Artificial Antiferromagnet Research Articles

Angular Sensor Using Tunneling Magnetoresistive Junctions With an Artificial Antiferromagnet Reference Electrode and Improved Thermal Stability

Magnetic tunneling junctions (MTJs) are fabricated using CoFe-Ru-CoFe artificial antiferromagnet (AAF) sandwiches as a hard-magnetic reference layer and plasma-oxidized aluminum as a tunnel barrier. Tailoring the magnetic properties of the artificial antiferromagnet reference layer allows an on-chip magnetization (initialization) of individual junctions, which makes it possible to build monolithic bridges in a Wheatstone arrangement without multiple mask process steps or on-chip heating elements. The functionality of an angular field sensor based on this concept is demonstrated in detail. The thermal stability of such a sensor is investigated and the limitations of the concept are discussed.

Thermally induced changes of magnetic coupling in a pinned artificial antiferromagnet used in magnetic tunnel junctions

The thermally induced changes of the magnetic coupling in an artificial antiferromagnet exchange biased by an antiferromagnetic layer in a magnetic tunnel junction (Mn–Ir/Co–Fe/Ru/Co–Fe/AlOx/Ni–Fe) is investigated for annealing temperatures up to 450 °C. Beside the usual increase of the tunneling magnetoresistance (TMR) by annealing (maximum TMR 43.9% at 325 °C) the degradation of the artificial antiferromagnet causes a well-defined 90° coupling of the Co–Fe layers in a narrow temperature window around 325 °C. At higher temperature, both Co–Fe layers are coupled ferromagnetically but the exchange biasing by the Mn–Ir is still present. This behavior results from an interplay of different coupling mechanisms of the two Co–Fe layers: (1) indirect antiferromagnetic interlayer exchange coupling across the Ru spacer, (2) ferromagnetic coupling by pinholes in the Ru spacer, and (3) exchange coupling between the lower Co–Fe layer and the antiferromagnet Mn–Ir. The importance of each contribution is discussed with respect to thermally activated diffusion processes.

Rotation angle sensors based on spin valve structures: A modeling approach

Spin valve systems are widely considered in magnetic rotation angle sensors. We present a simple magnetostatic model to simulate the electrical and magnetic signal of typical spin valve systems as a function of external magnetic field and rotation angle. To match simulation and experiment only a few parameters are needed, which can be determined by a single measurement. Two types of systems are treated and discussed: a simple spin valve and a spin valve with an “artificial antiferromagnet,” often used to enhance the exchange bias. The model shows a very good correspondence of the predictions to the experimental results of the systems investigated. In order to quantitatively compare experiment and model, we introduce a useful concept to quantify the quality of the angle rotation curves: the total harmonic distortion.

Compensation of magnetostatic interactions in magnetic tunnel junctions with artificial antiferromagnets

We study the magnetostatic coupling between the magnetically hard and soft electrodes of tunnel junction devices employing artificial antiferromagnetic structures as hard subsystems. The coupling is found to depend drastically on the thickness and the stacking sequence of the ferromagnetic layers of the artificial antiferromagnet. By adjusting these two parameters we can cancel out the net magnetostatic coupling.

Observation of the bulk spin-flop in an Fe/Cr superlattice

The layer magnetisation reorientation transition (spin-flop, SF) was studied in an artificial layer antiferromagnet (AF), namely in MgO(0 0 1)/[ 57Fe(2.6 nm)/Cr(1.3 nm)] 20 epitaxial superlattice (SL) by synchrotron Mössbauer reflectometry and Kerr effect (SMOKE). The SF occurs simultaneously in the entire SL stack (bulk SF) in an increasing field of H SF=13 mT along the easy direction parallel to the layer magnetisations. It is recognised by the kink in the SMOKE loop and by the sharp up-rise of the AF Bragg peak in the delayed Mössbauer reflectivity. The moderate value of observed H SF is compared with estimations from a spin-chain model and interpreted as due to intraplane domain-wall motion during SF.

Enhanced uniaxial anisotropy in an artificial antiferromagnet (AAF) using thin TbCo seed layers

When increasing the uniaxial anisotropy in (one of) the ferromagnetic layers in an artificial antiferromagnet, an enhancement of the rigidity of the artificial antiferromagnet is achieved, which is of importance for angular sensors. A new concept, i.e. a controlled uniaxial anisotropy induced by a thin TbCo seed, has been successfully implemented in a Co–Ru–Co artificial antiferromagnet, which serves as hard layer in spin-valve devices.

Evolution of the dielectric breakdown in Co/Al2O3/Co junctions by annealing

The temperature and dielectric stability of magnetic tunnel junctions are important requirements for magnetic memory devices and their integration in the semiconductor process technology. We have investigated the changes of the tunneling magnetoresistance (TMR), the barrier properties (height, thickness, and asymmetry) and the dielectric stability upon isochronal annealing up to 410 °C in Co/Al2O3/Co junctions with an artificial antiferromagnet as a pinning layer. Besides a small decrease of the TMR signal after annealing up to 230 °C, a strong decrease between 300 and 350 °C is found. According to Auger and transmission electron microscopy investigations, this decrease is mainly due to interdiffusion of the metallic layers. The dielectric breakdown is characterized by voltage ramp experiments. The size-averaged breakdown voltage improves from 1.35 V for the as prepared junctions to 1.55 V by annealing at 300 °C. At higher temperatures the breakdown voltage decreases strongly to 0.8 V (at 380 °C). Simultaneously, the typical breakdown process changes from few sudden current jumps to a large number of small steps. The breakdown properties are discussed within a statistical model and related to structural changes of the barrier.

