Spin Tunnel Junctions Research Articles

Spin tunnel junction recording head

Micro-cracks in a magnetized ferromagnetic material cause stray fields that can be observed using giant magnetoresistive (GMR) sensors. This work investigates the applicability of GMR sensors to the non-destructive evaluation of micro-cracks via the observation of stray fields. For this purpose, our measurement setup is assessed using a fast new sensitivity analysis based on adjoint states, employing the finite-element method. A model for the GMR sensor is developed and verified. We are able to resolve micro-cracks with an opening of 3 μm and a depth of 30 μm. GMR positioning inaccuracies are analyzed.

Effects of two in-plane fields on the magnetization reversal mechanism in magnetic tunnel junction elements

One of the potential applications of the spin tunnel junction is as magnetic random access memory (MRAM) elements. For MRAM application purposes, two in-plane perpendicularly applied fields are required during magnetization reversal. In this article, Lorentz transmission electron microscopy has been used to study the magnetization reversal mechanism of tunnel junction elements under the influence of two in-plane perpendicular fields. Four hundred NiFe/MnFe/NiFe/Al2O3/NiFeCo elements with differing shapes and sizes have been observed. When an in-plane Y field was applied to the hard axis followed by sweeping the X field along the easy axis, it was found that the number of elements in which 360° domain walls are observed to form decreases as the strength of the Y field increases, and that the magnetization reversal mechanism in the tunnel junction elements gradually changes from a domain wall motion mechanism to a moment rotation mechanism as the Y field increases. As expected, the reversal field (X field) reduces as the Y field increases.

Magnetocapacitance effect of spin tunneling junctions

Magnetocapacitance effect of coercive differential spin tunneling junctions Co(100 Å)/Al2O3(∼20 Å)/Co(500 Å) fabricated onto a glass substrate by ion-beam mask sputtering was investigated. The impedance was measured by a four-probe method at room temperature in the frequency range from 120 Hz to 1 MHz. It is found that the effective capacitance changes with the application of an external magnetic field. At high frequencies, the magnetocapacitance ratio is as large as the dc magnetoresistance ratio. However, at low frequencies, capacitance changes cannot be observed because the measurement sensitivity is too low. The magnetocapacitance effect was hence found to be a promising tool for high frequency magnetic sensing.

Effect of natural oxidation conditions on low resistance spin tunnel junctions

Spin dependent tunnel junctions with AlOx barrier were fabricated by in situ natural oxidation of a 7 Å (or 8 Å) thick Al film. Junctions were deposited on top of preannealed, ion-beam smoothed, 600 Å thick Al bottom electrodes. By changing the oxidation time from 10 min to 10 h with the oxygen pressure of 10 and 100 Torr, the top-pinned junctions have resistances varying from 31 to 406 Ω μm2 and the tunneling magnetoresistance (TMR) varies from 8.5% to 19.6% for the as deposited state. For the bottom-pinned junctions, the oxygen pressure was varied from 0.5 to 100 Torr, and oxidation time ranged from 5 min to 2 h. Bottom-pinned junction resistances as low as 10–12 Ω μm2 were obtained with TMR ranging from 14% to 17% for the junctions oxidized at the lower pressure (0.5 Torr). Rutherford backscattering analysis (RBS) indicates an O/Al ratio of 1.29±0.34 denoting incomplete oxidation of the Al for junctions oxidized at the lower pressures and short times. Junctions oxidized at higher pressures (⩾10 Torr) can reach 25%–30% TMR, with resistances ranging from 30 to 70 Ω μm2. RBS shows near stoichiometric Al2O3 oxide composition (O/Al=1.51±0.43) in these barriers.

Lokal'nye magnitnye sostoyaniya i sverkhtonkie vzaimodeistviya v magnitnykh sverkhreshetkakh i spin-tunnel'nykh perekhodakh

Lokal'nye magnitnye sostoyaniya i sverkhtonkie vzaimodeistviya v magnitnykh sverkhreshetkakh i spin-tunnel'nykh perekhodakh

