Yuping Shen

STRUCTURE AND MAGNETIC PROPERTIES OF GD3(FE1-XCOX)25CR4 COMPOUNDS

The structural and magnetic properties of Gd3(Fe1−xCox)25Cr4 compounds with x=0–0.6 have been investigated. The structures of all the compounds investigated are found to possess monoclinic symmetry and belong to the A2/m space group. Substitution of Co for Fe leads to a clear contraction of the unit-cell volume. The Curie temperature increases from 492 K for x=0 to 762 K for x=0.6. The composition dependence of the saturation magnetization at 5 K reaches a maximum around x=0.3. It is noteworthy that substitution of Co for Fe results in a significant change of the magnetocrystalline anisotropy of the Co sublattice, and changes the easy magnetization direction of Gd3(Fe1−xCox)25Cr4 compounds from basal plane to easy axis.

Syntheses and magnetic properties of Rm+nCo5m+3nB2n compounds

New compounds Pr3Co13B2 and Nd5Co19B6 have been synthesized successfully. They belong to the Rm+nCo5m+3nB2n family with m = 2, n = 1 and m = 2 and n = 3, respectively. Pr3Co13B2 adopts the hexagonal La3Ni13B2-type structure with lattice parameters a = 5.0672(3) Angstrom and c = 10.6850(6) Angstrom, while Nd5Co19B6 is isostructural to Lu5Ni19B6 With a = 5.1328(3) Angstrom and c = 16.6519(5) Angstrom. Magnetic measurements indicate that Pr3Co13B2 is ferromagnetic with a Curie temperature of 360 K. Its saturation magnetic moment at 5 K is 20.0 mu(B) fu(-1). Based on the results of the saturation magnetization, two kinds of Co sites with different magnetic moments are proposed. Pr3Co13B2 exhibits large uniaxial anisotropy with an anisotropy field of 90 A m(-1) at 5 K. The Nd5Co19B6 compound is ferromagnetic with a Curie temperature of 380 K. Its saturation magnetic moment and anisotropy field are 21.5 mu(B) fu(-1) and 340 A m(-1) at 5 K, respectively. No spin reorientation was detected from the temperature dependence of the magnetization of these compounds from 5 K to their Curie temperatures, and the behaviour of magnetocrystalline anisotropy was analysed using the single-ion model.

Structural and magnetic properties of R(Fe1−yCoy)12−xNbx compounds

The magnetic properties of RFe12−xNbx and R(Fe1−yCoy)11.3Nb0.7 compounds with R=Y, Tb, and Dy have been investigated in the concentration region defined by 0.6<x<0.8 and y⩽0.3, where the compounds are single phase with the ThMn12-type of structure. The Curie temperature TC of the RFe12−xNbx compounds is almost independent of the Nb content whereas the saturation magnetization Ms decreases with increasing Nb content. The spin-reorientation temperature Tsr of the TbFe12−xNbx and DyFe12−xNbx compounds decreases monotonically with increasing x. Substitution of Co for Fe in RFe11.3Nb0.7 leads to a remarkable increase of TC and the appearance of a maximum in the Co-concentration dependence of Ms. In contrast, Tsr decreases monotonically with increasing Co content for both R=Tb and Dy. The modification of T- and R-sublattice anisotropy originating from a change of the Nb content and from substitution of Co for Fe was analyzed by combining crystalline electric field theory and the individual-site model.

