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Surface engineering of magnetic and mechanical properties of Ta/Pt/GdFeCo/IrMn/Pt heterostructures by femtosecond laser pulses
Abstract
Irradiation of the heterostructure glass/Ta/Pt/GdFeCo/IrMn/Pt by 50 fs ultra-short laser pulses causes local thinning of the film down to ~12–30 nm depth. These pits are distinguished by Atomic Force Microscopy (AFM), while their stray magnetic fields were detected by Magnetic Force Microscopy (MFM). The crater was formed due to layer- by -layer evaporation of the material. No sign of melting was found in the vicinity of the crater at subthreshold laser fluence below 12–15 mJ/cm², but exceeding of the threshold caused destruction of the layers. Energy-dispersive X-ray spectroscopy (EDX) revealed depletion of Gd, Fe and Co, responsible for magnetic properties of the subthreshold crater. The amplitude of the local MOKE signal decreased down to 1.7 times in the irradiated area, while thinning of the ferromagnetic layer was 1.1 times. No proportional change of magnetization was caused by bias effect of IrMn layer. Decreased elastic modulus was found inside the subthreshold crater in the irradiated areas. Laser engineering of the surface of GdFeCo thin films opens the way for local control of energy balance between magnetic anisotropy, exchange coupling and Zeeman energy and creation of separated sectors for capture, storage and analysis of the ferromagnetic nanoparticles.
... With the advent of cheap, reliable and energy efficient lasers, their industrial use is constantly growing [1]. Laser-based tools are used in cutting, welding, surfacing, alloying, drilling and brazing [2][3][4]. These processes can be carried out on areas ranging from a millimeter to a micron, depending on the type, power, gas medium, and operating mode of the laser [5][6][7][8][9]. ...
... С появлением дешевых, надежных и энергоэффективных лазеров их применение в промышленности постоянно растет [1]. Инструменты на основе лазеров используются при резке, сварке, наплавке, легировании, сверлении и пайке [2][3][4]. Данные процессы можно проводить на областях в масштабе от миллиметрового до микронного размера, в зависимости от типа, мощности, газовой среды и режима работы лазера [5][6][7][8][9]. ...
В работе методом инструментального индентирования исследовались механические свойства режущей кромки вырубного штампа, модифицированной с помощью механического воздействия фрезерного инструмента и локального нагрева лазером. Построены профили твердости от глубины с помощью метода автоматизированного картографирования. Установлено, что обработка острия локальным нагревом с помощью лазера приводит к увеличению твердости сплава в 1,5 раза в области размером около 40 мкм.
The exchange bias effect in FeNi/FeMn/GdxCo100-x films with a pinning antiferromagnetic FeMn layer and an exchange-biased ferrimagnetic amorphous Gd-Co layer was studied. The FeNi layer allows the formation of the fcc lattice of FeMn, i.e. the antiferromagnetic γ-phase of FeMn. The Gd content was varied within 15–25 at. % in order to obtain different temperature dependences of the spontaneous magnetization Ms(T) of the Gd-Co layer. The magnetic properties of the films were determined from hysteresis loops measured in the temperature range 5–350 K. A correlation between Ms(T) and the temperature dependences of the exchange bias field HebGdCo(T) was established. In particular, it was shown that there is an inversion of the sign and sharp increase of the value of HebGdCo near the magnetic compensation state. The first one is a consequence of the minimization of the spontaneous magnetization of Gd-Co layer at the compensation temperature. The second one shows the predominance of the exchange interaction of the antiferromagnetic layer with the cobalt magnetic sub-system of the ferrimagnetic layer as a contribution to the interlayer exchange coupling.
