Magnetoresistance In Multilayers Research Articles

Tunnel magnetoresistance sensors with symmetric resistance-field response and noise properties under AC magnetic field modulation

Magnetoresistance (MR) devices with symmetric (even-function) resistance change to an external magnetic field is useful for various magnetic sensor applications, such as encoders for position and rotation detection and ultra-sensitive magnetic field sensors, when combined with a frequency modulation method. In this study, we developed tunnel magnetoresistance (TMR) sensors with even-function resistance–field (R–H) responses and investigated their noise properties. The developed TMR sensors achieved a higher MR ratio (∼120%) than those of conventional giant magnetoresistive multilayers exhibiting even-function R–H responses. The TMR sensors were realized by stabilizing the magnetization of the magnetic free layer (FL) in an anti-parallel configuration with respect to that of the reference layer. Orange-peel ferromagnetic coupling between the FL and a pinned layer through an AgSn spacer layer was used to control the strength of the unidirectional magnetic anisotropy of the FL, by which even-function R–H responses with negligible hysteresis were obtained. The sensors enabled us to study the intrinsic noise properties of TMR devices, particularly under an external AC modulation magnetic field. We found that the magnetic 1/f noise originating from thermal magnetization fluctuation was the dominant noise of the TMR sensors at low frequencies. We observed that the application of an AC modulation field to the TMR sensors increased the noise owing to the magnetic domain fluctuation and the transfer of magnetic 1/f noise to the modulation frequency.

Preliminary Study on the Magnetic Properties of GeMn Nanocolumn/Ge Multilayers

Ge0.94Mn0.06 nanocolumn thin film is a unique phase of GeMn diluted magnetic semiconductors (DMS) which exhibit Curie temperature (TC) > 400 K. The multilayers of Ge0.94Mn0.06 nanocolumns separated by nano-scaled spacers represent great interests for spintronic applications, such as spin valves or giant magneto-resistance (GMR) multilayers. In this article, we present the results obtained from the preliminary study on the exchange coupling in two types of GeMn nanocolumn/Ge multilayers. All the samples have been grown using molecular beam epitaxy (MBE). The superconducting quantum interference device (SQUID) magnetometer has been used to determine the magnetic properties of the samples. In the multilayer system Ge/[Ge0.94Mn0.06(40 nm)/Ge(d nm)]9/Ge0.94Mn0.06(40 nm)/Ge, no exchange coupling can be observed. Inversely, exchange coupling between the layers exists and depends on the thickness of the Ge spacers for the GeMn nanocolumns/Ge multilayer spin valve systems. The exchange coupling in the nanocolumns multilayer systems has been shown to be complex due to the leakage field induced by neighboring nanocolumns and the magnetic anisotropy of nanocolumns.

Ultrasensitive Magnetic Nanoparticle Detector for Biosensor Applications

Ta/Ru/Co/Ru/Co/Cu/Co/Ni80Fe20/Ta spin-valve giant magnetoresistive (GMR) multilayers were deposited using UHV magnetron sputtering and optimized to achieve a 13% GMR ratio before patterning. The GMR multilayer was patterned into 12 sensor arrays using a combination of e-beam and optical lithographies. Arrays were constructed with 400 nm × 400 nm and 400 nm × 200 nm sensors for the detection of reporter nanoparticles. Nanoparticle detection was based on measuring the shift in high-to-low resistance switching field of the GMR sensors in the presence of magnetic particle(s). Due to shape anisotropy and the corresponding demag field, the resistance state switching fields were significantly larger and the switching field distribution significantly broader in the 400 nm × 200 nm sensors as compared to the 400 nm × 400 nm sensors. Thus, sensor arrays with 400 nm × 400 nm dimensions were used for the demonstration of particle detection. Detection of a single 225 nm Fe3O4 magnetic nanoparticle and a small number (~10) of 100 nm nanoparticles was demonstrated. With appropriate functionalization for biomolecular recognition, submicron GMR sensor arrays can serve as the basis of ultrasensitive chemical and biological sensors.

Direct transfer of magnetic sensor devices to elastomeric supports for stretchable electronics.

A novel fabrication method for stretchable magnetoresistive sensors is introduced, which allows the transfer of a complex microsensor systems prepared on common rigid donor substrates to prestretched elastomeric membranes in a single step. This direct transfer printing method boosts the fabrication potential of stretchable magnetoelectronics in terms of miniaturization and level of complexity, and provides strain-invariant sensors up to 30% tensile deformation.

