Enhancement of MR ratios using thin oxide layers in PtMn and α-Fe2O3-based spin valves

Spin valves having thin oxide layers in the pinned and/or free layers were prepared by sputtering. MR ratios of the spin valves were increased from 8.1 to 11.9% by inserting the oxide layer into the pinned layer in Ta/PtMn/CoFe/Cu/CoFe/Ta spin valves. MR ratio of 13.9% and considerably large sheet ΔR of 2.55 Ω were obtained in the PtMn-based spin valves having the oxide layer in the pinned and free layer. Larger MR ratio of 17.3% and the sheet ΔR of 1.3 Ω were obtained in the PtMn-based dual-type spin valves having the oxide layer in both pinned layers. α-Fe₂O₃ based spin valves having thin oxide layers were also prepared. MR ratios of the spin valves were increased from 11.9 to 14.3% by inserting the oxide layer into the free layer in α-Fe₂O₃/CoFe/Cu/CoFe/Ta spin valves. The enhancement of the MR ratios may be attributed to the specular scattering effect of the conduction electrons by the thin oxide layers.     

Synthetic Co50Pt50/Ru/CoFe hard–soft trilayer in spin valves

The influence of deposition power and seedlayer on the properties of hard magnet Co₅₀Pt₅₀ was studied. Co₅₀Pt₅₀(/Co₉₀Fe₁₀)/Ru/Co₉₀Fe₁₀ trilayer was used as pining/pinned layer in spin valves. The influences of different hard layer, soft layer and free layer on exchange bias, interlayer coupling, and magnetoresistance (MR) ratio were studied. Weak antiferromagnetic interlayer coupling was obtained by adjusting the thickness of hard and soft layers. MR of a spin valve with structure Cr2/CoFe0.5/CoPt4/CoFe0.5/Ru0.8/CoFe2.2/Cu2.05/CoFe2.6/Cu1.1/Ta1 reached 10.68% (unit in nm), which is comparable to those of IrMn-based synthetic spin valves. The increment of the coercivity of the free layer is mainly due to the static magnetic interaction between the hard layer and the free layer.     

一种非连续纳米氧化层自旋阀的显微结构研究

利用高分辨电子显微学方法(HREM)研究了纳米氧化层镜面反射自旋阀多层结构Ta(35nm)Ni80Fe20(2nm)Ir17Mn83(6nm)Co90Fe10(15nm)NOL1Co90Fe10(2nm)Cu(22nm)Co90Fe10(15nm)NOL2Ta(3nm).该自旋阀的巨磁电阻(GMR)效应高达15%,较无此镜面反射纳米氧化层(NOL)的自旋阀提高近1倍,同时交换偏置场亦有所增强.高分辨显微结构分析表明,介于钉扎层与被钉扎层之间的氧化层(NOL1)并未完全氧化,即除氧化过程生成的CoFe氧化物 关键词： 自旋阀 纳米氧化层 高分辨电子显微学 巨磁电阻效应     

Pseudo spin-valves with Al or Nb as spacer layer: GMR and search for spin switch behaviour

The magnetoresistance of several Ferromagnet/Normal metal/Ferromagnet spin-valve type structures has been investigated using Al as normal spacer layer. A magnetoresistance ratio up to 4.1% at room temperature and 5.7% at 0.3 K is found for the sandwich with both Co layers, while slightly lower signals are found for the structures involving CoFe and NiFe layers. The magnetoresistance dependence for Co/Al/Co, Co/Al/CoFe and Co/Al/NiFe on the spacer layer thickness exhibits the familiar non monotonic behaviour with second peak slightly larger than the one reported for Cu based pseudo spin valves. At cryogenic temperatures, preliminary results on the onset of spin switch effects in Co/Al/Co and the full spin switch effect in Co/Nb/Co are also reported here.     

TaN underlayers for spin valves deposited directly on top of Si substrates

TaN underlayers for spin valves were studied, which were deposited directly on top of Si substrates. The experimental results obtained with the TaN underlayer were compared with those obtained with other (Ta, Mo, and MoN) underlayers. The spin valve structure was Si/Underlayer(tÅ)/NiFe(21 Å)/CoFe(28 Å)/Cu(22 Å)/CoFe(18 Å)/IrMn(65 Å)/Ta(25 Å). The TaN underlayer for a spin valve element exhibited good adhesion to the Si substrate. The XRD patterns of the annealed TaN on bare Si substrate at 900 °C showed no Ta silicide phases, which suggests that the TaN layer may also be used as a diffusion barrier between Si substrate and the ensuing spin valve active layers, as well as an underlayer. A spin valve element having TaN underlayer deposited directly on top of a Si substrate showed a high MR ratio of about 8.3% after annealing at 200 °C. It is concluded that it is advantageous to use a TaN underlayer if one wants to fabricate spin valve elements directly on top of Si substrates.     

