Journal of Magnetism and Magnetic Materials 115 (1992) 77-86

North-Holland

Morphological, structural and magnetic characteristics of Co-Ti and Co-Sn substituted Ba-ferrite particles for magnetic recording

Zheng Yang, Hua-Xian Zeng, De-Hua Han, Jian-Zhong Liu and Sheng-Li Geng

Research Institute of Magnetic Materials, Lanzhou University, Lanzhou 730000, China

Co-Ti substituted barium ferrite particles were prepared by the glass-ceramic method in the BaCO,-B,O,-Fe,O, glass

system. The crystallization process of glass flakes, the morphology and magnetic properties of doped barium, ferrite particles

have been investigated. The finely divided, hexagonal plate-like Co-Ti and Co-G substituted barium ferrite particles were

prepared by a method which combines the chemical coprecipitation process and the salt flux process. The formation process

of these particles was investigated. The correlation between the magnetic properties, the particle morphology and the

degrees of Co-Ti and Co-Sn substitutions of Ba-ferrite particles are studied. By the law of approach to saturation the

effective anisotropy field HA and crystalline anisotropy constant K, were estimated. It was found that BaFe,z_z,

Co,Sn,O,, with x = 1.10-1.40 have a very low temperature coefficient of coercivity at temperature range from 0 to 60°C

and suitable magnetic characteristics for magnetic recording.

1. Introduction

Perpendicular magnetic recording has been confirmed to be a promising technology achieving extremely high recording density [1,2]. It has been recently recognized that the single-domain fine- platelet barium ferrite particles substituted by Co-Ti are very well suited to the mass produc- tion of particulate media for perpendicular recording utilizing existing coating facilities [3]. It was shown, however, that the longitudinally, and even non-oriented Ba-ferrite media also have ex- cellent recording characteristics [4,5]. Various preparation methods that have been developed to obtain Ba-ferrite particles suitable for magnetic recording including the glass-ceramic method [6], liquid mix technique [7], chemical coprecipitation, and hydrothermal synthesis from melts [8]. The last method combined with the advantages of the coprecitation process and the salt flux process is regarded as a method suited for the low-cost production of Co-Ti and Co-Sn substituted Ba-

Correspondence to: Dr. Z. Yang, Research Institute of Mag-

netic Materials, Lanzhou University, Lanzhou 730000, China.

ferrite particles with a hexagonal, plate-like habit and narrow particle size distribution [9,10].

This paper reports an investigation of the mor- phological, structural and magnetic characteris- tics of BaFe,,_,,Co,Ti,O,, and BaFe,,_,, Co,Sn,O,, fine particles, prepared by two meth- ods: the glass-ceramic method and synthesis from melts.

2. Experimental

2.1. Preparation of samples

BaFe 12_zxCoxTi.0,9 (x = 0, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80 and 0.90) ferrite fine particles were prepared by the glass-ceramic method us- ing the glass system of 0.405BaCO,-0.265B,O,- 0.33Fe,O, (mole ratio) [ll]. In order to avoid crystallization during glass formation, the molten glass was poured between two steel rollers sepa- rated by 0.33 mm which rotate in opposite direc- tions at an angular speed of 80-100 rpm. The typical thickness of the amorphous glass flakes was about 0.1 mm. The amorphous flakes were heat-treated at 600-1000°C for crystallization,

0304~8853/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved

78 Z. Yang et al. / Co-Ti and Co-Sn substituted Ba-ferrite particles for magnetic recording

then crystallized Ba-ferrite particles were sepa- rated from the flakes using a weak acid.

