Ji Fu

Trap-assisted tunneling resistance switching effect in CeO2/La0.7(Sr0.1Ca0.9)0.3MnO3 heterostructure

We reported the resistance switching (RS) behavior in the epitaxially grown CeO2/ La0.7(Sr0.1Ca0.9)0.3MnO3 (CeO2/LSCMO) heterojunctions on SrTiO3 substrate. The CeO2/LSCMO device displayed improved switching characteristics as compared to that of metal/manganite device. The switching threshold voltage showed a strong dependence on the thickness of the CeO2 layer, where a minimum/maximum thickness was required for the appearance of the resistance switching. Both set and reset threshold voltages increase with the increase of the CeO2 layer thickness due to the trap-assisted electron tunneling effect. In the meantime, the defects or vacancies in the CeO2 films, in particular, the concentration of the defects or vacancies in the interface between CeO2 and LSCMO, have a significant impact on the switching effect. These results suggest that the electron tunneling accompanied by a trapping/detrapping process at the interface is likely responsible for the RS effect in the insulator/manganite system.

Contact resonance force microscopy for nanomechanical characterization: Accuracy and sensitivity

Contact Resonance Force Microscopy (CRFM), based on dynamic force microscopy, is a new promising technique for quantitative nanoscale mechanical characterization of various materials. In this work, we systematically investigated the measurement accuracy and sensitivity of CRFM both experimentally and numerically. For the accuracy study, we first evaluated the validity of the Euler-Bernoulli beam model used in CRFM and found that it is accurate enough for practical testing. Then, the influence of the tip location was also analyzed and results show that it can significantly affect the obtained indentation modulus. The measurement accuracy of CRFM was then compared with that of nanoindentation and it shows that CRFM has less relative testing errors than nanoindentation for modulus mapping but a larger data scattering for single-point measurements. As to the sensitivity study, we first conducted the cantilever-stiffness dependent sensitivity analysis using both numerical and experimental approaches, and sugge...

The asymmetric magnetization reversal in exchange biased granular Co/CoO films

A special kind of asymmetric magnetization reversal characterized by a sharp step in the second quadrant of the hysteresis loop has been investigated in the exchange biased Co/CoO films. This asymmetry only exists in the film where the small clusters grow to form islands. A bimodal distribution of the first-order reversal curves diagram demonstrates that the irreversible magnetization reversal is composed of the free part and exchange biased part, originating from the grains smaller and larger than 6 nm in the film, respectively. The finite-size effect of the antiferromagnetic anisotropy in the ultrafine grains is responsible for this asymmetry.

Temperature dependence of exchange bias and training effect in Co/CoO film with induced uniaxial anisotropy

The exchange bias effect and training effect of the Co/CoO film with induced uniaxial anisotropy were investigated as functions of temperature. It was found that both effects exhibited drastic differences along the easy and the hard axes. Along the easy axis, the magnetization reversal was dominated by domain wall motion throughout the whole temperature range. However, along the hard axis, the magnetization reversal was dominated by domain wall motion and domain rotation at temperatures below and above 150 K, respectively. The crossover of the two reversal modes characterized with significant asymmetry in the hysteresis loop was observed along the hard axis at 150 K due to the interplay between the exchange and uniaxial anisotropies. Significant difference of training effect in the two directions was observed and ascribed to the differences of the duration and intensity of the interaction between ferromagnetic and antiferromagnetic spins in the two magnetization reversal modes.

Magnetic and transport properties of cobalt doped La0.7Sr0.3MnO3

The magnetic and transport properties of La0.7Sr0.3Mn1−xCoxO3 (0 ≤ x ≤ 1) samples were systematically studied. A magnetic phase diagram was constructed according to the observed results. At the two ends (x ≤ 0.3 and x > 0.9), the samples exhibit the conventional ferromagnetism with the metallic conducting behavior, which can be interpreted in terms of the double exchange coupling between Mn (Co) ions. As for the intermediate Co-doping region (0.3 < x ≤ 0.9), the samples enter a mixed magnetic phase state, which displays the characteristics of the glassy states. According to the frequency-dependent results from the ac susceptibility, a representative frequency sensitivity factor K was calculated to be ∼0.1 for the typical La0.7Sr0.3Mn0.6Co0.4O3 sample, corresponding to the coexistence of superparamagnetic-like free spins and the reentrant spin glass. The results of the Curie-Weiss fitting of the dc magnetization suggest that the Co3+ ions are in the intermediate spin state with the increase of Co-doping le...

