more in-plane for an iron-rich surface (3Fe,1Pt). Our results could be used to study surface anisotropy effects on L10-FePt surfaces by mean of classical magnetic models, and they are very promising for the development of high density magnetic data storage such as magnetic memory.

Surface anisotropy induced spin wave nonreciprocity in epitaxial La0.33 Sr0.67 MnO3 film on SrTiO3 substrate

Spin wave propagation in perovskite La0.33 Sr0.67 MnO3 films epitaxially grown on a SrTiO3 substrate of (001) orientation was investigated using an all electrical spin wave spectroscopy technique. The spin wave nonreciprocity in amplitude, resonance frequency, and group velocity of the transmission spectra were observed. The origin of the spin-wave nonreciprocity is attributed to the out-of-plane surface anisotropy, with a value of 1.3 mJ/m2 at the interface with the substrate, as extracted from the theoretical model. The magnetic field dependence of the frequency shift is attributed to the perpendicular surface anisotropy. The important role of the surface anisotropy in the spin wave nonreciprocity was further confirmed by the angle dependent measurements of the spin wave transmission spectra.

High Spin to Charge Conversion Efficiency in Electron Beam-Evaporated Topological Insulator Bi2Se3.

Bi2Se3 is a well-established topological insulator (TI) having spin momentum locked Dirac surface states at room temperature and predicted to exhibit high spin to charge conversion efficiency (SCCE) for spintronics applications. The SCCE in TIs is characterized by an inverse Edelstein effect length (λIREE). We report an λIREE of ∼0.36 nm, which is the highest ever observed in Bi2Se3. Here, we performed spin pumping and inverse spin Hall effect (ISHE) in an electron beam-evaporated Bi2Se3/CoFeB bilayer. The Bi2Se3 thickness dependence of λIREE, perpendicular surface anisotropy (KS), spin mixing conductance, and spin Hall angle confirmed that spin to charge conversion is due to spin momentum locked Dirac surface states. We propose that the role of surface states in SCCE can be understood by the evaluation of KS. The SCCE is found to be high when the value of KS is small.

Surface anisotropy and spin wave resonance in submicron amorphous wires

Abstract The influence of surface magnetic anisotropy on ferromagnetic resonance (FMR) in submicron wires is investigated both theoretically and experimentally. An analytical formula for resonance fields of radial spin waves in a long axially magnetized circular cylinder is obtained. Three types of surface anisotropy with the easy direction along one of the cylindrical coordinates are considered. For sufficiently strong surface anisotropy with hard direction parallel to the cylinder axis the surface spin wave mode is observed. The theoretical results are verified by FMR measurements at six microwave frequencies from 9 to 69 GHz on glass-covered amorphous FeCrSiB wires with diameters from 541 to 1032 nm. From the bulk spin wave resonances the exchange stiffness constant about 6.5 10−7 erg/cm is obtained. The frequency dependence of the surface mode resonance field indicates that a perpendicular surface anisotropy with anisotropy constant Ks = 6.2 erg/cm2 is present at the metal/glass interface.

Effects of the antiferromagnetic spin structure on antiferromagnetically induced perpendicular magnetic anisotropy

Antiferromagnetic (AFM) thin films are promising materials for inducing perpendicular magnetic anisotropy (PMA) in adjacent ferromagnetic (FM) films. This study demonstrates that in a selected AFM spin structure with out-of-plane uncompensated moments, the magnitude of the induced PMA in its neighboring Co/Ni film could be significantly enhanced by the establishment of a collinearlike exchange interaction between the volume moments of the AFM film and the perpendicular magnetic FM film. Detailed magnetic hysteresis loops and x-ray analysis revealed a quench of perpendicular surface anisotropy in a monolayered ${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}/\mathrm{Co}/\mathrm{Ni}$ film due to the formation of antiparallel-like coupled Fe and Mn moments. By contrast, the establishment of a three-dimensional quadratic-type AFM spin structure of an ${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$ film triggered parallel-like out-of-plane uncompensated Fe and Mn moments at the interface and reinforced the PMA induced in the Co/Ni film.

