Effects of adding different types of carbon on the structure and magnetic properties of SmCo6.9Hf0.1 alloy

JOURNAL OF RARE EARTHS, Vol. 31, No. 12, Dec. 2013, P. 1168

Effects of adding different types of carbon on the structure and magnetic properties of SmCo6.9Hf0.1 alloy SUN Jibing (ᄭ㒻݉), BU Shaojing (ℹ㒡䴭)*, YANG Wei (ᴼ 㭛), WANG Hongshui (⥟⋾∈), CUI Chunxiang (የ᯹㖨), HE Chenhui (ԩ䖄䕝) (Key Laboratory for New Type of Functional Materials in Hebei Province, School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China) Received 21 June 2013; revised 25 October 2013

Abstract: In this paper, SmCo6.9Hf0.1 as-cast alloys and ribbons with the addition of either graphite (C) or carbon nanotubes (CNTs) were prepared by arc melting and melt-spinning, respectively. The effects of adding carbon on the structure and magnetic properties SmCo6.9Hf0.1 were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), magnetic force microscopy (MFM) and vibrating sample magnetometer (VSM). It was found that the microstructure and magnetic structure of SmCo6.9Hf0.1 ribbons were changed obviously due to the introduction of C or CNTs, although their crystal structure was characterized as the same Sm(Co,Hf)7 single phase, no matter carbon was added or not. As a result, the magnetic properties of carbon-contained ribbons were enhanced in a certain degree. This was considered to be related to the refined equiaxed grains, small domain size and the pinning effect of C or CNTs-rich regions. The magnetic properties of SmCo6.9Hf0.1(CNTs)0.05 ribbons reached Hc=12.5 kOe, Mr=57.0 emu/g and Mr/M2 T=0.788. Keywords: magnetically ordered materials; rapid-solidification; crystal structure; magnetic measurements; rare earths

The TbCu7-type Sm-Co intermetallics behave outstanding intrinsic magnetic properties, such as high Curie temperature (Tc=750–852 ºC), high anisotropy field (HA= 90–180 kOe at room temperature) and positive coercivity temperature coefficient, and so on. Since SmCo7 is a metastable phase, some stable elements can be introduced to obtain a single TbCu7-type phase[1–13]. Guo and co-workers[3,4] prepared a series of TbCu7-type SmCo7–xMx (M=Si, Cu, Ti, Zr, Ga, Ag, and Hf) alloys using arc melting method and found that the appropriate Hf substitution for Co could stabilize the TbCu7-type structure, concurrently enhance the crystalline magnetic anisotropy and saturation magnetization. Zhang et al.[14] prepared bulk nanocrystalline SmCo6.6Nb0.4 sintered magnet with TbCu7-type structure by spark plasma sintering (SPS) technique. The magnet exhibited good thermal stability with a coercivity of 0.48 T at 500 ºC and coercivity temperature coefficient ȕ of –0.169%/K. On the other hand, TbCu7-type structure is easier to be obtained by rapid solidification method. Aich et al.[15,16] studied the Sm-Co-Nb, Sm-Co-C and Sm-Co-Nb-C ribbons melt-spun at 40 m/s and concluded that Nb addition played an important role in stabilizing the TbCu7-type structure and tended to reduce the content of

long-range ordered Sm2Co17, while graphite carbon addition increased the coercivity by altering the morphology and producing a grain-boundary phase that effectively isolated the hard magnetic SmCo7 grains from one another. The (Sm0.12Co0.88)94Nb3C3 ribbons had a maximum coercivity of 9 kOe when annealed at 800 ºC. Chang et al.[17] found that SmCo7–xHfx (x=0.1 and 0.2) ribbons were composed of single TbCu7-type phase and SmCo6.9Hf0.1 ribbons performed the optimal magnetic properties of Br=6.4 kG, Hc=7.3 kG. Simultaneously, they prepared SmCo7–xHfxCy ribbons with TbCu7-type sturucture from the ingots under a wheel speed of 40 m/s and found that a slight graphite carbon addition (y=0.12) was helpful in refining the grain size of the ribbonsfrom 100–400 nm for SmCo7 to 10–80 nm for SmCo6.9Hf0.1 C0.12 ribbons. However, a small amount of face-centered cubic Co phase and Sm2Co17 phase existed in Ccontained ribbons, and even a slight nonmagnetic Sm2C3 phase for higher C addition of y=0.14 was present, which all reduced the coercivity of the ribbons. As a result, the maximum coercivity of 11.8 kOe was obtained in SmCo6.9Hf0.1C0.1 ribbons with an appropriate addition of graphite due to both fine grains and the exchange coupling effect.

