CoFe Free Layer Research Articles

The Influence of Edge Inhomogeneities on Vortex Hysteresis Curves in Magnetic Tunnel Junctions

Magnetoresistive sensors using a tunnel magnetoresistance (TMR) spin valve structure with CoFe and CoFeB free layer (FL) are investigated. Lateral dimension, thickness, and magnetic properties of the circular-shaped FL energetically favor the nucleation of a magnetic vortex. In the vortex configuration, the sensors show the expected hysteresis-free transfer curve. Distinct differences between CoFe and CoFeB in the vortex typical parameters are observed. The experimental results are compared to micromagnetic simulations. Influence of saturation magnetization ${M}_{s}$ on the vortex transfer curve is studied in order to reproduce the experimental data. However, only by adjusting ${M}_{s}$ iteratively the experimental data cannot be fully reproduced: The vortex annihilation ( ${H}_{\mathrm {an}}$ ) in CoFeB occurs at smaller fields than simulated. On the other hand, the change in TMR signal at ${H}_{\mathrm {an}}$ is significantly smaller in the experiment for CoFe. Discrepancies are most pronounced at ${H}_{\mathrm {an}}$ . This investigation focuses on the magnetic and electric properties at the edge of the FL structure. Reduction in ${H}_{\mathrm {an}}$ can be explained for 1.1 $\mu \text{m}$ FL diameters by introducing a magnetically disturbed edge. Reduced change in TMR signal at ${H}_{\mathrm {an}}$ can be understood by introducing an electrically inactive edge area. The analysis shows that for CoFe and CoFeB different edge inhomogeneity effects are present or edge inhomogeneities have different impacts.

Spin-orbit torque switching without an external field using interlayer exchange coupling.

Manipulation of the magnetization of a perpendicular ferromagnetic free layer by spin-orbit torque (SOT) is an attractive alternative to spin-transfer torque (STT) in oscillators and switches such as magnetic random-access memory (MRAM) where a high current is passed across an ultrathin tunnel barrier. A small symmetry-breaking bias field is usually needed for deterministic SOT switching but it is impractical to generate the field externally for spintronic applications. Here, we demonstrate robust zero-field SOT switching of a perpendicular CoFe free layer where the symmetry is broken by magnetic coupling to a second in-plane exchange-biased CoFe layer via a nonmagnetic Ru or Pt spacer. The preferred magnetic state of the free layer is determined by the current polarity and the sign of the interlayer exchange coupling (IEC). Our strategy offers a potentially scalable solution to realize bias-field-free switching that can lead to a generation of SOT devices, combining a high storage density and endurance with a low power consumption.

Challenging Issues for Terabit-Level Perpendicular STT-MRAM

Recently, many researchers have intensively studied p-STT MRAM as an alternative terra-bit-integration nonvolatile memory since current Si-based dynamic-random-access-memory has a physical scaling limit of less than 20 nm. p-STT MRAM has demonstrated a fast write time of ~10 ns, non-volatile memory operation, and extremely low power consumption. However, for satisfying a commercial specification as a terra-bit-integration nonvolatile memory, perpendicular magnetic tunneling junction (p-MTJ) of p-STT MRAM essentially needs to satisfy critical device performances for p-MTJ spin-valves such as a high TMR ratio of 150 %, a high Jex of > 0.7 erg/cm2, a good Ku of 1x107 erg/cm3, a good ¢ of ~ 74,and a low critical writing current (JC0) of 13.4 MA/cm2, which should satisfy the thermal stability such as an ex-situ annealing above 400 °C. In general, a CoFeB based p-MTJ spin-valve in p-STT-MRAM has been fabricated with the stacked structure of bottom electrode/ amorphous Ta seed layer/amorphous CoFeB free layer/MgO tunneling barrier/amorphous CoFeB pinned layer/capping layer/buffer layer/ synthetic anti-ferromagnetic layer (SyAF) /top electrode. In particular, nano-scale thick amorphous CoFeB layer has been applied for free and pinned layer. However, the mechanism by which the perpendicular magnetic anisotropy (PMA) characteristic for amorphous CoFeB based p-MTJ using amorphous Ta seed layer could be achieved has not been clarified. In our study, we will review its mechanism and the thermal stability limit for amorphous CoFeB based p-MTJ using amorphous Ta seed layer. In addition, we will present the thermal stability enhancement for amorphous CoFeB based p-MTJ using bcc crystalline seed layer and its mechanism. For amorphous CoFeB based p-MTJ using amorphous Ta seed layer, the B atoms were segregated at the interface between the MgO tunneling barrier and the CoFeB free and pined layer after the ex-situ annealing of 275 °C, producing interface-PMA (i-PMA), as shown in Fig. 1(a). However, the PMA characteristics for amorphous CoFeB based p-MTJ using amorphous Ta seed layer was rapidly degraded as the ex-situ annealing temperature increased above 400 °C, which the mechanism was not proved yet. Surprisingly, the crystalline structure of both CoFeB free layer and pinned layer sustained at the amorphous structure although the p-MTJ was ex-tu annealed at 275 °C, as shown in Fig. 2. In order to enhance the thermal stability of amorphous CoFeB based p-MTJ, a bcc crystalline seed layer is essentially necessary rather than amorphous Ta seed layer. During ex-situ annealing at 400 °C, the bcc crystalline seed layer could texture the amorphous CoFeB free and pinned layer into the bcc crystalline CoFe free layer, MgO tunneling, and CoFeB pinned layer, as shown in Fig. 3. As a result, amorphous CoFeB based p-MTJ could achieve the TMR of ~135 %, as shown in Fig. 4. In this presentation, I will challenging material issues for p-STT MRAM in the view of the perfect crystalline linearity for free layer, MgO tunneling barrier, and pinned layer, and SyAF layer.

