In data storage, magnetization reversal writes bits of information – strings of 1s and 0s – onto single domains in magnetic materials. The magnetization is usually reversed by applying a local magnetic field. To increase the amount of information stored in a device, researchers are making use of materials with increased magnetic energy densities but higher magnetic fields are needed to switch the magnetization here and such high fields are difficult to generate.

A team led by Edgar Bonet, Christophe Thirion and Wolfgang Wernsdorfer has now put forward a new method to overcome this limitation. “We used a constant applied magnetic field that was well below the switching field combined with a radio-frequency (RF) or microwave field pulse to reverse the magnetization of a 20 nm cobalt particle,” explained Thirion. “Importantly, we found that the phase of the magnetic field driving the magnetization reversal of the nanoparticle is an important parameter. Controlling the phase of this driving signal can thus increase its efficiency with no additional cost in terms of energy or time.”

Probing the fundamentals of magnetization

The researchers obtained their results using a pump-and-probe technique involving a pump magnetic field that causes the magnetic moments in the nanoparticle to precess at a certain, controlled frequency, amplitude and phase. The probe signal sent into the sample just afterwards may or may not reverse the nanoparticle’s magnetization, depending on the precessional state originally prepared by the pump. “By then checking the final state of the nanoparticle’s magnetization, we can see whether the probe succeeded in driving the reversal or not,” Bonet told

Such measurements are fundamentally important because varying the delay between the pump and probe pulses allows us to probe the very dynamics of magnetization itself on nanosecond time scales: free precession and “damping”, he said.

“Current microwave-assisted magnetization switching techniques do not take account of phase effects but our new work shows that these effects are important if we want to optimize magnetization reversal,” he added. “Any magnetic device, as long as it is small enough to be single domain, should behave in the same way as the cobalt nanoparticle we studied in our experiments, so our findings will be important for data-recording technologies in general, and especially for optimizing hard-drive writing times and in novel MRAM device technologies.”

The research is detailed in Physical Review Letters.