Uniaxial magnetic anisotropy in nanostructured Co/Cu(001): from surface ripples to nanowires

Numerical study of structural and magnetic properties of thin films obliquely deposited on rippled substrates

Structural modulation in thin films plays a substantial role in the formation of their magnetic properties. By producing topographic patterns in thin films, it is possible to engineer their magnetic response. Here, we report on the numerical study of the relationship between structural and static magnetic properties of thin films obliquely deposited on substrates with the sinusoidal surface. 3D Monte Carlo film growth simulations show that, under certain deposition conditions, an inhomogeneous columnar morphology can form in the films caused by the shadowing effect and the rippled substrate. Calculations of the demagnetizing tensors for these films demonstrate that their columnar structure is the source of the shape-induced uniaxial magnetic anisotropy that varies nonmonotonically with the deposition angle. Micromagnetic simulations of the generated films confirm the uniaxial character of the shape-induced anisotropy, and also show that magnetization reversal occurs via an incoherent rotation of magnetic moments.

Structural, electronic and magnetic properties of MxPt1-X, (M= Co, Ni and V) binary alloys

The structural, optical and magnetic properties of ordered M_xPt_1-x (M = Co, Ni and V) binary alloys have been investigated using Vienna ab initio Simulation Package (VASP) within the framework of Density Functional Theory (DFT) and the Generalized Gradient Approximation (GGA). Ab initio calculations have been performed to obtain the most stable structure for each of the three binary systems. In addition, the optical and electrical properties such as electronic band structure, density of states and partial density of states of M_xPt_1-x binary alloys have been investigated. Specifically, total energy minimization has been performed to calculate the equilibrium in-plane, a _o , out-of-plane, c _o , and volume, V _o , structural lattice parameters of M_xPt_1-x binary alloys. We found that a _o , c _o and V _o for CoPt, NiPt and VPt₃ equal to (a _o = 3.806 A, c _o = 3.707 A and V _o = 53.7 A³) (a _o = 3.84 A, c _o = 3.62 A and V _o = 53.64 A³) and (a _o = 3.88 A, c _o = 7.88 A and V _o = 118.71 A³) respectively. Furthermore, the magneto-crystalline anisotropy energies (MAE) have been calculated to get a deeper insight into magnetic characteristics of the M_xPt_{1- x} binary alloys. We found that MAE values for CoPt, NiPt and VPt₃ binaries are equal to 1.60, 0.231 and 0.0116 meV/unit cell respectively. These MAE values correspond to magneto-crystalline anisotropy constant K values equal to 4.8 ×10⁷, 6.9 ×10⁶ and 1.46 × 10⁵ erg/cm³. The obtained results reveal that CoPt and NiPt binary systems exhibit attractive optical and magnetic properties, which make both systems potential candidates for magneto-optical and optical-electronic devices. Our results are in good agreement with the previous experimental and theoretical findings.

Form and magnetic birefringence in undulated Permalloy/PET films

We report the measurement of form and magnetic birefringence in Permalloy (Ni80Fe20) films grown on rippled Poly(Ethylene Terephthalate), PET, substrates. Prior to Permalloy deposition, Laser Induced Periodic Surface Structures (LIPSS) were generated on the polymeric substrate by a nanosecond laser beam, developing an ordered rippled nanostructure. Due to their high transparency factor, we could investigate the behavior of linear polarized light transmitting at normal incidence on Permalloy/PET sample. The results show the existence of an optical axis parallel to the ripples direction, which yields an strong form birefringence effect arising from the laser patterning. Concerning the Permalloy thin film, the study of its in-plane magnetization was carried out measuring the Voigt magnetooptical effect. The obtained data in our samples reveal the appearance of two different mechanisms to reverse the magnetization, as the external magnetic field is parallel or perpendicular to the ripples direction. Accordingly, the transmitted light shows a magnetic birefringence depending on the relative orientation between the ripple direction, i.e. the optical axis of the LIPSS, and the in-plane magnetization of the Permalloy film.

