Electrodes In Magnetic Tunnel Junctions Research Articles

Controlling the magnetic alignment at the MnGa/Co2MnSi interface: A DFT study

The MnGa/Co2MnSi is a common electrode in Magnetic Tunnel Junctions (MTJ). We study the MnGa/Co2MnSi interfaces using spin-polarized Density Functional Theory. We have focused on the interface stability and the effect of the interface arrangements -at the atomic scale- on its electronic and magnetic properties. Depending on how the atomic species interact at the interface, the MnGa/Co2MnSi electrode can show either antiferromagnetic or ferromagnetic behavior, modulating the spin transfer through the system. Antiferromagnetic ordering appears due to Mn-Mn interlayer interactions across the MnSi-Mn interface and is thermodynamically stable for Co-poor and Mn-rich growth conditions. Mn atoms have magnetic moments of −2.97 μB and 2.31 μB. On the other hand, a ferromagnetic behavior could be engineered under Co-rich and Mn-poor conditions, as demonstrated by the interface formation energy analysis. Such magnetic ordering emerges at the Co-Ga interface since direct Mn-Mn interlayer interactions disappear; Mn atoms have magnetic moments of ∼ 3.00 μB and ∼ 2.8 μB for the Co2MnSi and MnGa films, respectively. Our findings lay the foundations to guide experimental efforts in designing electrodes that could potentially generate efficient tunneling behavior in perpendicular Magnetic Tunnel Junctions, which are the heart of novel MRAMs.

Sensitive metallic behavior in epitaxial NiCo2O4 films regulated by the film thickness

The transition metal oxide NiCo2O4 has aroused extensive research interest due to its metallic conductivity, but the precise nature of the conductivity remains elusive. In this study, the dependence of the metallic behavior in NiCo2O4 epitaxial films on temperature and film thickness are investigated for film thicknesses in the range of 7–155 nm. Our results show that a subtle lattice distortion caused by the substrate is necessary for metallic behavior in NiCo2O4 films. The widest temperature window for the metallic behavior is obtained for a 31-nm thick NiCo2O4 film. The NiCo2O4 films thinner than 31 nm are quasi-two-dimensional, and are affected by substrate clamping. In these films, conduction is constrained by electron–electron and electron–magnon scattering. However, NiCo2O4 films thicker than 31 nm are three-dimensional, exhibiting long-range structural order, with electrical transport constrained mainly by electron–phonon scattering, as well as electron–electron and electron–magnon scattering. Our study demonstrates that the metallic behavior in NiCo2O4 films can be tuned using the film thickness, which is of great significance for application of NiCo2O4 in development of alternative ferroelectric metal electrodes in magnetic tunnel junctions.

Tunnel magnetoresistance in magnetic tunnel junctions with FeAlSi electrode

(001)-oriented FeAlSi polycrystalline thin films with a flat surface and B2-ordered structure were grown on thermally oxidized SiO2 substrates using MgO buffer layers. The FeAlSi thin films composition-adjusted to the Sendust alloy exhibited a low coercivity (Hc) after the annealing process. We utilized these films as bottom electrodes of magnetic tunnel junctions (MTJs) and characterized their tunnel magnetoresistance (TMR) effect. The TMR effect was 35.9% at room temperature. In addition, the TMR ratio increased to 51.0% when a thin CoFeB layer was inserted into the FeAlSi/MgO interface, without degrading the small switching field of the FeAlSi electrode. These MTJs with a small switching field and relatively high TMR ratio using the FeAlSi electrode are promising for highly sensitive MTJ-based magnetic sensor devices.

Reduced Exchange Interactions in Magnetic Tunnel Junction Free Layers with Insertion Layers

Perpendicularly magnetized CoFeB layers with ultra-thin non-magnetic insertion layers are very widely used as the electrodes in magnetic tunnel junctions for spin transfer magnetic random access memory devices. Exchange interactions play a critical role in determining the thermal stability of magnetic states in such devices and their spin torque switching efficiency. Here the exchange constant of free layers incorporated in full magnetic tunnel junction layer stacks, specifically CoFeB free layers with W insertion layers is determined by magnetization measurements in a broad temperature range. A significant finding is that the exchange constant decreases significantly and abruptly with W insertion layer thickness. The perpendicular magnetic anisotropy shows the opposite trend; it initially increases with W insertion layer thickness and shows a broad maximum for approximately one monolayer (0.3 nm) of W. These results highlight the interdependencies of magnetic characteristics required to optimize the performance of magnetic tunnel junction devices.

