The interface between superconductor (S) and ferromagnet (F) is a hot spot in condensed matter physics. The interfacial coupling between the two produces more interesting physical phenomena.The magnetic near-neighbor effect at the S/F interface is caused by the exchange interaction between the electron spins on both sides of the interface, leading to the suppression of the magnetic order or the emergence of unconventional superconductivity. When a magnetic material is in close proximity to a superconductor, the magnetic field enters a region of only a few nanometers within the superconductor and destroys Cooper pairs, resulting in spatial changes in the superconducting behavior of the interface and affecting the macroscopic physical properties of the material on both sides. Currently, superconducting spintronics has become an emerging field that plays an important role in realizing dissipation-free spin logic and storage technologies.
Currently, the underlying mechanism of the magnetic proximity effect at the S/F interface in different material systems is controversial. Previously, it was observed that the superconducting transition temperature oscillates with the thickness of the ferromagnetic layer in S/F heterojunctions composed of metal alloys, suggesting that there may be a special mode of transmission of superconducting pairing waves in this system due to the strong exchange field. With the development of advanced thin-film preparation techniques, researchers have begun to study single-crystal oxide S/F heterointerfaces such as the high-temperature superconductor (YBa2Cu3O7)/spin-polarized semimetallic ferromagnet (La1-xCaxMnO3) interface. It is found that the interface has a reduced magnetic moment and the spin antiparallelism of the transition metal ions on both sides of the interface, and is affected by the electronic state of the magnetic layer, the thickness of the S layer, and the non-uniform domain structure. Observing the suppression of the superconducting transition temperature, the increase in transition width, and the spin-valve properties in S/F heterojunctions, the researchers found that this particular type of interface could be beneficial in the development of superconducting spintronic devices.
Guo Erjia, a researcher at the Institute of Physics, Chinese Academy of Sciences, and Jin Kuijuan, an academician at the Chinese Academy of Sciences, prepared Fe3N/VN heterojunctions on sapphire substrates using a pulsed laser deposition technique assisted by a radio-frequency nitrogen (RFN) atomic source, and characterized their structures. x-ray diffraction profiles showed that both Fe3N and VN films grew along the <111> crystalline phase and had good crystalline quality. High-resolution scanning transmission electron microscopy results show that the interfaces between the sapphire substrate and the heterojunctions and heterojunctions are characterized by atomic level flatness, ordered atomic arrangement and low chemical mixing. This study used electrical and magnetic characterization at low temperatures to characterize the resistance and magnetic moment of Fe3N/VN heterojunctions as a function of temperature versus magnetic field. It is found that the superconducting transition temperature of the Fe3N/VN heterojunction decreases by about 1.5 K, and both the Ginzburg-Landau coherence length and the mean free range increase by about 20%, influenced by the ferromagnetic Fe3N. Below the low field and superconducting transition temperature, the saturation magnetic moment, coercive field, and superconducting critical field of the Fe3N/VN heterojunction increase, suggesting that there may be a net magnetic moment introduced by the Fe3N near-neighbor effect in the VN interfacial layer.
Further, this study uses the neutron spectrometer of the China Scattered Neutron Source to measure the polarized neutron reflection spectra of Fe3N/VN heterojunctions. It is shown that a net magnetic moment of about 60.3 ± 2.4 kA/m exists in the region of about 5 nm near the interface in the VN film. At the same time, the direction of this magnetic moment is aligned with the direction of the magnetic moment in ferromagnetic thin films. It is found that the interface of VN has a net magnetic moment only when the VN is in the superconducting state, as indicated by the polarized neutron reflection spectra with variable temperature and magnetic field. This anomalous interfacial magnetic feature is different from the past law of anti-parallel spin alignment at YBa2Cu3O7/La1-xCaxMnO3 oxide interfaces and alloy interfaces. It is found by first-principles calculations that the Fe3N/VN interface has d-orbital reconstruction and interfacial charge transfer phenomena, as well as the spins between transition metal ions satisfy the Heisenberg direct exchange coupling, and the coupling constant J is about 4.28 meV. This work observes the unique magnetic nearest-neighbor effect of the all-nitride superconducting/ferromagnetic heterointerfaces, and is useful for the construction of the superconducting spintronics devices in the The work has been instrumental in the construction of superconducting spintronics devices, such as triple supercurrent spin valves and "π" Josephson junctions.
The related research results were published in National Science Review under the title of Syntropic spin alignment at the interface between ferromagnetic and superconducting nitrides. This work was supported by the "Quantum Regulation and Quantum Information" Special Project of the National Key Research and Development Program of China, the Joint Fund for Regional Innovation and Development and the Original Exploration Program of the National Natural Science Foundation of China, the Stable Support Program for Young Teams in Basic Research and the Special Assistantship Program of the Chinese Academy of Sciences, and the Postdoctoral Fellowship of China, among others.
