Positron Annihilation in Some Antiferromagnetic Materials

Abstract

It is known that the lifetimes and intensities in the lifetime spectra of positrons annihilating in molecular materials a r e sensitive to the state of aggregations (1,2) and exhibit sharp changes around solid-liquid phase transformations. In the present investigation, positron lifetimes and intensities have been studied a s a function of temperature in antiferromagnetic materials with a view to investigate the behaviour of these parameters with the magnetic phase transition from paramagnetic to antiferromagnetic state, which occurs around the Nee1 temperature (T ). It may be pointed out that the magnetic phase changes in various antiferromagnetic materials such a s oxides and alloys of the transition elements a r e also accompanied by changes in non-magnetic properties such a s crystal volume and symmetry, specific heat, thermal expansion, electrical resistivity, etc. (3). Such phase transitions also cause a significant change in the interaction of electrons and in ionic fields (3, 4).Since positron lifetimes a r e sensitive to such changes, it is interesting t o study positron annihilation in these materials. N Positron lifetimes were measured in the following compounds: COO, Cr203, MnTe, and V 2 0 3 . Samples in the form of pellets were pregared by pressing the Analar grade polycrystalline material under a pressure of 5000 p.s.i. A thin source of Na22 was sandwiched between two pellets and the delayed coincidence between 511 keV annihilation y and 1280 keV nuclear fast-slow coincidence technique. The spectra were recorded on a 1024 channel analyser. The samples were studied in vacuum and the temperature was kept constant within 0.25 C . At least two cycleg of temperature were studied to detect hysteresis effecfs i f any.was studied using the standard 0 1) Present address: