DESIGN AND PREPARATION OF NEW SOFTMAGNETIC AMORPHOUS FERROMAGNETS Bulk Amorphous Alloys;Amorphous Wires, Microwires and Nanowires

H. CHIRIAC and N. LUPU National Institute of Research and Development for Technical Physics 47 Mangeron Blvd., P.O. 3, P.O. Box 833, 700050 Iasi, Romania

Corresponding author: H. Chiriac e-mail: hchiriac@phys-iasi.ro

Abstract: DC and AC magnetic properties of Fe-based bulk amorphous alloys as thick melt-spun ribbons, cast rods and torroids are investigated. The influence of the prepara-tion conditions, composition, frequency, and treatments on the electrical and magnetic characteristics is discussed. The magnetic behaviour of amorphous wires and microwires is presented. The correlation between the diameter of the wires, the magnitude of the GMI effect and the surface magnetic permeability is discus-sed. Arrays of amorphous NiP and CoP nanowires with diameters near 200 nm have been obtained using electrodeposition. The microstructure and magnetic properties of the nanowire arrays strongly depend on the composition.

1. INTRODUCTION

The specific magnetic behaviour of amorphous soft magnets combined with good mechanical performances and high corrosion resistance make them competitive with their crystalline counterparts in applications as transformers, sensors, transducers, etc. Modern electronic devices such as power supplies and digital equipment for the tele-communications or IT industry demand magnetic cores or inductive components with compact volume, good magnetic properties, and the ability to fabricate in different geometries. Although there are a few limitations of magnetic metallic glasses in applications mainly due to their shape limitation (thin films, melt-spun ribbons with thicknesses usually no larger than 50 m, conventional wires or glass-coated micro-wires, powders), their main advantage is the wide range and the flexibility of compositions. Moreover, in the last decades they have been widely used as precursors for nanocrystalline or quasicrystalline alloys that exhibit remarkable magnetic and me-chanical properties owing to their very complex structure.

In this paper we will describe some of our recent materials developments related to the potential use of amorphous alloys.

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B. Idzikowski et al. (eds.), Properties and Applications of Nanocrystalline Alloys from Amorphous Precursors, 165–176.© 2005 Kluwer Academic Publishers. Printed in the Netherlands.

166 H. Chiriac and N. Lupu

2. BULK AMORPHOUS ALLOYS

Nowadays, there are tremendous efforts to prepare bulk amorphous alloys in different systems, with different geometries and good soft magnetic characteristics for specific applications [1-6], because these materials have a potential to open up the market either by replacing the conventional Fe-Si laminas and melt-spun amorphous or nanocrystalline ribbons or by triggering the development of new devices. Their main advantage consists of the ease of formation in bulk shapes by both casting and powder consolidation methods thanks to their large glass-forming ability.

2.1. Sample preparation

For the processing of bulk amorphous alloys, we prepared master alloys of 5-10 g each by arc melting pure elements (99.99%) in an argon atmosphere. The alloys were re-melted several times for homogenization. Melt-spun ribbons with thicknesses up to 200 m and widths of 2-4 mm were prepared by melt-spinning technique in an Ar atmosphere. Glassy rods with diameters no larger than 3 mm were prepared by suction casting method, whereas glassy rings having internal diameter, external diameter and height of 6, 10 and 1 mm, respectively, were obtained by mould casting method. The master alloy was melted in a quartz crucible using an induction coil system and pushed when melted with a pressure of 0.5-0.8 atm. in a water cooled Cu mould.

2.2. Large glass-forming ability

The ability to fabricate Fe56Co7Ni7Zr6M1.5Nb2.5B20 (M = Zr, Ti, Ta, Mo) bulk amorphous alloys by the conventional Cu mould casting method is settled on the large glass-forming ability of these alloys, i.e. the supercooled liquid regions achieving 80-90 K (Table I) in comparison with 30-40 K as previously observed for conventional Fe-based and Co-based metallic glasses.

