FAST RELAXATION OF HYPERFINE FIELD FORCED BY AN EXTERNAL RF MAGNETIC FIELD IN Fe-Ni ALLOYS

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FAST RELAXATION OF HYPERFINE FIELD

FORCED BY AN EXTERNAL RF MAGNETIC FIELD

IN Fe-Ni ALLOYS

M. Kopcewicz

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 12, Tome 37, Décembre 1976, page C6-107

FAST RELAXATION OF HYPERFINE FIELD FORCED

BY AN EXTERNAL RF MAGNETIC FIELD IN Fe-Ni ALLOYS

M. KOPCEWICZ

Institute of Experimental Physics, Warsaw University 00-681 Warszawa, Hoza 69, Poland

Résumé. — Nous examinons l'influence d'un champ magnétique de radiofréquence sur les propriétés d'alliages Fe-Ni ferromagnétiques. L'effet sur le spectre Môssbauer, des oscillations forcées du champ hyperfin en présence de radiofréquence est étudié dans des feuilles de permalloy et d'invar, en fonction de l'intensité du champ RF dans le domaine de fréquence 21-85 MHz. Nous observons un effondrement du spectre hyperfin qui se transforme en une raie unique, tandis qu'apparaissent simultanément des bandes RF latérales. L'effondrement du spectre est attribué à la rotation de l'aimantation sous l'effet de la radiofréquence, d'où résulte un renversement rapide de l'aimantation et donc du champ hyperfin, si bien que le champ moyen vu par le noyau Môssbauer s'annule.

Abstract. — The influence is studied of the rf magnetic field on the properties of ferromagnetic Fe-Ni alloys. The effect of fast relaxation of the hyperflne field forced by the rf field is investigated for permalloy and invar foils as a function of the rf field intensity in the frequency range of 21-85 MHz. The collapse of the hfs spectrum to a single line and the simultaneous appearance of rf sidebands has been observed. The collapse effect has been attributed to the rf induced rotation of internal magnetization which causes fast magnetization reversal leading to the fast relaxation of the hyperfine field as a result of which the average field at the Mossbauer nucleus is reduced to zero.

1. Introduction. — The effect of fast magnetization reversal in various magnetic materials has received considerable attention owing to its importance for better understanding of ferromagnetic materials and in view of its practical applications. The materials in which the direction of magnetization can be changed rapidly by an external magnetic field have found wide application in computing systems as components of logic and memory. A detail discussion of the magnetic flux reversal in various materials may be found, e. g., in [1-4]. The first study in which the effect of magnetization reversal caused by an external rf magne-tic field was observed in Mossbauer measurements was made by Pfeiffer [5]. To date only few papers have been published in which the observation of the effect of rf collapse of the Mossbauer hfs spectra to a single line due to fast relaxation of hyperfine field induced by external rf field was reported [5-8]. The rf collapse effect is due to the fast relaxation of the magnetic hyperfine field combined with the rapid reversal of internal magnetization induced by an external rf field. As a result the average field at Mossbauer nucleus is reduced to zero, and instead of the six line hfs spectrum a single line appears. This effect occurs when two conditions are fulfilled : (1) the frequency of the external magne-tic rf field is larger than the nuclear Larmor precession frequency, and (2) the switching time of the internal magnetization is comparable to or shorter than the oscillation period of the rf field applied. The first condition was veryfied in [5]. In the present study the

second condition is analysed in detail for Fe-Ni alloys The preliminary results of this study have been given in [8]. In the present report the rf collapse effect is studied as a function of the rf field intensity and fre-quency. The interpretation is made in terms of the ferromagnetic enhancement model.

Beside the rf collapse line, rf sideband lines appear in the spectra. Formation of rf sidebands when the rf field is applied to ferromagnetic materials was discussed in [51, [9], [10],

[HI-2. Experimental. — In the present investigation the rf collapse effect was studied for the following Fe-Ni alloys : permalloy (49.7 % Fe-50.0 % Ni-0.3 % Mn) and invar (61.98 % Fe-38.00 % Ni-0.02 % C). The Mossbauer measurements were performed for rf field frequencies ranging from 21 to 85 MHz as a function of the rf field intensity. Permalloy samples of thickness of 8,12,15 and 18 um, and invar samples of 18 um were used. All samples were annealed in hydrogen at 1130 °C for several hours, and the sample surfaces were carefuly cleaned with benzene before subjecting them to Mossbauer measurements. The samples were placed as stationary absorbers within the helical coil, which was a part of the rf resonance circuit of the rf power generator, in such a way that the rf magnetic field was applied in the plane of the absorber. The generator, constructed in our laboratory, delivered up to 90 W of rf output which corresponds to about 7 Oe of Htt intensity. In all experiments the sample was

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water-cooled because of the extensive rf heating. The Mossbauer measurements were performed at room temperature in a transmission geometry. The spectra were measured using a constant acceleration Nokia Electronics Co. Mossbauer spectrometer with an 800 channel analyser operating in the multiscaler mode. The 57Co source in a palladium matrix of initial acti- vity of 50 mCi was used.

