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Magnetic Materials: Hard
Magnetic Materials: Hard The prerequisite for permanent magnet materials that owe their hard magnetic properties to the presence of magnetocrystalline anisotropy is a crystal structure of less than cubic symmetry. The tetragonal structure of the AuCu type (L1 ) is a well-known example able to ! generate uniaxial magnetic anisotropy in compounds in which the magnetic moments are carried by cobalt, iron, or manganese atoms. As illustrated in the lefthand (or right-hand) part of Fig. 1, the unit cell of this structure can be obtained by atomic ordering of the two components (filled and open circles), an atomic ordering that leads to a small contraction of the c-axis. As discussed below, the attainment of this tetragonal structure for CoPt and FePt alloys is metallurgically different from that for MnAl alloys. The permanent magnets based on compounds of this tetragonal crystal structure are rather expensive for CoPt and FePt owing to the high price of platinum metal. The application of the latter magnets is therefore a restricted one. The MnAl magnets are low-cost, lowperformance magnets, but their widespread application suffers from the competition of hard ferrite magnets (see Alnicos and Hexaferrites).
1. Permanent Magnets based on Noble Metal Compounds At high temperatures the platinum–cobalt phase diagram is characterized by a range of complete solid solubility. The crystal structure is a disordered f.c.c. structure over the whole solid solubility range, in which cobalt and platinum atoms statistically occupy the crystallographic sites. This high-temperature phase is practically useless for permanent magnet applications because it does not show a sufficiently high magnetic anisotropy. Structural transformations take place, however, when the rapidly cooled solid solution Co VxPtx alloys are annealed at lower temperatures. The" f.c.c. phase gives rise to a disorder–order trans-
Figure 1 Schematic representation of an array of unit cells of the tetragonal AuCu structure type in which one component occupies the basal plane positions and the other component the equatorial plane positions. Switching of positions can lead to an antiphase boundary visible in the central part.
formation for comparatively high platinum concentrations (x 0.75) whereas for relatively low platinum concentrations (x 0.23) it transforms into a h.c.p. structure. The applicability of Co VxPtx alloys as " on the transpermanent magnet materials is based formation of the disordered f.c.c. phase to an ordered face-centered tetragonal (f.c.t.) phase occurring for alloys in the intermediate concentration range (40 x 75). Only the latter phase has a sufficiently high uniaxial magnetic anisotropy, the easy magnetization direction being along the tetragonal c-axis. The origin of the magnetic anisotropy and the role played by the orbital moment in transition metal compounds in CoPt has been explained by means of first principles band structure calculations using the local spin density approximation. For a more detailed discussion of this topic the reader is referred to the review by Buschow (1997). The Curie temperature of these materials is around 500 mC for the equiatomic composition and slightly decreases with platinum concentration. The maximum of the magnetic anisotropy energy EA l K j2K is found at the equiatomic composition, " the #maximum of the anisotropy field H l although A (2K j4K )\Ms is located at slightly higher x values " # because of the fairly strong decrease of the saturation magnetization, Ms, with x. For obtaining optimum values for the coercivity slightly higher platinum concentrations than the equiatomic composition are desirable because it is the anisotropy field rather than the anisotropy energy that determines the coercivity (see Coerciity Mechanisms). However, the energy product depends very strongly on the magnetization. For this reason, concentrations close to the equiatomic composition are generally chosen for practical applications in permanent magnets. As described in detail elsewhere (see Coerciity Mechanisms) high coercivities are usually obtained in magnet bodies composed of an assembly of sufficiently fine particles. CoPt alloys are known to have outstanding mechanical strength and for this reason the powder metallurgical route commonly used for the attainment of high coercivities in many other permanent magnet materials (see Magnets: Sintered) is not applicable here. In these alloys one may profit, however, from particulars of the phase diagram because the presence of the f.c.c.