Large MR Ratio Research Articles

Large Magnetoresistance and 15 Boolean Logic Functions Based on a ZnCoO Film and Diode Combined Device

AbstractMagnetoresistance (MR) effects not only have great applications in magnetic sensors, hard disk drive (HDD) read heads, and magnetic random access memory (MRAM), but also are expected to develop next‐generation computing systems. Recently, the MR ratio is found to be significantly increased in diode‐enhanced MR (DEMR) devices by coupling the Hall effect or anomalous Hall effect of magnetic materials and the nonlinear transport property of diodes. However, these devices cannot satisfy the requirement of high sensitivity at low magnetic field and small working current simultaneously. Here, large MR ratio (−6850% at 6 T) and high magnetic‐field sensitivity (−875% at 0.04 T) with small working current (<0.5 mA) are achieved at room temperature by combining ZnCoO film and diode. A theoretical model is established quantitatively, which can well explain the experimental observations and be used for evaluating the device performance. Based on this combined device, a logic unit with magnetoelectric symmetry is constructed by introducing a magnet as control bit. Through manipulating the control bit and tuning the working current, 15 Boolean logic functions can be carried out. This work will be beneficial for extending the functionality of spintronic logic‐in‐memory devices and provide a promising solution for realizing emerging computing systems.

Improvement of thermal stability of nano-granular TMR films by using a Mg-Al-O insulator matrix

A new metal-insulator nano-granular tunneling magnetoresistance (TMR) film made of (Fe-Co)-(Mg-Al-O) has been investigated. It is confirmed that the film has granular structure in which crystal Fe-Co granules are surrounded by an amorphous Mg-Al-O matrix. A large MR ratio of 11.8 % at room temperature is observed for a 42 vol.%(Fe0.6Co0.4)-(Mg-Al-O) film annealed at 395 °C. The electrical resistivity increases rapidly by annealing at above the changing point (500 °C). The changing point is about 300 °C higher than that of conventional (Fe-Co)-(Mg-F) nano-granular TMR films. The 42 vol.%(Fe0.6Co0.4)-(Mg-Al-O) film also exhibits less degradation in the MR ratio at high annealing temperatures such as 600 °C. These results suggest the (Fe-Co)-(Mg-Al-O) film is superior to the (Fe-Co)-(Mg-F) film in thermal stability.

Spin scattering asymmetric coefficients and enhanced specific interfacial resistance of fully epitaxial current-perpendicular-to-plane giant magnetoresistance spin valves using alternate monatomic layered [Fe/Co]n and a Ag spacer layer

Using current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) measurement, we have evaluated the bulk and interface spin scattering asymmetric coefficients, βF and γF/N and the specific interfacial resistance, AR*F/N, for exchange-biased spin-valves consisting of artificially ordered B2 structure Fe50Co50 and Ag spacer layer. Artificially epitaxial ordered Fe50Co50 superlattices have been successfully fabricated on MgO (001) substrate by alternate monatomic layer (AML) deposition at a substrate temperature of 75 °C. The structural properties of the full epitaxial trilayer, AML[Fe/Co]n/Ag/AML[Fe/Co]n, on the Ag electrode have been confirmed by in situ reflection high-energy electron diffraction and transmission electron diffraction microscopy. A considerably large resistance-area product change and MR ratio (ΔRA > 3 mΩμm2 and MR ratio ∼5%) were confirmed even at thin AML[Fe/Co]n layer at room temperature (RT) in our spin-valve elements. The estimated values of βF and γF/N were 0.80 and 0.84 ± 0.02, respectively, from the Valet–Fert theory analysis of ΔRA as a function of thickness of the ferromagnetic layer (3, 4, and 5 nm) on the basis of the two-current model.

Magnetoresistance and electronic structure of granular films with MgO or MgF2 matrices

The magneto-transport properties and electronic structures of FeCo-MgO, FeCoB-MgO, FeCo-MgF2 and FeCoB-MgF2 granular films were investigated. From the X-ray photoelectron spectroscopy (XPS) measurements, it was recognized that the oxide layer formed on the surface of granules in FeCo-MgO. Especially Fe oxides were preferentially formed. By using (Fe50Co50)80B20, the formation of Fe oxides on the granule surface was suppressed since B was oxidized instead of Fe. The MR ratio of FeCoB-MgO was approximately 2%. By using MgF2 matrix, the surface states of granules were drastically improved. XPS measurements indicated that an amount of metallic Fe in FeCoB-MgF2 increased compared with FeCoB-MgO. Reflecting the improvement of surface states of granules, a large MR ratio of approximately 6 % was obtained in FeCoB-MgF2.

