1998 MRS Spring Meeting & Exhibit

Chairs

David Bogy
Dept of Mechanical Engr
Univ of California-Berkeley
MC 1740
Berkeley, CA 94720-1740
510-642-6460

Tom Nolan 
Komag Inc 
San Jose, CA 95131 
408-576-2341

Kurt Rubin
IBM Almaden Research Center
MS K63/E3
San Jose, CA 95120-6099
408-927-2024

1998 Spring Exhibitor

* Invited paper

TUTORIAL 

STL: PHYSICS AND MATERIALS FOR ULTRAHIGH DENSITY RECORDING 
Sunday, April 12, 1:30-4:30 p.m. 
Pacific I 

This course will introduce various forms of magnetic recording heads and media and how they function together during the magnetic recording and readback processes. The design and fabrication of thin film media and metal-in-gap, thin film, magnetoresistive and giant magnetoresistive heads for high density recording will be described. Their operation together in magnetic disk and tape recording systems during both the record and readback processes will be discussed. Emphasis will be placed upon how recent advances in materials for media, heads and head-media interface are expected to affect the performance of advanced recording systems. 
Instructor: Mark H. Kryder, Carnegie Mellon University 

SESSION L1: HIGH MOMENT POLE MATERIALS 
Chair: James A. Bain 
Monday Morning, April 13, 1998 
Salon 2

8:30 AM *L1.1 
MICROSTRUCTURES AND PROPERTIES OF HIGH SATURATION SOFT MAGNETIC MATERIALS FOR ADVANCED RECORDING HEADS. Shan X. Wang, Jongill Hong, and Kyusik Sin, Dept. of Materials Science and Engineering, Stanford University, CA; Dept. of Electrical Engineering, Stanford University, CA. 

This paper will present recent development on sputtered FeXN-based (X=Ta, Rh, Mo, Al, etc.) high saturation materials and compare them with amorphous CoZr-based materials and electroplated NiFe- and CoFe-based materials in the context of advanced high density magnetic recording. In particular, correlations among processing, microstructure and magnetic properties under oblique incidence and in laminated structures will be discussed. Due to the extrinsic nature of coercivity, the mechanisms of soft magnetism are very complex and difficult to characterize. With the help of synchrotron radiation, pole figure analysis, transmission electron microscopy (TEM), torque magnetometry, and magnetic force microscopy (MFM), we can identify that (110) fiber texture plays a key role in the soft magnetism of FeXN films, in addition to the effects of film composition, stress, grain size and shape, and lattice spacing. Soft films, both single and laminated, usually display well defined bcc (110) textures even on sloping surfaces. In contrast, films with poor (110) textures and asymmetric pole figures tend to have relatively large coercivities, and in certain cases possess perpendicular anisotropy and stripe domains. Processing conditions promoting (110) texture, including substrate bias, lamination with AlN, and appropriate base layer, lead to soft magnetism in FeXN films. The addition of N and a third element, and lamination with insulating layers, result in significant increases in electrical resistivity, important to high frequency applications. The addition of N and X can also lead to enhanced pitting corrosion resistance. 

9:00 AM *L1.2 
ELECTRODEPOSITED MAGNETIC ALLOYS FOR THIN-FILM HEADS. Neil Robertson, IBM Storage Systems Division, San Jose, CA. 

Electodeposited alloys have been critical for the performance of thin-film magnetic recording heads. To meet future performance requirements for write heads new magnetic alloys need to be developed to deal with higher data rates and higher media coericivity. This paper will examine the requirements for future materials to meet the performance criteria A review of electrodeposited magnetic alloys will be presented. Data on magnetic and material properties of a specific new alloy NiFe (45/55) win be shown as well as magnetic recording performance data. 

