Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Field-induced magnetic reorientation and effective anisotropy of a
  ferromagnetic monolayer within spin wave theory   The reorientation of the magnetization of a ferromagnetic monolayer is
calculated with the help of many-body Green's function theory. This allows, in
contrast to other spin wave theories, a satisfactory calculation of magnetic
properties over the entire temperature range of interest since interactions
between spin waves are taken into account. A Heisenberg Hamiltonian plus a
second-order uniaxial single-ion anisotropy and an external magnetic field is
treated by the Tyablikov (Random Phase Approximation: RPA) decoupling of the
exchange interaction term and the Anderson-Callen decoupling of the anisotropy
term. The orientation of the magnetization is determined by the spin components
$\la S^\alpha\ra$ ($\alpha=x,y,z$), which are calculated with the help of the
spectral theorem. The knowledge of the orientation angle $\Theta_0$ allows a
non-perturbative determination of the temperature dependence of the effective
second-order anisotropy coefficient. Results for the Green's function theory
are compared with those obtained with mean-field theory (MFT). We find
significant differences between these approaches.

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The categories of this paper are: funct-an
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The categories of this paper are: plasm-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: supr-con
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The categories of this paper are: ao-sci
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The categories of this paper are: stat
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The categories of this paper are: econ
8
The categories of this paper are: hep-ex
3
The categories of this paper are: physics
1
The categories of this paper are: cs
5
The categories of this paper are: gr-qc
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The categories of this paper are: adap-org
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The categories of this paper are: quant-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: math
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The categories of this paper are: patt-sol