BaFe 12_ZxCoxTix019 (x = O-90) and Ba- Fe ,2_2xCoxSnx0,9 (x = O-1.40) ferrite fine parti- cles were prepared by a method which combines the chemical coprecipitation process and the salt flux process [9]. An aqueous solution of the metal chlorides containing Ba2+, Fe3+, Co*+ and Ti4+ or Sn4+ in the ratio required in the ferrite was stirred into an excess of an aqueous solution of NaOH and NaCO,. A suspension containing in- termediate precipitates was formed. The product of coprecipitation was filtered off, washed thor- oughly, dried and then mixed with salts (NaCl or NaCl-KCl). When this mixture is heated at an appropriate temperature, fine Co-Ti or Co-Sn substituted barium ferrite particles crystallize from the salt matrix. After the salts have been dissolved in water, a finely divided hexagonal, plate-like Ba-ferrite powder is obtained.

2.2. Measurement of characteristics

Differential thermal analysis (DTA) of the glass flakes was made with DuPont Thermal Analyzer type 1090. The identification of the crystalline phases in sample was carried out by means of X-ray diffraction patterns determined by a Rigaku X-ray diffractometer system Geigerflex D/Max- 3A. To obtain detailed information on the mor- phology and the particle size of Ba-ferrite pow- der, a set of micrographs has been taken by SEM and TEM. The mean dimension of the substi- tuted Ba-ferrite particles is related to the pure X-ray diffraction broadening p by the Scherrer equation [ll]. The instrumental broadening was estimated from X-ray diffraction patterns of a SiO, powder of large dimensions. For particles crystallized from salt melts the Fourier method was applied to the (110) and (107) lines, under the assumption that the diffraction profiles are Gaussian. The mean hexagonal diameter D and thickness t of particles have been determined from the apparent size &r (110) and cr (107) [7l.

Magnetization curves and M-H loops at dif- ferent temperature ranging from 77 to 673 K have been determined by using a TOE1 highly sensitive vibrating-sample magnetometer, model

VSM-5S-15 with a maximum applied field of 10 or 16 kOe. For magnetic measurements disc shaped samples with macroscopic density of 2.4 g/cm3 and a weight of 135 mg were used. The diameter/ thickness ratio of sample is kept about

In order to estimate the anisotropy field HA and the crystalline anisotropy constant K, of the Co-Ti and Co-Sn substituted Ba-ferrites, the law of approach to saturation was used [12]. A com- monly used type of this law can be written as

(1)

where Zt4, is the saturation magnetization, A the inhomogeneity parameter, and xP the high field susceptibility. B is the anisotropy parameter which can be expressed for crystals with hexago- nal symmetry as

B = Hi/15 = 4K;/15M,‘. (2)

It was found that a curve M vs l/H is linear in the magnetic field range from 8 to 13 kOe. This means that the parameters A and xP in eq. (1) could be neglected. The data may be accurately analyzed further by numerical methods.

A satisfactory method for determining the Curie temperature Tc is based on the following

b

2

Fig. 1. H/v vs. a2 curves of BaFe 10.3Co0.85’h&9 parti- cles.

Z. Yang et al. / Co-Ti and Co-Sn substituted Ba-ferrite particles for magnetic recording 79

relation derived from the free energy [13] when a temperature T is close to T,:

cua+pa3=H, (3)

a+pa2=H/a, (4)

where u is the magnetization, H a high magnetic field and (Y and /3 are constants. Plotting a series of isothermal curves H/a vs a2 as shown in fig. 1 for the Co-Ti substituted Ba-ferrite sample (x = 0.851, and then plotting the u intercepts versus temperatures gives T, as that temperature for which u vanishes at H = 0.

