Development of Advanced Magnetic Force Microscopy Tips for Magnetic Characterization

R.D. Gomez, Jon Orloff and Klaus EdingerUniversity of Maryland, College Park, MD andSy-Hwang LiouUniversity of Nebraska, Lincoln, NE

PROJECT SUMMARY:This research will emphasize novel means of the development of improved

magnetic tips for magnetic force microscopy for characterizing the magnetic properties of thin film magnetic materials. The improved tips will be achieved by utilizing a variety of magnetic coatings which can be tailored to the properties of the materials under investigation, and by using state-of-the-art technologies like ion-beam milling to fabricate tips capable of much higher spatial resolution and enhanced sensitivity.

INTRODUCTION:

The study of the magnetic properties of matter on a nanometer scale is of interest both from the scientific and the technological points of view. From a scientific standpoint, our present understanding of magnetism at these length scales is in its infancy at best. It is only in recent years that experimental tools and sufficient computing power have become available to tackle the complicated magnetic behavior occurring the microscopic length scales. In comparison with bulk magnetic studies where the effects of interfaces, defects, morphological grains and so on are treated as perturbations and lumped into global parameters, micromagnetic models attempt to incorporate these as locally varying parameters in combination with the relative contributions of the Zeeman, magnetostatic, magnetoelastic and local anisotropies. The goal of predicting the magnetic domain structures is thus an inherently complex problem, and we are just beginning the development of models starting from well-known magnetic alloys having the simplest geometrical configurations. The so called “standard problem #1”, inititiated by the National Institues of Science and Technology several years ago, consists of a rectangular 1um x 2um strip of a 50 nm Permalloy thin film with no crystalline anistropy and perfect edges. From a technological point of view, the ability to measure and control the properties of magnetic domains is of great interest in the areas of data storage, micro electromechanical systems (MEMS), magnetic ramdom acess memories (MRAMS), magneto-electronics (spin transistors), quantum computing and even as magnetic sensors for the industrial and biomedical applications. Often, the novel features of these devices, such as the giant magneto-resistance in field sensing applications or the ultimate areal density for recording media, have its origins upon the nascent micromagnetic domain configurations.

Magnetic force microscopy is arguably the most important imaging tool for studying a wide variety of local magnetic phenomena. Since its development in 1987, it has emerged as a powerful micromagnetic tool and has revealed magnetic processes with unprecented clarity, resolution and ease. It allows the direct visualization of magnetic domains and provides the experimental basis for theoretical modeling. The technique measures change of the interaction force between a magnetized probe and the local stray magnetic field from the sample, point by point, as the probe is scanned across the surface. The probe is a silicon-based cantilever with a ferromagnetic tip on the free end and placed in close proximity (~25 nm) with sample surface. The inherent resolution depends upon the confinement of the interaction at the end of the probe and sensitivity depends upon the ratio of the cantilever spring constant and the magnetic moment. At present, the resolution of the commercial MFM probes is about 50-100 nm with a force constant of about 0.01 N/m, which is roughly equivalent to resolving the field gradients from a 10-12 emu source at a distance of 50 nm.

Despite the impressive performance and widespread use of the MFM, there are important probe-related limitations that need to be overcome to realize the full potential of this technique. First of these is the enhancement of resolution and sensitivity. As is well known from microscopy, in order to measure something at a given scale it is necessary to have a probe whose fundamental size is well below the size of the object to be measured. In the case of atomic force microscopy, for example, in order to

characterize precisely and accurately a trench with a width of 0.1 micrometers and a depth of 0.2 micrometers, one needs a probe in the shape of a rod with a length of at least 0.2 micrometers and a well characterized diameter of less than 0.1 micrometers. In the case of magnetic force microscopy (MFM) the force between the probe and the sample is carried by the magnetic field. Obviously, the smaller the magnetically active area of the probe, the less it will be affected by areas from far away since the dipole nature of the field causes it to diminish rapidly with distance. Therefore, in order to make a high resolution MFM it would be necessary to create an extremely small magnetic probe. The smaller volume of the magnetic probe will result in a lower magnetic moment and a smaller interaction volume, and thus a weaker force. Hence, the lateral resolution of the MFM probe will also be limited by its sensitivity (e.g. the spring constant of the cantilever). The improvement in resolution would have to be complemented with an enhancement of the probe sensitivity.

