IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011 2563

Influence of Gamma Irradiation and Annealing on a Co-BasedAlloy by the Permeability Spectra Measurement

H. Song and D. G. Park

Korea Atomic Energy Research Institute, Daejeon 305-353, Korea

The effects of annealing and gamma irradiation on the magnetic properties of amorphous �������������� alloys have been inves-tigated by means of the changes of the permeability spectra. In general, there are no differences in the magnitude and shape of the realpart of the permeability curves between as-quenched amorphous and gamma-irradiated samples. However, a drastic increase, causedby gamma irradiation, is observed in the magnitude of the imaginary part of the permeability. This can be attributed to another relax-ation process by superposition due to the gamma-irradiation effect. The same trend was also found in the sample annealed at 380 � invacuum and in open air under a field of 3 Oe, which has a maximum value of the permeability due to the presence of ultrasoft magneticmaterials.

Index Terms—Amorphous alloys, annealing, gamma irradiation, giant magneto impedance (GMI).

I. INTRODUCTION

I N the past years, the giant magneto impedance (GMI) ef-fect in amorphous soft ferromagnetic alloys has attracted

much attention due to their potential for magnetic sensors ap-plications [1]–[3]. However, it is important to know the behav-iors of materials under the influence of radiations for effectivedesign of novel sensor systems. The GMI phenomenon is dueto the change in skin depth arising from the change in the mag-netic permeability of materials with applied magnetic fields.

In this paper, the annealing and gamma-irradiation effects onthe magnetic properties of amorphous alloysare studied by means of permeability measurement. In partic-ular, since the magnetization dynamics in ribbon annealed inweak fields, however, are not yet well understand, we also inves-tigate permeability spectra in ribbon annealed in a weak field.

II. EXPERIMENT

Samples used in this study were commercial amorphousribbon , of the size of .The amorphous state of the samples was confirmed by X-raydiffractometry. The samples were studied under the followingprocedures: gamma irradiated (No. 1), annealing at 380 ina vacuum for 1 h (No. 2), gamma irradiated after annealing at380 in a vacuum for 1 h (No. 3), annealing at 380 inopen air for 8 h (No. 4), and gamma irradiated after annealingat 380 in open air for 8 h (No. 5). The samples (Nos. 1, 3,and 5) were irradiated in the gamma facility of Jeongeup Ra-diation Science Institute at the Korea Atomic Energy ResearchInstitute. Gamma-ray irradiation was performed with a Co-60gamma-ray source. The irradiation dose rates used were 20,40, and 60 Gy/h. The maximum absorption dose was about 60Gy. The real and imaginary parts of the complex permeability,i.e, , were measured by an impedanceanalyzer in the frequency range of –10 MHz fields. Theamplitude of ac current applied to a small solenoid coil around

Manuscript received February 21, 2011; accepted May 23, 2011. Date ofcurrent version September 23, 2011. Corresponding author: H. Song (e-mail:hsong@kaeri.re.kr).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2011.2158573

the sample was set to a constant value during the frequencysweep in the permeability spectra.

III. RESULTS AND DISCUSSION

Fig. 1(a) and (b) shows the real and the imaginary parts ofthe spectra of the as-quenched sample marked as as-quenched.The real and imaginary parts of permeability show a typicalDebye-type relaxation at 40 kHz. The frequency dependenceof the real part of the permeability spectra is divided into threestages, as shown in Fig. 1(a): a constant value for the lower fre-quencies , rapid decrease in the intermediate re-gion of frequencies , and a saturationregion for the higher frequencies . The behaviorof permeability spectra can be explained by the equation of thedamped harmonic motion impeded by the defect or impurity [4]

(1)

where m is the effective mass of the domain wall, is the vis-cous damping parameter, is the restoring force of a domainwall, is the saturation magnetization, is the amplitudeof the applied field, is the angular frequency, and is thedomain wall bowing. Equation (1) can be solved for the sta-tionary, forced oscillation condition of the driving field, i.e.,

. As the acceleration of wall motion be-comes negligible, the permeability is given by the Debye-typerelaxation as

(2)

with

(3)

The relaxation frequency of magnetization corresponding tothe maximum is given in terms of the restoring force as (2).

