Chih-Ling Lee

Seed layer characterization for PtMn bottom spin-filter spin valves

The effect of seed layers on the giant magnetoresistance (GMR) response of bottom spin-filter spin valves (SFSVs) of the structure (seed layer)/PtMn/CoFe/Cu/CoFe/NiFe/Cu/Ta have been studied in detail. Four types of seed layers, NiFeCr, Ta/NiFeCr, NiFeCr/NiFe, and Ta/NiFe were used. The GMR response has been found to be very sensitive to the type and the thickness of the seed layers, which determine the crystallographic quality of the films and the degree of the fcc to fct phase transformation of the PtMn crystals in the films. Among the four, Ta/NiFeCr and NiFeCr/NiFe seed layers give the optimal GMR performance at a NiFeCr layer thickness of about 40–45 A.

Study of boron diffusion in MgO in CoFeB∣MgO film stacks using parallel electron energy loss spectroscopy

Boron diffusion in MgO has been investigated in annealed film stacks of sputtered CoFeB∣MgO using transmission electron microscopy and parallel electron energy loss spectroscopy. The analyses show significant B movement when the films are annealed, with the formation of BOx complexes. Characteristic diffusion lengths have been estimated in films annealed at the commonly employed temperature range of 300–400°C for the fabrication of magnetic tunnel junctions. An activation energy of 1.3eV (±0.4eV) has been extracted from these data that represent B diffusion in MgO through vacancies and defect states mediated by the formation of BOx complexes.

Crystallization and grain growth behavior of CoFeB and MgO layers in multilayer magnetic tunnel junctions

The relationship between crystallization, grain growth behavior, and the diffusion of B out of CoFeB has been investigated in annealed film stacks of sputtered CoFeB∣MgO using a combination of two dimensional x-ray diffraction, transmission electron microscopy, and parallel electron energy loss spectroscopy (PEELS). The analysis shows grain growth in MgO layers. It shows crystallization at approximately 350°C, and subsequent grain growth in CoFeB layers with annealing. The orientations of the grains of MgO and CoFe are definitively shown to be (002) in the out-of-plane direction. The MgO lattice is seen to have an in-plane tensile stress, while CoFe lattice is shown to have an in-plane compressive stress. CoFe grains are observed to be smaller than MgO grains, rather than being of equal size as previously understood. The physical process of B diffusion into MgO has also been investigated using PEELS and is determined that the diffusion of B through MgO is mediated through vacancies and defect states by th...

Optimization of high Bsat FeCo films for write pole applications

FeCo films and their lamination with ultrathin NiFe layers down to 5A were deposited using dc magnetron sputtering techniques. Soft magnetic FeCo films were obtained at an optimal target power of 500W and an optimal deposition pressure of 2mTorr with high saturation flux density, Bsat>2.4T, and low easy-axis coercivity, Hce⩽15Oe, and hard-axis coercivity, Hch⩽3Oe, at a film thickness of 2000A. While the magnetostriction remains at ∼4×10−6 the stress was further optimized by applying substrate bias at a controlled level ⩽50V without sacrificing film magnetic softness.FeCo films and their lamination with ultrathin NiFe layers down to 5A were deposited using dc magnetron sputtering techniques. Soft magnetic FeCo films were obtained at an optimal target power of 500W and an optimal deposition pressure of 2mTorr with high saturation flux density, Bsat>2.4T, and low easy-axis coercivity, Hce⩽15Oe, and hard-axis coercivity, Hch⩽3Oe, at a film thickness of 2000A. While the magnetostriction remains at ∼4×10−6 the stress was further optimized by applying substrate bias at a controlled level ⩽50V without sacrificing film magnetic softness.

Effect of microstructure on the oscillating interlayer coupling in spin-valve structures

It has been well established that the interlayer coupling in a spin valve is well described as a sum of terms associated with pinholes, magnetostatic (Neel), and oscillating exchange interlayer coupling [(OXC) or Ruderman–Kittel–Kasuya–Yosida]. We experimentally studied the effect of interface roughness on the OXC term. We systematically varied the microstructure by variation of the sputtering geometry, the sputtering pressure, the seed layer and by application of low-energy ion bombardment at the interfaces (“beam treatment”). It is found that smoothening the stacks leads to a stronger OXC, both when suppressing long-range (∼200 A) and short-range (∼20A) roughness. Neel coupling on the other hand, is found to be more sensitive to long-range waviness.