Self-organized ultrathin FePt nanowires produced by glancing-angle ion-beam codeposition on rippled alumina surfaces

Ultradense macroscopic arrays of ferromagnetic alloy nanowires exhibit unique properties that make them attractive both for basic physics studies and for prospective nanodevice applications in various areas. We report here on the production of self-organized equiatomic FePt nanowires produced by glancing-angle ion-beam codeposition on alumina nanoripple patterns at room temperature and subsequent annealing at 600 °C. This study demonstrates that periodically aligned FePt nanowires with tunable size (∼10-20 nm width and ∼0.5-10 nm height) can be successfully grown as a consequence of shadowing effects and low mobility of Fe and Pt on the rippled alumina surface. Moreover, the structure and magnetic properties of the FePt nanowires, which undergo a phase transition from a disordered A1 (soft) structure to a partially ordered L10 (hard) structure, can be modified upon annealing. We show that this behavior can be further exploited to change the effective uniaxial anisotropy of the system, which is determined by a strong interplay between the shape and magnetocrystalline anisotropies of the nanowires.

Reentrant surface anisotropy in the antiferromagnetic/ferromagnetic bilayer Mn/Co/Cu(001)

We investigated the magnetic anisotropy energy of monatomic surface-step atoms in antiferromagnetic/ferromagnetic (AF/FM) epitaxial Mn/Co bilayers grown on vicinal Cu(001) surfaces. The step-induced anisotropy of the Co/Cu(001) films was quenched upon submonolayer Mn deposition, but a reentrant uniaxial surface anisotropy was observed for Mn thickness (tMn) between 1 and 2 monolayers, which disappears for Mn thickness above 2 monolayers. In the Mn/Co/Cu(001) system, Mn films undergo a tMn-dependent transition from FM to AF in the 1-2 Mn monolayer thickness range, which entails the coexistence of FM and AF Mn phases in the film. The observation of a sizeable uniaxial anisotropy exclusively in the Mn-thickness range of coexistence of the FM and AF phases points out the crucial role of the boundaries between FM and AF regions within the Mn film. A symmetry-breaking mechanism of a magnetic type, rather than a purely geometric one, is therefore proposed as the origin of the reentrant anisotropy.

Rotatable magnetic anisotropy in Si/SiO2/(Co2Fe)(x)Ge(1-x) Heusler alloy films

Polycrystalline (Co2Fe)(x)Ge(1-x) Heusler alloy films are fabricated by sputtering on amorphous substrates and shown to possess three types of magnetic anisotropy. The nearly stoichiometric composition of x = 50 m.f.% shows a rectangular hysteresis loop and isotropic coercive and ferromagnetic resonance fields when the film is field-magnetized along any in-plane direction, thus predominantly possessing rotatable in-plane magnetic anisotropy. Higher-x compositions show evidence of two- and fourfold in-plane anisotropy superposed on the rotatable one. A qualitative model of the observed anisotropic magnetic properties is proposed. The model explains the rotatable anisotropy by taking into account dry friction for the in-plane rotation of the magnetization direction in a fine-grained polycrystalline film with the magnetic grain size smaller than the correlation length of the inter-grain exchange interaction. The observed two- and fourfold magnetic anisotropy contributions are attributed to partial texturing of the fine-grained films, even though the films are grown on amorphous SiO2 substrates. These results should be valuable for understanding and controlling the magnetic behaviour of highly spin-polarized Heusler alloy films used in various magnetic nanodevices.

Transition from ripples to faceted structures under low-energy argon ion bombardment of silicon: understanding the role of shadowing and sputtering

In this study, we have investigated temporal evolution of silicon surface topography under 500-eV argon ion bombardment for two angles of incidence, namely 70° and 72.5°. For both angles, parallel-mode ripples are observed at low fluences (up to 2 × 1017 ions cm-2) which undergo a transition to faceted structures at a higher fluence of 5 × 1017 ions cm-2. Facet coarsening takes place at further higher fluences. This transition from ripples to faceted structures is attributed to the shadowing effect due to a height difference between peaks and valleys of the ripples. The observed facet coarsening is attributed to a mechanism based on reflection of primary ions from the facets. In addition, the role of sputtering is investigated (for both the angles) by computing the fractional change in sputtering yield and the evolution of surface roughness. PACS: 81.05.Cy, 81.16.Rf, 61.80.Jh, 87.64.Dz.

Nanostructures on surfaces by ion irradiation

The bombardment of the surface of a solid by energetic ions often results in pronounced surface modifications, leading to characteristic topographical features. In this report, the development of specific morphological nanostructures on surfaces under ion irradiation is discussed. The following aspects will be emphasized: (i) on an atomic scale, the generation of isolated defects such as adatoms and surface vacancies due to single-ion impacts, and their possible clustering; (ii) the transition from such individual defects toward extended morphological features on the surface and suitable scaling relations to describe them; (iii) the formation of highly periodic structures with nanoscale dimensions such as nanodots and “ripple”-like features, and the dependence of these nanostructcures on various ion-irradiation parameters and substrate materials; (iv) the theoretical concepts proposed to model the observed patterns which are thought to be related to (and caused by) the interplay between ion erosion and diffusion of adatoms (vacancies), thus inducing a surface reorganization.

One-dimensional pattern of Au nanodots by ion-beam sputtering: formation and mechanism

Highly ordered one-dimensional arrays of nanodots, or nanobeads, are fabricated by forming nanoripples and nanodots in sequence, entirely by ion-beam sputtering (IBS) of Au(001). This demonstrates the capability of IBS for the fabrication of sophisticated nanostructures via hierarchical self-assembly. The intricate nanobead pattern ideally serves to identify the governing mechanisms for the pattern formation: nonlinear effects, especially local redeposition and surface-confined transport, are essential both for the formation and the preservation of the one-dimensional order of the nanobead pattern.

Applications of metal surfaces nanostructured by ion beam sputtering

We review results relative to the formation of regular nanoscale patterns on metal substrates exposed to defocused ion beam irradiation. Particular emphasis is placed on work which demonstrates the possibility of controllably modifying chemico-physical properties of the material by tailoring the nanoscale morphology during IBS patterning. Starting from the well-established results found on single-crystal model systems, we show how the controlled modification of the atomic step termination can deeply affect chemical reactivity or magnetic anisotropy. We then look in greater detail at the more recent attempts focused on the extension of IBS patterning on supported polycrystalline metal films, a promising class of systems in view of potential applications. A modification of the functional properties of metal films can also be obtained by forcing a shape anisotropy of the nanostructures. The modification of the optical response of polycrystalline metal nanowires supported on anisotropic templates produced by IBS provides a clear example of this.

Ion beam sputtering induced nanostructuring of polycrystalline metal films

The development of fine scale nanostructures in polycrystalline metal films by off-normal ion beam sputtering (IBS) follows similar mechanisms to those in random targets, i.e. the pattern results from the interplay of curvature-dependent-roughening and various smoothing processes. By grazing angle IBS of the deposited metal films it is possible to fabricate metallic nanoripples, nanowires, and nanorods onto semiconductor or insulator substrates without using a template. It has been found that the rms roughness of the as-deposited film is substantially reduced under ion bombardment before the development of nanoscale patterns. The structural anisotropy of the sputtered morphology can have a great influence on the local physical properties of the underlying material. In this paper, we shall review the experimental results on the formation and characteristics of the IBS ripples on polycrystalline metal films prepared by the physical vapor deposition (PVD) technique.

