Ultrafast pulsed laser deposition as a method for the synthesis of innovative magnetic films

Exchange-coupled monocomponent magnetic films constituted of disk-shaped Ni and Fe nanoparticles were produced by ultrafast pulsed laser deposition, in vacuum. These films show a peculiar cauliflower-like structure, made of granular agglomerates of nanoparticles sticking to one another with a significant shape and orientation anisotropy. Both as-deposited Ni and Fe films present hysteresis loops with a high in-plane remanence ratio (0.61 and 0.81 at 250 K, respectively), relatively low values of the saturation and coercive fields and a steep slope near coercivity. At temperature of 10 K and 250 K, the magnetization curves confirm the strong influence of the production technique on the topologic structure of these films, and consequently on their magnetic properties. In perspective, the striking and intriguing properties of these nanogranular films appear very promising for potential application as permanent magnets and in data storage technology.     

Peculiar properties of nanogranular Ni<Subscript>x</Subscript>Si<Subscript>100-x</Subscript> magnetic films obtained by ultrashort pulsed laser deposition

The use of ultrashort laser pulses enables the deposition of films composed of mono-component nanoparticles exhibiting similar shape and size. Films made of nickel (Ni) and silicon (Si) nanoparticles have been produced and investigated in view of the expected interesting properties, resulting from the uniform distribution of metallic, magnetic Ni particles among semiconductive, non-magnetic Si particles. The morphology of the deposited nanoparticles and the related magnetic and magneto-transport characteristics of the films have been studied for different Ni contents, evidencing properties deriving from the peculiar deposition technique and in particular the important role of the free volume inclusions and the particles tendency to not coalesce. PACS 75.75.+a; 81.07.-b; 81.16.Mk; 73.50.Jt     

Production of nanoparticles of different materials by means of ultrashort laser pulses

Ultrashort pulsed laser ablation in vacuum of different targets was performed in order to investigate the possibility of producing nanoparticles with controlled size and shape. A systematic morphology characterization of deposited products was performed for nickel and silicon as a function of laser pulse intensity and wavelength, at a fixed pulse repetition rate. The nanoparticles were investigated by atomic force microscopy, and clear trends for their size and shape anisotropy were evidenced. The best conditions to obtain nanosized particles of oblate ellipsoidal shape, with the minor axis below 10 nm, were determined in the case of nickel targets. Our results show that ultrashort pulse laser deposition can be considered as an interesting technique for the tailoring of nanogranular films with the desired particles dimension and shape, according to the peculiar properties required in specific applications. Moreover, the preliminary features are very promising from the point of view of the production of magnetoresistive films with specific anisotropy.     

Diamond-like-carbon films produced by magnetically guided pulsed laser deposition

We have compared the quality of carbon films deposited with magnetically guided pulsed laser deposition (MGPLD) and conventional pulsed laser deposition (PLD). In MGPLD, a curved magnetic field is used to guide the plasma but not the neutral species to the substrate to deposit the films while, in conventional PLD, the film is deposited with a mixture of ions, neutral species and clusters. A KrF laser pulse (248 nm) was focused to intensities of 10 GW/cm² on a carbon source target and a magnetic field strength of 0.3 T was used to steer the plasma around a curved arc to the deposition substrate. Electron energy loss spectroscopy was used in order to measure the fraction of sp³ bonding in the films produced. It is shown that the sp³ fraction, and hence the diamond-like character of the films, increased when deposited only with the pure ion component by MGPLD compared with films produced by the conventional PLD technique. The dependence of film quality on the laser intensity is also discussed. Received: 7 December 2000 / Accepted: 20 August 2001 / Published online: 2 October 2001     

Magnetic anisotropy in Ni–Si nanoparticle films produced by ultrashort pulsed laser deposition

