C. Tsang

Exchange induced unidirectional anisotropy at FeMn‐Ni80Fe20 interfaces

We have systematically investigated exchange coupled films of sputter deposited Ni80Fe20/FeMn and FeMn/Ni80Fe20 and obtained films with high exchange bias and low coercive forces. The variation of film properties with deposition conditions as well as with the permalloy and FeMn thicknesses have also been studied. The results demonstrated a strong dependence of exchange bias effect (HUA) on the amount of γ‐phase FeMn, the amount of impurities, as well as the abruptness of the transition profile at the interface. In addition, our results have also raised a number of new questions concerning the magnetic state of the interface and the origin of the interface coupling phenomena.

Magnetics of small magnetoresistive sensors (invited)

In the application of magnetoresistive sensors to magnetic recording two basic magnetic issues must be addressed: namely, the suppression of domain effects and the linearization of the magnetoresistive (MR) response. In this paper, we examine past results of the Barkhausen noise problem in small MR sensors, as well as methods for their suppression. Furthermore, we shall also discuss various transverse bias schemes: their principles of operation, their advantages and limitations in terms of bias sensitivities, bias uniformity, as well as their impacts on the linear and track resolution of the MR sensor.

Temperature dependence of unidirectional anisotropy effects in the Permalloy‐FeMn systems

We have systematically studied the temperature dependence of the exchange‐induced unidirectional anisotropy field (HUA) for the Permalloy‐FeMn system from room temperature up to ∼220 °C. Results show that HUA(T) decreases monotonically with T, vanishing around some critical temperature (Tc) which varies with sample configurations, film thicknesses, and deposition conditions, and in the best case is close to the Neel temperature of the bulk FeMn. The Permalloy/FeMn and the FeMn/Permalloy systems differ in their HUA(T) behavior, indicating a basic difference in the interface structures. Our results suggest a model where the interface comprises an ensemble of exchange paths, each with its own ordering temperature (To). The net HUA may then be regarded as an averaged behavior of all these To components. Using a special cooling procedure, we are able to study the distribution and behavior of these To components, with results showing that the FeMn/Permalloy system does exhibit a narrower and higher‐valued distr...

Study of domain formation in small permalloy magnetoresistive elements

Our past work has shown that the Barkhausen noise in the transverse response of small (10–100 μm) permalloy MR sensors originates from domain wall activities; namely, the systematic creation, intensification, and wall‐state transition of buckling domain structures. The cause and details of domain formation, however, remain unresolved. In this work, through direct domain observations and wall population studies, we will show that magnetostatic energy effects constitute the most basic cause of domain formation. Easy‐axis dispersion (δK) effects can also cause domains, but primarily for the shorter elements. Also, we will show how the magnetic configurations along the top and bottom edges of the element affect the geometry of the resultant buckling domains, and how fluctuations in these edge configurations produce fluctuations in the domain geometry and, consequently, in the MR response. Finally, we will show that domain activities are not suppressed by different orientations of the easy axis, although inter...

The origin of Barkhausen noise in small permalloy magnetoresistive sensors

The magnetoresistive (MR) response of permalloy sensors is increasingly plagued by jumps or noise of a magnetic origin (so‐called Barkhausen noise) as their sizes are reduced to tens of microns. The origin of this noise is not presently understood. In previous work (1) we reported a study of the Barkhausen noise by separately measuring the MR response and the domain processes involved, but detailed correlation between the two was still lacking. In this work, therefore, we performed simultaneous measurements of both the MR response and domain patterns on small MR elements of typically 12μm×24μm, enabling us to analyze the domain processes affecting the MR output. We have identified two classes of domain processes primarily responsible for noise in the MR output, namely, abrupt domain wall population reductions during early stages of demagnetization, and wall state transitions involving nucleation and expansion of N− wall segments around zero field. Additional insight into the N+→N− transitions has been obt...

Thermal annealing study of exchange‐biased NiFe‐FeMn films

We have studied the effects of thermal annealing (between 240 and 270 °C) on the magnetic and interface properties of exchange‐biased NiFe/FeMn thin film couples. With moderate annealing, there is a change in the temperature‐dependence of the exchange‐bias from an initial, linear shape to a convex shape, which better approximates the asymptotic behavior set by the temperature dependence of the sublattice magnetization in bulk FeMn. With extended annealing, the magnitude of the exchange‐bias at room temperature increases significantly, and the blocking temperature (where the exchange‐bias vanishes) increases beyond the Neel temperature of FeMn. Auger composition analysis shows that after extended annealing, there has been substantial interdiffusion of Ni into the FeMn, Mn into the NiFe, and Fe away from the interface; the original NiFe/FeMn interface becomes preferentially enriched in Mn and Ni, and deficient in Fe. Extended annealing apparently leads to the formation of a NiMnFe alloy that exhibits strong...