-to-f.c.t. phase transition provides ample means of obtaining sufficiently small f.c.t. particles. This is the reason that the generation of coercivity in CoPt alloys is a matter of controlling the nucleation and growth of f.c.t. particles during the phase transformation. This process involves heat treatments of the ingots under carefully selected conditions. Kaneko et al. (1968) have shown that the coercivity passes through a maximum when CoPt alloys are heat treated for variable aging times at temperatures sufficiently below the f.c.c.–f.c.t. transformation temperature, the optimum annealing temperature being around 680 mC. Furthermore, it has been found that 1
Magnetic Materials: Hard even better results are obtained when the first aging step is followed by a second aging step, provided the first aging step is kept sufficiently short to prevent overaging. The first annealing step is regarded as leading to the formation of a fine precipitate of the f.c.t. phase in the f.c.c. matrix, whereas the second step is held responsible for an increase of the magnetocrystalline anisotropy of the f.c.t. phase. This increase is probably associated with a more perfect atomic ordering of the cobalt and platinum atoms in the f.c.t. grains. By applying this two-step annealing process, coercivities can be obtained of greater than 700 kA mV" for alloys annealed first at 680 mC and subsequently at 600 mC for variable times. The corresponding energy products of the ingot magnets can reach values around 100 kJ mV$. The origin of the coercivity in f.c.t. CoPt alloys has been studied (Zhang and Soffa 1994) by means of Lorentz microscopy. It has been shown that the occurrence of antiphase boundaries (APB) is essential for the understanding of the coercivity in these materials. It is mentioned above that the f.c.t. phase precipitates from a disordered f.c.c. matrix phase on annealing. This phase transformation proceeds by means of a nucleation and growth mechanism, starting at a multitude of different locations in the alloy. The coalescence of the ordered regions during the growth process invariably leads to the occurrence of a large number of APBs. The atomic arrangement at APBs is shown schematically in Fig. 1. Zhang and Soffa (1994) showed by means of electron microscopy that the microstructure of fully ordered alloys can be characterized by profuse micro- and macrotwinning, leading to a dense net of APBs within the micro- and macrotwins. It is assumed that the coercivity is controlled by domain wall pinning at APBs and stacking faults. Behavior fairly similar to that described above for Co VxPtx alloys is found also for Fe VxPtx alloys. High " " obtained when coercivities for Fe VxPtx alloys are " alloys around the equiatomic composition are first homogenized at high temperatures and subsequently annealed at temperatures below the order–disorder transition temperature. Tanaka et al. (1997) made a fairly detailed investigation of the microstructures of heat-treated Fe VxPtx alloys and the resulting coerciv" that composition control is essential ities. It was shown for the attainment of high coercivities. In as-quenched alloys optimum coercivities are reached for 39.5 at.% platinum because the disordered f.c.c. phase disappears during quenching and can no longer act as nucleation centers for domain walls. Tanaka et al. (1997) further showed that alloys with optimum coercivities are reached after heat treatments in which the local stress associated with the cubic-to-tetragonal transformation is not yet released by twin formation, and the f.c.t. phase is still present in the form of nanoscale antiphase domains. The antiphase domain boundaries between the single-domain particles act as 2
pinning sites for domain wall motion and in this way generate high coercivities. An overlong aging treatment leads to particle growth so that these are no longer single-domain particles. Consequently, the coercivity is reduced. Investigations by Watanabe (1991) have made it clear that permanent magnets with hard magnetic properties even superior to those of CoPt alloys can be obtained for FePt alloys to which a small amount of niobium is added. The corresponding ingots are first homogenized at 1325 mC and then quenched in water. The desired high coercivity is obtained after a subsequent isothermal annealing treatment performed in a temperature range of 600–700 mC. The iron moments are somewhat higher than the cobalt moments in the iron alloys. This implies also that the remanences in the iron alloys are higher than in the cobalt alloys. This, in turn, leads to larger maximum energy products with (BH )max values higher than 160 kJ mV$. The platinum-based magnets are extremely expensive owing to the fact that they consist of roughly 75 wt.