Jan Čuda

Low-temperature magnetism of alabandite: Crucial role of surface oxidation

Abstract Manganese(II) monosulphide crystallizes into three different polymorphs (α-, β-, and γ-MnS). Out of these, α-MnS, also known as mineral alabandite, is considered the most stable and is widespread in terrestrial materials as well as in extraterrestrial objects such as meteorites. In this study, a low-temperature antiferromagnetic state of α-MnS was investigated using macroscopic magnetic measurements as induced and remanent field-cooled (FC) and zero-field-cooled (ZFC) magnetizations and magnetic hysteresis. Both natural alabandite and synthetic samples show: (1) Néel temperatures in a narrow temperature range around 153 K, and (2) a rapid increase of the magnetization around 40 K. An anomalous magnetic behavior taking place at about 40 K was previously ascribed to the magnetic transition from a high-temperature antiferromagnetic to a low-temperature ferromagnetic state documented for non-stoichiometric α-MnS slightly enriched in manganese. However, our detailed microscopic observations and, in particular, oxidation experiments indicate that the anomalous magnetic behavior around 40 K is caused by the presence of an oxide layer of ferrimagnetic hausmannite (Mn3O4) on the surface of α-MnS rather than being an intrinsic property of nearly stoichiometric α-MnS.

Thermally-induced solid state transformation of β‐Fe2O3 nanoparticles in various atmospheres

To date, iron oxides have become one of the most studied nanomaterials due to their interesting and aaplication appealing physical, chemical, and biological properties in comparison with their bulk counterparts. In general, four forms of iron(III) oxide can be distinguished depending on their crystallographic and magnetic properties. In this work, one of the rare phases of iron(III) oxide, β‐Fe2O3, prepared by the solid state reaction was explored for the thermal transformations in various ambient atmospheres, including O2, N2, and CO2 atmospheres. The thermally treated products were investigated employing X-ray powder diffraction and 57Fe Mossbauer spectroscopy.

In-field 57Fe Mössbauer spectroscopy below spin-flop transition in powdered troilite (FeS) mineral

Powdered troilite (FeS), extracted from the Cape York IIIA octahedrite meteorite, was investigated employing in-field 57Fe Mossbauer spectroscopy. The study identified a typical behavior of polycrystalline antiferromagnetic material under external magnetic fields. The in-field evolution of the 57Fe Mossbauer spectra showed that the spin-flop transition in the FeS system occurs at a field higher than 5 T.

Magnetic interaction in oxygenated alpha Fe-phthalocyanines

Alpha iron phthalocyanines (α-FePc) oxygenated at low temperatures were investigated with the help of 57Fe Mossbauer spectroscopy, magnetization measurements (SQUID) and X-ray diffractometry (XRD). Mossbauer spectroscopy revealed that upon oxygenation of α-FePc, new species were formed which could be associated with FeIIIPc oxygen adducts. Unexpectedly, magnetically split spectrum of oxygenated α-FePc was observed below 20 K. In-field Mossbauer spectra in a 5 T external magnetic field at 5K and magnetization measurements indicate antiferromagnetic coupling in oxygenated α-FePc.