RESEARCH

Establishment of the science principle of the grain boundary for Fe-based superconductors

High-temperature superconductors, including Fe-based superconductors, have fatal defects called “weak links” at grain boundaries, which significantly limits the supercurrent flow across the grain boundaries. For cuprates, a fabrication process that avoids the problem of “weak links” has been reported based on detailed studies of grain boundary. As a result, bismuth- and yttrium-based copper oxide superconducting wires are now commercially available.
On the other hand, Fe-based superconductors also have “weak links” at grain boundaries, although not as severe as that of cuprates, but the grain boundary characteristics have not been well investigated. Therefore, we are growing various iron-based superconductors on bi-crystal substrates to fabricate a single artificial grain boundary, and analyzing the grain boundary structure and studying its electromagnetic properties. The establishment of the science principle of the grain boundary for Fe-based superconductors is expected to advance applied research, especially the development of superconducting wires and bulk magnets.

Polycrystalline materials are agglomerates of single crystal grains. The space between grains is called grain boundary. The properties of a superconductor are determined by the product of intra-grain, inter-grain, and microstructure. To investigate grain boundary properties, an artificial single grain boundary is created by growing an epitaxial thin film of the material of interest on a bi-crystal substrate. Then, a small bridge for transport measurements across the grain boundary is structured. Critical current (inter-grain Ic) of the bridge is measured.

K. Iida et al., Grain boundary characteristics of Fe-based superconductors, Supercond. Sci. Technol. 33, 043001 (2020).
K. Iida et al., Grain boundary characteristics of oxypnictide NdFeAs(O,F) superconductors, Supercond. Sci. Technol. 32, 074003 (2019).
J. Hänisc and K. Iida, Grain Boundaries in Fe-Based Superconductors, Superconductivity From Materials Science to Practical Applications, 269-302 (2020), Eds. P. Mele et al, Springer Nature Switzerland.
T. Omura et al., Fabrication of grain boundary junctions using NdFeAs(O,F) superconducting thin films, Journal of Physics: Conference Series 1054, 012024 (2018).
K. Iida et al., Fe-based superconducting thin films on metallic substrates: Growth, characteristics, and relevant properties, Appl. Phys. Rev. 5, 031304 (2018).

Characterization of superconducting thin films grown on vicinal substrates

Epitaxial growth of tetragonal thin films on a vicinal substrate with the [001] direction tilted toward the [100] direction allows us to separate the in-plane and c-axis transport properties. We have evaluated the in-plane and c-axis resistivities of Fe-based superconductors in the normal state and evaluated the anisotropy of effective mass from the ratio of these resistivities.
On the other hand, it has been reported that many antiphase boundaries are introduced in the off-axis grown thin films. Therefore, the critical current characteristics in the presence of magnetic field are expected to be improved.

Schematic illustration of the thin films grown on normal and vicinal substrates. The resistivity measured in transverse and the longitudinal directions are called the transverse resistivity (ρ_T), and the longitudinal resistivity (ρ_L), respectively. From the resistivity measured in each direction, ρ_c can be calculated using the equation shown above.

M. Y. Chen et al., Inter-to intra-layer resistivity anisotropy of NdFeAs(O,H) with various hydrogen concentrations, Phys. Rev. Materials 6, 054802 (2022).
K. Iida et al., Anisotropy of the transport properties of NdFeAs(O,F) thin films grown on vicinal substrates, Supercond. Sci. Technol. 33, 044016 (2020).
H. Bryja et al., Deposition and properties of Fe(Se,Te) thin films on vicinal CaF₂ substrates, Supercond. Sci. Technol. 30, 115008 (2017).

Controlling the properties of functional materials by strain

It has been widely recognized that the physical properties of various materials change when strain is applied. There has been active research on controlling physical properties by static strain using lattice mismatch and thermal expansion mismatch between the thin film and substrate, and by dynamic strain using the inverse Piezo effect. We reported how the superconducting transition temperature Tc and the electronic state change by applying strain to Fe-based superconducting thin films using the thermal expansion coefficient and lattice mismatch. We also fabricated an iron-based superconducting thin film on a piezoelectric substrate and reported that Tc varies with the voltage, or mechanical pressure, applied to the piezoelectric substrate. However, piezoelectric devices operate slowly at low temperatures, making it difficult to apply large in-plane pressure to the thin film. To solve this problem and to apply a high level of strain, which is completely different from the conventional method, we came up with the idea of fabricating thin films on a flexible metal substrate. This method makes it possible to mechanically apply large strains to thin films even at low temperatures. In fact, we have controlled the properties of iron-based superconductors and inverse perovskite Mn nitrides as model materials. In the future, we plan to investigate the effects of strain not only on superconductivity but also on various functional materials.