Pulsed laser deposition (uPLD) in vacuum by means of subpicosecond laser pulses is a powerful, versatile technique for the production of films constituted by nanoparticles. On impact with the deposition substrate, the nanodrops ejected from the target assume an oblate ellipsoidal shape, solidifying with the major cross-section parallel to the substrate plane. These features and the difficult coalescence among the deposited nanoparticles are peculiar characteristics specific to the films obtained by uPLD. In the case of magnetic nanoparticle films obtained by means of this technique, a magnetization isotropy in the film plane and a hard magnetization axis orthogonal to the film plane are expected. This simple assumption, generated by the specific shape and orientation of the deposited nanoparticles, was not experimentally verified up to now. The present investigation represents the first experimental validation of magnetic anisotropy, determined by the peculiar morphology and topology of the constituent particles, in the uPLD Ni_xSi_100−x nanoparticle films. The in-plane isotropic magnetization behaviour, as well as the presence of a hard magnetization axis perpendicular to the sample surface were demonstrated for all investigated films. The difficult coalescence among the magnetic nanoparticles, even at high Ni volume fractions, is confirmed by the behaviour of the initial magnetization curve, typical for single-domain nanoparticles systems.     

In situ STM of pulsed laser nanostructured deposits: First stages of film formation

In the synthesis of nanostructured thin films the characterization of the growth processes plays a fundamental role for the control of the film and surface properties. Moreover when the deposition technique is based on the production and the assembling of nanoparticles/clusters the characterization of the precursor size distribution is of fundamental importance.We have designed a pulsed laser deposition (PLD) apparatus for the production of nanostructured thin films and surfaces, connected to a UHV variable temperature scanning tunneling microscope (STM). The whole system is devoted to the synthesis and in situ study of nanostructured and nanoporous functional metal and metal oxide films and surfaces. We have deposited W nanoparticles produced by a few hundreds laser pulses in order to investigate the initial mechanisms of the film growth. Different deposition conditions have been explored by controlling the laser generated plasma expansion through a background gas in the PLD chamber. STM measurements have been performed on W thin films deposited on different substrates to study both the size distribution and the aggregation of the precursors on the surface. Although substrate effects must be taken into account, the control of the background gas pressure and of the target-to-substrate distance allows to produce surfaces with different morphologies. This opens the possibility to tailor the material properties through the control of the size and deposition energy of the building nano-units.     

Orientation of hyperfine magnetic fields of α-iron films produced by laser deposition

Iron films were produced by pulsed laser deposition (PLD) of iron in Ar gas and M?ssbauer spectra of these films were obtained at room temperature. The orientation of the hyperfine magnetic field was found to vary depending on the pressure of the Ar gas. Iron films produced at low Ar pressures exhibited magnetic fields parallel to the substrate surface. The magnetic field became increasingly perpendicular to the substrate with increasing Ar pressure. Collisions with Ar gas molecules reduced the translational energies of laser-evaporated iron atoms and thus the orientation of crystals formed on the substrate varied depending on the Ar pressure.     

Heteroepitaxial film growth of layered compounds with the ZrCuSiAs-type and ThCr2Si2-type structures: From Cu-based semiconductors to Fe-based superconductors

FeAs-based layered superconductors such as F-doped LaFeAsO have recently been investigated intensively because of their high superconducting transition temperatures. Epitaxial films of these compounds are important to examine their intrinsic materials properties as well as to transfer them to device applications. In this review, we first present our research route from transparent p-type oxides semiconductors to the Fe-based superconductors. Then we review growth of epitaxial thin films for the layered oxychalcogenides and oxypnictides. Reactive solid-phase epitaxy technique was inevitable to prepare epitaxial thin films of the oxychalcogenides and Zn-based oxypnictides. On the other hand, epitaxial thin films of Mn-based oxypnictides were grown by standard pulsed laser deposition. These techniques, however, did not grow epitaxial thin films for LaFeAsO. Thus, we developed a modified pulsed laser deposition process and succeeded in obtaining epitaxial thin films of FeAs-based superconductors, LaFeAsO and cobalt-doped SrFe₂As₂.     

Properties of pulsed laser deposited nanocomposite NiO:Au thin films for gas sensing applications

Nanocomposite thin films formed by gold nanoparticles embedded in a nickel oxide matrix have been synthesized by a new variation of the pulsed laser deposition technique. Two actively synchronized laser sources, a KrF excimer laser at 248 nm and an Nd:YAG laser at 355 nm, were used for the simultaneous ablation of nickel and gold targets in oxygen ambient. The structural, morphological, and electrical properties of the obtained nanocomposite films were investigated in relation to the fluence of the laser irradiating the gold target. The nanocomposite thin films were tested as electrochemical hydrogen sensors. It was found that the addition of the gold nanoparticles increased the sensor sensitivity significantly.     

