An insight into the role of magnetic anisotropies in the behavior of thin films and arrays of nanoparticles

In this thesis work we discussed the properties of magnetic materials derived by the reduction of at least one of the spatial dimensions under the micrometer scale: in particular we analyzed the origin of some magnetic behaviors in magnetic nanostructures like thin films and arrays of nanodots. One of the important properties which is strongly affected by the size reduction and in which is contained most of the physical description of a magnetic system is the anisotropy energy term: a direction-dependent parameter which strongly contributes to the determination of the equilibrium state and magnetic behavior. We described various nanostructured systems concentrating prevalently on thin films and arrays of interacting nanoparticles and for each system the origin and the physical implications of magnetic anisotropy is discussed. In thin magnetic films, two types of magnetic anisotropies are presented: Perpendicular Magnetic Anisotropy which has a crystalline origin and competes with the shape anisotropy of the thin film producing a singular type of magnetic domains called “stripes” and the Rotatable Anisotropy (the easy magnetic direction is not fixed but could be rotated by means of an external magnetic field). We tried to give a better explanation and modeling of the Rotatable Anisotropy, making a parallelism between the static and dynamic experimental evidences. We performed also a description of the interaction of magnetic dots in arrays with different symmetry and with finite dimensions. In particular we discovered a peculiar space-dependent behavior that we called “Global Configurational Anisotropy”, that has a strong importance when the dimension of the array becomes comparable with the dimension of the nanoparticles

An insight into the role of magnetic anisotropies in the behavior of thin films and arrays of nanoparticles

In this thesis work we discussed the properties of magnetic materials derived by the reduction of at least one of the spatial dimensions under the micrometer scale: in particular we analyzed the origin of some magnetic behaviors in magnetic nanostructures like thin films and arrays of nanodots. One of the important properties which is strongly affected by the size reduction and in which is contained most of the physical description of a magnetic system is the anisotropy energy term: a direction-dependent parameter which strongly contributes to the determination of the equilibrium state and magnetic behavior. We described various nanostructured systems concentrating prevalently on thin films and arrays of interacting nanoparticles and for each system the origin and the physical implications of magnetic anisotropy is discussed. In thin magnetic films, two types of magnetic anisotropies are presented: Perpendicular Magnetic Anisotropy which has a crystalline origin and competes with the shape anisotropy of the thin film producing a singular type of magnetic domains called “stripes” and the Rotatable Anisotropy (the easy magnetic direction is not fixed but could be rotated by means of an external magnetic field). We tried to give a better explanation and modeling of the Rotatable Anisotropy, making a parallelism between the static and dynamic experimental evidences. We performed also a description of the interaction of magnetic dots in arrays with different symmetry and with finite dimensions. In particular we discovered a peculiar space-dependent behavior that we called “Global Configurational Anisotropy”, that has a strong importance when the dimension of the array becomes comparable with the dimension of the nanoparticles

How finite sample dimensions affect the reversal process of magnetic dot arrays

We investigate the magnetization reversal of a magnetic dot array by means of magneto-optical Kerr effect and magnetic force microscopy measurements as well as micromagnetic simulations. We find that the finite dimensions of the dot array introduce a global configurational anisotropy that promotes state transitions first in dots near the sample boundaries. From there, the reversal process expands towards the sample body by means of collective magnetization processes originating in the magnetostatic coupling between the dots. These processes are characterized by transition avalanches and the formation of magnetization chains. These findings are important in the development of applications that rely on a robust control of dot magnetization states in dot arrays

Towards III-V solar cells on Si: Improvement in the crystalline quality of Ge-on-Si virtual substrates through low porosity porous silicon buffer layer and annealing

