MAGNETIZATION PROCESSES AND δm(h) PLOTS FOR SINTERED

Transcription

1 MAGNETIZATION PROCESSES AND δm(h) PLOTS FOR SINTERED Nd-Fe-B-BASED PERMANENT MAGNETS A.G. Savchenko, V.P. Menushenkov Moscow State Institute of Steel and Alloys (Technological University) 1199, Moscow, Leninskij prospekt, Yu.E. Skuratovskij, A.V. Shakin, N.A. Dobrynin and A.S. Morgunov OAO Kompozit 17, Moscow region, Korolev, Pionerskaja street, ABSTRACT The hard magnetic properties, the behavior of the virgin magnetization curves, the minor loops and the direct field demagnetization behavior (recoil curves) are studied for the sintered Nd-Fe-B-based permanent magnets with the different hysteresis properties. The virgin remanent magnetization M r (H m ) and coercivity H ci (H m ) after magnetization in the field H m from thermally or direct field demagnetized states and the dc-demagnetization remanence (recoil remanence) M d (H d ) after demagnetization in the field H d from preliminary magnets saturating in a positive pulse field are discussed. In particular, an analysis of Wohlfarth s remanence relationship and Henkel plot: δm(h) = m d (h m ) [1 m r (h m )], where m r (h m ) = M r (H m )/M r (max), m d (h m ) = M d (H m )/M r (max) and h m = H m /H ci (max), are used to clarify the magnetization processes in different permanent magnets. The different character of the demagnetization curves starting from the magnetically saturated state and of the virgin curves after thermal demagnetization points that the coercivity of the all investigated Nd-Fe-B-based sintered magnets is definitely indicates a nucleation dominated behavior. Also in the magnets with the poor magnetic properties some domain wall pinning process occurs. 1. INTRODUCTION The macroscopic magnetic properties of the magnetic material depends on their microstructure or, in particular, are tightly related to the interactions between the grains. This intergrain interactions is very difficult to calculate ab initio. For that reason, an experimental technique, δm(h) or Henkel plot technique [1], is commonly adopted to investigate the intergrain interactions and their effects on the properties of Nd-Fe-B hard magnetic materials: the coercivity mechanisms in melt-spun ribbons [], the exchange spring behavior in nanocomposite hard magnetic materials [3], the effects of microstructure (size, shape and orientation of the grains) on the hard magnetic properties of sintered magnets []. In this paper, using the technique, developed in article R.W. Gao et al. [], we adopt the δm(h) plot technique to study the magnetization processes and coercivity mechanisms in several types of sintered Nd-Fe-B permanent magnets: prepared by conventional powder metallurgy method or by using the blending method [5], and with or without any additives.. EXPERIMENT The magnets, labeled shortly as A, B, C, E, H, P and X, were prepared by using the blending method [5], mixing a powder of the base alloys with a various quantity of the powders of master alloys with nominal compositions DyAl and/or Nd 3 Co (the alloys compositions and additive alloys quantity see in Table 1). The magnets K with the nominal composition Nd 1.5 Fe 75. Al.5 B 7. were prepared by the conventional powder metallurgy method. The mixture of the alloys (A H, P and X series) or crushed ingot (K series) were milled in isopropyl alcohol and then wet pressed in alignment magnetic field of 1 T. The shaped samples were dried and sintered in vacuum at 1 o C for 1 hour. After the demagnetization 1

