Liqing Pan

Structural and magnetic properties of Mn-doped CuO thin films

Highly (111) oriented Mn(6.6%–29.8%) doped CuO thin films have been fabricated on thermally oxidized silicon substrate by radio-frequency magnetron sputtering. X-ray photoelectronic spectroscopy and resistivity studies indicate that both Cu and Mn ions have 2+ valences in the film. Ferromagnetism has been observed for 15.2%–29.8% Mn doped CuO thin film with a transition temperature between 87.0 and 99.5K. The origin of ferromagnetism is analyzed in the context of competition among several interactions between Mn and Cu ions. Highly resistive nature of the films eliminates the carrier mediate mechanisms. The ferromagnetism arises from the ferromagnetic coupling between Mn ions mediated by Cu ions.

Structural and room-temperature ferromagnetic properties of Fe-doped CuO nanocrystals

Fe-doped CuO (Cu1−xFexO) nanocrystals (NCs) (x=0, 0.02, 0.05, 0.1, 0.15, 0.2, 0.25, and 0.3) are prepared by using the urea nitrate combustion method. X-ray diffraction (XRD) analysis confirmed the monoclinic structure of CuO. Single-phase structure is obtained for the 0%–20% Fe-doped CuO, whereas for the 25% and 30% Fe-doped CuO material, secondary phase, α-Fe2O3, is presented. Rietveld refinements of XRD data revealed that with an increase in Fe doping level, there is a monotonic increase in cation vacancies in the Fe-doped samples. X-ray photoelectron spectroscopy measurements on the Cu0.98Fe0.02O sample revealed that the Cu2+ sites are partly substituted by Fe3+ ions. The microstructure is investigated by high-resolution transmission electron microscopy. The magnetic hysteresis loops and the temperature dependence of magnetization of the samples indicated that the samples are mictomagnetic of ferromagnetic domains originated from ferromagnetic coupling between the doping Fe ions in Cu1−xFexO NCs rando...

Magnetoresistance and magnetocaloric properties involving strong metamagnetic behavior in Fe-doped Ni45(Co1−xFex)5Mn36.6In13.4 alloys

Here, we report the co-substitution of Fe and Co for Ni atoms on metamagnetic behavior, martensitic transformation, and transport and magnetocaloric properties in Ni45(Co1−xFex)5Mn36.6In13.4 (x = 0∼0.05) alloys. It is found that the introduction of Fe atoms stabilizes martensitic phase and shifts martensitic temperature (TM) to higher temperature. Meanwhile, the Curie temperature TC of parent phase notably decreases. Upon Fe doping, the low magnetization of martensitic phase keeps nearly unchanged while the magnetization of parent phase slightly decreases. As a result, the Fe-doped samples maintain strong metamagnetic behavior and show great MR and MCE in an extended temperature range around room temperature. The hysteresis loss is reduced upon Fe-doping, which leads to an enhancement of effective RC by 15%.

Ferromagnetism analysis of Mn-doped CuO thin films

Mn (6.6–29.8%)-doped CuO thin film fabricated on a thermally oxidized silicon substrate by radio-frequency magnetron sputtering has been reported. The films were structurally characterized using x-ray diffraction with Rietveld refinement. The analysis indicates that Mn uniformly substituted at the Cu position in the CuO lattice. 5% cation vacancies were detected at the Cu sites and are supposed to be responsible for the p-type electrical conduction of Cu1−xMnxO films. No evidence for large scale Mn aggregation was found in the composition range analyzed. The origin of ferromagnetism was analyzed in the context of competition among several interactions among Mn and Cu ions. A chain model was developed to simulate the ferromagnetic behavior with the random Mn distribution in the samples. The consistency between simulation and experiment strongly indicates that the ferromagnetism mainly arises from the super-exchange interactions of Mn–O–Cu–O–Mn coupling in the [] chain and Mn–O–Mn coupling contributes to the antiferromagnetism.

Metal-insulator transition in ferromagnetic Mn-doped CuO thin films

The ac susceptibility, electrical properties, and magnetotransport properties of the as-prepared MnxCu1−xO thin films (x=7%–29%) were studied in the temperature range of 2–300 K. The susceptibility measurement shows that ferromagnetic transition takes place for the samples with x≥14% below 100 K. The transport properties show the transition from variable-range-hopping to metalliclike behavior at the same transition temperature as the paramagnetism to ferromagnetism transition. The metal-insulator transition is a characteristic feature of magnetic ordering in this material.

