Sub-picosecond magnetisation manipulation via femtosecond optical pumping has attracted wide attention ever since its original discovery in 1996. However, the spatial evolution of the magnetisation is not yet well understood, in part due to the difficulty in experimentally probing such rapid dynamics. Here, we find evidence of a universal rapid magnetic order recovery in ferrimagnets with perpendicular magnetic anisotropy via nonlinear magnon processes. We identify magnon localisation and coalescence processes, whereby localised magnetic textures nucleate and subsequently interact and grow in accordance with a power law formalism. A hydrodynamic representation of the numerical simulations indicates that the appearance of noncollinear magnetisation via optical pumping establishes exchange-mediated spin currents with an equivalent 100% spin polarised charge current density of 10 7 A cm −2 . Such large spin currents precipitate rapid recovery of magnetic order after optical pumping. The magnon processes discussed here provide new insights for the stabilization of desired meta-stable states.

A new general method was developed suitable for measuring the lifetime of excited states in the condensed phase. This method is based on a multistep photoemission process in which the decay of an excited state generated by one pulse is measured by the use of a delayed probe pulse. The effective lifetime of a chargetransfer exciton state in tetracene was measured to be -20 ps.Photoemission of electrons from a condensed phase can occur even when the absorbed photon energy is lower than the ionization potential of that phase. In such cases, the mechanisms of photoionization may involve interactions between excited states or excited state-photon absorption phenomena, giving rise to superlinear intensity dependences of photoelectron yields. In general, it is difficult to identify the excited states which serve as intermediates in second or higher order photoionization processes, particularly if their lifetimes are in the subnanosecond range.We have developed a new general method for measuring picosecond lifetimes of transient electronically excited states in the condensed phase, and the same method can be used to measure subpicosecond lifetimes. This method is based on a multistep photoemission process in which the rate of decay of an excited state generated by a first pulse is measured by the use of a second (probe) pulse. We have used crystalline tetracene as a model system, and our provisional conclusion is that we have observed a charge-transfer state with an energy of 2.9 eV and an effective lifetime of -20 ps in the surface region ( N 15 A) of the crystal.Photoemission occurs via a single-photon ionization of this CT state.Photoemission in organic molecular crystals is competitive in rate with that of decay from higher excited states to lower excited states within'a molecular manifold. These rates can be in the subpicosecond range.' Tetracene is of interest because it has been well-studied, can be obtained in pure form, has a low fluorescence efficiency* (which makes it a good candidate for a nonfluorescence technique), and has a charge-transfer (CT) state3 whose lifetime has never been measured. In this paper, we show that using a multistep process for producing photoemission, it is possible to measure lifetimes.The experimental technique involves the use of our modified Millikan ~h a m b e r ,~ in which a small crystal ( 4 0 -p m diameter) of tetracene is suspended in nitrogen at 1 atm of pressure. The suspended crystal is negatively charged initially, and the loss of that chargeas a result of photoemission is the measured quantity. By measuring the rate of the loss of charge, the emission photocurrent is deduced. We measure the loss of charge by recording the field strength that is required to balance the crystallite against the force of gravity.4In a typical experiment, a suspended crystallite is excited by the first light pulse beam consisting of the 15 pulses/s, 355-nm ' harmonic of a YAG laser (20-ps fwhm as measured using a pentacene crystal as a detector, to be described more fully in another paper), an...