Nearing feasibility for Terahertz spintronics and all-optical spin manipulation

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

http://www.nextbigfuture.com/2016/12/nearing-feasibility-for-terahertz.html

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); http://dx.doi.org/10.1063/1.4958846

New route for switching magnets using light

Impression of an iron oxide crystal lattice. Red: spins of iron ions, Blue: oxygen ions. Green: electrons in their orbit responsible for the exchange interaction. The interaction keeps the spins aligned. A light pulse excites the electrons, changes the exchange interaction and thus releases the spins.
 http://phys.org/news/2015-09-route-magnets.html

An international team led by Radboud University physicists has discovered that reversing the poles of magnets must be possible without a heating or a magnetic field.. A strong pulse of light can have a direct effect on the strong quantum mechanical 'exchange interaction', therefore changing the magnetism (Nature Communications, 16 September 2015).

In 2007, Professor Rasing and his group at Radboud University showed for the first time that fast pulses of laser light can reverse the poles of magnets. This was a paradigm shift as, until then, physicists believed that light could never be strong enough to break the strong magnetic interaction forces. It can, however, and very local heating by the laser pulse in combination with differences in the response times of the constituent atoms can explain this phenomenon. The researchers have now discovered a new way in which light can manipulate magnetisation.

Directly on the electrons

In the article published by Nature Communications on September 16 the researchers show that the light can excite electrons, which in turn can directly influence the strength of the exchange interaction and therefore change the magnetisation. No heat is released in the process, which is good news for magnetic data storage applications as it means that the method requires little energy. Exchange interaction refers to the internal, quantum mechanical forces that make a magnet magnetic.

"We carried out our experiments in iron oxides, including hematite," says project leader Alexey Kimel. "The crystal structure of hematite is a good system to study this mechanism, as the iron ions are neatly separated by oxygen ions in the crystal lattice. Even so, exchange interaction takes place between the iron ions because the electrons interact through the oxygen ions. By exciting the electrons in the oxygen with a pulse of light, we can manipulate the exchange interaction between the iron spins, and perhaps even reverse their polarity in the near future."

Cool savings

Switching with no heat has the potential to revolutionise magnetic data storage. Huge amounts of heat are currently released in large data centres, and good cooling is becoming a big problem. Facebook, for example, is planning to build its new data centre in the north of Sweden for this very reason. "If we can store information using a new, cool method, data storage will be a lot cheaper," explains Kimel.

Measure what you do

The researchers also developed a magnetometer to measure the ultrafast changes they induce in a magnet. They use the freely propagating electromagnetic radiation in the Terahertz frequency range (1 THz = 1012 Hz) emitted by the spins of the magnet. By measuring the changes in this radiation, they are able to measure the effect of light on the magnetisation. "We have produced a magnetometer that measures at the femtosecond scale," says Rostislav Mikhaylovskiy, the first author of the article.

To be continued at FELIX and HFML

The researchers will conduct further studies into switching using light in the new FELIX laser lab and the adjacent HFML in Nijmegen. The strength of magnetic fields generated by HFML is comparable to that of the exchange interaction and the frequency of light waves generated by FELIX can be tuned to affect the electrons and change the strength of the exchange interaction in the most effective way. "This will certainly help us explore this mechanism in greater detail," says Theo Rasing.

 Explore further: Magnetization can surf on the top of a laser-induced sound wave

More information: "Ultrafast optical modification of exchange interactions in iron oxides." Nature Communications. 16 September 2015 DOI: 10.1038/ncomms9190

Abstract-Magnetization and phase transition induced by circularly polarized laser in quantum magnets

Shintaro Takayoshi, Hideo Aoki, and Takashi Oka
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.085150#abstract

We theoretically predict a nonequilibrium phase transition in quantum spin systems induced by a laser, which provides a purely quantum-mechanical way of coherently controlling magnetization. Namely, when a circularly polarized laser is applied to a spin system, the magnetic component of a laser is shown to induce a magnetization normal to the plane of polarization, leading to an ultrafast phase transition. We first demonstrate this phenomenon numerically for an S=1 antiferromagnetic Heisenberg spin chain, where a new state emerges with magnetization perpendicular to the polarization plane of the laser in place of the topologically ordered Haldane state. We then elucidate its physical mechanism by mapping the system to an effective static model. The theory also indicates that the phenomenon should occur in general quantum spin systems with a magnetic anisotropy. The required laser frequency is in the terahertz range, with the required intensity being within a prospective experimental feasibility.

DOI: http://dx.doi.org/10.1103/PhysRevB.90.085150

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Observed live with x-ray laser: electricity controls magnetism

http://www.chemeurope.com/en/news/147248/observed-live-with-x-ray-laser-electricity-controls-magnetism.html

0-03-2014: Data on a hard drive is stored by flipping small magnetic domains. Researchers from the Paul Scherrer Institute PSI and ETH Zurich have now changed the magnetic arrangement in a material much faster than is possible with today’s hard drives. The researchers used a new technique where an electric field triggers these changes, in contrast to the magnetic fields commonly used in consumer devices. This method uses a new kind of material where the magnetic and electric properties are coupled. Applied in future devices, this kind of strong interaction between magnetic and electric properties can have numerous advantages. For instance, an electrical field can be generated more easily in a device than a magnetic one. In the experiment, the changes in magnetic arrangement took place within a picosecond (a trillionth of a second) and could be observed with x-ray flashes at the American x-ray laser LCLS. The flashes are so short that you can virtually see how the magnetisation changes from one image to the next – similar to how we are able to capture the movement of an athlete with a normal camera in a series of images with a short exposure time. In future, such experiments should also be possible at PSI’s new research facility, the x-ray laser SwissFEL. The results will be published in the journal Science. They appear online in advance of print in Science Express on 6 March.

One common method of data storage uses materials in which different magnetic domains can be oriented in different directions. In other words, the tiny elementary magnets inside the material are aligned along two possible directions, which enables one bit to be saved in the material. A bit is the smallest unit of information, for which there are two possibilities, often referred to as 0 and 1. In the storage device, these correspond to the two different magnetic directions. In a real hard drive, which must store a large amount of information, there are many small areas that correspond to single bits. To change the information on the hard drive, the direction of the magnetism in one domain must be flipped. In modern consumer devices this is achieved using a small magnetic field.

An electric field can be generated in a small space more easily than a magnetic field, which means that, in principle, smaller storage devices can be constructed if magnetism is switched by electric fields. A strong connection between magnetic and electric properties is exhibited by so-called multiferroic materials, which have been one of the hottest topics in materials research for a number of years. Researchers from the Paul Scherrer Institute PSI and ETH Zurich have now studied the material TbMnO₃ and demonstrated that its magnetic arrangement can be changed by an electric field in a matter of picoseconds (10^-12 s = one trillionth of a second), which is considerably shorter than the time it takes for today’s hard drives to be switched. “This shows that multiferroic materials can be switched quickly enough electrically for them to be used in magnetic storage devices,” explains Urs Staub, a research group leader at PSI and one of the research project supervisors. “Electric switching could have numerous advantages. In order to generate a magnetic field, you need a coil through which a current flows. An electric field can be generated without current.

“The material we studied can’t be used in technical devices – you need very low temperatures and strong electrical fields to observe the relevant phenomena. However, the basic result probably also applies for materials that are more suitable for applications and will presumably consist of a combination of thin layers of different materials.”

Exposure time: 0.000 000 000 000 1 seconds

The experiment is based on the interaction between pulsed light produced by two lasers – terahertz light generated by a laser which can easily fit into a lab, and the radiation from the x-ray laser LCLS, a large-scale research facility at SLAC National Accelerator Laboratory in Menlo Park, California, that is roughly three kilometres in length. In the experiment, the material was illuminated with short flashes of terahertz-frequency light which were only a few picoseconds long. Light consists of an electric and a magnetic field, which periodically become stronger and weaker. The terahertz flashes were so short that the electric fields in them were only able to perform a few oscillations. With experiments at the LCLS, the researchers were able to demonstrate that the magnetic arrangement was distorted by the flash of light and – with a slight delay – this distortion followed the oscillation of the electrical field within the flash. The magnetic component of the light was too weak to influence the magnetic structure. The x-ray laser generates very short (100 femtoseconds = 0.000 000 000 000 1 seconds) and intense flashes of x-ray light which are so much shorter than the terahertz flash. This allows the x-rays to measure the magnetic distortion along the different stages of its motion, similar to how a camera with a fast shutter speed captures still images of rapid motions. Today, the LCLS is one of two facilities where such experiments are possible. In the future, they will also be possible at the x-ray laser SwissFEL, which is currently under construction at the Paul Scherrer Institute. “An experiment like this can only be conducted at an x-ray laser because only the pulses from the x-ray laser show the magnetic order and are short enough for you to follow the chronological sequences,” explains Staub.