Effect of post-annealing on martensitic transformation and magnetocaloric effect in Ni45Co5Mn36.7In13.3 alloys

The metamagnetic alloy Ni45Co5Mn36.7In13.3 was fabricated by conventional arc-melting technique. Subsequent annealing may relax the stress and modify the atom ordering, thus influencing the magnetic properties and martensitic transformation behaviors. Our studies demonstrate that post-annealing at temperatures ≤ 300 °C can lead to a significant change in the magnetic properties and martensitic temperature (TM). Annealing the sample at 300 °C for 3 h can cause a decrease of as much as 30 K in TM (from 319 to 289 K) while retaining strong metamagnetic behaviors. The field-induced metamagnetic transition is accompanied with a large magnetocaloric effect. With an increase in the annealing temperature, the magnitude of the effective magnetic entropy change decreases somewhat, while the refrigeration capacity shows a slight increase.

First order reversal curve diagrams of perpendicular magnetic anisotropy films

The exchange coupling and magnetic properties distributions in Co/Pd multilayer perpendicular magnetic anisotropy films with different magnetic properties are investigated using the experimental first order reversal curve (FORC) diagram with assistance of Landau–Lifshitz–Gilbert simulation. The simulated FORC diagrams of perpendicular magnetic anisotropy films with different exchange couplings and magnetic property distributions are quite different, which make FORC diagrams very powerful for characterizing perpendicular magnetic recording media.

Controlled synthesis, phase formation, growth mechanism, and magnetic properties of 3-D CoNi alloy microstructures composed of nanorods

3-D flower like CoNi alloys nanostructures composed of nanorods have been synthesized by template free hydrothermal method at relatively low temperature (120 °C). The detailed characterizations confirm the formation of good crystalline fcc CoNi alloy, average crystallite size of 18.8 ± 1.0 nm, lattice parameter of 3.531 ± 0.01 A, and the nearly equiatomic composition (Co50Ni50). Highly uniform flower like nano structures are built up with nanorod of diameter about 100 nm and length in range of 200–400 nm. The nanorods (building blocks of flower) have single crystalline nature with [111] preferred growth direction. The concentration of NaOH plays a vital role in formation of alloys and high concentration promotes the formation of CoNi alloy at low temperature. The concentration of NaOH also affects the morphology remarkably by changing the growth/reaction rate of CoNi nanostructures and results in hollow spheres to nanoplate flower of CoNi alloys. Based on the evolution of the morphology of the products, a step wise growth mechanism is rationally proposed for flower like nanostructures by considering the effects of kinetic parameters on growth. Magnetic measurements show Co50Ni50 flower like nanostructure have high saturation magnetization, coercivity, remanent magnetization, and high effective anisotropy constant of value 101.3 emu g−1, 210.5 Oe, 16.2 emu g−1, and 4.457 × 104 J m−3 respectively. The enhancements of coercivity and effective anisotropy constant are attributed to nanoscale effects such as shape/surface anisotropy.

Magnetically Actuated Wormlike Nanomotors for Controlled Cargo Release

Magnetically actuated nanomotor, which swims under externally applied magnetic fields, shows great promise for controlled cargo delivery and release in biological fluids. Here, we report an on-demand release of 6-carboxyfluoresceins (FAM), a green fluorescein, from G-quadruplex DNA functionalized magnetically actuated wormlike nanomotors by applying an alternating magnetic field. This field-triggered FAM releasing process can be easily controlled by multiple parameters such as magnetic field, frequency, and exposure time. In addition, the experimental results and the theoretical simulation demonstrate that both a thermal and a nonthermal mechanism are involved in the cargo releasing process.

Magnetoresistance and magnetocaloric effect in metamagnetic alloys Ni45Co5Mn36.5In13.5

Magnetoresistance (MR) and magnetocaloric effect around martensitic transformation were investigated in a Ni45Co5Mn36.5In13.5 alloy. The martensitic temperature (TM) locates at ∼260 K. An external magnetic field can drive TM to a lower temperature at a rate of ∼9.2 K/T. Associated with the field-induced metamagnetic behaviors, a large MR takes place. The maximal MR exceeds 80% under a field of 5 T around 235 K. More attractive is that the MR behavior is fully recoverable against magnetic field. We also studied the magnetocaloric effect associated with the martensitic transformation and found a large magnetic entropy change (ΔS) around 252 K.