If the lattice constant as of the substrate is smaller than the lattice constant af of the thin film, in-plane compressive strain is introduced into the thin film; if af < as, tensile strain is applied. When the iron-based superconductor BaFe1.8Co0.2As2 is grown on various substrates, the Tc varies with strain. Band calculations show that the Fermi surface becomes three-dimensional in the case of compressive strain.

K. Iida et al., Novel method to study strain effect of thin films using a piezoelectric-based device and a flexible metallic substrate, Appl. Phys. Express 12, 016503 (2019).
K. Iida et al., Hall-plot of the phase diagram for Ba(Fe_1-xCo_x)₂As₂, Sci. Rep. 6, 28390 (2016).
S. Trommler et al., Reversible shift in the superconducting transition for La_1.85Sr_0.15CuO₄ and BaFe_1.8Co_0.2As₂ using piezoelectric substrates, New J. Phys. 12, 103030 (2010).
K. Iida et al., Srong T_c dependence for strained epitaxial Ba(Fe_1-xCo_x)₂As₂ thin films, Appl. Phys. Lett. 95, 192501 (2009).

Evaluation of superconducting properties under high magnetic fields

The magnetic field that can be generated at a university laboratory is about 20 T at most in a DC magnetic field, but a larger magnetic field is required to evaluate the properties of high-Tc superconductors. The National High Magnetic Field Laboratory (NHMFL) in the United States has a hybrid magnet that can generate a 45 T DC magnetic field. NHMFL also has a number of 35 T magnets. We have been collaborating with Dr. Tarantini and Dr. Jaroszynski at NHMFL for more than 10 years and have evaluated transport properties of many superconductors.
Recently, we have also used the 25 T magnet at the High Field Center of the Institute for Materials Research, Tohoku University, to evaluate transport properties of cuprate superconductors.

The photo on the left shows the 25 T magnet at the Institute for Materials Research, Tohoku University. The angular dependence of the critical current density J_c of the HoBa₂Cu₃O₇ superconductor measured with this magnet at different temperatures is also shown. The external magnetic field was 24 T. The photo on the right shows a 45 T hybrid magnet at NHMFL. J_c-B characteristics of LnFeAs(O,F) thin film at 4.2 K and temperature dependence of the upper critical field H_c2 of doped BaFe₂As₂ thin film, measured with this magnet and 35 T Bitter magnet.

J. Hänisch et al., Microstructure, pinning properties, and aging of CSD-grown SmBa₂Cu₃O_7-δ films with and without BaHfO₃ nanoparticles, Supercond. Sci. Technol. 35, 084009 (2022).
S. Kauffmann-Weiss et al., Microscopic origin of highly enhanced current carrying abilities of thin NdFeAs(O,F) films, Nanoscale Adv. 1, 3036 (2019).
V. Grinenko et al., Selective mass enhancement close to the quantum critical point in BaFe₂(As_1-xP_x)₂, Sci. Rep. 7, 4589 (2017).
K. Iida et al., High-field transport properties of a P-doped BaFe₂As₂ film on technical substrate, Sci. Rep. 7, 39951 (2017).
S. Richter et al., Superconducting properties of Ba(Fe_1-xNi_x)₂As₂ thin films in high magnetic fields, Appl. Phys. Lett. 110, 022601 (2017).
C. Tarantini et al., Intrinsic and extrinsic pinning in NdFeAs(O,F): vortex trapping and lock-in by the layered structure, Sci. Rep. 6, 36047 (2016).
F. Kurth et al., Unusually high critical current of clean P-doped BaFe₂As₂ single crystalline thin film, Appl. Phys. Lett. 106, 072602 (2015).
J. Hänisch et al., High field superconducting properties of Ba(Fe_1-xCo_x)₂As₂ thin films, Sci. Rep. 5, 17363 (2015).
K. Iida et al., Oxypnictide SmFeAs(O,F) superconductor: a candidate for high-field magnet applications, Sci. Rep. 3, 2139 (2013).