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The impact of plasmonic surface lattice resonances on the magneto-optical properties and energy absorption efficiency has been studied in arrays of [Co/Gd/Pt]

The development of approaches that can efficiently control the magnetization of magnetic materials is central to the creation of fast and low-power spintronic devices. Spin transfer torque can be used to electrically manipulate magnetic order in devices, but is typically limited to nanosecond timescales. Alternatively, spin–orbit torque can be employed, and switching with current pulses down to ~200 ps has been demonstrated. However, the upper limit to magnetization switching speed remains unestablished. Here, we show that photoconductive switches can be used to apply 6-ps-wide electrical pulses and deterministically switch the out-of-plane magnetization of a common thin cobalt film via spin–orbit torque. We probe the ultrafast magnetization dynamics due to spin–orbit torques with sub-picosecond resolution using the time-resolved magneto-optical Kerr effect (MOKE). We also estimate that the magnetization switching consumes less than 50 pJ in micrometre-sized devices. The magnetization of a cobalt thin film can be reversed by spin–orbit torques using picosecond electrical pulses that are generated by photoconductive switches.

The understanding of measured jitter is improved in three ways. First, it is shown that the measured jitter is not only governed by written-in jitter and the reader resolution along the cross-track direction but by remanence noise in the vicinity of transitions and the down-track reader resolution as well. Second, a novel data analysis scheme is introduced that allows for an unambiguous separation of these two contributions. Third, based on data analyses involving the first two learnings and micro-magnetic simulations, we identify and explain the root causes for variations of jitter with write current (WC) (write field), WC overshoot amplitude (write-field rise time), and linear disk velocity measured for heat-assisted magnetic recording.

International audience; Since it was recently demonstrated in a spin-valve structure, magnetization reversal of a ferromagnetic layer using a single ultrashort optical pulse has attracted attention for future ultrafast and energy-efficient magnetic storage or memory devices. However, the mechanism and the role of the magnetic properties of the ferromagnet as well as the time scale of the magnetization switching are not understood. Here, we investigate single-shot all-optical magnetization switching in a GdFeCo/Cu/[Co x Ni 1−x /Pt] spinvalve structure. We demonstrate that the threshold fluence for switching both the GdFeCo and the ferromagnetic layer depends on the laser pulse duration and the thickness and the Curie temperature of the ferromagnetic layer. We are able to explain most of the experimental results using a phenomenological model. This work provides a way to engineer ferromagnetic materials for energy efficient single-shot all-optical magnetization switching.

New methods to induce magnetization switching in a thin ferromagnetic material using femtosecond laser pulses without the assistance of an applied external magnetic field have recently attracted a lot of interest. It has been shown that by optically triggering the reversal of the magnetization in a GdFeCo layer, the magnetization of a nearby ferromagnetic thin film can also be reversed via spin currents originating in the GdFeCo layer. Here, using a similar structure, it is shown that the magnetization reversal of the GdFeCo is not required in order to reverse the magnetization of the ferromagnetic thin film. This switching is attributed to the ultrafast spin current and can be generated by the GdFeCo demagnetization. A larger energy efficiency of the ferromagnetic layer single pulse switching is obtained for a GdFeCo with a larger Gd concentration. Those ultrafast and energy efficient switchings observed in such spintronic devices open a new path toward ultrafast and energy efficient magnetic memories., In GdFeCo/Cu/[Co/Pt] spin valve, the ultrafast demagnetization of the ferrimagnetic GdFeCo alloy, generated by a single femtosecond laser pulse, is shown to generate enough spin current to switch the magnetization of the ferromagnetic Co/Pt multilayer. By increasing the Gd concentration, the Co/Pt switching is found to be more energy efficient.