9:30 AM L1.3 
MICROSTRUCTURE AND MAGNETIC PROPERTIES OF REACTIVE SPUTTER DEPOSITED IRON-SILICIDE-NITRIDE FILMS. K. Uchiyama*, J. Megusar, and R.C. O'Handley, MIT, Department of Materials Science and Engineering, Cambridge, MA. *Permanent address, TDK Corporation, Nagano, JAPAN. films were sputter deposited from an Fe + SiN target in various flow rates of nitrogen up to 5 sccm onto an aluminum nitride buffered Silicon substrates. With no nitrogen flow, the films show the bcc structure with possible incorporation of some Si and N. At moderate nitrogen flow rates, a broad diffraction halo indicative of amorphous material, assumed to be Fe_1-xN_x-, is added to the bcc diffraction pattern. As nitrogen flow rate is increased to 5 sccrn, the size of the -Fe particles decreases to below 10 nm and the diffraction is dominated by the amorphous halo while the -Fe peaks are broadened: The magnetic properties reflect these changes in composition and microstructure. The coercivity is of order 30 to 45 Oe up to 0.5 sccm N₂ and the loops show a remanence which decreases with increasing nitrogen. Above 1 sccm, the coercivity vanishes and the loops show characteristics of superparamagnetism. The detailed structure of the loops indicates that the amorphous nitrogen matrix changes from ferromagnetic below, to paramagnetic above 1 sccm N₂. Reducing the amount of SiN in the target increases the saturatio magnetization and reduces the coercivity at low nitrogen flow rates. These films show magnetic properties attractive for use in high density write heads. 

SESSION L2: ANISOTROPIC MAGNETORESISTANCE (AMR), GIANT MAGNETORESISTANCE (GMR) AND EXCHANGE MATERIALS I 
Chair: Paulo J.P. Freitas 
Monday Morning, April 13, 1998 
Salon 2

10:30 AM *L2.1 
SURFACE EFFECTS IN THE GROWTH OF MAGNETIC MULTILAYERS WHICH EXHIBIT THE GIANT MAGNETORESISTANCE EFFECT. William F. Egelhoff, Jr., Magnetic Materials Group, National Institute of Standards& Technology, Gaithersburg, MD. 

A class of magnetic multilayers, known as spin valves, which exhibit the giant magnetoresistance (GMR) effect are likely to play a key role in ultrahigh density data-storage technologies of the coming decade. We have investigated the deposition and processing of a variety of spin valve structures with the aim of optimizing their properties. We have found that many of the magnetic and magnetoresisitive properties of spin valves are strongly influenced by surface and interface effects occurring during growth. These effects include the balance of surface and interface free energies, surface diffusion, interdiffusion at interfaces, low temperature deposition, the use of surfactants to modify growth, and specular electron scattering at surfaces. In some cases, it is possible to control these factors or to use them to manipulate the growth or improve post-growth processing of spin valves to improve their magnetic and magnetoresistive properties. In this manner we have been able to fabricate spin valves with some of the best properties ever observed, and these discoveries are being actively pursued by companies developing GMR products. This work is a good example of how the lessons of surface science can be used directly in the development of intelligent manufacturing processes. 

11:00 AM L2.2 
THERMAL STABILITY OF THE Cu-NiFe INTERFACE IN THE TEMPERATURE RANGE 200-400ºC. T.H. Westmore, J.E.E. Baglin, A.J. Kellock, V.R. Deline, IBM Almaden Research Center, San Jose, CA; E.L. Allen, Dept. of Materials Engineering, San Jose State Univ., San Jose, CA. 

The thermal interdiffusion kinetics for the thin film couples Cu-Ni and Cu-Ni₈₁Fe₁₉ were studied. The purpose was to model and predict the effects of thermal diffusion that could occur between such metal layers during processing of a GMR sensor head, and to correlate this with possible adverse effects on magnetic performance. Measurements were made on thick (90 nm) films that received heat treatments at temperatures between 225 and 400ºC. This enabled extrapolation to predict the nature of grain boundary diffusion and alloying effects during photoresist curing at 240ºC for sensor layers only 2.5 nm thick. Elemental depth profiles were obtained for Cu and Ni from AES sputter profiling, using a new technique to quantify the overlapping Cu and Ni signals. Additional data were obtained from XRD, RBS, and XTEM, and these structural data were correlated with changes in magnetic response of the films. It is concluded, from the resulting model, that Cu-Ni grain boundary diffusion is very fast, even at low temperatures, and is followed by bulk movement of Ni into Cu. The interaction is complex, and will be discussed quantitatively. It suggests specific processing paths to minimize degradation in this system. 

