###230% room-temperature magnetoresistance in CoFeB MgO CoFeB magnetic tunnel junctions###

230% room-temperature magnetoresistance in CoFeB MgO CoFeB
 magnetic tunnel junctions. Magnetoresistance (MR) ratio up to 230% at room temperature (294% at 20 K) has been observed in spin-valve-type magnetic tunnel junctions (MTJs) using MgO tunnel barrier layer fabricated on thermally oxidized Si substrates. We found that such a high MR ratio can be obtained when the MgO barrier layer was sandwiched with amorphous CoFeB ferromagnetic electrodes. Microstructure analysis revealed that the MgO layer with (001) fiber texture was realized when the MgO layer was grown on amorphous CoFeB rather than on polycrystalline CoFe. Since there have been no theoretical studies on the MTJs with a crystalline tunnel barrier and amorphous electrodes, the detailed mechanism of the huge tunneling MR effect observed in this study is not clear at the present stage. Nevertheless, the present work is of paramount importance in realizing high-density magnetoresistive random access memory and read head for ultra high-density hard-disk drives into practical use.

###70% TMR at room temperature for SDT sandwich junctions with CoFeB as free and reference Layers###

70% TMR at room temperature for SDT sandwich junctions with CoFeB as free and reference Layers. TMR at room temperature for SDT sandwich junctions with CoFeB as free and reference LayersSpin dependent tunneling (SDT) wafers were deposited using dc magnetron sputtering. SDT junctions were patterned and connected with one layer of metal lines using photolithography techniques. These junctions have a typical stack structure of Si(100)-Si/sub 3/N/sub 4/-Ru-CoFeB-Al/sub 2/O/sub 3/-CoFeB-Ru-FeCo-CrMnPt with the antiferromagnet CrMnPt layers for pinning at the top. High-resolution transmission electron microscopy (HRTEM) reveals that the CoFeB has an amorphous structure and a smooth interface with the Al/sub 2/O/sub 3/ tunnel barrier. Although it is difficult to pin the amorphous CoFeB directly from the top, the use of a synthetic antiferromagnet (SAF) pinned layer structure allows sufficient rigidity of the reference CoFeB layer. The tunnel junctions were annealed at 250/spl deg/C for 1 h and tested for magneto-transport properties with tunnel magnetoresistive (TMR) values as high as 70.4% at room temperature, which is the highest value ever reported for such a sandwich structure. This TMR value translates to a spin polarization of 51% for CoFeB, which is likely to be higher at lower temperatures. These junctions also have a low coercivity (Hc) and a low parallel coupling field (Hcoupl). The combination of a high TMR, a low Hc, and a low Hcoupl is ideal for magnetic field sensor applications.

###A perpendicular-anisotropy CoFeB MgO magnetic tunnel junction###

A perpendicular-anisotropy CoFeB MgO magnetic tunnel junction. Magnetic tunnel junctions (MTJs) with ferromagnetic electrodes possessing a perpendicular magnetic easy axis are of great interest as they have a potential for realizing next-generation high-density non-volatile memory and logic chips with high thermal stability and low critical current for current-induced magnetization switching1,2,3. To attain perpendicular anisotropy, a number of material systems have been explored as electrodes, which include rare-earth/transition-metal alloys4,5, L10-ordered (Co, Fe) Pt alloys3,6,7 and Co/(Pd, Pt) multilayers1,8,9,10. However, none of them so far satisfy high thermal stability at reduced dimension, low-current current-induced magnetization switching and high tunnel magnetoresistance ratio all at the same time. Here, we use interfacial perpendicular anisotropy between the ferromagnetic electrodes and the tunnel barrier of the MTJ by employing the material combination of CoFeB MgO, a system widely adopted to produce a giant tunnel magnetoresistance ratio in MTJs with in-plane anisotropy11,12,13. This approach requires no material other than those used in conventional in-plane-anisotropy MTJs. The perpendicular MTJs consisting of Ta/CoFeB/MgO/CoFeB/Ta show a high tunnel magnetoresistance ratio, over 120%, high thermal stability at dimension as low as 40 nm diameter and a low switching current of 49  A.