Foundation item: Project supported by General Program from the National Natural Science Foundation of China (51271072), National Science Fund for Young (51301056), Key Program from the Science and Technology Research for Colleges and Universities in Hebei Province (ZH2011202), and Hebei Natural Science Foundation of China (E2013202030) * Corresponding author: BU Shaojing (E-mail: [email protected]; Tel.: +86-22-60204555) DOI: 10.1016/S1002-0721(12)60422-0

SUN Jibing et al., Effects of adding different types of carbon on the structure and magnetic properties of …

In addition, carbon nanotubes (CNTs) are one dimensional nano-materials with hexcirclic structure. They are characterized by good thermal conductivity (~6000 W/m/K), vacuum stability (2800 ºC), good adsorbability and inertness, which make CNTs potentially a component of ideal pinning phase with good dispersivity. In this paper, graphite and carbon nanotubes (CNTs) were added into SmCo6.9Hf0.1 alloy respectively, and the effects of different carbon additions on the structure and magnetic properties were studied.

1 Experimental The graphite and multiwall CNTs (MWCNTs) prepared by chemical vapor deposition (CVD) were purchased from a commercial supplier, their macro morphologies and corresponding XRD patterns are shown in Fig. 1. The XRD peaks of both graphite and MWCNTs correspond to those in the PDF 411487 which describes a primitive hexagonal structure and space group of P63/mmc. However, the full width at half maximum (FWHM) of every diffraction peak of CNTs is much larger than that of graphite, implying the lower long range order of CNTs, whose characteristics are listed as follows: inner diameter of 5–10 nm, external diameter of 20–50 nm, and length of several to tens of micrometers, as shown in Fig. 1(d).

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The as-cast alloys with nominal compositions of SmCo6.9Hf0.1 (“SCH” for short), SmCo6.9Hf0.1C0.05 (“SCHC” for short) and SmCo6.9Hf0.1(CNTs)0.05 (“SCNTs” for short) were prepared from high purity (99.99%) elements with or without the addition of graphite (“C” for short) or CNTs by arc melting in a high purity argon atmosphere. Before arc melting, 8% extra Sm was added to compensate for the mass loss arising from the vaporization of Sm. The obtained ingots were then rapidly solidified by melt spinning in high purity argon at a chamber pressure of 0.6u105 Pa and a molybdenum wheel velocity of 40 m/s to get ribbon samples. The phase analysis was carried out on a Rigaku Dmax 2500 Pc X-ray diffractometer (XRD) with Cu KĮ radiation. The microstructure was observed by Hitachi S-4800 scanning electron microscope (SEM). The surface of the ribbons was polished and eroded using nitric acid alcohol or FeCl3 prior to the SEM observation. The magnetic measurements were made by Lake Shore 7407 vibrating sample magnetometer (VSM) with a maximum field of 20 kOe. All ribbon samples were magnetized by a 50 kOe peak pulse field prior to the VSM measurement. The magnetic structures of the ribbons were observed by NanoScope(R) IV magnetic force microscopy (MFM). The tip lift height was 160 nm, and the resonant frequency of the tips is about 73.0462 kHz.

Fig. 1 Macro morphology (a) and corresponding XRD pattern (b) of graphite and the macro morphology (c), inset of TEM image (d) and corresponding XRD pattern (e) of multiwall carbon nanotubes

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2 Results and discussion

the interplanar spacing of {111} is decreased while that of {002} is increased. According to Refs. [3,4,18,19], Hf prefers to occupy the 2e crystal position in Sm(Co,Hf)7 phase to form Hf-Hf dumbbell pair, which makes the lattice constant c increase and a decrease when x<0.2 in SmCo7–xHfx, resulting in the increase of the axial ratio c/a. It is well known that Sm(Co,Hf)7 phase has the P6/mmm space group and the lattice constants a=bc.

Fig. 2 shows the XRD patterns of SCH, SCHC and SCNTs as-cast alloys. From Fig. 2(1), we can see that the SCH alloy consists of TbCu7-type Sm(Co,Hf)7 main phase and HfCo2 minor phase. While the SCHC and SCNTs alloys are composed of Sm(Co,Hf)7 main phase and a small amount of HfC-type phase (Fig. 2(2, 3)). And it should be noted that neither HfCo2 nor pure graphite (or CNTs) appears in carbon-contained samples. Compared with Fig. 2(1), the 111 and 002 peaks of Sm(Co,Hf)7 phase in SCHC and SCNTs alloys shift right (see vertical lines A and B), which suggests the reduction of {111} and {002} interplanar spacing based on Bragg equation. As mentioned at the beginning, Co will be substituted by the bigger atom Hf in Sm(Co,Hf)7 phase[3,4]. Connecting with the above analysis of XRD results, we can conclude that after adding C or CNTs, the solid solubility of Hf in Sm(Co,Hf)7 phase decreases and leads all lattice constants (a, b and c) to decrease simultaneously. Subsequently, a part of Hf combines with C or CNTs to form HfC-type HfC or Hf(CNTs) phase. On the other side, the 200 and 111 peaks are separated more clearly in the latter two samples, indicating that the long range order degree of Sm(Co,Hf)7 phase is improved owing to the introduction of C or CNTs. The XRD patterns of SCH, SCHC and SCNTs ribbons in Fig. 3 indicate that the main phase of each ribbon is Sm(Co,Hf)7, and neither HfCo2 nor HfC-type phase is formed, meaning that nearly all Hf and C or CNTs dissolve into the Sm(Co,Hf)7-type phase. Compared with Fig. 3(1), the 111 peak of Sm(Co,Hf)7 phase in SCHC and SCNTs ribbons shifts right (see vertical line A), while the 002 peak shifts left (see vertical line B), which is much different from the results in Fig. 2, suggesting that the addition of carbon causes different changes in the interplanar spacing of {111} and {002}. To be specific,