Spin-dependent tunnelling in La 0.7Sr 0.3MnO 3/SrTiO 3 superlattices

La 0.7Sr 0.3MnO 3 is predicted to show half-metallic behaviour at low temperature, which gives rise to a metallic character for one spin direction and an insulating character for the other. This 100% polarisation of the conduction band should enhance the spin dependent tunnelling in manganite-based tunnel junctions. La 0.7Sr 0.3MnO 3/SrTiO 3 epitaxial superlattices were grown on LaAlO 3(001) substrates by metal–organic chemical vapour deposition (MOCVD). These multilayers consist of 15 epitaxial bilayers of La 0.7Sr 0.3MnO 3 and SrTiO 3. The junctions were patterned using UV lithography and Ar ion milling to carry out transport measurements in the current perpendicular-to-plane geometry (CPP). A temperature-independent non-linear I– V curve, which is characteristic of a tunnelling conduction mechanism, was observed below 50 K. At higher temperatures, the I– V curves are found to become linear and temperature-dependent. Up to 30 K, a constant tunnel magnetoresistance (TMR) (3%) is measured. The switching field is consistent with the film coercive field (a few 10s of mT). At higher temperatures, the TMR decreases rapidly. This temperature dependence is compared to the expected behaviour of a spin tunnel junction with half-metallic electrodes, with thermal activation or the loss of spin polarisation taken into account.

Resistance decrease in spin tunnel junctions by control of natural oxidation conditions

Spin-dependent tunnel junctions with AlOx barriers were fabricated by in situ natural oxidation of a 7 Å thick Al film. Oxygen pressure was varied from 0.5 to 100 Torr, and oxidation time ranged from 5 min to 2 h. Junction resistances as low as 10 to 12 Ω μm2 were obtained with corresponding tunnelling magnetoresistance values (TMR) ranging from 14% to 17%, for the junctions oxidized at the lower pressure (0.5 Torr). Rutherford backscattering analysis (RBS) indicates an O/Al ratio of 1.29±0.34 denoting incomplete oxidation of the Al. Junctions oxidized at higher pressures (⩾10 Torr) can reach 25% to 30% TMR, with resistances ranging from 30 to 70 Ω μm2. RBS shows near-stoichiometric Al2O3 oxide composition (O/Al=1.51±0.43) in these barriers.

Structure and chemistry of manganite based tunnel junctions

Half-metallic manganite (La0.7Ca0.3MnO3) based spin tunnel junctions with two barrier layers (NdGaO3 and La0.45Ca0.55MnO3) have been investigated structurally as well as chemically using high spatial resolution electron microscopy techniques. Both junctions were grown on a NdGaO3 substrate, which has better lattice match compared to a more traditional substrate such as strontium titanate (SrTiO3). Devices with a NdGaO3 barrier showed tunnel magnetoresistance (TMR) values up to 85% at 77 K, whereas the devices with a La0.45Ca0.55MnO3 barrier layer showed only 18% TMR at 77 K. The electron microscopy data show much clearer, sharper interfaces, structurally as well as chemically, for NdGaO3 than for the La0.45Ca0.55MnO3 barrier layer.

40% tunneling magnetoresistance after anneal at 380 °C for tunnel junctions with iron–oxide interface layers

Spin tunnel junctions fabricated with one interposed Fe–FeOx layer between the Al2O3 barrier and the top CoFe pinned electrode show large tunneling magnetoresistance (TMR) (40%) for anneals up to 380 °C. The annealing temperature TTMR*, where maximum TMR occurs, increases with the inserted Fe–FeOx layer thickness. For samples with thicker inserted layer, the pinned layer moment (which usually starts to decay below 300 °C in the normal junctions) increases with annealing temperature up to 380 °C and remains at a maximum until 450 °C. The large TMR at high temperature is related with the diffusion of extra Fe (from the Fe–FeOx layer) into the electrode interfacial region and the as-deposited paramagnetic FeOx decomposition into metallic Fe, and possibly the formation of some Fe3O4, which compensate the interface polarization loss associated with Mn interdiffusion. Rutherford backscattering spectrometry analysis confirms partial Fe diffusion into the top CoFe electrode after anneal. Meanwhile, x-ray photoelectron spectra for the Fe 2p core level show that the FeOx contribution in the upper part of the inserted layer decreases upon annealing, while it increases in the inner part near the barrier, suggesting the FeOx decomposition and the oxygen diffusion toward the inner metallic Fe and Al barrier. The study of R×A values and barrier parameters versus annealing temperature for samples with 7 and 25 Å Fe–FeOx also reflects the above structural changes in the inserted layer.

Annealing effect of magnetic tunnel junctions with one FeOx layer inserted at the Al2O3/CoFe interface

Spin tunnel junctions with one interposed Fe oxide layer between the Al2O3 barrier (tAl=8–9 Å) and the top CoFe pinned layer show large tunneling magnetoresistance (TMR) values (39%) after 40 min anneal at 380 °C. The as-deposited TMR is low and does not increase until 350 °C (<10%), but then increases sharply, peaking at 380°C. Further anneals at this temperature (380 °C), lead to TMR decrease to 20% with a diffusion constant of 34 min. At 360 °C, the diffusion constant is about 60 min. Samples without this inserted FeOx layer show TMR<5% after prolonged anneals at 380 °C. The different techniques utilized to probe the barrier and electrode changes during the annealing processes indicate that from the initial Fe–FeOx layer, part of the Fe diffuses into the CoFe electrode, and the remaining FeOx probably decomposes into a pure interfacial Fe layer at high temperature, responsible for the large TMR. The observed TMR values and barrier parameters, seem consistent with a standard CoFe/Al2O3/CoFe barrier formed at high temperature.

