The interface structure of Fe/MgO(100) magnetic tunnel junctions predicted by density functional theory (DFT) depends significantly on the choice of exchange and correlation functional. Bader analysis reveals that structures obtained by relaxing the cell with the local spin-density approximation (LSDA) display a different charge transfer than those relaxed with the generalized gradient approximation (GGA). As a consequence, the electronic transport is found to be extremely sensitive to the interface structure. In particular, the conductance for the LSDA-relaxed geometry is about one order of magnitude smaller than that of the GGA-relaxed one. The high sensitivity of the electronic current to the details of the interface might explain the discrepancy between the experimental and calculated values of magnetoresistance.
MgO based magnetic tunnel junctions are prepared to investigate the dielectric breakdown of the tunnel barrier. The breakdown is directly visualized by transmission electron microscopy measurements. The broken tunnel junctions are prepared for the microscopy measurements by focussed ion beam out of the junctions characterized by transport investigations. Consequently, a direct comparison of transport behavior and structure of the intact and broken junctions is obtained. Compared to earlier findings in Alumina based junctions, the MgO barrier shows much more microscopic pinholes after breakdown…
We investigated dependence of tunnel magnetoresistance effect in CoFeB/MgO/CoFeB magnetic tunnel junctions on Ar pressure during MgO-barrier sputtering. Sputter deposition of MgO-barrier at high Ar pressure of 10 mTorr resulted in smooth surface and highly (001) oriented MgO. Using this MgO as a tunnel barrier, tunnel magnetoresistance (TMR) ratio as high as 355% at room temperature (578% at 5K) was realized after annealing at 325 C or higher, which appears to be related to a highly (001) oriented CoFeB texture promoted by the smooth and highly oriented MgO. Electron-beam lithography defined d…
We compute thermal spin transfer torques (TST) in Fe-MgO-Fe tunnel junctions using a first principles wave function-matching method. At room temperature, the TST in a junction with 3 MgO monolayers amounts to 10^-7J/m^2/K, which is estimated to cause magnetization reversal for temperature differences over the barrier of the order of 10 K. The large TST can be explained by multiple scattering between interface states through ultrathin barriers. The angular dependence of the TST can be very skewed, possibly leading to thermally induced high-frequency generation.
We created epitaxial magnetic tunnel junctions of FeCo/MgO/EuS on MgO buffered Si (100). Tunnel magnetoresistance reached up to 64% at 4.2 K. An unexpected fast drop of magnetoresistance was recorded for MgO thickness above 1 nm, which is attributed to the forced nonspecular conductance across the EuS conduction band minimum located at the X point, rather than the desired Delta_1 conductance centered around the Gamma point.
The Mn$_3$Ga Heusler compound and related alloys are the most promising materials for the realization of spin-transfer-torque switching in magneto tunneling junctions. Improved performance can be achieved by high quality interfaces in these multilayered structured devices. In this context, the interface between Mn$_{1.63}$Ga and MgO is of particular interest because of its spin polarization properties in tunneling junctions. We performed a chemical characterization of the MgO/Mn$_{1.63}$Ga junction by hard x-ray photoelectron spectroscopy (HAXPES). The experiment indicated the formation of Ga-…
The electronic band structure in the epitaxial Fe/MgO/GaAs(001) tunnel junction has been studied by X-ray and ultraviolet photoelectron spectroscopy measurements. The Schottky barrier height (SBH) of Fe on MgO/GaAs heterostructure is determined to be 3.3+-0.1eV, which sets the Fe Fermi level about 0.3eV above the GaAs valence band maximum. This SBH is also exactly the same as that measured from Fe on MgO monocrystal. After Fe deposition, no band bending change is observed in MgO and GaAs underlayers. On the contrary, Au and Al deposition lead to clear variation of the band bending in both MgO …
We report on spin polarization reduction by incoherent tunneling in realistic single crystal Co2FeAl/MgO/Co50Fe50 magnetic tunnel junctions (MTJ) compared to reference Fe/MgO/Fe. A large density of misfit dislocations in the Heusler based MTJs has been insured by a thick MgO barrier and its 3.8% lattice mismatch with the Co2FeAl electrode. Our analysis implicates a correlated structural-transport approach. The crystallographic coherence, in the real space, is investigated using High Resolution Transmission Electron Microscopy phase analysis. The electronic transport experiments in variable tem…
