C. M. Wynn

Long-range magnetic order in the quasi-1D metalloporphyrin family of molecule-based magnets

Abstract The unusual magnetic properties of various members of the quasi-1D manganese-porphyrin family are reviewed. Typical members of the family consist of spin S = 2 Mn III porphyrin ions alternating with trans-μ 2 -bonded spin s = 1/2 acceptors [C 2 (CN) 4 ] − or [C 4 (CN) 6 ] − . All members exhibit 1D behavior at high temperatures as evidenced by their similarities to predictions for mixed quantumclassical ferrimagnetic chains. Below approximately 50 K, deviations from one-dimensionality are observed due to the effects of interchain interactions. The competition between interchain exchange, single-ion anisotropy, and dipolar interactions are considered when examining the remarkable differences in the low temperature magnetic states of the various compounds.

Effect of disorder on the linear and nonlinear magnetic susceptibilities of two manganeseporphyrin-based magnets

We analyze the effects of disorder on the magnetic properties of two manganeseporphyrins, [MnTPP][TCNE]⋅x(o-DCB) and [MnTPP][TCNE]⋅y(o-Xy) and (TPP=meso-tetraphenylporphyrinato, TCNE=tetracyanoethylene, o-Xy=o-xylene, o-DCB=o-dichlorobenzene, x⩽3, y⩽1). We present data for the in- and out-of-phase ac susceptibility and obtain the distribution of relaxation times using Cole–Cole analysis. The frequency dependence of the linear susceptibility suggests cluster glasslike behavior. The peak in the second harmonic of the ac susceptibility indicates the presence of a spontaneous magnetic moment. We propose that the unusual magnetic properties of these compounds reflect one-dimensional ferrimagnetic clusters interacting weakly through dipole-dipole interactions to form three-dimensional domains.

Canted antiferromagnetism and spin glasslike behavior in a family of two-dimensional organic/inorganic nanocomposites

We report magnetic properties of a new class of hybrid organic/inorganic layered materials, Cu2(OH)3(CmH2m+1COO), m=7, 9, and 11. The frequency dependence of the linear ac susceptibility and the irreversibility in the field-cooled/zero-field-cooled magnetization show that the geometrical frustration of the triangular lattice is strong enough to cause glassiness. The strong peak in the second harmonic of the nonlinear ac susceptibility indicates that frustration is not sufficient to prevent the system from developing a spontaneous moment. We propose that the interplay of Heisenberg antiferromagnetic exchange and Dzyaloshinskii–Moriya interaction leads to the unusual coexistence of glassiness and canted antiferromagnetism.

Magnetic ground state and its control in porphyrin-based magnets

Abstract Alternating metalloporphyrin-acceptor systems provide a ‘laboratory’ for study of quasi-one-dimensional magnetic phenomena. For example, the [octaethylporphyrinatomanganate(III)][hexacyanobutadiene], [MnOEP][HCBD], compound is a model alternating classical (spin 2) and quantum (spin 1/2) spin system. While the high temperature susceptibility is well fit to Seidens model for alternating S = 2 and S = 1/2 chains with antiferromagnetic exchange J/kB = 172 K, and g for [MnOEP] = 1.92, the low temperature data reveal a transition to a two-dimensional antiferromagnet, followed by a transition to a three-dimensional canted antiferromagnet. The similarly structured [tetraphenyl-porphyrinatomanganate(III)][tetracyanoethenide], [MnTPP][TCNE], also has ferrimagnetic behavior within the chains, though it has ferromagnetic interactions between the chains. We associate the change in sign of interchain exchange (from antiferromagnetic to ferromagnetic) with an increase in interchain separation and a change in ...

The influence of disorder on the magnetic phenomena in metalloporphyrin-based magnets

Abstract We investigate the effects of disorder in the quasi-one-dimensional metalloporphyrin family of molecule-based magnets. These materials are electron transfer salts consisting of linear chains of alternating S = 2 Mn -porphyrin donors and S = 1/2 bridging acceptors. Studies of the irreversibilities, and frequency dependence of the ac magnetic susceptibility suggest a cluster glass-like behavior in certain compounds at low temperatures. The role of the intercalated solvent and its relation to any disorder present in the compounds are discussed.

Magnetic order and disorder in a family of layered organic/inorganic nanocomposites

Abstract We report magnetic studies of a family of triangular quantum Heisenberg antiferromagnets (TQHAFs) with weak additional Dzyaloshinskii-Moriya interaction, Cu2(OH)3(CmH2m±1COO), m = 7, 9 and 11. The three compounds have nearly identical magnetic properties, despite the large differences in the interlayer separation, indicative of 2D behavior. While the high temperature dc susceptibility data suggest TQHAF behavior, at low temperatures the deviations from the TQHAF predictions imply a canted antiferromagnetic type of ordering, consistent with both the strong peak in the second harmonic of the nonlinear ac susceptibility (at ≈ 20 K) and the low saturation magnetization observed in the low temperature hysteresis curves. We propose that the interplay of Heisenberg and Dzyaloshinskii-Moriya exchanges leads to an unusual state with both 2D Ising-like canted antiferromagnetic and spin glass-like characteristics, a state in which order and disorder appear to coexist.

New High Tc Molecule-Based Magnets - Magnetic Behavior of M(TCNE)2·x(CH2Cl2) (M ˭ Mn, Fe)

Abstract We report and review studies of new partially crystalline members of the high-Tc molecule-based magnets family, M(TCNE)2·x(CH2Cl2), (M ˭ Fe, Mn, TCNE ˭ tetracyanoethylene). Remanent magnetization revealed high critical temperatures, Tc = 97 K for M ˭ Fe and 75 K for M ˭ Mn. Hysteresis curves for the Fe compound, for 5 ≤ T ≤ 80 K, have a spin-flop shape, indicating ferrimagnetic behavior. Field-cooled and zero-field-cooled magnetization reveals magnetic irreversibilities below Tc for both compounds. Static and dynamic scaling analyses of the dc magnetization and ac susceptibility for M ˭ Mn show a transition to a 3D ferrimagnetic state at Tc = 75 K, followed by a reentrant transition, to a spin glass state at Tg = 2.5 K. For M ˭ Fe static scaling analyses are consistent with a high-T transition to a correlated sperimagnet, while below ∼20 K there is a crossover to sperimagnetic behavior.