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Structural and magnetic properties of Pr2Co7 powders

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2012 IOP Conf. Ser.: Mater. Sci. Eng. 28 012014 (http://iopscience.iop.org/1757-899X/28/1/012014) View the table of contents for this issue, or go to the journal homepage for more

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MATERIAUX 2010 IOP Conf. Series: Materials Science and Engineering 28 (2012) 012014

IOP Publishing doi:10.1088/1757-899X/28/1/012014

Structural and magnetic properties of Pr2Co7 powders R. Fersi a,b 1, N. Mlikia and L. Bessaisb a Laboratoire Matériaux Organisation et Propriétés, Faculté des Sciences de Tunis, Université Tunis El Manar, 2092 Tunis, Tunisie. b CMTR, ICMPE, UMR7182, CNRS − Université Paris 12, 2-8 rue Henri Dunant F94320 Thiais, France. E-mail: [email protected] Abstract. The Pr2Co7 intermetallic compounds are known to crystallize in two polymorphic forms. A hexagonal of the Ce2Ni7 type structure and a rhombohedral of the Gd2Co7 type structure were obtained by mechanical milling and subsequent annealing in high vacuum. The highest intrinsic coercivity value 18 kOe at room temperature and magnetization ratio (M r/Mmax = 0.64) were determined for alloys milled during 5 h and annealed at 800°C for 30 min. The enhanced remanence (Mr/Mmax > 0.50) suggests strong exchange interaction between the adjacent crystallites. This is in good agreement with the small average crystallite size (D ≈ 50 nm). The high magnetic properties observed in these nanostructured Pr2Co7 intermetallic alloys have their origin in its relatively high uniaxial magnetocrystalline anisotropy field, and in the homogeneous nanostructure developed by mechanical milling process and subsequent annealing.

1. Introduction The study of magnetic properties of rare earth (R)-3d transition metals (M) intermetallics has been a topic of great interest both from a fundamental point of view as applied one. This interest is explained by the exceptional properties of these alloys which arise from the presence, in the same compounds, of the outer well delocalized 3d electrons and well localized and 4f electronic shell [1]. This combination of R and M elements can give rise to materials exhibiting high Curie temperature, given by the strong exchange interaction between 3d electrons of M metals, and a strong anisotropy provided by the rare earth 4f electrons. Among the family of intermettallics, R2Co7 (R= rare earth element) have attracted interest due to their applications as permanent magnets. They exhibit excellent magnetic properties such as high Curie temperature Tc, large magnetocrystalline anisotropy HA, and important saturation magnetization Ms [2, 3]. In the present work, we discuss the structural and magnetic properties of Pr 2Co7 powders produced by high-energy milling which we have optimized the grain size in order to obtain a high coercivity. 2. Experiment Pr2Co7 samples with praseodymium excess, were prepared by the technique of high energy ball milling and subsequent annealing from 600 °C to 1050 °C during 30 min. X-ray diffraction with Cu radiation has been carried out on a Brucker diffractometer with an internal Si standard to insure a unit 1

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cell parameter accuracy of ± 1 ×10-3. The intensities were measured from 2=20° to 110° with a step size of 0.04° and counting time of 22 s per scanning step. The structure refinement was performed by the Rietveld technique supported, in the assumption of Thompson-Cox- Hastings line profile, by the FULLPROF computer code. The detailed information was given in Ref. [4]. The magnetization was measured using a differential sample magnetometer (MANICS). When necessary, the samples were sealed in silica tubes to avoid oxidation upon heating. Thermomagnetic data were measured under an applied field of 1000 Oe with a heating rate of 10 K/min. The Curie temperature TC was determined from the M-T curves by extrapolating the linear part of the M-T curve and finding the temperature values of the intersection with the extended baseline [5,6]. For magnetic hysteresis measurements at T = 293 K, we used a Physical Properties Measurement System (PPMS) Quantum Design equipment in fields up to 9 kOe. 3. Results and discussion 3.1. Structure analysis Lets note that the Pr2Co7 compound was found to crystallize in two polymorphic forms: a high temperature phase modification of the hexagonal Ce2Ni7 structure and the low temperature phase modification of the rhombohedral Gd2Co7 type structure [7], can be obtained by stacking the hexagonal structural blocks for PrCo5 (structure of the CaCu5 type) and the cubic blocks PrCo2 (structure of the MgCu2) type along the common hexagonal (trigonal for PrCo2) axis.

