To study whether spin-reorientation transitions in 8-10 ML thick Ni/ Cu͑100͒ films take place by continuous or discontinuous rotation of the magnetization, we used spin-polarized low-energy electron microscopy to image the magnetization vector of magnetic domains during Ni growth. After substrate preparations that either promote or suppress bunching of atomic steps, we find strong evidence for either first-or second-order transition kinetics. The results are explained in terms of a magnetic phase diagram, taking the effect of topography on magnetic anisotropy contributions into account.Phase transitions are among the most fundamental phenomena in condensed-matter physics. They proceed either discontinuously ͑first order͒ or continuously ͑higher order͒. In general, changing the kinetic order of a phase transition is possible only under very special circumstances, i.e., close to a critical point of the phase diagram. In magnetism such special systems have attracted attention before: Some metamagnetic antiferromagnets are known where temperature determines the order of the metamagnetic phase transition ͓e.g., FeCl 2 ͑Ref. 1͔͒. In this Brief Report, we introduce topology as a new critical parameter and show that the thicknessdependent spin-reorientation transition ͑SRT͒ in ultrathin Ni films grown on Cu͑100͒ occurs close to a topology-driven critical point. The out-of-plane component of the magnetization changes either continuously over a thickness range of 0.6 ML equivalents or discontinuously within the experimental resolution ͑Ϸ1% ML͒ depending on substrate morphology. Subtle manipulation of the arrangement of atomic steps on the Cu͑100͒ substrate surface allows us to switch the kinetic order of this SRT between continuous and discontinuous changes of the magnetization direction.SRTs are well known to occur in many ultrathin ferromagnetic films as a function of temperature or thickness. 2 Epitaxial Ni/ Cu͑100͒ films are particularly interesting because an inverse SRT from planar to perpendicular orientation of the magnetization is observed 3-10 and the kinetic order of this transition is still under discussion. 3-6 In this system, starting from in-plane magnetization of thinner Ni films, thicknessdependent first-order transitions are manifested by nucleation and growth of domains that are magnetized perpendicular to the film plane. In contrast, continuous variation of the polar angle of the magnetization vector ͑spin canting͒ indicates SRTs with kinetics of second order. Using spin-polarized low-energy electron microscopy ͑SPLEEM͒, we show that by minor modifications of the preparation procedure of the Cu substrate we can tune the intrinsic magnetic anisotropy energy ͑MAE͒ of the deposited Ni film to such extent that the kinetic order of the phase transition switches.All experiments were performed in the SPLEEM at the Lawrence Berkeley National Laboratory at a base pressure below 5 ϫ 10 −11 mbar. The crystals were cleaned in the preparation chamber ͑p Ϸ 10 −10 mbar͒ by bombardment with 2 kV Ar ions while annealing...
The magnetic properties of polycrystalline Co/W multilayers have been investigated using angulardependent ferromagnetic resonance (FMR) and SQUID magnetometry. Two sets of samples with a constant thickness of Co layers (CoI: d Co ¼ 1 nm and CoII: d Co ¼ 2.5 nm) and varying W layer thickness (1 to 7 nm) were studied. The structure of the multilayers was analysed by X-ray reflectivity (XRR) and X-ray diffraction (XRD). We found a well-defined h110i texture and coherent structure of the multilayers. The saturation magnetization of Co is smaller than that of bulk Co which is explained by a reduced Co moment at the interfaces of more than 50%. A narrow FMR linewidth on the order of 200 Oe at 9.5 GHz is observed for CoI and CoII multilayers with d W % 1 nm that indicates the high magnetic homogeneity of the Co layers. The second and fourthorder magnetic anisotropy constants (K 2 , K 4? ) are found to be independent of the Co layer thickness. K 2 is inversely proportional to the W layer thickness, which is due to the changing lateral lattice mismatch with the W layers.
The thickness-dependent spin-reorientation transition (SRT) in ultrathin Ni films grown on a stepped Cu(100) substrate was studied by spin-polarized low-energy electron microscopy (SPLEEM). Magnetic contrast in SPLEEM is proportional to the scalar product of beam polarization and magnetization of the film. This dependence was exploited to determine the local 3D magnetization vector by evaluating magnetic contrast in images obtained using three orthogonal polarizations of the incident electron beam. Using this method, we were able to directly image the rotation of the magnetization vector in Ni films from in-plane and parallel to the steps to out-of-plane and perpendicular to the steps of the substrate. We found the SRT to proceed via two mechanisms: continuous growth of the out-of-plane component, accompanied by discontinuous reorientation of the in-plane component via domain nucleation. Published in
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