MgO Barrier Research Articles

Voltage Control of Magnetization and Its Chemical Properties

Magnetization control using an electric field [1] will be useful because of its expected ultra-low power consumption and coherent behavior. Several experimental approaches to realize it have been done using ferromagnetic semiconductors [2], materials with magnetostriction together with piezo-driver [3], multiferroic materials [4], ferromagnetic metal films sintered in a liquid electrolyte [5], and ultra-thin ferromagnetic layer in solid-state junctions[6-8].One of the critical issues in the electric field switching is a realization of bi-stable switching. Since the electric field does not break time reversal symmetry, it does not remove degeneracy of two magnetic states with opposite magnetization. Therefore, a selection of an arbitral magnetic state is not straightforward [9]. Here, we demonstrate a realization of the bi-stable switching using a coherent precessional magnetization toggle switching in nanoscale magnetic cells with a few atomic FeCo (001) epitaxial layers adjacent to MgO barrier [10].The control of the magnetization by voltage requires well controlled interface structure. We have investigated chemical states of the ferromagnetic atoms at interface with MgO using XAS and XMCD, and found several problems that may happen at the interfaces in voltage devices, i.e. segregation [11] and voltage controlled reversible oxidization/reduction [12]. We discuss the significance of those effects and the intrinsic effect.

Compact model of magnetic tunnel junction with stochastic spin transfer torque switching for reliability analyses

Spin transfer torque magnetic tunnel junction (STT MTJ) is considered as a promising candidate for non-volatile memories thanks to its low power, high speed and easy integration with CMOS process. However, it has been demonstrated intrinsically stochastic. This phenomenon leads to the frequent occurrence of switching errors, which results in considerable reliability issues of hybrid CMOS/MTJ circuits. This paper proposes a compact model of MTJ with STT stochastic behavior, in which technical variations and temperature evaluation are properly integrated. Moreover, the phenomenon of dielectric breakdown of MgO barrier which determines the lifetime of MTJ is also taken into consideration. Its accurate performances allow a more realistic reliability analysis involving the influences of ambient environment and technical process.

Leak current estimated from the shot noise in magnetic tunneling junctions

We performed the shot noise measurement in epitaxial Fe/MgO/Fe-based magnetic tunneling junctions (MTJs) with various MgO thicknesses between 1.1 nm and 1.625 nm. While the Fano factor to characterize the shot noise is very close to 1 in MTJs with MgO barrier thicker than 1.2 nm, the magnetic configuration-dependent reduction of the Fano factor for MTJs with thin MgO barrier was observed, which is mainly due to the existence of leakage current. By using a simple parallel circuit model, we demonstrated that the contribution of the leak current can be sensitively derived from the shot noise.

Electrical spin injection from ferromagnet into an InAs quantum well through a MgO tunnel barrier

The electrical spin injection from Ni81Fe19 (NiFe) into an InAs quantum well through a MgO tunneling barrier has been investigated for potential application to an InAs-based spin field-effect transistor. The insertion of a 2-nm-thick MgO tunnel barrier between NiFe and InAs increased the junction resistance by two orders of magnitude compared with that without a MgO barrier. The sample with a MgO barrier showed a clear nonlocal spin-valve signal at 1.4 K, possibly due to the alleviation of the impedance mismatching problem. The estimated spin polarization was 8.1%, which is higher than any reported in the literature for a NiFe/InGaAs Schottky junction.

Interfacial electronic transport phenomena in single crystalline Fe-MgO-Fe thin barrier junctions

Spin filtering effects in nano-pillars of Fe-MgO-Fe single crystalline magnetic tunnel junctions are explored with two different sample architectures and thin MgO barriers (thickness: 3–8 monolayers). The two architectures, with different growth and annealing conditions of the bottom electrode, allow tuning the quality of the bottom Fe/MgO interface. As a result, an interfacial resonance states (IRS) is observed or not depending on this interface quality. The IRS contribution, observed by spin polarized tunnel spectroscopy, is analyzed as a function of the MgO barrier thickness. Our experimental findings agree with theoretical predictions concerning the symmetry of the low energy (0.2 eV) interfacial resonance states: a mixture of Δ1-like and Δ5-like symmetries.

