Session L36: Focus Session: Nanomagnetism -- Tunnel Junctions

High temperature annealing induced superparamagnetism in CoFeB/MgO/CoFeB tunneling junctions

We have investigated the evolution of the magnetic transport properties as a function of short annealing time in CoFeB/MgO/CoFeB based MTJ junctions with a free layer of 2 nm. It is found that the hysteresis behaviors in MR loops disappear in samples annealed for 17 minutes. The linear region between MR and the applied field gradually increases. The MR loops without hysteresis can be well fitted by using the superparamagnetism theory, suggesting the formation of superparamagnetic particles in the free layer during the high temperature annealing. The control of MTJ properties with annealing time is desirable in magnetic field sensor productions.   

Noise Measurement in Magnetic Tunnel Junctions

An investigation has been carried out on the low frequency noise in magnetic tunnel junctions in various magnetic states. The noise measurements show similar 1/f spectra in either the parallel or antiparallel magnetic state. The transition region of applied magnetic field, switching from parallel to antiparallel and vice versa, indicates a 1/f$^{2}$ spectrum along with an increase in the low frequency noise. This 1/f$^{2}$ spectrum and noise increase is found to be an artifact resulting from the magnetic after effect which occurs in the magnetic switching regions. Removing the effects of the magnetic after effect, only 1/f spectra are measured. The shot noise at higher frequency where the 1/f noise is negligible is also found in the tunnel junctions. Though it appears to be linear with the applied current, the shot noise observed can be largely enhanced.   

Extended H\"{u}ckel theory based transport model for Fe-MgO-Fe magnetic tunnel junctions

Fe-MgO-Fe magnetic tunnel junction devices have attracted increased attention due to their preferential symmetry filtering of the half-metallic $\Delta _{1}$ band in Fe, which results in a high tunnel magnetoresistance ratio. Motivated by their applications in memory and sensor devices, we present an extended H\"{u}ckel theory (EHT) model coupled with the non-equilibrium Green's function formalism [1] to efficiently calculate the transport through these devices. We propose EHT parameters for MgO benchmarked with the k-resolved projected density of states calculated using the local density approximation of the density functional theory. We further optimize the MgO parameters to have the experimental band gap. These parameters are transferrable to various MgO thicknesses and the transport results match well with earlier calculations. [1] T. Z. Raza J. I. Cerda and H. Raza, in preparation. We are thankful to R. Hoffmann, R. Berger and S. Datta for useful discussions and NCN for computational resources.   

Impact of electrode crystallization and of impurity diffusion on barrier-resistance noise in CoFeB/MgO/CoFeB tunnel junctions

We have measured the changes of TMR and of barrier 1/$f$ resistance noise in sputtered, MgO-based MTJs as a function of annealing time at 250, 380, and 430 $^{\circ}$C. Although the behavior of TMR and of noise are anticorrelated at 380 and 430 $^{\circ}$C, the degree of crystallinity in CoFeB electrodes cannot account for drop in noise. An upper bound on electronic 1/$f$ noise from metal layers of materials stack is estimated to be many orders of magnitude less than measured value. Hence, electronic noise must be related to properties of barrier. Also, annealing at 250 $^{\circ}$C yields a steady decrease in noise while TMR remains mostly unchanged. Lastly, it is observed that parallel (P) state differential conductance begins to decrease at relatively short times of annealing at 430 $^{\circ}$C, suggesting that impurity diffusion is appreciable. Yet, noise continues to decrease around this time period. At longer times noise is essentially independent of time whereas P state differential conductance continues to decrease, indicating that impurities have a stronger effect on the conductance than on barrier noise. We conclude noise is attributed to oxygen vacancy defects in MgO tunnel barrier.   

Correlating microstructure and magnetization configurations with local spin transport in MgO-based nano-MTJs

Magnetic tunneling junctions (MTJs), the key components for many spin-based technologies, are commonly found on the order of 100nm in lateral dimensions due to the continuing trend of device miniaturization. Pinpointing the variations in transport properties due to local structural defects in these nano-MTJs is extremely difficult to accomplish using traditional experimental techniques. Here, we explore directly, the local structural-transport correlations on a series of 100nm by 100nm cross-sectional MgO MTJs by performing simultaneous structural characterization, magnetic imaging and \textit{in-situ} point contact tunneling experiments inside a transmission electron microscope. By changing the magnetic field at the specimen region, the two ferromagnetic electrodes in the MTJs can be controllably toggled between parallel and antiparallel (low and high resistance) states. I-V curves of the two resistance states are measured and quantitatively compared with the Simmons model. We found different barrier heights among nominally identical MTJs patterned from a single continuous film. The correlation of such barrier variations with the unique microstructure of individual devices will also be presented.   

