Researchers of the National Science Foundation center for Energy Efficient Electronics Science (E3S) at the University of California Berkeley have discovered a novel technique to reverse the magnetization of magnets at high speeds by shooting thin films of magnetic materials with very short and energetic laser pulses. The study, headed by Berkeley Electrical Engineering and Computer Sciences professor Jeffrey Bokor, demonstrated that a magnetic film comprising cobalt and platinum switches its magnetization in seven picoseconds (a picosecond is a trillionth of a second), a record for such magnetic materials. This result could lead to high-speed magnetic devices that would drastically reduce electronic power consumption.
In an electronic device, such as the RAM of a computer, the magnetization of a magnet can be used to store information. “Magnetization” is a term that indicates the magnetic strength of materials. For technological applications, magnetic films of only a few nanometers in thickness are used. One nanometer is a billionth of a meter, which is about 100,000 times thinner than an average strand of human hair. Thin magnetic films typically have only two directions their magnetization can point (called “up” and “down” for convenience) and can store 0’s and 1’s, respectively. Magnetic devices, called spintronic devices, have advantages over the silicon or other semiconductor transistors typically found in all computers and smartphones because magnets are non-volatile. This means that a magnet can retain its magnetic state without the need for external power. RAMs made out of silicon transistors, on the other hand, are volatile, meaning they will lose stored memory once the power supply is shut off. Devices based on spintronics could therefore lead to tremendous savings in energy, given the ubiquity of electronic gadgets.
Until now, spintronic devices have used magnetic stimuli to switch between the two magnetic states. External magnetic fields – like those from an electromagnet or the magnetic field around a current carrying wire – and currents of electrons with a particular direction of their spin (called spin-polarized currents) are examples of magnetic stimuli in spintronic devices. These switching techniques are quite slow—a few hundred or thousand picoseconds. This limitation in speed is a major drawback compared to the much faster silicon-based devices, and therefore magnetic RAMs (MRAMs) have still not made a mark in the consumer market, despite their energy efficiency.
The Berkeley study builds on pioneering experiments performed in the Netherlands in 2011 that demonstrated that an alloy of gadolinium, iron and cobalt (called GdFeCo) can reverse its magnetization really fast – a few picoseconds – when hit with a short and intense laser pulse, a non-magnetic stimulus. The physics of this high-speed switching between the two magnetic states depends on very fast heating of GdFeCo by the laser pulse. This is completely dissimilar to the switching of a magnet by a magnetic stimulus, and has so far only been observed in GdFeCo. However, GdFeCo belongs to a special class of magnetic materials called ferrimagnets, which are not practical for spintronic devices. An energy efficient and high-speed spintronic device needs ferromagnets, more commonly just called “magnets”, to switch in similar timescales as GdFeCo.
The team sought ways to quicken the magnetization reversal of a conventional (non-GdFeCo) magnet. They deposited a thin (ferro)magnet film with layers of cobalt and platinum – a popular magnet frequently used in older, slower spintronic devices – on top of a film of GdFeCo. “Such a stack of magnetic materials couples the magnetizations of GFC and cobalt-platinum magnet”, said Dr. Jon Gorchon, a post-doctoral researcher who formulated the experiments. “We predicted that the GdFeCo would transfer its high-speed switching capabilities to the ferromagnet through the strength of this coupling.”
The researchers then shot the magnetic stack with short laser pulses. A technique called magneto-optical Kerr effect (MOKE) microscopy was used to confirm that the two films switched after each pulse. They monitored the change in magnetization of their films with time after being hit with a laser pulse, and the time required for its reversal, using another technique called time-resolved magneto-optical Kerr effect (TR-MOKE). With some modifications to their experimental setup, the team was able to study the magnetizations of GdFeCo and cobalt-platinum separately. They observed, as expected, that first the GdFeCo reverses its magnetization around two picoseconds after the laser pulse hits. The cobalt-platinum follows soon, and switches its magnetization after five picoseconds, owing to its coupling with the GdFeCo.
“Such high-speed magnetization reversal in a ferromagnet is unprecedented, and is twenty times faster than previous records in spintronic devices. We can extend these results to any common ferromagnet”, says Professor Bokor. This discovery, published in the July 2017 edition of Applied Physics Letters, paves the way for fast spintronic devices for various logic and memory applications, and could greatly reduce the energy consumption of billions of electronic gadgets.