Resonance Widths in Polycrystalline Nickel-Cobalt Ferrites

Abstract

The width of the ferromagnetic resonance has been measured at 10 000 Mc/sec in polycrystalline mixed ferrites of the composition Co a Nii_ a Fe20 4 , with a between 0 and 0.04. The minimum line width is found for a=0.025 at 20°C. This, as well as the variation of line width with temperature, can be understood on the basis of known single crystal properties and the following assumptions: (a) line width is a linear function of \K\; (b) crystal anisotropy is an additive atomic property. S EVERAL papers 1-3 have been published recently on the crystal anisotropy of cobalt ferrite and of mixed ferrites containing cobalt. The points of interest here are the following: (1) cobalt ferrite has an exceptionally large crystal anisotropy and, uniquely among known ferrites, a preference for magnetization along the [100] directions (Xi>0); (2) Co+ 2 ions tend to occupy octahedral sites as do Ni +2 ions; (3) for either nickel or cobalt ferrite the anisotropy above room temperature is quite well described by a K x term alone, the magnitude decreasing smoothly to zero atjr c ; (4) the Curie temperatures of the two ferrites are nearly equal and high (about 600 °C). Under these conditions, one expects that a ferrite of the type Co a Nii_ a Fe 2 04 will have a crystal anisotropy constant Ki equal to the weighted algebraic sum of the values of the individual ferrites. In particular, a composition can be picked with Ki = 0 corresponding to a certain ao. For a<a 0 , Ki<0; for a>a 0 , Ki>0. Moreover, since \dKi/K\dT\ is greater for cobalt ferrite than it is for nickel ferrite, one expects the following qualitative behavior of Ki of the mixed ferrite as a function of temperature: (a) a<<Cao: |i^i| decreases for all tempera-

The Temperature Behavior of Resonant and Non-resonant Microwave Absorption in Ni-Zn Ferrites

“…It appears that anisotropy, and in particular magnetocrystalline anisotropy has a strong contribution to total linewidth. By measuring nickel ferrite with Co 2+ substitutions, Sirvetz & Saunders (1956) observed a minimum in linewitdth for the composition corresponding to the compensation of anisotropies (x = 0.025 in Co x Ni 1-x Fe 2 O 4 ), since nickel ferrite has a small negative contribution (single-ion contribution to anisotropy), while cobalt cations provide a strong positive contribution to the total magnetocrystalline anisotropy. More recently, Byun et al (2000) showed that in the case of Cosubstituted NiZnCu ferrites, ∆H increases for a Co composition higher than the magnetocrystalline anisotropy compensation point.…”

Switching Properties of Ferrites and Garnets

“…One, K' has its origin in the iron lattices, the other, K2, in the rare earth lattice. X = X' + '2 (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23) As shown in Sec. c, K Xis large; X has been discussed above and should be obtainable from resonance by application of the relation, Eq.…”

“…Since the nickel and cobalt ferrites have anisotropies of opposite sign, it is conceivable that a combination of these, in the correct proportions, would have a vanishing macroscopic anisotropy. Some evidence of this occurring is offered by the microwave measurements of Pippin (20) and Sirvetz (21). From their results, it appears that the resultant anisotropy may be found by again proportioning the anisotropy of the constituents by the amount each contributes to the total ferrite (see Table 3 These, together with Wolf's curves and Eq.…”

“…(6-18) can be written as S 2 X1 +"2 (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) From this equation it may be seen that K consists of two parts. One, K' has its origin in the iron lattices, the other, K2, in the rare earth lattice.…”

“…With these relationships a dependence of S on the w degree of substitution can be established. The first of these Is taken from [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] de Gennes et al (15) and the reasoning given above in connection with the gadolinium garnet. It gives the relative linewidth, (AH/H)x, as a function of the atomic fraction, x, of the yttrium sites occupied by rare earth ions: ( = X--H (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) Here AH/H is the relative linewidth for complete substitution (x = 1).…”