S. Cherifi

Tunnel magnetoresistance and robust room temperature exchange bias with multiferroic BiFeO3 epitaxial thin films

The authors report on the functionalization of multiferroic BiFeO3 epitaxial films for spintronics. A first example is provided by the use of ultrathin layers of BiFeO3 as tunnel barriers in magnetic tunnel junctions with La2∕3Sr1∕3MnO3 and Co electrodes. In such structures, a positive tunnel magnetoresistance up to 30% is obtained at low temperature. A second example is the exploitation of the antiferromagnetic spin structure of a BiFeO3 film to induce a sizable (∼60Oe) exchange bias on a ferromagnetic film of CoFeB at room temperature. Remarkably, the exchange bias effect is robust upon magnetic field cycling, with no indications of training.

Head-to-head domain-wall phase diagram in mesoscopic ring magnets

The nanoscale spin structure of head-to-head domain walls in mesoscopic ferromagnetic rings has been studied by high-resolution nonintrusive photoemission electron microscopy as a function of both ring width (100–730 nm) and film thickness (2–38 nm). Depending on the geometry, two types of head-to-head domain walls are found (vortex and transverse walls). The experimental phase diagram, which identifies the transition between the wall types, is compared to analytical calculations of the energy and micromagnetic simulations, which are found to agree well with the experimental results.

Observation of thermally activated domain wall transformations

The spin structure of head-to-head domain walls in Ni80Fe20 structures is studied using high-resolution photoemission electron microscopy. The quantitative phase diagram is extracted from these measurements and found to exhibit two phase boundaries between vortex and transverse domain walls. The results are compared with available theoretical predictions and micromagnetic simulations and differences to the experiment are explained, taking into account thermal excitations. Temperature-dependent measurements show a thermally activated transformation of transverse to vortex domain walls in 7 nm thick and 730 nm wide structures at a transition temperature between 260 °C and 310 °C, which corresponds to a nucleation barrier height for a vortex wall between 6.7×10−21J and 8.0×10−21J.

Structure and magnetism of self-organized Ge(1-x)Mn(x) nano-columns

We report on the structural and magnetic properties of thin Ge(1-x)Mn(x)films grown by molecular beam epitaxy (MBE) on Ge(001) substrates at temperatures (Tg) ranging from 80°C to 200°C, with average Mn contents between 1 % and 11 %. Their crystalline structure, morphology and composition have been investigated by transmission electron microscopy (TEM), electron energy loss spectroscopy and x-ray diffraction. In the whole range of growth temperatures and Mn concentrations, we observed the formation of manganese rich nanostructures embedded in a nearly pure germanium matrix. Growth temperature mostly determines the structural properties of Mn-rich nanostructures. For low growth temperatures (below 120°C), we evidenced a two-dimensional spinodal decomposition resulting in the formation of vertical one-dimensional nanostructures (nanocolumns). Moreover we show in this paper the influence of growth parameters (Tg and Mn content) on this decomposition i.e. on nanocolumns size and density. For temperatures higher than 180°C, we observed the formation of Ge3Mn5 clusters. For intermediate growth temperatures nanocolumns and nanoclusters coexist. Combining high resolution TEM and superconducting quantum interference device magnetometry, we could evidence at least four different magnetic phases in Ge(1-x)Mn(x) films: (i) paramagnetic diluted Mn atoms in the germanium matrix, (ii) superparamagnetic and ferromagnetic low-Tc nanocolumns (120 K 400 K) and (iv) Ge3Mn5 clusters.

Current-induced vortex nucleation and annihilation in vortex domain walls

We report observations of the effect of electrical currents on the propagation and spin structure of vortex walls in NiFe wires. We find that magnetic vortices are nucleated and annihilated due to the spin torque effect. The velocity is found to be directly correlated with these transformations and decreases with increasing number of vortices. The transformations are observed in wide elements, while in narrower structures the propagation of single vortex walls prevails.

Three-dimensional magnetic flux-closure patterns in mesoscopic Fe islands

We have investigated three-dimensional magnetization structures in numerous mesoscopic Fe/Mo(110) islands by means of x-ray magnetic circular dichroism combined with photoemission electron microscopy (XMCD-PEEM). The particles are epitaxial islands with an elongated hexagonal shape with length of up to 2.5 micrometer and thickness of up to 250 nm. The XMCD-PEEM studies reveal asymmetric magnetization distributions at the surface of these particles. Micromagnetic simulations are in excellent agreement with the observed magnetic structures and provide information on the internal structure of the magnetization which is not accessible in the experiment. It is shown that the magnetization is influenced mostly by the particle size and thickness rather than by the details of its shape. Hence, these hexagonal samples can be regarded as model systems for the study of the magnetization in thick, mesoscopic ferromagnets.

Quantitative determination of domain wall coupling energetics

The magnetic dipolar coupling of head-to-head domain walls is studied in 350nm wide NiFe and Co nanostructures by high resolution magnetic imaging. We map the stray field of a domain wall directly with sub-10-nm resolution using off-axis electron holography and find that the field intensity decreases as 1∕r with distance. By using x-ray magnetic circular dichroism photoemission electron microscopy, we observe that the spin structures of interacting domain walls change from vortex to transverse walls, when the distance between the walls is reduced to below (77±5)nm for 27nm thick NiFe and (224±65)nm for 30nm thick Co elements. Using measured stray field values, the energy barrier height distribution for the nucleation of a vortex core is obtained.

Tailoring of the structural and magnetic properties of MnAs films grown on GaAs—Strain and annealing effects

MnAs films were deposited by molecular-beam epitaxy on GaAs(001) and GaAs(111)B surfaces. Imaging of the temperature-dependent magnetic structure by x-ray magnetic circular dichroism photoemission electron microscopy, and the comparison with magnetization measurements by superconducting quantum interference device (SQUID) magnetometry, is used to study the impact of the different strain state of MnAs/GaAs(001) and of MnAs/GaAs(111)B films on the phase transition between ferromagnetic α-MnAs and paramagnetic β-MnAs, the spatial distribution of the two structural and magnetic phases, and the transition temperature. For the isotropically strained MnAs/GaAs(111)B films, the phase coexistence range is much wider than for the anisotropically strained MnAs/GaAs(001) films. The characteristic change of the saturation magnetization with film thickness is found to be a universal property of films with different epitaxial orientation, if at least one MnAs⟨112¯0⟩ direction is in the film plane. For MnAs/GaAs(001) fil...

Low-energy electron microscopy/x-ray magnetic circular dichroism photoemission electron microscopy study of epitaxial MnAs on GaAs

Epitaxial MnAs films on GaAs(001) substrates are studied at room temperature and in the completely ferromagnetic state below room temperature with low-energy electron microscopy, x-ray magnetic circular dichroism photoemission electron microscopy, and low-energy electron diffraction. The combination of these techniques shows a clear relation between the two-phase structure of the layers and their magnetic domain structure.

Tuning the domain wall orientation in thin magnetic strips using induced anisotropy

The authors report on a method to tune the orientation of in-plane magnetic domains and domain walls in thin ferromagnetic strips by manipulating the magnetic anisotropy of the system. Uniaxial in-plane anisotropy is induced in a controlled way by oblique evaporation of magnetic thin strips. A direct correlation between the magnetization direction and the domain wall orientation is found experimentally and confirmed by micromagnetic simulations. The domain walls in the strips are always oriented along the oblique evaporation-induced easy axis, irrespective of the shape anisotropy. The controlled manipulation of domain wall orientations could provide promising possibilities for recently proposed devices based on domain wall propagation.