Reducing energy dissipation while increasing speed in computation and memory is a long-standing challenge for spintronics research. In the last 20 years, femtosecond lasers have emerged as a tool to control the magnetization in specific magnetic materials at the picosecond timescale. However, the use of ultrafast optics in integrated circuits and memories would require a major paradigm shift. An ultrafast electrical control of the magnetization is far preferable for integrated systems. Here we demonstrate reliable and deterministic control of the out-of-plane magnetization of a 1 nm-thick Co layer with single 6 ps-wide electrical pulses that induce spin-orbit torques on the magnetization. We can monitor the ultrafast magnetization dynamics due to the spin-orbit torques on sub-picosecond timescales, thus far accessible only by numerical simulations. Due to the short duration of our pulses, we enter a counter-intuitive regime of switching where heat dissipation assists the reversal. Moreover, we estimate a low energy cost to switch the magnetization, projecting to below 1fJ for a (20 nm)^3 cell. These experiments prove that spintronic phenomena can be exploited on picosecond time-scales for full magnetic control and should launch a new regime of ultrafast spin torque studies and applications., Includes article + supplementary information. Latest version uses full name of the first author. Nature Electronics (2020)

The edge gradient of the thermal spot used to define bits in heat-assisted magnetic recording is a critical parameter in determining the quality of the magnetic transitions. We review the laser current modulation method used to measure this parameter and introduce the sideband ratio (SBR) approach as a fast, inexpensive technique for getting data on large volumes of heads or media. Mathematical derivation of the SBR method is given along with the details of implementation. Production level data on thousands of recording heads are used to illustrate its usefulness.

International audience; Spin-polarization and magnetic damping are measured for several polycrystalline films with each of them being made of a different single Co$_2$ Mn-based Heusler compound. As several epitaxial Co$_2$ Mn-based Heusler compounds are shown to be half-metal magnetic materials with full spin-polarization and ultralow magnetic damping, we explore here these properties but in polycrystalline films. Co$_2$ MnSi, Co$_2$ MnGe, and Co$_2$ MnGa thin films were grown on glass substrates and analyzed in situ by electron diffraction and spin-resolved photoemission and ex situ by transmission electron microscopy and ferromagnetic resonance. Despite the polycrystalline state of the films, they still exhibit high spin polarizations and very low magnetic damping coefficients. The latter are at least of the same order as the best epitaxial films using regular ferromagnetic materials. The key point to achieve such properties is to control the Heusler stoichiometry as best as possible.

All-optical helicity-dependent switching in ferromagnetic Co/Pt multilayers is investigated using magneto-optical microscopy and anomalous Hall effect measurements. A state diagram is built by studying the effect of pulse duration, fluence, and spot size. We use numerical solutions of the three-temperature model to explain that the all-optical helicity-dependent switching mechanism relies on the spin bath reaching temperatures close to the Curie point. Further insights into the reversal process are provided by the experimental demonstration of significant helicity-dependent reversal after a single laser pulse that reveals the involvement of direct angular momentum transfer. Moreover, based on the observation that longer pulse durations and larger spot sizes lead to enhanced reversal efficiency, we identify experimental conditions that lead to saturated magnetization reversal after just a few tens of laser pulses.

An optical pump-probe set-up that allowed short dwell time ( ~ 500 μs) heating with a high-temperature ramp rate ( ~ 106 K/s) was used to detect the magnetization change in heat assisted magnetic recording (HAMR) media. The temperature of the media was monitored by observing the Kerr signal. The pump power at zero magnetization allowed the determination of the power needed to attain the Curie temperature of the media (the Curie temperature was determined with a high-temperature magnetometer before the pump-probe experiments). HAMR media was then irradiated with a pump power to obtain 480°C for increasing exposure times. Atomic force microscopy (AFM) of these media surfaces revealed depressions or holes in the media surface [within the carbon overcoat (COC) layer] that increased in extent with cumulative exposure time. Media surfaces exposed to somewhat lower temperatures (450°C) and for shorter times had a swollen region that surrounded a much smaller depression. High-spatial resolution Raman spectroscopy was used to analyze these irradiated areas. An increased D-band was observed within the swollen portion of the media surface, while the overall Raman signal intensity decreased within the small depressed area. Using time and temperature irradiations along with AFM analysis of the depressions the activation energy for COC loss was determined to be 0.6 eV. These observations were attributed to COC failure through graphitization and oxidation. The failure mechanism leading to these observed changes and the possible relationship of the present results to the HAMR media COC thermal stability are discussed.