A comparison between the crystalline quality of Ge grown on bulk Si and on a low porosity porous Si (pSi) buffer layer using low energy plasma enhanced chemical vapor deposition is reported. Omega/2Theta coupled scans around the Ge and Si (004) diffraction peaks show a reduction of the Ge full-width at half maximum (FWHM) of 22.4% in presence of the pSi buffer layer, indicating it is effective in improving the epilayer crystalline quality. At the same time atomic force microscopy analysis shows an increase in root means square roughness for Ge grown on pSi from 38.5 nm to 48.0 nm, as a consequence of the larger surface roughness of pSi compared to bulk Si. The effect of 20 minutes vacuum annealing at 580°C is also investigated. The annealing leads to a FWHM reduction of 23% for Ge grown on Si and of 36.5% for Ge on pSi, resulting in a FWHM of 101 arcsec in the latter case. At the same time, the RMS roughness is reduced of 8.8% and of 46.5% for Ge grown on bulk Si and on pSi, respectively. The biggest improvement in the crystalline quality of Ge grown on pSi with respect to Ge grown on bulk Si observed after annealing is a consequence of the simultaneous reorganization of the Ge epilayer and the buffer layer driven by energy minimization. A low porosity buffer layer can thus be used for the growth of low defect density Ge on Si virtual substrates for the successive integration of III-V multijunction solar cells on Si. The suggested approach is simple and fast –thus allowing for high throughput-, moreover is cost effective and fully compatible with subsequent wafer processing. Finally it does not introduce new chemicals in the solar cell fabrication process and can be scaled to large area silicon wafers

Magnetization reversal in finite size dot arrays: Global Configurational Anisotropy

We analyzed with MFM and MOKE finite squared (rectangular) arrays of circular (elliptical) magnetic dots and performed simulations with MuMax, a GPU-based software. We showed as for limited size of the periodic arrays the transition of the magnetization during the reversal starts at the edges and corners of the array and propagates inside the pattern, so that in a restricted field range the magnetization results to be not uniformly distributed. While the shape of the dots (circular, elliptical, etc.) introduces a Configurational Anisotropy, we find that the finite array dimensions introduce an additional Global Configurational Anisotropy. Both effects originate at the demagnetizing interactions playing at different space scales: the dot and total array space scale, respectively. Simulations of dot arrays are often restricted to one dot assuming isolated non-interacting magnetization processes. Periodic boundary conditions are often used to incorporate interdot interactions, still limiting computations to a restricted number of dots and assuming infinite lattice periodicity. Then, configurational anisotropy is accounted for, but global configurational anisotropy is not. We show that mutual dot interactions together with finite array dimensions have a nonnegligible impact on the magnetization reversal of a dot array. We numerically and experimentally study the hysteresis properties of Permalloy (Py) arrays of 16x16 circular and elliptical dots, with thickness ranging between 10 and 25 nm and lateral size between 300 and 500 nm. In magnetooptical Kerr effect (MOKE) measurements, in-field magnetic force microscope (MFM) measurements and simulations, we find that global shape anisotropy steers the magnetization reversal of the array: the dots run through different magnetization states depending on the dot location and collective magnetization processes occur, leading to transition avalanches and formation of magnetization chains. Moreover, we find that imperfections as edge roughness and external perturbations, as the MFM measurement itself, anticipate the dots reversal path set by the global configurational anisotropy and promote field induced magnetization state changes. These findings are important in the development of applications that rely on a robust control of dot magnetization states in dot arrays

Magnetization reversal in magnetic dot arrays: Nearest-neighbor interactions and global configurational anisotropy

Various proposals for future magnetic memories, data processing devices, and sensors rely on a precise control of the magnetization ground state and magnetization reversal process in periodically patterned media. In finite dot arrays, such control is hampered by the magnetostatic interactions between the nanomagnets, leading to the non-uniform magnetization state distributions throughout the sample while reversing. In this paper, we evidence how during reversal typical geometric arrangements of dots in an identical magnetization state appear that originate in the dominance of either Global Configurational Anisotropy or Nearest-Neighbor Magnetostatic interactions, which depends on the fields at which the magnetization reversal sets in. Based on our findings, we propose design rules to obtain the uniform magnetization state distributions throughout the array, and also suggest future research directions to achieve non-uniform state distributions of interest, e.g., when aiming at guiding spin wave edge-modes through dot arrays. Our insights are based on the Magneto-Optical Kerr Effect and Magnetic Force Microscopy measurements as well as the extensive micromagnetic simulations

Perpendicular Magnetic Anisotropy in Fe–N Thin Films: Threshold Field for Irreversible Magnetic Stripe Domain Rotation