% platinum. Advantages of these magnets are that they can be produced as ingot magnets, avoiding the complicated powder metallurgical manufacturing route. The magnets are of high mechanical strength and of unequalled corrosion resistance. Because of price considerations they are produced only in small quantities and are mainly used for medical implants. Investigations (Liu et al. 1998) have shown that PtFetype permanent magnets can also be obtained in the form of thin films by combining Fe\Pt multilayering and rapid thermal processing. Energy products as high as 318 kJ mV$ (40 MGOe) have been reached and attributed to the presence of exchange coupling between the hard magnetic f.c.t. phase and soft magnetic f.c.c. phase. A detailed description of the exchange coupling mechanism and the concomitant remanence enhancement in nanostructured microstructures can be found in Magnets: Remanenceenhanced. When comparing the manufacturing routes of Alnico magnets (see Alnicos and Hexaferrites) and platinum alloy magnets one may notice that there is a striking similarity. In both cases the manufacturing benefits from the fact that an extended range of solid solubility exists at high temperatures and that on cooling this solid solution becomes supersaturated and leads to the precipitation of new phases. In both cases the decomposition of the supersaturated solid solution has to be performed at sufficiently low temperatures so that the resulting microstructure consists of fine grains and exhibits magnetic hardness. Interestingly, neither of the two parent solid solutions has a magnetic anisotropy of any significance. In the case of the platinum alloys the required magnetic anisotropy is obtained by the formation at low temperatures of a phase of lower symmetry (f.c.t.) than the parent phase (f.c.c.). In contrast to the latter, the former phase exhibits a fairly strong magneto-
Magnetic Materials: Hard crystalline anisotropy. Both phases have the same composition and the particle size is not very critical for the generation of anisotropy. Roughly speaking it can be said, therefore, that the main goal of finding optimum annealing treatments is to generate microstructures with grain sizes sufficiently small for generating coercivity. By contrast, the main goal of finding optimum annealing treatments in the case of Alnico alloys is to produce microstructures where not only the size but also the shape of the precipitated particles is at a premium, because not only coercivity but also shape anisotropy has to be generated owing to the absence of magnetocrystalline anisotropy. 2. MnAl Permanent Magnets
Temperature (°C)
Permanent magnets based on MnAl alloys owe their hard magnetic properties to the so-called τ-phase. This is an intermetallic compound with an f.c.t. structure (CuAu-type superstructure). It occurs in the composition range 51–58 at.% (67—73 wt.%) manganese. The manganese atoms are located predominantly at the 1a and 1c positions at (0, 0, 0) and (", ", 0), # # the respectively. The aluminum atoms occupy mainly 2e positions at (0, ", " ) in this crystal structure. Owing # to the fact that the#composition is richer in manganese than the equiatomic composition, not all the manganese atoms can occupy the former two sites. This has as a consequence in that the excess manganese atoms must be accommodated at the (0, ", ") positions, which # # they share with the aluminium atoms. Neutron diffraction results indicate that deviations from the ideal site occupancy in this type of MnAl alloy can lead to antiferromagnetic coupling between the manganese moments. This unfavorable feature has important
Mn (at.%)
Figure 2 Central portion of the Mn–Al phase diagram as reported by Liu et al. (1996). The shaded area of the homogeneity region of the β-phase corresponds to alloys that transform into β-Mn on quenching.
consequences for the saturation magnetization and for the hard magnetic properties of permanent magnets based on MnAl alloys. A second difficulty associated with MnAl alloys is the metastable nature of the τ-phase. In fact, this phase is not found in the Mn–Al phase diagram (see Fig. 2). It can be obtained by starting from the high-temperature equilibrium ε-phase occurring in this concentration range, which has an h.c.p. structure. However, the preparation of the f.c.t. τ-phase from the h.c.p. ε-phase is not easy. Formerly it was believed that the τ-phase forms from the ε-phase via an intermediate phase, εh, of orthorhombic structure (B19) by means of the reaction scheme h.c.p.(ε)
