Single-layer vanadium nitride (VN) and bilayer Pd_0.96Fe_0.04/VN and VN/Pd_0.92Fe_0.08 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 Pd_1−xFe_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 Pd_1−xFe_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)/Pd_0.92Fe_0.08(12 nm) and Pd_0.96Fe_0.04(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 Pd_0.96Fe_0.04/VN structure and at 6.1 K for the VN/Pd_0.92Fe_0.08 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/Pd_0.92Fe_0.08 structure. We propose epitaxial single-crystalline thin films of VN and heteroepitaxial Pd_1−xFe_x/VN and VN/Pd_1−xFe_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.

A thin-film superconductor(S)/ferromagnet(F) F1/S/F2-type Pd_0.96Fe_0.04(20 nm)/VN(30 nm)/Pd_0.92Fe_0.08(12 nm) heteroepitaxial structure was synthesized on (001)-oriented single-crystal MgO substrate utilizing a combination of the reactive magnetron sputtering and the molecular-beam epitaxy techniques in ultrahigh vacuum conditions. The reference VN film, Pd_0.96Fe_0.04/VN, and VN/Pd_0.92Fe_0.08 bilayers were grown in one run with the target sample. In-situ low-energy electron diffraction and ex-situ X-ray diffraction investigations approved that all the Pd_1−xFe_x and VN layers in the series grew epitaxial in a cube-on-cube mode. Electric resistance measurements demonstrated sharp transitions to the superconducting state with the critical temperature reducing gradually from 7.7 to 5.4 K in the sequence of the VN film, Pd_0.96Fe_0.04/VN, VN/Pd_0.92Fe_0.08, and Pd_0.96Fe_0.04/VN/Pd_0.92Fe_0.08 heterostructures due to the superconductor/ferromagnet proximity effect. Transition width increased in the same sequence from 21 to 40 mK. Magnetoresistance studies of the trilayer Pd_0.96Fe_0.04/VN/Pd_0.92Fe_0.08 sample revealed a superconducting spin-valve effect upon switching between the parallel and antiparallel magnetic configurations, and anomalies associated with the magnetic moment reversals of the ferromagnetic Pd_0.92Fe_0.08 and Pd_0.96Fe_0.04 alloy layers. The moderate critical temperature suppression and manifestations of superconducting spin-valve properties make this kind of material promising for superconducting spintronics applications.

We report on an investigation of magnetic resonance phenomenon in ferromagnetic thin films with controlled nonuniform composition. An epitaxial Pd-Fe thin film with linear distribution of Fe content across the thickness is used as the model material. Anomalous perpendicular standing spin waves are observed and quantified using the collective dynamic equation. Numerical analysis yields the exchange stiffness constant for the diluted Pd-Fe alloy $D=2A/{\mu }_0{M}_s=15\mathrm{T}·{\mathrm{nm}}^2$ and the ratio of the effective magnetization to the saturation magnetization $M_{\text{eff}}/M_s=1.16$. It is demonstrated that, overall, the engineering of thin films with nonuniform composition across the thickness can be used for high-frequency or low-field magnonic operations using exchange spin waves.

The results of experimental and theoretical studies of standing spin waves in a series of epitaxial films of the ferromagnetic Pd_1−xFe_x alloy (0.02 < x < 0.11) with different distributions of the magnetic properties across the thickness are presented. Films with linear and stepwise, as well as more complex Lorentzian, sine and cosine profiles of iron concentration in the alloy, and thicknesses from 20 to 400 nm are considered. A crucial influence of the magnetic properties profile on the spectrum of spin wave resonances is demonstrated. A capability of engineering the standing spin waves in graded ferromagnetic films for applications in magnonics is discussed.