Current-perpendicular-to-plane Spin Valves Research Articles

Interface Characterization of Current-Perpendicular-to-Plane Spin Valves Based on Spin Gapless Semiconductor Mn2CoAl

Employing the first-principles calculations within density functional theory (DFT) combined with the nonequilibrium Green’s function, we investigated the interfacial electronic, magnetic, and spin transport properties of Mn2CoAl/Ag/Mn2CoAl current-perpendicular-to-plane spin valves (CPP-SV). Due to the interface rehybridization, the magnetic moment of the interface atom gets enhanced. Further analysis on electronic structures reveals that owing to the interface states, the interface spin polarization is decreased. The largest interface spin polarization (ISP) of 78% belongs to the MnCoT-terminated interface, and the ISP of the MnMnT1-terminated interface is also as high as 45%. The transmission curves of Mn2CoAl/Ag/Mn2CoAl reveal that the transmission coefficient at the Fermi level in the majority spin channel is much higher than that in the minority spin channel. Furthermore, the calculated magnetoresistance (MR) ratio of the MnCoT-terminated interface reaches up to 2886%, while that of the MnMnT1-terminated interface is only 330%. Therefore, Mn2CoAl/Ag/Mn2CoAl CPP-SV with an MnCo-terminated interface structure has a better application in a spintronics device.

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(Ga0.5Sn0.5) (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.

The effect of film and interface structure on the transport properties of Heusler based current-perpendicular-to-plane spin valves

We present direct link between the transport properties of Co2MnSi and Co2FeMnSi Heusler based current-perpendicular-to-plane spin valves (CPP-SVs) and interface atomic structures resolved by aberration-corrected electron microscopy. The structure of the Co2FeMnSi electrodes is L21 but their interface with the CoSi spacer is disordered. In contrast to the Co2FeMnSi-electrodes, the Co2MnSi-electrodes have abrupt interfaces with the Ag spacer though their ordering is not fully L21. The magnetoresistance of the Co2MnSi-SV is over two orders of magnitude better than those of Co2FeMnSi-SV, demonstrating that the atomic interface ordering is crucial for the enhancement of the magnetoresistance in the Heusler CPP-SVs.

Structural, Magnetic and Magnetoresistive Properties of PTCTE Based Organic Spin Valves

Organic semiconductors are very promising materials in the field of spintronic devices like spin valves (SV) because of large spin relaxation time in ¿-conjugated molecules and the advantage of rather simple and low energy consuming device fabrication techniques. Most of the studies of spin injection in organic semiconductors focused on hole transport materials and one electron transport material: the tris(8-hydroxyquinoline) aluminum (Alq <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ). In this article, we present our first results on SV based on a perylene derivative, PTCTE (PTCTE: tetraethyl perylene 3, 4, 9, 10-tetracarboxylate). Perylene molecules were traditionally used as electron transport materials in optoelectronic devices like OLED (organic electroluminescent diodes) and solar cells. NiFe/PTCTE/Co devices were prepared by dc sputtering for the electrodes (with direct or off axis sample configuration for the upper electrode), and vacuum sublimation for the organic layer. CPP (current perpendicular to plane) devices geometry was obtained with millimetric contact masks determining a CPP spin valve area of 1 mm × 1 mm. Up to now, our best samples have a spin valve magnetic behavior and exhibit up to 3 % MR response at low temperature for a 300 nm organic spacer layer.

Magnetoresistance calculation in current-perpendicular-to-plane giant magnetoresistance spin valves with current-confined paths

We compared the magnetoresistance (MR) ratios of current-confined-path (CCP) current-perpendicular-to-plane (CPP) giant MR spin valves (SVs) calculated using the two-current-model. The simplified two-current model, which places the parasitic resistance external to the two-current network, has been inaccurate in determining the overall resistance of a SV in parallel magnetization configuration. The characteristics of the error in MR depend on the parasitic resistance value, the spin asymmetry coefficient, and the spacer layer resistivity. In comparison to the complete two-current model, we found that although the simplified model could generally predict the trend of various MR behaviors, it also overestimates MR for different CCP densities. A more comprehensive calculation on the CCP SVs was then performed using the spin-drift-diffusion equations based on the Valet–Fert model. It was found that the MR behavior upon considering spin relaxation in the ferromagnetic layers has a closer match with results predicted by the simplified model due to the strong reduction in spin accumulation in the parasitic layers. Spin relaxation in the spacer layer, however, shows a competing effect that tends to reduce the MR ratios. Our results in this paper are useful for accurate evaluation of MR performance in a CPP SV sensor with current-confined paths.

