IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 5 , SEPTEMBER 1989 4069

A I~SSBAUER STUDY OF COHO-SUBSTITUTED BARIUW FERRITE

D.G. Agresti and T.D. Shelfer Physics Department, University of Alabama at Birmingham, Birmingham, AL 35294

Y.K. How* and Y . J . Paig

Engineering, Auburn University, Auburn, AL 36849 hterials Science and Engineering Program, Department of Hechanical

Abstract

Well-resolved MGssbauer spectra have been obtained for CoHo-doped barium ferrite with variable doping (0 to 33% Fe), heat-treatment temperature (825- 1200"C), and sample temperature (90-300K). Substitution was observed to occur primarily on the 12K site, with little or no lattice distortion. A fitting model employing five independent components, each a sextet, yielded the magnetic hyperfine field (ionic moment) of each site as a function of doping and sample teqerature. A 10% reduction of the moment on the 2a site was observed upon doping, which results from its proximity to the 12K site. Other sextet parameters were generally constant or followed monotonic trends.

Introduction

As part of a program to optimize the properties of barium ferrite for use in perpendicular magnetic recording, we have produced samples af barium ferrite in which Fe3+ has been replaced by other ions of comparable size in a way that maintains charge balance clnd hence crystal stucture. These samples have been characterized by a variety of measurement techniques, including XRD, TEW, SEW, VSW, and "7Fe-N6ssbauer spectroscopy ( K 9 . l Ve report here in detail on some results obtained by 1(s on BaFeS+,z-~,CoZ',Wo4',0,, (designated COHO,).

Explicit information is obtained from XS on the ionic moment, local environment, and the population of each distinct crystallographic site as functions of sample temperature, degree of substitution, and other preparation conditions. This information may be used to assess sample quality and to infer local ionic interactions or to calculate magnetization and other bulk properties from the atomic point of view.

In contrast to the commonly investigated CoTi, <And other doped barium ferrites, we have discovered' that CoWo, spectra are well resolved (comparable to pure barium ferrite), suggesting that the COHO, system may serve as a testing ground for models attempting to quantitatively evaluate the effects of substitution on the properties of barium ferrite.

Sample Preparat iun

The preparation of samples ha5 been described previously.' Stoichametric ionic solutions were coprecipitdted in BaOH at 55OC (cold process) or dt 95°C (hot process), centrifuged, washed, dried, and then heated in air at a temperature. Theat, for 2 hours, and finally cooled at an adjustable rate. However, ab judged by XRD dnd 165, the cold process failed to produce COHO,, presumably because of the reduced solubility of Koa' dt the lower temperature.

HGssbauer absorbers of -20% Fe/cm2 were prepared by dispersing finely ground powder in melted paraffin, which wd5 then worked manually to achieve random urientation before shaping in d pill press.

Gssbauer Spectra Descriptive

Data obtained with a "20mCi F7Co(Rh) source moving in constant acceleration were collected into 1024 channels, each containing "2 3 ~ 1 0 ~ counts. This resulted in two mirror-symmetric 512-channel spctrd, edLh uf which wab fit as described in the next sectjun.

100

98

- 96 R c 100 0 v, v,

v

.-

.- E 98

F

5 100

VI t

96 Q, > .- - Q) K

98

96

~~

4f(VI), I I I 1 I 20 f I I I I 1

4f(iv) r I I 1 I 1 1 2 K f 1 I I I 1

2 b r l I I I I

-1 2 -6 0 6 12 Velocity (mm/s)

Fig. 1. 300K spectra for representative COHO, (Th,,+=1200"C) samples. The peak assignments are shown at the top.

Spectra were recorded for five COHO, (x = 0, 0.5, 0.9, 1.5, and 2.0) hot-process samples (Th,,,=1200"C) (Fig. 1). The relatively narrow lines are direct evidence that doping of CO=+ and HO"' for Fe3+ retains a unique environment around each FeJ' site, suggesting that these ions are incorporated only at FeJ+ sites with minimal distortion of the barium ferrite lattice, ab confjrmed by XRD. We see also that the strongest component in the x = 0 spectrum decreases with increasing doping, showing a strong preference for substitution at the corresponding Fe3+site (12K). The nearly constant positions of the lines shows that doping has little effect on the FeJ' moments, even dfter replacement of 1/3 of the FeS+ ions.