Controlling the relaxation of magnetization in magnetic nanostructures is key to optimizing magnetic storage device performance. This relaxation is governed by both intrinsic and extrinsic relaxation mechanisms and with the latter strongly dependent on the interactions between the nanostructures. In the present work we investigate laser induced magnetization dynamics in a broadband optical resonance type experiment revealing the role of interactions between nanostructures on the relaxation processes of granular magnetic structures. The results are corroborated by constructing a temperature dependent numerical micromagnetic model of magnetization dynamics based on the Landau-Lifshitz-Bloch equation. The model predicts a strong dependence of damping on the key material properties of coupled granular nanostructures in good agreement with the experimental data. We show that the intergranular, magnetostatic and exchange interactions provide a large extrinsic contribution to the damping. Finally we show that the mechanism can be attributed to an increase in spin-wave degeneracy with the ferromagnetic resonance mode as revealed by semianalytical spin-wave calculations.

Heat-assisted magnetic recording (HAMR) storage technology will thermally stress the lubricant film typically applied to the storage disk surface. Different lubricant loss and morphology change mechanisms have been hypothesized to occur during information writing. A loss to of the lubricant film will dramatically affect its overall mechanical and chemical performance of the head–disk interface, decreasing its reliability and its durability. Thus an all optical pump–probe method was used to study the effect of fast thermal transients (106 K/s) on a lubricant film on HAMR media. Thermal transients (Bhushan and Cheng in J Appl Phys 81:5390, 1997) with peak temperatures above the HAMR media Curie temperature (T c) were found to remove by evaporation the lubricant within the heated region creating a lubricant depression in the otherwise continuous film. No accumulation of lubricant volume was observed to take place in the cooler regions of the thermal spot, indicating that thermocapillary shear stress is not an important mechanism of lubricant thickness change with the optical spot size used (65 μm). The onset of lubricant loss was observed to begin at approximately 610 K and was totally removed at 823 K. The change in depth of the lubricant depression with time showed that no structural terms contributed to the disjoining pressure for the lubricant thickness range studied. From this change, the diffusion coefficient of the lubricant on the carbon overcoat surface was determined to be 1 × 10−13 m2/s by fitting Fick’s second law to the normalized lubricant thickness. The importance of these observations on the operating HAMR head–disk interface is discussed.

The ultimate speed limit of thermo-magnetic writing is investigated by time-resolved measurements of femtosecond laser pulse induced magnetization reversal in amorphous Gd23.1Fe71.9Co5.0. We prove the resulting reversal dynamics to be governed by the product of two independent processes-the temperature induced changes of the magnetization magnitude times the magnetic field driven reversal of its orientation. Based on this factorization, purely magnetic reversal times are derived from the data. These times show no variation with temperature, but tremendously decrease from several tens of nanoseconds down to only a few picoseconds with increasing excitation density. A minimum reversal time of about 2 ps is found that indicates the possibility of THz thermo-magnetic writing rates. However, the slow cooling of the heated sample limited the maximum achievable writing rates to the GHz range. Only slight improvement of the cooling rates is found when substrates with almost perfect electronic (gold) or phononic (sapphire) heat conductivity are used as heat sinks.

Due to the limits of conventional perpendicular magnetic recording, it appears that alternative technologies are needed at areal densities >500 Gb/in2. Heat-assisted magnetic recording (HAMR) is a promising approach to extend areal densities to 1 Tb/in2 and beyond. All of the unique components necessary for a working HAMR system have been demonstrated. Although HAMR permits writing on high Hc media with lower magnetic fields and can produce higher write gradients than conventional magnetic recording, head/media spacing and the development of high Hc media with small grains remains challenging

Femtosecond laser pulse induced magnetization reversal of α-Gd23.1Fe71.9Co5.0 was investigated by time-resolved measurements of the polar Kerr-rotation. The dynamics is found to follow the Bloch-equation via a relaxation time that does not depend on temperature but strongly decreases with increasing excitation density. A minimum value of (190±40)ps was obtained for the highest applicable pump-fluence. Comparing the reversal dynamics to temperature induced variations of magnetization within a single domain state and to the magnetization dynamics measured in remanence, the different contributions of transient temperature and domain formation to the magnetic response are separated.