Magneto-Optical and Spin-Transfer Switching Properties of Current-Perpendicular-to Plane Spin Valves With Perpendicular Magnetic Anisotropy

We evaluated the magneto-optical properties of spin-valve (SV) stacks with perpendicular magnets comprised of a Gd-Fe (4-30 nm) free layer and a Tb-Fe-Co/CoFe pinned layer, and the spin-transfer switching properties of current-perpendicular-to-plane (CPP) SVs with the perpendicular magnets. The Gd-Fe free layer had polar Kerr rotation thetask of 0.12deg at a thickness of 10 nm. The thetask reduced with a decrease in GdFe thickness and the sign of thetask switched from positive to negative at a Gd-Fe thickness of 6 nm or thinner. Coercivity (Hc) was maximum and saturation magnetization (Ms) was minimum at a thickness of 8 nm. These phenomena may be explained by an inhomogeneous Gd-Fe composition. The CPP spin valves with the Gd-Fe (10 nm) free layer had a magneto-resistance (MR) of 0.038%. The free layer of a CPP SV device was switched by a pulsed current, which exhibited intrinsic switching current densities of Jc0_P- AP= -3.3times107 A/cm2 and Jc0_ AP-P= 4.3times107 A/cm2 with a thermal stability of 75, which is above the required value of 40. Perpendicular magnets are very useful for obtaining large magneto-optical signals from nano-magnets driven by spin-transfer switching.

Magnetoresistance of FeCo Nanocontacts With Current-Perpendicular-to-Plane Spin-Valve Structure

We have achieved a magnetoresistance (MR) ratio of 7%-10% at a resistance area product (RA) of 0.5-1.5 Omegamum2 by ferromagnetic FeCo nanocontacts in Al nano-oxide-layer (NOL) with current-perpendicular-to-plane spin-valve (CPP-SV) structure. Conductive atomic-force-microscopy shows clear current-path regions of a few nanometers in size surrounded by the Al-NOL. The MR dependence on resistance area product (RA) is well explained by the current-confined-path model assuming that the spin-dependent scattering has an FeCo nanocontact origin, different from tunnel magnetoresistance (TMR). Resistance increases with increasing bias voltage, indicating joule heating by high-current density in nanocontacts, in contrast to TMR. The MR origin is mainly interpreted as spin-dependent scattering due to domain wall formed at ferromagnetic nanocontact

MR Enhancement in a Current Perpendicular-to-Plane Spin Valve by Insertion of a Ferromagnetic Layer Within the Spacer Layer

In this paper, we propose an alternative method to improve magnetoresistance by inserting a thin ferromagnetic (FM) layer into the nonmagnetic (NM) spacer of a basic spin-valve (SV) trilayer (FM1-NM-FM1), thus creating a penta-layer SV structure (FM1-NM-FM2-NM-FM1). We investigated the effect of increasing the resistivity (rho <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F2</sub> ) of the FM2 on overall magnetoresistance (MR). We performed both analytical and numerical studies on the MR of the current perpendicular-to-plane (CPP) structure using the phenomenological spin drift-diffusion models. For finite rho <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F2 </sub> , the MR profile is dependent on the intrinsic conductance polarization (alpha <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F2</sub> ) of FM2. We found that inserting FM2 enhances MR when alpha <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F2</sub> exceeds a critical value of alpha <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2C</sub> . It is found that MR can be doubled by inserting a FM layer with high alpha <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F2</sub> , such as the half-metallic Cr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O. We have numerically calculated MR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> and the corresponding rho <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F20</sub> values for different alpha <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F2 </sub> values. Finally, we studied the effect of spin relaxation on the MR of the CPP SV

Layer thickness effect on the magnetoresistance of a current-perpendicular-to-plane spin valve

