An international team of scientists, including researchers from Lancaster University, have discovered the speed of magnetising a material.
The research team consisted of scientists from Lancaster University, the University of California San Diego, the Moscow Institute of Physics and Technology, and Radboud University. Together they have addressed one of the most intriguing questions of magnetism: how fast can magnetisation be generated in a material?
The material becomes a ferromagnet just above room temperature
Scientists considered the common magnetic alloy of iron and rhodium (FeRh), which demonstrates a transition in both its structure and magnetism when heated just above room temperature. At room temperature, FeRh does not have a net magnetisation, due to its antiferromagnetic nature. However, when it is heated to just above room temperature, the material becomes a ferromagnet.
During their investigation, the team observed that FeRh undergoes a transition into its ferromagnetic phase in three stages:
- The excitation of the laser pulse, which induces a large number of tiny magnetic domains in the material;
- The magnetisation of all the domains, which aligns in one particular direction; and
- The individual domains grow to coalesce into a large single domain, where it can be said that the material has undergone a transition into its ferromagnetic phase.
Scientists have noted that the knowledge gathered in this study has prospective applications for utilising FeRh in future data-storage technology. This is due to the observations made during the ferromagnetic phase, such as the various stages involved and the corresponding time scales in inducing a well-defined magnetisation with a light pulse.
Utilising FeRh in data-storage technology
FeRh can potentially be utilised as the storage medium in heat-assisted magnetic recording (HAMR), which is a technology that employs both external heat and local magnetic fields to store information with a much higher density of tiny magnetic regions (or bits) where information is accumulated.
“Understanding the details of various stages involved in the fast emergence of magnetisation in a material helps scientists in developing ultrafast and energy-efficient magnetic data storage technologies,” explained Dr Rajasekhar Medapalli, Physicist at Lancaster University.
The speed of magnetising a material
The research team utilised intense ultrashort laser pulses to rapidly heat FeRh in a brief artificial stimulus, each pulse lasting only a quadrillionth of a second. Upon the interaction with the material, the laser pulse raised the temperature by a few hundred degrees Celsius at timescales shorter than a billionth of a second.
For decades researchers in the scientific community have condensed matter physics to utilise this ultrafast heat to control the magnetic phase transition in FeRh. However, it has been a challenge to experimentally detect this transition.
To overcome this issue, the team considered how time-varying magnetisation produces a time-varying electric field in a medium that should act as an emitter of radiation. The emitted radiation carries sensitive information about its origin, i.e., time-varying magnetisation in the sample.
Scientists employed the novel double pump time-resolved spectroscopy technique that was developed at Radboud University. They utilised two laser pulses for double pumping: while the first laser pulse serves as an ultrafast heater, the second one helps in generating an electric field. By detecting this field at multiple time-lapses between the two laser pulses, researchers were able to consider the speed of magnetising a material.