###A spin-valve-like magnetoresistance of an antiferromagnet-based tunnel junction###

A spin-valve-like magnetoresistance of an antiferromagnet-based tunnel junction. A spin valve is a microelectronic device in which high- and low-resistance states are realized by using both the charge and spin of carriers. Spin-valve structures used in modern hard-drive read heads and magnetic random access memoriescomprise two ferromagnetic electrodes whose relative magnetization orientations can be switched between parallel and antiparallel configurations, yielding the desired giant or tunnelling magnetoresistance effect1. Here we demonstrate more than 100% spin-valve-like signal in a NiFe/IrMn/MgO/Pt stack with an antiferromagnet on one side and a non-magnetic metal on the other side of the tunnel barrier. Ferromagneticmoments in NiFe are reversed by external fields of approximately50 mT or less, and the exchange-spring effect2 of NiFe on IrMn induces rotation of antiferromagnetic moments in IrMn, which is detected by the measured tunnelling anisotropic magnetoresistance3. Our work demonstrates a spintronic element whose transport characteristics are governed by an antiferromagnet. It demonstrates that sensitivity to low magnetic fields can be combined with large, spin-orbit-coupling-induced magnetotransport anisotropy using a single magnetic electrode. The antiferromagnetic tunnelling anisotropic magnetoresistance provides a means to study magnetic characteristics of antiferromagnetic films by an electronic-transport measurement.

###Co2MnSi Heusler alloy as magnetic electrodes in magnetic tunnel junctions###

Co2MnSi
 Heusler alloy as magnetic electrodes in magnetic tunnel junctions. As a consequence of the growing theoretical predictions of 100% spin-polarized half- and full-Heusler compounds over the past six years, Heusler alloys are among the most promising materials class for future magnetoelectronic and spintronic applications. We have integrated Co2MnSi
, as a representative of the full-Heusler compound family, as one magnetic electrode into magnetic tunnel junctions. The preparation strategy has been chosen so as to sputter Co2MnSi
 at room temperature onto a V-buffer layer, which assists in (110) texture formation, and to deposit the Al-barrier layer directly thereafter. After plasma oxidizing the Al-barrier layer, subsequent annealing leads (1) to the texture formation and (2) to the appropriate atomic ordering within the Co2MnSi
, and (3) homogenizes the AlO
 barrier. It is shown that the magnetic switching of the ferromagnetic electrodes is well controlled from room temperature down to 10K
. The resulting tunnel magnetoresistance-effect amplitude of the Co2MnSi
 containing magnetic tunnel junctions has been determined as a function of temperature and the spin polarization of the Co2MnSi
 Heusler compound has been estimated to be 61% at 10K
. Thus, the spin polarization of the Co2MnSi
 layer at 10K
 exceeds that of conventional transition metals.

 ###Giant magnetic tunneling effect in Fe/Al2O3/Fe junction###

 Giant magnetic tunneling effect in Fe/Al2O3/Fe junction. A giant magnetoresistance ratio of 30% at 4.2 K and 18% at 300 K was observed for the first time in an Fe/Al2O3/Fe junction. The conductance at room temperature was expressed well by G=96.2 (1 + 0.09 cos  )( -1), where   is the angle between the magnetizations of two iron electrodes. The dependence of the magnetoresistance ratio, saturated resistance and also the tunneling current on temperature were measured in the range 4.2 300 K. The results support the claim that the giant magnetoresistance is due to the magnetic tunneling of electrons between the electrodes through the thin Al2O3 insulator.

