Cao Shi-Xun

Magnetoresistance of Multiwalled Carbon Nanotube Yarns

We measure zero-Geld resistivity and magnetoresistance of multiwalled carbon nanotube yarns (CNTYs). The CNTYs are drawn from superaligned multiwalled carbon nanotube arrays synthesized by the low-pressure chemical vapour deposition method. The zero-Geld resistivity shows a logarithmic decrease from 2K to 300 K. In the presence of a magnetic Geld applied perpendicular to the yarn axis, a pronounced negative magnetoresistance is observed. A magnetoresistance ratio of 22% is obtained. These behaviours can be explained by the weak localization effect.

Fabrication, Structural and Magnetic Properties for Aligned MnBi

MnBi compound is fabricated under a magnetic field of 1T, and the c-axis of hexagonal MnBi crystal is aligned parallel to the magnetic field direction. The saturation magnetization Ms decreases with the increase of temperature. At temperatures below 200 K, the coercive field Hc is about 150 Oe, while the coercive field Hc increases with temperature above 200K. From 200K to 300K, the remnant magnetization Mr and the Mr/Ms increase with the temperature. Below 200 K, Mr and Mr/Ms reach roughly a constant value. However, there is an abnormal increase at 100 K in Hc, Mr and Mr/Ms, which comes from a spin-reorientation in MnBi. Magnetization results show the spin-reorientation for MnBi at about 91 K due to the variations of the anisotropy constants.

Ferromagnetic resonance in Co/Pt multilayers

Out-of-plane angular dependence of ferromagnetic resonance spectra was measured in sputtered Co/Pt multilayers and analyzed with the Landau-Lifshitz-Gilbert equation. The effective demagnetizing field 4πMeff is found to increase with decreasing tPt and decreasing 1/tCo, which can be considered as a result of interplay between the interlayer coupling and a low-dimensional effect. The g factor increases with increasing tPt and decreasing tCo, indicating contribution of spin-polarization of Pt atoms and additional contribution of orbital moment of Co atoms. The in-plane resonance line-width increases with decreasing tCo and increases with increasing tPt.

Nano-sized Domain Wall Pinning Effects in Dilute Cu-Doped Perovskite LaMn1-xCuxO3 Manganites

Magnetic properties of Cu-doped LaMn1-xCuxO3 (x=0.05-0.30) systems are carefully studied in the temperature range of 2–300 K. A visible unexpected drop is observed in the ac susceptibility and the zero-field cooled dc magnetization curves for the dilute x ≤0.10 near 100K, which depends on the measuring frequency and magnetic field. Measurements on frequency dependence of ac susceptibility, observation of magnetic relaxation, and the existence of critical field indicate that the anomaly can be attributed to the domain wall pinning effects. This is directly proven by the results of ball milled nano-sized powder counterparts compared with the bulk materials.

In-Situ Alignment of MnBi Crystals Induced by High Magnetic Field above Curie Temperature

Above Curie temperature, MnBi crystals are aligned in situ along the c-axis in a Bi matrix by a high fabrication magnetic field Hf of 10 T. Magnetic testing shows a pronounced anisotropy in magnetization in directions normal and parallel to the fabrication field, resulting from the alignment. The successful alignment may result from the fact that the easy magnetization direction is along the c-axis of MnBi and the high fabrication field of 10 T is large enough to rotate the MnBi crystal to this direction even though the temperature is above the Curie temperature.

Structural and Transport Properties for Al- and Fe-Doping La0.67Ca0.33MnO3Perovskite Manganese

The effects of Al and Fe ion doping in Mn sites was studied for the colossal La0.67Ca0.33MnO3 magnetoresistance material. It was found that when the Fe-doping amount x increases, the crystal cell structure has no obvious change, but the crystal cell volume decrease monotonically for Al-doping. Both resistances increase rapidly and the insulator–metal transition temperature moves to lower temperature and decreases linearly with Al-doping. The area for Al-doping is broader than Fe. At small amount of Al-doping, the resistance satisfies the metal transport property when T<TIM. The characteristic of the transport behaviour for Al- and Fe-doping can be explained by terminating the double exchange channel of Mn3+–O2−–Mn4+.

Reentrant Spin Glass Behaviour and CE-Type Antiferromagnetic Phase in Half Doping (La,Pr)1/2Ca1/2 MnO3 Manganites

The reentrant spin glass behaviour was observed in half doping (La,Pr)1/2Ca1/2MnO3 manganites with a CE-type antiferromagnetic structure. It shows sequential multiple magnetic transitions from the paramagnetic to the ferromagnetic, antiferromagnetic, spin glass (SG) transitions, which reflects the complex magnetic interaction in the ground state of manganites. This can be explained by the competition interaction between the ferromagnetic and the CE-type antiferromagnetic matrix, and the disorder due to the tolerance factor t and A-cation size variance ?2. The results reveals the coexistence of ferromagnetic clusters and SG clusters in the background of the antiferromagnetic matrix in the ground state of half doping (Pr,La)1/2Ca1/2MnO3 systems.