Ferromagnetic materials

Magnetic and electrical properties for La(1-x)Gd(x)0.7Ca(0.3)MnO3 were studied at ambient and high pressure. For x > 0.1 the ferromagnetic saturation moment M is smaller than that expected for complete ferromagnetic alignment of the Mn moments. Gd substitution lowers both the paramagnetic-ferromagnetic transition temperature T(C), the metal-insulator transition temperature T(MI) and dramatically increases the electrical resistivity rho. T(MI) is observed to be significantly lower than T(C) for x > 0.1. The disagreement of T(C) and T(MI) is a result of the A-site ion size difference which causes the coexistence of ferromagnetic La-Ca-Mn-O and antiferromagnetic Gd-Ca-Mn-O phases and leads to the anomalous electrical transport properties. Samples with the largest average A-site ionic radius <rA> show the highest T(C) or T(MI) and are least sensitive to pressure.

The techniques of x-ray and ultra-violet photoemission spectroscopies have been used to investigate the electronic structures in Fe-30%Ni and Fe-27.5%Ni alloys in both the bcc and fcc phases. The similarities between the Fe 3s core level spectra, and between the XPS and UPS EDC's of the valence band, from the bcc and fcc phases of the Fe-Ni alloys, provide evidence that the Fe local magnetic moment persists in the fcc phase, and is about the same magnitude as in the bcc phase. The binding-energies of the Fe-2p1/2 and Fe-2p3/2 core levels of Fe-30%Ni and Fe-27.5%Ni are shifted from the values in pure Fe by + 0.40eV +- 0.05eV. The binding-energy of the Fe-3p core level of Fe-30%Ni is shifted from the value in pure Fe by + 0.40eV +- 0.05eV. The intensity of the Ni-3p core level in XPS spectra is considerably decreased when Fe-30%Ni has undergone the martensitic phase transition. The EDC's of the valence bands of Fe-30%Ni, Fe-27.5%Ni (in both the bcc and fcc phases) and pure Fe agree with published data.

Characterization investigations were carried out for Ni-Fe alloys for three different concentrations of iron. The starting and ending temperatures of the fcc-bcc phase transformation both on heating and on cooling were obtained by measuring the resistance as a function of temperature; the results agree with previous work. We also measured the lattice constants of both structures by using a modified Debye-Scherrer method. For the fcc structure a is 3.5837 A, for the bcc structure a is 2.875 A, and we noticed a decrease of the lattice constant as the iron concentration is increased. Auger electron spectroscopy was carried out for the purpose of studying the surface segregation and a concentration analysis was attempted. The results show that Ni is preferentially sputtered but an annealing process can overcome this segregation. Magnetic measurements show a change of magnetization with temperature though the data is insufficient to obtain the average moment.