Comparison of RF bias, gas cluster ion beam, and ion beam in-situ beam treatment for enhancement of GMR in spin-valve stacks

In this paper, we present data comparing three different in-situ beam treatment (smoothing) techniques for enhancement of GMR properties. The three techniques were radio-frequency bias (RFB), gas cluster ion beam (GCIB), and ion beam (IB). All three were optimized for maximum enhancement of properties and resulted in an increase of GMR of about 0.5 to 1% (> 13.5% to > 14.5%) and a reduction in interlayer coupling of about 15 to 30 Oe (5 Oe to -25 Oe) when treating the CoFe/Cu interface of a synthetic pinned bottom spin valve. Smoothing the Ru/CoFe interface resulted in enhancement of GMR, but no change in the interlayer coupling. The optimum conditions for all three techniques corresponded to ion bombardment energies in the range of 10-60 eV. For RFB, the substrate bias voltage was optimized at 60 V, for IB, the extraction energy was 30 eV, and for GCIB, the equivalent energy/atom was in the same range. Our results indicate that all three smoothing methods are effective in improving the GMR properties with no fundamental advantage of any particular technique.

Determination and reduction of ion-beam etching induced magnetic dead layer

NiFe films with different thicknesses as were etched under several Ar+-ion-beam energy conditions. The functional dependence of the saturation magnetic flux on the remaining NiFe film thickness was used to determine the magnetic dead layer (MDL) thickness (tMDL). A tMDL of 24A was generated in the NiFe films etched using a 1200-eV Ar+-ion beam. A dual-energy (1200eV∕400eV) etching process was found to be effective in reducing tMDL to 16A without much throughput loss. A combination of optimal etching depth with an appropriate ion-beam energy is necessary in minimizing tMDL. The mechanism of MDL formation is discussed in terms of oxidation and surface roughening of the NiFe films.

Critical thickness effects of NiFeCr-CoFe seed layers for spin valve multilayers

In this article, we present data on the critical dependence of the magnetic, electrical and microstructural properties of spin-valves (SV) on seed-layer thicknesses. The SV structure is: seed-layer/PtMn 140 /spl Aring//CoFe 16 /Ru 8.5 /spl Aring/ /CoFe 21 /spl Aring/ /Cu 20 /spl Aring//CoFe 12 /spl Aring//NiFe 30 /spl Aring//Ta 30 /spl Aring/, where the seed layer is NiFeCr-CoFe or NiFeCr/NiFe. As the thickness of the bilayer seed layer is varied, it is found that a critical thickness boundary exists across which the film properties are radically different. The GMR ratio increased from 7% to 14% (a 100% change), the sheet resistance decreased by about 4 ohms/square and the crystalline texture transitioned from weak to extremely strong (111) texture. The critical thickness boundary is at a combined thickness of 37 /spl Aring/ to 40 /spl Aring/. These results suggest a mechanism at the boundary between NiFeCr and CoFe during film growth. A better lattice match between NiFeCr-CoFe, for example, NiFeCr 33 /spl Aring/ and CoFe 7 /spl Aring/, generates a strong (111) texture, which enhances the MR% as compared to NiFeCr 33 /spl Aring//CoFe 6 /spl Aring/. The H/sub 50/ (the field at 50% MR) also exceeds 2000 Oe. This also indicates enhancement of the PtMn fcc to fct transition based on the specifically combined thicknesses of NiFeCr-CoFe. With the NiFeCr-NiFe seed layer, the critical thickness effect is not observed within these thickness ranges.

Nano-oxide layer formed on ruthenium of synthetic pinned-structure spin valve by ion beam and cluster ion beam oxidation

In this paper, we report the performance of bottom synthetic spin-valve films with a newly developed ruthenium (Ru)-based nano-oxide layer (NOL). Two energetic oxidation techniques using an ion beam and a gas cluster ion beam were applied for the formation of the Ru NOL in the synthetic pinned layer. The giant magnetoresistance (GMR) was greatly enhanced from 13.8% to 15.7% with no sign of degradation in the pinning strength. In addition, these Ru NOL-bearing spin-valve films have demonstrated an excellent thermal stability, withstanding repeated thermal annealing cycles at elevated temperatures. This performance is clearly superior to that from the spin-valve films with a conventional CoFe-based NOL. The Ru NOL has shown a great potential for high-amplitude read sensor applications.

Sub-milli-Torr magnetron sputter deposition of magnetic thin films

In this paper, we evaluate the single films stress of various component layers in spin-valve or magnetic tunnel junction multilayers and the magnetic thickness loss due to interlayer atomic mixing in the magnetic thin films deposited by sub-milli-Torr magnetron sputtering with that from ion beam sputtering.