Crystallographic and spin polarized transport properties of (100) textured and (100) epitaxial Fe/MgO/Fe magnetic tunnel junctions are compared. Strong similarities in the transport properties show that structural coherence and magnetic quality at the 25 nm grain scale in textured junctions are sufficient to issue signatures of the spin polarized transport specific to a single crystal junction. This demonstrates that the lateral coherence of the Bloch tunneling wave function is identically limited in both systems. Our analysis leads to model the textured tunnel junction as a juxtaposition of n…
Thermal stability factor (delta) of recording layer was studied in perpendicular anisotropy CoFeB/MgO magnetic tunnel junctions (p-MTJs) with various CoFeB recording layer thicknesses and junction sizes. In all series of p-MTJs with different thicknesses, delta is virtually independent of the junction sizes of 48-81 nm in diameter. The values of delta increase linearly with increasing the recording layer thickness. The slope of the linear fit is explained well by a model based on nucleation type magnetization reversal.
The electronic structure and spin-dependent tunneling in epitaxial Fe/MgO/Fe(001) tunnel junctions are studied using first-principles calculations. For small MgO barrier thickness the minority-spin resonant bands at the two interfaces make a significant contribution to the tunneling conductance for the antiparallel magnetization, whereas these bands are, in practice, mismatched by disorder and/or small applied bias for the parallel magnetization. This explains the experimentally observed decrease in tunneling magnetoresistance (TMR) for thin MgO barriers. We predict that a monolayer of Ag epit…
Magnetic tunnel junctions with crystalline MgO tunnel barrier and amorphous CoFeB electrodes received much attention due to their high tunnel magneto resistance ratio at room temperature. One important parameter for achieving high tunnel magneto resistance ratios is to control the boron diffusion from the electrodes especially during post growth annealing. By high resolution transmission electron microscopy and electron energy loss spectroscopy techniques we show that the cap layer material adjacent to the electrodes and the MgO deposition method are crucial to control boron redistribution. It…
A magnetic tunnel junction Fe3O4/MgO/Fe with (001) layer orientation is considered. The junction magnetic energy is analyzed as a function of the angle between the layer magnetization vectors under various magnetic fields. The tunnel magnetoresistance is calculated as a function of the external magnetic field. In contrast with junctions with unidirectional anisotropy, a substantially lower magnetic field is required for the junction switching.
Magnetic tunnel junctions for spin-transfer torque switching were prepared to investigate the dielectric breakdown. The breakdown occurs typically at voltages not much higher than the switching voltages, a bottleneck for the implementation of spin-transfer torque Magnetic Random Access Memory. Intact and broken tunnel junctions are characterized by transport measurements and then prepared for transmission electron microscopy and energy dispersive x-ray spectrometry by focussed ion beam. The comparison to our previous model of the electric breakdown for thicker MgO tunnel barriers reveals signi…
We report tunnel magnetoresistance (TMR) ratios as high as 472% at room temperature and 804% at 5 K in pseudo-spin valve (SV) CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) annealed at 450oC, which is approaching the theoretically predicted value. By contrast, the TMR ratios for exchange-biased (EB) SV MTJs with a MnIr antiferromagnetic layer are found to drop when they are annealed at 450oC. Energy dispersive X-ray analysis shows that annealing at 450oC induces interdiffusion of Mn and Ru atoms into the MgO barrier and ferromagnetic layers in EB-SV MTJs. Mechanisms behind the different anne…
A possible mechanism for the resistive switching observed experimentally in Fe/MgO/V/Fe junctions is presented. Ab initio total energy calculations within the local density approximation and pseudopotential theory shows that by moving the oxygen ions across the MgO/V interface one obtains a metastable state. It is argued that this state can be reached by applying an electric field across the interface. In addition, the ground state and the metastable state show different electric conductances. The latter results are discussed in terms of the changes of the density of states at the Fermi level …
Magnetic tunnel junctions with the layer sequence Co$_2$Cr$_{0.6}$Fe$_{0.4}$Al/MgO/CoFe were fabricated by magnetron sputtering at room temperature (RT). The samples exhibit a large inverse tunneling magnetoresistance (TMR) effect of up to -66% at RT. The largest value of -84% at 20 K reflects a rather weak influence of temperature. The dependence on the voltage drop shows an unusual behavior with two almost symmetric peaks at $\pm600$ mV with large inverse TMR ratios and small positive values around zero bias.
