Characteristics Of Magnetic Tunnel Junctions Research Articles

Tunneling magnetoresistance of Co2MnGe∕Al2O3∕Co junctions

We report on the fabrication and characterization of magnetic tunnel junctions using the Heusler compound Co2MnGe as the bottom electrode and Co as counter electrode. The Co2MnGe phase in the cubic L21 structure belongs to the group of Heusler alloys with theoretically predicted full spin polarization at the Fermi level. From the maximum tunnelling magnetoresistance of 27% at low temperatures we derive a spin polarization P=36% for the Heusler alloy. We compare the properties of the Co2MnGe∕Al2O3∕Co junction with that of a Co∕Al2O3∕Co reference junction prepared under the same conditions and discuss the possible origin of the loss of full spin polarization for the Heusler alloy in the junction.

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.

Switching characteristics of magnetic tunnel junction with amorphous CoFeSiB free layer

The switching characteristics of magnetic tunnel junctions (MTJs) comprising amorphous ferromagnetic CoFeSiB free layer have been investigated. CoFeSiB was used for the free layer to enhance the switching characteristics. The typical junction structure was Si/SiO 2/Ta 45/Ru 9.5/IrMn 10/CoFe 7/AlO x /CoFeSiB ( t)/Ru 60 (in nm). CoFeSiB has low saturation magnetization ( M s) of 560 emu/cm 3 and high anisotropy constant ( K u) of 2,800 erg/cm 3. These properties caused low coercivity ( H c) and high sensitivity in MTJs. It was also confirmed in submicrometer-sized elements by micromagnetic simulation based on the Landau–Lisfschitz–Gilbert equation. By increasing CoFeSiB free layer thickness, the switching characteristics became worse due to increase of the demagnetization field.

Characterization of the magnetization vortex state in magnetic tunnel junctions patterned into nanometer-scale arrays

The aim of this work is to investigate the switching characteristics of magnetic tunnel junctions (MTJs) that contain two ferromagnetic layers in a magnetization vortex state as a function of the aspect ratio (length/width). Magnetic tunnel junctions NiFe30nm∕AlOx1.3nm∕Co15nm have been patterned into arrays of dots with identical geometries as in a single layer having a vortex state at low aspect (width/length) dots. The switching characteristics were studied using a vibrating sample magnetometer and magneto-optic Kerr effect techniques. The results show the relation between the switching fields and the sizes of the MTJ dots in which the switching of the two ferromagnetic layers is indistinguishable at low aspect ratio. It was found that as the aspect ratio increases, the switching becomes more distinguishable. This can be attributed to the magnetization vortices in the two ferromagnetic layers and the magnetostatic coupling (dipole-dipole interaction), which are size-dependent.

Effect of ion irradiation on the characteristics of magnetic tunnel junctions

The effect of disorder caused by MeV heavy ion irradiation on the tunnel magnetoresistance (TMR) of magnetic tunnel junctions is investigated. Two types of tunnel junctions, viz. Co/Al2O3/Ni80Fe20 and Co/Gd-doped Al2O3/Ni80 Fe20 are studied. Upon 70 MeV Si ion irradiation at a fluence of 5 × 1011 ions/cm2, the undoped junctions show relatively small but irreversible change, while the Gd-doped junctions show a huge reduction, in TMR. In both cases junctions were completely destroyed by 200 MeV Ag ion irradiation at a fluence of 1×1011 ions/cm2. The results are attributed to the modification of the barrier and the neighboring interfaces caused by the high energy density deposited by incident ions.

Switching characteristics of magnetic tunnel junctions with a synthetic antiferromagnetic free layer

The array of synthetic antiferromagnetic magnetic tunnel junctions (MTJs) consisting of the TiN/PtMn (15 nm)/CoFe (1.5 nm)/Ru (0.8 nm)/CoFe (1.5 nm)/AlO (1.2 nm)/NiFe (t/sub 1/ nm)/Ru (0.8 nm)/NiFe (t/sub 2/ nm)/Ta (10 nm)/TiN structure was fabricated into submicrometer dimensions. Magnetization switching field and magnetic domain structure were investigated by micromagnetic modeling as well as remanent magnetoresistive measurement. Domain structure was investigated to understand switching characteristics. The switching field depends on the thicknesses difference of two magnetic layers consisting of SAF free layer structure. When the thickness difference became 1.5 nm, the switching field was reduced to around 20 Oe with improved squareness.

Magnetotransport and interdiffusion characteristics of magnetic tunnel junctions comprising nano-oxide layers upon exposure to postdeposition annealing

Magnetic tunnel junction (MTJ) structures based on underlayer (CoNbZr)/bufferlayer (CoFe)/antiferromagnet (IrMn)/pinned layer (CoFe)/tunnel barrier (AlO x )/free layer (CoFe)/capping (CoNbZr) have been prepared to investigate thermal degradation of magnetoresistive responses. Some junctions possess a nano-oxide layer (NOL) inside either in the underlayer or bufferlayer. The main purpose of the NOL inclusion was to control interdiffusion path of Mn from the antiferromagnet so that improved thermal stability could be achieved. The MTJs with NOLs were found to have reduced interfacial roughness, resulting in improved tunneling magnetoresistance (TMR) and reduced interlayer coupling field. We also confirmed that the NOL effectively suppressed the Mn interdiffusion toward the tunnel barrier by dragging Mn atoms toward NOL during annealing.