2 curves measurements in as-sintered state (beneath the state labeled by letter W), the magnets were annealed at appropriate temperature for 5 min (beneath labeled by letter T). Table 1. The composition of the investigated magnets, prepared by using different processes Sample number Magnets composition Heat treatment A Nd 15.5 Dy 1.5 Fe 73.5 Co Al.5 B 7 +. wt. % DyAl + 1. Nd 3 Co 5 о С, 5 min B Nd 1 Fe 77.5 Co B.5 +. wt. % DyAl +.5 wt. % Nd 3 Co 55 о С, 5 min C Nd 1 Fe 77.5 Co B wt. % DyAl + 1. wt. % Nd 3 Co 57 о С, 5 min E Nd 15.5 Dy 1.5 Fe 73.5 Co Al.5 B wt. % Nd 3 Co 55 о С, 5 min H Nd 15.5 Dy 1.5 Fe 73.5 Co Al.5 B wt. % DyAl 55 о С, 5 min K Nd 1.5 Fe 75. Al.5 B о С, 5 min P Nd 15.5 Dy 1.5 Fe 73.5 Co Al.5 B wt. % DyAl 5 о С, 5 min X Nd 1.5 Dy 1.5 Fe 73.5 Co B wt. % DyAl 5 о С, 5 min The remanent magnetization and demagnetization curves were measured in as-sintered state and after heat treatment by using laboratory hysteresisgraph with an applied field of T. The saturated magnetization of samples is realized by using a pulse field of T. In particular, the scheme of the measurements (i) the demagnetization curves after partial magnetization from the thermally- or dc-demagnetized states and (ii) the partial demagnetization curves after saturation in pulse field were shown in Fig. 1. Fig. 1. The scheme of the measurements: (i) red lines - the virgin remanent magnetization M r (H m ) and correspondent coercivity H ci (H m ) after magnetization in the field H m from thermally demagnetized state and (ii) blue lines - the dc-demagnetization remanence (recoil remanence or remanence major loop) M d (H d ) is measured after demagnetization in the field H d from preliminary saturating the sample in a positive pulse field. πi s (= M s ), M r (max) and H ci (max) the saturation magnetization, saturation remanence and intrinsic coercivity respectively. 3. RESULTS AND DISCUSSION 3.1 The as-saturated hysteresis properties The hysteresis properties of samples prepared by different processes in as-sintered and heat treated states and measured at room temperature after saturation magnetization in pulse magnetic field are shown in Table. In particular, the data in Table show: (1) The properties of the magnets K series drastically lower then that one of the magnets series B, C, E, P and X, which may be connected also with the relatively higher B and Nd contents in the first one (see Table 1). () The addition of Nd 3 Co powder don t influenced on the coercivity of magnets comparing with the DyAl addition (samples E and H, P, X respectively). In particular, the sample H in as-sintered state and sample X in as-sintered and heat treated states demonstrates excellent hysteresis properties. (3) In the case of the combined DyAl and

3 Nd 3 Co addition the B r and H ci of the magnets obviously enhances when the Nd content in the base alloys lower and additives DyAl higher than the Nd 3 Co (see the properties of the samples A, B, C series in Table ). () Heat treatment drastically enhanced the hysteresis properties of the magnets with the DyAl addition the samples P and X series in Table. Table. The remanence M r (B r ), the coercivity H cb, the intrinsic coercivity H ci and the energy product (BH) max for the investigated magnets in as-sintered state (left value) and after proper heat treatment (right value) Sample number B r, T H ci, ka/m H cb, ka/m (BH) max, kj/m 3 A - / / / 7 - / 19 B 1.5 / / 913 / 53 / 3 C - / / 53 - / 5 - / 95 E - / 1. - / 5 - / - / 9 H 1.1 / / - 1 / - 7 / - K 1.15 / 1.1 / 573 / / 193 P / / / 39 / X 1. / / 111 / 7 5 / 3 3. Virgin magnetization-demagnetization curves On the Fig. the samples of virgin magnetization-demagnetization curves thermally or dcdemagnetized P series magnets in as-sintered (a) and heat treated (b) states are shown. 1 1 М,кГс М,кГc H,kЭ H,кЭ 1 1 М,кГс М,кГc H,kЭ H,кЭ (a) (b) Fig.. Virgin magnetization-demagnetization curves thermally (top row) or dc-demagnetized (bottom row) P series magnets in as-sintered (a) and heat treated (b) states. 3