We performed a theoretical study and analysis of the effect of modifying the layer thicknesses of a current-perpendicular-to-plane (CPP) spin valve multilayer on its magnetoresistance (MR) ratio. An increase in the ferromagnetic (FM) layer thickness results in (i) an increase in the spin-dependent component of its total resistance, thereby resulting in higher MR, but also leads to (ii) greater spin relaxation in that layer and (iii) an anomalous MR effect in the high resistance regime, both of which suppress the MR ratio. The interplay of these effects results in a complex MR dependence on FM thickness, instead of the simple monotonic MR increase predicted by the two-current model. It also explains the existence of an optimum FM thickness for maximum MR ratio, as evidenced by experimental data. Finally, we consider the MR dependence on the strength and spin selectivity of interfacial resistances, which can either arise naturally or be engineered in the spin valve structure. The study of the combined effects of the FM layer thickness and resistivity, the MR suppression in the high resistive limit, and the competitive spin-dependent scattering in the bulk and at the interfaces is essential for optimizing the structure and material of a practical CPP spin valve to achieve the maximum MR ratio.

Current-perpendicular spin valves with high-resistivity ferromagnetic metals for ultrahigh-density magnetic recording

We have observed a large specific magnetoresistance change (Δ RA, product of resistance change and element area) in metallic spin valves in the current-perpendicular-to-plane (CPP) mode by employing ferromagnetic Co–Fe–Al alloys as free and reference layers. The Δ RA is about five times larger than that for conventional materials. The results were analyzed with the semiclassical two-current theory, and the enhancement in Δ RA is ascribed to compatibility of relatively high resistivity and a large spin-asymmetry coefficient for the spin-dependent bulk scattering of the alloys. Since half-metallicity is not always necessary to realize such properties, this opens up new ways of developing CPP spin valve materials. The alloys show, respectively, low coercivity and high effective pinning field as the free and reference layers, which are eminently suitable for their practical application to a magnetic read sensor. Their applicability to the read heads for over 300 Gbit/in 2 recording density is also discussed.

Modeling and measurement of spin torques in current-perpendicular-to-plane giant magnetoresistive (invited)

This paper discusses some aspects of practical modeling of spin-torque effects in current-perpendicular-to-plane (CPP) giant magnetoresistive magnetic multilayers, with emphasis on considerations of primary importance for sensor and/or read head applications of CPP spin valves (SVs). These include an extension of the well-known Valet-Fert transport model [Phys. Rev. B 48, 7099 (1993)] to the general case of noncollinear magnetization vectors in CPP multilayer of arbitrary layer structure as well as the application of the model to realistic CPP-SV structures. Simple analytical models for the angular dependence of spin-torque critical currents in CPP-SV devices are reviewed and compared with numerical simulations which include thermal fluctuations. Finally, the paper also contains some recent high frequency spectral measurements of spin-torque-induced noise in dual spin-valve sensors.

Novel synthetic ferrimagnet pinned Layers for metallic CPP spin valves requiring high pinning-field and output

We have succeeded in enhancing the current-perpendicular-to-plane (CPP) magnetoresistance of the synthetic spin-valves by controlling the spin-dependent transport through the synthetic ferrimagnetic pinned layer using impurities. The new pinned layer overcomes the disadvantage of the synthetic pinned layer in CPP spin-valves for the magnetoresistance reduction. The experimental results are well described by our calculations based on the two-current theory. The proposed pinned layer, together with our newly developed magnetoresistive ferromagnetic materials, will be quite effective in producing CPP spin-valve sensors for ultrahigh-density data storage systems.

Sub-100-nm Current-Perpendicular-to-Plane Sensor Fabrication

Two sets of processes for sub-100-nm current-perpendicular-to-plane (CPP) spin-valves (SVs) were developed. In the first method, a CPP SV was fabricated by using via-hole instead of small cell patterning. This method makes it easy to characterize the CPP spin valve films. The shunting effect has been analyzed in this method. In the second method, a resist layer for planarization, ion-milling, and wet etching for self-opening via-hole in a self-aligned fashion by making use of the height difference between the cell and surrounding regions was introduced. This method is suitable for the actual recording heads with small dimension even without an ion-beam-deposition system. CPP sensors with size from 0.05 /spl mu/m/spl middot/0.05 /spl mu/m to 0.4 /spl mu/m/spl middot/ 0.5 /spl mu/m with different free- layer structures have been investigated by means of the first method.

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.