The magnetic anisotropy of ultra-thin films is very useful in magnetic devices where well-defined easy magnetisation axes are needed. Different ferromagnetic films on weakly magnetic substrates have been investigated in search of perpendicular magnetic anisotropy, which is of particular interest for applications in high-density magneto-optical storage. Once a system with perpendicular anisotropy is found, the goal is to understand how the magnetisation can spontaneously be oriented perpendicular to the surface, while the shape anisotropy tends to strongly orient the magnetisation in the plane of the film.

Such a system is Ni/Pt(001), which exhibits perpendicular magnetic anisotropy at room temperature for a large range of thickness. Magneto-optic Kerr effect (MOKE) measurements were performed in situ at the Surface Diffraction Beamline, ID03, in a geometry of the magnetic field slightly inclined with respect to the sample surface. The Ni film becomes magnetic at room temperature at 10 ML thickness, with a perpendicular easy axis. The polar MOKE cycle, which probes the perpendicular component of the magnetic field, is presented in Figure 97a. The coercivity increases with the film thickness and the anisotropy remains perpendicular, as showed for 20 ML in Figure 97b. At 40 ML, the decreasing polar MOKE signal indicates that the film is close to the point of switching toward in-plane anisotropy (Figure 97c).

Fig. 97: Polar MOKE signal measured for Ni films deposited on Pt(001), of thickness: (a) 10 ML; (b) 20 ML; (c) 40 ML.

The magnetic measurements were completed with an in situ crystallographic study, realised by surface diffraction at the ID03 beamline. A BCC(001) structure was identified for the Ni thin films, which arose from its very small lattice mismatch (3.7%) with the surface structure of Pt(001). Ni grows pseudomorphically on Pt(001), as proven by the similarity of the specular (00L) and non-specular (11L) rods, presented in Figure 98 for a 10 ML Ni film. Thickness fringes are clearly visible in the low L part. The BCC (001) Ni peak was identified at L = 2.6. The vertical cell parameter was measured, showing a tetragonal distortion of the BCC Ni structure (c/a = 1.07).

Fig. 98: (00L) and (11L) rods of a 10 ML Ni film deposited on Pt(001).

A comparison was realised with the system Co/Pt(001), previously studied at the ID03 beamline. For exactly the same crystallographic structure, the magnetic behaviour is completely different: as soon as the Co film becomes magnetic, it exhibits only in-plane anisotropy [1].

This difference can be understood on the basis of magneto-elastic effects. The bulk magnetostriction constants for FCC Co and FCC Ni are: Co = 7.5·10-5 and Ni = –4.6·10-5. Thus, Co and Ni behave oppositely under magnetic field: Co expands and Ni contracts in the direction of the applied magnetic film. We will presume here that the magnetostriction constants of the BCC thin films have the same signs as the bulk values.

The tetragonal distortion of the BCC structure of the Ni and Co films causes an increase of the elastic energy. Magneto-elastic effects will try then to orient the magnetisation in a way that compensates this increase. For the Co films this compensation is realised with in-plane magnetisation, while for Ni films the perpendicular magnetisation is needed.

These experimental results are in agreement with first principle calculations of uniaxial magneto-elastic anisotropy of Co and Ni ultra-thin films, realised by Burkert and co-workers [2]. They obtained a tetragonal distortion within the Bain path from FCC to BCC favors in-plane magnetic anisotropy for Co and perpendicular magnetic anisotropy for Ni.

In conclusion, BCC Ni ultra-thin films were grown pseudomorphically on the Pt(001) surface. Their perpendicular magnetic anisotropy, observed in a large range of thicknesses at room temperature, was understood on the basis of tetragonal distortion of the BCC structure and of magneto-elastic effects.



[1] S. M. Valvidares, T. Schroder, O. Robach, C. Quiros, T. L. Lee, S. Ferrer, Phys. Rev. B 70, 224413 (2004).
[2] T. Burkert, O. Eriksson, P. James, S. I. Simak, B. Johansson, L. Nordstrom, Phys. Rev. B, 69, 104426 (2004).

Principal Publication and Authors

I. Popa (a), O. Robach (b), C. Quiros (c), M.S. Valvidares (a), H. Kim (d), H. Isern (a), S. Ferrer (e), to be published.
(a) ESRF
(b) CEA – DRFMC/SI3M/PCM, Grenoble (France)
(c) Depto de Fisica, Faculdad de Ciencias, Universidad de Oviedo (Spain)
(d) LG Chem. Research Park, Daejeon (Korea)
(e) CELLS – Edifici Ciencies, Bellaterra (Spain)