Damping and spin mixing conductance in Ni80Fe20/CuIr structures: effect of Ir doping

Ni80Fe20(Py) thin films of different thicknesses have been sputtered on thermally oxidized Si substrates and capped with Pt or Cu doped with , and of Ir. Vibrating sample magnetometery has been used to measure the magnetization at saturation. Microstrip line ferromagnetic resonance (MS-FMR) has been used to investigate the Py and capping layer thickness dependence of the damping parameter enhancement due to the spin pumping. The experimental data have been analyzed using a model for spin pumping that includes the effective spin mixing conductance at the Py/HM (HM = Pt and CuIr) interface and the spin-diffusion length of HM. The spin mixing conductance (spin diffusion length) increases (decreases) as Ir doping increases. For samples capped with Pt (CuIr(6%)), the obtained values of spin mixing conductance and the spin diffusion length have been estimated to be 25 nm−2 (9.87 nm−2) and 1.05 nm (2.8 nm), respectively. MS-FMR measurements revealed that the effective magnetization varies linearly with the effective Py thickness due to the perpendicular surface anisotropy.

On the origin of perpendicular magnetic anisotropy in strained Fe–Co(–X) films

Very high magnetic anisotropies have been theoretically predicted for strained Fe–Co(–X) and indeed several experiments on epitaxial thin films seemed to confirm strain induced anisotropy enhancement. This study presents a critical analysis of the different contributions to perpendicular anisotropy: volume, interface and surface anisotropies. Tracing these contributions, thickness series of single layer films as well as multilayers with Au–Cu buffers/interlayers of different lattice parameters have been prepared. The analysis of their magnetic anisotropy reveals a negligible influence of the lattice parameter of the buffer. Electronic effects, originating from both, the Au–Cu interface and the film surface, outrange the elastic effects. Surface anisotropy, however, exceeds the interface anisotropy by more than a factor of three. A comparison with results from density functional theory suggests, that the experimentally observed strong perpendicular surface anisotropy originates from a deviation from an ideal oxide-free surface. Accordingly, tailored Fe–Co–X/oxide interfaces may open a route towards high anisotropy in rare-earth free materials.

Perpendicular Magnetic Anisotropy in Co<sub>2</sub> FeAl Thin Films: Effect of Annealing Temperature

Co2FeAl (CFA) thin films of various thicknesses (1 nm $\le d \le 10$ nm) have been grown on (001) MgO single-crystal substrates and annealed at different temperatures ( $T_{a}$ ). Their magnetic properties have been studied by vibrating sample magnetometry, broadband microstrip ferromagnetic resonance (MS-FMR), and Brillouin light scattering (BLS). The effective magnetization, deduced from the fit of MS-FMR and BLS measurements, linearly decreases with the inverse thickness ( $1/d$ ) for all the annealing temperatures. This allows quantifying perpendicular surface anisotropy constants of 1.46, 1.71, and 1.29 erg/cm2 for $T_{a} = 215$ °C, 265 °C, and 315 °C, respectively. The perpendicular magnetization is achieved for the CFA films below a critical thickness that increases with increasing $T_{a}$ . Moreover, the fourfold anisotropy fields, measured in the in-plane magnetized films, increase with $T_{a}$ and decrease with respect to the CFA inverse thickness.

The role of the non-magnetic material in spin pumping and magnetization dynamics in NiFe and CoFeB multilayer systems

We present a study of the effective magnetization Meff and the effective damping parameter αeff by means of ferromagnetic resonance spectroscopy on the ferromagnetic (FM) materials Ni81Fe19 (NiFe) and Co40Fe40B20 (CoFeB) in FM/Pt, FM/NM, and FM/NM/Pt systems with the non-magnetic (NM) materials Ru, Cr, Al, and MgO. Moreover, for NiFe layer systems, the influence of interface effects is studied by way of thickness dependent measurements of Meff and αeff. Additionally, spin pumping in NiFe/NM/Pt is investigated by means of inverse spin Hall effect (ISHE) measurements. We observe a large dependence of Meff and αeff of the NiFe films on the adjacent NM layer. While Cr and Al do not induce a large change in the magnetic properties, Ru, Pt, and MgO affect Meff and αeff in different degrees. In particular, NiFe/Ru and NiFe/Ru/Pt systems show a large perpendicular surface anisotropy and a significant enhancement of the damping. In contrast, the magnetic properties of CoFeB films do not have a large influence of the NM adjacent material and only CoFeB/Pt systems present an enhancement of αeff. However, this enhancement is much more pronounced in NiFe/Pt. By the introduction of the NM spacer material, this enhancement is reduced. Furthermore, a difference in symmetry between NiFe/NM/Pt and NiFe/NM systems in the output voltage signal from the ISHE measurements reveals the presence of spin pumping into the Pt layer in all-metallic NiFe/NM/Pt and NiFe/Pt systems.