Based on the formula d

4

(h  hk  k )  2

2

l

2 1/ 2

, 3a c the interplanar spacing d depends on both the crystal indices (hkl) and the lattice parameters (a and c). For {111} of Sm(Co,Hf)7 phase, the increase of c and decrease of a in SCHC and SCNTs ribbons make the d (d=(4/a2+ 1/c2)–1/2) decrease and 2ș increase, that is, the 111 XRD peak shifts right. However, for {002}, d=c/2, the increase of c makes the d increase and 2ș decrease, so the 002 peak in SCHC and SCNTs ribbons shifts left. On the other side, the 200 and 111 peaks are separated clearly in all three ribbon samples, indicating that the long range order degree of the Sm(Co,Hf)7-type phase is improved during the rapid solidification. Fig. 4 shows the SEM images of free face area (FFA) of SCH and the sticking roller face area (SRFA) of SCH, SCHC and SCNTs ribbons. It can be seen that the grains in SRFA are much smaller than those in FFA and coarse equiaxed grains are formed in FFA. Furthermore, the SRFA of SCH ribbons are composed of lamellar grains with a thickness of about 0.5–2 Pm; the grains in SRFA of SCHC ribbons display irregular blocks with a diameter of about 1–5 Pm; while those of SCNTs ribbons are equiaxed and less than 0.5 Pm. Because the surfaces of ribbons were polished and eroded prior to the SEM observation, the regions with higher contrast (appearing indark color in SEM images) correspond to the eroded grain boundaries and crystalline phases. Ref. [20] reports [

Fig. 2 XRD patterns of SCH (1), SCHC (2) and SCNTs (3) as-cast bulks

2

2

]

SUN Jibing et al., Effects of adding different types of carbon on the structure and magnetic properties of …

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Fig. 3 XRD patterns of SCH (1), SCHC (2) and SCNTs (3) ribbons

Fig. 4 SEM images of free face area of SCH (a) and the sticking roller face area of SCH (b), SCHC (c and c1) and SCNTs (d and d1) ribbons (Fig. 4(c1) and (d1) are the magnified image of Fig. 4(c) and (d), respectively)

that the genuine grain size of SCH and SCNTs is 150–450 nm and the phase containing CNTs tends to distribute at the boundaries. Combined with the analysis of Fig. 3(2), it can be deduced that the dark grain boundaries and the slight amount of dark points smaller than 500 nm appearing in the block grains in Figs. 4(c, c1) should be the possible regions where the C-rich phase

exists. So, the C-rich phase is easy to be eroded by nitric acid alcohol or FeCl3 and displays higher contrast in SEM images. By that analogy, the same thing should go for the CNTs-rich phase, which is eroded to show dark color at the intergranular positions in Figs. 4(d, d1). According to Fig. 3(2, 3), besides TbCu7-type main phase, no other C-rich or CNTs-rich phase exists, so we may

