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Applied Surface Science (v.258, #9)

Editorial Board (pp. ii).

Preface by Tapobrata Som; Shikha Varma (pp. 4101-4102).

Ion beam nanoscale fabrication and lithography—A review by J.E.E. Baglin (pp. 4103-4111).

► Ion beam lithography and patterning yield few-nanometer features with no proximity effect. ► Ion beam litho meets criteria for commercial adoption, e.g. CMOS beyond 22nm node. ► Feature edge roughness due to shot noise is overcome by guided self-assembly. ► Focused (∼1nm) beam patterns surfaces and seeds growth of 7nm diameter pillars. ► Throughput demands may be met using self-assembly and implant defect engineering.This review discusses some of the issues that will govern future commercial adoption of ion beam nanoscale fabrication and lithography, with special reference to some major fields of application. Selected recent research advances are highlighted, as they indicate new experimental successes, new insights into complex ion interaction mechanisms, and a fast evolving variety of advanced instrumentation and fabrication resources. It is evident that robust fabrication involving few-nanometer features will be enabled by strategic applications of ion beams, especially if they can be coupled with directed self-assembly.

Keywords: Nanofabrication; ITRS Roadmap; Self-assembly; Lithography; Focused-ion-beam; Ion-implantation

Multiple scattering causes the low energy–low angle constant wavelength topographical instability of argon ion bombarded silicon surfaces by Charbel S. Madi; Michael J. Aziz (pp. 4112-4115).

► Phase diagram of Ar⁺ bombarded Si surfaces in the linear regime of surface dynamics. ► Holes and perpendicular mode ripples are caused by multiple scattering events. ► Only ultra-smooth stable silicon surfaces are for incidence angles<50° from normal. ► Phase diagram characterized by diverging wavelength phase transition.We show that holes and perpendicular mode ripples that are generated at low argon ion beam energies and incidence angles on room temperature silicon targets (C.S. Madi, et al., Phys. Rev. Lett. 101 (2008) 246102; C.S. Madi, et al., J. Phys. Condens. Matter 21 (2009) 224010) are caused by multiple scattering events from the impingement of the primary ion beam on adjacent silicon shields. We show that in a geometry that minimizes these multiple scattering events, only ultra-smooth stable silicon surfaces are for incidence angles <50° from normal. We present a revised topographical phase diagram of 250–1000eV Ar⁺ ion bombarded silicon surfaces in the linear regime of surface dynamics in the absence of secondary scattering effects. It is characterized only by a diverging wavelength phase transition from parallel mode ripples to a flat stable surface as the incidence angle falls below about 50° from normal incidence, and a crossover to perpendicular mode ripples as the incidence angle crosses above about 80°.

Keywords: Sputtering; Nanoscale morphology evolution

In-situ study of magnetic thin films on nanorippled Si (100) substrates by Sarathlal K.V.; Dileep Kumar; V. Ganesan; Ajay Gupta (pp. 4116-4121).

► In-situ study of the magnetic properties of ultrathin Co films deposited on nanorippled Si (100) substrate. ► Co films on nanorippled surface exhibit a strong uniaxial magnetic anisotropy. ► Magnetic anisotropy decreases with thickness of the Co film. ► Co film on nanorippled Si surface exhibits a magnetic dead layer of 0.8nm.Magnetic properties of ultrathin Co film on nanorippled Si substrate have been studied. Low energy ion beam erosion of Si (100) substrate has been used to produce nanoripples with the wavelength and amplitude of modulations being 32nm and 1.2nm, respectively. In-situ magneto-optical Kerr effect measurements have been used to study the evolution of the hysteresis loop with film thickness. The results are compared with those of Co film deposited on polished Si substrate. Co films on nanorippled surface exhibit a strong uniaxial magnetic anisotropy with its easy axis along a direction normal to the ripple wave vector. The magnetic anisotropy exhibits a monotonous decrease with increasing film thickness. Co film on nanorippled Si surface exhibits a magnetic dead layer of 0.8nm because of possible intermixing of Co with Si to form non-magnetic silicide.

Keywords: Nanoripples; Co thin film; Magnetic anisotropy; Magnetic dead layer

Ion irradiation induced nano pattern formation on TiO₂ single crystal by Subrata Majumder; D. Paramanik; V. Solanki; I. Mishra; D.K. Avasthi; D. Kanjilal; Shikha Varma (pp. 4122-4124).

