ALBERTO BOLLERO REAL

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3.4 Development of exchange-coupled nanocrystalline magnets Nanocrystalline SmCo-based magnets have been widely studied since both SmCo 5 and Sm 2 Co 17 have higher Curie temperatures than other rare-earth transition-metal compounds used for permanent magnets and have many potential high-temperature applications [67-70]. Yan et al [71,72] have performed a systematic study of exchangecoupled nanocomposite SmCo 5 / Sm 2 Co 17 powders synthesised by intensive milling and subsequent annealing of as-cast alloy. A high coercivity of 2.40 T and a relative remanence J r / J s = 0.73 were obtained for a sample containing 40% SmCo 5 / 60% Sm 2 (Co 0.74 Fe 0.1 Cu 0.12 Zr 0.04 ) 17 [72]. 27
4 Systems under investigation 4.1 NdFeB The stoichiometric composition of the magnetically highly anisotropic Nd 2 Fe 14 B compound is Nd 11.8 Fe 82.3 B 5.9 . The lattice symmetry is tetragonal (space group P4 2 /mnm) with 68 atoms per unit cell. There are six crystallographically distinct transition metal sites, two different rare-earth positions, and one boron position (Fig. 4.1). Magnetisation curves measured at 300 K along different crystallographic directions show that the easy magnetisation direction is the c axis (right hand side in Fig. 4.1). Fig. 4.1: (Left) tetragonal unit cell of the Nd 2 Fe 14 B structure (from Herbst et al [73]); (right) magnetisation curves for single-crystal Nd 2 Fe 14 B at T = 300 K (from Chikazumi [1]). The intrinsic magnetic properties of Nd 2 Fe 14 B are: µ 0 H a = 6.7 T, J s = 1.60 T and T C = 312 o C [37,74]. Thus, the upper limit of (BH) max is 516 kJm -3 . Figure 4.2 shows the liquidus projection of the ternary Nd-Fe-B system. There are three stable ternary phases: Nd 2 Fe 14 B (φ), Nd 1+ε Fe 4 B 4 (η) and Nd 5 Fe 2 B 6 (ρ). The peritectic formation of the φ phase takes place at 1180 o C but it will not be complete in a standard casting process because here γ-Fe is the primary solidification product. 28
Page 1 and 2: Alberto Bollero Real Isotropic Nano
Page 3 and 4: Die vorliegende Dissertationsschrif
Page 5 and 6: when varying (i) the α-Fe content,
Page 7 and 8: liefert wichtige Informationen übe
Page 9 and 10: 6.1.2. Milled and melt-spun alloys
Page 11 and 12: µ 0 H no nucleation field [T] µ 0
Page 13 and 14: 1 Introduction J s = 0.5 can not be
Page 15 and 16: 2 Magnetism of R-T intermetallic co
Page 17 and 18: 2.2 Magnetic properties 2.2 Magneti
Page 19 and 20: 2.2 Magnetic properties classified
Page 21 and 22: 2.2 Magnetic properties sublattice
Page 23 and 24: 2.2 Magnetic properties where the m
Page 25 and 26: 2.2 Magnetic properties highly stru
Page 27 and 28: 3 The exchange-coupling mechanism A
Page 29 and 30: 3.1 Models enhancement to those obt
Page 31 and 32: 3.2 Wohlfarth’s relation and Henk
Page 33 and 34: 3.3 Magnetisation reversal processe
Page 35 and 36: 3.4 Development of exchange-coupled
Page 37: 3.4 Development of exchange-coupled
Page 41 and 42: 4.2 NdCoB 4.2 NdCoB The tetragonal
Page 43 and 44: 4.3 PrFeB e 1 p 1 Fig. 4.4: Liquidu
Page 45 and 46: 5 Experimental details The differen
Page 47 and 48: 5.1 Nanocrystalline powder processi
Page 49 and 50: 5.2 Structural characterisation and
Page 51 and 52: 5.2 Structural characterisation whe
Page 53 and 54: 5.3 Characterisation of magnetic pr
Page 55 and 56: 6.1 Thermal characterisation The te
Page 57 and 58: 6.2 Microstructural characterisatio
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Page 67 and 68: 6.5 Summary advantage of the Pr-con
Page 69 and 70: 7.1 Characterisation of the base al
Page 77 and 78: 7.3 Summary Magnetisation ( arb. un
Page 79 and 80: 8.1 Initial magnetisation processes
Page 81 and 82: 8.1 Initial magnetisation processes
Page 83 and 84: 8.2 Recoil loops at room temperatur
Page 85 and 86: 8.3 Analysis of exchange-coupling u
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8.3 Analysis of exchange-coupling u
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8.4 Summary An analysis of the init
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Fig. 9.1: Dependence of the lattice
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9.1 Reactive milling of a PrFeB all
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9.2 Reactive milling of Nd(Fe,Co)B
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9.3 Summary of J r = 0.91 T after a
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10 Conclusions and future work 10 C
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10 Conclusions and future work the
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10 Conclusions and future work spin
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10 Conclusions and future work (Mö
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REFERENCES [13] O. Gutfleisch, K. K
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REFERENCES [37] J.J. Croat, J.F. He
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REFERENCES [60] J. Bauer, M. Seeger
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REFERENCES [83] S. Hirosawa, Y. Mat
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REFERENCES [106] R.W. Lee, “Hot-p
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List of publications: Some parts of
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Acknowledgments The realisation of
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ALBERTO BOLLERO REAL

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