Physicists in the US have developed a warm compaction technique to make small, powerful bulk permanent magnets out of iron-platinum nanoparticles . The "exchange-spring" magnets consist of two magnetic phases – both "soft" and "hard" – and have exchange coupling between them, which makes the magnets stronger than ordinary magnets containing just a single phase. If perfected, the magnets could find use in a wide range of applications, from cell phones and data storage devices to hybrid cars, and could "revolutionize" our daily lives, say the researchers.
"Nanotechnology is not only good for creating novel materials like graphene but also for improving and re-birthing traditional materials like permanent magnets," team leader J Ping Liu of the University of Texas at Arlington told nanotechweb.org. Indeed, theory predicts that the energy product, or (BH)max, (the figure of merit for a magnet's strength) for exchange-coupled nanocomposite magnets could reach 100 MGOe. This is double the current highest value for single-phase magnets.
A high (BH)max requires the material to have a large magnetization and a large coercivity – the magnetic field needed to reduce the magnetization of a ferromagnetic material to zero. Exchange-spring magnets contain a magnetically hard phase, which has a high coercivity, and a soft phase with low coercivity and high magnetization. These two phases interact by exchange coupling. For the exchange coupling to be effective, however, the grain size of the hard and soft phases must be homogenously controlled at the nanoscale, which can be difficult using conventional top-down fabrication methods.
The group working on the bulk nanocomposite magnet project
Front from left: Chaubey, Rong, Nandwana and Yano; back from left: Poudyal, Liu and Wang. Credit: J Ping Liu
Researchers are therefore looking for alternative bottom-up approaches to overcome this problem. Recently, they have become interested in iron-platinum (FePt) nanoparticles made by chemical methods, because of the very small particle size (a few nanometres) and very narrow size distribution in these materials.
However, the biggest challenge here is turning these particles into bulk magnets. Conventional compaction techniques do not work for nanoparticles because these methods require extensive heat treatment at high temperatures, which causes excessive grain growth. Moreover, the smaller the nanoparticles, the more difficult it is to compact them.
Now, Liu and colleagues have developed a high-pressure, warm compaction method, inspired by a special technique routinely used for making car parts. Unlike hot pressing, warm compaction takes place at modest temperatures, where metallic powders are chemically stable and no excessive grain growth occurs.
The researchers mix together FePt and iron oxide nanoparticles in a ratio of 8:1 and then compact them under pressures of up to 3.8 GPa for 10 minutes at various temperatures. They then characterise the compacted samples using electron microscopy and X-ray diffraction. Magnetic measurements are made with a superconducting quantum interference device (SQUID) magnetometer with a maximum applied field of 70 KOe.
The team says that the best magnets are produced at temperatures of about 600 °C. A (BH)max of up to 16.3 MGOe can be reached, which is 25% higher than the value for conventional single-phase isotropic FePt magnets. Moreover, the samples have a density that is 95% of the theoretical density allowed for these materials, which makes them the densest bulk FePt magnets ever made with a nanoscale grain size.
"We also observed very interesting phenomena, like the pressure expedited phase transition from disordered face-centred-cubic to the so-called L10 structure in the material, at temperatures 100 °C lower than usual," explained Liu. This phase transition also makes consolidating the magnets easier.
The team is now working hard on solving a key issue in fabricating these nanocomposites – orienting the hard magnetic component, which would further increase the (BH)max of the material.
The researchers reported their work in J. Appl. Phys.
About the author
Belle Dumé is contributing editor at nanotechweb.org
Physicists in the US have developed a simple way to make nano-sized particles with potentially useful magnetic properties. J Ping Liu and colleagues at the University of Texas at Arlington added ordinary table salt to particles of iron-platinum particles and then heated them to produce nanoparticles that could be used as building blocks for magnetic recording media and in biomedical applications.
In 2000, Shouheng Sun and colleagues at IBM created iron-platinum particles that were just 4 nm across. This was a breakthrough because the face-centred tetragonal (fct) form of iron-platinum has excellent magnetic properties, such as a high coercivity -- the magnetic field needed to reduce the magnetization of the material to zero. However, the particles produced by the IBM team had a face-centred cubic (fcc) structure, which is not so useful for applications. Although it is possible to convert the fcc phase into the fct phase by heating it, this also causes the particles to sinter and form larger particles, which renders the material useless for applications.
Team Members (from left to right): Liu, Jin, Elkins, Poudyal, Nandwana, and Li (photo courtesy: J Ping Liu)
Liu and co-workers have now overcome this problem by adding a small amount of finely ground salt (sodium chloride) to the iron-platinum particles before they are heated (J. Phys. D38 2306). The salt keeps the iron-platinum nanoparticles apart so that they can undergo the phase transformation to the fct structure without sticking together and sintering. The team have produced particles with diameters of between 4 and 15 nm that have coercivities in excess of 3 Tesla. "For such small particles this is quite a remarkable result," says Kevin O'Grady of York University.
"Sodium chloride is an ideal medium since it is chemically stable up to the annealing temperatures and can be easily removed completely just by washing the particles in water," Liu told PhysicsWeb. "Moreover, production of the fct particles with this method can be easily scaled up and is very economic -- we tested with salt from a supermarket and it worked well."
News Release — 04 January 2005
FACULTY, STAFF IN THE NEWS
Department of Physics faculty members Narayan Poudyal, Baki Altuncevahir, Vamsi Chakka, Kanghua Chen, Truman D. Black, and J Ping Liu have recently co-authored and published a paper in Institute of Physics Publishing's Journal of Physics D: Applied Physics. The journal has an international readership among academic, government and corporate sectors and contains research across many areas of applied physics including theoretical, computational and experimental studies.