Short magnetic in-plane field pulses were used to pull the magnetic spin system of a NiFe film out of its equilibrium state and to initiate coherent magnetisation dynamics. This field induced dynamics was investigated by means of a time-resolved pump-probe experiment, measuring the linear Kerr effect with a balanced photodiode scheme. Precession frequencies of several GHz and relaxation-times in the nanosecond range were observed. Emphasis is on the generation of the short magnetic in-plane field pulses. They were generated using an ultrafast photoconductive switch based on GaAs and launched by a waveguide to the sample. The waveguide and the photoswitch were designed to yield as large as possible magnetic field strengths. The field pulses used in this work had a rise time of 10-60ps, a decay time of 500-700ps and a maximum field strength of 9Oe.

New results about relaxation dynamics of optically excited electrons in metals, mostly gold and nickel films, are presented. Emphasis is on electron temperature near the surface as well as on the range of energy transport by ballistic and diffusive electron motion in comparison to the optical penetration depth. The experiments focus on the interval between creation of an electron temperature and the time at which thermal equilibrium between electrons and lattice is reached. Results were obtained by time-resolved linear and second-harmonic reflectivity measurements carried out in pump-probe mode. It is shown that the two-temperature model is well suited to describe hot electron diffusion in metals and to extract electron–phonon coupling constants from experimental data, provided corrections for ballistic electron motion are incorporated. The electron–phonon coupling constant of gold was found to be independent of film thickness down to 10 nm. For noble metals, probe reflectivities near the interband transition were related to electron temperatures by a proper model for the dielectric function. For transition metals such relation between reflectivity and electron temperature is more difficult. A new pump-pump-probe technique was introduced which allows to study hot electron relaxation by probing the reflectivity in thermal equilibrium between electrons and lattice. Also these results can be well described by the two-temperature model. Finally, the interface sensitivity of the second harmonic was utilized to detect vibrational motion and thermal expansion of ultrathin nickel films on Cu(001).

Femtosecond laser pulses were applied to study the energy deposition depth and transfer to the lattice for Au, Ni, and Mo films of varying thickness. The onset of melting, defined here as damage threshold, was detected by measuring changes in the scattering, reflection and transmission of the incident light. Experiments were done in multi-shot mode and single-shot threshold fluences were extracted by taking incubation into account. Since melting requires a well-defined energy density, we found the threshold depends on the film thickness whenever this is smaller than the range of electronic energy transport. The dependence of the threshold fluence on the pulse length and film thickness can be well described by the two-temperature model, proving that laser damage in metals is a purely thermal process even for femtosecond pulses. The importance of electron–phonon coupling is reflected by the great difference in electron diffusion depths of noble and transition metals.

Ultrafast electron and magnetization dynamics of Ni were studied by pump-probe reflection second harmonic generation with an experimental time resolution of 40 fs. No delay between initial electronic heating and demagnetiza- tion was observed within this time resolution. Thus, there is no need to introduce an independent spin temperature. Once the electron gas is thermalized, the recovery of magnetization obeys the classical magnetization curve. This was demon- strated for delay times up to 1n s. The dependence of second harmonic yield on the azimuth of magnetization for selected pump-probe delays proved that the phase between even and odd contributions is zero.

The ultrafast magnetization dynamics of thin Ni and Co films on Cu(001) and of polycrystalline Ni surfaces was studied by pump-probe reflection second-harmonic generation, utilizing 150 fs/800 nm laser pulses. In all cases no delay between electron excitation and magnetization breakdown was observed within the experimental time resolution. An upper limit of such delay is 50 fs in case of bulk Ni surfaces. The recovery of magnetization follows the electron temperature relaxation during the first few picoseconds and thereafter cooling by regular thermal diffusion.