Thermally assisted writing for perpendicular MRAM

Spin valves composed of TbCo/CoFe/Cu/CoFe/TbFeCo were fabricated with perpendicular magnetization and GMR ratios of 4.5%. The (TbCo/CoFe) layers and (CoFe/TbFeCo) layers are referred to the free and the pinned layers, respectively. The compositions of two layers were chosen to have a lower Curie temperature (130 °C) but higher coercivity (13.2 kOe) of the free layer at room temperature than those of the pinned layer; therefore, the free layer is quite stable at room temperature but its magnetization can be easily switched at a relatively low temperature. Spin valves were patterned into 100-μm-wide cells and their coercivity was reduced with increasing writing current due to the temperature rise by current-heating. When the current density of the writing current was increased to 2.1×10⁶ A/cm², the required switching field for the free layer was only 10 Oe.     

O2 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 (H_cf) were also greatly influenced by the oxidation process. When only the Ta layer was oxidized, H₀ increases very slightly, and H_cf increases with the oxidation time. However, when the CoFe free layer was oxidized, a significant increase was found for H₀, and H_cf changes to decreasing. These results can be explained based on the Néel and RKKY coupling models. Received: 25 October 2001 / Accepted: 21 December 2001 / Published online: 3 June 2002     

Structure and magnetic properties of Fe4/Cun (n=2,4) superlattices from first-principles study

The structures and magnetic properties of Fe4/Cun (n=2, 4) superlattices have been investigated by the first-principles pseudopotential plane-wave method based on spin density approximation. Compared with the ideal fcc-Cu bulk structure, for the optimized Fe4/Cu2 model, obvious contraction of interlayer distances occurs on the interior Fe layers, whereas the interlayer distances of Fe layers in Fe4/Cu4 are expanded. The anti-parallel alignment magnetic moment and negative polarization of the interior Fe layer have been found in the Fe4/Cu2 model. This can be explained in terms of the magnetic-volume effect, and the moment of anti-parallel alignment attributes to the contracted interlayer distances between the interior Fe layers. The MR ratio has also been evaluated by means of the two-current model. The MR ratio of the Fe4/Cu2 model (4.89%) is much small than that of the Fe4/Cu4 one (23.65%).     

Studies of magnetization reversals about two kinds of MR curves observed in the spin valve using the (110) magnetite pinning layer

In the spin valves composed of Co/Cu/Co on the epitaxial (110) Fe₃O₄ as the pinning layer, we found out that shapes of magnetoresistance (MR) curves depended on thickness of the cobalt pinned layer (PL) with the field applied in the 〈110〉 direction of Fe₃O₄: (1) the flat-shaped MR curve showed low MR ratio under 2 nm thickness of cobalt pinned layer (PL): (2) the unusually shaped MR curve showed high MR ratio over 5 nm thickness of PL in spite of the hard direction of Co layers. We assumed that the synchronous magnetization reversal (SR) of PL and Fe₃O₄ would occur at the MR switching field due to 90° coupling between PL and Fe₃O₄ layers. Then, only occurrence of SR of PL cause the drastic change of the magnetization relative angle between FL and PL, indicating the observation of the unusually shaped MR curve having high MR ratio. On the other hand, the SR of cobalt free layer (FL) together with the PL flip also occur due to the large contribution of Néel-type ferromagnetic coupling between FL and PL, which lead to less changing the relative angle of FL and PL during magnetization processes, indicating the observation of a flat-shaped MR curve having low MR ratio. This dependence of PL thickness on MR curves might come from the balance of Néel (ferromagnetic) and stray field (antiferromagnetic) coupling due to magnetic free pole at edge of PL.     

Magnetoresistance study and interlayer coupling of CoFe/Os/CoFe thin films

The CoFe/Os/CoFe thin films were deposited on natural oxidized Si(1 0 0) substrates at room temperature by an ultra-high vacuum DC-magnetron sputtering system with a base pressure less than 1×10⁻⁸ Torr. The thickness of the ferromagnetic layers was 100 Å in all cases and a series of trilayers with Os spacer ranging from 3 to 20 Å was made. Effects of the Os layer thickness on the magnetoresistance (MR) and magnetic properties were investigated. The results showed that the magnetism switched from ferromagnetic (Os thickness=3, 5 Å) to antiferromagnetic (Os thickness=7–13 Å) and then ferromagnetic (Os thickness=20 Å) again. From the MR study, we see that the AMR ratio decreased from 4.64% to the minimum value 0.69% at 9 Å and then increased; GMR ratio increased from 0.01% to the maximum value 0.43% at 9 Å and then decreased. From the hysteresis loops, the results exhibited that coercivity increased from 16 Oe to the maximum value 92 Oe at 9 Å and then decreased, and squareness value decreased from 0.97 to the minimum value 0.17 at 9 Å and then increased. Dependence of saturation field on Os spacer-layer thickness for CoFe trilayers showed a maximum value 216 Oe at 9 Å. This suggests that the small GMR effect may be related to the small exchange coupling strength in CoFe/Os/CoFe thin films.     

