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Damage mechanisms and related performance degradations induced by ion beam etching (IBE) process in the fabrication of magnetic tunnel junctions (MTJs) were studied systematically. The loss in tunneling magnetoresistance (TMR) and coercive field (Hc) was investigated with different MTJ pillar sizes and IBE incident angles. It is found that IBE-induced damage is the formation of a surficial amorphous shell in the outer rim of an MTJ pillar. This amorphous shell is of low conductivity and TMR, mainly arising from the lattice damage of MgO barrier and partial oxidation of free/reference layers. Based on experimental and theoretical findings, we optimized the IBE process to reduce the damage as well as recover from degradation. As a result, the TMR loss ratio in comparison with a blanket film is reduced from about 18% to 7%, and Hc is increased from 1490 to 2280 Oe for the same stack.

We study the electric field (EF)-assisted magnetization switching in perpendicular magnetic free layer guided by an Oersted field using micromagnetic simulations. The EF is used to reduce the perpendicular magnetic anisotropy (PMA), and thus change the easy axis from the perpendicular to the in-plane. The Oersted field is used to guide the magnetization to the desired switching directions. The effects of various physical parameters, such as the damping constant, the PMA change, the PMA after EF, the saturation magnetization, and the Oersted field, on the switching times are examined. The simulation results indicate that the switching time decreases with the increase in the damping constant and the Oersted field. The EF efficiency has a significant effect on the switching performance. In order to make a fast and energy-saving switching, the PMA of the free layer needs to be properly controlled. Simulation results also show that a material with higher $M_{s}$ needs a lower EF efficiency for fast switching. For the merit of a fast switching, large damping constant ( $\alpha > 0.1$ ) should be used. In addition, large damping constant requires a small Oersted field for a reliable switching, which reduces the energy during writing. For $\alpha = 0.3$ , a fast total switching time within 3 ns can be achieved with an Oersted field as small as 1 mT.

We study the electric field (EF) effect on MgO/CoFeB/Ta/CoFeB/MgO free layers by varying the thickness of the top MgO layer. The two CoFeB/MgO interfaces oppose the change in magnetic anisotropy from each other and this can be understood by considering the voltage drop as well as the efficiency of the anisotropy modulation from both interfaces. These results are proven by monitoring both coercivity and anisotropy field as a function of the applied EF. From the fit to the model, we show that the bottom CoFeB/MgO interface has a higher EF efficiency than the top interface.

Electric field (EF)-assisted magnetization reversal is investigated in both top-pinned and bottom-pinned CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs). EF modulation in coercivity ( $H_{c}$ ) shows an increasing dependence with the thickness of the free layer ( $t_{\rm FL}$ ) at small $t_{\rm FL}$ values. This abnormal variation was attributed to possible Ta diffusion through the free layer to the interface with MgO barrier. It is found that the bidirectional switching is not achievable using unipolar EF only in our MTJ devices, which is due to small spin-transfer torque (STT) effect and switching uncertainty due to unipolar feature of EF-induced $H_{c}$ modulation. A bipolar external magnetic field should be applied to realize EF-controlled magnetic random access memory (MRAM). For quasistatic magnetization reversal, the required field can be as low as 10 Oe when $H_{c}$ is effectively modulated to nearly zero. Switching field as a function of switching time shows that the magnetization switching is dominated by thermal activation. Simulation results show that switching time is mainly determined by the damping constant and the EF efficiency. Different from STT-MRAM, a large damping constant is desired to achieve fast switching. Using a sweeping field method, we show that MTJs with a thermal stability factor as high as 58 can be reliably switched using an EF-modulated anisotropy scheme with a bipolar magnetic field as low as 10 Oe.

