Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Magnetoelectric coupling in the mixed erythrosiderite
  [(NH$_4$)$_{1-x}$K$_x$]$_2$[FeCl$_5$(H$_2$O)]   We present a study of the dielectric, structural, and magnetic properties of
the multiferroic or linear magnetoelectric substitution series
[(NH$_4$)$_{1-x}$K$_x$]$_2$[FeCl$_5$(H$_2$O)]. Pyroelectric currents, magnetic
susceptibilities, and thermodynamic properties were examined on large single
crystals of the erythrosiderite compounds and detailed magnetic-field versus
temperature phase diagrams are derived for three different substitution levels.
With increasing potassium concentration the material is tuned from a
multiferroic ($x \le 0.06$) to a linear magnetoelectric ($x \ge 0.15$) ground
state. In contrast to the respective pure parent compounds with $x=0$ or 1,
however, the ferroelectric or linear magnetoelectric polarization in none of
the substituted samples is switchable by external electric fields, because
these samples exhibit a significant electric polarization already above the
magnetic ordering transition. The polarization arises at a higher-lying
structural phase transition that is examined by THz spectroscopy and, on a
deuterated pure single crystal, by comprehensive neutron-diffraction
experiments. The structural phase transition is attributed to an ordering of
NH$_4^+$ tetrahedra but does not break inversion symmetry in the pure material,
while a finite K content causes pyroelectricity.

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The categories of this paper are: solv-int
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The categories of this paper are: physics
1
The categories of this paper are: cs
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The categories of this paper are: funct-an
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The categories of this paper are: q-alg
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The categories of this paper are: patt-sol
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The categories of this paper are: alg-geom
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The categories of this paper are: nucl-ex
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The categories of this paper are: math-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: hep-lat
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The categories of this paper are: mtrl-th
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The categories of this paper are: atom-ph
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The categories of this paper are: nlin
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The categories of this paper are: gr-qc
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The categories of this paper are: econ