Epitaxial strain and magnetic anisotropy in ultrathin Co films on W(110)

Spin and Alignment effects in O2 Chemisorption on Fe(110), Ni(111) and Co(0001) films grown on W(110)

O2 has a spin triplet multiplicity in its ground state while the effect of its electron spin on O2-surface interaction is not well understood. In the present study, the spin and/or alignment effects in O2 chemisorption on surfaces of Fe(110), Ni(111) and Co(0001) films grown on W(110) have been investigated with the use of a single spin-rotational state-selected O2 beam. The results indicate that the spin effects for the Fe and Ni films are similar in that the initial sticking probability (S0) of O2 is higher when spins of O2 and the film are antiparallel, and the spin dependence in S0, which amounts to 30-40 % at thermal energy, decays with increasing the O2 kinetic energy (E0). In case of the Fe/O2 system, however, considerable spin dependence was found to remain even at E0 > 0.2 eV and on oxidized surface. It has been also shown that the barrier for O2 chemisorption increases in the order of Fe(110) < Ni(111) < Co(0001) while the difference in the observed alignment effect among the samples can be understood based the difference in the barrier.

Dipolar magnetism in assembled Co nanoparticles on graphene

The magnetic properties of the assembled Co nanoparticles on graphene were studied using X-ray magnetic circular dichroism (XMCD), magneto-optical Kerr effects, and a modeling simulation. We demonstrate that the superparamagnetic nanoparticles reveal a ferromagnetic phase when they are assembled on graphene. The moderate increase of the XMCD asymmetry and magnetization with coverage for this assembly indicates a dipolar-mediated magnetism, which is further verified by a model simulation considering the dipolar interaction between neighboring nanoparticles. Furthermore, C K-edge spectra reveal visible dichroism at the π* state of graphene, which indicates the existence of a spin-polarized interface state, while the assembled Co nanoparticles reveal a ferromagnetic phase. These results suggest an efficient route to stabilize the ferromagnetic phase of nanostructures on graphene by tailoring dipolar interactions, which is essential to realize a higher efficiency of spin injection in graphene-based spintronics.

Thickness dependence of the triplet spin-valve effect in superconductor-ferromagnet-ferromagnet heterostructures

BACKGROUND

In nanoscale layered S/F1/N/F2/AF heterostructures, the generation of a long-range, odd-in-frequency spin-projection one triplet component of superconductivity, arising at non-collinear alignment of the magnetizations of F1 and F2, exhausts the singlet state. This yields the possibility of a global minimum of the superconducting transition temperature T c, i.e., a superconducting triplet spin-valve effect, around mutually perpendicular alignment.

RESULTS

The superconducting triplet spin valve is realized with S = Nb a singlet superconductor, F1 = Cu41Ni59 and F2 = Co ferromagnetic metals, AF = CoO x an antiferromagnetic oxide, and N = nc-Nb a normal conducting (nc) non-magnetic metal, which serves to decouple F1 and F2. The non-collinear alignment of the magnetizations is obtained by applying an external magnetic field parallel to the layers of the heterostructure and exploiting the intrinsic perpendicular easy-axis of the magnetization of the Cu41Ni59 thin film in conjunction with the exchange bias between CoO x and Co. The magnetic configurations are confirmed by superconducting quantum interference device (SQUID) magnetic moment measurements. The triplet spin-valve effect has been investigated for different layer thicknesses, d F1, of F1 and was found to decay with increasing d F1. The data is described by an empirical model and, moreover, by calculations using the microscopic theory.

CONCLUSION

The long-range triplet component of superconducting pairing is generated from the singlet component mainly at the N/F2 interface, where the amplitude of the singlet component is suppressed exponentially with increasing distance d F1. The decay length of the empirical model is found to be comparable to twice the electron mean free path of F1 and, thus, to the decay length of the singlet component in F1. Moreover, the obtained data is in qualitative agreement with the microscopic theory, which, however, predicts a (not investigated) breakdown of the triplet spin-valve effect for d F1 smaller than 0.3 to 0.4 times the magnetic coherence length, ξF1.

