Effect of selective Co addition on magnetic properties of Nd2(FeCo)14B/-Fe nanocomposite magnets

Nd2Fe14B/α-Fe-based hard/soft nanocomposite magnets with Co addition have been prepared by ball-milling and warm compaction. It was found that Co addition into the magnetically hard phase improves magnetic properties significantly, especially the remanence ratio and coercivity. The effect on the magnetic properties of the selective Co addition may be attributed to enhanced interdiffusion across the hard/soft interface that improves the interface conditions for effective interphase exchange coupling. By optimizing the Co content in the Nd15Fe79−xCoxB6 hard phase, an energy product value about 21 MG Oe can be obtained in the isotropic Nd2(FeCo)14B/α-(FeCo) nanocomposite magnets compared with 15 MG Oe of Nd2Fe14B/α-Fe nanocomposite magnets prepared under the same conditions with the same grain size and microstructure, owing to the strengthened intergranular exchange interactions. (Some figures may appear in colour only in the online journal) Increased demand for rare-earth permanent magnets coupled with shortages in rare-earth supply, especially the Nd–Fe–B based magnets for traction motor and wind turbine applications, necessitates reduction in the rare-earth content in these materials [1, 2]. Exchange-coupled hard/soft nanocomposite permanent magnets are a suitable approach to achieve this goal since the addition of a magnetically soft phase reduces the rare-earth content while enhancing the energy density by combining the large magnetocrystalline anisotropy of the hard phase and large magnetic flux density of the soft phase [3– 6]. The exchange-coupled nanocomposite magnets have the potential to double the energy products of the Nd–Fe–B singlephase magnets [4]. Our recent experimental work has demonstrated that energy product enhancement greater than 100% is achieved in Sm–Co/Fe nanocomposites with up to 30% of the Fe-based soft phase [7–9]. A prerequisite for the effective interphase exchange coupling is a homogeneous distribution of the soft phase with grain size smaller than a critical length (∼10 nm) [3–6, 10–12]. In addition, previous simulation and experimental investigations have shown that the grain boundaries with graded composition profiles promote exchange coupling between the hard and soft phases [13–19]. The effect of Co addition on magnetic properties of the Nd2Fe14B single phase has been extensively studied since the initial discovery of the Nd–Fe–B magnets [20–27]. It was found that Co substitution of Fe in the tetragonal phase greatly increases the Curie temperature while having minimal deleterious effects on the magnetization, magnetocrystalline anisotropy and coercivity. Chang et al [28] have investigated the effect of Co substitutions for Fe on magnetic properties of melt-spun Nd2Fe14B/α-Fe nanocomposite magnets and found that increasing Co content promoted grain coarsening with simultaneously enhanced exchange coupling. However, the underlying mechanism of these effects remains unclear. Unlike melt-spin processing, where partitioning on the various magnetic and possibly non-magnetic phases is a complex relationship between undercooling controlled by wheel speed and composition, mechanical milling 0022-3727/13/045001+05$33.00 1 © 2013 IOP Publishing Ltd Printed in the UK & the USA J. Phys. D: Appl. Phys. 46 (2013) 045001 C Bing Rong et al allows for controlled addition of soft phases to the hard phase. Subsequent solid-state, high pressure compaction (i.e. warm compaction) of the ball milled constituents into a nanocomposite magnet has been found to better control the interfacial properties in the Sm–Co/Fe system [7, 8]. In this paper, we report structural and magnetic properties for the Nd2(Fe,Co)14B/α-Fe nanocomposite system using a similar processing approach. Ingots of Nd15Fe79−xCoxB6 (x = 0–50) were prepared from high purity elemental constituents by arc melting under UHP argon. The Nd15Fe79−xCoxB6 ingots were further processed by grinding into powder samples with a size of ∼45μm. Commercial α-Fe powder samples with a size of ∼10μm were mixed together with the hard-phase powder and milled in a SPEX 8000M high-energy ball mill to form the nanocomposites. The weight ratio of soft phase to hard phase (denoted as y) was varied from 0% to 40%. The milling conditions included: 440 ◦C hardened steel balls, with a weight ratio of sample : balls ∼1 : 20–30, and milling times from 0.5 to 10 h. The as-milled powders were then annealed at temperatures from 525 to 600 ◦C under vacuum for 0.5 h. The optimal magnetic properties were found for a milling time of 4 h and annealing temperature of 550 ◦C. These are the default experimental conditions in this work if not otherwise specified. The annealed powders were then compacted using a warm compaction technique [29] at 500 ◦C under a quasiisostatic pressure of ∼2.5 GPa. The final bulk magnets having dimensions Ø6 mm × 1.5 mm were characterized for morphology and the crystalline structure using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-filtered TEM (EFTEM) and x-ray diffraction (XRD) using Cu Kα radiation. Magnetic properties were measured with a superconducting quantum interference device (SQUID) magnetometer with a maximum applied field of 70 kOe. To calculate the true energy product (BH)max of the bulk sample, we determined the demagnetization factor experimentally as described in [9]. Figure 1 shows the XRD patterns of the as-milled and annealed matrix-phase Nd15Fe79B6 (mostly the 2 : 14 : 1 phase) and the two-phase nanocomposite prepared from the mixture of Nd15Fe79B6 + 20 wt% α-Fe powders (y = 20). The patterns of both samples show very broadened peaks around 45◦ and 65◦ two theta, which is from a typical body-centred-cubic (bcc) structure, indicating that the as-milled NdFeB alloy consists of amorphous and bcc structured α-Fe phases. The XRD patterns of the annealed powders show that the 2 : 14 : 1 tetragonal structure was developed during the post-annealing at 550 ◦ C for 0.5 h. In addition, the α-Fe peaks were also observed in the two-phase system, indicating that a nanocomposite NdFeB/α-Fe was obtained after appropriate ball milling and post-annealing. It should be noted that the content of the soft phase will be somehow reduced after the annealing due to the equilibrium conditions. The Nd15Fe79−xCoxB6/α-Fe powders with varying Co content have very similar XRD patterns, as shown in figure 1. Figure 2 shows the demagnetization curves of the nanocomposite magnets prepared from the mixture of Nd15Fe79−xCox B6 + 20 wt% α-Fe compacted from the Figure 1. XRD patterns of as-milled Nd15Fe79B6 and Nd15Fe79B6 + 20 wt% α-Fe powders (the upper two) and the powders annealed at 550 ◦ C for 0.5 h (the lower two). The upper patterns show only α-Fe peaks and the lower patterns show the Nd2Fe14B peaks and the lowest pattern show both phases. Figure 2. Demagnetization curves of the Nd15Fe79−xCoxB6 + 20 wt% α-Fe nanocomposite magnets with variation of Co content in the ingot hard-phase alloy. annealed powders. It should be noted that maximum measuring magnetic field of 70 kOe was sufficient to saturate the samples, although the figure only shows a limited range of magnetic field. The coercivity of the nanocomposite magnet without Co substitution (x = 0) is only about 5.3 kOe. One can see clearly that the coercivity increases to 6.2 and 8.3 kOe just by increasing the Co content x in the Nd15Fe79−xCoxB6 raw materials in steps from 0 to 10 and 15, respectively. The remanence also increases from 97 to 108 emu g−1 by increasing