ANISOTROPY AND RELAXATION IN MANGANESE RICH MANGANESE FERRITES

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ANISOTROPY AND RELAXATION IN

MANGANESE RICH MANGANESE FERRITES

K. Závěta, P. Novák

To cite this version:

K. Závěta, P. Novák. ANISOTROPY AND RELAXATION IN MANGANESE RICH MANGANESE FERRITES. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-64-C1-66.

�10.1051/jphyscol:1971116�. �jpa-00213980�

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JOURNAL DE PHYSIQUE Colloque C I , supplkment atl no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1

-

64

ANISOTROPY AND RELAXATION

IN MANGANESE RICH MANGANESE FERRITE S

K.

ZAVETA

^and^{P. NOVAK}

Institute of Solid State Physics Czechoslovak Acad. Sci. Prague, Czechoslovakia

RhumB.

-

Des nlesures de rksonance ferromagnktique ont kt6 rkaliskes sur des Bchantillons monocristallins de ferrite de mangankse Mn,Fes-204 avec x = 1,013 et x = 1,46 dans la gamme de tempkrature de l'hklium liquide jusqu'b la tempkrature ambiante. Pour l'echantillon x = 1,46 la d6pendance angulaire du champ de rksonance a kt6 mesurke dans le plan (110) a des tempkratures de 2 OK, 4,2 OKet 77 OK. La variation de la largeur de raie en fonction de la temperature prksente un maximum a approximativement 10 OK, en accord avec des etudes anterieures. La dkpendance angulaire de la largeur de raie au voisinage de ce maximum est en accord avec l'hypothese d'une configuration de spins inclinks dans le sous-rkseau octakdrique de structure spinelle.

Abstract. - Measurement? of ferromagnetic resonance were performed on single crystal samples of manganese ferrites MnsFe3-,04 with x = 1.013 and x = 1.46 in the temperature range from liquid helium up to room temperatures.

For the sample x = 1.46 the angular dependence of the resonance field was measured in (110) plane at temperatures 2 OK, 4.2 OK and 77 O K . The temperature dependence of linewidth displays a maximum at approximately 10 OKin agreement with earlier studies. The angular dependence of linewidth in the vicinity of this maximum is consistent with the assump- tion of canted spin configurations in the octahedral sublattice of the spinel structure.

Introduction. - During ferromagnetic resonance studies on the Mn-ferrite system it was found earlier [I] that the manganese rich compositions display a maximum of linewidth a t about 10 OK. Clarke's explanation of the linewidth [2] based on the slow- relaxation type transitions between lowest lying levels of h4n3+ ions gives unfortunately a wrong angular dependence.

The contribution of Mn3+ ions to magnetocrystalline anisotropy in the cubic part of the Mn,Fe3 -,Oh system was also extensively studied [3], the experimental results being mostly based on static torque measurements [4]. The latest results indicate [5] that also higher anisotropy constants may be relevant.

Experimental. - Our measurements were per-

formed on single crystal spheres of Mn,Fe3-,04 fer- ^K, ^-¹⁹

rites ( x = 1.013 and 1.46) with a diameter of 0.2 t o

0.3 mm. Single crystals were grown by a modified -'"05' ⁵⁰ ¹⁰⁰ ¹⁵⁰ ^ZOO zso

Bridgman-Stockbarger method. The spheres were ^T^OK

oriented by a L~~~ method ; measurements FIG. 1. - Temperature dependence of the anisotropy fields and g-factor. In brackets the value of x in formula MnlFes-ZOd

were carried out in the (110) plane. _isindicated.

I n the first series of measurements. ~erformed a t the University of Maryland, we studied the temperature dependence of linewidth and resonance field in the direction of the three principal cubis axes, down t o liquid helium temperatures. The frequency was stabilized to the resonance frequency of the cylin- drical microwave cavity working in the TEoll mode and equalled appr. 35 GHz. The external static field was modulated a t a frequency of 44 Hz and the signal was detected by means of a lock-in amplifier.

The density of magnetocrystalline anisotropy energy was taken as

2 2 2

E, = Kl(a: a:

+

a: a:

+

a:

c13 +

K 2 ^a1^a,a, (I) with ai the directional cosines of magnetization. From the resonance conditions for the [100], [llO] and [lll] directions [6] the anisotropy fields K J M , K2/M and the g-factor were calculated (Fig. 1).

