By suppressing the growth of impurities, a melt spinning method was successfully applied to the synthesis of typical 1 : 2 : 2 layered CeMn 2 Si 2 and AlFe 2 B 2 compounds. X-ray diffraction analysis showed that CeMn 2 Si 2 and AlFe 2 B 2 had good purity and crystallized in ThCr 2 Si 2 -and AlFe 2 B 2 -type structures, respectively. The differences among three 1 : 2 : 2-type structures were also analyzed. The magnetic properties were investigated by magnetic measurements and electronic structure calculations. It was found that the Fe moment of AlFe 2 B 2 reaches 1.32 μ B at 5 K, which fits well with the calculated result of 1.44 μ B at 0 K, and that the isothermal magnetic entropy change reaches 7.2 J kg %1 K %1 at 5 T, which shows great potential for room-temperature refrigeration applications.

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A modified Metropolis method is performed to simulate the hysteresis of three-dimensional noninteracting nanoparticles. Unlike the method of Xu et al. [J. Appl. Phys. 89, 3403 (2001)] and that of Jalil [J. Appl. Phys. 93, 8050 (2003)], in which the particles are always restricted to stay in the minimum energy state in the former, or are allowed to fluctuate among five states around the minimum energy state in the latter, we completely consider the random thermal fluctuation of the magnetic moment in three-dimensional space at low temperature. Hence our method is more accurate compared with physical reality. The simulation results show that the hysteresis loops appear over a wider range of temperature compared with those of Xu et al. The remanence and coercivity of the system decrease as temperature increases and disappear at high temperature.

The dependence of remanence and coercivity on the exchange and dipolar fields in a three-dimensional simple cubic assembly of nanoparticles is studied by a modified Monte Carlo method in which the thermal fluctuation of magnetic moments of nanoparticles is completely considered. The long-range dipolar interaction is summed directly in real space during the simulation. It is found that the dipolar field elongates the hysteresis loop horizontally, but the exchange field elongates it vertically. The competition between dipolar and exchange interparticle interactions leads to complex dependences of the remanence and coercivity on the exchange and dipolar fields at low temperature. Both remanence and coercivity decrease with the increase of temperature. Some results are in agreement with the experimental measurements.

Size and polydispersity effect on the magnetization of densely packed magnetic nanoparticlesThe dipole-dipole interactions of monodisperse Fe 3 O 4 nanoparticles ͑NPs͒ can be directly controlled by a uniform SiO 2 shell with different thickness, i.e., different interspacings. Thus, the interacting strength of a serial of Fe 3 O 4 -SiO 2 NPs system can be revealed by fitting the blocking temperature T B measured at ac fields to the Vogel-Fulcher law. The interspacing over five times of diameter for less than 8.0 nm Fe 3 O 4 NPs is the critical value to achieve a noninteracting system. Furthermore, a general equation to evaluate critical interspacing for noninteracting magnetic NPs systems with different sizes and saturation magnetizations was calculated by Monte Carlo method.