Significant increase in magnetization within an ultrafast timeframe of 600 femtoseconds realized by the University of Tokyo

Generally, magnetization control in spintronics devices requires several nanoseconds, which is an order of magnitude slower than the time required for conventional semiconductor transistors. A research group led by Associate Professor Le Duc Anh, Associate Professor Masaki Kobayashi, Project Research Associate Takahito Takeda and Professor Masaaki Tanaka of the School of Engineering at the University of Tokyo, in collaboration with a research group led by Graduate Student Toshihide Sumi of the Graduate School of Science at the University of Tokyo, Assistant Professor Masafumi Horio and Professor Iwao Matsuda of the Institute for Solid State Physics at the University of Tokyo, and a research group led by Assistant Professor Kohei Yamamoto of the Institute for Molecular Science (IMS) at the National Institutes of Natural Sciences, Researcher Yuya Kubota and Group Director Makina Yabashi of RIKEN SPring-8 Center (RSC) and Senior Chief Researcher Shigeki Owada of Japan Synchrotron Radiation Research Institute (JASRI), successfully increased magnetization within a timeframe as short as 600 femtoseconds. The involved magnetization was achieved by irradiating a semiconductor quantum well (QW) structure containing ferromagnetic semiconductors (In,Fe)As with a 30 femtosecond pulsed laser. The obtained research findings indicate a new path toward the realization of spintronics and quantum devices capable of operating at ultra-high speeds with low power consumption. The findings have been published in Advanced Materials.

In the experiment, a pump lazer (infrared light) with an ultra-short duration was irradiated onto a QW structure comprising a ferromagnetic semiconductor (In,Fe)As and a non-magnetic semiconductor InAs. The time variation of the Fe magnetic moment within the QW was observed using a synchronized probe laser (X-ray free-electron laser, XFEL). When the ferromagnetic QW was magnetized, the XFEL beam reflected from the QW rotated the polarization plane (Kerr rotation). This rotation was detected using a rotating polarizer and photodetector. The intensity of the reflected XFEL beam detected by the photodetector indicated the magnetization of the ferromagnetic QW. The 30-femtosecond pulsed laser was used as the pump light, and within an incredibly short time span (600 femtoseconds), the level of magnetization increased. This represents the world's first demonstration of ultrafast magnetization control achieved through wavefunction (WF) manipulation.

Based on an analysis of the experimental results and theoretical simulations, carriers (electrons and holes) generated by the femtosecond pulsed laser light were concluded to not directly interact with the Fe magnetic moment within the ferromagnetic semiconductor layer. However, owing to the surface potential created by these spatial charges, the WF of 2D electrons confined within the QW and the corresponding electron density distribution changed rapidly. Consequently, the magnetic interactions between the Fe magnetic moments substantially increased at an ultrafast rate, leading to an enhancement in the magnetization (macroscopic magnetic order based on the total sum of all Fe magnetic moments).

In conventional ferromagnetic materials, a significant change in the electron densities of the d or f orbitals of materials is essential for magnetization enhancements. However, rapid and extensive modulations of the electron densities of materials through electrical means, such as the gate voltages of field-effect transistors, are extremely challenging. By contrast, the reported ultrafast magnetization control method based on WF engineering represents a breakthrough, as it focuses on controlling the WFs within semiconductors rather than conventionally altering the carrier density.

Journal Information
Publication: Advanced Materials
Title: Ultrafast Subpicosecond Magnetization of a 2D Ferromagnet
DOI: 10.1002/adma.202301347