Journal of Magnetism and Magnetic Materials 262 (2003) 162–165

The coercivity dependence of giant magneto-impedance effectin Fe–Cu–Nb–Si–B based metallic alloy ribbon at different

crystalline stages

Md. Kamruzzamana, I.Z. Rahmana,*, M.A. Rahmanb

aMagnetics Research Laboratory, Department of Physics, University of Limerick, Limerick, Irelandb Department of Electronic and Computer Engineering, University of Limerick, Limerick, Ireland

Abstract

We have studied the giant magneto-impedance (GMI) effect in one Fe-based FINEMET alloy ribbon with nominal

composition Fe73.5Cu1Nb3Si13.5B9 (at %). DTA experiments were conducted to identify the primary and secondary

crystallization temperatures. XRD, SEM and EDAX were performed to identify the phases at various crystalline stages.

Static magnetic properties of the ribbons were measured using a VSM. Coercivity was found to be a strong function of

annealing temperature, and this, in turn depended on the size and type of the crystalline phases. The maximum MI

effect of 103.4% was observed at annealing temperature of 6001C. It was found that a small change in DC coercivity as

a result of annealing greatly changed the MI ratios of the crystalline ribbons. Annealing above the secondary

crystallization temperature caused the precipitation of Fe2B and Fe3B phases, which induced magnetic hardening and

eliminates MI sensitivity.

r 2003 Elsevier Science B.V. All rights reserved.

PACS: 75.50.Kj; 81.40.Rs; 73.63.Bd; 75.47.m; 75.50.Cc; 75.60.Ej

Keywords: MI effect; Crystallization; Amorphous ribbon

The so-called Giant magneto-impedance (GMI)effect is the large variation of impedance in softmagnetic materials induced by a small DCmagnetic field in the presence of a relativelyhigh-frequency AC field [1,2]. It is well establishedthat the basic mechanism for MI phenomenon isthe change of magnetic penetration depth throughthe magnetic field induced modification of thetransverse magnetic permeability [1–4]. The high-

frequency impedance (Z) can be defined as

Z ¼ð1 iÞL

2lc

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2promj

q; ð1Þ

where, l is the ribbon width, L is the ribbon length,and c is the speed of light, r is the electricalresistivity, mf is the transverse magnetic perme-ability and oð¼ 2pf Þ is the angular frequency andf is the excitation frequency. The application ofaxial DC magnetic field (HDC), which is applied inthe longitudinal direction of the ribbons changesthe transverse permeability. The transverse do-main spins are reoriented toward the ribbonaxis. As the DC field increases, the transverse

*Corresponding author. Materials and Surface Science

Institute, University of Limerick, National Technological Park,

Limerick, Ireland. Tel.: +353-61-202205; fax: +353-61-202423.

E-mail address: zakia.rahman@ul.ie (I.Z. Rahman).

0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0304-8853(03)00042-8

permeability mf decreases and the skin depth dincreases. As a result, the effective cross-section ofthe conducting magnetic ribbon is modified at highfrequencies causing a large decrease in ACimpedance (Z). The MI ratios can be defined as

DZ

ZðHÞ¼

ZðHÞ ZðHmaxÞZðHmaxÞ

100ð%Þ; ð2Þ

where ZðHÞ is the impedance of the amorphousribbon at zero DC magnetic field, and ZðHmaxÞ isthe impedance at maximum DC field above whichthe variation of impedance is zero.

MI effect on Fe-based amorphous magneticribbons with nominal composition Fe73.5Cu1Nb3-

Si13.5B9 has been studied at different crystallinestages. The recrystallization temperature of theamorphous ribbon was determined from differen-tial thermal analysis (DTA). X-ray diffractometry(XRD) and scanning electron microscopy (SEM)were performed to identify the crystalline phases.The static magnetic coercivity has been determinedfrom the hystersis loop, measured using a VSMwhere a precision power supply was used withresolution of 1 mA per step. The VSM wascalibrated for measurement of low coercivity byusing data obtained from Alternating GradientMagnetometer from Princeton MeasurementsCorp. USA. For this purpose measurement on anumber of commercial samples of FINEMET wasused and compared with our VSM data to cancelout the magnetizing effect of electromagnet andthe earth’s magnetic filed. This way a relationshipwas establish to detect very low coercivity with theVSM. MI effect was measured by impedanceanalysis method [1] using Solartron 1260. Thecontacts of the ribbon to coaxial cables were madeusing silver paint. Effect of wire impedance andcontact resistance were taken into account whilemeasuring MI effect. The amorphous state of theas-cast ribbon was confirmed by XRD and SEMtechnique. From DTA (Fig. 1) it was observedthat the amorphous ribbon crystallized in twostages: the first stage of crystallization started at5061C and maximum reaction rate was found at5321C. The second stage of crystallization startedat 6701C and the maximum reaction rate wasfound to be at 703.31C that was completed at7651C. To induce primary and secondary crystal-

line phases, a series of ribbon samples wereannealed at different length of time and tempera-tures. Table 1 describes various samples atdifferent stages of crystallization. MI effect hasbeen studied in these series of samples (S1–S5) as afunction of excitation frequency up to 20 MHz andaxially applied DC magnetic fields in the range of0–4 kA m1 and annealing temperatures. It wasfound that Z first decreased slowly up to theannealing temperature 5321C, then increasedsharply up to 6001C, after that Z decreasedsharply to 7031C (Fig. 2). Similar behaviour wasalso observed for R (DC resistance) and X

(inductive reactance) values. This was due to thelocal rearrangement of the micro-voids, whichcaused the decrease in the electrical resistivityup to primary crystallization stage. The first

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

5.0

100 300 500 700

Primary Crystallization (532.09 ˚C)

Secondary Crystallization(703.3 ˚C)

A

B

S1

S2

S3

S4

S5

S6

∆T (

µV)

Temperature (˚C)

Fig. 1. DTA curve of the amorphous ribbon with nominal

composition Fe73.5Cu1Nb3Si13.5B9.