A methodology to quickly prepare CoFe nanofilms with large magnetic permeability and resonance frequency from simple salt solutions is demonstrated. As the microwave properties of thin films are largely determined by their surface morphology and composition, CoFe film with unique morphology and composition is proposed based on theoretical analysis and subsequently electrodeposited with suitable parameters. This approach reveals that Fe x Co1 - x (0.3 < x < 0.5 in atomic ratio) films consisting of sub-30 nm spherical nanoparticles, even in the form of bigger aggregated-nanoparticles, usually show a low coercivity (≤4240 A · m−1), moderate magnetic anisotropy (2900–8580 A · m−1), and high magnetic moment (≥1.4 T), permeability (>200) as well as resonance frequency (>1 GHz). Further experimental analyses show root causes of the phenomena. This methodology also provides useful references to rapidly identify microwave properties of thin films from their surface morphologies and main electrodeposition parameters.

Magnetic single-domain islands based on in-plane anisotropy (usually, shape anisotropy) and their dipole—dipole interactions have been investigated extensively in recent years. This has been driven by potential applications in magnetic recording, spintronics, magneto-biology, etc. Here, we propose a concept of outof-plane magnetic clusters with configurable domain structures (multi-flux states) via dipole—dipole interactions. Their flux stages can be switched through an external magnetic field. The concept has been successfully demonstrated by patterned [Co/Pd] islands. A [Co/Pd] multilayer exhibits a large perpendicular anisotropy, a strong physical separation, and uniform intrinsic properties after being patterned into individual islands by electron beam lithography. A threeisland cluster with six stable flux states has been realized by optimizing island size, thickness, gap, anisotropy, saturation magnetization, etc. Using [Co/Pd] multilayers, we have optimized the island structure by tuning magnetic properties (saturation magnetization and perpendicular anisotropy) using Landau—Liftshitz—Gilbert (LLG) simulation/calculation. Potential applications have been proposed, including a flexi-programmable logic device with AND, OR, NAND, and NOR functionalities and a magnetic domino, which can propagate magnetic current as far as 1 µm down from the surface via vertical dipole—dipole interactions.

Ferromagnetic nanofilms with high saturation magnetization (Ms), particularly FeCo alloys with the highest Ms value (theoretically 2.45T), are promising candidates for emerging compact high-performance magnetoelectronic, microwave, and spintronic devices. Thin films with diverse and specific dynamic microwave properties are essential components for these different applications. For example, a high-frequency mobile phone requires a ferromagnetic element of high resonant frequency (fr) ; [4] a thin single-band microwave absorber requires ferromagnetic films with large resonance-type permeability (m), whereas a broadband absorber requires films with wide frequency dispersion or large relaxation-type m. The demand for a wide range of functions poses challenges for both scientific understanding and fabrication of these thin films, in particular, for applications in gigahertz (GHz) range. Hence despite being one of the earliest themes in magnetism, the topic of ferromagnetic thin films for high-frequency applications still generates much interest from scientists and engineers. Although the influence of the composition of materials on static or dynamic magnetic properties is well studied and described mathematically, for example, by Snoek’s law and constant (S), 3, 15] most formulae are insensitive to the crystal structure or nanostructure of thin ferromagnetic films. The understanding of the influence of an altered structure on the static and dynamic magnetic properties is still limited. A notable example is the preparation of appropriate nanostructure or crystal texture of soft ferromagnetic thin films with high Ms, such as FeCo, for achieving low coercivity (Hc 50 Oe), as well as ultrahigh fr ( 5 GHz) or wide frequency dispersion ( 3 GHz) by simple diverse atom arrangements without the addition of other materials. 17] Due to the inherent limitations of pure homogeneous ferromagnetic single-layer films (e.g. undesirable magnetic softness, anisotropy, or crystallinity of FeCo films), and a lack of evidence on the relationship between the nanostructure or crystal texture and dynamic microwave properties, doping or insertion of nonmagnetic materials is often used to address the stringent requirements of functional devices. This has be achieved by adding AlOx to a FeCo(HfO) film to attain a high fr ( 3 GHz), utilizing FeCo–MnIr exchange-coupled multilayers for wideband microwave noise filtering, or by doping Zr or B into a ferromagnetic film to adjust the m type. However, there are two critical challenges associated with these methods. The foreign elements decrease the Ms of the ferromagnetic film and subsequently deteriorate the device efficiency or performance. Moreover, most of the modifications are tailored The application of a pure FeCo film directly to devices is limited by the intrinsic properties of a homogeneous monolayer of the alloy, despite it having the highest saturation magnetization. A feasible methodology based on alternating current-density electrodeposition to tune the microwave properties of FeCo films through diverse atom-stacking arrangements is reported. The properties range from a large relative resonance permeability (up to 728 in the real part) to ultrahigh resonant frequency (up to 5.2 GHz), and extremely wide frequency dispersion (1.5–6.0 GHz); these are not observed in other materials or as a result of conventional doping methods. They are attained using different single-layer, semi-multilayer, and multilayer FeCo nanofilms, fabricated by diverse stacking arrangements of Fe and Co atoms during deposition in a single bath. This technique firstly exemplifies the significant effect of atom arrangement on magnetic inhomogeneity, nanoscale morphology, composition, crystal texture, stress, and damping, which in turn largely alters both the static and dynamic magnetic properties of the ferromagnetic film. These thin FeCo films might be used directly in various devices operating in the gigahertz frequency range.

