D. Sander

Spin-Dependent Quantum Interference Within a Single Magnetic Nanostructure

Wave-Particle Duality The dual-wave nature of particles is nowhere more evident than in a confined space, where standing waves are formed with wavelengths that depend on particle energy. This so-called quantum interference has been observed in nanostructures using surface probes such as scanning tunneling microscopy. Now, Oka et al. (p. 843) use the spin-polarized version of this technique to study spin-dependent quantum interference on a triangular nanoscale cobalt island deposited on a copper surface. They observe the modulation of the magnetization, with the pattern depending on the energy of the interfering electrons. The experimental results are in good agreement with simulations, which indicate that the magnetization at a given energy and position largely depends on which of two electron spin states present dominates. Magnetization modulation is observed on a cobalt nanoisland using spin-polarized scanning tunneling microscopy. Quantum interference is a coherent quantum phenomenon that takes place in confined geometries. Using spin-polarized scanning tunneling microscopy, we found that quantum interference of electrons causes spatial modulation of spin polarization within a single magnetic nanostructure. We observed changes in both the sign and magnitude of the spin polarization on a subnanometer scale. A comparison of our experimental results with ab initio calculations shows that at a given energy, the modulation of the spin polarization can be ascribed to the difference between the spatially modulated local density of states of the majority spin and the nonmodulated minority spin contribution.

Direct observation of electron confinement in epitaxial graphene nanoislands.

One leading question for the application of graphene in nanoelectronics is how electronic properties depend on the size at the nanoscale. Direct observation of the quantized electronic states is central to conveying the relationship between electronic structures and local geometry. Scanning tunneling spectroscopy was used to measure differential conductance dI/dV patterns of nanometer-size graphene islands on an Ir(111) surface. Energy-resolved dI/dV maps clearly show a spatial modulation, indicating a modulated local density of states due to quantum confinement, which is unaffected by the edge configuration. We establish the energy dispersion relation with the quantized electron wave vector obtained from a Fourier analysis of dI/dV maps. The nanoislands preserve the Dirac Fermion properties with a reduced Fermi velocity.

The correlation between mechanical stress and magnetic properties of ultrathin films

The cantilever bending beam technique was applied to measure film stress, film magnetization and magneto-elastic coupling in nanometre Fe films grown epitaxially on W substrates. A simple optical deflection technique yielded sub-monolayer sensitivity for stress measurements and was used to determine magnetization and magnetostrictive properties of nanometre Fe films in situ. The combination of an electromagnet inside an ultra-high-vacuum chamber with a rotatable external magnet was employed to perform magneto-optical Kerr-effect measurements in the transversal, longitudinal and polar geometry in fields of up to 0.4 T. Examples for stress-driven structural changes in monolayer Fe films are discussed with respect to the unusual high coercivity found for sesquilayer Fe films and the re-orientation of the easy axis of magnetization in Stranski-Krastanov Fe films. The direct correlation between strain and magnetism was exploited to measure the magnetostrictive bending of the film-substrate composite. The magnitude and sign of the magneto-elastic coupling coefficient were found to depend on the film thickness, in contrast to the respective bulk values.

Stress, strain and magnetostriction in epitaxial films

The application of the cantilever bending technique to stress measurements at surfaces and in epitaxial films is elucidated. The role of elastic anisotropy in quantitative cantilever curvature analysis is discussed. The stress in Co monolayers is measured during epitaxial growth on Cu(001). The Co-induced stress is found to oscillate with a period of one atomic layer. Simultaneous stress and medium-energy electron diffraction identify maximum stress for filled Co layers. Strain relaxation in Co islands leads to the reduced stress contribution of 2.9 GPa in the partially filled top layer as compared to 3.4 GPa for the filled layers. The cantilever technique is also applied to measure magnetoelastic properties of nanometre thin epitaxial films. Our measurements reveal that the magnetoelastic-coupling coefficients in epitaxial Fe, Co and Ni films differ from the respective bulk values. It is proposed that the epitaxial misfit strain is of key importance for this peculiar magnetostrictive behaviour of ultrathin films.

Structure and morphology of Ni monolayers on W(110)

Scanning tunneling microscopy (STM ) is employed to investigate the structural and morphological properties of Ni monolayers (ML) deposited on W(110) at room temperature. We observe almost perfect layer-by-layer growth for the first three atomic layers. At a coverage of about 6 ML, a transition to three-dimensional growth is found. The analysis of STM images in the submonolayer regime indicates that the coalescence of the islands triggers the formation of a 8◊1 coincidence structure. In contrast to the preferential orientation expected from the twofold symmetric substrate, the islands in the first atomic layer have an irregular shape. Elongated Ni islands are observed in the second layer. The anisotropic island shape is ascribed to the anisotropic diVusion due to anisotropic strain in the underlying first layer. The edges of the Ni islands are oriented along the closed packed 110 directions of the growing Ni(111) film for Ni coverages above 5 ML.

