Magnetic properties of ball-milled FeAl nanograins

Transcription

1 phys. stat. sol. (a) 21, No. 15, (24) / DOI 1.12/pssa Magnetic properties of ball-milled FeAl nanograins L. F. Kiss *, 1, D. Kaptás 1, J. Balogh 1, L. Bujdosó 1, J. Gubicza 2, T. Kemény 1, and I. Vincze 1 1 Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, P.O. Box 49, 1525 Budapest, Hungary 2 Solid State Physics Department, Eötvös University, P.O. Box 32, 1518 Budapest, Hungary Received 27 June 24, accepted 9 November 24 Published online 6 December 24 PACS 75.5.Tt, a, y Ball milled alloys were investigated by X-ray diffraction, magnetic measurements and Mössbauer spectroscopy. No magnetic saturation is observed up to 5 T and neither the magnetization nor the Mössbauer measurements show a well-defined phase transition but a gradual disappearance of the magnetism with increasing temperature. The hyperfine field distributions and the X-ray grain size data enable the identification of Fe atoms with magnetic moments as some atomic layers thick surface of the nonmagnetic nanometric Fe Al grains. The unusual magnetic behaviour is attributed to the large magnetic anisotropy of the low dimensional magnetic surfaces. 1 Introduction The magnetic properties of nanosize objects are greatly influenced by their surface properties [1, 2]. Surface atoms may have significantly different properties (magnetic anisotropies, moments, etc.) from those of the bulk atoms. In real samples made by different preparation techniques (evaporation, ball milling, etc.) the separation of the surface and bulk properties is not simple because they are influenced by impurities, mixing and disordering of the components [3, 4]. In this paper the magnetic properties of ball-milled FeAl will be investigated, where a peculiar magnetic structure consisting of a nonmagnetic volume surrounded by a surface magnetic layer was found [5]. The structure of Fe Al alloys is based on the bcc lattice of α-fe [6] with two ordered phases around the stoichiometric FeAl and Fe 3 Al. FeAl and Fe 3 Al crystallizes to a CsCl-type (B2) and a D 3 -type crystal structure, respectively. FeAl is nonmagnetic, the bcc Fe Al solid solution and Fe 3 Al are ferromagnetic. The magnitude of the iron magnetic moments depends on the number of iron nearest neighbours: 2.2 µ B for five or more nearest Fe neighbours and the iron atoms with less than four nearest Fe neighbours are nonmagnetic [7]. Concentration dependence of the average magnetization yields 1.8 µ B for the saturation magnetic moment of Fe atoms with 4Fe 4Al first nearest neighbours. It is slightly larger than the room temperature value measured for the FeII sites by neutron diffraction [8] in Fe 3 Al (1.5 µ B ). In the stoichiometric ordered B2 structure, antiphase boundaries (APB, i.e. the replacement of an Al plane by an Fe plane) creates magnetic moments on the formerly nonmagnetic iron atoms since the number of the nearest Fe neighbours reaches 4 Fe along the boundary [9]. 2 Experimental The stoichiometric FeAl ingot was prepared by induction melting in a cold crucible. Ball milling was carried out in vacuum in a vibrating frame hardened-steel single ball vessel [3]. The X-ray diffracto- * Corresponding author: kissl@szfki.hu, Phone: (36-1) /1296, Fax: (36-1)

