Wednesday, December 7, 2016

Nearing feasibility for Terahertz spintronics and all-optical spin manipulation

The discovery of femtosecond demagnetization by laser pulses is 20 years old.

Terahertz spintronics and all-optical spin manipulation are becoming more and more feasible. The aim of this perspective is to point out where we can connect the different puzzle pieces of understanding gathered over 20 years to develop novel applications. Based on many observations in a large number of experiments. Differences in the theoretical models arise from the localized and delocalized nature of ferromagnetism. Transport effects are intrinsically non-local in spintronic devices and at interfaces. We review the need for multiscale modeling to address the processes starting from electronic excitation of the spin system on the picometer length scale and sub-femtosecond time scale, to spin wave generation, and towards the modeling of ultrafast phase transitions that altogether determine the response time of the ferromagnetic system. Today, our current understanding gives rise to the first usage of ultrafast spin physics for ultrafast magnetism control: THz spintronic devices. This makes the field of ultrafast spin-dynamics an emerging topic open for many researchers right now.

The ultimate way to gain control over magnetism is through coherent excitation with a light field. This implies an interaction of the laser field directly with the spin system. While coherent control seems feasible with ultrastrong THz field pulses, where the B-field amplitude reaches the Tesla range, there are reports that too much heat is deposited and the coherence is disturbed. For light in the visible region, coherent excitation of ferromagnetism and a corresponding model has been proposed by Bigot et al. In this detailed experiment, they extracted coherent signals that are only present as the laser pulse interacts with the sample, presented in Figure 11, for a CoPt3 film. One can picture a polarization that is driven by the light in a transient state. Those ultrashort polarization effects are also known from other material systems such as MnGaAs and manganites. They leave a typical fingerprint in the complex Kerr rotation that can be described in a Raman-type model. Other approaches have been developed for metals. An interesting pathway is to use this coherent polarization to trigger interactions with another part of the magnetic subsystem as, for example, the spin-polarized surface states in topological insulators, as seen in the different response for the components of the complex Kerr rotation from the Bi2Se3 family, (Bi0.57Sb0.43)2Te3 shown in Figure 11(b). It is believed that these processes are faster than the thermal demagnetization effect. Their investigation will shed light on the inverse Faraday effects and further ultrafast processes that happen faster than the scattering time of the electrons in a coherent state, ultimately leading to attosecond control of magnetization.

Ultrafast magnetism has arrived at the stage of quantitative prediction and understanding. Modeling becomes an important aspect for predictions: the understanding of how much power can be saved for all-optical writing to make it efficient within multiscale approaches leads to new ultrafast all-optical nanomemories addressing nanometer FePt grains. On all timescales, the spin-orbit interaction is one of the main players acting in two ways: resulting in switching asymmetries via magnetic-optics and the control of spin-flips. On the other hand, spin-orbit effects and spin-dependent transport can be controlled on THz time scales for applications. Ultrafast laser pulse based trigger and control of the spin currents and ultrafast spin waves set the stage for THz spintronics. We believe that the combination of ultrafast magnetism and spintronics has more interesting discoveries in fundamental physics and applications in future.

FIG. 11. Coherent control in ferromagnets and topological insulators. Copyright 2009 Macmillan Publishers Limited.140,144,150 Citation: J. Appl. Phys. 120, 140901 (2016);

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