11:15 AM L2.3 
MAGNETIC CHARACTERIZATION OF SPIN VALVES EXCHANGE BIASED BY COSYNTHETIC ANTIFERROMAGNETS. J.L. Leal, and M.H. Kryder, Carnegie Mellon Univ, Data Storage Systems Center, Dept of Electrical and Computer Engineering, Pittsburgh, PA. 

Spin valves with generic configuration substrate buffer Co Ru Co Cu Co NiFe, where the Cotrilayer is a synthetic antiferromagnet, were fabricated and characterized. Room temperature measurements show giant-magnetoresistance ratios above 5% and that fields in excess of 5000 Oe are needed to saturate the synthetic antiferromagnet. The in-plane stabilization of the synthetic antiferromagnet is performed by choosing the material and thickness of the buffer layer. It is shown that a thin Cr buffer layer induces an increase in the in-plane anisotropy of the synthetic antiferromagnet, by increasing the coercivity of the Co layers, contributing consequently to the overall stability of the spin valve system. The saturation field of the synthetic antiferromagnet is higher than 3000 Oe at 250 C, which yields the possibility of observing the spin valve effect at very high temperatures. The transfer curve of the patterned structure shows a much weaker magnetostatic coupling than that observed in standard spin valves of identical dimensions. This is a consequence of the particular magnetic configuration of the synthetic antiferromagnet, which promotes an almost vanishing magnetostatic field. 

11:30 AM L2.4 
EXCHANGE COUPLING PROPERTIES BETWEEN ORDERED ANTIFERROMAGNETIC PdPtMn AND NiFe FILMS. Y. Shimizu, A. Tanaka, K. Nagasaka, H. Kishi M. Oshiki, Fujitsu LTD, File Memory Fab., Kanagawa, JAPAN; H. Fujiwara, Ctr. for Mater. for Information Technol., Dept. of Metall. and Mater. Eng., Univ. of Alabama, Tuscaloosa, AL. 

Exchange coupling between ordered PdPtMn and NiFe bilayer films are studied to understand the pinning mechanism in this system. Ordered PdPtMn exhibits different pinning properties from those of disordered antiferromagnetic (AF) materials, such as, FeMn. In FeMn/NiFe system, Hc of the pinned layer is much smaller than the exchange bias field (Hua) when the FeMn is thicker than critical thickness of around 10 nm. On the other hand, in the PdPtMn/NiFe system, the pinning field appears when PdPtMn is thicker than 10 nm and Hc is almost comparable to the Hua. Hua becomes larger than Hc if PdPtMn is thicker than 25 nm. This large critical thickness of PdPtMn suggests that Ku/J is smaller than that of FeMn/NiFe bilayer system, where Ku is the anisotropy constant of the AF layer and J is a F-AF coupling constant. We investigated Hua and Hc behavior in the temperature range between 300C and room temperature in PdPtMn (25 nm) / NiFe (5-20 nm) bilayer films. The exchange bias field, Hua, appears below 280C and increases its value down to 150C. In this temperature range, Hc has a broad peek and these phenomena can be explained by thermal fluctuation model [1]. Below 150C Hua stays constant down to room temperature. However, Hc of NiFe increases monotonously as the films are cooled down. These phenomena can not be explained by simple thermal fluctuation model. They will be possible to explain if we assume a grater increase in J with decreasing temperature than Ku and the exchange stiffness of PdPtMn. A part of this work was performed under the management of ASET in the MITI's R&D Program supported by NEDO. 

11:45 AM L2.5 
SPIN VALVE SENSORS WITH A NEW HIGH CORROSION RESISTANCE MnRh EXCHANGE LAYER. A. Veloso and P.P. Freitas, INESC, Lisbon, PORTUGAL; IST, Department of Physics, Lisbon, PORTUGAL. 

