Perpendicular magnetic anisotropy of ultrathin FeCo alloy films on Pd(0 0 1) surface: First principles study

Using the full potential linearized augmented plane wave (FLAPW) method, thickness dependent magnetic anisotropy of ultrathin FeCo alloy films in the range of 1 monolayer (ML) to 5 ML coverage on Pd(0 0 1) surface has been explored. We have found that the FeCo alloy films have close to half metallic state and well-known surface enhancement in thin film magnetism is observed in Fe atom, whereas the Co has rather stable magnetic moment. However, the largest magnetic moment in Fe and Co is found at 1 ML thickness. Interestingly, it has been observed that the interface magnetic moments of Fe and Co are almost the same as those of surface elements. The similar trend exists in orbital magnetic moment. This indicates that the strong hybridization between interface FeCo alloy and Pd gives rise to the large magnetic moment. Theoretically calculated magnetic anisotropy shows that the 1 ML FeCo alloy has in-plane magnetization, but the spin reorientation transition (SRT) from in-plane to perpendicular magnetization is observed above 2 ML thickness with huge magnetic anisotropy energy. The maximum magnetic anisotropy energy for perpendicular magnetization is as large as 0.3 meV/atom at 3 ML film thickness with saturation magnetization of . Besides, the calculated X-ray magnetic circular dichroism (XMCD) has been presented.     

Perpendicular magnetic anisotropy of Fe-Nd alloy thin films

A study of the magnetic properties of Fe_{100 − x}Nd_x (18 x 50) films made by rf sputtering has been carried out. The perpendicularly magentized films were fabricated for compositions in the range from x = 35 to 50 at substrate temperatures between 210 to 290°C. The intrinsic perpendicular magnetic anisotropy constant K_u is maximum at about x = 40 where K_u is about 1 × 10⁷ erg/cm³ at room temperature. The temperature dependence of K_u implies that the origin of the perpendicular magnetic anisotropy may be related to some sort of atomic ordering of crystalline clusters in an amorphous matrix.     

Interface induced uniaxial magnetic anisotropy in amorphous CoFeB films on AlGaAs(001)

We demonstrate an isolated magnetic interface anisotropy in amorphous CoFeB films on (Al)GaAs(001), similar to that in epitaxial films but without a magnetocrystalline anisotropy term. The direction of the easy axis corresponds to that due to the interfacial interaction proposed for epitaxial films. We show that the anisotropy is determined by the relative orbital component of the atomic magnetic moments. Charge transfer is ruled out as the origin of the interface anisotropy, and it is postulated that the spin-orbit interaction in the semiconductor is crucial in determining the magnetic anisotropy.     

Plastic relaxation anisotropy in epitaxial films grown on miscut Si(001) substrates

A way of calculating the structural parameters of semiconductor epitaxial layers grown on miscut (0 0 1) substrates is developed. It is shown that the difference between the tensor of film elastic strains and the spherical tensor of strains translating the film’s crystal lattice into the substrate’s lattice is a tensor whose components are proportional to X-ray strains. The technique is used to analyze GaAs and Ge_0.2Si_0.8 films grown on (1 1 13) Si substrates. The anisotropy of the degree of plastic relaxation is established for lateral directions perpendicular and parallel to the interfacial steps in the GeSi film. It is proposed that noninteger Miller indices be used to denote the direction and value of the rotation of the film’s crystal lattice with respect to the substrate lattice.     

Exchange-coupling-induced fourfold magnetic anisotropy in CoFeB/FeRh bilayer grown on SrTiO3(001)

Exchange coupling across the interface between a ferromagnetic (FM) layer and an antiferromagnetic (AFM) or another FM layer may induce a unidirectional magnetic anisotropy and/or a uniaxial magnetic anisotropy, which has been extensively studied due to the important application in magnetic materials and devices. In this work, we observed a fourfold magnetic anisotropy in amorphous CoFeB layer when exchange coupling to an adjacent FeRh layer which is epitaxially grown on an SrTiO₃(001) substrate. As the temperature rises from 300 K to 400 K, FeRh film undergoes a phase transition from AFM to FM phase, the induced fourfold magnetic anisotropy in the CoFeB layer switches the orientation from the FeRh$\langle 110\rangle $ to FeRh$\langle 100\rangle $ directions and the strength is obviously reduced. In addition, the effective magnetic damping as well as the two-magnon scattering of the CoFeB/FeRh bilayer also remarkably increase with the occurrence of magnetic phase transition of FeRh. No exchange bias is observed in the bilayer even when FeRh is in the nominal AFM state, which is probably because the residual FM FeRh moments located at the interface can well separate the exchange coupling between the below pinned FeRh moments and the CoFeB moments.     