Multi-shot damage thresholds of thin Au and Ni films in the thickness range from 20 nm to 1500 nm for Au and 10 nm to 100 nm for Ni induced by 200-fs pulses at 400 nm have been investigated by light scattering. A dramatic difference in the dependence of damage threshold on film thickness for Au and Ni is observed. Both metals show a linear thickness dependence of the absorbed fluence at the damage threshold for thin films and a saturation as the film thickness increases, but the onset of saturation differs for both metals by one order of magnitude (50 nm for Ni and 500 nm for Au). This behavior can be explained by the different electron–phonon coupling in both metals. By applying the two-temperature model, we show that the energy transport after absorption of a fs-laser pulse is mainly determined by diffusion of hot electrons during the first picoseconds. This is in contrast to the case of ns-laser pulses, where the energy transport is governed by the heat diffusion in the lattice. The thermal diffusion lengths for Au and Ni are similar, whereas the electron diffusion length depends on the strength of the electron–phonon coupling, which differs by two order of magnitude for Au and Ni. This fact is responsible for the different thickness dependence of the ablation threshold for fs-pulses.

to \valunit{500}{nm} have been measured by femtosecond time-resolved pump-probe experiments. A conspicuous change of the relaxation behavior was found around \valunit{100}{nm} for pump pulse fluences of \valunit{1}{mJ/cm^{2}} . Thicker films show a nearly exponential decay of the transient linear reflectivity, which turns into a linear decay during the first \rangeunit{5}{7}{ps} for films with thicknesses of \valunit{100}{nm} or less. This observation is evidence of a mean free path of about \valunit{100}{nm} for hot electrons with temperatures around \valunit{1500}{K} .

The influence of surface magnetization of polycrystalline nickel samples on Second-Harmonic Generation (SHG) in reflection was investigated using pleosecond laser pulses of 600 nm. The effect was measured for different polarization combinations with samples under normal conditions. For the azimuthal rotation of the magnetization anisotropies of (13±2)% and (19±3)% were observed for s→P and p→P polarization, respectively. The Kerr angle observed by. SHG was found to be an order of magnitude larger (4±1)o than the linear Kerr angle. A hysteresis curve was recorded for p→P polarization. No influence of the oxide layer was noticed.

Optical Second-Harmonic Generation (SHG) in reflection from a polycrystalline copper surface in air was studied using femtosecond time-resolved pump and probe measurements at λ=625 nm. The observed time dependence of second-harmonic yield from the probe beam demonstrates, that SHG is a very sensitive technique for measuring transient electron temperatures of metals even when these are covered by an oxide layer. For polycrystalline copper, an electron-phonon energy transfer time of 2 ps was observed, corresponding to a coupling constant of 3.75×1017 W/m3 K at average lattice temperatures of about 500 K. The analysis of experimental data indicates that the time dependence of SHG is governed by the linear dielectric function e which, in turn, is affected by the electron temperature. There is no evidence for a temperature dependence of the nonlinear susceptibility χ(2).

Time-resolved x-ray diffraction with ultrashort x-ray pulses from a laser-produced plasma is used to study the lattice response of FeRh during a femtosecond laser-induced antiferromagnetic (AFM) to ferromagnetic (FM) phase transition. Pump-probe measurements at initial sample temperatures below as well as above the AFM-to-FM transition temperature and for different laser pump fluences allowed to disentangle the various contributions driving lattice expansion. In particular, the data reveal that the structural changes associated with the magnetic phase transition occur on a time scale of a hundred picoseconds.

The magnetization dynamics in a thin NiFe film was investigated by applying short in-plane magnetic field pulses while probing the response using a time-resolved magneto-optical Kerr effect setup. In-plane magnetic field pulses, with duration shorter than the relaxation of the system, were generated using a photoconductive switch and by subsequent propagation of current pulses along a waveguide. The field pulses with typical rise and decay times of 10–60 and 500–700 ps, respectively, have a maximum field strength of 9 Oe, by which Permalloy elements of 16 nm thickness and lateral dimensions of 10×20 μm were excited. The observed coherent precession of a ferromagnetic NiFe system had precession frequencies of several GHz and relaxation times on a nanosecond time scale. The dynamic properties observed agree well the Gilberts’s precession equation and the static magnetic properties of the elements

Spin transport equations in a nonhomogeneous ferromagnet are derived in the limit where the sd exchange coupling between the electrons in the conduction band and those in the d band is dominant. It is shown that spin diffusion in ferromagnets assumes a tensor form. The diagonal terms are renormalized with respect to that in normal metals and enhance the dissipation in the magnetic system while the off-diagonal terms renormalize the precessional frequency of the conduction electrons and enhance the nonadiabatic spin torque. To demonstrate what additional physics is included in the theory, we show that self-consistent solutions of the spin diffusion equations and the Landau-Lifshitz equations in the presence of a current lead to an increase in the terminal velocity of a domain wall which becomes strongly dependent on its width. We also provide a simplified equation that predicts damping due to the conduction electrons.

