Местоположение издательства:Piscataway, NJ, United States
Первая страница:502
Последняя страница:502
Аннотация:Amorphous magnetic materials have the pronounced soft magnetic properties,
high resistivity, giant magneto-impedance effect — the dependences
of the complex resistance on the magnetic field — and bistability
of magnetization state, that make it possible to create electrical machines
with enhanced efficiency [1], highly sensitive magnetic field sensors, in
particular, for geomagnetic applications or biomolecules sensing [2-5], and
binary elements for storage devices [6]. Moreover, the use of amorphous and
nanocrystalline materials for hyperthermia was also described in literature
[7]. The peculiarities of their behavior under magnetic field make it possible
to shed light on the formation mechanisms of various properties of magnetic
materials, which are one of the key issues in the field of physics of magnetic
phenomena and justifies the importance of amorphous materials investigations
from the fundamental science point of view. Complex micromagnetic
structure of amorphous microwires and the dependence of magnetic permeability
on magnetic field account for the nontrivial permeability distribution
over the cross-section of the sample. Nonuniform permeability distribution
over the sample volume contributes to the magnetic response of the system,
what makes its investigations crucial for further understanding of magnetic
properties formation on the microlevel. For the reconstruction of the permeability
distribution numerical calculations were combined with experimental
investigations. Data on impedance characteristics of the sample allows one
to restore the value of magnetic permeability. Frequency dependence of the
skin-depth makes it possible to involve different fractions of the wire into
the process of remagnetization by current. Thus, frequency dependences of
impedance allow one to obtain integral permeability values of wire shells
having different thickness. Assuming the additive character of magnetic
permeability, its distribution over the cross-section of the wire can be reconstructed
layer by layer. Experimental investigations of the impedance of
amorphous microwires were conducted using vector network analyzer
Agilent FieldFox N9923A in the frequency range from 2MHz to 6GHz.
External magnetic field was created by coil with the thickness of 4 cm and
diameter 15 cm having 1500 turns of the wire. Field magnitude was varied
in the range ±100 Oe. The sample of the amorphous microwire with the
length of 2cm was soldered to the substrate and then fixed in a measuring
cell that can be included into the coaxial transmission line. For the investigations
4 samples of amorphous microwires of Fe6.1Co57Ni10B15.9Si11 composition
having metal core diameters 7.6, 9.4, 14.4, 20.4 μm were chosen.
Static magnetic properties of the samples were investigated using vibrating
sample magnetometer VSM LakeShore 7400 Series at room temperature and
were used as reference parameters for further calculations. Calculations for
reconstruction of the permeability distribution were conducted using Matlab
software on the basis of inline functions. As a first step to permeability
restoration calculations of impedance and GMI effect were made based on
classical approach of constant permeability of cylindrical wire: Z=R*kR*-
J0(kR)/2*J1(kR), where k=(1+i)/δ, δ=c/sqrt(2πλμω). Values of calculation
parameters were chosen according to the experimental data. Comparison of
the values of GMI obtained in experiment and those obtained as a result of
calculations allows one to see the divergence between real experiment and
classical approach. The magnitude of this divergence depends on the experimental
circumstances – metal core diameter, frequency and so on. Classical
approximation considers permeability as a constant parameter, and its dependence
on the radial coordinate is not taken into account. In real microwire,
permeability is distributed extremely heterogeneous over the cross-section of
the microwire. With increasing frequency of the flowing current the fraction
of the sample volume involved in the remagnetization process decreases
due to the skin effect. As the permeability reduces when approaching the
surface of the wire in reference to the permeability in zero field used for
calculations, measured values of GMI also strongly decrease. Thus, the
difference between the calculations based on the classical approximation
and experimental data confirms the initial assumptions of this research and
the importance of its implementation. Experimental results for frequency
dependences of impedance and GMI effect were processed according to the
described methodology. For each frequency and, thus, for each skin depth
the value of effective magnetic permeability was obtained. Assuming the
additive character of the permeability function, the contribution of each layer
into magnetic response was restored. The dependence of the permeability on
the radial coordinate was then obtained. The authors acknowledge support
from M.V. Lomonosov Moscow State University Program of Development
and Russian Fund for Basic Research No 18-02-00137. YA and NP were
supported with Russian Fund for Basic Research No 19-32-90089.