Article ID: 1003-6326(2005)02-0358-03
Structure and magnetic properties of Pr0.1CexTb0.9-xFe1.9 alloys
LI Song-tao(李松涛), LIU He-yan(刘何燕), MENG Fan-bin(孟凡斌),
LU Zun-ming(卢遵铭), QU Jing-ping(曲静萍), LI Yang-xian(李养贤)
(School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China)
Abstract: The structural and magnetic properties of Pr0.1CexTb0.9-xFe1.9 alloys were investigated by using X-ray diffraction, AC susceptibility, VSM and standard strain gauge techniques. The lattice parameter exhibits positive deviation from Vegards law with the increasing Ce content in the range of 0.4-0.6. Curie temperature decreases linearly with increasing Ce content. The saturation magnetization and magnetostriction decrease when 0.2≤x〈0.4 and x>0.6, while abnormally increase with Ce content increasing from 0.4 to 0.6. These abnormal changes of lattice parameter, saturation magnetization and magnetostriction can be attributed to the valence fluctuation of Ce ions.
Key words: magnetostriction; laves phase; Pr0.1CexTb0.9-xFe1.9 alloy; valence fluctuation CLC number: TM274
Document code: A
1 INTRODUCTION
Terfenol-D(Tb0.27Dy0.73Fe2) exhibits large magentostriction and minimized magnetic anisotropy at room temperature[1], so it is widely used in actuators and transducers. But the main raw materials of Terfenol-D are expensive Tb and Dy. According to the single-ion model[2], CeFe2 and PrFe2 compounds have larger magnetostriction than TbFe2 and DyFe2 at 0K. In addition, Ce and Pr are much cheaper than Tb and Dy. So Ce-based and Pr-based compounds may be candidate materials for applications. But the magnetostriction of CeFe2 alloy only shows 6×10-5 at 4.2K due to the mix-valence behaviors of Ce ions. The PrFe2 compound cannot be fabricated at ambient pressure because of the bigger atomic radius of Pr. It is interesting that Ce has high atomic bonding energy and the substitution of Ce for Pr can enhance the formation of Laves phase compounds with high Pr content[3, 4]. So much effort has been done on the Pr-Fe and Ce-Fe compounds[5-10]. The investigations on Ce-based intermetallics show that there is valence fluctuation of Ce ions[11] and the Ce ions contribute a lot to magnetostriction when they fluctuate towards trivalence. It is also found that the Pr substitution for Tb can enhance the magnetostriction[12, 13].
In our previous work, the heavy rare earth Tb and Dy substitution for Ce in CexR1-xFe2 compounds result in the valence fluctuation of Ce ions towards localized state[14, 15], while light rare earth such as Pr results in the delocalized tendency of Ce ions[3]. In order to obtain light rare earth magnetostrictive materials with large magntostriction, and investigate the effect of valence fluctuation of Ce ions, the Pr content is chosen as 0.1, and the structure and magnetic properties of Pr0.1Cex-Tb0.9-xFe1.9 alloys are all studied.
2 EXPERIMENTAL
The Pr0.1CexTb0.9-xFe1.9(x=0.2-0.8) alloys were prepared by arc melting under a high purified-argon atmosphere. The purities of rare earth and Fe were 99.9% and 99.99%, respectively. The alloy buttons were melted four times and then wrapped in a stainless steel foil and vacuum annealed in a sealed quartz capsule at 850℃ for 72h.
The structure analysis was performed by X-ray diffraction (Philips XPert MPD) using CuKα radiation at room temperature. The lattice parameters were determined by least-squares fitting to X-ray peaks, and the accuracy is estimated to be ±0.0002nm. Curie temperature was determined by measuring the temperature dependence of AC susceptibility. The magnetization was obtained with a VSM at room temperature. The anisotropic magnetostriction was measured using a standard strain gauge in directions of parallel (λ∥) and perpendicular (λ⊥) to the magnetic field up to 950kA/m at room temperature.