3. Results and discussion

3.1. BaFe,, _ ,,Co,Ti,O,, particles prepared by glass-ceramic method

The DTA data of as-quenched glass flakes show that in the case of x = 0 the glass transition occurs at 502°C and near 540 and 640°C two exothermic peaks were observed due to crystal- lization. The as-quenched glass flakes were X-ray amorphous and the crystalline phases were formed after heating the glass flakes at 600 N 1000°C for 10 h. X-ray diffraction patterns are shown in fig. 2. X-ray diffraction analysis shows that the magnetic crystalline phase BaFe,,O,, was precipitated after heating at 600 N 650°C and the BaB,O, crystallites and a third crystalline phase were formed after heating at 700°C and disappeared at about 875°C. The d-spacings of the third phase were identical to those of the Ba,B,O, (JCPDS, International Centre for Diffraction Data, 24-84). In the glass flakes heated at 900°C for 10 h only the BaFe,,O,, phaseDwas observed. Its lattice constants are a = 5.86 A, c = 23.04 A. It was shown that the sam- ples which were then slowly cooled and quenched

29

Fig. 2. X-ray diffraction patterns of the heated glass flakes

(x = 0); (A) BaB,O,; (01 Ba,B,O,.

in air from the molten glass also consist of single-crystalline phase BaFe,,O,, [14].

Table 1 gives the effect of the crystallization temperature T,, on specific magnetization uiO (at H = 10 kOe) and coercivity H, for heated glass flakes (x = 0). The theoretical yield of BaFe,,O,, is 45 wt% in a completely crystallized glass [6], so that the maximum value of uiO should be 32.4 emu/g, supposing uiO = 72 emu/g for BaFe,,O,,. It shows that only part of the iron in the glass crystallizes to Ba-ferrite BaFe,,O,,.

Transmission electron micrographs (fig. 3) of Ba-ferrite particles (x = 0 and 0.80) obtained by

Table 1

The effect of crystallization temperature on cl,, and H, of heated glass flakes (x = 0)

T,, [“Cl 500 600 650 700 800 830 850

ml0 [emu/g1 2.8 6.6 26.7 26.4 23.4 22.0 20.0

H, Noel 420 1140 2550 2650 3120 4300 2960

700 650

,.600

Z. Yang et al. / Co-Ti and Co-& substituted Ba-ferrite particles for magnetic recording

Fig. 3. TEM photograph of Co-Ti substituted barium-ferrite

particles.

leaching the glass flakes show that the particles Fig. 5 shows the effect of Co-Ti substitution are hexagonal platelet crystals. The particle size on (via and H, for leached BaFe 12_2XCoXTi.X019 decreases with increasing X, an interesting aspect particles. The coercivity H, decreases with in-

is the agglomeration of particles under their mu- tual interaction.

In table 2 the effect of crystallization tempera- ture on (~,a, H, and average particle size of the leached BaFe,,O,, p articles is summarized. As shown in table 2, the average particle diameter D and thickness t measured by TEM photograph increase with increasing T,. In the table 2 the apparent particle diameter D[220] and thickness t[006] determined by the Scherrer equation are given too. The dependence of H, on T,, (tables 1 and 2) can thus be interpreted as a result of the dependence of particle size on T,. From table 2 we might deduce that the coercivity H, of Ba-fer- rite particles changes considerably with changes of its thickness rather than its diameter. The above mentioned results show that when the glass flakes were heated at low temperatures (550°C < T, < 650°C) a small number of BaFe,,O,, parti- cles were crystallized, and most of them are su- perparamagnetic. This conclusion had been war- ranted by the Mossbauer spectra measured at room temperature and 77 K respectively for sam- ples heated at 600°C for 10 h (fig. 4). When T,, = 700-800°C Ba-ferrite crystals formed are single-domain particles. When T, > 850°C the Ba-ferrite crystals formed are partly multidomain particles. Table 3 gives the effect of the Co-Ti substitution x on H, for the glass flakes heated at different temperature. The results show that by choosing the amount of substitution, X, and crystallization temperature it is easy to obtain barium-ferrite powder with the desired character- istics for magnetic recording.

