Slawomir Podsiadlo

Paramagnetism and antiferromagnetic d–d coupling in GaMnN magnetic semiconductor

The magnetization of Ga1−xMnxN (x<0.1) crystals was measured as a function of the magnetic field and temperature. Paramagnetic behavior typical of spin S=5/2 expected for Mn2+ (d5) magnetic centers was observed in the temperature range of 2 K<T<300 K. On the other hand, antiferromagnetic coupling between Mn ions was clearly visible. The nearest neighbor (NN) coupling constant JNN/kB=−1.9 K was estimated from the temperature dependence of the magnetization.

Raman scattering study of Ga1−xMnxN crystals

Raman spectra of Ga1−xMnxN crystals grown by the resublimation method have been investigated. New bands around 300 and 667 cm−1, as well as a broad structure near 600 cm−1, not observed in undoped GaN have been found. The temperature dependence of major Raman bands has been measured. The simple model of GaN lattice dynamics has been presented, and the observed bands have been assigned to disorder-activated phonon modes, in good agreement with the calculated phonon density of states.

Possible origin of ferromagnetism in (Ga,Mn)N

Ferromagnetic behavior of GaN doped with Mn (Ga1−zMnzN) grown by the ammonothermal and chemical transport methods is discussed in terms of a second phase (ferromagnetic one) produced during the growth process. The reference manganese nitride samples grown by the same method as (Ga,Mn)N reveal room-temperature ferromagnetic behavior, depending on the growth details. Different MnxNy phases are suggested to be responsible for ferromagnetic behavior of (Ga,Mn)N.

Preparation of silicon nitride powder from silica and ammonia

Abstract Silicon nitride powders were obtained in a reaction of SiO 2 powders with NH 3 . The influence of the following parameters on the quality of Si 3 N 4 was determined: silica particle size, temperature and furnace heating programme, sample heating time, ammonia flow rate and application of the selected mineralising agents. The powders of silica were characterised by: optical microscopy, SEM, X-ray phase analysis and thermal analysis. The products of the syntheses were tested by: optical microscopy, SEM, X-ray phase analysis, IR spectroscopy and classic quantitative analysis. The investigations resulted in elaboration of a method of preparation of crystalline silicon nitride powders containing more than 96% Si 3 N 4 . The discussion on the influence of magnesium nitride additive on the Si 3 N 4 synthesis process has also been carried out.

Magnetic Properties of (Ga,Fe)N

Wurtzite (Ga,Fe)N bulk crystals were, for the first time, successfully grown by AMMONO and chemical transport methods. The magnetization measurements of (Ga,Fe)N crystals revealed the coexistence of paramagnetic and ferromagnetic contributions. The paramagnetic component was shown to be growth condition dependent. The Brillouin-type behavior was observed in the samples obtained by both methods. Some (Ga,Fe)N samples (especially those codoped with Si) grown by the chemical transport method show a Van Vleck–type paramagnetic behavior.

Raman scattering study of gallium nitride heavily doped with manganese

Abstract Small MnxGa1−xN crystals grown by the resublimation method have been characterised by Raman scattering technique. It has been shown, that for small concentration of manganese the concentration of free electrons in the material is high as in typical free standing GaN crystals, but for higher contents of manganese the concentration of free carriers in the alloy has been substantially reduced. The measured Raman spectra have been compared with the calculated phonon density of states, and it has been shown that the Raman peaks correspond to the peaks of the phonon density of states. In consequence the new Raman peaks found in the material are ascribed to disorder activated phonon modes.

Low-temperature thermal expansion of Mg3N2

Abstract Lattice parameter and thermal-expansion coefficient for magnesium nitride, Mg 3 N 2 , were determined using X-ray powder diffraction at the B2 beamline in Hasylab (DESY) in the temperature range from 11 K up to 304.5 K. The relative change of lattice parameter on raising the temperature in the studied range is about 0.14%. At 300 K, its value is 9.9644(1) A and the linear thermal expansion coefficient derived from a polynomial approximation is a =1.1×10 −5 K −1 .