Layer Of Magnetic Tunnel Junction Research Articles

Defect-Mediated Properties of Magnetic Tunnel Junctions

Defects play an important role in the properties of metal oxides which are currently used as barrier layers in magnetic tunnel junctions (MTJs). We study the effect of O vacancies on the interlayer exchange coupling (IEC) and tunneling magnetoresistance (TMR) in Fe-MgO-Fe tunnel junctions. Measurements of IEC in fully epitaxial Fe-MgO-Fe(001) tunnel junctions show IEC is antiferromagnetic (AFM) for small MgO thickness but changes sign and then vanishes for large barrier thickness. First-principles calculations based on density functional theory demonstrate that the presence of neutral O vacancies (F-centers) in the MgO barrier can explain this behavior. Resonant tunneling through the F-centers makes IEC AFM for thin barriers but with increasing MgO thickness the resonance contribution to IEC is reduced resulting in the ferromagnetic (FM) coupling typical for perfect MgO barriers. First-principles calculations also show that O vacancies can affect TMR. F-centers produce occupied localized s-states and unoccupied resonant p-states in the gap of MgO. We demonstrate that F-centers affect the conductance by either resonant transmission or nonresonant scattering of tunneling electrons both causing a substantial reduction of TMR compared to the ideal case

Difference between chemical structures of the interface at the Al-oxide tunneling barrier prepared by plasma or by radical oxidation

We have studied chemical structures of the interface between the Al-oxide tunneling barrier and the underlying Co 90Fe 10 layer in magnetic tunnel junctions when a 1-nm thick metallic Al barrier was oxidized by two different methods: plasma oxidation and radical oxidation. Our chemical analyses confirmed that the underlying CoFe layer was unavoidably attacked by oxygen during the oxidation and that this left different oxide states at the AlO x /CoFe interface, depending on the oxidation method. The radical oxidation required long oxidation time for optimizing tunneling performance and resulted in a large amount of oxygen at the interface, which, in turn, resulted in the formation of mostly α-Fe 2O 3 and Al 2O 3. Conversely, the plasma oxidation required a relatively short oxidation time for optimization and left FeO as a dominant phase at the interface. Our results also show that the thermal treatment helped AlO x , an oxygen-deficient phase, to be re-oxidized and transformed into Al 2O 3, the thermodynamically stable stoichiometric phase. The oxygen that diffused from the reduced CoFe layer into the barrier is likely responsible for this oxygen enrichment. We show that such differences in the chemical structure of the interface are critical clues to understanding what causes the change in tunneling properties of magnetic tunnel junctions.

Ｃｏ２Ｃｒ０．６Ｆｅ０．４Ａｌ薄膜を用いたエピタキシャル強磁性トンネル接合の製作とトンネル磁気抵抗特性

Fully epitaxial magnetic tunnel junctions (MTJs) were fabricated with a full-Heusler alloy Co2Cr0.6Fe0.4Al (CCFA) thin film and a MgO tunnel barrier. Cross-sectional high-resolution transmission electron microscope observations clearly showed that all layers of CCFA/MgO/Co50Fe50 MTJ layer structures were grown epitaxially and were single-crystalline. The fully epitaxial CCFA/MgO/Co50Fe50 MTJs, where we used the difference in the coercive forces to form the antiparallel magnetization configurations between the lower and upper electrodes, demonstrated high tunnel magnetoresistance (TMR) ratios of 90% at room temperature (RT) and 240% at 4.2 K. This result suggested that a CCFA film composition close to the stoichiometric one is essential for obtaining high spin polarizations in CCFA thin films. The fully epitaxial CCFA/MgO/Co50Fe50 MTJs with exchange-biasing, where a Co50Fe50 upper electrode was used in a synthetic ferrimagnetic Co50Fe50/Ru/Co90Fe10 trilayer exchange-biased with an IrMn layer through the Co90Fe10/IrMn interface, exhibited high TMR ratios of 109% at RT and 317% at 4.2 K. From the obtained TMR ratio of 317% at 4.2 K, a high tunneling spin polarization of 0.88 at 4.2 K was estimated for epitaxial CCFA films with the B2 structure. These results confirm the promise of an epitaxial MTJ using a Co-based full-Heusler alloy as a key device structure for utilizing the potentially high spin polarization of this material system.