Second, is the development of specialized probes whose properties are optimized for a given specimen and free from instrument-induced distortions. Since the MFM relies on a mutual interaction, it is inherently invasive. Thus, it is easy to imagine that measurement process itself could cause irreversible changes to the system and the measured image may not reflect the intrinsic state of the sample. Conversely, the probe’s moment itself may change as it moves in varying fields, which would cause a nonlinear instrument response. This would render the interpretation of images to be quite complicated and equivocal. To overcome this problem, it is necessary to use probes whose properties are compatible with the sample at hand. For instance in one extreme, a sample with very low coercivity such as garnets or Permalloy, would require the use of high sensitivity, low moment probe. The low probe moment ensures that fringing field is much lower that the sample coercivity, and increased sensitivity is to compensate the reduction in the interaction forces. On the other extreme, for the case of permanent magnet samples, the probe would have to be relatively stiffer and its coercivity must be higher than the stray field generated at the sample surface. In cases where no known materials exist that could withstand the strength of the specimen field, it may be judicious to use superparamagnetic probes and interpret the images accordingly. Most interesting problems are likely to have requirements that are midway between these extreme cases, while certain applications, such as understanding the magnetic evolution of soft magnetic materials at high fields, might require a low moment probes with high coercivity.

Finally, the MFM requires the fundamental understanding of the magnetic characteristics of the probes themselves, which can be incorporated into theoretical models of image interpretation. At present the generally accepted model for MFM assumes a point dipole at the tip apex. This picture is adequate in qualitative descriptions which treat the images as representations of the distribution of magnetic charges from the divergence of the volume magnetization or the normal component of the surface magnetization. Several sophisticated theoretical descriptions for image representation have been proposed in the literature which take into consideration the finite volume of the tip. Unfortunately, because of the absence of direct experimental evidence of the probe’s magnetization distribution, the models simply provide possible explanations of observed contrast formation rather than offer precise magnetization reconstruction.

The goal of this work is address the aforementioned limitations of conventional MFM probes, by improving the resolution and sensitivity, by developing processes to tailor probes with predefined moment and coercivity, and by developing characterization and calibration methods for incorporation into theoretical models of image reconstruction. In this work, we will use our combined resources in thin film preparation, micromachining using focused ion beam (FIB) facility and expertise in magnetic force microscopy. By using this collaborative approach, the composition, probe size, cantilever mechanical sensitivity will be systematically studied and tailored over a wide range of parameters. The main feature that differentiates this project from other probe development efforts is the nature of the collaboration. This effort spans a very broad based development --- exploring the interrelationships of film chemistry, micromachining, MFM implementation and theory.

II. STATE OF THE ART IN MFM IMAGINGThe efficacy of the MFM as a micromagnetic tool and the need for advanced tips

is best illustrated by considering a case study of a micron-sized NiFe (Permalloy) island. Permalloy is one of the most important and well known magnetic alloys, owing to its unique characterisitics that make it ideally suited for field generation and detection. In fact, nearly all magnetic recording systems use Permalloy in one or more components and it is likely to be a major ingredient in the next generation of magnetic devices. Nevertheless, despite the enormous volume of work in studying this material, it is only now that we are beginning to understand the dependence of the magnetic properites with the material shape, edge irregularities, defects and pinning centers, and magnetization history.

The set of MFM images in figure 1 shows the micromagnetic evolution of a NiFe square element with a 3 um lateral dimension and about 26 nm thick. (some discussion on the MFM). Image is sensitive to change in M rather than M itself. Remanence image shows 7 domain with crosstie structure, evolving showing the pinning effects of the xtie. It progressive expands until a transtion at 60Oe is observed. This is interesting BUT: contrast is weak. Apart from the domain walls and xtie contrast, the interior regions show little or weak contrast. The resolution is limited, unable to measure the width of the 90 and 180 degree walls. Perturbation due to probe field evident in image at 60 Oe, which limits the increments that the field that could be applied in the evolution studies. Most importantly, the internal magnetization of the probe is unknown which makes it very difficult to recontruct the magnetization of the sample. If we have an ideal probe, what can we do? Can measure the domain walls, can derive the ratio anisotropy versus exchange field, can determine the precise switching and reversal behavior, i.e., nucleation at edges or at some interior regions. Can determine the micromagnetic everywhere and not only near the edges.

PROGRAM OBJECTIVESThe questions that will be addressed are:

What are the factors that determine the experimental resolution limit of magnetic force microscopy and how much can it be improved through the use of advanced MFM tips?What are the ultimate limits on magnetic grain sizes, and what are the factors that influence them?

We propose to address these questions by systematically using a variety of coating chemistry, thin film fabrication techniques, and focused ion beam (FIB) technology combined with theoretical modeling to:(i) understand the micromagnetics of coated thins on Si and SiN3 AFM probes, both as

function of chemistry and geometry,(ii) develop new models and experimental methods to characterize the MFM tips, and to

quantify their interaction (and perturbation) with the sample,(iii) elucidate the relationship between probe size and MFM resolution,(iv) understand and control the mechanical properties of micro-machined AFM probes.We will then use the MFM to investigate, in detail, the evolution of magnetic domain structures in nanostructured thin films with a spatial resolution previously unobtainable. We expect that the proposed studies will play a central role in our continuing efforts to elucidate magnetization reversal mechanisms and to develop predictive models of the reversal process. The proposed studies will also guide our efforts to develop new thin film media for high-density information storage and for magnetic sensor applications.

