Materials Science and Engineering. B5 (1990) 409-412 409

Short Communication

Research needs and opportunities in magnetic materials*

GARETH THOMAS

l)qmrttnent oj" MateriaZs A~'ience. Universi O' of ('aliJbrnia at Berkeley, Berkeley, (',4 94720 (U.S.A.)

It is becoming more and more widely realized that the magnetic behavior of materials has not received the attention it deserves from materials scientists. This is in contrast to the widespread and increasing scientific and technological atten- tion focused on electronic, optical and other physical properties. Only a handful of materials science programs that address magnetic materials exist in universities in the United States.

Magnetic materials have been and are finding extensive applications in a variety of techno- logical areas. Many of the new applications are placing stringent requirements on various aspects of these materials. In many cases, the problems can be reduced to conventional materials-related phenomena and solutions can be found by carry- ing out systematic studies. In many other cases, the technological challenges are completely new and new experiments, ideas and theories have to be generated to explain the observed phenomena. In order to respond to these challenges, a cadre of materials scientists and engineers is needed who are well versed in the materials, the physics and the engineering of magnetic materials. Currently, there is a scarcity of such trained personnel and the requisite equipment. Consequently, with the support of the U.S. Department of Energy, Basic Energy Sciences, Division of Materials Sciences, a Workshop was held in Santa Rosa, CA, Novem- ber 14-18, 1988, to address the topic of mag- netic materials from the materials sciences viewpoint [1J. Emphasis was placed on basic research needs to understand the fundamentals of structure-property-processing interrelations and to identify promising new areas of research. Attendance was limited to 35 people, mostly from the materials sciences, with a significant

*Summary reporl on U.S. DOE Workshop on Magnetic Materials, Santa Rosa. CA, November 14-18, 1988.

number of attendees from the physical sciences. There were also three DOE observers [21.

The mission of the Workshop was to identify research needs and opportunities, addressed through a series of seminal lectures, which have been published recently [31. The Workshop was held in an informal atmosphere in order to foster forward looking ideas and to maximize inter- actions among the participants. A significant portion of the workshop was dedicated to topical discussions in subgroups. Each participant was informally attached to a particular subgroup, although participation in the discussions of all the subgroups was encouraged and was indeed the case. The overall aim of the subgroup discussion sessions was to provide ample time and oppor- tunity for individuals with different strengths to interact with one another.

Overall emphasis was placed in three broad categories

( 1 ) New or improved experimental methods for characterizing magnetic structures and the micro- structure." One prime area of concern was the characterization of the magnetic and structural properties of surfaces and interfaces. This aspect was discussed in several of the discussions and the subgroup meetings. It is thus fair to say that a thorough understanding of the structural aspects and the magnetic properties of surfaces and inter- nal interfaces, be it in a polycrystalline material or in a multilayered heterostructure, is one broad area of research that presents research needs and opportunities.

(a) Instrumentation needs." Considerable ad- vances have been made in the area of structural and analytical characterization of materials, with concentrated efforts developed at several national research centers now equipped with sophisticated characterization tools (e.g. National Center for Electron Microscopy, Lawrence

0921-51()7/90/$3.50 © Elsevier Sequoia/Printed in The Netherlands

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Berkeley Laboratory; Arizona State University; University of Illinois Center for Microanalysis). However, high spatial resolution magnetic characterization is at best in its infancy. There are two techniques that require serious consideration as tools for characterizing the magnetic structure. One of them is the differential phase contrast Lorentz electron microscopy (DPCLEM) and the other is secondary electron spin polarization (SEMPA). The DPCLEM technique, when car- ried out on a dedicated STEM fitted with a field emission gun (FEG), has the potential to reveal the characteristics of the domain wall, the mag- netization of local regions with respect to their spatial location (i.e. whether near a defect or interface etc.). It was clearly identified that these two techniques have tremendous potential to- wards a better understanding of the fine scale magnetic structure, surface magnetism and the interaction of magnetism and microstructure. However, in order to achieve high resolution magnetic imaging a special field-free objective lens is required (otherwise the lens field saturates the specimen), in addition to the FEG and DPCLEM capabilities. Since no TEM/STEM microscopes are currently built in the U.S.A. we must rely on foreign manufacturers to provide such microscopes.