Element-selective nanosecond magnetization dynamics in magnetic heterostructures.

We have developed a new original technique to study the magnetization reversal dynamics of thin films with element selectivity in the nanosecond time scale. X-ray magnetic circular dichroism measurements in pump-probe mode are carried out taking advantage of the time structure of synchrotron radiation. The dynamics of the magnetization reversal of each of the layers of complex heterostructures (like spin valves or tunnel junctions) can be probed independently. The interlayer coupling in the studied systems has been shown to play a key role in the determination of the magnetization reversal of each individual layer.

Plasma oxidation of thin aluminum layers for magnetic spin-tunnel junctions

This article presents results of a study initiated to characterize the plasma-oxidation process of very thin Al films, a technology commonly used to produce good barrier layers for magnetic spin-tunnel junctions. The behavior of oxygen in the oxidizing Al layer is determined using both quantitative (Rutherford backscattering spectrometry, transmission electron microscopy) and qualitative (x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry) analytical techniques. We have applied in situ XPS and experimented with O218 to unravel details of the oxidation mechanism. In addition, the influence of the oxygen pressure on the oxidation rate was established, both with and without a plasma being present. From optical emission spectra it is concluded that this pressure has a minor effect on the relative abundance of excited species in the oxygen plasma. When combined, these data constitute the basis of a model that distinguishes several steps in the plasma oxidation of Al. At the start, oxygen penetrates rapidly throughout the total Al layer, followed by a period of increasing oxygen concentration but constant oxide thickness. Finally, the Co underlayer becomes involved in the oxidation process, which marks the deterioration of the spin-tunnel junction. Evidence is obtained that for the thicker initial Al layers the Co electrode layer starts to oxidize before completion of the Al oxidation. This explains why for 0.8-nm-thick Al films the highest tunnel-magnetoresistance effect is obtained for stoichiometric Al2O3, whereas for 1.5 nm Al this occurs while the oxide is still substoichiometric.

Magnetization reversal of patterned spin–tunnel junction material: A transmission electron microscopy study

Electron beam lithography and reactive ion etching have been used to pattern micron-size magnetic elements in the free layer of spin–tunnel junctions. The magnetization reversal processes of elements with dimensions in the range from 15×1 μm2 to 1×1 μm2 have been studied using Lorentz microscopy in the transmission electron microscope. Under the application of a field parallel to the bias direction, elongated elements reverse by the growth and subsequent annihilation of a quasiperiodic domain structure which evolves from the ends of the elements. Similar processes occur in both halves of a magnetization cycle. By contrast, the reversal of square elements involves the formation of more complex domain structures which differ significantly according to the direction in which the field in applied.

Compositional, structural, and electrical characterization of plasma oxidized thin aluminum layers for spin-tunnel junctions

In this paper we present results on how the plasma oxidation of a thin (1.5 nm) Al layer proceeds. Transmission electron microscopy of a Co/Al–oxide multilayer was used to determine the thickness of the oxides and Rutherford backscattering spectrometry and elastic recoil detection were utilized in order to determine the oxygen content. The oxide was also characterized via ac impedance measurements. These measurements indicated that the oxidation of Al on Co occurs in three discrete steps.

Interface chemistry and structure of multiply oxidized barrier spin-tunnel junctions

Nb/Fe/Al 2 O 3 /CoFe/Nb spin-tunnel junctions fabricated using a multiple oxidation technique have been characterized using high spatial resolution electron microscopy techniques. Junction magnetoresistance values up to 6.2% at room temperature and 9.2% at 77 K were obtained for junctions fabricated using this technique. Energy dispersive x-ray spectroscopy and electron energy loss spectroscopy were used to study the chemistry and interface structure of the barrier layer; elemental mapping showed the degree of chemical homogeneity across the layers and high spatial resolution electron energy loss spectroscopy revealed changes in the oxidation state and d-shell occupancies of Fe and Co across the layers, which need to be considered when modelling the spin-tunneling effect. Pinholes across the barrier were also observed by high resolution electron microscopy.

