Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The square-lattice spiral magnet Ba_2CuGe_2O_7 in an in-plane magnetic
  field   The magnetic structure of Ba_2CuGe_2O_7 is investigated by neutron
diffraction in magnetic fields applied along several directions in the $(a,b)$
plane of the crystal. In relatively weak fields, $H\lesssim 0.5$~T, the
propagation vector of the spin-spiral rotates to form a finite angle with the
field direction. This angle depends on the orientation of $H$ itself. The
rotation of the propagation vector is accompanied by a re-orientation of the
plane of spin rotation in the spiral. The observed behaviour is well described
by a continuous-limit form of a free energy functional that includes exchange
and Dzyaloshinskii-Moriya interactions, as well as the Zeeman energy and an
empirical anisotropy term.

16
The categories of this paper are: stat
19
The categories of this paper are: eess
5
The categories of this paper are: gr-qc
37
The categories of this paper are: supr-con
29
The categories of this paper are: funct-an
9
The categories of this paper are: nlin
0
The categories of this paper are: hep-ph
3
The categories of this paper are: physics
13
The categories of this paper are: quant-ph
28
The categories of this paper are: alg-geom
2
The categories of this paper are: math
30
The categories of this paper are: q-alg
22
The categories of this paper are: chao-dyn
15
The categories of this paper are: nucl-ex
32
The categories of this paper are: atom-ph
25
The categories of this paper are: patt-sol