B19(εh)
f.c.t.(τ)
where the h.c.p. structure (ε) transforms first into the orthorhombic εh-phase by an atomic ordering reaction. Subsequently, the metastable f.c.t. ferromagnetic τphase is formed by way of shear transformation. However, this reaction scheme is rather unlikely in view of the observation by electron microscopy that precipitates of the εh-phase in undercooled ε do not act as nucleation centers for τ. The occurrence of a massive transformation has been proposed by Hoydick et al. (1997). It is furthermore important to realize that the metastable τ-phase, after an annealing time that depends on alloy composition and the annealing temperature, tends to decompose into the two stable phases with β-Mn ( β) and Cr Al type structure (γ ). ) has to be chosen # For this reason, the annealing& time carefully. Various heat treatments have been proposed to obtain and preserve the τ-phase, as discussed in more detail in the review of Mu$ ller et al. (1996). In several investigations (Otani et al. 1977, Pareti et al. 1986) it has been shown that doping with carbon can lead to significant improvements in the kinetics associated with the formation and decomposition of the τ-phase. In the carbon-doped alloys, an incubation period is involved with the formation of the τ-phase. The equilibrium phases β and γ do not form simultaneously with the τ-phase, but# form subsequently. The upshot is that the retardation of the formation allows for better process control because it also delays the formation of the undesirable β- and γ -phases. Carbon # effect on the doping has, furthermore, a beneficial saturation magnetization, although there is a substantial decrease in Curie temperature. The partial occupation of aluminum sites by manganese atoms at (0, ", ") implies that many antiphase # in the crystal lattice of the τdomain boundaries #occur phase. As shown in Fig. 3, these antiphase domain boundaries can act as nucleation sites for Bloch walls, which leads to a relatively easy magnetization reversal and hence to low values of the coercivity and remanence. The magnetic properties of the τ-phase in the 3
Magnetic Materials: Hard
Figure 3 Schematic representation of an antiphase boundary and the corresponding reversal in magnetization direction across the boundary.
MnxAl100-xCy
y
when coupled antiparallel to those of the main lattice, can cause a reduction in the net saturation moment. The anisotropy field shows a slight increase with manganese content. According to Pareti et al. (1986) this increase in HA is not a real increase in anisotropy but merely reflects the fact that the anisotropy constant, K l MsHA\2, remains approximately constant since M"s decreases with manganese concentration. The effect of carbon addition and aluminum concentration on the anisotropy field is shown in Fig. 4. An important improvement in the magnet manufacturing is due to Otani et al. (1977). They have shown that the hard magnetic properties can be significantly enhanced when using a high-temperature extrusion process. In this process microstructures are realized consisting of very fine grains while also the number of antiphase boundaries is strongly reduced. The manufacturing route involves the following steps. The alloys (typically 70.0 wt.% Mn, 29.5 wt.% Al, 0.5 wt.% C) are given a homogenizing treatment at 1100 mC for 1 h. After quenching the ingots to 500 mC they are annealed at 600 mC for 30 min. The hot extrusion is performed at 700 mC with a pressure of 80 kg mmV#. Finally, the extruded material is aged at 700 mC for 10 min. The energy product can reach values close to 56 kJ mV$ (7 MGOe). Compared with the energy products reached in rare earth-based magnets (see Rare Earth Magnets: Materials) these values are very low. However, one has to take into consideration that the raw materials costs are much lower. Moreover, the production cost is much lower because the whole processing route can be performed in air. Bibliography
x
Figure 4 Dependence of the room temperature anisotropy field, HA, on carbon and manganese content in MnxAl VxCy with "!! 68 x 72 wt.% and 0 y 0.6 wt.%. Constant anisotropy field values are represented by solid lines. The shaded area corresponds to alloys above the carbon solubility limit (after Pareti et al. 1986).
