We describe how a classic electrostatics experiment can be modified to be a four-point probe lab experiment. Students use the four-point probe technique to investigate how the measured resistance varies as a function of the position of the electrodes with respect to the edge of the sample. By using elementary electromagnetism concepts such as the superposition principle, the continuity equation, the relation between electric field and electric potential, and Ohm's law, a simple model is derived to describe the four-point probe technique. Although the lab introduces the students to the ideas behind the Laplace equation and the methods of images, advanced mathematics is avoided so that the experiment can be done in trigonometry and algebra based physics courses. In addition, the experiment introduces the students to a standard measurement technique that is widely used in industry and thus provides them with useful hands-on experience.
Some members of the modified iron titanate family show remarkable tolerance to radiation and are well suited for radhard electronics. Of particular interest are solid solutions of ilmenite-hematite (IH) represented by (1−x) FeTiO 3 .xFe 2 O 3 where x varies from 0 to 1; and pseudobrookite, Fe 2 TiO 5 (PsB). These multifunctional oxides can be both ferrimagnetic and wide bandgap semiconductors, and can be exploited in a variety of ways in radhard electronics, microelectronics and spintronics technologies. In this paper we emphasize the potential applications of the modified Fe-titanates with special emphasis on: (a) response of the non-linear current-voltage (I-V) characteristics to a magnetic field; (b) how the introduction of a biasing voltage might be used to produce bipolar currents in circuits and fabrication of voltage tunable varistors; and (c) the response of non-linear current-voltage characteristics when irradiated with neutrons, protons and heavy Fe-ions. Based on these observations, we will identify a few applications for which we can make use of the unique multifunctional nature of modified Fe-titanates.
The temperature and magnetic-field dependence of the magneto-optic Kerr effect was used to determine the temperature and field dependence of the magnetization of amorphous Gd x Si 1Ϫx films for composition x near the metal-insulator ͑MI͒ transition at fields up to 25 T. Amorphous Gd x Si 1Ϫx was previously shown to exhibit spin glass freezing and suppression of the magnetization well below the noninteracting Brillouin function for Gd 3ϩ at fields to 6 T at 4 K, indicative of strong mixed ferromagnetic and antiferromagnetic interactions. The Kerr rotation was found to be proportional to the directly measured magnetization at low fields. The high-field Kerr data show that the magnetization is not saturated even at 25 T and 4 K, reaching a value of approximately 0.6 of the expected saturation magnetization value. The data indicate extremely strong indirect exchange interactions despite the localization of electrons at the MI transition; the strong disorder might be the cause of the observed near perfect balancing of these interactions. The Kerr ellipticity was small.
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