Hall coefficient behavior in TiFe, TiCo, TiNi, and their alloys

Abstract

Abstract Hall coefficient and resistivity measurements on the 3d transition metal compounds TiFe, TiCo, and TiNi, and on several of their alloys, were made at 295°, 77°, and 4.2°K. These materials contain between 6 and 7 valence electrons per atom (Nve). The Hall coefficient behavior was characterized by values as large as +35 × 10−4 cm3/C (in TiFe), by a strong temperature dependence at all compositions studied, and, with increasing Nve, by changes in sign from positive to negative and back to positive again. Negative Hall coefficients have not been observed before in this range of Nve values in any other 3d materials. The discussion showed, first of all, that a phenomenological two-band model could account qualitatively for the dependence of the Hall data on temperature and on Nve. It was also demonstrated that a small group of positive carriers was present in TiFe, not exceeding 1 40 per atom in number, which had a room-temperature mobility greater than 122 cm2/V-sec, and it was shown that this conclusion did not depend on the number of bands actually present. Secondly, it was shown that Mattheiss's theoretical band model calculation for non-magnetic Cr, Mo, and W contained a specific feature—a band maximum at which the wave function had a surprisingly p-like (rather than d-like) character—which could account for the small number of high-mobility holes in TiFe. The model was then applied to the remainder of the Hall data, and it was not difficult to say which of the four bands was responsible for the several major features of the experimental data. Finally, a comparison of the present results with earlier data on Cr and its alloys (Nve between 5.9 and 6.6) revealed distinctive differences in the temperature- and composition dependence of the Hall coefficient. These differences were explained using Lomer's proposal that antiferromagnetic ordering in Cr changes Mattheiss's fourband model to a two-band model with a considerably lower carrier concentration. It was assumed that chemical ordering in TiFe has essentially the same effect, and it was then shown that the Hall effect behavior was related to the different ways that the magnetic and chemical ordering depend on temperature and composition in the two sets of materials. It was concluded that weak-field Hall coefficient studies on transition metal elements and compounds, and their alloys, can be quite useful, especially in cases where low carrier mobilities and lack of single-crystal samples prevent application of sophisticated, high magnetic field techniques. This is due to the fact that the weak-field Hall coefficient is more sensitive than had been realized to the numbers and mobilities of the carriers of all the bands which cross the Fermi level.