Impact of interfacial roughness on spin filter tunneling.

The impact of interface roughness on spin filter tunneling is considered at low biases as functions of temperature and barrier parameters. Roughness reduces the maximum achievable spin polarization, which results from tunneling ``hot spots'' (thin regions of the barrier) having intrinsically reduced spin filtering efficiency. Surveying a range of experimentally reasonable roughness and mean barrier thickness values allows us to conclude that roughness values greater than 10{\%} of the mean barrier thickness have an adverse impact on the spin polarization. Atomic-scale roughness may thus be critical for achieving 100{\%} spin polarization in spin filter tunnel junctions at low biases.   

Observation of Peak/Dip Switches of Electric Conductance at Zero Bias Voltage in Pt/Neon/Pt+Fe Tunneling Junction

We conducted a tunneling experiment with Pt as one electrode and Pt+Fe as another, where the Fe on Pt is less than a monolayer. The insulation is solid neon and the experiments are carried out at temperatures of 2.3 K and 4.2 K with thickness of the solid neon about 10 angstroms. The tunneling junction is formed by using Lorentz force to push one electrode wire toward the other, which is perpendicular to the first one, while the solid neon had been deposited on them in advance. Since the Pt wire is thin (0.001 inch in diameter) the junction can be regarded as a point. Both I -- V and dI/dV -- V are recorded with lock -- in amplifier. We found that at 2.3 K the dip at zero bias voltage on conductance dI/dV can switch to a peak and back to a dip again at different trials while all the experimental parameters are kept unchanged. As the bias voltage is scanned consecutively, the overall conductance dI/dV increases due to, we suspect, the temperature increase/decrease at tunneling point because of the Ohm heat/cooling pump although such increase/decrease cannot be shown by a nearby thermal couple thermometer. At temperature of 4.2 K, however, such phenomenon has not been observed. The competition between Kondo effect and Coulomb blockade is a reasonable explanation for this peak/dip switch.   

Determining Conditions for Microwave-Frequency Gain in Magnetic Tunnel Junctions

Spin transfer torque from a microwave current flowing through a magnetic tunnel junction (MTJ) can excite resonant magnetic dynamics and hence a resonant resistance oscillation. When an additional DC current is applied to the device, the MTJ produces an oscillating voltage at the same frequency as the input microwave signal. This oscillating voltage increases with DC bias and can become larger than the input signal, thereby providing gain near the magnetic resonance frequency. We have investigated this phenomenon both theoretically and by measuring the high-frequency response of MTJs using network analyzer measurements. We will report the device parameters that are required to achieve gain and discuss the implications of our measurements for determining the magnitude and direction of the spin torque in MTJs at high bias.   

Magnetic tunnel junction based microwave detector

Besides being high sensitive magnetic field sensors, magnetic tunnel junction (MTJ) also has potential as a microwave magnetic field detector. The free magnetic layer in a MTJ precesses at a large angle under the excitation of microwave magnetic field at ferromagnetic resonance, while the pinned magnetic layer remains fixed due to the large damping. Consequently, the alternating relative angle between the free magnetic layer and the pinned magnetic layer induces a change of magnetoresistance in the MTJ. By detecting the average change of the magnetoresistance, one is able to determine both the magnitude and the frequency of microwave. We found that the sensitivity of such detector depends on both tunneling magnetoresistance at high bias and the damping of the free magnetic layer. This work was supported by NSF DMR Grant No. 08242249.   

Electromotive force and huge magnetoresistance in magnetic tunnel junctions with zinc-blende MnAs nano-magnets

For nanostructures such as magnetic nanowires or spin valves, it is theoretically predicted that an electromotive force (emf) arises from a time-varying magnetization in a static magnetic field [1]. This reflects the conversion of magnetic energy to electrical energy. Here we show that such an emf can indeed be induced by a \textit{static} magnetic field in magnetic tunnel junctions containing zinc-blende (ZB) MnAs quantum nano-magnets. The ZB MnAs nanomagnets are coupled to a NiAs-structure hexagonal MnAs top electrode through an AlAs tunnel barrier, and to a GaAs:Be bottom electrode through a GaAs barrier. Under a static magnetic field, an emf of up to 7 mV was observed for a time scale of 10$^{2}\sim $10$^{3}$ sec. This emf is induced by a co-tunneling process of electrons and magnetization of ZB MnAs nanomagnets subject to a strong Coulomb blockade of 50 meV. Huge magnetoresistance of up to 100,000{\%} is observed for certain bias voltages. Our results strongly suggest that Faraday's Law of induction must be generalized to account for purely spin effects in magnetic nanostructures [2]. The author thanks S. Ohya, M. Tanaka, S.E. Barnes and M. Maekawa for their collaboration.\\[4pt] [1] S. E. Barnes et al. APL 89, 122507 (2006); PRL 98, 246601 (2007).\\[0pt] [2] P. N. Hai et al., Nature 458, 489 (2009).   

