The effect of oxidation on magnetic properties of amorphous Co75.26−xFe4.74(BSi)20 + x alloys

MAPERIALS SCIENCE d!k ENCIMEERINC ELSEVIER

B

Materials Science and Engineering B40 (1996) lo- 18

The effect of oxi’dation on magnetic properties of amorphous CO 75.26-~~~4.74

(BSi)20+x

a11oYs

C.K. Kim Depnrtment

of M~tetYcrls

Scimce

rind Engineering,

Massnchrsetts

Institute

of Technology,

Cm&itlge,

hf.4

0213.9, us,.i

Received 1 August 1995

Abstract A comparative oxidation study of several amorphous Co,,,, _ x Fe,,,, (BSi),, + x alloys was carried out. A reentrant magnetization behavior and field-induced ani:otropy were obtained after oxidation of the amorphous Co-rich ribbons. During this. oxidation, the ribbons were found to develop surface oxides which are primarily non-magnetic borosilicate or a combination of borosilicate and magnetic oxides such Co0 or FeO. Beneath this lies about lOO- 1000 8, thick Co-rich magnetic alloy which may be either hcp or fee in its crystal structure. The thickness of Co crystallized layer is determined by the type of the surface oxides. The oxidation products such as surface and internal oxides (oxygen impurity faults) appear to affect magnetization behavior of Co rich amorphous alloys significantly. We have determined the amount of metalloids (a critical concentration) which is necessary to form a continuous layer of the most thermodynamically stable oxide, in our case borosilicate, on the surface. We also observed that there is a good correlation between reentrant magnetization and thickness of Co layer. The best reentrant M-H loop was obtained in ribbons with a surface borate-rich borosilicate since this ensures the conditions such as (1) metalloid depletion in the substrate and (2) formation of oxygen impurity faults in Co grains, that are required for strong reentrant magnetization behavior. Keyrvordst

Surface oxide; Internal oxidation; Reentrant magnetization

1. Introduction Cobalt-rich amorphous glasses (CoFeBSi system) which have almost zero magnetostriction have been of central interest due to their soft magnetic properties and therefore technological application to the field of electronics [1,2] and magnetic recording heads [3-51. The application of cobalt-rich amorphous materials sometimes requires a magnetic annealing below the crystallization temperature. During a low temperature heat treatment, the magnetic property changesin amorphous alloys may occur due to either crystallization of the substrate or surface oxide formation. These microstructural changes will affect soft magnetic properties due to induced crystalline or magnetoelastic anisotropy [6]. Above a crystallization temperature, it is widely believed that metallic glasses undergo a homogeneous devitrification transformation. This process has been categorized as either primary, eutectic or polymorphous crystallization

[7,8]. There

is also some recognition

the literature that the crystallization

in

process can be

accelerated well below the bulk crystallization temperature by oxidation of the surface which changes the sub-surface composition and thus alters its crystallization kinetics [9,10]. Although abundant works have been reported on the magnetic property changes in Co-rich amorphous alloys in terms of crystallization [l 1- 151,relatively little information has been reported from the viewpoint of oxidation [16]. Recently, we observed reentrant magnetization behavior in annealed (oxidized in air) Co-rich amorphous alloys [17,18]. It was found that this behavior is attributable to a domain wall configuration that remains pinned for increasing values of applied field up to the threshold value (pinning field: Hpinning) at which the pinned condition is disabled causing a step change in flux. A schematic description of domain wall configurations observed by Kerr magneto-optical microscopy and a resultant reentrant hysteresis loop are shown in Fig. 1. During this observation, we found that there is a good correlation between the magnitude of pinning field and the nature of surface oxides. For example, 0921-5107/96/S15.00 0 1996 - Elsevier Science S.A. All rights reserved

C.K.