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Figure 1. Figure Rietvield analysis for X-ray diffraction pattern of Pr2Co7 annealed at 750 °C for 30 min. Fig.1 presents, as an example, the analysis result of XRD pattern of Pr 2Co7 sample annealed at 750°C using Rietveld analysis. The result of the structure refinement performed for the Pr 2Co7 alloys show the presence of main phase with the hexagonal structure. The lattice constants were a = 5.078 Å and c = 24.471 Å. This result suggests that the Pr2Co7 structure is isotopic with the crystal structure of Ce2Ni7 described by Cromer and Larson [8]. When the annealing temperature was increased from 700°C to 1050°C, we have seen emerging the Pr2Co7 rhombohedral phase at the expense of the hexagonal phase of Pr2Co7. However, we note minor quantities of the oxide phases appearing due to selective oxidation of Pr (PrO, PrO2, Pr2O3). For the sample annealed at 1050 °C, the structure

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refinement show the presence of main phase with the rhombohedral Pr 2Co7 alloys. The lattice constants were a = 5.070 Å and c =36.691Å. A comparison with X-ray reflexions of the Gd2Co7 compound showed that Pr2Co7 and Gd2Co7 are isotopic. The structure of Gd2Co7 has been described by Bertaut, Lemaire and Schweizer (1965) [9]. 3.2. Intrinsic magnetic properties: Curie temperature and magnetic moments The Curie temperature, also called the magnetic ordering temperature, is a direct measure of the exchange interaction, which is the origin of ferromagnetism. This interaction depends manifestly on the interatomic distance. Generally, the Curie temperature in rare-earth transition-metal intermetallic compounds is governed by three kinds of exchange interactions, namely, the 3d-3d exchange interaction (JCo-Co) between the magnetic moments of the Co sublattice, 4f - 4f exchange interaction (JPr-Pr) between the magnetic moments within the Pr sublattice and the intersublattice 3d-4f exchange interaction (JPr-Co) [10]. The interactions between the rare earth spins 4f-4f are assumed to be weak and negligible in comparison with the other two types of interactions [11]. The thermal variation of magnetic susceptibility obtained at 293 K for the two samples, indicate that Pr2Co7 phase is ferromagnetic with a Curie temperature around 600 K. These results are in good agreement with literature [10,12]. However, the low temperature in Pr2Co7 compounds is due to the predominance of the Co-Co interactions. 3.3. Extrinsic magnetic properties: coercivity The hysteresis loops of Pr2Co7 have been measured at T = 293 K (see Fig.2). One can see that for annealing temperature (Ta) equal to 800°C, the coercive field rich 18 kOe. It is well established that the coercivity decreases with increasing grain size i.e. with increasing annealing temperature, which is the reason of the relatively small HC value for Ta = 1050°C.

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Figure 2. Magnetization vs 0 H of Pr2Co7 annealed at 800°C, measured at 293 K. We have obtained for Ta = 293 K, Hc = 18 kOe, Mr = 41.5 uem/g and Mr/Mmax= 0.64. Enhanced remanence ratio (Mr/Mmax > 0.50) suggests magnetic exchange interactions between adjacent crystallites in these nanostructured powders [13,14]. The high magnetic properties observed in these nanostructured Pr2Co7 intermetallic alloys might have their origin in their relatively high uniaxial magnetocrystalline anisotropy field and in the homogeneous nanostructure developed by mechanical milling process and subsequent annealing.

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4. Conclusion Depending on the annealing temperature, we have shown that, by mechanical milling we have elaborated Pr2Co7 intermetallic compounds in two polymorphic forms a hexagonal of the Ce 2Ni7 type structure and a rombohedral of the Gd2Co7 type structure. This compounds is a highly anisotropie uniaxial ferromagnet with the easy magnetization direction parallel to the c-axis. No saturation reached at room temperature for magnetic applied field of 90 kOe. High coercivity of about 18 kOe at 293K is obtained after 5h of mechanical-milling and annealed at 800°C for 30 min. Pr2Co7 alloys are good candidate for applications as permanent magnet. References [1] Deryagin A. V 1979 J. Phys. C 5 165. [2] Buschow K. H. J, Less-Common J 1973 Met. 33 311. [3] Kirchmayr H and Polldy C 1978 J. Magn. Magn. Mater. 8 l. [4] Rietveld H 1967 Acta Crystallogr. 22 151. [5] Bessais L, Djega-Mariadassou C, and Beaunier P 2006 J. Appl. Phys. 99 093906. [6] Khazzan S, Mliki N and Bessais L 2009 J. Appl. Phys. 105 103904. [7] Khan Y 1974 Acta Crystallogr. 30 1533. [8] Cromer and D. T, Larson A. C 1959 Acta Crystallogr. 12 855. [9] Bertaut E. F, Lemaire R, Schweizer J 1965 Bull. Soc. Fr. Mineral. Cristallogr. 88 580–585. [10] Buschow K. H. J 1977 Rep. Prog. Phy. 40 1179. [11] Givord D and Lemaire R 1974 IEEE Trans. Magn. 10 109. [12] Zhaoy X, Zhang Z, Yao Q, Liu J, Ren W, Lui W and Geng D 2006 J. Mater . Sci . Technol. 22 843. [13] Fischer R and Kronmüller H 1996 Phys. Rev. B 54 7284. [14] Fischer R and Kronmüller H 1999 J. Magn. Magn. Mater. 191 225.

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