We introduce a theoretical and experimental framework that enables quantitative measurements of the temperature and magnetic field profiles governing the thermo-magnetic write process in heat assisted magnetic recording. Since our approach allows the identification of the correct temperature dependence of the magneto-crystalline anisotropy field in the vicinity of the Curie point as well, it provides an unprecedented experimental foundation to assess our understanding of heat assisted magnetic recording.

The problem of the dynamic response of a magnetic system to pulsed laser heating was addressed. The magnetic system is modeled at the atomistic level, using a classical spin Hamiltonian with the Heisenberg form of the exchange interaction and a local uniaxial anisotropy. The material parameters used corrersponds to permalloy. It was shown that the rapid decrease of the magnetization is predicted naturally by a simple atomistic model based Landau-Lifshitz-Gilbert (LLG) equation with Langevin dynamics.

Time-resolved pump-probe measurements of the magnetic phase change from paramagnetic to ferromagnetic and back in FeRh thin films are presented. Data are compared with simulations of laser-induced magnetization dynamics using the Landau-Lifshitz-Gilbert equation with a time dependent magnetization density, responding to the evolution of the thermal profile throughout the film. We demonstrate that the observed magneto-optical transients should be interpreted in terms of an interplay between the local magnetic moment, nonlocal domain growth or alignment, and magnetization precession as launched by the varying demagnetizing fields. Kerr rotation and reflectivity are shown to provide a complementary view on the phase transition.

In multi-layered solids, an acoustic wave is partially reflected and partially transmitted at boundaries, which renders a too complex wave pattern to be predicted with analytical models. A Finite Element Method (FEM) based numerical model is developed to predict the acoustic wave propagation in multi-layered solids, where an ANSYS acoustic fluid element is adopted to solve this problem. The model is applied to study the pump-probe transient reflectivity measurements on Heat Assisted Magnetic Recording (HAMR) media, where the thermo-elastic waves are isolated and then subtracted from the composite reflectivity change measurement. As a result, the reflectivity change caused by the thermal decay is separated from the thermo-elastic waves, allowing a more accurate prediction and measurement of the thermal properties of HAMR media.Copyright © 2005 by ASME

Contains fulltext : 57617.pdf (Publisher’s version ) (Open Access) We fabricate iron nanolines by depositing an atomic beam of iron through a far-off resonant laser standing wave (SW) onto a glass-ceramic substrate. The laser SW is tuned 200 MHz above the D-5(4)-->F-5(5)o Fe-56 transition at a vacuum wavelength of 372.099 nm. The resulting nanolines exhibit a period of 186 nm, a height above the background of 8 nm and a full width at half maximum of 95 nm. These nanostructures cover a surface area of similar or equal to1.6x0.4 mm(2), corresponding to similar or equal to8600 iron lines with a length of similar or equal to400 mum. (C) 2004 American Institute of Physics.

We have observed a phase transition from a uniaxial to a biaxial nematic phase in a lyotropic liquid crystal as a function of decreasing film thickness. The results, obtained by optical second-harmonic generation experiments in a wedged cell geometry, are supported by additional Z -scan measurements and can be interpreted by wall-induced ordering effects.

Summary form only given. Hybrid recording has already been realized in laser beam assisted perpendicular magnetic recording in audio recordable MiniDiscs. In the year 2000 alone, more than 20 million drives and 220 million disks were shipped. Hybrid recording is expected to be a very strong candidate in the near future in the very high density region, exceeding terabits/in/sup 2/. Here, we report on hybrid recording with the capability to overcome two of the main serious problems; the paramagnetic limit in high density region and the writing speed limit caused by ferromagnetic resonance around 2 to 5 GHz.

