1 Introduction

Permanent magnetic materials including no heavy rare earths (Dy or Tb), low cost, and capable of operating at elevated temperatures (≥ 200 °C) are needed for applications in advanced power systems such as motors, hybrid vehicles, and wind generators [1,2,3]. Sm-Co and Nd-Fe-B rare-earth permanent magnets are preferred materials in these systems because these magnets have high energy products (> 20 MGOe), which can satisfy the weight or size limitation of these systems [4, 5]. However, the Nd-Fe-B-based magnet presents poor high-temperature magnetic properties resulting from their low Curie temperature (312 °C) and large negative thermal coefficients for coercivity β and remanence α, and hence, the addition of significant amounts of heavy rare-earth elements Dy or Tb is needed to increase the thermal stability of the Nd-Fe-B-based magnets [6, 7]. Unfortunately, Dy and Tb are essential elements on basis of supply risk and the importance of clean energy [8]. In contrast, the Sm-Co-based magnets have excellent thermal stability in the upper temperature range due to their high Curie temperatures (from 680 to 920 °C) and a large magnetocrystalline anisotropy [9], but their energy product is below that of the Nd-Fe-B-based magnet at room temperature (RT) resulting from their rather low saturation magnetization [10]. Furthermore, the rare-earth permanent magnet supplies have aroused extensive attention in recent years. It is ascribed to that the rare-earth permanent magnets hold the key to improving the efficiency and property of equipment in power generation, conversion, transportation, and other economic energy-use areas [11]. These phenomena have promoted the lean rare earth or non-rare-earth magnets becoming an investigation hotspot in the sector of permanent magnetic materials [12,13,14].

Nowadays, nanohybrid magnets composed of hard magnetic phases featuring high coercivity and soft magnetic phases featuring high saturation magnetization at nanoscale aroused broad attention because they were expected to become a new type of strong magnet with less rare-earth metal content [12, 15,16,17,18,19,20,21]. Recently, we have succeeded in yielding bulk anisotropic SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe multiphase nanohybrid magnets with high energy product (25 MGOe) and high-content soft phases (~ 30 wt%) [22]. Moreover, these magnets possess a good thermal stability (β(RT-250 °C) =  −0.28% °C−1) resulting from the synergetic effect exerted by the soft-phase α-Fe and α-Fe(Co), and the hard-phase Nd2Fe14B and SmCo7 [22]. On basis of the discoveries, the bulk SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe multiphase nanohybrid magnets, having high magnetic properties and low cost, feature great practical application potential; thus, it is very necessary to further investigate this type of multiphase nanohybrid magnet.

We all know that the composition and structure of materials significantly affect the physical and chemical performances of materials [4, 6, 23, 24]. Hence, changing the composition or structure of the precursors is an alternative approach to produce the bulk SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe multiphase nanohybrid magnets with optimized magnetic performance. Here, we focus on fabricating the bulk SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe multiphase nanohybrid magnet by high-pressure thermal compression (HPTC) deformation of the mixtures of Nd9Fe83.2Ti0.8Nb1B6 amorphous-nanocrystalline powder and SmCo-based amorphous-nanocrystalline powder precursor. Precursors used in this work are different from previous studies. In the previous work, the precursors were the mixture of Nd9Fe85.5Cu1.5B4 amorphous powder and SmCo-based amorphous-nanocrystal powder [22]. The microstructure, magnetic performances, and thermal stability of the magnets yielded from HPTC deformation of the precursors were studied in this investigation. We find that the bulk anisotropic SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe multiphase nanohybrid magnet exhibits an energy product of 22.5 MGOe at RT and 14.8 MGOe at 200 °C under the microstructure with a (00 l) texture of SmCo7 phase and fine nanograin sizes for all components. Moreover, these obtained magnets display a good thermal stability with a small β(RT-250 °C) =  −0.25% °C−1 and a good thermal stability of nanostructure. Resultants exhibit obtained magnets feature high high-temperature applications potential.