III.

Preliminary ResultsA magnetic probe can be fabricated by coating an AFM tip made of Si 3N4 or Si

with an appropriate material of high permeability. Such a probe can be successfully used for MFM. But since an AFM probe consists of a structure with dimensions of many micrometers having a sharp point, the inherent resolution for MFM is not high - the magnetic material covers too large an area. If such a tip were fabricated and then machined using a high resolution ( < 0.05 micrometer) FIB, it is possible to remove the magnetic material everywhere except at the apex of the probe, as shown in Fig.1. This has to be done with great care to avoid damaging the magnetic material at the tip apex, and we believe it can be done much better by developing a coating the Si3N4 or Si probe with a "stop-layer" of material having a thickness of only a few tens of nm, so that the tip can be imaged at high resolution with the FIB prior to the micromachining step. The stop-layer can subsequently be removed by chemical means after the micromachining step.

The figure below shows an example a sharp probe we created using FIB. The original probe was pyramidal with facets (having half angles of 25o front, 17o side and 10o front) which tapered to a point roughly 30 nm. The probes are batch-fabricated using selective etching techniques, so that the resulting facets are more or less imposed by the crystallographic property of the material. It is quite difficult to prepare probes that have arbitrary geometry, e.g., very high aspect ratios. However, as shown in the figure, a very

sharp protrusion can be prepared by ultra high precision FIB milling of the commercial probe. In this example, the nominal radius of the tip is about 300 nm with a length of about 2 microns and a diameter of about 100 nm.

Left: FIB micromachined MFM tip. Right. Schematic of the geometry of untrimmed commercial MFM tip (Digital Instrument spec).

One would suspect that a probe were coated with a magnetic material, then it is conceivable that a small amount of magnetic material would be left at the protrusion, and thereby satisfy our requirement of small magnetic volume, i.e., an ultra high resolution probe. However, things are not so simple. In some cases, as our initial results suggest, the MFM contrast are very different from what one would expect by merely reducing the effective volume size while keeping the magnetization unchanged. A number of important questions emerged from our initial work. First, we suspect that the process of material removal changes the magnetostatic energy of the magnetic film, which causes it to remagnetize in some, thus far, unknown manner. Secondly, we suppose that a reasonable alternative is to micromachine first and later deposit the magnetic layer. We have successfully implemented this approach and the results are very promising. In both cases, the nascent issue is how to control the films under those conditions. The exact nature of the remagnetization process of micromachined magnetic films and the micromagnetics of magnetic thin films deposited on protrusions can be understood and potentially controlled, only through a careful systematic study. Finally, there is the issue of ion implantation. The process of FIB causes a certain dose of gallium (or other atoms) to be implanted on the probe. While the use of FIB micromachining on a coarser (1 micrometer) scale is already being used commercially for the finishing step in the production of read-write heads for magnetic disk drives, the effects on the magnetic properties on probes at the submicron length scales are still unexplored. Therefore, while a major portion of this study will be devoted to determining the relationship between probe tip morphology and the field distribution associated with the tip, substantial emphasis will be placed on extending the limits of FIB techniques on magnetic thin films.

Expected Outcome of This Research :The purpose of the proposed research is to develop improved methods for understanding

and characterizing the magnetic properties of nanostructured materials. This research should have significant implications for a variety of advanced technologies, including high-temperature permanent magnets, extremely high-density information storage applications and magnetic sensor applications. In each of these cases, the atomic-scale structure of the materials plays a dominant role in the macroscopic magnetic behavior, especially as regards the thermal and magnetic stability. The magnetic properties of nanostructured materials are usually controlled by the behavior of the fundamental “magnetic building blocks” - magnetic grains or clusters - and their interactions. The typical magnetic grain-size of these technology important materials is in the range of 5-10 nm. To have a better understanding these magnetic materials, it will be necessary to control and characterize the structure of the materials on the nanoscale level. We expect that the proposed activities will also have impacts as follows: The advanced MFM tip can serve as a small and sensitive magnetic sensor or a local magnetic field source for a variety of applications. It may be possible to use a similar approach for the improvement of other long-range scanning-probe force microscopy applications, e. g., to obtain images of ferroelectric domains.

(1) "Magnetic coating materialsThe control of the size and shape of magnetic materials"

A. Fabrication and Characterization of Nanostructured Magnetic Clusters The goals of this research are to synthesize and understand magnetic single

domain particles and thin films with nanoscale magnetic particles that suitable for MFM applications. The desired properties include a very large magnetic anisotropy or the superparamagnetic instability inherent in extremely small magnetic particles.