Table I. The evolution of the glass transition temperature (Tg), crystallization temperature (Tx) and supercooled liquid region ( Tx = Tx – Tg) on the difference between the main components atomic radii

M = Zr M = Ta M = Ti M = Mo

(rZr – rM)/rM (%) – 7 8 13(rM – rFe)/rFe (%) 27 18 16 10(rM – rB)/rB (%) 63 52 50 42

Tx = Tx – Tg (K) 80 85 85 90

Tg (K) 773 768 733 743 Tx (K) 853 853 823 833

By increasing the difference between the atomic radii of Zr and addition elements (rZr = 0.16 nm, rTi = 0.147 nm, rMo = 0.139 nm), the supercooled liquid region ( Tx) is increased despite of the decrease in the difference in atomic radii between the main components (rFe = 0.126 nm; rB = 0.098 nm) and the addition elements. Consequently,

Design and Preparation of New Soft Magnetic Amorphous Ferromagnets 167

we expect a different magnetic behaviour as a function of the nature of the addition element.

2.3. DC and AC magnetic properties

DC-magnetic measurements were carried out using an extraction DC-fluxmetric method. Primary and secondary coils were wound on the torroids using enamel-coated copper wire. Magnetic characteristics as AC magnetic permeability ( e), electrical resistivity ( RT), saturation induction (Bs) and coercive field (Hc), as well as their variation vs. temperature were measured using an AC fluxmetric method in fields up to 25 kA/m. The saturation magnetostriction constant ( s) was measured by the small angle magnetization rotations (SAMR) method.

The hysteresis losses for a given frequency were determined by the area within the hysteresis loop. They were assumed to be constant with frequency.

The eddy current losses are defined by

fBd

WRT

e

2)ˆ(

where d is the thickness of the sample, B̂ is the peak value of the sinusoidal flux density, RT is the electrical resistivity, is the density, and f is the frequency. is a geometrical coefficient [7]. For sheets of thickness d, 6 , while for rods of diameter d, 16 [8]. For ring shaped samples, is calculated using the following formula [9]:

w

h

h

w58.1tanh663.01

6,

where w and h represent the width and the height of the torroid, respectively. For the Fe56Co7Ni7Zr7.5Nb2.5B20 amorphous torroid with Dext = 10 mm, Dint = 6 mm andh = 1 mm, is 6.1065.

Table II presentes the electrical resistivity and magnetic characteristics of Fe56Co7Ni7Zr6M1.5Nb2.5B20 bulk amorphous alloys.

Table II. Magnetic characteristics and room temperature electrical resistivity as a function of the addition element in amorphous Fe56Co7Ni7Zr6M1.5Nb2.5B20 cast rods and melt-spun ribbons

0Ms (T) Hc (A/m) e (500 Hz) TC (K) RT ( m) s ( 10–6)

M = Zr 1.01 9.5 21500 554 1.51 7 M = Ti 0.89 6.06 18000 560 2.08 9 M = Ta 1.06 6.1 19000 603 1.87 9 M = Mo 1.07 7.43 17000 560 1.76 10

Whereas the saturation magnetization, 0Ms, does not change significantly by the partial substitution of Zr, the effective magnetic permeability, e, decreases drastically. The Curie temperature increases for the alloy containing Ti probably because of the increase in the strength of the interactions between the unfilled 3d (for Ti

168 H. Chiriac and N. Lupu

and Fe) and 4d (for Zr and Nb) electronic shells. This also seems to be the cause for the largest values of the electrical resistivity for the samples with 1.5 at.% Ti. The coercive field, Hc, decreases when 1.5 at.% Zr is replaced with Mo, Ta or Ti. From these results one can conclude that Ti seems to be the most suitable element to improve the softness of the bulk amorphous samples prepared in (Fe, Co, Ni)70(Zr, M, Nb)10B20

multicomponent system. Moreover, the magnetic properties of the amorphous torroids are comparable with

those of FeSiB and CoFeSiB melt-spun amorphous ribbons (Table III), often used as magnetic cores, shielding elements, and inductive components.

Figure 1 presents the variation of the DC magnetic permeability as a function of the applied field for the Fe56Co7Ni7Zr7.5Nb2.5B20 amorphous torroid. DC slowly de-creases by increasing the applied field above 5 A/m, but increases very rapidly when the applied field approaches to zero.

Table III. DC and AC magnetic characteristics in the as-cast state for bulk amorphous Fe-based torroids and amorphous Fe-based and Co-based melt-spun ribbons

Sample / Geometry / Thickness 0Ms (T) Hc (A/m) DC (105) AC, 5 00 (104) AC, 1 0k (104)

Fe56Co7Ni7Zr7.5Nb2.5B20

Torroid/1 mm 1.01 1.44 1 2.15 0.56

Fe77.5Si7.5B15

Melt-spun ribbon/30 m1.56 3.2 – – –

Co68.25Fe4.5Si12.25B15

Melt-spun ribbon/30 m0.76 0.48-160 – 1 1

AC, 5 00 is the relative magnetic permeability at 500 Hz, whereas AC, 1 0k represents the relative magnetic permeability at 10 kHz

Figure 1. DC-magnetic permeability vs.applied field for Fe56Co7Ni7Zr7.5Nb2.5B20

amorphous torroid

Figure 2. Magnetic permeability as a func-tion of the AC-applied field frequency, f,for Fe56Co7Ni7Zr6M1.5Nb2.5B20 (M = Zr, Ti, Ta, Mo) bulk amorphous torroids

The large values of both DC and AC magnetic permeability measured for Fe56Co7Ni7Zr6M1.5Nb2.5B20 (M = Zr, Ti, Ta, Mo) bulk amorphous alloys in the as-cast state are strongly related to the large electrical resistivities of 180–230 10–8 m, in comparison with 110 and 150 10–8 m for FeSiB and CoFeSiB melt-spun amorphous ribbons. Coercive fields (Hc) in the as-cast state are below 8 A/m, whereas the saturation induction (Bs) reaches 0.9-1.2 T for Fe56Co7Ni7Zr6M1.5Nb2.5B20 (M = Ti,

Design and Preparation of New Soft Magnetic Amorphous Ferromagnets 169

Ta, Mo) amorphous torroids. The Curie temperature varies between 550 and 605 K depending on the addition element (M), that being a very important point to be considered when defining new materials for applications.

The frequency dependence of the AC-magnetic permeability for Fe56Co7Ni7Zr6M1.5Nb2.5B20 bulk amorphous alloys is presented in Fig. 2. The magnetic permeability of Fe56Co7Ni7Zr7.5Nb2.5B20 amorphous torroid increases by about 50% by decreasing the frequency from 500 Hz to 5 Hz and decreases about 4 times when the frequency increases to 10 kHz. The partial substitution of Zr results in a less pronounced frequency variation of the magnetic permeability, indicating these bulk amorphous alloys with additions as suitable candidates for frequency applications.

The decrease of the permeability with the increase of the frequency is especially due to the classical eddy current losses and anomalous (excess) eddy current losses. The hysteresis losses also contribute to the deterioration of the magnetic characteristics vs. frequency, but their contribution is significantly smaller (Fig. 3). The most signi-ficant increase of the classical eddy current losses is observed for the amorphous torroids containing Zr, for which the hysteresis losses are also the largest. Consequently, the magnetic permeability of Fe56Co7Ni7Zr7.5Nb2.5B20 amorphous torroid decreases more rapidly in comparison with the magnetic permeability of the Fe56Co7Ni7Zr6M1.5Nb2.5B20

(M = Ti, Ta, Mo) bulk amorphous torroids. The eddy current losses are smaller when adding Ti and Ta in comparison with the ring-shaped samples containing Zr or Mo, but they are comparable with those reported in the literature for other metallic glasses with large glass-forming ability [9, 10].

Figure 3. Frequency dependence of hysteresis losses (Wh) and classical eddy current losses (We) for the as-cast Fe56Co7Ni7Zr6M1.5Nb2.5B20

amorphous torroid measured at f= 50 Hz and maximum flux density T5.0B̂

Figure 4. The hysteresis loop shape dependence on the treatment, for Fe56Co7Ni7Zr7.5Nb2.5B20 melt-spun amor-phous ribbons, 120 m thickness

Thermal and thermomagnetic treatments may strongly affect the magnetic properties of amorphous alloys, especially the shape of the hysteresis loop, the magnetic permeability and the coercive field. The modification of the shape of the hysteresis loop after thermal and thermomagnetic treatments at relaxation temperature (748 K) for Fe56Co7Ni7Zr7.5Nb2.5B20 thick amorphous ribbons (120 m) are shown in Fig. 4. The di-rection of the applied magnetic field is relative to the long axis of the ribbon. One

