C.-X. Ji

Over 70% tunneling magnetoresistance at room temperature for a CoFe and AlOx based magnetic tunnel junction

More than 70% tunneling magnetoresistance (TMR) ratio has been observed at room temperature for a CoFe and AlOx based magnetic tunnel junction. The annealing of the epitaxial bottom electrode, Si (001)/Ag fcc (200)∕Co84Fe16 bcc (200), at 400°C prior to fabricating the tunnel barrier and the upper electrode is crucial for achieving this high TMR ratio. Moreover a high output voltage could be obtained for this magnetic tunnel junction due to its high V1∕2, the bias voltage at which the TMR ratio is reduced to half of that near the zero bias. The rationale for obtaining this high TMR ratio is discussed, and there are reasons to believe that the TMR ratio for this junction could be further improved.

Structure and electronic properties of self-assembled Pt silicide nanowires on Si(100)

We investigated the formation of Pt silicide nanowires on a Si(100) surface using scanning tunnelling microscopy and high-resolution photoemission spectroscopy. Pt silicide nanowires with a tetragonal Pt2Si structure are formed along the step edges of Si(100). Pt-induced c(4 × 2) reconstructions also appear adjacent to the tetragonal Pt2Si nanowires. Formation of the Pt2Si nanowires is attributed to the anisotropic lattice mismatches between the tetragonal Pt2Si structure and Si(100). Scanning tunnelling spectroscopy data show that the nanowires are metallic. The stoichiometry of Pt silicide is confirmed by high-resolution photoemission spectroscopy.

Growth and physical property of epitaxial Co70Fe30 thin film on Si substrate via TiN buffer

Epitaxial Co70Fe30 films with the bcc structure were grown on a Si(001) substrate with TiN as a buffer by sputtering technique. The x-ray diffraction results confirmed the epitaxial nature of the films and the crystallographic relationship was determined as Co70Fe30(002)⟨110⟩∕∕TiN(002)⟨100⟩∕∕Si(004)⟨100⟩. The surface morphology characterized by atomic force microscopy on our films revealed that smooth surfaces could be obtained at growth temperatures below 350°C. The strain state of 60nm epitaxial Co70Fe30 films was studied as a function of growth temperature. Magnetization hysteresis loops of the films grown at 300°C were measured using superconducting quantum interface device magnetometer.

Oxidation of tunnel barrier metals in magnetic tunnel junctions

The oxidation of an ultrathin metal layer (<1nm) to form an oxide tunnel barrier is of critical importance for the fabrication of magnetic tunnel junctions (MTJs) with low product of resistance and area (R×A). Nonuniform and excessive or insufficient oxidation will occur by using conventional plasma, air, or O2 and noble gas mixtures as oxidation methods. An oxidation method was investigated to oxidize only an ultrathin layer of metal (such as Y) without oxidizing adjacent ferromagnetic thin film layers. We have now demonstrated that a gas mixture of H2O∕H2 with a fixed chemical potential of oxygen determined by the relative amounts of the two gases can oxidize Y and Ta thin layers while simultaneously keeping a Co ferromagnetic layer completely unoxidized. This universal method can be used to preferentially oxidize a host of other metals with high tendency to form oxides, such as Zr, Hf, Nb, rare earth metals, etc. and may allow us to access the feasible lower limit of barrier thickness in MTJs.

Origin of the dependence of magnetoresistance on the composition of Co100−xFex electrodes in magnetic tunnel junctions

The tunneling magnetoresistance value of a Co100−xFex (4nm)∕AlOx 1.7nm∕Co100−xFex (4nm) magnetic tunnel junction has been demonstrated to depend on the composition of the Co100−xFex electrodes. The interface roughness, crystal structure, and tunneling spin polarization versus the composition of the Co100−xFex electrode were studied to address the origin of this compositional dependence. Ab initio calculations of s-like electron spin polarization predict a composition dependence similar to that observed experimentally. The combined experimental and computational results show that the trends in Co100−xFex tunneling magnetoresistance are modified slightly by the interface roughness but mainly determined by the s-like electron spin polarization values associated with different compositions and crystal structures.

Origin of inverse tunneling magnetoresistance in a symmetric junction revealed by delaminating the buried electronic interface

Electrical properties of modern electronic devices are usually controlled by the physical and chemical structure of one or more buried material interfaces. Accessing these buried interfaces by energetic ion milling can destroy this structural information. We report a delamination technique that exposes pristine buried interfaces for x-ray photoemission spectroscopy. We use this technique to show that unusual inverse tunneling magnetoresistance in a nominally symmetric (Co,Fe)/AlOx/(Co,Fe) magnetic tunnel junction devices is attributable to subtle over-oxidation of the lower AlOx/CoFe interface. Ion-milling investigation of the same samples misleads by chemically reducing the signature Fe oxide species during milling.

Thermal stability of the interfaces between Co-, Ni-, and Fe-based ferromagnets in contact with selected nitrides MN (M=Al, B, Nb, Ta, Ti, and V)

Nitride tunnel barriers have potential applications in magnetic tunnel junctions (MTJs). Thermal stability of the interfaces between Co-, Ni-, and Fe-based ferromagnets and these nitride tunnel barriers is critical to device performance. With guidance from low-temperature ternary isothermal phase diagrams of the Co–M–N, Ni–M–N, and Fe–M–N systems (M=Al, B, Nb, Ta, Ti, and V), the interfaces in Co∕MN, Ni∕MN, and Fe∕MN structures were evaluated in terms of two criterions: the phases in contact must (1) be in equilibrium with each other (i.e., connected by a stable tie line) and (2) have negligible mutual solubility in the phase diagram at the temperatures of interest. Of the investigated interfaces, Co∕AlN, Co∕BN, Co∕NbN, Co∕TaN, Co∕TiN, Ni∕BN, Ni∕TaN, Fe∕BN, Fe∕NbN, Fe∕TaN, and Fe∕TiN were found to be thermodynamically stable. However, in light of some simplifications made in this analysis, the current evaluation of interfacial stability serves as a useful step in preselecting candidate nitride-based MTJ t...