Preparation and magnetic properties of (Zn–Sn) substituted barium hexaferrite nanoparticles for magnetic recording

Journal of Magnetism and Magnetic Materials 187 (1998) 129—135

Preparation and magnetic properties of (Zn—Sn) substitutedbarium hexaferrite nanoparticles for magnetic recording

H.C. Fang!, Z. Yang!,", C.K. Ong!,*, Y. Li#, C.S. Wang"

! Department of Physics, National University of Singapore, 10 Lower Kent Ridge Crescent, Singapore 119260, Singapore" Research Institute of Magnetic Materials, Lanzhou University, Lanzhou, 730000 China

# Department of Materials Science, National University of Singapore, Singapore 119260, Singapore

Received 15 December 1997; received in revised form 16 February 1998

Abstract

Zn—Sn substituted barium ferrite particles BaFe12~2x

ZnxSn

xO

19with 0)x)1.1 have been prepared by a chemical

co-precipitation method for the first time. X-ray diffraction (XRD) and transition electron microscopy (TEM) have beencarried out to determine the particle structure and morphology. Magnetic properties have been measured usinga vibrating sample magnetometer (VSM) with an applied field up to 80 kOe. The specific saturation magnetizationM

4was determined using the law of approach to saturation, and the effective anisotropy field H

!and anisotropy constant

K1

were also estimated. It was found that the particle size could be effectively decreased and coercivity H#

easilycontrolled, by varying x without significantly decreasing saturation magnetization. In particular, BaFe

12~2xZn

xSn

xO

19with x"0.7—1.1 has suitable magnetic characteristics and a particle size small enough for high-density magneticrecording. ( 1998 Elsevier Science B.V. All rights reserved.

PACS: 75.50.Ss; 81.40.Z

Keywords: Substituted barium ferrite; Co-precipitation; Nanoparticle; Approach to saturation law; Magnetic recording

1. Introduction

In order to meet future high-density recordingrequirements such as HDTV, high-density mag-netic tapes and floppy disks, high-coercivity mag-netic particulate media with small particle size are

*Corresponding author. Fax: #65 777 6121; e-mail:[email protected].

required [1]. The hexagonal ferrite BaFe12

O19

andits substituted derivatives have been considered ascandidates with the most potential because of theirchemical stability and suitable magnetic charac-teristics [2,3]. It has been shown that advancedbarium ferrite tapes, which utilize very smallparticles (diameter D"40—50 nm, aspect ratio"3.5—4) of high coercivity (H

#"1750—2060 Oe) offer

superior high-density recording performance. Incomparison with advanced metal powder (MP)

Information Storage: Basic and Applied

0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved.PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 1 3 9 - 5

tapes, advanced barium ferrite tapes offer evenhigher signal-to-noise ratio and are capable ofsupporting densities above 160 Kfci [4]. Examplesinclude Toshiba’s 3.5 in 4 MB floppy disk, Profes-sional Hi-8 mm tape, and Sony’s DAT soft tape.Nevertheless, it should be noted that the extremelyhigh-uniaxial anisotropy of the pure barium ferritecompound precludes its application in the field ofmagnetic recording. Consequently, the pure phaseis usually doped with other cations in order toreduce its magnetocrystalline anisotropy.

Several methods have been developed to preparedoped and undoped barium ferrite particles [5—10],such as by the glass—ceramic method, chemicalco-precipitation, liquid mix technique and hydro-thermal processes. Using the glass—ceramicmethod, a small particle size can be obtained, butthe magnetic properties are poor, because it is verydifficult to completely prevent the formation ofintermediate phases such as Fe

2O

3, BaCO

3or

BaFe2O

4[11]. In addition, a high temperature is

required. The chemical co-precipitation methodis a cheap and easy choice for mass production[12]. Particles prepared by this method have bettermagnetic properties. The main disadvantage is thatthe particle size is not small enough for high-den-sity recording application, however this problemcan be overcome by choosing suitable dopingcations.

In this paper, Zn—Sn substituted barium ferritewas fabricated using the chemical co-precipitationmethod, and investigated by differential thermalanalysis (DTA), X-ray diffraction (XRD), transitionelectron microscopy (TEM) and magnetic measure-ments. The approach to saturation law was used todetermine zero-field saturation magnetization M

4,

anisotropy field H!, inhomogeneity constant A and

high-field differential susceptibility s1. The depend-

ence of substitution concentration x on particle size,saturation magnetization, coercivity and other char-acteristics have been determined and discussed.

2. Experimental procedure

BaFe12~2x

ZnxSn

xO

19nanoparticles with 0)

x)1.1 were prepared by chemical co-precipita-tion. An aqueous solution of the metallic chlorides

containing Ba2`, Fe3`, Zn2` and Sn4`, in theratio required for the ferrite was stirred into anexcess of aqueous solution of NaOH and Na

2CO

3.