concentration range 67–73 wt.% manganese have been investigated by Pareti et al. (1986). It has been shown that an increasing manganese concentration in the homogeneity range of the τ-phase leads to an increase in the Curie temperature but it lowers the saturation magnetization. The increase of TC is the result of a general increase of the exchange interaction when magnetic manganese replaces nonmagnetic aluminum. The decrease of the saturation magnetization originates from the excess manganese atoms occupying former aluminum sites. These manganese moments, 4
Buschow K H J 1997 Magnetism and processing of permanent magnet materials. In: Buschow K H J (ed.) Handbook of Magnetic Materials. Elsevier, Amsterdam, Vol. 10, Chap. 4 Hoydick D P, Palmiere E J, Soffa W A 1997 Microstructural development in Mn–Al base permanent magnet materials: new perspectives. J. Appl. Phys. 81, 5624–6 Kaneko H, Homma M, Suzuki K 1968 A new heat treatment of Pt–Co alloys of high-grade magnetic properties. Trans. JIM 9, 124–9 Liu J P, Luo C P, Liu Y, Sellmyer D J 1998 High energy products in rapidly annealed nanoscale Fe\Pt multilayers. Appl. Phys. Lett. 72, 483–5 Liu X J, Kainuma R, Ohtani H, Ishida K 1996 Phase equilibria in the Mn-rich portion of the binary system Mn–Al. J. Alloys Compds. 235, 256–61 Mu$ ller Ch, Stadelmaier H H, Reinsch B, Petzow G 1996 Metallurgy of the magnetic τ-phase in Mn–Al and Mn–Al–C. Z. Metallkd. 87, 594–7 Otani T, Kato N, Kojima S, Kojima K, Sakomoto Y, Konno I, Tsukahara M, Kubo T 1977 Magnetic properties of Mn–Al–C permanent magnets. IEEE Trans. Magn. MAG-13, 1328–30 Pareti L, Bolzoni F, Leccabue F, Ermakov A E 1986 Magnetic anisotropy of MnAl and MnAlC permanent magnets. J. Appl. Phys. 59, 3824–8 Tanaka Y, Kimura N, Hono K, Yasuda K, Sakurai T 1997
Magnetic Materials: Hard Microstructure and magnetic properties of Fe–Pt permanent magnets. J. Magn. Magn. Mater. 170, 289–97 Watanabe K 1991 Permanent magnetic properties and their temperature dependence in the Fe–Pt–Nb alloy system. Mater. Trans. JIM 32, 292–8
Zhang B, Soffa W A 1994 The structure and properties of L1 ! ordered ferromagnetic Co–Pt, Fe–Pt, Fe–Pd and Mn–Al. Scr. Metall. 30, 683–8
K. H. J. Buschow
Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 4813–4817 5
1. Permanent Magnets based on Noble Metal Compounds At high temperatures the platinum–cobalt phase diagram is characterized by a range of complete solid solubility. The crystal structure is a disordered f.c.c. structure over the whole solid solubility range, in which cobalt and platinum atoms statistically occupy the crystallographic sites. This high-temperature phase is practically useless for permanent magnet applications because it does not show a sufficiently high magnetic anisotropy. Structural transformations take place, however, when the rapidly cooled solid solution Co VxPtx alloys are annealed at lower temperatures. The" f.c.c. phase gives rise to a disorder–order trans-
Figure 1 Schematic representation of an array of unit cells of the tetragonal AuCu structure type in which one component occupies the basal plane positions and the other component the equatorial plane positions. Switching of positions can lead to an antiphase boundary visible in the central part.
formation for comparatively high platinum concentrations (x 0.75) whereas for relatively low platinum concentrations (x 0.23) it transforms into a h.c.p. structure. The applicability of Co VxPtx alloys as " on the transpermanent magnet materials is based formation of the disordered f.c.c. phase to an ordered face-centered tetragonal (f.c.t.) phase occurring for alloys in the intermediate concentration range (40 x 75). Only the latter phase has a sufficiently high uniaxial magnetic anisotropy, the easy magnetization direction being along the tetragonal c-axis. The origin of the magnetic anisotropy and the role played by the orbital moment in transition metal compounds in CoPt has been explained by means of first principles band structure calculations using the local spin density approximation. For a more detailed discussion of this topic the reader is referred to the review by Buschow (1997). The Curie temperature of these materials is around 500 mC for the equiatomic composition and slightly decreases with platinum concentration. The maximum of the magnetic anisotropy energy EA l K j2K is found at the equiatomic composition, " the #maximum of the anisotropy field H l although A (2K j4K )\Ms is located at slightly higher x values " # because of the fairly strong decrease of the saturation magnetization, Ms, with x. For obtaining optimum values for the coercivity slightly higher platinum concentrations than the equiatomic composition are desirable because it is the