0018-9464/89/0900-4069$01.0001989 IEEE

Five Cob, ( ~ ~ 0 . 9 ) samples were prepared with Thrrt = 1200, 1025, 975, 925, and 825%. The spectra (Fig. 2) show that both Fe3+ moments and t h e sites occupied by t h e s u b s t i t u e n t s are i n s e n s i t i v e t o The,,. However, the component l i n e s broaden with decreasing Thrrt, showing t h a t The,, = 1200°C produces samples with t h e least d isorder around t h e i ron s i t e s ; that is, t h e best q u a l i t y from t h e atomic poin t of view.

I

( +* Theat= 975OC

-1 2 -6 0 6 12

Velocity (mm/s)

Fig. 2. 30@K spectra f o r representa t ive C O H ~ . ~ samples with d i f f e r i n g The,,.

A s a benchmark, spectra were obtained for a pure (x=0) barium f e r r i t e sample (cold process) from 300K t o 90K (Fig. 3). Spectra were also obtained f o r a Cok, a m p l e (x=0.9, Th,,,=1200"C) a t 50K i n t e r v a l s over t h e same temperature range. The laajor t rend is the SRI& in both cases: t h e 12K mnent increases f a s t e r as temperature is lowered than t h a t of t h e o ther sites.

The c e n t r a l doublet in Figs. 1 and 2 (6-8% of t h e t o t a l a rea) is present i n 1[2issbauer spectra of a l l hot- process, but not cold-process, samples. Comparison of t h e x = 0 spectrum of Fig. 1 with t h e 300K spectrum of Fig. 3 shows that t h i s doublet corresponds t o a paramagnetic f e r r i c contamination (giving unident i f ied e x t r a peaks i n t h e XRD p a t t e r n ) t h a t does not compromise t h e q u a l i t y of t h e barium f e r r i t e produced. In f a c t , t h e doublet remains unchanged, t h a t is paramagnetic, down t o a temperature of 22K.

rossbauer Spectra - Analysis

Fe" occurs i n bar iun f e r r i t e i n f i v e d i s t i n c t c rys ta l lographic si tes, designated 12K, 4 f I u , 4fU1, Za, and ab, i n the ratio, 6:2:2:1:1, r e s p . , with ion ic mments of var iab le s t r e n g t h and d i r e c t i o n (parallel or a n t i p a r a l l e l t o t h e c -ax is ) . Fhch cont r ibu tes a magnetic hyperf ine s e x t e t , i n proportion t o s i t e population t i m e s recoi l - f ree f r a c t i o n , t o the t o t a l I[ijssbauer spectrum. Vith t h e assignments shown i n Pig. 1, four of these sites may be i d e n t i f i e d v isua l ly . The weak 2a component is buried under t h e 4f pedks. with a cunsequence that, u n t i l r e c e n t l y , ' , 2 only four d i s t i n c t sites have been used i n f i t t i n g barium f e r r i t e spectra.

In t h e present work, we use f i v e independent s e x t e t s , each of which c o n s i s t s of six Lorentzian d i p s with area r a t i o s f ixed a t 3:2:1:1:2:3. For each s e x t e t , t o t a l area, isomer s h i f t , quadrupole in te rac t ion , and magnetic f i e l d are var ied i n t h e f i t , as well as peak width and a broadening parameter t o account f o r a s l i g h t f i e l d d i s t r i b u t i o n ( r e s u l t i n g i n width r a t i o s , Vt2B: VtB: V: V: VtB: Vt2B) , which are assumed t o be t h e same f o r a l l s e x t e t s . Thus, with background and geometric e f f e c t , t h e r e are 24 var iab le parameters (28 with t h e c e n t r a l doublet) .

Resul t s of Analysis

Analysis of t h e pure barium f e r r i t e spectra demonstrates t h a t t h e f i t t i n g procedure reso lves a l l f i v e c rys ta l lographic sites. The monotonic t renda for hyperfine f i e l d s versus temperature seen i n Fig. 4 show minimal c o r r e l a t i o n among t h e f i e l d values , which d i f f e r a t 300K, on average, by less than 0 . 1 T from those of Evans et Site populat ion (sextet areas) and local environnent (quadrupole in te rac t ion) are near ly independent of temperature. A s temperature is lowered, isomer s h i f t s increase i n accordance with t h e second-order Doppler e f f e c t , although t h e t rend f o r t h e 2b si te is not s i g n i f i c a n t l y more exaggerated than t h e o thers , cont ra ry t o expectat ions." F ina l ly , a weighted sum of f i e l d values (6:-2:-2:1:1) p r e d i c t s the published v a r i a t i o n of s a t u r a t i o n magnetization with temperature3 and demonstrates that t h e ion ic moment f o r each si te is proport ional t o t h e hyperf ine f i e l d .

