MgO Barrier Magnetic Tunnel Junctions Research Articles

Large tunnel magnetoresistance in magnetic tunnel junctions with magnetic electrodes of metastable body-centered cubic CoMnFe alloys

A magnetic tunnel junction (MTJ) is a key device in spintronics applications, such as magnetoresistive random access memory (MRAM). Current standard magnetic material for MTJs is a body-centered cubic (bcc) FeCo(B), which shows large tunnel magnetoresistance (TMR) and high perpendicular magnetic anisotropy (PMA), both are crucial for high-memory capacities MRAM. It is demanded that magnetic materials showing TMR as well as PMA higher than FeCo(B) for further advance. One of alternative magnetic materials may be bcc Co since past ab-initio studies predict that bcc Co show large TMR effect in MgO-barrier MTJs and large PMA due to magneto-elastic effect for those strained state. However, bcc Co is thermodynamically unstable; thus, it is aimed to explore metastable bcc Co based alloys with high stability. Here, we propose metastable bcc Co based alloy with adding both Mn and Fe as a candidate. We study in-plane magnetized MTJs utilizing metastable bcc CoMnFe films as electrodes and demonstrate large TMR effect of 350% at 300 K and of 1002% at 5 K. This experimental finding will open a route to advanced bcc Co based magnetic material for MRAM applications.

Tunnel magnetoresistance exceeding 100% in magnetic tunnel junctions using Mn-based tetragonal alloy electrodes with perpendicular magnetic anisotropy

We studied MgO barrier magnetic tunnel junctions (MTJs) comprising perpendicularly magnetized MnGa and FeCoB electrodes. In those perpendicular (p-) MTJs, we utilized thin metastable bcc CoMn alloys as an interlayer between MnGa and MgO to enhance the tunnel magnetoresistance (TMR) effect. Moreover, we investigated the effect of a thin Mg interlayer between MnGa and CoMn. Owing to the interlayer engineering, we achieved a TMR ratio over 100%, the highest value observed for p-MTJs with a tetragonal MnGa electrode. Our study contributes to the further development of spintronic devices using p-MTJs with various Mn-based tetragonal alloy electrodes.

Structural and magnetic properties of CoIrMnAl equiatomic quaternary Heusler alloy epitaxial films designed using first-principles calculations

• CoIrMnAl was computationally designed as one of the advanced materials for spintronics. • We obtained B2 ordered CoIrMnAl thin films with the lattice constant similar to that predicted in fully ordered alloy. • Magnetization at 10 K and Curie temperature measured were about 70% of the predicted values in fully ordered alloys. • Experimental data were well explained with ferrimagnetism due to Co-Mn swap disorders predicted from ab-initio calculation. MgO-barrier magnetic tunnel junctions with half-metallic Heusler alloy electrodes have attracted considerable attention for spintronics applications. However, there remain a couple of issues related to materials that should be resolved before practical use. Recently, quarterly equiatomic Heusler alloys have attracted attention as advanced Heusler alloys. CoIrMnZ (Z = Al, Si, Ga, and Ge) half-metallic Heusler alloys were designed and predicted to have moderate Curie temperatures and to be lattice-matched with the MgO barrier, which is advantageous compared to traditional Co 2 Heusler alloys T. T [T. Roy et al., J. Magn. Magn. Mater. 498 (2020) 166092]. Here, we experimentally investigated the structure and magnetic properties of thin films composed of one of these alloys, CoIrMnAl, fabricated by sputtering deposition. We successfully obtained films with the B 2 chemical ordering, even without a post-annealing process. The lattice constants for the films annealed at 500–600 ∘ C were approximately equal to the predicted values. The magnetization at 10 K was close to 500 kA/m, and the Curie temperature was approximately 400 K, which were approximately 70% of the values predicted for the fully ordered structure. The magnetic properties observed in the B 2-ordered films were well explained by ferrimagnetism that appeared in the B 2-ordered CoIrMnAl with full-swap disorders of Co-Ir and Mn-Al and almost full-swap disorder of Co-Mn, which was predicted from the first-principles calculations.

