Hall-effect and resistivity experiments were carried out on natural crystals of pyrrhotite, in the temperature range from 77 to 450°K. Because of the magnetic anisotropy, samples were cut and measured in three different orientations of the hexagonal crystals. For samples cut with their long axis perpendicular to the c axis and with the magnetic field applied parallel to the c axis (paramagnetic case), the sign of the Hall coefficient changes from n to p type with increasing temperature. The reversal takes place at 165°K. The Hall coefficient in the exhaustion range has a value ^10~2 cm 3 /C which corresponds to 10 20 cm -3 carrier concentration. For samples with both their long axis and the magnetic field perpendicular to the c axis (ferromagnetic case), no reversal of the Hall coefficient was observed and the material was p type for all temperatures. The curve of the Hall resistivity pH versus the magnetic induction, for constant temperature, exhibits the typical form for ferromagnetic materials.
We report here measurements on natural crystals of pyrrhotite from room temperature to beyond the Néel point. This material is known as a ferrimagnetic semiconductor, and also for its magnetic anisotropy. The isotherms of the Hall effect are given for samples cut in such a way that the magnetic field is applied along the c axis and perpendicular to it. The sign of the ordinary Hall effect proves that, in the measured temperature region, the conductivity is of p type. The energy gap can be derived from the ordinary Hall effect, and is ten times smaller along the c axis than in a direction perpendicular to it. The spontaneous coefficient Rs falls with increasing temperature, as happens in other antiferromagnetic and ferrimagnetic materials.
Magnetite, a ferrimagnetic semiconductor of inverse spinel struct,ure has attracted much attention due to a phase transition occurring at low temperatures. The transition temperature reported differs for the various properties under investigation. I n the present paper measurements are reported of the Hall effect and resistivity in the range from 77 to 380 OK.The 105 "K Hall resistivity isotherm and the ones for higher temperatures follow the usual trend for ferromagnetic materials. Contrary, the lower temperature isotherms, up to 100 OK, exhibit irregular behaviour, involving sign revcrsal. If we accept the average of the two temperatures, i.e. the value of 103 OK, as our transition temperature we observe that it lies, by some degrees, below the values found in theliterature. Oniy part of the difference can be attributed to a systematic error of our thermocouple. The previously mentioned sign reversal shifts to stronger magnetic fields as the temperature increases from 77 t o 100 OK.Magnetit, ein ferrimagnetischer Halbleiter mit inverser Spinelstruktur hat durch seinen Phaseniibergang bei niedrigen Temperaturen starke Beachtung gefunden. Die veroffentlichten Ubergangstemperaturen unterscheiden sich bei der Untersuchung verschiedener Eigenschaften. I n der vorliegenden Arbeit wird iiber Messungen des Hall-Effekts und der Leitfiihigkeit im Temperaturbereich zwischen 77 bis 380 "K berichtet. Die Isothermen des Hallwiderstandes bei 105 "K und hoheren Temperaturen folgen dem iiblichen Verlauf fur ferromagnetische Materialien. In) Gegensatz dazu zeigen die Isothermen fur tiefe Temperaturen bis 100 "K irregulares Verhalten, einschlieDlich Vorzeichenumkehr. Wenn man den Mittelwert der beiden Temperaturen, d. h. den Wert von 103 "K als in diesem Fall giiltige nbergangstemperatur akzeptiert, stellt man fest, daD er etwas unterhalb der Werte liegt, die in der Literatur angegeben werden. Nur ein Teil dieser Differenz kann einem systematischen Fehler des Thermoelements zugeschrieben werden. Die oben erwahnte Vorzeichenumkehr verschiebt sich zu starkeren Magnetfeldern, wenn die Temperatur von 77 bis 100 "K gesteigert wird.
This is a report of measurements of the Hall resistivity on natural single crystals of pyrrhotite. When the applied magnetic field is normal to the c axis (ferrimagnetic sample) the Hall resistivity exhibits oscillations with an increasing magnetic field. Such oscillations were detected when the sample was kept at 77°K. The period and the damping of the oscillations permit the calculations of the cross section of the Fermi surface, normal to the field, the cyclotron effective mass of the carriers, and also the Fermi energy. The same sample at room temperature does not exhibit oscillations. In the paramagnetic sample, cut so that the applied field is parallel to the c axis, no oscillations are detected.
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