Han Xiufeng

Perpendicular magnetic tunnel junction and its application in magnetic random access memory

Recent progresses in magnetic tunnel junctions with perpendicular magnetic anisotropy (PMA) are reviewed and summarized. At first, the concept and source of perpendicular magnetic anisotropy (PMA) are introduced. Next, a historical overview of PMA materials as magnetic electrodes, such as the RE—TM alloys TbFeCo and GdFeCo, novel tetragonal manganese alloys Mn—Ga, L10-ordered (Co, Fe)/Pt alloy, multilayer film [Co, Fe, CoFe/Pt, Pd, Ni, Au]N, and ultra-thin magnetic metal/oxidized barrier is offered. The other part of the article focuses on the optimization and fabrication of CoFeB/MgO/CoFeB p-MTJs, which is thought to have high potential to meet the main demands for non-volatile magnetic random access memory.

Reversible and reproducible giant universal electroresistance effect

After the prediction of the giant electroresistance effect, much work has been carried out to find this effect in practical devices. We demonstrate a novel way to obtain a large electroresistance (ER) effect in the multilayer system at room temperature. The current-in-plane (CIP) electric transport measurement is performed on the multilayer structure consisting of (011)-Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT)/Ta/Al-O/metal. It is found that the resistance of the top metallic layer shows a hysteretic behavior as a function electric field, which corresponds well with the substrate polarization versus electric field (P—E) loop. This reversible hysteretic R—E behavior is independent of the applied magnetic field as well as the magnetic structure of the top metallic layer and keeps its memory state. This novel memory effect is attributed to the polarization reversal induced electrostatic potential, which is felt throughout the multilayer stack and is enhanced by the dielectric Al-O layer producing unique hysteretic, reversible, and reproducible resistance switching behavior. This novel universal electroresistance effect will open a new gateway to the development of future multiferroic memory devices operating at room temperature.

Magnetization Reversal Mechanism for CoFeB Ferromagnetic Nanotube Arrays

CoFeB nanotube arrays are fabricated in anodic aluminum oxide (AAO) membranes and track etched polycarbonate (PCTE) membranes by using an electrochemical method, and their magnetic properties are investigated by vibrating sample magnetometry. The coercivity Hc and remanent squareness SQ of these CoFeB nanotube arrays are derived from hysteresis loops as a function of angle between the field and tube axis. The Hc(θ) curves for CoFeB nanotube arrays in AAO and PCTE membranes show M-type variation, while they change shape from M to mountain-type as the tube length increases. However, the overall easy axis perpendicular to tube axis does not change with tube length. The different angular dependences are attributed to different magnetization reversal mechanisms.

Interstitial vortex in superconducting film with periodic hole arrays

The response of superconducting Nb films with a diluted triangular and square array of holes to a perpendicular magnetic field are investigated. Due to small edge-to-edge separation of the holes, the patterned films are similar to multi-connected superconducting islands. Two regions in the magnetoresistance R(H) curves can be identified according to the field intervals of the resistance minima. Moreover, in between these two regions, variation of the minima spacing was observed. Our results provide strong evidence of the coexistence of interstitial vortices in the islands and fluxoids in the holes.

Role of defects in magnetic properties of Fe-doped SnO2 films fabricated by the Sol—Gel method

This paper obtains the room temperature ferromagnetism in Sn1-xFexO2 films fabricated by the Sol-Gel method. X-ray diffraction results show that Fe doping inhibits the growth of SnO2 and Fe3+ ions occupy the Sn sites. The measurement of resistance excludes the free carrier inducing ferromagnetism. Moreover, the temperature dependence of magnetization has been better fitted by the Curie Weiss law and bound magnetic polaron (BMP) theory. An enhancement of ferromagnetism is achieved by annealing the samples with x = 7.1% in H-2, and a decrease of oxygen flow rate. All these results prove that the BMP model depending on defects can explain ferromagnetism in diluted magnetic oxides.