Pseudo Spin Valves Using a (1 1 2)-Textured D0$_{22}$ Mn$_{2.3-2.4}$Ga Fixed Layer

We demonstrate pseudo spin valves with a (112)-textured DO_22 MnGa (MnGa) tilted magnetization fixed layer and an in-plane CoFe free layer. Single D0_22 MnGa films exhibit a small magnetoresistance (MR) typically observed in metals. In MnGa/Cu/ CoFe spin valves a transition from a negative (-0.08%) to positive (3.88%) MR is realized by introducing a thin spin polarizing CoFe insertion layer at the MnGa/Cu interface and tailoring the MnGa thickness. Finally, the exchange coupling between the MnGa and CoFe insertion layer is studied using a first-order reversal curve (FORC) technique.

Effects of target bias voltage in magnetic tunnel junctions grown by ion beam deposition

Magnetic tunnel junctions (MTJs) with an AlOx barrier were fabricated by a biased target ion beam deposition (BTIBD) sputtering technique using a low energy ion source (0–50 eV) and voltage biased targets. The BTIBD system applies a bias voltage directly onto the desired targets, providing enough sputtering energy and avoiding overspill contamination during film deposition. The successful deposition of AlOx-MTJs demonstrated the capability of the BTIBD to make multilayer structures with good film quality. MTJ thin film surface roughness and intermixing between layers are among the key problems leading to low tunneling magnetoresistance (TMR) performance. Here we study the effects of bias voltage on MTJ properties via the measurements of the Néel coupling field and TMR. We suggest that a lower bias voltage reduces the intermixing that occurs when a top CoFe free layer is deposited on an AlOx barrier, but produces relatively high surface roughness. On the other hand, higher energy deposition enhances both interlayer mixing and surface flattening. Such understanding of the bias voltage effects on film properties could be used to optimize MTJ performance.