Inelastic tunneling conductance and magnetoresistance investigations in dual ion-beam sputtered CoFeB(110)/MgO/CoFeB (110) magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) comprising Ta(5)/NiFe(5)/IrMn(15)/CoFeB(5)/Mg(1)/MgO(3.5)/ CoFeB(5)/Ta(5)/Ag(20) (thickness in nm) with (110) oriented CoFeB layers are grown using dual ion beam sputtering. The tunnel magnetoresistance (TMR) of MTJs is found to be significantly bias dependent and exhibits zero bias anomaly (ZBA) which is attributed to the presence of magnetic impurities or diffusion of Mn from antiferromagnetic IrMn in the barrier. Adjacent to the ZBA, two peaks at 24 ± 3 mV and 34 ± 3 mV are also observed, which differ both in intensity as well as their position in the antiparallel and parallel magnetic states, suggesting that they are due to magnon excitations. In addition to this, a phonon peak at 65 ± 3 mV is also observed. The effect of temperature on the inelastic and elastic tunneling contributions is studied in detail in 25–300 K range using the Glazman and Matveev model. Ten series of localized states are found to be involved in hopping conduction in the forbidden gap of MgO barrier. The effect of presence of such inelastic channels is found to be insignificant at low temperatures yielding sizeable enhancement in TMR.

Magnetotransport in MgO-based magnetic tunnel junctions grown by molecular beam epitaxy (invited)

The strong impact of molecular beam epitaxy growth and Synchrotron Radiation characterization tools in the understanding of fundamental issues in nanomagnetism and spintronics is illustrated through the example of fully epitaxial MgO-based Magnetic Tunnel Junctions (MTJs). If ab initio calculations predict very high tunnel magnetoresistance (TMR) in such devices, some discrepancy between theory and experiments still exists. The influence of imperfections in real systems has thus to be considered like surface contaminations, structural defects, unexpected electronic states, etc. The influence of possible oxygen contamination at the Fe/MgO(001) interface is thus studied, and is shown to be not so detrimental to TMR as predicted by ab initio calculations. On the contrary, the decrease of dislocations density in the MgO barrier of MTJs using Fe1−xVx electrodes is shown to significantly increase TMR. Finally, unexpected transport properties in Fe1−XCox/MgO/Fe1−XCox (001) are presented. With the help of spin and symmetry resolved photoemission and ab initio calculation, the TMR decrease for Co content higher than 25% is shown to come from the existence of an interface state and the shift of the empty Δ1 minority spin state towards the Fermi level.

Structural comparison between MgO/Fe(001) and MgO/Fe(001)–p(1×1)O interfaces for magnetic tunneling junctions: An Auger electron diffraction study

Abstract Magnetic tunnel junctions based on MgO(0 0 1) barriers and ferromagnetic electrodes, such as Fe/MgO/Fe, represent a very popular and widely investigated subject in the field of spin-electronics because of the large values of magnetoresistance shown by these systems. In this paper, the structural properties of MgO thin films grown onto Fe(0 0 1) and MgO/Fe(0 0 1)–p(1 × 1)O surfaces, with MgO thickness ranging from 2 to 14 equivalent monolayers, have been investigated by means of Auger electron diffraction. The structural order and the crystal quality of the MgO films result practically independent from the template, the latter being either the clean Fe(0 0 1) surface or the oxidized Fe(0 0 1)–p(1 × 1)O one. This is confirmed by numerical simulations, showing that, apart from the first two MgO layers close to the interface, the structure is unaffected by the choice of the starting surface. By a structural point of view, we can conclude that Fe(0 0 1)–p(1 × 1)O is a good candidate for the role of bottom electrode for the realization of MgO-based magnetic tunnelling junctions, also considering its higher chemical stability and reproducibility if compared to the clean Fe(0 0 1) surface.