Spin-polarized electron tunneling in bcc FeCo/MgO/FeCo(001) magnetic tunnel junctions

In combining spin- and symmetry-resolved photoemission, magnetotransport measurements and ab initio calculations we detangled the electronic states involved in the electronic transport in Fe(1-x)Co(x)(001)/MgO/Fe(1-x)Co(x)(001) magnetic tunnel junctions. Contrary to previous theoretical predictions, we observe a large reduction in TMR (from 530 to 200% at 20 K) for Co content above 25 atomic% as well as anomalies in the conductance curves. We demonstrate that these unexpected behaviors originate from a minority spin state with Δ(1) symmetry that exists below the Fermi level for high Co concentration. Using angle-resolved photoemission, this state is shown to be a two-dimensional state that occurs at both Fe(1-x)Co(x)(001) free surface, and more importantly at the interface with MgO. The combination of this interface state with the peculiar density of empty states due to chemical disorder allows us to describe in details the complex conduction behavior in this system.     

Rare earth free exchange spring magnet FeCo/FePt(001): Giant magnetic anisotropy and energy product

Using the full potential linearized augmented plane wave (FLAPW) method, we have investigated the thickness dependent magnetic properties of rare earth free exchange spring magnet FeCo/FePt(001). The FeCo adlayer thickness is increased from one monolayer (ML) to four ML coverage. It is observed that the FeCo adlayers and Fe atoms in FePt substrate show almost half metallic behavior, while an ordinary metallic feature is found in Pt atoms. The average magnetization increases with FeCo thickness and the estimated maximum energy product reaches 66 MGOe in FeCo(4 ML)/FePt(001). A giant perpendicular magnetocrystalline anisotropy (MCA) energy of 18.20 meV/cell is found in pure FePt(001) and it becomes 17.35 meV/cell even in FeCo(4 ML)/FePt(001). In addition, we find very large coercivity field in FeCo/FePt(001) systems. For instance, the calculated maximum coercivity field in FeCo(4 ML)/FePt(001) is about 188 kOe. Both energy product and coercivity field calculations may imply that the FeCo/FePt can be utilized for potential rare earth free exchange spring magnet material.     

Perpendicular anisotropy and magneto-optical Kerr effect of (Ni/Pd) multilayers

Nickel film, with total thickness t_Ni in the range 1000-2000 Å, is known to exhibit perpendicular magnetic anisotropy (PMA), if the film has been deposited at room temperature. This phenomenon is due to the magneto-elastic (ME) effect. The same is also true for the (Ni/Pd)_n multilayers, where n is the period (n≥3). In this paper, we have made two kinds of multilayers: one, which does not have a Pd cap layer, belongs to the A-group, and the other, which has, belongs to the B-group. The polar Kerr rotation θ_k, the polar Kerr ellipticity ε_k, and the figure of merit (θ_k)²R, where R is the reflectance, were measured for the two wavelengths, i.e. λ=633 and 442 nm, respectively. The effective PMA energy K_⊥ was measured from the vibrating sample magnetometer. It was found that the most favorable multilayer for the magneto-optical (MO) application exists among the A-group samples: i.e. the t_Ni=1300 Å, t_Pd=50 Å (seed layer), and n=1 sample. We obtained θ_k=−9.76 min, ε_k=−9.13 min, (θ_k)²R=1.51 (rad)² at λ=442 nm, and K_⊥=3.21×10⁶ erg/cc for this optimal multilayer. Finally, the effects of the Pd seed layer on PMA and MO are also studied.     

