Mechanical loads improved properties of magnetic microwires

Scientists from Immanuel Kant Baltic Federal University found out that mechanical impact on amorphic ferromagnetic wires during their heat treatment—annealing—improved magnetic properties of these composites. Such materials thanks to the ability to change their magnetization quickly can be used for creation of protective magnetic marks and trackers. That's why new information about how to control their properties effectively, can broaden their usage in engineering. Results of the research are published in magazine Research Bulletin.

Amorphic ferromagnetic wires—thin metal threads—with glass covering are promising composite materials. They possess a range of useful properties: high strength, sustainability to corrosion, biocompatibility and also controlled magnetic properties. The last characteristic enables to use them for production of protective magnetic marks and magnetoelastic trackers—devices for transformation of mechanical impact (compression, spread, bending, pressure and others) into internal stress or current.

You can change magnetization of amorphic ferromagnetic wires thanks to the fact that their metallic core is made from domains—blocks that are situated one after another and different as far as direction of magnetic moment is concerned. Magnetic moment is a quantity that determines the ability of a material to become magnetized. By this between domains of different direction exist a kind of boundaries, called walls. Under the influence of external magnetic fields, they move: one of domains, that has more favorable condition in the concrete environment, "moves" wall towards neighboring domain and so "captures" it. The quicker boundaries between domains move, the quicker the material changes magnetization and the better it is suitable for practical use. That is why scientists study factors that influence the speed of movement of domain walls.

Scientists from Immanuel Kant Baltic Federal University (Kaliningrad), ITMO University (Saint- Petersburgh) together with colleagues from Spain studied how conditions of formation of amorphic ferromagnetic wires, exactly annealing conditions, influence properties of obtained composites.

As test samples authors use microwires with core from composition of cobalt, iron, nickel, boron, silicon and molybdenum, covered with glass covering. Diameter of samples was 24 micrometers (about twice thinner than human hair) and length—10 centimeters. Microwires were placed into muffle furnace, that was heated up to 200, 300 or 350 °C for different periods of time—from one minute to an hour, Apart from temperature, authors studied the influence of mechanical impacts. To achieve this aim, they hang loads on some samples during annealing, that create internal stress equal to the force of 125 megapascal (1274 kilogram on square centimeter) or 250 megapascal (2549 kilograms on square centimeter).

The experiment showed that composite wasn't destroyed under the influence of such stresses: both glass covering and metal heart maintained integrity and didn't acquire any defects.

After annealing scientists estimated magnetic properties of microwires. To achieve this aim, they placed samples into the magnetic field and determined how quickly domain walls in material moved under its influence. It turned out that the speed of their movement depended on the period of annealing: after treatment during 2 minutes it was 710 meters per second, after 30 minutes—1724 meters per second. Longer period of annealing led to the decrease of speed with which domain walls moved.

The analysis of microwires that experienced mechanical stress during annealing, showed that such influence helped to start movement of domain walls in the wire with the help of smaller magnetic field, than for samples, that didn't experience stress.

Thus, for example, in microwave that was annealed during 5 minutes by 300°С, mobility of domain walls increased in approximately three times during application of tension of 250 megapascal, as compared with composite, annealed in similar conditions, but without application of any stress. During longer annealing (about one hour) and higher temperature (350 °С) the effect was the same. That means that mechanical impact has a positive effect on properties of samples regardless of temperature and period of annealing.

"We managed to understand how conditions of annealing influenced magnetic properties of amorphic ferromagnetic wires. This study is developed by our colleagues under the guidance of professor Arkady Zhukov in Spain. It will help to create microwires with controlled properties, that are demanded in modern magnetic devices. In future we are planning to use the tested protocol of change of wires' properties for creation of composite multiferroic materials. Group of ITMO University will continue to model dynamic motion of domain border and change of microstructure using microwires as model objects",—tells Valeria Rodionova, Doctor of Physical and Mathematical Sciences, Director of Research and Education Center. "Smart materials and biomedical applications" of Immanuel Kant Baltic Federal University.

Provided by Immanuel Kant Baltic Federal University