Enhanced magnetic performance of aligned wires assembled from nanoparticles: from nanoscale to macroscale

Magnetic wires in highly dense arrays, possessing unique magnetic properties, are eagerly anticipated for inexpensive and scalable fabrication technologies. This study reports a facile method to fabricate arrays of magnetic wires directly assembled from well-dispersed α″-Fe16N2/Al2O3 and Fe3O4 nanoparticles with average diameters of 45 nm and 65 nm, respectively. The magnetic arrays with a height scale of the order of 10 mm were formed on substrate surfaces, which were perpendicular to an applied magnetic field of 15 T. The applied magnetic field aligned the easy axis of the magnetic nanoparticles (MNPs) and resulted in a significant enhancement of the magnetic performance. Hysteresis curves reveal that values of magnetic coercivity and remanent magnetization in the preferred magnetization direction are both higher than that of the nanoparticles, while these values in the perpendicular direction are both lower. Enhancement in the magnetic property for arrays made from spindle-shape α″-Fe16N2/Al2O3 nanoparticles is higher than that made from cube-like α″-Fe16N2/Al2O3 ones, owing to the shape anisotropy of MNPs. Furthermore, the assembled highly magnetic α″-Fe16N2/Al2O3 arrays produced a detectable magnetic field with an intensity of approximately 0.2 T. Although high-intensity external field benefits for the fabrication of magnetic arrays, the newly developed technique provides an environmentally friendly and feasible approach to fabricate magnetic wires in highly dense arrays in open environment condition.


Introduction
Magnetic wire-like structures with high aspect ratios, a link between nanoscale objects and the macroscale world, play important roles in both fundamental research and the development of modern materials [1,2]. The high aspect ratio endows the material with anisotropic properties and ensures that magnetization prefers to align with the long axis of the wires [3][4][5]. A longitudinal magnetic anisotropy has been widely reported to be related to strong shape anisotropy because the coercive fields in the wire direction are lower than those perpendicular to it [4,[6][7][8]. Anisotropic magnetic wires also have higher magnetic moments than their spherical counterparts and therefore open up new possibilities for applications [5,9]. As a consequence, magnetic wire-like structures are of great interest in the development of new-generation spintronic devices [3], sensors [10], data storage technologies [11][12][13], biological and harsh environment applications [7,14], as well as many other potential applications [1,5,15]. To exploit their collective properties and the various applications in functional devices, many methods have been developed to produce wire-like structures [1,16].
The approaches used to obtain high aspect ratio magnetic wires can be generally classified into two main categories, i.e. direct synthesis and assembly methods [5]. The direct synthesis method includes solid-state, α 00 -Fe 16 N 2 /Al 2 O 3 MNPs with a shell thickness of 4.8 nm were prepared from cube-like Fe 3 O 4 MNPs by the surface coating of Ale 2 O 3 , reduction with H 2 gas and then nitridation with NH 3 gas, as detailed elsewhere [46]. The Fe 3 O 4 MNPs with cube-like structures were prepared via a large-scale liquid precipitation method (US Patent no. 5843610, Toda Kogyo Co. Ltd, Japan) as described in our previous report [47]. An epoxy resin (3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate) was used as an MNP binder.
The colloidal dispersion of MNPs was prepared by using the bead-mill dispersion apparatus (dual axial type bead-mill, Kotobuki Industries, Co. Ltd, Japan), as described in our previous papers [48,49]. The precursor solutions for the wire fabrication were made from the mixture of epoxy resin (1.0 wt%) and MNPs (1.5 wt%) suspended in toluene with NH 4 OH added to adjust the pH to approximately 9. Scanning electron microscope (SEM) images for core-shell α 00 -Fe 16 N 2 /Al 2 O 3 and Fe 3 O 4 and MNPs before and after dispersion are shown in figure 1. Although the dispersion process was well controlled, sizes of α 00 -Fe 16 N 2 /Al 2 O 3 MNPs were still decreased after dispersion owing to their strong aggregation effect. Average diameters after dispersion are 45.3 and 64.7 nm for α 00 -Fe 16 N 2 /Al 2 O 3 and Fe 3 O 4 MNPs, respectively. To determine the implications of the results obtained in this study, spindleshaped core-shell α 00 -Fe 16 N 2 /Al 2 O 3 MNPs 110 nm in length and 18 nm in width (detailed characteristics were reported in our previous paper [46]) were also used to fabricate wires to reveal the effect of shape anisotropy.

