Magnetic nanoparticles for magnetocaloric applications should combine small coercivity, low demagnetization temperature, and high pyromagnetic coefficients while keeping the magnetization as high as possible. The strong dependence of the magnetic properties of cobalt-zinc mixed ferrite with specific dopant species enables this material to be considered a promising candidate for magnetocaloric applications. On this basis, pure and Dy-doped Co0.7Zn0.3Fe2O4 cobalt-zinc ferrite nanocrystals have been synthesized by conventional and modified (i.e., flow rate controlled addition of reactants) coprecipitation routes. The modified approach allows the control of ferrite crystal growth at the nanoscale and hence tuning of the corresponding magnetic properties. The magnetic properties of the produced nanocrystals were determined as a function of their structure, nominal dopant concentration, and crystal size. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy analyses suggested both the actual incorporation of the dopants into the host ferrite lattice and the promoting effect on crystal size of the flow rate at which the reactants are contacted. The average crystallite size varied from 13 nm (no control of flow rate) to 28 nm when the ferrite was synthesized at 1 ml/min. Doping caused the maximum magnetization of the ferrite to decrease; this parameter dropped from 60 emu/g (nondoped ferrite) to 55 emu/g when the ferrite was doped with 0.01 at. % of Dy. The maximum magnetization of the Dy (y=0.01) Co–Zn ferrite went up to 62 emu/g when the synthesis was carried out under flow-controlled conditions. The presence of 0.01 at. % Dy in the ferrite caused the demagnetization temperature to decrease from 350 °C (nondoped ferrite) to 320 °C. The demagnetization temperature was further decreased down to 308 °C when the ferrite powders were synthesized under flow rate controlled conditions.

CoxZn1−xFe2O4 (0.5 ≤ x ≤ 1.0) nanocrystals have been synthesized by conventional and a modified size-controlled coprecipitation method and characterized by X-ray diffraction, transmission electron microscopy, vibrating sample magnetometry, and Mössbauer spectroscopy. The control of the oversaturation conditions in the reacting solution, favored by the control of the flow rate at which the reactants were contacted, promoted heterogeneous nucleation and hence crystal growth, as evidenced by XRD measurements. The size-controlled synthesis route also allowed the tuning of the maximum magnetization and the coercivity, which increased by a factor of five. The demagnetization temperature was found to be strongly dependent on the Co-content and crystal size and varied between 106 °C and 540 °C. Mössbauer spectroscopy confirmed the strong influence of the synthesis conditions on the superparamagnetic fraction in the samples.

Pure and Zn1-xCoxO nanoparticles have been synthesized by a simple sol-gel method at low temperature where neither a chelating agent nor subsequent annealing was required. The effect of Cobalt atomic fraction, ‘x’ ≤ 0.0625, on the structural and magnetic properties of the doped ZnO powders was evaluated. X-ray diffraction and Fourier-transform infrared spectroscopy analyses evidenced the exclusive formation of the ZnO-wurtzite structure; no isolated Co-phases were detected. The linear dependence of cell parameters a and c with ‘x’, suggested the actual replacement of Zn by Co ions in the oxide lattice. Micro Raman spectroscopy measurements showed a band centered at 534cm-1, which can be assigned to a local vibrational mode related to Co species, in addition to the normal modes associated with wurtzite. The intensity and broadening of this band at 534 cm-1 were enhanced by increasing ‘x’. In turn, the other bands corresponding to A1 (E2, E1) and E2High modes were red shifted at higher Co contents. Room-temperature magnetization measurements revealed the paramagnetic behavior of the Co-doped ZnO nanoparticles.

Development of new materials includes establishing relationships between their different components and the physical and chemical properties pertinent to a particular application. In this work, the relationship between several magnetic nanoproperties of cobalt–zinc ferrite as functions of the contents of sodium hydroxide (NaOH) and gadolinium (Gd) are studied using statistically designed experiments. The results point towards the possibility of optimizing the material’s coercivity while leaving the rest of the properties statistically unaffected. This study helps to establish the material’s feasibility to be used in magneto-caloric applications.

A Co 2 (pzdc) 2 (bpy)(H 2 O) m (pzdc: pyrazine-2,3dicarboxylate; bpy: 4,4′-bipyridine) porous coordination polymer (PCP) was studied for CO 2 uptake and concomitant structural changes at ambient temperature. Extended structural characterization included evaluation of lattice parameter changes upon CO 2 adsorption and in situ synchrotron X-ray powder diffraction data. The PCP effective pore size increased by ∼2% with gas uptake over the pressure range of 1−50 atm, allowing the adsorption capacity to double. Furthermore, the hysteretic behaviors seen during CO 2 adsorption at moderate pressures are commensurate with the structural changes from synchrotron data. The adsorption and hysteresis occur with rotation of the linking carboxylate groups, and yet only minor changes in unit cell volume (ΔV ≈ 6 Å 3 ) are observed. This contrasts the findings for [Cu 2 (pzdc) 2 (bpy)] n , where a combination of pillar bpy rotations and significant lattice expansion (ΔV ≈ 68 Å 3 ) takes place upon hysteretic adsorption of CO 2 . In situ high-temperature X-ray diffraction revealed that the Co(II)-based material has good thermal stability up to ca. 200 °C. Finally, the CO 2 uptake also appears to be at a physisorption level, with adsorbent−adsorbate interactions that are ca. 30% stronger than what has been reported for CO 2 adsorption onto [Cu 2 (pzdc) 2 (bpy)] n .