ALBERTO BOLLERO REAL

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4.3 PrFeB In addition to the mentioned advantages, at present, Pr is cheaper than Nd (about 2/3 of the price) resulting in lower costs of the final magnet. Table 4.2 summarises the intrinsic magnetic properties and important microstructural parameters of Nd 2 Fe 14 B and Pr 2 Fe 14 B and, for comparison, α-Fe. Figure 4.6 shows the evolution of the domain wall width, δ w , with temperature. Compound T C (K) µ 0 H a (T) K 1 (MJm -3 ) J s (T) δ w (nm) d c (µm) reference Nd 2 Fe 14 B 585 6.7 5 1.60 4.2 0.3 [20,37] Pr 2 Fe 14 B 565 8.7 5 1.56 ∼4 ∼0.3 [16,83] α-Fe 1043 - - 2.16 30 - [84] Table 4.2: Intrinsic magnetic properties (Curie temperature T C , anisotropy field H a = (2K 1 + 4K 2 )/J s , anisotropy constant K 1 , saturation polarisation J s ), domain wall width δ w and critical single-domain particle size for magnetically uniaxial materials d c = 72µ 0 (AK 1 ) 1/2 / J s 2 of Nd 2 Fe 14 B and Pr 2 Fe 14 B. α-Fe has been added for comparison. Fig. 4.6: Calculated domain wall width δ w in Nd 2 Fe 14 B, Pr 2 Fe 14 B and Dy 2 Fe 14 B (from Givord and Rossignol [18]). 33
5 Experimental details The different techniques used in the present work will be shortly explained in this Section. Additionally, specific experimental details will be given. The first part will focus on three different powder processing techniques: melt-spinning, high energy ball milling and reactive milling in hydrogen. The resulting magnets will be isotropic due to a randomly oriented grain structure. The hot deformation process will be shown as a way of producing highly dense anisotropic magnets. Characterisation of the materials after the different processing steps is fundamental in order to evaluate the efficiency of the chosen parameters during the experiment and, at the end, the quality of the resulting magnet. The employed techniques have been grouped into, first, those related to structural characterisation and, second, those ones necessary for characterisation of magnetic properties. 5.1 Nanocrystalline powder processing In the following a brief summary of the different methods of processing alloys into particles with submicron or smaller grains is given. The aim of these processes is to create, at the end, a microstructure consisting of crystallites which are in size equal to or even significantly smaller than the corresponding single-domain size and, therefore, can resist effectively the formation and propagation of domain walls on reversal magnetisation and/or can show an effective exchange-coupling. “Nanocrystalline” is usually defined by grain sizes ≤ 50 nm. In the following this term will be used for a broader range in grain sizes (< 500 µm). 5.1.1 Melt-spinning Melt-spinning is one of the principal fabrication routes for magnetic materials [37]. It involves ejection of a molten starting alloy through a crucible orifice onto the surface of a rapidly rotating copper wheel, to form a fine ribbon. The ribbon is either amorphous (overquenching) or is composed of many randomly oriented nanocrystallites (direct-quenching) [85-88]. The process is carried out in an inert atmosphere (usually argon) because the rare- 34
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 and 38: 3.4 Development of exchange-coupled
Page 39 and 40: 4 Systems under investigation 4.1 N
Page 41 and 42: 4.2 NdCoB 4.2 NdCoB The tetragonal
Page 43: 4.3 PrFeB e 1 p 1 Fig. 4.4: Liquidu
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
Page 65 and 66: 6.4 Hot workability necessary for N
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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