The Interlayer Exchange Cu as a Function of Nonmagnetic On Giant Magnetoresistance Multilayers

This paper presents a study of the magnetic multilayer structure and a key advantage of Molecular Beam Epitaxy preparation technique, and additional epitaxial over layers to study interlayer exchange coupling and the resulting reduction of growth induced defect are discussed.

Magnetothermoelectric figure of merit of Co/Cu multilayers

The switching of the magnetic configuration of giant magnetoresistance multilayers not only changes the electrical and thermal conductivities but also the thermopower. We study the magnetotransport and the magnetothermoelectric properties of Co/Cu multilayer devices in a lateral thermal gradient. The Seebeck coefficient reaches values up to −18 μV/K at room temperature and shows a magnetic field dependence up to 28.6% upon spin reversal. In combination with thermal conductivity data of the same Co/Cu stack, we find a magnetothermoelectric figure of merit of up to 65%. Furthermore, a magneto-power factor of up to 110% is derived.

Influence of Permalloy Layer and Ti Spacer Thicknesses on Magnetic and Magnetoresistive Properties of Fe19Ni81/Ti/Tb-Co Films

Fe19Ni81/Tb35Co65 magnetic films having unidirectional anisotropy were investigated with focus on the influence of the Fe19Ni81 layer and Ti spacer thicknesses on magnetic and magnetoresistive properties of the multilayered structures. Principles of the optimization of the properties of magnetoresistive multilayers with unidirectional anisotropy were proposed.

Observation of Stress Assisted Magnetization Reversal in a Spin Valve Structure

In order to reduce a power consumption of a perpendicular magnetoresistive random access memory, a stress assisted magnetization reversal method in which an in-plane tensile stress changes the magnetic anisotropy of a magnetic material has been proposed. A spin valve structured giant magnetoresistive multilayer of Cu(15 nm)/Terfenol-D(10 nm)/Cu(3 nm)/Tb-Fe-Co(30 nm)/Cu(15 nm) was prepared and its perpendicular magnetization was measured by the anomalous Hall effect. When a mechanical stress of 279 MPa was applied to the multilayer, a direction of the magnetization of only the free layer, a recording layer, was changed from perpendicular to in-plane.

Magnetic properties of SrTiO3/Pr2/3Ca1/3MnO3 multilayers

Nanoscale ferromagnetic (FM) clusters embedded within insulating (I) layers of the antiferromagnetic Pr2/3Ca1/3MnO3 (PCMO) are known to improve the magnetoresistance ratio of FM-I magnetoresistive multilayers. Here, we study the magnetic properties of perovskite superlattices comprised of five PCMO layers of variable thickness t separated by 5nm-thick spacers of SrTiO3 (STO). Several multilayers (1.5 nm ≤ t ≤ 8 nm) were grown on (001) STO substrates by magnetron sputtering. We show that the magnetic moment of PCMO due to the FM inclusions presents a maximum for t ~ 3 nm, accompanied by an abrupt increase of coercivity and exchange bias field. The nonmagnetic nature of the neighboring STO layers demonstrates that the observed behavior is not related to any interlayer exchange coupling, but the geometrical matching between layer thickness and FM domain size is the key driver for the enhancement of the FM moment and anisotropy energy. These results open a new door for the optimization of perovskite based spintronic devices.

Stress-assisted Magnetization Reversal for a Perpendicular GMR Multi-layer

A Stress-Assisted Magnetization Reversal (SAMR) method has been demonstrated in a perpendicular GMR multilayer to reduce a power consumption during the magnetization-reversal process in a perpendicular magenetoresistive random access memory. A giant magnetoresistive multilayer of Cu(15nm)/Terfenol-D(10nm)/Cu(3nm)/TbFeCo(30nm)/Cu(15nm) was prepared. The perpendicular components of magnetization reversal in the GMR multilayer were observed during the SAMR process by using the anomalous Hall effect. As a result, the magnetization of a free layer was shifted from perpendicular to in-plane under the application of a stress of 279MPa.