A new high corrosion resistance exchange material, Mn_xRh_1-x (MnRh) is described, that requires no post deposition anneal to stabilize the antiferromagnetic phase, in contrast with MnNi. Spin valve sensors prepared with this exchange layer show good corrosion resistance, thermal stability up to 250 ºC, and good exchange coupling characteristics. Spin valves with structures Si/Ta(80 )/NiFe(45 )/CoFe(4 )/Cu(22 )/CoFe(20 )/MnRb(100-170 )/Ta(30 ) were prepared by magnetron sputtering. The as-deposited coupon samples show exchange fields H 550 Oe, with a pinned layer coercivity of 77 Oe, a free layer coercivity < 2 Oe, and a coupling field of 10 Oe. Magnetoresistance (MR) is about 8 %, with sensitivity of 3.5 to 7 %/Oe. The blocking temperature T_b of the as-deposited samples is 235-240 ºC. The corrosion resistance of the exchange material was studied by potentiodynamic scans carried out in a 0.1N sodium sulfate electrolyte (PH=7) to evaluate atmospheric corrosion resistance, and compared with that of NiFe films. The MnRh and NiFe films exhibit corrosion potentials of -0.039V and -0.093V respectively, indicating that the MnRh films have a better corrosion resistance than NiFe. Unshielded sensors with trackwidths W = 3.5-6 m, and height h = 2 m were fabricated. The sensors show well linearized MR transfer curves, without hysteresis nor Barkhausen noise. Sequential 5 hours anneals were carried out in vacuum at different temperatures up to 280ºC, followed by furnace cooling to room temperature in a magnetic field of 500 Oe applied along the easy axis of the sensors (transverse to sensor trackwidth). Sensors are thermally stable under anneal up to 250ºC. Tape head devices are being fabricated. 

SESSION L3: ANISOTROPIC MAGNETORESISTANCE (AMR), GIANT MAGNETORESISTANCE (GMR) AND EXCHANGE MATERIALS II 
Chair: William F. Egelhoff 
Monday Afternoon, April 13, 1998 
Salon 2

1:30 PM *L3.1 
SECOND HARMONIC MAGNETO-OPTIC KERR EFFECT FOR STUDYING MAGNETIC SURFACES AND INTERFACES. Charles T. Rogers, Department of Physics, University of Colorado, Boulder CO; Thomas M. Crawford, and Thomas J. Silva, National Institute of Standards and Technology, Boulder CO. 

The second harmonic magneto-optic Kerr effect (SH-MOKE) is an optical technique that is sensitive to surface and interfacial regions in magnetic films. It has a roughly two orders of magnitude larger intensity change and polarization rotation than is found for the linear magneto-optic Kerr effect (MOKE). Because it is one of the few experimental techniques available that is specifically surface and interface sensitive (due to the nonlinear electron response associated with the rapidly varying electronic potentials near these regions), it is ideally suited for studying the fundamentally and commercially interesting giant magnetoresistance systems and other magnetic films and multilayers. The second harmonic signal can be used to determine the direction of magnetization as a function of external magnetic field and thus can be used to measure coercivity and anisotropy fields. Further, because SH-MOKE is studied using a pulsed optical excitation source (passively mode-locked Ti:Sapphire laser producing 50 femtosecond pulses in our system), the technique is easily operated in a time-resolved mode. The magnetic response can be studied versus time by synchronizing the incident optical pulse with an applied magnetic pulse. We will discuss details of the experimental system and its application to static and dynamic measurements of soft ferromagnetic films and multilayers. 

2:00 PM L3.2 
MAGNETIZATION PROCESS OF F/AF SYSTEMS. C. Hou, H. Fujiwara, F. Ueda*, H.S. Cho, MINT Center and Department of Physics, University of Alabama, Tuscaloosa, AL *Data Storage and Retrieval System Div., Hitachi, Ltd., Kozu, Odawara, JAPAN. 