CoFe/NiFeSiB/CoFe 자유층을 갖는 이중장벽 자기터널접합의 바이어스전압 의존특성

이 연구에서는 Ta 45/Ru 9.5/IrMn 10/CoFe <TEX>$3/AlO_x$</TEX>/자유층/<TEX>$AlO_x$</TEX>/CoFe 7/IrMn 10/Ru 60(nm) 구조를 갖는 이중장벽 자기터널접합(double-barrier magnetic tunnel junction: DMTJ)를 다루었다. 자유층은 <TEX>$Ni_{16}Fe_{62}Si_8B_{14}\;7nm$</TEX>, <TEX>$Co_{90}Fe_{10}(fcc)$</TEX> 7 nm 및 <TEX>$CoFet_1$</TEX>/NiFeSiB <TEX>$t_2$</TEX>/CoFe <TEX>$t_1$</TEX>으로 구성하였으며 두께 <TEX>$t_1,\;t_2$</TEX>는 변화시켰다. 즉 TMR비와 RA를 개선하기 위하여 부분적으로 CoFe층을 대체할 수 있는 비정질 NiFeSiB층이 혼합된 자유층 CoFe/NiFeSiB/CoFe을 갖는 DMTJ를 연구하였다. NiFeSiB(<TEX>$t_1=0,\;t_2=7$</TEX>)만의 자유층을 갖는 DMTJ는 터널자기저항(TMR)비 28%, 면적-저항곱(RA) <TEX>$86k{\Omega}{\mu}m^2$</TEX>, 보자력(<TEX>$H_c$</TEX>) 11 Oe 및 층간 결합장(<TEX>$H_i$</TEX>) 20 Oe를 나타내었다. <TEX>$t_1=1.5,\;t_2=4$</TEX>인 경우의 하이브리드 DMTJ는 TMR비 30%, RA <TEX>$68k{\Omega}{\mu}m^2$</TEX> 및 <TEX>$H_c\;11\;Oe$</TEX>를 가졌으나 <TEX>$H_i$</TEX>는 37 Oe로 증가하였다. 원자현미경(AFM)과 투과전자현미경(TEM)측정을 통하여 NiFeSiB층 두께가 감소하면 <TEX>$H_i$</TEX>가 증가하는 것을 확인하였다. 비정질 NiFeSiB층이 두꺼워지면 보통 계면의 기복을 유도하는 원주형성장(columnar growth)를 지연시키는데 유효하였다. 그러나 NiFeSiB층이 얇으면 표면거칠기는 증가하고 전자기적 Neel 결합 때문에 Hi는 커졌다. The typical double-barrier magnetic tunnel junction (DMTJ) structure examined in this paper consists of a Ta 45/Ru 9.5/IrMn 10/CoFe7/<TEX>$AlO_x$</TEX>/free layer/AlO/CoFe 7/IrMn 10/Ru 60 (nm). The free layer consists of an <TEX>$Ni_{16}Fe_{62}Si_8B_{14}$</TEX> 7 nm, <TEX>$Co_{90}Fe_{10}$</TEX> (fcc) 7 nm, or CoFe <TEX>$t_1$</TEX>/NiFeSiB <TEX>$t_2$</TEX>/CoFe <TEX>$t_1$</TEX> layer in which the thicknesses <TEX>$t_1$</TEX> and <TEX>$t_2$</TEX> are varied. The DMTJ with an NiFeSiB-free layer had a tunneling magnetoresistance (TMR) of 28%, an area-resistance product (RA) of <TEX>$86\;k{\Omega}{\mu}m^2$</TEX>, a coercivity (<TEX>$H_c$</TEX>) of 11 Oe, and an interlayer coupling field (<TEX>$H_i$</TEX>) of 20 Oe. To improve the TMR ratio and RA, a DMTJ comprising an amorphous NiFeSiB layer that could partially substitute for the CoFe free layer was investigated. This hybrid DMTJ had a TMR of 30%, an RA of <TEX>$68\;k{\Omega}{\mu}m^2$</TEX>, and a of 11 Oe, but an increased of 37 Oe. We confirmed by atomic force microscopy and transmission electron microscopy that increased as the thickness of NiFeSiB decreased. When the amorphous NiFeSiB layer was thick, it was effective in retarding the columnar growth which usually induces a wavy interface. However, if the NiFeSiB layer was thin, the roughness was increased and became large because of the magnetostatic <TEX>$N{\acute{e}}el$</TEX> coupling.

Characteristics of magnetic tunnel junctions comprising ferromagnetic amorphous NiFeSiB layers

Magnetic tunnel junctions (MTJs), which consisted of amorphous ferromagnetic Ni 16Fe 62Si 8B 14 free layers, were investigated. NiFeSiB has a lower saturation magnetization ( M s: 800 emu/cm 3) than Co 90Fe 10 and a higher anisotropy constant ( K u: 2700 erg/cm 3) than Ni 80Fe 20. By increasing the free layer thickness, the tunnel magnetoresistance (TMR) ratio of up to 41% was achieved and it exhibited a much lower switching field ( H sw) than the conventionally used CoFe free layer MTJ. Furthermore, by inserting a thin CoFe layer (1 nm) at the tunnel barrier/NiFeSiB interface, the TMR ratio and switching squareness were enhanced.