Low frequency noise in asymmetric double barrier magnetic tunnel junctions with a top thin MgO layer

Low frequency noise has been investigated at room temperature for asymmetric double barrier magnetic tunnel junctions (DBMTJs), where the coupling between the top and middle CoFeB layers is antiferromagnetic with a 0.8-nm thin top MgO barrier of the CoFeB/MgO/CoFe/CoFeB/MgO/CoFeB DBMTJ. At enough large bias, 1/f noise dominates the voltage noise power spectra in the low frequency region, and is conventionally characterized by the Hooge parameter αmag. With increasing external field, the top and bottom ferromagnetic layers are aligned by the field, and then the middle free layer rotates from antiparallel state (antiferromagnetic coupling between top and middle ferromagnetic layers) to parallel state. In this rotation process αmag and magnetoresistance-sensitivity-product show a linear dependence, consistent with the fluctuation dissipation relation. With the magnetic field applied at different angles (θ) to the easy axis of the free layer, the linear dependence persists while the intercept of the linear fit satisfies a cos(θ) dependence, similar to that for the magnetoresistance, suggesting intrinsic relation between magnetic losses and magnetoresistance.

Dependence of Interlayer AF Coupling on Ferromagnetic Layer Thickness in [Pt/Co]5/Ru/[Co/Pt]5 Multilayers

We study magnetization reversal in the interlayer coupled [Pt/Co]5/Ru/[Co/Pt]5 multilayers (MLs) by means of the measurement of extraordinary Hall effect (EHE). Fitting experimental data to a simple model, we determine the interlayer coupling strength for various thicknesses of the ferromagnetic layers at a fixed Ru spacer thickness. It is found that the dependence of interlayer coupling strength on the Pt layer thickness is much stronger than the previous report in the ferromagnetic/nonmagnetic/ferromagnetic multilayers.

Correlation between Electroresistance and Magnetoresistance in Slight Oxygen-Deficient Nd0.67Sr0.33MnO3-δ Polycrystalline Ceramics

A colossal electroresistance effect is found in Nd0.67Sr0.33MnO3-δ polycrystalline ceramics with a very slight oxygen deficiency δ = 0.05. The electroresistance behaviour of Nd0.67 Sr0.33MnO3-δ (δ = 0.05) displays the striking similarity to its counterpart magnetoresistance. It reveals that the boundary effect plays an important role in electroresistance and the same physical origin is likely responsible for both magnetoresistance and electroresistance effects in Nd0.67 Sr0.33MnO3-δ polycrystals.

Spin Transport in Multi Wall Carbon Nanotubes with Co Electrodes

The magnetic field dependent transport behaviour of Co contacted multi-wall nanotubes is investigated. A sample with three Co electrodes has been measured by two-channel method with an in-plane magnetic field. When the in-plane magnetic field is perpendicular to the tube, high positive magnetoresistance up to 30% is obtained at low temperature from 3 K to 25 K and with field parallel to the tube, negative magnetoresistance up to 15% is observed only from the high resistance junction. The detailed positive and negative magnetoresistance behaviour also changes with temperature.

Spin-Flip Behaviour Observed in (Pt/Co)5/Ru/(Co/Pt)5 Films

(Pt/Co)5/Ru/(Co/Pt)5 films with different Ru thicknesses were prepared by magnetron sputtering. Oscillatory behaviour of effective perpendicular anisotropy and remanence in the (Pt/Co)5/Ru/(Co/Pt)5 films as a function of the thickness of Ru has been first observed, from which we can obtain the conclusion that the synthetic spin in the (Pt/Co)5/Ru/(Co/Pt)5 films rotates from the vertical direction to the in-plane one periodically with the change of the thickness of the spacer layer of Ru. The oscillatory period is about two atomic layers of Ru and different from the long and short oscillatory periods of the interlayer antiferromagnetic coupling in Co/Ru/Co, Co/Cu/Co, and Fe/Cr/Fe, etc.