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Abstract:
The specific heat capacity of mostly single-crystalline samples of 21
boracite compounds M3B7O13X withM a 3d transition metal (Cr, Mn, Fe, Co,
Ni, Cu, Zn) or Mg and X a halogen (Cl, Br, I) is determined. In
combination with magnetic susceptibility data the magnetic ordering of
the M2+ ions at TN is investigated in detail. The fully
ferroelectric/fully ferroelastic structural phase transitions at higher
temperatures are measured by differential scanning calorimetry. In the
Cr-Br, Cr-I, Cu-Cl, andCu-Br compounds, previously unknown magnetic
phases were found. Magnetic order in the boracites is characterized by
the quantum and classical spin states of the M2+ ions, a variable degree
of structural distortion, orbital effects, and competing exchange
interactions. The Cu-Cl, Cu-Br, and Ni-Cl boracites exhibit broad maxima
of magnetic specific heat and of magnetic susceptibility above TN caused
by low-dimensional or frustrated magnetic interactions. Co boracites
display additional broad anomalies below TN originating from continuous
spin reorientations and effective S = 1/2 ground states. Indications for
spin reorientations are also observed for Fe boracites. New phases
appear in high magnetic fields for some Co and Fe boracites, which is
not the case for the Mn compounds. Stronger magnetic frustration is
deduced for the cubic Cr compounds. For the latter compounds and Ni-I
boracite magnetostructural phase transitions are observed.
