F. Casoli

Morphology evolution and magnetic properties improvement in FePt epitaxial films by in situ annealing after growth

This paper presents a detailed study of structure, morphology, and magnetic properties in FePt thin films epitaxially grown on MgO(100) at intermediate temperatures (≅380°C). It focuses on the effects obtained by in situ annealing the FePt films after deposition. We have demonstrated that the annealing allows one to complete the ordering, thus obtaining an epitaxial L10-FePt film with large perpendicular anisotropy (up to 3×107erg∕cm3) and high perpendicular squareness and remanence ratio (both higher than 0.85). At the same time, we have found a peculiar morphology evolution by increasing the annealing time: the interconnected mazelike grains evolve towards interconnected dotlike grains of reduced size (down to 10nm). Due to the interconnection at the grains basis, the increase in the film perpendicular coercivity with the annealing process is moderate (up to 4.1kOe), leading to an increasing anisotropy/coercivity ratio with the annealing time. As a further step towards the understanding of the propertie...

Growth and characterization of epitaxial Fe-Pt films

The magnetic and structural properties of Fe/sub 53/Pt/sub 47/ deposited by alternated-layer sputtering on MgO (100), heated at temperatures up to T=550/spl deg/C, were investigated. The effect of a subsequent thermal annealing performed at the same temperature was found to be essential for obtaining 10-nm FePt thin films with a high degree of ordering and high squareness.

Spin configurations in hard/soft coupled bilayer systems: Transitions from rigid magnet to exchange-spring

We investigate equilibrium properties of an exchange-spring magnetic system constituted of a soft layer (e.g. Fe) of a given thickness on top of a hard magnetic layer (e.g. FePt). The magnetization profile M(z) as a function of the atomic position ranging from the bottom of the hard layer to the top of the soft layer is obtained in two cases with regard to the hard layer: i) in the case of a rigid interface (the FePt layer is a single layer), the profile is obtained analytically as the exact solution of a sine-Gordon equation with Cauchys boundary conditions. Additional numerical simulations also confirm this result. Asymptotic expressions of M(z) show a linear behavior near the bottom and the top of the soft layer. In addition, a critical value of the number of atomic planes in the soft layer, that is necessary for the onset of spin deviations, is obtained in terms of the anisotropy and exchange coupling between the adjacent plane in the soft layer. ii) in the case of a relaxed interface (the FePt layer is a multilayer), the magnetization profile is obtained numerically for various Fe and FePt films thicknesses and applied field.

Achieving Giant Magnetically Induced Reorientation of Martensitic Variants in Magnetic Shape‐Memory Ni–Mn–Ga Films by Microstructure Engineering

Giant magnetically induced twin variant reorientation, comparable in intensity with bulk single crystals, is obtained in epitaxial magnetic shape-memory thin films. It is found to be tunable in intensity and spatial response by the fine control of microstructural patterns at the nanoscopic and microscopic scales. A thorough experimental study (including electron holography) allows a multiscale comprehension of the phenomenon.

Arrays of ordered nanostructures in Fe-Pt thin films by self-assembling of polystyrene nanospheres

Large-area arrays of dots having diameter ranging in the interval 80–400 nm were obtained by polystyrene nanosphere (PN) lithography in L10-FePt thin films. Fe53Pt47 thin films (thickness 10 nm) were epitaxially deposited by rf sputtering on a MgO(100) substrate heated at 400 °C to promote the formation of the L10 tetragonal phase. Patterned films were obtained by assembling PN nanospheres monolayer with starting mean diameter of 100 and 500 nm on a continuous thin film; subsequently, the PNs size has been reduced by reactive ion etching, obtaining a final dots diameter of 80 and 400 nm, respectively. The patterning process resulted to disorder the tetragonal phase therefore reducing the coercive field. A post-annealing at 550 °C for 1 h resulted to be effective in re-inducing the precipitation of the ordered phase. The effect of patterning (i.e., dot diameter and mutual distance) on the magnetic properties is discussed in the light of the presence of the high-anisotropy L10 tetragonal phase, by means of ...