Summary form only given. The understanding of coherent switching of a magnetic thin film system is required for practical applications such as the future MRAM memory cell. The Landau-Lifshitz dynamics describing the precessional motion of the magnetic spin system is studied by excitation of lithographically structured micron-sized Permalloy elements by ultra-short magnetic field pulses. The shortest time needed for the reversal of the magnetisation is given by the half of one full precessional oscillation but for this a complete suppression of the magnetisation ringing after the switching process is needed. This requires stopping of the precessional motion at exactly this time, which in practice means stopping this motion on a time scale much shorter than the typical damping time. We will present a technique that enables us to arbitrarily control the magnetisation precession on a picosecond timescale. The method is based on two independently triggered GaAs photoswitches, excited by femtosecond laserpulses. The femtosecond-laserpulse-system allows us to measure the in-plane components of the magnetisation by means of magnetisation induced second harmonic generation (MSHG) while the polar component is probed by linear MOKE.

The development of femtosecond lasers has opened the way to creating external stimuli such as optical or magnetic field pulses that are much shorter than fundamental timescales such as spin-lattice relaxation or precession times and can be used to change the magnetization direction on an ultrafast timescale. In this chapter, two such approaches are discussed: precessional switching by exploiting specifically designed magnetic field pulses, generated by photoconductive switches, and ultrafast thermomagnetic writing using femtosecond lasers in combination with an external static field. Consequences for applications in magnetic storage (MRAM and MO recording) are discussed.

The dynamics of the optically induced phase transition from the antiferromagnetic to the paramagnetic state in FeBO3 is observed using a pump-probe magneto-optical Faraday technique employing 100 fs laser pulses. At the pump energy of 1.55 eV phonon-assisted transitions dominate in the absorption of light and ultrafast heating of the lattice occurs. The quenching of the magnetic order is caused by an increase of the magnon temperature due to energy transfer from the heated lattice. The heating time of the magnon system is around 700 ps, which is a factor of 20 faster than previously reported phonon-magnon interaction times.

Since the invention of the first magnetic memory disk in 1954, much effort has been put into enhancing the speed, bit density and reliability of magnetic memory devices. In the case of magnetic random access memory (MRAM) devices, fast coherent magnetization rotation by precession of the entire memory cell is desired1,2,3,4,5,6, because reversal by domain-wall motion is much too slow. In principle, the fundamental limit of the switching speed via precession is given by half of the precession period. However, under-critically damped systems exhibit severe ringing7,8 and simulations show that, as a consequence, undesired back-switching of magnetic elements of an MRAM can easily be initiated by subsequent write pulses, threatening data integrity. We present a method to reverse the magnetization in under-critically damped systems by coherent rotation of the magnetization while avoiding any ringing. This is achieved by applying specifically shaped magnetic field pulses that match the intrinsic properties of the magnetic elements. We demonstrate, by probing all three magnetization components9,10, that reliable precessional reversal in lithographically structured micrometre-sized elliptical permalloy elements is possible at switching times of about 200 ps, which is ten times faster than the natural damping time constant.

We have developed a method to vary the shape of magnetic field pulses on a picosecond time scale and to monitor the magnetization response in all three dimensions by using time-resolved linear and nonlinear magneto-optical techniques. With this, we have measured the temporal response of rectangular 10/spl times/20 /spl mu/m and elliptical 8/spl times/16 /spl mu/m, 8 nm thin Permalloy (Ni/sub 80/Fe/sub 20/) elements due to in-plane magnetic field pulses with varying length and amplitudes. The data shows coherent precession of the magnetic spins in the elliptical elements, for large and small angle excitations. The shaping of the in-plane field pulses allows the control of the magnetization-motion on a picosecond timescale, so that stopping of the precessional motion of the underdamped magnetic system after one full precessional oscillation could be achieved. The large angle excitation showed a 160/spl deg/ switching of the magnetization within 300 ps. Further, it is demonstrated that the present technique can successfully be applied to improve the data integrity in magnetic random access memories (MRAMs).

