Dirk Visser

Magnetic properties of tapiolite (FeTa2O6); a quasi two-dimensional (2D) antiferromagnet

The possibilities of two-dimensional (2D) short-range magnetic correlations and frustration effects in the mineral tapiolite are investigated using bulk-property measurements and neutron Laue diffraction. In this study of the magnetic properties of synthetic single-crystals of tapiolite, we find that single crystals of FeTa2O6 order antiferromagnetically at TN = 7.95 ± 0.05 K, with extensive two-dimensional correlations existing up to at least 40 K. Although we find no evidence that FeTa2O6 is magnetically frustrated, hallmarks of two-dimensional magnetism observed in our single-crystal data include: (i) broadening of the susceptibility maximum due to short-range correlations, (ii) a spin-flop transition and (iii) lambda anomalies in the heat capacity and d(χT)/dT. Complementary neutron Laue diffraction measurements reveal 1D magnetic diffuse scattering extending along the c* direction perpendicular to the magnetic planes. This magnetic diffuse scattering, observed for the first time using the neutron Laue technique by VIVALDI, arises directly as a result of 2D short-range spin correlations.

Magnetic order in the organic–inorganic layer compound (CD3PO3)Mn·D2O: a neutron and synchrotron X-ray powder diffraction study

Both long- and short-range magnetic order in the organic–inorganic layer compound (CD3PO3)Mn·D2O has been studied by temperature-dependent powder neutron diffraction. Above 15 K the magnetic intensity is dominated by diffuse scattering due to two-dimensional short-range order while at lower temperatures Bragg diffraction indicates three-dimensional order. The long-range ordered magnetic structure is similar to that of AMnPO4·D2O (A = K, ND4). The crystal structure of (CD3PO3)Mn·D2O has been refined from neutron and synchrotron X-ray powder diffraction, including location of all D atoms.

Muon spin relaxation studies of magnetic ordering in the molecular-based ferrimagnets PPh4MnIIFeIII(C2O4)3 and (n-C4H9)4NFeIIFeIII(C2O4)3

Muon spin relaxation measurements are reported on two molecular-based ferrimagnets, PPh4MnIIFeIII(C2O4)3 and (n-C4H9)4NFeIIFeIII(C2O4)3, at temperatures from 8 to 100 K and applied fields up to 2.5 T using pulsed (ISIS) and continuous (PSI) muon sources. In zero field, the initial asymmetry in the muon depolarization falls sharply, and the muon relaxation rate diverges, in the vicinity of the transition to long-range order (Tc) measured by bulk susceptibility. The onset of both effects takes place significantly about Tc, indicating low-dimensional short-range fluctuations in these larger materials. No coherent muon precession is observed in either compound, with pulsed or continuous muons.