J. Sanchez Marcos

Effect of Ni2+(S= 1) and Cu2+(S=½) substitution on the antiferromagnetic ordered phase Co2(OH)PO4 with spin glass behaviour

The hydroxyphosphates with formula Co1.7M0.3(OH)PO4 (M = Ni, Cu) have been prepared from hydrothermal synthesis. The compounds have been characterized by X-ray powder diffraction and spectroscopic measurements. Diffuse reflectance data of both phases show bands belonging to the two chromophores, octahedral and trigonal bipyramidal, of the Co(II) ions together with others associated to the chromophores of nickel(II) and copper(II) ions. Magnetization measurements of Co1.7Ni0.3(OH)PO4 show the presence of two maxima at ca. 62 and 5 K, respectively. The first peak was attributed to a three-dimensional antiferromagnetic ordering and the second one reveals the existence of a spin glass-like state. This behaviour was confirmed from the ac measurements obtained at different frequencies and applied fields. The Co1.7Cu0.3(OH)PO4 phase exhibits only one maximum at 65 K in both χm and ac measurements associated to the three-dimensional antiferromagnetic interactions; however, the existence of spin glass behaviour is not observed. ZFC-FC curves for the cobalt–nickel and cobalt–copper compounds show irreversibility just below TN indicating the existence of higher spin decompensations than in the Co2(OH)PO4 phase. This difference and its influence in the magnetic anomalies observed at low temperatures is the more important effect in the magnetic measurements of the ordered Co2(OH)PO4 phase substituted at about 15% of the Co2+ (S = ) by Ni2+ (S = 1) or Cu2+ (S = ½) ions. The origin of the frustration, which is necessary to achieve a disordered ground state, is investigated in these kinds of spin glass compounds. Taking into account the possible magnetic exchange pathways, which are essential to install competition and ensure cooperativeness of the freezing process, correlations between structural and magnetic properties have been performed.

Magnetic structures of (Co2-xNix)(OH)PO4 (x = 0.1, 0.3) spin glass-like state in antiferromagnetically ordered phases

Compounds of the general formula Co2?xNix(OH)PO4 (x = 0.1, 0.3) have been synthesized under mild hydrothermal conditions. Neutron powder diffraction, susceptibility and heat capacity measurements were carried out on polycrystalline samples. The cobalt?nickel compounds are ordered as three-dimensional antiferromagnets with ordering temperatures of 70 and 64?K for x = 0.1 and x = 0.3, respectively. The magnetic study shows a spin glass-like state below 11 and 5?K for Co1.9Ni0.1(OH)PO4 and Co1.7Ni0.3(OH)PO4, respectively. Specific heat data present peaks at 68 and 61?K for Co1.9Ni0.1 and Co1.7Ni0.3, respectively. These peaks show broad shoulders between approximately 15 and 40?K. The lack of any distinguishable anomaly below 10?K supports the spin glass nature of the low temperature transitions. Refinement of room temperature neutron diffraction data indicates that the Ni(II) ions are in octahedral co-ordination with the practical absence of these ions in the trigonal bipyramidal sites. The magnetic structures of Co2?xNix(OH)PO4 consist of ferromagnetic arrangements between the octahedral chains and trigonal bipyramidal dimers within the xz plane with the magnetic moments along the z axis. The ferromagnetic layers are disposed antiparallel to one another along the y direction establishing the three-dimensional antiferromagnetic order (TN?70?K for Co1.9Ni0.1 and ?64?K for Co1.7Ni0.3). The different exchange pathways, the anisotropy of the Co(II) ions and the frustration of the magnetic moments in the trigonal bipyramidal geometry could be responsible for the freezing process.

Influence of Ce?H bonding on the physical properties of the hydrides CeCoSiH1.0 and CeCoGeH1.0

The hydrides CeCoSiH(1.0) and CeCoGeH(1.0) which crystallize like the parent antiferromagnetic compounds CeCoSi and CeCoGe in the tetragonal CeFeSi-type structure, have been investigated by specific heat and thermoelectric power measurements and (1)H nuclear magnetic resonance (NMR). CeCoSiH(1.0) is an intermediate valence compound whereas CeCoGeH(1.0) can be considered as a nearly trivalent cerium compound. This behaviour is corroborated by the occurrence of a slight broadening of the (1)H NMR signal in the sequence [Formula: see text]. The band structure calculations performed on these hydrides reveal the existence of strong bonding Ce-H interaction, found to be larger in CeCoSiH(1.0) than in CeCoGeH(1.0).

High temperature electrical resistivity in U1−xLaxPt compounds

Abstract We present electrical resistivity measurements on U 1− x La x Pt ( x =0, 0.1, 0.2, 0.5, 0.8, 0.9, 1) between 2 and 1000 K. The compounds exhibiting ferromagnetic behaviour ( x ≤0.3) display a maximum at low temperatures and some of them a minimum at higher temperatures. For the non-ordered compounds ( x ≥0.5) the resistivity increases continuously with temperature. A common feature of the U-rich compounds is the existence of a negative logarithmic temperature dependence of the total resistivity above the maximum, whereas a combination of a linear term plus a logarithmic term is found for the others. Under a Kondo-like framework, a breakdown of the Mattiessen rule is suggested.