The occurrence of bulk superconductivity at ~22 K is reported in polycrystalline samples 1.INTRODUCTIONOver the last two years much progress has been made in establishing superconductivity unambiguously in MFe 2 As 2 (M=Ba,Sr,Ca,Eu) systems [1][2][3][4][5][6]. The pristine sample that has a Spin Density Wave (SDW) ground state is nudged into a superconducting (SC) state by electron/ hole doping and application of pressure [2,7,8,9,10]. Band structure calculations point to the fact that SDW state arises on account of the special 2D geometry of Fermi surface that is unstable to nesting [11,12]. Also associated with or preceding the magnetic transition is a tetragonal to orthorhombic structural transition, which is suppressed in the superconducting state. The strong interplay of structure, magnetism and electronic structure have been investigated recently in the Co substituted BaFe 2-x Co x As 2 system [13].The temperature composition phase diagrams determined for the different chemical substitutions at different sites in BaFe 2 As 2 [14,15,16] show a generic behaviour as a function of the concentration of the substituent, viz., a systematic suppression of the SDW transition, followed by co-existence of SDW and SC and the occurrence of a superconducting dome. Several transition metal (TM) substitutions with electrons in excess of Fe forming, BaFe 2-x TM x As 2 have been studied but the maximum T C has remained at ~25 K [17]. A much higher T C of 38 K and ~35 K, were however observed by optimal hole doping in the Ba 1-x K x Fe 2 As 2 [2] system and in BaFe 2 As 2 by application of high pressure [10]. A systematic investigation on the role of hydrostaticity, in the pressure dependent resistivity study of BaFe 2 As 2 , has revealed that uniaxial pressure favours the occurrence of high T C at 36 K whereas a lower T C of 29 K occurs under truly hydrostatic pressure [18]. Consistent with this finding are results that indicate that strained crystals of BaFe 2 As 2 and SrFe 2 As 2 display superconductivity at ambient pressure [19]. A compilation of structural data from several compounds of the related ReOFeAs (Re=rare-earth) superconducting family, indicates that T C is optimized at a particular Fe-As distance [20] and/or at a particular Fe-As tetrahedral angle [21], indicating that the local structure of the FeAs 4 tetrahedra plays a crucial role in determining T C . Devising schemes to effect structural distortions by chemical substitution that would lead to higher T C in the BaFe 2 As 2 system will be useful.Thus motivated, we examine the effect of Ru substitution at the Fe site in BaFe 2 As 2 . At the outset, it is clear that Ru is isoelectronic to Fe and being larger in size should 3 introduce steric effects, affecting the Fe-As bond length leading to distortions of the FeAs 4 tetrahedral motifs. In addition, the larger radius of the 4d electron shell should increase the metal-metal overlap in the Fe/Ru layer and increase the hybridization of metal atom with As leading to significant alterations in the electr...

High coercivity (9.47 kOe) has been obtained for oleic acid capped chemically synthesized CoFe(2)O(4) nanoparticles of crystallite size approximately 20 nm. X-ray diffraction analysis confirms the formation of spinel phase in these nanoparticles. Thermal annealing at various temperatures increases the particle size and ultimately shows bulk like properties at particle size approximately 56 nm. The nature of bonding of oleic acid with CoFe(2)O(4) nanoparticles and amount of oleic acid in the sample is determined by Fourier transform infrared spectroscopy and thermogrvimetric analysis, respectively. The Raman analysis suggests that the samples are under strain due to capping molecules. Cation distribution in the sample is studied using Mossbauer spectroscopy. Oleic acid concentration dependent studies show that the amount of capping molecules plays an important role in achieving such a high coercivity. On the basis of above observations, it has been proposed that very high coercivity (9.47 kOe) is the result of the magnetic anisotropy, strain, and disorder of the surface spins developed by covalently bonded oleic acid to the surface of CoFe(2)O(4) nanoparticles.

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