4 In the result of there analyses on the Fig. 3 the normalized values of the correspondent virgin remanent magnetization m r (h m ) = M r (H m )/M r (max) curves and intrinsic coercivity h c (h m ) = H ci (H m )/H ci (max) curves as a function of the respective normalized values of h m = H m /H ci (max) for the magnets B, K, P and X in as-sintered and heat treated states are plotted. 1, 1, Br/Br(max) Hci/Hci(max) -,5 1,5,,5 3, 3,5, -,5 1,5,,5 3, 3,5, 1, 1, Br/Br(max) Hci/Hci(max) -,5 1,5,,5 3, 3,5,,5 5, H m/h ci(max) (a) -,5 1,5,,5 3, 3,5,,5 5, (b) Fig. 3. The relatives virgin remanent magnetization M r (H m )/M r (max) (a) and intrinsic coercivity H ci (H m )/H ci (max) (b) curves as a function of the respective values of H m /H ci (max) for the some investigated thermally (top row) or dc-demagnetized (bottom row) magnets. It s possible to conclude, that: (1) the shape of the virgin magnetization curve depends on the technological (as-sintered or heat treated state) and demagnetization (thermal- or dcdemagnetization) history of the magnets; () the shape of the m r (h m ) and h c (h m ) curves is strong indicates that the coercivity of the Nd-Fe-B sintered magnets is genuinely nucleationcontrolled []. Nucleation of reversal domain at the Nd Fe 1 B grain surface determine the coercivity in the Nd-Fe-B-base sintered magnets. (3) The virgin remanent magnetization curves m r (h m ) of the magnets B and especially K series differ then that one of the magnets series P and X. The former shows the kink in the field h m ½ H ci (max) and the magnet K almost in the magnetizing field equal to the correspondent H ci (max) has remanence slightly higher than ½ B r (max). It is obvious for nucleation-controlled magnets that in the thermally demagnetized state the domain walls move freely inside the grains of Nd Fe 1 B and quickly annihilated from grains resulting in a drastic increase of magnetization and thermal remanence at relatively low fields. The above mentioned kinks on the m r (h m ) curves, Fig. 3a, correlate with the kinks on the correspondent initial magnetization curve. It is an indication of domain wall pinning process in this samples. It is interesting to note that the kinks observed only for the magnets BW and K which contain low or does not contain any heavy rare earth

5 elements. These are also reflected on the h c (h m ) curves. The dependences of the relatives reversible magnetization [M(H m )-M r (H m )]/M r (max) vs H m /H ci (max) for the some investigated thermally- or dc-demagnetized magnets are shown on the Fig.. As we see, the dependences of reversibly magnetized volume on the magnetization field H m confirms mentioned above and the proposal, that the magnets with the better hysteresis characteristics cooperatively (but not grain-by-grain) magnetized. (B(H)-Br(Hm))/Br(max) PW PT XW XT BW BT KW KT (B(H)-Br(Hm))/Br(max) PW PT XW XT BW BT KW KT -,5 1,5,,5 3, 3,5,,5 5, (a) -,5 1,5,,5 3, 3,5,,5 5, (b) Fig.. The relatives reversible magnetization [M(H m )-M r (H m )]/M r (max) curves as a function of the respective values of H m /H ci (max) for the some investigated thermally (a) or dc-demagnetized (b) magnets. () The virgin intrinsic coercivity h c (h m ) of all samples is found to be strongly dependent on applied field, Fig. 3b. Initially h c (h m ) increases slowly and then more rapidly with the applied field. But in the above mentioned magnets P and X after thermal demagnetization it is practically negligible until some a critical field where it start increasing rather drastically. Compare the h c (h m ) curves for thermally- and dc-demagnetized magnets P and X it s possible to conclude that there domain walls not only move freely inside the grains but also annihilate on the grain boundaries make more difficult there creation during remagnetization. For the magnets B and K such critical fields are close to the fields where the shoulder appears on the correspondent m r (h m ) curves and also indicate that in this magnets inside the grains exist somehow domain wall pinning process. The slow, but continuous increasing of h c (h m ) observed for the magnets B and K series is too indicative of the fact that domain walls move freely inside the grains. 3.3 dc-demagnetization remanence The major demagnetization curves with the family of recoil curves for the magnets P and K series in as-sintered state are shown in Fig. 5. Each recoil curve was generated from a fully magnetized magnet by their partial demagnetizing in a dc-reverse field and then following the subsequent magnetization into the first quadrant. The same graphs were obtained for other series magnets and also in heat treated state. It was found that the smaller volume fraction of low coercivity grains in enhanced coercivity P series magnet (DyAl substituted) tends to lower reversibility compared to its rapid magnetization when thermally demagnetized, Fig., and higher reversible magnetization in magnet K. As we see on the Fig. 5, the recoil curves for P magnet is visibly more flat than for K magnet with lowered coercivity. It means that in magnet K we have easy domain-wall 5

