The synthesis and structural and magnetic characterization of 16 compounds AM(II)Fe(III)(C(2)O(4))(3) (A = N(n-C(3)H(7))(4), N(n-C(4)H(9))(4), N(n-C(5)H(11))(4), P(n-C(4)H(9))(4), P(C(6)H(5))(4), N(n-C(4)H(9))(3)(C(6)H(5)CH(2)), (C(6)H(5))(3)PNP(C(6)H(5))(3), As(C(6)H(5))(4); M(II) = Mn, Fe) are reported. X-ray powder diffraction profiles are indexed in R3c or its subgroup P6(5)22 or P6/mmm to derive unit cell constants. The structures of all the compounds consist of two-dimensional honeycomb networks [M(II)Fe(III)(C(2)O(4))(3)(-)](infinity). The M(II) = Fe compounds behave as ferrimagnets with T(c) between 33 and 48 K, but five exhibit a crossover from positive to negative magnetization near 30 K when cooled in a field of 10 mT. The compounds exhibiting this unusual magnetic behavior are those that have the highest T(c). Within the set N(n-C(n)()H(2)(n)()(+1))(4)Fe(II)Fe(III)(C(2)O(4))(3) (n = 3-5), T(c) increases with interlayer separation and the low-temperature magnetization changes from positive (n = 3) to negative (n = 4, 5). In the M = Mn(II) compounds, the in-plane cell parameter a(0) is approximately 0.03 Å greater than in the corresponding M = Fe(II) ones while the interlayer separation (c(0)/6) is on average 0.08 Å smaller. All members of the M(II) = Mn series have magnetic susceptibilities showing broad maxima at 55 K characteristic of two-dimensional antiferromagnetism, but the magnetization of several of the salts increases sharply below 27 K due to the onset of spin canting, the magnitude of which varies significantly with A.

No abstract

The phases A F e ~~F e ~~~ (C2O4I3 (A = NPrn,; 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.

No abstract

No abstract

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.

Bimetallic tris-oxalato-salts (n-C n H 2nϩ1 )PPh 3 M II Fe III (C 2 O 4 ) 3 (n = 3-7, M II = Mn, Fe) were prepared and the structures investigated by powder X-ray diffraction in order to study the evolution of the structure and magnetic properties as a function of alkyl chain length. The compounds all have the same two-dimensional honeycomb structure of M II and Fe III bridged by oxalate, with the organic cations lying between the metal-oxalate layers, whose separation ranges from 9.48 Å (n = 3) to 11.10 Å (n = 7) for the Fe II salts and 9.37 to 10.81 Å for Mn II . The compounds all behave as ferrimagnets, with magnetic parameters similar to the corresponding AM II Fe III (C 2 O 4 ) 3 with A = NR 4 ϩ , PPh 4 ϩ and T c s almost insensitive to interlayer separation. The Mn II salts exhibit uncompensated magnetisation below T c and the Fe II ones show Néel type N ferrimagnetism, with negative magnetisation at low temperature, the magnitude of which is influenced by the preparation conditions, due to vacancies in the Fe II sublattice.

Critical exponents of magnetization p below TN in the weakly ferromagnetic layer compounds MnGH%+IFQ.H20 have been measured by SQUm magnetometly for n = 2-4. In all three compounds crossovers a~ observed in p as follows @I, &): 0.21(2), 0.73(2) (n = 2); o.Ia(l), 0.42(6) (n = 3 , o.ia(i), -0.6 (n = 4). The crossover o c m at values of the reduced temperature t = (TN -T ) / T N that become smaller as the separation between the magnetic layers increases.