Tunnel Magnetoresistance Ratio Research Articles

Effects of the radio-frequency sputtering power of an MgO tunneling barrier on the tunneling magneto-resistance ratio for Co2Fe6B2/MgO-based perpendicular-magnetic tunnel junctions

For Co2Fe6B2–MgO based p-MTJ spin valves with [Co/Pt]n–SyAF layers ex situ annealed at 350 °C and 30 kOe for 30 min, the tunneling magneto-resistance (TMR) ratio strongly depended on the radio-frequency (RF) sputtering power in a 0.65–1.15 nm thick MgO tunneling barrier, achieving a TMR ratio of 168% at 300 W.

Giant tunnel magneto-resistance in graphene based molecular tunneling junction.

We propose and theoretically investigate a class of stable zigzag graphene nanoribbon (ZGNR) based molecular magnetic tunneling junctions (MTJs). For those junctions having pentagon-connecting formations, huge tunnel magneto-resistance (TMR) is found. Different from most of the other proposed molecular junctions, the huge TMR in our structures is generic, and is not significantly affected by external parameters such as bias voltage, gate voltage, length of the molecule and width of the ZGNRs. The double pentagon-connecting formation between the molecule and ZGNRs is critical for the remarkable TMR ratio, which is as large as ∼2 × 10(5). These molecular MTJs behave as almost perfect spin filters and spin valve devices. Other connecting formations of the ZGNR based MTJs lead to much smaller TMR. By first principles analysis, we reveal the microscopic physics responsible for this phenomenon.

High tunneling magnetoresistance ratio in perpendicular magnetic tunnel junctions using Fe-based Heusler alloys

Heulser alloys Fe2Cr1−xCoxSi (FCCS) with different Co compositions x have been predicted to have high spin polarization. High perpendicular magnetic anisotropy (PMA) has been observed in ultra-thin FCCS films with magnetic anisotropy energy density up to 2.3 × 106 erg/cm3. The perpendicular magnetic tunnel junctions (p-MTJs) using FCCS films with different Co compositions x as the bottom electrode have been fabricated and the post-annealing effects have been investigated in details. An attractive tunneling magnetoresistance ratio as high as 51.3% is achieved for p-MTJs using Fe2CrSi (FCS) as the bottom electrode. The thermal stability Δ can be as high as 70 for 40 nm dimension devices using FCS, which is high enough to endure a retention time of over 10 years. Therefore, Heusler alloy FCS is a promising PMA candidate for p-MTJ application.

Manipulation of magnetization switching and tunnel magnetoresistance via temperature and voltage control.

Magnetization switching between parallel and antiparallel alignments of two magnetic layers in magnetic tunnel junctions (MTJs) is conventionally controlled either by an external magnetic field or by an electric current. Here, we report that the manipulation of magnetization switching and tunnel magnetoresistance (TMR) in perpendicularly magnetized CoFeB/MgO/CoFeB MTJs can be achieved by both temperature and voltage. At a certain range of temperature, coercivity crossover between top and bottom magnetic layers is observed in which the TMR ratio of the MTJs is almost unmeasurable. Furthermore, the temperature range can be tuned reversibly by an electric voltage. Magnetization switching driven by the voltage reveals an unconventional phenomenon such that the voltage driven coercivity changes with temperature are quite different for top and bottom CoFeB layers. A model based on thermally-assisted domain nucleation and propagation is developed to explain the frequency and temperature dependence of coercivity. The present results of controlling the magnetization switching by temperature and voltage may provide an alternative route for novel applications of MTJs based spintronic devices.

Magnetic properties of the CoxZn1−xO (x = 0.20, 0.86, 0.92) films and its tunnel magnetoresistance

CoxZn1−xO (X = 0.20, 0.86, 0.92) films were synthesized using the radio frequency (RF) co-sputtering method. Though the same synthesis method was used, these samples exhibited different structures and magnetic properties. The Co0.20Zn0.80O film sample was crystalline, but its magnetic property was antiferromagnetism. Amorphous phase, room temperature ferromagnetism and anomalous Hall effect were shown in the sputtered and annealed Co0.86Zn0.14O and Co0.92Zn0.08O samples. Through x-ray photoelectron spectroscopy (XPS), the ferromagnetism of the high Co content amorphous films originates from metallic Co. The antiferromagnetism of Co0.20Zn0.80O crystalline film can be explained by an indirect-exchange interaction model. The electric conductivity is interpreted with the spin-dependent variable-range hopping model at low temperature. A tunneling magnetoresistance (TMR) ratio of 16.4% was observed at 77 K in Co(40 nm)/ZnO(4 nm)/Co0.92Zn0.08O(75 nm) magnetic tunnel junctions (MTJs). The MTJs can resist up to room temperature with the TMR ratio of 1.6% at 10 kOe.