2 3334 L. F. Kiss et al.: Magnetic properties of ball-milled FeAl nanograins grams were measured by a Philips X pert powder diffractometer using CuK α radiation. The grain size of the granules, D were determined from the full widths at half maximum of the X-ray diffraction profiles by the modified Williamson-Hall procedure [1] to be 21 ± 2, 13 ± 2 and 8 ± 1 nm after 1, 1 and 1 hours of ball milling, respectively. In this paper the detailed temperature and magnetic-field dependence of the magnetic properties of the sample ball milled for 1 h will be reported. The bulk magnetization measurements were performed by a Quantum Design MPMS-5S SQUID magnetometer with a maximum field of 5 T. 57 Fe Mössbauer spectra were recorded by a constant acceleration spectrometer between 4.2 and 3 K, and in external magnetic fields using a 7 T Janis superconducting magnet. Standard procedures were used for the evaluation of the spectra: the ordered B2 component was fitted with a single Lorentzian line and after subtracting this curve from the measured spectra, the remaining part was described by binomial distributions [11]. 3 Results and discussion Figure 1 shows the results of the magnetization measurements. The low-field magnetization showing a broad peak as a function of temperature and different behaviour in the ZFC and FC states hints at the freezing of a spin glass. The difference between the ZFC and FC curves persists up to a field of about.1 T (with decreasing peak temperature), at higher fields the ZFC and FC thermomagnetic curves coincide showing no distinct features (Fig. 1b). The temperature dependence of the magnetization curves does not show the transformation to the paramagnetic state. The magnetization cannot be saturated even in 5 T (Fig. 1c) which is expected for spin glasses with antiferromagnetically coupled magnetic moments. The Mössbauer spectra in zero field and 5 T for the 1 h ball-milled FeAl alloy are shown in Fig. 2. At low temperature the single line corresponding to the ordered nonmagnetic phase is absent but according to XRD the crystal structure is bcc with slightly increased lattice parameter. There appears a broad, magnetic component which has a double-peaked hyperfine field (hf) distribution (Fig. 3a). Fe hyperfine fields with similar structure were observed in off-stoichiometric B2 ordered Fe Al alloys and the highfield peak (shaded area in Fig. 3a) was attributed to Fe atoms with magnetic moment [12]. The low-field 1. FeAl 1h.8.6 FC 1 Oe.4.2. ZFC T(K) T 3 FC 3T 2 1T ZFC 1.1T a) T(K) b) K 5 K K 2 K 3 K B (T) Fig. 1 Low-field (in 1 Oe) behaviour of the 1 h ball-milled FeAl alloy after zero-field (ZFC, continuous line) and field (FC, broken line) cooling (a), temperature dependence of the magnetization in different fields (b), and magnetization vs. external field at different temperatures (c). c)

3 phys. stat. sol. (a) 21, No. 15 (24) / T 4.2K 11.5K 5T Fig. 2 Temperature dependence of the Mössbauer spectra for 1 h ball-milled FeAl in zero and 5 T field. The positions of the second and fifth lines for the high field peak are marked for 4.2 K. Full, broken and dotted lines are fitted curves of the full spectra, the magnetic and paramagnetic components, respectively. 2Γ is the full width at half maximum of the paramagnetic line. 2K 5K 1K 2K (a) 2Γ 3K (b) velocity [mm/s] part of the hf distribution is related to nonmagnetic Fe atoms with magnetic neighbours, i.e. it is due to conduction electron polarization. Examining the grain size dependence of the fraction of the magnetic Fe atoms, p M (which is the spectral weight of the shaded area in Fig. 3a) it has been shown recently that the magnetic iron atoms mostly belong to grain boundaries in the ball-milled samples [5]. The average hf of the high-field part of the distribution, B m (Fig. 3b) is attributed to Fe atoms with 4Fe 4Al nearest neighbours. It is somewhat lower than the hf of the FeII sites in Fe 3 Al (23.4 T, extrapolated from the room temperature value [13]), which is explained by the lower magnetic moment of the neighbouring Fe atoms. p(b) (1-2 T -1 ) T 5 T 2 4 (a) B (T) (T) B m (T) B + m (5T) T (K) (b) Fig. 3 Hyperfine distributions for 1 h ballmilled FeAl alloy in zero and 5 T field at 4.2 K (a). Shadowing marks the hyperfine field of Fe atoms with localized moments. Temperature dependence of the hf of the localized Fe magnetic moments in zero field (B m, dots) and in 5 T (B m +, circles) (b).