Based on requirements for spin-transfer torque (STT)-magnetic random access memory (MRAM), fundamental challenges in current CoFeB- and MgO-based STT-MRAM with perpendicular magnetic anisotropy (PMA) are addressed when scaling down to sub-20 nm. Electric-field (EF)-induced PMA modulation has been proposed to significantly reduce the switching current. As EF alone is unable to induce the magnetization reversal but PMA reduction, additional mechanisms should be applied to realize EF switching. We discuss difficulties in the implementation of currently reported two switching approaches and propose a new approach using Oersted field guided EF switching in MRAM arrays. Our latest experimental results on the EF effect show that the spin reorientation transition can be realized at an EF of 0.8 V/nm for the top-pin CoFeB/MgO magnetic tunnel junctions (MTJs) with the thermal stability factor of as high as 57 in the absence of EF. However, the bidirectional switching of the free layer cannot be achieved at a fixed biasing field, which is attributed to small STT effect and switching uncertainty due to unipolar feature of EF-induced PMA modulation. Perspective of EF-MRAM is analyzed based on currently published data on EF efficiency. We show that the EF switching is very promising for future MRAM applications in spite of challenges in materials and MTJ stack engineering.

Ultrathin (Co 90 Fe 10 /Pt) 8 superlattices composed of alternating CoFe (thickness 0.21 nm) and Pt (thickness 0.23 nm) monolayers were deposited by magnetron sputtering. Superlattices deposited at ultralow sputtering power and high Ar pressure showed perpendicular magnetic anisotropy (PMA) whereas superlattices deposited at higher power and lower pressure and a cosputtered Pt-CoFe alloy film exhibited in-plane magnetic anisotropies. X-ray diffraction and atomic force microscopy showed that the PMA was mainly contributed by the interface anisotropy at Pt/CoFe interfaces. The PMA constant of the superlattice (Co 90 Fe 10 /Pt) 8 deposited under optimized conditions is as high as 6 × 10 6 erg/cm 3 .

We present a detailed study of spin-transfer torque induced noise in self-biased differential dual spin valves (DDSV) which could be potentially used as magnetic read-heads for hard-disk drives. Micromagnetics studies of DDSV were performed in all the major magnetic configurations experienced by read-heads and we show that in every case, self-biased DDSV provide a much stronger stability against spin-transfer torque noise than conventional spin valves. Provided are also insights on the influence of the dipolar interlayer coupling, shape anisotropy, exchange bias and relative orientation between the 2 free layers. Our results demonstrate the viability of DDSV read-heads for future hard disk drives generations.

We investigate the switching of electric-field (EF)-controlled magnetic tunneling junction free layer (FL) magnetization assisted by Oersted fields using micromagnetic simulations. The effects of several physical parameters, such as damping constant, magnetic anisotropy, as well as the EF efficiency and the applied Oersted fields on the switching of the FL magnetization are examined. Successful and reliable magnetization switching depends on all these parameters. The switching time is mainly determined by the time required for the magnetization of the FL to turn in-plane due to the demagnetizing field created after applying the EF.