We investigated the relationship between the tunnel magnetoresistance (TMR) ratio and the electrode structure in MgO-barrier magnetic tunnel junctions (MTJs). The TMR ratio in a MTJ with Co40Fe40B20 reference and free layers reached 355% at the post-deposition annealing temperature of Ta=400 degree C. When Co50Fe50 or Co90Fe10 is used for the reference layer material, no high TMR ratio was observed. The key to have high TMR ratio is to have highly oriented (001) MgO barrier/CoFeB crystalline electrodes. The highest TMR ratio obtained so far is 450% at Ta = 450 degree C in a pseudo spin-valve M…
We present a detailed study of the spin-torque diode effect in CoFeB/MgO/CoFe/NiFe magnetic tunnel junctions. From the evolution of the resonance frequency with magnetic field at different angles, we clearly identify the free-layer mode and find an excellent agreement with simulations by taking into account several terms for magnetic anisotropy. Moreover, we demonstrate the large contribution of the out-of-plane torque in our junctions with asymmetric electrodes compared to the in-plane torque. Consequently, we provide a way to enhance the sensitivity of these devices for the detection of micr…
By use of first-principles electronic structure calculations, we predict that the magnetoresistance of the bcc Co(100)/MgO(100)/bcc Co(100) and FeCo(100)/MgO(100)/FeCo(100) tunneling junctions can be several times larger than the very large magnetoresistance predicted for the Fe(100)/MgO(100)/Fe(100) system. The origin of this large magnetoresistance can be understood using simple physical arguments by considering the electrons at the Fermi energy travelling perpendicular to the interfaces. For the minority spins there is no state with $Δ_1$ symmetry whereas for the majority spins there is onl…
Giant tunnel magnetoresistance (TMR) ratios of 417% at room temperature (RT) and 914% at 3 K were demonstrated in epitaxial Fe/MgO/Fe(001) exchanged-biased spin-valve magnetic tunnel junctions (MTJs) by tuning growth conditions for each layer, combining sputter deposition for the Fe layers, electron-beam evaporation of the MgO barrier, and barrier interface tuning. Clear TMR oscillation as a function of the MgO thickness with a large peak-to-valley difference of ~80% was observed when the layers were grown on a highly (001)-oriented Cr buffer layer. Specific features of the observed MTJs are s…
We observe both dc voltage rectification and frequency conversion that occur when a reference microwave current is injected to a MgO based magnetic tunnel junction (MTJ). The rectification that is spin-transfer torque dependent is observed when the frequency of the input microwave current coincides with the resonance frequency of the magnetization of the active layer. In addition, we demonstrate that frequency conversion is the result of amplitude modulation between the reference signal and the resistance of the MTJ that is fluctuating at the resonance frequency of the magnetization of the act…
Ultra-thin magnetic tunnel junctions with low resistive MgO tunnel barriers are prepared to examine their stability under large current stress. The devices show magnetoresistance ratios of up to 110 % and an area resistance product of down to 4.4 ohm micrometer squared. If a large current is applied, a reversible resistance change is observed, which can be attributed to two different processes during stressing and one relaxation process afterwards. Here, we analyze the time dependence of the resistance and use a simple model to explain the observed behavior. The explanation is further supporte…
The authors studied an effect of ferromagnetic (Co20Fe60B20 or Fe) layer insertion on tunnel magnetoresistance (TMR) properties of MgO-barrier magnetic tunnel junctions (MTJs) with CoFe/Pd multilayer electrodes. TMR ratio in MTJs with CoFeB/MgO/Fe stack reached 67% at an-nealing temperature (Ta) of 200 degree C and then decreased rapidly at Ta over 250 degree C. The degradation of the TMR ratio may be related to crystallization of CoFe(B) into fcc(111) or bcc(011) texture result-ing from diffusion of B into Pd layers. MTJs which were in-situ annealed at 350oC just after depo-siting bottom CoFe…
