TEMPERATURE DEPENDENCE OF MAGNETIZATION REVERSAL OF SINTERED Nd-Fe-B MAGNETS

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1 TEMPERATURE DEPENDENCE OF MAGNETIZATION REVERSAL OF SINTERED Nd-Fe-B MAGNETS K. Kuntze, D. Kohake, R. Beranek, S. Stieler, C. Heiden To cite this version: K. Kuntze, D. Kohake, R. Beranek, S. Stieler, C. Heiden. TEMPERATURE DEPEN- DENCE OF MAGNETIZATION REVERSAL OF SINTERED Nd-Fe-B MAGNETS. Journal de Physique Colloques, 1985, 46 (C6), pp.c6-253-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1985 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C6, supplément au n 9, Tome 46, septembre 1985 page C6-253 TEMPERATURE DEPENDENCE OF MAGNETIZATION REVERSAL OF SINTERED Nd-Fe-B MAGNETS K. Kuntze, D. Kohake, R. Beranek, S. Stieler and C. Heiden Magnetfabrik Dortmund der TEW, Ostkirohstrasse 177, D-4600 Dortmund 41, F.R.G. + Institut fur Angewandte Physik der Justus-Liebig-Univevsit&t Giessen, Heinrieh-Buff-Ring 16, D-6300 Giessen, F.R.G. Résumé - Le retournement de l'aimantation de plusieurs aimants Nd2Fel4B a été mesuré à des températures comprises entre 4,2 K et 300 K, le chaap extérieur étant appliqué parallèlement ou perpendiculairement à l'axe c. Il semble que la constriction de la courbe d'aimantation, visible au voisinage de H = 0, puisse avoir plusieurs causes : à des températures supérieures à 150 K, elle a été attribuée à l'oxydation de l'échantillon. Au-dessous de 150 K, d'autres effets semblent jouer un rôle : une phase riche en bore (NdFe4B4> dont la température de Curie est environ 50 K et la réorientation de spins dans Nd2Fei4B. Abstract - Magnetization reversals of several Nd,Fe,,B magnets were measured at temperatures between 4.2K and 300K, the external field being applied parallel or perpendicular to the c-axis. It appears, that the constriction in the magnetization curve, visible near H=0, may have several causes: At temperatures above 150K it was found to be associated with sample oxidation. Below 150K, additional effects seem to play a role: A boron- rich phase (NdFe.B,) with Curie temperature at ca. 50K, and the spin reorientation. I - Introduction This paper reports on some investigations carried out on sintered Nd? Fe.,B magnets to study their apparent softening at low temperatures, which becomes visible by the appearence of a constriction in the magnetization curve near H=0. Ho et al. /I/ explained this step in the hysteresis loop with the spin reorientation, which was found to set in below about 140K /1,2,3,4/. While evidence for the existence of this anisotropy anomaly has been established by measuring the anisotropy field by the singular point detection method /3/ and also by deriving the anisotropy constants K. and K from the magnetization curves /1,2,4/, another cause for the existance of a shoulaer in the hysteresis curve seems to be the formation of other phases, associated for instance with oxidation. That surface oxidation can produce steps in the magnetization curve is already known from Sm-Co magnets /5/. II - Experimental procedure To study the magnetization behaviour at different temperatures, a vibrating sample magnetometer with a dewar was used, that contained an electric heater. The equipment was inserted into a helium cryostat, allowing to produce temperatures between 4.2K and 300K. The magnetic field was generated either by a superconducting 5T magnet or by a 15T Bitter magnet /6/. We used polycrystalline samples in the shape of spheres with diameter between 2mm and 3mm, that were prepared from commercial magnets of Sumitomo Special Metals Ltd. or of Colt Industries. They were mounted either with their c-axis parallel or perpendicular to the external field. Article published online by EDP Sciences and available at

3 C6-254 JOURNAL DE PHYSIQUE I11 - Results Measurements were carried out on three different samples with different surface treatment. Sample I (Sumitomo) was electropolished and measured immediately afterwards. Then it was oxidized for 5.5 hours at 150'~ and measured again. Fig. 1 shows a plot of the magnetization versus temperature. The sample was mounted with the c-axis either parallel or perpendicular to the applied field, which was held constant resulting in an internal between 0.3T and 0.4T for both cases. The temperature was varied from the low end upwards. From the two components of M at 4.2K one deduces an angle of about 28 between the magnetization vector and the c-axis. A similar result was obtained by Ho et al. /l/. They obtained a somewhat Fig. 1 Magnetization versus temperature for cllh and c LH for an internal field of 0.3TLp Hif0.4T (sample I). Fig. 2 Magnetization curves (sample I) for temperatures between 4.2K and 300K, c //H. Curves are shifted vertically for better separation. Solid: polished, dotted: oxidized.