Fabrication technique
The precursor solutions with the MNP suspension were shaken in an ultrasonic bath (Sine Sonic UA-100, 36 kHz, 100 W) for 30 min before fabrication. The solutions with a volume of 30 ml were then transferred into quartz beakers with inner diameters of 35 mm and inner heights of 15 mm. The beaker was fixed to another quartz beaker, with an inner diameter of 45 mm, a sieve mesh bottom and a hood. The hood was connected to a flexible tube for pumping nitrogen gas with a constant flow rate of 1 l min −1 , which was controlled by a mass flow meter. The input of nitrogen gas assisted the evaporation of the solvent in the precursor solutions and flowed out from the bottom of the outside beaker. The packed beaker system, including the solutions, was placed at the centre position of a superconducting magnet (Oxford Instruments, Spectromag-1000) through a bore tube (50 mm). A holder was placed at the bottom of the outside breaker to fix the packed system, as shown in figure 2a. The maximum strength of the EMF was 15.0 T in the vertical direction at the centre position. A detailed description of the superconducting magnet system has been reported elsewhere with the magnetic field spatial distribution along the centre axis shown in figure 2b [50]. The solutions were maintained in the strong

Characterization
The morphologies of the MNPs and the fabricated arrays were observed using an SEM (Hitachi S-5000, Japan). Their crystalline structures were examined using X-ray diffraction (XRD; D2 Phaser, Bruker, Germany), while assigned Miller indices of the peaks were obtained from the JCPDS database. Their magnetic properties were evaluated using a superconducting quantum interference device (SQUID, Quantum Design, Tokyo, Japan), which was operated at 300 K. Magnetization was measured as a function of the applied field from 0 to 50 kOe.  [35,51,52], the assembled wires in this study were not single MNP chains and contained many MNPs in the cross-section of every single wire. Thus, the average diameter of the wires was larger than that of the composite MNPs. The diameters of wires, of the order of 1 µm, are difficult to estimate because they are overlapped with each other. The average length of the assembled wires is estimated to be approximately 10 mm. Therefore, the ratio between length and width is of the order of 10 4 . The microscope images show that the assembled wires were nearly straight and parallel to each other. royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 7: 191656 Figure 4 shows the XRD patterns of dispersed MNPs, featuring α 00 -Fe 16 N 2 /Al 2 O 3 and Fe 3 O 4 crystalline structures, and those of their assembled arrays under an EMF of 15.0 T. Different from MNPs, the XRD patterns of the fabricated arrays possess a slightly uphill baseline and obvious noise on baselines. This difference probably originates from the addition of the epoxy resin during the production of arrays. Similar phenomena were also observed for fabricating α 00 -Fe 16 N 2 contained fibre sand carbon nanotube films in our previous studies [44,53,54]. The XRD peaks of (202), (220), (004) and (400) from crystalline α 00 -Fe 16 N 2 /Al 2 O 3 MNPs are also visible for their assembled arrays in the 2θ range of 25°-70°, while peaks of (111), (311), (222), (400), (422), (511) and (400) from single-crystalline Fe 3 O 4 MNPs (as reported in our previous paper [47]) are shown for their assembled arrays in the same 2θ range. The patterns indicate that the fabricated arrays retained the inherent crystalline properties of their composite MNPs.

