Materials Science and Engineering, B3 (1989) 435-441 ,435

Interrelationships between Structure and Property in Magnetic Materials

R. RAMESH

Bell Communications Research, Red Bank, NJ 07701 (U.S.A.)

G. THOMAS

Department of Materials Science and Mineral Engineering and National Center for Electron Microscopy, Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, University of California, Berkeley, ~4 94720 (U.S.A.)

(Received March 10, 1989)

Abstract

The basics of structure-property-processing relationships in magnetic materials are outlined. It is shown that both the microstructure and the magnetic domain structure have to be understood to reveal details of the reversal mechanisms. This enhanced understanding will enable tailoring of the magnetic properties.

I. Introduct ion

Magnetic properties can be broadly classified into two categories: (i) those that are independent of the microstructure, i.e. intrinsic properties such as saturation magnetization, anisotropy field etc.; (ii) extrinsic properties, i.e. those that are influenced by the microstructure of the magnet, such as the intrinsic coercivity He, remanence Br, energy product (BH)max , squareness ratio, per- meability etc. Some of these extrinsic properties are illustrated in the schematic plot in Fig. 1. In this figure, a virgin curve and hysteresis loop for a typical hard magnet are presented. Hard magnets

I ndt~liOn, B

P

Br, Remnont Mognetizotion

0 I ~R I I

0 [ : Virgin Mo~lefizoticn Cu¢~.~,O L If: Hysteresis Loop, LMNPQRL

Fig. 1. Schematic virgin curve and hysteresis loop for a hard magnetic material, with the relevant properties marked on the plot.

are characterized by large values of H c and (BH)max. The values of H c and (BH)max can be varied over several orders of magnitude by suit- able alterations of the composition and/or the processing conditions, i.e. by tailoring the micro- structure. In addition to the shape of the virgin curve, both H c and (BH)max reflect the magnetiza- tion reversal process in the magnet. Thus to improve the magnetic properties of any given magnet or to synthesize new magnets with better properties, it is essential to understand the inter- relationships between microstructure-magnetic property-processing iteratively, as illustrated in Fig. 2.

COl'1~SoTlOel

~TAIL[O ~,STORV I rEED BACK i

1 CHA~GES ;

r'

ITERATIVE i i ' n ~ l QF

Fig. 2. A block diagram of structure (and microstructure)- property-processing interrelationships, also showing the role of materials characterization.

0921-5107/89/$3.50 © Elsevier Sequoia/Printed in The Netherlands

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In general, magnets can be classified, base0 on the reversal mechanisms, into two types: (i) coher- ent (or incoherent) rotation magnets, wherein the H c equals the anisotropy field (taking into account the demagnetizing field); (ii) magnets in which reversal is controlled by domain wall pro- cesses. Only the latter will be examined here, since in many technologically relevant magnets magnetization reversal occurs by domain wall processes. Domain-wall-based magnetization reversal processes consist of two steps: (a) wall nucleation and (b) wall propagation. Both steps depend sensitively on the microstructure. In general, one of these two steps will determine ft~. Consequently, magnets are classified as either nucleation controlled or domain wall pinning controlled. Figures 3(a) and 3(b) show typical virgin curves and hysteresis loops for the two types, from which clear distinctions between the two cases can be made. The microstructural features for these two types of magnets are also different. In general, four different types of mi- crostructures are observed, as illustrated sche- matically in Fig. 4. Figure 4(a) corresponds to elongated single-domain particles, embedded in a non-ferromagnetic matrix. The magnetic particles in this case are prevented from interacting with one another through exchange interactions. The microstructure in Fig. 4(b) corresponds to a typi-

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O.

bH

IHnl > IHpl

Hp n

f , ,

J IHnl < IHpl

(d)

Fig. 4. Schematic illustrations of different:microstructures encountered in hard magnetic materials: (a) elongated single- domain particles in a non-magnetic medium; (b) precipita- tion-hardened magnet; (c) two-phase microstructure consisting of non-magnetic dispersoids in a ferromagnetic matrix; (d) nucleation-type magnet.

Fig. 3. (a) Schematic virgin curve and hysteresis loop for a nucleation-type magnet; (b) similar plot for a pinning-type magnet.

