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

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.

Hybrid quantum-classical Monte Carlo study of a molecule-based magnet

“…Within MF theory, the Curie-Weiss temperature Θ obtained from the high-temperature susceptibility χ ≈ C/(T −Θ) is always smaller in magnitude than T c [4] whereas experimentally it can be twice as large [3,20]. While |Θ| should remain close to its MF value even in the presence of fluctuations, T c is reduced by about half from its MF value.…”

“…Second, unlike the model studied by Nakamura [6], the model studied in this paper does not rely on interlayer interactions to produce magnetic compensation. So it can explain the appearance of magnetic compensation in the A = N(n-C n H 2n+1 ) 4 family: as n increases from 3 to 5, the interlayer separation grows from 8.2Å to 10.2Å [3] but magnetic compensation appears only for n = 4 and 5, probably because defects in the bimetallic layer are created by the shortest cation. Third, Eq.…”

“…With λ = −147 K [5] and T c = 45 K, we find that |λ|/T c ≈ 3.3. The bimetallic oxalates should lie quite close to the phase boundary in Fig.7 because some exhibit a compensation point but others do not [3]. In Fig.2, we used the parameter values λ = −13.3J and L cf = 0.3, corresponding to T c = 4.24J and |λ|/T c ≈ 3.13, which is close to the expected ratio.…”

“…For the Fe(II)Fe(III) bimetallic oxalates, however, the presence of magnetic compensation below the ferrimagnetic transition temperature T c does depend on the choice of A [3]. For several cations, the magnetization in a small field is positive just below T c ≈ 45 K but then become negative below T comp ≈ 32 K. This effect was explained by Fishman and Reboredo [4], who used meanfield (MF) theory to solve an effective Hamiltonian that includes spin-orbit coupling on the Fe(II) sites.…”

Polarized neutron diffraction and Mössbauer spectral study of short-range magnetic correlations in the ferrimagnetic layered compounds(PPh4)[<

“…Short-range antiferromagnetic correlations have been studied in the layered compounds (PPh 4 3 ͔ show no magnetic Bragg scattering; the lack of such scattering indicates the absence of long-range magnetic order. However, a broad asymmetric feature observed at a Q of ca.…”