Journal of Magnetism and Magnetic Materials 272–276 (2004) e217–e219

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An influence of the local environment on local magneticmoments and hyperfine fields in Fe3�xCrxSi

Anna Goa,*, Maria Pugaczowa-Michalskab, Ludwik Dobrzy !nskia,c

a Institute of Experimental Physics, University of Bia!ystok, Lipowa 41, Bia!ystok 15-424, Polandb Institute of Molecular Physics, Polish Academy of Science, Smoluchowskiego 17, Pozna !n 60-179, Poland

cThe Soltan Institute for Nuclear Studies, Otwock-Swierk 05-400, Poland

Abstract

We present the results of ab initio calculations of Fe3Si substituted by chromium. An influence of the local

environment on the distribution of hyperfine magnetic fields and spin-density distributions are discussed. Results of

calculations confirmed that the presence of chromium atom in the nearest neighborhood causes strong decrease of the

hyperfine field on iron as well decrease of magnetic moment of Fe in (A,C)- and B sites.

r 2004 Elsevier B.V. All rights reserved.

PACS: 71.20.Be; 75.50.Bb; 75.20.Hr; 76.80.+y

Keywords: Transition metals and alloys; Iron alloy; Chromium alloy; Local magnetic moment; Hyperfine field

Effect of alloying with Cr on magnetic and electronic

properties of Fe3Si has been studied in Refs. [1–5].

According to the experimental data [1] the Fe3�xCrxSi

alloys crystallise in the DO3-type structure in the range

0pxp0:4: Two non-equivalent Fe sites, which are (A,C)

and B, are in the perfectly ordered structure. As a

consequence of the different nearest environments of

atoms at these sites the different electronic and magnetic

properties connected with these positions are observed

[6,7]. In Fe3�xCrxSi chromium can be found at (A,C)

and B positions with the weak preference for the B site

[1–3]. The results of study of the site preferences of Cr in

Fe3Si; which were examined using ab initio calculation

in [5], indicate, however, the existence of preference of

the B-site occupation by chromium.

In the present paper we concentrate essentially on the

discussion of the hyperfine magnetic fields obtained

from ab initio calculations. These results are compared

with the experimental results of M .ossbauer spectroscopy

at room temperature [2,3].

onding author. Tel.: +48-85-745-7249; fax: +48-

3.

address: annago@alpha.uwb.edu.pl (A. Go).

$ - see front matter r 2004 Elsevier B.V. All rights reserve

/j.jmmm.2003.12.1184

The electronic structure for ordered Fe3�xCrxSi alloys

have been obtained using the TB-LMTO method [8].

The calculations were carried out for the exchange-

correlation potential in the form of von Barth and Hedin

[9] and the Langreth–Mehl-Hu non-local exchange-

correlation [10] was added. The experimental lattice

parameter [1], which is linearly decreasing with concen-

tration of Cr and the supercell structure with 32 atoms

[5] were used. The calculations were performed for at

least 250 k-points in the irreducible wedge of the

Brillouin zone. The Fermi contact contribution to the

hyperfine field discussed in this paper is determined by

the spin density at the position of the nucleus of the

examined atom [11]

B ¼ 83pmB½rmð0Þ � rkð0Þ�; ð1Þ

where mB is the Bohr magneton and the term in brackets

is the electronic spin density at the iron nucleus.

Calculated dependence of hyperfine fields for Fe(A,C)

and Fe(B) on the concentration of Cr atom is shown in

Fig. 1. Our result reveals the regular sequence of

decreasing hyperfine field for both iron atoms at the

(A,C)- and the B-sites with the decrease of the number of

nearest neighborhood (n.n.) iron atoms. Every addi-

tional Cr atom as the first n.n. reduces the hyperfine

d.

ARTICLE IN PRESS

Fig. 1. Hyperfine magnetic fields of iron (a) on B-site (b) on

(A,C)-sites.

Fig. 2. Spin density profiles along ½1 0 0� direction for two iron

atoms (a) on A,C-sites (b) on B-site.

A. Go et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e217–e219e218

field. Similar, although much weaker, effect is observed

when concentration of Cr increases. The calculated

hyperfine fields are in good agreement with experimental

data [2,3,7].

In Fe3Si the transition metal impurities like chromium

perturb the neighboring iron atoms strongly enough to

hybridize the 3d, 4s and 4p atomic orbitals. The effects

of short-range order on the local magnetic moment and

hyperfine field show that this perturbation is local and

therefore, especially efficient when Cr atoms occupy the

first neighboring sites to the iron atoms at both (A,C)-

and B-positions. To illustrate the role of Cr perturbation

on the spin density of iron we plotted the spin densities

of Fe for the case with one Cr in the n.n. shell and

compared it with the case with no Cr in the n.n.

surroundings (Fig. 2). Both the core, which dominates

the central region and valence contributions are

suppressed by the presence of Cr in the neighborhood.

This effect is mainly driven by the negative polarization

of Cr.

The magnetic moments depend on the position which

given atom occupies in the unit cell and on the

environment. Since for B-position the n.n. shell is

strongly dominated by iron atoms, magnetic moments

for these sites do not change drastically with concentra-

tion. The magnetic moment of Fe(B) ranges from

2; 322 mB for the case with four Fe n.n.’s and four Cr

to 2; 615 mB for the case with all iron n.n.’s. The

moments for Fe(A,C) are lower since four Fe atoms at

most occupy first n.n. shell. For n.n. configuration

f4FeðBÞ; 4SiðDÞg the value of moment of Fe(A,C) equals

1:560 mB and for f4CrðBÞ; 4SiðDÞg it takes the negative

value �0:289 mB: The latter negative value of moment

responds to the parallel alignment of Fe and Cr

magnetic moments. For pure Fe3Si we calculated one

configuration with iron atom moved to D-site, in which

case magnetic moment of Fe (A,C) surrounded by

f4FeðBÞ; 1FeðDÞ; 3SiðDÞg is 1:729 mB: The calculated

value of hyperfine field is 26:08 T; which is in good

agreement with experimental one [7].

In most cases the magnetic moments of Cr are

oriented antiparallel to the Fe moments. For B-sites

the moments range from �1:949 to �0:51 mB: For (A,C)-

sites the magnetic moments of Cr are from �1:337 to

�0:032 mB: A small negative moment is also induced on

Si atoms ranging from �0:122 to �0:025 mB: The larger

value corresponds to the larger number of iron n.n.’s.

The small negative polarization of Si was suggested also

in Ref. [12]. The values of magnetic moments and the

observed tendency of the decrease of the absolute values

of magnetic moments with the decrease of the average

ARTICLE IN PRESSA. Go et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e217–e219 e219

magnetic moment on the n.n. shell are in agreement with

experimental results [1–3].

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