Fabrication of fully epitaxial magnetic tunnel junctions with a Co 2MnSi thin film and a MgO tunnel barrier

Fully epitaxial magnetic tunnel junctions (MTJs) were fabricated with a Co-based full-Heusler alloy Co 2MnSi (CMS) thin film having the ordered L2 1 structure as a lower electrode, a MgO tunnel barrier, and a Co 50Fe 50 upper electrode. Reflection high-energy electron diffraction patterns observed in situ for each layer in the MTJ layer structure during fabrication clearly indicated that all layers of the CMS lower electrode, MgO tunnel barrier, and Co 50Fe 50 upper electrode grew epitaxially. The microfabricated fully epitaxial CMS/MgO/Co 50Fe 50 MTJs demonstrated relatively high tunnel magnetoresistance ratios of 90% at room temperature and 192% at 4.2 K.

Fabrication of exchange-biased epitaxial magnetic tunnel junctions with a Heusler alloy Co 2Cr 0.6Fe 0.4Al thin film

Exchange-biased epitaxial magnetic tunnel junctions (MTJs) with a Heusler alloy Co 2Cr 0.6Fe 0.4Al (CCFA) lower electrode, a MgO tunnel barrier, and a Co 50Fe 50 upper electrode were fabricated. Reflection high-energy electron diffraction observations indicated that all layers of the CCFA/MgO/Co 50Fe 50 MTJ layer structure were grown epitaxially. The fabricated epitaxial MTJs exhibited clear exchange-biased tunnel magnetoresistance (TMR) characteristics with high TMR ratios of 102% at room temperature and 291% at 4.2 K. From the TMR ratios, a high effective spin polarization of 0.86 at 4.2 K was obtained for the epitaxial CCFA films with the B2 structure.

High tunnel magnetoresistance in fully epitaxial magnetic tunnel junctions with a full-Heusler alloy Co2Cr0.6Fe0.4Al thin film

Fully epitaxial magnetic tunnel junctions (MTJs) were fabricated with a Co-based full-Heusler alloy Co2Cr0.6Fe0.4Al (CCFA) thin film, whose composition was close to the stoichiometric one, and a MgO tunnel barrier. Cross-sectional high-resolution transmission electron microscope observations indicated that all layers of the CCFA∕MgO∕Co50Fe50 MTJ layer structure were grown epitaxially and were single crystalline. The microfabricated CCFA∕MgO∕Co50Fe50 MTJs exhibited high tunnel magnetoresistance (TMR) ratios of 90% at room temperature and 240% at 4.2K. A high tunneling spin polarization of 0.79 at 4.2K was obtained for the epitaxial CCFA films from the TMR ratios.

Microscopic structures of MgO barrier layers in single-crystal Fe∕MgO∕Fe magnetic tunnel junctions showing giant tunneling magnetoresistance

The microscopic structures of MgO(001) barrier layers in magnetic tunnel junctions showing giant tunneling magnetoresistance were characterized by in situ scanning tunneling microscopy. The MgO thin films formed exceedingly flat surfaces, and their terraces were made even flatter by annealing after deposition. This flattening of MgO surfaces apparently promotes coherent transport of electrons, which should enhance the tunneling magnetoresistance ratio. Local tunneling spectroscopy revealed that an annealed MgO layer has a critical thickness between 3 and 5 ML (monolayer), and a continuous film without pinholes can be formed over the thickness.

자기터널접합을 활용한 고집적 MRAM 소자 기술

Ferromagnetic amorphous and layers have been devised and incorporated as free layers of magnetic tunnel junctions (MTJs) to improve MRAM reading and writing performance. The NiFeSiB and CoFeSiB single-layer film exhibited a lower saturation magnetization ($Ms

Enhancement of writing margin for low switching toggle magnetic random access memories using multilayer synthetic antiferromagnetic structures

We developed synthetic antiferromagnets (SAFs) with a multilayer structure where more than three magnetic layers were coupled in an antiparallel configuration for use as a free layer of toggle magnetic random access memories, with the aim of reducing the switching field. The SAFs consisted of a magnetic layer of NiFe∕CoFe with a spacer layer of Ru and were deposited on the Al-oxide barrier layer in magnetic tunnel junctions. Conventional bilayer SAFs did not realize a large writing margin with a low spin flop field (Hflop). On the other hand, multilayer SAFs increased the saturation field (Hs) significantly with increases in both the number of magnetic layers (N) and the antiferromagnetic coupling (AFC) strength between the inner magnetic layers while maintaining a low Hflop. This is because strong AFCs involved in the inner magnetic layers and weak AFCs of the outer magnetic layers determine Hs and Hflop of these, respectively. To control an AFC of each spacer layer, we chose an appropriate thickness for each Ru layer in the multilayer SAFs according to the Ru crystalline structure that was responsible for its AFC strength.