Several materials (Ta, Cu, Al, Ru) are investigated for the gap layer (GL) in a differential dual spin valve (DDSV) as a function of GL thickness in terms of the differential effect and interlayer coupling. High field measurements in current in the plane geometry show that similar GMR effect can be obtained for two spin valves (SVs) with Ta, Ru, and Cu GLs, while for Al GL, GMR ratio of the upper SV decreases as Al thickness increases. For Ta GL, the two free layers (FLs) may switch either simultaneously or separately, depending on which FL switches first due to the weak ferromagnetic (FM) Neel coupling between the FLs. For Ru GL, the switching behavior depends on the thickness of Ru, due to the oscillation of interlayer exchange coupling. Cu GL causes additional GMR resulted from the FL/GL/FL structure. Detailed investigation on thickness dependence of Ru GL shows that FM interlayer coupling is achieved with Ru thickness of 1.6, 2.4, and 4 nm. The good differential effect can be obtained till Ru = 8 nm, above which MR of the upper SV decreases. Patterned DDSV sensors with Ru GL = 2.4 nm shows an overall FM coupling between the FLs, implying FM exchange coupling is larger than the antiferromagnetic magnetostatic coupling at edges.

Miniaturization is very important in magnetic recording. Here, we have micromagnetically evaluated the respective effects of the free layer (FL) thickness and width on the downtrack and crosstrack responses for single spin valves (SSVs) and differential dual spin valves (DDSVs). Our micromagnetic results show that the linear resolution scales down with the free layer thickness for DDSVs, as is the case for SSVs found by the reciprocity theory. We have also found that the crosstrack resolution scales down with free layer width in both SSVs and DDSVs (correspondingly, the side reading increases, which needs to be mitigated). The corresponding scalings of the signal amplitudes with FL thickness and width in SSVs and DDSVs are also discussed in terms of the shape anisotropy of the free layers. Finally, we have found that side shields can provide bias fields for the free layers and, meanwhile, the improvement in track density and the reduction in side reading are not compromised; this bias point of view of side shields is an additional appreciation of the usual roles played by side shields.

A finite element method (FEM) model, with refinements for the free layers, is proposed for GMR-type differential readers (GDRs). Our purpose is to explore the underlying reasons for the effects of the gap length and bit length on the GDR responses and for the origin of the undershoots in the GDR pulses. Our major findings are that the maximum field within the free layers of GDRs increases with decreasing gap length due to flux concentration; and that the GDR pulse and the associated undershoot effect obtained from the FEM model proposed are consistent with those obtained from our micromagnetic modeling. It has been revealed, for the first time, that the undershoot effect in differential readers are related to that the downtrack profile of the mean field near a transition is non-monotonic.

For future magnetic recording, one challenge is that the shield to shield spacing (SSS) of readers cannot be scaled down to achieve the linear density requirement. A differential dual spin valve (DDSV) has been proposed to improve the linear resolution as it does not rely on the magnetic shields to diminish the interference from adjacent bits. The side reading becomes more and more challenging as the track width shrinks to below SSS to accommodate high track density, in particular for a DDSV reader in which no magnetic shield or larger SSS is used. A self-biased DDSV structure is proposed to replace the conventional abutted junction stabilization scheme. In the self-biased DDSV readers, two side shields are put at the both sides of the sensor across the track direction in the replacement of the hard bias (HB). The stray fields from the two free layers in the DDSV sensor bias each other to stabilize the domain structure of the free layers. Preliminary analysis shows that the self-bias DDSV is also robust against the spin torque-induced magnetic instability. Simulation results indicate that the self-biased DDSV reader has much better reading sensitivity for the opposing fields generated by magnetic transitions than HB-stabilized sensor. It is found that for DDSV readers, the mag-noise is less significant due to a larger signal field and thicker free layer. Current perpendicular to the plane (CPP)-DDSV sensors have been fabricated and show very good differential effect in the real operating mode. The magnetoresistance performance of individual spin valve in CPP-DDSV sensors has been obtained through measurements with applied field and pinning field parallel to the easy axis of the free layer.