 ###Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions###

 Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions. The tunnel magnetoresistance (TMR) effect in magnetic tunnel junctions (MTJs)1,2 is the key to developing magnetoresistive random-access-memory (MRAM), magnetic sensors and novel programmable logic devices3,4,5. Conventional MTJs with an amorphous aluminium oxide tunnel barrier, which have been extensively studied for device applications, exhibit a magnetoresistance ratio up to 70% at room temperature6. This low magnetoresistance seriously limits the feasibility of spintronics devices. Here, we report a giant MR ratio up to 180% at room temperature in single-crystal Fe/MgO/Fe MTJs. The origin of this enormous TMR effect is coherent spin-polarized tunnelling, where the symmetry of electron wave functions plays an important role. Moreover, we observed that their tunnel magnetoresistance oscillates as a function of tunnel barrier thickness, indicating that coherency of wave functions is conserved across the tunnel barrier. The coherent TMR effect is a key to making spintronic devices with novel quantum-mechanical functions, and to developing gigabit-scale MRAM.

 ###Giant tunneling magnetoresistance in Co2MnSi Al O Co2MnSi magnetic tunnel junctions###

 Giant tunneling magnetoresistance in Co2MnSi Al O Co2MnSi
 magnetic tunnel junctions. Magnetic tunnel junctions (MTJs) with a stacking structure of Co2MnSi Al O Co2MnSi
 were fabricated using magnetron sputtering system. Fabricated MTJ exhibited an extremely large tunneling magnetoresistance (TMR) ratio of 570% at low temperature, which is the highest TMR ratio reported to date for an amorphous Al O tunneling barrier. The observed dependence of tunneling conductance on bias voltage clearly reveals the half-metallic energy gap of Co2MnSi
. The origins of large temperature dependence of TMR ratio were discussed on the basis of the present results.
The authors give special thanks to Dr. S. Yuasa of the Japanese National Institute of Advanced Industrial Science and Technology (AIST) for helpful discussion and advice. This study was supported by the IT Program of the Research Revolution 2002 (RR2002) under the title  Development of Universal Low-Power Spin Memory,  by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan, by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology (JST) Corporation, by the New Energy and Industrial Technology Development Organization (NEDO) grant program, and by a Japan Society for the Promotion of Science (JSPS) Research Fellowship for Young Scientists.

###Giant tunneling magnetoresistance in spin-filter van der Waals heterostructures###

Giant tunneling magnetoresistance in spin-filter van der Waals heterostructures. An electrical current running through two stacked magnetic layers is larger if their magnetizations point in the same direction than if they point in opposite directions. These so-called magnetic tunnel junctions, used in electronics, must be carefully engineered. Two groups now show that high magnetoresistance intrinsically occurs in samples of the layered material CrI3 sandwiched between graphite contacts. By varying the number of layers in the samples, Klein et al. and Song et al. found that the electrical current running perpendicular to the layers was largest in high magnetic fields and smallest near zero field. This observation is consistent with adjacent layers naturally having opposite magnetizations, which align parallel to each other in high magnetic fields.

###Giant tunneling magnetoresistance up to 410% at room temperature in fully epitaxial Co MgO Co magnetic tunnel junctions with bcc Co(001) electrodes###

 Giant tunneling magnetoresistance up to 410% at room temperature in fully epitaxial Co MgO Co
 magnetic tunnel junctions with bcc Co(001) electrodes. Fully epitaxial Co(001) MgO(001) Co(001)
 magnetic tunnel junctions (MTJs) with metastable bcc Co(001) electrodes were fabricated with molecular beam epitaxy. The MTJs exhibited giant magnetoresistance (MR) ratios up to 410% at room temperature, the highest value reported to date. Temperature dependence of the MR ratio was observed to be very small compared with fully epitaxial Fe MgO Fe
 and textured CoFeB MgO CoFeB
 MTJs. The MR ratio of the Co MgO Co
 MTJ showed larger bias voltage dependence than that of the epitaxial Fe MgO Fe
 MTJs, which probably reflects the band structures of bcc Co and Fe for the k =0
 direction.
This study was partly supported by the New Energy and Industrial Technology Development Organization (NEDO).