Origin of interfacial perpendicular magnetic anisotropy in MgO/CoFe/metallic capping layer structures.

Spin-transfer-torque magnetic random access memory (STT-MRAM) attracts extensive attentions due to its non-volatility, high density and low power consumption. The core device in STT-MRAM is CoFeB/MgO-based magnetic tunnel junction (MTJ), which possesses a high tunnel magnetoresistance ratio as well as a large value of perpendicular magnetic anisotropy (PMA). It has been experimentally proven that a capping layer coating on CoFeB layer is essential to obtain a strong PMA. However, the physical mechanism of such effect remains unclear. In this paper, we investigate the origin of the PMA in MgO/CoFe/metallic capping layer structures by using a first-principles computation scheme. The trend of PMA variation with different capping materials agrees well with experimental results. We find that interfacial PMA in the three-layer structures comes from both the MgO/CoFe and CoFe/capping layer interfaces, which can be analyzed separately. Furthermore, the PMAs in the CoFe/capping layer interfaces are analyzed through resolving the magnetic anisotropy energy by layer and orbital. The variation of PMA with different capping materials is attributed to the different hybridizations of both d and p orbitals via spin-orbit coupling. This work can significantly benefit the research and development of nanoscale STT-MRAM.

Co2Fe6B2/MgO-based perpendicular spin-transfer-torque magnetic-tunnel-junction spin-valve without [Co/Pt]n lower synthetic-antiferromagnetic layer

We design a Co2Fe6B2/MgO-based p-MTJ spin-valve without a [Co/Pt]n lower synthetic-antiferromagnetic (SyAF) layer to greatly reduce the 12-inch wafer fabrication cost of the p-MTJ spin-valve. This spin-valve achieve a tunneling magnetoresistance (TMR) of 158% and an exchange field (Hex) of 1.4 kOe at an ex situ annealing temperature of >350 °C, which ensures writing error immunity. In particular, the TMR ratio strongly depends on the body-center-cubic capping-layer nanoscale thickness (tbcc), i.e., the TMR ratio peaks at tbcc = 0.6 nm.

Impact of MgO Thickness on the Performance of Spin-Transfer Torque Nano-Oscillators

A magnetic tunnel junction (MTJ) stack incorporating an MgO wedge was deposited over a 200 mm wafer and nanofabricated into circular nanopillars with a diameter of 200 nm. Due to the variable MgO thickness, we could obtain MTJ nanopillars with resistance $\times $ area (RA) ranging from less than $1~\Omega \mu \text{m}^{2}$ up to $15~\Omega \mu \text{m}^{2}$ and tunnel magnetoresistance ratios up to $\sim 100$ %. It was observed that the ferromagnetic coupling ( $H_{F})$ displayed by the transfer curves increases for smaller values of RA. This variation is attributed to the orange-peel coupling, which is caused by the MgO roughness, with a larger impact for thinner barriers. The RF output of the nanopillars caused by the spin-transfer torque-induced magnetic precession of the free layer was measured as a function of bias current and external magnetic fields for nanopillars with different RA values. Results concerning typical devices in the thin MgO region ( $1.3~\Omega \mu \text{m}^{2})$ and in a thicker MgO region ( $2.7~\Omega \mu \text{m}^{2})$ are shown to illustrate the dependence found. It was observed that the higher output power ( $\sim 300$ nW) and the smaller linewidth ( $\sim 90$ MHz) could be obtained with the higher RA region. These results suggest that there must be an optimum RA value for the production of nano-oscillator devices, which depends on the tradeoffs between the need for large breakdown voltages and strong spin polarization of the bias current (favored for larger RA values) and between the need of endurance under large current densities (favored for smaller RA values). Provided the breakdown voltage of the MgO barrrier can be made large enough, nanopillars based on MTJ stacks with an intermediate RA can provide a solution for the production of large output power spin-transfer nano-oscillators.

Prediction and Optimization of Linearity of MTJ Magnetic Sensors Based on Single-Domain Model

In the fabrication of linear magnetic tunnel junction (MTJ) sensors, the utilization of magnetic bias field along the easy-axis direction makes the response of MTJ nonlinear, and the Neel and magnetostatic coupling usually induces a strong interlayer coupling field along the hard-axis direction. In this paper, we investigated the influence of hard-axis and easy-axis bias fields on the linearity of MTJ magnetic sensor. The prediction and optimization of linearity were done by applying an appropriate hard-axis bias field in the specific easy-axis bias field and the practical tunnel magneto-resistance (TMR) ratio, and the model was verified by experiments. This paper can give guidance to improve the linearity and to maximize the practical TMR ratio of the MTJ sensors under a given linearity requirement.

Tunneling magnetoresistance based on a Cr/graphene/Cr magnetotunnel junction**Project supported by the National Natural Science Foundation of China (Grant Nos. 10874143, 10974166, and 11574260), the Program for New Century Excellent Talents in University, China (Grant No. NCET-10-0169), the National Innovation Foundation for Graduate, China (Grant No. 201310530003), and the Computational Support from Shanghai Super-computer Center, China.

Using the density functional theory and the nonequilibrium Green’s function method, we studied the finite-bias quantum transport in a Cr/graphene/Cr magnetotunnel junction (MTJ) constructed by a single graphene layer sandwiched between two semi-infinite Cr(111) electrodes. We found that the tunneling magnetoresistance (TMR) ratio in this MTJ reached 108%, which is close to that of a perfect spin filter. Under an external positive bias, we found that the TMR ratio remained constant at 65%, in contrast to MgO-based MTJs, the TMR ratios of which decrease with increasing bias. These results indicate that the Cr/graphene/Cr MTJ is a promising candidate for spintronics applications.

Large magnetocapacitance effect in magnetic tunnel junctions based on Debye-Fröhlich model

The frequency dependence of tunneling magnetocapacitance (TMC) in magnetic tunnel junctions (MTJs) is investigated theoretically and experimentally. According to the calculation based on Debye-Fröhlich model combined with Julliere formula, the TMC ratio strongly depends on the frequency and it has the maximum peak at a specific frequency. The calculated frequency dependence of TMC is in good agreement with the experimental results obtained in MgO-based MTJs with a tunneling magnetoresistance (TMR) ratio of 108%, which exhibit a large TMC ratio of 155% at room temperature. This calculation also predicts that the TMC ratio can be as large as about 1000% for a spin polarization of 87%, while the TMR ratio is 623% for the same spin polarization. These theoretical and experimental findings provide a deeper understanding on AC spin-dependent transport in the MTJs and will open up wider opportunities for device applications, such as highly sensitive magnetic sensors and impedance-tunable devices.

Large tunnel magnetoresistance ratio in Fe/O/NaCl/O/Fe

Magnetic tunnel junction (MTJ) is an important device element for many practical spintronic systems. In this paper, we propose and theoretically investigate a very attractive MTJ Fe(001)/O/NaCl(001)/O/Fe(001) as a two-terminal transport junction. By density functional theory total energy methods, we establish two viable device models: one with and the other without mirror symmetry across the center plane of the structure. Large tunnel magnetoresistance ratio (TMR) is predicted from first principles, at over 1800% and 3600% depending on the symmetry. Microscopically, a spin filtering effect is responsible for the large TMR. This effect essentially filters out all the minority spin channels (spin-down) from contributing to the tunnelling current. On the other hand, transport of the majority spin channel (spin-up) having Δ1 and Δ5 symmetry is enhanced by the FeO buffer layer in the MTJ.

Perpendicularly magnetized ferrimagnetic [Mn50Ga50/Co2FeAl] superlattice and the utilization in magnetic tunnel junctions

The ferrimagnetic superlattice (SL) [MnGa/Co2FeAl]n exhibiting perpendicular magnetic anisotropy opened a new method for spintronics materials used in magnetic random access memory, because of the high anisotropy, small damping constant and tunable magnetization. In this work, we fabricated SLs with different MnGa composition and studied the MnGa composition dependence of the structure and magnetic properties of the SLs. Furthermore, we fabricated fully perpendicular magnetic tunnel junctions with SLs as both top and bottom electrodes. A clear tunnel magnetoresistance (TMR) effect with TMR ratio of 1.3% at room temperature was observed.