Fig. 1 shows the permeability spectra of the real and imaginaryparts in the annealed sample marked as Nos. 2 and 4. Thermaleffects of permeability are clearly observed in the low and inter-mediate regions of permeability spectra but the behavior is char-acterized by the annealing time. The annealed sample at 380in a vacuum for 1 h show almost the same permeability as theas-quenched sample, but the annealed sample at 380 in open

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2564 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011

Fig. 1. (a) Real and (b) imaginary parts of the permeability spectra of�� �� � �� amorphous ribbon with nonirradiation (as-quenched),irradiation (No. 1), annealing in a vacuum (No. 2), irradiation after annealingin a vacuum (No. 3), annealing in open air (No. 4), irradiation after annealingin open air (No. 5).

air for 8 h show lower permeability than the as-quenched samplein the low-frequency region. In the intermediate region, the an-nealed samples show a slower decreasing rate compared to theas-quenched sample. The decreasing rate shows the dependenceof the annealing time. The annealed sample for 8 h was decreasedat the most slowest rate. In the high-frequency region, all thecurves merged into the same low permeability value irrespectiveof the annealing time. The other trend is observed in the imag-inary part of permeability, as shown in Fig. 1(b). The profile of

in Fig. 1(b) shifts toward the high-frequency region as the an-nealing time increases. The relaxation frequency of magnetiza-tion corresponding to the maximum increased with the an-nealing time with the factor of 4 for the annealed sample for 8 h.The thermal effects on permeability spectra are not clearly ob-served in the high-frequency region. The annealed alloy is knownto have an ultrafine structure embedded in an amorphous matrix.Such samples exhibit very high permeability, very small domainwall, and local magneto crystalline anisotropy [5]. The decreasein the distance between pining edges results in an increase in therestoring force owing to the small domain wall. The increase inrelaxation frequency due to annealing in the intermediate-fre-quency region is attributed to the increase in the restoring force

. As increasing the restoring force, the permeability shifts tohigher frequencies.

The gamma-irradiation effects of permeability are shown inFig. 1 marked as Nos. 1, 3, and 5. A negligible effect of gamma

Fig. 2. (a) Real and (b) imaginary parts of the permeability spectra of�� �� � �� amorphous ribbon with nonirradiation and irradiation afterannealing in a vacuum.

irradiation is observed in the real part of the permeability. How-ever, the other trend is observed in the imaginary part of perme-ability, as shown in Fig. 1(b) marked as Nos. 1, 3, and 5. The per-meability was increased in the low-frequency region, still main-taining the original relaxation frequency, which was observedin an unirradiated sample. This reflects that another relaxationprocess begins to be involved in the low-frequency region. Thus,complex permeability can be described by the superposition oftwo relaxations as

(4)

where and are, respectively, the magnitude and relaxationin the low frequency and and are those of the high-fre-quency relaxation.

The irradiation effects on permeability spectra are not clearlyobserved in the high-frequency region. At the higher frequencies,the skin effect is stronger, domain wall movements are stronglydamped by eddy currents, and the magnetization rotations as-sociated with spin rotation dominate the process [6][7]. To findthe effect of gamma irradiation on dose, the gamma ray has beenirradiatedwith various dosages from 20 to 60 Gy.Fig. 2 shows thedependenceofpermeabilityon the gamma-ray dose. A negligible

SONG AND PARK: INFLUENCE OF GAMMA-IRRADIATION AND ANNEALING ON CO-BASED ALLOY BY THE PERMEABILITY SPECTRA MEAS. 2565

effect of gamma irradiation is observed in the real part of thepermeability. However, the other trend is observed in Fig. 2(b).The imaginary part of permeability shows a linear variation withthe increasing dose. That is, it was increased in the low-frequencyregion and it begins to saturate in the relaxation frequency. Thepinned domain wall motions are responsible for the variationof the low-frequency relaxation because the domain walls havea threshold field to be displaced due to pining at defects. Theincrease of permeability in low frequency is attributed to theincrease in the number of defects caused by gamma irradiation.

IV. CONCLUSION

The annealing and gamma-irradiation effects on the magneticproperty of an amorphous alloy has been in-vestigated by permeability measurements. The sample annealedat 380 in air for 8 h has a maximum value of the perme-ability due to the presence of ultrasoft magnetic materials. The

magnetic properties of samples are drastically increased undergamma irradiation and show the dependence of the gamma dosein the imaginary part of permeability. It can be attributed to an-other relaxation process by the gamma irradiation. A negligibleeffect of gamma irradiation is observed in the real part of thepermeability.

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