Table 1

Heat treatment and induced precipitate phases of different

samples

Sample

name

Annealing Physical state and

phases

Temperature

(1C)

Time

(h)

S1 Unannealed 0 Amorphous

S2 510 1 Dispersed fine

particles a-Fe and

Fe3Si

S3 532 1 Clusters of fine a-Fe

and Fe3Si

S4 600 1 Network of a-Fe

Clusters with Fe3Si

S5 703 1 Mixed phases of

a-Fe, Fe3Si, FeB,

Fe2B

M. Kamruzzaman et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 162–165 163

crystallization transition corresponding to 5321Cobtained from the DTA curve is due to theprecipitation of a-Fe and Fe3Si grains in amor-phous matrix as observed from XRD study. TheZ; R; X values of the ribbons increased verysharply due to the electron scattering from thegrain boundaries of a-Fe and Fe3Si. Furtherincreases of annealing temperature at or abovesecondary crystallization temperature (7031C)caused Z; R; X of the ribbon to decrease verysharply. This is due to the complete structuralrearrangement of the annealed ribbons. In as-castribbons the amorphous structure is stabilized dueto short-range magnetic interaction between TM–M alloying elements. After secondary crystalliza-tion long-range order sets in and the mobility ofthe conduction electron increases thus causing theZ; R and X to decrease. Fig. 3 shows the plot ofMI ratio as a function of applied axial DCmagnetic field for amorphous and crystalline

ribbons. As the ribbons were annealed, the MIspectra appeared with double peak, for example at5101C where the maximum MI value reachedB54%. As the annealing temperature gets higher,two things happen, first the peak values changesand secondly, the separations of the peaks change.In annealed ribbons at Ta ¼ 5101C the doublepeak appears with the magnetic field intensitydifference of 960 A m1. At 5321C, the peak valueof the MI ratio reached B88% with magnetic fieldintensity difference of 320 A m1. The magneticfield intensity difference of 206 A m1 was ob-served at 6001C and peak value of the MI ratiowas B103.4%. As the annealing temperatureincreased to 7001C, the MI ratio dropped tonearly zero and the double peak completelydisappeared as shown in Fig. 3. Similar behaviourwas also observed in the plot of MR and MX ratiovs. axially applied DC magnetic field. Measure-ment of DC coercivity as a function of annealingtemperature revealed the change in MI behaviourupon crystallization (Fig. 4). During primarycrystallization, the coercivity of the ribbon de-creased due to the formation of magnetically softernanoparticles or crystallites of a-Fe as expectedfrom the random anisotropy model [5].

When the ribbon was annealed above secondarycrystallization temperature, magnetic hardeningcaused the sharp increase in coercivity. It is wellknown that the MI effect is closely related to themagnetic domain structure. The domain wallmovement contributes to the change in transversemagnetic permeability during the interactionbetween ac excitation field and axially applied

0

20

40

60

80

100

120

0 510 532 600 700

Max

. of M

I Rat

io

00.10.20.30.40.50.60.70.80.91

Coe

rciv

ity (H

C)

A m

-1

Temperature (°C)

Max. MI Effect

Coercivity

Poly. (Max. MI Effect)

Poly. (Coercivity)

Fig. 4. Comparison between temperature dependence coerciv-

ity and MI ratio for annealed ribbons.

0

5

10

15

20

25

0 200 400 600 800

Temperature (˚C)

Impe

danc

e (Z

) in

(Ω

)

At Hdc=0At Hdc=2722 A/m

Fig. 2. Annealing temperature dependence of the impedance of

ribbons at 6.31MHz frequency.

-20

0

20

40

60

80

100

120

-5000 -3000 -1000 1000 3000 5000

MI

Rat

io (

%)

S1

S2

S3

S4

S5

DC Magnetic field strength (H) (A m-1)

Fig. 3. Axially applied magnetic field dependence of the

magneto-impedance (MI) ratio of amorphous and annealed

ribbons.

M. Kamruzzaman et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 162–165164

DC magnetic field. In as-cast ribbon the coercivityis high due to the induced quenching stressesduring fabrication of the ribbon. This can createirregular domain pattern due to variation of localmagnetoelastic anisotropy. As a result, the domainwall movement occurs in different directions andits net effect on transverse permeability is very low.This leads to the MI sensitivity value towards zeroin as-cast ribbons.

Magnetically softer regions in samples com-posed of nanosize grains can help to form stripedomain of regular pattern [6,7]. In sample S3magnetically softer regions expanded in theamorphous matrix and led to increased MI ratio.A network of magnetically softer regions can havehigher exchange energy and stripe domains canform leading to increased transverse permeabilityand hence higher MI ratio. This explains theobserved increase in HC and highest MI ratio withsingle peak in sample S4. The disappearance of thedouble peak is due to near-zero stress anisotropyvalue in the sample. In sample S5 further increasein the coercivity value caused domain wall pinning

and led to decrease in MI sensitivity. It wasobserved that MI effect was present in ribbonsannealed at a temperature below the secondaryrecrystallization temperature. Annealing abovesecondary recrystallization temperature causedmagnetic hardening, which led to a sharp increasein coercivity and completely eliminated the MIsensitivity in the crystalline ribbons.

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[6] Md. Kamruzzaman, PhD thesis, University of Limerick,

2003.

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M. Kamruzzaman et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 162–165 165