1. Fabrication of Nanostructured FilmsThere are many ways to fabricate nanostructured magnetic films. We describe

here a few of the methods that we have developed.a. Granular Magnetic Films

Granular Magnetic Films have the form of small crystallites dispersed in a matrix. The magnetic properties of these films can be controlled by either changing the size of crystallites and/or the separation distance between the crystallites[1,2]. The average size of the Fe crystallites is 4-5 nm and the Fe crystallites are mostly isolated in an amorphous SiO2 matrix.

b. Nanocrystalline FilmsNanocrystalline films such as, CoxPr1-x and other alloys that have very high magnetic

anisotropy energy has been systematically studied. These films have high coercivities, about 2.5 to 4 kOe, and other favorable properties. The nanostructure and their magnetic properties can be also tailored by annealing, for example the coercivity at room temperature as high as 45 kOe were achieved. The nanocrystallite in the film is about 5 nm for a 28 nm thick PrCo// Cr film annealed at 500 oC. [3]

c. Nanoparticulate FilmsIt is known that an assembly of very fine noninteracting high anisotropy magnetic

particles is magnetically hard; i.e., it has a large coercivity. This effect is due to the fact that the particles are single domains and that magnetization reversal takes place only by rotation of the magnetization vectors against strong anisotropy forces. In an earlier report, we showed that the CoPt film containing nanometer-size particles has the coercivity as high as 30 kOe [4]. Figs. 1a and 1b are the AFM and MFM images from a CoPt sample.

As shown in Fig. 1(a), the 5nm-thick film contains well-separated nanometer-size crystallites as indicated in the AFM image. The sizes of the crystallites are in the range of 100nm to 400nm. The height of crystallites is in the range of 20nm to 80nm. The MFM image, Fig. 1b, was obtained using a CoPt MFM tip magnetized parallel to the sample surface. The light and dark contrast corresponds to the strength of the stray-field gradient on the sample surface. The lighter color represents a frequency shift in the MFM tip when the magnetization of the sample and that of the MFM tip are repulsive. As shown in Fig. 1b, crystallites with one light and dark area are single-domain (as indicated by "S"); the grain that may contain a few crystallites with two or more light and dark areas are multi-domain (as indicated by "M"). The size of a single-domain crystallite is between 100-200nm.

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Figure 1. The topographic (a) and magnetic (b) images of a 5 nm thick CoPt film on a

quartz substrate, annealed at 750 oC for 3 hours. "M" indicates a multi-domain grain. "S" indicates a single domain grain.

At first, we will identify and fabricate nanostructured magnetic films suitable for our purposes. The main focus will be on materials with very high anisotropy energy and stable magnetic properties over a wide range of temperature. Some promising candidates are CoPt, Nd2(Co, Fe)14B, and AlNiCo. We will attempt to alter the microstructure and microchemistry of these and other phases to optimize the structural and magnetic parameters.

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"The advanced cantilevers The development of new methods for the characterization of MFM tips"

B. Development of Advanced MFM tips The goal of this research is to obtain higher resolution and/or sensitivity in MFM

images, and to develop improved methods of interpreting MFM images.

There are many possible ways to improve the resolution of magnetic imaging, but one of the most productive routes is through the development of advanced tips for MFM measurements. We describe here a few of the tip innovations that we have developed.

1. Novel Tip CoatingsWe have developed two types of tip coatings which markedly improve the quality

of the data taken and the ease of interpretation of the resulting images. The first tip coating developed (“soft” MFM tips) is based on superparamagnetic Fe particles embeded in a SiO2 matrix. The Fe particles can rotate freely in the presence of the stray magnetic field from the sample, and thus cannot cause the sample magnetization to reverse (which has been a severe problem with conventional “soft” MFM tips). The force between the tip and the sample is always attractive, so that interpretation of the images is relatively straightforward (similar to Bitter techniques). These tips have been used to demonstrate high resolution imaging in magnetic recording heads and in nanostructured magnetic films [5].

The second tip coating developed (high-coercivity MFM tips) is based on permanent magnetic materials, such as CoPt alloys. [6] We have developed a very high coercivity (larger than 10 kOe) tip coating which is useful for investigating high stray field materials and devices. It is most useful however for obtaining MFM images with a magnetic field applied to the sample. As long as the applied field is smaller than the coercivity of the tip, the interpretation of the images is relatively straightforward. It may ultimately make possible more complete studies of the dynamics of the reversal process.

So far, we have demonstrated the capability of making these tips, but there are many materials-processing parameters which need to be fine tuned to further improve the magnetic properties of the tips. For example, we will need to systematically study how the thickness of the magnetic coating on the tip affects the tip-sample interactions.