Theory of tunnel magnetoresistance and spin filter effect in magnetic tunnel junctions

Tunnel magnetoresistance (TMR) and spin filter effect in magnetic tunnel junctions are studied by using numerical simulations based on the linear response theory. In Fe/MgO/Fe junction, TMR is calculated for both clean and disordered systems. Large MR ratio obtained in clean junctions is attributed to the momentum and symmetry filter effect of MgO barrier. It is shown that lattice distortions within MgO and at the interfaces decrease the MR ratio because the disorder weakens the filter effects and causes hybridization between Bloch wave functions having different momenta and symmetries. The spin filter effect in junctions containing a ferromagnetic insulator LaNi0.5Mn0.5O3 is also investigated. It is shown that a large spin filter effect is obtained in LaNiO3/LaNi0.5Mn0.5O3/LaNiO3 junctions although the tunnel barrier height is spin independent.

Study on anomalous magnetoresistance in nano-Fe 3O 4/Ag granular system

The possibility of AgCl remainder was confirmed in the production process of nano-Fe 3O 4/Ag granular system. The AgCl impurities contributed to large negative MR-ratio of about −4 and −6% at 300 K and 1 T for x* = 0.05 and 0.20 samples, respectively, where x* is the nominal Ag volume fraction of V Ag/ ( V F e 3 O 4 + V Ag ) . It was suggested that the formation of insulating AgCl barriers were effective for the large MR ratio in nano-Fe 3O 4 system.

Electrical and magnetic properties of nano-oxide added spin valves

The nano-oxide layer (NOL) inside the spin valve can smooth the surface topography, which results in two effects: suppressing the ferromagnetic Néel magnetostatic coupling and enhancing the RKKY exchange coupling between the free layer and the pinned layer. As a consequence, MR ratio is increased in the spin valve with NOL layer inside the pinned layer or on top of the free layer. Because of the enhancement of the specular reflectivity in the NOL added spin valves, the oscillation of the interlayer coupling field with respect to the thickness of the spacer layer and even the thickness of the cap layer has been observed. In terms of the performance of both electrical and magnetic properties of the spin valves, CoFe–O turns out to be the best materials inside the pinned layer. But, as a cap layer, ZnO is the best choice because of its crystalline growth on top of the free layer CoFe, which causes the enhancement of the MR ratio. About 4.5% of MR ratio has been achieved in a NOL added spin valve in the current-perpendicular-to-plane configuration. Large MR ratio observed in the NOL added CPP sensor is due to the increment of the interface scattering and current confined path in the NOL added pinned layer.

Nanoconstricted structure for current-confined path in current-perpendicular-to-plane spin valves with high magnetoresistance

We have successfully observed a nanoconstricted structure for current-confined-path (CCP) effect in current-perpendicular-to-plane-giant-magnetoresistance (CPP-GMR) spin valves. By inserting an AlCu nano-oxide layer (NOL) formed by ion-assisted oxidation (IAO) between a pinned layer and a free layer, the MR ratio was increased while maintaining a small area resistance product (RA). The cross-sectional high-resolution transmission electron microscopy image of the sample with RA=380mΩμm2, ΔRA=16mΩμm2, and MR ratio=4.3% showed that an amorphous oxide layer is a main part of the NOL that blocks the electron conduction perpendicular to plane. Some parts of the NOL are punched through crystalline, metallic channels having a diameter of a few nanometers, which are thought to work as nanoconstricted electron conduction paths between the pinned layer and the free layer. Nano-energy-dispersive-x-ray-spectrum analysis also showed that Cu is enriched in the metallic channels, whereas Al is enriched in the amorphous oxide region, indicating that the metallic channel is made of Cu and the oxide is made of Al2O3. The nanoconstricted structure with good segregation between the metallic channel and the oxide layer enables us to realize a large MR ratio in CCP-CPP spin valves.

Magnetoresistance in the Hard/Cu/Soft Sandwiches: Dependence on Layer Thickness and Field Annealing

NiFe(2nm)/Cu(tCu)/Co(1.5nm) sandwiches were deposited on Ta buffer layer by magnetron sputtering method. Small angle X-ray diffraction analysis was made to investigate the structural characterizations of the sandwiches. It has been obtained that MR value showed a maximum at tCu=2nm when Cu spacer thickness changed, which is contributed by the effect of shutting and interlayer coupling. The Cu layer was deposited at two different rates. Larger MR ratio was observed with a lower deposition rate. The result was discussed in terms of magnetic reversal fields. In addition, the sandwiches were subjected to field annealing at different annealing temperatures. MR ratio has exhibited a largest increase after annealing at 175oC with a magnetic field of 750Oe applied.

