Most people know about the magnets used to stick messages and decorations on their refrigerator door, but they are also key to the functioning of essential devices like electric motors and electronics. Permanent magnets store information to save data on CDs and hard drives, as well as improving electrical power in power generators and batteries.
Unlike other artificial magnets, a permanent magnet acquires its properties through the interaction of its spins (quantum property) with a magnetic field. When this field is removed, the material maintains a residual magnetisation. From a technological point of view, the ability to maintain magnetism even after removing the magnetic field is important because it means that permanent magnets can store information, which is why they are used in electronics.
The strongest permanent magnets are composed of rare earth elements - a set of chemical elements, normally found in nature as mixed ores - such as neodymium and samarium. Such materials are difficult to extract and manipulate, with high costs and great environmental impact. Because of these difficulties, an efficient use of rare earths is necessary in the production of a new generation of permanent magnets that achieve a more diverse technological functionality at a lower cost and environmental impact.
This study, carried out by researchers from the Physics Institute (IF) at USP in collaboration with ISIS researchers, identified theoretical approaches and experiments to describe permanent magnets, which, in the future, can assist in research to develop more advanced sustainable materials.
“It is important, in the interest of physics, to understand how electrons participate within quantum mechanics and how quantum mechanics explains magnetism, and, on the other hand, to study the application of how new rare earth elements could provide new alternatives for producing permanent magnets with lower cost and environmental impact”, says Julio Larrea, professor at IF and coordinator of the Laboratory of Quantum Materials in Extreme Conditions (LQMEC). This type of study is of particular interest to Brazil, as the country has the third largest reserves of rare earths, but does not explore this wealth due to the cost of extraction technologies.
The beginning
The study started with the neodymium-cobalt magnet (NdCo5), with a neutron scattering experiment carried out on the MARI spectrometer at ISIS. “We hoped to obtain the parameters of the interactions in the material that contribute to its magnetism”, says Fernando de Almeida Passos, first author of the research.
In addition to this investigation, they also studied (Sm,Y)Co5 magnets to understand how replacing elements changes the magnetic properties of the material, and more specifically, the interactions responsible for the aspects of permanent magnets. “We do what we call chemical substitution, changing the samarium and adding an atom of another element, which in this case is yttrium.”
“It is also close to rare earths, but has a different electron configuration”, explains Fernando Passos. “The alloy goes through processes of synthesis, casting, characterisation and measurement of magnetic properties.”
From the detailed analysis of their neutron measurements, considering all interactions between electrons and spins, in addition to the crystalline symmetry formed in the permanent magnet, the researchers obtained the parameters of the crystal field (interactions between atomic charges) and the internal magnetic field. This helps to better understand what happens to these compounds at the atomic level and what are the best theoretical tools to investigate permanent magnets.
By using neutron scattering experiments, they were able to infer the different quantum states and energy scales associated with them. These are responsible for physical properties such as the orientation of magnetic moments and magnetic anisotropy (how much energy it takes to change the direction of the magnetic moments) within a magnet. The results also confirmed the importance of orbital hybridisation effects for the magnetic properties of NdCo5 magnets.
A better understanding of rare earth magnets opens ways to identify more suitable approaches to describe the material and create new, better-performing magnets.
For more information: email Julio Larrea at larrea@if.usp.br
