L. Le Guyader

Ultrafast heating as a sufficient stimulus for magnetization reversal in a ferrimagnet

The question of how, and how fast, magnetization can be reversed is a topic of great practical interest for the manipulation and storage of magnetic information. It is generally accepted that magnetization reversal should be driven by a stimulus represented by time-non-invariant vectors such as a magnetic field, spin-polarized electric current, or cross-product of two oscillating electric fields. However, until now it has been generally assumed that heating alone, not represented as a vector at all, cannot result in a deterministic reversal of magnetization, although it may assist this process. Here we show numerically and demonstrate experimentally a novel mechanism of deterministic magnetization reversal in a ferrimagnet driven by an ultrafast heating of the medium resulting from the absorption of a sub-picosecond laser pulse without the presence of a magnetic field.

Controlling optical transmission through magneto-plasmonic crystals with an external magnetic field

The magneto-optical properties of surface-plasmon polaritonic crystals on ferromagnetic substrates have been studied. The resonant optical transmission of such magneto-plasmonic nanostructures can be efficiently controlled with the applied static magnetic field. The effect is explained by the influence of magneto-optical effects on surface-plasmon polariton waves supported by the metal/magnetic-dielectric interface and, in particular, on the plasmonic bandgap formation.

Nanoscale sub-100 picosecond all-optical magnetization switching in GdFeCo microstructures.

Ultrafast magnetization reversal driven by femtosecond laser pulses has been shown to be a promising way to write information. Seeking to improve the recording density has raised intriguing fundamental questions about the feasibility of combining ultrafast temporal resolution with sub-wavelength spatial resolution for magnetic recording. Here we report on the experimental demonstration of nanoscale sub-100 ps all-optical magnetization switching, providing a path to sub-wavelength magnetic recording. Using computational methods, we reveal the feasibility of nanoscale magnetic switching even for an unfocused laser pulse. This effect is achieved by structuring the sample such that the laser pulse, via both refraction and interference, focuses onto a localized region of the structure, the position of which can be controlled by the structural design. Time-resolved photo-emission electron microscopy studies reveal that nanoscale magnetic switching employing such focusing can be pushed to the sub-100 ps regime.Abstract The recently discovered magnetization reversal driven solely by a femtosecond laser pulse hasbeen shown to be a promising way to record information at record breaking speeds. Seeking toimprove the recording density has raised intriguing fundamental question about the feasibility tocombine the ultrafast temporal with sub-wavelength spatial resolution of magnetic recording. Herewe report about the rst experimental demonstration of sub-di raction and sub-100 ps all-opticalmagnetic switching. Using computational methods we reveal the feasibility of sub-di raction mag-netic switching even for an unfocused incoming laser pulse. This e ect is achieved via structuringthe sample such that the laser pulse experiences a passive wavefront shaping as it couples andpropagates inside the magnetic structure. Time-resolved studies with the help of photo-emissionelectron microscopy clearly reveal that the sub-wavelength switching with the help of the passivewave-front shaping can be pushed into sub-100 ps regime.

Demonstration of laser induced magnetization reversal in gdfeco nanostructures

Magnetization switching by a single femtosecond laser heat pulse is demonstrated for out-of-plane domains with sizes down to 200 nm in GdFeCo nanostructures. A complex magnetic domain configuration was revealed with a photoemission electron microscope employing x-ray magnetic circular dichroism at the Fe L3 edge and consisted of in-plane magnetized rims and out-of-plane domains, which results from the structuring process. No influence of this complex domain pattern on the switching efficiency of the structures was detected, constituting an important step towards the application of laser induced magnetization switching in storage devices.

Dipolar energy states in clusters of perpendicular magnetic nanoislands

We investigated the energy states in compact clusters of ferromagnetic islands with perpendicular anisotropy arranged on a triangular lattice. Due to their finite nature, we were able to determine the energies of all possible cluster states using dipolar energy calculations. We employed photoemission electron microscopy to observe the magnetic states in arrays of clusters of monodomain Co/Pt multilayer islands and following demagnetization, we observed a shift in the energy distribution to lower energies as the dipolar coupling increased. These multistate island clusters not only provide model arrangements of frustrated Ising-type nanomagnets but are also interesting for data storage applications.

