Superparamagnetic Fe3O4 nanocrystals were prepared by a chemical coprecipitation method with a thin thickness-adjustable silica layer coated on the surface by hydrolysis of tetraethyl orthosilicate. The silica-coated Fe3O4 nanocrystals were well dispersed and consisted of a 6–7 nm diameter magnetic core and a silica shell about 2 nm thick, according to transmission electron microscopy observations. Fourier transform infrared spectra revealed that amino (–NH2) groups were successfully covalently bonded to the silica-coated Fe3O4 and then carboxyl (–COOH) groups were functionalized to the surface through the reaction of –NH2 and glutaric anhydride. The synthesized nanocrystals have a cubic spinel structure as characterized by x-ray diffraction, electron diffraction and high-resolution transmission electron microscopy. Their magnetic properties were carefully investigated by a SQUID magnetometer. The results showed that the nanocrystals were superparamagnetic and the blocking temperature TB shifted from 131 K down to 92 K after they were coated with a thin nonmagnetic layer, since this layer can effectively suppress the magnetic dipolar interaction between particles; the chemically inert silica layer can limit the outside environment effect on the Fe3O4 cores quite well due to the excellent magnetic reproducibility of the coated nanocrystals after ageing for 7 months at room temperature. In addition, the dependence of their high-field specific magnetization on temperature has a T2 relationship. These functionalized silica-coated Fe3O4 superparamagnetic nanocrystals have great potential in biomagnetic applications.

The crystal structure, magnetic and magnetocaloric properties of (Ho1−x Y x )5Pd2 (x = 0, 0.25, and 0.5) compounds are investigated. All the compounds crystallize in a cubic Dy5Pd2-type structure with the space group Fd3m and undergo a second order transition from spin glass (SG) state to paramagnetic (PM) state. The spin glass transition temperatures T g decrease from 26 K for x = 0 to 13 K for x = 0.5. In the PM region, the reciprocal susceptibilities for all the compounds obey the Curie–Weiss law. The paramagnetic Curie temperatures ( θ p ) for Ho5Pd2, (Ho0.75Y0.25)5Pd2, and (Ho0.5Y0.5)5Pd2 are determined to be 32 K, 30 K, and 22 K, respectively, and the corresponding effective magnetic moments ( μ eff ) are 10.8 μ B /Ho, 10.3 μ B /RE, and 7.5 μ B /RE, respectively. Magnetocaloric effect (MCE) is anticipated according to the Maxwell relation, based on the isothermal magnetization curves. For a magnetic field change of 0–5 T, the maximum values of the isothermal magnetic entropy change − Δ S M of the (Ho1−x Y x )5Pd2 (x = 0, 0.25, and 0.5) compounds are determined to be 11.5 J · kg − 1 · K − 1 , 11.1 J · kg − 1 · K − 1 , and 8.9 K J · kg − 1 · K − 1 , with corresponding refrigerant capacity values of 382.3 J · kg − 1 , 336.2 J · kg − 1 , and 242.5 J · kg − 1 , respectively.