Recently, extensive studies on ferromagnetic-antiferromagnetic (F-AF) exchange coupling have been carried out^1-6 because of its high potential for the applications in both MR and GMR heads. We studied the hysteresis loop shift H_p and the coercivity H_c in the F/AF system as functions of AF layer thickness, and the coupling strength which is controlled by inserting thin Cu spacer or contaminating the interface between F and AF layers. The degree of contamination was controlled by varying the waiting time between sputtenng the F and AF layers. We also measured what we call the  H_p0, which is defined as , where  the initial susceptibility in the transverse direction. The results from the systems, NiFe / [Cu(t_cu) or impurity] / FeMn and NiFe/FeMn(t_FeMn) all showed H H_p + H_c, no matter how H_p and H_c behaved individually. Reported data on the NiFe/NiMn⁵ have also been found to follow this relationship. Very recently, Z. Qian⁶, etc. found the same relationship for the NiFe/NiO system and proposed a model in which the existence of a uniaxial interfacial anisotropy was assumed. Here, we report that, assuming the rotation mechanism for the magnetization process in both F and AF layers, the above relationship is generally verified by means of multiple variable energy minimization for arbitrary anisotropy distribution both in direction and in strength in the AF layer, and for arbitrary F-AF coupling strength. The interfacial uniaxial anisotropy mentioned above is explained by the switching of some part of the interfacial spins of the AF layer along with the switching of the magnetization of the F layer. The results obtained above imply that the rotation mechanism is dominating in the magnetization process, contrary to the fact that domain wall motion usually dominates in single F layers. On the other hand, our experimental results for NiO/Co system always showed H_p0>H_p + H_c, which implies that magnetization processes, such as domain wall motion or incoherent rotation, are still domianate in the NiO/Co system as in single F layers. 

2:15 PM L3.3 
INFLUENCE OF CRYSTAL LATTICE DEFECTS ON DOMAIN WALL NUCLEATION AND MOTION IN EXCHANGE-BIASED FILMS AND SPIN VALVES. V.I. Nikitenko, V.S. Gornakov, L.M. Dedukh, Inst of Solid State Physics, Academy of Sciences, Moscow District, RUSSIA; A.J. Shapiro and R.D. Shull, National Inst of Standards and Technology, Gaithersburg, MD; A. Chaiken, Hewlett-Packard, Palo Alto, CA. 

Epitaxial and polycrystalline NiFe/NiO bilayer films have been prepared by sputtering and characterized using both conventional magnetometry and the magneto-optic indicator film (MOIF) technique.¹ Epitaxial films grown simultaneously with polycrystalline films consistently show a smaller exchange bias field despite their greater crystalline perfection. MOIF images show that the screw and edge dislocations caused by plastic deformation of the MgO substrates serve as nucleation centers for domain walls. Bilayers with a reverse order of deposition (NiFe directly on the MgO substrate, NiO on top) show magnetization behavior similar to the polycrystalline bilayers with the usual order of deposition. A comparison is also made with epitaxial NiFe films grown directly on the MgO, which show domain patterns typical of films with uniaxial anisotropy. Magnetization and magnetoresistance behavior of polycrystalline NiFe/Co/Cu/Co/NiFe/NiO spin valves was also investigated. Unusual features in the magnetization process are explained in terms of domain wall formation in the antiferromagnetic layer. ¹ V.S. Gornakov et al., J. Appl. Phys. 81, 5215 (1997). 

2:30 PM L3.4 
EXCHANGE PROPERTY CORRELATION WITH DEPOSITION GAS SPECIES AND INTERFACE STRUCTURE OF ION BEAM DEPOSITED Ta (50 A)/NiFe (x A)/FeMn (y A)/Ta (50 A) LAYERS. H. Hegde *, J. Wang *, S.B. Sant ** *Veeco Instruments Inc., Plainview, NY; ** Veeco Technology Center, San Jose, CA. 

In the quest for giant magnetoresistance exchange-biased spin valves, the exchange coupling at the interface between the ferromagnetic layer and the antiferromagnetic layer is critical. We have correlated the exchange property, in ion beam deposited Ta (50 A)/NiFe (x A)/ FeMn (y A)/Ta (50 A) layers, as a function of the deposition gas species (Ar or Xe) and resultant interface structure. High resolution transmission electron microscopy and surface sensitive analytical techniques have been employed to understand the role of interface roughness, preferred orientation, defect analyses and grain size of individual layers, nature and incorporation of the inert gas species, towards optimization of the exchange behavior. Under certain conditions, exchange fields >300 Oe were consistently obtained for NiFe and FeMn thickness of 50 A each. To the best of our knowledge, exchange coupling strength of 0.24 ergs/cm2 so obtained are the highest for such structures. 

2:45 PM L3.5 
SIGNIFICANT IMPROVEMENTS IN MAGNETORESISTANCE OF ION BEAM SPUTTERED NiFe FILMS BY ANNEALING IN STABLE STRUCTURES. Mustafa Pinarbasi, IBM Corporation, Storage Systems Division, San Jose, CA. 