###Giant tunnelling magnetoresistance at room temperature with MgO (100) tunnel barriers###

Giant tunnelling magnetoresistance at room temperature with MgO (100) tunnel barriers. Magnetically engineered magnetic tunnel junctions (MTJs) show promise as non-volatile storage cells in high-performance solid-state magnetic random access memories (MRAM)1. The performance of these devices is currently limited by the modest (< 70%) room-temperature tunnelling magnetoresistance (TMR) of technologically relevant MTJs. Much higher TMR values have been theoretically predicted for perfectly ordered (100) oriented single-crystalline Fe/MgO/Fe MTJs. Here we show that sputter-deposited polycrystalline MTJs grown on an amorphous underlayer, but with highly oriented (100) MgO tunnel barriers and CoFe electrodes, exhibit TMR values of up to  220% at room temperature and  300% at low temperatures. Consistent with these high TMR values, superconducting tunnelling spectroscopy experiments indicate that the tunnelling current has a very high spin polarization of  85%, which rivals that previously observed only using half-metallic ferromagnets2. Such high values of spin polarization and TMR in readily manufactureable and highly thermally stable devices (up to 400  C) will accelerate the development of new families of spintronic devices.

###Inverse Tunnel Magnetoresistance in $\mathrm{Co}/{\mathrm{SrTiO}}_{3}/{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$: New Ideas on Spin-Polarized Tunneling###

Inverse Tunnel Magnetoresistance in $\mathrm{Co}/{\mathrm{SrTiO}}_{3}/{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$: New Ideas on Spin-Polarized Tunneling. We report tunnel magnetoresistance (TMR) measurements on Co/SrTiO3/La0.7Sr0.3MnO3 junctions. The half-metallic La0.7Sr0.3MnO3 electrode is used as a spin analyzer. The large ( 50%) inverse TMR indicates a negative spin polarization of Co, in agreement with the density of states (DOS) of the d band in Co. The bias dependence of the TMR, with a maximum inverse TMR at  0.4V and a crossover to normal TMR above +0.8V, reflects the structure of this DOS. Our results demonstrate that the choice of the insulating barrier can strongly influence and even reverse the spin polarization of tunneling electrons.

###Large Magnetoresistance at Room Temperature in Ferromagnetic Thin Film Tunnel Junctions###

Large Magnetoresistance at Room Temperature in Ferromagnetic Thin Film Tunnel Junctions. Ferromagnetic-insulator-ferromagnetic tunneling has been measured in CoFe/Alzo3/Co or NiFe
junctions. At 295, 77, and 4.2 K the fractional change in junction resistance with magnetic field,
b,R/R, is 11.8%, 20%, and 24%, respectively. The value at 4.2 K is consistent with Julliere's model
based on the spin polarization of the conduction electrons of the magnetic films. b,R/R changes little
with a small voltage bias, whereas it decreases significantly at higher bias ()0.1 V), in qualitative
agreement with Slonczewski's model. These junctions have potential use as low-power field sensors
and memory elements.

###Large magnetotunneling effect at low magnetic fields in micrometer-scale epitaxial ${\mathrm{La}}_{0.67}$${\mathrm{Sr}}_{0.33}$Mn${\mathrm{O}}_{3}$ tunnel junctions###

Large magnetotunneling effect at low magnetic fields in micrometer-scale epitaxial ${\mathrm{La}}_{0.67}$${\mathrm{Sr}}_{0.33}$Mn${\mathrm{O}}_{3}$ tunnel junctions. We have used a self-aligned lithographic process to fabricate magnetic tunnel junctions of La0.67Sr0.33MnO3 down to a few micrometers in size. We have obtained a magnetoresistance ratio as large as 83% at low magnetic fields of a few tens of Oe, which correspond to the coercivities of the magnetic layers. Transmission-electron-microscopy analysis has revealed the heteroepitaxial growth of the trilayer junction structure, La0.67Sr0.33MnO3/SrTiO3/La0.67 Sr0.33MnO3. We have observed current-voltage characteristics typical of electron tunneling across an insulating barrier. The large magnetoresistance is likely due to the nearly half-metallic electronic structure of the manganites.