Magnetic tunnel junctions with a B2 -ordered CoFeCrAl equiatomic Heusler alloy

The equiatomic quaternary Heusler alloy CoFeCrAl is a candidate material for spin-gapless semiconductors (SGSs). However, to date, there have been no experimental attempts at fabricating a junction device. This paper reports a fully epitaxial (001)-oriented MgO barrier magnetic tunnel junction (MTJ) with CoFeCrAl electrodes grown on a Cr buffer. X-ray and electron diffraction measurements show that the (001) CoFeCrAl electrode films with atomically flat surfaces have a $B2$-ordered phase. The saturation magnetization is $380\phantom{\rule{0.16em}{0ex}}{\mathrm{emu}/\mathrm{cm}}^{3}$, almost the same as the value given by the Slater--Pauling--like rule, and the maximum tunnel magnetoresistance ratios at 300 K and 10 K are 87% and 165%, respectively. Cross-sectional electron diffraction analysis shows that the MTJs have MgO interfaces with fewer dislocations. The temperature- and bias-voltage-dependence of the transport measurements indicates magnon-induced inelastic electron tunneling overlapping with the coherent electron tunneling. X-ray magnetic circular dichroism (XMCD) measurements show a ferromagnetic arrangement of the Co and Fe magnetic moments of $B2$-ordered CoFeCrAl, in contrast to the ferrimagnetic arrangement predicted for the $Y$-ordered state possessing SGS characteristics. Ab initio calculations taking account of the Cr-Fe swap disorder qualitatively explain the XMCD results. Finally, the effect of the Cr-Fe swap disorder on the ability for electronic states to allow coherent electron tunneling is discussed.

Magnetoresistance and spin-torque effect in flexible nanoscale magnetic tunnel junction

Flexible electronics or hybrid electronics exhibit great potential for widespread applications in future wearable electronics. In this work, we fabricated flexible nanoscale MgO-barrier magnetic tunnel junctions (MTJs) using a transfer printing process. The magnetic transport measurements reveal that the fabricated devices possess excellent performance with a tunnel magnetoresistance ratio of ∼130% under different strained conditions. In addition, we also studied the spin-torque diode effect under different strained conditions and found that the resonant frequency and rectified voltage remain almost unchanged. These results demonstrate that the nanoscale MTJs have good strain endurance, which provides the feasibility to flexible spintronic storage and microwave applications.

Fabrication of magnetic tunnel junctions with a metastable bcc Co3Mn disordered alloy as a bottom electrode

We fabricated MgO barrier magnetic tunnel junctions (MTJs) with a Co3Mn alloy bottom and FeCoB top electrodes. The (001)-oriented epitaxial films of the metastable bcc Co3Mn disordered alloys obtained showed saturation magnetization of approximately 1640 emu cm−3. The transmission electron microscopy showed that the MgO barrier was epitaxially grown on the Co3Mn electrode. Tunnel magnetoresistance of approximately 150% was observed at room temperature after the annealing of MTJs at 350 °C, indicating that bcc Co3Mn alloys have relatively high spin polarization.

Reliability enhancement due to in-situ post-oxidation of sputtered MgO barrier in double MgO barrier magnetic tunnel junction

We have investigated the effects of in-situ post-oxidation (PO) of a sputtered MgO barrier in a double-MgO-barrier magnetic tunnel junction (MTJ) and found that the short error rate was significantly reduced, the magnetoresistance (MR) ratio was increased approximately 18%, and the endurance lifetime was extend. In addition, we found that the distribution of breakdown number (a measure of endurance) exhibits trimodal characteristics, which indicates competition between extrinsic and intrinsic failures. This improvement in reliability might be related to the suppression of Fe and Co diffusion to the MgO barrier, as revealed by electron energy-loss spectroscopy (EELS) analysis.

High Performance MgO-barrier Magnetic Tunnel Junctions for Flexible and Wearable Spintronic Applications

The magnetic tunnel junction (MTJ) using MgO barrier is one of most important building blocks for spintronic devices and has been widely utilized as miniaturized magentic sensors. It could play an important role in wearable medical devices if they can be fabricated on flexible substrates. The required stringent fabrication processes to obtain high quality MgO-barrier MTJs, however, limit its integration with flexible electronics devices. In this work, we have developed a method to fabricate high-performance MgO-barrier MTJs directly onto ultrathin flexible silicon membrane with a thickness of 14 μm and then transfer-and-bond to plastic substrates. Remarkably, such flexible MTJs are fully functional, exhibiting a TMR ratio as high as 190% under bending radii as small as 5 mm. The devices‘ robustness is manifested by its retained excellent performance and unaltered TMR ratio after over 1000 bending cycles. The demonstrated flexible MgO-barrier MTJs opens the door to integrating high-performance spintronic devices in flexible and wearable electronics devices for a plethora of biomedical sensing applications.

Flexible MgO Barrier Magnetic Tunnel Junctions.

Flexible MgO barrier magnetic tunnel junction (MTJ) devices are fabricated using a transfer printing process. The flexible MTJ devices yield significantly enhanced tunneling magnetoresistance of ≈300% and improved abruptness of switching, as residual strain in the MTJ structure is released during the transfer process. This approach could be useful for flexible electronic systems that require high-performance memory components.

