Ultrafast and low-energy switching in voltage-controlled elliptical pMTJ

Switching magnetization in a perpendicular magnetic tunnel junction (pMTJ) via voltage controlled magnetic anisotropy (VCMA) has shown the potential to markedly reduce the switching energy. However, the requirement of an external magnetic field poses a critical bottleneck for its practical applications. In this work, we propose an elliptical-shaped pMTJ to eliminate the requirement of providing an external field by an additional circuit. We demonstrate that a 10 nm thick in-plane magnetized bias layer (BL) separated by a metallic spacer of 3 nm from the free layer (FL) can be engineered within the MTJ stack to provide the 50 mT bias magnetic field for switching. By conducting macrospin simulation, we find that a fast switching in 0.38 ns with energy consumption as low as 0.3 fJ at a voltage of 1.6 V can be achieved. Furthermore, we study the phase diagram of switching probability, showing that a pulse duration margin of 0.15 ns is obtained and a low-voltage operation (~ 1 V) is favored. Finally, the MTJ scalability is considered, and it is found that scaling-down may not be appealing in terms of both the energy consumption and the switching time for the precession based VCMA switching.Comment: There are 28 pages and 5 figure

Enhancement of thermovoltage and tunnel magneto-Seebeck effect in CoFeB based magnetic tunnel junctions by variation of the MgAl2_2O4_4 and MgO barrier thickness

We investigate the influence of the barrier thickness of Co$_{40}$Fe$_{40}$B$_{20}$ based magnetic tunnel junctions on the laser-induced tunnel magneto-Seebeck effect. Varying the barrier thickness from 1nm to 3nm, we find a distinct maximum in the tunnel magneto-Seebeck effect for 2.6nm barrier thickness. This maximum is independently measured for two barrier materials, namely MgAl$_2$O$_4$ and MgO. Additionally, samples with an MgAl$_2$O$_4$ barrier exhibit a high thermovoltage of more than 350$\mu$V in comparison to 90$\mu$V for the MTJs with MgO barrier when heated with the maximum laser power of 150mW. Our results allow for the fabrication of improved stacks when dealing with temperature differences across magnetic tunnel junctions for future applications in spin caloritronics, the emerging research field that combines spintronics and themoelectrics

Perpendicular magnetic anisotropy of CoFeB\Ta bilayers on ALD HfO2

Perpendicular magnetic anisotropy (PMA) is an essential condition for CoFe thin films used in magnetic random access memories. Until recently, interfacial PMA was mainly known to occur in materials stacks with MgO\CoFe(B) interfaces or using an adjacent crystalline heavy metal film. Here, PMA is reported in a CoFeB\Ta bilayer deposited on amorphous high-kappa dielectric (relative permittivity kappa=20) HfO2, grown by atomic layer deposition (ALD). PMA with interfacial anisotropy energy K-i up to 0.49 mJ/m(2) appears after annealing the stacks between 200 degrees C and 350 degrees C, as shown with vibrating sample magnetometry. Transmission electron microscopy shows that the decrease of PMA starting from 350 degrees C coincides with the onset of interdiffusion in the materials. High-kappa dielectrics are potential enablers for giant voltage control of magnetic anisotropy (VCMA). The absence of VCMA in these experiments is ascribed to a 0.6 nm thick magnetic dead layer between HfO2 and CoFeB. The results show PMA can be easily obtained on ALD high-kappa dielectrics

Determination of spin-dependent Seebeck coefficients of CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars

We investigate the spin-dependent Seebeck coefficient and the tunneling magneto thermopower of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ) in the presence of thermal gradients across the MTJ. Thermal gradients are generated by an electric heater on top of the nanopillars. The thermo power voltage across the MTJ is found to scale linearly with the heating power and reveals similar field dependence as the tunnel magnetoresistance. The amplitude of the thermal gradient is derived from calibration measurements in combination with finite element simulations of the heat flux. Based on this, large spin-dependent Seebeck coefficients of the order of (240 \pm 110) \muV/K are derived. From additional measurements on MTJs after dielectric breakdown, a tunneling magneto thermopower up to 90% can be derived for 1.5 nm MgO based MTJ nanopillars

Process development for integration of CoFeB/MgO-based magnetic tunnel junction (MTJ) device on silicon

