ON THE RELATIVE EFFICIENCY OF COBALT IN THE LINEWIDTH REDUCTION OF MICROWAVE FERRITES

(1)

HAL Id: jpa-00214474

https://hal.archives-ouvertes.fr/jpa-00214474

Submitted on 1 Jan 1971

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

ON THE RELATIVE EFFICIENCY OF COBALT IN THE LINEWIDTH REDUCTION OF MICROWAVE

FERRITES

S. Banerjee, P. Baba, B. Evans, S. Hafner

To cite this version:

S. Banerjee, P. Baba, B. Evans, S. Hafner. ON THE RELATIVE EFFICIENCY OF COBALT IN THE LINEWIDTH REDUCTION OF MICROWAVE FERRITES. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-145-C1-147. �10.1051/jphyscol:1971147�. �jpa-00214474�

(2)

JOURNAL DE PHYSIQUE Colloque C 1, suppk!ment au no 2-3, Tome 32, Fe'vrier-Mars 1971, page C 1

-

¹⁴⁵

ON THE RELATIVE EFFICIENCY OF COBALT

IN THE LINEWIDTH REDUCTION OF MICROWAVE FERRITES *

by S. K. BANERJEE* and P. D. BABA Ampex Corporation, Redwood City, Calif.

and B. J. EVANS and S . S. HAFNER University of Chicago, Chicago, Illinois

Ri5sum6. - Nous avons obtenu la largeur de raie de resonance ferrimagnktique AH B 300 OKpour que1ques valeurs de la substitution du CoZ+ dans les ferrites de Li, Li-A1 et Li-Ti. Les changes du rendement en rkduction du A H dans ces trois ferrites s'expliquent par les variations de symktrie du champ 15lectrostatique aux sites du Co2+. +am&me thkorie phkno- menologique peut aussi expliquer l'effet obtenu auparavant pour la magnetite et les ferrltes du NI et du Mn.

Abstract. - We have observed the ferrimagnetic resonance linewidth, AH, at 300 OKfor various values of Coz+-subs- tituti0n.h lithium (Li) ferrite, lithium-aluminum (Lj-A1) ferrite and lithium-t~fanium (Li-Ti) ferrite. The variability in the efficiency in linewidth reduction in the above ferrites is explatned by the variation of electrostatic fields at the Co2+-sites.

The same phenomenological theory can also explain the effects previously observed in magnetite, nickel ferrite and manganese ferrites.

Introduction. - The contribution of positive anisotropy (-I- K,) per atom of Co2+ is variable [la-el from one host ferrite to another. In addition, Co2+

substitution can sometimes [2] be totally ineffective.

We have attempted to clarify the role of Co2" in lithium ferrites with different diamagnetic substitutions and then draw some conclusions about spinels in general.

Experimental observations. - Polycrystalline sin- tered ferrites (for sintering conditions, see Table 1) were used to obtain resonance linewidth minima at frequencies of 8.9 and 2.8 GHz at room temperature.

The linewidths at half-power points were plotted as a function of Co2+ substitutio~l and a minimum in the linewidth was interpreted as the point of anisotropy cancellation, all other variables including porosity, having a negligible variation. No second phases were detected by optical microscopy.

In figure 1 the variation of resonance linewidth in the different ferrites as a function of Co2+ substitution is shown. From torque measurements in pure Li-ferrite Stel'mashenko [lb] has confirmed that at 300 ^OK, a substitution of 0.015 atoms per formula unit of Co2+

indeed causes the first order anisotropy constant (K,) to go to zero. The linewidth minimum for Li-A1 ferrite at 2.8 GHz occurs at a substitution level of 0.007 Co2' per formula unit.

For the first sample of Li-Ti ferrite, Li-Ti (I), no

t Present address : The Franklin Institute, Philadelphia, Pa.

(*) Work supported by Lincoln Laboratory. M. I. T.