Antiferromagnetic interlayer exchange coupling in epitaxial Fe/MgO/Fe trilayers with MgO barriers as thin as single monolayers

Comparative studies of the magnetic properties of epitaxial Fe/MgO/Fe trilayers, which were directly grown on an MgO(001) crystal and a homoepitaxial MgO buffer layer, were performed to examine the interlayer exchange coupling as a function of the MgO spacer thickness. Measurements of the magneto-optic Kerr effect at room temperature demonstrated that there was a strong antiferromagnetic coupling (AFC) between the Fe layers, which was mediated by the insulating MgO barrier. A measurable AFC was found in a range of MgO thickness (dMgO) from a single monolayer (2 Å) to 6 Å, which had a maximum strength of −1.15 ergs/cm2 at dMgO = 2.7 Å for the Fe/MgO/Fe trilayers that were prepared on the buffered sample and was notably weaker for the trilayers that were prepared directly on the MgO(001).

Damping parameter and interfacial perpendicular magnetic anisotropy of FeB nanopillar sandwiched between MgO barrier and cap layers in magnetic tunnel junctions

We systematically investigated the Gilbert damping (α) and interfacial perpendicular magnetic anisotropy (Ki) in magnetic tunnel junctions with a MgO-barrier/FeB/MgO-cap layered structure using the spin-torque diode effect. By increasing the MgO cap thickness, α decreased, whereas Ki increased monotonically. Values down to 0.0054 for α and up to 3.3 erg/cm2 for Ki were obtained for a MgO cap thickness of 0.6 nm. The small α and large Ki suggest that MgO-capped FeB is a suitable free layer for spintronics devices such as spin-torque oscillators and spin-torque magnetic random access memories.

Co/Pt multilayer-based magnetic tunnel junctions with a CoFeB/Ta insertion layer

We investigate properties of magnetic tunnel junctions (MTJs) having a Co/Pt multilayer as a recording layer. A CoFeB layer is inserted between MgO barrier and the recording layer in order to enhance the tunnel magnetoresistance ratio. We show that an additional layer of Ta inserted between CoFeB and Co/Pt multilayer is effective in improving the MTJ properties after annealing. A high effective magnetic anisotropy energy per unit area over 1.2 mJ/m2 is obtained after annealing at 300 °C. Using a 1.6 nm-thick CoFeB insertion layer, both high thermal stability factor of 92 and high tunnel magnetoresistance ratio of 91% are achieved in a MTJ with 17 nm in diameter.

Tunnel magnetoresistance effect in magnetic tunnel junctions using Fermi-level-tuned epitaxial Fe2Cr1−xCoxSi Heusler alloy

This paper reports a systematic investigation on the structural and magnetic properties of Fe2Cr1−xCoxSi Heusler alloys with various compositions of x by co-sputtering Fe2CrSi and Fe2CoSi targets and their applications in magnetic tunnel junctions (MTJs). Fe2Cr1−xCoxSi films of high crystalline quality have been epitaxially grown on MgO substrate using Cr as a buffer layer. The L21 phase can be obtained at x = 0.3 and 0.5, while B2 phase for the rest compositions. A tunnel magnetoresistance (TMR) ratio of 19.3% at room temperature is achieved for MTJs using Fe2Cr0.3Co0.7Si as the bottom electrode with 350 °C post-annealing. This suggests that the Fermi level in Fe2Cr1−xCoxSi has been successfully tuned close to the center of band gap of minority spin with x = 0.7 and therefore better thermal stability and higher spin polarization are achieved in Fe2Cr0.3Co0.7Si. The post-annealing effect for MTJs is also studied in details. The removal of the oxidized Fe2Cr0.3Co0.7Si at the interface with MgO barrier is found to be the key to improve the TMR ratio. When the thickness of the inserted Mg layer increases from 0.3 to 0.4 nm, the TMR ratio is greatly enhanced from 19.3% to 28%.

