Giant Magnetoresistance Multilayers Research Articles

Mechanisms by which oxygen acts as a surfactant in giant magnetoresistance film growth

The mechanisms by which oxygen acts as a surfactant in giant magnetoresistance multilayers have been elucidated for the first time. Three-dimensional atom probe analysis of Cu/CoFe multilayers reveals the elemental distributions at the atomic level. Interfacial intermixing and oxygen impurity levels have been quantified for the first time. Both with and without oxygen the intermixing is greater at the CoFe-on-Cu interface than at the Cu-on-CoFe one and for both interfaces, oxygen reduced the intermixing. The oxygen largely floats to the growing surface and is incorporated at grain boundaries. The oxygen also reduces conformal roughness and grain boundary grooving, indicating a reduction in long-range surface diffusion.

Noise power spectral density in single-strip NiFeCo-Cu GMR sensors

Noise power spectral density measurement were performed on single-strip patterned NiFeCo-Cu giant magnetoresistance (GMR) multilayers with various width ranging from 2 /spl mu/m to 20 /spl mu/m. The measurements were carried out over two frequency ranges from 0.1 Hz to 100 Hz and 10 Hz to 10 kHz. Noise spectrum was obtained for various de bias fields in the presence or of an external alternating field. Noise power spectral density measurements indicate that the noise is 1/f type. In the absence of an external alternating field the 1/f noise slope showed no field dependence in both frequency ranges. The slope of the 1/f noise is approximately 1 at close to zero bias field in all samples investigated. The 11f noise slope measured in the presence of the alternating field shows field dependent noise behavior of a value of about 1.1 in the region close to zero bias but fall off at large bias field. The 1/f noise is also analyzed using the Hooge parameter /spl alpha/.

Annealing effects and degradation mechanism of NiFe/Cu GMR multilayers

Structural, transport and magnetic properties of sputtered Ni 80Fe 20/Cu multilayers showing giant magnetoresistance (GMR) were studied using X-ray reflectometry and diffraction, transport measurements, ferromagnetic resonance (FMR), and magneto-optical Kerr effect. In particular, mechanisms of the GMR degradation at elevated temperatures were investigated. Multilayers with an individual layer thickness of 2 nm show a sharp drop of the GMR after annealing at about 250°C. Whereas below this temperature grain growth and defect reduction contribute to a partial improvement of the GMR, above ∼250°C interdiffusion between Ni and Cu appears to lead to layer intermixing and to the degradation of transport and magnetic properties. Moreover, the initial 〈1 1 1〉 texture sharpens, and strong tensile stresses arise in the layer stack. We correlated the structural alterations to changes in the magnetic properties such as the strength of the antiferromagnetic coupling (bilinear and biquadratic) and the magnetic anisotropy. Above 250°C an increasing magnetic inhomogeneity of the Permalloy layers can be inferred from the FMR linewidth broadening.

Surfactant-mediated growth of giant magnetoresistance multilayers

A series of experiments have been conducted to evaluate the magnetotransport properties of rf-diode-sputter-deposited giant magnetoresistive multilayers with either copper or copper-silver-gold nonferromagnetic (NFM) conducting layers. The study revealed that rf-diode-deposited multilayers utilizing ${\mathrm{Cu}}_{80}{\mathrm{Ag}}_{15}{\mathrm{Au}}_{5}$ as the NFM conducting layer possess significantly superior giant magnetoresistance to otherwise identical device architectures that used pure copper as the NFM conducting layer. To explore the origin of this effect, copper and ${\mathrm{Cu}}_{80}{\mathrm{Ag}}_{15}{\mathrm{Au}}_{5}$ films of varying thickness have been grown under identical deposition conditions and their surface morphology and roughness have been investigated. Atomic-force microscopy revealed significant roughness and the presence of many pinholes in thin pure-copper films. The surface roughness of the ${\mathrm{Cu}}_{80}{\mathrm{Ag}}_{15}{\mathrm{Au}}_{5}$ layers was found to be much less than that of pure copper, and the alloying eliminated the formation of pinholes. Using an embedded-atom-method alloy potential, molecular-dynamics simulations have been used to investigate the role of silver and gold upon the multilayer growth process. The smoother growth surface of ${\mathrm{Cu}}_{80}{\mathrm{Ag}}_{15}{\mathrm{Au}}_{5}$ was found to predominantly result from the addition of silver, which acts as a surfactant during growth. Molecular statistics estimates of atom migration energy barriers indicated that both silver and gold have significantly higher mobilities than copper atoms on a flat copper surface. However, gold was found to be incorporated in the lattice whereas silver tended to segregate (and concentrate) upon the free surface, enhancing its potency as a surfactant. The atomic-scale mechanism responsible for silver's surface-flattening effect has been explored. We found that silver, when present at a ledge edge, reduces the Ehrlich-Schwoebel barrier for copper, promoting a step-flow growth mode. Gold was also found to reduce the Ehrlich-Schwoebel barrier, but its potency was less than that of silver due to its lower surface concentration. These observations suggest that small alloy additions can be used to manipulate the energy barriers that fundamentally control atomic assembly during vapor deposition, and provide a potentially powerful means of controlling the structure of thin films.