Insertion of a Specular Reflective and Transmissive Nano-Oxide Layer into Giant Magnetoresistance Spin-Valve Structure

We have studied the influence of the insertion of a nano-oxide layer (NOL) into a magnetic GMR spin-valve. It was found that the spin-valve with NOL has a higher GMR ratio than that of the normal spin-valve without NOL. Naturally formed NOL without vacuum break shows a uniform layer, which effectively suppresses the current shunt, resulting in the reduction of the sheet resistance of GMR. The NOL spin-valve also shows a lower interlayer coupling (Hin) than that of the optimal normal spin-valve, which is consistent with AFM measurement showing lower roughness of NOL formed CoFe surface. Based on the advantage of NOL, we succeeded in lowering Hin while maintaining GMR ratio by insertion of NOL inside the CoFe free layer, where the free layer consists of CoFe/NOL/CoFe/NOL/Capping layer.

Insertion of a Specular Reflective and Transmissive Nano-Oxide Layer into Giant Magnetoresistance Spin-Valve Structure

We have studied the influence of the insertion of a nano-oxide layer (NOL) into a magnetic GMR spin-valve. It was found that the spin-valve with NOL has a higher GMR ratio than that of the normal spin-valve without NOL. Naturally formed NOL without vacuum break shows a uniform layer, which effectively suppresses the current shunt, resulting in the reduction of the sheet resistance of GMR. The NOL spin-valve also shows a lower interlayer coupling (Hin) than that of the optimal normal spin-valve, which is consistent with AFM measurement showing lower roughness of NOL formed CoFe surface. Based on the advantage of NOL, we succeeded in lowering Hin while maintaining GMR ratio by insertion of NOL inside the CoFe free layer, where the free layer consists of CoFe/NOL/CoFe/NOL/Capping layer.

Bias Voltage Dependence of Magnetic Tunnel Junctions Comprising Double Barriers and CoFe/NiFeSiB/CoFe Free Layer

The typical double-barrier magnetic tunnel junction (DMTJ) structure examined in this paper consists of a Ta 45/Ru 9.5/IrMn 10/CoFe 7/AlOx/free layer/AlOx/CoFe 7/IrMn 10/Ru 60 (nm). The free layer consists of an Ni16Fe62Si8 B14 7 nm, Co90Fe10 (fcc) 7 nm, or CoFe t1/NiFeSiB t2/CoFe t1 layer in which the thicknesses t1 and t2 are varied. The DMTJ with an NiFeSiB-free layer had a tunneling magnetoresistance (TMR) of 28%, an area-resistance product (RA) of 86 k Omega mum2, a coercivity (Hc) of 11 Oe, and an interlayer coupling field (Hi) of 20 Oe. To improve the TMR ratio and RA, a DMTJ comprising an amorphous NiFeSiB layer that could partially substitute for the CoFe free layer was investigated. This hybrid DMTJ had a TMR of 30%, an RA of 68 k Omegamu m2, and a Hc of 11 Oe, but an increased Hi of 37 Oe. We confirmed by atomic force microscopy and transmission electron microscopy that Hi increased as the thickness of NiFeSiB decreased. When the amorphous NiFeSiB layer was thick, it was effective in retarding the columnar growth which usually induces a wavy interface. However, if the NiFeSiB layer was thin, the roughness was increased and Hi became large because of the magnetostatic Neacuteel coupling

The influence of nano-oxide layer on magnetostriction of sensing layer in bottom spin valves

The magnetostriction coefficient (λs) of ultrathin sputtered polycrystalline as-deposited and annealed Ta∕Ni81Fe19(t)∕Ta films was studied as a function of the thickness. λs and magnetoresistance (MR) of bottom-type spin valves (SVs) with nano-oxide layer (NOL) added in the pinned layer were investigated by using NiFe, Co90Fe10, and CoFe∕NiFe∕CoFe layers as free layer (FL), respectively. λs of SV with NOL increased slightly except that of CoFe FL. NOLs were added at different positions to study the effects of NOL on λs of CoFe FL. All λs of CoFe FL change from negative to positive and its absolute value also increases significantly with CoFeOx related NOL added below. Our λs and surface roughness results indicated that the structure of the film not the roughness dominates λs of ultrathin FL in SVs.

Soft magnetic characteristics of an ultrathin CoFeNi free layer in spin-valve films

We have investigated the soft magnetic characteristics of an ultrathin Co-rich CoFeNi free layer in spin-valve films. By addition of Ni to a Co-rich CoFe free layer, magnetostriction ( λ ) of the films increased positively with Ni concentration, in contrast to which a Co 90Fe 10 free layer showed a negatively large λ . However, Ni addition also caused an increase in coercivity of the easy axis direction ( H c e.a.). To avoid this problem, a slight decrease in the Co contents of a CoFeNi free layer was found to be effective for decreasing H c e.a.. In order to satisfy both the small λ and H c e.a., a free layer of (Co 86Fe 14) 88−94Ni 12−6 proved to be an optimum composition in spin-valve films. Moreover, the zero λ composition of the CoFeNi free layer was changed by a high-conductance Cu layer deposited on the free layer, which was considered to come from the lattice strain of a free layer.