2. Point-Dipole MFM TipsWe have developed a new tip in which the active magnetic material (in this case,

the amorphous 2605Co alloy ) is deposited only on the very end of the tip. The resultant smaller active region of the tip (500 nm in dimensions) results in improved spatial resolution. Figure 2 shows a scanning electron microscope image of this tip. The amorphous 2605Co alloy is a soft magnetic material that is used as the core of high power transformers. The amorphous alloy is structurally and magnetically isotropic. A coating of an amorphous magnetic material is expected to be less sensitive to the shape of

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the tip than a crystalline magnetic material. The magnetization of the 2605Co amorphous alloy is about twice that of Permalloy. The coercivity field of the tips is about 3 Oe which is smaller than the typical stray magnetic field near written bits in hard disk recording media.

Fig. 2 MFM cantilever with a magnet of size 500nm on the tip.

We compared the images obtained using a standard “soft” thin film tip and this new “point” tip. Both tips were coated with same magnetic material and thus have soft magnetic behavior (i.e. the tip magnetic moment follows the stray field of the sample). The left side of Figure 3 shows MFM images from the same area of a reference sample (a recorded tri-bit on a standard hard disk, provided by Paul Rice of NIST [7]). The tri-bit pattern is marked by the arrow in Fig. 3. The top of Fig. 3 shows the corresponding AFM images. For a “soft” MFM tip, the interaction of the stray field from the recording disk results in a magnetic force on the tip which is always attractive (as shown in dark line). Figures 3a and 3b are the images from the standard thin-film tip, while Fig. 3c was taken using the point tip. The difference between Figs. 3a and 3b is that the tip was initially magnetized in opposite directions, in both cases perpendicular to the sample surface. It is noted that Figs. 3a and 3b are not identical; this is a consequence of the fact that the extand area of the tip coating are farther from the sample, and thus see only a weak stray field. The stray field is not sufficient to switch the magnetization in the whole tip coating, which degrades the resolution, and makes interpretation of the image more difficult. Consequently, the tri-bit pattern written on the disk could not be resolved. Only one or two transitions (depending on the magnetization direction of the tip) are observed. In contrast, Fig. 3c shows that the magnetization of the point tip follows the stray field of the sample, with essentially no effects due to the magnetic interactions from the extand area of the tip. It shows clearly that the point tip provides a better resolved MFM image. This observation is consistent with a theoretical analysis which considered the point tip response as a function of the tip geometry and the tip-sample spacing. [8,9] This analysis shows that the point dipole tip has a resolution of 0.6r (r is the radius of curvature of the apex of the tip), while a conventional tip only has a resolution of 1.25r.

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Fig. 3 A comparison of the MFM images with the thin-film tip (2a and 2b) and the point tip ( 2c). The details are explained in the text.

3. Ultrafine Tips

Figure 4 shows a scanning electron microscopy image of a very sharp MFM tip that produced by focused ion beam milling (Produced by Professor Jon Orloff of the University of Maryland). The tip was first coated with a magnetic film then machined by a focused ion beam source, so that there is a nanometer-size magnetic particle on the very end of the tip. The smaller size of the magnetic particle result in much improved lateral resolution.

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Fig. 4 An MFM cantilever machined by a focused ion beam source.

As shown in the Fig. 5, the domain configuration of a 150nm-thick epitaxial (110) Fe film obtained by a MFM tip that was machined by a focused ion beam source. The image was performed with a vertically magnetized tip in a zero applied magnetic filed to the sample. The full width at half maximum of the Bloch domain wall width was measured to be 60-70nm that agrees well with the calculated value for that of bulk Fe. This domain image is clearly has higher resolution than the recently published results [10]. The above MFM tip innovations undoubtedly lead to an improved imaging capability. We propose a systematic investigation of these advanced tip technology that has been demostrated for high-resolution MFM images. In this project, we will continue to improve the magnetic properties of the MFM tips. We propose a systematic investigation of tip-sample interactions. We will have a better understanding of magnetic images and will improve the resolution of magnetic imaging.

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Fig. 5 The MFM domain image of a 150nm-thick epitaxial (110) Fe film obtained by a MFM tip that was machined by a focused ion beam source. The arrows indicate the magnetization direction. 180o Bloch domain wall is clear observed. The full width at half maximum of the Bloch domain wall width was measured to be 60-70nm that agrees well with the calculated value for that of bulk Fe.

