Scientists produce a magnetic nanocrystal with many potential applications.

Spinels are oxides with chemical formulas of type AB₂OR_{4 4}, where A is a divalent metal cation (positive ion), B is a trivalent metal cation and O is oxygen. Spinels are valued for their beauty, which is derived from the spatial configurations of molecules, but spinels in which the trivalent cation B consists of the element chromium (Cr) are interesting for a reason that has nothing to do with the Aesthetics: They have magnetic properties with a host of potential technological applications, including gas sensors, drug carriers, data storage media, and components of telecommunications systems.

A study by Brazilian and Indian researchers investigated a peculiar type of spinel: the zinc-doped manganese chromite. Nanoparticles of this material, described by the formula Mn_0.5Zn_0.5Cr₂OR_{4 4} [where manganese (Mn) and zinc (Zn) compose the A-site divalent cation]They were synthesized in the laboratory and characterized by calculations based on functional density theory (DFT), a method derived from quantum mechanics that is used in solid state chemistry and physics to solve complex crystal structures.

The structural, electronic, vibratory and magnetic properties of the material were determined by X-ray diffraction, neutron diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. A study report has been published in the Magazine of Magnetism and Magnetic Materials with the title “Structural, electronic, vibratory and magnetic properties of MnCr2O4 nanoparticles substituted with Zn2 +”.

The Brazilian scientists who participated in the study are affiliated with the Center for Research and Development of Functional Materials (CDMF), one of the Research, Innovation and Dissemination Centers (RIDC) supported by the São Paulo Research Foundation (FAPESP).

A paramagnetic to antiferromagnetic phase transition was established at 19 degrees Kelvin (-254.15 degrees Celsius). Paramagnetic materials are attracted to an external magnetic field because their atoms or molecules each have an electron with an unpaired spin. Magnetic materials have several organized unpaired electrons, and the cumulative effect of these electrons produces magnetic attraction. In antimagnetic or antiferromagnetic materials, the spins of all the electrons are paired, so that for each rotating electron, there is one electron turned down. As a result, they do not respond significantly to the presence of a moderate external magnetic field.

“Our interest in this material is due to its magnetic properties,” said Elson Longo, one of the study’s authors. Longo is a professor emeritus in the Department of Chemistry of the Federal University of São Carlos (UFSCar) in the state of São Paulo, Brazil, and the principal investigator of CDMF.

“Conventional studies consider the magnetic properties generically, from the point of view of the system as a whole, while we have developed a quantum mechanical method to determine the magnetic properties based on the morphologies of the surfaces of the crystalline structure of a material Longo said. “Even before synthesizing any material, we can theoretically predict its magnetic properties. In this specific case, we were expecting zinc to promote an increase in surface area with magnetic properties, and this, in fact, happened.”

According to Longo, to be properly understood, a crystal must be considered on three different scales. “At a long distance, we have the entire crystal. At a short distance, we have the smallest possible group of atoms. At a medium distance, we have two or more groups interacting. If a group is perfectly ordered, it will not show behavior paramagnetic, let alone magnetic behavior, because for every rotating electron, there will be a compensating rotating electron. However, if any changes are made, if the chemical bond angles are altered, for example, then unpaired electrons can appear, and the material can become paramagnetic or even magnetic, “he said.

This disturbance can also occur as a result of medium distance interactions. Magnetism, therefore, can be produced by changes at short and medium distances.

The same material can show different properties depending on variations in certain parameters, which has to do with how the material is synthesized.

“CDMF is conducting studies that focus on identifying very cheap materials with bactericidal and fungicidal properties. One of the applications would be the production of packaging to extend the shelf life of food products.

“Another approach is the identification of inorganic materials with anti-cancer properties. A third line of research aims to find photodegradation materials capable of decomposing organic molecules and converting them into carbon gas and water. These materials could be used to clean rivers contaminated by pollutants” Longo said.