Table 2

Crystallization temperature effect on H,, (T,,, and particle size (D, t) of leached BaFe,,O,, particles

T, [“Cl 700 750 800 850 900

H, lOe1 2610 2900 4100 2860 1450

(rIo [emu/s1 48.5 48.1 46.4 45.0 50.1

t [Al 75-100 400-500 750-1000 1000-1130 _

D [AI 400-600 1800-2500 2500-3000 2500-3000

tl0061 (A) 250 440 1160 1510 2350 012201 (A) 600 1100 1130 1440 _

Z. Yang et al. / Co-Ti and Co-& substituted Ba-ferrite particles for magnetic recording 81

f1 I I I I I I

.:.c:.:... ‘. -._:-

: ;’ : . A:;. :, 3 . .

‘8; . .:<‘.._. :f$,: .:s.... . . . . .

. . . . . . . .

: :’ . . . ..; C.

. :....:

. . 5. :;. . . . r’

:. .Y .. .,

. . y .: R.T :: . . .

. :

. . . . . ’ . . . . .Z

I

1. : . : .,.

: : . . ::’ .

:: : <.. :. f.’ .:’ . ..: . .

: :< . . . . :. . . . ‘:

. . . : . . . . . . . . . . + : *

. .:. .- . ‘C

._ . :

.,. _ .:. . . . .

-. .i . . . :: . . . . i I’_.

. .

. . _.. :

. . . . : . .- . . *. .

::. . . .

77 “I

Fig. 4. M&barter spectra of the sample after heated at 600°C for 10 h recorded at RT and 77 K.

Table 3 Co and Ti substitution effect on H, of glass flakes heated at different temperature T,, for 10 h

Co-Ti substitution x

0 0.30 0.40 0.50 0.60 0.70 0.80 0.90

H, lOe1 700°C 750°C 800°C 850°C

3300 4200 3230 3800 2370 3150 2840 2690 1940 2310 2450 2410 1500 1800 1860 1440 930 1300 1520 1520 570 980 1090 850 330 530 580 700 160 175 300 400

Fig. 5. Co-Ti substitution effect on H, and (rts for Ba-ferrite particles.

creasing substitution x, and the decrease of uiO with increasing x is relatively small.

3.2. BaFe,, _ 2xCo,Ti,0,9 particles crystallized from salt melts

To clarify the formation process of BaFe,,O,, crystallites from a molten salt, three powder sam- ples were prepared: a coprecipitated powder (sample A), and its mixtures with NaCl (sample B) and with NaCl-KC1 (sample C). These sam- ples have been heated in air at temperature T, ranging from 600 to 900°C for two hours. The X-ray diffraction analysis shows that intermediate phases, e.g. a-Fe,O, and BaFe,O,, cannot be identified during the reaction process. Up to 600°C all samples are amorphous. At tempera- tures 600-630°C the BaFe,,O,, phase begins to crystallize [14].

In order to make a comparison between three samples, fig. 6 shows the relative heights J(114) of the X-ray diffraction peak (114) for samples A, B and C as a function of T,,. The magnetic properties vs T,, for these samples are summa- rized in table 4. From the results we can see that with an increase of T,, both J(114) and magneti- zation ci6 increased, but with different rates. It is clear that the salt flux process accelerated the process of formation of Ba-ferrite phase. The lower the melting point of the salt flux, the lower is the temperature at which the BaFe,,O,, phase begins to crystallize. There is a similarity between BaFe,,O,, and BaFe,,,Co,,Ti,,,O,, particles. Fig. 7 gives the magnetic properties vs T,, for

BaFe,,.,Co,.sTi,.sO,,. The magnetization in- creases with the increase of Th. It was found that the heating temperature T,, influences both the amount of formed magnetic particles as well as their size and morphology. A very fine divided and uniform Ba-ferrite powder was obtained by synthesis from salt melts. Fig. 8 gives the SEM photograph of BaFe,,O,, particles (A) and TEM photograph of BaFe,,,,Co,,,Ti,,,O,, particles (B). In table 5 the mean particle size and 3014) of BaFe,,,Co,,,Ti,,O,, ferrite particles as a func- tion of Th are summarized. The 5(114), mean diameter D and thickness t of the Co-Ti substi- tuted Ba-ferrite particles increase with an in-

