Molecule-based Materials as model magnets: case studies on

Molecule-based materials as model magnets: case studies on thermodynamic properties
Stefan Süllow
Institute of Physics for Condensed Matter
TU Braunschweig
In recent years, the interdisciplinary field of molecule-based magnetic materials has received
much attention, especially in context with quantum magnetism and the prospective to obtain
multifunctional materials. In terms of materials research, the ability to chemically tailor new
systems has provided an avenue to obtain new classes of model magnets. Here, we present
three case studies on molecule-based magnets, highlighting their model character through
detailed comparisons of our experimental investigations of their thermodynamic properties
with theoretical modeling. First, we discuss the pressure response of the structural and magnetic properties of XPM2Cl2, X=Fe, Co, Ni, PM = pyrimidine = C4N2H4 in relation to the results of electronic structure calculations. Further, we present a specific heat study of the bimetallic ferromagnetically coupled chain compound MnNi(NO2)4(en)2 (en = ethylenediamine =
C2N2H4). Here, we observe a peculiar specific heat anomaly at low temperatures. Calculations
for a finite S1 = 1 / S2 = 5/2 mixed spin chain indicate that it is a consequence of the mixed
spin character, resulting from a splitting of the spin excitation spectrum into an acoustic and
an optical magnon mode. Finally, we present a magnetization study of the staggered S = ½
antiferromagnetically coupled Heisenberg chain system CuPM(NO3)2(H2O)2]n. For this material, because of the Dzyaloshinskii-Moriya interaction and a staggered g tensor, with respect
to the magnetic properties there are two main crystallographic directions, a’’ and c’’. While
along c’’ the effects of the staggering are most prominent, along a’’ the system behaves like a
perfect S = 1/2 AFM Heisenberg chain. From a comparison of a high-field magnetization
study with the results of calculations obtained from exact diagonalization of a finite linear
spin chain and based on the Bethe ansatz, for both crystallographic directions we can perfectly
reproduce the magnetization. This way, we demonstrate the qualitatively different response
along a’’ and c’’ as consequence of an alternating local symmetry of the Cu ions.