18 research outputs found
Anisotropic Dependence of Giant Magneto-Impedance of Amorphous Ferromagnetic Ribbon on Biasing Field
The magneto-impedance (MI) in amorphous ribbon of nominal composition
Fe73.5Nb3Cu1Si13.5B9 has been measured at 1MHz and at room temperature for
different configurations of exciting a.c and biasing d.c. fields. A large drop
in both resistance and reactance is observed as a function of d.c magnetic
field. When the d.c and a.c fields are parallel but normal to the axis of
ribbon, smaller magnetic field is needed to reduce the impedance to its small
saturated value compared to the situation when fields are along the axis of
ribbon. Larger d.c. field is required to lower the impedance when the d.c field
acts perpendicular to the plane of the ribbon. Such anisotropy in
magneto-impedance is related to the anisotropic response of the magnetization
of ribbon. The large change of impedance is attributed to large variation of
a.c permeability on the direction and magnitude of the dc biasing field.Comment: 12 pages, 7 figures, to be published in "International Journal of
Modern Physics B
Ferromagnetic Wires Composite Media with Tunable Scattering Spectra at Microwaves
We demonstrate composite media with ferromagnetic wires that exhibit a
frequency region at the microwave regime with scattering spectra strongly
dependent on an external magnetic field or stress. These tunable composite
materials have recently been proposed theoretically; however, no direct
experimental verification has been reported. We used composite materials with
predominantly oriented CoFeCrSiB glass-coated amorphous wires having large
magnetoimpedance at GHz frequencies. The free space measurements of reflection
and transmission coefficients were conducted in the frequency range 1-8 GHz in
the presence of an external static magnetic field or stress applied to the
whole sample. In general, the transmission spectra show greater changes in the
range of 10dB for a relatively small magnetic field of few Oe or stress of 0.1
MPa. The obtained results are quantitatively consistent with the analytical
expressions predicted by the effective medium arguments. The incident
electromagnetic wave induces an electrical dipole moment in each wire, the
aggregate of which forms the effective dipole response of the whole composite
structure in the radiative near or far field region. The field and stress
dependences of the effective response arise from a field or tensile stress
sensitivity of the ac surface impedance of a ferromagnetic wire. In the
vicinity of the antenna resonance the variations in the magneto-impedance of
the wire inclusions result in large changes of the total effective response. A
number of applications of proposed materials is discussed including the field
tunable microwave surfaces and the self-sensing media for the remote
non-destructive evaluation of structural materials
Off-diagonal impedance in amorphous wires and application to linear magnetic sensors
The magnetic-field behaviour of the off-diagonal impedance in Co-based
amorphous wires is investigated under the condition of sinusoidal (50 MHz) and
pulsed (5 ns rising time) current excitations. For comparison, the field
characteristics of the diagonal impedance are measured as well. In general,
when an alternating current is applied to a magnetic wire the voltage signal is
generated not only across the wire but also in the coil mounted on it. These
voltages are related with the diagonal and off-diagonal impedances,
respectively. It is demonstrated that these impedances have a different
behaviour as a function of axial magnetic field: the former is symmetrical and
the latter is antisymmetrical with a near linear portion within a certain field
interval. In the case of the off-diagonal response, the dc bias current
eliminating circular domains is necessary. The pulsed excitation that combines
both high and low frequency harmonics produces the off-diagonal voltage
response without additional bias current or field. This suits ideal for a
practical sensor circuit design. The principles of operation of a linear
magnetic sensor based on C-MOS transistor circuit are discussed.Comment: Accepted to IEEE Trans. Magn. (2004
Stress effect on magnetoimpedance (MI) in amorphous wires at GHz frequencies and application to stress-tunable microwave composite materials
The effect of tensile stress on magnetoimpedance (MI) in CoMnSiB amorphous
wires at microwave frequencies (0.5-3 GHz) is investigated both experimentally
and theoretically. In the presence of the dc bias magnetic field of the order
of the anisotropy field, the impedance shows very large and sensitive change
when the wire is subjected to a tensile stress: 100% and 60% per 180 MPa for
frequencies 500 MHz and 2.5 GHz, respectively. It is demonstrated that this
behavior owes mainly to the directional change in the equilibrium magnetization
caused by the applied stress and field, which agrees well with the theoretical
results for the surface impedance. This stress effect on MI is proposed to use
for creating microwave stress-tunable composite materials containing short
magnetic wires. The analysis of the dielectric response from such materials
shows that depending on the stress level in the material, the dispersion of the
effective permittivity can be of a resonant or relaxation type with a
considerable change in its values (up to 100% at 600 MPa). This media can be
used for structural stress monitoring by microwave contrast imaging
Optomagnetic composite medium with conducting nanoelements
A new type of metal-dielectric composites has been proposed that is
characterised by a resonance-like behaviour of the effective permeability in
the infrared and visible spectral ranges. This material can be referred to as
optomagnetic medium. The analytical formalism developed is based on solving the
scattering problem for considered inclusions with impedance boundary condition,
which yields the current and charge distributions within the inclusions. The
presence of the effective magnetic permeability and its resonant properties
lead to novel optical effects and open new possible applications.Comment: 48 pages, 13 figures. accepted to Phys. Rev. B; to appear vol. 66,
200
Effect of transition layers on the electromagnetic properties of composites containing conducting fibres
The approach to calculating the effective dielectric and magnetic response in
bounded composite materials is developed. The method is essentially based on
the renormalisation of the dielectric matrix parameters to account for the
surface polarisation and the displacement currents at the interfaces. This
makes it possible the use of the effective medium theory developed for
unbounded materials, where the spatially-dependent local dielectric constant
and magnetic permeability are introduced. A detailed mathematical analysis is
given for a dielectric layer having conducting fibres with in-plane positions.
The surface effects are most essential at microwave frequencies in
correspondence to the resonance excitation of fibres. In thin layers (having a
thickness of the transition layer), the effective dielectric constant has a
dispersion region at much higher frequencies compared to those for unbounded
materials, exhibiting a strong dependence on the layer thickness. For the
geometry considered, the effective magnetic permeability differs slightly from
unity and corresponds to the renormalised matrix parameter. The magnetic effect
is due entirely to the existence of the surface displacement currents.Comment: PDF, 33 pages, 10 figure