S.A. Romero

The (SmZr)Co

Detailed microstructural characterization of magnets and homogenized as-cast alloys, which included X-ray diffraction Rietveld analysis, has indicated that the so-called platelet or lamellae phase is (SmZr)₁(CoFeCu)₃ with the PuNi₃ structure and lattice parameters a~0.5 nm and c~2.4 nm. The structural and magnetic properties of the (SmZr)Co₃ phase were investigated. The microstructure shows two phases differing in their Zr/Sm ratio. Magnetization curves for the samples (Sm_0.33Zr _0.67)Co₃, (Sm_0.33Zr_0.67)Co _2.97Fe_0.03, and (Sm_0.67Zr_0.33)Co₃ are consistent with the two-phase microstructure observed. Room temperature coercivity values of these samples are low (ap1 kOe.)

_3

The Callen-Liu-Cullen (CLC) modification of the Stoner-Wohlfarth model was found able to describe properly the hysteresis curves of isotropic Sm(CoFeCuZr)z magnets. The SW-CLC model uses three parameters, and all of them have physical meaning. One of the parameters is related to the saturation magnetization, another to the anisotropy field, and another is 1/d, which evaluates the interaction between grains or particles. The model was applied for several magnets, indicating an anisotropy field of 6-7 T, which is compatible with other methods for anisotropy field determination. The model also gives insight into the abnormal temperature dependence of the coercivity found in SmCo 2:17 magnets. For compositions with a low z, the parameter 1/d is significant. These compositions with a low z are those showing the most abnormal coercivity behavior with temperature.

Phase in Sm(CoFeCuZr)

SmCo5 samples were investigated under the following conditions: ‘as-sintered’ at 1150 ◦ C (low coercivity—0.1–0.5 T), heat-treated at 850–880 ◦ C (coercivity ∼2.5–3.0 T) and after heat treatment at 750 ◦ C during 25 days (low coercivity—0.1–0.5 T). Transmission electron microscopy investigation showed that the increase of coercivity at 850–880 ◦ C is not due to second phases precipitated at grain boundaries in the samples nor to the elimination of stacking faults or dislocations. After the heat treatment during 25 days at 750 ◦ C, no clear evidence of eutectoid decomposition of SmCo5 phase was found, but a slight change of lattice parameters was detected.

_rm z

Magnets with composition Sm(CobalFe0.2Cu0.1Zrx)8 (x=0;0.02;0.04;0.06,0.08) were studied using x-ray diffraction Rietveld analysis. In addition to the majority 2:17 and 1:5 phases, the magnets present a rhombohedral (SmZr)1(CoFeCu)3 (PuNi3 structure) phase, whose volume fraction increases with Zr addition. Higher Zr contents decrease coercivity and lead to the appearance of impurity phases such as 6:23 (Th6Mn23 structure), 2:7R (Gd2Co7 structure), and 5:19R (Ce5Co19 structure) in the magnets. The 1:3R phase was reproduced in a Fe57-doped sample of Sm0.33Zr0.67Co3 and its Mossbauer spectra, at several temperatures, were adjusted using four sextets and a nonmagnetic doublet. The beneficial effect of small additions of Zr on the coercivity of the magnets is discussed.

Magnets

Sm(CoFeCuZr)z commercial magnets are manufactured by powder metallurgy techniques. Microstructural investigations of Sm(CoFeCuZr)z magnets have shown that, increasing the Zr content, some impurity phases may appear. An alloy with composition (at%): 60.5% Co – 15.5% Fe – 11.5% Zr - 8.5% Sm - 4% Cu, homogenized at 1050oC, was investigated. Three main phases were identified: rhombohedral 1:3 (ZrSm)1(CoFeCu)3, hexagonal 1:7 (SmZr)1(CoFeCu)7 and cubic 6:23 (Zr)6(CoFe)23. Knowledge of possible phases present in 2:17-type magnets allows a better understanding of the nanocrystalline microstructure responsible for high coercivity of these magnets.

Estimate of the anisotropy field in isotropic SmCo 2:17 magnets with the Stoner-Wohlfarth CLC model

The achievement of high coercivity in 2:17 magnets depends upon the presence of Zr, although there is some disagreement as to its role. The important role of Zr in stabilizing the high temperature 1:7 phase has been emphasized. It has been proposed that the function of the Zr might be to improve the imperfect shape of the cell walls and thereby increase coercivity. A Rietveld refinement of an X-ray diffraction spectrum was performed using Topas software. This analysis revealed that the sample consisted mainly (~70%) of a rhombhohedral 1:3 phase with lattice parameters a = 0.5 nm and c = 2.4 nm.