► Nanodots fabrication on rutile TiO₂ (110) single crystal by Ar ion sputtering. ► Sputtering reduces inter planar separation along [110] direction of TiO₂. ► Compressive stress generation in the ion sputtered TiO₂ lattice.Nanodots have been fabricated on rutile TiO₂(110) single crystals using Ar ion beam. Ion beam sputtering creates oxygen vacancies, leading to a 45nm thick Ti rich layer, on the surface. Post-sputtering, rutile TiO₂ also exhibits a decrease in the inter planar separation along [110] direction. Additionally, blueshift in the E_g Raman mode, representing the vibrations of oxygen atoms along c-axis, is also observed. Both these results suggest the development of a compressive stress along c-axis upon sputtering. Enhancement in intensity of A_1g Raman mode also indicates modification in TiO vibrational influence.

Regularly spaced conducting or magnetic stripe formation in nano ripples by P. Karmakar (pp. 4125-4128).

► Fabrication of spatially resolved nanostructures by low energy ion beams. ► Magnetic–nonmagnetic stripe. ► Coexistence of nano scale nonconducting–semiconducting zones on Si. ► Ion induced Pt wire.Conducting–insulating and magnetic–nonmagnetic segments are produced using pre-fabricated ion induced nano ripples. Ripple structures of Si are formed by oblique angle keV energy oxygen or argon ion bombardment. Further oxygen ion implantation leads to insulating–semiconducting zone formation whereas Fe ion implantation helps to produce nonmagnetic–magnetic stripes. Conducting Atomic Force Microscopy, Magnetic Force Microscopy and Energy dispersive X-ray spectroscopy shows the coexistence of semiconducting–insulating and magnetic–nonmagnetic zones. Grazing angle keV ion bombardment fabricates parallel metal ripples from polycrystalline thin films. C-AFM measurements show that the crests of the ripples are regularly spaced unidirectional conducting wires.

Keywords: Nano ripple; C-AFM; Sputtering

Pit formation on the Ge (100) surfaces by normal incident Si⁻ ion implantation by S.A. Mollick; S. Karmakar; A. Metya; D. Ghose (pp. 4129-4134).

► Micron size sputter-pits of different shapes are formed during energetic Si ion implantation in Ge substrate. ► The growth of the morphological structures is explained by the local gradient-dependent erosion. ► It is observed that Si concentration is less at the pit bases compared to the outside surface. ► The difference is attributed to the lower mobility of Si in Ge.We have observed micron size pit formation on Ge surface due to bombardment of 26keV Si⁻ ion at normal incidence in the fluence range 1×10¹⁸ and 7×10¹⁸ions/cm². Scanning electron microscopy (SEM) and atomic force microscopy (AFM) are used to follow the evolution of the surface morphology. The pits are of various shapes, e.g., crescent-shaped, kidney-like or circular structures. The two-field continuum model developed for small slope approximations can describe the pit formation and growth at the very beginning of ion bombardment. The growth of the pits at late times (high fluence) can be explained by the gradient dependent erosion mechanisms due to primary ion beam as well by secondary flux of particles originating from steep slopes. Energy dispersive X-ray analysis attached to SEM is employed to obtain the chemical information of the pitted surface. The depletion of Si at the bottom of the pits is explained due to lower diffusivity of Si in Ge.

Keywords: Si ion implantation; Ge; Morphology; Pit formation; Simulation

Evolution of ripple morphology on Si(100) by 60-keV argon ions by Sandeep Kumar Garg; V. Venugopal; T. Basu; O.P. Sinha; S. Rath; D. Kanjilal; T. Som (pp. 4135-4138).

► This paper deals with 60keV argon ion induced evolution of ripple morphology on silicon surface at room temperature. ► A large angular window (0–75°), in terms of ion incident angle, was chosen for this study. ► Ripple formation in the angular window of 45°–75°. Ripple wavelength decreases with increasing incident angle. ► The results are attributed to the viscous flow mechanism.In this paper, we report on evolution of ripple morphology on Si(100) surface due to 60keV Ar⁺-ion implantation to the fluence of 2×10¹⁸ionscm⁻² and over a large incident angular window of 0–75°. Room temperature implantations were carried out by using a uniform current density of 20μAcm⁻². Atomic force microscopic (AFM) studies indicate that ripple morphology starts to appear at an incident angle 45° and becomes more prominent at higher incident angles. AFM studies also reveal that while the ripple wavelength decreases with increasing angle of incidence, the amplitude increases with the same. We also observe a systematic variation in the surface roughness with incident angle. Micro-Raman studies show that the sub-surface silicon layer becomes amorphous whose depth keeps reducing at higher incident angles. The results are attributed to viscous flow mechanism.

