A Study of the Hard Magnetic Properties in Different Hard Magnetic Materials G

A STUDY OF THE HARD MAGNETIC PROPERTIES IN DIFFERENT HARD MAGNETIC MATERIALS G. Hadjipanayis, S. Nafis, W. Gong

To cite this version:

G. Hadjipanayis, S. Nafis, W. Gong. A STUDY OF THE HARD MAGNETIC PROPERTIES IN DIFFERENT HARD MAGNETIC MATERIALS. Journal de Physique Colloques, 1988, 49 (C8), pp.C8-657-C8-658. 10.1051/jphyscol:19888298. jpa-00228470

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A STUDY OF THE HARD MAGNETIC PROPERTIES IN DIFFERENT HARD MAGNETIC MATERIALS

G. C. Hadjipanayis, S..Nafis and W. Gong

Kansas State University Manhattan, KS 66506, U.S.A.

Abstract. - The magnetic properties of several hard magnetic materials have been investigated to determine any trends in their magnetic characteristics. The materials studied were Nd16Fe77Bs,SmCo5, Smz (Co, Fe, Cu, Zr)17rstrontium ferrite magnets and single domain particles of Ba-ferrite and Cu-Mn-Al. The initial magnetization curves, field dependence of coercivity and remanence curves were measured.

Introduction (a) SmCos; Nd-Fe-B; ferrite magnets: the density of defects is very low inside the grains but very high Hard magnetic materials are characterized by large at around grain boundaries where it is believed the coercive fields. Since 1948 the progress in hard mag- domain walls are pinned. netic materials has been remarkable [I]. The magnetic properties were measured with a vi- However, despite of the progress made it appears brating sample magnetometer and a SQUID magne- that there is a confusion about the models used to de- tometer in the temperature range of 4.2-300 K and in scribe magnetic hardening. It is not clear when and fields up to 55 kOe. The samples had a spherical shape where the models of single domain particles, domain to correct for the demagnetizing factor. wall pinning [2] and nucleation of reverse domains [3] are more appropriate.-- - Our feeling- is that there is a Results and discussion great need for some systematic studies, both theoreti- Figure 1 shows that the initial magnetization curves cal and experimental, to understand better the funda- of Nd-Fe-B, SmCob and ferrite magnets increase mental behavior of magnetic hysteresis. steeply at low fields. This group of magnets 4s known In this study we present some preliminary results on as the "nucleation type" or "localized domain wall initial magnetization curves, field dependence of coer- pinning" magnets where the domain walls move easily civity and remanence curves of various magnets in an inside the grains and are pinned (or nucleated) at grain attempt to clarify further the origin of masetic hys- boundaries [2]. For the precipitation hardened mag- teresis. nets (REC-26, H-27) the initial curve shows a small change with field up to a critical field (close to the Experimental coercive field) above which it increases drastically to saturation. Surprisingly a similar behavior was also The hard magnetic alloys chosen for these studies are: observed in the single domain Ba-ferrite powders. Tp Cu-Mn-A1 magnets showed a slow but steady increase (i) single domain particle type: of magnetization with field as expected for a random array of non-interacting single domain particles. (a) Cu-Mn-Al: microstructure consists of two finely dispersed phases, a ferromagnetic Heusler phase and the non-magnetic 7 Cu-A1 phase [4]; (b) Baiferrite Fine Powders: the size of the powders is 200 A and therefore they behave as single domain particles;

(ii) uniform domain wall pinning: (a) Smz (Co, Fe, Cu, Zr),, (REC-26; H-27) [5]: these are precipitation hardened magnets having a fine cellular microstructure (500-1000 A) with the cell interior having the 2:17 phase and the cell walls the 1:5 phase [6]. Domain wall pinning takes place at the cell boundaries; H (Koe)

(iii) nucleation or localized domain wall pinning Fig. 1. - Initial magnetization curves.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888298 C8 - 658 JOURNAL DE PHYSIQUE

For the nucleation type magnets the coercivity in- In figure 3 the solid line is the theoretical predic- creases first slowly and linearly with field followed by tion of the remanence relationship. It is obvious that a steeper increase and a saturation at higher fields none of the magnets studied obeys this relationship. (Fig. 2). A slightly different behavior has been ob- The smaller deviations occur for the Cu-Mn-A1 and served in SmCos where a small but steep increase of ferrite powders indicating the existence of interactions Hc has been observed at low fields and no saturation between the ferromagnetic part~cles.The large devia- of Hc is achieved in the applied field used (N 16 kOe) . tions observed in nucleation type magnets are expected For the precipitation hardened magnets and fine ferrite because the domain walls meet different barriers in the powders the coercivity behavior is similar to that of initial magnetization state than in the demagnetized M (H) described earlier. Similar measurements were state. However, we were surprised to see the large also reported by Durst et al. [7]. Saturation of Hc deviations in precipitation hardened magnets. These is achieved faster in H-27 and in the ferrite powders deviations should not be seen for these magnets where indicating a more uniform coercivity range. That was a uniform cellular microstructure is observed and a expected for both these magnets because of the uni- uniform domain wall pinning is expected at the cell form size and microstructure. A rather intersting re- boundary. sult has been obtained in Cu-Mn-A1 where the coercivity increases slowly and steadily with field like the magnetization. This results indicates a range of coer- civities in this magnet although the microstructure is quite uniform [4].

Fig. 3. - Fits to remanence relationship (Eq. (1)).

Acknowledgments

This work was supported by NSF DMR-8607023.

[I] Livingston, J. D., General Electric, Report No. Fig. 2. - Field dependence of coercivity. 8OCRD139 (1980). [2] Hadjipanayis, G. C., J. Appl. Phys. 63 (1988) 3310. [3] Kneller, E., Magnetism and Metallurgy, Eds. Wohlfarth [8] has first shown that for an assembly A. E. Berkowitz and E. Kneller (Academic Press, of non-interacting single domain particles the following New York) Vol. 1 (1969) 365. relationship is obeyed [4] Narita, K., Koga, S. and Motowaki, Y., Appl. Phys. .Lett. 36 (1980) 862. MD(H) = MR (Hmax) - ~MR(H) (1) [5] Commercial Smn (Co, FE, Cu, Zr),, Magnets; where MR (H) is the initial remanent magnetization, REG26 made by TDK, Japan; H-27 made by Hi- MD (H) the demagnetizing remanence after applica- tachi, Japan. tion of the maximum forward field H,, followed by [6] Hadjipanayis, G. C., Hazelton, R. C., Lawless, a reverse field H. McCurie et al. [9] have derived a K. R. and Horton, L. S., IEEE Trans. magn. similar relationship for domain walls interacting with MAG-18 (1982) 1480. a uniform distribution of defects. Deviations from the [7] Durst, K. D. and Kronmuller, H., J. Magn. Magn. relationship will occur for intervting single domain Mater. 68 (1987) 63. particles or if the reversing domain walls do not inter- [8] Wohlfarth, E. P., J. Appl. Phys. 29 (1958) 595. act with the same density and distribution of spin sites [9] McCurie, R. A. and Gaunt, P., Proc. Int. Conf. as on the forward trip. on Magnetism, Nottingham (1964) p. 780.