Scientists Describe Distribution of Cobalt Nanoparticle Size with Their Magnetic Properties

Scientists of Boreskov Institute of Catalysis joined efforts with the colleagues from Ekaterinburg, France, and Belgium to describe the distribution of cobalt nanoparticle size with the help of nuclear magnetic resonance (NMR) in the inner field of sample. It is a relatively new method, and the researches proved it to be efficient in practice. Determining the size of synthesized cobalt nanoparticles is necessary for increasing the efficiency of their use.

Cobalt nanoparticles are used in catalysis, in particular in the Fischer-Tropsch process, for obtaining synthetic fuels, in magnetic resonance imaging (MRI), in the devices of communication, accumulators and coatings absorbing and reflecting radiation at selected frequencies.

The first works on the subject were published in 2013 in ACS Catalysis and in 2016 in Nature Communications, but they used the correction factor of 2.5 for comparing the obtained nanoparticle sizes with the data of electronic microscopy. In the new work the data are in complete agreement with the theory.

“When synthesizing the cobalt nanoparticles, a problem appears concerning description of their size, since the particle diameters in the sample always vary within certain limits. Usually this problem is solved with the methods of electronic microscopy, but in this case only a very small share of nanoparticles present in the sample is investigated at a time. As a result it is difficult to understand to what extent the obtained data correspond to the real distribution. We used the strong inner magnetic field of the cobalt nanoparticles and described their size distribution by observing the changes in magnetic moment with increasing and decreasing the temperature”, says Ilya Yakovlev, junior researcher in the Group of Solid-State NMR Spectroscopy of BIC.

Macroscopic cobalt particles can exist in different magnetic states depending on their size. Multidomain particles of more than 70 nm in diameter are considered to be true ferromagnets. If the particle diameter is lower, its energetically favorable state is single-domain one. Each type of particles has its own resonance frequency within the radio-frequency band, which is visible in the NMR-spectra.

With size decreasing the particles keep their own one-way magnetic moment. But the smallest particles (5-10 nm in diameter) feature the effect of magnetic moment flip due to the temperature changes. This state of the particle is called superparamagnetic. The experiment lasts for 10-20 microseconds, and if during this time the magnetic moment of a small particle flips then the obtained information is deleted, i.e. no signal from such particles is received.

The speed of flip of the magnetic moment of the particles and their transition into superparamagnetic state depends directly on temperature: the higher it is the more probable is the flip of magnetization of the particles. Thus, with decreasing temperature (in the experiment down to 30 K) the researchers were able to observe a higher number of particles. The difference in the spectra obtained at high and low temperatures can show how many particles of certain size are there in the sample. An important peculiarity of the method is that the data are obtained for the whole sample at once, as opposed to the microscopy methods that provide information only on its small part.

The paper named Superparamagnetic behaviour of metallic Co nanoparticles according to variable temperature magnetic resonance is published in Physical Chemistry Chemical Physics.