Effect of Iron Substitution on the High-temperature Properties of Sm(Co, Cu, Ti) z Permanent Magnets
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Effect of Iron Substitution on the High-temperature Properties of Sm(Co,Cu,Ti)z Permanent Magnets Jian Zhou, Ralph Skomski, and David J. Sellmyer Department of Physics and Astronomy and Center for Materials Research and Analysis University of Nebraska, Lincoln, NE 68588 Wei Tang and George C. Hadjipanayis Department of Physics and Astronomy, University of Delaware, Newark, DE 19716 ABSTRACT Recently, Ti-substituted Sm-Co permanent magnets have attracted renewed attention due to their interesting high-temperature coercivity. Our presentation deals with the effect of iron substitutions on the magnetic properties of the materials. X-ray diffraction shows that the investigated Sm(Co,Fe,Cu,Ti)z materials (z = 7.0 - 7.6) are two-phase magnets, consisting of 1:5 and 2:17 regions. The iron content affects both the coercivity and the magnetization. Depending on composition and heat treatment, some samples show a positive temperature coefficient of the coercivity in the temperature range from 22 ºC to 550 ºC. Moderate amounts of iron enhance the room-temperature coercivity. For example, the room-temperature coercivity of Sm(Co6.0Fe0.4Cu0.6Ti0.3) is 9.6 kOe, as compared to 7.6 kOe for Sm(Co6.4Cu0.6Ti0.3). At high temperatures, the addition of Fe has a deteriorating effect on the coercivity, which is as high as 10.0 kOe at 500 ºC for Sm(Co6.4Cu0.6Ti0.3). The room-temperature magnetization increases on iron substitution, from 73 emu/g for Sm(Co6.4Cu0.6Ti0.3) to 78 emu/g for Sm(Co6.0Fe0.4Cu0.6Ti0.3). The observed temperature dependence is ascribed to the preferential dumbbell-site occupancy of the Fe atoms. INTRODUCTION The demand for permanent magnets with high-temperature applications above 450 ºC has attracted much attention in recent years. Usually, the best room- temperature permanent magnets are made by Nd-Fe-B since it has a relatively high saturation magnetization and moderate intrinsic coercivity. These two properties lead to a high energy product (BH)max, which is a key figure of merit for a hard magnet. However Nd-Fe-B has large temperature coefficients of Hc and Br. Therefore, (BH)max drops below 10 MGOe when T > 200 ºC, and even more for higher temperatures, which makes it unsuitable for high-temperature applications. Another type of commercially available permanent magnets is 2:17-type Sm-Co magnets. The complicated heattreatment of Sm(Co, Fe, Cu, Zr)z leads to a cellular microstructure that is Fe-containing SmCo2:17 phases surrounded by Cu-rich SmCo5 grain boundaries. Zr plays a key role in the formation of this type of structure by helping the precipitation of the Cu-rich phase. Although the maximum energy product of Sm2Co17 is not as high as that of the Nd-Fe-B at room temperature, the low temperature coefficients of Hc and Br make the energy product larger at elevated temperature, which makes the 2:17 type Sm-Co based magnets a good choice for hightemperature applications [1, 2]. Our previous studies have been focused on SmCo-based alloys with Ti-substitution and their possible use as high temperature permanent
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