Fe∖MgO∖Cu-Phthalocyanine 복합구조 계면구조와 그 전자기적 특성

MgO 기반 스핀소자에 유기장벽 Cu-Phthalocyanine(CuPc)가 삽입된 무기<TEX>${\backslash}$</TEX>유기 터널 접합 소자 Fe<TEX>${\backslash}$</TEX>MgO(001)<TEX>${\backslash}$</TEX>CuPc<TEX>${\backslash}$</TEX>Co의 자기 저항 현상과 그 계면 특성의 상관관계에 대한 연구가 진행되었다. 특히 1.6 nm MgO(001)<TEX>${\backslash}$</TEX>x nm CuPc(x = 0~5) 계면의 전자기적 특성을 스핀 편극된 준안정상태 He 원자 분광계(Metastable Helium De-excitation Spectroscopy, MDS)를 이용하여 규명하였다. 에피 성장된 MgO(001) 위에 적층된 약 1.6 nm 두께의 CuPc 층상구조의 표면에서, MgO(001) 하지층의 표면과는 달리, up-spin band와 down-spin band의 비대칭성이 현저해지는 것으로 관찰되었다. 이 결과는 실온과 저온(77 K)에서 ~10 %와 30 %로 각각 측정된 자기저항 현상과 복합장벽을 통과하는 스핀거동을 이해하는데 중요한 단초를 제공해 준다. The influence of insertion of an ultra-thin Cu-Phthalocyanine (CuPc) between MgO barrier and ferromagnetic layer in magnetic tunnel juctions (MTJs) was investigated. In order to understand the relation between the electronic and structural properties of Fe<TEX>${\backslash}$</TEX>MgO<TEX>${\backslash}$</TEX>CuPc, the surface (or interface) analysis was carried out systematically by using spin polarized metastable He de-excited spectroscopy for the CuPc films grown on the Si(001)<TEX>${\backslash}$</TEX>5 nm MgO(001)<TEX>${\backslash}$</TEX>7 nm Fe(001)<TEX>${\backslash}$</TEX>1.6 nm MgO(001) multilayer structure as the thickness of CuPc increases from 0 to 5 nm. In particular, for the 1.6 nm CuPc surface, a rather strong spin asymmetry between up- and down-spin band appears while it becomes weaker or disappears for the CuPc films thinner or thicker than ~1.6 nm. Our results emphasize the importance of the interfacial electronic properties of organic layers in the spin transport of the hybrid MTJs.

Spin-dependent tunneling in magnetic tunnel junctions with Fe nanoparticles embedded in an MgO matrix

We investigated the transport properties in magnetic tunnel junctions with Fe nanoparticles (nominal thickness: tFe=0.17–1.37nm) and double MgO barriers. A disappearance of the hysteresis of the Fe nanoparticle magnetization due to its room-temperature superparamagnetic behavior and a large increase in the electrical resistance due to the Coulomb blockade effect were observed for tFe≤0.97nm. We found an increase in the tunnel magnetoresistance when the tFe decreases, which can be understood as enhancement of the spin polarization for the Fe particles that are smaller in size. Dip and shoulder structures that appeared in the first differential conductance spectra are also discussed.

Heteroepitaxy and ferromagnetism of EuO/MgO (001): A route towards combined spin- and symmetry-filter tunneling

We demonstrate the high-quality heteroepitaxy of ultrathin EuO films with bulklike ferromagnetism directly on MgO (001), an important step towards combining the spin-filter tunnel effect in magnetic insulators and symmetry-filter tunneling through single-crystalline MgO barriers. Despite a large compressive lattice mismatch, EuO grows fully relaxed on MgO (001) and adopts its bulk lattice parameter from the first monolayer on. This initial heteroepitaxial growth mode is discussed in terms of different electrostatic atomic configurations of the oxides interface. Single-crystalline EuO/MgO (001) thus can be envisioned as highly effective double spin-selective tunnel barriers for spintronics applications.

