Light Reading

Xiaoying Zheng | April 3, 2019

Light Reading

As technology continues to evolve, data writing is growing as an area of interest. Simultaneously, the emerging field of spintronics studies how the spin and magnetic properties of an electron can be used for information processing. All Optical Switching (AOS) is a related technique that reverses magnetization of a magnetic material with short laser pulses. It allows one optical signal to control another optical signal through interfering with the natural magnetization of the signal. In the past decade, since its discovery in gadolinium/iron/cobalt alloys, AOS technology has seen an influx of attention and a boom in research, as it shows potential in increasing the speed and energy efficiency of data writing in spintronic memory devices. This type of data memory relies on setting the spin of electrons. Each electron can assume two states: up and down. These can be utilized to represent data, as different patterns of spin can represent bits of information. Because AOS allows information to travel at near light speed, it has wide appeal in its efficiency if possible. The AOS mechanism still has flaws, causing it to lose some speed and energy efficiency, but researchers have begun to solve some of the issues.

In the past decade, researchers discovered that AOS in Rare Earth-Transition Metals (RE-TM) alloys can be performed in a single pulse process driven by heat. AOS mechanisms in rare earth transition metal alloys, as well as their existence in ferromagnetic thin films and multilayers, were shown. Additionally, AOS has been connected to two promising spintronics research areas, namely racetrack memory and next-generation magnetic random-access memory. Racetrack stores magnetic signals in regions of nanowires that are oppositely oriented, called racetracks. Electric pulses are then applied to the nanowires, which create so-called domain walls between them. The magnetic orientation of each region is then used to store bits of data. If AOS is integrated successfully in racetrack memory, it has the potential to store a high volume of information compared to existing storage devices such as flash drives. However, even though AOS was found to be possible in these methods, it requires many pulses to switch memory states, which bars this type of AOS from being used in fast spintronic devices. Only a thermal single-pulse AOS mechanism can be successfully integrated into spintronic devices.

Searching for new AOS materials

In recent work done by a team at Eindhoven University of Technology, it was found that the thermal single-pulse AOS mechanism needed for spintronic integration existed in multilayers made of a platinum/cobalt/gadolinium (Pt/Co/Gd) synthetic-ferrimagnetic stack, stacks of alloys that display weak permanent magnetism. A member of the team, professor M. L. M. Lalieu, has led research demonstrating that this Pt/Co/Gd stack is ideal for integrating AOS with spintronics and racetrack memory. In this study, Lalieu shows that clear single-pulse AOS in Pt/Co/Gd racetracks is possible, suggesting that the domain walls (which separate magnetic domains) of the optically written domains are chiral Neél walls, meaning they can be moved along the racetrack with the Spin Hall Effect (SHE), a means of transport that relies on the accumulation of homogeneity in spin direction. Additionally, the SHE efficiency in this Pt/Co/Gd racetrack and domain wall velocities are both predicted to be high. Domain wall velocity can be used to predict speed of memory devices, making its value integral to gaining insight regarding the mechanism of AOS in spintronics.

In order to affirm that single-pulse AOS can be found in Pt/Co/Gd wires, tantalum/platinum/cobalt/gadolinium/platinum stacks were deposited on silicon substrates coated with silicon dioxide (SiO2) at room temperature and formed into wires. Each wire had a structure called a Hall Cross. The magnetization in a Hall Cross is measured in order to investigate AOS in these wires. The Anomalous Hall Effect (AHE), a phenomenon that occurs in ferromagnetic solids that measures magnetization, was used in order to do this. To identify single pulses in the AHE measurement, the team utilized slow, repeated laser-pulses. It was shown that the magnetization in the Hall Cross region oscillates between positive and negative states of spin. When measurements with AOS were repeated for a longer time, a one-hundred percent success rate of the AOS was shown, which means that single-pulse AOS of magnetization is, indeed, present in the Pt/Co/Gd wires.

It was expected that the domain walls in the Pt/Co/Gd were chiral Neél walls, which means that the two domain walls were symmetrical with respect to spin organization. These domain walls could move through the wire by means of electrical current through SHE and the accumulation of a common spin. The direction of motion experienced by these domain walls could be determined by the sign of magnetization and chirality of domain walls. In the top layer of the wire, the Pt layer, this motion was reported to go against the direction of electron flow.

Spintronics on the fly

On-the-fly data writing, which is data writing that is changed as the process of writing data is being carried out, has been established through a combination of SHE-driven transport of optically written domains, or domains that are coded with optical signals, and single pulse AOS in racetrack memory. In this type of measurement, AOS is used for writing a domain in a Pt/Co/Gd wire with two Hall Crosses while a current is also flowing through the wire. Because both the domain walls that have the written domains share the same chirality, they move in the same direction, along the direction of the current, immediately as they are written. In this, the full domain is transported through the wire. It is then recorded using AHE.

SHE efficiency and domain wall chirality in optically written domains were also analyzed quantitatively through performing field-driven SHE-assisted domain wall velocity measurements. In this method, an out-of-plane field is applied while a current is sent through the wire. Depending on the polarity of the current, domain wall motion is affected by SHE (either assisted or hindered), so an increase or decrease in velocity is seen. The wires used for these measurements have two Hall Crosses that are exposed to gallium ion irradiation. When the measurement is started, the applied field causes the domain to expand through the wire and pass the two Hall Crosses. This is recorded by a switch in the AHE signal.

It has been demonstrated that thermal single-pulse AOS- and SHE-induced domain wall motion can be combined in a racetrack to utilize the chiral Neél nature of domain walls for efficient and smooth motion of optically written domains. Because of this, the Pt/Co/Gd racetrack is an ideal mechanism for integrating AOS into spintronics. The integration of AOS with racetrack memory could potentially pave the way for integrated photonic memory devices.

This discovery holds much power in the world of spintronics, as it seems that a potential mechanism for creating efficient photonic memory devices is finally plausible. However, many professionals in the field are still skeptical about the application of AOS to widely accessible memory devices. “I don’t think any of these ideas are reality yet,” stated a Yale electrical engineering professor who asked not to be named. “They have remained in the laboratory or on paper for now.” However, if perfected, AOS could alter the field of data writing and memory, offering society more efficient memory devices.