We study IrCrMnZ (Z=Al, Ga, Si, Ge) systems using first-principles calculations from the perspective of their application as the electrode materials of MgO-based MTJs. These materials have highly spin-polarized conduction electrons with partially occupied $Δ_1$ band, which is important for coherent tunneling in parallel magnetization configuration. The Curie temperatures of IrCrMnAl and IrCrMnGa are very high (above 1300 K) as predicted from mean-field-approximation. The stability of ordered phase against various antisite disorders has been investigated. We discuss here the effect of "spin-orb…
We demonstrate the strong impact of the oscillator agility on the line broadening by studying spin transfer induced microwave emission in MgO-based tunnel junctions with current. The linewidth is almost not affected by decreasing the temperature. At very low currents, a strong enhancement of the linewidth at low temperature is attributed to an increase of the non linearity, probably due to the field-like torque. Finally we evidence that the noise is not dominated by thermal fluctuations but rather by the chaotization of the magnetization system induced by the spin transfer torque.
The knowledge of chemical and magnetic conditions at the Co40Fe40B20 / MgO interface is important to interpret the strong annealing temperature dependence of tunnel magnetoresistance of Co-Fe-B / MgO / Co-Fe-B magnetic tunnel junctions, which increases with annealing temperature from 20% after annealing at 200C up to a maximum value of 112% after annealing at 350C. While the well defined nearest neighbor ordering indicating crystallinity of the MgO barrier does not change by the annealing, a small amount of interfacial Fe-O at the lower Co-Fe-B / MgO interface is found in the as grown samples,…
We report on systematic ab-initio investigations of Co and Cr interlayers embedded in Fe(001)/MgO/Fe(001) magnetic tunnel junctions, focusing on the changes of the electronic structure and the transport properties with interlayer thickness. The results of spin-dependent ballistic transport calculations reveal options to specifically manipulate the tunnel magnetoresistance ratio. The resistance area products and the tunnel magnetoresistance ratios show a monotonous trend with distinct oscillations as a function of the Cr thickness. These modulations are directly addressed and interpreted by mea…
Flexible electronic devices require the integration of multiple crucial components on soft substrates to achieve their functions. In particular, memory devices are the fundamental component for data storage and processing in flexible electronics. Here, we present flexible MgO barrier magnetic tunnel junction (MTJ) devices fabricated using a transfer printing process, which exhibit reliable and stable operation under substantial deformation of the device substrates. In addition, the flexible MTJ devices yield significantly enhanced tunneling magnetoresistance (TMR) of ~300 % and improved abrupt…
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$μ$V in comparison to 90$μ$V for the MTJs with MgO barrier when heated with the maximum laser power…
We present a comparison of the tunnel magneto-Seebeck effect for laser induced and intrinsic heating. Therefore, Co$_{40}$Fe$_{40}$B$_{20}$/MgAl$_2$O$_4$ and Co$_{25}$Fe$_{55}$B$_{20}$/MgO magnetic tunnel junctions have been prepared. The TMS ratio of 3\,\% in case of the MAO MTJ agrees well with ratios found for other barrier materials, while the TMS ratio of 23\,\% of the MgO MTJ emphasizes the influence of the CoFe composition. We find results using the intrinsic method that differ in sign and magnitude in comparison to the results of the laser heating. The intrinsic contributions can alter…
We demonstrate spin torque induced auto-oscillation in MgO-based magnetic tunnel junctions. At the generation threshold, we observe a strong line narrowing down to 6 MHz at 300K and a dramatic increase in oscillator power, yielding spectrally pure oscillations free of flicker noise. Setting the synthetic antiferromagnet into autooscillation requires the same current polarity as the one needed to switch the free layer magnetization. The induced auto-oscillations are observed even at zero applied field, which is believed to be the acoustic mode of the synthetic antiferromagnet. While the phase c…