Time‐to‐breakdown characteristics of magnetic tunnel junctions

To investigate the reliability of the MTJs on the roughness of insulating tunnel barrier, we prepared two MTJs with the different uniformity of barrier thickness. Namely, the one has uniform insulating barrier thickness; the other has non-uniform insulating barrier thickness as compared to different thing. As to depositing amorphous layer CoZrNb under the pinning layer IrMn, we achieved MTJ with uniform barrier thickness. First of all, we performed the breakdown-voltage measurement of two junctions and obtained the result having a similar breakdown-voltage about 1.4 V at two all junctions. And then, the time dependence dielectric breakdown (TDDB) measurements of two junctions are carried out under constant voltage stresses. The Weibull fit of our data shows clearly that time to breakdown (tBD) scales with the thickness uniformity of tunnel barrier. And also, Assuming a linear dependence of log(tBD) on stress voltages, we meet with result about the different lifetime of the two MTJs with uniform barrier thickness or non-uniform barrier thickness. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of magnetic tunnel junctions (MTJ) with magnetostrictive free layer materials

Abstract Magnetic tunneling junctions (MTJs) incorporating magnetostrictive free layer materials are developed and characterized for the application as highly sensitive strain sensors. Gauge factors of the order of 400–600 have already been demonstrated compared to typical values of 2–4 for metal film based strain sensors. Here, different magnetostrictive materials such as amorphous FeCoSiB alloy and polycrystalline Ni films have been investigated in order to characterize and optimize the MTJ based sensor properties.

Characteristics of magnetic tunnel junctions consisting of amorphous CoNbZr layers

Magnetic tunnel junctions comprising amorphous CoNbZr layers have been investigated. Co85.5Nb8Zr6.5 (in at. %) layers were employed to substitute traditionally used Ta layers with an emphasis given on understanding underlayer effect. The typical junction structure was SiO2/CoNbZr or Ta 2/CoFe 8/IrMn 7.5/CoFe 3/Al 1.6+oxidation/CoFe 3/CoNbZr or Ta 2 (nm). For both as-deposited state and after annealing, the CoNbZr-underlayered structure showed superior surface smoothness up to the tunnel barrier than Ta-underlayered one (0.16 vs 0.34 nm). Ultrasmooth layer structure was proven beneficial for reducing temperature-dependent magnetoresistance properties. A longer annealing degraded the properties most likely due to Mn interdiffusion toward the bottom ferromagnetic electrode as confirmed by Auger electron spectroscopy. By a slight change in the bottom electrode thickness (CoNbZr 4/CoFe 10), the tunneling magnetoresistance ratio can be increased up to 32% after 10 min annealing at 300 °C.

Interface characterization of magnetic tunnel junctions by using tunneling spectroscopy

Junctions doped with a small mount of Al between the top ferromagnetic electrode and the insulator were fabricated. The tunnel magnetoresistance (TMR) ratio increased at the doped Al thickness of 0.2 nm after annealing at 250 °C. Inelastic-electron-tunneling spectroscopy (IETS) has been applied to investigate the spin-dependent tunneling process for the tunnel junctions. The IET spectrum subtracting the spectrum at parallel magnetization configuration from that at antiparallel configuration showed a peak around 20 mV of the bias voltage especially for the junction after annealing at 250 °C. Over the corresponding voltage the TMR ratio as a function of the bias decreased; however, the influence was relatively small.

Characterization of magnetic tunnel junctions using IETS

In this paper inelastic electron tunneling spectroscopy measurements are presented on tunnel junctions containing both magnetic and nonmagnetic electrodes. Magnon excitations have been found near zero voltage; phonons are observed between 30 and 110 mV. Therefore, a model description of the transport properties of magnetic tunnel junctions at elevated temperatures or non-zero bias voltage should include both inelastic contributions.

Spin dependent tunneling

During the past four years, researchers made significant advances in fabricating magnetic tunnel junctions with reproducible magnetic and magnetotransport properties. Important developments include optimization of oxidation processes, discovery of new class of magnetic tunnel junctions, combination of spin dependent tunneling with the Coulomb blockade effect, and a better theoretical understanding of the I– V characteristics of magnetic tunnel junctions. These developments make them promising candidates for magnetic random access memories.

Hrem Characterization of Magnetic Tunnel Junctions and Discontinuous Multilayers

There is much current interest in the magnetotransport properties of systems consisting of two (or more) magnetic metal layers separated by thin insulating layers. Traditional magnetic tunnel junctions (MTJs) are comprised of simple ferromagnet-insulator-ferromagnet trilayer structures and the conductance depends on the relative alignments of the magnetizations in the two ferromagnets. In the case of discontinuous metal/insulator multilayers, negative magnetoresistance (MR) results from spindependent tunneling. In both types of systems, the tunneling phenomena are strongly influenced by the microstructure of the films, particularly the metal/insulator interfaces and the nature and uniformity of the thin oxide barrier layers. In this study, we have used cross-sectional HREM to characterize a variety of magnetic tunnel junctions and discontinuous multilayers.The MTJs were prepared by rf and dc magnetron sputtering onto thermally oxidized (100) silicon wafers at room temperature. The magnetic layers consisted of thin films of Co, Fe and/or CoFe alloys with thicknesses ∼ 30-50nm, and the barriers included MgO, HfO2, CoO, SiO2 as well as Al2O3 (thicknesses in the range 2-10nm).

Shape-anisotropy-controlled magnetoresistive response in magnetic tunnel junctions

We show that shape anisotropy can be used to control the response characteristics of magnetic tunnel junctions. By varying the junction shape, the resistance versus field curve was made to vary from a nonhysteretic linear curve with a high-field sensitivity (0.3%/Oe) to a hysteretic response curve with high squareness.