Properties of sputtered full Heusler alloy Cr2MnSb and its application in a magnetic tunnel junction

We deposit 30 nm thick Cr2MnSb films on a MgO(0 0 1) substrate by magnetron sputtering. X-ray diffraction reveals that the annealing promotes the crystallization of Cr2MnSb from amorphous at as-deposited state to partially crystallized phase, and that the crystallinity improves by raising the annealing temperature. The crystallized phase exhibits full B2 and partial L21 ordering after the annealing above 300 °C. The magnetic and magneto-transport properties reveal that the annealed Cr2MnSb shows magnetic ordering up to ~350 K with small magnetic moment, 0.3µB/formula unit for the sample annealed at 300 °C. We adopt Cr2MnSb as one of the electrodes in magnetic tunnel junctions (MTJs), and observe the switching of junction resistance, when the magnetization direction is reversed by sweeping an external magnetic field.

Spin injection and pumping generated by a direct current flowing through a magnetic tunnel junction

A charge flow through a magnetic tunnel junction (MTJ) leads to the generation of a spin-polarized current which exerts a spin-transfer torque (STT) on the magnetization. When the density of applied direct current exceeds some critical value, the STT excites high-frequency magnetization precession in the "free" electrode of MTJ. Such precession gives rise to microwave output voltage and, furthermore, can be employed for spin pumping into adjacent normal metal or semiconductor. Here we describe theoretically the spin dynamics and charge transport in the CoFeB/MgO/CoFeB/Au tunneling heterostructure connected to a constant-current source. The magnetization dynamics in the free CoFeB layer with weak perpendicular anisotropy is calculated by numerical integration of the Landau-Lifshitz-Gilbert-Slonczewski equation accounting for both STT and voltage controlled magnetic anisotropy associated with the CoFeB|MgO interface. It is shown that a large-angle magnetization precession, resulting from electrically induced dynamic spin reorientation transition, can be generated in a certain range of relatively low current densities. An oscillating spin current, which is pumped into the Au overlayer owing to such precession, is then evaluated together with the injected spin current. Considering both the driving spin-polarized charge current and the pumped spin current, we also describe the charge transport in the CoFeB/Au bilayer with the account of anomalous and inverse spin Hall effects. An electric potential difference between the lateral sides of the CoFeB/Au bilayer is calculated as a function of distance from the CoFeB|MgO interface. It is found that this transverse voltage signal in Au is large enough for experimental detection, which indicates significant efficiency of the proposed current-driven spin injector.

Nanoscale Tantalum layer impacting magnetic properties of tunnel junction-based molecular devices

Abstract

Tunneling Anisotropic Magnetoresistance in Fe Nanoparticles Embedded in MgO Matrix

The tunnel magnetoresistance (TMR) effect is related to the relative orientation of the magnetizations of the two ferromagnetic electrodes in magnetic tunnel junctions (MTJs). The tunnel anisotropic magnetoresistance (TAMR) effect is related to the orientation of the magnetization with respect to the current direction or the crystallographic axes. Beyond the TMR, the TAMR is not only present in MTJs in which both electrodes are ferromagnetic but may also appear in tunnel structures with a single magnetic electrode. We investigated the magnetotransport properties in an Au/MgO/Fe nanoparticles/MgO/Cu tunnel junction. We found that both the TMR and TAMR can appear in tunnel junctions with Fe nanoparticles embedded in an MgO matrix. The TMR is attributed to spin-dependent tunneling between Fe nanoparticles, so the device resistance depends on the magnetization directions of adjacent Fe nanoparticles. The TAMR is attributed to the interfacial spin–orbit interaction, so the device resistance depends on each magnetization direction of an Fe nanoparticle. This is the first observation of the TAMR in Fe nanoparticles embedded in an MgO matrix.

