Fabrication Of Magnetic Tunnel Junctions Research Articles

The role of structure on magneto-transport properties of Heusler Co2MnSi films deposited on MgO(001)

We present an experimental study identifying structural reasons that degrade spin-polarization of Co2MnSi thin films deposited on MgO(001) substrates. Through the fabrication of magnetic tunnel junctions, we measure a range of values for tunneling magneto-resistance (TMR) ratios following post-deposition annealing and epitaxial crystallization of the Heusler film. These TMR ratios reflect qualitatively the change in spin polarization of the Co2MnSi thin films. Low-temperature annealing results in low spin-polarization due to a high fraction of an amorphous phase. As annealing temperatures increase, the fraction of L21 and B2 chemically ordered phases increases, thus improving significantly the spin-polarization. However, for samples annealed at higher temperatures, significant degradation in the cubic magneto-crystalline anisotropy is observed, which we attribute to the detection of manganese diffusion into the MgO substrate. This Mn diffusion is manifested in a reduction of the value of the TMR ratio, namely, the spin polarization. Additionally, the maximum TMR ratio measured here, approximately 65% at room-temperature, is limited because the semi-coherent interface of Co2MnSi with the MgO substrate terminates with a Mn-Si layer.

Fabrication of magnetic tunnel junctions with a bottom synthetic antiferro-coupled free layers for high sensitive magnetic field sensor devices

Magnetic tunnel junctions with a Ni80Fe20/Ru/Co40Fe40B20 synthetic antiferro-coupled bottom free layer and an MgO barrier layer have been fabricated. Double annealing process was carried out in order to obtain linearity against magnetic field with hysteresis-free resistance response. The effect of the annealing temperature and NiFe thickness in the free layer on the magnetic field sensor performance was investigated. We have observed a very high sensitivity of 25.3%/Oe while keeping linearity.

Fabrication of Magnetic Tunnel Junctions with a Synthetic Ferrimagnetic Free Layer for Magnetic Field Sensor Applications

Magnetic tunnel junctions (MTJs) with a CoFeB/Ru/Ni80Fe20 synthetic ferrimagnetic free layer and an MgO barrier layer were fabricated. The effect of the shape and thickness of the free layer on the magnetic field sensor characteristics was systematically investigated. We achieved a high sensitivity of 4.8%/Oe in the MTJ with a 70-nm-thick Ni80Fe20 layer and an aspect ratio of 1.0. Here, sensitivity is defined as TMR/(2Hk), where TMR is tunnel magnetoresistance ratio in the MTJ and Hk is a magnetic anisotropy field of the free layer. Furthermore, we successfully increased the detection field range up to 230 Oe while keeping high sensitivity and linearity.

Fabrication of Magnetic Tunnel Junctions with a Synthetic Ferrimagnetic Free Layer for Magnetic Field Sensor Applications

Fabrication of Magnetic Tunnel Junctions with a Synthetic Ferrimagnetic Free Layer for Magnetic Field Sensor Applications

Charge transport and magnetic ordering in laser ablated Co2FeSi thin films epitaxially grown on (1 0 0) SrTiO3

We have successfully deposited epitaxial thin films of the Heusler alloy Co2FeSi on (1 0 0) cut SrTiO3 single crystal substrates by pulsed laser deposition. X-ray diffraction reveals the L21 cubic crystal structure for the films grown at 400 and 600 °C, with a better crystallographic quality at the higher growth temperature. The magnetic moment in saturation of the 600 °C grown film is 5.0 µB/fu at 5 K with the easy axis of magnetization in the (1 0 0) plane. Inplane hysteresis measurements suggest a soft magnetic state with a coercive field HC ≈ 250 Oe at 5 K for the 600 °C film which decreases on lowering the growth temperature. The normalized magnetization of thin films follows the well-known Bloch T3/2 law due to spin wave excitation. Electrical resistivity measurements on the films with the L21 crystal structure show a pronounced metallic behaviour down to the lowest temperature of measurement (≈5 K), and enhanced metallicity in the 600 °C film as indicated by its lower residual resistivity (≈0.16 µΩ cm) and higher residual resistivity ratio (RRR ≈ 1280). These results suggest that films grown at the higher temperature may be better suited for the fabrication of magnetic tunnel junctions.

Magnetoresistance of perpendicularly magnetized tunnel junction using L10-CoNiPt with low saturation magnetization

Investigations of the structural and magnetic properties of thin Co50-xNixPt50 (x = 0, 10, 15, 37.5) films and fabrication of magnetic tunnel junctions (MTJs) using Co50Pt50 and Co35Ni15Pt50 electrodes were performed. X-ray diffraction analyses revealed that 20-nm-thick CoPt and CoNiPt films were epitaxially grown with (001)-orientation with an L10-chemical order parameter of 0.66–0.82. CoNiPt with various Ni contents magnetized perpendicularly; the saturation magnetization reduced to 157 emu/cm3 when the Ni content was increased to 37.5%. Magnetotransport measurements under a magnetic field applied perpendicular to the film plane revealed a tunnel magnetoresistance ratio of 10% and 1% at 10 K and 300 K, respectively, for MTJ using Co35Ni15Pt50 electrodes.

Study of boron diffusion in MgO in CoFeB∣MgO film stacks using parallel electron energy loss spectroscopy

Boron diffusion in MgO has been investigated in annealed film stacks of sputtered CoFeB∣MgO using transmission electron microscopy and parallel electron energy loss spectroscopy. The analyses show significant B movement when the films are annealed, with the formation of BOx complexes. Characteristic diffusion lengths have been estimated in films annealed at the commonly employed temperature range of 300–400°C for the fabrication of magnetic tunnel junctions. An activation energy of 1.3eV (±0.4eV) has been extracted from these data that represent B diffusion in MgO through vacancies and defect states mediated by the formation of BOx complexes.

