RIKEN Researchers Induce Global Phase Change Using Electrical Current

Micromagnetic simulations of current-induced magnetic phase transitions in an open-boundary system. (a) Ground-state magnetic phase diagram of the Hamiltonian [Eq. (1)] with respect to the magnetic-field h [33, 50]. The color wheel specifies the magnetization direction on the x−y plane. The brightness of the color represents the z component of the magnetization; that is, local magnetizations pointing fully in the z direction are displayed as white. (b) and (c) Snapshots of the evolution of the current-induced magnetic phase change at h = 0.02 (b) and 0.025 (c). The magnitude of the DC spin-polarized current js is 0.01. The unit of time is 1/(γJ) (see Materials and Methods). (d) Magnetic-field-dependent energy hierarchies of the helical skyrmion and ferromagnetic phases in the Hamiltonian [Eq. (1)]. Credit: Physical Review B (2022). DOI: 10.1103/PhysRevB.106.144425

Researchers from the RIKEN Center for Emergent Matter Science have discovered that an electrical current can induce a global phase change in an alloy made of manganese and silicon. This discovery has important implications for low-power computer memory, as it opens up new routes for controlling the properties of microscale materials. The team's results were published in the journal Physical Review B.

Understanding Phase Transitions

The atoms or molecules in a material can interact with one another in many ways and to different degrees. Solids can have many so-called phases of matter, each defined by the relative physical arrangement of the atoms or molecules or by the alignment of their magnetic properties. Changes between these phases are potentially a useful way to store data. For example, creating tiny magnetic swirls known as skyrmions has been proposed as an energy-efficient way of creating high-density computer memory.

Inducing Global Phase Changes with Electrical Current

"It's well known that such global phase transitions can be induced by changing environmental parameters, such as the temperature, magnetic field or pressure," explains Fumitaka Kagawa from the RIKEN Center for Emergent Matter Science. "But it wasn't certain whether an electric current could induce global phase changes."

The team created an 18-micrometer-long bar of manganese–silicon alloy and connected electrical contacts to it. Electrical measurements confirmed the emergence of a skyrmion phase at a temperature of around −250°C.

When the team passed a current through the bar, they observed a change in the material's properties that was indicative of a switch between a skyrmion and a non-skyrmion state. Their results were supported by numerical calculations.

Non-Equilibrium Steady State

The team ruled out the possibility that this change occurred due to the current heating the material, so it was not a thermodynamic phase transition. Also, the same results were not seen in much larger samples, indicating that the confined geometry plays an important role in the non-thermodynamic phase change.

"A sample under a strong current is generally in what is called a non-equilibrium steady state, which is not well understood by today's well-established theories of thermodynamics and statistical mechanics," says Kagawa. "So our results make clear that dramatic phenomena such as phase changes can occur even in these poorly understood regimes."

Implications for Low-Power Computer Memory

The ability to induce a global phase change using an electrical current could have important implications for low-power computer memory. Creating skyrmions has been proposed as an energy-efficient way of creating high-density computer memory. This discovery opens up new routes for controlling the properties of microscale materials.

Journal Information: Takuro Sato et al, Nonthermal current-induced transition from skyrmion lattice to nontopological magnetic phase in spatially confined MnSi, Physical Review B (2022). DOI: 10.1103/PhysRevB.106.144425