The mole ratio of Fe and dopant Zn—Sn over Bawas chosen to be 11, which had previously beenfound to be the optimum value [13]. All the re-agents were of analytical grade. A suspensioncontaining intermediate precipitates was formedduring mixing. The product of the co-precipitationwas filtered off, washed thoroughly with deionizedwater until no NaCl could be detected and the pHvalue was less than 8, and then dried at a temper-ature of 60°C. When heated at an appropriatetemperature (760—800°C), fine crystallized Zn—Snsubstituted barium ferrite hexagonal particles wereobtained.

The formation and crystallization temperatureswere determined using a ‘Dupont 1600’ differentialthermal analyzer. The identification of the crystal-line phases in the samples was carried out by X-raydiffraction, using a Philips PW1729 diffractometerwith a copper target. The mean dimensions ofthe substituted barium ferrite particles were deter-mined by the pure diffraction line broadening b andby using the Scherrer equation. The instrumentalbroadening was estimated from X-ray diffractionpattern of SiO

2powders with large dimensions.

The Fourier method was applied to the (1 1 0) and(1 0 7) lines of barium ferrite particles, under theassumption that the diffraction profiles are Gaus-sian. The mean hexagonal diameter D and thick-ness t of the particles were determined from theapparent size e(1 1 0) and e(1 0 7) [14]. To obtaindetailed information on the morphology and toverify the particle size of the BaFe

12~2xZn

xSn

xO

19powder, a set of micrographs were taken by TEM(JEM-100cx II).

Magnetic properties were measured using anOxford superconducting vibrating sample mag-netometer (VSM) with a maximum applied field of80 kOe, at room temperature (292 K). The samplesused for the VSM measurements were cold pressedand disk shaped platelets with a diameter of 3 mmand a height of 1 mm.

To determine the zero-field saturation magneti-zation M

4and estimate the anisotropy field H

!and

crystalline anisotropy constant K1

of the Zn—Snsubstituted barium ferrite, the law of approach to


130 H.C. Fang et al. / Journal of Magnetism and Magnetic Materials 187 (1998) 129–135

saturation (LAS) was used [15]:

M"M4(1!A/H!B/H2)#s

1H, (1)

where M4

is the zero-field saturation magneti-zation, A the inhomogeneity parameter, B theanisotropy parameter and s

1the high-field differen-

tial susceptibility. As for hexagonal symmetry,B may be expressed as

B"H2!/15"4K2

1/15M2

4. (2)

When the demagnetization factor for the shortcylinder (N"0.59]4p for our samples [16]) wastaken into consideration, it was found that thecurve M versus 1/H2 is linear at magnetic fieldsranging from 13 to 22 kOe. This means that theparameters of A and s

1in Eq. (1) can be safely

neglected in this range. Thus, H!

and K1

can becalculated from Eq. (2). To obtain the values of M

4,

A and s1, data from the high-field (40—80 kOe)

section of M—H curve were fitted to Eq. (1), makingthe assumption that the term B/H2 has no effect inthis range.

3. Results and discussions

The XRD patterns of all the as-co-precipitatedproducts show that they are amorphous. DTA datashow an exothermic peak at 760°C for undopedbarium ferrite, indicating the transition to crystal-linity. With the increase of doping concentration x,the peak shifts a little to higher temperatures. Whenx"1.1, it reaches 800°C.

XRD patterns of some samples are given inFig. 1. Clearly in the case of an undoped sample(x"0), although the magnetic phase BaFe

12O

19is

formed when heated at 750°C for 2 h, there is stilla small residual amount of ferrous oxide (see pat-tern b). All the other samples were heated at 800°Cand no other phases were apparently detectable. Itis clear that with the increase of x, the relativeintensities decrease and peaks begin to broaden,indicating a decrease in particle size. Table 1 showsthe diameter (D), thickness (t) and aspect ratio ofdifferent x for BaFe

12~2xZn

xSn

xO

19platelet par-

ticles, measured by XRD line broadening accordingto the Sherrer equation. They are all smaller thanthe critical value of a single domain. Thickness

Fig. 1. X-ray diffraction patterns of BaFe12~2x

ZnxSn

xO

19for-

med by co-precipitation, and in the case of (b)—(g), heat-treated:(a) as-precipitated; (b) 750°C of 2 h (x"0); (c) 800°C for 2 h(x"0); (d) 800°C for 2 h (x"0.3); (e) 800°C for 2 h (x"0.5);(f) 800°C for 2 h (x"0.7) and (g) 800°C for 2 h (x"1.1).