Magnetic domain patterns on strong perpendicular magnetization of Co/Ni multilayers as spintronics materials: I. Dynamic observations

Materials with perpendicular magnetic anisotropy can reduce the threshold current density of the current-induced domain wall motion. Co/Ni multilayers show strong perpendicular magnetic anisotropy and therefore it has become a highly potential candidate of current-induced domain wall motion memories. However, the details of the mechanism which stabilizes the strong perpendicular magnetization in Co/Ni multilayers have not yet been understood. In the present work, the evolution of the magnetic domain structure of multilayers consisting of pairs of 2 or 3 monolayers (ML) of Ni and 1 ML of Co on W(110) was investigated during growth with spin-polarized low-energy electron microscopy. An interesting phenomenon, that the magnetic domain structure changed drastically during growth, was revealed. In the early stages of the growth the magnetization alternated between in-plane upon Co deposition and out-of-plane upon Ni deposition. The change of the magnetization direction occurred within a range of less than 0.2 ML during Ni or Co deposition, with break-up of the existing domains followed by growth of new domains. The Ni and Co thickness at which the magnetization direction switched shifted gradually with the number of Co/Ni pairs. Above 3-4 Co/Ni pairs it stayed out-of-plane. The results indicate clearly that the Co-Ni interfaces play the important role of enhancing the perpendicular magnetic anisotropy.

Tuning of uniaxial magnetic anisotropy in amorphous CoFeB films

We demonstrate that the uniaxial magnetic anisotropy (UMA) of amorphous CoFeB films can be tuned by crystallinity and orbital moment ratio, combining the results of magnetization reversal and ferromagnetic resonance with high-resolution transmission electron microscopy, x-ray-absorption near-edge structure and x-ray magnetic circular dichroism. Isotropic polycrystalline buffers of tungsten (W), tantalum (Ta), and copper (Cu) between CoFeB and Si(100) substrates have direct and crucial bearing on the interfacial microstructure and orbital moment ratio. Compared with Ta and Cu buffer, CoFeB with W buffer exhibits obvious UMA and has lower crystallinity at the interface and higher orbital moment. Amorphous phase distributed homogeneously in CoFeB film grown on W buffer contributes to improve the easy-axis squareness with a sharp magnetization reversal. Our demonstrations not only realize effective tuning of UMA in amorphous CoFeB, but also provide an appealing alternative buffer (W) for CoFeB-based magnetic tunnel junctions.

Magnetic Domain Structure and Spin Reorientation Transition in the System Co/Au/W(110)

Ultrathin epitaxial Co layers grown on Au(111) show strong perpendicular magnetic aniso-tropy resulting in out-of-plane magnetization in thin films. With increasing Co thickness the increasing contribution of the shape anisotropy leads to a reorientation transition of the magnetization which has already been subject of many other studies. In the dynamic in-situ experiments reported here bulk Au(111) is replaced by epitaxially grown Au(111)/W(110). The high time resolution (0.05 ML Co deposition per acquisition cycle) allows to extract several mechanisms which occur during the spin reorientation transition.

Imaging spin-reorientation transitions in consecutive atomic Co layers on Ru(0001)

By means of spin-polarized low-energy electron microscopy, we show that the magnetic easy axis of one to three atomic-layer thick cobalt films on Ru(0001) changes its orientation twice during deposition: One-monolayer and three-monolayer thick films are magnetized in plane, while two-monolayer films are magnetized out of plane. The Curie temperatures of films thicker than one monolayer are well above room temperature. Fully relativistic calculations based on the screened Korringa-Kohn-Rostoker method demonstrate that only for two-monolayer cobalt films does the interplay between strain, surface, and interface effects lead to perpendicular magnetization.