ErLEED 150

Thin epitaxial films of the palladium-rich Pd1−xFex alloy were synthesized and extensively studied as a tunable ferromagnetic material for superconducting spintronics. The (001)-oriented MgO single-crystal substrate and the composition range of x = 0.01–0.07 were chosen to support the epitaxial growth and provide the films with magnetic properties spanning from very soft ferromagnet for memory applications to intermediately soft and moderately hard for the programmable logic and circuit biasing, respectively. Dependences of the saturation magnetization, Curie temperature and three magnetic anisotropy constants on the iron content x were obtained for the first time from the analyses of the magnetometry and ferromagnetic resonance data. The experimental results were discussed based on existing theories of dilute ferromagnetic alloys. Simulation of the hysteresis loops within the Stoner-Wohlfarth model indicates the predominant coherent magnetic moment rotation at cryogenic temperatures. The obtained results were compiled in a database of magnetic properties of a palladium-iron alloy in a single-crystal thin-film form considered as a material for superconducting spintronics.

A. Esmaeili, I. V. Yanilkin, A. I. Gumarov, I. R. Vakhitov, B. F. Gabbasov, R. V. Yusupov,
D. A. Tatarsky, and L. R. Tagirov
Science China Materials volume 64, pages 1246–1255 (2021)

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_0.92 Fe_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_0.96 Fe_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_0.92 Fe_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.

I. Yanilkin, W. Mohammed, A. Gumarov, A. Kiiamov, R. Yusupov, and L. Tagirov
Nanomaterials 2021, 11, 64

In the paper we report on growth conditions, morphology, crystallographic structure and magnetic anisotropy of 20 nm thick, palladium-rich Pd_1−xFe_x (0.01 < x < 0.1) alloy films grown on (001)-oriented MgO single-crystal substrate. Molecular beam deposition at ultra-high vacuum conditions has been utilized along with the three-step procedure to achieve the epitaxy conditions for the synthesized films. The scanning electron microscopy and atomic force microscopy have shown flat and smooth morphology of the films studied in a wide range of lateral scales. In situ low energy electron diffraction and ex situ X-ray diffraction measurements confirmed that our Pd_1−xFe_x alloy films are epitaxial. Ferromagnetic resonance investigations have shown the in-plane four-fold magnetocrystalline anisotropy characteristic for single-crystalline films with the cubic structure. The whole set of our measurements testifies the epitaxial cube-on-cube growth and excellent magnetic homogeneity of Pd_1−xFe_x films on MgO substrate obtained within the three-step deposition process.

A. Esmaeilia, I.V. Yanilkina, A.I. Gumarova, I.R. Vakhitova, B.F. Gabbasova, A.G. Kiiamova,
A.M. Rogova, Yu.N. Osina, A.E. Denisova, R.V. Yusupova, and L.R. Tagirova
Thin Solid Films 669 (2019) 338–344

A method of band mapping providing full control of the three-dimensional k is described in detail. Angle-dependent very-low-energy electron diffraction is applied to determine the photoemission final states along a Brillouin zone symmetry line parallel to the surface; photoemission out of these states is then utilized to map the valence bands in the constant-final-state mode. The method naturally incorporates the non-free-electron and excited-state self-energy effects in the unoccupied band, resulting in an accuracy superior over conventional techniques. Moreover, its intrinsic accuracy is less limited by lifetime broadening. As a practical advantage, the method provides access to a variety of lines in the Brillouin zone using only one crystal surface. We extensively tested the method on Cu. Several new aspects of the electronic structure of this metal are determined, including non-free-electron behavior of unoccupied bands and missing pieces of the valence band.

V.N. Strocov, R. Claessen, G. Nicolay, S. Hüfner, A. Kimura, A. Harasawa, S. Shin, A. Kakizaki, P. O. Nilsson, H. I. Starnberg, P. Blaha
Phys. Rev. B. 63, 205108 ff.

We present an experimental method to determine the electronic E(k) band structure in crystalline solids absolutely, i.e., with complete control of the three-dimensional wave vector k. Angle-dependent very-low-energy electron diffraction is first applied to determine the unoccupied states whose k is located on a high-symmetry line parallel to the surface. Photoemission via these states, employing the constant-final-state mode, is then utilized to map the valence bands along this line. We demonstrate the method by application to Cu, and find significant deviation from free-electron-like behavior in the unoccupied states, and from density-functional theory in the occupied states.

V. N. Strocov, R. Claessen, G. Nicolay, S. Hüfner, A. Kimura, A. Harasawa, S. Shin, A. Kakizaki, P. O. Nilsson, H. I. Starnberg, P. Blaha
Phys. Rev. Lett. 81 (22), pp. 4943ff.

The present ray-tracing scheme employs the electrostatic potential calculation by finite-difference solution of the Laplace equation in three dimensions. The run time reduction, achieved by using successively refined grids and optimizing the iteration cycle, allows one to perform calculations on a personal computer. The code is applied to a standard four-grid LEED optics, operated in the retarding field mode, to calculate electron trajectories in the case of off-symmetry incidence. In particular, dependences of incidence on the sample rotation and primary energy are calculated. A simple empirical formula is derived, which allows accurate experimental determination of without extensive calculations. Ray-tracing calculations are further applied to analyse the influence of residual asymmetries of the electron optics.

V.N. Strocov
Meas. Sci. Technol. 7, pp. 1636-1642