82 Z. Yang et al. / Co-Ti and Co-G substituted Ba-ferrite particles for magnetic recording

600 700 800

Fig. 6. X-ray intensity J(114) vs. T, for samples A, B and C.

crease of the heating temperature T,,. The mag- netic behavior of a magnetic particle assembly is a function of the particle size and its morphology, so the dependence of H, on T,, can be inter- preted as a result of the dependence of particle size on T,. The M-H loops measured at room temperature and 77 K [9] show that a small amount of magnetic particles has been crystal- lized even in the sample heated at 600°C for 2 h, and most of them are superparamagnetic. The samples heated at 750450°C are composed mainly of single-domain particles. A steep de- crease of H, of the sample heated at 900°C was caused by an increase of multi-domain particles.

It was shown that the substitution of Fe ions

I I 600 700 800 900

Th( “C)

Fig. 7. H,(T), u,,(T) and ur,(T) curves of BaFe,,,,,

Co,,sTi,,,O,, particles.

by Co-Ti ions tends to reduce both the size and the H, of Ba-ferrite particles. In table 6 the composition dependence of magnetic properties for BaFe,,_,_,Co,Ti,O,, is summarized. It was found that it is easy to control the H, of Ba-fer- rite particles by changing the Co-Ti substitution and reaction temperature.

Fig. 9 shows the temperature dependence of coercivity for BaFe,,_,,Co,Ti,O,, (x = 0, 0.80, 0.85 and 0.901 particles. The coercive force H, of BaFe,,O,, particles increases monotonously with an increase of temperature at temperatures -196-120°C. On the contrary, the H, value of the Co-Ti substituted particles has a minimum at about -50°C. It seems that this minimum of coercive force may be associated with the contri- bution of the crystalline anisotropy of Co ions

D51.

Table 4

The effect of the heating temperature T, on the magnetic properties of samples

T, [“Cl Sample A

[T16

[emu/s1 or

[emu/al 4

loel

Sample B

Cl6

[emu/al fir

[emu/g1

Sample C

Ul6

lemu/gl or

lemu/gl

600 0.65 0.044 380 0.81 0.089 820 0.90 0.11 990

630 0.80 0.13 1815 1.28 0.33 8250 1.34 0.35 2700

670 16.6 8.50 5450 3.18 16.6 5050 62.8 32.9 4760

700 59.6 31.8 5100 60.2 31.5 5080 63.1 33.0 4850

750 61.4 33.3 5380 62.9 33.4 5280 63.3 32.9 5070

800 61.9 33.2 5780 63.8 33.5 5320 63.7 33.2 5080

850 62.5 33.5 5800 63.8 34.0 5400 64.0 33.5 5120

900 62.3 33.8 5800 63.6 33.8 5390 64.2 33.5 5120

2. Yang et al. / Co-Ti and Co-Sn substituted Ba-ferrite particles for magnetic recording 83

Fig. 8. SEM and TEM photographs of BaFe,,O,, and

BaFe ,0,4C00~sTo,,,0,, particles.