IrMn基反铁磁自旋阀的巨磁电阻效应

用第一性原理方法研究了在微观尺度具有三重对称磁结构的IrMn合金的反铁磁自旋阀(AFSV)的电子输运.研究表明:基于有序L1₂相IrMn合金的Co/Cu/IrMn自旋阀的巨磁电阻(GMR)效应具有三重对称性,可以利用这一特性区分反铁磁材料的GMR与传统铁磁材料的GMR.基于无序γ相IrMn合金的IrMn(0.84 nm)/Cu(0.42 nm)/IrMn(0.42 nm)/Cu(0.42 nm)(111) AFSV的电流平行平面构型的GMR约为7.7％,大约是电流垂直平面构型的GMR(3.4％)的两倍,明显大于实验中观测到的基于共线磁结构的FeMn基AFSV的GMR. 关键词： 反铁磁自旋阀 巨磁电阻效应 非共线磁结构 电流平行平面结构     

Effect of NiAl underlayer and spacer on magnetoresistance of current-perpendicular-to-plane spin valves using Co2Mn(Ga0.5Sn0.5) Heusler alloy

We investigated the effect of a NiAl underlayer and spacer on magnetoresistive (MR) properties in current-perpendicular-to-plane spin valves (CPP-SVs) using Co₂Mn(Ga_0.5Sn_0.5) (CMGS) Heusler alloy ferromagnetic layers. The usage of a NiAl underlayer allowed a high temperature annealing for the L2₁ ordering of the bottom CMGS layer, giving rise to a MR ratio of 10.2% at room temperature. We found that the usage of a NiAl spacer layer also improved the tolerance of the multilayer structure against thermal delamination, which allowed annealing to induce the L2₁ structure in both the bottom and top CMGS layers. However, the short spin diffusion length of NiAl resulted in a lower MR ratio compared to that obtained using a Ag spacer. Transmission electron microscopy of the multilayer structure of CPP-SVs showed that the atomically flat layered structure was maintained after the annealing.     

Temperature dependence of giant magneto-resistance in PtMn- and Fe2O3-based specular spin valves

Temperature dependence of the giant magneto-resistance (MR) was measured for spin valves with and without nano-oxide layer (NOL). In spin valves with NOL, the MR ratio increased more remarkably on lowering the temperature than in those without NOL. The temperature dependence of MR ratio and that of the resistivity were explained by using two-current model. The MR ratio enhanced with NOL is attributed to the increase of the mean free path of up-spin electrons.     

Magnetic and transport properties of sputtered Fe/Si multilayers

The structural, magnetic and transport properties of sputtered Fe/Si multilayers were studied. The analyses of the data of the X-ray diffraction, resistance and magnetic measurements show that heavy atomic interdiffusion between Fe and Si occurs, resulting in multilayers of different complicated structures according to different sublayer thicknesses. The nominal Fe layers in the multilayers generally consist of Fe layers doped with Si, ferromagnetic Fe-Si silicide layers and nonmagnetic Fe-Si silicide interface layers, while the nominal Si spacers turn out to be Fe-Si compound layers with additional amorphous Si sublayers only under the condition either for the series or for the series multilayers. A strong antiferromagnetic (AFM) coupling and negative magnetoresistance (MR) effect, about 1%, were observed only in multilayers with iron silicide spacers and disappeared when -Si layers appear in the spacers. The dependences of MR on and on bilayer numbers N resemble the dependence of AFM coupling. The increase of MR ratio with increasing N is mainly attributed to the improvement of AFM coupling for multilayers with N. The dependence of MR ratio is similar to that in metal/metal system with predominant bulk spin dependent scattering and is fitted by a phenomenological formula for GMR. At 77 K both the MR effect and saturation field increase. All these facts suggest that the mechanisms of the AFM coupling and MR effect in sputtered Fe/Si multilayers are similar to those in metal/metal system. Received: 11 February 1998 / Revised: 9 March 1998 / Accepted: 9 March 1998     

Microstructural and magnetic characteristics of IrMn exchange-biased tunnel junctions

Tunneling magnetoresistance values above 20% and 40% were obtained for as-deposited and annealed tunnel junctions, Ta/NiFe/Cu/NiFe/IrMn/CoFe/Al-oxide/CoFe/NiFe/Ta, respectively. Exchange biasing field increased from 270 to 550 Oe after annealing resulting from sharpening of the IrMn/CoFe interface. dV/dI vs. V curves showed asymmetric profiles, which were due to asymmetry of the CoFe/Al-oxide interfaces and difference in microstructure of the CoFe layers.     

