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We have demonstrated magnetodielectric (MD) coupling in an Ising-chain magnet Sr2Ca2CoMn2O9 via detailed investigation of ac susceptibility and dielectric constant as a function of temperature, magnetic field, and frequency. Sr2Ca2CoMn2O9 consists of spin-chains made of the regular stacking of one CoO6 trigonal prism with two MnO6 octahedra. The (Co2+–Mn4+–Mn4+) unit stabilizes a (↑↓↑) spin-state along the chains that are distributed on a triangular lattice. This compound undergoes a partially disordered antiferromagnetic transition at TN ∼ 28 K. The dielectric constant exhibits a clear peak at TN only in the presence of an external magnetic field (H ≥ 5 kOe), evidencing the presence of MD coupling, which is further confirmed by field-dependent dielectric measurements. We argue that spatial inversion symmetry can be broken as a result of exchange-striction along each spin chain, inducing uncompensated local dipoles. At low temperatures, a dipolar relaxation phenomenon is observed, bearing strong similarities to the blocking effect typical of spin dynamics in this compound. Such a spin-dipole relationship is referred to as a "multiblocking" effect, in relation to the concept of magnetodielectric "multiglass" previously introduced for related materials. [ABSTRACT FROM AUTHOR]

It has been well established that a fine tuning in cobalt ferrite nanocrystal size within the single domain region would lead to the achievement of extremely high coercivity values at room-temperature. If so, the high coercivity and chemical stability expected for this ferrite will increase its attractiveness for magneto-optical recording applications. The present work addresses the development of a size-sensitive phase separation method for cobalt ferrite nanocrystals that is based on selective dissolution of the superparamagnetic fraction and subsequent size-sensitive magnetic separation of single domain nanoparticles. Ferrite nanocrystals synthesized under size-controlled conditions were first contacted with acidic solutions under precise conditions of acid concentration and contact time, followed by a magnetically assisted phase separation in water. Produced nanocrystals were characterized by x-ray diffraction, transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), vibrating sample magnetometry (VSM), and Mβssbauer spectroscopy techniques. The attained coercivity value of 9.4 kOe was mainly attributed to the enlargement of the average crystal size within the single domain region coupled with the removal of the superparamagnetic fraction in the ferrite powders. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]