Kim / Maierials

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B40 (1996) METGLAS

ribbons with a borosilicate surface oxide after annealing showed a strong pinning. In contrast, cobalt oxide formation on the ribbon surface was associated with a very weak pinning, In addition, we observed that the nature of surface oxide can be controlled either by alteration of substrate metalloid (B and Si) concentrations or ribbon casting conditions. Therefore, in this paper magnetization behavior of Co-rich amorphous materials is described from the viewpoint of oxidation.

lo-

18

270%

Coercivitv

Test (425%

1kHz)

20.5

2. Experimental

details 201 0

Amorphous Co,,,,, _ x Fe,,, (BSi),, tx alloys with various compositions were made by planar flow casting at the Metglas Products Division of Allied-Signal Corporation. Compositions were verified by chemical analysis with X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS). These ribbons are characterized by near-zero magnetostriction (1~ 1 x 10 - 6, and high d.c. permeability as the as-cast state. Coercivities of ribbons with various compositions to reach 1 Oe at 425 “C were measured (Fig. 2). Differential scanning calorimetry indicates that the onset of crystallization occurs at 465 “C and maximum peak appears at 535 “C. Ribbons with a cross-section of 20 pm x 3 mm were cut to a length of 40 mm. Then two step annealing (first

Demagnetized State

Fig. 1. Schematic illustration of the domain configurations in a ribbon with a pinned wall loop for various points on the magnetization curve. 0: demagnetized state; E: immediately after field exceeds wall pinning threshold (Hpinning); F: near saturation; A: field decreasing toward demagnetized state; 0: demagnetized state; B: immediately after negative reentrant reversal; C: negative saturation; and D: approaching demagnetized state from negative saturation. Wall pinning threshold is indicated as HpinninE.

I I 5

I 110.8 10 15 20 Time (min.) to reach 1 Oe 0 (B+Si) Content (at%) 0 B/(B+Si)

25

Fig. 2. Coercivity test: metalloid content vs. time to reach 1 Oe at 425 "C.

and second) was carried out. Samples were first annealed for various time (3 min-7 h), temperature (250410 “C) and atmosphere in the presence of a 60 Oe longitudinal magnetic field that saturated them along their length. Maximum annealing temperature was set well below the crystallization temperature, 465 “C. Then, the field induced anisotropy was measured by hysteresis loop tracing. Next, the second annealing was performed at temperatures as low as 320 “C in a field free environment right after the first annealing at 380 “C for 30 min with a 60 Oe longitudinal field. After the second annealing, the reentrant magnetization behaviour was obtained. Thermogravimetry was initially performed with Netzsch STA 409 for the study of oxidation kinetics. However, no change of mass was observed for material annealed up to 60 min at 380 “C due to its thin gauge of specimen (thickness: 18 pm; weight: 17 mg). Therefore, Auger depth profiling was conducted to measure the thickness of oxide and Co crystallization depth. Resistivity measurements using a RLC meter at 1 GHz were also made to determine the crystalline cobalt layer thickness after annealing. Then the resistivity data were matched with the Co crystallization thickness determined by Auger depth profiling. Historically, many studies of crystallization, relaxation and field annealing of Co-rich amorphous alloys are based on macroscopic techniques such as differential scanning calorimetry (DSC), X-ray scattering, resistivity measurement, magnetometry, and dilatometry [19-221. Consequently, such macroscopic characterization techniques often provide obscure or even misleading information since they average over structural features in three dimensions. For the purpose of detailed microstructural observation, transmission electron microscope (TEM) studies are unique in allowing the structural morphology, the

12

C.K. Kim / Materials Science rind Engineering 840 (1996) lo- 18

crystal structure and chemical composition of small regions of the samples down to the microscopic level (50 A) to be determined simultaneously. We have used TEM (model: JEOL 200 CX and Akashi 002B operating at 200 keV) to obtain direct microstructural evidence of the oxidation and crystallization in amorphous Co-rich alloys. STEM observatio:l was made to obtain local compositional information down to 20 8, scale using a VG HB5 FEG-STEM with LINK LZ-5 X-ray detector. Auger electron spectroscopy (AES) was carried out to measure the depth of surface crystallization, oxide thickness and a surface chemistry, using a PerkinElmer 660 scanning Auger microprobe.

