Z. Celinski

Ferromagnetic resonance linewidth of Fe ultrathin films grown on a bcc Cu substrate

The ferromagnetic resonance (FMR) linewidth ΔH is a parameter used to characterize ultrathin film quality. The frequency‐independent part, ΔH(0), reflects the contribution of magnetic inhomogeneities. The linear part is caused by the intrinsic Gilbert damping mechanism. Recently we have found that the lattice reconstructions of substrate templates strongly affects the FMR linewidth. bcc Fe(001) films (∼10 ML) were grown on unreconstructed fcc Ag(001), bcc Cu(001), and on surface reconstructed bcc Cu(001) templates. Fe layers grown on unreconstructed Cu templates exhibited an isotropic FMR linewidth. Fe layers grown on reconstructed Cu templates, on the contrary, showed an anisotropic behavior in both the zero‐frequency FMR linewidth and the intrinsic damping parameter. Other magnetic parameters such as uniaxial and 4‐fold anisotropies are also discussed.

Exchange coupling in Fe/Cu, Pd, Ag, Au/Fe trilayers

Abstract Epitaxial trilayer Fe/Cu, Pd, Ag, Au/Fe structures have been grown by MBE. The growth of bcc Fe(001), Cu(001), lattice expanded fcc Pd(001) and fcc Ag(001) and Au(001) layers were monitored and characterized by RHEED patterns and RHEED intensity oscillations. Ferromagnetic Resonance (FMR) and Surface Magneto-Optical Kerr Effect (SMOKE) were used to investigate the exchange coupling in these trilayers using a wide range of Cu, Pd, Ag and Au interlayer thicknesses. It is shown that Cu and Pd interlayers exhibit long range antiferromagnetic coupling. Both interlayers exhibit unique oscillatory behavior. The exchange coupling through bcc Cu exhibits only one crossover to antiferromagnetic coupling. The strength of the antiferromagnetic coupling reveals an oscillatory behavior with periodicity of 10 monolayers. In the region of ferromagnetic coupling the exchange coupling through lattice expanded fcc Pd shows an oscillatory behavior with periodicity of 4 monolayers. Ag(001) interlayers show a rapid decrease in the ferromagnetic coupling with increasing Ag interlayer thickness. Au(001) interlayers behave similarly to those of Ag(001). However a weak antiferromagnetic coupling was observed in Au(001).

Using ferromagnetic resonance to measure the magnetic moments of ultrathin films

Abstract A technique that uses ferromagnetic resonance (FMR) to measure the saturation magnetization of ultrathin ferromagnetic films is described. It has been used to measure the layer averaged magnetic moments of ultrathin Fe films located in Ag/Fe/Ag, Au/Fe/Ag, Cu/Fe/Ag, Pd/Fe/Ag, and Ni/Fe/Ag structures relative to that of an Au/5.7 ML Fe/Ag reference film. The ratios obtained using method have a total measurement error of a little over 1%. The measurements were carried out to investigate theoretical predictions that Fe atoms located at or near surfaces and interfaces should possess enhanced magnetic moments compared those of Fe atoms in the bulk. All of the bcc Fe(001) films used in our work were approximately 5 monatomic layers (ML) thick and were all grown on bulk fcc Ag(001) substrates. The covering layers of Cu, Pd, and Ag were all 7 ML thick while the covering layers of Ni were 2–3 ML thick and the covering layers of Au were 20 ML thick. All FMR measurements were carried out at a temperature of 77 K. An Ag/5.5 ML Fe/Ag specimen and an Ag/10.9 ML Fe/Ag specimen (the thick Fe specimen) were determined to have a moment of ratio of 1.06 ± 0.01. This compared well to a layer averaged, ground state, moment ratio of 1.05 calculated for the two films using results obtained from published first principles calculations. Ratios of 1.08 ± 0.01 and 1.10 ± 0.01 were obtained for Au/5.7 Fe/Ag and Cu/5.8 Fe/Ag structures, respectively, when compared to the thick Fe specimen. In a similar manner, ratios of 1.11 ± 0.01 and 1.12 ± 0.01, respectively, were determined for Pd/5.6 Fe/Ag and Pd/5.7 Fe/Ag sandwiches. Finally, the layer averaged moment of a 2 Ni/5.7 Fe/Ag structure was in a ratio of 1.15 ± 0.01 with the thick Fe specimen while two 3 Ni/5.7 Fe/Ag sandwiches were in ratios of 1.24 ± 0.01 and 1.20 ± 0.01, respectively. All measured values were in excellent agreement with published first-principles calculations and with the layer averaged moments obtained from polarized neutron reflection (PNR) studies carried out on many of our specimens.

Magnetic anisotropies in single and multilayered structures (invited)

Ultrathin magnetic metallic structures provide a variety of systems in which unique magnetic properties can be engineered. The investigation of magnetic anisotropies in ultrathin structures has brought exciting results to the basic studies of magnetism in systems with reduced dimensionality and to the engineering of new magnetic materials. Molecular‐beam epitaxy (MBE) techniques turned out to be particularly useful in the formation of new stable and metastable structures whose sharply defined interfaces, lattice relaxations, and lattice reconstructions have resulted in a wide range of interesting magnetic properties. In this presentation the present authors’ recent studies of single and multilayered structures composed of bcc Fe(001), bcc Cu(001), lattice‐expanded Pd(001), fcc Co(001), and fcc Cu(001) will be summarized. Besides presenting their interesting magnetic properties the following techniques will be highlighted: reflection high‐energy electron diffraction (RHEED) and ferromagnetic resonance (FMR...