anisotropy field rather than the anisotropy energy that determines the coercivity (see Coerciity Mechanisms). However, the energy product depends very strongly on the magnetization. For this reason, concentrations close to the equiatomic composition are generally chosen for practical applications in permanent magnets. As described in detail elsewhere (see Coerciity Mechanisms) high coercivities are usually obtained in magnet bodies composed of an assembly of sufficiently fine particles. CoPt alloys are known to have outstanding mechanical strength and for this reason the powder metallurgical route commonly used for the attainment of high coercivities in many other permanent magnet materials (see Magnets: Sintered) is not applicable here. In these alloys one may profit, however, from particulars of the phase diagram because the presence of the f.c.c.-to-f.c.t. phase transition provides ample means of obtaining sufficiently small f.c.t. particles. This is the reason that the generation of coercivity in CoPt alloys is a matter of controlling the nucleation and growth of f.c.t. particles during the phase transformation. This process involves heat treatments of the ingots under carefully selected conditions. Kaneko et al. (1968) have shown that the coercivity passes through a maximum when CoPt alloys are heat treated for variable aging times at temperatures sufficiently below the f.c.c.–f.c.t. transformation temperature, the optimum annealing temperature being around 680 mC. Furthermore, it has been found that 1
Magnetic Materials: Hard even better results are obtained when the first aging step is followed by a second aging step, provided the first aging step is kept sufficiently short to prevent overaging. The first annealing step is regarded as leading to the formation of a fine precipitate of the f.c.t. phase in the f.c.c. matrix, whereas the second step is held responsible for an increase of the magnetocrystalline anisotropy of the f.c.t. phase. This increase is probably associated with a more perfect atomic ordering of the cobalt and platinum atoms in the f.c.t. grains. By applying this two-step annealing process, coercivities can be obtained of greater than 700 kA mV" for alloys annealed first at 680 mC and subsequently at 600 mC for variable times. The corresponding energy products of the ingot magnets can reach values around 100 kJ mV$. The origin of the coercivity in f.c.t. CoPt alloys has been studied (Zhang and Soffa 1994) by means of Lorentz microscopy. It has been shown that the occurrence of antiphase boundaries (APB) is essential for the understanding of the coercivity in these materials. It is mentioned above that the f.c.t. phase precipitates from a disordered f.c.c. matrix phase on annealing. This phase transformation proceeds by means of a nucleation and growth mechanism, starting at a multitude of different locations in the alloy. The coalescence of the ordered regions during the growth process invariably leads to the occurrence of a large number of APBs. The atomic arrangement at APBs is shown schematically in Fig. 1. Zhang and Soffa (1994) showed by means of electron microscopy that the microstructure of fully ordered alloys can be characterized by profuse micro- and macrotwinning, leading to a dense net of APBs within the micro- and macrotwins. It is assumed that the coercivity is controlled by domain wall pinning at APBs and stacking faults. Behavior fairly similar to that described above for Co VxPtx alloys is found also for Fe VxPtx alloys. High " " obtained when coercivities for Fe VxPtx alloys are " alloys around the equiatomic composition are first homogenized at high temperatures and subsequently annealed at temperatures below the order–disorder transition temperature. Tanaka et al. (1997) made a fairly detailed investigation of the microstructures of heat-treated Fe VxPtx alloys and the resulting coerciv" that composition control is essential ities. It was shown for the attainment of high coercivities. In as-quenched alloys optimum coercivities are reached for 39.5 at.% platinum because the disordered f.c.c. phase disappears during quenching and can no longer act as nucleation centers for domain walls. Tanaka et al. (1997) further showed that alloys with optimum coercivities are reached after heat treatments in which the local stress associated with the cubic-to-tetragonal transformation is not yet released by twin formation, and the f.c.t. phase is still present in the form of nanoscale antiphase domains. The antiphase domain boundaries between the single-domain particles act as 2