Current-perpendicular-to-the-plane giant magnetoresistance in structures with half-metal materials laminated between CoFe layers

A current-perpendicular-to-the-plane spin-valve (CPP-SV) structure having half-metal materials laminated between CoFe layers was investigated. A CPP-SV film having a CoMnGe free layer had a resistance-change-area product ΔRA of 0.9 mΩ μm2, almost the same as that of a CPP-SV film having a simple CoFe free layer. But the ΔRA of a CPP-SV film with a CoFe/CoMnGe/CoFe sandwiched free layer was 1.6 mΩ μm2. CPP-SV films having CoFe/iron-added-magnetite/CoFe sandwiched layers were also investigated and found to have a ΔRA of ≈2 mΩ μm2. We found that a CoFe/half-metal/CoFe sandwiched layer was a stronger source of spin-dependent scattering than a CoFe layer and simple half-metal layer.

O 2 plasma oxidation effect on magnetic properties of spin valves

The use of nano-oxide to improve the performance of spin valves has been extensively studied. But most of the investigations so far have been carried out on different samples. This may make some of the conclusions drawn from the experiments inconsistent because of the fluctuation in deposition conditions and device structures. In this work, the effect of nano-oxide on the properties of spin valves has been investigated through post-growth oxidation of the same sample in oxygen plasma for different rf powers and durations. The sample investigated was a bottom spin valve with the structure Si/SiO2/Ta/NiFe/IrMn/CoFe/Cu/CoFe/Ta. A relative increase of 20% and 12% was obtained in the giant magnetoresistance (GMR) ratio of as-deposited and annealed samples, respectively. It was found that, at a fixed rf power, there is a peak of the GMR ratio as the oxidation time increases. A higher peak value of the GMR ratio was obtained for lower rf power, although the required oxidation time is longer. This result can be well understood by considering both the enhanced specularity at the insulator/metal interface and the loss of magnetic effective thickness of the free layer by the oxidation. Magnetic parameters such as the interlayer coupling field (H0) and the coercivity of the free layer (Hcf) were also greatly influenced by the oxidation process. When only the Ta layer was oxidized, H0 increases very slightly, and Hcf increases with the oxidation time. However, when the CoFe free layer was oxidized, a significant increase was found for H0, and Hcf changes to decreasing. These results can be explained based on the Neel and RKKY coupling models.

Spin-filter spin valves with nano-oxide layers for high density recording heads

A new spin-filter spin valve with nano-oxide specular layers with structure of Ta/NiFe/IrMn/CoFe/NOL1/CoFe/Cu/CoFetfl/CutCu/NOL2/Ta was deposited using a Nordiko 9606 physical vapor deposition system. The data clearly show that the magnetoresistive (MR) ratio has been significantly improved for spin valves with thinner free layers. The MR ratio remains larger than 12% even when the CoFe free layer is as thin as 1 nm. An optimized MR ratio of ∼15% was obtained when tfl was about 1.2 nm and tCu about 1.5 nm, and was a result of the balance between the increase in the electron mean free path difference and current shunting through the conducting layer. It is also found that the Cu enhancing layer can improve soft magnetic properties of the CoFe free layer due to the low atomic intermixing observed between Co and Cu. The CoFe free layer of 1–4 nm exhibited coercivity of ∼3 Oe after annealing in a static magnetic field. This kind of spin valve with a very thin soft CoFe free layer is particularly attractive for ultra high density read head applications.

スピンバルブ構造をもつ極薄ＣｏＦｅフリー層の磁歪

An ultrathin CoFe free layer is very important for future high density recording head such as those using specular spin-valve films, because a CoFe free layer gives a higher MR ratio than a conventional NiFe/CoFe free layer. In order to realize a single CoFe free layer, we investigated the magnetostriction (λs) of an ultrathin CoFe free layer in spin-valve films. We compared the magnetostriction dependence on the underlayer structure among Cu, Ru, and Ru/Cu underlayers on a Ta buffer layer. For an Ru/Cu double-layered underlayer, λs becomes positively large when the Ru underlayer thickness is increased; on the other hand, it becomes negatively large when the Cu underlayer thickness is increased. This tendency can be explained by a model in which the λs is changed by the lattice elastic strain, where the volume ratio of Ru/Cu changes the lattice strain of an ultrathin CoFe free layer. The sensitivity of λs to the Cu underlayer thickness becomes large with increasing Co concentration in the CoFe free layer. This indicates that λs of a CoFe free layer becomes sensitive to the elastic strain with increasing Co concentration.