Some specific research topics are proposed here: If we are to improve currently-used magnetic coating materials, we will need to improve the crystalline orientations of the magnetic material, in order to control the anisotropy of the magnetic material at the tip. If this can be done, the magnetization direction of the tip will be well defined, which is important in the interpretation of the magnetic images. The crystalline orientations of the magnetic material will be controlled by using different growth conditions and different underlayers. Such techniques have been extremely successful for fabrication of nanostructured magnetic films. We propose to study tip-sample interactions by systematically varying the thickness of magnetic layer on the tip. We propose to fabricate advanced tips with a well-defined magnetic size and well-defined and controlled tip magnetic properties. We propose to more fully characterize the tips and their coatings to achieve more complete quantitative understanding of magnetic images. The characterization methods used will be similar to those employed by Gomez, by Babcock and by Proksch. [11-13] We propose to continue to search for better magnetic tip coating materials.

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IV. EXPECTED OUTCOMES:

To improve the ultimate resolution of MFM (to obtain a resolution of 10 nm or perhaps even better). The advanced MFM tips can be used for magnetic characterization of nanometer-size features, such as magnetoresistive sensors and random-access memories etc.. The advanced MFM tips can be used by other research groups. We will collaborate with national laboratories, universities, and companies. The advanced MFM tips can be used for detecting electrical currents on nanometer scale. For example, it may be possible to map the current density in superconductors or integrated circuits. The advanced MFM tips serve as small and sensitive magnetic sensors for a variety of applications. It may be possible to use a similar approach for the improvement of other long-rangescanning-probe force microscopy applications, e. g., to obtain images of ferroelectricdomains. We will better understand the magnetic behavior of single-domain particles, and better understand the connection between nanostructure and that behavior We will determine the ultimate size for magnetic devices. We will develop advanced materials which will ultimately lead to improve devices in a variety of applications Students and postdocs will gain knowledge and skills appropriate for a wide range of

career choices.

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V. REFERENCES:

1. S. H. Liou, C. L. Chien, “Granular Meter Films as Recording Media”, Appl. Phys. Lett., 52, 512(1988).

2. S.S. Malhotra, Y. Liu, J. X. Shen, S. H. Liou and D. J. Sellmyer; ``Thickness Dependence of the Magnetic and Electrical Properties of Fe:SiO2 Nanocomposite Films”, J. Appl. Phys., 76, 6304(1994)

3. S. S. Malhotra, Y. Liu, Z. S. Shan, S. H. Liou, D. C. Stafford, and D. J. Sellmyer; “High Coercivity PrCoCr Thin Film for Longitudinal Magnetic Recording Media”, J. of Magnetism and Magnetic Materials, 161, 316(1996).

4. S. H. Liou, Y. Liu S. S. Malhotra, M. Yu and D. J. Sellmyer; ``Magnetic Properties of Nanometer-Size CoPt Particles'', J. Appl. Phys. 79, 5060(1996).

5. S. H. Liou, S. S. Malhotra, John Moreland and P. F. Hopkins,“High Resolution Imaging of Thin-Film Recording Heads by Superparamagnetic Magnetic Force Microscopy Tips”, Appl. Phys. Lett., 70, 135(1997).

6. S. H. Liou, and Y. D. Yao, "Development of High Coercivity Magnetic Force Microscopy Tips" to appear at J. Magn. and Magn. Mater. 1998

7. Paul Rice and Stephen E. Russek, and Bill Haines, “Magnetic Imaging Reference Sample”, IEEE Trans on Magn. Vol. 32, 4133(1996).

8. D. Rugar, H. J. Mamin, P. Guethner, S.E. Lambert, J.E. Stern, I. McFadyen, T. Yogi; J. Appl. Phys. 68, 1169(1990).

9. P.Grutter, H.J. Mamin, and D. Rugar; “Magnetic Force Microscopy(MFM)” in Surface Science: Scanning Tunneling Microscopy II, eds. By R. Wiesendanger and H.-J. Guntherodt, (Springer Verlag Berlin, 1992,1995)

10. A.D. Kent , U. Ruediger, J. Yu, S. Zhang, P. M. Levy, Y. Zhong, S. S. P. Parkin; "Magnetoresistance due to Domain Wall in Micron Scale Fe Wires with Stripe Domains", IEEE Trans. on Magn. 34, 900(1998).

11. R. D. Gomez, 12. K. L. Babcock, V. B. Elings, J. Shi, D. D. Awschalom, and M. Dugas, “Field-

dependence of Microscopic Probes in Magnetic Force Microscopy”, Appl. Phys. Lett., 69, 705(1996).

13. Roger Proksch, George D. Skidmore, E. Dan Dahlberg, Sheryl Foss, J. J. Schmidt, Chris Merton, Brian Walsh, and Matt Dugas, “Quantitative Magnetic Field Measurements with the Magnetic Force Microscope”, Appl. Phys. Lett., 69, 2599(1996).

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VI. FACILITIES:

The PI’s of this proposal have a wide variety of deposition systems and characterization tool for making nanostructured materials and for characterizing them structurally, magnetically, and electronically. A brief list of existing capabilities follows.

