Safia Ouazi

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.

Atomic-scale engineering of magnetic anisotropy of nanostructures through interfaces and interlines

The central goals of nanoscale magnetic materials science are the self-assembly of the smallest structure exhibiting ferromagnetic hysteresis at room temperature, and the assembly of these structures into the highest density patterns. The focus has been on chemically ordered alloys combining magnetic 3d elements with polarizable 5d elements having high spin–orbit coupling and thus yielding the desired large magneto-crystalline anisotropy. The chemical synthesis of nanoparticles of these alloys yields disordered phases requiring annealing to transform them to the high-anisotropy L10 structure. Despite considerable efforts, so far only part of the nanoparticles can be transformed without coalescence. Here we present an alternative approach to homogeneous alloys, namely the creation of nanostructures with atomically sharp bimetallic interfaces and interlines. They exhibit unexpectedly high magnetization reversal energy with values and directions of the easy magnetization axes strongly depending on chemistry and texture. We find significant deviations from the expected behaviour for commonly used element combinations. Ab-initio calculations reproduce these results and unravel their origin.

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 Response and Spin Polarization of Bulk Cr Tips for In-Field Spin-Polarized Scanning Tunneling Microscopy

The magnetic properties of bulk Cr tips have been investigated by spin-polarized scanning tunneling spectroscopy (SP-STS). To extract the properties of the Cr tips, we performed low-temperature SP-STS measurements on a well-known model system: nanometric Co islands on Cu(111). Our experiments indicate the existence of uncompensated magnetic moments at the apex of the Cr tips, which rotate in the direction of the applied vertical magnetic field and become aligned with it at approximately 2 T. We extracted a tip spin polarization of 45% at the Fermi energy. We showed that the tip spin polarization can change with a modification of the tip apex.

Electronic states of moiré modulated Cu films

We examined by low-energy electron diffraction and scanning tunneling microscopy the surface of thin Cu films on Pt(111). The Cu/Pt lattice mismatch induces a moiré modulation for films from 3 to about 10 ML thickness. We used angle-resolved photoemission spectroscopy to examine the effects of this structural modulation on the electronic states of the system. A series of hexagonal- and trigonal-like constant energy contours is found in the proximity of the Cu(111) zone boundaries. These electronic patterns are generated by Cu sp-quantum well state replicas, originating from multiple points of the reciprocal lattice associated with the moiré superstructure. Layer-dependent strain relaxation and hybridization with the substrate bands concur to determine the dispersion and energy position of the Cu Shockley surface state.

Probing the spinor nature of electronic states in nanosize non-collinear magnets

Non-collinear magnetization textures provide a route to novel device concepts in spintronics. These applications require laterally confined non-collinear magnets (NCM). A crucial aspect for potential applications is how the spatial proximity between the NCM and vacuum or another material impacts the magnetization texture on the nanoscale. We focus on a prototypical exchange-driven NCM given by the helical spin order of bilayer Fe on Cu(111). Spin-polarized scanning tunnelling spectroscopy and density functional theory reveal a nanosize- and proximity-driven modification of the electronic and magnetic structure of the NCM in interfacial contact with a ferromagnet or with vacuum. An intriguing non-collinearity between the local magnetization in the sample and the electronic magnetization probed above its surface results. It is a direct consequence of the spinor nature of electronic states in NCM. Our findings provide a possible route for advanced control of nanoscale spin textures by confinement.

Comparing XMCD and DFT with STM spin excitation spectroscopy for Fe and Co adatoms on Cu2N/Cu(100)

We report on the magnetic properties of Fe and Co adatoms on a Cu2N/Cu(100)-c(2 x 2) surface investigated by x-ray magnetic dichroism measurements and density functional theory (DFT) calculations including the local coulomb interaction. We compare these results with properties formerly deduced from STM spin excitation spectroscopy (SES) performed on the individual adatoms. In particular we focus on the values of the local magnetic moments determined by XMCD compared to the expectation values derived from the description of the SES data. The angular dependence of the projected magnetic moments along the magnetic field, as measured by XMCD, can be understood on the basis of the SES Hamiltonian. In agreement with DFT, the XMCD measurements show large orbital contributions to the total magnetic moment for both magnetic adatoms.

Switching Fields of Individual Co Nanoislands

We explore the magnetization reversal process of individual Co nanoislands grown on Cu(111) by low temperature spin-polarized scanning tunneling microscopy (spin-STM) and spectroscopy (spin-STS). We measure hysteresis loops of the differential conductance of single Co islands in magnetic fields of up to 4 T. From such hysteresis loops we extract the magnetic switching field of single Co islands as a function of island size. Tentatively we analyze the size dependence of the switching field using the venerable model of thermally assisted coherent magnetization reversal. We present evidence for the failure of that model to explain our experimental results. We propose that the magnetization reversal process within individual Co nanoislands on Cu(111) is a non-coherent process.