Pump-pulse induced magnetization reversal of amorphous ${\mathrm{Gd}}_{23.1}{\mathrm{Fe}}_{71.9}{\mathrm{Co}}_{5.0}$ 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.

We study the factors that ultimately limit the performance of an external enhancement resonator for optical second-harmonic generation (SHG). To describe the resonant SHG process we introduce a theoretical model that accounts for the intensity-dependent cavity loss that is due to harmonic generation and that also includes a realistic assumption about the shape and the frequency width of the laser mode. With the help of this model we optimized the performance of a doubling cavity based on a lithium triborate (LBO) crystal. This cavity was used for frequency doubling the output of a single-frequency titanium-doped sapphire laser at 850 nm. We were able to push the total second-harmonic conversion efficiency to 53% (a 1.54-W pump resulted in 820 mW of second-harmonic light), which to our knowledge is the best result ever reported for a LBO-based doubling cavity.

We deposit a laser-collimated chromium beam onto a substrate through a laser standing-wave (SW) tuned above the atomic resonance at either of the two 52Cr transitions 7S3→7P3o at 427.600 nm or 7S3→7P4o at 425.553 nm. In both of these cases, the resulting pattern on the surface consists of nanolines with a period of that of the SW. We extend the range of periods accessible to laser-focused atom deposition by superimposing the structures grown at both these resonances. The resulting beating pattern exhibits a period of 44.46±0.04 μm as determined with a polarizing optical microscope. This structure provides a link between nanoscopic and macroscopic worlds and could potentially become a calibration standard for length metrology.

The optical pump-probe technique using ultrashort laser pulses with a photon energy of 1.55 eV was used to study the dynamics of the antiferromagnet-paramagnet phase transition in FeBO3. The Faraday magneto-optical effect was measured with a time resolution of 100 fs, and signal transients were observed as functions of sample temperature. The rate of photoinduced phase transition was shown to be limited by the phonon-magnon relaxation rate with a characteristic time of 700 ps. The subpicosecond dynamics of Faraday rotation is not associated with the destruction of magnetic order but is caused by electron photoexcitation and recombination.

The 3rd topical conference Nonlinear Optics at Interfaces (NOPTI 2001) was held from October 15 to 19, 2001 at the Faculty of Science at the University of Nijmegen, The Netherlands. NOPTI 2001 followed the very successful meetings at Kassel (1993) and Berlin (1998) in bringing together active researchers in this field to discuss recent developments and applications of nonlinear optical techniques for surface and interface studies. The conference programme consisted of 15 invited lectures, 25 oral contributions and 35 posters, covering recent advances in timeresolved studies of surface chemistry, electron and spin dynamics, higher-order mixing processes, nano-optics, magnetization-induced nonlinear optical effects and other areas. The field of nonlinear optics in general, and time-resolved nonlinear optical studies in particular, has greatly benefited from the strong development of solid-state pulsed laser sources. This is particularly clear in the strong development of magnetization-induced second harmonic generation and its application to the studies of ultrathin magnetic films. This new area would never have developed so strongly if we were still limited to Q-switched nanosecond laser sources. This is a nice example of the mutually beneficial interaction between science and engineering, a topic that was so beautifully addressed by Nicolas Bloembergen in his well attended evening lecture on the “Synergy of Science and Technology in Nonlinear Optics”. We were indeed very pleased to have both the founding father of the field of nonlinear optics, Nicolas Bloembergen, and the pioneer of the nonlinear optical probing of interfaces, Ron Shen, actively participate at NOPTI 2001, in addition to many other leading experts in the field. NOPTI 2001 was chaired by Th. Rasing, University of Nijmegen, and was organized by the local committee consisting further of A. Kirilyuk, J. Hohlfeld, A. Fasolino andM. van Breemen, all from Nijmegen. The programme committee were M. Aeschlimann, J. Hohlfeld, B. Jerome, B. Koopmans, Th. Rasing and H. Zacharias. NOPTI 2001 gratefully acknowledges financial support from the University of Nijmegen, the NSRIM Institute of the University of Nijmegen, Spectra Physics, Coherent and Springer-Verlag. Finally the editors would like to thank all the authors and reviewers for their contributions and cooperation.