Magnetic properties and microstructure in NdFeB strip-cast permanent magnets

Magnetic properties and microstructure in NdFeB strip-cast permanent magnets

Journal of Non-Crystalline Solids 287 (2001) 140±144 www.elsevier.com/locate/jnoncrysol Magnetic properties and microstructure in NdFeB strip-cast p...

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Journal of Non-Crystalline Solids 287 (2001) 140±144

www.elsevier.com/locate/jnoncrysol

Magnetic properties and microstructure in NdFeB strip-cast permanent magnets H. Chiriac, M. Marinescu * National Institute of R&D for Technical Physics, 47 Mangeron Blvd., 6600 Iasi, Romania

Abstract Nd8 Fe73 Co5 Hf 2 B12 alloys were cast as strips (continuous ribbons with thickness t ˆ 100±160 lm† by a chill-disk melt spinning technique …vdisk ˆ 3 m=s† from a master alloy prepared by arc-melting (A) and from two previously prepared alloy components (B). Samples obtained by the B procedure had, in the as-cast state, magnetic properties with a coercivity, i Hc ˆ 813 kA=m (not saturated at 1.6 T applied ®eld), remanence, l0 Mr ˆ 0:5 T, and maximum energy product, …BHmax ˆ 107 kJ=m3 †. By comparison, the magnetic properties of A type samples obtained by annealing 5 min at 700°C are i Hc ˆ 513 kA=m, l0 Mr ˆ 0:52 T, …BHmax ˆ 63 kJ=m3 †. Based on dM and irreversible susceptibility plots we suggest a smaller exchange coupling of the grains in sample A as compared to sample B. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 75.50.Kj; 75.50.Ww

1. Introduction Data on strip cast NdFeB alloys indicate that the strip casting technique could be used to produce reliable materials for bonded magnets or to directly produce thin plate-shaped permanent magnets for diverse applications [1]. It has been shown [2] that using the quenching technique to produce strips of NdFeB alloys, the RE content can be decreased, without the formation of a-Fe, to 28.5 wt%, as compared to 33 wt% for the conventional casting process. The structural morphology of these materials obtained by milling the strip cast precursors and containing cellular Nd2 Fe14 B phase and small regions of a Nd-rich lamellar phase is suitable for bonded magnets [2]. Other studies have

shown that NdFeB strip cast alloys with low Nd and high B content …Nd3:5 Dy1 Fe73 Co3 Ga1 B18:5 † have the following magnetic parameters: Hc ˆ 400 kA=m, Br ˆ 1:15 T and …BH †max ˆ 131:6 kJ=m3 [3] and thin platelet magnets (thickness t ˆ 100±240 lm† could be fabricated. The present paper presents our results on the magnetic properties of Nd8 Fe73 Co5 Hf 2 B12 strip cast alloys along with their dependence on the microstructure and the casting process. The intrinsic magnetic interactions between the crystallites that determine the macroscopic properties are evaluated through dM plots and pro®le of the irreversible susceptibility.

2. Experimental procedures * Corresponding author. Tel.: +40-32 130 680; fax: +40-32 231 132. E-mail address: [email protected] (M. Marinescu).

Nd8 Fe73 Co5 Hf 2 B12 ingots were prepared either by (A) arc-melting the constituent elements or (B)

0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 5 5 0 - 6

H. Chiriac, M. Marinescu / Journal of Non-Crystalline Solids 287 (2001) 140±144

induction-melting two alloyed components: Nd8 Fe69 Co5 B18 and Nd8 Fe78 Co5 B6 Hf 3 . Continuous ribbons 100±160 lm thick were cast by the melt spinning technique at a speed 3 m/s of the chill-disk and under 1 atm pressure of argon in the chamber …pch ˆ 60 kPa† to ensure a smooth surface of the strip free of super®cial gas pockets. X-ray di€raction (XRD) investigations were performed on samples and the average size and shape of the grains were computed and evaluated through a program based on Warren±Averbach analyses [4]. The oxygen contents of samples A and B was determined by an oxygen determinator (LECO EF50) based on a solid-state IR adsorption cell. Magnetic hysteresis, MH, loops along with minor loops and thermomagnetic measurements were performed with a vibrating sample magnetometer (VSM) in a maximum applied ®eld of 1.6 T. The strength and type of the magnetic interactions between the crystallites of the constituent phases are evaluated through dM and irreversible susceptibility, virrev , plots calculated from the recoil loops.

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Fig. 1. Thermomagnetic curves on Nd8 Fe73 Co5 Hf 2 B12 (A) and (B) samples in maximum applied ®eld of 0.8 T.

