R. Varga

Magnetocaloric effect in melt spun Ni50.3Mn35.5Sn14.4 ribbons

We determined the magnetic entropy change and refrigerant capacity of melt spun Ni50.3Mn35.5Sn14.4 ribbons around both the structural and the magnetic transitions for a field of 20kOe. The maximum entropy changes at the structural and magnetic transitions were of 4.1 and −1.1Jkg−1K−1. Ribbons studied show a larger refrigerant capacity around the magnetic transition (46Jkg−1) than around the structural transition (26Jkg−1), suggesting that the temperature range at the magnetic transition is more adequate for a refrigerant cycle than that at the structural transition.

Temperature dependence of the switching field and its distribution function in Fe-based bistable microwires

The switching field distribution for magnetization reversal in a single Barkhausen jump of a bistable Fe-based amorphous microwire as well as its temperature dependence have been investigated in the temperature range from 77 to 450 K. Two processes have been identified to be responsible for the temperature dependence of the switching field: magnetostrictive volume domain wall pinning on stresses and relaxation effects due to local structural rearrangements. While at low temperatures, pinning on the atomic level defects plays the dominant role, magnetostrictive pinning becomes more important at intermediate temperatures. A simple model is proposed considering both energy contributions that fits reasonably well with experimental data and allows us to interpret additionally the observed temperature dependence of the switching field fluctuations.

Domain Wall Propagation in Thin Magnetic Wires

We present the domain wall dynamics in amorphous glass-coated microwires. It is described by the linear dependence of the domain wall velocity on the applied magnetic field. For higher Ni content microwires, the domain wall dynamics consists of two regions: at low field, the domain wall has low mobility and a negative critical propagation field. At higher fields, the domain wall mobility increases as a result of the domain wall structure change. The domain wall dynamics in this range is described by positive critical propagation field and high domain wall mobility, which allows the domain wall to reach a very high velocity of more than 10\thinspace 000 m/s. Such a fast domain wall exceeds the sound velocity in microwires. The interaction of the domain wall with the phonons is observed when the domain wall approaches the sound velocity limit.

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.

Nanocrystalline glass-coated FeNiMoB microwires

The evolution of the structure of glass-coated Fe40Ni38Mo4B16 amorphous microwire with thermal treatments and its interplay with magnetism has been studied. As shown by x-ray diffraction, a primary crystallization process resulted into formation of γ-(Fe,Ni) nanocrystallites embedded in a residual amorphous matrix. The evolution of the saturation magnetization and the switching field after different thermal treatment was studied. Amorphous glass-coated microwires based on FeNi exhibit magnetic bistability even in the nanocrystalline state. This is explained by the high magnetoelastic anisotropy, which is also responsible for magnetic hardening after annealing at the temperatures above 670 K.

Frequency dependence of the single domain wall switching field in glass-coated microwires

The frequency dependence of the switching field in glass-coated FeNiMoB microwires has been studied in the temperature range from 77 to 373 K. Two contributions to the domain wall switching mechanism were recognized: a magnetoelastic contribution coming from the magnetoelastic interaction of the magnetic moments with the stresses, and a relaxation contribution coming from the structural relaxation of the atomic level defects. The structural relaxation results in the unusual increase in the switching field at low frequencies, whereas the increase in the switching field at high frequencies was assigned to the frequency dependence of the magnetoelastic contribution, which obeys the power law Hsw~f1/3.

Kerr-effect based Sixtus-Tonks experiment for measuring the single domain wall dynamics

Here we present the Kerr-effect-based Sixtus-Tonks experiments to study the single domain wall dynamics. It combines the advantage of the classical Sixtus-Tonks experiments with the quick optical method to register the domain wall propagation. Instead of the pick-up coil, the reflection of the broken laser beam from the microwire surface is used. The change of the reflected signal has much smaller relaxation time than that of the pickup coil, that allows us to study the propagation of small and fast domain walls. We compare the measurement obtained by the classical and Kerr-effect-based Sixtus-Tonks experiment.

Pinning field distribution in the amorphous CoFeSiB wire

A method for obtaining the pinning field distribution from the amplitude dependence of susceptibility is introduced. The method is tested on the amorphous non-magnetostrictive CoFeSiB wire. The effect of magnetic relaxation on the pinning field distribution was also studied. It was found that stabilization at higher temperatures leads to the unification of the restoring pressure on the domain walls and consequently to the formation of maxima in the pinning field distribution.

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).

Structural and Magnetic Characterization of Half-Metallic Co 2 MnAl Heusler Alloy

Magnetic and structural characterization of full Heusler Co2MnAl alloys prepared by rapid quenching from pure elements was studied. This technique allows a fast production of relatively a large amount of Heusler alloys without the necessity of further thermal treatment. The Heusler alloys have been prepared by the melt-spinning method in the form of ribbons with homogeneous chemical composition (Co = 46.1%, Mn = 29.4%, and Al = 24.5%). The X-ray diffraction shows a single-phase B2 crystalline structure with lattice parameter α = 5.671 Å. Magnetic measurements reveal an anisotropic character of the Co2MnAl ribbon with easy magnetization axis well defined in parallel direction with respect to the ribbon plane. Curie temperature was estimated from magnetization measurement to be 710 K.