Crystallization characteristics of a middle CoFeB layer in a double MgO barrier magnetic tunnel junction

The crystallization characteristics of a middle CoFeB free layer in a magnetic tunnel junction (MTJ) with double MgO barriers were investigated by tunneling magnetoresistance (TMR) measurements of patterned cells across an 8-inch wafer. The MTJ structure was designed to have two CoFeB free layers and one bottom pinned layer, separated by MgO tunnel barriers. The observed resistance showed three types of TMR curves depending on the crystallization of the middle CoFeB layer. From the analysis of TMR curves, coherent crystallization of the middle CoFeB layer with the top and bottom MgO barriers was found to occur non-uniformly: About 80% of the MTJ cells in the wafer exhibited coherent crystallization of the middle CoFeB layers with the bottom MgO tunnel barrier, while others had coherent crystallization with the top MgO tunnel barrier or both barriers. This non-uniform crystallization of the middle CoFeB layer in a double MTJ was also clearly observed in tunneling electron microscopy images. Thus, control of the crystallization of the middle CoFeB layer is important for optimizing the MTJ with double MgO barriers, and especially for the fabrication of double barrier MTJ on a large area substrate.

Linear nanometric tunnel junction sensors with exchange pinned sensing layer

Highly sensitive nanosensors with high spatial resolution provide the necessary features for high accuracy imaging of isolated magnetic nanoparticles. In this work, we report the fabrication and characterization of MgO-barrier magnetic tunnel junction nanosensors, with two exchange-pinned electrodes. The perpendicular magnetization configuration for field sensing is set using a two-step annealing process, where the second annealing temperature was optimized to yield patterned sensors responses with improved linearity. The optimized circular nanosensors show sensitivities up to 0.1%/Oe, larger than previously reported for nanometric sensors and comparable to micrometric spin-valves. Our strategy avoids the use of external permanent biasing or demagnetizing fields (large for smaller structures) to achieve a linear response, enabling the control of the linear operation range using only the stack and thus providing a small footprint device.

Yoke-shaped MgO-barrier magnetic tunnel junction sensors

Yoke-shaped sensors based on MgO-barrier magnetic tunnel junctions have been designed, fabricated, and studied; they show a good linear tunneling magnetoresistance (TMR) response. A nearly-perpendicular configuration of two ferromagnetic electrodes was set by two-step annealing together with shape anisotropy. The low-frequency noise characterization shows these yoke-shaped TMR sensors have relatively low magnetic 1/f noise. The field sensitivity is up to 27%/mT, while the field detectivity reaches 4.6 nT/Hz at 10 Hz and 460 pT/Hz at 1 kHz. These TMR sensors may be useful for applications such as biomagnetic detection.

Magnetotransport properties of dual MgO barrier magnetic tunnel junctions consisting of CoFeB/FeNiSiB/CoFeB free layers

We report the transport properties of a dual MgO barrier magnetic tunnel junction (DMTJ) where a FeNiSiB layer was inserted in a CoFeB free layer. Upon post-deposition annealing at 330 °C, the tunneling magnetoresistance (TMR) ratio of the DMTJ with a hybrid CoFeB/FeNiSiB/CoFeB free layer reached 195% which is higher than the TMR ratio of 121% from the DMTJ with the single CoFeB free layer. The bias voltage dependence profile was more symmetric for the hybrid case. Boron depth profiling result suggests that the FeNiSiB layer dragged boron atoms more to it rather than letting them diffuse toward CoFeB/MgO interfaces, resulting in improved MTJ performances.

Low magnetisation alloys for in-plane spin transfer torque devices

Magnetic properties of thin films of three in-plane alloy systems, namely, Co40Ni40−xCrxB20, Co40Fe40−xNixB20, and Co40Fe40−xCrxB20 are investigated with a view to reducing the critical current density (jc) for spin transfer torque switching in MgO barrier magnetic tunnel junction nanopillars. In all three systems, the saturation magnetisation, Ms, decreases with increasing Cr or Ni substitution. The Co40Fe40−xCrxB20 alloy system is found to be the best in terms of reducing Ms, while maintaining a high tunnel magnetoresistance ratio. Ferromagnetic resonance experiments revealed that the damping coefficient of CoFeCrB alloy remains unchanged with increasing Cr content, but jc is reduced by a factor of four to 4.9 × 105 A/cm2 by using a Co40Fe32.7Cr7.3B20 free layer while maintaining a magnetoresistance of around 60 %.