Mechanisms of inert gas impact induced interlayer mixing in metal multilayers grown by sputter deposition

Control of interfacial roughness and chemical mixing is critical in nanomaterials. For example, multilayers composed of ∼20 Å conductive layer sandwiched between two ∼50 Å ferromagnetic layers can exhibit giant magnetoresistance (GMR). This property has caused a tremendous recent increase in hard disk storage capacity, and can potentially result in a new generation of nonvolatile magnetic random access memories. It has been established that good GMR properties can be obtained when the interfacial roughness and interlayer mixing of these multilayers are low. However, flat interfaces in nanoscale multilayers are not thermodynamically stable, and cannot be obtained using thermal energy deposition processes such as molecular-beam epitaxy. Hyperthermal energy sputter deposition techniques using either plasma or ion-beam gun are able to create nonequilibrium flat interfaces, and have been shown to produce better GMR multilayers. In these processes, however, inert gas ions or neutrals with energies between 50 and 200 eV can impact the growth surface. This may be a major source for interlayer mixing. By using a molecular dynamics technique and a reduced order model, the composition profile across the thickness of multiply repeated Ni/Cu/Ni multilayers has been calculated as a function of the energy and the relative flux of the inert gas ions or neutrals as well as the layer thickness. The results indicate that the 50–200 eV inert gas impact caused atomic exchange between adjacent atomic layers near the surface. The probability of exchange increased with impact energy, but decreased with the number of overlayers. The exchange between Ni overlayer and Cu underlayer atoms was much more significant than that between Cu overlayer and Ni underlayer atoms. As a result, the Ni on Cu interfaces were much more diffuse than the Cu on Ni interfaces, in good agreement with experiments. At very high inert gas flux and impact energy, an increased probability for the underlying Cu atoms to be exchanged to the surface resulted in significant Cu surface segregation.

Growth of giant magnetoresistance multilayers: Effects of processing conditions during radio-frequency diode deposition

The magnetotransport properties of giant magnetoresistance multilayers are significantly effected by the atomic-scale structure of the interfaces between the nonferromagnetic conducting and ferromagnetic (FM) metal layers. The interfacial roughness and the extent of intermixing at these interfaces are both known to be important. A combination of experimental and multiscale modeling studies have been used to investigate control of interface structure during multilayer growth by rf diode deposition and the consequences of such control for magnetotransport. Experiments were conducted to evaluate the dependence of the magnetotransport properties of NiFeCo/CoFe/CuAgAu multilayers upon the growth conditions (background pressure, input power), and to link the roughness of vapor-deposited copper layers to the same process parameters. These experimental studies reveal the existence of intermediate background pressure (20 mTorr) and plasma power (175 W) that resulted in the highest magnetoresistance and a strong sensitivity of copper layer surface roughness to both the power and pressure at which deposition was conducted. By using a combination of modeling technologies, the deposition process conditions have been linked to the atomic fluxes incident upon the sample surface. This was then used to determine the atomic-scale roughness of the film. Energetic metal atoms (and inert gas ions) were found to have very strong effects upon interfacial structure. The models revealed an increase in interfacial roughness when metal (or inert gas ion) translational energy was decreased by either reducing the plasma power and/or increasing the background pressure. Because high-energy metal impacts activated atomic jumping near the impact sites, high plasma power, low background pressure process conditions resulted in the smoothest interface films. However, these conditions were also conducive to more energetic Ar+ ion bombardment, which was shown by molecular dynamics modeling to induce mixing of the FM on the copper interface. Intermediate plasma powers/background pressures result in the most perfect interfaces and best magnetotransport. The insights gained by the modeling approach indicate a need to avoid any energetic ion bombardment during the early growth stages of each new layer. This could be accomplished by operating at low power and/or high pressure for the first few monolayers of each layer growth and may provide a growth strategy for further improvement in magnetotransport performance.