A. Insufficient amount of B and Si (< 21% ) to form a continuous layer of borosilicante

I I

Co-Fe,B ,Si Alloy

Internal Oxidation

I I Co crystallites

B. Sufficient amount of B and Si (~21% ) to form a continuous layer of borosilicate

I

Co-Fe,B ,Si Alloy

Fig. 3. Schematic illustrationof oxidationbehaviorof Co-richamorphousalloys.

3. Results and discussion 3.1 e Oxidation alloys

forehand to control magnetic properties of Co-rich amorphous alloys since resultant oxidation products will affect magnetic behavior significantly.

mechanisms on Co -rich amorphous

The formation of continuous borosilicate layer (rich in borate in this study: B,O, depends on the concentration of B and Si and the diffusion of oxygen. This means that a continuous layer of borosilicate is formed at and above a certain concentra!ion of B and xi, This content is known as the critical minimum concentration which has been discussed in detail by Wagner [23,24]. When the affinity of element A for oxygen is much higher than that of element B, the minimum (critical) concentration for exclusive formation of the most thermodynamically stable oxide of element A is obtained as:

XA(min)

=

Y3,oy 162

nk, D A (

“2 (1) dl0y

1

where V;,;,,,: molar volume of alloy; Z,: valence of element A; k,: oxidation rate constant; and Dalloy: diffusivity of A in the alloy. This equation suggests that the critical concentration is a function of temperature and ambient partial pressure of oxygen, since oxidation rate constant and diffusivity depend on them. When the concentration of the alloying element A (boron and silicon) in substrate is higher than XA(min), i.e. above the critical concentration, the surface oxide formed during oxidation of Co-rich alloys is a non-magnetic borosilicate. When the total metalloid concentration is below .the critical concentration, instead of a continuous layer of borosilicate, the surface oxides consist of borosilicate and magnetic oxides such as COO, Fe0 or Fe,O, which are less thermodynamically stable than borosilicate. A schematic description of oxidation behavior of Co-rich amorphous alloys is given in Fig. 3. Therefore, it is of central importance to determine the critical concentration be-

3.2. Experimental detemimtion of the criticd concentraiorz in Co-kh ar~oi~photcs alloys Amorphous CO~~.,~_ x Fe,,,, (BSi)20 + x alloys with four different compositions (CO,~,~~Fedc,, Si2,, B13.5, co 74.26 Fe4.74 si2.1 B18.9, co74.76 Fe4.74 si2d B18.1, co75.26 Fe,,,, Si2,3 B,,,,) were chosen to investigate the type of surface oxide during oxidation. From experimental results, the critical concentration of B and Si is determined as 21% of B and Si. We observed that co 73.66 Fe,.,, Si2., b.5 and Co74.26 Fed,74h B,s.~ oxidized to form a borosilicate on the surface (Fig, 4a) while surface oxldes of CO,~,~~Fe,,,, SL4 B ,s.I and co 75,26 Fe,.,, Si2., J3,7.7consisted of cobalt oxides and small islands of borosilicates (Fig. 4b). Consequently, at least 21% of B and Si is necessary to form a continuous layer of borosilicate during oxidation. 3.3. Intemd

oxidatioil

Internal oxidation is the process by which oxygen diffuses into an alloy and causes sub-surface precipitation of oxides of alloying elements at appropriate locations [25]. We observed two types of internal oxidation in this study. When Co0 forms on the surface, B and Si segregate to the Co grain boundaries, which provide the energetically favorable sites for these solutes. In this case, internal oxidation occurs at the Co grain boundaries since oxygen also penetrates along Co grain boundaries. Oxide formation at these sites results in the formation of a thin layer of borosilicate which surrounds the crystallized Co grains (Fig. 5). Another type of internal oxidation is observed when a borate-rich borosilicate surface oxide forms. This type of internal oxidation is clearly favoured by a surface