FIG. 1. - Variation of resonance linewidth (cersteds) with cobalt substitution in lithium ferrite [Li], lithium-aluminum

ferrite [Li-All and lithium-titanium ferrites [Li-Ti].

decrease in linewidth at 2.8 GHz is observed. At substitutions greater than 0.005 atoms of Co2

*

per formula unit, the linewidth increases monotonically, essentially as reported [2] before. Because of a lower iron content, TABLE 1

Sintering Conditions of the Ferrite Samples

Ferrite Chem. Formula Temperature Duration Atmosphere

- -

-

- -

Li Li.5- .5xFe2.5 - . 5 ~ C ~ ~ 0 4 1 150OC 5 h 1 atm. 0,

Li-A1 Li.~-.~~A~.ssFez.~-.s~Co~04 1 200 OC 5 h 1 atm. O2

Li-Ti (1) Li.8,- . s ~ ~ ~ . 7 ~ ~ 1 . 4 5 - .5xC0x04 1 150 OC 5 h 1 atm. O2 Li-Ti (2) ^Li.85- . ~ ~ ~ ~ , 7 ~ ~ 1 . 4 5 - .sxC0~04 1 175 OC

5B

^{1 atm.}^0,

12

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971147

(3)

C 1 - 146 S. K. BANERJEE AND P. D. BABA AND B. J. EVANS AND S. S. HAFNER it may be argued, that only a minute amount of Co2+

is required to obtain anisotropy cancellation. However, for the second, Li-Ti (2) ferrite, no minimum is seen even when Co2 + is added in steps of 0.0025 atoms per formula unit. Cobalt cannot be present as Co3+

because Roiter and Paladino [3] have shown that Co3+

is difficult to maintain in the spinel structure. Further- more, the approximately 200 Oe increase of linewidth seen for Li-Ti (1) ferrite cannot be explained on the basis of Co3+.

The above observations led us to explore whether a trigonal electric field, so necessary for the effecti- veness of cobalt doping, was present in the Li-Ti ferrite and if so, how its magnitude varies with x.

-1.6 -1.2 -0.8 -0.4 0 0.4 0.8 1.2 1.6 VELOCITY ( MM / S j

FIG. 2. - Spectra of Li.s+.5sFe2.5-1.5xTiz04 in the parama- gnetic state. For (ax), x = 0 ; (bl), x = 0.4 ; (cI), x = 0.7.

Host ferrite - Fe304 Li.5Fe2.504 Li.sFe2.504 Li.5Fe1.95A1.5504 NiFe,.gMn.104

We therefore observed the Mossbauer spectra (Fig. 2) above the Curie points to obtain information on the total quadrupole splitting. Two interesting features can be noted. First, by making our measurements above the Curie point, we have, for the first time, observed the quadrupole splitting in pure lithium ferrite. Secondly, the doublet seems to be due to two (tetrahedral and octahedral iron) quadrupole split doublets, with the quadrupole splitting greater than the individual linewidths. As more titanium is added, the quadrupole splitting does not vanish, but contra- rily, it continues to increase. Whether the increased splitting is due to an increase in the beneficial trigonal distortion of the octahedral sites or to an increase of the local distortions [4] caused by the fluctuation of ionic charges (Lif, Fe3+, Ti4+) at equivalent neighboring sites, cannot be quantitatively decided. But, in view of the different ferromagnetic resonance linewidth behavior (Li vs. Li-Ti) it js very likely that the observed increase in average quadrupole splitting is mostly due to ionic charge fluctuations.

Discussion. - The above theory of charge fluctuations can describe the

+

Kl contribution of Co2+

ions in other magnetic spinels too. In Table I1 below, the compounds are listed in order of decreasing f K, contribution. A reference to figure 3 will explain how the desirable negative trigonal field is spoiled by increas- ing non-trigonal fields. Octahedral sites in magnetite have nearly cubic site symmetry with a pure trigonal distortion causing a beneficial twofold degenerate level. However, in lithium ferrite some of the octahedral cation sites have Fe3+ and Li+ neighbors, the latter causing an additional monoclinic contribution to the trigonal field and thus, a small tendency to split the degenerate P,, orbitals with a concomi- tant decrease in the

+

K, contribution from Co2+.

