1,721,007 research outputs found
Formation and mobility of oxygen vacancies in RuSr2GdCu2O8
No full textOxygen vacancies are introduced in the RuO2 and possibly CuO2 planes of RuSr2GdCu2O8 by annealing in vacuum above 600 K. The diffusive jumps of the O vacancies are accompanied by a reorientation of the local distortion, and are probed by measuring the elastic energy loss and modulus versus temperature at 1–10 kHz. An intense acoustic absorption peak develops near 670 K at 1 kHz and finally stabilizes after heating up to 920 K in vacuum. The analysis of the peak shows a barrier for the O diffusion of .1.4 eV, and a slowing down of Curie-Weiss type, with TC54002470 K, due to the interaction among the O vacancies. A secondary peak is attributed to O vacancies trapped at defects in the RuO2 planes, or to vacancies in the CuO2 planes. No sign of structural phase transformation is found up to 920 K
Synthesis and magnetic characterization of Ni nanoparticles and Ni nanoparticles in multiwalled carbon nanotubes
No full textNickel nanoparticles were synthesized by reduction of nickel acetylacetonate in a monosurfactant system. These nanoparticles, mostly amorphous,
were used as catalyst for the growth of multiwalled carbon nanotubes by the catalytic decomposition of methane at 500 ◦C. TEM analysis reveals
a wide size distribution of the diameter of the particles centred around two main values. A detailed characterization of the magnetic properties of
the Ni nanoparticles and Ni nanoparticles carbon nanotubes embedded is hereby presented. Both the systems show superparamagnetic behaviour
above the blocking temperature TB. Magnetization data are well fitted by an equation formed by two weighted Langevin functions and display the
correct scaling of M/MS versus H/T for superparamagnetic nanoparticles. The hysteresis loops obtained below TB agree with the ferromagnetism
of single-domain particles, confirmed also by the expected temperature dependence of the coercive field HC. Differences in the coercive fields at
increasing and decreasing applied magnetic fields are to be ascribed to a NiO layer originating an exchange bias with inner Ni. The smallness of
the coercive fields difference (around 5 Oe) confirms that the used preparation method produces a very thin NiO layer around Ni nanoparticles
Thermodynamic, electric and magnetic properties of Pr7Co6Al7 intermetallic compound
No full textThe low temperature heat capacity and magnetization were measured for the Pr,Co,Al, intermetallic compound. A ferromagnetic transition was observed at 4.5 K with a saturation value of the magnetic moment of the Pr atom in the ordered state of 1.1 mu(B) at 2.5 K. In the paramagnetic region the compound follows the Curie-Weiss law with mu(eff) = 3.59 mu(B) and theta(P) = +5 K. Above the magnetic transition an estimate of the electronic coefficient of the specific heat was obtained
Weak ferromagnetic fluctuations in GdSr2RuCu2O8 compound
No full textIt has been investigated the static magnetic response of the polycrystalline rutheno-cuprate oxide GdSr2RuCu2O8. The susceptibility χ shows an upturn at TC ∼ 134 K which indicates the onset of a magnetic instability. Above that transition we have analyzed within a spin mode coupling theory the dependence of χ-1 in the form of ∼ (T - TC)γ. The main finding is that the value of the critical exponent γ can be understood as mainly due ferromagnetic fluctuations, assuming that the system is close to its quantum paramagnetic instability and that the magnetization comes primarily from itinerant moments, rather than moments localized to the atomic sites
Crystal and magnetic structure of Cr- and Ni-substituted (La0.50Ca0.50)MnO3
No full textThe crystal and magnetic structure of (La0.50Ca0.50)(Mn1−xBx )O3 (x = 0.00, 0.03, 0.08; B = Cr, Ni) has been investigated between 5 and 300 K by means of dc magnetic measurements and neutron powder diffraction followed by Rietveld refinement. In the pristine compound an orthorhombic to monoclinic phase transition is detected on cooling, accompanied by a CE-type antiferromagnetic (AFM) ordering arising. Ni2+ and Cr3+ substitutions have similar effects on the structural and magnetic properties of (La0.50Ca0.50)MnO3, despite the fact that these ions are
characterized by different external electronic configurations. After substitution, the orthorhombic to monoclinic phase transition is hindered. As a consequence, charge and orbital orderings are suppressed, as is the superexchange; double exchange takes place inducing
ferromagnetic (FM) interactions. No evidence for stable magnetic interaction between Cr3+ or Ni2+ and the neighbouring Mn ions was detected. Nevertheless, in the Ni-substituted samples a detectable quantity of monoclinic phase forms during cooling, inside which AFM interactions