Preface: phys. stat. sol. (a) 205/8

AbstractA conference series on magnetism has been organized in Germany under the auspices of Forschungszentrum Karlsruhe. The first meeting of this series (SCM2001) was held in September 2001. The Third Seeheim Conference on Magnetism (SCM2007) took place from 26 to 30 August 2007. The aim of the conference was to bring together scientists working in the field. SCM2007 was focused on the magnetism of nanostructured materials, interfaces, multilayers and nanoparticles. The most recent results were presented in the form of plenary lectures, invited and contributed talks and posters. A total number of 230 papers correlating nanostructures and magnetic properties were selected for presentation. Throughout the meeting 75 talks were given; the remaining 155 papers were scheduled for poster presentation.The topical areas were:1. Hard and soft magnetic materials– Permanent magnets and ferrites– Amorphous alloys– Nanocrystalline materials– Domain walls, modeling and hysteresis2. Magnetic memories– MRAM– TMR, GMR– CPP‐GMR3. Domain walls, modelling and hysteresis4. Spin dependent transport properties– Magneto‐resistance– Magneto‐impedance– Giant magneto‐resistance (multilayers, magnetic tunnel junctions, ultrafine particles, other materials, magnetic semiconductors)– Magneto‐optics– Magneto‐resistance in transition metal oxides5. Thin films and particles– Superlattices– Ultrathin films– Multilayers– Ultrafine particles– Dots– Current induced magnetization, spin torque, injection and accumulation– Surfaces and interfaces– Magnetic fluids and nanoparticles6. Magnetic interactions– Exchange interactions (multilayers, transition metals and compounds, rare earth and compounds)– Crystal field anisotropy– Magneto‐elastic interactions7. Magnetization processes– Dynamic effects in nanomaterials– Relaxation, spin waves and solitons8. Applications– Recording media and devices– Magnetic sensors and memory– Magneto‐optics– Measurement techniquesThere were more than 200 registered participants from 37 countries, representing academic institutions and industry.We look forward to welcoming again all the participants of SCM2007 at the fourth Seeheim conference, planned for 2010. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low energy ion beam assisted deposition of a spin valve

The spin dependent electron transport in giant magnetoresistive (GMR) multilayers is significantly affected by the atomic scale structure of their interfaces. Devices with atomically flat and chemically sharp interfaces are preferred for magnetic sensor and memory applications. Recent atomic simulations of the atom-by-atom assembly of these devices indicate that near optimal interfacial structures can be created using low energy, ion assisted vapor deposition techniques with ion energies in the 5–10eV range. A recently developed biased target ion beam deposition system has been used to experimentally test this hypothesis. Prototypical Ta∕NiFe∕Co∕Cu∕Co∕FeMn∕Cu spin valve structures were first grown using (simultaneous) argon ion assistance during deposition of the Co∕Cu∕Co trilayer part of the spin valve multilayer. Assisting ion energies of around 10eV resulted in structures with a 30% higher magnetoresistance ratio and significantly reduced coupling field compared to samples grown with no ion assistance or with ion energies above 15eV. These results are consistent with the atomistic simulation predictions. Other promising ion assistance schemes identified by the simulations were then used to deposit the Ta, NiFe, FeMn, and the top copper layer. A near optimal strategy was identified that resulted in the further improvement of the GMR ratio.

Role of heterostructures and multiple magnetic phases in the low-field magnetization of Fe–Cr giant magnetoresistive multilayers

Zero-field-cooled (ZFC) and field-cooled (FC) magnetizations along with ac magnetization versus temperature and M-H loop measurements are reported for two series of ion beam sputtered Fe–Cr giant magnetoresistive (GMR) (maximum of 33%) multilayers where the interface roughness is different. The ZFC and FC magnetization data follow different curves below an irreversibility temperature (Tirr). The FC data show a T3∕2 thermal demagnetization behavior at lower temperatures with a very small spin-wave stiffness constant but they go as 1∕T at higher temperatures (above Tirr). The ZFC data show maxima at Tm(H), from which we find the glass transition temperature Tg using the de Almeida and Thouless plot. ac magnetization versus T also shows broad peaks roughly near the same Tg. This behavior is interpreted in terms of the coexistence of spin-glass∕superparamagnetic, FM and AF phases. Most of the above exotic low-field features are absent in molecular beam epitaxy grown Fe–Cr multilayers (GMR >50%) with very smooth interfaces. The relatively rough interfaces in the present sputtered samples give rise to frustrations of Fe and Cr moments, leading to spin-glass-like behavior.

Relaxation of thermo-remanent magnetization in Fe–Cr GMR multilayers

The time decay of the thermo-remanent magnetization (TRM) in Fe–Cr giant magnetoresistive (GMR) multilayers has been investigated. The magnetization in these multilayers relaxes as a function of time after being cooled in a small magnetic field of 100 Oe to a low temperature and then the magnetic field is switched off. Low-field ( < 500 Oe ) magnetization of these samples has shown history dependence. This spin-glass-like behavior may originate from structural imperfections at the interfaces and in the bulk. We find that the magnetization relaxation is logarithmic. Here the magnetic viscosity is found to increase first with increasing temperature, then it reaches a maximum around T g , and further it decreases with increasing temperature. This behavior is different from that of conventional spin glasses where the logarithmic creep rate is observed to increase with temperature. The dynamical effects of these multilayers are related to the relaxation of thermally blocked superparamagnetic grains and magnetic domains in the film layers.

