We have investigated the effect of oleic acid concentration on the physicochemical properties of solvothermally derived cobalt ferrite nanoparticles (CFO NPs). Without the oleic acid, agglomerated nanoplatelets with a crystallite size of about 19 nm were obtained, according to X-ray diffraction (XRD) and transmission electron microscopy (TEM). However, the addition of oleic acid decreases the size of the CFO NPs and at critical concentration, which was determined to be 0.25 M, well-dispersed, nonagglomerated spherical particles of about 6 nm were obtained. A further increase in the oleic acid concentration affected the particle size only slightly, with a relatively constant surface coverage of the oleic acid ligand. The results of our study indicate that particle-size control was achieved by bridging bidentate interactions between the oleic acid molecules and the metal atoms on the surface of the NPs, as determined by Fourier transform infrared spectra. These interactions affected the surface strain of NPs considerably, but kept the initial cation redistribution according to the Raman spectra. The room temperature magnetic measurements revealed that oleic acid enables us to effectively control the magnetic behavior of the CFO, which changes from ferrimagnetic to superparamagnetic at a critical concentration. Interparticle interactions are further interpreted by using low-temperature magnetic measurements, which also showed decreased surface anisotropy for samples prepared with oleic acid concentration above the critical value. An investigation of the treatment time showed that the capping with oleic acid is already achieved after 1 h of synthesis, but in order to improve the crystallization and consequently achieve the desired magnetic response a synthesis time of at least 4 h is required.

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The ESR spectra of a TDAE 1 -C 2 60 single crystal show the existence of antiferromagnetic (AFM) correlations between the unpaired spins on neighboring C 2 60 ions along the c axis and spin canting which leads, below T c , to weak ferromagnetism along a direction perpendicular to the c axis. This suggests that a Dzyaloshinsky-Moriya type mechanism is responsible for ferromagnetic ordering rather than itinerant ferromagnetism. The incomplete orientational ordering of the C 2 60 ions leads to a distribution of exchange coupling constants resulting in spin-glass type behavior which coexists with long range ferromagnetic ordering below T c . The observation of AFM correlations may help to understand why the same basic molecule C 60 can support such different phenomena as ferromagnetism (in TDAE-C 60 ) and superconductivity (in, e.g., K 3 C 60 ).

To check on the nature of the weak magnetic order in polycrystalline magnetoelectric Pb(Fe1∕2Nb1∕2)O3 the X-band, Q-band, and far infrared electron paramagnetic resonance (EPR) spectra have been measured between 4 and 600K and compared with magnetic susceptibility and magnetization data. The asymmetric line shapes can be simulated at higher temperature by thermally fluctuating superparamagnetic nanoclusters. The pronounced temperature dependence of the position of the spectra demonstrates the presence of an internal magnetic field which is small but nonzero even at room temperature, i.e., far above the antiferromagnetic transition. The electronic spin-spin exchange has been found to be in the terahertz range. The magnetization data reveal a weak ferromagnetism even above 300K and a break in the temperature dependence of susceptibility at the paramagnetic to ferromagnetic transition.

The site and charge disordered solid solution 0.8Pb(Fe1∕2Nb1∕2)O3–0.2Pb(Mg1∕2W1∕2)O3 is a magnetoelectric relaxor which shows broad and frequency dependent maxima both in the electric as well as in the magnetic susceptibilities. The Vogel-Fulcher-type electric relaxor freeze-out is accompanied by a significant magnetic anomaly demonstrating magnetoelectric coupling. Thus, a magnetoelectric effect is observed in a system exhibiting only mesoscopic and no long range order.

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Here, we report on the observation of a weak ferromagnetic transition at TN=122K in a K3Fe5F15 system which is ferroelectric and ferroelastic below Tc=490K. The magnetization and the susceptibility continuously increase with decreasing T down to 2K. The Mössbauer spectra show a spontaneous magnetic ordering and at least three sites corresponding to Fe2+ and Fe3+. The ratio between Fe2+ and Fe3+ is 60:40. At 6K, there are two magnetically ordered sextets with internal fields of 585 and 263kOe. The “slim” hysteresis loops observed are as well characteristic of weak ferromagnetism. At 122K and at low frequencies, the system shows dielectric anomaly characteristic of magnetoelectric behavior.