Capping layer-tailored interface magnetic anisotropy in ultrathin Co2FeAl films

Co2FeAl (CFA) thin films of various thicknesses (2 nm ≤ d ≤ 50 nm) have been grown on (001) MgO single crystal substrates and then capped with Cr, V, and Ta. Their magnetic and structural properties have been studied by x-ray diffraction (XRD), vibrating sample magnetometry, and broadband microstrip ferromagnetic resonance (MS-FMR). The XRD revealed that the films are epitaxial with the cubic [001] CFA axis normal to the substrate plane and that the chemical order varies from the B2 phase to the A2 phase when decreasing the thickness. The deduced lattice parameters showed that the Cr-capped films exhibit a larger tetragonal distortion, as compared with the films capped with V or Ta. The presence of magnetic dead layers has been observed in CFA samples capped with V and Ta but not in the case of the Cr-capped ones. The effective magnetization, deduced from the fit of MS-FMR measurements, increases (decreases) linearly with the CFA inverse thickness (1/d) for the Cr-capped (Ta-capped) films while it is constant for the V-capped ones. This allows quantifying the perpendicular surface anisotropy coefficients of −0.46 erg/cm2 and 0.74 erg/cm2 for Cr and Ta-capped films, respectively. Moreover, the fourfold and the uniaxial anisotropy fields, measured in these films, showed different trends with a respect to the CFA inverse thickness. This allows inferring that a non-negligible part of the fourfold magnetocrystalline term is of interfacial origin.

Inclusion of surface anisotropy in the micromagnetic analysis of exchange-coupled hard/soft bilayers

The effects of surface anisotropy and interface anisotropy in soft and hard layers have been considered in the frame of a previously developed micromagnetic continuum model for the study of demagnetization processes in exchange-coupled bi- and multi-layers of soft/hard-type with planar and perpendicular anisotropy, in order to extend and generalize it. In particular, fields applied along the easy direction and strong interface ferromagnetic coupling have been considered. The theoretical results have been applied to a bilayer with perpendicular anisotropy in the hard layer and positive perpendicular surface anisotropy in the soft layer, in order to compare them with those obtained by considering the equivalent volume anisotropy in the soft layer. Moreover, the separate effects of positive perpendicular surface and interface anisotropies in the bilayer have been compared. We have deduced the nucleation field equation and obtained plots of the nucleation field as a function of the soft layer thickness and of the surface and interface anisotropies. Demagnetization curves have been also calculated. The nucleation process can be reversible (exchange-spring (ES) regime) or irreversible (rigid magnet (RM) regime) depending on magnetic and geometrical parameters. The critical equation, which allows determining the boundary between RM and ES regions, has been deduced. Corresponding phase diagrams in the plane of soft and hard layer thickness have been obtained as a function of surface and interface anisotropies.

Magnetic and structural properties of Co2FeAl thin films grown on Si substrate

The correlation between magnetic and structural properties of Co_{2} FeAl (CFA) thin films of different thickness (10 nm<d< 100 nm) grown at room temperature on MgO-buffered Si/SiO2 substrates and annealed at 600\lyxmathsym{\textdegree}C has been studied. XRD measurements revealed an (011) out-of-plane texture growth of the films. The deduced lattice parameter increases with the film thickness. Moreover, pole figures showed no in-plane preferential growth orientation. The magneto-optical Kerr effect hysteresis loops showed the presence of a weak in-plane uniaxial anisotropy with a random easy axis direction. The coercive field measured with an applied field along the easy axis direction and the uniaxial anisotropy field increase linearly with the inverse of the CFA thickness. The microstrip line ferromagnetic resonance measurements for in-plane and perpendicular applied magnetic fields revealed that the effective magnetization and the uniaxial in-palne anisotropy field follow a linear variation versus the inverse CFA thickness. This allows deriving a perpendicular surface anisotropy coefficient of -1.86 erg/cm2

