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

Detailed bulk magnetization and magnetic susceptibility measurements are reported for representative examples of the series of ferrimagnetic tris-oxalato-ferrate(II,III) salts with general formula AFeIIFeIII(C2O4)3 (A = quaternary ammonium, phosphonium, or arsonium). The compounds all crystallize in two-dimensional hexagonal honeycomb lattices, but while some show conventional low field positive magnetization at low temperature (Néel type Q), others show a large negative magnetization below a compensation temperature T comp (Néel type N) under the same measurement protocol. Isothermal and temperature-dependent magnetizations are measured after zero field and field cooling. Temperature-dependent hysteresis measurements are also presented. In compounds that exhibit negative magnetization a discontinuity in the magnetization is observed at T t (T comp < T t < T c) which correlates with the onset of a giant anisotropy and is indicative of a magnetostrictive transition. In compounds that exhibit positive magnetization, hysteresis behavior and frequency-dependent ac susceptibility data indicate a glassy magnetic order at low temperatures, possibly arising from the frustration of random anisotropy domains below a characteristic blocking temperature or imbalance and disorder of Fe(II) and Fe(III).

The ac-susceptibilities of the isostructural metal-organic magnets, M"(N(CN)&(M = Ni (l), Co (2) and Fe (3)) have been studied as a function of temperature and pressure. Large variation in behaviors has been observed: the transition temperature initially increases in all three compounds and at higher pressures that for (1) saturates, for (2) decreases and for (3) increases continuously. These behaviors are due to the competition between antiferromagnetic (tZg tf cg) and ferromagnetic (tZg tf t2g and e tf eg) interactions to the most dominant exchange pathway, M,,.N=C-N,-M. A mechanism for the magnetic ordering is proposed.

No abstract