###Negative Spin Polarization of $\mathrm{F}{\mathrm{e}}_{\mathrm{3}}{\mathrm{O}}_{\mathrm{4}}$ in Magnetite/Manganite-Based Junctions###

Negative Spin Polarization of $\mathrm{F}{\mathrm{e}}_{\mathrm{3}}{\mathrm{O}}_{\mathrm{4}}$ in Magnetite/Manganite-Based Junctions. Epitaxial oxide trilayer junctions composed of magnetite (Fe3O4) and doped manganite (La0.7Sr0.3MnO3) exhibit inverse magnetoresistance as large as  25% in fields of 4 kOe. The inverse magnetoresistance confirms the theoretically predicted negative spin polarization of Fe3O4. Transport through the barrier can be understood in terms of hopping transport through localized states that preserve electron spin information. The junction magnetoresistance versus temperature curve exhibits a peak around 60 K that is explained in terms of the paramagnetic to ferrimagnetic transition of the CoCr2O4 barrier.

###NiFe2O4: A Versatile Spinel Material Brings New Opportunities for Spintronics###

NiFe2O4: A Versatile Spinel Material Brings New Opportunities for Spintronics. Ultrathin layers of the versatile spinel oxide NiFe2O4 can be conductive or insulating depending on the growth conditions. Conductive NiFe2O4 (NFO) electrodes are inserted into conventional magnetic tunnel junctions containing La2/3Sr1/3MnO3/SrTiO3 (LMSO/STO) bilayers, and insulating and ferrimagnetic NiFe2O4 layers are used as barriers to define a spin filter (see figure). The ability of such heterostructures to highly spin-polarize electrons injected from a non-magnetic electrode is demonstrated.

###Room-Temperature Tunnel Magnetoresistance and Spin-Polarized Tunneling through an Organic Semiconductor Barrier###

Room-Temperature Tunnel Magnetoresistance and Spin-Polarized Tunneling through an Organic Semiconductor Barrier. Electron spin-polarized tunneling is observed through an ultrathin layer of the molecular organic semiconductor tris(8-hydroxyquinolinato)aluminum (Alq3). Significant tunnel magnetoresistance (TMR) was measured in a Co/Al2O3/Alq3/NiFe magnetic tunnel junction at room temperature, which increased when cooled to low temperatures. Tunneling characteristics, such as the current-voltage behavior and temperature and bias dependence of the TMR, show the good quality of the organic tunnel barrier. Spin polarization (P) of the tunnel current through the Alq3 layer, directly measured using superconducting Al as the spin detector, shows that minimizing formation of an interfacial dipole layer between the metal electrode and organic barrier significantly improves spin transport.

###Spin polarized tunneling in ferromagnetic junctions###

Spin polarized tunneling in ferromagnetic junctions. Spin polarized tunneling studies by Tedrow and Meservey in the early 1970s that showed the spin conservation in electron tunneling gave rise to the possibility of spin sensitive tunneling between two ferromagnetic (FM) films. Julli re put forward a quantitative model (1975) showing that tunneling in FM/I/FM junctions should lead to a large magnetoresistance (JMR). This conjecture was realized with repeatable results only in 1995, and since then JMR values >30% have been achieved at room temperature. This recent success has led to several fundamental questions regarding the phenomenon of spin tunneling, besides showing tremendous potential for applications as nonvolatile magnetic memory elements, read head and picotesla field sensors. We briefly review the experimental results and the current theoretical understanding of FM I FM tunneling: its dependence on bias, temperature and barrier characteristics. The influence of inelastic tunneling processes, metal at the interface and material properties on the JMR is discussed. Early theories are reviewed and their relationship to the linear response theory is presented. The future direction, both from the point of fundamental physics as well as applications, is also covered.

