Karsten Falk

Three-dimensional magnetic ordering in manganese(III)–porphyrin–TCNE complexes

Abstract We have investigated Mn(III)–porphyrin–TCNE complexes showing peculiar magnetic properties at low temperatures. The tetraphenylporphyrin ligand is substituted on the periphery of the phenyl rings with different groups. Static and dynamic susceptibility measurements have been carried out on the samples.

DSC Monitoring of the Spin Crossover in Fe(II) Complexes

AbstractHeat flow to [Fe(bzimpy)2](ClO4)2⋅0.25H2O complex (bzimpy=2,6-bis(benzimidazol-2-yl)pyridine) (I) was measured between 300 and 460 K by differential scanning calorimetry. This exhibits a well-developed peak characteristic of the first-order phase transitions at temperature 403 K. The enthalpy and entropy of transition from low-spin to high-spin state has been determined to be ΔH=17 kJ mol−1 and ΔS=43.0 Jmol−1 K−1. Heat flow to [Fe(bzimpy−1H)2]⋅H2O complex (bzimpy−1H=deprotonated bzimpy) (II) was measured between 300 and 580 K. The spin crossover in this system is accompanied with liberation of crystal water on the first heating. To monitor the structural changes during the spin crossover, powder diffraction data have been collected as a function of temperature.

Spin Flop Transition in the Substituted Mn-Porphyrin Magnet [Mn(OC12H25)4TPP][TCNE]*2PhMe

Abstract The modificated Mn-porphyrin magnet [Mn(OC12H25)4TPP][TCNE]* solvent orders ferromagnetically at Tc = 22 K. Close to Tc transition to the spin glass-like state occurs. The coercive field Hc at T = 2.3 K equals 21 kOe but decreases quickly with temperature. In the paper the low temperature magnetization curves are investigated. The observed field induced magnetization jump we identify as the spin flop which is the transition from the longitudinal to the transverse spin glass state. The transition may come from the sign change of the anisotropy constant. The critical field for the transition decreases with temperature and equals zero for T > 7.5 K. Above 7.5 K magnetic uniaxial anisotropy is lost and a single ion anisotropy favours the ferromagnetic ordering of spins.

Manganese (III)-porphyrin-TCNE-systems with three-dimensional magnetic ordering at ∼ 20 K

Tetrakis-(para-R-phenyl)-porphyrinatomanganese(III)tetracyanoethenides with different R groups in the para-position of the phenyl-rings show interesting magnetic properties at low temperatures. In an area of 20 K they undergo a transition to a three-dimensional-ordered magnetic phase. Below these critical temperatures the compounds show typical magnetic behaviour like coercivity and remanence. AC-susceptibility measurements give hints for reentrant-spin-glass behaviour.

Mn–Porphyrin Complexes with Long Range Magnetic Ordering

We investigated one-dimensional donor-acceptor chains, built of [Mn III R 4 TPP] + cations (R4TPP = substituted tetraphenylporphyrin) and [TCNE] - radical anions (TCNE = tetracyanoethylene) which can exhibit a phase transition to a three-dimensional magnetically ordered state at low temperatures. This transition can be followed by measurements of the dynamic magnetic susceptibility which in this case shows a maximum in the real as well as in the imaginary part. Magnetization measurements give information about characteristic properties like remanence and coercivity (hysteresis), and spin flop behaviour. Despite the fact of large interchain distances between the columns one explanation for the magnetic ordering is a dipolar interaction in combination with the high magnetic anisotropy of the Mn III ions.

Recent results on some columnar paramagnetic metallomesogens

The liquid crystalline and selected physical properties of some vanadyl and manganese(III)-porphyrin-TCNE complexes are discussed. These materials form linear chains from a magnetic point of view and columnar discotic LC phases. Some of the manganese complexes exhibit a bulk magnetic phase below ∼20 K. The discussed materials are studied by means of DSC, X-ray powder diffraction, dielectric spectroscopy, reversal current characterization and magnetic susceptibility measurements. A broader view on some physical properties of columnar paramagnetic metallomesogens is given.

Non-Linear Magnetic Behavior of a Tetranuclear Copper(II) Cluster

Abstract A tetranuclear copper(II) cluster, namely [Cu4OCl6(mor)4], mor—morpholine, has been investigated by the magnetization measurements at the temperature as low as 1.74 and 2.73 K, respectively. A numerical derivative yielded the differential magnetic susceptibility. The low-temperature magnetization data were fitted to a two dimensional function M= f(B,T) and the following set of magnetic parameters has been obtained: g = 2.351, Ja/k = − 6.33 K and Jb/k = − 0.76 K. These parameters determine the spacing of energy levels in a tetranuclear cluster of the C3v symmetry.