Keywords: Ion sputtering; Self-organized nanostructures; Ripples

Formation of self-organized nanostructures on semi-insulating InP by 100keV Ar⁺-ion irradiation by Jyoti Ranjan Mohanty; Tanmoy Basu; D. Kanjilal; T. Som (pp. 4139-4143).

► Nanostructure formation on InP surface at room temperature without substrate rotation by 100keV argon ion. ► A large angular window (0–75°), in terms of ion incident angle, was studied. Dot formation in the angular window of 30–75°. ► RMS surface roughness, the dot size, and the dot height decreases with increasing angle of incidence. ► But dot density increases with increasing angle of incidence (for a particular fluence).We report on formation of self-organized nanodots on semi-insulating InP surface due to bombardment by 100keV Ar⁺ ions incident both normally and off-normally (0°, 30°, 60°, and 75° with respect to the surface normal) for three fluences viz. 1×10¹⁷, 5×10¹⁷, and 1×10¹⁸ionscm⁻² at room temperature and without any substrate rotation. The novelty of our work is that we have studied pattern formation, at this energy range, in a systematic manner as a function of incident angle of ions. It is seen that average dot-size, -height, and inter-dot distance decrease with increasing angle of incidence (for a given fluence), while dot density increases. This trend is followed for all the fluences under consideration. RMS surface roughness shows a decreasing trend with increasing angle of incidence. This is indicative of surface smoothening due to enhanced ion induced surface diffusion at higher incident angles. Dot formation is attributed to preferential sputtering. We do not observe any transition from dot to ripple pattern unlike low energy experiments performed on semi-insulating InP substrates. Although evolution of dot patterns on both n- and p-type InP surfaces, at intermediate energies, are known there are subtle differences in terms of variations in dot-size and -density for semi-insulating InP substrates.

Keywords: InP; Self-organized nanostructures; Preferential sputtering; Mounds

Nanostructures on GaAs surfaces due to 60keV Ar⁺-ion beam sputtering by V. Venugopal; Sandeep Kumar Garg; Tanmoy Basu; Om Prakash Sinha; D. Kanjilal; S.R. Bhattacharyya; T. Som (pp. 4144-4147).

► Effect of angle of ion incidence and ion fluence on the surface nanostructures formed due to 60keV Ar⁺-ion beam sputtering of p-type GaAs(100) is examined using an ion current density of 10–12μA/cm². ► As a function of angle of incidence the surface topography changes from ‘dots+holes’ (0°)→‘smooth’ (30°)→‘dots’ (60°). For 60° off-normal ion incidences ripple formation is suppressed. ► Effectiveness and limitations of existing theories of ion induced pattern formation to explain the observed nanostructures are discussed.The effect of 60keV Ar⁺-ion beam sputtering on the surface topography of p-type GaAs(100) was investigated by varying angle of incidence of the ion (0–60°) with respect to substrate normal and the ion fluence (2×10¹⁷–3×10¹⁸ions/cm²) at an ion flux of 3.75×10¹³ions/cm²-s. For normal incidence and at a fluence of 2×10¹⁷ions/cm², holes and islands are observed with the former having an average size and density of 31nm and 4.9×10⁹holes/cm², respectively. For 30° and 45° off-normal incidence, in general, a smooth surface appears which is unaffected by increase of fluence. At 60° off-normal incidence dots are observed while for the highest fluence of 3×10¹⁸ions/cm² early stage of ripple formation along with dots is observed with amplitude of 4nm. The applicability and limitations of the existing theories of ion induced pattern formation to account for the observed surface topographies are discussed.

Keywords: Ion beam sputtering; GaAs(1; 0; 0); Holes; Dots; Ripples; Smooth surface

Formation of nanodots on GaAs by 50keV Ar⁺ ion irradiation by Tanuj Kumar; S.A. Khan; U.B. Singh; S. Verma; D. Kanjilal (pp. 4148-4151).