take place. The amount of this secondary monoclinic phase decreases on increasing Ni substitution; the global FM magnetic moment decreases as well, due to a spin-cluster glass-type state arising. As a result a FM state is found to coexist with a spin-cluster glass-type state
Ferromagnetic interactions in Nd7Co6Al7
No full textThermodynamic, magnetic and electrical properties of the Nd7Co6Al7 intermetallic phase have been extensively studied. The compound crystallises in the tetragonal Pr7Co6Al7 type structure (space group P4/mbm) and undergoes a ferromagnetic transition at 15.5 K. From the hysteresis loop the values of the remanence and of the coercive field were obtained. Magnetic heat capacity measurements allow to obtain the magnetocaloric effect related to the magnetic transition. An estimate of refrigerant capacity of the compound is obtained
Physical characterization of RuSr2GdCu2O8 sintered samples: An analysis of the magnetic measurements performed by a SQUID magnetometer
No full textIn this paper we present the resistive and magnetic characterization of sintered RuSr2GdCu2O8 samples. We show how the physical properties, related both to the superconducting and magnetic state, are affected by the final annealing step: one week in flowing oxygen. In doing so we perform a detailed analysis of the complexity of the dc magnetic measurements performed on such type of samples below the superconducting transition: in particular we discuss the structural and experimental reasons that make magnetic measurements in such a system so difficult. These reasons come from the simultaneous presence of two magnetic lattices and of the diamagnetism related to the superconducting state. The situation justifies, at least in part, the wide spread of results presented in the literature and the controversial visibility of the Meissner effect. We remark moreover the not negligible effect of the residual field and how it is possible to take it, at least in the zero field cooled magnetic procedure, into account
OBSERVATION OF PHONON-FLUXON SCATTERING IN YBA2C3O7 THROUGH THERMAL-DIFFUSIVITY MEASUREMENTS
No full textFerromagnetic nanoclusters observed by ac and dc magnetic measurements in RuSr2GdCu2O8 samples.
No full textBoth ac and dc magnetic measurements have been performed on polycrystalline RuSr2GdCu2O8 samples, aimed at studying the magnetic ordering of such a phase. Linear ac susceptibility measurements give clear evidence of the antiferromagnetic transition at a temperature T=TN, which has been already widely observed in such system with different techniques. In addition, nonlinear (third-order) susceptibility shows that another magnetic structure exists at a temperature near but slightly higher than TN. Shape and sign of this small signal are compatible with the presence of ferromagnetic nanoparticles exhibiting superparamagnetic behavior. Such possibility has been confirmed by magnetization decay measurements performed at temperatures lower than the blocking temperature TB of the superparamagnetic system. A fit of magnetization decay measurements with a model describing the slow dynamic of magnetic nanoparticle systems gives the size distribution of the nanoparticles and a mean volume of the order of V=2.5×10−19 cm3, in accord with previous estimates. The scenario of RuSr2GdCu2O8 as a phase separated material, already proposed to explain some peculiarities of its magnetic behavior, is here confirmed
Ageing effect on the magnetocaloric properties of Gd, Gd5Si1.9Ge2.1 and on the eutectic composition Gd75Cd25
No full textAbstract
The ageing effects due to the interaction with typical working fluids (water and air) of magnetic refrigerant materials have been analysed for up to one year. Among the many compounds reported to exhibit a sufficiently high magnetocaloric effect, we decided to artificially age Gd, which represents the first choice high magnetocaloric element, Gd5Si1.9Ge2.1, belonging to the series Gd5(SixGe4-x)4 which for x ~ 0.5 displays a giant magnetocaloric effect, and finally, the eutectic Gd75Cd25 alloy, presenting an almost constant adiabatic temperature rise in an interesting temperature span (260-280 K). Magnetothermal results give evidence that corrosion and corrosion/erosion processes take place with different results on the refrigerant properties of all the materials. The adiabatic temperature rise is strongly reduced due to surface oxidation which lowers thermal conduction while the effect on the refrigerant capacity is definitely smaller. The effects of corrosion/erosion processes are confirmed by quantitative chemical analysis performed on the refrigerant fluid before and after the ageing process. Employing working fluids with a reduced corrosive effect but with comparable thermal properties may be a viable way of avoiding corrosion damage; alternatively the use of additives to decrease the corrosive properties of the fluids is strongly suggested
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