Néel Temperature ofα-Manganese Alloys with Nonmagnetic Impurities

Abstract

Magnetic moments of the four sites align noncollinearly with different magnitudes. Because the magnetic structure is complex, the magnetism of α-Mn has not entirely been understood. The electrical resistivity for α-Mn takes a minimum at TN. Variations of the Neel temperature as a function of the concentration of transition metal impurities were studied by Williams and Stanford by means of resistivity measurements. The results have been explained by a spin density wave theory, as in the case of Cr alloys. In the present paper, the variations of TN as a function of the concentration of nonmagnetic impurities such as Al, Si, Ga, Ge and Sn were studied mainly by the measurements of the electrical resistivity. The measurements of magnetic susceptibility, neutron diffraction and Mossbauer effect were also performed supplementally for several specimens. Each ingot was prepared by melting together proper amounts of 4N Mn and of impurity material such as Al, Si, Ga, Ge or Sn in an Al2O3 crucible under an Ar+H2 atmosphere. The specimens for the Mossbauer experiment were made of enriched Sn material. The ingots were annealed at 500 or 550◦C in an Ar+H2 atmosphere for 4 days. The specimens were confirmed to be in the α-phase using an X-ray powder diffractometer, although small amounts of manganese oxides were found in some specimens. The electrical resistivity measurements were performed by a conventional four-terminal method. Temperature variations of magnetic susceptibility in a magnetic field of 5 kOe were measured using a vibrating sample magnetometer. Powder neutron diffraction experiments were conducted using a T-1-1 spectrometer installed at JRR-3M in JAERI. Besides temperature variations of scattering intensities for magnetic Bragg reflections, the intensities of about 10 nuclear Bragg reflections were also studied at room temperature, to determine the site preference of the impurity atoms among the four kinds of sites, site I to IV. This experiment reveals that Al and Sn prefer the site II, while Si and Ge do the site