► Creation of nanodots by energetic ion. ► Nanodots are formed on GaAs surface by 50keV Ar⁺ ion irradiation. ► Characterization using atomic force microscopy and micro-Raman spectroscopy is reported. ► Average dot size, density of dots and rms roughness varied are analyzed with fluence. ► Strain production and relaxation occurred with irradiation.Nanopatterning of semi-insulating GaAs (100) by 50keV Ar⁺ ion irradiation at an angle of 50° with respect to surface normal is investigated systematically. The samples were analyzed using atomic force microscopy and Raman spectroscopy as a function of fluence. Irradiation with fluence of 1×10¹⁷ions/cm² results in the formation of nano-sized dots on the surface of GaAs which ripens at a fluence of 3×10¹⁷ions/cm² and shows fragmentations at a fluence of 7×10¹⁷ions/cm². The average diameter of dots varies between 20 and 35nm in the fluence range. It is also observed that the density of dots first decreases with increase in fluence and later increases due to fragmentation of dots. Non-linear behavior of surface roughness is also observed with increase in fluence. Raman spectroscopy shows that the ion irradiation leads to amorphization of the irradiated surfaces in this range of fluence and relaxations and smoothing of the surface via decreased strain.

Keywords: Ripples; Sputtering; Atomic force microscopy (AFM)

Growth process of GaAs ripples as a function of incident Ar-ion dose by D. Datta; Shyamal Mondal; S.R. Bhattacharyya (pp. 4152-4155).

► Ripple morphology on GaAs is a promising topic of research. ► We report here the physical mechanism of ripple formation on GaAs sputtered by 60keV Ar-ions at an angle of 60°. ► The results are described by the existing formalism of ripple pattern formation on surfaces.We report periodic ripple formation on GaAs sputtered by 60keV Ar ions at an angle of 60° over a large range of ion doses from 1×10¹⁷ to 1×10¹⁹ions/cm² under Atomic Force Microscopy (AFM) study. Initially in the dose range between 1×10¹⁷ and 4×10¹⁷ions/cm², only very small roughness is formed on the surface and from the dose of 5×10¹⁷ions/cm², the ripples start to form and attain a well-defined structure at a dose around 9×10¹⁷ions/cm², remain stable and then from a dose of 4×10¹⁸ions/cm², the ripple structures become very rough, periodicity breaks down and step-like features become prominent all over the surface. Parameters like rms roughness, ripple wavelength, amplitude etc. are measured from the AFM image analysis. The results are discussed with the help existing formalism with the understanding of preferential sputtering of one of the components of GaAs.

Keywords: PACS; 68.35.Bs; 79.20.Rf; 64.60.HtGallium Arsenide; Ripple morphology; Atomic Force Microscopy; Sputter-induced nanopattern

MeV Au²⁺ ions induced surface patterning in silica by P. Santhana Raman; K.G.M. Nair; M. Kamruddin; A.K. Tyagi; A. Rath; P.V. Satyam; B.K. Panigrahi; V. Ravichandran (pp. 4156-4160).

► Irradiation of 1.8MeV Au²⁺ ions at normal incidence on silica samples under room temperature (doses: from 5×10¹⁶ions/cm² to 2×10¹⁷ions/cm²). ► Formation of periodic surface pattern with a wavelength of 1.35μm only on samples irradiated to a dose of 1×10¹⁷ions/cm² and more. ► Surface segregation of irradiated gold occurs in samples with dose more than 1×10¹⁷ions/cm². ► Possible role of surface stresses, due to incorporation of metal atoms, in the formation of the observed periodic surface pattern.This paper reports the formation of self-organised surface morphological features on silica irradiated with MeV energy gold ions. Amorphous silica substrates were irradiated with 1.8MeV gold ions at normal incidence at room temperature to various doses in the range of 5×10¹⁶ions/cm² to 2×10¹⁷ions/cm². The formation of a periodic surface pattern with a wavelength of 1.35μm was observed at an irradiation dose of 1×10¹⁷ions/cm². The observation of surface segregation of gold at around the same dose suggests possible role of surface stresses caused by the incorporation of metal atoms in the formation of the observed periodic surface morphology.

Keywords: Surface pattern; Ion irradiation; MeV ions; Silica

A surface layer of altered composition can play a key role in nanoscale pattern formation induced by ion bombardment by R. Mark Bradley; Patrick D. Shipman (pp. 4161-4170).