A Fully Functional 64 Mb DDR3 ST-MRAM Built on 90 nm CMOS Technology

A spin torque magnetoresistive random access memory (ST-MRAM) holds great promise to be a fast, high density, nonvolatile memory that can enhance the performance of a variety of applications, particularly when used as a non-volatile buffer in data storage devices and systems. Towards that end, we have developed a fully functional 64 Mb DDR3 ST-MRAM built on 90 nm CMOS technology. The memory is organized in an 8-bank configuration that can sustain 1.6 GigaTransfers/s (DDR3-1600). We have run standard memory tests, such as a March6N pattern, on the full 64 Mb at 800 MHz with 0 fails for greater than 10 5 cycles. Full functionality was also verified from 0°C to 70°C with no significant change in performance. The bits are magnetic tunnel junctions (MTJs) having an MgO tunnel barrier and a magnetic free layer made of a CoFeB-based alloy with an in-plane magnetization, but with an out-of-plane anisotropy reduced by more than 50% due to an enhanced perpendicular surface anisotropy. To enable the 64 Mb performance, we developed an MTJ stack that has low switching voltage (Vsw), high breakdown voltage (Vbd), and excellent switching reliability with tight distributions. The ST switching distribution has σ ≈ 10%, and we found excellent agreement with a single Gaussian distribution down to an error rate . For our optimized material, the Vsw/Vbd ≈ 0.3, and the separation between Vsw and Vbd is ≈ 25σ. The energy barrier to magnetization reversal (Eb) was characterized using both time-dependent coercivity and higher temperature to accelerate reversal. We found the average Eb ≈ 70kbT.

Ferromagnetic resonance in submicron amorphous wires

Ferromagnetic resonance in glass-coated amorphous wires with the diameter of metallic core varying from 25 μm to 133 nm is investigated. The microwave frequencies of 49.1 and 69.7 GHz are used and static magnetic field is applied either parallel or perpendicular to the long wire axis. In agreement with theoretical predictions the resonance curves of submicron wires substantially differ from the curves of the bulk wires. Depending on the symmetry and intensity of microwave electric and magnetic fields in the sample vicinity the circumferential and/or dipolar resonance modes can be excited. In bulk wires the resonance fields of the two modes coincide. In submicron wires, however, their resonance fields differ, indicating the metallic character of the circumferential mode and the insulator character of the dipolar mode. In wires with diameters 717 and 869 nm radial standing spin wave resonances are observed in parallel field configuration. The experimental results for the parallel field configuration can be well explained by the rigorous theoretical model. From the fit of experimental data the exchange stiffness constant A = 8.2 10−12 J/m and perpendicular surface anisotropy constant Ks = 6 × 10−4 J/m2 are obtained. The resonance curves measured in the transversal field configuration can be well explained in the frame of the skin effect and quasistatic approximations for the bulk and submicron wires, respectively. In submicron wires, however, an additional resonance of unknown origin is observed at higher magnetic fields.

Ferromagnetic resonance study of polycrystalline cobalt ultrathin films

We present room-temperature ferromagnetic resonance (FMR) studies of polycrystalline ∥Pt∕10nmCu∕tCo∕10nmCu∕Pt∥ films as a function of Co layer thickness (1⩽t⩽10nm) grown by evaporation and magnetron sputtering. FMR was studied with a high-frequency broadband coplanar waveguide (up to 25 GHz) using a flip-chip method. The resonance field and the linewidth were measured as functions of the ferromagnetic layer thickness. The evaporated films exhibit a lower magnetization density (Ms=1131emu∕cm3) compared to the sputtered films (Ms=1333emu∕cm3), with practically equal perpendicular surface anisotropy (Ks≃−0.5erg∕cm2). For both series of films, a strong increase of the linewidth was observed for Co layer thickness below 3 nm. For films with a ferromagnetic layer thinner than 4 nm, the damping of the sputtered films is larger than that of the evaporated films. The dependence of the linewidth can be understood in terms of the spin-pumping effect, from which the interface spin-mixing conductance is deduced.