Study on Magnetoresistance for Trilayer with Fe<sub>3</sub>O<sub>4</sub> Film

High quality Fe3O4 thin film was synthesized by the magnetron sputtering method using Ar-H2 gas. Giant magneto resistance of the trilayer using Fe3O4 was investigated, where the observed MR ratio was less than 1%. A phenomenological calculation was made to make the origin of the small MR ratio clear, using a parallel current model assuming five passes. It is concluded that the non-magnetic layer surrounded by two magnetic layers is necessary to have the same order resistivity and similar thickness with Fe3O4 layer in order to obtain the large MR ratio for the trilayer using Fe3O4.

垂直磁気記録用フロントヨーク積層型ＴＭＲヘッド解析

A tunneling magnetoresistive (TMR) element with a magnetic tunneling junction has the advantage of large MR ratio compared with a GMR element, and has high potential for applications such as magnetic random access memory and magnetic read heads for high-density recording. This paper describes the design and performance of yoke-type read heads with the TMR element. Magnetic field calculations were carried out for a front-yoke stacked-type TMR head. The head has the advantage of a high signal-to-noise ratio, and produces Lorentzian waveforms without any distortion in combination with perpendicular magnetic recording media.

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.

Magnetoresistance properties of Co/Cu multilayer films with FeN and Co layers

MR behavior is observed in the multilayer of FeN/Co/Cu/Co structure utilizing the difference of the coercive force of two ferromagnetic layers, FeN/Co and Co layer. The mixed phase of FeN layer, fabricated by using the reactive sputtering method, is α-Fe (BCC) and ε-Fe 3N (HCP) when MR behavior is observed. A large MR ratio (%) of 3.2% at 25 Oe is observed in the FeN(250 Å)/Co(70 Å)/Cu(25 Å)/Co(70 Å)/Cu(25 Å)/Co(70 Å)/FeN(250 Å) multilayer film.

ＣｏｘＦｅ１−ｘ／Ａｌ‐ｏｘｉｄｅ／ＣｏｘＦｅ１−ｘ接合の磁気抵抗効果

The relationship between CoxFe1-x(X= 100, 90, 75, 60, 50) and the tunneling magnetoresistive effect was investigated. CoxFe1-x, which have high saturated magnetization, were selected as the ferromagnetic materials. The fabrication process was successful in a TMR junction with large MR ratio. The maximum MR ratio for Co75Fe25 junctions yielded 40.7%. The MR ratios for Co75Fe25 junctions were almost constant as a function of the junction area, which meant an MR ratio over 40% didn't contain a geometrically enhanced MR effect. Vh, defined as the point where the TMR decreased to half its 1mV bias value, lay between 360 and 455 mV. The bias dependence of TMR was related to barrier height in all CoxFe1-x junctions. As barrier height increased, weaker bias dependence was obtained.

GMR Revolution Sensors for Automobiles

In order to apply Giant Magneto-Resistive (GMR) elements to revolution sensors for automobiles, we studied an annealing effect upon GMR elements. After the GMR elements were annealed with the optimized condition, they showed sufficient temperature stability for automobile applications, as well as large MR ratio. The GMR elements were integrated with a sensing processing IC without any damage during the GMR wafer process. Sensitive sensor properties were obtained, especially the repeatability was three times better than that of a semiconductor Hall revolution sensor, which was due to that the integration of the GMR element with IC prevented noise and the signal output of the GMR element was one order higher than that of the semiconductor Hall revolution sensor.

Top, bottom, and dual spin valve recording heads with PdPtMn antiferromagnets

We fabricated top-type (Substrate/free layer/Cu/pinned layer/antiferromagnetic layer) and bottom-type (Sub./antiferro/pinned/Cu/free) spin valve heads using PdPtMn antiferromagnets. Bottom type spin valve head with a buffer layer of Ta/NiFe showed similar exchange bias field and MR ratio as top-type spin valve head. Dual spin valve (DSV) films with two pinning layers of PdPtMn had an enhanced MR ratio by 40% compared to the top-type spin valve. Patterned DSV elements with 3 /spl mu/m MR height showed relatively large deviation from the optimum bias state. In order to adjust the bias point, asymmetric DSV elements with Cu layers of different thicknesses were investigated. Asymmetric DSV with the structure of Ta/NiFe/PdPtMn/CoFe/Cu(t/sub 1/)/CoFe/NiPe/CoFe/Cu(t/sub 2/)/CoFe/PdPtMn (t/sub 1//spl ne/t/sub 2/) showed an improvement for just bias using sense current magnetic field. The possibility of reducing the PdPtMn critical thickness for exchange coupling was investigated using ultra high vacuum (UHV) deposition environment. UHV technique is significantly useful to obtain exchange bias field (H/sub ua/) of PdPtMn(t)/ferro at t<15 nm. The thickness of pinning PdPtMn was reduced from 25 nm to 13 nm and we fabricated single spin valve with total thickness of 26.5 nm which exhibited large MR ratio over 8.0%, sufficient H/sub ua/ (>500 Oe), large sheet resistance change (/spl Delta//spl rho///sub total/=1.6 /spl Omega/), and good thermal stability up to 300/spl deg/C. The total thickness of DSV was also reduced from 80 to 53 nm. Single and dual spin valves with thin PdPtMn pinning layer are very promising sensors for ultra high density recording applications.

