# Microstructure, Magnetic Properties, and Application of FINEMET-Type Alloys with Co Addition
###### Abstract  
The choice of materials for cores of electrotechnical devices is currently related to energy saving and global warming problems. Nanocrystalline alloys are emerging as materials for cores in these devices in addition to amorphous materials already commonly used due to their better magnetic properties at high operating frequencies. The thermal stability of the magnetic properties of cores is also an important criterion. Keeping these criteria in mind, a study of microstructure and magnetic properties was carried out in this work, and FeCoNbBCu-type material was selected for use as the core of a choke operating in a DC/DC converter in interleaved topology. On the basis of the conducted studies, it was found that good magnetic properties and the best thermal stability were shown by Fe\({}_{85}\)Co\({}_{25}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) alloy. Using RALE software, the technical parameters of the choke core were determined and compared with the same parameters for a choke core made of FINEMET-type alloy. FeCo-based nanocrystalline alloys; microstructure; magnetic properties; magnetic cores +
Footnote †: journal: Applied Sciences  
Acticle Microstructure, Magnetic Properties, and Application of FINEMET-Type Alloys with Co Addition  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON], M. Microstructure, Magnetic Properties, and Application of FINEMET-Type Alloys with Co Addition  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], M.  
Nanocrystalline FINEMET-type alloys show excellent soft magnetic properties at room temperature and low-frequency conditions up to 20 kHz. This is related to their microstructure and depends on the exchange coupling between \(\alpha\)-FeSi grains through amorphous boundaries [16]. This interaction significantly weakens with increasing temperature, and it is finally suppressed when the temperature approaches the Curie temperature of the amorphous matrix of these alloys [17]. For higher frequencies, a rapid increase in core losses is observed [18]. This behavior limits the use of this type of material for the cores of devices operating at elevated temperatures and high frequencies. HITPERM and NANOPERM alloys behave much better under these conditions, but they are not used in the industry due to their high price because of the presence of elements such as Zr, Hf, and Nb in their composition. Moreover, the presence of these elements not only increases the material cost, but also results in a major decrease in saturation induction. Therefore, other ways to improve the magnetic properties of core materials have been sought, especially under extreme operating conditions. It is known that the stability of the soft magnetic properties towards high temperatures is significantly improved with an addition of cobalt [19] because the Co atoms enhance the Curie temperature to about 1100 K depending on the chemical composition of the alloys [20]. A lot of research results have shown that the partial substitution of Co for Fe is an effective way to improve high-temperature and high-frequency characteristics [5; 21; 22]. Therefore, in this paper, the authors propose to study FINEMET-based material in which some Fe atoms are replaced by Co atoms. Optimal properties have been found for alloys with equal amounts of Fe and Co atoms [23]. Due to the high price of cobalt, in this work, we used FeCoNbBCu materials with a reduced Co content for the choke core in order to reduce the cost of the device. For selected nanocrystalline Fe\({}_{83-x}\)Co\({}_{x}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) (x = 6, 25, 58 at.%) materials, microstructure and magnetic properties measurements were carried out. Using values such as maximum induction and core losses, basic parameters for the choke core were calculated using RALE design software. In order to obtain the most stable inductance characteristics over a wide range of variations in rated current, tested materials in the form of thin ribbons were used to build multigap choke cores.