A thermally activated slow change in the microscopic magnetic domain structure of metallic wires, known as creep, which is induced by external forces has been studied by Hideo Ohno and Shunsuke Fukami of Tohoku University, Japan, and colleagues. Their findings reveal that creep motion induced by current is very different from that by magnetic fields. The research could have implications for future high-performance magnetic memory devices; Nature Physics.
"Creep means a very slow motion of interface or boundary between two different regions under very weak driving forces, and it is seen in various systems including superconductors, ferromagnetic, ferroelectric, and soft materials, bacterial colonies, Earth's plates.
The team made a wire using ferromagnetic cobalt-iron-boron, CoFeB, and looked at the universality class of the magnetic domain wall "creep". They applied a range of magnetic field or electric current intensities while keeping the temperature of the device constant and could thus derive a scaling exponent. They found different temperature-independent scaling exponents, that is, universality class, between the field- and current-driven creeps, meaning that current has a different effect on the magnetic structure to the magnetic field case.
The difference observed between magnetic field and current results was not seen in earlier studies on metallic systems, while it was seen on a semiconductor system. The implication of this finding is that the actions of a magnetic field and current on the domain wall are fundamentally different from each other irrespective of the material, and although magnetic field-driven creep can be understood with a present theory, the current-driven creep cannot be accounted for by any established theories.
From detailed investigations of the behavior of the domain wall under the application of a current, the researchers showed that the current gives rise to an adiabatic spin-transfer torque acting on the domain wall. This force is of a different symmetry to the torque induced by a magnetic field. In other words, for sample in which the stack structure is designed so that the adiabatic spin-transfer torque predominates, the universal characteristics of creep are manifested regardless of whether the material is a metal or semiconductor and irrespective of its microscopic structure. This provides new fundamental insights into the creep motion of elastic interfaces and could be exploited in new devices.
"From a physics point of view, the theoretical model that can explain our observation on the current-induced creep is of particular interest," team member Shunsuke Fukami told Materials Today. "From an application point of view, we would like to explore material systems that effectively exhibit the adiabatic spin-transfer torque for moving the domain wall with a low current density and high speed, which is required for high-performance magnetic memory devices."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".