pinning sites for domain wall motion and in this way generate high coercivities. An overlong aging treatment leads to particle growth so that these are no longer single-domain particles. Consequently, the coercivity is reduced. Investigations by Watanabe (1991) have made it clear that permanent magnets with hard magnetic properties even superior to those of CoPt alloys can be obtained for FePt alloys to which a small amount of niobium is added. The corresponding ingots are first homogenized at 1325 mC and then quenched in water. The desired high coercivity is obtained after a subsequent isothermal annealing treatment performed in a temperature range of 600–700 mC. The iron moments are somewhat higher than the cobalt moments in the iron alloys. This implies also that the remanences in the iron alloys are higher than in the cobalt alloys. This, in turn, leads to larger maximum energy products with (BH )max values higher than 160 kJ mV$. The platinum-based magnets are extremely expensive owing to the fact that they consist of roughly 75 wt.% platinum. Advantages of these magnets are that they can be produced as ingot magnets, avoiding the complicated powder metallurgical manufacturing route. The magnets are of high mechanical strength and of unequalled corrosion resistance. Because of price considerations they are produced only in small quantities and are mainly used for medical implants. Investigations (Liu et al. 1998) have shown that PtFetype permanent magnets can also be obtained in the form of thin films by combining Fe\Pt multilayering and rapid thermal processing. Energy products as high as 318 kJ mV$ (40 MGOe) have been reached and attributed to the presence of exchange coupling between the hard magnetic f.c.t. phase and soft magnetic f.c.c. phase. A detailed description of the exchange coupling mechanism and the concomitant remanence enhancement in nanostructured microstructures can be found in Magnets: Remanenceenhanced. When comparing the manufacturing routes of Alnico magnets (see Alnicos and Hexaferrites) and platinum alloy magnets one may notice that there is a striking similarity. In both cases the manufacturing benefits from the fact that an extended range of solid solubility exists at high temperatures and that on cooling this solid solution becomes supersaturated and leads to the precipitation of new phases. In both cases the decomposition of the supersaturated solid solution has to be performed at sufficiently low temperatures so that the resulting microstructure consists of fine grains and exhibits magnetic hardness. Interestingly, neither of the two parent solid solutions has a magnetic anisotropy of any significance. In the case of the platinum alloys the required magnetic anisotropy is obtained by the formation at low temperatures of a phase of lower symmetry (f.c.t.) than the parent phase (f.c.c.). In contrast to the latter, the former phase exhibits a fairly strong magneto-
Magnetic Materials: Hard crystalline anisotropy. Both phases have the same composition and the particle size is not very critical for the generation of anisotropy. Roughly speaking it can be said, therefore, that the main goal of finding optimum annealing treatments is to generate microstructures with grain sizes sufficiently small for generating coercivity. By contrast, the main goal of finding optimum annealing treatments in the case of Alnico alloys is to produce microstructures where not only the size but also the shape of the precipitated particles is at a premium, because not only coercivity but also shape anisotropy has to be generated owing to the absence of magnetocrystalline anisotropy. 2. MnAl Permanent Magnets
Temperature (°C)
Permanent magnets based on MnAl alloys owe their hard magnetic properties to the so-called τ-phase. This is an intermetallic compound with an f.c.t. structure (CuAu-type superstructure). It occurs in the composition range 51–58 at.% (67—73 wt.%) manganese. The manganese atoms are located predominantly at the 1a and 1c positions at (0, 0, 0) and (", ", 0), # # the respectively. The aluminum atoms occupy mainly 2e positions at (0, ", " ) in this crystal structure. Owing # to the fact that the#composition is richer in manganese than the equiatomic composition, not all the manganese atoms can occupy the former two sites. This has as a consequence in that the excess manganese atoms must be accommodated at the (0, ", ") positions, which # # they share with the aluminium atoms. Neutron diffraction results indicate that deviations from the ideal site occupancy in this type of MnAl alloy can lead to antiferromagnetic coupling between the manganese moments. This unfavorable feature has important
Mn (at.%)
Figure 2 Central portion of the Mn–Al phase diagram as reported by Liu et al. (1996). The shaded area of the homogeneity region of the β-phase corresponds to alloys that transform into β-Mn on quenching.