2 Experimental

In this investigation, the HPTC precursors were consisted of Nd9Fe83.2Ti0.8Nb1B6 amorphous-nanocrystalline powder and SmCo-based amorphous-nanocrystalline powder. To gain Nd9Fe83.2Ti0.8Nb1B6 amorphous-nanocrystalline powder, the Nd9Fe83.2Ti0.8Nb1B6 alloy ingot was fabricated through arc melting in an argon atmosphere employing industrial pure Nd, Ti, Nb, and Fe metals and Fe3B alloy, firstly. Then, the alloy ingots were adopted for preparing ribbons through melt spinning at a tangential speed of 26 m/s. Finally, the ribbons were milled into amorphous-nanocrystalline powder applying a high-energy mill (SPEX 8000 M) in an Ar atmosphere. The weight ratio of the specimen to the ball was 1:20 and the milling time was 1.5 h (h). For obtaining the SmCo-based amorphous-nanocrystalline powders, industrial SmCo5 (powder size ~ 40 m), Fe (powder size < 5 m), and Co (powder size < 5 m) materials were blended and subjected to high-energy ball milling, where the milling time was 4 h and the sample-to-ball weight ratio was 1:20. In SmCo5 + FeCo (a weight ratio of Fe and Co was 65:35) abrasive material, the weight fraction of FeCo load was 30%. The HPTC precursors were the blends of the Nd9Fe83.2Ti0.8Nb1B6 amorphous-nanocrystalline and SmCo-based amorphous-nanocrystal powder with a weight ratio of 30:70. The precursor was then consolidated into a bulk cylinder with a relative density of 80% in an Ar atmosphere at RT. The bulk specimens were placed in a steel tube and the HPTC deformation was performed on the specimen unit in vacuum adopting Gleeble 3800 machine. The HPTC deformation process is as follows: deformation temperature T = 700 °C, deformation time t = 30 s (s), height reduction (strain) = 79%, and maximal stress σ = 700 MPa. The disc-shaped HPTC-deformed specimen with a mean diameter of about 15 mm and a thickness of about 1.2 mm were generated, and the specimen density was detected to be ρ = 8.2 g/cm3 by Archimedes’ principle.

Transmission electron microscopy (TEM) and X-ray diffraction (XRD) with Co Kα radiation were employed for characterizing the microstructure of the HPTC-deformed specimens. The volume fraction and grain size of the nanocrystals were detected from the determined XRD pattern employing the Rietveld refinement program via the HighScore Plus software, where the goodness of fitting < 2, a profile R-value Rp < 10, and a weighted profile R-value Rwp < 10 were acquired. The RT and high-temperature magnetic properties of HPTC-deformed specimens were determined employing a vibrating specimen magnetometer (VSM) with a maximal magnetic field of 21 kOe, where the demagnetization effect was corrected by traditional methods [18]. Thermal stability was characterized through temperature coefficients of remanence α and coercivity β just as many reported works [24, 25].

3 Results and Discussion

3.1 Structural Analyses

Figure 1 exhibits the XRD pattern of HPTC-deformed magnet precursors. XRD analyses prove the precursors are blends of Nd9Fe83.2Ti0.8Nb1B6 amorphous-nanocrystalline powder and SmCo-based amorphous-nanocrystalline powder (Fig. 1c). The Nd9Fe83.2Ti0.8Nb1B6 amorphous-nanocrystalline powder is a blend of amorphous and crystal phases of α-Fe with a grain size of 5–7 nm (Fig. 1a), and the SmCo-based amorphous-nanocrystalline powder is a blend of amorphous and crystalline phases of α-Fe(Co) with a grain size of 5–7 nm (Fig. 1b). Therefore, the precursors for the HPTC-deformed magnets are a blend of amorphous and crystal phases of α-Fe and α-Fe(Co) with a fine nanograin size (Fig. 1c). These results indicate that the precursors in this work are different form that in Ref. 22.

