Magnetic tunnel junctions with an equiatomic quaternary CoFeMnSi Heusler alloy electrode (original) (raw)
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Co 40 Fe 40 B 20 / MgO single and double barrier magnetic tunnel junctions ͑MTJs͒ were grown using target-facing-target sputtering for MgO barriers and conventional dc magnetron sputtering for Co 40 Fe 40 B 20 ferromagnetic electrodes. Large tunnel magnetoresistance ͑TMR͒ ratios, 230% for single barrier MTJs and 120% for the double barrier MTJs, were obtained after postdeposition annealing in a field of 800 mT. The lower TMR ratio for double barrier MTJs can be attributed to the amorphous nature of the middle Co 40 Fe 40 B 20 free layer, which could not be crystallized during postannealing. A highly asymmetric bias voltage dependence of the TMR can be observed for both single and double barrier MTJs in the as-deposited states and after field annealing at low temperature. The asymmetry decreases with increasing annealing temperature and the bias dependence becomes almost symmetric after annealing at 350°C. Maximum output voltages of 0.65 and 0.85 V were obtained for both single and double barrier MTJs, respectively, after annealing at 300°C, a temperature which is high enough for large TMR ratios but insufficient to completely remove asymmetry from the TMR bias dependence.
We investigated the I-V curves and differential tunneling conductance of two, CoFeB/MgO/CoFeB-based, magnetic tunnel junctions (MTJs): one with a low tunneling magnetoresistance (TMR; 22%) and the other with a high TMR (352%). This huge TMR difference was achieved by different MgO sputter conditions rather than by different annealing or deposition temperature. In addition to the TMR difference, the junction resistances were much higher in the low-TMR MTJ than in the high-TMR MTJ. The low-TMR MTJ showed a clear parabolic behavior in the dI/dV-V curve. This high resistance and parabolic behavior were well explained by the Simmons' simple barrier model. However, the tunneling properties of the high-TMR MTJ could not be explained by this model. The characteristic tunneling properties of the high-TMR MTJ were a relatively low junction resistance, a linear relation in the I-V curve, and conduction dips in the differential tunneling conductance. We explained these features by applying the coherent tunneling model.
Solid State Communications, 2005
We measured differential tunneling conductance (dI/dV, d 2 I/dV 2) spectra of spin-valve-type magnetic tunnel junctions (MTJs) with a MgO(001) tunnel barrier layer and amorphous CoFeB ferromagnetic electrodes that show 315% magnetoresistance (MR) ratio at 4.3 K. The dI/dV spectra showed clear reduction in the conductance at around G400 mV for a parallel magnetic configuration. Such anomalous spectra have never been observed for MTJs with an amorphous Al-O barrier. The d 2 I/dV 2 spectra showed several distinct peaks between 5 and 100 mV. Magnon excitations are assigned to an origin of those peaks and thought to be a dominant process to reduce MR at finite bias voltage.
Tunnel Magnetoresistance Effect in CoFeB/MgAlOx/CoFeB Magnetic Tunnel Junctions
IEEE Transactions on Magnetics, 2000
Magnetic tunnel junctions (MTJs) with the core structure of CoFeB MgAlO x CoFeB were fabricated using magnetron sputtering technique. The MgAlO x tunnel barrier was obtained by plasma oxidation of an Mg/Al bilayer in an Ar + O 2 atmosphere. Series of MTJs were fabricated with different Mg layer thicknesses ( Mg ), and Al layer thickness was fixed at 1.3 nm. The annealing effect on the tunneling magnetoresistance (TMR) ratio was investigated, and TMR ratio of 65% at room temperature (RT) was shown when it was annealed at 375 C with the Mg = 0 5 nm. The temperature dependence of conductance can be fit by the magnon-assisted tunneling model by adding spin independent tunneling contribution for the samples investigated here, and the spin independent conductance varies with Mg , possibly due to less oxidation for thicker Mg layer.
Journal of Applied Physics, 2007
Growth and magnetic characterization of thin films of Co2Cr0.6Fe0.4Al and Co2MnSi full-Heusler compounds are investigated. Thin films were deposited by magnetron sputtering at room temperature directly onto oxidized Si wafers. These Heusler films are magnetically very soft and ferromagnetic with Curie temperatures well above room temperature. Polycrystalline Co2Cr0.6Fe0.4Al Heusler films combined with MgO barriers and CoFe counter electrodes are structured to magnetic tunnel junctions and yield almost 50% magnetoresistance at room temperature. The magnetoresistance shows a strong bias dependence with the maximum occurring at a voltage drop well above 1V. This special feature is accompanied by only a moderate temperature dependence of the tunnel magnetoresistance.
Magnetic tunnel junctions with a B2 -ordered CoFeCrAl equiatomic Heusler alloy
Physical Review Materials, 2019
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 3 a junction device. This paper reports a fully epitaxial (001)-oriented MgO barrier magnetic tunnel 4 junction (MTJ) with CoFeCrAl electrodes grown on a Cr buffer. X-ray and electron diffraction 5 measurements show that the (001) CoFeCrAl electrode films with atomically flat surfaces have a 6 B2-ordered phase. The saturation magnetization is 380 emu/cm 3 , almost the same as the value 7 given by the Slater-Pauling-like rule, and the maximum tunnel magnetoresistance ratios at 300 8 K and 10 K are 87% and 165%, respectively. Cross-sectional electron diffraction analysis shows 9 that the MTJs have MgO interfaces with fewer dislocations. The temperature-and bias-voltagedependence 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 B2ordered 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. I. INTRODUCTION A spin-gapless semiconductor (SGS) is a material in which the Fermi level is located at a zero-energy gap state for a majority spin band and at an energy gap for a minority spin band.[1-3] SGSs belong to the class of half-metals that have fully spin-polarized carriers at the Fermi level, so they exhibit a huge magnetoresistance (MR) and low spin relaxation (the so-called Gilbert damping). These physical properties are ideally suited to solid-state spintronic devices, and are commonly observed in half-metals.[4-10] In addition to such physical properties, SGSs could be used to realize devices with new functionalities, such as reconfigurable magnetic tunnel diodes and transistors,[11] which use their gapless electronic characteristics. Therefore, it is of fundamental and technological importance to investigate
Advanced Materials Research, 2015
The massive tunnel magnetoresistane (TMR) is expected when Fe3Al Heusler alloy are used for magnetic electrodes in magnetic tunnel junction (MTJ). We demonstrate the Fe3Al/MgO/Fe3Al MTJ, which have good lattice matching interfaces. The electronic and transport properties of these system are systematically investigated by first principle calculation. The electronic band structure of Fe3Al Heusler alloy and MgO are represented for discussion spin filtering effect. Besides, the transmission conductance is calculated to study the spin tunneling effect. We found that the massive TMR is clearly achieved at zero-bias condition, and the TMR is decreased when the finite bias are applied. Thus, among ferromagnetic materials, the Fe3Al Heusler alloy will be good alternative to bcc-Fe based electrodes with MgO barrier for the MTJs beyond.