Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Orientation of the electric field gradient and ellipticity of the
  magnetic cycloid in multiferroic BiFeO3   The paper deals with the hyperfine interactions observed on the 57Fe nucleus
in multiferroic BiFeO3 by means of the 14.41-keV resonant transition in 57Fe,
and for transmission geometry applied to the random powder sample. Spectra were
obtained at 80 K, 190 K and at room temperature. It was found that iron occurs
in the high spin trivalent state. Hyperfine magnetic field follows distribution
due to the elliptic-like distortion of the magnetic cycloid. The long axis of
the ellipse is oriented along <111> direction of the rhombohedral unit cell.
The hyperfine magnetic field in this direction is about 1.013 of the field in
the perpendicular direction at room temperature. This ratio diminishes to 1.010
at 80 K. Axially symmetric electric field gradient (EFG) on the iron atoms has
the principal axis oriented in the same direction and the main component of the
EFG is positive. Our results are consistent with the finding that iron magnetic
moments are confined to the [1-21] crystal plane.

36
The categories of this paper are: cmp-lg
9
The categories of this paper are: nlin
15
The categories of this paper are: nucl-ex
24
The categories of this paper are: dg-ga
1
The categories of this paper are: cs
18
The categories of this paper are: econ
3
The categories of this paper are: physics
23
The categories of this paper are: solv-int
7
The categories of this paper are: hep-th
16
The categories of this paper are: stat
34
The categories of this paper are: chem-ph
17
The categories of this paper are: q-fin
25
The categories of this paper are: patt-sol
28
The categories of this paper are: alg-geom
0
The categories of this paper are: hep-ph
20
The categories of this paper are: acc-phys