170 H. Chiriac and N. Lupu

observes that the rectangularity of the hysteresis loop as well as the saturation magnetization and coercive field are changing with the direction of the applied magnetic field. The thermal treatment without magnetic field, just below glass transition temperature (Tg = 773 K), leads to a slight increase in the coercive field and to the elongation of the hysteresis loop. The presence of an external magnetic field results in better magnetic properties leading to the magnetic softness of the amorphous samples. The best response is obtained when the applied field is parallel to the long axis of the ribbon because of the easy axis induced on the parallel direction with the applied field. When the external field is perpendicular to the ribbon long axis the induced anisotropy is smaller owing to the competition of the magnetoelastic anisotropy induced during the melt-spinning preparation process and oriented parallel with the long axis. Consequently, the coercive field increases. Further experimental work along this line is expected in the near future.

3. MAGNETIC AMORPHOUS WIRES

In addition to bulk amorphous alloys with soft magnetic properties, the low dimension soft magnetic amorphous materials also represent a very exciting field of research for both fundamental studies and applications. In this category, amorphous wires and glass-covered microwires, and very recently amorphous nanowires are extremely important.

3.1. Amorphous wires and glass-covered microwires

Amorphous wires with diameters up to 200 m and glass-covered amorphous metallic wires having diameters of the metallic core of 1 to 80 m and the glass cover thickness of 1 to 20 m, with positive, negative, and nearly zero magnetostriction, have been extensively studied over the last years, and their magnetic properties have been found to be extremely attractive for sensors applications [11]. Their specific magnetic properties are strongly dependent on composition through the manetostriction value, on the magnitude and the direction of the induced anisotropies either during the preparation process or by annealing, on the magnetic permeability, on their diameter and on the internal and superficial stresses distribution. In the following we will focus on the influence of the composition and dimensional characteristics on the magnetic properties, mainly on the GMI effect, of nearly zero magnetostrictive Co-based amorphous wires and glass-covered wires

3.1.1. Preparation

Amorphous wires with nominal compositions Co72.5–xFexSi12.5B15 (x = 5.5-6.2) were obtained by two different methods, namely the in-rotating water spinning and the glass-coated melt-spinning [11], at the National Institute of Research and Development for Technical Physics of Iasi, Romania. The diameter of the conventional amorphous wires (CAW) ranges from 90 to 150 m, whereas the glass-covered amorphous wires (GCAW) consists of a metallic core having diameters between 15 and 80 m, coated by a glass cover ranging from 5 to 20 m. The saturation magnetostriction constant ( s)

Design and Preparation of New Soft Magnetic Amorphous Ferromagnets 171

changes the sign from negative ( s = –0.23 10–6 for x = 5.5) to small positive ( s = +0.05 10–6 for x = 6.2) values when the Fe content increases, passing through zero for x ~ 6.0.

3.1.2. The influence of the composition and microwires diameter on the GMI effect

GMI measurements were performed in the high frequency range (100 kHz-10 MHz) for driving AC currents up to 5 mA using a digital oscilloscope coupled with a computer, which allowed automatic frequency control, data acquisition, and process-ing. For comparison, we carried out GMI measurements on CAW and GCAW having different compositions and saturation magnetostriction constants. Additionally, we investigated the influence of the cold rolling process on the GMI response of the CAW.

GMI effect in low magnetostrictive amorphous wires is mainly influenced by two factors: the specific circumferential magnetic domain structure and dynamic magnetization processes [12].

Iac passes only through a superficial layer of the wires, owing to the skin effect that occurs at frequencies of the order of MHz. The depth of this layer is determined by the magnetic penetration depth m, given by

m ,

where is the electrical resistivity, the frequency, and the circular permeability. The circular magnetization is driven by the circular AC field created by Iac while m isusually of the order of several micrometers at megahertz frequencies.

The glass cover induces an additional anisotropy, which strongly influences the magnitude of the GMI effect in low magnetostrictive amorphous glass-covered wires [13]. From the point of view of sensing applications, the amorphous glass-covered wires are often preferred against the conventional amorphous wires due to their more reduced dimensions, i.e. the diameter of the metallic core varies from a few micrometers to a few tens of micrometers [11, 14].

