Magnetic Properties of Nanocrystalline Fe 50 Co 50 Powders
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Magnetic Properties of Nanocrystalline Fe50Co50 Powders Shashishekar Basavaraju and Ian Baker Thayer School of Engineering, Dartmouth College, Hanover, NH 03748, U.S.A.
ABSTRACT Nanocrystalline stoichiometric FeCo powders were prepared by mechanically alloying elemental Fe and Co powders using a high-energy ball mill. The microstructural evolution was studied as a function of milling time and subsequent annealing using X-ray diffractometry and differential scanning calorimetry. The magnetic behavior of the specimens was characterized using a vibrating sample magnetometer and a magnetic force microscope. A reduction in grain size coupled with an increase in coercivity was observed as function of milling time. The smallest grain size of 4 nm, which exhibited a coercivity of 122 Oe and magnetization of 2 T at room temperature, was obtained after 240 h of milling. The reduction in grain size during milling was not accompanied by enhanced soft magnetic properties. INTRODUCTION Modern electronic devices in power supplies, digital communications equipment and automobiles demand magnetic cores or inductive components with compact volumes and excellent magnetic properties [1-3]. To realize such performance, core materials must exhibit a combination of soft magnetic properties such as low coercivity (Hc), large saturation magnetization (Ms), large resistivity and good thermal stability [1-5]. Nanocrystalline FeCo alloys with superior soft magnetic properties are excellent candidates for such applications [1-3]. The magnetic properties of FeCo nanocrystalline materials are affected in a complicated manner by microstructural features such as the grain size and internal strain [1, 4-8]. While the structure of nanocrystalline materials prepared by mechanical alloying is well researched, the magnetic properties and their relation with the microstructure are still under investigation. In this work, nanocrystalline FeCo powders were fabricated by high-energy ball milling and their microstructural characteristics and magnetic behaviors were investigated. EXPERIMENTAL DETAILS Equiatomic amounts of 50 µm, 99.08 % Fe and 50 µm, 99.5 % Co powders were mechanically milled in a Szegvari 01HD attritor at 700 rpm using hardened steel balls for 5, 20, 30, 60, 120 and 240 hrs, under an argon atmosphere. A ball to powder ratio of 11:1 was used. Milled samples were subsequently annealed for 1 h and 10 h in an argon atmosphere at 400, 600, 800 and 900 ºC. Both the milled and annealed FeCo powders were studied using Cu kα radiation on a Siemens D5000 diffractometer (XRD) and a Perkin - Elmer DSC7 differential scanning calorimeter (DSC). The magnetic behaviors of the powders were investigated using a Veeco Dimension 3100 magnetic force microscope (MFM) and a Lakeshore 7304 vibrating sample
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magnetometer (VSM). The magnetization and coercivity were measured in an applied field of 10 kOe. Possible phase changes were investigated using the DSC at a constant heating rate of 20 °C/min under argon. RESULTS AND DISCUSSION The initial e
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