Step-decorated Ferromagnetic Fe Nanostripes on Pt(997)

We investigate the step decoration growth and magnetic properties of Fe grown on a Pt(997) vicinal single crystal by means of reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM), and the surface magneto-optical Kerr effect (SMOKE). Pt(997) has 2 nm wide, (111)-oriented terraces separated by ordered, monoatomic steps along the $[1\overline{1}0]$ direction. The Fe is grown at room temperature as wedges with thickness ranging from 0 to 4 monolayers (ML). RHEED and STM show that Fe stripes form at low coverage due to step decoration. SMOKE data taken along a wedge indicate that the Fe is ferromagnetic above 0.2 ML with its magnetic easy axis canted from surface normal direction. As temperature is increased near the Curie temperature, there is an irreversible spin reorientation to the perpendicular out-of-plane direction. The canting can be understood as arising from a competition between an in-plane, step-induced magnetic anisotropy and a perpendicular surface anisotropy associated with extended terraces due to the polarization of the proximal Pt. Above 1.7 ML the perpendicular easy axis reorients to in-plane, along the step direction, due to the dominance of the shape anisotropy. The magnetic anisotropy energy is extracted by fitting the temperature dependence of the coercivity data.

A study of perpendicular Ag hyperfine fields induced at the interface inFe∕Agmultilayer systems

In a series of experiments the hyperfine fields of Ag at the interface with Fe were measured in $\mathrm{Fe}∕\mathrm{Ag}$ multilayers using the low temperature nuclear orientation technique. The thickness of the Fe layers was chosen so that the in-plane shape anisotropy would dominate over the perpendicular surface anisotropy. However, for multilayers with a well ordered multilayer structure as measured by x-ray diffraction, the Ag magnetic hyperfine fields were found to be nearly perpendicular to the layer plane. The out-of-plane magnetic structure of the induced Ag magnetic moments has been also supported by vibrating sample magnetometry measurements on the same samples. In addition, the magnitude of the induced Ag hyperfine fields are significantly reduced by roughness. Calculations using WIEN97 confirm that the Ag hyperfine fields are strongly affected by the number of nearest Fe neighbors and their distance.

Magnetic interface anisotropy in Ni/Fe/Ni(1 1 1) trilayers

Abstract The direction of the spins of Ni capping layers and the 57Fe x interlayer have been studied in Ni/57Fe x /Ni(1 1 1) films by magneto-optic-vector Kerr and Mossbauer spectroscopy. It was found that the magnetization direction of the Ni follows the orientation of that of the Fe, and that there is an unusual rotation of the magnetization from the in-plane to out-of-plane in the pseudomorphic range of the Fe growth. The effect of the competition between perpendicular surface anisotropy and the magnetostatic energy is discussed.

Magnetization and ferromagnetic resonance studies in Co/V multilayers

The magnetic properties of evaporated Co/V multilayer films have been studied by superconducting quantum interference device magnetometer and ferromagnetic resonance (FMR). The magnetization follows a quasi-linear temperature dependence, which reverts to a T 3/2 dependence further into the bulk. We attribute the quasi-linear temperature behavior to surface spin-wave modes created by a perpendicular surface anisotropy at the interface. The FMR spectra are obtained with the applied magnetic field parallel and perpendicular to the film plane at 300 K. The FMR linewidth of uniform mode, Δ H ⊥, for the perpendicular geometry, depends on the thickness of Co layer as t Co −2. From both FMR and torque magnetometer measurements, a positive value for Co/V interface anisotropy is obtained.

Structural, magnetic and Mössbauer studies of Fe–Cu granular films

X-ray diffraction, magnetization and Mössbauer effect investigations have beenperformed for the sputtered Fe0.2Cu0.8 thin films. A segregation forming isolatedbody-centred cubic Fe grains in the face-centred cubic Cu matrix with anaverage diameter between 1 and 40 nm took place upon annealing in thetemperature range from 100 to 500̂C. In addition, the magnetic coercive fieldis found to be enhanced and reaches a value of 37 mT for the sampleannealed at 400̂C. A perpendicular surface anisotropy constant KS = 0.04mJ m−2 is deduced. Surface spin configurations are discussed for the granular filmsinvestigated.