C.K. Kim

1 Materials

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and Enginecrhg

B40 (1996)

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13

are either fee or hcp Co-rich phase. These crystallites are highly faulted when the metalloid content is rich in boron (CO~~,~~Fe4,74B,, (designated as Si-0), CO,~,~~ Fe,,,, Bis,9 (Si-2)) (Fig. 6a) and less faulted or unfaulted when the metalloid content is rich in Si (CO~~,?~ b.,, Si,,., B PA (Si-13)) (Fig. 6b). The nature of these faults turned out to be oxygen impurity faults. The detailed explanation regarding the structure of oxygen faults was published elsewhere [26]. These different densities in oxygen impurity faults appear to result from the difference in the porous nature of the surface oxide formed during annealing. Electron diffraction patterns obtained from these oxides (which are Fourier transforms of local spatial atomic correlations) indicated that they retain amorphous structures, but with different packing densities. In the case of Si-0 and Si-2 ribbons, the oxide is largely B203 type and its diffraction pattern indicates only weak atomic correlations, i.e. it is highly disordered and porous. In contrast, for the Si-13 ribbons, the oxide is predominantly SiOZ with a compact structure showing more short range order in its electron diffraction pattern. It is well known that Si02 is a protective oxide, i.e. a barrier to further diffusion. Therefore, the diff%Aty of oxygen penetration to the Si-13 ribbon may be responsible for the observed fault-free hcp Co structure (which is the stable structure of pure cobalt at room temperature). Si-0 and Si-2 ribbons which oxidize to the porous BzO, phase contain larger oxygen concentrations in the

(b) Fig. 4a. TEM bright field image of the amorphous borosilicate oxide formed on the crystallized substrate CO,~,?~Fe,.,j Si,., B,,,g during annealing at 380 “C for 90 min. Fig. 4b. TEM bright field image of the crystalline cobalt oxide and amorphous borosilicate oxide formed on the crystallized substrate CO,~,~~F~~,,~S~~,~B,,,, during annealing at 380 “C for 90 min. Borosilicate islands are indicated by arrows.

oxide which is permeable to oxygen. The crystallization products we observed under the surface borosilicate

Fig. 5. Crystallized Co substrate (Co,,,,, Fe,,,,$, B,,,,) annealed at 400 “C for 12 min. The micrograph shows almost fault-free Co crystallites and internal oxidation around them. Internal oxidation is indicated by arrows.

C.K. Kim / Materials Science and Engineering B40 (1996) IO- 18

annealing. Upon cooling, the introduced oxygen stabilizesthe fee structure against transformation to the hcp phase and condensesas oxygen impurity faults in fee {ill} planes. The effect of internal oxidation (oxygen impurity faults) on magnetic properties will be discussedlater.

Ribbons with compositions (CO~~.~(, Fe,.,, Si2,1B,9,5, co 74,,6Fe,,,, S&, B,,.,) were made with oxygen gas flowed around the nozzle during a casting. Interestingly, the Co0 was formed although B and Si content are above the critical content. It can be explained in terms of the altered critical concentration due to increased oxygen pressure [27]. According to Birks et al. [28], the critical content given previously can be alternatively expressedas

(4

where g*: volume fraction of oxide; y: stoichiometric constant from oxide AOy, Do; D,: diffusivity of oxygen and element A, and V: volume. According to this equation, conditions which increase the inward flux of oxygen (increase Xgr”‘cc by flowing oxygen) will allow the formation of external borosilicate at higher metalloid concentrations such as B t Si = 22 or 23%. Consequently, oxidizing gas during a casting favours Co0 formation during oxidation (Fig. 7).