合成反铁磁Ni81Fe19/Co90Fe10/Ru/Co90Fe10/Ni81Fe19的研究

采用直流磁控溅射方法制备了一系列的合成反铁磁及以其为自由层的自旋阀.研究发现,在Ni₈₁Fe₁₉与Ru层之间插入适当厚度的Co₉₀Fe₁₀层后,可有效地提高合成反铁磁两磁性层间的反铁磁耦合强度,得到具有饱和场H_s更高、饱和磁化强度M_s更低、热稳定性更好的合成反铁磁.另外,以这种合成反铁磁作自旋阀的自由层时,可有效提高自旋阀的稳定性. 关键词： 合成反铁磁 退火 自旋阀     

Stability of the magnetoresistance for NiO-containing Co/Cu/Co spin valves naturally placed in air

The Co/Cu/Co spin valves with an additional NiO layer at the top (TSV) and under the bottom (BSV) were set in air at room temperature for about four years. Effects of time on the stability, the magnetoresistance (MR), and microstructures were investigated. Results show that the MR of TSV is more stable than that of BSV. The MR of BSV decreases 12.5% in the first 1000 days, while it increases 4% after another 360 days, while the MR of TSV keeps almost unchanged within the range of measurement error. Different time-dependent behavior of the MR value for BSV and TSV was ascribed to the different contribution of the roughness at NiO/Co and Co/Cu interfaces. PACS 68.60.Dv; 61.10.Eg; 68.35.Ct; 73.43.Qt; 75.25.+z     

Reduction in critical current density of current-induced magnetization switching

We have investigated the current-induced magnetization switching in an exchange-biased spin valve structure. By using an unpatterned antiferromagnetic layer to pin the fixed Co layer, we obtained a lower switching current density by a factor of 5 than a simple spin valve structure. For the application, it is important to know how to keep the spin polarization when the thicker layer is pinned by an antiferromagnet. The unpatterned pinned ferromagnetic lead can be a good solution for spin-transfer-torque-activated device. The effect of Cu buffer layer on the top of the thin Co and Ru buffer layer under the thick Co layer on the current-induced magnetization switching in cobalt-based trilayer spin valves was also investigated. The experimental results showed that the Ru buffer layer in combination with Cu buffer layer could induce a decrease in the critical switching current by 30%, and an increase in the absolute resistance change by 35%, which is caused by an improvement of a microstructure of a thicker Co polarizer.     

Annealing effect on magnetoresistance in NiO-Co-Cu based spin valves

We have fabricated two sets of NiO-Co-Cu based spin valves by the magnetosputtering technique with different deposition parameters. Magnetoresistance (MR) measurements show that the MR value for the NiO layer under the bottom of Co/Cu/Co spin valve (BSV) is larger than that for the NiO layer at the top of Co/Cu/Co (TSV). The MR value of BSV decreases with increasing annealing temperature in air or in vacuum, which disappears at the blocking temperature of NiO, i.e., about 250 °C. There is maximum MR value for TSV annealing at a temperature range from room temperature to 350 °C. The different thermal behavior for BSV and TSV is explained by the competition between the interface roughness of NiO/Co, which was determined by the grazing incident X-ray reflectivity and X-ray diffuse scattering, and the coupling effect between Co layer and NiO layer. PACS 68.60.Dv; 61.10.Eg; 68.35.Ct; 73.43.Qt; 75.25.+z     

Three-dimensional noncollinear antiferromagnetic order in single-crystalline FeMn ultrathin films

We present experimental evidence for a three-dimensional noncollinear antiferromagnetic spin structure in ultrathin single-crystalline fcc Fe50Mn50 layers using magnetic circular dichroism photoelectron emission microscopy and x-ray magnetic linear dichroism. Layer-resolved as-grown domain images of epitaxial trilayers grown on Cu(001) in which FeMn is sandwiched between ferromagnetic layers with different easy axes reveal the presence of antiferromagnetic spin components in the film plane and normal to the film plane. An FeMn spin structure with no collinear order in the film plane is consistent with the absence of x-ray magnetic linear dichroism in Fe L3 absorption in FeMn/Co bilayers.