► Hexagonal arrays of nanodots can form when a binary compound is ion bombarded. ► We develop a theory that explains the genesis of these arrays. ► The species with the higher sputter yield is concentrated at the peaks of the nanodots. ► For a range of the parameters, the nanodot arrays coarsen as time passes.We review and augment our recent work which demonstrates that a surface layer of altered composition can have a crucial effect on pattern formation induced by ion bombardment of a solid surface. First, we discuss a theory that explains the genesis of the strikingly regular hexagonal arrays of nanodots that can form when the binary material GaSb is bombarded at normal incidence. In the theory, the coupling between a surface layer of altered stoichiometry and the topography of the surface is the key to the observed pattern formation. For a certain range of the parameters, we find that nanodot arrays with strong short-range hexagonal order emerge spontaneously. Well-ordered arrays of nanoholes develop in another range of the parameters. A closely related theory governs the dynamics of the surface of an elemental material that is seeded with impurities during ion bombardment. If the incident impurity flux exceeds a critical value, the surface layer in which impurities are present destabilizes the surface, leading to the formation of a disordered array of nanodots.

Keywords: Ion bombardment; Pattern formation; Nanotechnology; Binary materials; Impurities

Hydrodynamic approach to surface pattern formation by ion beams by Mario Castro; Rodolfo Cuerno (pp. 4171-4178).

► Unified physical framework for ion induced surface patterns. ► Non-phenomenological approach to continuum models. ► Emphasis on relevance of stress and viscous flow against traditional explanations based on sputtering and surface diffusion. ► Detailed calculations allow to predict morphological transitions and ripple velocities.On the proper timescale, amorphous solids can flow. Solid flow can be observed macroscopically in glaciers or lead pipes, but it can also be artificially enhanced by creating defects. Ion Beam Sputtering (IBS) is a technique in which ions with energies in the 0.1–10keV range impact against a solid target inducing defect creation and dynamics, and eroding its surface leading to formation of ordered nanostructures. Despite its technological interest, a basic understanding of nanopattern formation processes occurring under IBS of amorphizable targets has not been clearly established, recent experiments on Si having largely questioned knowledge accumulated during the last two decades. A number of interfacial equations have been proposed in the past to describe these phenomena, typically by adding together different contributions coming from surface diffusion, ion sputtering or mass redistribution, etc. in a non-systematic way. Here, we exploit the general idea of solids flowing due to ion impacts in order to establish a general framework into which different mechanisms (such as viscous flow, stress, diffusion, or sputtering) can be incorporated, under generic physical conservation laws. As opposed to formulating phenomenological interfacial equations, this approach allows to assess systematically the relevance and interplay of different physical mechanisms influencing surface pattern formation by IBS.

Keywords: Ion Beam Sputtering; Hydrodynamics; Solid flow; Pattern formation; Stability; Viscous flow; Stress; Erosion

Non-local linear stability of ion beam eroded surfaces by S.N. More; R. Kree (pp. 4179-4185).

► Extension of Bradley-Harper theory (BHT) avoiding gradient expansions. ► Extension of BHT including ion-induced surface mass redistribution. ► More complex scenarios of pattern formation than in BHT, including both type I and type II instabilities. ► Ion-beam induced pattern formation does not need erosion. ► Bifurcation scenarios depend sensitively upon statistical shapes of collision cascades.Continuum theories of spontaneous pattern formation at solid surfaces during ion irradiation exist in many variants, but all of them are based upon low order gradient expansions of an underlying non-local theory and are formulated as partial differential equations. Here we reconsider the non-local theory based upon a simple Gaussian erosive crater function of Sigmund's theory of sputtering, which is also a basic ingredient of most of the existing continuum theories. We keep the full non-locality of the crater function in a linear stability analysis of a flat surface. Without gradient expansion the evolution of the height profile is governed by an integral equation. We show that low order gradient expansions may be misleading and that the bifurcation scenarios become significantly more complex, if the non-locality is taken into account. In a second step, we extend our analysis and include mass redistribution due to ion-induced drift currents of collision cascade atoms. The model is based upon results from kinetic theory and uses a simple phenomenology. Both erosion and mass redistribution share the same non-local features, as they are both caused by the collision cascade. If mass redistribution is the dominant pattern forming mechanism, we show that the resulting bifurcation scenarios may provide explanations for many of the recent, seemingly contradictory experimental results of pattern formation on Si surfaces.