At 10 Tb/in2 areal density and bit aspect ratio (BAR) of 4, the 1 T bit length is only 4 nm. By proportional scaling, it requires the head media spacing (HMS) to be around 2 nm. In such a small spacing, it is extremely hard to squeeze in the medium overcoat and lubricant for having a reliable head disk interface. This small HMS demand is mainly determined by the readback process for the reader to have large signal intensity and high signal resolution. The differential dual spin valve (DDSV) reader incorporates a metallic gap layer between the free layers of two spin valves. The opposite pinned reference layers generate the differential operation and reproduce the pulse signal from the transition of perpendicular magnetizations. With the readback resolution defined by the thicknesses of gap layer and two free layers, the DDSV reader gains more resolution than the conventional reader. Unlike conventional reader also using two shields to define signal resolution, the shields of DDSV reader is only for the elimination of inter-symbol interference. This allows the shield to shield spacing of DDSV reader to be much larger, which has smaller shunting effect and promotes the signal strength. With stronger signal intensity and higher readback resolution, the DDSV reader is able to reproduce good quality signal at much higher HMS than the conventional reader.

The effect of itinerant spin moment (m) dynamic in spin transfer switching has been ignored in most previous theoretical studies of the magnetization (M) dynamics. Thus in this paper, we proposed a more refined micromagnetic model of spin transfer switching that takes into account in a self-consistent manner of the coupled m and M dynamics. The numerical results obtained from this model further shed insight on the switching profiles of m and M, both of which show particular sensitivity to parameters such as the anisotropy field, the spin torque field, and the initial deviation between m and M.

Microstructure, static magnetic properties and microwave permeability of sputtered FeCo films were examined. Fe 60 Co 40 films (100 nm in thickness) deposited on glass substrates exhibited in-plane isotropy and a large coercivity of 161.1 Oe. When same thickness films were deposited on 2.5 nm Co underlayer, well-defined in-plane anisotropy was formed with an anisotropy field of 65 Oe. The sample had a static initial permeability of about 285, maximum imaginary permeability of 1255 and ferromagnetic resonance frequency of 2.71 GHz. Cross-sectional TEM image revealed that the Co underlayer had induced a columnar grain structure with grain diameter of 10 nm in the FeCo films. In comparison, FeCo films without Co underlayer showed larger grains of 70 nm in diameter with fewer distinct vertical grain boundaries. In addition, the Co underlayer changed the preferred orientation of the FeCo from (1 0 0) to (1 1 0). The improvement in soft magnetic properties and microwave behavior originates from the modification of the film microstructure, which can be well understood by the random anisotropy theory.

We explore the lattice and the electronic band structures matching between the half-metallic Heusler alloys (half-Heusler NiMnSb and full-Heusler Co 2 MnSi) and several hypothetical non-magnetic Heusler alloys by using first principle calculations. The lattice and band structure matching are almost perfectly satisfied between the two materials of similar crystal structures: (i) NiMnSb and XYSb and (ii) Co 2 MnSi and X 2 YSi, where X , Y =Ni or Cu. Owing to the high interface spin scattering asymmetry, these materials are promising to realize a high giant magnetoresistance at room temperature.

A conventional read head with the sensor embedded between two magnetic shields would hit the wall to provide a resolution required for a bit length less than 10 nm. A differential dual spin valve (DDSV) is proposed as an alternative for recording density of 10 Tb/in2. Our simulation results show that a DDSV can work well for a bit length down to 4 nm without magnetic shields. An excellent differential effect for a DDSV was achieved under a uniform field. Two resistance transitions are observed due to the inconsistent switching of free layers of the two spin valves. The inconsistent switching results from the interlayer coupling and its effect on DDSV performances can be eliminated by applying a vertical bias field. Signal to noise ratio (SNR) of a DDSV reader is analyzed mainly based on the mag-noise. We show that the mag-noise may not be a crucial limiting factor of SNR, because of the relative high available field at 10 Tb/in2 and thicker free layer in the DDSV reader. The specifications of the DDSV reader for 10 Tb/in2 are also derived. Our results demonstrate that the DDSV reader is very promising for 10 Tb/in2 applications.