Spin-filter specular spin valves

Both a thin free layer and high magnetoresistance (MR) ratio are required in spin valves for high magnetic density recording heads. In traditional spin valve structures, reducing the free layer normally results in a reduction in MR. We report here on a spin-filter specular spin valve with structure Ta 3.5 nm/NiFe 2 nm/IrMn 6 nm/CoFe 1.5 nm/Nol/CoFe 2 nm/Cu 2.2 nm/CoFe tF/Cu tSF/Nol2/Ta 3 nm, which is demonstrated to maintain MR ratio higher than 12% even when the CoFe free layer is reduced to 1 nm. The semiclassical Boltzmann transport equation was used to simulate MR ratio. An optimized MR ratio of ∼14.5% was obtained when tF was about 1.5 nm and tSF about 1.0 nm as a result of the balance between the increase in electron mean free path difference and current shunting through conducting layer. It is found that the Cu enhancing layer not only enhances the MR ratio but also improves soft magnetic properties of CoFe free layer due to the low atomic intermixing observed between Co and Cu. The CoFe free layer of 1–4 nm exhibits a low coercivity of ∼3 Oe even after annealing at 270 °C for 7 h in a field of 1 kOe. Furthermore, the interlayer coupling field Hint between free layer and pinned layer can be controlled by balancing the Rudermann–Kittel–(Kasuya)–Yosida and magnetostatic coupling. Such a thin soft CoFe free layer is particularly attractive for high density read sensor application.

NOL specular spin-valve heads using an ultrathin CoFe free layer

This paper reports the film and head performance of specular spin valves with nano-oxide layer (NOL-SPSV). A large MR ratio of 17% was obtained by using an ultrathin CoFe free layer with a high conductance Cu layer, which decreases the sense current field of a free layer and brings good soft magnetic characteristics. Prototype heads with a read track width of 0.47–0.61 μm were fabricated by using NOL-SPSV films with an MR ratio of 14–15%, H ua∼400 Oe, and H c∼5 Oe. High output signal voltage of 8–11 mV/μm was realized in the NOL-SPSV heads.

High temperature magnetoresistance in (Co, CoFe and NiFe)/Cu/CoFe/IrMn spin-valves

The MR characteristics of temperature variations ranging from room temperature to 570 K for IrMn spin-valves which consist of free layer Co, CoFe and NiFe were studied. Co-SV had the highest MR value in all temperature ranges among them and even at 510 K the MR ratio held more than half of room temperature value. Whereas CoFe-SV had a slightly higher MR than NiFe-SV below 450 K, above 450 K CoFe-SV showed a lower MR ratio than NiFe-SV. MR loops of large coercivity such as CoFe free magnetic layer collapsed in the lower elevated temperature compared to NiFe and Co. Small coercivity of the free magnetic layer would be preferable to spin-valves.

Oxygen-enhanced IrMn spin valves deposited by ion-beam and magnetron sputtering

Enhancement of giant magnetoresistance properties of single (bottom) and dual IrMn-based spin valves through exposure of part of the CoFe pinned layer to O2 is reported. Under optimal conditions, a ΔR/R of 10.4% [Hua=460 Oe, Hf=5.1 Oe, and Hc=4.7 Oe for a free and pinned layer thickness (permalloy equivalent) of 50 Å each] for an ion beam sputtered single spin valve, and a ΔR/R of as high as 20.5% for a magnetron sputtered dual spin valve having a 30 Å thick CoFe free layer are observed, compared to a value of 6.5% and 10.6% for the corresponding spin valve without O2 exposure, respectively. Transmission electron microscopy results reveal the presence of a thin (10 Å) crystalline oxygen-containing layer near the IrMn–CoFe pinned layer interface as a result of O2 exposure. X-ray reflectivity data show smoother interfaces for the spin valves subjected to O2 exposure, consistent with the lower Hf and smaller sheet resistance observed for these samples. The enhanced ΔR/R thus can be attributed to improved growth after O2 exposure.