In lithium-aluminum ferrite, the local symmetry for Co2+ has not changed except that the total negative anisotropy is much less due to a lower iron content.

As a result, only a small amount of Co2+ is required.

In nickel ferrite, the Ni2+ and Fe3 ⁺ions are neighbors

TABLE I1

Efectiveness of Co2+ in anisotropy cancellation at 300 OK

K, of host ferrite

(ergs

.

~ m - ~ ) -

- 1.1 x 105

-

7 x 104

Atoms/

formula unit of c o 2 + necessary

for cancellation

- 0.01 0.015 0.015 0.007 0.025 0.06

No cancellation

obtained

+

^K1

contribution per Co2+ atom

(ergs. cm- 3,

- 1.1 x 107 0.5 x lo7

Reference - Bickford [ l a ] Stel'mashenko [I b]

Present work Pippin & Hogan [Ic]

Sirvetz & Saunders [Id]

Pearson [le]

Present work

(4)

ON THE RELATIVE EFFICIENCY OF COBALT IN THE LINEWIDTH REDUCTION C 1 - 1 4 7

of Co2+, the P,, orbitals are no longer degenerate and Co2+ atoms contribute a small

+

^K,to the sys- tem and this requires more Co2 ⁺to obtain a minimum in anisotropy. MnFe204 is predominantly a c i nor- mal ⁾⁾ferrite and hence an inversion of the sign of the trigonal field is expected [5]. This causes the ground states to be occupied by an equal number of spins of opposite signs and again the net contribution to AL.S and -t K, is small. Lithium-titanium ferrite is an unsuitable host because octahedral Co2+ has neighboring atoms, Li', Fe3+ and Ti4+. This generates strong nontrigonal fields, the orbitals P,, are nonde- generate and Co2+ is ineffective in anisotropy cancellation.

Acknowledgements. - We are happy to acknow- Iedge the detailed discussions we have had with Profes- sor J. Smit of the University of Southern California on the subject matter of this paper. We are also gra- teful to Professor Robert L. White of Stanford Univer- sity and Mr D. H. Temme of Lincoln Laboratory, M. I. T., for helpful discussions, and to Messrs.

M. Chan and J. Tolvanen of Ampex Corporation for various measurements.

MAGNETITE L I T H I U M FERRITE

NICKEL FERRITE

\+id.L

\\<+:::

lo). Cubic

'+-

f Trigona/ ^/C).Trigonol f Ofher /Medium)

MANGANESE FERRITE LITHIUM-Ti FERRITE

'

\

\ , , , (

;

*\\qt

^{d - l}^{+ I}

d o (dl. Inverted Trigonol (e). + Ofher

/L urge/

FIG. 3. - Ground state orbitals for various spinel ferrites.

Splittings are approximate.

References BICKFORD (L. R.), BROWNLOW (J. M.), PENOYER

(R. F.), Proc. IEE, 1957, B 104, 238.

STEL'MASHENKO ( M . A.), Soviet Phys.-Solid State, 1967, 9 , 1137.

PIPPIN (J. E.). HOGAN CC. L.). Trans. IRE-MTT. , ,

1958, 6 , 17.

SIRVETZ ( M . H.), SAUNDERS ( J . H.), Phys. Rev., 1956, 102. 366.

PEARSO; ( R . F.), PYOC. Phys. Soc. (London), 1959, 7 4 , 505.

[2] Final Report No WF-2430, Contract No Nobsr 77618, Motorola, Inc., 1961, 31.

[3] ROITER (B, D.), PALADINO (A. E.), J. Amer. Cerarn.

SOC., 1962, 45, 128.

[4] BANERJEE (S. K.), O'REILLY (W.), GIBB (T. C.), GREEN-

WOOD (N. N.). J. ,, Phvs. Chem. Solids. 1967.

28, 1323.

[5] SUIT (J.), LOTGERING (F. K.), VAN STAPELE ( R . P.), J. Phys. Soc. Japan, 1962, 17 Suppl. B-1, 268.