The consensus of the subgroup was that the potential of the DPCLEM and SEMPA tech- niques must be pushed to the limit. Clearly, it is equally important that more of the research laboratories involved in magnetics research are equipped with these sophisticated magnetic characterization tools. In addition, other tech- niques such as atomic force microscopy (AFM), ultrahigh vacuum techniques such as Auger spec- troscopy, X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), reflection electron microscopy (REM), neutron scattering, M6ssbauer spectroscopy etc., were also discussed, mainly for the structural, chemical and electron information they provide. The characterization of local bonding configurations in amorphous magnetic materials is another aspect that requires considerable experimental and theoretical work. The fine structure in (EELS) electron energy loss spectra can poten- tially be used to obtain nearest neighbor bonding information. This technique is still in its infancy and considerable experimental and computa- tional (modeling) work needs to be done.

Another area of instrumentation is to utilize

high resolution magnetic measurements bettel. such as with (SQUIDS) that are capable of measuring the properties of, for example, indb vidual magnetic particles. This aspect of instru- mentation is clearly an important research tool for research in the magnetic recording industry. If such information can be obtained as a function of spatial location, it would be a Large step towards mapping out the spatial magnetization distribu- tion in thin films, particulate media etc.

(2) Processing: This is the second broad area that requires considerable attention. In fact the whole area of materials synthesis and processing needs to be boosted in the U.S.A. as pointed out in a recent NRC report [4j. The processing tech- niques used for magnetic materials are generally driven by the specific application. For example, the hard magnetics industry uses conventional ceramic processing to produce bulk magnets while the recording industry primarily uses par- ticulate media or thin films. Several issues, both on the fundamental and the technological level, were addressed in the general area of materials processing methods.

In sintered nucleation-type permanent mag- nets, the structural defects, interfaces such as grain boundaries etc., are the main sites of wall nucleation and the initiation of magnetization reversal. The effect of processing parameters on the macroscopic properties and the microscopic mechanisms of reversal need to be established with consistency. Grain boundary and interfacial engineering are aspects that can be applied to the nucleation-type magnets. In pinning-type mag- nets, the nature of the pinning sites, the effect of processing variables (such as annealing tempera- ture, composition, alloying element partitioning) on the efficiency of the pinning sites needs to be determined. Microanalytical characterization is important in the case where elemental partition- ing is utilized to obtain optimum magnetic hardness.

An understanding of the magnetic hardening mechanism is another top priority in hard mag- netic materials. In the conventional precipitation- hardened magnets, wherein pinning takes place in the volume of the magnets, the issues are much clearer than that in magnets typified by the sintered Fe-Nd-B system. Design for high intrin- sic coercivity and energy product are generally the main guidelines, supported by the tempera- ture coefficients of the intrinsic coercivity and remanence. However, it was also emphasized that

processing issues involving cost lowering and technological adaptability (e.g. resin-bonded magnets) were also topics that were addressed. In tandem, the search for new hard magnetic phases in binary or ternary alloys has to be carried out. The use of novel processing techniques may be a possible route to achieving these goals. Some promising results seem to be forthcoming by using mechanical alloying techniques.

Recording rnaterials: The area of recording materials requires considerable attention and presents another broad area for research oppor- tunities.

(a) Particulate media: The details of particulate recording media are discussed in the article by Berkowitz and White [3]. Some of the main issues addressed are related to surface magnetism, anis- otropy of surfaces, effect of surface coatings (e.g. cobalt on v-Fe20~), engineering of surface mag- netic properties etc. This is another area that requires inter-disciplinary research involving structural and magnetic characterization of the surfaces, techniques for computing the magnetic properties of surfaces etc. As mentioned earlier, it would be of great value if the properties of indi- vidual particles were to be characterized, without the influence of the magnetostatic interactions. Indeed, effort in this direction is just underway at the Center for Magnetic Recording Research, UC San Diego.

On the technological side, the control of par- ticle size, shape and defect content through the use of novel processing techniques such as sol-gel precipitation, needs to be systematically investi- gated and likewise coating surfaces with powders by such methods now seems to be controllable. It may be noted that such novel liquid phase pro- cessing routes are being employed in other areas of research and development, such as in struc- tural ceramics.

Since recording media materials are primarily hard magnetic materials, research into the poten- tial of other hard magnetic materials as potential recording media should be encouraged. This requires systematic studies into the processing of hard magnetic compounds as particulates, con- trol over their surface magnetic properties, prep- aration in controllable and suitable morphologies etc. Similarly, thin films of permanent magnetic materials, such as the RE systems, could also find potential use as recording media.

