Epitaxial growth and superconducting properties of thin-film PdFe/VN and VN/PdFe bilayers on MgO(001) substrates

Abstract

Single-layer vanadium nitride (VN) and bilayer Pd0.96Fe0.04/VN and VN/Pd0.92Fe0.08 thin-film heterostructures for possible spintronics applications were synthesized on (001)-oriented single-crystalline magnesium oxide (MgO) substrates utilizing a four-chamber ultrahigh vacuum deposition and analysis system. The VN layers were reactively magnetron sputtered from a metallic vanadium target in Ar/N2 plasma, while the Pd1− x Fe x layers were deposited by co-evaporation of metallic Pd and Fe pellets from calibrated effusion cells in a molecular beam epitaxy chamber. The VN stoichiometry and Pd1− x Fe x composition were controlled by X-ray photoelectron spectroscopy. In situ low-energy electron diffraction and ex situ X-ray diffraction show that the 30 nm thick single-layer VN as well as the double-layer VN(30 nm)/Pd0.92Fe0.08(12 nm) and Pd0.96Fe0.04(20 nm)/VN(30 nm) structures have grown cube-on-cube epitaxially. Electric resistance measurements demonstrate a metallic-type temperature dependence for the VN film with a small residual resistivity of 9 μΩ·cm at 10 K, indicating high purity and structural quality of the film. The transition to the superconducting state was observed at 7.7 K for the VN film, at 7.2 K for the Pd0.96Fe0.04/VN structure and at 6.1 K for the VN/Pd0.92Fe0.08 structure with the critical temperature decreasing due to the proximity effect. Contrary to expectations, all transitions were very sharp with the width ranging from 25 mK for the VN film to 50 mK for the VN/Pd0.92Fe0.08 structure. We propose epitaxial single-crystalline thin films of VN and heteroepitaxial Pd1− x Fe x /VN and VN/Pd1− x Fe x (x ≤ 0.08) structures grown on MgO(001) as the materials of a choice for the improvement of superconducting magnetic random access memory characteristics.

Synthesis, Characterization, and Magnetoresistive Properties of the Epitaxial Pd0.96Fe0.04/VN/Pd0.92Fe0.08 Superconducting Spin-Valve Heterostructure

“…Table 1 contains the data on the residual resistance ratio RRR (the ratio of the resistance R 300K at room temperature to the resistance R 10K at 10 K), the superconducting transition temperature T c (according to the criterion of the middle of the transition), and the width of the superconducting transition according to the criterion of 10–90. The structural quality of our 30 nm thick VN film, manifested in RRR ~5.1 >> 1 and low resistivity at room temperature ~42.5 μΩ·cm [ 50 ], is one of the best [ 41 , 44 , 64 , 65 , 66 ].…”

“…Table 1 contains the data on the residual resistance ratio RRR (the ratio of the resistance R 300K at room temperature to the resistance R 10K at 10 K), the superconducting transition temperature T c (according to the criterion of the middle of the transition), and the width of the superconducting transition according to the criterion of 10-90. The structural quality of our 30 nm thick VN film, manifested in RRR~5.1 >> 1 and low resistivity at room temperaturẽ 42.5 µΩ•cm [50], is one of the best [41,44,[64][65][66]. The most notable feature of the T c measurement results is a small width of the superconducting transition of the MgO/Pd 0.96 Fe 0.04 (20 nm)/VN(30 nm)/Pd 0.92 Fe 0.08 (12 nm)/Si (S4) sample of about 40 mK according to the 10-90% criterion, and the absence of "tail" of the resistive transition towards low temperatures.…”

“…Particular bilayer samples formed by the vanadium nitride and the Pd 1−x Fe x alloy on MgO(001) were grown in the course of developing the approach to the synthesis of such heterostructures: MgO(001)/VN/Pd 0.96 Fe 0.04 was studied in [ 49 ] with a focus at the magnetic anisotropy of epitaxial Pd 0.96 Fe 0.04 on VN; MgO(001)/Pd 0.96 Fe 0.04 /VN and MgO(001)/VN/Pd 0.92 Fe 0.08 bilayers were investigated in [ 50 ] with the focus on their electrical properties. In this work, we aimed to synthesize fully epitaxial, three-layer pseudo-spin-valve (spin-valve without exchange bias, but with different coercivities of the functional ferromagnetic layers) heterostructure MgO(001)/Pd 0.96 Fe 0.04 (20 nm)/VN(30 nm)/Pd 0.92 Fe 0.08 (12 nm).…”

Ab initio Investigation of Impurity Ferromagnetism in the Pd1−xFex Alloys: Concentration and Position Dependences

“…The current revival of interest is fueled by the potential use as a material of a choice for a weak and soft ferromagnet in superconducting Josephson magnetic random-access memory (MRAM) based on Josephson junctions [4][5][6][7][8][9][10][11][12][13]. The Pd-rich ferromagnetic Pd 1−x Fe x (0.01 < x < 0.1) alloy is used there in the form of a thin film fabricated utilizing magnetron sputtering [5][6][7][8][9][10][11][13][14][15], molecular-beam epitaxy (MBE) [15][16][17][18][19][20][21][22][23], and ion-beam implantation [24].…”

Functional nanostructures for electronics, spintronics and sensors

“…For example, Arutyunov et al [11] presented an advanced technology including lift-off electron-beam lithography followed by ultra-high-vacuum deposition of materials that was used for fabrication of nanostructured quasi-1D chains of Josephson junctions. This was followed by the work of Mohammed et al [12] who presented a smart vacuum technology for the design of hetero-epitaxial S/F nanostructures. The elaborated and fabricated nanostructures can be utilized in superconducting memory and logic circuits as Josephson magnetic random access memory (MRAM) elements for a superconducting computer.…”