Fig. 1
figure 1

XRD patterns of the as-milled Nd9Fe83.2Ti0.8Nb1B6 amorphous-nanocrystalline powders (a), as-milled SmCo-based amorphous-nanocrystalline powder (b), and blends of as-milled Nd9Fe83.2Ti0.8Nb1B6 amorphous-nanocrystalline powder and SmCo-based amorphous-nanocrystalline powder (c)

Full size image

Figure 2 displays the XRD patterns of HPTC-deformed magnets determined on the specimen surface perpendicular to the pressure direction. XRD analyses prove the HPTC-deformed magnets comprise SmCo7, Nd2Fe14B, α-Fe(Co), and α-Fe crystalline phases (Fig. 2a), as described in the previous work [22]. In further analyses of the XRD spectrum, weight fractions of SmCo7, α-Fe(Co), Nd2Fe14B, and α-Fe phases are determined as about 56%, 14%, 24%, and 6%, respectively, and the mean grain sizes of SmCo7, Nd2Fe14B, α-Fe(Co), and α-Fe phases are determined as about 12 nm, 15 nm, 13 nm, and 13 nm, separately. These outcomes prove the HPTC-deformed magnet majorly contains four crystal phases, namely, SmCo7 and Nd2Fe14B hard magnetic phases and α-Fe(Co) and α-Fe soft magnetic phases. Resultants show that the SmCo7 and Nd2Fe14B hard magnetic phases for the HPTC-deformed magnets come from the amorphous matrix in the HPTC process. We conclude that the multiphase nanohybrid magnet is produced by HPTC deformation of blends of Nd9Fe83.2Ti0.8Nb1B6 amorphous-nanocrystalline powder and the SmCo-based amorphous-nanocrystalline powder.

Fig. 2
figure 2

XRD patterns of the HPTC-deformed bulk SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe multiphase nanohybrid magnets determined on the specimen surface perpendicular to the pressure direction (a). The vertical lines imply powder diffraction data of isotropic Nd2Fe14B, SmCo7, α-Fe, and α-Fe(Co) crystals. (b, c) The magnified view of the XRD pattern in plane (a). Individual XRD peaks isolated from the pattern employing the PeakFit software

Full size image

Through further detailed analyses of XRD patterns, we discovered that, for the HPTC-deformed magnet, the SmCo7 hard-phase nanocrystals possess a (00 l) texture along the pressure direction (Fig. 2a, c). SmCo7 nanocrystal texture is expressed by the improved intensity of (002) diffraction peak, where the ratio of I(002) / I(111) = 64% exceeds that I(002)/ I(111) = 30% of isotropic SmCo7 crystals (see the Inorganic Crystal Structure Database ICSD:168,273). However, the (00 l) texture of Nd2Fe14B hard-phase nanocrystals cannot be found, which is indicated by the I(004)/I(220) = 71% is similar to that I(004)/I(220) = 70% of isotropic Nd2Fe14B crystals (see ICSD:067,224) (Fig. 2a, b). These results demonstrate that the bulk anisotropic SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe multiphase nanohybrid magnet can be prepared by HPTC technology, even though the HPTC-deformed magnets only exhibit the (00 l) texture of SmCo7 hard-phase nanocrystals, and the Nd2Fe14B hard-phase nanocrystals do not display the (00 l) texture.

TEM observations were employed for further characterizing the morphology of the HPTC-deformed magnets. Figure 3a shows that the HPTC-deformed magnets are composed of two regions, which are determined to be SmCo7/α-Fe(Co) and Nd2Fe14B/α-Fe regions, separately. In the SmCo7/α-Fe(Co) regions (Fig. 3b), the microstructure comprises approximately equiaxed SmCo7 and α-Fe(Co) nanocrystals featuring a mean grain size of about 12 nm (inset of Fig. 3b). The SmCo7 nanocrystals display a (00 l) texture along the direction of pressure, implied by stronger (002) diffraction points in the selected region electron diffraction (SAED) pattern (inset of Fig. 3b). These results coincide with the XRD studies. In the Nd2Fe14B/α-Fe regions (Fig. 3c), the microstructure consists of approximately equiaxed α-Fe and Nd2Fe14B nanocrystals featuring a mean grain size of ~ 15 nm (inset of Fig. 3c). The Nd2Fe14B nanocrystals show arbitrary orientation, which is indicated by the SAED pattern (inset of Fig. 3c) in keeping with XRD resultants. These findings prove the HPTC-deformed magnet possesses a (00 l) texture of SmCo7 hard magnetic nanocrystals and small nanograin sizes for all components.