Table 1Particle size of BaFe

12~2xZn

xSn

xO

19and BaFe

12~2xCo

xSn

xO

19measured by XRD line broadening

BaFe12~2x

ZnxSn

xO

19BaFe

12~2xCo

xSn

xO

19

x D (nm) t (nm) D/t D (nm) t (nm) D/t

0 95 76 1.2 — — —0.1 88 66 1.3 — — —0.2 73 32 2.3 196 70 2.80.3 70 25 2.8 — — —0.4 — — — 134 64 2.10.5 55 18 3.1 — — —0.6 — — — 81 33 2.60.7 45 14 3.1 — — —0.8 — — — 87 32 2.70.9 42 16 2.6 — — —1.0 — — — 76 27 2.81.1 43 14 3.1 — — —1.2 — — — 66 22 3.0

t decreases much faster than diameter D, resultingin an increase in aspect ratio. Table 1 also showsthe X-ray particle size of BaFe

12~2xCo

xSn

xO

19fabricated by the same method and reported by


H.C. Fang et al. / Journal of Magnetism and Magnetic Materials 187 (1998) 129–135 131

Fig. 2. TEM micrographs of particles of BaFe12~2x

ZnxSn

xO

19; (a) x"0; (b) x"0.3; (c) x"0.5 and (d) x"0.7.

Yang [17]. We conclude that doping with Zn—Sn ismore effective for decreasing particle sizes thanwith Co—Sn.

Fig. 2 shows the morphology of BaFe12~2x

ZnxSn

xO

19particles with x"0, 0.3, 0.5, and 0.7

taken by TEM. The particles are hexagonal plateletcrystals. It is very interesting that they stack on topof each other due to their magnetic attraction.Because particle sizes obtained by XRD linebroadening are the mean values, those derived fromTEM measurement are comparatively larger thanthose measured by XRD, as has also been reportedby Pernet [14]. Nevertheless, with increasing x,particles become smaller and thinner, which agreeswell with the XRD measurements.

Table 2 lists the lattice parameters of Zn—Sn sub-stituted barium ferrite. Because of radii of the Zn2`

(0.74 A_ ) and Sn4` (0.71 A_ ) ions are larger than thatof Fe3` (0.64 A_ ), as x increases, both a and c par-ameters increase.

Table 2Lattice parameters of BaFe

12~2xZn

xSn

xO

19measured by XRD

x a (A_ ) c (A_ ) c/a

0 5.8778 23.1236 3.930.2 5.8859 23.1937 3.940.7 5.8948 23.2318 3.941.1 5.9070 23.2683 3.94

The effects of Zn—Sn substitution on specificsaturation magnetization M

4and coercivity H

#for

BaFe12~2x

ZnxSn

xO

19particles are shown in

Fig. 3. Coercivity H#decreases linearly and rapidly

with increasing substitution x in contrast to thedecrease of specific magnetization M

4(the deter-

mination of M4will be discussed in the following

section). This is preferable for magnetic record-ing because one can easily control H

#to meet

the requirements of different magnetic recording



Fig. 3. The effect of Zn—Sn substitution on coercivity H#

and saturation magnetization M4

for BaFe12~2x

ZnxSn

xO

19particles.

systems, while still maintaining a relatively highvalue of M

4. The fact that M

4decreases very slowly

for x(0.7 may be due to the strong preferentialoccupancy of Zn2` into the 4f

1(tetrahedral) sublat-

tice sites, which have a negative contribution toM

4[18]. However, as the doping amount is further

increased (x'0.7), the subsequent decrease in theamount of Fe3` ions in the tetrahedral sites reduc-es the tetrahedral (A)—octahedral (B) interactionpath number. The negative B—B interaction thenbecomes dominant over the A—B interaction andsome Fe3` ions in the octahedral up-spin siteschange their spin direction. Thus, M

4decreases

sharply.As is well known, the coercivity of the substituted

barium ferrite particles is attributed to both thecrystalline anisotropy and the shape anisotropy asexpressed by

H#"C(2K

1/M

4!NM

4), (3)

where C is a constant and N is the demagneti-zation factor. The compositional dependences ofthe effective anisotropy constant K

1(x), anis-

otropy field H!(x) and the coercivity H

#(x) for

BaFe12~2x

ZnxSn

xO

19particles are shown in

Fig. 4. H!and K

1are estimated from the linearity

of M versus 1/H2 in the considered magnetic field

Fig. 4. Effect of x (BaFe12~2x

ZnxSn

xO

19) on anisotropy con-

stant K1

and anisotropy field H!.

range, together with Eq. (2). For x"0, the resultsagree well with the reported value of 17 kOe and3.2]106 erg/cc, respectively [19]. The effective crys-talline anisotropy constant K

1decreases rapidly

with increasing x because of the substitution ofFe3`. Since the Zn2` ion has strong preference forthe 4f

1tetrahedral site, which is ineffective in the

reduction of the magnetocrystalline anisotropy,the rapid decrease of K

1may be attributed to the

occupancy of Sn4` ions in the 2b octahedral sites[20]. It is well known that 2b site has the greatestcontribution to the anisotropy.