Heterostructures composed of ferromagnetic layers that are mutually interacting through a nonmagnetic spacer are at the core of magnetic sensor and memory devices. In the present study, layer-resolved ferromagnetic resonance was used to investigate the coupling between the magnetic layers of a Co/MgO/Permalloy magnetic tunnel junction. Two magnetic resonance peaks were observed for both magnetic layers, as probed at the Co and Ni L3 x-ray absorption edges, showing a strong interlayer interaction through the insulating MgO barrier. A theoretical model based on the Landau-Lifshitz-Gilbert-Sloncz…
Spin-pumping generates pure spin currents in normal metals at the ferromagnet (F)/normal metal (N) interface. The efficiency of spin-pumping is given by the spin mixing conductance, which depends on N and the F/N interface. We directly study the spin-pumping through an MgO tunnel-barrier using the inverse spin Hall effect, which couples spin and charge currents and provides a direct electrical detection of spin currents in the normal metal. We find that spin-pumping is suppressed by the tunnel-barrier, which is contrary to recent studies that suggest that the spin mixing conductance can be enh…
Magnetite (Fe3O4) is an eligible candidate for magnetic tunnel junctions (MTJs) since it shows a high spin polarization at the Fermi level as well as a high Curie temperature of 585°C. In this study, Fe3O4/MgO/Co-Fe-B MTJs were manufactured. A sign change in the TMR is observed after annealing the MTJs at temperatures between 200°C and 280°C. Our findings suggest an Mg interdiffusion from the MgO barrier into the Fe3O4 as the reason for the change of the TMR. Additionally, different treatments of the magnetite interface (argon bombardment, annealing at 200°C in oxygen atmosphere) during the pr…
We report the intrinsic critical current density (Jc0) in current-induced magnetization switching and the thermal stability factor (E/kBT, where E, kB, and T are the energy potential, the Boltzmann constant, and temperature, respectively) in MgO based magnetic tunnel junctions with a Co40Fe40B20(2nm)/Ru(0.7-2.4nm)/Co40Fe40B20(2nm) synthetic ferrimagnetic (SyF) free layer. We show that Jc0 and E/kBT can be determined by analyzing the average critical current density as a function of coercivity using the Slonczewski's model taking into account thermal fluctuation. We find that high antiferromagn…
We investigated the dependence of giant tunnel magnetoresistance (TMR) on the thickness of an MgO barrier and on the annealing temperature of sputtered CoFeB/MgO/CoFeB magnetic tunnel junctions deposited on SiO2/Si wafers. The resistance-area product exponentially increases with MgO thickness, indicating that the quality of MgO barriers is high in the investigated thickness range of 1.15-2.4 nm. High-resolution transmission electron microscope images show that annealing at 375 C results in the formation of crystalline CoFeB/MgO/CoFeB structures, even though CoFeB electrodes are amorphous in th…
We studied the thermoelectric coefficients (Seebeck and thermal conductance)of FeCo|MgO|FeCo(001) magnetic tunnel junctions (MTJs) from first principles using a generalized Landauer-Büttiker formalism. FeCo|MgO|FeCo(001) MTJs usually yield smaller thermoelectric effects compared with epitaxial Fe|MgO|Fe(001) MTJs. The (magneto-) Seebeck effect is sensitive to the details of the FeCo$|$MgO interfaces. Interfacial oxygen vacancies (OVs) can enhance the thermoelectric effects in MTJs greatly. We also compute angular dependent Seebeck coefficients that provide more information about the transport …
Current-driven magnetization switching in low-resistance Co40Fe40B20/MgO/Co40Fe40B20 magnetic tunnel junctions (MTJs) is reported. The critical-current densities Jc required for current-driven switching in samples annealed at 270C and 300C are found to be as low as 7.8 x 10^5 A/cm^2 and 8.8 x 10^5 A/cm^2 with accompanying tunnel magnetoresistance (TMR) ratios of 49% and 73 %, respectively. Further annealing of the samples at 350C increases TMR ratio to 160 %, while accompanying Jc increases to 2.5 x 10^6 A/cm^2. We attribute the low Jc to the high spin-polarization of tunnel current and small …
We study the annealing stability of bottom-pinned perpendicularly magnetized magnetic tunnel junctions based on dual MgO free layers and thin fixed systems comprising a hard [Co/Ni] multilayer antiferromagnetically coupled to thin a Co reference layer and a FeCoB polarizing layer. Using conventional magnetometry and advanced broadband ferromagnetic resonance, we identify the properties of each sub-unit of the magnetic tunnel junction and demonstrate that this material option can ensure a satisfactory resilience to the 400$^\circ$C thermal annealing needed in solid-state magnetic memory applica…