International audienceThe magnetic properties of an iron nitride thin film obtained by ion implantation have been investigated. N+22+ ions were implanted in a pristine iron layer epitaxially grown on ZnSe/GaAs(001). X-ray diffraction measurements revealed the formation of body-centered tetragonal N-martensite whose cc-axis is perpendicular to the thin film plane and cc-parameter is close to that of α′α′-Fe8N. Magnetic measurements disclosed a weak perpendicular magnetic anisotropy (PMA) whose energy density KPMAKPMA was assessed to about 105J/m3. A sharp decline of the in-plane magnetocrystalline anisotropy (MCA) was also observed, in comparison with the body-centered cubic iron. The origin of the PMA is attributed to the MCA of N-martensite and/or stress-induced anisotropy. As a result of the PMA, weak magnetic stripe domains with a period of about 130nm aligned along the last saturating magnetic field direction were observed at remanence by magnetic force microscopy. The application of an increasing in-plane magnetic field transverse to the stripes HtransHtrans highlighted a threshold value (μ0Htrans≈0.1μ0Htrans≈0.1T) above which these magnetic domains irreversibly rotated. Interestingly, below this threshold, the stripes do not rotate, leading to a zero remanent magnetization along the direction of the applied field. The interest of this system for magnetization dynamics is discussed

Robustness of majority gates based on nanomagnet logic

We studied the evolution of the magnetic state of a majority logic gate consisting of a cluster of five dipolarly-coupled nanomagnets, fabricated by e-beam lithography, under the application of a clocking field, using a combination of magneto-optical Kerr effect and magnetic force microscopy. The data were interpreted by advanced GPU-based micromagnetic simulations, where, in addition to the single ideal-shaped gate, a 3 x 3 array of "realistic gates", whose shape is directly derived from scanning electron microscopy images, is considered. A fairly good agreement between measurements and simulations has been achieved, showing that asynchronous switching of nominally identical gates may occur, because of unavoidable structural and morphological imperfections. Moreover, a slight misalignment of 1 degrees-2 degrees of the clocking field with respect to the hard axis of the dots may be detrimental for the correct logic operation of the gates. It follows that reliable, error-free and reproducible operations in future magnetologic devices would require tight control and precision of both the lithographic process and the direction of the clocking field. Moreover, a significant improvement could be insured by a stronger dipolar coupling between the dots, for instance increasing their thickness and/or using materials with larger magnetization. (C) 2018 Elsevier B.V. All rights reserved

Stripe domains reorientation in ferromagnetic films with perpendicular magnetic anisotropy

International audienceFerromagnetic thin films with moderate perpendicular magnetic anisotropy (PMA) are known to support weak stripe domains provided film thickness exceeds a critical value. In this work, we performed both an experimental and theoretical investigation of a peculiar phenomenon shown by weak stripe domains: namely, the stripe domains reorientation when a dc magnetic field is applied in the film plane along the direction perpendicular to the stripes axis. We focus on bct α ′-Fe 8 N 1−x thin films obtained by N 2 irradiation of α-Fe films epitaxially grown on ZnSe/ GaAs(001). By using different ion implantation and heat treatment conditions, we show that it is possible to tune the PMA values. Magnetic force microscopy and vibrating sample magnetometer measurements prove the existence of weak stripe domains at remanence, and of a threshold field for the reorientation of the stripes axis in a transversal field. Using a one-dimensional model of the magnetic stripe domains, where the essential parameter is the maximum canting angle of the stripe magnetization out of the film plane, the various contributions to the magnetic energy can be separately calculated. A linear increase of the reorientation threshold field Stripe domains reorientation in ferromagnetic films with PMA 2 on the PMA is obtained, in qualitative agreement with experimental data in our Fe-N films, as well as in other thin films with weak stripe domains. Finally, we find that also the rotatable anisotropy field linearly increases as a function of the PMA magnitude

In-plane rotation of magnetic stripe domains in Fe1-xGax thin films

International audienceIn this work we report the appearence of a large perpendicular magnetic anisotropy (PMA) in Fe1−xGax thin films grown onto ZnSe/GaAs(100). This arising anisotropy is related to the tetragonal metastable phase in as-grown samples recently reported [M. Eddrief {\it et al.}, Phys. Rev. B {\bf 84}, 161410 (2011)]. By means of ferromagnetic resonance studies we measured PMA values up to ∼ 5×105 J/m3. PMA vanishes when the cubic structure is recovered upon annealing at 300∘C. Despite the important values of the magnetoelastic constants measured via the cantilever method, the consequent magnetoelastic contribution to PMA is not enough to explain the observed anisotropy values in the distorted state. {\it Ab initio} calculations show that the chemical ordering plays a crucial role in the appearance of PMA. Through a phenomenological model we are able to explain that an excess of next nearest neighbour Ga pairs (B2-like ordering) along the perpendicular direction arises as the source of PMA in Fe1−xGax thin films