1. Two multiple gun sputter systems 2. Four magnetometers for measuring magnetization, (Quantum Design SQUID, Lake3. Shore AC/DC susceptometer, Lake Shore High temperature virbration sample

magnetometer, Micro-Mag alternating gradient force magnetometer). Samples can be studied at temperatures from 5 K to 400 K in fields up to 9 T.

4. Zeiss polarizing microscope with CCD camera for magnetic domain observations. The ultimate resolution of this system is 0.5 microns.

5. Atomic force microscopy (Nanoscope IIIa, with frequency detection module for MFM) 6. CMRA Central Service Facilities: (a.) X-Ray Materials Characterization Facility,

Electron Microscopy Facility; a EOL2010 TEM, a JEOL JSM840A SEM. The Department of Physics maintains a well-staffed machine shop, electronic shop and there is a glass-blower in the Department of Chemistry as well as another machine shop to which we have access.

VII. ORGANIZATION/EDUCATION/COLLABORATIONSThe proposed project will benefit the research group in magnetism at the

University of Nebraska. This group has 9 faculty and about 25 postdoctoral, graduate and undergraduate researchers. Many of our graduate and undergraduate students and postdoctoral are already employed in the magnetic storage industry. For examples, F. Foong (Liou’s student) are with Applied Magnetic Inc.; S.S. Malhotra and D. C. Stafford (Liou’s students) are with HMT Technology Corporation; A. Tsoukatos (Liou’s postdoctoral) is with the Materials Research Corporation.

It is worthwhile to note that we are also collaborating with researchers at IBM, Digital Instruments Inc., Park Scientific Inc., Quantum Peripherals of Colorado, the National Institute of Standards and Technology and the University of Maryland. The research proposed here will also have great impact on the development of advanced MFM tips which will be used by collaborators at national laboratories , companies and other universities.

VIII. CREDENTIALS OF PARTICIPANTSLiou’s research interests include nano-engineering of magnetic films and studies

of interfaces and particle size effects, applications of scanning probe microscopy. He has more than 140 articles in refereed journals and books, 34 invited presentations, and 1 patent. Liou has worked as a guest research scientist in the group of nanoprobe imaging for magnetic technology at the National Institute of Standards and Technology for one year. He is currently collaborating with groups at Digital Instruments Inc., Park Scientific Inc., IBM, Quantum Peripherals of Colorado, the National Institute of

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Standards and Technology and the University of Maryland in the developing advanced MFM tips

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Sy-Hwang LiouDepartment of Physics and Astronomy and Center for Materials Research and AnalysisUniversity of Nebraska, Lincoln, Nebraska, 68588-0111Tel. 402-472-2405, Fax: 402-472-2879, E-mail: sliou@unlinfo.unl.edu

Educational Background1974 B.Sc. in Physics, Soochow University, Taiwan1979 M.Sc. in Physics, Florida Institute of Technology, Melbourne, Florida1981 M.Sc. in Physics, Johns Hopkins University1985 Ph.D. in Physics, Johns Hopkins UniversityHonors and AwardsOutstanding paper of the Center for Electronics and Electrical Engineering (NIST) 1988Professional Experience1995-1996: Guest Research Scientist, NIST, Colorado1993-present: Associate Prof. of Physics, University of Nebraska1990-1994: Co-Editor, Applied Physics Communications 1988-1993: Assistant Prof. of Physics, University of Nebraska1986-1988: Post-doctoral Staff, AT&T Bell Laboratory1985-1986: Post-doctoral Fellow, Johns Hopkins UniversityResearch: Nano-Engineering films - fundamental studies of interfaces and particle size effects. Magnetic Force Microscopy - magnetic domain images and advanced MFM tips for

improving magnetic images and local probes. High Tc oxide superconductors and magnetic oxides – microstructural characterization, static

and dynamic properties of flux motion, and growth of complex oxide compounds. Author of more than 140 articles in refereed journals, 30 invited presentations, and 1 patent. Selected Publications Related to This Proposal:1. “High Resolution Imaging of Thin-Film Recording Heads by Superparamagnetic

Magnetic Force Microscopy Tips”, S. H. Liou, S. S. Malhotra, John Moreland and P. F. Hopkins, Appl. Phys. Lett., 70, 135(1997).

2. “Magnetization Reversal Behavior in Cobalt Rare-Earth Thin Films”, S. S. Malhotra, Z. S. Shan, D. C. Stafford, S. H. Liou and D. J. Sellmyer; IEEE Trans. on Magn., 32, 4019(1996).

3. “Magnetic Properties of Nanometer-Size CoPt Particles’’, S. H. Liou, Y. Liu S. S. Malhotra, M. Yu and D. J. Sellmyer; J. Appl. Phys. 79, 5060(1996).

