Metastable Magnetic Epitaxial Thin Films: Fid-Bau Portal

Metastable Magnetic Epitaxial Thin Films

Willis, R. F.

Abstract We have studied the ferromagnetic behaviour of the metastable phases of f.c.c. cobalt and iron by clamping the lattice spacing in thin films epitaxed to appropriate single-crystal metal substrates/1/. This allows one to effectively “tune” the lattice strain such that changes in lattice volume are reflected in changes in both the magnitude of the magnetic moments and the magnetic exchange coupling between spin moments. This translates into new metastable magnetic systems with unusual properties, i.e., the magnetic moments are predicted to change discontinuously with varying atomic volume and there exist ranges of coexistence of different magnetic phases/2,3,4,5/. However, to date there has been very little experimental evidence to corroborate these views, principally due to the difficulties inherent in measuring the magnitudes of the magnetic moments in ultrathin epitaxial layers. Recently, we have succeeded in applying spin-polarized neutron reflectance at grazing angle of incidence to study the ferromagnetic properties of thin magnetic films of Co and Fe down to the monolayer thickness level/6/. The technique allows us to determine the magnitude of the magnetic moment per atom, μ, in ferromagnetically ordered layers, as a function of temperature, thickness and applied magnetic field/1/. The results are unusual in that, whereas the f.c.c. Co(001) layers remain ferromagnetic down to a single monolayer with μ ⋍ μbulk ⋍ 1.8/μB with no sign of a reduced Tc for temperatures up to 450K, the f.c.c. Fe(001) films are ferromagnetically ordered only at temperatures below this value (their Tc values being thickness dependent). Also, the Fe(001) films exhibit strong magnetic anisotropy effects which appear to be strain-related, and dependent on the number of layers. Detailed LEED measurements/7/ have determined the degree of strain as a function of thickness. Perhaps most surprising, whereas the f.c.c. Co(001) films exhibit spontaneous magnetization within the film plane for all layer thicknesses, the f.c.c. Fe(001) films show in-plane magnetization only for the first monolayer, the succeeding layers up to 7 monolayers showing strong spin-anisotropy oriented normal to the interface with remanence strongly dependent on the layer thickness/8/. The 4 monolayer film shows maximum volume expansion and a moment of the order of μ = 1 to 1.5μB in agreement with the recently predicted “low-moment” ferromagnetic ground state in the f.c.c. bulk iron phase/3,4,5/. The unusual uniaxial magnetic anisotropy of the magnetic exchange interaction in these strained lattice films produces behavior different to that which might be expected in a strictly two-dimensional ferromagnetic system. For example, the much higher Tc threshold for the cobalt films would indicate that the spin-wave energy gap is greater than that in the f.c.c. iron films, the latter showing reduced Curie temperatures as low as 150K compared with a bulk phase value, 900 to 1200K /9/. The significance of this latter result for Surface Science is that it offers the opportunity to investigate possible magnetocatalytic behaviour in surface chemical reactions by “tuning” Tc through an appropriate temperature range simply by varying the thin-film thickness.