0

0 O 0

0 0 0

0 0

0 0 0

0 m 0

(a)

(b)

t

O '

o

(c

cal precipitation-hardened alloy, in which coher- ent (or semi-coherent) precipitates are uniformly dispersed in the matrix. A typical example of this is the Sm-Co-Ce-Cu (with zirconium additions) system. The microstructure in Fig. 4(c) corre- sponds to a dispersion of non-magnetic or weakly magnetic, almost spherical dispersoids in a fer- romagnetic matrix. Magnetic hardening in this case is similar to the Orowan dislocation pinning mechanism of work hardening of alloys. In Fig. 4(d), the case of a nucleation-type magnet is shown, in which the nucleation of reverse domains occurs at defect sites such as grain boundaries. Hence, in general, magnets can be classified, based on the reversal mechanism, into wall nucleation controlled and wall pinning con- trolled. In the first case, the step that controls the magnetization process is the nucleation of a domain wall, after which propagation of the wall across the grain occurs almost instantaneously. In the second case, walls either already exist in the magnet or can be nucleated relatively easily, but are strongly pinned at certain sites. Unpinning of the walls is the important step in this case. Struc- ture-property interrelationships for these two cases will be discussed below.

2. Pinning-type magnets

Typical examples of pinning magnets are the C o - S m - C e - C u system, AlNiCo, CuNiFe, CuMnA1, F e - C r - C o etc. The microstructure- property relationships bear a striking resem- blance to the precipitation-hardened aluminum alloys. The characteristic microstructure consists of a two-phase mixture of a precipitate in a matrix phase. The main feature to be borne in mind is that the magnetic properties (i.e. the crystalline anisotropy, saturation magnetization) of these two phases should be very different. In addition, the spacing of the second-phase particles is an important parameter for optimization of the magnetic properties. Thus processing variables that influence these properties are those related .to the nucleation and growth of the precipitates. The aging temperature, time, method of aging (single vs. step aging) etc. are the most important parameters. For example, for the Sm-Co-Ce-Cu system, zirconium and copper partition such that they increase the difference in magnetic proper- ties between the matrix and the precipitate [1]. The grain size is not expected to have a signifi- cant effect on the magnetic properties, since

437

domain walls already exist in the magnets. The virgin curve and hysteresis loops of these magnets reflect the microstructural condition of the mag- net, as illustrated in Fig. 3(b). The virgin curve is shallow at low fields, since the applied field is not sufficient to unpin the walls from the precipitates. When the applied field is larger than the unpin- ning field of the particular precipitate, an increase in the permeability is observed because of the movement of the walls. In real magnets, however, a spectrum of pinning sites will exist and hence the sharp rise shown in Fig. 3(b) will be smeared out [2]. On reversal of the field direction, walls nucleate relatively easily, but are pinned immedi- ately at the nucleation sites. Unpinning of the walls requires a reverse field equal to Hp.

The behavior of pinning-type magnets is rela- tively well understood and modelled [3]. A characteristic feature of the models is that the predicted H c depends on the anisotropy con- stants and exchange parameters of the matrix and the precipitate phase, thus requiring a large dif- ference between the intrinsic properties of the two phases for optimum H c and energy product.

3. Nucleation-type magnets

Typical examples of nucleation-type magnets are the sintered Fe-Nd-B magnets, MnBi, barium ferrite and SmC%. These magnets acquire their high H c and (BH)max from the large magnetocrystalline anisotropy Of the magnetic phase. The upper limit in this case is the aniso- tropy field, given by HA = 2 K I / M s, where K1 is the first anisotropy constant and M s is the satura- tion magnetization. Magnetization reversal in these magnets requires the nucleation of a domain wall against this magnetocrystalline anisotropy. Because the energy required to nucle- ate reversal against a high magnetocrystalline anisotropy is very high, reversal nucleates at structural and/or chemical defects where the local magnetocrystalline anisotropy is lower than that in the bulk. Typical examples of such defect sites are grain boundaries, two-phase interfaces, antiphase boundaries etc. Thus one typical desir- able microstructure consists of grains of the hard magnetic phase that are exchange isolated from one another by a thin layer of a non-magnetic phase. However, the presence of the non-mag- netic "grain boundary" phase will also be a poten- tial site for wall nucleation. The volume fraction of this phase will determine the fraction of grain

