Zhang Shao-Ying

Structure, magnetic properties and giant magnetoresistance of YMn6Sn6-xGax (x=0-0.6) compounds

Structure, magnetic and transport properties of YMn6Sn6-xGax (0≤x≤0.6) compounds with a HfFe6Ge6-type structure were investigated. It was found that the Ga substitution leads to a contraction of the unit-cell volume. A transition from an antiferromagnetic to a ferromagnetic (or ferrimagnetic) state can be observed for samples (0.1≤x≤0.2) with increasing temperature. The antiferro-ferromagnetic transition for samples with x≤0.2 can also be induced by an external field. The required field is very low and decreases with increasing Ga concentration. More Ga concentration (x≥0.3) leads to the samples being ferromagnetic in the whole temperature range below the Curie temperature. The Ga substitution weakens the interlayer magnetic coupling between the Mn spins. Corresponding to the metamagnetic transition, a magnetoresistance as large as 32% under a field of 5 T was observed at 5 K for the sample with x=0.2.

Magnetic properties and magnetocaloric effects of Mn5Ge3−xGax

We report on the magnetic properties and magnetocaloric effects of Mn5Ge3−xGax compounds with x=0.1, 0.2, 0.3, 0.4, 0.6 and 0.9. All samples crystallize in the hexagonal Mn5Si3-type structure with space group P63/mcm and order ferromagnetically. The Curie temperature of these compounds decreases with increasing x, from 306K (x=0.1) to 274K (x=0.9). The average Mn magnetic moments increases with increasing Ga content, reaching a maximum value at x=0.6. The magnetic entropy changes in these compounds are determined from the temperature and field dependence of the magnetization using the thermodynamic Maxwell relation. The Ga substitution has two kinds of influence on the magnetocaloric effect (MCE) of Mn5Ge3. One is that the magnitude of the magnetic entropy change decreases, the other is that the MCE peak becomes broadened.

Structure and superconductivity of Mg(B1-xCx)2 compounds

In this paper, we report on the structural properties and superconductivity of Mg(B1-xCx)(2) compounds. Powder X-ray diffraction results indicate that the samples crystallize in a hexagonal AlB2-type structure. Due to the chemical activity of Mg powders, a small amount of MgO impurity phase is detected by X-ray diffraction. The lattice parameters decrease slightly with the increasing carbon content. Magnetization measurements indicate that the non-stoichiometry of MgB2 has no influence on the superconducting transition temperature and the transition temperature width. The addition of carbon results in a decrease of T-c and an increase of the superconducting transition width, implying the loss of superconductivity.In this paper, we reported the structural properties and superconductivity of Mg(B1-xCx)2 compounds. Powder x-ray diffraction results indicate that the samples crystallize in a hexagonal AlB2-type structure. Due to the chemical activity of Mg powders, a small amount of MgO impurity phase was detected by x-ray diffraction. The lattice parameters decrease slightly with increasing carbon content. Magnetization measurements indicate the non-stoichiometry of MgB2 has no influence on the superconducting transition temperature and the transition temperature width. The addition of carbon results in a decrease of Tc and an increase in the superconducting transition width, implying the loss of superconductivity.

Magnetic properties of TbMn6Sn6-xGax (x=0.0-1.2) compounds

Effects of Ga substitution for Sn on the structure and magnetic properties of TbMn6Sn6-xGax (x=0.0-1.2) compounds have been investigated by means of x-ray diffraction, magnetization measurement and Sn-119 Mossbauer spectroscopy. The substitution of Ga for Sn results in a decrease in lattice constants and unit-cell volumes. The magnetic ordering temperature decreases monotonically with increasing Ga content from 423 K for x=0.0 to 390 K for x=1.2. At room temperature, the easy magnetization direction changes from the c-axis to the ab-plane. This variation implies that the substitution of Ga for Sn leads to a decrease in the c-axis anisotropy of the Tb sublattice. An increase in the non-magnetic Ga concentration results in a monotonic decrease of the spontaneous magnetization M-s at room temperature. Since there are three non-equivalent Sn sites, 2c (0.33, 0.67,0), 2d (0.33, 0.67,0.5) and 2e (0,0,0.34) in the TbMn6Sn6-xGax compounds, the Sn-119 Mossbauer spectra of the TbMn6Sn6 and TbMn6Sn5.4Ga0.6 compounds can be fitted by three sextets. The hyperfine fields (HFs) decrease in the order of HF(2d)>HF(2e)>HF(2c), which is in agreement with the magnetic structure.

Magnetic properties and magnetoresistance of HfFe6Ge6-type Dy1-xGdxMn6Ge6 (x=0.1-0.6) compounds

The magnetic properties and magnetoresistance effects of Dy1-xGdxMn6Ge6 (x=0.1-0.6) compounds have been studied by magnetic properties and resistivity measurements in applied magnetic fields up to 5T. The compounds with x=0.1, 0.2, 0.4 and 0.5 order antiferromagnetically at 425, 428, 430 and 432K, respectively and there are second magnetic phase transitions below 100K. The compound with x=0.6 exhibits a transition from ferrimagnetic to antiferromagnetic, then to ferrimagnetic state again with decreasing temperature. Furthermore, it displays a field-induced metamagnetic transition and its threshold field decreases with increasing temperature. The magnetoresistance curve of the compound with x=0.6 in applied magnetic fields up to 5T is presented and the magnetoresistance effects are related to the metamagnetic transitions.

