Watching the first steps of magnetic information transport

Figure: Watching the ultrafast spin Seebeck effect

News from Oct 15, 2018

In conventional electronics, information is encoded in bits (0 or 1) by the presence or absence of electron charges. A promising new approach - spintronics - aims to use the electron ‘spin’ as an information carrier. This method takes advantage of the orientation (up or down) of the electron spin to encode information. The speed at which electronics operate continues to increase and is expected to work at terahertz speeds in the future. To be competitive and compatible with charge-based electronics, spintronic operations must, therefore, also work at these high frequencies.

An elementary but vital spintronic operation is the transport of spin-based information from a magnetic metal layer into an attached nonmagnetic metal layer (see figure). It was discovered only a few years ago that this transfer can happen simply by heating the magnet and metal to different temperatures. When heating the magnetic layer, hot electrons move into the colder nonmagnetic metal, thereby carrying magnetic information across the interface of the two layers.

What is remarkable is that this transfer still occurs when the magnetic layer is an electrical insulator—meaning electron currents cannot move across the interface. The spin transfer happens instead from the torque exerted by the immobile spins of the magnetic layer onto the spins of the neighbouring mobile electrons in the metal layer. This phenomenon is called the spin Seebeck effect.

In the framework of the CRC/TRR 227 at the Freie Universität Berlin, a team of scientists from Germany, England and Japan aimed to discover just how quickly the spin transfer can happen. “Answering this question is not only interesting for potential applications in future high-speed information technology. It is also relevant to understand the elementary steps that lead to the emergence of the spin current”, says physicist Dr. Tom Seifert, who conducted the experiments at the Fritz Haber Institute of the Max Planck Society in Berlin.

In their experiment, the researchers used a pulse from a femtosecond laser to heat up a metal film on top of a magnetic insulator in less than one millionth of a millionth of a second (see figure). The metal itself then emitted an electromagnetic pulse caused by the spin current flowing into it - behaving like an ultrafast spin-amperemeter. Using the emitted pulse, the researchers observed the formation of the spin current caused by the spin Seebeck effect. Once heated, the electrons in the metal hit the metal-insulator interface and are reflected back. During this scattering event, the magnet exerts torque on the incident electron’s spin, aligning it a little more parallel to the magnetization M of the insulator. Thus, spin information of the magnetic insulator is transported into the metal (see figure at time 0 femtoseconds).

The researchers made a surprising observation - the spin transport does not begin immediately, taking about 200 femtoseconds to peak. The reason is that the laser pulse excites relatively few electrons, but they receive a lot of energy and collide with ‘cold’ electrons, redistributing the energy. This avalanche-like process heats up a large number of electrons which also hit the interface, becoming a part of the spin transport (see figure at time 100 femtoseconds). “The photoexcited electrons need to multiply their numbers to generate sizeable spin transport”, says theorist Dr. Joseph Barker, who conducted simulations of the spin dynamics at the Tohoku University in Sendai, Japan.

Finally, the electrons cool down by transferring heat to the atomic lattice of the metal, and after 1000 femtoseconds, the spin transport finishes (see figure). In effect, the instantaneous spin current is also a measure of the effective temperature of the electrons in the metal. “Our ultrafast amperemeter also acts like an ultrafast thermometer. This is very useful for studying spin and electron dynamics in a broad range of materials which hold a great potential for applications in spintronics and terahertz photonics”, notes Dr. Tom Seifert.