Magnets made of elongated single domain (ESD) particles were commercialized under the name Lodex in the 1960s. However, they fell out of favour as their performance was surpassed by the rare-earth magnets a decade later. Thomas Maurer of the Laboratoire Léon Brillouin in Saclay, near Paris, and colleagues of the Laboratoire ITODYS (Paris) and the Laboratoire de Physique et Chimie des Nano-Objets (Toulouse) have now brought ESD particle magnets back to life.
Simple fabrication process
The magnetic nanowires are made of 3d transition metals that have a potentially high Curie temperature (above which the material loses its magnetism) and high coercivity (the magnetic field needed to reverse the magnetization of a ferromagnetic material). Moreover, the magnets are made using a simple, low-temperature chemical process that could easily be scaled up for industry.
The CoNi nanowire magnets could fill the gap between rare-earth magnets, such as SmCo, and AlNiCo Thomas Maurer, Laboratoire Léon Brillouin
Maurer and co-workers made their magnetic wires by reducing cobalt and nickel salts in an organic solvent at 170 °C for 30 minutes. The wires are produced with a very high yield that could reach several grams per litre of solvent. They are around 240 nm long and 7 nm wide.
The researchers characterized their wires using SQUID and vibrating sample magnetometer techniques. They found a relatively high coercivity of 3.6 kOe for the as-prepared materials. However, since the most interesting property of these nanoobjects is their shape anisotropy – their length is much bigger than their width – Maurer's team aligned the particles in a solvent under an applied field of 10 kOe to see if their magnetic properties would improve. The coercivity indeed increased to 6.5 kOe at 140 K, and to 4.8 kOe at room temperature.
Competitive with SmCo and AlNiCo
If made into bulk magnets, the materials would have an "energy product" – the figure of merit for a magnetic material – of around 12 MGOe. This is not as good as that of neodymium iron boride (NdFeB) permanent magnets, which have energy products of over 40 MGOe. However, the CoNi nanowire magnets could fill the gap between rare-earth magnets, such as samarium-cobalt (SmCo) and aluminium nickel cobalt (AlNiCo), which have energy products of around 25 and 8 MGOe, respectively.
But, although rare-earth magnets do have higher coercivities, they soften when heated, while the new CoNi wires could resist temperatures far above room temperature. AlNiCos, on the other hand, have lower coercivities at these temperatures.
According to the French team, the wires have such good properties because they are highly crystalline, something that is needed for high coercivity and resistance to high temperatures.
Applications
"Potential applications range from high operating temperature permanent magnetic technology to magnet recording devices since their properties are already twice as good as magnetic tape recording materials," Maurer told nanotechweb.org.
The scientists now plan to synthesize new types of nanowire with varied shapes and compositions to further increase the coercivity. "Another objective is to elaborate much denser materials to obtain more efficient permanent magnets for industrial applications," said Maurer.
His team also recently discovered some other interesting phenomena, like an unusual temperature dependence of the nanowire coercivity coexisting with a strong exchange bias at low temperatures. "Our aim is to also use neutron diffraction techniques to investigate the static and dynamic magnetic properties of individual nanowires," he added.
The work was reported in Appl. Phys. Lett..