Bottom Spin Valves Research Articles

Magnetoresistive Properties of Dy-Based Bottom Spin Valve

Dysprosium-based bottom spin valves with the thickness of the dysprosium layer comparable to the spatial period of the magnetic helicoid were fabricated by magnetron sputtering. Perfect fiber crystal texture was obtained in Dy layer sputtered on Ta underlayer. The inversion of magnetoresistive curves with temperature decreasing is observed. The explanation of these features is based on the paramagnet-antiferromagnet transition in dysprosium nanolayer and on the deformation of helicoidal magnetic structure by an applied magnetic field. It has been determined that at the same temperature and field the smaller is the thickness of the ultra-thin dysprosium layer, the greater is its magnetic moment. The Neel temperature was estimated for dysprosium nanolayer in Ta/Dy/Ta structures where the smallest thickness of the dysprosium layer was 10 nm. It has been revealed that the temperature of transition from paramagnetic to antiferromagnetic state is different in the film regions in which helicoid axis is close to parallel or perpendicular to the film plane.

Top and Bottom Spin Valves With Ni-Fe-Mn Antiferromagnetic Layer

Structure, magnetic and magnetoresistive properties of spin valves with Ni-Fe-Mn antiferromagnet as a pinning layer have been studied. A technique of fabrication of spin valves with an enhanced thermal stability and improved hysteretic characteristics has been elaborated.

Спиновый клапан с нижним расположением упорядоченной антиферромагнитной фазы Ni–Fe–Mn

Спиновый клапан с нижним расположением упорядоченной антиферромагнитной фазы Ni–Fe–Mn

Giant Magnetoresistive Sensors for DNA Microarray

Giant magnetoresistive (GMR) sensors are developed for a DNA microarray. Compared with the conventional fluorescent sensors, GMR sensors are cheaper, more sensitive, can generate fully electronic signals, and can be easily integrated with electronics and microfluidics. The GMR sensor used in this work has a bottom spin valve structure with an MR ratio of 12%. The single-strand target DNA detected has a length of 20 bases. Assays with DNA concentrations down to 10 pM were performed, with a dynamic range of 3 logs. A double modulation technique was used in signal detection to reduce the 1/f noise in the sensor while circumventing electromagnetic interference. The logarithmic relationship between the magnetic signal and the target DNA concentration can be described by the Temkin isotherm. Furthermore, GMR sensors integrated with microfluidics has great potential of improving the sensitivity to 1 pM or below, and the total assay time can be reduced to less than 1 hour.

Effects of gas-cluster ion beam processing on physical, magnetic, and giant magnetoresistance properties of α-Fe 2O 3 bottom spin-valves

The effects of gas-cluster ion beam (GCIB) processing accelerated by a few thousands of volts on the surface roughness, crystalline orientation texture, magnetic, and giant magnetoresistance (GMR) properties of exchange biased α-Fe 2O 3 bottom spin-valves have been investigated. It was found that all of these properties were strongly influenced by the GCIB processing conditions, such as gas sources including Ar, H 2, O 2, and their mixture combinations, and ion cluster doses. The properties of both GCIB processed and unprocessed areas on 50 and 100-nm thick α-Fe 2O 3 coated Si(1 0 0) wafers with GMR multi-layers of Co(3)/Cu(2.5)/Co(3)/Ta 2O 5(2.5 nm) were compared. The GMR ratio, Δ R/ R 0, was consistently increased by up to 20–30% for the areas that were GCIB processed with condition “P1(10 14 Ar gas)+P4(5×10 14 Ar+H 2 gases) ion cluster ions/cm 2” (Δ R/ R 0=14%) over the unprocessed α-Fe 2O 3 layer (Δ R/ R 0=11.2%), and decreased by up to 80% when using the process with the heaviest ion does, “P3” (Δ R/ R 0=2.4%). In addition, GCIB processing changed the magnetic properties of GMR spin-valve including exchange bias field, pinned, and free-layer coercivities significantly. The free-layer coercivity was decreased dramatically with “P1+P4” GCIB processing from 160 Oe to as low as 27 Oe and the exchange bias field increased from 72 Oe up to ∼125 Oe. The decrease of the GMR ratio, pinned coercivity, and exchange bias field when the “P3” condition process was used are thought to be due to surface damage and composition changes of the α-Fe 2O 3 anti-ferromagnetic layer. X-ray photoelectron spectroscopy (XPS) analysis of the α-Fe 2O 3 thin film surface showed a 4.6% reduction of the oxygen content (unprocessed: 65 kcps (kilo-counter per second), “P3”: 62 kcps) after application of the P3, the heaviest process condition. Atomic force microscopy (AFM), X-ray diffraction (XRD), and cross-sectional transmission electron microscopy (XTEM) analyses suggest that the increase of the GMR ratio and the improved magnetic properties of optimized GCIB-processed α-Fe 2O 3 spin-valve are due to the reduction of surface roughness and improvement of the crystalline orientation texture resulting from a more “defect-free” or denser α-Fe 2O 3 layer.

