Ferromagnetic semiconductors (FMSs) combine the unique properties of semiconductors and magnetism, making them ideal candidates for developing spintronic devices that integrate both semiconductor and magnetic functionalities. However, one of the key challenges in FMSs has been achieving high Curie temperatures (TC) that enable their stable operation at room temperature. Though previous studies achieved a TC of 420 K, which is higher than the room temperature, it was insufficient for effectively operating the spin functional materials, highlighting the demand for an increase in TC among FMSs. This challenge has been featured among the 125 unsolved questions selected by the journal Science in 2005. Materials such as (Ga,Mn)As exhibit low TC, limiting their practical use in spintronic devices. While adding Fe to narrow bandgap semiconductors like GaSb seemed promising, incorporating high concentrations of Fe while maintaining crystallinity proved difficult, restricting the attainable TC.
To overcome these limitations, a team of researchers led by Professor Pham Nam Hai from Institute of Science Tokyo, Japan, developed a high-quality (Ga,Fe)Sb FMS using the step-flow growth method on vicinal GaAs (100) substrates with a high off-angle of 10°. Their findings are published in Volume 126, Issue 16 of Applied Physics Letters on April 24, 2025. Utilization of the step-flow growth approach allowed them to incorporate a high concentration of Fe while maintaining excellent crystallinity, resulting in a TC of up to 530 K—the highest reported so far for FMSs.
The team utilized magnetic circular dichroism spectroscopy measurements to confirm the intrinsic ferromagnetism in the (Ga0.76,Fe0.24)Sb layer based on the spin-polarized band structure of FMS. In addition, the team employed Arrott plots, a standard technique for extrapolating the TC from magnetization data. This method helped identify the magnetic transition points, offering a more precise understanding of the material's ferromagnetic behavior at varying temperatures.
"In the conventional (Ga,Fe)Sb samples, maintaining crystallinity at high Fe doping levels was a persistent issue. By applying the step-flow growth technique on vicinal substrates, we successfully addressed this challenge and achieved the world's highest TC in FMSs," says Prof. Hai.
Furthermore, the researchers also investigated the long-term stability of their sample by measuring the magnetic properties of a thinner (Ga,Fe)Sb (9.8 nm) layer stored in open air for 1.5 years. Despite a slight reduction in TC from 530 K to 470 K, the material retained significant ferromagnetic properties, showing its potential for practical applications. Additionally, the material exhibited a large magnetic moment per Fe atom (4.5 μB/atom), which is close to the ideal value for Fe³⁺ ions in a zinc blende crystal structure (5 μB/atom). This is twice that of α-Fe metal, highlighting the superior magnetic properties of the material.
"Our results demonstrate the feasibility of fabricating high-TC FMSs that are compatible with room temperature operations, which is a crucial step towards the realization of spintronic devices," adds Prof. Hai.
Overall, the study highlights the effectiveness of film formation using step-flow growth on vicinal substrates in producing high-quality, high-performance FMSs with higher Fe concentrations. By overcoming the bottleneck of low TC, the study represents a significant step forward toward the realization of spin-functional semi-conductor devices that can operate at room temperature.
