We have systematically investigated the magnetic properties and magnetocaloric effect (MCE) in RMnO3 (R=Dy, Tb, Ho and Yb) single crystals. Above a critical value of applied field (Hc), RMnO3 undergo a first-order antiferromagnetic (AFM) to ferromagnetic (FM) transition below the ordering temperature (T R N ) of R 3+ moment and a second-order FM to paramagnetic (PM) transition above T R N . Both H and T dependence of M shows that the system is highly anisotropic in the FM as well as PM states and, as a result, the magnetic entropy change (∆SM ) is extremely sensitive to the direction of applied field and can be negative (normal MCE) or positive (inverse MCE). For hexagonal HoMnO3 and YbMnO3 systems, a very small inverse MCE is observed only for H parallel to c axis and it decreases with increasing H and crosses over to normal one above Hc. On the other hand, for orthorhombic DyMnO3 and TbMnO3, though the inverse MCE disappears above Hc along easy-axis of magnetization, it increases rapidly with H along hard-axis of magnetization for T T R N . Except for YbMnO3, the values of ∆SM , relative cooling power and adiabatic temperature change along easy-axis of magnetization are quite large in the field-induced FM state for a moderate field strength. The large values of these parameters, together with negligible hysteresis, suggest that the multiferroic manganites could be potential materials for magnetic refrigeration in the low-temperature region.

We have investigated the magnetic and magnetocaloric properties of HoMnO3 single crystal. HoMnO3 displays a series of complicated phase transitions due to the long range ordering of Mn3+ and Ho3+ moments. Field variation in magnetization generates a metamagnetic transition and produces an entropy change of 13.1 J/kg K at 7 T in the vicinity of antiferromagnetic ordering temperature of Ho3+. The values of adiabatic temperature change (∼6.5 K) and relative cooling power (∼320 J/kg) for a field change of 7 T are also appreciable to consider HoMnO3 as a magnetic refrigerant at low temperature.

Earlier attempts to obtain technetium complexes with cysteine always resulted in the formation of a product contaminated with polymeric species. A pure product, which could be chemically characterized and adopted for radiopharmaceutical preparation, has now been obtained by using cystine as the precursor of cysteine. This method has been extended to prepare the corresponding rhenium chelate, isolated as the tetraphenylphosphonium salt [Ph(4)P](+)[{ReO(Cys)(2)}(-){HReO(Cys)(2)}].4H(2)O. The X-ray crystal structure of this compound revealed the presence of both neutral and anionic chelated species. In [HReO(Cys)(2)], the cysteine carboxylate moiety is unidentatedly bound to rhenium, while the carboxylic acid of the second cysteine remains as free COOH. The coordination environment around rhenium in the anionic species [ ReO(Cys)(2)(-)] is similar, the only difference being that the uncoordinated carboxylate moiety is present as a COO(-) anion. The thiolate, amine coordination of the ligand with the metal is present in both the chelate units. The compound crystallized in an orthorhombic system with the space group P2(1)2(1)2(1), and having four formula units in each cell. The crystal data are a = 9.700(2) Å, b = 12.836(3) Å, and c = 36.228(3) Å. The rhenium chelate has been structurally correlated with the technetium chelates through comparable spectroscopic and chromatographic data. The technetium-99m analogue of this rhenium chelate exhibited renal tubular transport and renal retention, which makes this radiopharmaceutical useful for evaluation of the clinical status of renal patients.

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