CEMS Evidence of the Presurface Zone Enhancement of the Spin-Density in Chromium

Abstract

Subject classification: 75.25.þz; 76.80.þy Spin-density enhancement in the presurface zone of a single crystal (110) Cr was observed by using the 119 Sn Mö ssbauer effect in the conversion electron mode (CEMS). The observed enhancement is by a factor of two smaller and by a factor of 30-50 deeper than theoretical predictions. The spin structure is different from the one in bulk chromium and can be explained in terms of higher-order harmonics.The antiferromagnetism of chromium is constituted by incommensurate spin-density waves (SDWs). Their importance stems from the fact that they are closely related to the Fermi surface and its topology [1]. SDWs set in as a linearly polarized electron structure at the Né el temperature which in bulk chromium is equal to $313 K [2, 3]. The structure consists of a sinusoidal modulation of the magnetic moment m ¼ m 0 sin a, with a ¼ Qr, where Q is a wave vector, r a position vector and m 0 the amplitude of the SDWs equal to $0.6m B at 4.2 K. Q is related to the periodicity of the SDWs L by Q ¼ ð2p=a À 2p=L), where a is the lattice constant. L is a continuous function of temperature T with a value of $7.8 nm at RT. SDWs can exist along any of the [001] crystallographic axes, i.e. three different domains can coexist in an equilibrium state (the so-called 3Q state).Theoretical calculations predict that the surface properties of Cr are different. In particular, the magnetic moment should be enhanced in comparison with its bulk value. According to the tight-binding calculations of Allan, the (001) Cr surface has a moment of 2.8m B [4]. Self-consistent tight binding calculations by Victoria and Falicov [5] predict a value of 3m B , whereas ab initio FLAPW calculations by Fu and Freeman [6] give a value of 2.5m B for the top layer of bulk Cr. Enhanced magnetic moment was also predicted for (110) Cr monolayers on substrates other than Cr [7, 8] as well as for Cr monolayers sandwiched between Fe on one side and vacuum on the other [9].Experimental evidence in favour of such an enhancement was recently supplied by 119 Sn Mö ssbauer spectroscopic measurements carried out on epitaxial Cr/Sn multilayers [10]. A hyperfine magnetic field of 11-13 T was recorded which is twice as large as in a single crystal bulk Cr [11].To obtain such information on bulk chromium, a $100 mm thick foil of a single crystal (110) Cr was implanted at 65 keV with the dose of 10 16 119 Sn ions per cm 2 . The profile of the tin concentration in the Cr matrix as found by Auger Electron Spectroscopy (AES) is shown in Fig. 1. According to the TRIM code, the mean projected range for the ions is 15.6 nm, which corresponds to $2L at RT and is illustrated in

The Debye temperature of σ-phase Fe–Mo compounds as determined with Mössbauer spectroscopy

“…Among about 50 known examples of σ revealed so far to exist in a binary alloy systems, its physical properties were systematically studied only in two systems viz. Fe-Cr and Fe-V [2]. In the case of σ-FeMo compounds, papers available in the literature were, to our best knowledge, exclusively devoted to determining its crystallographic structure and ordering of atoms over the lattice sites [1,[3][4][5][6] while its physical properties still remain to be discovered.…”

“…Temperature dependence of the average centre shift, <CS>, as found for the σ-Fe 47 Mo 53 compound. The solid line represents the best-fit to the experimental data in terms of equation(2).…”

Structural and hyperfine characterization of σ-phase Fe–Mo alloys

“…It was not possible to obtain samples in a pure σ-FeMo alloys by using the procedure that was previously successfully applied in the case of Fe-Cr and Fe-V alloys i.e. an isothermal annealing of ingots obtained by melting constituents 11 . The Fe-Mo ingots treated in that way were always two-phase, the second phase being µ.…”

“…Even the data on its crystallographic structure in the whole concentration range of its occurrence is not complete. In these circumstances systematic studies aimed at filling this gap are justified, all the more so such studies have been recently carried out for this phase in Fe-Cr and Fe-V systems 11 . A comparison of corresponding data obtained for different binary alloy systems containing iron, Fe-X, where the σ-phase can be formed, is of interest per se, and, additionally, it may contribute to a better understanding of its formation mechanism.…”

Application of the Mössbauer Spectroscopy to Study Harmonically Modulated Electronic Structures: Case Study of Charge- and Spin-Density Waves in Cr and Its Alloys

“…To verify these predictions 119 Sn conversion electron spectra (CEMS) were recorded at room on a single-crystal (110) Cr foil implanted with 119 Sn ions. The range of the implantation was 16 nm what corresponds to 2 a[21][22][23]25]. The measured spectrum14 alongside with the corresponding histogram of the hyperfine field distribution is displayed in The spectrum measured in a transmission mode on a similar sample but doped with the 119 Sn atoms by diffusion and the related histogram are added for the sake of comparison.…”