The temperature dependence of linewidth taken as the field difference between the maxima of the absorption derivative curve is plotted in figure 2 for the composition Mn,.4,Fe,.,404. The other composition with x = 1.013 displayed also a local maximum of linewidth a t approximately the same temperature, but much less pronounced and well bellow the linewidth of the smooth background.

Because we were interested in more detailed descrip- tion of the magnetocrystalline anisotropy, the complete angular dependences of resonance fields were measured. We used the JES-3 BQ spectrometer JEOL with K-band cavity operated a t appr. 24 GHz. The measurements were only performed at 77, 4.2 and 2 OK. The results for two temperatures are displayed in figure 3 (together with calculated curves described further).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971116

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ANISOTROPY AND RELAXATION IN MANGANESE C 1 - 6 5

/

and the equation of equilibrium

MH(8,) sin (8 - 8,)

+

(3)

4

I where q , 8 are the spherical angles of magnetization,

I I 8, is the angle between the external magnetic field

o! 50 100 150 200 and the [001] direction, E, has the form (1) and y,

250

T OK 300 Kl, K2 are calculated as before.

FIG. 2. -Temperature dependence of linewidth for Mn1.46Fe1.5404.

FIG. 3. - Angular dependence of the resonance field in the (110) plane. Full lines represent tht results of theoretical calcu- lation, dashed line was obtained using conventional linear

approximation.

Discussion. - As was shown previously [3] an agreement of theory with experiment can be reached, if the anisotropy of manganese rich manganese ferrites is ascribed primarily to the one-ion contributions of the Mn3+ ions subject to a tetragonal crystal field.

In order to describe the situation more exactly, the dynamic Jahn-Teller effect of Mn3' has to be consi- dered [8].

The measured angular dependence of the resonance field in (110) plane was evaluated in the follo- wing way. We have taken the resonance condition [6]

in the (110) plane

The two unknown quantities H(8,) and 8 were determined from eqs (2), (3) by the itteration proce- dure. The usual way of evaluation of H(8,), in which the quantities proportional to (Ki/M)< (K~/M)~...

are neglected, was found insufficient in our case.

From the agreement between the measured and calculated curves (Fig. 3) we can conclude that in contrast to the analysis performed for static measurements [51, our results can be well described by K, and K2 only.

As was already mentioned, Clarke's model for the linewidth does not agree with experiment. If, however, the noncollinear spin arrangement, which probably exists in manganese rich manganese ferrites, is addi- tionally taken into account, a dramatic change occurs.

Assuming that the spins are randomly distributed over a cone surface, each spin making an angle /3 with the resulting magnetization, the [001] linewidth increases with increasing j3 and at angles of about 30°

becomes even the largest of the three, as is seen from

FIG. 4. - Theoretically calculated dependence of the ratio AHIAH [ I l l ] on the canting angle B, for a temperature at which

the maximum linewidth occurs.

Fig. 4, presenting our calculations. This case describes well the experimental situation (see Fig. 2) and .the magnitude of the canting angle is in reasonable agreement with susceptibility measurements [7] and with the analysis of magnetocrystalline anisotropy [3].

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Acknowledgement. - The authors express their Dr S. Bhagat and Dr S. Krupitika for valuable~discus- gratitude to the University of Maryland for supporting sions. Our thanks are also due to Mr E. Lee and Mr part of this study during the stay of one of us and to M. MarySko for the help with the measurements.

References

[I] CLARKE (B. H.) and TEALE (R. W.), J. Appl. Phys., KRUPIEKA (S.) et al., Czech. J. Phys., B, 1968, 18,

1964, 35, 892 S. 1016.

[2] CLARKE (B. H.), J. Phys. Chem. Sol., 1966, 27, 353. [5] NOVAK ^(P.)et al., paper at this Conference.

[3] KRUPIEKA (S.) et al., Proc. Int. Conf. Magnetism, [6] ARTMAN (J. O.), Phys. Rev., 1957, 105, 62.

Nottingham, 1964, p. 650. [7] MORUZZI (V.), J. Appl. Phys., 1961, 32, 59 S.

[4] PALMER (W.), 2. Appl. Phys., 1962, 33, 1201 S. [S] NOVLK (P.), Czech. J. Phys., 1970 B 20, 259.