Table 5

The mean particles’ size and A1141 of BaFe,,,,Co,,,To,,sO,,

as a function of heating temperature T,

T, [“Cl J(114) E,(llO) &,(107) D t

[mm1 [Al [Al [Al [Al 670 46 390 190 450 160

700 52 440 220 510 180

750 56 490 240 570 200 800 70 520 280 610 240

850 76 680 320 790 270

900 149 g F(O08) = 1500 4000 1500

Table 6

Composition dependence of magnetic properties in

BaFe,,_,_,Co,Ti,O,,

x

0 0.70

0.75

0.80

0.85

0.90

0.80

0.80

0.80

0.80

Y “16 a, a,6/ar 4

[emu/g1 [emu/g1 Del 0 63.8 34.0 0.53 5400

0.70 62.1 27.0 0.44 1040

0.75 60.0 23.3 0.39 730 0.80 57.0 22.5 0.40 660

0.85 58.3 22.0 0.37 510

0.90 57.9 19.5 0.34 370

0.20 60.2 30.3 0.50 1950

0.40 59.2 28.4 0.48 1940

0.75 62.1 28.6 0.46 1280

0.80 61.3 25.9 0.42 750

Information about the magnetic anisotropy field HA and the crystalline anisotropy constant K, of Co-Ti substituted Ba-ferrite particles is very important to clarify their magnetization re- versal process. The anisotropy field HA and ef-

Table 7

Some intrinsic magnetic properties of BaFe,,_,,Co,Ti,O,, particles

x B HA

[lo6 Oe] We1 :A6 erg/cm’] 4

Del 0 RT 350 18 16.4 3.0 5120 _

0.80 RT 327 2.2 5.7 0.95 722 613 77 K 445 3.8 7.6 1.7 990

0.85 RT 319 1.3 4.4 0.72 495 591 77 K 414 3.0 6.7 1.4 860

0.90 RT 316 1.2 4.2 0.68 362 586 77 K 399 2.7 6.4 0.73 850

Z. Yang et al. / Co-Ti and Co-Sn substituted Ba-ferrite particles for magnetic recording

BaFe12-2xC~Tix019

/m’ I3 lOOC*

7**7* o, l -•- 0

(5 .” *_A-*-*-*

2”

-i,

x=0 A**- / -5

\

/ /Of 4 8 Jo ,/y

o&8,00°' -3 3 500- -

\

0’ 7 40 ’ 7

0.85 ,“-,A 0’ /’

\ _vvfl+ 2

0.96 -1

C I I I I I 0 -200 -1.00 0 100

T("C)

Fig. 9. Temperature dependence of coercivity for Co-Ti sub- stituted Ba-ferrite particles.

fective crystalline anisotropy constant K, were roughly estimated by using eqs. (1) and (2). In the calculation of K, from eq. (2) no correction of the particles’ shape anisotropy was made. In table 7 are summarized the Curie temperature Tc, M,, and the anisotropy parameter B, HA and K, measured at room temperature and 77 K. It can be seen that with an increase of the Co-Ti substi- tution x the saturation magnetization M,, T,, HA and K, decrease.

3.3. BaFe,, _ ,,Co,Sn,O,, p articles crystallized from salt melts

From fig. 9 we can see that Co-Ti substituted barium ferrite has a large temperature coefficient of coercivity, which was determined as AHJAT = [ H,(6o”C) - H,(o”C)]/60, where H,(6o”C) and

H,(o”C) are the coercivities at 60 and 0°C respec- tively [15]. In order to improve the margin for operating temperature changes and for the high- density recording performance, the morphology, structure and temperature dependence of mag- netic properties of BaFe,,_,,Co,Sn,O,, parti- cles with x = 0 - 1.4 have been investigated. Co- Sn substituted barium ferrite particles were pre- pared by two methods [93: chemical coprecipita- tion and synthesis from salt melts. They are called method A and method B, respectively, in the following description.

Table 8 Composition dependence of lattice constants and mean parti- cle size of BaFe,2_2,Co,Sn,01s particles

X a [Al c [Al D &I t [Al 0.20 5.903 23.214 1960 700 0.40 5.902 23.215 1340 640 0.60 5.900 23.287 810 330 0.80 5.901 23.306 870 320 1.10 5.902 23.356 760 270 1.20 5.901 23.360 660 220 1.40 5.902 23.402 670 200

X-ray diffraction patterns show that Ba-

Fe i2 -&o,Sn,% p articles with x = 0 N 1.40 prepared by both methods A and B have the same crystal structure, the so-called magneto- plumbite structure M. In table 8 we summarize the composition dependence of lattice constants and the mean particle size D and t, determined by X-ray diffraction broadening [lo], of Co-Sn substituted barium ferrite particles. With the in- crease of Co-Sn substitution x the lattice param- eter a, however, remains constant, while the lat- tice c increases slowly. The fact that the radius of the Sn4+ ion is larger than that of the Ti4+ ion [16] may cause the increase of lattice parameter c. From table 8 we can see a decrease of the mean hexagonal diameter D and mean thickness t of particles with the increase of the substitution X.

