Ultrafast precessional magnetization reversal by picosecond magnetic field pulse shaping

Abstract

Since the invention of the first magnetic memory disk in 1954, much effort has been put into enhancing the speed, bit density and reliability of magnetic memory devices. In the case of magnetic random access memory (MRAM) devices, fast coherent magnetization rotation by precession of the entire memory cell is desired, because reversal by domain-wall motion is much too slow. In principle, the fundamental limit of the switching speed via precession is given by half of the precession period. However, under-critically damped systems exhibit severe ringing and simulations show that, as a consequence, undesired back-switching of magnetic elements of an MRAM can easily be initiated by subsequent write pulses, threatening data integrity. We present a method to reverse the magnetization in under-critically damped systems by coherent rotation of the magnetization while avoiding any ringing. This is achieved by applying specifically shaped magnetic field pulses that match the intrinsic properties of the magnetic elements. We demonstrate, by probing all three magnetization components, that reliable precessional reversal in lithographically structured micrometre-sized elliptical permalloy elements is possible at switching times of about 200 ps, which is ten times faster than the natural damping time constant.

Irreversible magnetization switching using surface acoustic waves

“…This regime will be named "sustained switching, conditional reversal". It relies on the same mechanism as the precessional switching using tailored ultrafast magnetic field pulse which was used in garnets 17,31 but the lower precession frequencies make the adjustment of T SAW less constraining. The optimal value of T SAW for reversal is quite experiment-dependent though, as it depends on both the decay time of the SAW (related to the number of teeth in the IDT), and to the magnetization damping α.…”

“…A short perturbation (e.g an optical 15,16 , acoustic 3 , or ultra-fast magnetic 17 or electric 18 field pulse) then modifies the micromagnetic parameters enough to change the effective field seen by the magnetization, and send it precessing. If the precession amplitude is sufficiently large, the magnetization can switch to another potential valley, where it will remain if the perturbation lasts an odd multiple of half the precession period 17 , or if damping eventually prevents M from oscillating between the two minima ("ringing" phenomenon).…”

Femtosecond Photomagnetic Switching of Spins in Ferrimagnetic Garnet Films

“…This generally involves starting a precession, stopping it, and keeping the magnetization stable along the desired new orientation. Coherent control and even precessional switching on the time scale of a few hundred picoseconds have been demonstrated by different methods based on the shaping of magnetic field pulses [3,7,8]. However, the limitations and complexity of these techniques have stimulated the search for alternative ways to create the equivalent of strong, ultrashort, and localized magnetic field pulses [9,10].…”

“…It circumvents the need for complex devices for the generation of ultrafast magnetic field pulses [2,3], and could bring the switching speed into the femtosecond regime. The mechanism that couples spins with light, the spin-orbit interaction, is also responsible for the magnetocrystalline anisotropy.…”

Exploring nanomagnetism with soft X-ray microscopy

“…to understand the time scale on which magnetization can evolve upon excitation e.g. with ultrafast optical pulses [8,9].…”