The interaction of subpicosecond laser pulses with magnetically ordered materials has developed into a fascinating research topic in modern magnetism. From the discovery of subpicosecond demagnetization over a decade ago to the recent demonstration of magnetization reversal by a single 40 fs laser pulse, the manipulation of magnetic order by ultrashort laser pulses has become a fundamentally challenging topic with a potentially high impact for future spintronics, data storage and manipulation, and quantum computation. Understanding the underlying mechanisms implies understanding the interaction of photons with charges, spins, and lattice, and the angular momentum transfer between them. This paper will review the progress in this field of laser manipulation of magnetic order in a systematic way. Starting with a historical introduction, the interaction of light with magnetically ordered matter is discussed. By investigating metals, semiconductors, and dielectrics, the roles of nearly free electrons, charge redistributions, and spin-orbit and spin-lattice interactions can partly be separated, and effects due to heating can be distinguished from those that are not. It will be shown that there is a fundamental distinction between processes that involve the actual absorption of photons and those that do not. It turns out that for the latter, the polarization of light plays an essential role in the manipulation of the magnetic moments at the femtosecond time scale. Thus, circularly and linearly polarized pulses are shown to act as strong transient magnetic field pulses originating from the nonabsorptive inverse Faraday and inverse Cotton-Mouton effects, respectively. The recent progress in the understanding of magneto-optical effects on the femtosecond time scale together with the mentioned inverse, optomagnetic effects promises a bright future for this field of ultrafast optical manipulation of magnetic order or femtomagnetism.

/pdf/ultrafast-optical-manipulation-of-magnetic-order-2ms2t5k1zo.pdf

Ultrafast optical manipulation of magnetic order

Inorganic nanoparticles exhibit size-dependent properties that are of interest for applications ranging from biosensing and catalysis to optics and data storage. They are readily available in a wide variety of discrete compositions and sizes. Shape-selective synthesis strategies now also yield shapes other than nanospheres, such as anisotropic metal nanostructures with interesting optical properties. Here we demonstrate that the previously described photoinduced method for converting silver nanospheres into triangular silver nanocrystals--so-called nanoprisms--can be extended to synthesize relatively monodisperse nanoprisms with desired edge lengths in the 30-120 nm range. The particle growth process is controlled using dual-beam illumination of the nanoparticles, and appears to be driven by surface plasmon excitations. We find that, depending on the illumination wavelengths chosen, the plasmon excitations lead either to fusion of nanoprisms in an edge-selective manner or to the growth of the nanoprisms until they reach their light-controlled final size.

Controlling anisotropic nanoparticle growth through plasmon excitation

We experimentally demonstrate that the magnetization can be reversed in a reproducible manner by a single 40 femtosecond circularly polarized laser pulse, without any applied magnetic field. This optically induced ultrafast magnetization reversal previously believed impossible is the combined result of femtosecond laser heating of the magnetic system to just below the Curie point and circularly polarized light simultaneously acting as a magnetic field. The direction of this opto-magnetic switching is determined only by the helicity of light. This finding reveals an ultrafast and efficient pathway for writing magnetic bits at record-breaking speeds.

/pdf/all-optical-magnetic-recording-with-circularly-polarized-2osbqqkcfz.pdf

All-optical magnetic recording with circularly polarized light.

We describe the operation of, and demonstrate logic functionality in, networks of physically coupled, nanometer-scale magnets designed for digital computation in magnetic quantum-dot cellular automata (MQCA) systems. MQCA offer low power dissipation and high integration density of functional elements and operate at room temperature. The basic MQCA logic gate, that is, the three-input majority logic gate, is demonstrated.

/pdf/majority-logic-gate-for-magnetic-quantum-dot-cellular-4pab5ag4pu.pdf

Majority Logic Gate for Magnetic Quantum-Dot Cellular Automata

All magnetically ordered materials can be divided into two primary classes: ferromagnets and antiferromagnets. Since ancient times, ferromagnetic materials have found vast application areas, from the compass to computer storage and more recently to magnetic random access memory and spintronics. In contrast, antiferromagnetic (AFM) materials, though representing the overwhelming majority of magnetically ordered materials, for a long time were of academic interest only. The fundamental difference between the two types of magnetic materials manifests itself in their reaction to an external magnetic field-in an antiferromagnet, the exchange interaction leads to zero net magnetization. The related absence of a net angular momentum should result in orders of magnitude faster AFM spin dynamics. Here we show that, using a short laser pulse, the spins of the antiferromagnet TmFeO3 can indeed be manipulated on a timescale of a few picoseconds, in contrast to the hundreds of picoseconds in a ferromagnet. Because the ultrafast dynamics of spins in antiferromagnets is a key issue for exchange-biased devices, this finding can expand the now limited set of applications for AFM materials.

Laser-induced ultrafast spin reorientation in the antiferromagnet TmFeO3.

We have prepared a grooved indium tin oxide (ITO) surface with groove widths of ∼40–90 nm and a variable groove separation up to 36 μm using atomic force microscopy nanolithography Twisted nematic pixels with 4-n-pentyl-4-cyanobiphenly (5CB) liquid crystal were prepared using a rubbed polyimide counter plate The 5CB molecules align along the direction of the grooves, but no pretilt angle was observed on the ITO surface The surface anchoring energy appeared not to depend on the groove separation nor on the scan force The lateral correlation length of 5CB on ITO was measured to be 22 μm If a grooved surface is scanned in another direction, liquid crystal molecules follow the direction of the last scan

/pdf/submicron-liquid-crystal-pixels-on-a-nanopatterned-indium-34bvisspki.pdf

Submicron liquid crystal pixels on a nanopatterned indium tin oxide surface

Picosecond control of coherent magnetisation dynamics in permalloy thin films by picosecond magnetic field pulse shaping

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).

Picosecond precessional magnetization reversal by magnetic field pulse shaping

The as-deposited characteristics and the thermal stability at 300 °C of CoFe and Ru based ultrathin multilayers have been investigated and structural and magnetic characteristics have been correlated. These layer systems constitute the basis of the hard artificial antiferromagnetic subsystem used in current sensors. The coercive field of 2 nm CoFe (3.5 kA/m) is found to increase linearly by a factor of 13 upon increasing the Ru seed layer thickness from 5 to 40 nm. The weakening of the exchange coupling due to the sharpening of the crystallite boundaries is likely the origin. Magnetization-induced second-harmonic generation measurements show that the CoFe/Ru interface remains magnetically active after 36 h with a magnetic contrast of 0.17, indicating that no strong mixing at the interface is taking place. In accordance, the saturation field of the antiferromagnetic coupling in Ru5/CoFe2/Ru0.7/CoFe2/Ru (nm) does not change during 20 h of annealing. However, slowly a remanent magnetization develops which is...

/pdf/investigation-of-the-as-deposited-characteristics-and-the-4woqc0jmkl.pdf

Investigation of the as-deposited characteristics and the thermal stability of CoFe and Ru based multilayers

The second harmonic light scattering by uniformly and non-uniformly magnetized magnetic dots is theoretically investigated for both s- and p-polarized incident light. It is shown that the contribution of in plane and out of plane magnetization components of dots can be separated by a polarization analysis of the scattered light at the second harmonic frequency.

Second harmonic light scattering by magnetic dots

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