4 3336 L. F. Kiss et al.: Magnetic properties of ball-milled FeAl nanograins 2Γ [mm/s] Fig. 4 Full width at half maximum (2Γ) of the paramagnetic line in the Mössbauer spectra as a function of temperature for 1 h ball-milled FeAl. Empty circles mark the values for the well-ordered nonmagnetic B2 structure which is obtained after annealing the ball-milled sample at 72 K. Continuous and broken lines are guides to the eye T (K) 3 The gradual magnetic transition and the great influence of the external field on this transition is evidenced by the temperature dependence of these spectra (Fig. 2) in zero field and in 5 T. The magnitude of B m, i.e. that of the Fe magnetic moment decreases linearly with temperature (Fig. 3b). The temperature dependence of the full width at half maximum (2Γ) of the paramagnetic line (Fig. 4) shows no well defined transition temperature. The low-field peaks of the bulk magnetization (Fig. 1a) are not reflected at all in the Mössbauer data. It is remarkable that superparamagnetic relaxation starts already at 5 K. Even small magnetic fields influence greatly the magnetic state of these alloys. In line with the non-saturating field-dependent magnetization (Fig. 1c), highly anisotropic behaviour is shown by the Mössbauer spectra in applied magnetic fields (Fig. 2). Information on the direction of the Fe magnetic moments is given by the relative intensity of the second and fifth lines, I 2,5 of the spectra. In 5 T magnetic field (as seen in Fig. 2) I 2,5 significantly decreases from I 2,5 = 2 (random orientation of the magnetic moments) observed in zero field. It signals the increased alignment of the canted moments under the influence of the external field. A decreasing width of the hf field distribution with increasing field was also found, as it is expected for a strong anisotropy dominated ferromagnet. If a linear extrapolation of the magnetic field dependence were assumed, saturation, i.e. perfect collinearity would be reached in about 14 T. This value is substantially larger than the characteristic (<1 T) magnetic anisotropy field of the related bcc alloys. The value of the average magnetic moment, µ Fe obtained from the magnetization measurements is related to p M (magnetic fraction), µ Fe (moment of the magnetic iron atoms) and θ (angle between magnetic moment and applied field) as µ Fe = µ Fe p M cos θ. All these quantities depend on the applied field and temperature, explaining the peculiar temperature behaviour of the average magnetization (Fig. 1). 4 Conclusion Ball milling of FeAl results in the formation of a magnetic shell around the nonmagnetic grain containing about two adjacent iron layers. The behaviour of the system is ferromagnetic but saturation is not reached even in 5 T external field because of the large magnetic anisotropy of these layers. Acknowledgements This work was supported by the Hungarian Research Fund (OTKA T31854). References [1] R. Skomski, J. Phys.: Condens. Matter 15, R841 (23). [2] P. Grünberg, Physics Today, May 21, p. 31. [3] J. Balogh, L. Bujdosó, D. Kaptás, T. Kemény, I. Vincze, S. Szabó, and D. L. Beke, Phys. Rev. B 61, 419 (2). [4] J. Balogh, D. Kaptás, T. Kemény, I. Vincze, and G. Radnóczi, Phys. Rev. Lett. 82, 415 (1999).

5 phys. stat. sol. (a) 21, No. 15 (24) / [5] L. F. Kiss, D. Kaptás, J. Balogh, L. Bujdosó, T. Kemény, I. Vincze, and J. Gubicza, Phys. Rev. B 7, 1248 (24). [6] O. Kubaschewski, Iron-Binary Phase Diagrams (Springer, Berlin, 1982), p. 5. [7] T. M. Srinivasan, H. Claus, R. Viswanathan, P. A. Beck, and D. Bardos, Phase Stability in Metals and Alloys, edited by Rudman (Stringer and Jaffe, McGraw-Hill, N. Y., 1967), p [8] S. J. Pickart and R. Nathans, Phys. Rev. 123, 1163 (1961). [9] R. Besson, A. Legris, and J. Morillo, Phys. Rev. B 64, (21). [1] T. Ungár and A. Borbély, Appl. Phys. Lett. 69, 3173 (1996). [11] I. Vincze, Nucl. Instrum. Methods 199, 247 (1982). [12] G. P. Huffman, J. Appl. Phys. 42, 166 (1971). [13] M. B. Stearns, Phys. Rev. 168, 588 (1968).