Synthesis and crystal growth

Abstract:

We report muon-spin rotation, heat capacity, magnetization, and AC magnetic susceptibility measurements of the magnetic properties of the layered spin-1/2 antiferromagnet NaNiO2. These show the onset of long-range magnetic order below T-N = 19.5K. The temperature dependence of the muon precession frequency suggests 2D XY magnetic ordering. Rapid muon depolarization persisting to about 5 K above TN is consistent with the presence of short-range magnetic order. Our AC susceptibility measurements suggest that magnetic clusters persist above 25 K, with their volume fraction decreasing with increasing temperature, and that there is a slowing of spin fluctuations at T-sf = 3 K. A partial magnetic phase diagram has been deduced. (c) 2006 Elsevier B.V. All rights reserved.

Abstract:

We report muon-spin rotation, heat capacity, magnetization, and AC magnetic susceptibility measurements of the magnetic properties of the layered spin-1/2 antiferromagnet NaNiO2. These show the onset of long-range magnetic order below TN=19.5K. The temperature dependence of the muon precession frequency suggests 2D XY magnetic ordering. Rapid muon depolarization persisting to about 5K above TN is consistent with the presence of short-range magnetic order. Our AC susceptibility measurements suggest that magnetic clusters persist above 25K, with their volume fraction decreasing with increasing temperature, and that there is a slowing of spin fluctuations at Tsf=3K. A partial magnetic phase diagram has been deduced. © 2006 Elsevier B.V. All rights reserved.

Abstract:

Nanocrystalline CuFe2O4 and CuFe2O 4/xSnO2 nanocomposites (x=0, 1, 5 wt%) have been successfully synthesized by one-pot reaction of urea-nitrate combustion method. The transmission electron microscope study reveals that the particle size of the as synthesized CuFe2O4 and CuFe2O 4/5 wt%SnO2 are 10 and 20 nm, respectively. The SnO 2 coating on the nanocrystalline CuFe2O4 was confirmed from HRTEM studies. The resultant products were sintered at 1100 °C and characterized by XRD and SQUID for compound formation and magnetic studies, respectively. The X-ray diffraction pattern shows the well-defined sharp peak that confirms the phase pure compound formation of tetragonal CuFe2O4. The zero field cooled (ZFC) and field cooled (FC) magnetization was performed using SQUID magnetometer from 2 to 350 K and the magnetic hysteresis measurement was carried out to study the magnetic properties of nanocomposites.

Abstract:

We have studied the magnetization of a series of spin-charge-ordered La2-x Srx Ni O4+δ single crystals with 0≤x≤0.5. For fields applied parallel to the ab plane there is a large irreversibility below a temperature TF1 ∼50 K and a smaller irreversibility that persists up to near the charge-ordering temperature. We observed memory effects in the thermoremnant magnetization across the entire doping range. We found that these materials retain a memory of the temperature at which an external field was removed and that there is a pronounced increase in the thermoremnant magnetization when the system is warmed through a spin reorientation transition. © 2006 The American Physical Society.

Abstract:

A shallow-to-deep instability of hydrogen defect centres in narrow-gap oxide semiconductors is revealed by a study of the electronic structure and electrical activity of their muonium counterparts, a methodology that we term 'muonics'. In CdO, Ag2O and Cu2O, paramagnetic muonium centres show varying degrees of delocalization of the singly occupied orbital, their hyperfine constants spanning 4 orders of magnitude. PbO and RuO 2, on the other hand, show only electronically diamagnetic muon states, mimicking those of interstitial protons. Muonium in CdO shows shallow-donor behaviour, dissociating between 50 and 150 K; the effective ionization energy of 0.1 eV is at some variance with the effective-mass model but illustrates the possibility of hydrogen doping, inducing n-type conductivity as in the wider-gap oxide, ZnO. For Ag2O, the principal donor level is deeper (0.25 eV) but ionization is nonetheless complete by room temperature. Striking examples of level-crossing and RF resonance spectroscopy reveal a more complex interplay of several metastable states in this case. In Cu2O, muonium has quasi-atomic character and is stable to 600 K, although the electron orbital is substantially more delocalized than in the trapped-atom states known in certain wide-gap dielectric oxides. Its eventual disappearance towards 900 K, with an effective ionization energy of 1 eV, defines an electrically active level near mid-gap in this material. © 2006 IOP Publishing Ltd.