Magnetoresistance and magnetization switching characteristics of magnetic tunnel junctions with amorphous CoFeSiB single and synthetic antiferromagnet free layers

To obtain low switching field (Hsw) we introduced amorphous ferromagnetic Co70.5Fe4.5Si15B10 single and synthetic antiferromagnet (SAF) free layers in magnetic tunnel junctions (MTJs). The switching characteristics for MTJs with structures Si∕SiO2∕Ta 45/Ru 9.5/IrMn 10/CoFe 7∕AlOx∕CoFeSiB 7 or CoFeSiB (t)∕Ru 1.0/CoFeSiB (7−t)∕Ru 60 (in nanometer) were investigated and compared to MTJs with Co75Fe25 and Ni80Fe20 free layers. CoFeSiB showed a lower saturation magnetization of 560emu∕cm3 and a higher anisotropy constant of 2800erg∕cm3 than CoFe and NiFe, respectively. An exchange coupling energy (Jex) of −0.003erg∕cm2 was observed by inserting a 1.0nm Ru layer in between CoFeSiB layers. In the CoFeSiB single and SAF free layer MTJs, it was found that the size dependence of the Hsw originated from the lower Jex experimentally and by micromagnetic simulation based on the Landau-Lisfschitz-Gilbert equation. The CoFeSiB SAF structures showed lower Hsw than that of NiFe, CoFe, and CoFeSiB single structures. The CoFeSiB SAF structures were proved to be beneficial for the switching characteristics such as reducing the coercivity and increasing the sensitivity in micrometer- to submicrometer-sized elements.

Fabrication of fully epitaxial magnetic tunnel junctions using cobalt-based full-Heusler alloy thin film and their tunnel magnetoresistance characteristics

Cobalt-based full-Heusler alloy thin films have recently attracted much interest as highly desirable ferromagnetic electrodes for spintronic devices because of the half-metallic ferromagnetic nature theoretically predicted for some of these alloys and because of their high Curie temperatures, which are well above room temperature (RT). In this study, Co-based full-Heusler alloy thin films of Co2Cr0.6Fe0.4Al (CCFA) and Co2MnGe (CMG) were epitaxially grown on MgO-buffered MgO (001) substrates using magnetron sputtering. The films were deposited at RT and subsequently annealed in situ at temperatures ranging from 400 to 600 °C. X-ray pole figure measurements of the CCFA films indicated that these films were epitaxial and crystallized in the B2 structure. X-ray pole figure measurements of the annealed CMG films showed (111) peaks with four-fold symmetry, which provides direct evidence that these films were epitaxial and crystallized in the L21 structure. Furthermore, cross-sectional transmission electron microscope images of a fabricated CMG film indicated that it was single-crystalline. The annealed films of CCFA and CMG had sufficiently flat surface morphologies with roughness of about 0.23 nm rms for 100 nm thick CCFA films and 0.26 nm rms for 45 nm thick CMG films. Using these epitaxially grown thin films, we fabricated fully epitaxial magnetic tunnel junctions (MTJs) consisting of a Co-based full-Heusler thin film of either CCFA or CMG as a lower electrode, a MgO tunnel barrier and a Co50Fe50 (CoFe) upper electrode. All layers were successively deposited in an ultrahigh vacuum chamber through the combined use of magnetron sputtering and electron beam evaporation. Reflection high-energy electron diffraction patterns observed in situ for each layer during preparation clearly indicated that all layers grew epitaxially in both the CCFA/MgO/CoFe and CMG/MgO/CoFe MTJ layer structures. The microfabricated epitaxial CCFA/MgO/CoFe MTJs demonstrated relatively high tunnel magnetoresistance (TMR) ratios, for MTJs using a full-Heusler alloy, of 42% at RT and 74% at 55 K. On the other hand, the microfabricated epitaxial CMG/MgO/CoFe MTJs showed strongly temperature-dependent TMR characteristics with typical TMR ratios of 14% at RT and 70% at 7 K. These results confirm the promise of epitaxial MTJs as a key device structure for clarifying and utilizing the potentially high spin-polarization of Co-based full-Heusler alloy thin films.