The surface modifications in a multilayer thin-film structure (50-nm alternating layers of Si and Al) induced by a single Gaussian-shaped femtosecond laser pulse (350 fs, 1028 nm) in the air are investigated by means of atomic-force microscopy (AFM), scanning electron microscopy (SEM), and optical microscopy (OM). Depending on the laser fluence, various modifications of nanometer-scale metal and semiconductor layers, including localized formation of silicon/aluminum nanofoams and layer-by-layer removal, are found. While the nanofoams with cell sizes in the range of tens to hundreds of nanometers are produced only in the two top layers, layer-by-layer removal is observed for the four top layers under single pulse irradiation. The 50-nm films of the multilayer structure are found to be separated at their interfaces, resulting in a selective removal of several top layers (up to 4) in the form of step-like (concentric) craters. The observed phenomenon is associated with a thermo-mechanical ablation mechanism that results in splitting off at film–film interface, where the adhesion force is less than the bulk strength of the used materials, revealing linear dependence of threshold fluences on the film thickness.
This manuscript and Supplementary Materials can freely be downloaded from the web-sites: http://rdcu.be/nXCT or http://www.nature.com/articles/srep39133
In this paper, we establish connections between the thresholds and mechanisms of the damage and white-light generation upon femtosecond laser irradiation of wide-bandgap transparent materials. On the example of Corning Willow glass, evolution of ablation craters, their quality, and white-light emission were studied experimentally for 130-fs, 800-nm laser pulses. The experimental results indicate co-existence of several ablation mechanisms which can be separated in time. Suppression of the phase explosion mechanism of ablation was revealed at the middle of the irradiation spots. At high laser fluences, air ionization was found to strongly influence ablation rate and quality and the main mechanisms of the influence are analysed. To gain insight into the processes triggered by laser radiation in glass, numerical simulations have been performed with accounting for the balance of laser energy absorption and its distribution/redistribution in the sample, including bremsstrahlung emission from excited free electron plasma. The simulations have shown an insignificant role of avalanche ionization at such short durations of laser pulses while pointing to high average energy of electrons up to several dozens of eV. At multi-pulse ablation regimes, improvement of crater quality was found as compared to single/few pulses.
The present invention provides an appropriately produced magnetic material sensor having a small size.
The magnetic material sensor of this invention includes: a magnetoresistive effect film, formed using magnetic films; a current source, for supplying to the magnetoresistive effect film a current having a magnitude and a direction that can change the magnetization directions of the magnetic films; and a detector, for detecting the resistance of the magnetoresistive effect film.
We present an experimental and numerical study of the damage and ablation thresholds at the surface of a dielectric material, e.g., fused silica, using short pulses ranging from 7 to 300 fs. The relevant numerical criteria of damage and ablation thresholds are proposed consistently with experimental observations of the laser irradiated zone. These criteria are based on lattice thermal melting and electronic cohesion temperature, respectively. The importance of the three major absorption channels (multi-photon absorption, tunnel effect, and impact ionization) is investigated as a function of pulse duration (7–300 fs). Although the relative importance of the impact ionization process increases with the pulse duration, our results show that it plays a role even at short pulse duration (<50 fs). For few optical cycle pulses (7 fs), it is also shown that both damage and ablation fluence thresholds tend to coincide due to the sharp increase of the free electron density. This electron-driven ablation regime is of primary interest for thermal-free laser-matter interaction and therefore for the development of high quality micromachining processes.
Pump-pulse induced magnetization reversal of amorphous Gd23.1Fe71.9Co5.0 showed a subpicosecond magnetization collapse followed by a slower reversal. The reversal dynamics is well described by the Bloch equation via a reversal time that does not depend on temperature, but strongly decreases with increasing pump fluence. A comparison to data obtained in external saturation field and in remanence opened the way to separate the contributions due to temperature induced effects within single domains from those related to field induced domain formation.
