Low-field hysteresis in the magnetoimpedance of amorphous microwires
M. Ipatov,* V. Zhukova, A. Zhukov, J. Gonzalez, and A. Zvezdin
†
Departamento de Física de Materiales, Facultad de Qímicas, Universidad del País Vasco, San Sebastián, Spain
Received 14 January 2010; revised manuscript received 8 March 2010; published 19 April 2010
The phenomena of low-field hysteresis of the magnetoimpedance MI in zero-magnetostrictive amorphous
wires are studied theoretically and experimentally. We developed a mathematical model for magnetization
reversal and impedance field dependence. The presented model considers the low-field hysteresis and the effect
of circular bias magnetic field. It is demonstrated that the hysteresis originates from a nonzero angle between
the anisotropy easy axis and transversal plane. The bias field, which is produced by current running through the
wire, considerably affects the MI dependence making it anhysteretic and highly asymmetric. The validity of the
model is confirmed by the experiments. The main characteristics of the studied amorphous wire such as
anisotropy field H
A
, angle between the anisotropy easy axis with the transversal direction , and Gilbert
damping constant
G
were obtained from the experiment in accordance with the presented model.
DOI: 10.1103/PhysRevB.81.134421 PACS numbers: 75.47.-m, 75.30.Gw, 75.50.Kj, 75.60.Jk
I. INTRODUCTION
The giant magnetoimpedance effect GMI consists of
significant change of the impedance of a magnetically soft
conductor upon the application of an external magnetic field.
The GMI effect is intrinsically related with magnetic softness
and requires low magnetic anisotropy constant and high cir-
cumferential magnetic permeability. In particular, these con-
ditions are fulfilled in magnetically soft amorphous wires
with low and negative magnetostriction constant
s
-10
-7
.
1
It was reported the sensitivity to magnetic field as
high as 1 Oe Ref. 2 and up to hundreds of percents of the
impedance changes in amorphous wires
1,3,4
that, combined
with low cost and simple fabrication method, have made
them very attractive for prospective application where local-
ized weak magnetic field is especially important such as bio-
medical, geological, environmental, navigation, and indus-
trial highly sensitive magnetic field sensing.
Recently a novel family of amorphous wires with reduced
dimensionality—glass-coated microwires consisting on
much thinner metallic ferromagnetic nucleus usually of the
order of 1–30 m coated by glass—has been developed.
4,5
These microwires fit much better for utilization in magnetic
sensors mostly because of their thinner dimensions and
therefore lower effect of demagnetizing stray fields. At cer-
tain conditions such microwires exhibit quite good magnetic
softness, high GMI Refs. 5 and 6 and stress-impedance
7
effects.
Another emerging application of amorphous microwires
is tunable and self-sensing composite materials with micro-
wave electromagnetic properties depending on the imped-
ance of the microwires embedded in the dielectric matrix.
8–12
The use of microwires with high GMI and stress impedance
effects in composites gives the possibility to realize the ma-
terials which dielectric permittivity is determined by the
structural scaling, external stimuli or internal state of the
material. For example, a material with self-monitoring prop-
erties could be able to evidence invisible structural damages,
defects, excessive loadings, local stress, and temperature dis-
tribution, thus considerably facilitating the in situ health
monitoring of large scale objects such as infrastructure
bridges, buildings, etc..
Obviously, for both GMI applications in magnetic field
sensors and tunable composites, the highly sensitive MI and
its low hysteresis are required. Consequently, improvement
of these parameters is essential for these applications. At the
same time, the hysteresis up to 100 A/m or even higher was
found in amorphous microwires
13
that considerably limit the
sensor’s precision.
14
Though the MI effect has been rather extensively studied
over the last two decades, most of the performed investiga-
tions were devoted to MI in high fields above 1 kA/m. To
the best of our knowledge, the problem of low-field hyster-
esis in amorphous wires was consider only in a few
works.
15,16
It these papers it was shown that the MI hyster-
esis is related with static circumferential magnetization and
the application of circumferential dc bias field H
B
is required
to suppress this hysteresis. Nevertheless, a complete model
describing all aspects of MI dependence such as hysteresis,
asymmetry induced by the bias field, influence of anisotropy
constant, and so on has not been given. Thus, rigorous theo-
retical and experimental studies of MI effect with consider-
ation for the low-field anomalies are of considerable interest
and importance.
In the paper we developed the mathematical model for the
magnetization reversal and MI field dependence for zero-
magnetostrictive amorphous microwires and compared it
with the experiment. The paper is organized as follows. First,
we considered the equilibrium magnetization state in the sur-
face layer of the wire. We assume i a nonzero angle be-
tween the anisotropy easy axis and transversal plane and ii
the presence of a dc circular field that is created by the bias
current. Second, the tensors of magnetic permeability and
surface impedance are discussed. Finally, the model is com-
pared with the experimental measurements from which the
main characteristics of the microwire such as anisotropy field
H
A
, angle between the anisotropy easy axis with the trans-
versal direction , and Gilbert damping constant
G
were
obtained.
II. MAGNETIC STRUCTURE
The outer shell of amorphous glass-coated microwires
with vanishing magnetostriction is characterized by the cir-
PHYSICAL REVIEW B 81, 134421 2010
1098-0121/2010/8113/1344218 ©2010 The American Physical Society 134421-1