비정질 CoFeSiB 단일 및 합성형 반강자성 자유층을 갖는 자기터널접합의 자기저항 효과

To obtain low switching field () we introduced amorphous ferromagnetic single and synthetic antiferromagnet (SAF) free layers in magnetic tunnel junctions (MTJs). The switching characteristics for MTJs with structures 45/Ru 9.5/IrMn 10/CoFe 7/AlOx/CoFeSiB 7 or CoFeSiB (t)/Ru 1.0/CoFeSiB (7-t)/Ru 60 (in nm) were investigated and compared to MTJs with and free layers. CoFeSiB showed a lower saturation magnetization of and a higher anisotropy constant of than CoFe and NiFe, respectively. An exchange coupling energy () of was observed by inserting a 1.0 nm Ru layer in between CoFeSiB layers. In the CoFeSiB single and SAF free layer MTJs, it was frond that the size dependence of the originated from the lower experimentally and by micromagnetic simulation based on the Landau-Lisfschitz-Gilbert equation. The CoFeSiB SAF structures showed lower than that of NiFe, CoFe and CoFeSiB single structures. The CoFeSiB SAF structures were proved to be beneficial far the switching characteristics such as reducing the coercivity and increasing the sensitivity in micrometer to submicrometer-sized elements.

Characteristics of the Al/sub 2/O/sub 3/ barrier with CoFeB pinned Layer in magnetic tunnel junctions

The magnetoresistance (MR) ratio of magnetic tunnel junctions (MTJs) depends on the structure and characteristics of the interface between ferromagnetic electrode and insulating layer. In order to understand the role of an amorphous layer as an electrode, the amorphous CoFeB and the crystalline CoFe were used for the pinned layers in MTJs and their properties were compared. When the CoFeB was used for the pinned layer, the MR ratio was higher than that of the CoFe pinned layer after annealing process. The reason for the higher MR of the CoFeB pinned layers are considered due to the chemically sharp interface formation. The chemically sharp interface means that B from the pinned layer gathers excess oxygen and Mn diffusion from the antiferromagnetic layer during the annealing is reduced by the amorphous CoFeB layer.

Magnetization switching and tunneling magnetoresistance effects of synthetic antiferromagnet free layers consisting of amorphous NiFeSiB

A synthetic antiferromagnet (SAF) structure comprising of ferromagnetic amorphous Ni16Fe62Si8B14 layers has been devised and employed as a free layer of magnetic tunnel junctions (MTJs) to enhance cell switching performance. We observed −0.03erg∕cm2 of exchange coupling energy (Jex) by inserting a 0.5 nm Ru layer in between NiFeSiB layers. In Si∕SiO2∕Ta 45∕Ru 9.5∕IrMn 10∕CoFe 7∕AlOx1.5∕(single NiFeSiB 7) or [NiFeSiB(t)∕Ru 0.5∕NiFeSiB(7−t)]∕Ru 60(nm) MTJ structures, we found size dependence of the switching field originating from the lower Jex both experimentally and by simulation. The NiFeSiB SAF structure showed lower switching field than traditional CoFe and CoFeB SAF structures. This is because NiFeSiB possesses low saturation magnetization (Ms=800emu∕cm3) and high anisotropy constant (Ku=2,700erg∕cm3). These properties were proven beneficial for the switching characteristics such as reducing the coercivity (Hc) and increasing the sensitivity in micrometer to submicrometer sized elements.