Tilted elementary magnets

Magnetic materials which can be used to store data can have different magnetic arrangements. In today’s hard drives, the magnetic areas are arranged ferromagnetically, which means that the elementary magnets or, to use the technical term, magnetic moments are all pointing in same direction within the area encoding one bit. In the material studied in the experiment, the moments are arranged in rows but in such a way that two neighbouring moments are slightly rotated with respect to each other as opposed to being parallel. If you move from one moment to the next, the direction of the moments keeps turning and overall the sequence of magnetic moments forms a cycloid. Generally speaking, there are two directions in which the moments can turn, clockwise and anticlockwise – and these could correspond to the two values of a bit. To change between “0” and “1”, the magnetic moments would have to change the turning direction within the sequence, which is equivalent to rotating the entire sequence of magnetic moments by 180 degrees.

Positive and negative – offset from each other

The multiferroic material also has another property: electric polarisation, which means that the positive and negative charges are shifted slightly against each other. The interior of the material is constructed from atoms that have fixed positions in a three-dimensional structure. As there are just as many negative charges (electrons) as positive ones (atomic nuclei) in the atoms, the entire material is electrically neutral. Some of the electrons, however, are not bound rigidly to the atomic nuclei. These electrons can be displaced with respect to the atomic nuclei, which means that one side of the material is positively charged, the other negatively. In other words, the material is electrically polarised. In everyday life, electrically polarised materials are primarily known thanks to the piezoelectric effect used to produce sparks in lighters or sound in loudspeakers, for instance.

Electrically and magnetically linked

In TbMnO₃, the electrical polarisation is linked to the magnetic arrangement, which means that if the magnetic moments turn in one direction, this always corresponds to an alignment of the electric polarisation; if you reverse the polarisation, the rotational direction of the magnetic moments also turns around. The researchers studied this coupling in their experiment. Using the alternating electric field of the terahertz pulse, they influenced the electric polarisation and observed the extent to which the magnetic arrangement followed the alternating field. Although the electric field was too weak to actually turn the sequence of magnetic moments by 180 degrees, the scientists were able to observe that it was turned by around four degrees in time with the electrical field. “This procedure is also important for possible applications,” explains Teresa Kubacka, a doctoral student in the Ultrafast Dynamics Group at ETH Zurich and first author of the paper. “The terahertz pulse is designed in such a way that it influences the magnetic arrangement only in this particular way. If the magnetic arrangement in a device could be changed so specifically, much less energy would be wasted and the material would not heat up as much.”

Precision measuring

It is the first time that it was possible to measure such a rapid change in a multiferroic material so precisely. The angle by which the magnetic moments were turned, was determined using the short flashes from the LCLS x-ray laser in a scattering experiment. It involved sending the x-ray beam through the sample studied and observing the directions in which the x-ray light was deflected by the sample. In the case of this material, there are directions in which the light is deflected by the atomic structure and others where the deflection is caused by the magnetic moments. If the magnetic arrangement is changed, the intensity of the deflected x-ray light changes. In the experiment, the researchers measured the intensity of the deflected x-ray beam at different times for a selected direction. Then they calculated how the magnetic moments react to the electric field within the terahertz flash.

Experimental challenges

“One of the challenges of the experiment was to create the terahertz flashes with the correct frequency and guarantee that enough of their intensity reaches the sample. Such pulses were not created directly by a laser, but rather with the aid of special organic crystals hit by laser pulses with another frequency. At ETH Zurich, we are also working on facilities that generate terahertz pulses and working together with the specialists from PSI and LCLS we were able to adapt the lasers available at the LCLS to our experiment’s needs,” says Kubacka.

Original publication:
Large-amplitude spin dynamics driven by a THz pulse in resonance with an electromagnon T. Kubacka et al., Science Express, 6 March 2014