Thin NiFe films are used as sensing layers in magneto-resistive heads. The change in the resistance of the NiFe films in response to a magnetic field, dR/R, is a key material parameter that determines the signal of the MR heads. The dR/R of the NiFe films have been improved by depositing the films at elevated temperatures or annealing them at high temperatures. These previous studies used bare NiFe films that are deposited directly on substrates and then annealed in vacuum ovens. In sensors, however, NiFe films are always in contact with other layers such as tantalum spacer layer. But Ta/NiFe interface is stable only at temperatures below 300ºC. As a result, the instability of the Ta/NiFe interface prevents the annealing of these films in sensor forms and a stable interface material is required to take advantage of the improvements upon annealing. The experiments carried out in this study show that alumina films provide an excellent interface with NiFe that is stable at temperatures as high as 550ºC. The films have been deposited using ion beam sputtering and annealing is done on Ta, tantalum oxide, and alumina encapsulated NiFe films. The studies in the as deposited state show that there is an 8  reduction in the magnetic moment of NiFe per Ta/NiFe interface compared to the alumina/NiFe interface. This significant intermixing in Ta/NiFe interface seems to be constant up to 250ºC and starts to increase at higher temperatures. For Ta/NiFe/Ta samples annealed at 400ºC, the dR/R decreases to 25 of its original value while all soft magnetic properties are lost. In alumina/NiFe/alumina samples however the dR/R of the samples improved 35 upon annealing while coercivity and magnetostriction of the films stayed nearly constant. The X-ray studies have shown nearly a factor of two increase in grain size and significant enhancement in the (111) texture of the alumina encapsulated NiFe films upon annealing that accompanies the improvement in dR/R. This study shows that alumina films make very stable interface with NiFe therefore making it possible to anneal NiFe films for sensor applications. 

3:00 PM L3.6 
HEAT DISSIPATION MATERIAL APPLICATION FOR MR HEAD. Xiao-feng Zhang, X. Yan, Rong-fu Xiao, Hong Kong University of Science and Technology, Department of Physics, HONG KONG. 

The requirement that a magneto-resistive(MR) head dissipate the heat produced by resistive heating in the sense element without excessive temperature rise is one of the factors limiting the electric current density in the head. This, in turn, limits the signal to noise ratio attainable for a particular head design. Thermal constraints will cripple high resolution MR head unless careful thermal management is included in the design process. The purpose of this investigation is to determine thermal conductivity of heat dissipation material AlN through new technique, which is especially designed for heat transfer measurement in thin film structure. It has been demonstrated to be convenient and accurate approach of obtaining thermal conductivity coefficient for a variety of MR thin film gap material. In this experiment AlN material shows better heat dissipation performance than that of Al₂O₃ that is currently used in MR head industry. The measurement sample was made of two-layer patterned film and deposited on Silicon substrate. Meanwhile ion-milling technique was involved in patterned film preparation. In MR head industry, AlN definitely can reduce temperature rise more than 15% than existing material Al₂O₃ does. This, in turn, can improve signal to noise ratio significantly. Industry can continuously push for narrower track and smaller device geometric, and more and more signal can be attainable. Therefore AlN is easy to prepare using sputtering technique.

SESSION L4: SPIN TUNNEL JUNCTIONS 
Chair: Shan X. Wang 
Monday Afternoon, April 13, 1998 
Salon 2

3:45 PM *L4.1 
MAGNETIC TUNNEL JUNCTIONS WITH LOW RESISTANCE, HIGH CURRENT DENSITY AND GOOD UNIFORMITY. Hisanao Tsuge, Tsutomu Mitsuzuka, Atsushi Kamijo and Kazuhiro Matsuda, NEC Corporation, Fundamental Research Laboratories, Kawasaki, JAPAN. 

We have developed a controllable fabrication technique for magnetic tunnel junctions, including in situ deposition of a junction trilayer (e.g. Fe/Al₂O₃/CoFe) and tunnel barrier formation with in situ natural oxidation of an Al layer. The trilayer was patterned into junction structures with dimensions of 2x2m² to 40x40m² using photolithography and ion milling.