Magnetic Tunnel Junctions (MTJ) with Large Tunneling Magnetoresistance (TMR) and Small Saturation Fields

There is a continuing need for more sensitive magnetic sensors. We report here an approach that leads to MTJ structures at the wafer level that have TMR values in excess of 100{\%} and saturation field (B$_{sat})$ values below 1 Oe. The ratio appears to be the largest ever reported. The approach we have used is to fabricate and anneal MTJs of a rather conventional type, SiO2/5Ta/30Ru/5Ta/2CoFe/15IrMn/2CoFe/0.9Ru/ 3CoFeB/2MgO/ 5CoFeB/5Ta/10Ru (thicknesses in nm), then etch down into the free layer of the MTJ, and deposit a thick (100 nm) and very soft magnetic film to lower B$_{sat}$. We have found that incorporating the soft film in the initial structure lowers the TMR significantly upon annealing. Maintaining a large TMR depends on depositing the soft film after annealing. A static field of 1.8 Oe is applied perpendicular to the sweep field ($i.e$. in the hard axis) to suppress the hysteresis of 1.3 Oe. This technique is described in Ref. 1.$^{ }$ The sweep field is in the easy axis of the free layer and the side field is in the hard axis of the free layer. The low-field loop is linear, non-hysteretic, and extrapolates to saturation at 0.8 Oe. The measured TMR is 118{\%}. 1) X. Liu, C. Ren, and G. Xiao 92, 4722 (2002).   

Effect of Metallic interlayers in MgO-based Magnetic Tunnel Junctions

Since the theoretical prediction and experimental observations of the giant tunneling magnetoresistance effect in magnetic tunnel junctions (MTJs) with a single-crystalline MgO(001) barrier, these MTJs have been extensively studied due to their broad potential applications in spintronic devices. This presentation covers very recent progress in theoretical calculations in a few select topics related to MgO-based MTJs. Specifically, we focus on the Layer-KKR first-principles method based theoretical studies of electronic structure and spin-dependent transport properties of MgO-based MTJs with different metallic interlayers, including structures of Fe(001)/Mg/MgO/Fe and Fe(001)/Co/MgO/Fe, as well as comparisons with recent experiments. An important role of the non-magnetic Mg interlayer is identified to be preserving the preferential transmission of the majority-spin states with $\Delta_1$ symmetry, which dominates the spin-dependent transport of MTJs with MgO barriers.   

A novel reduced symmetry oxide (Mg$_{3}$B$_{2}$O$_{6}$) for magnetic tunnel junctions based on FeCo or Fe leads

Magnetic tunnel junctions with high TMR values, such as Fe$\mid$MgO$\mid$Fe, capitalize on spin filtering in the oxide due to the band symmetry of incident electrons. However, these structures rely on magnetic leads and oxide regions of the same cubic symmetry class. This raises the question of whether reducing the oxide symmetry can enhance spin filtering. A new magnetic tunnel junction (FeCo$\mid$Mg$_{3}$B$_{2}$O$_{6}$$\mid$FeCo) is presented that uses a reduced symmetry oxide region (orthorhombic) to filter spins between two cubic magnetic leads. Symmetry analysis of coupling between states in the cubic leads and the orthorhombic oxide indicates that majority carrier tunneling through the oxide should be favored over minority carriers. Complex band structure analysis of Mg$_{3}$B$_{2}$O$_{6}$ shows that the relevant evanescent states in the band gap are due to boron p states and that there is sufficient difference in the decay rates of the imaginary bands for spin filtering to occur. Electronic transport calculations through a Fe$\mid$Mg$_{3}$B$_{2}$O$_{6}$$\mid$Fe magnetic tunnel junction are also performed to address the possible influence of interface states. Some recent experimental studies of FeCoB$\mid$MgO$\mid$FeCoB junctions, with B diffusion into the MgO region, indicate that this new type of junction may have already been fabricated. The prospect of developing a general class of magnetic tunnel junctions based on reduced symmetry oxides is also examined.