4. “Magnetic Properties and Magnetization Reversal of CoSm//Cr Thin Films”, Z. S. Shan, S. S. Malhotra, S. H. Liou, Y. Liu, M. J. Yu and D. J. Sellmyer, J. of Magnetism and Magnetic Materials, 161, 323(1996).

5. “High Coercivity PrCoCr Thin Film for Longitudinal Magnetic Recording Media”, S. S. Malhotra, Y. Liu, Z. S. Shan, S. H. Liou, D. C. Stafford, and D. J. Sellmyer, J. of Magnetism and Magnetic Materials, 161, 316(1996).

6. “Metal-Insulator Composites Having Improved Properties and Method for Their Preparation”, C.L. Chien, Gang Xiao, S.H. Liou; U.S. Patent 4,973,525 Nov. 27, 1990.

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Technical DiscussionThe utility and the neccessity of developing advanced probes is best illustrated in the case

study of a 3 micron x 3 micron Permalloy element. mentioned in the Introduction,

a function of The individual magnetic transitions that encode data bits are comprised of magnetic grains which are typically on the order of 20 nm or less. The readback noise is directly relnt ated to the sharpness of the transitions which in turn is a function of the grain size as well as the interaction between individual grains. As areal storage density in magnetic recording increases, the transition spacing becomes shorter, making the transition edge noise increasingly significant. One solution is to reduce the individual grain sizes, which is currently implemented by the industry. However, it is not possible to arbitrarily reduce the grain dimensions before reaching a fundamental limit where the particles are no longer ferromagnetic but become superparamagnetic. Estimates based on bulk samples put this limit at about 15 nm grain size, although there is a strong sentiment in the community that in thin films, this limit might be smaller. Clearly, a systematic investigation of this behavior, using a magnetic imaging instrument with sub 10 nm resolution will elucidate the exact nature of this hard constraint.

The magnetic properties are highly contingent upon the exact stoichiometry, crystallinity and film thickness. In the case of nanostructured materials, the situation becomes even more interesting, since the additional effects of edges and geometry-induced demagnetizing fields come into play.

Motivation: Scientific and Technological PerspectivesOur current level of understanding of these artificially fabricated magnetic structures is

very limited in comparison with our knowledge of bulk and single crystal magnetic systems. Issues such as the mechanisms of magnetization reversal, dynamics of local moment switching, thermodynamic stability and magneto-transport are but a few open topics that require intensive exploration. Predictive numerical calculations such as micromagnetic simulations have proven to be quite useful in describing the mechanism of domain motion and reversal. However, these are based on solving coupled LLG equations and require empirically-derived quantities such as exchange interaction parameter, along with modifications to include magnetization pinning at defect sites. Similarly, theoretical models on magneto-transport effects such as anisotropic and giant magneto-resistance work well in describing the average or aggregate behavior, but have not yet achieved the sophistication to predict magneto-transport behavior locally. Part of the difficulty is that there are not enough experimental data on magnetization distributions at the microscopic level and their correlation with transport behavior. Direct investigations of domain configuration and wall motion as a function of external excitations such as magnetic field or heat, would allow us to derive the important input parameters for the various theoretical models as well as enable us to specifically assess the role of imperfections and defects in pinning the magnetic structures.

Another interesting area of research involves the studies of micromagnetic domains in patterned thin film structures. There a growing belief that the next generation of ultra-high density media would patterned nanostructures. There is an impetus in developing patterned

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media as well as minute reading and writing elements capable of addressing each island containing the encoded information. At the heart of the issue is the understanding of the magnetic switching mechanisms involved in ferromagnetic islands with ten nanometer dimensions. Additionally, the traditional methods of reading magnetic transitions using amorphous or giant magnetoresistance effects would have to be optimized to increase the signal to noise ratio as the active areas are reduced. The “reading” performance of these devices are directly linked with the behavior of domain walls, pinning effects, and the overall micromagnetics of the system. Therefore, a crucial starting point for optimization is the direct understanding of the magnetic behavior at the at nanometric length scales.

Looking ahead, there is considerable interest in the community towards quantum computation as implemented using magnetic spins as the computational variables, known as qubits. With a quantum computer, it is envisaged that the quantum mechanical interaction between individual spins of an ensemble can be used to implement certain algorithms with greatly enhanced efficiency, and thus offer tremendous improvement over conventional computers. The realization of this machine hinges on the possibility of preparing the initial state of the ensemble, accomplished by a direct manipulation of the individual spins, and of course, the capability of reading the final outcome by interrogating the individual qubits. Thus, one major leap towards the realization of this machine is the ability to detect electronic spins, as manifested by their magnetic moments. Magnetic imaging at the nanometric length scales

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