Magnetic Structures of Heterometallic M(II)–M(III) Formate Compounds

A study
of the magnetic structure of the [NH2(CH3)2]n[FeIIIMII(HCOO)6]n niccolite-like compounds,
with MII = CoII (2) and MnII (3) ions, has been carried out using neutron
diffraction and compared with the previously reported FeII-containing compound (1). The inclusion of two different
metallic atoms into the niccolite-like structure framework leads to
the formation of isostructural compounds with very different magnetic
behaviors due to the compensation or not of the different spins involved
in each lattice. Below TN, the magnetic
order in these compounds varies from ferrimagnetic behavior for 1 and 2 to an antiferromagnetic behavior with
a weak spin canting for 3. Structure refinements of 2 and 3 at low temperature (45 K) have been carried
out combining synchrotron X-ray and high-resolution neutron diffraction
in a multipattern approach. The magnetic structures have been determined
from the difference patterns between the neutron data in the paramagnetic
and the magnetically ordered regions. These difference patterns have
been analyzed using a simulated annealing protocol and symmetry analysis
techniques. The obtained magnetic structures have been further rationalized
by means of ab initio DFT calculations. The direction of the magnetic
moment of each compound has been determined. The easy axis of the
MII for compound 1 (FeII) is along
the c axis; for compound 2 (CoII), the moments are mainly within the ab plane; finally,
for compound 3 (MnII), the calculations show
that the moments have components both in the ab plane
and along the c axis.