Magnetic characterization and modeling of FeMn/Co/Ru/Co artificial antiferromagnets

FeMn/Co/Ru/Co artificial antiferromagnets for use in giant magnetoresistive spin-valve elements were fabricated and magnetically characterized. The magnetization behavior of the films can be understood by coherent rotation of the magnetizations of the layers. A simple model is presented which reproduces all the basic characteristics of the magnetization loops, e.g., shape, switching fields, and magnitude of the pinning field as a function of thickness. The model is also extended to a complete spin-valve system, including the effects of coupling between the biased layer and the free layer.

Giant magnetoresistance increase in a hard–soft spin valve structure with the growth of a semiconductor layer

Magnetic and transport properties of a hard–soft spin valve structures have been investigated. A first series of sandwiches composed of an artificial antiferromagnetic (AAF) Co/Ru/Co sandwich decoupled from a soft Fe/Co buffer layer as follows: Fe 50 Å/Co 5 Å/Cu 30 Å/Co 30 Å/Ru 5 Å/Co 30 Å/Cu 20 Å/Cr 20 Å has been prepared. This sandwich presents a giant magnetoresistance (GMR) of 1.7% and an exchange coupling strength of approximately −1.73 erg/cm 2. Afterwards, we have grown a second series of sandwiches in which the Cu/Cr capping layer has been replaced by a 15-Å thin semiconductor layer of ZnS, covered by a soft ferromagnetic layer of Co 5 Å/Fe 50 Å. Surprisingly, the giant magnetoresistance for the last sandwiches has been increased by a factor of 2, up to 4%. To explain this non-expected result, we have performed atomic force microscope imaging at the semiconductor layer surface. The results show that the semiconductor layer is not homogeneous and contains a non-negligible density of pin-holes, that are responsible of a direct magnetic coupling between the upper 30 Å Co layer of the AAF and the Co 5 Å/Fe 50 Å bilayer. This coupling induces a strong asymmetry between the magnetic layers of the AAF and consequently an enhancement of the GMR.

Switching stability of magnetic tunnel junctions with an artificial antiferromagnet

Avoiding fatigue in the switching of magnetic tunnel junctions is crucial for their long-term use in nonvolatile magnetic memories. We compare the switching stability of two types of junctions with different soft layers: Fe or Ni81Fe19, both with Co dusting at the barrier interface. The magnetically hard electrode is a Co/Cu/Co artificial antiferromagnet. While the tunneling magnetoresistance (TMR) remains unchanged after 104 cycles in a 4 kA/m rotating field, it decreases by more than 45% due to uniaxial switching. Fringing fields of domain walls in the soft layer and an intrinsic instability of Co/Cu/Co are identified as the main reasons. Magnetization reversal by two perpendicular switching pulses avoids this magnetic degradation and maintains a full TMR signal.

Sensor properties of a robust giant magnetoresistance material system at elevated temperatures

The temperature dependence of the giant magnetoresistance (GMR) ratio, resistance and exchange-biasing field for a spin valve comprising an Ir19Mn81-biased artificial antiferromagnet (AAF) has been studied up to 325 °C. Up to 200–250 °C the temperature effects are reversible, at higher temperatures gradual irreversible changes are observed, probably due to atomic diffusion. The magnetoresistance effect is even at 200 °C still higher than for anisotropic magnetoresistance sensors at room temperature. The resistance of the multilayer shows a maximum around 250 °C. We found that this is due to the peculiar behavior of Ir–Mn, which has a negative temperature coefficient of the resistance. This provides a possibility to tune the temperature coefficient for the complete multilayer by varying the thickness of the Ir–Mn layer. The relative decrease of the exchange-biasing field as a function of temperature is much smaller for spin valves with AAF than for conventional spin valves (without AAF). Furthermore, it was demonstrated that the GMR ratio can be increased to 12% at room temperature by using a dual spin valve with two AAFs.

Spin valve structures with artificial antiferromagnets

An artificial antiferromagnetic (AAF) structure as substitute for the ferromagnetic layer may overcome the limitations of sensitivity and dynamic range of a standard spin valve (SV). The AAF consists of two ferromagnetic films (CoFe or NiFe in this article) antiferromagnetically coupled across a thin nonmagnetic layer (Ru). Improved sensitivities combined with high magnetoresistance (MR) ratios and low coercivities and interlayer coupling fields are obtained in NiFe based AAF SVs. The differences between the observed reversal mechanism of SVs with parallel and crossed easy axes configurations have been studied by Lorentz microscopy and correlated with their different MR and sensitivity characteristics.

