Elsevier

Advanced Powder Technology

Volume 26, Issue 6, November 2015, Pages 1618-1623
Advanced Powder Technology

Original Research Paper
Preparation and characterization of magnetic films of well-dispersed single domain of core–shell α″-Fe16N2/Al2O3 nanoparticles

https://doi.org/10.1016/j.apt.2015.09.005Get rights and content

Highlights

  • Ferromagnetic films were constructed from single domain nanoparticles.

  • Disperse core–shell α″-Fe16N2/Al2O3 nanoparticles were used.

  • External magnetic field was used to align magnetic nanoparticles orientation.

  • XRD analysis showed a vertical alignment of magnetic nanoparticles in the film.

  • Magnetic coercivity and remanence were increased by 22.7% and 55.2%, respectively.

Abstract

Ferromagnetic thin films of single domain core–shell α″-Fe16N2/Al2O3 nanoparticles (NPs) with average size of 47 nm were prepared by spin coating of the NPs which were well-dispersed to the primary NPs size in toluene by low energy bead-mill dispersion. The magnetic orientation (perpendicular to the Si substrate) was set by applying a magnetic field of 1.2 T during the film formation with a thickness of ∼1 μm followed by fixation of the NPs with resin. The SEM and XRD results showed that densely packed assemblies of the NPs were formed and aligned perpendicularly in the film. Based on the magnetic hysteresis curves of the NPs film, the application of the magnetic field during film formation increase the magnetic coercivity (Hc) and remanence (Mr) values of the resulted films by 22.7% and 55.2%. These results demonstrate the potential for nanostructuring bulk magnetic materials comprised of single domain core–shell α″-Fe16N2/Al2O3 NPs.

Introduction

Synthesis of ferromagnetic nanoparticles (NPs) with a single magnetic domain has attracted much attention for the formation of bulk magnet preparation [1], [2], [3], [4]. Recent reports have shown that these ferromagnetic NPs with single magnetic domain, by magnetically oriented assembly, will give higher magnetic performance than that of large MNPs with many magnetic domains [5], [6], [7], [8]. However, most of these ferromagnetic NPs with the highest magnetocrystalline anisotropy usually composed of rare-earth elements, such as Nd, Sm, and Dy [8], [9], [10], [11]. Therefore, synthesis of single domain ferromagnetic NPs without rare-earth component and alignment of their magnetic orientation are still become a challenge in the magnet development.

α″-Fe16N2 NPs have been reported as the ferromagnetic NPs with the highest magnetic moment and uniaxial magnetic anisotropy among ferromagnetic NPs [12]. Moreover, due to these excellent magnetic properties, these α″-Fe16N2 NPs have a great potential for the rare-earth-free magnetic material applications. However, compared to the rare-earth magnetic materials, such as Nd2Fe14B and SmCo5, these α″-Fe16N2 NPs still have relatively low magnetic coercivity value (Hc) and more studies are still necessary to enhance magnetic coercivity of the α″-Fe16N2 NPs. Therefore, synthesis of single domain α″-Fe16N2 NPs and alignment of their magnetic orientation may become a prospective method to increase Hc value of α″-Fe16N2 NPs.

However, since this α″-Fe16N2 NP is a hard magnet material, these particles will easy to agglomerate during the preparation process, for that it will be difficult to synthesize α″-Fe16N2 NPs with a single magnetic domain and in well-disperse condition. Thus, dispersion process after NPs synthesis will be necessary prior to further magnetic alignment process.

Recently, our group successfully synthesized spherical core–shell typed α″-Fe16N2 NPs via a gas phase method using α-Fe, and Al2O3 or SiO2 as the core and shell, respectively [13], [14], [15]. The synthesized core–shell magnetic NPs primarily constituted single phase α″-Fe16N2 and had a relatively high magnetic performance without any hard aggregation of the NPs observed. The core–shell α″-Fe16N2/Al2O3 NPs has been bead-milled to break up aggregates into the primary NPs with a single magnetic domain.

Based on this successfulness to prepare well-dispersed single magnetic domain α″-Fe16N2 NPs, as the continuation for the next step process, the magnetic alignment process are performed in a film form. Preparation of these α″-Fe16N2 NPs in the film form are selected and more preferable to demonstrate the magnetic alignment process, because in the film form, the particle will be easier to align in a single direction rather than in the bulk form. Therefore, the evaluation on this magnetic alignment will be easier. Moreover, the preparation of these α″-Fe16N2 NPs in the film form allow these particles to be applied in many applications, such as magnetic sensor, magnetic recording media, and especially for the construction of rare-earth-free bulk magnetic material with high magnetic performance. In addition, the formation of α″-Fe16N2 NPs in the film form has not been reported elsewhere.

In this study, the formation of α″-Fe16N2 magnetic films using α″-Fe16N2 NPs is reported. Core–shell α″-Fe16N2/Al2O3 NPs were used as a basic material. For a model process, spin-coating method was used due to its simplicity and high processing speed. An external magnetic field was applied during spin-coating to align the magnetic orientation of the assembled NPs. The evaluated magnetic performance of the prepared α″-Fe16N2 NPs films demonstrates the potential of magnetic NPs to be used as a base material for magnetically oriented film. This study also creates possibilities for the construction of bulk magnetic materials by using magnetic NPs.

Section snippets

Methods

The detailed routes on magnetic alignment of core–shell α″-Fe16N2/Al2O3 NPs in the film form from core–shell α-Fe/Al2O3 NPs are shown in Fig. 1. First, core–shell α-Fe/Al2O3 NPs was synthesized by one-step radio-frequency thermal plasma process. This synthesis was explained in our previous reports [16]. To synthesize core–shell α-Fe/Al2O3 NPs, precursor containing pentacarbonyl iron (purity 99.5%, BASF) and alumina (1 μm, purity 99.9%, Kojundo Chem. Lab. Co. Ltd.) were used. The produced

Physicochemical properties of core–shell α″-Fe16N2/Al2O3 NPs

The physicochemical properties of the core–shell α″-Fe16N2/Al2O3 NPs prepared by nitridation of core–shell α-Fe/Al2O3 NPs using a plasma method are shown in Fig. 2. As observed from the SEM image in Fig. 2(a) and bottom-left inset of Fig. 2(a), respectively, the as-prepared core–shell α″-Fe16N2/Al2O3 NPs are slightly agglomerated without significant sintering and featured average particle size and shell thickness of 47 nm and ∼4.5 nm, respectively. The obtained particle size is appropriate for

Conclusions

Nanostructured spherical core–shell α″-Fe16N2/Al2O3 NPs films were prepared by spin-coating of well-dispersed core–shell α″-Fe16N2/Al2O3 NPs in toluene. The spherical α″-Fe16N2 NPs were oriented under an applied magnetic field of 1.2 T during spin-coating. XRD results confirmed the successful alignment of the core–shell α″-Fe16N2/Al2O3 NPs in the film. Kerr effect evaluation showed that the spin-coated core–shell α″-Fe16N2/Al2O3 NPs had a higher Hc of 3.5 kOe than the as-prepared α″-Fe16N2/Al2O3

Acknowledgments

We gratefully acknowledge the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan for providing Scholarships (C.W.K. and R.Z.). The authors thanks to Professor Dr. Toshiro Takabatake and Associate Professor Dr. Takahiro Onimaru from Department of Quantum Matter, Graduate School of Advanced Sciences of Mater, Hiroshima University, for SQUID measurements and Associate Professor Dr, Naoki Ishimatsu from Department of Physics, Graduate School of Science, Hiroshima

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