Morphology and crystalline structure
For the highly magnetic isotropic α 00 -Fe 16 N 2 MNPs and their arrays, the (004) peak has a vertical direction along the c-axis direction, while the (220) peak is in the horizontal direction [54]. When the EMF was applied along the array direction, the (004) and (220) diffraction peaks increased and decreased in the array direction, respectively. The XRD patterns were obtained in the parallel direction of the arrays. By contrast, the (004) and (220) diffraction peaks decreased and increased in the vertical direction of the arrays, respectively, as shown in figure 4. This result is consistent with our previous report on α 00 -Fe 16 N 2 /Al 2 O 3 films synthesized under various EMF conditions [54]. Therefore, the XRD patterns suggest that the strong EMF leads to increasing the alignment of the c-axis of the MNPs in the array direction.    Compared with the dispersed α 00 -Fe 16 N 2 MNPs, the shape of the M-H loop for the fabricated α 00 -Fe 16 N 2 array appeared to be more rectangular and spindle-shaped when the measured field was applied parallel and perpendicular to the array direction, respectively, as clearly shown in figure 5.     The induced magnetic field strength of the arrays fabricated from α 00 -Fe 16 N 2 /Al 2 O 3 and Fe 3 O 4 MNPs was directly measured using a magnetometer (TM-801, Kanetec, Japan). The magnetic values under the bottom of inner beakers used for fabrication were approximately 0.2 T and higher than the detection lower limit (less than 0.1 T). The generated detectable magnetic field suggests that the fabricated arrays from isotropic MNPs possess the potential application in tiny magnets [57].

Arrays fabricated from spindle-shaped MNPs
The fabricated wires possess a shape anisotropy and cause the difference of the magnetic performance in the parallel and perpendicular directions of the wires. The enhancement of magnetic properties for the fabricated straight wires in the parallel direction of the wires is due to the increased magnetic dipoledipole interaction between MNPs in the wire direction [37]. In a new scenario, if elongated MNPs with shape anisotropy can rotate freely in a solvent, an EMF can make them orient along a magnetic easy axis oriented to the magnetic line and inevitably induce the formation of arrays. Thus, the shape anisotropy of MNPs can increase the alignment of single-domain MNPs with magnetic easy axes under an EMF [37,58]. The shape anisotropy will be enhanced if the external magnetic field is applied to spindle-shape MNPs. Figure 7 presents the comparison of the magnetic property for the dispersed spindle-shape core-shell α 00 -Fe 16 N 2 /Al 2 O 3 MNPs and their aligned arrays under EMFs of 0.8 and 15 T with the magnetic field, as measured by SQUID, parallel to the wire direction. The magnetic properties of the spindle-shaped α 00 -Fe 16 N 2 /Al 2 O 3 MNPs 110 nm in length and 18 nm in width have been described in detail elsewhere [46]. The ratio of Mr/Ms and Hc increased with the EMF strength, as summarized in table 2. The increase in EMF strength caused the increase in the alignment of MNPs in arrays and resulted in an enhancement of magnetic performance of the fabricated arrays. This result is consistent with our previous report on the alignment of α 00 -Fe 16 N 2 /Al 2 O 3 MNPs in polymer films [54], as well as other investigations on the EMF intensity effect on wire-like structures via the direct synthesis under a weak EMF (less than 0.5 T) [33,51,[59][60][61] and strong EMF (less than 1.4 T; 0-10 T) [8].
The ratio of Mr/Ms increased 15.0% and 44.4% under 0.8 and 15 T, respectively, while the Hc increased 11.7% and 19.0% for the two cases, respectively. The increase in Mr/Ms and Hc for the spindle-shaped α 00 -Fe 16 N 2 /Al 2 O 3 are both higher than those for the cube-like one (34.7% and 6.4%) under the same EMF of 15 T. The increased enhancement of magnetic properties is due to the spindle-shape anisotropy, which increases the alignment of single-domain MNPs along the EMF direction. Competing interests. The authors declare that there are no competing interests involved. Funding. This work was supported by JSPS (Japan Society for the Promotion of Science) KAKENHI (grant nos. 26709061 and 16K13642). This work was partly supported by the Center for Functional Nano Oxide at Hiroshima University (Japan). Table 2. Summary of magnetic properties at 300 K for the arrays assembled from spindle-shape α 00 -Fe 16 N 2 /Al 2 O 3 MNPs under EMF of 0.8 and 15 T with the measured magnetic field applied parallel to the arrays.