(b) Fig. 6a. TEM bright field images of co 74,26Fe,,,, Si,,, B,,,, (identified as Si-2 at 380 “C for 60 min. Fig. 6b. TEM bright field image of co 74,26Fe,,,, Si,,,, B,,, (identified as Si-13 at 380 “C for 60 min.

the crystallized substrate material) during annealing the crystallized substrate material) during annealing

crystallized sub-surface layer. This is confirmed by STEM analysis. Oxygen enters the hightemperature-stable fee phase through porous B,O, during

To probe long term magnetic properties during the service, low temperature oxidation (accelerated aging) experiments were conducted on a sample. (Co74.26 h74 Si2,,B,,,,) cast under standard inert gas atmosphere for l-62 h at 60 “C and 90% relative humidity. We observed that the samples aged under these conditions showed extensive amorphous Co0 formation for aging times from 5 h up to 62 h although the substrates remain in an amorphous state. Amorphous Co0 formed during acceleratedaging developed into crystalline Co0 after subsequent annealing above 380 “C. Magnetically, the accelerated aged samples exhibit a significant loss of pinning behavior. The loss of threshold depends on the time span of accelerated aging, i.e. specimenswhich underwent longer accelerated aging before annealing, exhibit more loss of threshold. Therefore, these accelerated aging experiments suggest that the substantial amount of Co0 formation appears to be responsible for the loss of

C.K. Kin? 1 Materials Science and Engineering B40 (1996) lo-18

Reaction

1.5

Coordinate (a)

rc I thermodynamic driving force

c activation e rgy control (kinetic contml)

Tin-M

c

COntrol

(b)

Fig. 7. Crystalline coo formed on the substrate co 73.66 Fed.74%, B19.5 during annealing at 400 “C for 12 min. This ribbon was cast with oxygen flowed around the nozzle.

pinning threshold. More detailed explanation for the cause of threshold loss will be given later. Another observation is that the aged samples generally have less faulted crystallites. This can be attributed to the growth mechanism of Co0 which is quite different from that of borosilicate. It is known that Co0 grows via cation outward diffusion through oxide layer [29] while inward oxygen diffusion through oxide layer appears to be responsible for borosilicate growth [30]. Consequently when borosilicate forms, the mobile species through oxide are oxygens. These oxygen atoms penetrate the porous borate-rich borosilicate layer and the oxygen is indeed supplied to the Co substrate resulting in oxygen faulted structure. In contrast, Co cations move outwardly through the oxide in the case of Co0 formation. Therefore, most oxygen is consumed to form Co0 at the oxide/air interface leading to the less-faulted Co strcture due to reduced supply of oxygen. It should be questioned why accelerated aging over a long time leads to the formation of Co0 instead of borosilicate. During aging the dominant oxidation mechanism appears to be changed from borosilicate formation to Co0 formation with increasing time. It can be explained in terms of the competition between thermodynamical driving force and activation energy (Fig. 8). At the initial stage of the aging, borosilicate formation is expected since the thermodynamic driving force for borosilicate (AGO at 25 “C; kcal/mole of O2

Fig. 8a. Schematic illustration of the variation of free energy for borosilicate and cobalt oxide formation. Fig. 8b. Schematic oxide growth rate showing that Co0 outgrows borosilicate with increasing time.

consumed, B,O,: -188 kcal, SiO,: -204 kcal, COO: -102 kcal) is substantially larger than that of COO. However, once oxide is formed, its growth rat’e is governed by kinetics. Since the activation energy barrier of Co0 growth is much smaller than that of borosilicate (Fig. 8a), Co0 eventually outgrows borosilicate (Fig. 8b). In other words, the growth rate of Co0 is fast while that of borosilicate is slow. 3.6. The effect of irztemal oxidation (oxygen impurity faults) on magnetic properties of Co-rich amorphous alloys

There is a correlation among the density of oxygen faults, field induced anisotropy and reentrant magnetization behavior. The detailed report on the relation between magnetic properties and microstructures will be found elsewhere [18,31]. The field induced anisotropy which developed during magnetic annealing is roughly proportional to the density of faulted crystallites of the surface region [31]. This observation is consistent with one in Perminvar and N&Fe made by Nesbitt et al. [32-351. There is also a relationship between field induced anisotropy and wall pinning. The samples which exhibited stronger field induced anisotropy after the first annealing resulted in more consistent pinned wall switching after the second annealing. The best reentrant magnetization behavior was observed when the ribbon surface consisted of highly