4 higher value for the temperature, at which the two components of M start to deviate from the curves, that can be extrapolated from the behaviour at higher temperatures. This might be caused by a different composition of the samples, for instance with respect to the dysprosium content. The curves obtained for the oxidized sample differ from the curves of the polished magnet. The magnetization curves of the electropolished sample I (c.f. fig. 2) exhibit a step near H=O. The magnitude of this step is almost constant for temperatures between look and 300K, but more pronounced for the 4.2K-curve. The steps are bigger for the oxidized magnet. The coercive field H is smaller for the oxidized sample but seems to become larger than that for the p&lished sample for a temperature range below about 100K. Fig. 3 Magnetization curves (sample I) for temperatures between 4.2K and 300K, clh. Solid: polished, dotted: oxidized. The magnetization curves for MA of the same sample are shown in fig. 3. Here oxidized and polished sample exhibit exactly the same behaviour. It should be noticed, that H has a maximum value at about 150K. A non monotonous behaviour of the coercive fo$ce has been reported also by Grossinger et al. /7/. The pronounced step of the magnetization near H=O at 4.2K might be caused by the spin reorientation, which should produce a step in the ML, but probably not in the M// component. -l r -2 """"""""' l''''d Fig. 4 Magnetization curves (sample 11) for T=100K and C 1 H. Solid: polished, dotted: oxidized.

5 C6-256 JOURNAL DE PHYSIQUE Sample I1 (Swnitomo) was stored at room temperature for about four months after preparation. The magnetization curve measured then at look showed a pronounced constriction. This step however completely disappeared after polishing the magnet (c.f. fig 4). Again, the coercive field at look is larger for the oxidized sphere. Fig. 5 shows the magnetization curves obtained for a sample (111, Colt), that after polishing was stored two days under hexane. Again it appeares, that for the oxidized sample the constriction in the magnetization curves is enhanced K : p, Hi LT1 Fig. 5 Magnetization curves (sample 111) for temperatures between 4.2K and 300K, c /l H. Solid: polished, dotted: oxidized. The fact, that M appears larger for the oxidized sample for T=4.2K and look may result from a different calibration of the two magnetometers. The results lead to the conclusion, that over the entire temperature range from 4.2K to 300K the existence of additional phases like oxides plays an important role concerning the magnetization reversal of Nd2Fe14B magnets. Most of our polished samples exhibited shoulders even at room temperature. This may be explained by the fact, that only a thin surface layer was removed by polishing, still leaving oxidized material in the sample. At lower temperatures an additional mechanism sometimes seems to generate a larger shoulder in the magnetization curve. This can be clearly seen from fig. 2 and 5, where the 4.2K curves exhibit such a large step. Responsible for this might be the NdFe4B4 phase, that was found in those magnets. To check this hypothesis, a sample was prepared, consisting mainly of NdFe B Fig. 6 shows the magnetization versus temperature for this sample, measured at $n4'external field of 1T from 4.2K upwards. ja Curie temperature near 50K can be deduced. It thus appeares possible, that this phase may be responsible for the large step in the magnetization curve seen at 4.2K. Further, more detailed investigations concerning this effect are under way.

6 T CKI Fig. 6 Magnetization versus temperature for a sample mainly containing the NdFe4B4 phase at a constant external field of 1T. Curie temperature is about 50K. References /I/ W. W. Ho, H. Y. Chen, Y. Y. Yang, J. Wang, B. Liao, C. Lin, F. Xing, Z. Liu and J. Lan, Proceedings of the 8th Int. Workshop on REM, Dayton, USA (1985), p. 635 /2/ D. Givord, H. S. Li and R. Perrier de la Bdthie, Solid State Comm., 11 (1984), p. 857 /3/ R. Grossinger, P. Obitsch, X. K. Sun, R. Eibler, H. R. Kirchmayr, F. Rothwarf and H. Sassik, Mat. Lett., 6A&B (1984), p. 539 /4/ Y. Ming, Z. Chao, 2. Wen, Y Hong and 2. Gong, Proceedings of the 8th Int. Workshop on REM, Dayton, USA (1985), p. 529 /5/ E. Adler, H. J. Marik, Proceedings of the 5th Int. Workshop on REM, Roanoke, USA (1981), p. 335 /6/ A Bitter magnet of the high field facility of the Technical University of Braunschweig was used. /7/ R. Grijssinger, H. R. Kirchmayr, R. Krewenka, K. S. V. L. Narasimhan and M. Sagawa, Proceedings of the 8th Int. Workshop on REM, Dayton, USA (1985), p. 565