C. Y. Hsu

Complex capacitance spectroscopy as a probe for oxidation process of AlOx-based magnetic tunnel junctions

Proper as well as under- and over-oxided CoFe−AlOx−CoFe magnetic tunnel junctions (MTJs) have been systematically investigated in a frequency range from 102 to 108Hz by complex capacitance spectroscopy. The dielectric relaxation behavior of the MTJs remarkably disobeys the typical Cole–Cole arc law probably due to the existence of imperfectly blocked Schottky barrier in the metal-insulator interface. The dielectric relaxation response can be successfully modeled on the basis of Debye relaxation by incorporating an interfacial dielectric contribution. In addition, complex capacitance spectroscopy demonstrates significant sensitivity to the oxidation process of metallic Al layers, i.e., almost a fingerprint of under, proper, and over oxidation. This technique provides a fast and simple method to inspect the AlOx barrier quality of MTJs.

Interfacial and quantum well effects on ac magnetotransport of La0.7Sr0.3MnO3/La1.4Sr1.6Mn2O7 composites

The field-dependent complex impedance (CI) spectroscopy is employed to clarify the origins of ac magnetoimpedance (MI) effects at room temperature near the conduction threshold of [La0.67Sr0.33MnO3(113)]1−x∕[La1.67Sr1.33Mn2O7(327)]x composites. The field-dependent CI spectra in 113-327 granular composites are further transformed to field-dependent magnetoconductance (MC) spectra. The frequency-dependent high-field and low-field MC spectra can be well interpreted by including the trap-state contributions in 113-327 interfaces and 327 grain boundaries, together with the leakage-region-induced spin-dependent quantum well state effect. The principle of this work could be applied to study the magnetoresistance and MI effects of magnetic granular composites.

Enhanced antiferromagnetic saturation in amorphous CoFeB-Ru-CoFeB synthetic antiferromagnets by ion-beam assisted deposition

The interlayer coupling of CoFeB-based synthetic antiferromagnets (SyAFs), modulated by the ion-beam assisted deposition (IBAD) has been systematically investigated under different assisted deposition voltage from 0 to 140 V. We observe that proper IBAD voltage can significantly enhance the antiferromagnetically coupled saturation field from 280 to 1000 Oe and retain the amorphous structure of CoFeB layers. This approach provides a convenient method to enhance the magnetic coupling of SyAFs, which is useful for the magnetic tunnel junctions fabrication and magnetoresistive random access memory development.

Tunable magnetic order of Co nanoparticles and magnetotransport in Co/ZnO nanocomposites

We demonstrate tunable magnetic order of cobalt nanoparticles in Co∕ZnO nanocomposites. High-density electronic states in ZnO formed during high vacuum annealing help generate bound and free charge carriers, which in turn enable the stable magnetic ordering of Co nanoparticles in the Co∕ZnO nanocomposites in a tunable manner. This is demonstrated by the following experimental observations: (i) enhanced spontaneous magnetization and coercivity, (ii) transition from semiconducting to metallic electrical-transport, and (iii) transverse magnetotransport transition from negative magnetoresistance to the anomalous Hall effect. The work explores a route to manipulate the magnetic order of magnetic nanoparticles by means of intentionally generated defects in oxides.

Interfacial defects controlled electrical and magnetotransport in Co/ZnO nanocomposites

This work demonstrates, by frequency-dependent electrical measurements, significant variation associated with interfacial defects in Co/ZnO nanocomposites (NCs) for electrical and magnetic transport. Radio-frequency impedance measurements showed that heat treatment in an ultrahigh vacuum (UHV) increased interfacial defect density. Interfacial defects-assisted transport reduced the magnetoresistance (MR) ratio from 5.04% for the as-grown sample to 0.17% following heat treatment in UHV. The evolution of the onset frequency of power-law dispersion conductivity revealed that the increase in the interfacial defect density in Co/ZnO NCs upon heat treatment was consistent with the change in MR.

