Indian Journal of ChemistryVol. 21A, July 1982, pp. 714-715

Synthesis & Characterisation of DopedGamma Ferric Oxide

M R ANANTHARAMAN, S S SHEWALE, K SESHAN &H v KEER*

Solid State Laboratory. Department of Chemistry, Indian Instituteof Technology, Bombay 400076

Received 25 January 1982; revised and accepted 3 April 1982

y-Fe20J samples doped with Mg/Co/Ni/Cu have been prepared.The doped compositions are all monophasic and their magneticproperties have been explained on the basis of the added dopantions.

Maghemite (y-Fe203), a useful material for magnetictape recording", is a vacancy ordered spinel having thestructure Fe3 + [Fd/~ 0 1/3]01 -, where the vacancies arefound exclusively on the octahedral sites (B Site)", Thiscompound is highly difficult to form and is unstabletowards heating; it is converted into y-Fe203 at 673 K.

In the present note, efforts have been made tostabilise the structure of y-Fe203 by doping it withsmall amounts of magnesium, cobalt, nickel andcopper. The effect of this substitution on the variousproperties is also discussed.

The y-Fe203 prepared by the decomposition ofacicular ferrous oxalate? under nitrogen atmospherecontaining water vapour has been shown to retain theacicular shape of the ferrous oxalate and found to havethe magnetic properties required for a recordingmaterial. Essentially a similar method has beenfollowed here to prepare y-Fe203 doped with thedifferent ions mentioned.

Mixed oxalates of iron and the dopant (Mg/CofNi/Cu) were co-precipitated from a solution of thecorresponding sulphates in required proportions,using oxalic acid. The medium for precipitation wasstarch and water in order to yield needle-like crystals ofthe oxalate. The dopant concentration was variedbetween I and 16 atomic per cent.

The decomposition scheme and the products wereana lysed using the techniques like OTA, X-ray powderdiffraction, magnetic hysteresis loop parameters andtransmission electron microscopy.

The DT A of all the mixed oxalates in moistnitrogen" (PH20 = 35 torr) shows two characteristicendothermic peaks, corresponding to dehydration (- 495 K) and decomposition (- 710 K). The dopedcompounds were, therefore. prepared by isothermallydecomposing the mixed oxalates around 710 K for 30min and then oxidising the products at 475 K in a

714

Table I-Lattice Parameter and Dopant CompositionsComposition Lattice parameter. A(atomic X) -----.-----------.---

Cobalt Nickel Magnesium Copper

1 8.33 8.32 8.33 8.322 8.35 8.32 !U4 8.324 8.35 8.32 8.33 8.346 8.34 8.32 8.33 8.368 8.35 8.32 8.34 8.33

10 8.34 8.32 8.33 8.3216 8.34 8.32 8.34 •

* Biphasic

Table 2-Magnetic Properties of Doped y-Fez03Dopant Cobalt Nickel Magnesium Copper

(atomic %)-- -- -----a, He a, He a, He a, He

I 94 270 83 225 89 320 76 3002 71 382 86 270 100 270 80 2704 74 371 78 250 76 230 94 3206 72 562 75 230 91 270 80 2508 90 810 73 230 81 210 77 250

10 93 675 81 250 80 200 69· 23016 81 675 74 250 86 200 55· 250

a, in ernu/g and He in oersteds• Unsaturated

stream of air. All the compositions were found to bemonophasic and there was practically no change in thelattice parameter, from that of pure y-Fe203, ondoping (cf. Table I).

The magnetic properties of the compounds are listedin Table 2. A few interesting points emerge from thesedata. The saturation magnetisations (a.) of the dopedcompounds are generally higher than that of pure y-Fe203' One would expect a slightly lower value of as ifthe dopant compounds exist as a mixture along with y-Fe203' This is indeed shown by one compound (Ii>atomic per cent CUI + doped y-Fe203) which isbiphasic and shows lower as value (55 emu/g) ascompared to that of pure y-Fe203 (74 emu/g). Thehigher o; values, therefore, are an indication of the factthat the doped ions have entered the vacant B sites,since Ni2 +, Co2 + and to a larger extent Cu2 + havestronger B site preference", Though the addition of amagnetic ion to the B site is expected to increase the netmagnetisation, the increase with diamagnetic MgI +

addition is surprising. This is probably due to thedisplacement of some amount of Fe3 + ions fromtetrahedral (A) to octahedral sites by the Mg2 + ions

which may to some extent be occupying the A sites. Asexpected, only the cobalt substituted compounds haverelatively high coercive force resulting from theiruniaxial anisotropy. Further, the doped compoundsretain the required acicular shape.

The problem of confirming the presence of thedopant ions in the lattice and their ability to stabilizethe y-Fe203 structure remains to be solved. However,the results obtained so far give a fair indication of thepossibility of doping various ions in y-Fe203 via theoxalate method.

NOTES

The authors are grateful to (Late) Prof. A.B. Biswasfor his guidance and to Indian National ScienceAcademy for financial assistance.References1 Mollard P, Collomb A, Devnyi J, Rousset A & Paris J, IEEE

Trans, 11 (3) {I 975) 894.2 Bate G & Alstad J K, IEEE Trans, Mag, 15 (l969) 821.3 Anantharaman M R, Seshan K, Chakrabarty D K & Keer H Y,

Bull Maler Sci, 3 (3) (1981) 275.4 Anantharaman M R, Shewale S S, Rao Y S, Seshan K & Keer H

Y, Indian J Chern, 21A (1982) in press.5 Greifer A P, IEEE Mag, 5 (4) (1969) 774.

715