Shihui Ge

Relationship between the surface chemical states and magnetic properties of CeO2 nanoparticles

Hydrogen reduction treatments with variations in isothermal reduction temperature, duration, and partial pressure of hydrogen (PH2) are carried out on CeO2 nanoparticles (NPs) to investigate the relationship between the surface chemical states and magnetic properties of the samples. The phenomenon of surface reduction degree dependence of the ferromagnetism of CeO2 NPs is observed. Semiquantitative calculated concentrations of surface Ce3+ ions from x-ray photoelectron spectroscopy data demonstrate that ferromagnetism does not relate to the surface oxygen vacancies but to the surface Ce3+/Ce4+ pairs. The higher the number of surface Ce3+/Ce4+ pairs, the more robust the ferromagnetism appears.

Study of the local structure of metastable crystalline iron-boron alloys

The microstructure of metastable crystalline Fe 100-x B x ( 1 \leq x \leq 9 ) alloys, produced by rapidly quenching from the melt, has been studied with different techniques. Studies performed with Moss-bauer effect (ME) spectroscopy and nuclear magnetic resonance (NMR) are reported here, along with preliminary high resolution electron microscopy (HREM) and selected area electron diffraction (SAED) observations, as well as initial extended X-ray absorption fine structure (EXAFS) studies. The alloys, which appeared as bcc single phase upon X-ray diffraction (XRD) analyses, were found to be heterogeneous on a microscopic scale. Both the Mossbauer effect and nuclear magnetic resonance results showed that the alloys could not be considered as random solid solutions of boron in iron. Based on the hyperfine field values at the57Fe,10B, and11B sites, on the relative population of probes at the different sites, and on the temperature dependence of the hyperfine fields, it was concluded that very small regions with an orthorhombic Fe 3 B-like structure exist in the bulk. The high resolution electron microscopy bright field image observations showed regions 5-15 A in diameter, embedded in the α-Fe matrix, and having fringe spacings different from those of bcc α-Fe. The selected area electron diffraction patterns contained weak, broad spots which were somewhat more consistent with those from orthorhombic Fe 3 B than tetragonal Fe 3 B. The initial extended X-ray absorption fine structure analyses were consistent with the results provided by the other techniques. A comparison is made with a previous description in which these alloys were considered as random solid solutions. Our results are discussed in terms of the short-range order (SRO) for orthorhombic and tetragonal Fe 3 B, as well as bcc α-Fe.

Grain size dependence of coercivity in magnetic metal-insulator nanogranular films with uniaxial magnetic anisotropy

Excellent soft magnetic properties and appropriate uniaxial magnetic anisotropy field have been achieved in a wide metal volume fraction (x) range for as-deposited (Fe65Co35)x(SiO2)1−x granular films fabricated by magnetron sputtering. With decreasing x from 0.86 to 0.53, the coercive force of easy axis Hce decreases clearly and shows the minimum value (Hce=0.85 Oe) at x=0.53. More importantly, not only nanoscale grain-size D contributing to small coercivity is proved, but also a D3 dependence of Hce is observed in the range of 0.53<x<0.86, which is similar to the results of Suzuki et al. reported for nanocrystalline alloys. Based on the random anisotropy model of Suzuki and Herzer, the grain-size D dependence of coercivity Hce in our metal-insulator granular film system was analyzed. Analyze results indicate that strong coherent uniaxial anisotropy which dominates over the random anisotropy ⟨K1⟩ of magnetic grains obtained by the random anisotropy model can be responsible for the D3 dependence of the coe...

Room temperature ferromagnetism in Sn1−xVxO2 films prepared by sol-gel method

The structure and magnetic properties of Sn1−xVxO2 (x=0.02–0.22) thin films fabricated on Si (111) substrate using a sol-gel method and spin coating technique have been investigated. All the samples have pure rutile polycrystalline structure and exhibit room temperature ferromagnetism. The magnetic moment per V reaches 2.92μB for the Sn0.98V0.02O2 film and drops rapidly as V content is increased. X-ray photoelectron spectroscopy study reveals that vanadium is in V4+ chemical state. Various annealing treatments were performed to explore the origin of the ferromagnetism. It is found that the ferromagnetism of Sn0.98V0.02O2 film disappears after annealing in a rich-oxygen atmosphere and occurs again after annealing in a low vacuum condition. Furthermore, an annealing in Sn vapor leads to the decrease in ferromagnetism. These results confirm that the oxygen vacancies play a critical role in introducing ferromagnetism of Sn1−xVxO2 films; therefore, the origin of the ferromagnetism in our samples can be underst...

HYPERFINE FIELDS AND MAGNETISM IN THE ND2FE14B COMPOUND - A COMBINED MOSSBAUER AND NMR-STUDY

A combined nuclear magnetic resonance (NMR) and Mossbauer effect (ME) study of Nd2Fe14B has been carried out, which uses the Fe hyperfine field (HF) values obtained by NMR experiments in the ME fitting process. Taking the advantages of both NMR in determining the HF values at different Fe sites and ME in determining the intensity ratios of subspectra, the determination of the Fe HF values and their assignment can be made more precisely and more reliably. It can be deduced from the present result that the existence of a nearest Fe neighbor leads to an increase in the Fe HF of about 15 kOe/Fe, while a nearest B neighbor atom reduces it at a larger rate. The nearest Nd neighbor atom seems to have no significant effect on the Fe HF. The analysis concerning the various contributions to the HF and their correlation with the atomic magnetic moment demonstrates that the hyperfine interaction study for the case of Fe provides a simple means to measure Fe sublattice magnetization and relevant properties of the R2Fe14B system.