Ripple formation on silicon by medium energy ion bombardment

The formation of a self-organized nanoscale ripple pattern after off-normally incident ion bombardment on the surface of amorphous materials, or on semiconductors like silicon that are easily amorphized by ion bombardment, has attracted much attention in recent years from the point of view of both theory and applications. As the energy of the impinging ions increases from low to medium, i.e. several hundred eV to a few tens of keV, the ratio of amplitude to wavelength of the generated ripple pattern becomes so large that inter-peak shadowing of the incident ion flux takes place. Morphologically, the sinusoidal surface profile starts to become distorted after prolonged ion bombardment under such conditions. Structural and compositional modifications of the ripple morphology generated under shadowing conditions include the formation of a thicker amorphous layer with high incorporation of argon atoms in the form of nanometer sized bubbles around the middle part of the front slope of the ripple facing the ion beam, as compared to the rear slope. The present paper reviews recent developments in the experimental study of morphological, structural and compositional aspects of ripple patterns generated on a silicon surface after medium keV (30-120 keV) argon bombardment mainly at an angle of ion incidence of 60°.

Self-organised synthesis of Rh nanostructures with tunable chemical reactivity

Nonequilibrium periodic nanostructures such as nanoscale ripples, mounds and rhomboidal pyramids formed on Rh(110) are particularly interesting as candidate model systems with enhanced catalytic reactivity, since they are endowed with steep facets running along nonequilibrium low-symmetry directions, exposing a high density of undercoordinated atoms. In this review we report on the formation of these novel nanostructured surfaces, a kinetic process which can be controlled by changing parameters such as temperature, sputtering ion flux and energy. The role of surface morphology with respect to chemical reactivity is investigated by analysing the carbon monoxide dissociation probability on the different nanostructured surfaces.

Robust nanopatterning by laser-induced dewetting of metal nanofilms

We have observed nanopattern formation with robust and controllable spatial ordering by laser-induced dewetting in nanoscopic metal films. Pattern evolution in Co film of thickness 1≤h≤8 nm on SiO(2) was achieved under multiple pulse irradiation using a 9 ns pulse laser. Dewetting leads to the formation of cellular patterns which evolve into polygons that eventually break up into nanoparticles with unimodal size distribution and short range ordering in nearest neighbour spacing R. Spatial ordering was attributed to a hydrodynamic thin film instability and resulted in a predictable variation of R and particle diameter D with h. The length scales R and D were found to be independent of the laser energy. These results suggest that spatially ordered metal nanoparticles can be robustly assembled by laser-induced dewetting.

Carbon monoxide dissociation on Rh nanopyramids

CO dissociation on rhomboidal faceted nanopyramids, produced on Rh(110) by fine-tuning of ion irradiation conditions, has been studied by high resolution core-level spectroscopy. We find that this morphology presents a large efficiency towards CO dissociation, a process which is inhibited on flat (110) terraces. We also measured the reactivity of nanostructures bound by different artificial step distributions identifying the sites responsible for the molecular bond disruption in the undercoordinated (n=6) edges running along the [11[over ]2] equivalent directions, with CO sitting in on-top configuration.

Isolating the step contribution to the uniaxial magnetic anisotropy in nanostructured Fe/Ag(001) films

We have investigated the possibility of isolating the step-induced in-plane uniaxial magnetic anisotropy in Fe/Ag(001) films on which nanoscale surface ripples were fabricated by the ion sculpting technique. For rippled Fe films deposited on flat Ag(001), the steps created along the ripple sidewalls are shown to be the only source of uniaxial anisotropy. Ion sculpting of ultrathin magnetic films allows one to selectively study the step-induced anisotropy and to investigate the correlation between local atomic environment and magnetic properties.

Self-organized formation of rhomboidal nanopyramids on fcc(110) metal surfaces

We report on the far from equilibrium self-organized morphologies obtained after Xe ion irradiation of the Rh(110) and Cu(110) surfaces. Here we experimentally identify by means of high resolution LEED a novel interfacial state characterized by a rhomboidal pyramid islanding with majority steps oriented along nonequilibrium low-symmetry directions. The formation of the novel rhomboidal pyramid state and the transition to the well-known rippled phases results from a delicate interplay of kinetic processes which are controlled by acting on temperature, ion flux, and impact energy.