Effect of gaas intermediary layer thickness on the properties of (Ga,Mn) as tri-layer structures

We have investigated the magnetic properties, current–voltage characteristics, and tunnel magnetoresistance of (Ga,Mn)As/GaAs/(Ga,Mn)As tri-layers having different thickness of the intermediary GaAs layer (1–7 nm). When the thickness of GaAs layer is less than 6 nm, the two (Ga,Mn)As layers couples ferromagnetically. For the tri-layers with thicker GaAs, the GaAs acts as an effective barrier for the carrier transport and decouples the two (Ga,Mn)As layers, which results in tunnel magnetoresistance ratio as high as 100% at 5 K.

Co2Fe6B2/MgO-Based Perpendicular Spin-Transfer-Torque Magnetic-Tunnel-Junction Spin-Valve without [Co/Pt]n lower Synthetic-Antiferromagnetic Layer

In recently years, the research on magnetic tunnel junctions (MTJs) using CoFeB magnetic layer and MgO tunneling barrier layer has been performed for realizing perpendicular-spin-transfer-torque magnetic-random-access-memory (p-STT-MRAM). In particular, to integrate a terra-bit-level nonvolatile memory-cell, the p-MTJ needs to achieve critical device performances such as a high tunneling magnetoresistance (TMR) ratio of ~150%, a high thermal stability (°â = E/KBT) of 74, and a low critical current density (Jo ) of 13.4 MA/cm2, where E, Ms , kB , and T are the energy barrier, the saturation magnetization, the Boltzmann constant, and the temperature, respectively.[1] In addition, to achieve CoFeB/MgO based p-MTJs grown on 12-inch TiN/W electrode, a design of [Co/Pt]n synthetic-anti-ferro-magnetic (SyAF) layer being operated at the ex-tu annealing above 350 oC has been intensively studied.[2] However, the [Co/Pt]n SyAF layer essentially has thicker a [Co/Pt]n SyAF layer since a [Co/Pt]n lower SyAF should couple antiferromagnetically with a [Co/Pt]n upper SyAF layer and couple ferromagnetically with a Co2Fe6B2 pinned layer at the same time.[3] Thicker SyAF layer has the problem of a high fabrication cost for p-MTJ spin-valves in the mass production of p-STT MRAM. Thus, we designed a Co2Fe6B2/MgO based p-MTJ spin-valve without a [Co/Pt]n lower SyAF layer, which can largely reduce the fabrication cost since the total multi-layer number (n) of [Co/Pt]n for the Co2Fe6B2/MgO-based p-MTJ spin-valve.Figures 1(a) and (b) show the spin-valve structure of a [Co/Pt]n SyAF layer with lower SyAF layer, without lower SyAF layer, respectively. The [Co/Pt]n SyAF layer without a lower SyAF layer is much thinner than that with a lower SyAF layer, as you can see in figure 1. In addition, the upper SyAF layer of p-MTJ without a lower SyAF layer is much thinner than that of p-MTJ with a lower SyAF layer because of fitting for antiferromagnetically coupling between lower and upper SyAF layer. And we studied the TMR ratio dependency on bcc nanoscale capping layer thickness of two structures by using current in-plane tunneling (CIPT) technique. In result, the maximum TMR ratios of two structures were both over 150%. In case of the p-MTJ with a [Co/Pt]n lower SyAF layer, the TMR ratio rapidly increased from 19% to 159% when the bcc nanoscale capping-layer thickness (tbcc ) increased from 0.3 to 0.6 nm. Then, it abruptly decreased from 159% to 2% when tbcc decreased from 0.6 to 0.73 nm. Also, for the p-MTJ spin-valve without a [Co/Pt]n lower SyAF layer, the TMR ratio very rapidly increased from 5% to 158% when tbcc increased from 0.3 to 0.6 nm. Then, it abruptly decreased from 158% to 2% when tbcc decreased from 0.6 to 0.73 nm.In the conference, we will present the effect of Co2Fe6B2/MgO-based perpendicular spin-transfer-torque magnetic-tunnel-junction spin-valve without [Co/Pt]n lower synthetic-antiferromagnetic layer and review the advantage of p-MTJ without lower SyAF layer. In addition, we report the physical and magnetic properties of p-MTJ by using vibrating-sampling-magnetometer (VSM), transmission-electron-microscopy (TEM). Furthermore, we review the mechanism why the magnetic properties of p-MTJs are varied by the thickness of SyAF layer. * This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No.2014R1A2A1A01006474) and the Brain Korea 21 PLUS Program in 2014. Reference [1-3] [1] K. C. Chun, H. Zhao, J. D. Harms, T. H. Kim, J. P. Wang, and C. H. Kim, IEEE J. Solid-State Circuits 48, 598, 2013.[2] S. Ikeda, H. Sato, H. Honjo, E. C. I. Enobio, S. Ishikawa, M. Yamanouchi, S. Fukami, S. Kanai, F. Matsukura, T. Endoh and H. Ohno, in IEEE International Electron Devices Meeting (IEDM) IEEE, 2014, pp. 33.2.1. [3] J. G. Park, T. H. Shim, K. S. Chae, D. Y. Lee, Y. Takemura, S. E. Lee, M. S. Jeon, J. U. Baek, S. O. Park, and J.P. Hong, in IEEE International Electron Devices Meeting (IEDM) IEEE, 2014, pp. 19.2.1. Figure 1