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infer that C or CNTs dissolve into the lattice and the boundaries of partial Sm(Co,Hf)7 phase. The dissolved C or CNTs increase the lattice distortion energy and make the eroding easier. This implies that C or CNTs addition probably changes the crystalline process by stimulating the nucleation and preventing the grain growth. Particularly, CNTs addition has a stronger effect in refining grains than C addition, as can be observed by comparing Fig. 4(c) with (d). The demagnetization curves of SCH, SCHC and SCNTs ribbons are shown in Fig. 5. It can be seen that the coercivity (Hc) and remanence (Mr) increase with the addition of C or CNTs, and SCHC ribbons have the highest maximum magnetization (M2 T) under 20 kOe. Though both C and CNTs are non-magnetic or weakmagnetic additives, their introduction make the M2 T and Mr of SCH ribbons increase by more than 10 and 14 emu/g respectively, which is similar to the results of Ref. [17]. However, Ref. [17] ascribes the increase of magnetization to the precipitated face-centered cubic Co phase in C-contained ribbons. But in our research, no Co phase can be found in Figs. (3, 4). So the increase of magnetization might be mainly contributed by the doped C or CNTs. It is known that the anisotropy of Sm(Co,M)7 alloys mainly arises from the Co atoms at the 2c sites in the lattice of Sm(Co,M)7 phase[3,4]. At the same time, Hf at the 2e site also contributes a strong anisotropic field. On the other side, the Slater-Betle curve illustrates that the ferromagnetic Co atoms will behave the paramagnetism when the distance between two adjacent atoms is out of the appropriate range. In the present study, the addition of C or CNTs is assumed to increase the distance between Co atoms at the 2c and 2e sites, resulting in the improvement of magnetic moment. As is well known, the saturated magnetization is insensitive to the microstructure of alloy and mainly determined by the chemical composition and crystal structure. Though the ferromagnetism of CNTs is an open and controversial subject, Stamenov et al.[21] have proposed that CNTs can display weak ferromagnetism and their orbital magnetic moment can approach as high as 10–20 PB. From the above, the

M2 T enhancement of SCHC and SCNTs ribbons in this paper is supposed to have relations with the combined action of CNTs or C with Sm as well as Co. Moreover, the Mr/M2 T of SCHC and SCNTs ribbons increases by 18.7% and 37.5% respectively from 0.573 of SCH ribbons, implying that the addition of C or CNTs leads to the improvement of squareness of the demagnetization curve. Even more remarkably, there is a one-fold or two-fold increase in Hc of SCH ribbons due to the C or CNTs additions, respectively. Because the main phase of all three types of ribbons is nearly single Sm(Co,Hf)7, the ribbon coercivity is basically determined by the nucleation field (Hn) of the Sm(Co,Hf)7 phase. At the same time, the C-rich regions distributed mainly in the grains and grain boundaries and CNTs-rich regions mainly in the grain boundaries also play an important role in improving the coercivity of C or CNTs-contained ribbons by pinning the domain wall motion. Particularly, due to the good dispersibility and special structure of CNTs, the CNTsrich regions in ribbons promote the formation of fine equiaxed grains and produce a uniform pinning effect, resulting in an excellent coercivity performance. Ref. [17] provided the MFM images of SmCo6.9Hf0.1 and SmCo6.9Hf0.1C0.12 ribbons, showing that the domain size of C-contained ribbons was smaller than that of C-free ribbons. Here, Fig. 6 displays the magnetic domain morphology of SCH and SCNTs ribbons. According to the formula for magnetic domain width[22], in which the domain width W=(2× the total test line length)/(ʌ× the number of intersections), the mean domain width values are calculated as 395 nm for SCH ribbon and 140 nm for SCNTs ribbon, respectively. It is illustrated that the magnetic domain size of ribbon decreases significantly after adding CNTs. As a result, more domain walls emerge, which create much greater moving resistance. And this is considered to be the main reason for the ribbons to achieve higher coercivity. Thus, it further confirms our conjecture above that the addition of C or CNTs enhances the coercivity of the ribbons through the pinning effect by changing the domain structure. It should be mentioned that the residual transverse lines in Fig. 6(b) can not be avoided because of the magnetic field of the ribbons higher than that of the probe.

3 Conclusions

Fig. 5 Demagnetization curves of SCH, SCHC and SCNTs ribbons

In summary, the structure and magnetic properties of SmCo6.9Hf0.1 alloys with graphite or CNTs addition were studied. It was found that the SmCo6.9Hf0.1 as-cast alloy consisted of Sm(Co,Hf) 7 and HfCo 2 , while the SmCo6.9Hf0.1C0.05 and SmCo6.9Hf0.1(CNTs)0.05 as-cast alloys were composed of the Sm(Co,Hf)7 main phases and a small amount of HfC-type phase. The main phase of

SUN Jibing et al., Effects of adding different types of carbon on the structure and magnetic properties of …

Fig. 6 Magnetic domain morphology of SCH (a) and SCNTs (b, c) ribbons

ribbons melt-spun at 40 m/s was nearly single Sm(Co,Hf)7-type phase no matter whether C or CNTs was added or not. The addition of C or CNTs made the coercivity of SmCo6.9Hf0.1 ribbons increase by over one or two times, the remanence by 37.5% or 54.9% and the remanence ratio by 18.7% or 37.5%, respectively. The magnetic properties of SmCo6.9Hf0.1(CNTs)0.05 ribbons reached Hc=12.5 kOe, Mr=57.0 emu/g and Mr/M2 T=0.788. The refined equiaxed grains, small domain size and the pinning effect of C or CNTs-rich regions were considered as the major factors that enhance the remanence and coercivity.

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