Antiparallel pinned NiO spin valve sensor for GMR head application (invited)

NiO antiferromagnetic material possesses certain advantages for spin valve applications and has attracted considerable attention. Some of the key advantages are its insulating properties, very high corrosion resistance, less sensitivity to composition, and its low reset temperature. This material, however, has a low blocking temperature which prevents its application to simple spin valve designs. The use of this material in spin valve structures required significant improvements in thermal stability, blocking temperature, and the spin valve design. In the present study, the blocking temperature and the blocking temperature distribution of the NiO films have been improved by depositing the films reactively using ion beam sputtering. A number of improvements in the processing method and deposition system had to be made to allow full NiO spin valve deposition for mass production. Another critical part was the use of antiparallel pinned design in place of the simple design to improve the thermal stability of the NiO spin valves as read elements at disk drive temperatures. The selection of the ferromagnetic pinned layers and the Ru spacer thickness in AP-pinned spin valves has significant impact on the behavior of the devices. These spin valves are all bottom type, NiO/PL1/Ru/PL2/Cu/Co/NiFe/Ta, where the metallic portion of the spin valve is deposited on top of the NiO AF layer. The PL1 and PL2 are ferromagnetic layers comprising NiFe and Co layers. Read elements have been made using these spin valves that delivered areal densities of 12 Gbit/in. These topics and other improvements which resulted in successful use of NiO spin valves as GMR heads in hard disk drives will be discussed.NiO antiferromagnetic material possesses certain advantages for spin valve applications and has attracted considerable attention. Some of the key advantages are its insulating properties, very high corrosion resistance, less sensitivity to composition, and its low reset temperature. This material, however, has a low blocking temperature which prevents its application to simple spin valve designs. The use of this material in spin valve structures required significant improvements in thermal stability, blocking temperature, and the spin valve design. In the present study, the blocking temperature and the blocking temperature distribution of the NiO films have been improved by depositing the films reactively using ion beam sputtering. A number of improvements in the processing method and deposition system had to be made to allow full NiO spin valve deposition for mass production. Another critical part was the use of antiparallel pinned design in place of the simple design to improve the thermal stability of ...

Design and performance considerations in high areal density longitudinal recording (invited)

In this work we discuss various design and performance issues in very high areal density longitudinal magnetic recording. We show that low flying height and small read gap are the two most important factors for high linear resolution, while small write gap and good P1–P2 alignment are important for high‐track density operations. Next, we discuss the on‐track signal‐to‐noise performance in terms of peak shift and peak jitter. For nonlinear peak shift, the proximity‐effect transition shift is found to be significant. For peak jitter, major sources of jitter are discussed, and an analytical description of jitter due to head and electronics noise is developed. Applying this result shows a signal‐to‐noise advantage.

Magnetics of nonlaminated. bilaminated, and multilaminated permalloy stripes

Using the magnetoresistive response technique and the micro‐Kerr domain imaging technique, we have studied the magnetics of 2000‐A‐thick nonlaminated, bilaminated, and multilaminated permalloy stripes with a length of 2400 μm, widths varying from 100 to 2 μm, and with the uniaxial anisotropy easy axis transverse to the length of the stripes. Results show that stripes from all three types of films exhibit non‐single‐domain remanent states that are hysteretic in their longitudinal magnetic responses. For the nonlaminated structures, closure domains are formed for the minimization of transverse demagnetization energy. For the laminated structures, longitudinally magnetized regions along the edges are formed as a basic feature of the flux‐closure configuration in the laminated films. Furthermore, for the multilaminated structures, the large number of degrees of freedom and the low wall energies have created an abundance of low‐energy remanent states that might cause the small‐field response of the structure t...

Nonlinear transition shifts in magnetic recording due to interpattern proximity effects

A high‐precision measurement of the proximity shift on a magnetic transition due to the presence of an old transition in the background is presented. Using peak detection electronics, this interpattern shift was deduced by measuring the distance between a reference and a test transition in a multibit pattern, with and without a background transition in the vicinity of the test transition. The measurement process was carefully designed to achieve high accuracy. The proximity shift of a 3380‐type write head on disks with different remenent‐thickness product for cases of both easy and hard transition types was measured for both the background and the data transitions. Results showed three distinct transition‐shift behaviors depending on the background‐to‐test intertransition distance: a gradual decay in the far region (≳2 μm), a rapid rise in the intermediate region, and asymptotic saturation at the near region when the two transitions are almost on top of each other. The asymptotic shift magnitude agrees well with independently measured hard‐transition shifts, and good agreement with theoretical modeling is obtained for shifts in the far and intermediate regions.A high‐precision measurement of the proximity shift on a magnetic transition due to the presence of an old transition in the background is presented. Using peak detection electronics, this interpattern shift was deduced by measuring the distance between a reference and a test transition in a multibit pattern, with and without a background transition in the vicinity of the test transition. The measurement process was carefully designed to achieve high accuracy. The proximity shift of a 3380‐type write head on disks with different remenent‐thickness product for cases of both easy and hard transition types was measured for both the background and the data transitions. Results showed three distinct transition‐shift behaviors depending on the background‐to‐test intertransition distance: a gradual decay in the far region (≳2 μm), a rapid rise in the intermediate region, and asymptotic saturation at the near region when the two transitions are almost on top of each other. The asymptotic shift magnitude agrees we...