Ferromagnetism of Pd(001) substrate induced by antiferromagnetic CoO

Our first-principles study has revealed unexpected spin polarization of the Pd(001) substrate in contact with antiferromagnetic CoO overlayers. We give an evidence that the ferromagnetism of Pd is caused by the zigzag positions of Co atoms with respect to the Pd interface, resulted from the lattice-mismatch driven structural relaxation. Because of the itinerant nature of its 4d electrons, we see that the ferromagnetic properties of Pd are highly sensitive to the local environment and can be enhanced further by increasing the thickness of CoO overlayer film or/and by applying an additional uniaxial pressure along c-axis exerted externally on the bottom layers of the Pd substrate. Our finding provides new functionality for the interfacial moments of the CoO/Pd system, which can be accessed experimentally, e.g., by the magneto-optical Kerr effect (MOKE) or/and by element-resolved X-ray magnetic circular dichroism (XMCD) measurement.     

Growth of ultrathin Pd films on Al(001) surfaces

High-energy ion backscattering spectroscopy (HEIS) and X-ray photoelectron spectroscopy (XPS) were used to determine the growth mode and the interface structure of ultrathin Pd films deposited on Al(001) surfaces at room temperature. Measured Al and Pd surface peak areas for MeV He⁺ ions incident normal to the surface show that Pd atoms intermix with and displace Al substrate atoms. The mixing continues for Pd coverages from 0–5 monolayers, at which point a Pd metal film begins to grow on the alloy surface. XPS measurements of the Pd 3d photopeaks show a chemical shift that is consistent with the formation of an AlPd-like compound during the mixing phase, and Pd metal thereafter. HEIS results further reveal that the alloyed overlayer as well as the Pd metal film have some degree of axial alignment with respect to the Al substrate. The XPS intensity measurements are consistent with this two-stage growth model.     

Magnetoresistance anisotropy of strontium ruthenate films grown coherently on a TiO2-terminated SrTiO3(001) substrate

The SrRuO₃ films with a thickness of 80 nm have been coherently grown on a TiO₂-terminated SrTiO₃(001) substrate. Biaxial mechanical stresses induce a considerable difference between the unit cell parameters of the SrRuO₃ layer in the substrate plane (??3.904 ?) and along the normal to the substrate surface (??3.952 ?). The electrical resistivity of the SrRuO3 film decreases practically linearly with increasing magnetic field strength H when the latter is parallel to the current I _b and the projection of the easy magnetization axis in the substrate plane. At T = 4.2 K, ??₀ H = 14 T, and the magnetic field oriented along the hard magnetization axis, the negative anisotropic magnetoresistance of the grown layers reaches 16% and exerts a notice-able effect on the response of electrical resistivity of the SrRuO₃ film to the magnetic field.     

Thickness-dependent magnetic anisotropy in obliquely deposited Fe(001)/Pd thin film bilayers probed by VNA-FMR

The thickness-dependent magnetic anisotropy of obliquely deposited Fe(001)/Pd thin films on Mg(001) is investigated by fitting the field-dependent resonant field curve using the Kittel equation.In this study, three Fe film samples with thicknesses of 50 monolayers(ML), 45 ML, and 32 ML deposited at 0°, 45°, and 55°, respectively, are used.The magnetic anisotropy constant obtained from ferromagnetic resonance(FMR) spectra exhibits a dominant fourfold magnetocrystalline anisotropy(MCA) at the normal deposition angle with larger Fe thickness.However, the in-plane uniaxial magnetic anisotropy(UMA) is induced by a higher oblique deposition angle and a smaller thickness.Its hard axis lies between the [100] and [010] directions.The FMR data-fitting analysis yields a precise measurement of smaller contributions to the magnetic anisotropy, such as in-plane UMA.Due to MCA, when the magnetic field is weaker than the saturated field,the magnetization direction does not always align with the external field.The squared frequency-dependent resonant field measurement gives an isotropic Landé g-factor of 2.07.Our results are consistent with previous experiments conducted on the magneto-optical Kerr effect(MOKE) and anisotropic magnetoresistance(AMR) systems.Thus, a vector network analyzer ferromagnetic resonance(VNA-FMR) test-method for finding UMA in obliquely deposited Fe(001)/Pd bilayer ferromagnetic thin films, and determining the magnetic anisotropy constants with respect to the film normal deposition, is proposed.     

