Microstructures and magnetic alignment of L10 FePt nanoparticles

Abstract

Chemically ordered FePt nanoparticles were obtained by high temperature annealing a mixture of FePt particles with NaCl. After the NaCl was removed with de-ionized water, the transformed FePt nanoparticles were redispersed in cyclohexanone. X-ray diffraction patterns clearly show the L1 0 phase. Scherrer analysis indicates that the average particle size is about 8 nm, which is close to the transmission electron microscopy ͑TEM͒ statistical results. The coercivity ranges from 16 kOe to more than 34 kOe from room temperature down to 10 K. High resolution TEM images reveal that most of the FePt particles were fully transformed into the L1 0 phase, except for a small fraction of particles which were partially chemically ordered. Nano-energy dispersive spectroscopy measurements on the individual particles show that the composition of the fully transformed particles is close to 50/ 50, while the composition of the partially transformed particles is far from equiatomic. TEM images and electron diffraction patterns indicate c-axis alignment for a monolayer of L1 0 FePt particles formed by drying a dilute dispersion on copper grids under a magnetic field. For thick samples dried under a magnetic field, the degree of easy axis alignment is not as high as predicted due to strong interactions between particles.

Prospects for nanoparticle-based permanent magnets

“…However, chemical methods require high-temperature annealing to obtain the desired crystalline ordering. Annealing of self-assembled fcc FePt nanoparticles in a very large magnetic field to produce simultaneously the L1 0 -ordering and easy-axis alignment was unsuccessful, presumably because the temperature required for a higher ordering is above their Curie temperature [44,45]. The surfactant-assisted chemical method performed at a moderate temperature of ~350 °C and subsequent alignment using a magnetic field of 20 kOe could obtain only partially ordered and aligned L1 0 -FePt nanoparticles [46].…”

Novel Nanostructured Rare‐Earth‐Free Magnetic Materials with High Energy Products

“…These values are comparable to those of SmCo5 and L10-ordered FePt and CoPt nanoparticles smaller than 10 nm. [13][14][15][16][17]24,25,27,28] Although nanoparticles are densely packed and stick to each other in the film, as shown in figure S1 in the Supporting Information, they nevertheless exhibit an appreciable room-temperature coercivity of about 4.5 kOe. This indicates that interparticle interactions and even physical contact do not destroy the coercivity.…”

“…In fact, this annealing step has been an important issue in controlling the size-distribution, self-assembly, easyaxis alignment, and nanostructure, not only in permanent magnetism but also in magnetic recording and other areas. [17][18][19][21][22][23][24][25] Easy-axis alignment of nanoparticles is particularly important to obtain high-energy products in permanent magnets because the energy product is quadratic in the magnetization. The annealing process and the associated sintering can be avoided by fabricating directly ordered nanoparticles using nonequilibrium cluster deposition, and this method can also be used to align the easy axes prior to deposition.…”

Assembly of uniaxially aligned rare-earth-free nanomagnets

“…For example, thermal annealing of soft FePt nanoparticles having disordered face center cubic structure in a very large magnetic field to produce hard L1 0 -ordering with aligned easy axes was unsuccessful, presumably due to the fact that the temperature required for higher ordering is above Curie temperature T c . 13,14 Since nanoparticle growth and crystallization of HfCo 7 nanoparticles occur directly during the gas-aggregation process in the present study, it is possible to align the easy axes by applying a magnetic field to HfCo 7 nanoparticles with a set of permanent magnets before deposition as shown in Fig. 5(a).…”

“…2,11,12 However, the easy-axis alignment process for the improvement of M r /M s using a magnetic field during the growth of nanoparticles is strongly hindered by the requirement of formation at high temperature (above 500 C) for obtaining the desired crystalline ordering. 13,14 In the present study, we have overcome the abovementioned problems by producing rare-earth-free permanentmagnet nanoparticles (HfCo 7 ) using a single-step gasaggregation-type cluster-deposition method without the requirement of a high-temperature thermal annealing and subsequently aligned their easy axes via applying a magnetic field, prior to deposition on substrates. Note that the HfCo 7 intermetallic phase can play a key role in the ongoing search for alternative permanent-magnet alloys due to its high T c of about 600 K, J s of above 10 kG, and non-cubic crystal structure.…”