Study of the Insulating Layer and Interfaces of MgO-Based Magnetic Tunnel Junctions by Impedance and Capacitance Spectra

We have systematically investigated the complex impedance (CI) and complex capacitance (CC) spectra of MgO-based magnetic tunnel junctions (MTJs) due to successive annealing. The variation of the tunnel magnetoresistance (TMR) ratio due to annealing can be related to the change of interfacial trap states and textured MgO based on the analysis of CI spectra by an equivalent circuit model. The results of CC spectra are also consistent with the analysis of CI and observed TMR ratio variation. The CI and CC techniques are also powerful to characterize the oxide-based MTJs

ac impedance techniques to study oxidation process of tunnel barriers in CoFe–AlOx–CoFe magnetic tunnel junctions

The complex impedance spectra of CoFe–AlOx–CoFe tunnel junctions with under-, proper-, and overoxidized tunnel barriers have been investigated by ac impedance techniques. Two sets of parallel resistance (R) and capacitance (C) elements and a R element in series, modeling the impedance contributions of the metal-oxide interfaces and bulk insulating layers, are employed to describe the impedance spectra of under- and proper-oxidized junctions. This model, however, reveals a discrepancy for overoxidized junctions. This discrepancy can be reconciled by including a third set of parallel RC element, which suggests the appearance of overoxidized CoFeOx layer upon the bottom electrode. From further analysis of interfacial capacitance as a function of oxidation time, the bottom interfacial capacitance widely diverges from the top interfacial capacitance and can be related to the oxidation process of tunnel barrier. The analyzing results of impedance technique are also consistent with the results by complex capacit...

The Kondo effect and carrier transport in amorphous Cr-doped In2O3 thin films

Understanding the interaction between spin of the charge carriers and local magnetic moments in diluted magnetic oxide is an important issue for applications in spintronic devices. This study examines amorphous Cr-doped In2O3 diluted magnetic oxide thin films for the existence of the Kondo effect and a general s-d scattering effect on the magneto transport, as well as for the well known 3D weak localization effect that explains the low temperature transport behavior of transparent conducting oxides. The carrier transport behavior at low temperature can be accurately described and well fit by a combination of these effects. At temperatures lower than the minimum resistivity temperature, the Kondo effect dominates the magnetoresistance effect and is responsible for the enhancement of resistivity.

Effects of submonolayer Mg on CoFe–MgO–CoFe magnetic tunnel junctions

CoFe–MgO–CoFe magnetic tunnel junctions (MTJs) were fabricated by means of ion beam sputtering of MgO as well as subsequent thin films on CoFe. We demonstrated that when a submonolayer of Mg was deposited on the CoFe layer prior to the deposition of MgO, profound improvement in the performance of the CoFe–Mg–MgO–CoFe MTJ was achieved compared to that without the Mg layer. ac impedance measurement indicated that the interfacial Mg layer resulted in fivefold increase in the magnetoresistance ratio from 0.76% for MTJs without it to 4% for that with it. Complex-capacitance (CC) spectra of MTJs showed that the contribution in the relaxation frequency by textured MgO increased. This indicated that the submonolayer Mg improved the texture of the subsequently deposited MgO. Bias-dependent complex-impedance (CI) spectra reveal that interfacial defects were also suppressed due to the submonolayer of Mg. Results obtained from x-ray reflectivity and transmission electronic microscopy were also consistent with those o...

Characterization of Conduction Properties of La

The electrical transport characterization of dual-layer perovskite, La1.4Sr1.6Mn2O7 (327), has been systematically studied by the complex impedance technique. The complex resistivity spectra, under a dc bias (Vdc ) from 0 to 400 mV, have been analyzed by an equivalent circuit model (ECM), including a resistance (R) component and two sets of parallel R and capacitance (C) components in series. The electrical elements represent the 327 grain and 327 two different (extrinsic and intrinsic) grain boundaries (GBs) contributions, respectively. The analyzing results by the ECM demonstrate the R decreases, but C increases with rising Vdc for extrinsic phase of 327 GBs. The behavior can be assigned to the trap-states dominated conduction. For intrinsic phase of 327 GBs, R is almost independent on the Vdc but C decreases with rising Vdc. The transport behavior can be attributed to the decrease of electric dipoles rotation by increasing Vdc. It is here noted that the R element of 327 grains is almost independent on Vdc. The results indicate that two phase GBs is a predominant effect on the electrical transport for 327 and exists different transport behavior on Vdc