Nanowire based exchange coupled permanent magnets

Xiao, John Q.Permanent magnets (PMs) are found in many applications such as electrically controlled brake systems, robotics, wind power generators, magnetic bearings, magnetic relays, etc. Performance of these devises strongly depends upon the properties of PMs, whose figure of merit is defined as magnetic energy product (BH)max. The magnetic energy product represents the ability to do work with a unit volume of the PM. The efficiency of PM based devices can be increased by increasing the (BH)max of PMs. ☐ Two possible approaches exist to increase the (BH)max significantly: (1) finding new materials with higher (BH)max and (2) developing an exchange couple PM by coupling a soft phase of high magnetization with a hard phase of high coercivity. Although the idea of exchange coupled PM was proposed 26 years ago, it has not been commercialized yet due to the challenges in the fabrication process. This dissertation focused on addressing some of the challenges. This includes (1) fabrication of soft magnetic material suitable for exchange coupled PMs and (2) low temperature synthesis to reduce the interdiffusion between soft and hard phase of exchange coupled PM. ☐ Soft magnetic material must possess certain features to be suitable for exchange coupled PMs. One such feature is to have high nucleation field (HN). The nucleation field depends upon the size and the volume fraction of the soft phase in the soft/hard composite. Studies have suggested that soft magnetic nanowires inside hard magnetic matrix is an optimum geometry to achieve the highest (BH)max. ☐ We used electrospinning method to fabricate soft magnetic Fe65Co35 nanowires with ultra-small diameter and high magnetization. The diameter and diameter distribution were controlled by the viscosity and surface tension of electrospinning solutions. Role of surface tension in determining the electrospun nanowire diameter is debatable. We found that surface tension plays a critical role to control diameter distribution for ultra-small (20- 50 nm) magnetic nanowires. ☐ Hard magnetic materials such as NdFeB, L10 FePt, and SmCo5 require high annealing temperatures to develop the hard-magnetic phase. Consequently, one typically needs to treat soft/hard nanocomposites at elevated temperatures to develop hard magnetic phase. This high temperature treatment often induces interdiffusion between hard and soft phases, which significantly deteriorate the PM performance. Interdiffusion can be reduced by decreasing the processing temperature. A recently reported FePt crystals can be transformed into hard magnetic L10 FePt at 400 °C, which is much lower than the traditionally required 600 °C. We used this precursor and electrospun Fe65Co35 nanowires to develop exchange coupled PM at 400 °C. It is found that, low synthesis temperature indeed reduces the interdiffusion that leads to good magnetic properties of exchange coupled PMs. ☐ Another approach to restrict the interdiffusion is to find chemical processes to couple soft and hard magnetic materials. In collaboration with Prof. Ren at Temple University, we have developed a room temperature solution process method to exchange couple Fe65Co35 nanowires with MnBi. This process uses organic ligand to bridge Fe65Co35 and MnBi. The magnetic properties of the composite show that the solution process method is promising and can also be used to fabricate exchange coupled PMs.University of Delaware, Department of Physics and AstronomyPh.D