Titanium nitride as a seed layer for Heusler compounds

Titanium nitride (TiN) shows low resistivity at room temperature (27 μΩ cm), high thermal stability and thus has the potential to serve as seed layer in magnetic tunnel junctions. High quality TiN thin films with regard to the crystallographic and electrical properties were grown and characterized by x-ray diffraction and 4-terminal transport measurements. Element specific x-ray absorption spectroscopy revealed pure TiN inside the thin films. To investigate the influence of a TiN seed layer on a ferro(i)magnetic bottom electrode in magnetic tunnel junctions, an out-of-plane magnetized Mn2.45Ga as well as in- and out-of-plane magnetized Co2FeAl thin films were deposited on a TiN buffer, respectively. The magnetic properties were investigated using a superconducting quantum interference device and anomalous Hall effect for Mn2.45Ga. Magneto optical Kerr effect measurements were carried out to investigate the magnetic properties of Co2FeAl. TiN buffered Mn2.45Ga thin films showed higher coercivity and squareness ratio compared to unbuffered samples. The Heusler compound Co2FeAl showed already good crystallinity when grown at room temperature on a TiN seed-layer.

STATE OFCoANDMnIN HALF-METALLIC FERROMAGNETCo2MnSiEXPLORED BY MAGNETIC CIRCULAR DICHROISM IN HARD X-RAY PHOTOELECTRON EMISSION AND SOFT X-RAY ABSORPTION SPECTROSCOPIES

The half-metallic Heusler compound Co2MnSi is a very attractive material for spintronic devices because it exhibits very high tunnelling magnetoresistance ratios. This work reports on a spectroscopic investigation of thin Co2MnSi films as they are used as electrodes in magnetic tunnel junctions. The investigated films exhibit a remanent in-plane magnetization with a magnetic moment of about 5 μBwhen saturated, as expected. The low coercive field of only 4 mT indicates soft magnetic behavior. Magnetic dichroism in emission and absorption was measured at the Co and Mn 2p core levels. The photoelectron spectra were excited by circularly polarized hard X-rays with an energy of 6 keV and taken from the remanently magnetized film. The soft X-ray absorption spectra were taken in an induction field of 4 T. Both methods yielded large dichroism effects. An analysis reveals the localized character of the electrons and magnetic moments attributed to the Mn atoms, whereas the electrons related to the Co atoms contribute an itinerant part to the total magnetic moment.

A Density Functional Theory Study of the Fe(001)/Fe<sub>3</sub>O<sub>4</sub>(001) Interface

Magnetite is a very promising candidate for hard electrode in magnetic tunnel junction devices due to its electronic and magnetic properties. Addition of a buffer iron layer improves required properties. Here we report on the results of our DFT study of the impact of the additional Fe buffer layer on the electronic and magnetic properties of magnetite.

Co/Ni multilayers with perpendicular anisotropy for spintronic device applications

This letter presents a study of perpendicular anisotropy in Co/Ni multilayers, which could constitute a thick polarizer in spin torque oscillators or a magnetic electrode in magnetic tunnel junctions (MTJ) with perpendicular anisotropy. Perfectly square perpendicular loops are observed for as-deposited Co/Ni multilayers with various sublayer thicknesses and bilayer repetition numbers using a Pt buffer layer. An anisotropy energy of 1.0 × 106 erg·cm−3 is obtained for 9 nm thick Co/Ni multilayers. For Co/Ni multilayers deposited on MgO, no perpendicular magnetization component is observed in the as-deposited state, but it develops (even in 2.1 nm Co/Ni multilayers) after annealing at 250 °C.

Effect of oxidizing the ferromagnetic electrode in magnetic tunnel junctions on tunneling magnetoresistance

The ferromagnetic layer in magnetic tunnel junctions (MTJs) was oxidized with varying O2 concentrations, and the corresponding effect on spin-dependent transport was studied. As expected from our previous results for MTJs with an over-oxidized AlOx tunnel barrier, a partially oxidized ferromagnetic layer plays an important role in spin-dependent transport. As the temperature is lowered, the junction resistance increases dramatically, and the tunneling magnetoresistance (TMR) is strongly suppressed. Increasing the O2 concentration enhances the increase of resistance and suppression of TMR. This work supports our previous conclusion that oxidizing the ferromagnetic layer generates localized magnetic moments, which act as a scattering center for spin-polarized tunneling electrons.

Negative Tunneling Magnetoresistance by Canted Magnetization inMgO/NiOTunnel Barriers

The influence of the insertion of an ultrathin NiO layer between the MgO barrier and the ferromagnetic electrodes in magnetic tunnel junctions has been investigated from measurements of the tunneling magnetoresistance and via x-ray magnetic circular dichroism (XMCD). The magnetoresistance shows a high asymmetry with respect to bias voltage, giving rise to a negative value of up to -16% at 2.8 K. We attribute this effect to the formation of noncollinear spin structures at the interface of the NiO layer as inferred from XMCD measurements. The magnetic moments of the interface Ni atoms tilt from their easy axis due to exchange coupling with the neighboring ferromagnetic electrode, and the tilting angle decreases with increasing NiO thickness. The experimental observations are further supported by noncollinear spin density functional calculations.