The readback resolution of conventional CPP spin valve reader is mainly determined by the shield to shield spacing and partially affected by the head media spacing change. As the linear density grows continuously, the bit length will be reduced to sub-10 nm or even sub-5 nm in the future. In such, the shield to shield spacing has to be below 20 nm or even 10 nm. It gradually becomes not feasible to squeeze all the functional layers used by current reader, such as: AFM layer, SAF, tunnel barrier, free layer, and capping layer, in such a small spacing. The differential dual spin valve (DDSV) read head is one of the new reader structures that can provide better readback resolution. This work studied the sensitivity function of DDSV reader without and with shields, then, identified the waveform characteristics of readback signal at different head media spacing (HMS) and gap layer (GL) thickness. The readback resolution of DDSV reader is mainly determined by the thickness of the gap layer between the two free layers. The two shields are still necessary for the DDSV reader in order to have the free layers picking up more localized media flux. The shield to shield spacing at 35 nm helps to constrain the free layers to pick up media flux mainly from ±5 nm region.

We studied exchange coupling in the CoFe/Ru/CoFe synthetic antiferromagnetic structure with systematical replacement of the crystalline CoFe with amorphous CoFeB. Antiferromagnetic exchange coupling intensity decreases with an increase in the replacement in the bottom magnetic layer, which indicates that exchange coupling intensity could be tuned by the replacement. The origin of weakening antiferromagnetic exchange coupling is attributed to the amorphous CoFeB replacement inducing incomplete crystallization and disordered orientation in the Ru layer.

Commercial electron-beam resists are modified into semimetallic resists by doping with 1–3 nm metal nanoparticles, which improve the resolution, contrast, strength, dry-etching resistance, and other properties of the resist. With the modified resists, fine resist nanopatterns from electron-beam lithography are readily converted into 5–50 nm, high-quality multilayers for metallic nanosensors or nanopatterns via ion-beam etching. This method solves the problem of the fabrication of fine (

A multilayer structure magnetic force microscopy (MFM) tip coated with antiferromagnet/ferromagnet/Ru/ferromagnet/Ru/ferromagnet/antiferromagnet on one side of the tip has been fabricated and evaluated. The ferromagnet used was CoFe. The central CoFe layer is antiferromagnetically (AFM) coupled with the two-side CoFe layers via ultra-thin Ru layers through optimizing the Ru layer thickness. The strong antiferromagnetic coupling between the CoFe layers leads to a more stable magnetization configuration and confined tip stray field as compared to that of conventional single-layer coated tips. The former improves the signal-to-noise ratio of the detected signals and the latter leads to an improvement of spatial resolution. The performance of this kind of tip has been evaluated by imaging magnetic patterns and comparing them with the images taken by both ferromagnetically coupled multilayer tips and commercial tips. Micromagnetic modeling has been performed to gain an understanding of the experimental results.

A double exchange biased magnetic force microscopy tip, which is coated with an antiferromagnet (IrMn)/ferromagnet (CoFe)/antiferromagnet (IrMn) multilayer on one side of a bare tip, has been fabricated and studied. In this design, the CoFe layer can be very thin due to its high moment, which leads to high spatial resolution without sacrificing the signal intensity. The exchange coupling between IrMn and CoFe stabilizes the magnetization of the latter which enables this kind of tip to have a better sensitivity and thermal stability as compared to conventional single ferromagnet layer tips. The performance of the double exchange biased tips has been evaluated through imaging the magnetic patterns recorded on longitudinal magnetic recording media. Micromagnetic modeling has been conducted to understand the underlying mechanism of the tip performance improvement.