To get stable perpendicularly magnetized tunnel junctions at small device dimensions, composite free layers that comprise two MgO/FeCoB interfaces as sources of interface anisotropy are generally used. Proper cristallisation and annealing robustness is typically ensured by the insertion of a spacer layer of the early transition metal series within the FeCoB layer. We study the influence of the spacer thickness and growth condition on the switching metrics of tunnel junctions thermally annealed at 400$^\circ$C for the case of 1-4 Å Ta spacers. Thick Ta spacer results in a large anisotropies ind…
We prepared magnetic tunnel junctions with one ferromagnetic and one superconducting Al-Si electrode. Pure cobalt electrodes were compared with a Co-Fe-B alloy and the Heusler compound Co2FeAl. The polarization of the tunneling electrons was determined using the Maki-Fulde-model and is discussed along with the spin-orbit scattering and the total pair-breaking parameters. The junctions were post-annealed at different temperatures to investigate the symmetry filtering mechanism responsible for the giant tunneling magnetoresistance ratios in Co-Fe-B/ MgO/ Co-Fe-B junctions.
We report the successful electrical creation of spin polarization in p-type Si at room temperature by using an epitaxial MgO(001) tunnel barrier and Fe(001) electrode. Reflection high-energy electron diffraction observations revealed that epitaxial Fe/MgO(001) tunnel contacts can be grown on a (2 x 1) reconstructed Si surface whereas tunnel contacts grown on the (1 x 1) Si surface were polycrystalline. Transmission electron microscopy images showed a more flat interface for the epitaxial Fe/MgO/Si compared to that of the polycrystalline structure. For the Fe/MgO/p-Si devices, the Hanle and inv…
Group-IV-based ferromagnetic semiconductor Ge1-xFex (GeFe) is one of the most promising materials for efficient spin injectors and detectors for Si and Ge. Recent first principles calculations (Sakamoto et al., Ref. 9) suggested that the Fermi level is located in two overlapping largely spin-polarized bands formed in the bandgap of GeFe; spin-down d(e) band and spin-up p-d(t2) band. Thus, it is important to clarify how these bands contribute to spin injection and detection. In this study, we show the first successful observation of the tunneling magnetoresistance (TMR) in magnetic tunnel junct…
The capacitance of MgO based magnetic tunnel junctions (MTJs) has been observed to be magnetic field dependent. We propose an equivalent circuit for the MTJs with a parallel-leaky capacitance (Cl) across the series combination of geometric and interfacial capacitance. The analysis of junctions with different tunneling magnetoresistance values suggests higher Cl for low TMR junctions. Using Cole-Cole plots the capacitive nature of MTJs is manifested. Fitting with Maxwell-Wagner capacitance model validates the RC parallel network model for MTJs and the extracted field dependent parameters match …
By carrying out density functional theory analysis within the Keldysh non-equilibrium Green's functional formalism, we have calculated the nonlinear and non-equilibrium quantum transport properties of Fe/MgO/Fe trilayer structures as a function of external bias voltage. For well relaxed atomic structures of the trilayer, the equilibrium tunnel magnetoresistance ratio (TMR) is found to be very large and also fairly stable against small variations in the atomic structure. As a function of external bias voltage, the TMR reduces monotonically to zero with a voltage scale of about 1V, in agreement …
Here we demonstrate that both, tunnel magneto resistance (TMR) and resistive switching (RS), can be observed simultaneously in nano-scale magnetic tunnel junctions. The devices show bipolar RS of 6 % and TMR ratios of about 100 %. For each magnetic state, multiple resistive sates are created depending on the bias history which provides a method for multi-bit data storage and logic. The electronic transport measurements are discussed in the framework of a memristive system. Differently prepared MgO barriers are compared to gain insight into the switching mechanism.