Fig. 10 shows the dependence of the Curie temperature T, and saturation magnetization us at room temperature on the Co-Sn substitution x for doped barium-ferrite particles prepared by

3d 0.40 0.80 1.20

Fig. 10. Dependence of a, and T, on x for Co-Sn doped Ba-ferrite particles.

Z. Yang et al. / Co-Ti and Co-Sn substituted Ba-ferrite particles for magnetic recording 85

method A. The dependence of both T, and a, versus x may be fitted by the following linear relations:

T, = 720.5 - 35.3x, (5)

a, = 68.1 - 7.9x. (6)

It was found that the rates of decrease of Tc and a, at room temperature are lower than that reported in ref. [7], and we got particles with x = 1.20-1.40 which have magnetic characteristics suitable for magnetic recording and excellent temperature stability (see fig. 13). This discrep- ancy is probably associated with a different preparation method of samples which causes the different chemical composition and the cation distribution of Co and Sn ions in the samples.

As well known, the coercivity of the doped barium ferrite particles is attributed to both the crystalline anisotropy and the shape anisotropy, as expressed by

H,=C(2K,/M,-NM,) =C(H,-H,), (7)

where C is a constant, M, is the saturation mag- netization and N is the demagnetization coeffi- cient. The composition dependence of the effec- tive anisotropy field H,(X), the effective crys- talline anisotropy constant K,(x) and the coerciv- ity H,(X) for BaFe,,_,,Co,Sn,O,, particles pre- pared by method A have been studied (fig. 11). The effective crystalline anisotropy constant K, decreases rapidly with increasing the Co-Sn sub-

T(X) Fig. 12. Temperature dependence of H, for Co-% doped

Ba-ferrite particles.

stitution x in contrast to the composition dependence of magnetization us (x). Fig. 11 shows that the dependences of H, and HA vs x are similar. It means that the crystalline anisotropy is the dominant factor to determine the magnetization reversal process of particles.

The temperature dependence of the coercivity H, CT) for particles with different Co-Sn substi- tution x is given in fig. 12. It is clear that in the case of pure barium ferrite particles (x = 0) the coercivity increases monotonously with increasing temperature. However, the Co-Sn substituted

G 6000

s? E?

10

.4000 Liz

0 0.40 0.80 1.20 x' I I I 4 I I

Fig.’ 11. Dependence of K,, HA and H, on x for Co-Sn

doped Ba-ferrite particles.

G2.50 < PI F2.0

a $00

a

i

0

-1.004 I I I I I

0. 20 0.60 1.00 X

Fig. 13. The dependence of AH, /AT on x for Co-Sn doped

particles.

86 Z. Yang et al. / Co-Ti and Co-.% substituted Ba-ferrite particles for magnetic recording

Table 9

Composition dependence of magnetic characteristics in

BaFe ,2_zxCoxSnx019 particles

x Ulh a, ;1

H, [emu/g1 lemu/gl lOe1

0.20 61.2 32.3 53 3970

0.40 61.9 32.8 53 2730

0.60 61.7 30.9 50 2320

0.80 62.0 30.9 50 1890

0.90 61.0 30.3 50 1650

1.10 60.5 29.7 49 1290

1.20 59.9 28.4 47 990

1.40 57.7 24.9 43 660

particles exhibit a minimum in H,, which in- creases with X. The H,(T) curves of particles with x = 1.10-1.40 have a flat region at tempera- tures 270-420 K. Fig. 13 shows that the tempera- ture coefficient of coercivity AHJAT is depend- ent on the substitution X. It was found that BaFe 12-2xCoxSnxO19 particles with x = l.lO- 1.40 have not only a suitable coercivity for mag- netic recording, but also a very low temperature coefficient of AHJAT. In table 9 is summarized the composition dependence of magnetic charac- teristics for Co-Sn doped particles prepared by method B.