###Spin-dependent tunneling conductance of Fe | MgO | Fe sandwiches###

Spin-dependent tunneling conductance of Fe | MgO | Fe sandwiches. We present first-principles based calculations of the tunneling conductance and magnetoconductance of
epitaxial Fe(100)uMgO(100)uFe(100) sandwiches. Our results indicate that tunneling is much more interesting
and complicated than the simple barrier model used previously. We obtain the following general results: ~1!
Tunneling conductance depends strongly on the symmetry of the Bloch states in the electrodes and of the
evanescent states in the barrier layer. ~2! Bloch states of different symmetry decay at different rates within the
barrier. The decay rate is determined by the complex energy bands of the same symmetry in the barrier. ~3!
There may be quantum interference between the decaying states in the barrier. This leads to an oscillatory
dependence of the tunneling current on ki and a damped oscillatory dependence on barrier thickness. ~4!
Interfacial resonance states can allow particular Bloch states to tunnel efficiently through the barrier. For
Fe(100)uMgO(100)uFe(100) our calculations indicate that quite different tunneling mechanisms dominate the
conductance in the two spin channels. In the majority channel the conductance is primarily via Bloch electrons
with small transverse momentum. One particular state with D1 symmetry is able to effectively couple from the
Fe into the MgO. In the minority channel the conductance is primarily through interface resonance states
especially for thinner layers. We predict a large magnetoresistance that increases with barrier thickness.

###Theory of tunneling magnetoresistance of an epitaxial Fe/MgO/Fe(001) junction###

Theory of tunneling magnetoresistance of an epitaxial Fe/MgO/Fe(001) junction. Calculation of the tunneling magnetoresistance ~TMR! of an epitaxial Fe/MgO/Fe~001! junction is reported.
The conductances of the junction in its ferromagnetic and antiferromagnetic configurations are determined
without any approximations from the real-space Kubo formula using tight-binding bands fitted to an ab initio
band structure of iron and MgO. The calculated optimistic TMR ratio is in excess of 1000% for an MgO barrier
of '20 atomic planes and the spin polarization of the tunneling current is positive for all MgO thicknesses. It
is also found that spin-dependent tunneling in an Fe/MgO/Fe~001! junction is not entirely determined by states
at the G point (ki50) even for MgO thicknesses as large as '20 atomic planes. All these results are explained
qualitatively in terms of the Fe majority- and minority-spin surface spectral densities and the complex MgO
Fermi surface.

###Tunnel magnetoresistance of 604% at 300K by suppression of Ta diffusion in CoFeB MgO CoFeB pseudo-spin-valves annealed at high temperature###

Tunnel magnetoresistance of 604% at 300K
 by suppression of Ta diffusion in CoFeB MgO CoFeB
 pseudo-spin-valves annealed at high temperature. The authors observed tunnel magnetoresistance (TMR) ratio of 604% at 300K
 in Ta Co20Fe60B20 MgO Co20Fe60B20 Ta
 pseudo-spin-valve magnetic tunnel junction annealed at 525 C
. To obtain high TMR ratio, it was found critical to anneal the structure at high temperature above 500 C
, while suppressing the Ta diffusion into CoFeB electrodes and in particular to the CoFeB MgO
 interface. X-ray diffraction measurement of MgO on SiO2
 or Co20Fe60B20
 shows that an improvement of MgO barrier quality, in terms of the degree of the (001) orientation and stress relaxation, takes place at annealing temperatures above 450 C
. The highest TMR ratio observed at 5K
 was 1144%.
This work was supported in part by  High-Performance Low-Power Consumption Spin Devices and Storage Systems  program under Research and Development for Next-Generation Information Technology of MEXT. The authors wish to thank Y. Ohno, I. Morita, and T. Hirata for their technical support in MTJ fabrication and valuable discussions.