Coherent control of surface plasmon polariton mediated optical transmission

We demonstrate the coherent control of the surface plasmon polariton mediated optical transmission through arrays of nano-holes in a gold film on a garnet substrate by a standing surface acoustic wave (SAW) induced by a sequence of ultrashort laser pulses. The transmission of the pump pulse through the holes results in a periodic pattern of shock-waves leading, via interference, to a standing SAW. The resulting modulation frequency is determined by the periodicity of the arrays. The polarization of the transmitted light is modulated as well via a strain-induced linear dichroism.

Deterministic character of all-optical magnetization switching in GdFe-based ferrimagnetic alloys

Using photo-emission electron microscopy with X-ray magnetic circular dichroism as a contrast mechanism, new insights into the all-optical magnetization switching (AOS) phenomenon in GdFe based rare-earth transition metal ferrimagnetic alloys are provided. From a sequence of static images taken after single linearly polarized laser pulse excitation, the repeatability of AOS can be measured with a correlation coefficient. It is found that low coercivity enables thermally activated domain wall motion, limiting in turn the repeatability of the switching. Time-resolved measurement of the magnetization dynamics reveal that while AOS occurs below and above the magnetization compensation temperature TM, it is not observed in GdFe samples where TM is absent. Finally, AOS is experimentally demonstrated against an applied magnetic field of up to 180 mT. PACS numbers: 75.78.Jp, 68.37.Yz, 75.70.Kw, 75.50.Gg 1 ar X iv :1 41 2. 03 96 v1 [ co nd -m at .m tr lsc i] 1 D ec 2 01 4

Reversible switching between bidomain states by injection of current pulses in a magnetic wire with out-of-plane magnetization

The influence of current pulses on the domain structure of a 2μm wide wire composed of a soft out-of-plane magnetized magnetic material is studied by high spatial resolution nonintrusive magnetic imaging. The injection of current pulses (1012A∕m2) leads to stable magnetic states composed of two domains with opposite magnetization direction separated by a domain wall parallel to the wire. The direction of the magnetization in the domains is reversed back and forth by applying successive current pulses with opposite polarity. The formation and control of the domain states by the current is attributed to the effect of the Oersted field, which is calculated to be large enough to induce the switching.

Control of the magnetization in pre-patterned half-metallic La0.7Sr0.3MnO3 nanostructures

The evolution of the magnetization configurations in highly spin polarized La0:7Sr0:3MnO3 (LSMO) thin film elements (20-60 nm in thickness) as a function of external magnetic field and temperature is studied by direct magnetic imaging using x-ray magnetic circular dichroism photoemission electron microscopy. The sample structuring is done via a pre-patterning process using a Cr mask layer. The LSMO grows amorphous on the Cr layer for the 20 nm thick film but polycrystalline at larger thicknesses. Temperature dependent studies allow for a direct comparison of the properties of the strained and unstrained LSMO regions on a single sample and show that the polycrystalline areas exhibit a higher TC compared to the epitaxial areas. The single crystalline areas are largely magnetically decoupled from the matrix. The magnetic switching between domain states and domain wall spin structures is determined for LSMO ring elements of varying size and thickness. We find that the magnetic field values required to depin domain walls or to nucleate domains increase with decreasing ring width due to the increasing role of shape anisotropy and edge defects. Both transverse and vortex domain walls are stable spin configurations at room temperature and at zero field. In particular, we demonstrate that the desired domain wall type can be selected by applying an appropriate field sequence.

The effect of magnetocrystalline anisotropy on the domain structure of patterned Fe2CrSi Heusler alloy thin films

The effects of magnetic anisotropy on domain structures in half-metallic Heusler alloy Fe2CrSi thin film elements were investigated using high resolution x-ray magnetic circular dichroism photoemission electron microscopy. The transition of the dominating contribution from the magnetocrystalline anisotropy to the shape anisotropy is observed in square-shaped elements when reducing the size below 2.0–2.5 μm. In particular, we identify in disk-shaped Heusler elements the vortex state as the ground state. The shape-anisotropy dominated, well-defined magnetization configuration shows the potential of the Fe2CrSi Heusler alloy for applications in vortex-core- or domain-wall-devices, where the high spin polarization is desirable.