[en] Experimental results on amorphous rare earth and transition metal alloys suggest the presence of Fe-rich clusters. A model is proposed in which the magnetic units are magnetic clusters. The magnetization of the clusters decreases with the increase of temperature. In this model, there are two critical temperatures, T_c^system and T_c^cluster. T_c^cluster is the Curie temperature of the magnetic clusters, which is also the Curie temperature of the sample. T_c^system is the measurement of the strength of interactions between clusters. Between T_c^cluster and T_c^system, the system exhibits superparamagnetism with strong cluster interactions. The strong cluster interactions result in the ferromagnetic state below the critical temperature (T_c^system), which is called a cluster ferromagnetism. Our experimental data (magnetization curves and susceptibility values of amorphous Y₆₀Fe₃₀Al₁₀ and Nd₆₀Fe₃₀Al₁₀ ribbons) support the cluster ferromagnetic model. The zero temperature coercivity and the relationship between T_block and T_c^system are also discussed in this article. [copyright] 2001 American Institute of Physics

[en] Ni_0.5Mn_0.5-xSb_x (x = 0.1, 0.2, 0.3, and 0.4) ingots fabricated by arc-melting high-pure metals have been studied to determine their structure and magnetic properties and to identify the magnetocaloric effect. X-ray diffraction analyses reveal that the sample with x = 0.2 is close to a single phase in a cubic structure while the others have the secondary phases. This influences strongly the magnetic properties of Ni_0.5Mn_0.5-xSb_x. With increasing Sb content, the Curie temperature (T_C) increases from ∼ 210 (for x = 0.1) to 435 K (for x = 0.4). Among the studied alloys, two samples, x = 0.2 and 0.3, have the greatest saturation magnetization values, which were recorded at 300 K. Under an applied field of 12.0 k Oe, the maximum magnetic entropy changes are about 1.0 and 0.5 J·kg · K^-1 for x = 0.2 and 0.3, respectively. Detailed analyses related to isothermal M-H curves in the vicinity of T_C by using the modified Aerate method reveal that these samples undergo a second-order phase transition with critical exponents of β = 0.40 ± 0.01 and γ = 1.27 ± 0.08 for x = 0.2 and of β = 0.69 ± 0.09 and γ = 0.85 ± 0.10 for x = 0.3. The differences in the critical parameters are likely related to the presence of Ni-related secondary phases.

[en] The structure and magnetic properties of high-energy-milled R₄₀Fe₃₀Co₁₅Al₁₀B₅ (R = Nd, Pr) have compounds been investigated by means of x-ray diffraction, high-resolution transmission electron microscopy, magnetic measurements, and Moessbauer spectroscopy. The x-ray diffractograms of the as-milled alloys are typical for the amorphous state. Nanocrystalline R₂(Fe,Co,Al)₁₄B coexisting with R₆(Fe,Co)_13-xAl_1+x is observed after recrystallization at 750 deg. C. The mechanically milled amorphous samples exhibit a relatively moderate coercivity of ∼6.5 kOe at room temperature and a Curie temperature (T_C) ∼ 650 K. After subsequent annealing, both systems show hard magnetic behaviours such as a record-high coercivity of 29 kOe with approximately the same T_C as for amorphous as-milled alloys