(b) Thin film media: Thin film magnetic media, for both media and heads are another top priority

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area that presents several research opportunities. Clearly, there is a need to mimic the semicon- ductor heterostructures research field, in which the trend is to grow thin films of the required materials epitaxially.

In the thin film magnetic materials area, evaporation and sputtering have been in use for a long time. However, more sophisticated thin film fabrication techniques such as molecular beam epitaxy (MBE) are just beginning to be exploited. However, as shown by the work of Prinz and coworkers, the time is now ripe for a systematic evaluation of the processing of thin film magnetic media and for the integration of semiconducting and magnetic devices.

One of the advantages of growing thin epitaxial films is the potential for obtaining novel or new physical properties due to the reduced dimen- sions. The large surface-to-volume ratio of thin films may, under suitable conditions, enable unusual structures to be stabilized or composi- tions that are not usually stable in the bulk form. The mismatch strains induced at the interfaces can also be utilized to stabilize metastable phases.

An attractive form of epitaxial thin films is compositionally modulated superlattices. For example, the A1-Ni-Co, Cu, Ni-Fe or the Fe- Cr-Co systems have been extensively studied in the sixties and seventies. These systems undergo solid state transformation by spinodal decom- position, which produces a compositionally modulated microstructure. The optimization of the compositional modulation yields the best magnetic properties. With the advent of modern thin film growth techniques, this process can also be accomplished by artificial layering. A further level of structural and chemical complication arises when the structure and/or the chemical properties of the different layers are not similar. In such cases, it is essential that these layers are isolated by suitable buffer layers. Suitably archi- tectured heterostructures can also be possible candidates for three-dimensional recording media, similar to the three-dimensional semicon- ductor heterostructures. This can be a possible mechanism to increase the recording density.

The sputtered films, currently in use, are not single-crystalline and generally are found to have a columnar microstructure. The role of this granularity on the magnetic properties, especially on the magnetization reversal is still not clear. It is evident, however, that control of the intergranular regions is critical for the improvement of the

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signal-to-noise ratio, through a reduction in cross-talk.

In order to unravel the scientific and techno- logical aspects of heterostructures, collaborative research involving materials scientists and crystal growers, solid state physicists and electrical engineers, and chemists is needed. This calls for the establishment of either a large research pro- gram in a single institution or collaborative programs among institutions that can provide complementary skills. The ultimate goal of such efforts would be to integrate materials with differ- ent properties and applications in a single hetero- structure so that multiple tasks can be performed in parallel. In this sense, there is a need to emulate the efforts that have been made with semicon- ductor heterostructures in which epitaxial metal- lic layers have been successfully integrated with semiconductor structures.

(3) Microstructure-property interrelationships: A general requirement for all the materials of interest is to ascertain interrelationships between microstructure composition and the magnetic properties. This needs an approach involving systematic modifications of the critical processing parameters, characterizing the local structure, composition microstructure and magnetic struc- ture along with measurement of the relevant

physical (both intrinsic and extrinsic) properties. Clearly, there is also the need for appropriate models that can address the theoretical aspects of the problem. Modeling and theoretical studies are required at two levels. At the fundamental level, models that can explain magnetic properties such as the origin of magnetocrystalline anisotropy, magnetostriction etc., are required. On the pheno- menological side, models that explain reversal mechanisms in sintered magnets, particulate media, or thin films need considerable attention. The detailed mechanism(s) of reverse magnetiza- tion are still not elucidated.

In summary, magnetic materials research is an inter-disciplinary endeavor involving materials scientists, physicists, electrical engineers and chemists. Avenues should be found to enhance the interaction between these groups to respond to the challenges that exist in this area of research.

1 Supported by the Director, Office of Energy Research~ Office of Basic Energy Sciences, Malerials Sciences Divi- sion of the U.S. Department of Energy under contract No. DE-ACO3-76SF 00098.

2 R. Heckel, J. Darby and D. Keefer. 3 Mater. Sci. Eng., B3 (1989). 4 R. Price and S. Maurizi (eds.), Materials Science and Engi-

neering Jor the 1990s; Maintaining Competitiveness in the Age o[Materials, 1989.