Surface flattening processes of metal layer and their effect on transport properties of magnetic tunnel junctions with Al–N barrier

In order to form ultrathin insulating layer in magnetic tunnel junctions (MTJs), two surface flatting processes of metal films are investigated. Oxygen-additive sputter-deposition process was applied to the bottom Cu electrode and the Al layer to be nitrided. Dry-etching process was applied for the surface treatment of lower Co–Fe layer. As a result, the surface roughness of stacked ultrathin Al layer to be nitrided is reduced from 3.2Å to 1.7Å, and the tunnel magnetoresistance (TMR) ratio of the MTJs increases from 1% to 26% while maintaining resistance-area product (R×A) less than 5×102Ωμm2 in the Co–Fe∕Al(6A)–N∕Co–Fe MTJs. We conclude that the decrease of the surface roughness of Al layer is one of the key factors to realize high performance MTJs with low R×A and high TMR ratio.

Thin skins for magnetic sensitivity

Understanding and tuning the insulating tunnel barrier layer in magnetic tunnel junctions is key to developing commercial spintronic devices. A naturally self-assembled insulating layer on bilayer manganites provides a highly sensitive model system.

Enhancement of tunneling magnetoresistance by inserting an amorphous nonmagnetic FeZr layer in magnetic tunnel junctions

An amorphous metallic FeZr layer is inserted inside the bottom magnetic layer in exchange-biased CoFe/Al2O3/CoFe magnetic tunnel junctions (MTJs). At room temperature, the tunneling magnetoresistance (TMR) increases from 25% to 36% before annealing, and from 45% to 52% after annealing. The junction resistance also increases with the FeZr thickness. The higher quality of the MTJ is attributed to the improved Al-oxide barrier due to the amorphousness of the FeZr layer.

Nitridation Process of Al Layer by Microwave-Excited Plasma and Large Magnetoresistance in Co-Fe/Al-N/Co-Fe Tunnel Junctions— As a Comparison With Oxidization Process

Nitridation and oxidization processes of metal Al film to form very thin insulating layer in magnetic tunnel junctions (MTJs) are investigated, using microwave-excited plasma. The plasma nitridation process provides wider controllability than the plasma oxidization for the formation of ultrathin insulating layer in MTJs, because of the slow nitriding rate of metal Al films, comparing with the oxidizing rate of them. As a result, high tunnel magnetoresistance (TMR) ratios of 49% and 44% with respective resistance-area product (R/spl times/A) of 3/spl times/10/sup 4/ /spl Omega//spl mu/m/sup 2/ and 6/spl times/10/sup 3/ /spl Omega//spl mu/m/sup 2/ are obtained in the Co-Fe/Al-N/Co-Fe MTJs. These values are comparable to those of MTJs with Al-O barriers. The remarkable feature of the nitridation process is its larger amount of plasma exposure, needed to obtain similar resistance in MTJs, comparing with the oxidization process. It means wider controllability of the plasma nitridation for the formation process of very thin tunnel barriers. We conclude that Al-N is a hopeful barrier material to realize MTJs with high TMR ratio and low R/spl times/A for high-performance MRAM cells.

Transport property studies on magnetic tunnel junctions with wedge-shaped barrier

Systematic studies on the transport property were carried out in FM (NiFe, Co, FeCo)/AlOx/NiFe magnetic tunnel junctions (MTJs) by oxidizing a wedge-shaped Al layer, resulting in over-, optimized, and under-oxidized barriers. An interesting resistance peak can be generally observed in the over-oxidized region, which was attributed to the significant change in oxidation level of the bottom electrode. This argument was supported by the barrier thickness and height fittings. Combining the studies of tunnel magnetoresistance (TMR) and the bias dependence as a function of the Al layer thickness, we can conclude that, in the MTJ exhibiting maximum TMR, the Al layer has not been fully oxidized to reach stoichiometric Al2O3, and the bottom electrode has already been slightly oxidized. A sizable TMR ratio can still be observed in under-oxidized region, even with very thick Al layer and short oxidation time, which implies that no pure metallic Al exists near the bottom ferromagnet/insulator (FM/I) interface. This is consistent with the grain boundary oxidation model for post-oxidizing the Al layer in MTJs.

Magnetic tunnel junctions with lowMs free layers

An amorphous soft magnet such as CoZrNb was considered as a free layer of magnetic tunnel junctions (MTJs) for high-density MRAMs. The MTJs exhibited a tunnel magnetoresistance (MR) of about 7.5% as the free layer thickness ranged from 2 to 15 nm. When CoFe filtering was added in the free layer, the MR ratio reached 35%.