Magnetron and pulsed laser deposition of silver and gold nanoparticles and discontinuous films and their optical properties

Ag and Au nanoparticles are obtained by magnetron sputtering and pulsed laser deposition under different conditions, and the features of their absorption spectra associated with plasmon resonances are investigated. Optimal deposition conditions for obtaining small (5?C10 nm) silver nanoparticles with a high density of surface distribution include an increased argon pressure (2.5 × 10^?2 Torr) and a low discharge voltage (100 V). Gold nanoparticle arrays obtained by pulsed laser deposition at a temperature of 200°C in vacuum are more uniformly distributed on the substrates than those deposited at room temperature in argon. It is shown that the maximum of the plasmon absorption shifts toward shorter wavelengths with a decrease in the equivalent thickness of metal films and depends not only on this thickness but also on the type of substrate, which is responsible for the morphology of nanoparticle arrays.     

Optical and structural properties of pulsed laser ablation deposited ZnO thin film

A limited number of reports exists in the literature concerning the systematic study of the structural and optical properties of ZnO thin films, produced by pulsed laser ablation, in correlation with the deposition parameters adopted. In this paper we present a characterization of a sample prepared by this technique and studied by photoelectron spectroscopy and X-ray diffraction. The dielectric function of both target and films has been deduced by reflection electron energy loss spectroscopy.     

Pulsed laser deposition of complex oxide heteroepitaxy

The modern studies of complex oxides have been mainly driven by the development of advanced growth and characterization techniques, which provide researchers unprecedented access to new insights and functionalities of these materials. Epitaxial growth of thin films and related architectures offers a pathway to the discovery and stabilization of a wide spectrum of new possibilities in conjunction with the availability of high quality materials that produced with larger lateral sizes and being grown constrainedly. Compared with conventional growth techniques, such as sputtering, spin coating, sol–gel processes, metal-organic chemical vapor deposition, molecular beam epitaxy and so on, no other single advance in the creation of oxide materials has had as pronounced an impact as pulsed laser deposition. In pursuit of the fruitful functionalities and exciting physical phenomena among complex oxides, pulsed laser deposition technique has played an important role to fulfill the flurry of complex oxides in recent decades. In this article, we focus on the details of the growth of epitaxial oxide thin films and the related polymorphs, as well as recent advances in control of the oxide heteroepitaxy via pulsed laser deposition.     

Co掺杂ZnO薄膜的结构和磁学性能

研究了用单束脉冲激光沉积法制备的Co掺杂ZnO薄膜的结构和磁学性能。XRD表征结果表明制备的薄膜是具有沿c轴择优取向的纤锌矿点阵结构。然而,进一步的高分辨电子显微镜结果显示整个样品上的晶体取向并不完全相同。很难说明形成了单晶。结果分析表明Co占据了部分Zn的格点,并对电子结构产生了影响。室温下观察到了磁滞回线,显示掺杂Co可以实现ZnO的磁性翻转,但磁性比较小。该薄膜与我们以前用双束脉冲激光沉积法制备的Co掺杂ZnO薄膜具有相似的性能,提示我们其内部的机制可能相似。     

Carrier storage in Ge nanoparticles produced by pulsed laser deposition

In this work, we report on the electrical characterization of Ge nanoparticles (NPs) produced by pulsed laser deposition (PLD) at room temperature (RT) in Ar gas inert atmosphere using a shadowed off‐axis deposition geometry. Our results show that functional thin films of crystalline Ge NPs embedded between thin alumina films can be obtained on p‐type Si(100) substrates following a low temperature and short rapid thermal annealing (RTA) treatment. Metal–oxide–semiconductor (MOS) structures with and without Ge NPs embedded in the alumina were prepared for the electrical measurements. The results indicate a strong memory effect at relatively low programming voltages (±4 V) due to the presence of Ge NPs. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nanosecond laser ablation and deposition of silver,copper, zinc and tin

Nanosecond pulsed laser deposition of different metals (Ag, Cu, Sn, Zn) has been studied in high vacuum at a laser wavelength of 355 nm and pulse length of 6 ns. The deposition rate is roughly similar for Sn, Cu and Ag, which have comparable cohesive energies, and much higher for the deposition of Zn which has a low cohesive energy. The deposition rate for all metals is strongly correlated with the total ablation yield, i.e., the total mass ablated per pulse, reported in the literature except for Sn, for which the deposition rate is low, but the total ablation yield is high. This may be explained by the continuous erosion by nanoparticles during deposition of the Sn films which appear to have a much rougher surface than those of the other metals studied in the present work.     