Time-domain magnetization dynamics of sputtered GdFeCo (30 nm) amorphous alloy films was measured by pump-probe method using high-power ultra-short pulse fiber laser. The effective g -factor g <sub>eff</sub> and effective damping constant alpha<sub>eff</sub> of the GdFeCo films were estimated by using a numerical calculation of Landau-Lifshitz-Gilbert equation. The precessional frequency took a maximum near the magnetization compensation composition C <sub>M</sub> of the GdFeCo, while the estimated g <sub>eff</sub> and alpha<sub>eff</sub> increased around the angular momentum compensation composition C <sub>A</sub>. The compositional dependences of g <sub>eff</sub> and alpha<sub>eff</sub> were roughly described by a mean-field model. The g <sub>eff</sub> and alpha<sub>eff</sub> were also estimated from the ferromagnetic resonance (FMR) spectra, and the data from the FMR spectra agreed well with those from the pump-probe measurement except for the composition near C <sub>M</sub>. The FMR method was unable to excite the magnetization near C <sub>M</sub> because of the small net magnetization.
We report extensive laser-induced damage threshold measurements on dielectric materials at wavelengths of 1053 and 526 nm for pulse durations tau ranging from 140 fs to 1 ns. Qualitative differences in the morphology of damage and a departure from the diffusion-dominated tau1/2 scaling of the damage fluence indicate that damage occurs from ablation for tau~50 ps. We find a decreasing threshold fluence associated with a gradual transition from the long-pulse, thermally dominated regime to an ablative regime dominated by collisional and multiphoton ionization, and plasma formation. A theoretical model based on electron production via multiphoton ionization, Joule heating, and collisional (avalanche) ionization is in quantitative agreement with the experimental results.
Time-resolved pump-probe measurements show ultrafast and heat accumulation demagnetization in Co/Pd superlattices on glass substrates. A model of demagnetizing fields and micromagnetic simulations are applied to examine the evolution of a demagnetized cylinder into a switched state.
Gd-Co/Ti multilayers were prepared by magnetron sputtering deposition onto glass substrates. It was found, that the amount of cobalt determined by energy dispersive X-ray spectroscopy (EDX) analysis decreased from 77.9 down to 74.4 at% with a decrease of the GdCo layer thickness from 250 nm to 3 nm. For relatively thick layers this compositional change correlates well with the observed changes in the compensation temperature. However, for thinner Gd-Co layers the Curie temperatures (TC) taken in conjunction with the composition dependence of TC would require a significant change in composition much more important than the one observed by EDX. The reason for this discrepancy between the results of EDX analysis and the "effective" composition obtained by means of magnetic analysis is the sharp decrease in TC as a result of finite size effect.
The possibility of mapping the elasticity modulus during scanning of the material surface with a piezoresonance probe is proposed. An analytical model is proposed that describes the force of an elastic interaction between the surface and an indenter in the form of a truncated cone as a function of a displacement. Within the framework of this model, dependences were obtained that allow the elasticity-modulus value to be determined, provided that the data on the average force of a semicontact interaction, the amplitude, and the frequency of probe resonance oscillations are available. The obtained dependences were used for mapping the elasticity modulus of the boundaries of deposited aluminum and copper films on glass, as well as for a TGZ2 test structure. Experiments were performed using a NanoScan 3D scanning nanohardness tester, and a resolution of 0.2 μm was attained. This technique is applicable to materials that allow only an elastic deformation and can be used in instruments that can operate in a range of contact forces providing predominantly elastic deformations.
We have fabricated flexible exchange biased heterostructures with magnetostrictive Fe81Ga19 alloy as the ferromagnetic layer and Ir20Mn80 as the antiferromagnetic layer on polyethylene terephthalate substrates. The mechanical strain can modify both the strength and the orientation of the uniaxial anisotropy, giving rise to the switching between the easy and hard magnetization directions. Different from the previously reported works on rigid exchange biased systems, a drastic decrease in exchange bias field was observed under a compressive strain with magnetic field parallel to the pinning direction, but only a slightly decrease was shown under a tensile strain. Based on a Stoner-Wohlfarth model calculation, we suggested that the distributions of both ferromagnetic and antiferromagnetic anisotropies be the key to induce the mechanically tunable exchange bias.