16

C K. Kim /j Marekds

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J/m3, Permalloy K , z lo? J/m” or amorphous Co-rich alloys K, 5 10 J/m”). Another possibility is that oxygen impurity faults give rise to breaking a cubic symmetry of a Co lattice in (111) direction. Consequently, suppression of a long range ordering of Co atoms may favor local pair ordering in the field direction. Similar mechanisms were reported by Chikazumi et al. in Ni,Fe and Fe,Al superlattices [36,37] and by Chicn et al. in Pt,Co superlattice [38].

Fig. 9. TEM bright Beld image of the crystallized substrate COXES h7, % BIT,, during annealing at 400 “C for 12 min. Microstructural variations of faulting and internal oxidation are associated with the surface oxide variation. The boundary between faulted and less-faulted region is indicated by arrows.

oxygen faulted cobalt grains. Nesbitt et al. reported that constricted hysteresis loops found in Perminvar were related to oxygen impurity faults [32]. Sometimes, we observed localized microstructural variations of faulting and internal borosilicate in a specimen (Fig. 9). They are ascribable to the local variation of surface oxide. Magnetically, the specimens with non-uniform faulting of Co crystallites exhibit unstable pinning behavior (bimodal distribution of pinning field). Three kinds of M-H loop were observed in this study and the difference in surface oxide formation appears to be responsible for them (Fig. 10). It is tempting to suggest that these oxygen impurity faults may form on those { 11 l} planes which bear a particular orientation relative to the direction of the magnetization; however, there is no evidence of this yet. The details of how the oxygen faults select field induced anisotropy axis are unclear. We speculate that there are two possible contributions from these oxygen impurity layers. The atomic arrangement near an oxygen fault plane may resemble that near the { 111) oxygen plane of antiferromagnetic Coo. Or a 111 I} oxygen fault plane may be locally coordinated like a { 11 l} oxygen plane in ferrimagnetic Co,O, spine1 ferrite. This local spinel-like planar defect may have stronger magnetic anisotropy energy (for cobalt ferrite, K, 2 2.9 x lo5 J/m3) than the parent alloy (for Perminvar, K, z 10”

When the content of B and Si is above the critical concentration, metalloids oxidize first since metalloid elements (B and Si) have higher free energy of oxide formation than metals. Consequently, the dominant mechanism for oxidation appears to be the selective oxidation of I3 and Si at the surface leading to depletion of the underlying amorphous alloy of metalloids critical to glass stability. In other words, this reaction lowers local crystallization temperature beneath the oxide and leads to crystallization of this unstable amorphous alloy at temperatures well below the bulk crystallization temperature, 465 “C. This has been confirmed by in situ

6

0.7 - 0.8 oc Pinning field

H

0)) 4c Iimodal distribu&~ mfpinning field : Jnstnblcpinning 7

(c>

L

f lelow0.4Oe

H

7 Fig. 10. Three kinds of wall pinning observed in this study: (a) a strong wall pinning with a surface borosilicatc oxide; (b) unstable wall pinning with a mixture of surface oxides (borosilicatei-cobalt oxide); (c) a weak wall pinning with a surface cobalt oxide.

C.K.

Kim / Materials

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B40 (1996)

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1.00

Metalloid Crystallized

depletion zone

:

II

Amorphous zone

3

0.90

0 -

0.80

:

0.70

3

0.60

.I2 c 2

050 0.40

2

030

.Y5

0.20

(II

0.10 0.00

Crystalline

Co Thickness (A)

Fig. 12. Pinning threshold (ITpinning) vs. thickness of crystalline cobalt layer.