100

98

n 6\" v 96

c 100 0 cn VI

cn 98 c

+ Q,

.-

.- E

F

> 96 .- 100 -

a, [r

98

96

+ +

300K t

-12 -6 0 6 12

Velocity (mm/s)

Fig. 3. Spectra f o r pure barium f e r r i t e <cold-process, Th,,,=12000C), as a funct ion of sample temperature.

55 I

407 1

Gonc lusi on

Vith this work, substituted barium ferrite spectra have been successfully analyzed for the first time in a way that distinguishes all five crystallographic sites. The consistency of the sextet parameters for each site as a function of doping is very encouraging and, with a sufficiently well-resolved spectrum, should allow a detailed study of the effect of substitution and preparation conditions on ionic moments, their environments, and their interactions.

In the future, ne plan to acquire spectra for a series of absorber thicknesses for pure and doped barium ferrite. Drawing on our previous experience,E we will analyze these with the proper thick-absorber lineshape function in order to determine the recoil- free fraction €or each site, which, as the discussion relating to Fig. 5. shows, is necessary to calibrate sextet areas in terns of site populations. This will allow bulk llagnetic properties to be calculated from the N&sbauer results, e.g. to combine H6ssbauer and VSlI measurements to determine the CO=+ moment in CoXo,.

n I- W

x a,

a, c % L a, Q x I

.- %

.-

(+---e- " ._,._

45

40 50 100 150 200 250 300

Temperature (K)

Fig. 4. Variation of magnetic hyperfine fields (site moments) with temperature for pure barium ferrite.

The five-site fitting model (plus the central doublet) applied to the 300K COHO, spectra was equally successful in resolving all five sites. Fig. 5 indicates the trend for site population versus doping, clearly showing the primary substitution on the 12K site. However, the relative area for the 4frv site increases faster than can be explained by doping considerations alone, and probably results from differences in site recoil-free fractions. The sm11 2b area also shows the importance of the recoil-free fraction, which is reduced for this site due to an unusually large vibration amplitude.4 From the field values shown in Fig. 6, we conclude that the site muments are nearly independent of doping except for the 2a site, which is nearest the substituted 12K site (3.0 A, versus 3.5 A for the next nearest Fe"' site4).

For CoNo, samples at 300K, Thrrlt has little effect on the sextet parameters (except width) for the 12K and 4f sites. However, due to the poorer spectral resolution for T,,,, < 1200"C, these values are erratic for the 2a and 2b sites. Analysis of the temperature- dependent CoNo, - (Th.,,,=l200"C) spectra shows that site population and local environment are essentially unaffected by sample temperature.

n R W

U

U

cu > 0

E)

.- -U

6o r-- 1 40L 20

" 0 0.5 1 1.5 2

Doping concentration, x Fig. 5. Sextet areas (normalized to 1002)

for 380K COHO, spectra versus doping.

111

c21

E91

I 4 1

151

h 55 i i a, c .- 5 451 Q x T

0 0.5 1 1.5 2 Doping concentration, x

Fig. 6. Nagnetic hyperfine fields for 300K CoNo, spectra versus doping.

References

Y.K. Hong, Y. J . Paig, D.G. Agresti, and T.D. Shelfer, "Synthesis and characterization of modified barium ferrite particles," J. Appl. Phys., vol. 61, pp.3872-3874, April 1987. B. J. Evans, F. Grandjean, A.P. Lilot, R.H. Voge?, and A. Gerard, "s7Fe hyperfine interact ion parameters and selected magnetic properties of high purity HFelzOls (N=Sr,Ba)," J. N a p . Nagn. Nat.,

J. J. van Loef and P. J. ]I. Franssen, "The %ssbauer effect in the hexagonal ferrite Ba0.6Fez&," Phys. Lett., vol. 7, pp.225-226, December 1963. X. Obradors, A. Collomb, N. Pernet, D. Samaras, and J.C. Joubert, "X-ray analysis of the structural and dynamic properties of BaFelzO19 hexagonal ferrite at room temperature," J. Solid State Chem., vol. 56, pp. 171-181, February 1985. D.G. Agresti, N. Belton, J. Vebb, and S . Long, "Determination of transmission integral parameters by simultaneous fits to thick-absorber niissbauer spectra: applicaton to lEIEuZ&," i n "K6ssbauer Effect Methodology," vol. 9, pp. 225-243 (Plenum Press, 1974).

vol. 67, pp.123-129, Nay 1987.

*Dr. Hong's current address is: Central Research Center, Oriental Chemical Industry, Inchon, Korea.