All nitride magnetic tunnel junctions based on nitrogen-doped CoFe electrode and AlNx insulating barrier

The dependence of the structural and magnetic properties of nitrogen-doped CoFe (CoFeN) films as a function of the nitrogen gas flow rate was analyzed to investigate its potential use as ferromagnetic electrodes in magnetic tunnel junctions (MTJs). The addition of a small amount of nitrogen into the CoFe film was strongly associated with the formation of low coercivity and low magnetization in the CoFeN film. We also discuss the electrical and microstructural properties of the as-grown and postannealed CoFeN/AlN/CoFeN MTJs devices developed by using only nitride materials of CoFeN electrodes and AlNx tunneling barriers.

Pt/Co/oxide and oxide/Co/Pt electrodes for perpendicular magnetic tunnel junctions

This letter presents a study of perpendicular magnetic anisotropy in oxide/Co/Pt structures, which could constitute the upper magnetic electrode of magnetic tunnel junctions. The growth of cobalt layers on SiO2 substrates shows that all 0.6 nm thick Co films are superparamagnetic, whereas perpendicular magnetic anisotropy is obtained for 1.5 nm films after annealing. Co layers grown on various Al and Mg oxides prepared by sputtering also exhibit perpendicular magnetic anisotropy after annealing. Combined with inverse Pt/Co(CoFeB)/oxide stackings, these structures allow preparing tunnel junctions with thicker magnetic electrodes and much better thermal stability than those based on standard Pt/Co multilayers.

Influence of ion bombardment induced patterning of exchange bias in pinned artificial ferrimagnets on the interlayer exchange coupling

Artificial ferrimagnets have many applications as, e.g., pinned reference electrodes in magnetic tunnel junctions. It is known that the application of ion bombardment (IB) induced patterning of the exchange bias coupling of a single layer reference electrode in magnetic tunnel junctions with He ions is possible. For applications as, e.g., special types of magnetic logic, a combination of the IB induced patterning of the exchange bias coupling and the implementation of an artificial ferrimagnet as reference electrode is desirable. Here, investigations for a pinned artificial ferrimagnet with a Ru interlayer, which is frequently used in magnetic tunnel junctions, are presented. It is shown that in this kind of samples the exchange bias can be increased or rotated by IB induced magnetic patterning with 10keV He ions without a destruction of the antiferromagnetic interlayer exchange coupling. An IrMn∕Py∕Co∕Cu∕Co stack turned out to be more sensitive to the influence of IB than the Ru based artificial ferrimagnet.

Origin of the dependence of magnetoresistance on the composition of Co100−xFex electrodes in magnetic tunnel junctions

The tunneling magnetoresistance value of a Co100−xFex (4nm)∕AlOx 1.7nm∕Co100−xFex (4nm) magnetic tunnel junction has been demonstrated to depend on the composition of the Co100−xFex electrodes. The interface roughness, crystal structure, and tunneling spin polarization versus the composition of the Co100−xFex electrode were studied to address the origin of this compositional dependence. Ab initio calculations of s-like electron spin polarization predict a composition dependence similar to that observed experimentally. The combined experimental and computational results show that the trends in Co100−xFex tunneling magnetoresistance are modified slightly by the interface roughness but mainly determined by the s-like electron spin polarization values associated with different compositions and crystal structures.

Sign of tunneling magnetoresistance in CrO2-based magnetic tunnel junctions

Half-metallic ferromagnets are potentially useful as electrodes in magnetic tunnel junctions (MTJs) because of the possibility for 100% spin polarization near the Fermi energy. We report tunneling magnetoresistance (TMR) behavior of MTJs having epitaxial CrO2(100) as one electrode, CoFe as second electrode, and MgO as tunnel barrier, i.e., TiO2(100)∕CrO2(100)∕MgO∕CoFe. When CrO2 is exposed to air, several monolayers of oxide surface decompose and negative MR is observed. Conversely, sample surfaces protected by thin Mg layers show small but positive MR consistent with intrinsic spin polarization. Thus, the TMR sign can be reversed simply by subtle modifications to the CrO2∕MgO interface.

Double Perovskites with Ferromagnetism Above Room Temperature

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