Keywords: Pattern formation; Sputtering; Continuum theory; Ion beam erosion; Bifurcation; Si surface

Ripples and dots generated by lattice gases by Géza Ódor; Bartosz Liedke; Karl-Heinz Heinig; Jeffrey Kelling (pp. 4186-4190).

► Patterns generated by binary lattice gases. ► Efficient simulations of large system. ► Wavelength growth using power spectrum density.We show that the emergence of different surface patterns (ripples, dots) can be well understood by a suitable mapping onto the simplest nonequilibrium lattice gases and cellular automata. Using this efficient approach difficult, unanswered questions of surface growth and its scaling can be studied. The mapping onto binary variables facilitates effective simulations and enables one to consider very large system sizes. We have confirmed that the fundamental Kardar–Parisi–Zhang (KPZ) universality class is stable against a competing roughening diffusion, while a strong smoothing diffusion leads to logarithmic growth, a mean-field type behavior in two dimensions. The model can also describe anisotropic surface diffusion processes effectively. By analyzing the time-dependent structure factor we give numerical estimates for the wavelength coarsening behavior.

Keywords: Ripple; Dot; KPZ; Kuramoto–Sivashinsky; Mullins–Herring; Lattice gas; Coarsening; Scaling; PSD

Fabrication of metallic stamps for injection moulding applications by combining proton beam writing and UV lithography by P. Malar; Zhao Jianhong; J.A. van Kan (pp. 4191-4194).

► Proton beam writing and UV lithography are used to expose AR-P 3250 resist. ► The processing conditions of AR-P 3250 resist have been optimized. ► Smooth and tall resist structures are demonstrated in AR-P 3250. ► These structures have been successfully transferred into high quality Ni moulds. ► Potential application can be found in nanoimprint lithography.In this paper, we present the results of resist evaluation for the fabrication of metallic stamps used in the injection moulding of micro/nano fluidic devices. The resist was evaluated in terms of its suitability to combine direct proton beam writing (PBW) and UV lithography techniques for incorporating few tens of micron and micron- to millimeter sized features respectively. In a first step PBW is used to generate the fine features with smooth sidewall profiles in AR-P 3250, here the resist shows negative behavior. Following PBW, masked i-line UV lithography was used to create larger features to complete the device design, here the AR-P 3250 acts as a positive resist. After developing, the resist was used to generate the final mould through Ni electroplating, resulting in a high quality metallic stamp.

Keywords: Proton beam writing; UV lithography; Ni mould fabrication

Proton beam writing on PMMA and SU-8 films as a tool for development of micro-structures for organic electronics by Mihir Sarkar; Neeraj Shukla; Nobin Banerji; Y.N. Mohapatra (pp. 4195-4198).

► A proton beam writing set-up has been optimized for micro-patterning. ► Micro-channel and grating patterns are fabricated in PMMA, SU-8. ► Possible applications of the patterns are discussed.Proton beam writing is a maskless lithographic technique for the fabrication of 3D micro and nano structures in polymers. The fabricated structures find application in micro fluidics, optics, biosensors, etc. We use proton beam writing for micro-patterning in polymers which will facilitate fabrication of test structures for micro-components of micro-fluidic devices, organic thin film transistors (OTFT) and organic light emitting diodes (OLED). In this paper we report fabrication of varying width micro channels in PMMA and SU-8 films used as positive and negative resists respectively. The patterns were written using 2MeV proton beam focused down to around 1 micron. We have achieved clean periodic micro-channels of width varying from few micrometers to wider ones in both the resists. Being a mask less lithography it provides an efficient way of reducing turnaround time for test structures with several channel widths and patterns being conveniently written at the same development cycle. Possible applications of the patterned structures in OLED/TFT are discussed. Additional structures like checkered board are also fabricated. Optimized fluence for both the resist has been determined.

Keywords: Deep ion beam lithography (DIBL); PMMA; SU-8; Micro-channel; Micro-patterning

Properties of nanopatterned pins generated in a superconductor with FIB by Gorky Shaw; Pabitra Mandal; Biplab Bag; S.S. Banerjee; T. Tamegai; Hermann Suderow (pp. 4199-4202).