Fig. 3
figure 3

Characterization of synthesized bulk SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe multiphase nanohybrid magnets produced through HPTC. (a) The bright-field TEM image of the magnet including the SmCo7/α-Fe(Co) region and the Nd2Fe14B/α-Fe region. (b) The bright-field image of the SmCo7/α-Fe(Co) region and corresponding SAED pattern and statistic distribution of the nanograin size (insets). (c) The bright-field image of the Nd2Fe14B/α-Fe region and corresponding SAED pattern and statistic distribution of the nanograin size (insets). TEM images were determined on the specimen surface parallel to the pressure direction

Full size image

SmCo7 hard magnetic nanocrystals are oriented along their easy magnetizing direction in obtained magnets. It results from the preferential nucleation and growth of SmCo7 nanocrystals from SmCo amorphous matrix under the HPTC process, which introduces strain-energy anisotropy, facilitating the directional nucleation and growth of nanocrystals along the minimal strain-energy direction [16,17,18, 26, 27]. By contrast, the Nd2Fe14B hard magnetic nanocrystals do not have any orientation, which is different from those previous studies [17, 22, 27, 28]. This result mainly ascribes to that the material composition and microstructure of Nd-Fe-B precursors are different from those reported work’s precursors [17, 22, 27, 28]. A further deep study for this phenomenon will be conducted in the future research work. Based on the XRD measurements and the TEM analyses, small grain sizes of nanocrystals for each component were realized in the HPTC-deformed magnets. The small grain sizes result from the dispersion of ultrafine α-Fe(Co) and α-Fe nanograins (5–7 nm) in an amorphous matrix before HPTC deformation and rapid deformation time (~ 30 s) during HPTC deformation process, as demonstrated in previous studies [16,17,18, 22].

3.2 Magnetic Properties

After characterizing synthesized nanohybrid magnets, we investigated the magnetic properties of the HPTC-deformed magnet. The magnet presents obvious magnetic anisotropy with 4πMr∥/4πMr⊥ = 1.26 (Table 1), which is further demonstrated by the easy-axis and hard-axis magnetic behavior parallel and perpendicular to the pressure direction (Fig. 4). For the HPTC-deformed magnet along the pressure direction at RT, the maximum energy product of (BH)max = 22.5 MGOe was obtained (Table 1). This (BH)max comes from the high remanence ratio 4πMr/4πMs = 0.87 and coercivity Hci = 4.4 kOe (Table 1), where 4πMr and 4πMs represent the remanence and saturation magnetization along the pressure direction, respectively. The high value of 4πMr/4πMs = 0.87 along easy axis comes from the (00 l) texture of SmCo7 nanocrystalline and excellent exchange coupling between small nanocrystalline, where the exchange coupling occurs between small nanocrystalline of α-Fe(Co) and SmCo7 as well as between the small Nd2Fe14B and α-Fe nanocrystals [29,30,31]. The Hci = 4.4 kOe comes from the synergetic effect exerted by SmCo7 and Nd2Fe14B nanocrystals as well as the pinning-type coercivity mechanisms in nanostructured permanent magnets [32,33,34]. Besides, we believe the interfaces between SmCo7/α-Fe(Co) regions and Nd2Fe14B/α-Fe regions significantly affect magnetic performances of this kind of multiphase nanohybrid magnet, and we will carry out such studies in the future work.

Table 1 Magnetic properties of bulk SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe multiphase nanohybrid magnets produced through HPTC. These magnetic properties came from the hysteresis loop given in Fig. 4
Full size table
Fig. 4
figure 4

Hysteresis loops of the bulk SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe multiphase nanohybrid magnet measured parallel (∥) and perpendicular (⊥) to the pressure (P) direction at RT

