ALBERTO BOLLERO REAL

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3.4 Development of exchange-coupled nanocrystalline magnets Hirosawa et al [56] obtained the critical dimension of about 10 nm for a mutually coupled Fe 3 B/Nd 2 Fe 14 B mixture with melt-spun (Nd,Dy) x Fe 80.5-x-y Co y B 18.5 M 1 (3 ≤ x ≤ 5 and 0 ≤ y ≤ 5) alloys where M represents additives (Al, Si, Cu, Ga, Ag, Au) used to reduce the grain size. In this way, a coercivity of 0.6 T and a remanence of 0.98 T were obtained in Nd 3 Dy 2 Fe 70.5 Co 5 Ga 1 B 18.5 . Ping et al [57] argumented that Co and Ga atoms control the grain growth of Fe 3 B precipitates and contribute to the formation of the Fe 3 B/Nd 2 Fe 14 B nanocomposite with ultrafine grains, which leads to a high coercivity and an improved maximum energy product. Uehara et al [58] explained how the addition of Cr contributes to the magnetic properties of these nanocomposite magnets produced by melt-spinning, providing a high coercivity. This is in good agreement with a previous study performed by Hirosawa and Kanekiyo [59] on Fe/Nd 2 Fe 14 B nanocomposite magnets produced from melt-spun Nd-Fe- B-Co-Cr alloys with low-Nd and high-B concentration; they obtained magnetic properties between (J r ,µ 0 H c ) = (1.20 T, 0.43 T) and (0.86 T, 0.77 T), the large coercivity being due to a refinement of microstructure and an increase in the volume fraction of the hard magnetic Nd 2 (Fe,Cr) 14 B phase produced by the partial replacement of Fe by Cr. Bauer et al [60] studied the relationship between the magnetic properties and the Fe/Nd 2 Fe 14 B volume ratio, finding that a composite magnet containing 30 vol% bcc-Fe exhibits a maximum remanence of J r = 1.25 T and an energy product of (BH) max = 185 kJm -3 . Melsheimer et al [61] compared the magnetic properties of three different Nd-Fe-Co-B magnets produced also by the melt-spinning procedure: a nanocrystalline exchange-coupled Nd 2 (Fe 1-x Co x ) 14 B single-phase magnet, Nd 2 (Fe 1-x Co x ) 14 B + (Fe 1-x Co x ) composite magnets and a decoupled Nd 2 (Fe 1-x Co x ) 14 B magnet with an overstoichiometric Nd content. As it was expected, they observed an enhanced remanence and a decrease in coercivity for both exchange-coupled magnets in comparison to the decoupled material. They obtained a uniform grain size (22- 25 nm) for the single phase magnet, and a mixture of very small grains of a soft magnetic phase (< 20 nm) and small hard magnetic grains ( ≈ 35 nm) for the composite magnet. The single-phase and the composite magnets exhibited maximum energy products of up to 175 kJm -3 , at T = 300 K, whereas only 93 kJm -3 was obtained for the decoupled magnet. In Fig. 3.6, hysteresis loops of a decoupled and an exchange-coupled magnet with compositions Nd 15 (Fe 57.5 Co 18.5 )B 9 and Nd 12 (Fe 62 Co 20 )B 6 , respectively, are reproduced from that work. 25
3.4 Development of exchange-coupled nanocrystalline magnets Fig. 3.6: Hysteresis loops of a decoupled (squares) and an exchange-coupled (circles) magnet (from Melsheimer et al [61]). In a study about nanocomposites of Fe/Nd 2 Fe 14 B produced by mechanical alloying, Crespo et al [62] reported values of µ 0 H c ≈ 0.37 T and (BH) max ≈ 92 kJm -3 for Nd 8 Fe 88 B 4 - based samples after optimum heat treatment, exhibiting average grain sizes of 33 nm and 28 nm for Nd 2 Fe 14 B and bcc-Fe, respectively, whereas the addition of Zr (or Si) results in a refinement of the microstructure with average grain sizes of 20 nm (21 nm) and 19 nm (21 nm) for Nd 2 Fe 14 B and bcc-Fe, respectively. This leads to improved magnetic properties: µ 0 H c ≈ 0.44 T and J r ≈ 1.03 T with Zr addition, and µ 0 H c ≈ 0.40 T and J r ≈ 1.06 T with Si addition (both of them after heat treatment at T = 650°C). Neu et al [63] performed a study about nanocrystalline single-phase Nd-Fe-B powders produced by intensive milling and heat treatment, obtaining a remanence of J r = 0.92 T and a maximum energy product of (BH) max = 132 kJm -3 for a microstructure with an average grain size of about 30 nm. Further Co addition improved the remanence to J r = 0.95 T, and the energy density to (BH) max = 140 kJm -3 , due to a smaller grain size of 21 nm. Recently, several experimental studies have focused on optimising the mean grain size of the soft magnetic phase in order to achieve full exchange-coupling by the hard phase [16,64-66]. Goll et al [16] obtained a remanence of J r = 1.17 T and a maximum energy product of (BH) max = 180 kJm -3 in a nanocomposite Pr 2 Fe 14 B-based magnet containing 30 vol% α-Fe produced by melt-spinning. The average grain sizes of the hard and the soft magnetic grains were about 20-30 nm and 15 nm, respectively. 26
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: 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 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
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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
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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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