Formation, structure and magnetic properties of TbFe12−xNbx compounds

TbFe12-xNbx compounds (x=0.55, 0.6, 0.65, 0.7, 0.75 and 0.8) with the ThMn12-type of structure have successfully been synthesized. The structural and magnetic properties of these compounds have been investigated by means of X-ray diffraction and magnetic measurements. With Nb content increasing from x=0.5 to x=0.85 the lattice parameters monotonously increase and the crystal structure does not change. The saturation magnetization decreases monotonously with increasing Nb content. The easy magnetization direction at room temperature has been found in the plane for all compounds. For all compounds investigated, an anomaly has been observed in the M-T curves measured in a field of 0.05 T, which corresponds to a change of the easy magnetization direction. The Curie temperature T-C is almost independent of the Nb content while the spin reorientation temperature T-sr decreases clearly with increasing Nh content

Spin reorientation and magnetohistory of DyFe12-xNbx compounds

Structural and magnetic properties, especially the magnetocrystalline anisotropy of DyFe12-xNbx compounds with x = 0.55-0.85, have been investigated. The easy magnetization directions at room temperature for all the compounds are along the c-axis. With decreasing temperature the magnetocrystalline anisotropy changes from easy axis to easy cone at Tsr2, then to easy plane at Tsr1. A spin phase diagram has been constructed for DyFe12-xNbx. The change of magnetocrystalline anisotropy was further investigated by the angular dependence of magnetization with respect to the magnetic field at various temperatures. The temperature dependence of the cone angle was determined for DyFe11.3Nb0.7. It is noteworthy that with increasing Nb content both Tsr1 and Tsr2 decrease monotonically while the Curie temperature is almost independent of Nb content. The cone angle increases monotonically with decreasing temperature from Tsr2 and discontinuously comes up to 90° at Tsr1. The temperature dependence of the cone angle can be quite well defined in terms of crystal field theory. In addition, an obvious magnetohistory effect was observed for all compounds at low temperature. The critical field of magnetohistory was found to be smaller than 0.5 T for DyFe11.3Nb0.7.

Structure and magnetic properties of ErFe11-xGaxTi

ErFe11-xGaxTi compounds with ThMn12-type structure were synthesized for x = 0, 1, 2, 3, 4, 5. The crystalline structure and Ga occupancy in the compounds were analysed in terms of the Rietveld method. With increasing Ga content, the lattice constant and unit-cell volume increase monotonically. Ti atoms are found to occupy 8i sites for all the compounds. Bond length and Ga occupancy factor are given. The Curie temperature increases first, going through a maximum and then decreases with increasing x while saturation magnetization decreases monotonically. Spin re-orientation from the uniaxial to the easy-cone type is detected for all the compounds as the temperature decreases from 300 to 5 K. A tentative spin phase diagram is constructed for the ErFe11-xGaxTi. In the ErFe11-xGaxTi compounds, a magneto-history effect is observed which can be attributed to irreversible movement of narrow domain walls that are easily pinned.

Magnetocrystalline anisotropy and exchange interaction in YCo12-xTix compounds

Magnetic and structural properties of YCo12-xTix compounds have been investigated by x-ray diffraction and magnetic measurements. X-ray diffraction patterns of aligned powder samples indicate that the easy magnetization direction at room temperature is along the c-axis for all the compounds. The lattice parameters increase monotonously while Curie temperature TC decreases with increasing Ti content. It was found that all the compounds investigated are strong ferromagnets by analysis of magnetic moments using a magnetic valence model. The values of anisotropy fields Ba at room temperature and 1.5 K decrease with increasing x. The easy-axis anisotropy of the YCo12-xTix compounds is mainly contributed by the Co atoms at 8i sites according to the individual-site-anisotropy model.

Magnetic properties of the ErFe/sub 10-x/Ga/sub x/Si/sub 2/ compounds

The ErFe/sub 10-x/Ga/sub x/Si/sub 2/ compounds with the ThMn/sub 12/-type structure were synthesized for x=0.0, 0.5, 1.0, 1.5 and 2.0. With increasing Ga content, the lattice constant c and the unit-cell volume V increase monotonically. The lattice constant, a, and the Curie temperature increase first, go through a maximum and then decrease with increasing x. Spin re-orientation from the easy-cone to the uniaxial type is detected for all the compounds as the temperature increases from 5 to 300 K. A tentative spin phase diagram is constructed for ErFe/sub 10-x/Ga/sub x/Si/sub 2/. The saturation magnetization decreases monotonically which can be understood in terms of a simple dilution model.