Nanowire based exchange coupled permanent magnets
Date
2018
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
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.
Description
Keywords
Applied sciences, Electrospinning, Exchange coupling, Low temperature synthesis, MNBI, Permanent magnets