3. Results The XRD Warren±Averbach analyses detected a nanocrystalline structure of the as strip cast samples as follows: strips cast from type A ingots contained ellipsoidal grains of a(Fe, Co) in a matrix of Nd2 Fe14 B grains with an interval of the grain size from 5 to 20 nm and a reduced fraction of the remaining amorphous phase. After an isothermal treatment at 700°C for 5 min, an average size of about 60 nm for 2:14:1 crystallites and 20 nm for the a(Fe, Co) phase was obtained. Strips cast following the second procedure (B), contained almost spherical shaped grains of 2:14:1 and a(Fe, Co) phases with a diameter of about 20 nm. We mention that both types of samples were cast under the same experimental conditions and the analysed strips had the same thickness, t ˆ 110 lm. XRD data did not detect any iron or hafnium borides even though the starting alloy contained an excess of boron as compared with stoichiometric composition cB ˆ 6 at:%. Further investigation by transmission electron microscopy

Fig. 2. DSC scan on Nd8 Fe73 Co5 Hf 2 B12 (B) as cast strips (25 K/min heating rate).

is needed. The oxygen content is rather high in both sorts of samples with an average of (0.0121  0.0005)% for A-type specimen and (0.0143  0.0005)% for B-type specimen. Thermomagnetic curves measured in a maximum applied ®eld of 0.8 T are presented in Fig. 1 and show the presence of Nd2 …Fe; Co†14 B (®rst drop) and a(Fe,Co) (second drop) phases in differing volume ratios for samples A and B. Although the calorimetric investigations using

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di€erential scanning calorimeter (DSC) indicates phase transitions (Fig. 2 shown by the arows) and we assume that metastable phases such us Nd3 Fe62 B14 or Nd2 Fe23 B3 are in the sample B, from the thermomagnetic curves, there is no evidence for Nd3 Fe62 B14 phase (TC…Nd3Fe62B14† ˆ 210°C [5]) with or without Co substitution for Fe whereas Nd2 Fe23 B3 phase is not identi®able since its Curie temperature TC…Nd2Fe23B3† ˆ 382°C [6] is close to that of Nd2 …Fe; Co†14 B phase. 4. Discussion It is well known [7] that grain sizes on nanometer scales ensure optimum hard magnetic properties for sub-stoichiometric Nd2 Fe14 B alloys. To produce such sizes, di€erent alloying elements such as Hf are added to suppress grain growth by forming very small grains (e.g. Fe2 Hf or hafnium boride phases) at the boundaries of the crystallites of the main phases. Co is soluble in both a-Fe and Nd2 Fe14 B phases and so 5 at.% Co was substituted for Fe to increase the Curie temperature by about 50°C [8]. The applied magnetic ®eld (0.8 T) during thermomagnetic measurements is not sucient to result in saturation in either case. The magnetization increases to a maximum at about 200°C. Appar-

(a)

ently, when the Curie temperature of the 2:14:1 hard phase is reached, a greater magnetization in the case of the (A) sample results. From these results, and considering a decreased number of a-Fe peaks for B samples on X-ray patterns, we conclude that the procedure of obtaining strips by casting the molten alloyed components decreases a-Fe formation. In this case, we assume that the temperature at which grain nucleation occurs during solidi®cation of the strips on the rotating disk, is close to the peritectic temperature for 2:14:1 formation. Fig. 3 shows the hysteresis curves of Nd8 Fe73 Co5 Hf 2 B12 strips A and B in a maximum applied ®eld of 1.6 T. Coercivity as large as i Hc ˆ 813 kA=m (and i Hc ˆ 947 kA=m at 2.8 T applied ®eld) is obtained for B samples, a coercivity unusually large for sub-stoichiometric Nd2 Fe14 B alloys. We suggest that the remaining amorphous phase detected by XRD and DSC (Fig. 2) contributes to the coercive force through domain wall pinning. Remanence, Mr , and maximum energy product, …BH †max , are certainly limited by the relatively small applied magnetic ®eld: l0 Mr ˆ 0:5 T, …BH †max ˆ 107 kJ=m3 . By comparison, the magnetic properties of the A samples obtained by annealing 5 min at 700°C are l0 Mr ˆ 0:52 T, …BH †max ˆ i Hc ˆ 513 kA=m, 63 kJ=m3 . Fig. 3(a) shows the change of the

(b) Fig. 3. Comparison between the hysteresis curves of Nd8 Fe73 Co5 Hf 2 B12 (A) and (B) type of strips.

H. Chiriac, M. Marinescu / Journal of Non-Crystalline Solids 287 (2001) 140±144

hysteresis with the annealing conditions of the Nd8 Fe73 Co5 Hf 2 B12 strips obtained from the ingot cast by arc-melting. To have an insight into the intrinsic interactions between the grains that determine the magnetic properties at a macroscopic level, we made M plots that carry the qualitative information on the magnetizing type (exchange) or demagnetizing type (dipolar) of interactions between the grains [9] dM ˆ Md …H †=Mr …Hmax †

‰1

2 Mr …H †=Mr …Hmax †Š;

where Mr …H † is the remanent magnetization of an initial demagnetized probe as a function of the applied ®eld and Md …H † denotes the remanent magnetization of a previously magnetized probe at l0 Hmax applied ®eld of 1.6 T. The main contribution to deviation from Wohlfarth's relation [10], Md …H † ˆ Mr …1† 2Mr …H † for which dM ˆ 0, is due to particle interactions [9]: preponderance of exchange interactions is associated with dM > 0 and preponderance of dipolar interactions is associated with dM < 0. Fig. 4 shows that the exchange interactions dominate for Nd8 Fe73 Co5 Hf 2 B12 B sample while dipolar interactions are present in A sample. The strength of the magnetic interactions between grains of di€erent phases can be evaluated from the pro®le of the irreversible susceptibility,

Fig. 4. dM plots on Nd8 Fe73 Co5 Hf 2 B12 (A) annealed and (B) as cast strips. Lines are drawn as guides for the eye.

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virr ˆ dMd …H †=dH , calculated as the ®rst derivative of the remanent magnetization during the demagnetization process. One single peak indicates a well coupled system whereas a peak of greater width or two peaks denotes two phases with a smaller magnetic coupling (see Fig. 5). The ®eld corresponding to the maximum of virr is an average of the nucleation ®eld of the reverse domains at which the irreversible rotation of the magnetic moments in the hard phase occurs in an avalanche. The di€erence between i Hn ˆ 670 kA=m and the coercivity i Hc ˆ 513 kA=m is larger in Nd8 Fe73 Co5 Hf 2 B12 A sample annealed 5 min at 700°C compared to the as-cast B sample where the nucleation ®eld is almost equal to the coercive ®eld (see Fig. 5). Based on the fact that Hn is larger than i Hc we suggest that for A sample, the grain size of the soft phase exceeds the critical size (about 40 nm) under which exchange coupling between the hard and soft regions occurs. This result con®rms a smaller exchange coupling of the grains, as deduced from the dM plots for Nd8 Fe73 Co5 Hf 2 B12 , in sample A as compared to sample B. This phenomenon is visible on hysteresis curves only after a higher annealing temperature (10 min/700°C in Fig. 3(a)), as an accentuated concave shape of the hysteresis in the second quadrant.

Fig. 5. Irreversible susceptibility virr calculated as ®rst derivative of demagnetization remanence of (A) annealed and (B) as cast Nd8 Fe73 Co5 Hf 2 B12 strips. Lines are drawn as guides for the eye.

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5. Conclusion

References

Nd8 Fe73 Co5 Hf 2 B12 strips obtained by quenching the molten two alloys present magnetic properties with a coercivity, i Hc ˆ 813 kA=m, not saturated at 1.6 T applied ®eld for sub-stoichiometric Nd2 Fe14 B alloys while the samples prepared by conventional ingot casting have an expected coercivity (e.g. i Hc ˆ 513 kA=m attained by annealing 5 min at 700°C). Samples cast by both methods with the same thickness, t ˆ 110 lm, have di€erent structural morphologies. Exchange interactions, coupling the soft and hard regions in Nd8 Fe73 Co5 Hf 2 B12 strips, are revealed by dM and irreversible susceptibility plots.

[1] C.S. Marinescu, H. Chiriac, M. Marinescu, Micro rotaryencoder using noncrystalline magnetic materials, accepted for presentation at EMSA 2000, Dresden. [2] Y. Hirose, H. Hasegawa, S. Sasaki, in: Proceedings of the xvth International Workshop on Rare-Earth Magnets and Their Applications, vol. 1, Dresden, 1998, p. 77. [3] H. Kanekiyo, S. Hirosawa, J. Appl. Phys. 83 (11) (1998) 6265. [4] B.E. Warren, in: X-ray Di€raction, Dover, New York, 1990, p. 324. [5] L. Withanawasam, A.S. Murthy, G.C. Hadjipanayis, R.K. Lawless, R.F. Krause, J. Magn. Magn. Mater. 140±144 (1995) 1057. [6] D.B. deMooij, K.H.J. Buschow, Philips J. Res. 41 (1986) 400. [7] E.F. Kneller, R. Hawig, IEEE Trans. Magn. MAG-27 (1991) 3588. [8] H. Chiriac, M. Marinescu, in: Proceedings of the XV International Symposium on Magnetic Anisotropy and Coercivity in RE-TM Alloys, Dresden, 1998, p. 75. [9] P.E. Kelly, K. O'Grady, P.I. Majo, R.W. Chantrell, IEEE Trans. Magn. 25 (1989) 3881. [10] E.P. Wohlfarth, J. Appl. Phys. 29 (1958) 595.

Acknowledgements The authors acknowledge the contribution of Dr R. Scholl from Fraunhofer Institut Angewandte Materialforschung Dresden with the results on oxygen content and support with DSC investigation.