This technique uses nanocluster assembly to produce model soft magnetic materials with simpler chemical composition than existing materials and well-controlled nanostructure, and to use these materials to improve understanding of the fundamental mechanisms responsible for the soft magnetic properties. An inert-gas condensation deposition chamber was developed, and transition-metal, rare-earth, and alloy nanoparticles with mean grain size D from 5 - 50 rim were deposited.
The figure shows a transmission electron micrograph of IGC-Fe nanoparticles. Deposition conditions were optimized to be able to produce 10-40 mg/h of nanoparticles with a size dispersion ΔD/D~0.1-1.0. The integrated system includes in-situ compaction and a transfer chamber to move samples to an inert-gas atmosphere without exposure to air. Ni, FeCo, Gd, GdN, Gd1-xFex, and Tb clusters were studied, as well as Gd:GdN compacts. Grain sizes varied-from less than 5 nm to over 50 nm by a combination of changing the deposition conditions and post-deposition annealing. This is a powerful method for studying grain-size dependence in soft magnets because it can produce large enough quantities of materials for extensive structural characterization and magnetic measurements, while maintaining precise control over the nanostructures.
Gd-based nanomaterials were studied extensively to examine the mechanisms responsible for exchange coupling changes at the ferromagnetic transition. The 293 K room-temperature Curie temperature of Gd enabled tracking of the magnetic behavior through the ferro-magnetic transition without changing the nanostructure by applying high temperatures. The coercivity exhibited unexpected temperature dependence, including non-zero magnetization and coercivity temperature (K) on the order of 100 Oe, well above the Tc of Gd.
Similar behavior was observed in GdN measured at 100 Oe (open squares) and the nanocompacts. GdN has a lower Tc and allows coercivity (closed circles) to measure at temperatures far above Tc, again temperature for an IGC-Gd compact without changing the nanostructure.
Coercivity was observed in the nitride sample at temperatures up to 400 K, which is significantly higher than the Tc of 60 K. The cause of this unexpected behavior was difficult to identify. X-ray diffraction, energy-dispersive x-ray spectroscopy, and electron diffraction did not indicate the presence of any secondary phases. Atomic Absorption Spectroscopy (AAS) showed that very small (0.5-2 wt. %) amounts of Fe were present in the samples. The culprit ultimately was identified as failure of the sputtering gun to confine the plasma to the target.
The decrease in the coercivity suggests that there are two phases. One is the Gd or Gd-N phase that orders at its expected Tc, although Tc may be depressed from the bulk value if the grain size is small. The second phase is ordered above Tc. When the Gd or GdN phase orders, anisotropy averaging decreases the coercivity. It remains unclear why very small amounts of iron have such a significant effect on the magnetic properties; however, the system enables investigation of the behavior of the exchange averaging as the ferromagnetic temperature is reached.
To understand the mechanism, Gd-Fe samples were fabricated with Fe concentrations ranging from 1 wt. % to 40 wt %. The intermediate part of this range has not been studied because of the limited solubility of Fe in Gd. Samples were made using IGC and melt spinning (another non-equilibrium fabrication technique) to overcome the limited solubility and understand how iron affects the magnetic properties. X-ray diffraction shows only peaks from hcp-Gd crystallites in Gd1-xFex for x up to 0.70. A combination of x-ray diffraction, electron microscopy, and electron diffraction suggested that there is a glassy GdFe phase in which hcp-Gd crystallites are embedded. The glassy Gd-Fe phase is magnetic at temperatures up to at least 400 K.
Determining the details of the nanostructure is critical to understanding the origin of the magnetic behavior. This is especially complex because the second phase is not evident in x-ray or electron diffraction. Studies show significant changes in the positions of the Gd and Fe atoms as the composition is changed. Coercivity data for different values of x were taken. Importantly, these data are taken at 310 K, above Tc of Gd. The shape of the loop also indicates the sensitivity of the magnetic properties to the presence of iron.
This work was done Diandra Leslie- Pelecky of the University of Nebraska for the Office of Naval Research.
This Brief includes a Technical Support Package (TSP).
Cluster-Assembled Soft Magnets for Power Electronics Applications
(reference ONR-0013) is currently available for download from the TSP library.
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