Fabrication of FeOx thin films and the modulation of transport and magnetic properties by resistance switching in Au/α-Fe2O3/Pt heterostructure

It was found that by using the same α-Fe2O3 target in fabrication process, an oxygen-deficient and an oxygen-sufficient atmosphere facilitated the formation of Fe3O4 and α-Fe2O3, respectively. The Au/α-Fe2O3/Pt heterostructure showed a memristive bipolar resistance switching. A clear difference was observed in the transport and magnetic properties between the two resistance states. The enhanced conductivity and magnetization in the Au/α-Fe2O3/Pt heterostructure are believed to result from an enhanced electron hopping between Fe3+-Fe2+ pairs and Fe3+-O-Fe2+ double exchange coupling effect. The enhanced Fe2+ comes from a redox transition between two phases (one behaves like Fe3O4 and the other like α-Fe2O3), due to the electrochemical migration of oxygen vacancies.

An elastography method based on the scanning contact resonance of a piezoelectric cantilever

PURPOSE Most tissues may become significantly stiffer than their normal states when there are lesions inside. The tissues modulus can then act as an identification parameter for clinic diagnosis of tumors or fibrosis, which leads to elastography. This study introduces a novel elastography method that can be used for modulus imaging of superficial organs. METHODS This method is based on the scanning contact-resonance of a unimorph piezoelectric cantilever. The cantilever vibrates in its bending mode with the tip pressed tightly on the sample. The contact resonance frequency of the cantilever-sample system is tracked at each scanning point, from which the samples modulus can be derived based on a beam dynamic model and a contact mechanics model. Scanning is performed by a three-dimensional motorized stage and the whole system is controlled by a homemade software program based on LabVIEW. RESULTS Testing on in vitro beef tissues indicates that the fat and the muscle can be easily distinguished using this system, and the accuracy of the modulus measurement can be comparable with that of nanoindentation. Imaging on homemade gelatin phantoms shows that the depth information of the abnormalities can be qualitatively obtained by varying the pressing force. The detection limit of this elastography method is specially examined both experimentally and numerically. Results show that it can detect the typical lesions in superficial organs with the depth of several centimeters. The lateral resolution of this elastography method∕system is better than 0.5 mm, and could be further enhanced by using more scanning points. CONCLUSIONS The proposed elastography system can be regarded as a sensitive palpation robot, which may be very promising in early diagnosis of tumors in superficial organs such as breast and thyroid.

Exchange bias of CoO1−δ/(NiFe,Fe) system with blocking temperature beyond Néel temperature of bulk CoO

The exchange bias effects in pulsed laser deposited CoO1−δ/(NiFe,Fe) bilayers were investigated. An anomalously high blocking temperature (TB) of 325 K was obtained for the as-deposited CoO1−δ/ferromagnet bilayers, which is 33 K higher than the Neel temperature of the bulk CoO (TN = 292 K). The amount of oxygen vacancies δ in the CoO1−δ film can be controlled by a post-annealing treatment in the O2 atmosphere. The TB of the CoO1−δ/ferromagnet bilayers increases with an increasing δ from δ = 0 to δ = 0.16. Our results indicate that the high TB of CoO1−δ/ferromagnet bilayers originates from the enhancement of the magnetic interaction in the CoO1−δ layer owing to O2− vacancies.

The manipulation of magnetic properties by resistive switching effect in CeO2/La0.7(Sr0.1Ca0.9)0.3MnO3 system

The bipolar resistance switching behavior was observed in the epitaxially grown CeO2/La0.7(Sr0.1Ca0.9)0.3MnO3 (CeO2/LSCMO) heterojunctions on SrTiO3 substrate using pulsed laser deposition technology. It was found that the magnetization of CeO2/LSCMO heterojunction varies with the resistance state of the device when the external triggered voltage is higher than the set and reset voltages. The magnetization could be reversibly changed by exerting external set and reset voltages on the junction. The electron tunneling accompanied by a trapping/detrapping process at the interface is likely responsible for the modulation of the magnetization in this insulator/manganite device.

Contact resonance force microscopy with higher-eigenmode for nanoscale viscoelasticity measurements

Nanoscale viscoelastic properties are essential for polymeric materials in their wide applications in nanotechnology. Here we proposed a contact resonance force microscopy (CRFM) method for viscoelasticity measurements by utilizing a cantilevers higher-eigenmode (n > 3). Numerical analysis results show that, compared to its lower eigenmodes, a cantilevers higher eigenmode is more sensitive to contact damping and less affected by contact stiffness variations. This tendency is then verified by nanoscale viscoelasticity mapping on a polystyrene (PS)/polymethyl methacrylate (PMMA) copolymer thin film using a compliant cantilevers different eigenmodes. Results show that higher-eigenmode CRFM can provide better imaging contrast and is thus suggested for viscoelasticity measurements.