Giant magnetoresistance and microstructure of CoCu and FeCoCu granular films

Abstract Co Cu and FeCo Cu granular films were prepared by DC (or RF) magnetron sputtering and post-deposition annealing. The influence of annealing conditions (temperature, duration) on GMR has been investigated systematically. It is found that the cumulative isothermal annealing at an optimal temperature (moderately high) promotes the precipitation of magnetic grains but limits their excessive growth, and therefore, leads to more homogenous and smaller particles, which is in favor of the GMR improvement. By a combined study of TEM, magnetization and GMR, it is believed that the GMR in these granular films mainly originates from the spin-dependent scattering of conducting electrons in the surfaces between magnetic particles and Cu matrix; especially, the very fine particles with a diameter of a few nm may be most effective for spin-dependent scattering.

Fabrication and Magnetic Properties of Fe65Co35–ZnO Nano-Granular Films

A series of nano-granular films composed of magnetic metal (Fe65Co35) granules with a few nanometers in size and semiconductor oxide (ZnO) have been fabricated by a magnetron sputtering method, and excellent soft magnetic properties have been achieved in a wide metal volume fraction (x) range for as-deposited samples due to the exchange coupling between FeCo granules (a ferromagnetic interaction in nano-scale). In a wide range (0.53 <x < 0.71), the films exhibit coercivity HC not exceeding 15 Oe, along with high resistivity. Especially for the sample with x = 0.67, coercivities in hard and easy axes are 1.43 and 7.08 Oe, respectively, 4πMS = 9.85 kg, and ρ reaches 2.06 × 103 μΩ cm. The dependence of complex permeability μ = μ′ − jμ″ on frequency shows that the real part μ′ is more than 100 below 1.83 GHz and that the ferromagnetic resonance frequency reaches 2.31 GHz, implying the promising for high frequency application. The measured negative temperature coefficient of resistivity reveals that may be the weak localized electrons existing in samples mediate the exchange coupling.

Magnetic properties and the tunneling magnetoresistance effect in Co−MgF2 granular films

Co−MgF2 granular films were deposited on glass substrates by rf co-sputtering at room temperature (RT). The influence of the Co volume fraction, fv, of these granular films on the tunneling magnetoresistance (TMR) and magnetic properties was studied systematically. In a magnetic field of 1.2 T, the TMR value at RT initially increases gradually with decreasing fv, reaches its maximum value of −8% for fv=0.38, and then decreases. The corresponding magnetization curves indicate a change from ferromagnetism to superparamagnetism. A minimum in the coercivity, Hc, (11 Oe) is obtained in the Co50(MgF2)50 granular film which also has a large zero field resistivity. This magnetically soft granular film may consequently be a good candidate for high frequency applications. These variations of the TMR and magnetic properties can be ascribed to gradual changes in the film microstructure with decreasing fv, from interconnected metallic Co grains to nano-scaled Co particles dispersed in a crystallized insulating MgF2 ma...

The influence of microstructure on tunnelling magnetoresistance in Fe-SiO2 granular films

The influence of film thickness on the microstructure and tunnelling magnetoresistance (TMR) in Fe-SiO2 granular films has been systematically studied using x-ray diffraction, transmission electron microscopy (TEM) and Mossbauer spectroscopy. The results indicate that the average Fe granule size and the size distribution, as well as the interface between the granules and SiO2 matrix, sensitively depend on the film thickness, which modifies the TMR effect. For the very thin films, the TEM image shows an indistinct interface between the Fe particles and SiO2 matrix, and the corresponding Mossbauer spectrum shows the existence of a strong non-magnetic Fe2 SiO4 component, which is responsible for the small TMR of this film. While for the thickest film, the long sputtering time makes the Fe granules grow so large as to aggregate some of them, which also leads to the small TMR. The optimum thickness in our samples is 0.55 µm, which gives the largest TMR value of -3.3% at room temperature under a field of 1.6 T due to the uniform size of the small particles and the few in the Fe2 SiO4 phase. Our results give evidence that small Fe particles with a narrow size distribution and without oxidation favours the TMR effect.

Investigation on the intergranular interaction of the soft magnetic granular films by δM(H) curves

(Fe65Co35)x(SiO2)1−x granular films with various volume fraction (x) of Fe65Co35 alloy were fabricated by magnetron sputtering, and the interaction between granules was explored by δM(H) plots. Research results show that δM(H) is positive for the samples in a wide range of 0.42<x<0.86, indicating that exchange coupling exists among granules. Further research revels that δM(H)max increases gradually with decreasing x, reaching the maximum value at x=0.57, then decreases after x<0.57. Since δM(H)max can effectively reflect the intensity of exchange coupling, the intergranular exchange coupling also increases first, then decreases as x decreases. The microstructure is analyzed with help of the high-resolution transmission electron microscopy, and results show that the variation of exchange coupling with metal volume fraction x is dominated by the change of microstructure of samples. The microstructure of the sample with x=0.57 satisfy the optimization for realizing exchange coupling, leading to excellent sof...