Introduction Currently, there is a rising demand for rare-earth elements for a broad range of industrial applications. However, the risk of supply of these strategic materials due to political restrictions, the high environmental impact of their extraction as well as price oscillations have motivated many researchers in finding viable alternatives. Specifically, in the case of permanent magnet applications the research is focusing on finding novel materials that can replace currently used RE permanent magnet in a wide part of the spectrum of industrial necessities where the high performance of rare-earth permanent magnets are not strictly required. We propose nanometric cobalt ferrite (CoxFe3-xO4) as candidate to achieve this goal. On one hand cobalt ferrite, thanks to its large cubic magnetic anisotropy, is a hard magnet. On the other hand, the reduction to the nanoscale may offer unique advantage, which can significantly improve the characteristic of the material. Discussion A family of monodisperse cobalt ferrite nanoparticles (NPs) with average size covering a broad range (from 4 to 60 nm) was synthesized by thermal decomposition of metal-organic precursors. Metal precursors, surfactants and synthetic parameters were settled in order to fine tuning the particle size while preserving a narrow particle size distribution. The morphological characterization of the nanoparticles revealed an evolution of the particle shape from spheres to octahedrons through cubes for intermediated sizes. Subsequently, the evolution of magnetic properties was studied as a function of particles size and shape, particularly focusing on those determining the best performance as permanent magnet. While saturation and remnant magnetization maintain constant values in the whole investigated particle size range, the coercive field exhibits a non-monotonic behaviour with two distinct maxima values for low and room temperature, respectively (Figure 1). In addition, we evaluated the (BH)max product, the figure of merit of permanent magnets, obtaining the maximum value ever reported in the literature for cobalt ferrite NPs (i.e., 2.1 MGOe (18 MJm-3) for 40 nm NPs). The collected data gave us the opportunity to discuss the feasibility of the application of this material for the realization of permanent magnet. Figure 1: (a) B and 4?M vs. H loops for 40 nm NPs at RT and (b) (BH)max at 5K (black solid circles) and RT (red empty circles) as a function of the particle size. Conclusions and/or Outlook This investigation allowed us to establish, at least on the basis of (BH)max, the potentiality of cobalt ferrite NPs for the realization of RE free permanent magnet. Indeed, if the possibility of orienting the magnetic anisotropy axes of the nanograins is taken into account, (BH)max as large as 8 MGOe (60 kJ/m3) can be in principle obtained. This value makes cobalt ferrite NPs a viable alternative to replace RE-based permanent magnet at least in the intermediate region of the energy product map where the latter are currently employed simply because standard ferrites do not have large enough (BH)max. Acknowledgement: Research supported by EU-FP7 NANOPYME Project (No. 310516) [www.nanopyme-project.eu]
Exploring the magnetic properties of ferrite nanoparticles for the development of rare-earth-free permanent magnet
2015
Abstract
Introduction Currently, there is a rising demand for rare-earth elements for a broad range of industrial applications. However, the risk of supply of these strategic materials due to political restrictions, the high environmental impact of their extraction as well as price oscillations have motivated many researchers in finding viable alternatives. Specifically, in the case of permanent magnet applications the research is focusing on finding novel materials that can replace currently used RE permanent magnet in a wide part of the spectrum of industrial necessities where the high performance of rare-earth permanent magnets are not strictly required. We propose nanometric cobalt ferrite (CoxFe3-xO4) as candidate to achieve this goal. On one hand cobalt ferrite, thanks to its large cubic magnetic anisotropy, is a hard magnet. On the other hand, the reduction to the nanoscale may offer unique advantage, which can significantly improve the characteristic of the material. Discussion A family of monodisperse cobalt ferrite nanoparticles (NPs) with average size covering a broad range (from 4 to 60 nm) was synthesized by thermal decomposition of metal-organic precursors. Metal precursors, surfactants and synthetic parameters were settled in order to fine tuning the particle size while preserving a narrow particle size distribution. The morphological characterization of the nanoparticles revealed an evolution of the particle shape from spheres to octahedrons through cubes for intermediated sizes. Subsequently, the evolution of magnetic properties was studied as a function of particles size and shape, particularly focusing on those determining the best performance as permanent magnet. While saturation and remnant magnetization maintain constant values in the whole investigated particle size range, the coercive field exhibits a non-monotonic behaviour with two distinct maxima values for low and room temperature, respectively (Figure 1). In addition, we evaluated the (BH)max product, the figure of merit of permanent magnets, obtaining the maximum value ever reported in the literature for cobalt ferrite NPs (i.e., 2.1 MGOe (18 MJm-3) for 40 nm NPs). The collected data gave us the opportunity to discuss the feasibility of the application of this material for the realization of permanent magnet. Figure 1: (a) B and 4?M vs. H loops for 40 nm NPs at RT and (b) (BH)max at 5K (black solid circles) and RT (red empty circles) as a function of the particle size. Conclusions and/or Outlook This investigation allowed us to establish, at least on the basis of (BH)max, the potentiality of cobalt ferrite NPs for the realization of RE free permanent magnet. Indeed, if the possibility of orienting the magnetic anisotropy axes of the nanograins is taken into account, (BH)max as large as 8 MGOe (60 kJ/m3) can be in principle obtained. This value makes cobalt ferrite NPs a viable alternative to replace RE-based permanent magnet at least in the intermediate region of the energy product map where the latter are currently employed simply because standard ferrites do not have large enough (BH)max. Acknowledgement: Research supported by EU-FP7 NANOPYME Project (No. 310516) [www.nanopyme-project.eu]I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