Effect of interface intermixing on the magnetoresistive and the exchange coupling in bottom‐ and top‐spin valves

AbstractWe investigated the interfacial mixing phenomena associated with magnetoresistance and exchange coupling in a top spin valve (T‐SV) and bottom spin valve (B‐SV) at IrMn/CoFe and CoFe/IrMn interfaces. The difference in the surface free energies of IrMn and CoFe is the major factor determining the degree to which interfacial mixing occurs, but it depends on the deposition sequence. A B‐SV featuring a gradational IrMn/CoFe interface had a small Hex in the as‐deposited state, but showed improved exchange coupling on annealing as a result of limited interdiffusion. By contrast, a T‐SV featuring a sharp CoFe/IrMn interface had a high Hex in the as‐deposited state, but suffered a reduction in exchange coupling on annealing, due to a moderate level of interdiffusion. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Brillouin light scattering of CoFe/IrMn-based top and bottom spin-valves

Abstract The spin-wave Brillouin light scattering (BLS) was observed from CoFe/IrMn-based bottom and top spin-valve structures. The magnitude of perpendicular magnetic anisotropy of these structures was investigated by means of BLS. The experimental spin-wave frequency was simulated using a simple model which includes crystallographic, exchange, in-plane uniaxial, and in- and out-of-plane anisotropies and takes the exchange interaction between the two ferromagnetic layers into account. This enabled us to determine, in addition to the other magnetic parameters, both in- and out-of-plane anisotropy constants near the surface. It was found that the top spin-valve is in a strong out-of-plane anisotropy whereas in-plane anisotropy is dominant for the bottom spin-valve.

Comparison Between Top and Bottom NiO-Pinning Spin Valves: Correlation Between the Extraordinary Hall Effect and Resistivity

Comparison of the extraordinary Hall effect (EHE) on top and bottom NiO-pinning Co/Cu/Co spin valves (SVs), has been made in order to investigate the relationship Rs~rhon between the extraordinary Hall coefficient Rs and the resistivity rho. Bottom and top spin valves have stronger and weaker specular reflection effects which mainly originate from smoother and rougher Co/NiO interface, respectively. The scaling law Rs~rho2, which implies that the side-jump mechanism is the dominant scattering process, works fairly well (n=1.93plusmn0.02) in the bottom NiO-pinning spin valve, while it deviates much (n=1.32plusmn0.04) in the top one

Temperature dependence of magnetoresistive properties in bottom spin valve films employing very thin Cu spacers

Temperature dependence of magnetoresistive properties in bottom spin valve films having very thin Cu spacers are reported. NiFeCr55 Å/NiFe10 Å/IrMn70 Å/CoFe tÅ/Ru4 Å/CoFe( t+3)Å/Cu/CoFe tÅ/NiFe10 Å/Ta50 Å bottom spin valve films were deposited using a DC magnetron sputter deposition system. Magnetoresistance (MR) ratio reached a maximum of 13.5% and 11.9% at the Cu thickness of 10.4 Å, when the thickness of the CoFe layers t was 20 and 10 Å, respectively. Unlike the top spin valves reported earlier, the dip in the MR ratio was not observed when the interlayer coupling between the free layer and reference layer became zero. Sheet resistance change (DR s) reached a maximum of 4.22 Ω/□ at the Cu spacer thickness of 10 Å when the CoFe thickness t was 10 Å. Temperature dependences of MR ratio, DR s, interlayer coupling field ( H i), and sensitivity showed mostly monotonic decrease as the temperature was increased up to 200 °C. It turns out that DR s for the film having 10.4 Å of Cu spacer thickness at 200 °C was larger than the DR s for the film having 20 Å of Cu spacer thickness at 40 °C. This suggests a high output voltage of the spin valve sensor made of the thin Cu spacer even at high operating temperature. These very thin Cu spacers could be utilized for very small devices where the interlayer coupling field is dominated by high demagnetizing fields.