If we extrapolate the curve of K1, it predicts that

the uniaxial anisotropy will disappear at x"1.6,i.e. BaFe

12~2xZn

xSn

xO

19with x'1.6 will have

planar anisotropy. However, for Co—Sn substitu-tion, the reported value is x"2.4 [17]. This meansthat the Zn—Sn combination is also more effectivein decreasing the anisotropy field than Co—Sn andshows the importance of counter ions on the dop-ing of barium ferrite.

Fig. 4 also shows that the dependence of H!and

H#on x are very similar, indicating the crystalline

anisotropy is the dominant factor in the determina-tion of the magnetization reversal processes forthese particles.

The high-field section of the M—H curves fordifferent value of x are shown in Fig. 5 and datafrom the 40—80 kOe range were applied to Eq. (1) in



Fig. 5. High-field section of isothermal magnetizationM—H curve for Zn—Sn substituted barium ferrite(BaFe

12~2xZn

xSn

xO

19) as a function of doping concentration

x at room temperature.

order to determine M4, A, and s

1. In Eq. (1), the

A/H term is related to the existence of in-homogeneities in the microcrystals which reducethe mobility of the magnetization, and whichtheoretically should disappear at high enoughmagnetic fields [21]. Above 40 kOe, the B/H2

term is negligible as B is of the order of 107. s1

is due to the contribution of non-collinear spinsin the magnetic structure, and it appears fre-quently in M-type doped barium ferrite as a conse-quence of the disruption of the collinear uniaxialmagnetic structure on doping with non-magneticcations.

The fitting results of the effect of substitutionx on the inhomogeneity constant A and the high-field susceptibility s

1of BaFe

12~2xZn

xSn

xO

19are

shown in Fig. 6. A decreases with increasing x,which is probably due to the decreasing of theparticle size. Neel reported that the term in A/Hshould be largely attributed to the presence of voids[22], and that the more porous the sample, themore difficult it is to magnetize it to saturation dueto the interaction of surface magnetic charges. Thesamples used for the VSM measurements wereshort cylinders cold-pressed from powders, andsince the particle size decreases with increasing x,

Fig. 6. Effect of substitution x (BaFe12~2x

ZnxSn

xO

19) on the

inhomogeneity constant A and the high-field differential suscep-tibility s

1.

the cavity volume fraction of the samples will alsodecrease. Therefore, the decrease of A may be at-tributed to this effect.

The increase of s1

with decreasing particle sizehas been observed experimentally in pure bariumferrite nanoparticles, and was thought to be theresult of the enhancement of the finite-size effectand of surface-spin canting [23]. However, as forZn—Sn substituted barium ferrite, the increase ofs1

versus x could also result from another impor-tant factor: the substitution of Fe3`. When Fe3` issubstituted by the non-magnetic cations Zn2` andSn3`, part of the super-exchange interactions be-tween Fe3` ions via O2` will disappear, and conse-quently the magnetic structure collinearity willweaken [24]. From Table 1, we see that there isa sharp decrease both of particle diameter D andthickness t from x"0 to x"0.5. When x'0.5,D and t remain almost constant. If the surface-spincanting plays a dominant role, there should bea rapid decrease of M

4and increase of s

1before

x"0.5. However, if we look at Figs. 3 and 6, it canbe seen that the sharp decrease in M

4and the large

increase in the high-field differential susceptibilitys1, do not appear until x'0.7. This means that

the disappearance and suppression of Fe3`—Fe3`super-exchange interactions, accompanied by a de-crease of magnetic structure collinearity, is themain reason for the sharp increase of s

1.



4. Conclusions

Zn—Sn substituted barium ferrite (BaFe12~2x

ZnxSn

xO

19where 0)x)1.1) have been prepared

for the first time using a chemical co-precipitationmethod. It was shown that particle size, determinedby XRD and verified by TEM, could be effectivelydecreased by doping with a Zn—Sn combination.The magnetic properties were measured by VSMand were calculated according to the approach-to-saturation law. It was found that the coercivitycould be easily controlled by changing the substi-tution concentration x, without significantly de-creasing M

4. The fitted results of the high-field

susceptibility s1

also show that with increasing x,s1

increases, mainly due to the disappearanceof some super-exchange effects. BaFe

12~2xZn

xSn

xO

19particles with x"0.7—1.1 have good

characteristics (M4

around 50 emu/g, H#"1700—

3000 Oe, D"40—45 nm, and D/t"3) and are suit-able for high-density magnetic recording.

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Preparation and magnetic properties of (Zn–Sn) substituted barium hexaferrite nanoparticles for magnetic recording