Local modifications of magnetism and structure in FePt (001) epitaxial thin films by focused ion beam : Two-dimensional perpendicular patterns

Focused ion beam was utilized to locally modify magnetism and structure of L10 FePt perpendicular thin films. As a first step, we have performed a magnetic, morphological, and structural study of completely irradiated FePt films with different Ga+ doses (1×1013–4×1016 ions/cm2) and ion beam energy of 30 keV. For doses of 1×1014 ions/cm2 and above a complete transition from the ordered L10 to the disordered A1 phase was found to occur, resulting in a drop of magnetic anisotropy and in the consequent moment reorientation from out-of-plane to in-plane. The lowest effective dose in disordering the structure (1×1014 ions/cm2) was found not to affect the film morphology. Taking advantage of these results, continuous two-dimensional (2D) patterns of perpendicular magnetic structures (250 nm dots, 1 μm dots, 1 μm-large stripes) were produced by focused ion beam without affecting the morphology. The 2D patterns were revealed by means of magnetic force microscopy, that evidenced peculiar domain structures in the ca...

Brillouin light scattering study of magnetic-element normal modes in a square artificial spin ice geometry

We report the results, from experimental and micromagnetic studies, of the magnetic normal modes in artificial square spin ice systems consisting of ferromagnetic-monodomain islands. Spin-wave properties are measured by Brillouin light scattering. The mode spectra contain several branches whose frequencies are sensitive to the magnitude and in-plane orientation of an applied magnetic field. We also identify soft modes that exhibit different behaviours depending on the direction of the applied magnetic field. The obtained results are well described with micromagnetic simulations of independent magnetic elements arranged along two sublattices.

Ferromagnetic Resonance Study of Fe/FePt Coupled Films With Perpendicular Anisotropy

We have studied exchange coupled FePt/Fe magnetic layers using the technique of ferromagnetic resonance (FMR). The FePt layers show strong uniaxial perpendicular anisotropy, growing in the L10 epitaxial phase on MgO (100). We have considered the case for two thicknesses of Fe: 2 nm and 3.5 nm, which exhibit rigid magnet and exchange-spring behavior. All FePt thicknesses are limited to 10 nm. The FMR results display multipeaked spectra, where we have identified three Fe resonance lines in the RM sample and an extra two in the ES sample. Angular FMR studies show a strong uniaxial anisotropy induced in the Fe layer via the strong exchange coupling with the FePt film. An additional uniaxial component is also observed with an easy axis inclined by about 50 deg from the film normal. Supplementary magnetic measurements aid with the magnetic characterization. We also discuss the elements for the theory of FMR in these exchanged coupled systems.

Electronic and magnetic properties of iron doped zirconia: Theory and experiment

We systematically investigated, both theoretically and experimentally, Zr1−xFexO2−y ranging from diluted (x ≈ 0.05) up to large (x ≈ 0.25) Fe concentration. By atomic layer deposition, we grew thin films of high-κ zirconia in cubic phase with Fe uniformly distributed in the film, as proven by time of flight secondary ion mass spectrometry and transmission electron microscopy measurements. Iron is in Fe3+ oxidation state suggesting the formation of oxygen vacancies with y concentration close to x/2. By ab-initio simulations, we studied the phase diagram relating the stability of monoclinic vs. tetragonal phase as a function of Fe doping and film thickness: the critical thickness at which the pure zirconia is stabilized in the tetragonal phase is estimated ranging from 2 to 6 nm according to film morphology. Preliminary results by X-ray magnetic circular dichroism and alternating gradient force magnetometry are discussed in comparison to ab initio data enlightening the role of oxygen vacancies in the magnet...

Epitaxial Fe3Pt/FePt nanocomposites on MgO and SrTiO3

We have exploited the pseudomorphic growth of the magnetically soft Fe3Pt phase on top of L10-FePt to obtain fully epitaxial soft/hard nanocomposites on both MgO(100) and SrTiO3(100). The magnetic properties of this new nanocomposite system, driven by the soft/hard exchange-coupling, can be tailored by varying soft phase thickness, soft phase magnetic anisotropy and substrate. Coercivity is strongly reduced by the addition of the soft phase, a reduction which is definitely affected by the nominal composition of the soft phase and by the substrate choice; similarly is the magnetic phase diagram of the composite system. Coercive field decreases down to 21% of the hard layer value for Fe3Pt(5 nm)/FePt(3.55 nm) nanocomposites on SrTiO3; this maximum coercivity reduction was obtained with a nominal atomic content of Fe in the soft phase of 80%.