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

Spin-transfer torque magnetic random access memory (STT-MRAM) has become a promising candidate for future nonvolatile and universal memory because it features non-volatility, fast access time, almost unlimited programming endurance and zero standby power [1-2]. The magnetic tunneling junction (MTJ) is the fundamental building element of STT-MRAM. An MTJ consists of two ferromagnetic layers (a free layer, FL and a reference layer, RL) separated by a thin tunneling dielectric film. The magnetization of the FL can be set as parallel or anti-parallel with the RL by a spin polarized current, which leads to low or high resistance state of the MTJ. The applications of STT-MRAM have been successfully demonstrated [3-4]. However, like all the other nanotechnologies, STT-MRAM suffers from process variations and environment fluctuations, such as thermal fluctuations, which significantly affect the performance and stability of MTJ devices. Several studies have been performed to address the impact of the process variations on the reliability of STT-MRAM and the thermal fluctuation effects on the magnetization switching [5-6]. These works either need costly Monte-Carlo simulations with complex macro-magnetic and SPICE models or do not integrate both effects of the thermal fluctuation and process variations. Wang et al [7] developed a compact MTJ switching model for MTJs derived from the macro-magnetic modeling to simulate the statistical electrical and magnetic properties of MTJ due to both thermal fluctuation and process variations; however it is restricted for in-plane MTJs.

The domain structures in NiFe elements were studied by magnetic force microscopy measurement and micromagnetic modeling. The remanent states in the elements were dependent on the direction of the saturation field. The “S” and “U” states were observed at remanence by applying the saturation field at different directions. The “S” and “U” states are metastable: magnetic force microscopy tip field-induced switching from the “S” and “U” states to the flux closure configuration was observed.

The nano-oxide layer (NOL) inside the spin valve can smooth the surface topography, which results in two effects: suppressing the ferromagnetic Neel 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.

We report the self-assembly of microsized nanowalls and nano-/micropatterns of magnetic iron oxides upon the oxidation of electrodeposited Fe-based films when annealed in air at temperatures ranging from 300 to 450 °C. Scanning electron microscope studies indicate that these nanowalls have the thinnest wall of down to ∼8 nm with a length up to ∼2 μm and a height reaching ∼6 μm; the square patterns have an area of 0.06−1.0 μm2. The generated nanowalls can be used to detect O2 or H2 at high temperatures via their resistance change. The simple spontaneous synthesizing procedure reported here can be applied as well to other electro-/electroless-deposited metal films to form zero-, one-, and two-dimensional metal-based nanostructures.

Exchange bias (Hex) and coercivity (Hc) of the NiFe layer in the NiFe/IrMn system can be tuned by inserting either spacer layers such as Cu, Ru, and Ta or ferromagnetic layers such as CoFe into IrMn since the antiferromagnetic spin structure of IrMn is altered in consequence. Hex usually decreases while Hc increases when the thin discontinued spacer layers are inserted into IrMn. The crossover from the unidirectional exchange-biased coupling to the uniaxial exchange-spring coupling has been successfully observed in the NiFe/[IrMn/CoFe(t)]3/IrMn (here, 3 means the number of the repeated periods of IrMn/CoFe double layer) system when the thickness of CoFe layer t exceeds 1 nm. It indicates that much of the physics and reversal mechanisms in the exchanged-biased systems are similar to that observed in exchange-spring systems where the hard layer replaces the antiferromagnetic layer as the biasing layer. Uniaxial spin valves (both of current in the plane and current perpendicular to the plane modes) and uniax...

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.

Electron beam lithography and ion beam etching have been used to pattern a wire-like array in FeMn/NiFe bilayers. The variation of hysteresis loops with the etching depth in FeMn layer has been presented, and it has been found that with increasing etching depth the coercivity increases and M – H loops show an asymmetric kink. Detailed studies of the magnetic behaviors of the asymmetric kink in the patterned sample with 3.5 nm thick FeMn layer have been performed, and a magnetization component perpendicular to the wire direction has been observed.

