P. Klein

Study of the Switching Field in Amorphous and Nanocrystalline FeCoMoB Microwire

We have studied the frequency dependence of switching field in a wide range of frequencies in amorphous and nanocrystalline microwires with nominal composition Fe40Co38Mo4B18. Samples were heat treated for 1 h at different temperatures in a wide temperature range 20-600°C. Three regions in the frequency dependence of the switching field were identified. Drop of switching field at low frequencies up to 50 Hz is explained in term of structural relaxation. Above 50 Hz the magnetoelastic contribution of the switching field is dominant. The magnetoelastic contribution of the switching field can be fitted by the power law (H sw ¿ ~ f1/n), giving exponent n equal 2 for frequency below 1000 Hz for all studied samples. Above 1000 Hz, the switching field reflects the structure of microwire being highly frequency dependent in as-cast sample and sample annealed at 450°C (where the microwire is quite inhomogeneous) while its frequency dependence is very weak for other annealing temperatures. Moreover, power exponent n gives non-physical values (~ 100) in this range.

Bistable FeCoMoB microwires with nanocrystalline microstructure and increased Curie temperature

Novel nanocrystalline glass-coated Fe40Co38Mo4B18 microwires are introduced. They combine the advantages of nanocrystalline alloys exhibiting simultaneously increased Curie temperature and magnetic bistability, which is required for modern sensoric and spintronic devices. Positive magnetostriction of the crystalline FeCo grains results in a magnetic bistability, whereas good soft magnetic properties remain stabilized. As a result of the mechanical stress induced by the glass coating, the optimum temperature range for thermal treatment is enhanced up to 600 °C.

Influence of Fixation on Magnetic Properties of Glass-Coated Magnetic Microwires for Biomedical Applications

The control of biomechanical processes in the tissue-implant interface and thermal changes created by friction or inflammatory processes in the implant and its environment represent the key validating processes of the postimplanting process. It is crucial for a patient and their health to minimize the invasiveness of the temperature measuring processes and the inner mechanical stress in the implant-tissue interface. For the purpose of these measurements, amorphous magnetic glass-coated microwires are the most suitable. Compared with other sensors, such as radio frequency identification sensors, the microwires have a significant advantage due to their dimensions (~2 cm × 50 μm) (because of which the sensor almost does not interfere with the inner implant structures), their production is relatively cheap, and only ~ 20 mm microwire is needed for the functional sensor. This paper is concerned with the testing of more types of microwire fixation in an implant and the impact of the fixation; it deals with necessary magnetic properties of a microwire and their dependence on the temperature. Microwire made of master alloy Fe78W5B17 was created and fixed in four ways: 1) on one end; 2) on two ends; 3) in the middle; and 4) along its full length. The results show that the optimal way of fixation is the one along the full length of a microwire; however, the final signal is influenced by both, the type and volume of the applied fixation material. The highest sensitivity of the designed microwire was in the range of 120-140 °C with no fixation and only with the full length fixation, this sensitivity decreased to 40-50 °C, which is a level close to the level required for biomedical applications (35-42 °C).

Enhancing the velocity of the single domain wall by current annealing in nanocrystalline FeCoMoB microwires

The effect of current annealing on the velocity of the single domain wall, sDW, and its dynamic stability is investigated in the 80–400 K temperature range for FeCoMoB nanocrystalline microwires. In addition to the heating effect of thermal treatments in a conventional field-free furnace, current annealing induces circumferential magnetic anisotropy, which significantly modifies the sDW dynamics characteristics. Such an anisotropy leads to an increased high domain wall velocity with a maximum measured value of about 8 km s −1 . An unexpected increase in the domain wall velocity and mobility at 175 K is ascribed to the compensation of the anisotropy distribution. Finally, the domain wall dynamics exhibit very stable behaviour in a wide range of temperatures (200–400 K).

Mechanical Stress Dependence of the Switching Field in Amorphous Microwires

Glass-coated magnetic microwires belong to a unique rank among materials that have high potential to be used in new technologies. We have studied the possibility to employ amorphous bistable microwires as a sensor for bending of carbon fiber composite. The measurement of switching time instead of switching field drastically increases the sensitivity of such sensors up to 380% for positive bending and 125% for negative bending. The results show that microwires applied on the surface structures of the materials can be used as sensors of mechanical stresses or bending.

Magnetic Properties of Glass-Coated Amorphous and Nanocrystalline FeMoBCu Microwires

We have studied temperature dependence of the switching field in bistable amorphous and nanocrystalline FeMoBCu microwires with low Curie temperature. Using a low Curie temperature of amorphous matrix, a huge variation of the switching field (over 400%) in a quite narrow temperature range (20 °C-150 °C) has been obtained. Such a variation is attributed to superparamagnetic behavior of separated crystalline grains in the early stage of nanocrystallization.

Tailoring the Switching Field Dependence on External Parameters in Magnetic Microwires

We have studied the effect of thermal treatment on the sensitivity and stability of the switching field in bistable glass-coated Fe-Ni-Si-B microwire. We have found that annealing at 300°C/1 hour leads to the increase in the sensitivity of the switching field to the applied external stress. Moreover, the switching field fluctuation decreases after such treatment as a result of domain structure stabilization through the structural relaxation.

Effect of Current Annealing on Domain Wall Dynamics in Bistable FeCoMoB Microwires

We have studied effect of current annealing on domain wall dynamics of FeCoMoB microwires. It was showed that 10 minutes of current annealing corresponds to 1 hour of classical annealing in furnace. Moreover, electrical current flowing through microwire produces Oersted magnetic field and therefore circular magnetic anisotropy is induced during annealing. As a result, induced circular magnetic anisotropy prefers vortex domain wall with velocities up to 3 km/s that can be observed in the current annealed nanocrystalline FeCoMoB microwires with much higher temperature stability.

Magnetically Bistable Microwires: Properties and Applications for Magnetic Field, Temperature, and Stress Sensing

Amorphous glass-coated microwires with positive magnetostriction are characterized by the magnetic bistability where the switching between the two stable magnetic states appears at the switching field. The switching field is sensitive to the external parameters like magnetic field, temperature, mechanical stress, etc., which gives us possibility to employ the microwires as a miniaturized sensing elements for the mentioned parameters.

Nanocrystalline Magnetic Glass-Coated Microwires Using the Effect of Superparamagnetism Are Usable as Temperature Sensors in Biomedical Applications

For the control and study of the post-implantation biomechanical processes of TiAl6V4 implants, the ability of wireless in vivo measurement of various parameters (i.e., temperature in this case) at the tissue–implant interface is required. Compared with other types of sensors, which consist of sensing and transmitting elements, nanocrystalline magnetic glass-coated microwires combine both of these features into unit. Thanks to a Pyrex coating, the microwires are biocompatible, and due to their size, they do not intervene into the surface structures of implants. The studied as-cast microwire has a low Curie temperature due to the high amount of molybdenum, and is not magnetically bistable at room temperature. In order to create its bistability at room temperature and enhance its temperature sensitivity in the range from 37 °C up to 42 °C, the microwire is specially annealed under axial stress above crystallization temperature. Extreme temperature sensitivity in the required temperature range is achieved using the superparamagnetism effect; moreover, due to this effect, the switching field increased in an almost linear way. The temperature dependence of the switching field is employed to sense the temperature in two TiAl6V4 samples produced by additive manufacturing and representing implants with different types of fixations of the microwire onto the surface. Sensitivity up to 0.01 K is achieved.