Full size image

Figure 5 exhibits the demagnetizing curves for HPTC-deformed magnets measured along the pressure direction at different high temperatures. It is suggested that the magnets present good single-phase magnetic behavior at the different high temperatures and the energy product of 14.8 MGOe is achieved at 200 °C (Figs. 5 and 6). The thermal stability of the HPTC-deformed magnet was investigated by measuring its remanence () and coercivity () temperature coefficients [24, 25]. The HPTC-deformed magnets exhibit a low (RT-250 °C) =  −0.02% °C−1 and β(RT-250 °C) =  −0.25% °C−1. The β(RT-250 °C) =  −0.25% °C−1 (Fig. 6) is less than β(RT-250 °C) =  −0.26% °C−1, β(RT-250 °C) =  −0.42% °C−1 or β(RT-250 °C) =  −0.4% °C−1 for single-phase SmCo5, Nd-Fe-B or Nd-Dy-Fe-Co-B magnet, respectively [1, 35]. In particular, the β(RT-250 °C) =  −0.25% °C−1 in this investigation is smaller than the values of −0.28% °C−1 and −0.3% °C−1 for our previous works [22, 36]. It is mainly contributed to that the Nd2Fe14B/α-Fe regions have a small grain size and Nd2Fe14B phase contains Ti and Nb elements [37, 38]. These results demonstrate that changing the precursors for the HPTC-deformed magnets can further improve the temperature stability of the multiphase nanohybrid magnets.

Fig. 5
figure 5

Demagnetization curves for the HPTC-deformed magnets measured along the direction of pressure at different high temperatures

Full size image
Fig. 6
figure 6

Dependence of (BH)max, Hci, Br, and 4πMs on the measurement temperatures for the HPTC-deformed magnets measured along the direction of pressure. These magnetic properties came from the hysteresis loop given in Fig. 5

Full size image

The thermal stability of nanostructure for the nanostructure materials is of great importance for their practical applications. For studying the thermal stability of HPTC-deformed magnet nanostructures, the (BH)max along the pressure direction was obtained at RT after long-run annealing at Ta = 400 °C in a vacuum chamber at p < 10−4 Pa (Fig. 7a). These values of (BH)max are no degradation after annealing at different times at Ta = 400 °C, and thus suggesting the HPTC-ed magnet with a stable nanostructure morphology. This result has been further verified by Fig. 7b, which shows a rigorous single-phase demagnetization behavior after being heat treatment. These results demonstrate the HPTC-deformed magnet possesses a stable nanostructure for the potential applications.

Fig. 7
figure 7

(a) (BH)max and (b) demagnetizing curves along the pressure direction measured at RT after different annealing times at Ta = 400 °C

Full size image

3.3 Discussion

The anisotropic bulk SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe magnets featuring an improved thermal stability and good magnetic properties were fabricated with HPTC technology. However, the maximum energy product (22.5 MGOe) for the HPTC-deformed magnet is still low as compared with the sintered and hot deformed single-phase magnets such as SmCo5, Sm2Co17, and Nd2Fe14B magnets. In order to promote the practical applications of multiphase nanohybrid magnets, enhanced energy product is of great significance for the bulk SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe multiphase nanohybrid magnets. For enhancing the maximum energy product for the HPTC-deformed magnet, it is necessary to further enhance the Br and the Hci for the obtained magnets simultaneously. Many previous studies show that further aligning hard-phase nanograins and engineering their interface structure could be further enhanced the Br and the Hci, thus promoting the synthesized magnets having enhanced energy products [39,40,41]. Therefore, higher energy products for the SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe multiphase nanohybrid magnets could be achieved by further enhancing the hard-phase texture along the easy direction of magnetization and adjusting the interface structure between nanocrystals or the different regions.

4 Conclusions

Bulk anisotropic SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe multiphase nanohybrid magnets were prepared using HPTC technique. The microstructure, magnetic properties, and thermal stability of HPTC-deformed magnets have been investigated. We discovered HPTC-deformed magnets with a low β(RT-250 °C) =  −0.25% °C−1 exhibit a maximal energy product of 22.5 MGOe at RT and 14.8 MGOe at 200 °C. This work shows that the HPTC-deformed bulk anisotropic SmCo7/α-Fe(Co) + Nd2Fe14B/α-Fe multiphase nanohybrid magnet features a high high-temperature application potential. Moreover, this work gives a method of further improving the thermal stability of multiphase nanohybrid magnets.