Thickness Dependence of Giant Magnetoresistance in Spin Valves: Influence of Interface and Bulk Scattering

Using the semiclassical Boltzmann theory, we have investigated the influences of pure interface, nonpure interface, bulk, and interface plus bulk scattering on the giant magnetoresistance (GMR) dependence on layer thickness of the bottom spin valve (BSV), synthetic antiferromagnetic (SAF) spin valve, and dual spin valve (DSV). Our work shows consistency with experiment and theory and may provide some useful guidelines for electrical and structural optimizations of spin valves

Effect of new materials as under and cap layers on thermal stability of synthetic bottom spin valves

We investigated the thermal stability of the Ta-, ZrAl- and TiAl-based synthetic bottom spin valves. In the as-deposited state, the MR ratios of ZrAl- and TiAl-based synthetic SVs (SSV) are similar to that of the Ta-based SV. However, after annealed at 400 °C for 10 min, the MR ratio of Ta-based SV decreased by 42.5% (from 7.43% to 4.27%). On the other hand, the MR ratio of ZrAl- and TiAl-based SSVs increased 0.3% (from 7.37% to 7.39%) and 13.3% (from 7.47% to 8.46%), respectively. Auger depth profile clearly showed that ZrAl- and TiAl-based SSVs have higher interdiffusion resistance than Ta-based SV. Especially, diffusion of Mn from antiferromagnet layer to pinned layer was suppressed in TiAl-based SVs. The root-mean-square (RMS) roughness of the TiAl layer (0.11 nm) was much smaller than those of the ZrAl (0.16 nm) and Ta layer (0.43 nm). Due to high affinity of Ti or Al with oxygen, very stable and dense TiAl-oxides were formed at the surface during annealing. Thermal stability of TiAl- and ZrAl-based SSVs was enhanced due to surface uniformity and high passivity of TiAl- and ZrAl-oxides.

Inverse giant magnetoresistance inFe∕Cu∕Gd1−xCoxspin-valves

For bottom spin-valves of $\mathrm{NiO}∕\mathrm{Fe}∕\mathrm{Cu}∕{\mathrm{Gd}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}$, giant magnetoresistance has been measured as a function of thickness and composition of $\mathrm{GdCo}$ layers as well as temperature. For all spin-valves involved here, the giant magnetoresistance has been attributed to contributions of spin-dependent scattering at interfaces and in bulk. The interfacial contribution produces positive giant magnetoresistance ratio for various compositions of $\mathrm{GdCo}$ alloys. However, the bulk contribution produces negative one for the Co contents from $50\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%$ to a critical value ${x}_{\mathrm{C}}(R)$ (between $69\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%$ and $77\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%$) and positive one for higher Co contents. It is suggested that the interfacial asymmetric factor of spin-dependent scattering is larger than 1.0 at the $\mathrm{Cu}∕\mathrm{GdCo}$ for various alloy compositions of $\mathrm{GdCo}$ and that the bulk asymmetric factor in $\mathrm{GdCo}$ layer is smaller and larger than 1.0 for Co contents below and above ${x}_{\mathrm{C}}(R)$, respectively. For spin valves with Co contents below and/or above ${x}_{\mathrm{C}}(R)$, the giant magnetoresistance ratio varies nonmonotonically and/or monotonically as a function of the $\mathrm{GdCo}$ layer thickness (temperature) for a specific temperature (a specific $\mathrm{GdCo}$ layer thickness), respectively. The change of the bulk asymmetric factor in the $\mathrm{GdCo}$ layer with the alloy composition can be attributed to the variation of either the spin alignment of Co and Gd atoms or the spin polarization of the $\mathrm{GdCo}$ layer.

Current-induced hysteretic switching and reversible transition of magnetization in spin-valve structures with current constraint paths in spacer layers

Current-induced magnetization dynamics has been investigated in circular spin-valvelike nanopillars (less than $100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter) with a nano-oxide spacer layer. From the magnetoresistance measurements at room temperature, both the hysteretic switching and the reversible transition of the free layer magnetization between the parallel alignment and the antiparallel alignment are observed for current-perpendicular-to-the-plane bottom spin valves with nano-oxide spacer layer insertion. The findings that may reveal the fundamentals of the spin-transfer torque effect indicate that out present understanding based on the phenomenological description of the spin-transfer torque is incomplete.

Microstructure and interface evolution of PtMn bottom spin-filter spin valves induced bystress and unidirectional field annealing

The microstructure evolution in PtMn-based bottom spin-filter spin valves was investigatedto clarify the alteration of the sensor performance caused by different annealingtreatments during manufacturing. Neither unidirectional field annealing (UDA)nor stress annealing (SA) for 38 h, which simulates the final quality test of thefinished read-write head, have any significant influence on the texture of the sensor stack. UDA causes the appearance of domain boundaries in thePtMn layer as a consequence of the fcc–fct transformation together with some disturbancein the functional layers of the sensor, neither of which affect the giant magneto-resistiveperformance. The SA treatment causes an inter-diffusion in the NiFeCr/NiFe seedlayers, some grain growth in PtMn, more serious disturbance of the functionallayers, and complete amorphization of the Ta cap layer. In comparison to otherstudies of spin valves, the intermixing of seed layers in the present study doesnot appear to influence the PtMn/CoFe interface. The alteration of the sensorperformance seems to be mainly caused by the inter-diffusion of the functional layers.