3 RESULTS AND DISCUSSION
The X-ray diffraction patterns of Pr0.1-CexTb0.9-xFe1.9 alloys are shown in Fig.1. All the homogenized samples exhibit a perfect cubic Laves phase with MgCu2-type structure.
Fig.1 XRD patterns of Pr0.1CexTb0.9-xFe1.9 alloys
The dependence of lattice parameters on Ce content is shown in Fig.2. The lattice parameters exhibit positive deviation from Vegards law when 0.4≤x≤0.6. It is well known that the volume of unit cell is related to the valence of elements in the compound. The positive deviation from Vegards law is related to the valence fluctuation of Ce ions towards trivalence. In our previous work, with the increasing Ce content, the Ce4f electrons in CexTb1-xFe2 compounds drift towards localized state[15], while drift towards delocalized state in PrxCe1-xFe compounds[3]. In this work, the valence of Ce ions in Pr0.1CexTb0.9-xFe1.9 alloys fluctuates towards localized state only in the range of 0.4-0.6. Anomalous behaviors can be also observed from magnetization and magnetostriction in this range.
Fig.2 Ce content dependence of lattice parameter for Pr0.1CexTb0.9-xFe1.9 alloys
The dependence of Curie temperature (TC) on Ce content is shown in Fig.3. It shows that Curie temperature decreases linearly with the increasing Ce content. This can be attributed to the fact that Curie temperature of CeFe2 (235K) is much lower than that of TbFe2 (704K).
Fig.3 Dependence of curie temperature (TC) on Ce content for Pr0.1CexTb0.9-xFe1.9 alloys
The dependence of magnetization on the applied magnetic field is shown in Fig 4.
Fig.4 Magnetization at room temperature of Pr0.1CexTb0.9-xFe1.9 alloys
The saturation magnetization Ms was taken as the value of Pr0.1CexTb0.9-xFe1.9 alloys at the largest available magnetic field of 800kA/m. It can be clearly seen that Ms decreases with Ce content increasing from 0.2 to 0.4, but abruptly increases when x=0.6. And then Ms drops again when x>0.6. The fact may be related to the valence fluctuation of Ce ions. The moments of the Ce ion with +3 valence and Fe in normal RFe2 are 2.54μB and 1.77μB, respectively[11]. But the Ce and Fe moments in CeFe2 alloy are -0.14μB and 1.17μB, respectively from a polarized neutron study. The lattice parameter show the greatest deviation from Vegards law at x=0.6, and the moment of the Ce ion will increase a lot with the fluctuation tendency to +3 valence, which may contribute to the Ms at room temperature. Table 1 shows the values of Ms, Mr, and Hc.
The dependence of magnetostriction on magnetic field is shown in Fig.5.
Table 1 Saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc)
of Pr0.1CexTb0.9-xFe1.9 alloys
Fig.5 Magnetostriction curves of Pr0.1CexTb0.9-xFe1.9 alloys at room temperature
It can be seen that the magnetostriction decreases with the increasing Ce content when 0.2≤x〈0.4 and x>0.6, but increases abnormally with Ce content increasing from 0.4 to 0.6. According to the single-ion model, the trivalent Ce ions have large magnetostriction, but the CeFe2 compound shows 6.0×10-5 magnetostriction at 4.2K and nearly 0 at the room temperature, which is ascribed to the mix valence (+3.29) of Ce ions and the low Curie temperature of CeFe2. It is known from the dependence of the lattice parameter on Ce content that the valence of Ce ions in Pr0.1CexTb0.9-xFe1.9 alloys is almost unchanged when x〈0.4 and x>0.6. So the rapid decrease in magnetostriction when 0.2≤x〈0.4 and x>0.6 is ascribed to the Ce substitution for Tb, that is the Ce ion dilutes the single-ion magnetostriction of Tb ion. The sudden increase of magnetostriction when 0.4≤x≤0.6 is ascribed to the valence fluctuation of Ce ions towards localized state, which has been confirmed from the change of lattice parameter. It is also found that when x〈0.5, the magnetostriction doesnt saturate at the magnetic field of 950kA/m, while reaches saturation easily when x is larger than 0.6 even at low magnetic field.