6 nucleation-annihilation and substantial domain-wall motion at both reverse and forward fields М,кГс М,кГс H,кЭ H,кЭ (а) (б) Fig. 5. Major demagnetization curves with recoil curves for the pulse magnetized magnets P (a) and K (b) series in as-sintered state. The recoil curves characterized the irreversible demagnetization. The higher value M(-H m ) - M d - the higher reversibly demagnetized magnets volume. It can be found that structurally more uniform magnets (magnets of the P and X series, Fig. ) demonstrate relatively low reversible demagnetization, especially in heat treated state. That results support the results and conclusions of initial magnetization investigation. 1.. Fig.. The relatives reversible demagnetization M(-H m ) - M d / M r (max) curves as a function of the respective values of H m /H ci (max) for the investigated thermally demagnetized magnets in a-sintered and heat treated states. [Bd(H)-B(H)]/Br(max) H/Hci 3. Henkel plots As mentioned in [7], since Wohlfarth s classic paper [] the interactions in magnetic materials have been characterized in term of the remanent magnetizations. For noninteracting, uniaxial, single domain particles he showed that m ( H ) = 1 m ( H ), d r where H = H m, m d (H) = M d (H)/M r (max) and m r (H) = M r (H)/M r (max) are the dcdemagnetization remanence and the isothermal remanence, normalized to saturation

7 remanence respectively. A quantitative estimation of interaction can be obtained in terms of the Henkel plot [1] which exhibit m d (H) as a function of m r (H), or δm(h)-plot (curve) which is defined as [1]: [ 1 m ( )] δ M ( H ) = m ( H ) H. d r Henkel [1] noted that δm(h) plot curves indicate the type and strength of the dominant interaction mechanism between grains: (i) δm(h) > suggests that the intergrain exchange coupling interactions is dominant and promoting the magnetized state [], and (ii) δm(h) < are usually taken to indicate, that the intergrain interactions have the effect to stabilize the demagnetized state, for example, by formation of flux closure structures (the magnetostatic interaction is dominant []). The delta-m(h) or Henkel plots of the all investigated samples are shown in Fig. 7., 1,, 1, PW PT XW XT BW BT KW KT 1, 1, PW PT XW XT BW BT KW KT δmr 1, δmr , 1, H/Hci (a) - 1, 1, H/Hci (b) Fig. 7. δm(h) plots of same investigated Nd-Fe-B-based permanent magnets after thermal (a) or dc-demagnetization (b). As seen from the Fig. 7, the characteristics of δm(h) plots for magnets are as follows: (1) For the thermally demagnetized magnets the corresponding values of δm(h), Fig. 7(a), are positive if the applied field not higher than H ci (max), while for the magnets BW and K with the increasing of applied field δm(h) increases and reaches a maximum slower then for the other one. For the dc-demagnetized magnets δm(h) plots quite differ. Fig. 7(b) exhibits that δm(h) climbs from zero to the maximum value as the field H m increases from zero to H ci, and then descends to the minimum value as the field H m excess H ci. The same shape of δm(h) curve has been observed and reported in Refs. [3,,7,9] for the nanostructured hard magnetic alloys. The variations of δm(h) for thermally demagnetized magnets implies that the strength of magnetic interactions are related to the magnetization state. But it is obvious that the exchange coupling interaction between the grains, responsible for the positive δm(h) plot in common theory [], in sintered multiphase Nd-Fe-B magnets is weak. Also have no reason to propose the existence of an exchange coupling between the hard grains and the soft boundary phases []. Only the exchange interaction between the domains with different orientations in multidomain grains may be responsible for the positive δm(h) values. () The values of δm(h) for the magnets with the better hysteresis characteristics (A, H, P and X) are higher than for the magnets BW, ET and especially K, but in contrast with the data [3,,7,9], for all thermally demagnetized magnets the values of applied field corresponding to the maximum δm(h) not equal to the coercivity of the magnets. The shape of δm(h) curves 7