Mechanisms of Ag as a surfactant in giant magnetoresistance multilayer growth and thermal stability

The mechanisms played by Ag as a surfactant in giant magnetoresistance multilayers were investigated using interface sensitive x-ray anomalous scattering techniques. Analysis on [Cu∕Ni70Co30]20 and [Cu∕Ag∕Ni70Co30]20 multilayers revealed that 6Å thick NiCu and 6Å thick CuNi3Co intermixing regions are formed at the Ni70Co30-on-Cu interfaces of undoped and Ag-doped multilayers, respectively. The Cu-on-Ni70Co30 interfaces in both multilayers are sharp. Annealing causes severe diffusion across both types of interfaces in the undoped multilayer. But the interfaces in the Ag-doped multilayer do not change significantly upon annealing, except that Ag atoms diffuse into the whole Ni70Co30 layer and some parts of the Cu layer. The results suggest that addition of Ag during the deposition suppresses interfacial intermixing. X-ray diffuse scattering profiles show that the interfacial lateral correlation length of the Ag-doped multilayer is longer than that of the undoped multilayer and does not change significantly after annealing, suggesting that the addition of Ag gives rise to smoother interfaces and results in a good thermal stability.

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.

Optimization of magnetoresistive sensitivity in electrodeposited FeCoNi/Cu multilayers

Giant magnetoresistive (GMR) multilayers with high GMR sensitivity might find application in low-end magnetic sensors. Electrodeposition of FeCoNi/Cu GMR multilayers directly onto n-type silicon was carried out from a single electrolyte and their magnetotransport properties were investigated. Inhomogeneous dissolution of the ferromagnetic (FM) layer during Cu sublayer deposition has an important role in determining the shape of the GMR characteristics; lower dissolution promotes sharper interfaces and larger slopes of the GMR versus field curve at low fields. An increase of the Fe content in the FM sublayers reduces coercivity but promotes selective dissolution and degrades interface sharpness. Electrodeposition conditions are optimized to achieve a maximum GMR of 9% saturated at <0.5 kOe, and a maximum GMR sensitivity of over 0.11%/Oe in the field range 5-15 Oe with a saturation GMR of 6.1%.

Giant-Magnetoresistive/Superconducting Contacts and Josephson Junction Devices

We have been investigating superconductor-normal metal-superconductor (SNS) (Proximity-Effect) Josephson Junctions in which the normal metal is a magnetic heterostructure in which the magnetic character can be actively controlled. As part of this program we have investigated the use of giant magnetoresistive (GMR) Cu-Py multilayers as the barrier in variable thickness planar bridge SNS junctions in addition to in-plane S-GMR contacts. In this paper we report on the latest results from our devices which have been fabricated using our established focussed ion beam (FIB) processes.

A charge transfer ionic–embedded atom method potential for the O–Al–Ni–Co–Fesystem

Magnetic tunnel junctions (MTJs) require the growth of a thin(∼20 Å) dielectric metaloxide layer, such as Al2O3, on a ferromagnetic metal layer, such as Co, CoFe, or CoNiFe. The atomic assemblymechanisms that combine to form a uniformly thin metal oxide layer on these metalsurfaces are not well understood. The application of molecular dynamics simulations to thegrowth of metal and metal oxide multilayers that involve more than one metalelement has not been possible using the conventional interatomic potentials. Arecently proposed modified charge transfer ionic–embedded atom method potentialappears to correctly enable the charge transfer between oxygen and numerous metalelements to be modelled in a format amenable for molecular dynamics studies. Herewe parametrize this charge transfer ionic–embedded atom method potential forthe quinternary O–Al–Ni–Co–Fe system so that a direct molecular dynamicssimulation of the growth of the tunnelling magnetoresistive multilayers can be realized.

Evidence of spin-wave demagnetization in Fe–Cr giant magnetoresistance multilayers

The temperature dependence of the saturation magnetization of Xe ion-beam sputtered Fe–Cr multilayers has been measured between 5 and 300K in the presence of a field greater than the saturation field (Hsat). With the application of Hsat, the zero-field antiferromagnetically coupled ferromagnetic Fe layers are forced to align ferromagnetically. We find that the thermal demagnetization of such magnetically composite structure follows the Bloch formula for spin waves with anharmonic term in the magnon dispersion relation, viz., M(T)=M(0)[1+AT3∕2+BT5∕2]. Comparisons are presented between the multilayers and bulk crystalline Fe.