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boundaries that will be "laced" with it. Nuclea- tion-type magnets are more sensitive to the grain size, as illustrated by classic experiments by Becker [4]. Although the role of defects in wall nucleation has been quite commonly recognized, there have been very few attempts at systemati- cally determining the nature of these defects. And there are not many detailed ab initio models available (for a specific type of defect), in contrast to the pinning-type magnets. This is probably because the exact nature of the defects where domain wall nucleation occurs is not precisely defined. In general, H~ has been correlated to the anisotropy field with good success. In more recent models, the nucleation field (which is equal to H~ plus the demagnetizing field) has been shown to be linearly related to the anisotropy field. However, these models relate the macro- scopically measured properties, but have not been correlated to the microstructural details. So the role of magnetic interactions between par- ticles in a polycrystalline sample also remains to be clarified. The theories and experiments relat- ing to nucleation-type magnets have been dis- cussed in a recent review article [5].

One aspect that needs considerable attention is the understanding of surfaces and interfaces such as grain boundaries. Structural characterization of interfaces in other types of materials is being carried out routinely using high resolution trans- mission electron microscopy (TEM) and analyti- cal electron microscopy [6]. Characterization of surfaces is dealt with elsewhere [7]. In the case of sintered materials such as Fe-Nd-B, MnZn fer- rite or barium ferrite, the microstructure fre- quently consists of grain boundary phases (crystalline or amorphous), isolated second-phase particles introduced during the processing steps or products of solid state phase transformations. The structure, composition, spatial distribution and magnetic properties of each of these phases needs to be determined to understand the mag- netic properties of the sample. This will also enable the effect of different processing treat- ments on the microstructure and the properties to be understood. Once the role of these interfaces is clearly established, such as for the sintered Fe-Nd-B magnets, it will also be possible to engineer them to improve the properties. A typi- cal example is in the MnZn ferrite system to which calcium is added to increase the grain boundary resistivity. It has been found by con- vergent beam electron diffraction (Fig. 5) that the

~ - - , , , , ~ i ¸ ¸ ~ v •

Fig. 5. (a) [001] CBED pattern from inside the grain in CaO- doped MnZn ferrite showing 4 mm symmetry; (b) [001] CBED pattern from the grain boundary region showing the absence of 4 mm symmetry, suggesting local distortions.

symmetry of the crystal close to the grain bound- ary is tetragonal, suggesting the existence of a local strain. Calcium segregation has also been detected by X-ray microanalysis [8]. It was also shown by Lorentz microscopy (Fig. 6) that the domain walls interact strongly with the grain boundary regions and are pinned at these inter- faces. Thus, although CaO addition improves resistivity and reduces eddy current losses for high frequency recording, its presence is detri- mental for high permeability. Thus clean ferrites must be synthesized. Barium ferrite may be a good candidate since it does not require doping for high frequency applications.

Fig. 6. Lorentz images (Fresnel mode) and corresponding schematic illustrations of interaction of a moving domain wall with the grain boundary region in CaO-doped MnZn ferrite.

X-ray microanalysis (using an ultrathin win- dow detector) in conjunction with microdiffrac- tion (Fig. 7) has enabled the characterization of the grain boundary phase in sintered Fe-Nd-B magnets. Likewise, for melt-spun Fe-Co-Nd-B magnets, these techniques and high resolution imaging have been instrumental in characterizing phases whose volume fraction is below the detect- ability limit for X-ray diffraction. Similarly, other non-TEM techniques have been employed with considerable success in characterizing the struc- ture and microstructure [9].

In magnetic materials, it is equally, if not more, important to understand the magnetic domain structure in different conditions of magnetization.

439

Conventionally, Lorentz microscopy, Bitter tech- niques, Kerr effect microscopy etc. have been used to examine the magnetic domain structure. These techniques are, however, limited in the spatial resolution, which is about 100 A at best and is not sufficient to image the domain walls in some of the hard magnetic materials. Addi- tionally, they do not provide information about the direction of magnetization. The recently developed differential phase contrast Lorentz microscopy (see ref. 10 for details) has been suc- cessfully employed to examine magnetic detail with a spatial resolution reaching 20 A. This technique can reveal details of domain wall pin- ning in precipitation-type magnets, or wall nucleation events in nucleation-type magnets.