► Using Ion-beam Induced Nanopatterning, we have generated regions with artificial pinning centers in superconducting samples. ► By comparing the magnetization response associated with artificial and natural pinning centers we unravel remarkable difference in response associated with the two types of pins. ► In the nanopatterned sample we observe a large drive dependent enhancement in pinning which depends on the field range at which the experiment is performed. ► The pattered regions are found to sustain highly non-uniform field gradients whose behavior just inside and outside the patterned region exhibit very different character.We have used focused ion-beam to nanopattern an array of artificial pinning centers on the surface of a superconductor. By investigation of the bulk as well as local magnetization responses we find that the pinning properties of the artificially generated pins differ significantly from those in an unpatterned superconductor with natural pinning centers. The gradients in the magnetic field distribution inside the nanopatterned region have very different field dependences compared to those outside it. The non-equilibrated nature of the flux distribution due to artificial pins is associated with a magnetization response which is dependent on the rate at which magnetic field is swept during the measurements. The array of artificially generated pins produces large increases in pinning force at large drives and at high fields, which potentially could help in enhancing the current carrying capability of superconductors especially at high fields.

Keywords: Nanoscale; Patterning; Focused ion beam; Superconductivity; Vortex state; Pinning

200MeV Ag¹⁵⁺ ion induced surface modification and transport behaviour in manganite based thin film devices by Ashish Ravalia; Megha Vagadia; P.S. Vachhani; R.J. Choudhary; D.M. Phase; K. Asokan; D.G. Kuberkar (pp. 4203-4206).

► Surface modification. ► Track like defects formation. ► Suppression in resistivity.We report the effect of 200MeV Ag¹⁵⁺ ion irradiation with fluence of ∼5×10¹¹ ions/cm² on the structure and surface morphology of thin film devices of La_0.6Pr_0.2Sr_0.2MnO₃ (named as LPSMO) (p)/SrNb_0.2Ti_0.8O₃ (named as SNTO) (n) manganite grown by Pulsed Laser Deposition (PLD) technique. The n-type manganite layers studied were of thickness 50, 100 and 200nm. After ion irradiation, the structural studies revealed improved crystallinity in the LPSMO/SNTO devices with the thickness of 50 and 200nm. But the 100nm films showed the formation of regular tracks like feature in their surface morphology after irradiation. All the irradiated films show suppression of resistivity at all the temperatures but it was different rate in 100nm LPSMO films which may be attributed to the formation of linear track like defects.

Keywords: SHI irradiation; Surface morphology; Manganite; Devices

Swift heavy ion irradiation induced modifications in magnetic and dielectric properties of Mn–Ca ferrite by S.N. Dolia; P.K. Sharma; M.S. Dhawan; Sudhish Kumar; Arun S. Prasad; A. Samariya; S.P. Pareek; R.K. Singhal; K. Asokan; Y.T. Xing; M. Alzamora; E. Saitovitach (pp. 4207-4211).

► Effects of 200MeV Ag¹⁵⁺ ion irradiation of nanocrystalline Mn–Ca ferrite investigated using utmost techniques. ► Characterization results confirmed formation of cubic spinel structure and spherical morphology consistent with the crystalline diameter. ► After irradiation, the saturation magnetization, coercivity, and blocking temperature enhance appreciably. ► SHI irradiation induced modifications in surface states of the nanoparticles and cationic re-distribution identified as probable reasons. ► Irradiation caused increase in dielectric constant, attributed to surface modifications through slight crystal growth.The effects of 200MeV Ag¹⁵⁺ ion irradiation on structural, magnetic and dielectric properties of nanocrystalline Mn–Ca ferrite have been investigated. The specimens characterized using XRD confirmed the formation of cubic spinel structure. The TEM images revealed the spherical morphology consistent with the crystalline diameter. After irradiation by 200MeV Ag ions, saturation magnetization, coercivity, and blocking temperature were enhanced appreciably which could be attributed to swift heavy ion (SHI) irradiation induced modifications in surface states of the nanoparticles, slight increase in crystallite size and cationic re-distribution. The dielectric measurements revealed that both dielectric constant ( ɛ_r) and the loss tangent (tan δ) disperse normally as a function of frequency of the externally applied electric field. The increase in ɛ_r and tan δ on irradiation could be attributed to the surface modifications through slight crystal growth and hence the availability of sufficient number of Fe²⁺/Mn³⁺ ions particularly at the octahedral site on grain boundaries showing a fair agreement with the magnetization results.

Keywords: SHI irradiation; Nanocrystalline ferrite; Superparamagnetism; Dielectric properties

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Applied Surface Science (v.258, #9)