ＣｏＦｅ／Ｃｕ多層膜のＣＰＰ（Ｃｕｒｒｅｎｔ Ｐｅｒｐｅｎｄｉｃｕｌａｒ ｔｏ ｔｈｅ Ｐｌａｎｅ）‐ＧＭＲ特性

Multilayer GMR films have a larger MR ratio than spin-valve films and, therefore, have great potential for use as read sensors in HDD (hard disk drive) heads for ultra-high-density recording. Moreover, the MR ratio of multilayer GMR films is twice as large when the sense current flows perpendicular to the plane (CPP) than when it flows in the plane (CIP). We investigated the GMR properties of CPP elements of a Co0.9Fe0.1/Cu multilayer in a structure where the GMR film is under the top terminal layer and connected to the bottom terminal layer through a micron-sized contact hole. An MR ratio of 13.1% and saturation field (Hs) of 150 Oe were obtained. The field sensitivity of 0.1 %/Oe was much larger than that of CIP elements. The value of Hs of the CPP elements was almost constant as a function of the element dimension, because the demagnetization field was insensitive of the sensor size in our CPP element structure and small enough even for submicron elements.

超高真空成膜によるＰｄＰｔＭｎスピンバルブ膜のＭＲ特性

We investigated the relation between the background pressure (PB.G.) of the sputtering chamber and the unidirectional anisotropy field (Hua) of spin-valve films with PdPtMn antiferromagnetic layers fabricated by an ultrahigh vacuum (UHV) sputtering process. Hua increased with decreasing PB.G. until PB.G. reached 2 × 10-7 Pa. The UHV sputtering process helped to reduce the PdPtMn thickness from 25 nm, which was needed in the conventional high-vacuum (HV) sputtering process, to under 15 nm while preserving the magnitude of Hua. We fabricated a spin-valve structure with total thickness of 26.5 nm exhibiting a large MR ratio over 8.0%, sufficient Hua (>500 Oe), and large sheet resistance change (Δρ/ttotal = 1.6 Ω).

Annealing effect on magnetic characteristics on (La,Sr)MnO/sub 3/ sputtered films

A perovskite type of (La,Sr)MnO/sub 3/ films were deposited on SiO/sub 2//Si substrates with amorphous surface by using the facing targets sputtering apparatus. Although the film as-deposited at P(O/sub 2/) of 0.3 mTorr was not fully crystallized and the MR ratio was as small as 0.03% at the applied field of 5 kOe, it increased up to 2.5% for the film post-annealed at 850/spl deg/C in air for 10 min. The partial O/sub 2/ pressure P(O/sub 2/) strongly affected their crystallinity as well as the Mn content C/sub Mn/. The film as deposited at P(O/sub 2/) of 0.03 mTorr was composed of crystallites with excellent (100) orientation at relatively low substrate temperature T/sub s/ up to 500/spl deg/C and the large MR ratio of 3.8% was attained even at room-temperature measurement. Since this MR response exhibits excellent linearity, these LSMO films seem to be applied for magnetic sensor devices. It is clarified that the annealing process in preparation of LSMO film effectively increases the MR ratio of the film with insufficient crystallinity, and is not effective for the fully crystallized film.

Spin valve films with PdPtMn antiferromagnetic layer fabricated using ultrahigh vacuum sputtering process

We investigated the background pressure (P/sub BG/) dependence of the unidirectional anisotropy field (H/sub ua/) of spin valve films with a PdPtMn antiferromagnetic layer fabricated by an ultrahigh vacuum (UHV) sputtering process. The H/sub ua/ increased with decreasing P/sub BG/ until P/sub BG/ reached 2/spl times/10/sup -7/ Pa. The UHV deposition, such as below 2.0/spl times/10/sup -7/ Pa of P/sub BG/, improved the fct [111] texture of PdPtMn, the fcc[111] of NiFe/CoFeB/Cu/CoFeB layers, enlarged the grains of spin valve films, and reduced an absorption of residual oxygen component into the spin valve films. Consequently, the exchange coupled CoFeB/PdPtMn layers exhibited large H/sub ua/ of 620 Oe, in spite of a quite thin PdPtMn (15 nm) as an ordered antiferromagnetic material, and the spin valve films show a large MR ratio of 7.8% and a sheet resistance change of 1.6 /spl Omega/.