consequences for the saturation magnetization and for the hard magnetic properties of permanent magnets based on MnAl alloys. A second difficulty associated with MnAl alloys is the metastable nature of the τ-phase. In fact, this phase is not found in the Mn–Al phase diagram (see Fig. 2). It can be obtained by starting from the high-temperature equilibrium ε-phase occurring in this concentration range, which has an h.c.p. structure. However, the preparation of the f.c.t. τ-phase from the h.c.p. ε-phase is not easy. Formerly it was believed that the τ-phase forms from the ε-phase via an intermediate phase, εh, of orthorhombic structure (B19) by means of the reaction scheme h.c.p.(ε)
B19(εh)
f.c.t.(τ)
where the h.c.p. structure (ε) transforms first into the orthorhombic εh-phase by an atomic ordering reaction. Subsequently, the metastable f.c.t. ferromagnetic τphase is formed by way of shear transformation. However, this reaction scheme is rather unlikely in view of the observation by electron microscopy that precipitates of the εh-phase in undercooled ε do not act as nucleation centers for τ. The occurrence of a massive transformation has been proposed by Hoydick et al. (1997). It is furthermore important to realize that the metastable τ-phase, after an annealing time that depends on alloy composition and the annealing temperature, tends to decompose into the two stable phases with β-Mn ( β) and Cr Al type structure (γ ). ) has to be chosen # For this reason, the annealing& time carefully. Various heat treatments have been proposed to obtain and preserve the τ-phase, as discussed in more detail in the review of Mu$ ller et al. (1996). In several investigations (Otani et al. 1977, Pareti et al. 1986) it has been shown that doping with carbon can lead to significant improvements in the kinetics associated with the formation and decomposition of the τ-phase. In the carbon-doped alloys, an incubation period is involved with the formation of the τ-phase. The equilibrium phases β and γ do not form simultaneously with the τ-phase, but# form subsequently. The upshot is that the retardation of the formation allows for better process control because it also delays the formation of the undesirable β- and γ -phases. Carbon # effect on the doping has, furthermore, a beneficial saturation magnetization, although there is a substantial decrease in Curie temperature. The partial occupation of aluminum sites by manganese atoms at (0, ", ") implies that many antiphase # in the crystal lattice of the τdomain boundaries #occur phase. As shown in Fig. 3, these antiphase domain boundaries can act as nucleation sites for Bloch walls, which leads to a relatively easy magnetization reversal and hence to low values of the coercivity and remanence. The magnetic properties of the τ-phase in the 3
Magnetic Materials: Hard
Figure 3 Schematic representation of an antiphase boundary and the corresponding reversal in magnetization direction across the boundary.
MnxAl100-xCy
y
when coupled antiparallel to those of the main lattice, can cause a reduction in the net saturation moment. The anisotropy field shows a slight increase with manganese content. According to Pareti et al. (1986) this increase in HA is not a real increase in anisotropy but merely reflects the fact that the anisotropy constant, K l MsHA\2, remains approximately constant since M"s decreases with manganese concentration. The effect of carbon addition and aluminum concentration on the anisotropy field is shown in Fig. 4. An important improvement in the magnet manufacturing is due to Otani et al. (1977). They have shown that the hard magnetic properties can be significantly enhanced when using a high-temperature extrusion process. In this process microstructures are realized consisting of very fine grains while also the number of antiphase boundaries is strongly reduced. The manufacturing route involves the following steps. The alloys (typically 70.0 wt.% Mn, 29.5 wt.% Al, 0.5 wt.% C) are given a homogenizing treatment at 1100 mC for 1 h. After quenching the ingots to 500 mC they are annealed at 600 mC for 30 min. The hot extrusion is performed at 700 mC with a pressure of 80 kg mmV#. Finally, the extruded material is aged at 700 mC for 10 min. The energy product can reach values close to 56 kJ mV$ (7 MGOe). Compared with the energy products reached in rare earth-based magnets (see Rare Earth Magnets: Materials) these values are very low. However, one has to take into consideration that the raw materials costs are much lower. Moreover, the production cost is much lower because the whole processing route can be performed in air. Bibliography
x
Figure 4 Dependence of the room temperature anisotropy field, HA, on carbon and manganese content in MnxAl VxCy with "!! 68 x 72 wt.% and 0 y 0.6 wt.%. Constant anisotropy field values are represented by solid lines. The shaded area corresponds to alloys above the carbon solubility limit (after Pareti et al. 1986).
concentration range 67–73 wt.% manganese have been investigated by Pareti et al. (1986). It has been shown that an increasing manganese concentration in the homogeneity range of the τ-phase leads to an increase in the Curie temperature but it lowers the saturation magnetization. The increase of TC is the result of a general increase of the exchange interaction when magnetic manganese replaces nonmagnetic aluminum. The decrease of the saturation magnetization originates from the excess manganese atoms occupying former aluminum sites. These manganese moments, 4
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Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 4813–4817 5