Methods of communication and dissemination of information have changed dramatically with the emergence of the Internet and mobile phones. To sustain this revolution, we need reliable mass storage devices which would store information not only in large amount in small space but also for long time. Therefore, realizing high performance memory technologies is very critical for this revolution. This work contributes towards the development of one such technology; Magnetic Random Access Memory (MRAM) based on Magnetic Tunnel Junction (MTJ). The research conducted in this study is primarily focused on the process development for integrating MTJ on silicon. The film stack explored in this work is CoFeB/MgO-based. The relevant issues in this integration such as smooth bottom electrode preparation, low thermal budget, process chemistry and parameters, and MTJ patterning involving ion-milling have been addressed in this work. Ta and NiCr are evaluated as candidates for bottom electrode. Spin-on Glass (SOG)-based low temperature Inter Level Dielectric (ILD) process is developed. MTJ devices with varying sizes with four terminal contacts for on wafer testing have been designed and fabricated using the process developed. The devices exhibited Resistance-Area (RA) product in the range of 1-5 k_Um2. Recent literature on MgO-based MTJ devices has reported values in a range of 0.1 – 1000 k_um2. This data confirms the electrical integrity of the MTJ fabricated. The RA values have been observed to be unchanged on application of magnetic field (+-300Oe). Detailed investigations have been carried out to find possible causes for the absence of magnetic response from these junctions. These include XRD analysis of the MTJ stack for CoFeB crystallization and STEM-PEELS studies to investigate the chemical composition. “Neel coupling” or “Orange peel coupling” due to interface roughness is thought to be one of the main possible causes for magnetically inactive junctions. Suggestions for future are given on the basis of the results from the process and the experiments. In summary, a process has been developed for fabricating MTJ on silicon yielding desired values for junction resistivity. The magnetic response is extremely sensitive to film roughness at nanoscales and will require control of roughness at each step starting with wafer specification. It is concluded that with a control of surface roughness and recommended modifications in MTJ films, a CMOS compatible process for fabricating MTJ is plausible at RIT. (Refer to PDF file for exact formulas

Realization of CoFeB|MgO|CoFeB magnetic tunnel junction devices through materials analysis, process integration and circuit simulation

Spin based magnetic tunnel junctions (MTJs) consist of two ferromagnetic thin films separated by a nonmagnetic insulating barrier. The MTJ exhibits two switchable resistive states, making them ideal candidates for non-volatile memory. The discovery of high Tunneling Magnetoresistance (TMR) in MgO based MTJs has brought spintronics into the forefronts of modern technology. A device structure CoFeB|MgO|CoFeB achieved by physical vapor deposition (PVD) has revolutionized the hard-drive industry to go beyond densities of gigabyte per square inch. There is increasing interest in the application of these devices toward other technical areas, such as sensors, logic and reconfigurable computing. In these structures, the thicknesses of the layers are in the order of a few nanometers. For integration of these devices in other platforms, particularly on silicon, to augment the well-developed CMOS technology, it is imperative to (1) investigate processing constraints, (2) develop appropriate physical models, and (3) build circuit models for effective circuit implementation. The work presented in this dissertation focuses on these three important aspects for the realization of CoFeB|MgO|CoFeB MTJs on silicon. A systematic annealing study has been carried out to investigate the role of boron in the device structure. It has been shown using electron energy loss spectroscopy (EELS), and 2D x-ray diffraction (2D XRD) that boron diffuses into MgO with an activation energy of 1.30.4 eV and facilitates the crystallization of CoFe with (200) out-of-plane oriented crystals, with MgO as a template. The grain size of CoFe has been definitively shown to be smaller than the grain size of MgO, which were otherwise believed to be the same. A process temperature of 385°C has been determined to be the optimum limit of processing. A low temperature (\u3c385°C) process employing standard integrated circuit fabrication techniques has been developed. The partial crystallization of CoFe necessitates the modification of the tunneling model. A new model that combines the Julliëre\u27s, free electron and tight-binding model with the probabilistic distribution of grains on either side of the tunneling barrier has been proposed. This model explains the variation of TMR as a function of temperature in devices made by PVD. A generalized circuit macromodel has been developed representing field-switchable magnetic tunnel junctions (MTJs) characterized by two distinct voltage-dependent resistance values in parallel and antiparallel states. General-purpose subcircuit implementations are designed for a switchable voltage-dependent resistor capable of implementation using any version of SPICE. Transient simulation of a flash-comparator circuit using multiple MTJs in series is successfully demonstrated showing the robustness of the model

Comparison of the magneto-Peltier and magneto-Seebeck effects in magnetic tunnel junctions

Understanding heat generation and transport processes in a magnetic tunnel junction (MTJ) is a significant step towards improving its application in current memory devices. Recent work has experimentally demonstrated the magneto-Seebeck effect in MTJs, where the Seebeck coefficient of the junction varies as the magnetic configuration changes from a parallel (P) to an anti-parallel (AP) configuration. Here we report the study on its as-yet-unexplored reciprocal effect, the magneto-Peltier effect, where the heat flow carried by the tunneling electrons is altered by changing the magnetic configuration of the MTJ. The magneto-Peltier signal that reflects the change in the temperature difference across the junction between the P and AP configurations scales linearly with the applied current in the small bias but is greatly enhanced in the large bias regime, due to higher-order Joule heating mechanisms. By carefully extracting the linear response which reflects the magneto-Peltier effect, and comparing it with the magneto-Seebeck measurements performed on the same device, we observe results consistent with Onsager reciprocity. We estimate a magneto-Peltier coefficient of 13.4 mV in the linear regime using a three-dimensional thermoelectric model. Our result opens up the possibility of programmable thermoelectric devices based on the Peltier effect in MTJs