Annealing Effects on Py/Cu GMR Multilayer Films with Limited Number of Sublayers

The influence of annealing on giant magnetoresistance (GMR) effect and magnetization reversal processes has been investigated in Py/Cu (Py = Ni 83 Fe 17 ) sputter-deposited multilayers (MLs) with a limited number of magnetic sublayers (N ≤ 6) and equal sublayer thicknesses both for Py and Cu (t Cu = 2 nm, t Py = 2 nm). Based on the magnetization and magnetoresistance measurements it has been shown that the magnetic behavior of such MLs is strongly influenced by annealing driven disruption of magnetic layers (lateral decoupling) and nonmagnetic layers (magnetic bridging between Py layers). A comparison of experimental hysteresis curves with model dependences indicates that annealing causes a change of the magnetization reversal from a local to an absolute energy minimum mode leading to quasi-two-state magnetoresistance characteristics.

Improving magnetoresistance characteristics by toppings

Often magnetoresistance characteristics of robust giant magnetoresistance multilayers are not ideal in all respects (e.g., signal amplitude, hysteresis and field offset). Sometimes this can be corrected by means of external biasing magnets; however, for most applications this is not desirable. We have developed a new concept, which provides a new degree of freedom to improve the characteristics of GMR (or TMR) multilayers by the use of toppings without the need for external components or extra materials. With this method offset compensation, hysteresis elimination and the combination of both have been experimentally demonstrated in exchange-biased spin valves.

The low energy ion assisted control of interfacial structure: ion incident angle effects

It is difficult to maintain smooth unmixed interfaces during the vapor deposition of giant magnetoresistive multilayers, especially as the number of layers is increased. Recent atomistic modeling of normal incident angle ion impacts with model Ni/Cu/Ni multilayer surfaces have indicated that low energy (3 eV) inert gas ion bombardment significantly reduced the roughness of Cu on Ni interfaces without causing interlayer mixing. Higher ion energies (12 eV or above) were necessary to achieve similar smoothing of the Ni on Cu interface, but this was accompanied by extensive mixing between the surface Ni atoms and the underlying Cu atoms by an impact ion induced exchange mechanism. Here, molecular dynamics simulations have been used to explore the effect of varying the incident ion angle during 12 eV Ar + and Xe + ion bombardment of a model Ni on Cu surface. The results indicate that increasing the incident ion angle up to 70° reduced the probability of intermixing while retaining most of the surface flattening effect seen in the previous study. A low combination of interfacial roughness and intermixing was obtained for an incident ion angle of approximately 60°. The heavier Xe + ions were found to transfer more momentum to the surface atoms than the lighter Ar + ions, and resulted in significantly more mixing at all angles of incidence.

Study of magnetic multilayers by RBS with nanometer resolution

The interface roughness of thermally grown giant magnetoresistance (GMR) multilayers was investigated with nanometer depth scale by Rutherford backscattering spectroscopy (RBS) using a high-resolution magnetic spectrograph. Co/Cu multilayers were prepared by electron beam evaporation under ultra-high vacuum conditions in an MBE system. The samples were measured by RBS under several scattering angles to obtain the concentration depth profiles. Ion beam mixing effects deduced from the experimental data and theoretical models were used to unfold the experimental spectra, giving the initial interface roughness. It is shown that the interface roughness, comparable to the amount of surface roughness, is probably not responsible for the extremely weak GMR effect, found in this type of samples.