Tunable linear magnetoresistance in MgO magnetic tunnel junction sensors using two pinned CoFeB electrodes

MgO-barrier magnetic tunnel junction sensors with both CoFeB layers pinned by IrMn have been fabricated, which show a tunneling magnetoresistance (TMR) of up to 255% at room temperature. The perpendicular configuration for magnetic field sensing is set using a two-step field annealing process. The linear TMR field range and sensitivity are tuned by inserting an ultrathin Ru layer between the upper IrMn and the top-pinned CoFeB layer. The field sensitivity reaches 26%/mT, while the noise detectivity is about 90 nT/Hz at 10 Hz for a 0.3 nm Ru insertion layer. The bias dependence of the noise suggests that this is a useful design for sensor applications.

Tunnel Magnetoresistance Properties of Double MgO-Barrier Magnetic Tunnel Junctions With Different Free-Layer Alloy Compositions and Structures

We investigated tunnel magnetoresistance (TMR) properties in double MgO-barrier magnetic tunnel junctions (DMTJs) with (Co25Fe75)100-xBx free-layers with x = 15, 20, and 25 (in at.%) sputtered at different power. The TMR ratio of DMTJs increased with sputtering power, which is ascribed to the B composition reduction in the CoFeB free-layer with increasing sputtering power. The x-ray diffraction measurement showed that crystallization into (001)-oriented texture of CoFeB film sandwiched between the two MgO-layers as well as an improvement of (001) orientation of top MgO-barrier are realized. The TMR ratio over 200% was obtained in DMTJs with a (Co25Fe75)85B15 free-layer sputtered at 0.88 and 1.77 W/cm2.

Tunnel magnetoresistance and spin torque switching in MgO-based magnetic tunnel junctions with a Co/Ni multilayer electrode

We have fabricated MgO-barrier magnetic tunnel junctions with a Co/Ni switching layer to reduce the demagnetizing field via interface anisotropy. With a fcc-(111) oriented Co/Ni multilayer combined with an FeCoB insertion layer, the demagnetizing field is 2 kOe and the tunnel magnetoresistance can be as high as 106%. Room-temperature measurements of spin-torque switching are in good agreement with predictions for a reduced critical current associated with the small demagnetization for antiparallel-to-parallel switching. For parallel-to-antiparallel switching the small demagnetization field causes spatially nonuniform reversal nucleated at the sample ends, with a low energy barrier but a higher switching current.

Impedance spectroscopy of micron sized magnetic tunnel junctions with MgO tunnel barrier

We have studied the magnetoimpedance of micron sized magnetic tunnel junction sensors with 1.7 nm MgO tunnel barrier. We performed ac impedance spectroscopy in the frequency range between 100 Hz–40 MHz as a function of applied magnetic field in the sensing direction. We model our devices with a simple RLC circuit. Fitting the model to our data results in frequency independent R, L, and C, and our low frequency results are in agreement with dc measurements. Despite excellent agreement with published result on interface capacitance for MgO barrier magnetic tunnel junctions similar to ours we do not observe any magnetocapacitance in our devices.

Advances and Future Prospects of Spin-Transfer Torque Random Access Memory

Spin-transfer torque random access memory (STT-RAM) is a potentially revolutionary universal memory technology that combines the capacity and cost benefits of DRAM, the fast read and write performance of SRAM, the non-volatility of Flash, and essentially unlimited endurance. In order to realize a small cell size, high speed and achieve a fully functional STT-RAM chip, the MgO-barrier magnetic tunnel junctions (MTJ) used as the core storage and readout element must meet a set of performance requirements on switching current density, voltage, magneto-resistance ratio (MR), resistance-area product (RA), thermal stability factor (¿) , switching current distribution, read resistance distribution and reliability. In this paper, we report the progress of our work on device design, material improvement, wafer processing, integration with CMOS, and testing for a demonstration STT-RAM test chip, and projections based on modeling of the future characteristics of STT-RAM.

Tunnel magnetoresistance properties and film structures of double MgO barrier magnetic tunnel junctions

The authors fabricated double MgO barrier magnetic tunnel junctions (MTJs) with 3-nm-thick Co40Fe40B20 free layer. When annealed at 350 °C, tunnel magnetoresistance (TMR) ratio at room temperature was 130%, much lower than that (297%) of single MgO barrier MTJs processed and annealed under the same condition. The middle CoFeB free layer sandwiched between the two MgO barriers was found to be mostly amorphous. Replacement of the Co40Fe40B20 free layer by a highly oriented Co50Fe50 layer and a composite Co50Fe50/Co40Fe40B20 layer led to the enhanced TMR ratios up to 165% and 212% at annealing temperature of 350 °C, respectively.