Hybrid High-Temperature Nanostructured Magnets
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Hybrid High-Temperature Nanostructured Magnets David J. Sellmyer, J. Zhou, H. Tang, and R. Skomski Behlen Laboratory of Physics and Center for Materials Research and Analysis University of Nebraska Lincoln, NE 68588-0113, USA ABSTRACT The hysteretic behavior of two-phase permanent magnets for high-temperature applications is examined. A variety of systems have been synthesized and investigated, including Sm-Co-Cu-Ti bulk magnets, SmCo5:Cu-Ti thin-film materials, and mechanically milled Sm-CoZr magnets. The hybrid character of the material leads to very high room-temperature coercivities, between 30.2 and 43.6 kOe, and to the survival of a comparatively large part of the coercivity at high temperatures (12.3 kOe at 500 °C for SmCo6.5Cu0.8Ti0.3). The coercivity reflects the structure and chemical composition of the material. When ferromagnetic grains are separated by a ferromagnetic boundary phase, the boundary phase acts as a pinning center, but when the grain-boundary phase has a comparatively low Curie temperature, the high-temperature magnetism of the system is that of a weakly interacting ensemble of magnetic particles. In spite of some residual paramagnetic exchange coupling, which is discussed in this work, this mechanism enhances the coercivity. INTRODUCTION Recently there has been a resurgence of interest in Sm-Co-based permanent-magnet alloys. The reasons for this are twofold. First, Sm2Co17 and phases based on this composition have the highest known Curie temperature (Tc) and thus have the best chance of creating a practical magnet for use at high temperatures such as 500 EC. Second, SmCo5 has the highest-known uniaxial anisotropy constant which leads generally to maximum values of coercivity. SmCo5 also is a relatively high-temperature magnet. In the quest for new nanocomposite magnets based on the exchange coupling of hard and soft phases with nanoscale dimensions, the use of SmCo5 as the hard phase has the major benefits of being both extremely hard and a high-temperature magnet. The clear challenge for workers in the design of high-temperature and high-energy product nanoscale magnets is to synthesize hybrid systems that exploit the properties of at least two phases to create magnets with much improved figures of merit including operating temperature and coercivity. In this paper we present results of recent work in our laboratories based on controlling the nanostructure and phase mixtures of hybrid or composite magnets based on SmCo5 (1:5), Sm2Co17 (2:17), SmCo7 (1:7) and related compounds. First, we discuss a series of Sm-Co-Cu-Ti alloys containing 2:17 and 1:5 phases. This system has a cellular structure and has produced the highest known coercivity (Hc) at 500 EC (12.3 kOe). Second, we outline results on SmCo5:CuTi nanocomposite magnets prepared in thin-film form with sputtering methods. These magnets have led to Hc values above 40 kOe at 295 K. Third, the technique of mechanical milling has been used to prepare two-phase magnets containing SmCo5. These systems also have large Hc
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