Structural and magnetic properties of Dy2Co17-xMnx compounds

Using x-ray diffraction and magnetic measurements, we have studied the structural and magnetic properties of Dy2Co17-xMnx (x = 0 ~ 5) compounds with a rhombohedral Th2Zn17-type structure. With an increasing Mn concentration x, the unit-cell volume V was found to increase linearly. The Curie temperature TC decreases linearly and the saturation magnetization Ms at 5K first increases slightly for x 1 with a further increase of Mn concentration x. In compounds for x = 1 ~ 3, a spin reorientation was found. A magnetic diagram of the compounds is given.

Effect of Co substitution on magnetic properties and magnetic entropy changes in LaFe11.83Si0.94Al0.23 compounds

Effect of Co substitution on magnetic properties and magnetic entropy changes in LaFe11.83Si0.94Al0.23 compounds has been investigated by means of magnetization measurements. X-ray diffraction shows the prepared compounds to be single phase with the cubic NaZn13-type structure. Substitution of Co for Fe leads to an increase of Curie temperature of the material. The magnetic entropy changes in LaFe11.83Si0.94Al0.23 and LaFe11.03Co0.80Si0.94Al0.23 compounds are 21.8J/(kg.K) to 16.9J/(kg.K) under a magnetic field change of 0–5T at Curie temperature, respectively. Giant magnetic entropy changes are attributed to the higher magnetization and the rapid change in magnetization at Curie temperature.

Magnetic Properties and Magnetoresistance Effect of YMn6Sn6-xCrx (x = 0-0.8) Compounds

The magnetic properties and magnetoresistance effect of YMn6Sn6-xCrx (x = 0 - 0,8) compounds have been experimentally studied by magnetic properties and resistivity measurements in the applied field range 0 - 5 T. The compound (x = 0.8) displays a ferromagnetic behaviour, while the compounds (x = 0 - 0.4) display an antiferromagnetic behaviour in the whole ordering temperature range. The compounds (x = 0.5, 0.6) experienced a transition from an antiferromagnetic state to a ferromagnetic state with increasing temperature. The compound with x = 0.8 is rapidly saturated in the lower magnetic field with saturation magnetization of 35.92 emu/g. The compounds (x = 0 - 0.6) display a field-induced metamagnetic transition, and the threshold fields decrease with increasing Cr content. The cell-volume V of compounds (x = 0 - 0.8) increases, and the ordering temperature decreases with the increasing Cr content. A large magnetoresistance effect was observed for the compounds (x = 0.4, 0.5), and the maximum absolute value at 5 K are 32 6 and 24% under 5 T for x = 0.4 and x = 0.5, respectively.

Remarkable improvement of the coercivity of TbMn6Sn6 compound by melt-spinning process

The melt-spinning process has been carried out to improve the hard-magnetic properties of the TbMn6Sn6 compound. For the TbMn6Sn6 ribbons quenched at a rate of 40m/s and annealed at 545K for 30min, the highest coercivity of about 0.6T is achieved at room temperature, which is much higher than that of the TbMn6Sn6 ingot. Both the ingot and the ribbon coercivities will increase with decreasing temperature. For ribbons, a greater improvement of coercivity has been made at lower temperatures. Microstructural studies show the uniform nanocrystalline distribution in the TbMn6Sn6 ribbons and a small amount of Tb-rich phase in grain boundaries. The observed remarkable improvement of magnetic hardening in ribbons is believed to arise from the uniform nanoscale microstructure and the domain-wall pinning at the grain boundaries.

Magnetotransport properties and magnetocaloric effects of Mn1.95Cr0.05Sb0.95Ga0.05 compound

The magnetotransport properties and magnetocaloric effects of the compound Mn1.95Cr0.05Sb0.95Ga0.05 have been studied. With decreasing temperature, a spontaneous first-order magnetic phase transition from ferrimagnetic (FI) to antiferromagnetic (AF) state takes place at Ts=200K. A metamagnetic transition from the AF to FI state can be induced by an external field, accompanied by a giant magnetoresistance effect of 57%. The magnetic entropy changes are determined from the temperature and field dependence of the magnetization using the thermodynamic Maxwell relation. Mn1.95Cr0.05Sb0.95Ga0.05 exhibits a negative magnetocaloric effect and the absolute values of ΔSMmax (T,ΔH) are 4.4, 4.1, 3.6, 2.8 and 1.5 J/(kg.K) for magnetic field changes of 0–5T, 0–4T, 0–3T, 0–2T and 0–1T, respectively.