Contactless current detection with GMR sensors based on an artificial antiferromagnet (AAF) subsystem

The advantages of the newly developed giant magneto-resistive (GMR) sensor scheme based on artificial antiferromagnetic (AAF) subsystem which was already demonstrated in angle detection, also promises a successful application in contactless current sensing. Wheatstone bridge structures were formed by combining sensors with opposite response characteristics. The sensor linearity was improved by an additional bias field influencing the slope and the maximum range of the signal curve. In our contactless sensing device, currents up to 40 A were measured reliably. Depending on the measuring range, excess currents up to 10 times of the nominal range do not change the sensitivity and bridge offset of the sensor.

Robust giant magnetoresistance material system for magnetic sensors

A new, robust giant magnetoresistance (GMR) material system is described that comprises an exchange-biased artificial antiferromagnet. For the exchange biasing Ir18Mn82 is used because of its high blocking temperature. Experiments have demonstrated that this material can withstand magnetic fields larger than 150 kA/m and annealing at temperatures above 275 °C without irreversible damage. At higher temperatures the GMR effect starts to decrease due to atomic diffusion; the exchange-biasing effect, however, does not disappear, which means that this is no longer the limiting factor. The excellent magnetic and thermal robustness, combined with the unambiguous asymmetric magnetoresistance curve, makes this GMR material system very suitable for application in (e.g., automotive) sensors.

Magnetic tunnel sensors with Co-Cu artificial antiferromagnetic (AAF) hard subsystem

Micron-size magnetic tunnel junctions are studied, in which the pinning of the hard magnetic reference layer is accomplished by the so-called Artificial Antiferromagnetic Subsystem (AAF). In the AAF, two Co layers are antiferromagnetically coupled by a 1 nm thick Cu layer, thus giving rise to stability windows larger than 2 kA/m for the AAF. Al/sub 2/O/sub 3/-tunnel barriers of 1 and 1.5 nm Al thickness were put on top of the AAF and at its opposite surface Fe detection layers are located. The signal could be enhanced by a factor of two to 22% by inserting a 1 nm Co-coating between the barrier and the Fe. Thus sensors were realized with signals four times higher than for the GMR counterparts.

Structure and Sensor Properties of a Robust GMR Material System

ABSTRACTRecently we have developed a robust GMR material system, based on an artificial antiferromagnet which is exchange-biased with Ir-Mn. By combining magnetoresistance and magnetization measurements with Transmission Electron Microscopy analyses we show that the degree of [111] texture has direct effects on the sensor properties of the GMR multilayer. The degree of texture turns out to depend not only on the buffer layer, but also on the thickness of the Ir-Mn layer. The strongest texture is found for Ir-Mn layer thicknesses of around 4 nm. For this thickness the largest exchange-biasing field and GMR ratio are also obtained (after cooling in a magnetic field), but the blocking temperature is significantly reduced. If attention is paid to the material structure by choosing the right materials, layer thicknesses and stack order, a robust GMR multilayer system can be made that can withstand high temperatures (> 200°C) and strong magnetic fields (> 200 kA/m), as has been demonstrated experimentally.

Giant Magnetoresistive Sensors for Industrial Applications

Giant magnetoresistance (GMR) is the extraordinarily large change in resistance in ultrathin magnetic multilayer films under an applied magnetic field. Its origin is spin dependent scattering in these films with layer thicknesses of only a few nanometers. Besides fundamental interest in spin dependent transport properties, the focus on these materials results from a variety of attractive applications.Here, we describe a GMR sensor concept which is tailor-made for the contactless measurement of angles and positions and we show the accompanied gain in performance compared to conventional Hall or Permalloy sensors. The basic scheme of this sensor is a so called spin-valve set-up. Two adjacent magnetic layers are designed such that one is magnetically hard while the other is soft. The soft magnetic layer changes the direction of its magnetisation under an applied external field with respect to the fixed magnetisation of the hard subsystem, which consists of an artificial antiferromagnet (AAF). The sensor signal, i.e. the change in resistance ΔR/R varies as the cosine of the angle α between the magnetisation directions in both magnetic layers.

Magnetic properties of ferrimagnetic rare-earth—transition-metal films and multilayers

In this paper we present a brief report on magnetization studies of three different types of rare-earth—transition-metal ferrimagnetic thin films. The first consists of compositionally modulated Gd 0.15Fe 0.85/Gd 0.40Fe 0.60 where the two compositions are on different sides of the magnetic compensation point (Gd 0.26Fe 0.74) at room temperature. The compositionally modulated layers therefore form an artificial (macroscopic) superlattice antiferromagnet which exhibits both spin-flop transitions and enhanced in-plane anisotropy owing to classical dipolar effects. We also report magnetization measurements on a homogeneous film of Tb 0.23Fe 0.77, which exhibits large perpendicular anisotropy typical of amorphous TbFe thin films as well as a spin-flop transition near 60 kOe. Finally, we present data on a film of Gd 0.40Fe 0.60(300 Å)/Fe(10 Å)/Pd(100 Å)/Fe(30 Å) which demonstrates a practical method of determining the strength of ferromagnetic coupling between two magnetic layers from simple magnetization measurements.