Thermally activated deterioration processes in Co/Cu GMR multilayers

The magnitude of the giant magnetoresistance (GMR) observed in multilayers is known to change irreversibly at elevated temperatures. To improve the thermal stability of devices, a fundamental understanding of the GMR and its correlation to the temperature-induced structural and morphological changes in a given system is mandatory. We therefore investigated the structural and magnetic properties of sputtered Co/Cu multilayers in the pre- and postannealed states (temperature regime for annealing up to 750 °C) by in situ x-ray diffraction, transport measurements, ferromagnetic resonance (FMR), and magneto-optical Kerr effect (MOKE). We were able to identify a sequence of distinct structural changes each of which sets in above a characteristic critical temperature. These critical temperatures depend strongly on the thickness of the individual layers. The structural alterations observed range from interfacial sharpening through texture reorientations up to the formation of a granular state, and are associated with distinct in/decreases of the GMR signal. Using FMR and MOKE we determined in-plane magnetic anisotropies and interlayer coupling for as-grown and annealed samples.

Finite-size effects in giant magnetoresistance multilayers

It is of interest to determine the characteristic length scale that determines giant magnetoresistance (GMR). In order to understand this behavior, GMR multilayers of Co/Ru and Co/Cu have been studied at a temperature of 4.2 K. The total thickness of Co/Ru multilayers has been varied from 96 to 1654 Å and Co/Cu from 77 to 2712 Å by increasing the number of bilayers (N). It has been observed that GMR increases with the number of bilayers and more than 20 bilayers for Co/Ru and 50 for Co/Cu are needed to reach the saturation value.

Theoretical study of perpendicular giant magnetoresistance in multilayers

Theoretical study of perpendicular giant magnetoresistance in multilayers

Atomic Assembly of Giant Magnetoresistive Multilayers

ABSTRACTThe emergence of metal multilayers that exhibit giant magnetoresistance (GMR) has led to new magnetic field sensors, and approaches for nonvolatile random access memories. Controlling the atomic scale structure across the many interfaces within these multilayers is central to improve the performance of these devices. However, the ability to manipulate atomic arrangements at this scale requires an understanding of the mechanisms that control heterometal film growth during vapor deposition. It is important to develop methods that enable prediction of the effects of deposition conditions upon this structure. Atomistic simulation approaches have been combined with deposition reactor models to achieve this. We have applied these approaches to analyze the atomic scale structure of sputter deposited CoFe/Cu/CoFe giant magnetoresistive multilayers similar to those used for magnetic field sensing. Significant intermixing is revealed at the CoFe-on-Cu interface, but not at the Cu-on-CoFe interface. Recent experiments verified these predictions. The insights provide a basis for the development of processes that inhibit thermally activated atomic diffusion while allowing the controlled use of the metal atom impact energy and inert gas ions to manipulate the structure of interfaces.

Composition-Morphology-Property Relations For Giant Magnetoresistance Multilayers Grown By RF Diode Sputtering

ABSTRACTA series of experiments have been conducted to evaluate the magnetotransport properties of RF diode sputter deposited giant magnetoresistive (GMR) multilayers with either copper or copper-silver-gold nonferromagnetic (NFM) conducting layers. The study revealed that RF diode deposited multilayers utilizing Cu80Ag15Au5 as the NFM conducting layer posses significantly superior giant magnetoresistance to otherwise identical device architectures that used pure copper as the NFM conducting layer. To explore the origin of this effect, copper and Cu80Ag15Au5 films of varying thickness have been grown under identical deposition conditions and their surface morphology and roughness investigated. Atomic force microscopy revealed significant roughness and the presence of many pinholes in thin pure copper films. The surface roughness of the Cu80Ag15Au5 layers was found to be much less than that of pure copper, and the alloying eliminated the formation of pinholes. Molecular statics estimates of activation barriers indicated that both silver and gold have significantly higher mobilities than copper atoms on a flat copper surface. However, gold is found to be incorporated in the lattice whereas silver tends to segregate (and concentrate) upon the free surface, enhancing its potency as a surfactant. The atomic scale mechanism responsible for silver's surface flattening effect has been explored.

Optimization processes of giant magnetoresistance characteristic for mixed magnetic multilayers

Up to now, giant magnetoresistance (GMR) multilayers usually consist of only one type of double layer (e.g., permalloy/copper). In this work, we report on results obtained on mixed systems, i.e., multilayers containing simultaneously the first and second maximum of the antiferromagnetic coupling. Both a phenomenological numerical approach as well as experimental results demonstrate, that such systems can be used for tailoring new shapes of GMR curves.