Thermal Stability Enhancement of Magnetic Perpendicular-Magnetic Tunnel Junctions Using Double MgO Interface Structure

Perpendicular spin-transfer-torque magnetic-random-access-memory (p-STT MRAM) has been investigated widely as a next generation memory, due to their capability of non-volatility, low power consumption, write endurance and high speed operation. Many efforts have been done to develop of p-STT MRAM, especially CoFeB/MgO-based perpendicular magnetic tunnel junction (p-MTJ) have widely studied because it shows high tunneling magnetoresistance (TMR) ratio caused by Δ1coherent tunneling effect and relatively low current density required than that of in-plane easy axis MTJ. To accommodate the p-MTJ to p-STT MRAM, p-MTJ should be increased the thermal stability (Δ=KuV/kBT where Ku is the anisotropy energy density, V is the volume of recording layer, kB is Boltzmann constant and T is absolute temperature respectively) of 74 to assure the data retention for 10 years. Thus, in this work, we investigated the effect of double MgO interface CoFeB-MgO based p-MTJ on the thermal stability. In CoFeB-MgO based p-MTJs, thermal stability is improve with increasing the thickness of CoFeB free (recording) layer [1]. However, magnetic anisotropy become in plane when CoFeB thickness over 1.3 nm [2]. With double MgO interface, CoFeB free layer maintain perpendicular magnetic anisotropy even the thickness is over 1.3 nm and lead to higher thermal stability. We prepared CoFeB-MgO based p-MTJs utilizing magnetron sputtering on TiN electrode constructed 12-inch Si wafers. Afterword p-MTJs were annealed at several ex-situ annealing temperatures (Tex ) for 1 hour under perpendicular magnetic field of 3T. And we analyzed p-MTJ magnetic properties by using Vibrating-Sample-Magnetometer (VSM), crystallinity by observing high resolution cross-sectional TEM (x-TEM) images and TMR ratio by using a current-in-plane tunnelling (CIPT) technique. Fig. 1 shows the anisotropy energy density (Kut) of p-MTJs as a function of CoFeB thickness and spacer thickness. Single MgO interface p-MTJ with Ta seed layer exhibited Δ=21 at Tex =275°C as shown in Fig. 1 (A). On the other hand, single MgO interface p-MTJ with bcc seed layer showed Δ=26 even after it was annealed at 400°C, as shown in Fig. 1 (B). We could improve D and annealing temperature at onetime using bcc seed layer, actually improving of annealing temperature is also required to accommodate p-STT-MRAM. Kut of Double MgO interface p-MTJ with bcc seed layer are shown in Fig. 1 (C). It showed almost two times higher Δ (61) than that of single MgO interface at Tex=400°C, and almost independent of spacer thickness. Fig. 2 shows the x-TEM images of single and double MgO interface p-MTJ spin-valves. We could obtain well crystallinity MgO barrier layer at both of single and double MgO interface p-MTJs, and those TMR ratio were 163 and 141% respectively. TMR ratio of double interface p-MTJ was decreased because of increasing of resistance area product (RA), which caused by bottom MgO layer. In the conference, we will present how thermal stability of p-MTJ using MgO double interface structures was improved and review those mechanisms by analyzing magnetic property, crystallinity of MgO and TMR ratio using VSM, x-TEM and CIPT respectively. * This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (No. 2014R1A2A1A01006474 & No. 1004608) and Brain Korea 21 PLUS Program in 2014.