An efficient electrical spin injection into an InGaAs/GaAs quantum well light emitting diode is demonstrated thanks to a CoFeB/MgO spin injector. The textured MgO tunnel barrier is fabricated by two different techniques: sputtering and molecular beam epitaxy (MBE). The maximal spin injection efficiency is comparable for both methods. Additionally, the effect of annealing is also investigated for the two types of samples. Both samples show the same trend: an increase of the electroluminescence circular polarization (Pc) with the increase of annealing temperature, followed by a saturation of Pc …
We fabricate fully epitaxial Fe/ZnO/MgO/Fe magnetic tunnel junctions (MTJs) with a bilayer tunnel barrier, in which ZnO has a metastable rock-salt crystal structure. We observe a high magnetoresistance ratio up to 96% at room temperature (RT) and find that these MTJs have asymmetric current-voltage characteristics, and their rectifying performances are largely dependent on the magnetization alignments of the Fe electrodes. Diode responsibilities at a zero-bias voltage ($β_{0}$), which is an important performance index for harvesting applications, are observed up to 1.3 A/W at RT in the antipar…
Magnetocrystalline anisotropy at Heusler alloy$|$MgO interfaces have been studied using first principles calculations. It is found that Co terminated Co$_{2}$FeAl$|$MgO interfaces show perpendicular magnetic anisotropy up to 1.31 mJ/m$^2$, while those with FeAl termination exhibit in-plane magnetic anisotropy. Atomic layer resolved analysis indicates that the origin of perpendicular magnetic anisotropy in Co$_{2}$FeAl$|$MgO interfaces can be attributed to the out-of-plane orbital contributions of interfacial Co atoms. At the same time, Co$_{2}$MnGe and Co$_{2}$MnSi interfaced with MgO tend to …
We present an {\it ab-initio} study of the spin-transfer torque in a Fe/MgO/FePt/Fe magnetic tunnel junctions. We consider a FePt film with a thickness up to six unit cells, either in direct contact with the MgO spacer or with an intercalated ultra-thin Fe seed layer. We find that in the FePt layer the torque is not attenuated as strongly as in the case of pure Fe. Moreover, in FePt the torque alternates sign at the Fe and Pt atomic planes throughout the stack for all FePt thicknesses considered. Finally, when Fe is intercalated between MgO and L$1_0$-FePt, the torque is sharply attenuated and…
Recently, it has been shown that magnetic tunnel junctions with thin MgO tunnel barriers exhibit extraordinarily high tunneling magnetoresistance (TMR) values at room temperature1, 2. However, the physics of spin dependent tunneling through MgO barriers is only beginning to be unravelled. Using planar magnetic tunnel junctions in which ultra-thin layers of magnetic metals are deposited in the middle of a MgO tunnel barrier here we demonstrate that the TMR is strongly modified when these layers are discontinuous and composed of small pancake shaped nanodots. At low temperatures, in the Coulomb …
We investigate bias voltage effects on the spin-dependent transport properties of Fe/MgAl${}_2$O${}_4$/Fe(001) magnetic tunneling junctions (MTJs) by comparing them with those of Fe/MgO/Fe(001) MTJs. By means of the nonequilibrium Green's function method and the density functional theory, we calculate bias voltage dependences of magnetoresistance (MR) ratios in both the MTJs. We find that in both the MTJs, the MR ratio decreases as the bias voltage increases and finally vanishes at a critical bias voltage $V_{\rm c}$. We also find that the critical bias voltage $V_{\rm c}$ of the MgAl${}_2$O${…