Analysis of bilinear and biquadratic exchange coupling in Fe/Ag/Fe(001) trilayers

Ferromagnetic resonance (FMR) and surface magneto‐optical Kerr effect (SMOKE) studies of the exchange coupling in Fe/Ag/Fe(001) structures are presented. The interfaces in these structures can be improved significantly by growing the first Fe(001) layer at a raised substrate temperature. The exchange couplings in Fe/Ag/Fe trilayers were studied as a function of the interlayer thickness and temperature. The improved interfaces in the Fe/Ag/Fe system revealed new features in the exchange coupling which were absent in samples grown entirely at room temperature. Quantitative data from the FMR and SMOKE measurements are compared. The magnetization loops for Fe/Ag/Fe trilayers can be explained well only by including the simultaneous presence of bilinear and biquadratic exchange coupling. It is shown that the exchange coupling through Ag(001) exhibits long wavelength oscillations.

In-Situ Techniques for Studying Epitaxially Grown Layers and Determining their Magnetic Properties

Ultrathin films of bcc Fe (001) on Ag (001) and Fe/Ni (001) bilayers on Ag were grown by molecular beam epitaxy. A wide range of surface science tools (RHEED, REELFS, AES, and XPS) were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with the easy axis perpendicular to the film surface were observed in all ultrathin structures studied. In sufficiently thin samples the saturation magnetization was oriented perpendicular to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. Fe/Ni bilayers were also investigated. Ni grows in the pure bcc structure for the first 3–6ML and then transforms to a new structure which exhibits unique magnetic properties. Transformed ultrathin bilayers possesses large in-plane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.

Surface magneto‐optical Kerr effect for ultrathin Ni‐Fe bilayers

Surface magneto‐optical Kerr effect is used to measure magnetic hysteresis loops for Ni‐Fe bilayers previously studied by ferromagnetic resonance [Phys. Rev. 38, 12879 (1988)]. The approach to saturation in the [11] direction confirms the existence of enhanced fourfold in‐plane magnetic anisotropy, attributed to the effect of arrays of misfit dislocations arising during the epitaxial growth of bcc Ni overlayers on ultrathin layers of bcc (001)Fe. The [11] magnetization curves for 10 monolayers (ML) of Ni on 7 ML of Fe shows a breaking of symmetry between positive and negative fields. This is ascribed to spin configurations frozen during growth.

Ferromagnetic and antiferromagnetic exchange coupling in ultrathin Fe(001)/Pd(001)/Fe(001) structures

A new phase of fcc Pd with an expanded lattice (5%) was grown on Fe(001). This structure is maintained up to 12 monolayers (ML) of Pd and then it undergoes a complex lattice reconstruction. Ultrathin Fe/Pd, Pd/Fe bilayers, and Fe/Pd/Fe trilayers with Pd thicknesses between 4 and 18 ML were studied using ferromagnetic resonance and Brillouin light scattering. The metallic Pd and particularly its lattice‐expanded structure is believed to have a strong tendency to become ferromagnetic. This tendency was observed in Fe/Pd/Fe trilayers with Pd thicknesses less than 6 ML. The exchange coupling remains ferromagnetic up to 12 ML of Pd and then it changes to a weak antiferromagnetic coupling. The exchange coupling in the ferromagnetic region exhibits a distinct oscillatory behavior superimposed on a decreasing background.

The temperature dependence of the bilinear and biquadratic exchange coupling in Fe/Cu, Ag/Fe(001) structures

Abstract The exchange coupling through non-magnetic metallic layers is described by bilinear (− J 1 cos( θ )) and biquadratic ( J 2 cos 2 ( θ )) exchange coupling terms. In simple metals such as Cu(001) and Ag(001) the bilinear exchange coupling exhibits strong short wavelength oscillations (∼ 2 ML). The exchange coupling through non-ferromagnetic interlayers in this case is strongly affected by the interface roughness. Realistic interfaces consist of finite terraces which results in variations of the interlayer thickness. Slonczewski showed that in this case the biquadratic exchange term can be produced by frustrations of the local magnetic moments. In the presented studies we used the MOKE and FMR techniques to investigate the temperature dependence of the bilinear and biquadratic exchange coupling in Fe/Cu/Fe(001) and Fe/Ag/Fe(001) structures. Quantitative studies of the exchange coupling were carried out in the temperature range 77–400 K. By growing the first Fe layer at various substrate temperatures one can prepare samples possessing interfaces with a variable atomic terrace width. That way we can controlle the strength of the biquadratic exchange coupling. Two limits were investigated: (a) J 2 ⪡ J 1 and (b) J 2 ∼ J 1 . It was found that the biquadratic coupling scales linearly with the strength of the bilinear exchange coupling. The results will be discussed within the framework of existing theories.

Diffuse polarised neutron reflection studies of ultrathin bcc Fe(001) epitaxial films

Abstract Polarised neutron reflection (PNR) has recently been developed as a highly sensitive magnetometric probe of ultrathin epitaxial ferromagnetic films. In this contribution we demonstrate the importance of accounting for the effect of diffuse scattering from the sample substrate and overlayers in using PNR to accurately estimate magnetic moments and illustrate these results by analysing the beam profile obtained from sandwiched ultrathin bcc Fe films prepared by MBE.