4. Summary and future directions

The structure-property relationships of pin- ning-type magnets are relatively better under- stood. The main guidelines are to maximize the difference in magnetic properties between the matrix and the precipitate and to obtain the opti- mum size and distribution of the precipitates. Alloying additions and heat treatments which help in attaining this goal are to be sought,

For nucleation-type magnets, it is generally agreed that wall nucleation occurs at defect sites. Unfortunately, the exact nature of these sites and the potency of different sites are still poorly understood. This limits a comparison of experi- mental results with theoretical predictions. On the theoretical side, ab initio models must be developed. These models have to be verified using the composition, temperature or field dependence of H c. On the experimental side, completely characterized model systems have to be used to study the mechanistic aspects. Since it is clear that in nucleation-type magnets the defect sites are at grain boundaries and interfaces, attempts have to be made to engineer these inter- faces, to "strengthen them magnetically". It may be noted that grain boundary engineering has been very successful for nickel-based superalloys wherein intergranular embrittlement has been overcome, at least partially, by the addition of boron. Engineering of the interfaces may mean that new and novel processing routes have to be adopted.

It is known that the Hc values of nucleation- type magnets have a higher temperature depen- dence than those of pinning-type magnets. Thus

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4- B . e B o Ra.g~- IB.e3B ~ , t . sne -~

STRUCTURE: fcc LATTICE PARAMETER: 5.24

PARAMAGNETIC AT 300K 411

211

100

II0

Fig. 7. Bright field image, low energy end X-ray microanalysis and microdiffraetion patterns from the grain boundary neodymium-rich phase in the sintered Fe-Nd-B system.

new processing methods have to be designed to alter the microstructure of the typical nucleation- type magnets, such as Fe-Nd-B, so that the domain walls are pinned inside the grain. Exis- tence of a composition range can be used to induce the system to undergo a solid state trans- formation (which unfortunately is not possible for the Nd2Fet4 B phase, which is a linear compound). However, non-magnetic or preferably magnetic dispersoids with the desired size, aspect ratio and magnetic properties can be introduced inside the matrix phase to induce pinning. Some success has been achieved in recent experiments on the Fe-Nd-B system. New processing techniques to produce panicles with fewer surface defects [11], compared with the conventional powder metal- lurgy processed panicles, are also needed. For example, gas atomization can provide spherical particles which can be processed further.

There is still much to be learned using tradi- tional materials science emphases, i.e. structure- property-processing interrelationships, that have

been established iteratively. The examples chosen here are merely representative of typical prob- lems solved or still to be solved. Improved characterization techniques will aid in this en- deavour.

Acknowledgments The authors wish to acknowledge stimulating

discussions with and valuable suggestions made by Dr. J. D. Livingston. We also acknowledge the support of the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Depart- ment of Energy under Contract DE-AC03- 76SF00098. One of the authors (R.R.) acknowl- edges the support and encouragement of P. L. Key, J. H. Wernick and J. M. Rowell.

R e f e r e n c e s

I L. K. Rabenberg, Ph.D. thesis, University of California, Berkeley, CA, 1983 (available through University Micro-

fiche International, Ann Arbor, MI). 2 J.J. Becker, 1EEE Trans. Magn., 12 (1976) 965. 3 H. Kronmuller, J. Magn. Mag. Mater., 7 (1978) 341; H.

Kronmuller and H. R. Hilzinger, J. Magn. Mag. Mater., 2 (1976)3.

4 J. J. Becket, 1EEE Trans. Magn., 5 (1969) 211; J. J. Becker, 1EEE Trans. Magn., 7(1971) 644.

5 J.D. Livingston, IEEE Trans. Magn., 23 (1987) 2109. 6 G. Thomas, in J. A. Pask and A. G. Evans (eds.), Ceramic

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Microstructures, Plenum, New York, 1988, p. 55. 7 J.C.H. Spence, Mater. Sci. Eng., B3 (1989) 421. 8 A. H. Tsunekawa, A. Nakata, T. Kamijo, K. Okutani,

R. K. Mishra and G. Thomas, IEEE Trans. Magn., 15 (1979) 1855.

9 R. Ramesh and G. Thomas, Acta Metall., 36 (12) (1988) 3137.

10 J.N. Chapman, Mater. Sci. Eng., B3 (1989) 355. 11 L.C. De Jonghe, Mater. Sci. Eng., B3(1989) 427.