The axial DC field dependences of the impedance ( = 10 MHz) for Co72.5-xFexSi12.5B15 (x = 5.5; 5.95; 6.0; 6.2) CAW and GCAW are illustrated in Fig. 5. It is important to note that the GMI curves exhibit only one central peak for positive magnetostrictive GCAW with x = 6.2, independent of the frequency [15], due to the non-formation of the circumferential magnetic domains structure in the positive magnetostrictive glass-covered amorphous wires [16]. The impedance shows two maxima whose positions change to high dc fields with the frequency increase, for negative magnetostrictive GCAW (x < 6) [15]. This displacement is mainly caused by the supplementary induced anisotropy as the effect of the negative magnetostriction constant. For nearly zero magnetostrictive composition (x = 6) the GMI behaviour is very sensitive to the ac field frequency. The two maxima on the impedance curves correspond to the anisotropy field, Hk.

It is worthwhile to note that the best response of the GMI effect is obtained for x = 5.95 in the case of GCAW and at x = 6 for CAW. This different behaviour, which is

172 H. Chiriac and N. Lupu

strongly dependent on the saturation magnetostriction constant value, is caused by the variation of the penetration depth of the ac current, i.e. the magnetic domain structure and the existence or non-existence of the closure surface magnetic domains in GCAW.

Figure 5. GMI curves for CAW and GCAW with different compositions and saturation magnetostriction constants measured at 10 MHz

The influence of the metallic core diameter on the GMI effect for both CAW and GCAW with the nominal composition Co68.18Fe4.32Si12.5B15 (x = 5.95) is presented in Fig. 6. The GMI response monotonously decreases with the increase of the metallic core diameter.

Figure 6. DC field dependence of the impedance for Co68.18Fe4.32Si12.5B15 CAW and GCAW as a function of the metallic core diameter ( = 10 MHz)

This effect is ascribed to the decrease of the volume of the external circumferential magnetic domains with respect to the volume occupied by the axially magnetized metallic inner core. The GMI effect diminishes much stronger for GCAW because the volume occupied by the axially magnetized central magnetic single domain is comparable with the volume occupied by the circumferentially magnetized magnetic domains on the surface.

The GMI response is strongly affected by the type of treatment applied on the conventional wires, and especially if cold rolling treatments are present, as shown in Fig. 7. The cold rolling process modifies the distribution of the stresses induced in

-2000 -1000 0 1000 20000

20

40

60

80

100

120

140

160

180

GCAW

tg = 7 m

d = 30 m

d = 40 m

d = 50 m

d = 60 m

d = 70 m

d = 80 m

Imp

eda

nce

()

Hdc

(A/m)

-2000 -1000 0 1000 200020

40

60

80

100

CAW

d = 90 m

d = 100 m

d = 110 m

d = 120 m

d = 130 m

d = 140 m

d = 150 m

Impedance,

Z (

)

Hdc (A/m)

Design and Preparation of New Soft Magnetic Amorphous Ferromagnets 173

Figure 7. DC field dependence of the impedance for Co68.18Fe4.32Si12.5B15 CAW as a function of the applied treatment ( = 10 MHz)

the superficial layer during the preparation. Consequently, the GMI curves are broad-ened. The reduction of the diameter to 100 m results in the increase in the GMI response and the broadening of the field dependence (Fig. 7a). This increase is even much more pronounced when subsequent thermal treatments are applied. The super-ficial polishing of the conventional wire combined with specific thermal treatments give rise to a supplementary increase of the GMI response, but narrow the field dependence. The reduction of the diameter to 25 m by cold rolling (Fig. 7b) leads to the increase of the GMI response of about 3 times and the important broadening of the field depend-ence, which can be partially reduced by choosing a suitable combination of thermal and thermomechanical treatments.

3.2. Nanowires

Nowadays, arrays of magnetic nanowires are widely investigated in order to understand their magnetic behaviour and to estimate their potential use in applications such as patterned recording media, nano-sensors and nano-devices [17, 18].

