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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.

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.

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.

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.

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.

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.

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.

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.

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.

An experimental scheme (double pump/reflectivity probe using femtosecond laser pulses) enables the investigation of nonequilibrium electron dynamics at metal surfaces by measuring the equilibrated surface temperature. The competition between electron-phonon coupling and hot-electron transport gives rise to a reduced equilibrated temperature when the two pump pulses overlap in time, and provides a way of accurately determining the electron-phonon coupling constant. These observations have important consequences for femtosecond photochemical investigations.

Ultrafast magnetization dynamics of nickel has been studied for different degrees of electronic excitation, using pump-probe second-harmonic generation with 150 fs/800 nm laser pulses of various fluences. Information about the electronic and magnetic response to laser irradiation is obtained from sums and differences of the SHG intensity for opposite magnetization directions. The classical M(T)-curve can be reproduced for delay times larger than the electron thermalization time of about 280 fs, even when electrons and lattice have not reached thermal equilibrium. Further we show that the transient magnetization reaches its minimum approx. 50 fs before electron thermalization is completed., Comment: 8 pages, 5 figures, revtex

Contains fulltext : 57910.pdf (Publisher’s version ) (Open Access) We deposit a laser-collimated chromium beam onto a substrate through a quasiperiodic laser standing-wave (SW) tuned above the atomic resonance at the Cr-52 transition S-7(3)-->P-7(4)o at 425.55 nm. This SW is created by interference of five laser beams crossing in one point at mutual angles of 72degrees. The resulting chromium pattern on the substrate surface mimics the geometry of the SW and it is thus itself quasiperiodic. On a surface area of 0.2x0.2 mm(2) the spatial Fourier spectrum of the measured patterns is decagonal. Besides being of fundamental interest, this quasiperiodic nanofabrication via atom optics can find its applications in photonics.