8 with a maximum in the vicinity of the coercive field, like in Ref. [3,,7,9], has been observed only for the dc-demagnetized magnets, Fig. 7(b). (3) The shape of δm(h) plots for the thermally- or dc-demagnetized magnets both with high coercivity (P and X) and with enhanced B r and (BH) max (C) quite similar and differ than for the rest (especially BW and K) magnets.. SUMMARY We have been study the magnetization and demagnetization behavior of the several types of sintered Nd-Fe-B permanent magnets: prepared by conventional powder metallurgy method or by using the blending method [5], and with or without any additives, using the δm(h) plot technique to characterized the magnetization processes and coercivity mechanisms. It has been shown that the coercivity of the magnets depends on the technological prehistory of the magnets and is definitely nucleation-controlled. However in the magnets with the poor magnetic properties some domain wall pinning process occurs. The positive δm(h) plots indicate that in investigated sintered Nd-Fe-B magnets the exchange type interactions are dominant and support the magnetization state, but it is nether intergrain interactions nor an exchange coupling between the hard grains and the soft boundary phases. McMichael et al. [11] noted that it is impossible safely draw the conclusions about interaction using the δm(h) plot technique when domain wall motion is involved in magnetic hysteresis. Nevertheless the δm(h) plot technique (modified no ordinary) can be used for the sintered Nd-Fe-B magnets characterization and the effects of microstructure and additives determination. In particular it was found also, that for the thermally demagnetized magnets the values of applied field corresponding to the maximum δm(h) not equal to the coercivity, similar to it has been observed in dc-demagnetized magnets. But the shape of δm(h) curves for dc-demagnetized magnets are the same like for the nanostructured Nd-Fe-B-based hard magnetic alloys. REFERENCES [1] Henkel O. Remanenzverhalten und Wechselwirkung in hartmagnetischen Teilchenkollektiven. Phys. Status Solidi, 19, 7, [] K. H. Muller etc. Deviations from Wohlfarth s remanence relatioship in NdFeB magnets. J. Magn. Magn. Mater., 199, -7, [3] Panagiotopoulos I., Withanawasam L., Hadjipanayis G.C. Exchange spring behavior in nanocomposite hard magnetic materials. J. Magn. Magn. Mater., 199, 15, [] Gao R.W. etc. Hard magnetic property and δm(h) plot for sintered NdFeB magnet. J. Magn. Magn. Mater.,,, [5] Savchenko A.G., Menushenkov V.P. Phys. Met. and Metalography (Rus.), 1, 91, Suppl. 1, S. [] Sagawa M. etc. New material for permanent magnets on a base of Nd and Fe. J. Appl. Phys., 19, 55, 3-7. [7] Emura M., Cornejo D.R., Missell F.P. Reversible and irreversible magnetization in hybrid magnets. J. Appl. Phys.,, 7, [] Wohlfarth E.P. J. Appl. Phys., 195, 5, [9] Zhang H.-W. Hard magnetic properties in isotropic nanocomposite Pr Fe 1 B/α-Fe ribbons. J. Magn. Magn. Mater., 3, 7, -33. [] Folk L., Street R., Woodward R. Investigation of interaction mechanisms in meltquenched NdFeB. J. Appl. Phys., 199, 75, [11] McMichael R.D., Vajda F., Torre E.D. Demagnetized-state dependence of Henkel plots. II. Domain wall motion. J. Appl. Phys., 199, 75,