HYSTERESIS OF THE MICROWAVE ABSORPTION OF BaFe12O19 POWDER WITH VARIOUS DEGREE OF ANNEALING

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HYSTERESIS OF THE MICROWAVE ABSORPTION OF BaFe12O19 POWDER WITH VARIOUS DEGREE

OF ANNEALING

K. Hempel, H. Kmitta

To cite this version:

K. Hempel, H. Kmitta. HYSTERESIS OF THE MICROWAVE ABSORPTION OF BaFe12O19 POWDER WITH VARIOUS DEGREE OF ANNEALING. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-159-C1-160. �10.1051/jphyscol:1971152�. �jpa-00214479�

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

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159

HYSTERESIS OF THE MICROWAVE ABSORPTION OF BaFe,,O,, POWDER WITH VARIOUS DEGREE OF ANNEALING

K. A. HEMPEL (*) and H. KMITTA

Institut fiir Angewandte Physik der Universitat Miinster, Germany

R6sum6. - On a mesure a 9,4 GHz et la temperature ambiante I'absorption magnetique de poudre de BaFelzOls moulue de ferrite de barium fritte et traitee aux temperatures entre 300 OC et 1 000 OC. Lorsqu'on varie le champ magnetique continu cycliquement entre

+

25 kOe on observe des pertes aux champs faibles considerables avec un maximum a 2 kOe environ qui montrent un hysteresis prononce, en dehors de la resonance ferromagnetique qui se montre a 15 kOe environ. Ces pertes et la largeur de raie de la resonance ferromagnetique dkcroissent en temperature de traitement crois- sante. On voit par des mesurages le long des boucles d'hystersis intkrieures que l'absorption aux hyperfrequences depend distinctement de l'6tat ferromagnetique de l'echantillon.

Abstract. - The magnetic absorption of BaFelz01~ powder, milled from sintered barium ferrite and annealed at temperatures between ³⁰⁰OC and 1 ⁰⁰⁰OC was measured at 9.4 GHz and room temperature. When the dc field is cycled between f 25 kOe considerable low field losses with a peak at about 2 kOe are observed which show a pronounced hysteresis, apart from the FMR occuring at about 15 kOe. The low field losses and the line width of the FMR decrease with increasing annealing temperature. Measurements along minor hysteresis loops show that the microwave absorption depends strongly on the ferromagnetic state of the sample.

Hysteresis of the microwave absorption of ferromagnetic single domain crystals, especially of ferrites with strong anisotropy, may be well understood as a consequence of the hysteresis of the ferromagnetic resonance frequency according to the Stoner- Wohlfarth model (Mikhaylovskiy [I]). Assuming a polycrystalline sample consisting of non-interact- ing single domain crystallites with positive uniaxial anisotropy and a microwave frequency sufficiently below the natural ferromagnetic resonance frequency a pronounced additional absorption peak at

H

--

^HA/2

should appear if the dc field direction is opposite to that of previous saturation of the sample (Hem- pel [2, 31). The usual ferromagnetic resonance which is the only absorption observed if the dc field is paral- lel to that direction occurs in this case around H = HA (HA = 2 K,/M, : anisotropy field). The rea- son for the additional absorption is that the resonance frequency of the homogeneous precessional mode is lowered to zero by the ^{{negative ⁾⁾field and therefore becomes equal to the signal frequency for HA/2 5 H d HA. The values of the externally applied field H for which the resonance frequency is equal to zero are those at which the discontinuous jumps of the magnetization take place.

In real ferrites with strong anisotropy like BaFel,O,, the crystallites do not behave like single domain particles for all values of H, and especially the magnetic reversal is well known as to take place mainly by inhomogeneous processes such as nucleation and movement of Bloch walls (Zijhtra [4]). Therefore the predictions of the single domain theory as to hysteresis of the microwave absorption cannot be taken as a description of experimental results on technical available materials. In principle, one would have to deal with the problem of microwave absorption of partially magnetized ferrites which has recen- tly been treated by Schlomann [5] for the case of weak anisotropy and some simple domain configura- tions.

(*) Present address : Institut fiir Werkstoffe der Elektro- technik, Technische Hochschule Aachen, Germany.