3.2.1. Samples preparationIn this work, arrays of about 200 nm diameter amorphous Ni100–xPx and Co100–xPx

nanowires were obtained by electrodeposition into the nanometre-sized pores of tracketched polycarbonate or anodic alumina oxide (AAO) membranes using a two-electrode electrochemical cell. The pore diameters and the thickness of both the polycarbonate and AAO membranes were 200 nm and about 6 µm, respectively. Prior to the electrodeposition of NiP and CoP nanowires in polycarbonate or AAO membranes, a 800 nm Ag film was deposited by thermal evaporation on the membrane surface to act as a substrate and working electrode [19]. The metallic layer was insulated from electrolyte solution by a special insulator film. Thus, the metallic layer used as a cathode was in direct contact with the electrolyte through the membrane pores. A platinum wire was used as an anode. The current densities ranged between 0.1 and 0.6 mAcm–2.

For comparison, Co80P20 amorphous thin films having 200 nm in thickness were electrodeposited on alumina substrates covered with a thin layer of Cr/Ag deposited in the process of vacuum evaporation.

174 H. Chiriac and N. Lupu

3.2.2. Structure and morphology

Figure 8 shows the X-ray diffraction patterns of the electrodeposited Ni80P20

nanowire arrays and of the AAO membrane, on which the Ag thin film was deposited, respectively. The presence of just one broad peak onto the X-ray diffraction pattern of the nanowire arrays and no additional sharp peaks are features characteristic of the amorphous phase.

Figure 8. X-ray diffraction data obtained from the electrodeposited Ni80P20 nanowire arrays

Figure 9. SEM micrograph of electrodeposited Co80P20 200 nm diameter nanowire arrays

The morphology of the electrodeposited Co80P20 200 nm diameter nanowire arrays, obtained after the dissolution of the AAO membrane in chloroform, is shown in Fig. 9.

3.2.3. Magnetic results

The reduced hysteresis loops as well as the temperature dependence of the reduced magnetiation were carried out using a vibrating sample magnetometer (VSM) in a maximum applied field of 1260 kA/m. The measurements were conducted on the as-deposited samples and with the external field applied in the plane.

The magnetic properties of the amorphous NiP and CoP nanowire arrays are strongly dependent on the P content, i.e. electrodeposition conditions. Figure 10 presents the hysteresis loops of the electrodeposited Co80Fe20 and Ni100–xPx 200 nm diameter nanowire arrays as a function of P content. The data for Ni100–xPx nanowire arrays were normalized to pure Ni magnetization. The coercive field value of about 5 kA/m measured for the in-plane field direction for amorphous Co80P20 nanowire arrays was comparable with the ones published recently on amorphous CoP 100 nm cylinder arrays [20].

Significant differences in the coercive field were observed depending on the samples morphology and the direction of the external field, as shown in Fig. 11. While the coercive field reaches 0.2 kA/m for Co80P20 amorphous thin films with the field parallel on the film plane, the 200 nm nanowire arrays show coercive fields of 1.5 kA/m when the external field is parallel to the long axis of the nanowires. This behaviour is given by the presence of magnetostatic interactions between nanowires, as proposed previously [21].

Design and Preparation of New Soft Magnetic Amorphous Ferromagnets 175

-100 -50 0 50 100-0.50

-0.25

0.00

0.25

0.50

H parallel

Co80

P20

thin layer

nanowire arrays

Ma

gn

etic m

om

en

t (e

mu

)

H (kA/m)

Figure 10. Magnetic hysteresis loops of electrodeposited Ni100–xPx and Co80P20 (inset) 200 nm diameter nanowire arrays

Figure 11. Magnetic hysteresis for both sample morphology in two applied field direction for Co80P20 amorphous thin film and nanowire arrays

Figure 12. Temperature dependence of the reduced magnetization of electrodeposited Ni100–xPx 200 nm diameter nanowire arrays

The temperature dependence of the reduced magnetization as a function of P content in Ni100–xPx nanowire arrays is presented in Fig. 12. Both samples containing 10.56 and 16 wt% P are amorphous, but the increase of the P content over 15 wt.% diminishes the ferromagnetic behaviour of NiP amorphous alloys. Consequently, the magnetization and Curie temperature decreased, whereas the coercive field increased. However, the value of Hc is still below the one of the Ni electrodeposited nanowire arrays. Thus, the magnetic properties of the electrodeposited NiP and CoP nanowire arrays are very sensitive to the P content, i.e. the electrodeposition conditions.

176 H. Chiriac and N. Lupu

ACKNOWLEDGMENTS

This work was supported in part by the Romanian Ministry of Education, Research and Youth – Department of Research under Contract CERES 3/15.10.2001.

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