REFERENCES
[1]Clark A E, H T Savage. Magnetostriction of Rare-Fe2 compound under compressive stress [J]. Magnetism and Magnetic Materials, 1982, 31-34: 849-851.
[2]Clark A E. Ferromagnetic Materials [M]. Amsterdam: North Holland Publications, 1980. 531-589.
[3]TANG C C, ZHAN W S, LI Y X, et al. Synthesis and magnetic properties of PrxCe1-xFe2 compounds [J]. Phys: Condens Matter, 1997, 9: 9651-9659.
[4]TANG C C, DU J, LI Y X, et al. Magnetostriction in (CexTb1-x)0.5Pr0.5Fe2 compounds [J]. Appl Phys Lett, 1998, 73: 692-694.
[5]LIU H Y, MENG F B, LI S T, et al. Structure, magnetic properties and magnetostriction of Pr0.15Pb0.3-Dy0.55Fe1.85-xSix alloys [J]. Physica B, 2004, 351: 102-105.
[6]ZHAO X G, LI J Y, LIU S C, et al. Magnetic properties and thermal stability of PrFe2 compound [J]. Alloys Comp, 1997, 258: 39-41.
[7]TANG C C, WU G H, LI Y X, et al. The electronic structure and magnetic properties of Ce1-xYxFe2 compounds [J]. Acta Phys Sin, 2001, 50: 132-138.
[8]LI Y X, HAO Y M, WANG B W, et al. Growth and magnetostriction of oriented polycrystalline Pr0.15-TbxDy0.85-xFe2 (x=0-0.85) [J]. IEEE Transactions on Magnetics, 2001, 37: 2696-2698.
[9]Chaboy J, Piquer C, Garcia L M, et al. X-ray absorption spectroscopy study of the instability of ferromagnetism in CeFe2: Effects of Co and Al substitutions [J]. Appl Phys, 2000, 87: 6809-6811.
[10]Fukuda H, Fujii H, Kamura H, et al. Near the collapse of ferromagnetism in CeFe2 under high pressures [J]. Magnetism and Magnetic Materials, 2002, 226-230: 1200-1202.
[11]TANG C C, LI Y X, DU J, et al. Effects of rare-earth substitution in CeFe2: mixed-valence and magnetic properties [J]. Phys: Condens Matter, 1999, 11: 2027-2034.
[12]LI Y X, TANG C C, DU J, et al. Magnetostrictive and magnetic properties of the pseudobinary compounds PrxTb1-xFe2 and Pr0.15TbxDy0.85-xFe2 [J]. Appl Phys, 1998, 83: 7753-7756.
[13]REN W J, ZHANG Z D, ZHAO X G, et al. Structure and magnetostriction of Tb1-xPrxFe1.93B0.15 alloys [J]. Magnetism and Magnetic Materials, 2004, 269: 281-285.
[14]TANG C C, CHEN D F, LI Y X, et al. Magnetic properties in Laves phase CexDy1-xFe2 intermetallics [J]. Appl Phys, 1997, 82: 4424-4427.
[15]TANG C C, ZHAN W S, CHEN D F, et al. Anomalous magnetic properties of cerium ions in the compounds CexR1-xFe2 (R=Tb, Dy) [J]. Phys: Condens Matter, 1998, 10: 2797-2804.
Foundation item: Project(50271023) supported by the National Natural Foundation Science Foundation of China; Project(02017) supported by Key Foundation of Education Ministry
Received date: 2004-11-16; Accepted date: 2005-01-18
Correspondence: LI Yang-xian, Professor; Tel: +86-22-60204342; Fax: +86-22-60202214; E-mail: admat@jsmail.hebut.edu.cn
(Edited by LONG Huai-zhong)