Near field properties of nanoparticle arrays fabricated by laser annealing of thin Au and Ag films

In this work we show the properties of the electromagnetic field in the vicinity of a monolayer nanoparticle array on SiO₂ substrate. The nanoparticle array is produced by a simple experimental procedure, where thin gold and silver films are deposited on a substrate by pulsed laser deposition technique and they are annealed by nanosecond laser pulses. At certain conditions the laser annealing leads to a homogeneous decomposition of the film into nanoparticles with diameters in the range of few tens of nanometers. Using FDTD simulations the near field distribution in array structures taken from SEM images are obtained. The distribution shows presence on “hot spots” where the near field intensity is enhanced more than two orders of magnitude compared to the incident one. The existence of enhanced field intensity is assumed to be the main reason on enhancement of the Raman scattering signal obtained experimentally using the produced structures as active substrates.     

Pulsed laser deposition of Mg–Al layered double hydroxide with Ag nanoparticles

Powdered layered double hydroxides (LDHs)—also known as hydrotalcite-like (HT)—compounds have been widely studied due to their applications as catalysts, anionic exchangers or host materials for inorganic or organic molecules. Assembling thin films of nano-sized LDHs onto flat solid substrates is an expanding area of research, with promising applications as sensors, corrosion-resistant coatings, components in optical and magnetic devices. The exploitation of LDHs as vehicles to carry dispersed metal nanoparticles onto a substrate is a new approach to obtain composite thin films with prospects for biomedical and optical applications. We report the deposition of thin films of Ag nanoparticles embedded in a Mg–Al layered double hydroxide matrix by pulsed laser deposition (PLD). The Ag-LDH powder was prepared by co-precipitation at supersaturation and pH = 10 using aqueous solutions of Mg and Al nitrates, Na hydroxide and carbonate, and AgNO₃, having atomic ratios of Mg/Al = 3 and Ag/Al = 0.55. The target to be used in laser ablation experiments was a dry pressed pellet obtained from the prepared Ag-LDH powder. Three different wavelengths of a Nd:YAG laser (266, 532 and 1064 nm) working at a repetition rate of 10 Hz were used. X-Ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and secondary ions mass spectrometry (SIMS) were used to investigate the structure, surface morphology and composition of the deposited films.     

Microstructure and properties of Co-Sm-O nanogranular films

The effect of annealing on the structure and physical properties of Co-Sm-O alloy films prepared using pulsed plasma deposition is investigated. It is found that Co-Sm-O films in the initial state possess superparamagnetic properties due to the presence of small-sized magnetic nanoparticles surrounded by dielectric layers of samarium oxide in the film structure. Upon annealing, the Co-Sm-O films undergo structural transformations and exhibit a number of magnetic properties (including those inherent in both soft and hard magnetic materials).     

Pulsed laser deposition of hard magnetic films

Pulsed laser deposition from elemental targets was used to prepare highly textured hard magnetic Nd–Fe–B and Fe–Pt films with coercivities of 2 T and 5.2 T, respectively. In situ methods such as reflection high energy electron diffraction and Auger electron spectroscopy were applied to analyse film composition and structure during growth. Optimisation of the hard magnetic properties is discussed together with the specific advantages of pulsed laser deposition. PACS 81.15.Fg; 75.50.Ww; 75.50.Vv     

Films of Mn12-acetate by pulsed laser evaporation

Films of the molecular nanomagnet, Mn₁₂-acetate, have been deposited using pulsed laser deposition and its novel variant, matrix assisted pulsed laser evaporation. The films have been characterized by X-ray photoelectron spectroscopy, mass spectrometry and magnetic hysteresis. The results indicate that an increase in laser energy and/or pulse frequency leads to fragmentation of Mn₁₂-acetate, whereas its chemical and magnetic integrity is preserved at low laser energy (200 mJ). This technique allows for the fabrication of patterned thin films of molecular nanomagnets for fundamental and applied experiments.