High tunneling magnetoresistance ratios (TMR) up to 44.6% were observed at room temperature on magnetic tunnel junctions (MTJs) with Ta20/Ni 81 Fe 19 20/Co 90 Fe 10 2/Al(1 nm)-oxide/Co 90 Fe 10 2/Ni 81 Fe 19 7/Fe 50 Mn 50 20/Ta5 (the thickness of other layers is in nm) structures fabricated by using an inductively coupled plasma sputtering system operating at a base pressure of 5×10 −10 Torr. High field sensitivity up to 6.1%/Oe has been achieved in these MTJs with dimensions of 120 μm×120 μm by using extremely soft ferromagnetic (FM) Ni 81 Fe 19 /Co 90 Fe 10 electrode. Small ferromagnetic coupling field (11 Oe) between the top and bottom FM electrodes, high breakdown electrical field (1.2×10 7 V/cm), and high V h (340–414 mV) at which the TMR decreases to 50% of its initial value, were obtained as the surface roughness of the bottom FM electrode ( R ms A ) and Al-oxide ( R ms A ) layer have been improved.

Characteristics of silicon nitride (SiNx:H) films, grown by plasma enhanced chemical vapor deposition (PECVD) on various metals such as Ta, IrMn, NiFe, Cu, and CoFe at various temperatures down to 100 °C, were studied using measurements of BHF etch rate, surface roughness and Auger electron spectroscopy (AES). The results were compared with those obtained for SiNx:H films on Si. The deposition rate of SiNx:H films increased slightly as deposition temperature decreased, and showed a weak dependence on the underlying materials. The surface of the nitride films deposited on all underlying materials at lower temperatures (below 150 °C) became rougher. In particular, a bubble-like surface was observed on the nitride film deposited on NiFe at 100 °C. At higher deposition temperatures (above 200 °C), SiNx:H films on all the above metals had small RMS values, except for films on Cu which cracked at 250 °C. BHF (10:1) etch rate increased dramatically for nitride films deposited below 150 °C. For different underlying films, the BHF etch rate was quite different, but exhibited the same trend with decrease in deposition temperature. AES measurements showed that Si and N concentrations in the SiNx:H films were only slightly different for the various deposition temperatures and underlying materials. AES depth profile of nitride films indicated that both surface O content and the depth of oxygen penetrating into SiNx:H increased for low temperature-deposited films. However, there was no observed oxygen signal from within the films, even for films deposited at 100 °C, and both Si and N concentrations were uniform throughout the film.

An atomistic model based on a classical spin Hamiltonian and a Landau-Lifshitz-Gilbert (LLG) equation was utilized to simulate and gain understanding of the magnetic, interfacial, and reversal properties of perpendicular anisotropy L10-FePt/Ag/L10-FePt pseudo spin valves, with different interfacial roughness, representing the experimentally observed behavior of the interface where the Ag spacer layer was postannealed at different temperatures. Simulation results showed that the influence of the Ag spacer on the independent switching of the FePt layers became stronger with a greater degree of interlayer mixing under higher temperature treatment. This was the result of an increased magnetic polarization of Ag with a decrease in Ag spacer thickness. Furthermore, with greater intermixing the magnetization reversal of the harder fixed FePt layer also changed from a coherent reversal process to one which took place via nucleation and propagation of reversed domains.

Perpendicular magnetic anisotropy L1 0 -FePt/MgO/[Co/Pd] 30 magnetic tunnel junctions (MTJs) with ultra-thin L1 0 -FePt layer of thickness 1 to 4 nm were fabricated and studied. The ultra-thin L1 0 -FePt behaved as the free layer while the thicker and harder [Co/Pd] 30 behaved as the fixed layer. The interlayer coupling within the MTJ with L1 0 -FePt layer thickness of 4 nm was attributed mainly to magnetostatic coupling and direct coupling due to pinholes. The magnitude of the interlayer coupling field in the L1 0 -FePt/MgO/[Co/Pd] 30 MTJ was insignificant compared to the L1 0 -FePt based pseudo spin valves reported earlier. There was also no sign of dipolar stray field coupling, owing to the single domain characteristic property of the fixed [Co/Pd] 30 layer.