High thermal stability tunnel junctions

The thermal stability of CoFe/Al2O3/CoFe/MnIr spin-tunnel junctions fabricated by ion beam, using two oxidation methods is studied for annealing temperatures up to 450 °C. Tunnel magnetoresistance (TMR) is 40% after annealing at 300 °C, and a TMR of 15% is still achieved after annealing at 380 °C. The TMR decay with anneal depends on the oxidation process (O2 beam or remote plasma) and on the thickness of the MnIr exchange layer. Overoxidation of the Al, or higher kinetic energy of the O2 ions, during oxidation, lead to faster thermal degradation of the TMR. Thinner MnIr retards thermal degradation. Rutherford backscattering shows a strong interdiffusion of Mn into the CoFe electrode above 300 °C, but no significant changes are detected at the CoFe/Al2O3/CoFe interfaces, even after annealing at 435 °C (the junction resistance remains high confirming barrier stability). The pinned layer moment decreases upon annealing as expected from Mn incorporation. Also, the strong TMR decrease may indicate polarization loss when Mn reaches the Al2O3/CoFe interface. At 410 °C, the TMR is still 5%, with the fast switching characteristics of the free layer unchanged.

Investigation of an 18 Å Al 2O 3 layer in a magnetic multilayer system by Rutherford and resonant scattering

Spin tunnel junctions, consisting of a metallic multilayer with a thin insulating buffer layer, in most cases Al 2O 3, show a large tunneling magnetoresistance (TMR) and are therefore of high interest for future magnetic read heads and non-volatile memories. The properties of these junctions are strongly influenced by the quality of the insulating buffer layer and its behavior during thermal treatment. Measurements with conventional RBS using devices in the as-deposited state and after different heat treatments at temperatures up to 200°C revealed changes in the stoichiometry and O distribution in the buffer layer that can be correlated with changes in the respective magnetic properties. The application of the 16 O( 4 He, 4 He) 16 O resonant scattering resonance with incident energies ranging from 3.01 to 3.35 MeV enabled the separate profiling of O in all parts of the sample: the partly oxidized sample surface, the aluminum oxide layer and the silicon dioxide layer covering the Si substrate. Simulations of the energy dependence of the O scattering yield showed that the oxygen is fully retained in the aluminum oxide buffer layer and no diffusion into adjacent metallic layers has occurred. The results indicate also that even Al 2O 3 layers with significantly lower thicknesses can be profiled with this method.

Strongly reduced bias dependence in spin–tunnel junctions obtained by ultraviolet light assisted oxidation

For future implementation of ferromagnetic tunnel junctions, we need a better understanding of the influence of the insulating barrier preparation method on the junction resistance, tunnel magnetoresistance (TMR), and its voltage bias dependence. In this letter, we focus on the bias dependence of junctions (Co–Al2O3–Ni80Fe20) prepared by ultraviolet light assisted in situ oxidation in an O2 ambient. For an initial Al thickness of 1.3 nm, the resistance times area product of the junctions is 60 kΩ μm2, while showing up to 20% TMR at 5 mV bias. The decrease of TMR with bias voltage up to 1 V is remarkably small leading to V1/2, for which half of the low-bias TMR remains, well over 0.6 V.

Spin-tunnel-junction thermal stability and interface interdiffusion above 300 °C

Spin tunnel junctions (CoFe/Al2O3/CoFe/MnIr) were fabricated with tunneling magnetoresistance (TMR) of 39%–41% after anneal at 300 °C, decreasing to 4%–6% after anneal at 410 °C. Junction resistance decreases from (0.8–1.6) to (0.5–0.8) M Ω μm2 during anneal. The pinned-layer moment decreases by 44% after anneal at 435 °C, but the free-layer moment does not change. The current–voltage characteristics change significantly and become asymmetric above 300 °C. Rutherford backscattering analysis (RBS) shows that above 300 °C, strong interdiffusion starts at the CoFe/MnIr interface with Mn moving into CoFe, causing the electrode moment to decrease. Mn eventually reaches the Al2O3/CoFe interface contributing to the TMR decrease. RBS analysis of a separate CoFe/Al2O3/CoFe structure shows only minor structural changes at the CoFe/Al2O3 interfaces after anneal at 435 °C, possibly leading to a second mechanism for the loss of interface polarization and TMR.

Storage mechanism of spin-tunneling junctions

This paper concerns a microstructured spin-tunneling junction in which the minor loop's slope has a dependency on its past magnetic histories. The above storage mechanism inspired the authors to design and fabricate a new type of solid state spin-tunneling random access memory. This memory operates on the general principle of storing a binary datum in the `hard’ magnetic component and sensing its remanent state by switching the `soft’ magnetic component in such a way that the magnetic state of the `hard’ component remains unaltered.