Magnetoelastic coupling in Ni and Fe monolayers on Cu(001)

The correlation between mechanical stress and magnetic anisotropy of Ni and Fe films on Cu(001) is investigated. The magnetoelastic coupling and the film stress during the growth are measured in situ with a highly sensitive optical bending beam technique. For Ni a dramatically reduced magnetoelastic coupling of B1=3.5 MJ/m3 is found for films thinner than 10 ML, roughly one third of the bulk value of 9.4 MJ/m3. This change is explained by a strain correction to the magnetoelastic coupling. The influence of the interfaces does not significantly contribute to the magnetoelastic coupling. A very small magnetoelastic coupling of 0.4 MJ/m3 for Fe films in the range from 12 ML to 25 nm is attributed mainly to the crystallographic orientation of the bcc–Fe.

Strain dependence of the magnetic properties of nm Fe films on W(100)

The thickness dependence of the magneto-elastic coupling B1, the intrinsic film stress, and the magnetic in-plane anisotropy K4 of Fe films on W(100) are measured with an in situ combination of a highly sensitive optical deflection technique with magneto-optical Kerr-effect measurements. We find that both B1 and K4 depend strongly on the Fe film thickness. The thickness dependence of B1 can be described by considering a second order magneto-elastic coupling constant D=1 GJ/m3 as a strain dependent correction of B1. We tentatively ascribe the deviation of K4 from its bulk value to the tetragonal lattice distortion caused by an effective tensile in-plane strain of 5.3% in the pseudomorphic region and of 0.2% in thicker films.The thickness dependence of the magneto-elastic coupling B1, the intrinsic film stress, and the magnetic in-plane anisotropy K4 of Fe films on W(100) are measured with an in situ combination of a highly sensitive optical deflection technique with magneto-optical Kerr-effect measurements. We find that both B1 and K4 depend strongly on the Fe film thickness. The thickness dependence of B1 can be described by considering a second order magneto-elastic coupling constant D=1 GJ/m3 as a strain dependent correction of B1. We tentatively ascribe the deviation of K4 from its bulk value to the tetragonal lattice distortion caused by an effective tensile in-plane strain of 5.3% in the pseudomorphic region and of 0.2% in thicker films.

Structure and perpendicular magnetization of Fe/Ni(111) bilayers on W(110)

Scanning tunneling microscopy and low energy electron diffraction show that high quality fcc Ni(111) films can be prepared on W(110). The subsequent coverage of this Ni template by monolayers of Fe leads to a Fe/Ni bilayer with striking magnetic properties. The Fe cap layer induces a spin reorientation of the easy axis of magnetization from in-plane to perpendicular to the film, as checked with the magneto-optic Kerr effect. At higher Fe coverages, an in-plane magnetization of the bilayer is found, which is proposed to be caused by the fcc to bcc transition in the Fe layer.

Stress oscillations in a growing metal film

The stress in Co monolayers has been measured during epitaxial growth on Cu(0 0 1). The Co-induced stress is found to oscillate with a period of one atomic layer. Simultaneous stress and medium energy electron diffraction identify maximum stress for filled Co layers. Strain relaxation in Co islands leads to a reduced stress contribution of 2.9 GPa in the partially filled top layer as compared to 3.4 GPa for the filled layers. The corresponding variation of the elastic energy is 1 meV per Co atom. Atomic scale calculations reveal that the size-dependent mesoscopic mismatch is the driving force for stress relaxation in Co islands. 2002 Elsevier Science B.V. All rights reserved.

Magnetic Hysteresis Loop of Single Co Nano-islands

Spin-dependent scanning tunneling spectroscopy has been performed on single Co islands on Cu(111) at 7 K in fields of up to 4 T. The differential conductance shows a hysteretic behavior as a function of magnetic field. Symmetric hysteresis curves of the differential conductance are obtained which identify an abrupt switching of the Co island magnetization along the sample normal at fields around 1.5 T, and a reversible change of the spin orientation of the Cr-tip apex with increasing magnetic field. Our result allows a clear-cut assignment of the differential conductance curves in terms of parallel and antiparallel states of the spin orientation between tip and sample.