Cluster-Assembled Iron-Platinum Nanocomposite Permanent Magnets
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0962-P04-03
Cluster-Assembled Iron-Platinum Nanocomposite Permanent Magnets Xiangxin Rui1, Zhiguang Sun2, Yingfan Xu2, David J. Sellmyer2, and Jeffrey E. Shield1 1 Mechanical Engineering, University of Nebraska-Lincoln, Lincoln, NE, 68588 2 Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, NE, 68588
ABSTRACT Exchange-spring nanocomposite permanent magnets have received a great deal of attention for their potential for improved the energy products. Predicted results, however, has been elusive. Optimal properties rely on a uniformly fine nanostructure. Particularly, the soft magnetic phase must be below approximately 10 nm to ensure complete exchange coupling. Inert gas condensation (IGC) is an ideal processing route to produce sub-10 nm clusters method. Two distinct nanostructures have been produced. In the first, Fe clusters were embedded in an FePt matrix by alternate deposition from two sources. Fe cluster content ranged from 0 to 30 volume percent. Post-deposition multi-step heat treatments converted the FePt from the A1 to L10 structure. An energy product of approximately 21 MGOe was achieved. Properties deteriorated rapidly at cluster concentrations above 14 volume due to uncoupled soft magnetic regions (from cluster-cluster contacts) and cooperative reversal. The second nanostructure, designed to overcome those disadvantages, involved intra-cluster structuring. Here, Fe-rich Fe-Pt clusters separated by C or SiO2 were fabricated. Phase separation into Fe3Pt and FePt and ordering was induced during post-deposition multi-step heat treatments. By confining the soft and hard phases to individual clusters, full exchange coupling was accomplished and cooperative reversal between clusters was effectively eliminated. An energy product of more than 25 MGOe was achieved, and the volume fraction of the soft phase was increased to greater than 0.5 while maintaining a coercivity of 6.5 kOe. The results provide new insight into developing high energy product nanostructured permanent magnets. INTRODUCTION The exchange-spring phenomenon has received significant attention because of its potential to increase the energy densities of permanent magnets [1,2]. Combining soft and hard magnetic phases together at the nanoscale results in a very high remanent magnetization while maintaining sufficient coercivity, producing a high energy product. To ensure effective exchange coupling, the dimension of the soft magnetic phase should be on the order of twice the domain wall width of the hard magnetic phase, which is usually less than 10 nm. Practically, it is difficult to obtain an ideal nanostructure. For conventional processing methods like melt-spinning [3,4] or mechanical milling[5], the sizes of grains are larger than 15 nm. This results in incomplete exchange coupling, leading to early magnetic reversal and dramatic losses of coercivity. Materials processing utilizing gas aggregation offers the opportunity to produce a uniformly fine-scale structure below 10 nm [6]. Here, we combine gas aggregation with conventional
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