Effect of H2 sputter gas on interfacial mixing in spin valves

We introduced the use of hydrogen during the deposition of spin valves to achieve better control of interfacial mixing, especially for bottom spin valves (B-SV). The hydrogen was introduced during deposition of the CoFe or IrMn layer depending on whether it was a B-SV or a top spin valve (T-SV). The magnetoresistance ratio and Hex increased to 15% and 30%, respectively, when hydrogen (10vol%) was introduced for a B-SV. By contrast, the hydrogen effect for a T-SV was small. Using hydrogen (10vol%), the surface of the SV was smoother (1.01nm) than without hydrogen (1.94nm). In addition, the SV with hydrogen had a well-developed (111) texture and larger grains. By introducing hydrogen, we could control the microstructure and reduce the intermixing between CoFe and IrMn, especially for B-SVs.

The effect of cyclic annealing treatments on magnetoresistance in IrMn‐pinned top and bottom spin valves

IrMn-pinned top spin valve (TSV) and bottom spin valve (BSV) films were prepared by DC magnetron sputtering onto thermally oxidized Si (111) substrates at room temperature under a magnetic field of above 100 Oe. The effect of cyclic annealing treatments on magnetoresistive and structural properties in the films was investigated. The exchange coupling field (Hex) of TSV and BSV is stabilized after the second annealing cycle and it is thought that these films reveal high thermal stability. The magnetoresistance (MR) ratio and Hex of TSV were 5.6% and 430 Oe, respectively. For BSV the Hex value (1180 Oe) is much higher while MR ratio (3.6%) is reduced with respect to TSV. The blocking temperatures of the TSV and BSV were 250 °C and 270 °C, respectively. The results concerning magnetic properties together with structural investigations suggest the Hex is mainly dependent on the quality of fcc (111) crystalline texture while MR is sensitive to both crystalline texture and interface roughness. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Stability in In-Stack Biased CPP SV Heads

In this paper, we present a quantitative micromagnetic modeling analysis of head stability in in-stack biased current perpendicular to plane (CPP) spin valve (SV) heads at deep submicron track widths. It is found that sufficient pinning on the in-stack bias layer is the key for stability. The narrower track-width heads have better stability. For in-stack biased bottom SV heads, the conventional structure has serious stability problem due to weakened or reversed bias field at free layer track edge. Controlling CPP edge profile is crucial for stability. We also studied the effect of current field on head performance. Depending on the current direction, current field can either induce instability or assist head stability.

Optimization of Nano-Oxide Layer in CIP Spin Valves

The influence of nano-oxide layer (NOL) on interlayer coupling (Hin) and magnetoresistance (MR) of bottom spin-valves (SV) were studied by inserting NOL at four positions: 1) before the Cu spacer; 2) in the middle of the Cu spacer; 3) after the Cu spacer; and 4) after the Cu cap layer. The NOL smoothens the surface of the interface at position 1), 3), and 4), except for 2). Surface flatten by NOL is associated with CoFeOx. Ten percent of MR enhancement can be achieved with NOL at the position 3) and 4). Weak antiferromagnetic interlayer coupling was observed with NOL at position 3). A 27.2% MR ratio has been reached in dual spin valve with two NOLs in two pinned layers.

Interface or bulk scattering in the semiclassical theory for spin valves

By taking into account spin asymmetries of the interface transmissions and the bulk mean free paths, we have treated pure interface, non-pure interface, bulk, and interface plus bulk scattering within the semiclassical Boltzmann theory. First, the optimizations of NOL (nano-oxide-layers) insertions in bottom, synthetic, and dual spin valves and the variations of the giant magnetoresistance (GMR) with the thickness of the free layer have been examined. For non-pure interface, bulk, and interface plus bulk scattering, qualitative trends of GMR versus NOL positions in spin valves are similar to each other. For pure interface scattering, there is no optimized NOL insertion positions and the blocking effect of the NOL inserted in the spacer remains effective as other three kinds of scattering. The GMR ratio for bulk scattering simply approaches zero when the free layer thickness becomes short; in contrast, for interface scattering or interface plus bulk scattering, the GMR ratio is nonzero at zero thickness of the free layer. Second, the relationships between GMR and specular and diffusive scattering have been explored. As far as specular reflection is concerned, our results imply that for a realistic bottom spin filter spin valve, Ta/NiFe/IrMn/CoFe/Cu/CoFe/Cu/Ta, roughness of the surfaces of Ta and the interfaces of Ta/NiFe, NiFe/IrMn, pinned layer/spacer, and spacer/free layer may lead to large GMR. We also find that the enhancement of GMR due to surface specular reflection is only a pure interface effect. The dependences of GMR on the specular transmissions roughly follow square relations. The trends of GMR against the spin-down diffusive scattering depend on the values of the spin-up transmission. Finally, impurity scattering was investigated and our semiclassical results are in qualitative agreement with the experiments and the quantum theory.