A magnetometry study of Co oxidation in Co/MgO bilayers grown by sputtering

One of the critical challenges during fabrication of magnetic tunnel junctions is to avoid the magnetic bottom electrode oxidation, considered as a spin depolarization source that lowers the tunneling magnetoresistance effect. In this paper, we present a study of the magnetic properties of Co/MgO bilayers as a function of several sputtering growth parameters by using a superconducting quantum interference device magnetometer. We demonstrate the high sensitivity of the magnetic properties to the oxidation of the Co layer and identify the relevant parameters to optimize the MgO growth process.

Enhanced antiferromagnetic saturation in amorphous CoFeB-Ru-CoFeB synthetic antiferromagnets by ion-beam assisted deposition

The interlayer coupling of CoFeB-based synthetic antiferromagnets (SyAFs), modulated by the ion-beam assisted deposition (IBAD) has been systematically investigated under different assisted deposition voltage from 0 to 140 V. We observe that proper IBAD voltage can significantly enhance the antiferromagnetically coupled saturation field from 280 to 1000 Oe and retain the amorphous structure of CoFeB layers. This approach provides a convenient method to enhance the magnetic coupling of SyAFs, which is useful for the magnetic tunnel junctions fabrication and magnetoresistive random access memory development.

Suppression of orange-peel coupling in magnetic tunnel junctions by preoxidation

We have found that preoxidation of the bottom Co electrode in magnetic tunnel junctions (MTJs) very effectively suppresses orange-peel coupling. The result is a free layer that is much softer. Work by others has demonstrated that preoxidation is compatible with high-quality MTJ fabrication.

30-nm scale fabrication of magnetic tunnel junctions using EB assisted CVD hard masks

30-nm scale fabrication of magnetic tunnel junctions (MTJs) was demonstrated. A scanning electron microscope (SEM) was used for chemical-vapor deposition (CVD) of carbon hard masks. Using electron beam (EB)-CVD, less than several 10-nm scale carbon pillar could be formed on MTJ films. Argon ion milling, of which incident angle from the normal of the film plane was determined 45/spl deg/ and 75/spl deg/, was utilized to pattern the MTJs. TMR properties of 80-nm scale MTJs were successfully measured using DC-four-probe at room temperature.

Oxidation of tunnel barrier metals in magnetic tunnel junctions

The oxidation of an ultrathin metal layer (<1nm) to form an oxide tunnel barrier is of critical importance for the fabrication of magnetic tunnel junctions (MTJs) with low product of resistance and area (R×A). Nonuniform and excessive or insufficient oxidation will occur by using conventional plasma, air, or O2 and noble gas mixtures as oxidation methods. An oxidation method was investigated to oxidize only an ultrathin layer of metal (such as Y) without oxidizing adjacent ferromagnetic thin film layers. We have now demonstrated that a gas mixture of H2O∕H2 with a fixed chemical potential of oxygen determined by the relative amounts of the two gases can oxidize Y and Ta thin layers while simultaneously keeping a Co ferromagnetic layer completely unoxidized. This universal method can be used to preferentially oxidize a host of other metals with high tendency to form oxides, such as Zr, Hf, Nb, rare earth metals, etc. and may allow us to access the feasible lower limit of barrier thickness in MTJs.

Change of the barrier potential shape in magnetic tunnel junctions due to an anneal treatment

A very important process step in the fabrication of magnetic tunnel junctions (MTJs) is the application of a modest anneal step in the presence of a high magnetic field. Roughly, a doubling of the magnetoresistance (MR) ratio is commonly observed. We show that both AlOx as well as TaOx MTJs with Co90Fe10 electrodes have similar oxidation time and anneal temperature dependencies of the MR ratios. In both cases, the maximum MR ratio shifts to higher oxidation times with annealing. TaOx MTJs are, in this sense, good model systems. From photoconductance experiments we find that for TaOx MTJs, this shift in maximum MR is accompanied by a similar shift of the zero crossing of the oxidation time dependent barrier asymmetry. This directly supports the point of view that for obtaining the highest MR ratio one should anneal MTJs that would be characterized as “slightly overoxidized” in the as-deposited state. We argue that this result can be understood by a homogenization of the oxygen distribution in the barrier, and∕or a change of the bottom barrier-electrode interface.

Growth and properties of high-Curie-temperature Sr2CrReO6 thin films

Thin films of ordered double-perovskite Sr2CrReO6 have been grown epitaxially onto SrTiO3 substrates by sputtering, and their microstructural, electrical, and magnetic properties are investigated. It is found that a wide growth-temperature range of at least 700–800°C can yield epitaxial films with superior magnetic properties and atomically flat surfaces. These films exhibit Curie temperatures Tc of up to 620K, and saturation magnetizations Ms of 0.9μB∕formula unit (f.u.), which is comparable to the predicted value of 1μB∕f.u. for the fully ordered half-metallic material. Owing to the wide temperature window for growth of high-quality films, the surface morphology and coercivity of Sr2CrReO6 films are highly controllable, which is favorable for fabrication of magnetic tunnel junctions.

Study of Longitudinal Stabilization Using In-Stack Biasing

The in-stack stabilization of unshielded and shielded magnetic tunnel junction (MTJ) sensors have been studied experimentally by quasi-static and recording tests as well as theoretically by micromagnetic modeling. Results showed the viability of in-stack stabilization over a range of design and material conditions. Performance tradeoff studies and design optimization results led to the fabrication of MTJ read heads with good magnetic stability and high readback sensitivity.