X-ray absorption spectroscopy and magnetic circular dichroism studies of L1-Mn-Ga thin films

Tetragonally distorted Mn3−xGax thin films with 0.1&amp;lt;x&amp;lt;2 show a strong perpendicular magnetic anisotropy and low magnetization and thus have the potential to serve as electrodes in spin transfer torque magnetic random access memory. Because a direct capping of these films with MgO is problematic due to oxide formation, we examined the influence of a CoFeB interlayer and of two different deposition methods for the MgO barrier on the formation of interfacial Mn-O for Mn62Ga38 by element specific X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). A highly textured L10 crystal structure of the Mn-Ga films was verified by X-ray diffraction measurements. For samples with e-beam evaporated MgO barrier no evidence for Mn-O was found whereas in samples with magnetron sputtered MgO, Mn-O was detected, even for the thickest interlayer thickness. Both XAS and XMCD measurements showed an increasing interfacial Mn-O amount with decreasing CoFeB interlayer thickness. Additional element specific full hysteresis loops determined an out-of-plane magnetization axis for the Mn and Co, respectively.

Epitaxy of MgO magnetic tunnel barriers on epitaxial graphene

Epitaxial growth of electrodes and tunnel barriers on graphene is one of the main technological bottlenecks for graphene spintronics. In this paper, we demonstrate that MgO(111) epitaxial tunnel barriers, one of the prime candidates for spintronic application, can be grown by molecular beam epitaxy on epitaxial graphene on SiC(0001). Ferromagnetic metals (Fe, Co, Fe20Ni80) were epitaxially grown on top of the MgO barrier, thus leading to monocrystalline electrodes on graphene. Structural and magnetic characterizations were performed on these ferromagnetic metals after annealing and dewetting: they form clusters with a 100 nm typical lateral width, which are mostly magnetic monodomains in the case of Fe. This epitaxial stack opens the way to graphene spintronic devices taking benefits from a coherent tunnelling current through the epitaxial MgO/graphene stack.

ChemInform Abstract: MgO(001) Barrier Based Magnetic Tunnel Junctions and Their Device Applications

AbstractReview: 210 refs.

Tunneling magnetoresistance effect in MnGa based perpendicular magnetic tunnel junction with Fe/Co interlayer

In order to enhance the magnetoresistance (MR) of perpendicular magnetic tunnel junctions (pMTJs) based on MnGa alloys, a single ferromagnetic layer such as Fe and Co was previously inserted between MnGa and MgO barrier. In this study, to further enhance the spin-filter effect, we introduced a Fe/Co bilayer as an interlayer in the MnGa/MgO interface. Compared to the single Co interlayer, an apparent MR ratio enhancement was obtained when Fe layer thickness was around 0.3 nm for pMTJs with MnGa compositions of Mn57Ga43, Mn62Ga38, and Mn70Ga30, and the maximum MR ratio reaches 50% at room temperature. In addition, inverted magnetoresistance loops were observed due to the antiparallel alignment of the magnetic moments of Co and MnGa layers separated by the thin Fe layer.

Role of MgO barriers for spin and charge transport in Co/MgO/graphene nonlocal spin-valve devices

We investigate spin and charge transport in both single and bilayer graphene non-local spin-valve devices. Similar to previous studies on bilayer graphene, we observe an inverse dependence of the spin lifetime on the carrier mobility in our single layer devices. This general trend is only observed in devices with large contact resistances. Furthermore, we observe a second Dirac peak in devices with long spin lifetimes. This results from charge transport underneath the contacts. In contrast, all devices with low ohmic contact resistances only exhibit a single Dirac peak. Additionally, the spin lifetime is significantly reduced indicating that an additional spin dephasing occurs underneath the electrodes.