In this paper we report on measurements of the microwave absorption of BaFe,,O,, a t 9.4 GHz and room temperature. Because of the high anisotropy field (HA

-

17 kOe) the signal frequency amounts only to about 20 % of the natural precessional frequency. The samples were prepared by milling powder from commercially available barium ferrite with grain orientation (Koerox 300, Fried.

Krupp Widia Fabrik) and after extracting all the grains with diameter larger than some 10 p embedding it in fractions without any grain orientation into a plastic matrix. Before embedding the fractions were annealed for 2 hours in air a t 300 OC, 400 OC,

...

10000C and then slowly cooled down. The final ferrite density of the samples was about 0.1 1 g/cm3.

The samples were ground roughly t o spherical form (diameter 4 to 5,4 mm) and put into a HI,, reflexion type resonator which was part of a conventional X-band spectrometer with the signal frequency stabilized to the resonance frequency of the cavity.

The rf field was perpendicular to the dc field ; the variation of the output voltage of the microwave bridge was plotted continuously as a function of the slowly varying dc field.

In figure 1 the absorption of several samples (redu- ced to the same ferrite mass) is shown which is measured when the dc field is varied from 25 kOe to 0, the field direction reversed and the dc field then varied from 0 to 25 kOe again. The small discontinuities at H = 0 are caused by a dc drift of the spectrometer during the reversal sequence of the electro-magnet.

Without annealing and a t low annealing temperatures large low field losses and a rather broad F M R at H

-

15 kOe are observed. The low field losses exhi- bit a pronounced dependence on the field direction ; the low field peak at H

--

2 kOe of the unannealed sample occuring for the ⁽⁽opposite ¹⁾ field direction (with respect to the direction of previous saturation) is even higher than the FMR peak. With increasing annealing temperature the low field losses and their hysteresis became drastically smaller ; above 900 OC no hysteresis can be detected within the experimental limits of our spectrometer. Simultaneously the FMR line becomes considerably narrower. The position of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971152

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K. A. HEMPEL AND H. KMITTA

FIG. 1. - Field dependence of the magnetic absorption (redu- ced to equal ferrite mass) for different annealing temperatures.

1 : without annealing ; 2 : 400 OC ; 3 : 500 OC ; 4 : 800 OC ; 5 : 1 000 OC. Aro = VSWR of the sample cavity at H-VSWR a t H + c o .

the FMR line may be explained by the modification of the anisotropy field HA

-

¹⁷kOe by the shape anisotropy of the particles.

These results show that the behaviour of the low field losses cannot be explained by the single domain theory which predicts an additional absorption peak a t H

--

8 kOe. It is obvious that the milling process induces many physical imperfections such as local strain etc. into the ferrite particles which gradually disappear with increasing annealing temperature.

As the domain configuration within the particles and the pecularities of the wall movement under the influence of the external field will depend on the kind and density of the defects we may assume that the field dependence of the microwave absorption reflects - in an essentially unknown manner - the field dependence of the domain structure of the crystallites. That mainly the wall processes are responsible for the hysteresis of the microwave absorption is confirmed by the fact that these effects could only be observed to a considerable degree if the crystallites of the starting material were sufficiently large so that multidomain structures were more likely to form.

In figures 2 and 3 the microwave absorption along various paths within the outer hysteresis loop of the unannealed sample is shown. The field dependence

[ I ] MIKHAYLOVSKIY (L. K.), POLLAK (B. P.), BALAKOV (V. I?.), KHANAMIROV (A. E.), Radio Eng. EZec- tvon. Phys. (USSR), 1965, 10, 1495.

[2] HEMPEL (K. A.), 2. Angew. Phys., 1970, 28, 280.

FIG. 2. - Field dependence of the magnetic absorption of the unannealed sample along the outer hysteresis loop and

along the virgin curve.

FIG. 3. - Field dependence of the magnetic absorption of the unannealed sample along a sequence of minor loops all

starting from the negative remanence (point A).

of the absorption starting from the demagnetized state differs markedly from that starting from the negative remanence or from the saturation which again shows the close connection between microwave absorption and domain configuration. Also the absorption along some minor loops with the negative remanence as the common starting point reflects the ferromagnetic history of the sample in a remarkable manner.

rences

[3] HEMPEL (K. A.), Habilitationsschrift Universitat Miinster 1968.

[4] ZIJLSTRA (H.), Z. Angew. Phys., 1966,21,6.

[ 5 ] SCHLOMANN (E.), J. Appl. Phys., 1970, 41, 204.