Fig. 11. Auger depth profile showing surface oxides, underneath depleted metalloid region (crystallized zone) followed by region with original metalloid content (amorphous zone), sputtering rate (10 nm/min). Note that borosilicate oxide formation leads to a much thicker crystallized Co layer (a), compared to Co0 formation (b).

STEM microanalysis and Auger depth profiling. Consequently, after sufficient time (e.g. 30 min at 380 “C, well below the bulk T,: 465 “C) a crystalline, magnetic, metallic layer (10 to 100 nm in thickness) of predominantly fee or hcp cobalt (depending on starting alloy composition) forms beneath the oxidized surface. However, this borosilicate oxide does not play any role magnetically, and simply leads to the depletion of metalloids. In contrast, when the content of B and Si is below the critical concentration, a major oxide formation during oxidation is COO. Consequently, a substantial amount of B and Si still remains in the matrix after oxidation although some of them segregate at Co grain boundaries. Consequently, this reaction does not lower local crystallization temperature much and eventually leads to a thinner Co crystallization depth compared to a borosilicate formation. Comparative Auger depth profiles for different surface oxide formation are given in Fig. 11. 3.8. The effect of the oxidation magnetization behavior

crystallization, leading to no reentrant magnetization behavior. Etching of the surface faulted Co layer makes the wall pinning disappear, which again suggests that the wall pinning is associated the faulted Co crystalline layer. We also observed that there is a good correlation between pinning field and thickness of crystalline Co layer after second annealing (Fig. 12). According to Auger depth profile analysis, the semihard Co layer thickness grows in proportion to the borosilicate oxide layer thickness with an approximate relationship

Of Lrystalline

co

layer

= 3. %ur~ace

oxide

(Fig.

13).

In contrast, in case of cobalt oxide formation, the crystallized Co layer is much thinner and almost kept constant regardless of Co0 thickness since the metalloids (B and Si) are not effectively removed from an amorphous substrate during oxidation. We, therefore, believe that surface oxide, borate-rich and porous borosilicate, are necessary to our pinned wall process since they set up the conditions such as (1) metalloid depletion in the substrate and (2) formation of oxygen impurity faults in Co grains, that are required for strong reentrant magnetization behavior.

4. Summary Oxidation products such as surface oxides and internal oxygen precipitates have a critical effect on the

on the reentrant

We have found evidence that the semi-hard Co crystalline layer is necessary for the stabilization of the wall pinning. Annealing in N, and Ar at 380 “C for 60 min resulted in neither surface oxide formation nor Co

0

100

Borosilicate

200

300

400

oxide thickness (A)

Fig. 13. Borosilicate thickness vs. crystalline Co layer measured by Auger depth profiling.

18

C.K. Kim ! Materials Science and Engineering B40 (1996) IO-18

magnetization behavior of Co-rich amorphous alloys. The strong field induced anisotropy is observed in oxidized ribbons with the high density of oxygen impurity faults inside Co grains. This type of internal oxidation is clearly favored by a surface oxide (B,03 rich borosilicate) which is permeable to oxygen. The best reentrant M-H loop is obtained in specimens with a borosilicate surface oxide since this ensures the conditions (metalloid depletion) in the Co crystallized layer that are required for a strong pinning. In contrast, Co0 formation is associated with a weak reentrant magnetization behavior since metalloid depletion does not occur effectively. Therefore, the nature of the surface oxide is of central importance to obtain reentrant magnetization in Co-rich amorphous alloys. It is observed that the surface oxide formation is governed by the metalloid content. The critical (minimum) concentration for exclusive formation of the most thermodynamically stable oxide (borosilicate) is determined as B + Si = 21%. The critical concentration is altered by increasing oxygen pressure during a ribbon cast, which leads to Co0 formation.

Acknowledgements The author would like to thank Dr. R.C. O’Handley and Mr. W.K. Ho for their valuable advice. Financial support from Sensormatic Electronics Corporation for this research is gratefully acknowledged.

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