MOCVD growth, structure and magnetic properties of Fe films grown on GaAs (001) substrates

Thin iron films have been grown on (001) GaAs substrates by low pressure metal organic chemical vapor deposition (LP-MOCVD) at different temperatures with the pressure of 150 Torr. X-ray diffraction (XRD) analysis showed that all films have only one strong diffraction peak (110). The surface of Fe film became smooth with increasing the growth temperature. Magnetization measurements showed that the Fe films grown at different temperatures were ferromagnetic with easy axis parallel to the film surface and hard axis perpendicular to the substrates. The field dependence of magnetization along two axes showed a remarkable difference, implying that the samples have strong magnetic anisotropy. Furthermore, when the applied magnetic field is perpendicular to the Fe surface, a sharp jump in the hysteresis loop could be observed, followed by a broad shoulder, which is related to the interface effect, the existence of carbon and the formation of 180^°/90^° magnetic domains.     

Epitaxial Fe films on ZnSe(001): effect of the substrate surface reconstruction on the magnetic anisotropy

It is well known that Fe films deposited on a c(2 × 2)-reconstructed ZnSe(001) surface show a strong in-plane uniaxial magnetic anisotropy. Here, the effect of the substrate reconstruction on the magnetic anisotropy of Fe has been studied by in situ Brillouin light scattering. We found that the in-plane uniaxial anisotropy is strongly reduced for Fe films grown on a (1 × 1)-unreconstructed ZnSe substrate while the in-plane biaxial one is nearly unaffected by the substrate reconstruction. Calculations of magnetic anisotropy energies within the framework of ab initio density functional theory reveal that the strong suppression of anisotropy at the (1 × 1) interface occurs due to complex atomic relaxations as well as the competing effects originating from magnetocrystalline anisotropy and dipole-dipole interactions. For both sharp and intermixed c(2 × 2) interfaces, the magnetic anisotropy is enhanced compared to the (1 × 1) case due to the further lowering of symmetry. The theoretical results are in agreement with the experimental findings.     

Perpendicular magnetic anisotropy of Pd/Co2MnSi/NiFe2O4/Pd multilayers on F-mica substrates

Pd/Co₂MnSi(CMS)/NiFe₂O₄(NFO)/Pd multilayers were fabricated on F-mica substrate by magnetron sputtering.The best PMA performance of the multilayer structure Pd(3 nm)/CMS(5 nm)/NFO(0.8 nm)/Pd(3 nm)was obtained by adjusting the thickness of the CMS and NFO layers.F-mica substrate has a flatter surface than glass and Si/SiO₂ substrate.The magnetic anisotropy energy density(K_eff)of the sample deposited on F-mica substrates is 0.6711 Merg/cm³(1 erg=10^-7 J),which is about 30%higher than that of the multilayer films deposited on glass(0.475 Merg/cm³)and Si/SiO₂(0.511 Merg/cm³)substrates,and the R_Hall and H_C are also significantly increased.In this study,the NFO layer prepared by sputtering in the high purity Ar environment was exposed to the high purity O₂ atmosphere for 5 min,which can effectively eliminate the oxygen loss and oxygen vacancy in NFO,ensuring enough Co-O orbital hybridization at the interface of CMS/NFO,and thus effectively improve the sample PMA.     

Electron states of ultrathin Pd and Cu films on (001)Ag

We present the results of a theoretical calculation of the electronic structure of a monolayer of Pd and Cu deposited on the (001) surface of Ag. The adsorbate atoms are placed in a central site with the same lateral periodicity as a parallel plane of the substrate. The method of calculation is a parametrized LCAO with hopping integrals determined through a fit to bulk data. Charge transfer effects are taken approximately into account by assuming the intraatomic parameters to vary as the valence orbital ionization potentials. Energy bands and local densities of states are given and the nature and location of the adsorbate states are discussed. The results are compared with the experimental data: good agreement is found for the Pd on Ag system. For Cu on Ag we find that our assumptions do not explain the photoemission data and suggest that some clustering of Cu atoms occurs during the deposition.