4. Conclusions

After heating the glass flakes at 600-650°C the magnetic crystalline phase BaFe,,O t9 is precipi- tated. Two intermediate nonmagnetic phases BaB,O, and Ba,B,O, are formed during heat- ing, and these phases disappeared at tempera- tures T, > 875°C. By choosing the amount of substitution x and crystallization temperature it is easy to obtain barium-ferrite powder with the desired characteristics for magnetic recording. The finely divided hexagonal plate-like Co-Ti and Co-Sri substituted barium-ferrite particles with magnetization ur6 = 58-61 emu/g and H, =

500-1200 Oe obtained by synthesis from salt melts is suitable for magnetic recording. The fact that the anisotropy constant K, decreases rapidly with increase of Co-Sn substitution x leads to a re- markable decrease of the coercivity for Co-Sn substituted barium-ferrite powder. Co-Sn substi- tuted Ba-ferrite particles with x = 1.10-1.40 have not only suitable magnetic characteristics for magnetic recording, but also a very low tempera- ture coefficient of coercivity in comparison with that of Co-Ti substituted particles. These parti- cles are a good candidate for mass production of the particulate perpendicular magnetic recording media.

References

[l] S.I. Iwasaki, IEEE Trans. Magn. MAG-20 (1984) 657.

[2] S. Yamamoto, Y. Nakamura and IS. Iwasaki, IEEE

Trans. Magn. MAG-23 (19871 2070.

[3] T. Fujiwara, IEEE Trans. Magn. MAG-21 (1985) 1480.

[4] J. Takahashi, K. Itoh and S. Ogawa, IEEE Trans. Magn.

MAG-22 (1986) 713.

[5] D.E. Speliotis, Digests of the INTERMAG’ 1989, JA-01.

[6] B.T. Shirk and W.R. Buessem, J. Am. Ceram. Sot. 53 (1970) 192.

[7] M. Pernet, X. Obradors, M. Vallet, T. Hernandez and P.

Germi, IEEE Trans. Magn. MAG-24 (1988) 1898.

[8] H. Hibst, Angew. Chem. Int. Ed. Engl. 21 (1982) 270.

[9] Zheng Yang, Zeng Hua-Xian, Yan Ming-long and Geng

Sheng-Ii, J. Magn. Sot. Jpn. Sl (1989) 363.

[lo] Zeng Hua-xian, Han De-hua, Geng Sneng-Ii and Zheng Yang, Proc. PMRC’91 (to be published).

[ll] H.P. Klug and L.E. Alexander, X-ray Diffraction Proce-

dures for Polycrystalline and Amorphous Materials (John

Wiley, New York, 1954) Chapter 9.

[12] R. Grossinger, Phys. Stat. Sol. (a) 66 (1981) 665.

[13] K.P. Belov, Magnetic Transition (Boston Tech. Publ.,

Boston, 1965) p. 34.

[14] Zheng Yang, Liu Jian-zhong, Zheng Pei, Geng Sheng-Ii

and Chen zi-yu, IEEE Trans. Magn. MAG-23 (19871

3131.

[15] O.Kubo, T. Nomura, T. Ido and H. Yokoyama, IEEE Trans. Magn. MAG-24 (1988) 2859.

[16] H. Kojima, Ferromagnetic Materials, vol. 3, ed. E.P. Wohlfarth (North-Holland, Amsterdam, 1982) p. 317.