Ferromagnetic semiconductors (FMSs) combine the unique properties of semiconductors and magnetic materials, making them ideal candidates for the construction of next-generation spintronic devices with both electronic and spin functions. However, FMSs have long faced a key bottleneck in practical applications - the Curie Temperature (TC) is too low, which is difficult to meet the requirements of stable operation at room temperature. Although the TC has been increased to 420 K, which is higher than room temperature, it is still insufficient to support efficient spin-functional devices. This conundrum was even listed by the journal Science as one of the world's 125 unsolved scientific questions in 2005.
Materials commonly used in previous research, such as (Ga, Mn)As, are difficult to be applied to practical devices due to the low Curie temperature. However, the attempt to improve TC by introducing Fe into narrow bandgap semiconductors such as GaSb is limited by the degradation of crystal quality caused by high concentration doping, and it is difficult to achieve a material design that combines high TC and high crystal quality.
In response to this challenge, the team of Professor Pham Nam Hai of Tokyo Institute of Technology in Japan proposed an innovative solution. They successfully fabricated high-quality (Ga, Fe)Sb ferromagnetic semiconductor films using a step-flow growth method on a high-declination angle (10°) GaAs (100) step substrate. This method significantly improved the crystallinity of the material at high Fe doping concentrations, so that the samples with Fe content of up to 24% still maintained good crystal structure, and the TC reached 530 K, setting a new record in the field of FMSs. The research was published in Applied Physics Letters.
Figure: Ferromagnetic semiconductors achieve a Curie temperature of 530 K, helping the development of room-temperature spintronic devices (Source: Phys)
The team verified the intrinsic ferromagnetism of the material through magnetic circular dichroism (MCD) spectroscopic measurements, and analyzed the magnetic transition behavior in combination with the Arrott plot method, so as to more accurately evaluate the magnetic evolution of the material at different temperatures.
"Conventional (Ga,Fe)Sb materials have always been difficult to maintain crystal quality while increasing Fe content," Professor Hai said. Through the combination of step-flow growth and step substrates, we were able to break through this technical barrier and achieve the highest TC to date among FMSs for the first time.”
The researchers also evaluated the long-term stability of the material. They tested a thin film sample (9.8 nm thick) exposed to the environment for 1.5 years, and although the TC decreased (from 530 K to 470 K), it still retained significant ferromagnetism, showing good practical potential. More remarkably, the material has a magnetic moment of up to 4.5 μB/atom per unit Fe atom, which is close to the ideal theoretical value of Fe³⁺ (5 μB/atom) and twice that of the pure metal α-Fe, demonstrating its excellent magnetic properties.
"Our study shows that FMSs with high Curie temperatures are fully achievable, which is critical for the operation of spintronic devices at room temperature," said Prof. Hai.”
In summary, this study not only made a key breakthrough in material design and fabrication methods, but also laid an important foundation for the realization of a new generation of room-temperature spin functional semiconductor devices, demonstrating the great potential of step-flow growth technology to improve the performance of FMSs.