The electronic transport and spincaloric properties of epitaxial magnetic tunnel junctions with half-metallic Co$_2$MnSi Heusler electrodes, MgO tunneling barriers, and different interface terminations are investigated by using first-principles calculations. A new approach to spincaloric properties is presented that circumvents the linear response approximation inherent in the Seebeck coefficient and compared to the method of Sivan and Imry. This approach supports two different temperatures in the two electrodes and provides the exact current and/or voltage response of the system. Moreover, it…
We present results on all-MgB2 tunnel junctions, where the tunnel barrier is deposited MgO or native-oxide of base electrode. For the junctions with MgO, the hysteretic I-V curve resembles a conventional underdamped Josephson junction characteristic with critical current-resistance product nearly independent of the junction area. The dependence of the critical current with temperature up to 20 K agrees with the [Ambegaokar and Baratoff, Phys. Rev. Lett. 10, 486 (1963)] expression. For the junctions with native-oxide, conductance at low bias exhibits subgap features while at high bias reveals t…
We study the effect of strain on magnetic tunnel junctions (MTJ) induced by a diamond like carbon (DLC) film. The junction resistance as well as the tunnel magnetoresistance (TMR) reduces with the DLC film. Non-equilibrium Green's function quantum transport calculations show that the application of biaxial strain increases the conductance for both the parallel and anti-parallel configurations. However, the conductance for the minority channel and for the anti-parallel configuration is significantly more sensitive to strain, which drastically increases transmission through a MgO tunnel barrier,…
Alloying Fe electrodes with V, through reduced FeV/MgO interface mismatch in epitaxial magnetic tunnel junctions with MgO barriers, notably suppresses both nonmagnetic (parallel) and magnetic (antiparallel) state 1/f noise and enhances tunnelling magnetoresistance (TMR). A comparative study of the room temperature electron transport and low frequency noise in Fe1-xVx/MgO/Fe and Fe/MgO/Fe1-xVx MTJs with 0 <= x <= 0.25 reveals that V doping of the bottom electrode for x < 0.1 reduces in nearly 2 orders of magnitude the normalized nonmagnetic and magnetic 1/f noise. We attribute the enhanced TMR …
We report on room temperature low frequency noise due to magnetic inhomogeneities/domain walls (MI/DWs) in elliptic submicron FeCoB/MgO/FeCoB magnetic tunnel junctions with an area between 0.0245 and 0.0675μm2. In the smaller area junctions we found an unexpected random telegraph noise (RTN1), deeply in the parallel state, possibly due to stray field induced MI/DWs in the hard layer. The second noise source (RTN2) is observed in the antiparallel state for the largest junctions. Strong asymmetry of RTN2 and of related resistance steps with current indicate spin torque acting on the MI/DWs in th…
In this work we have analysed the deterimental effect on conductance caused by diffusion of hydrogen atoms in interstitial voids during fabrication process of magnesium oxide barrier of an Fe MgO Fe magnetic tunnel junction, using first principle calculations. This diffusion of hydrogen atoms in interstitial voids in barrier disturbs a certain kind of symmetry possessed by magnesium oxide often termed as delta 1 state symmetry. Distortion in delta 1 state symmetry tempers the condutance through the magnetic tunnel junction specially in parallel configuration, which in turn reduces the Tunnelli…
Motivated by observation of very high tunnel magnetoresistance (TMR) in Fe-MgO-Fe magnetic tunnel junction devices, we propose a theoretical model for these devices based on a single-band tight-binding approximation. An effort is made to capture the band dispersions over the two dimensional transverse Brillouin zone. In the transport direction, spin dependent Hamiltonian is prescribed for Delta_1 and Delta_5 bands. Non-equilibrium Green's function formalism is then used to calculate transport. Features like voltage dependence of TMR are captured quantitatively within this simple model and the …