We report the compositional and temperature dependence of magnetic compensation in amorphous ferrimagnetic GdxFe93−xCo7 alloy films. Magnetic compensation is attributed to the competition between antiferromagnetic coupling of rare-earth (RE) with transition-metal (TM) ions and ferromagnetic interaction between the TM ions. The low-Gd region of x between 20 and 34 was found to exhibit compensation phenomena characterized by a low saturation magnetization and perpendicular magnetic anisotropy (PMA) near the compensation temperature. Compensation temperature was not observed in previously unreported high-Gd region of x=52–59, in qualitative agreement with results from recent model calculations. However, low magnetization was achieved at room temperature, accompanied by a large PMA with coercivity reaching ~6.6 kOe. The observed perpendicular magnetic anisotropy of amorphous GdFeCo films probably has a structural origin consistent with certain aspects of the atomic-scale anisotropy. Our findings have broadened the composition range of transition metal-rare earth alloys for designing PMA films, making it attractive for tunable magnetic anisotropy in nanoscale devices.
The scratch test is a very powerful technique for measuring hardness at very shallow penetration depths. In the present work, the scratching protocol and data analysis were extended by combining the initial surface profile, the instrument compliance, thermal drift and indenter displacement during the test, which allows reconstruction of the complete indenter trajectory during the scratch test. The analysis of such a diagram leads to the in situ estimation of pile-up sizes and their influence on the area of contact between the indenter and the material. As a result, criteria for a scratch width estimation algorithm can be formalized as well as scratching contact depth can be derived. Based on the indenter apex shape characterized with scanning probe microscopy, it has been shown that lateral elastic recovery takes place during the scratch test. The proposed analysis was applied to the measurement of scratch hardness for several materials known for different piling and elastic recovery behavior during mechanical testing.
We study the femtosecond laser ablation properties of borosilicate glass using atomic force microscopy and laser pulses of 200 fs duration, centered at 780 nm wavelength. We show that both single-shot and multishot ablation threshold fluences can be determined by studying the diameter and the depth of single-shot ablated craters. The linear relationship between the square of the crater diameter and the logarithm of the laser fluence in the form of D2=2w02ln(F0∕FthN=1) provides the single-shot ablation threshold, FthN=1, whereas the linear relationship between the ablation depth and the logarithm of laser fluence in the form of ha=αeff−1ln(F0∕FthN>1) provides the multishot ablation threshold, FthN>1. The results depict a multishot ablation threshold of ≈1.7 J∕cm2 independent of the atmospheric conditions. The slopes of the linear fits also provide a precise estimate of the beam radius at the surface, w0≈5.9 μm, and the “effective optical penetration depth,” αeff−1≈238 nm in air. The method is systematic, provides results that are consistent with the literature, and eliminates uncertainties because of instrument sensitivities. We also show that threshold measurement based on the extrapolation of volume to zero, a method used often in previous studies, is somewhat questionable. Finally, the measured dimensions of ablated craters reveal that the ablation volume per unit input energy is about 1.3–1.5 μm3∕μJ at an intermediate fluence regime of 10<F0av<40 J∕cm2. This value represents an order of magnitude larger ablation efficiency when compared to the ablation of glass with nanosecond ultraviolet laser pulses.
High-pulse energy picosecond laser systems provide industrial-level reliability combined with the performance features required for many ultrafast material processing applications. Current diodepumped solid-state systems provide pulse energies into the 100's of microjoules at 1064 nm wavelength and pulse widths of approximately 10 picoseconds, which allows effective “cold” ablation of most materials with high efficiency. Average power of these systems is scalable up to 50 W and will further increase into the multi-100 W range, allowing for very high speed picosecond micromachining.
This article first reviews the basics of picosecond micromachining, gives a short perspective on the development of these lasers over the last few decades, then summarizes their current performance. We review some of the applications that are enabled with these lasers, before we discuss future developments in this rapidly changing field.
Ablation experiments in several glasses with single and multishot irradiation by laser pulses in the 10-fs pulse duration domain are presented; physical and technological implications are discussed. We demonstrate that these short pulses offer the potential for lateral and vertical machining precision of the order of 100 nm.
Femtosecond, picosecond and nanosecond laser ablation of solids
- Chichkov