Co2FeAl based magnetic tunnel junctions with BaO and MgO/BaO barriers

We succeed to integrate BaO as a tunneling barrier into Co2FeAl based magnetic tunnel junctions (MTJs). By means of Auger electron spectroscopy it could be proven that the applied annealing temperatures during BaO deposition and afterwards do not cause any diffusion of Ba neither into the lower Heusler compound lead nor into the upper Fe counter electrode. Nevertheless, a negative tunnel magnetoresistance (TMR) ratio of -10% is found for Co2FeAl (24 nm) / BaO (5 nm) / Fe (7 nm) MTJs, which can be attributed to the preparation procedure and can be explained by the formation of Co- and Fe-oxides at the interfaces between the Heusler and the crystalline BaO barrier by comparing with theory. Although an amorphous structure of the BaO barrier seems to be confirmed by high-resolution transmission electron microscopy (TEM), it cannot entirely be ruled out that this is an artifact of TEM sample preparation due to the sensitivity of BaO to moisture. By replacing the BaO tunneling barrier with an MgO/BaO double layer barrier, the electric stability could effectively be increased by a factor of five. The resulting TMR effect is found to be about +20% at room temperature, although a fully antiparallel state has not been realized.

Electronic and transmission properties of magnetotunnel junctions of cobalt/iron intercalated bilayer two dimensional sheets

Density functional theory and the nonequilibrium Green's function method are used to study the electronic properties and tunneling magnetoresistance (TMR) of magnetotunnel junctions (MTJs) based on Co/Fe intercalated bilayer graphene (bi-Gr), bilayer hexagonal boron nitride (bi-h-BN), and bilayer Gr-h-BN (bi-GBN). The spin-polarized bands around the Fermi energy of the two dimensional bilayer sheets are modulated by the intercalated cobalt. The TMR ratio reaches 169.94% and 173.00% for cobalt and iron intercalated Ni|bi-GBN|Ni MTJs, respectively. We observe that the Co/Fe intercalated bi-GBN is a promising candidate as a spacer in MTJs for spintronics.

Electronic and magnetic properties of off-stoichiometric Co2MnβSi/MgO interfaces studied by x-ray magnetic circular dichroism

We have studied the electronic and magnetic states of Co and Mn atoms at the interface of the Co2MnβSi (CMS)/MgO (β = 0.69, 0.99, 1.15, and 1.29) magnetic tunnel junction (MTJ) by means of x-ray magnetic circular dichroism. In particular, the Mn composition (β) dependences of the Mn and Co magnetic moments were investigated. The experimental spin magnetic moments of Mn, mspin(Mn), derived from x-ray magnetic circular dichroism weakly decreased with increasing Mn composition β in going from Mn-deficient to Mn-rich CMS films. This behavior was explained by first-principles calculations based on the antisite-based site-specific formula unit (SSFU) composition model, which assumes the formation of only antisite defect, not vacancies, to accommodate off-stoichiometry. Furthermore, the experimental spin magnetic moments of Co, mspin(Co), also weakly decreased with increasing Mn composition. This behavior was consistently explained by the antisite-based SSFU model, in particular, by the decrease in the concentration of CoMn antisites detrimental to the half-metallicity of CMS with increasing β. This finding is consistent with the higher tunnel magnetoresistance ratios which have been observed for CMS/MgO/CMS MTJs with Mn-rich CMS electrodes.

Impact of seed layer on post-annealing behavior of transport and magnetic properties of Co/Pt multilayer-based bottom-pinned perpendicular magnetic tunnel junctions

The magnetic and transport properties of Co/Pt multilayer-based bottom-pinned perpendicular magnetic tunnel junctions (pMTJs) on Ru, Hf, and Ru/Hf seed layers (SLs) were investigated after annealing at different temperatures. The perpendicular synthetic antiferromagnetic (pSAF) layer on the Ru SL was found to be thermally robust (after annealing at 400 °C for 30 min). A high tunnel magnetoresistance (TMR) ratio of 100% was achieved at a low resistance-area product (5.5 Ω·µm2) and was stable up to 350 °C. For the stack on Ru SL, TMR degradation after annealing was caused by the degradation of the pMTJ (CoFeB/MgO/CoFeB), while in the Hf and Ru/Hf SL, both the pMTJ and pSAF were affected.