Fabrication of pseudo-spin-valves and 100 nm sized periodic elements for magnetic memory application

Co–Cu–Co pseudo spin-valves (PSV’s) and Co–Cu multilayers have been fabricated and studied for the giant magnetoresistance (GMR) effect. Attention is focused on optimizing the low field response. They are deposited on Si (001) wafers without removing the natural silicon-dioxide layer, using S-research sputtering guns in high vacuum. A buffer layer of Cr–Cu was used before depositing the PSVs. Realization magnetic switching at low field for the soft layer and high field for the hard-layer makes the structure suited for MRAM application. The low and high switching fields are typically 100 and 2200 Oe, respectively. The magnetic measurement using SQUID magnetometry and VSM confirms the switching fields in the hysteresis loops of the magnetization. The GMR multilayers have a low switching field of 60 Oe at low temperatures. They are patterned into two-dimensional 100 nm sized periodic dot-array using ultraviolet interferometric lithography. The purpose is to develop processes of fabricating GMR structures over large areas with nanometer feature sizes for high-density memory devices. The coercive field for the patterned structures is enhanced by a factor of four at low temperatures and by a small fraction at higher temperatures. The magnetization remains to be in plane as in un-patterned case.

Role of the density of states in the giant magnetoresistance of magnetic multilayers

The role of the density of states in the giant magnetoresistance (GMR) of ferromagnet 1 (FM1)/nonmagnetic (NM=Cu)/ferromagnet 1 (FM1) sandwiches has been investigated by inserting a thin layer of the second ferromagnetic material (FM2=Co) at the FM1/Cu interfaces. As the thickness of the inserted layer increases, the GMR increases to a value seen for purely Co/Cu/Co sandwiches. By using differing FM1 materials [e.g., Py (permalloy (Ni80Fe20), Ni, Fe] we observe the exponential increase in GMR as more Co is inserted to be dependent on the FM1 material behind it. This is explained by realizing that although the GMR is dominated by the scattering caused by defects like those at the FM2 material/Cu interface, the nature of the scattering depends on the density of states of the FM1 material. This result is discussed in terms of advanced theories on the origin of the GMR in magnetic multilayers.

Atomistic simulations of low energy ion assisted vapor deposition of metal multilayers

The properties of giant magnetoresistance multilayers are a sensitive function of the vapor deposition process used for their synthesis. The highest magnetoresistance occurs when deposition results in interfaces that are flat and chemically separated. Molecular dynamics simulations have been used to explore the potential benefits of low energy xenon ion assistance during the physical vapor deposition of Ni/Cu/Ni multilayers grown in the [111] direction from thermalized metal fluxes characteristic of molecular beam epitaxy. The simulations indicated that the roughness of the interfaces was significantly reduced as the ion energy was increased from 0 to 5 eV. However, increasing the ion energy above 2 eV also resulted in significant copper–nickel intermixing at the nickel on copper interface. Interface flattening without intermixing could be achieved using a modulated low energy ion assistance strategy in which the first half of each new material layer was deposited without ion assistance, while the remainder of the layer was deposited with an optimum low ion energy assistance of 4 eV. Modulated low energy ion assistance during thermalized metal atom deposition was found to be a promising approach for creating metal multilayers with improved magnetoresistance.

Energetic Inert Gas Atom Impact Effects During Ion Beam Multilayer Deposition

AbstractNew magnetic field sensors and non-volatile magnetic random access memories can be built from giant magnetoresistive multilayers with nanoscale thickness. The performance of these devices is enhanced by decreasing the atomic scale interfacial roughness and interlayer mixing of the multilayers. During ion beam sputtering, inert gas neutrals with energies between 50 and 200 eV impact the growth surface. A molecular dynamics method has been used to study the effects of these impacts on the surface roughness and interlayer mixing of model nickel/copper multilayers. The results indicate that impacts with energy above 50 eV cause mixing due to the exchange of Ni atoms with Cu atoms in an underlying Cu crystal. The extent of the mixing increases with impact energy, but decreases as the number of the Ni monolayers above the Cu crystal increases. While Xe and Ar impacts have a similar mixing effect at low energies, heavier Xe ions/neutrals induce more significant mixing at high energies.