Simon G. Carling

Molecular-based mixed valency ferrimagnets (XR4)FeIIFeIII(C2O4)3(X = N, P; R =,n-propyl, n-butyl, phenyl): anomalous negative magnetisation in the tetra-n-butylammonium derivative

The phases AFeIIFeIII(C2O4)3(A = NPrn4; NBun4; PPh4) have been synthesised and characterised chemically, structurally and magnetically; they behave as ferrimagnets, but the tetrabutylammonium salt shows a highly unusual negative magnetisation at low temperature.

Crystal structure and magnetic properties of the layer ferrimagnet N(n-C5H11)4MnIIFeIII(C2O4)3

The crystal structure and magnetic properties of the molecular-based ferrimagnet N(n-C5H11)4MnIIFeIII(C2O4)3 have been determined. The compound is orthorhombic, space group C2221, a= 9.707(3), b= 16.140(3), c= 19.883(7)A(120 K), Z= 4[R 0.047 for I > 2σ(I)]. The structure consists of hexagonal layers of alternating MnII and FeIII bridged by C2O42–, separated by layers containing only N(n-C5H11)4+ with the alkyl chains extended, though with the terminal bonds twisted towards the gauche conformation. The terminal CH3 are embedded in the hexagonal pockets formed by three C2O42–. Since both metal ions have 3d5 configuration with 6A1 ground states the magnetic properties in the paramagnetic region mimic those of a two-dimensional antiferromagnet. Below TN= 27 K an uncompensated moment estimated as 8.78 × 10–5µB atom–1 arises, the direction of which was identified as parallel to the c axis by single-crystal magnetization measurements.

A Monte Carlo study of honeycomb lattice ferrimagnets

Abstract Layered, bimetallic oxalate materials of general formula AM(II)M′(III)(C2O4)3 can be synthesised with a wide range of transition metal ions M(II) and M′(III) and organic cations A. Depending on the nature of these ions, a variety of magnetic behaviours have been found. In particular, compounds AFe(II)Fe(III)(C2O4)3 are ferrimagnets that show both compensated (Neel type N) and non-compensated (Neel type Q) behaviour at low temperatures. We present the results of magnetic Monte Carlo studies on a honeycomb lattice, Ising model ferrimagnet that provide insight into the exchange interactions involved in these compounds.

The anhydrous alums as model triangular-lattice magnets

We show that materials based on the yavapaiite layered structure are of potential interest as realizations of a model quasi-two-dimensional triangular-lattice antiferromagnet. The structure type is such that magnetic ions occupy a regular or very slightly distorted triangular lattice in well separated layers. We report the magnetic susceptibility versus temperature behaviour of three compounds: , which has an equilateral triangular lattice, and and which both have isosceles triangular lattices. A comparison of the behaviour of these three compounds identifies the effect of distortion and the spin value on the properties of the triangular-lattice antiferromagnet. , which we have made for the first time, has S = 1/2, and may prove to be the best example of the S = 1/2 triangular-lattice antiferromagnet yet discovered.

Dimensionality crossovers in the magnetization of the weakly ferromagnetic two-dimensional manganese alkylphosphonate hydrates MnCnH2n+1PO3.H2O, n=2-4

Critical exponents of magnetization beta below TN in the weakly ferromagnetic layer compounds MnCnH2n+1PO3.H2O have been measured by SQUID magnetometry for n=2-4. In all three compounds crossovers are observed in beta as follows ( beta 1, beta 2): 0.21(2), 0.73(2) (n=2); 0.18(1), 0.42(6) (n=3), 0.18(1), approximately 0.6 (n=4). The crossover occurs at values of the reduced temperature epsilon =(TN-T)/TN that become smaller as the separation between the magnetic layers increases.

Dimensionality crossovers in the magnetization of the canted antiferromagnets NH4MnPO4.H2O and ND4MnPO4.D2O

Abstract Critical exponents of the magnetization below T N measured by SQUID magnetometry and neutron diffraction are reported for the two-dimensional canted antiferromagnets NH 4 MnPO 4 .H 2 O and ND 4 MnPO 4 .D 2 O. Both compounds exhibit a crossover in the power law dependence of magnetization on temperature. In the former we find β 1 0.21(3), β 2 0.40(7), T N 17.5(1) K; in the latter β 1 0.20(1), β 2 0.39(4), T N 17.5(3) K. The crossover between the two exponents occurs at a reduced temperature of 0.07(1) in the hydrogenated compound and 0.030(5) in the deuterated one. The values of β 1 are compared with those predicted by two dimensional (2D) models of exchange and the change from β 1 to β 2 is attributed to a crossover in lattice dimensionality from 2d–3d.

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.

POLARIZED NEUTRON DIFFRACTION STUDY OF THE EXTENDED HONEYCOMB MOLECULAR NETWORK D20-P(C6D5)4MNFE(C2O4)3

The MII and MIII magnetic ions in the extended molecular network P(C6D5)4MnFe(C2O4)3 form a two-dimensional honeycomb magnetic lattice. The Mn2+ and Fe3+ ions alternate in the extended network which is formed by the oxalate (C2O4) ligands. These hexagonal layers are separated and charge compensated by large [P(C6D5)4]+ ions, positioned in between the honeycomb layers. P(C6D5)4MnFe(C2O4)3 orders magnetically at TN=27(1) K. A full neutron spin polarization study of the neutron scattering cross section has been carried out which allows the unambigious separation of the magnetic cross section from the total diffraction process. The magnetic structure can be described with the magnetic Shubnikov group R3c. The magnetic moments are antiferromagnetically aligned along the c axis while the Mn2+ and Fe3+ ions form an antiferromagnetic alignment on the honeycomb lattice.

Magnetic and structural characterisation of the layered materials AMnFe(C2S2O2)3

Abstract We present structural and magnetic data on the series of compounds AMnIIFeIII(C2S2O2)3 with A=N(CnH2n+1)4 (n=3–5) or P(C6H5)4. The tetra-alkyl ammonium compounds are all ferromagnets with TC=10 K, whilst the tetraphenyl phosphonium compound is a ferrimagnet with TN=15 K. EXAFS spectroscopy shows that there is significant disorder in the orientation of the dithio-oxalate moiety, while high resolution X-ray powder diffraction measurements prove that the dithio-oxalate materials are structurally analogous to the layered oxalates.

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.