3. Results and discussion.

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The typical results obtained for the rf collapse and sideband effects for permalloy are presented in figures 1,2, 3 and 4, and for invar in figures 5, 6 and 7. Since the fast relaxation of

the hyperfine field forced by an external rf field (collapse effect) is of primary interest for this study, the most of the discussion will be devoted to this effect.

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FIG. 1,

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Mossbauer spectra obtained for 12 pm permalloy foil : (A) with n o rf field ; (B) to (E) with an 85 MHz rf field of

4.3, 5.8, 6.5 and 7.0 Oe, respectively.

In figures 1,2,3 and 4 the Mossbauer spectra obtained for 12 pm thick permalloy foil at frequencies of 85, 64, 33.5 and 21 MHz, respectively, are shown as a function of the rf field intensity. As can be seen, application of the rf field causes considerable changes in the shape of permalloy Mossbauer spectra. Instead of the six line pattern (Fig. lA), complex spectra appear for increasing rf field (Figs. lC, ID, 2B-2E) which have a shape typical for the incomplete relaxation of hyperfine field. When the rf field intensity is further increased, the spectrum collapses to a single line, which isomer shift is very similar to that observed normally for permalloy of the composition used in this study (Figs. lE, 2F). In figure 1E the collapse is not yet

complete since the line observed is considerably broadened, but as can be seen from figure 2G a further increase of the rf field causes narrowing of the collapsed line. Beside the rf collapse line rf sideband lines appear in the spectra (Fig. 2) also showing a collapsed shape. The sideband lines are separated from the central collapsed line by

+

nw,,, where

&,,

is the frequency of the external rf field (for a detailed discussion of rf sideband effect see [9-111). The rf sidebands are not visible in figure 1 because the first sidebands appear outside the velocity range used. As can be seen from figure 2 the rf sidebands appear distinctly at a quite high rf field applied (Figs. 2F, 2G), but the effect of

I I I I

-10 -5 0 5 10rnrn/,

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FIG. 2. - Mossbauer spectra obtained for 12 pm peralloy foil :

(A) with no rf field ; (B) to (G) with a 64 MHz rf field of 3.3, 4.3, 4.8, 5.3, 5.7 and 6.0 Oe, respectively.

relaxation of hyperfine field is of primary importance for lower rf fields. Effects similar to that presented in figure 1 for a 12 pm sample were observed for 8, 15 and 18 pm permalloy samples, thus the rf collapse effect is not very sensitive to the sample thickness, at least for the sample thicknesses studied.

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FAST RELAXATION OF HYPERFINE FIELD FORCED BY AN EXTERNAL rf MAGNETIC FIELD C6-109

64 MHz (Fig. 2) and 46 MHz show that for lower frequencies the complete collapse occurs at a considerably lower rf field intensity. The rf collapse is complete for the 5.2 Oe rf field at 46 MHz and 6.0 Oe at 64 MHz (Fig. 2G), but is not yet complete for the 7.0 Oe field at 85 MHz (Fig. 1E). For lower rf field intensities the relaxation of the hyperfine field is too slow and does not follow the external rf field oscillations. In this case the spectra show a complex shape, which changes from the six line pattern (Figs. lB, 2B) to unresolved hfs spectra (Figs. lC, ID, 2C-2E). As can be seen from figures 1 and 2 the relaxation time is strongly dependent on the intensity of the driving rf field which forces the relaxation.

Very interesting results were obtained for the rf field frequency only slightly higher than (33.5 MHz, Fig. 3),

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FIG. 3. - Mijssbauer spectra obtained for 12 pm permalloy foil : (A) with no rf field ; (B) to (H) with a 33.5 MHz rf field of

1.7, 2.0, 2.7, 3.5, 5.0, 6.0 and 6.7 Oe, respectively.

and comparable to (21 MHz, Fig. 4) the nuclear Larmor frequency in permalloy which is about 21 MHz. The successive spectra obtained at a 33.5 MHz rf field show that for low rf fields (up to 2 Oe, Fjgs. 3B and 3C) it is possible to distinguish the particular Mossbauer lines. No collapse effect is seen, and the rf sideband effect dominates. This agrees well with the studies of the rf sideband effect [9] which show that this effect increases considerably with decreasing rf field frequency. For intermediate rf fields (2.7-5.0 Oe, Figs. 3D- 3F) the Mossbauer pattern is completely washed out and no lines can be seen in the spectra. In this case the induced hyperfine field relaxation is relatively slow and produces a very broad partly collapsed pattern, together with the rf sidebands which are also partly collapsed and broadened by the relaxation of hyperfine

field. The resulting Mossbauer lines are very weak and broad what leads to the disappearance of these lines in the statistical spread of experimental points. For higher rf fields (above 6 Oe, 3G, 3H) the Mossbauer pattern begins to be restored since the collapse effect dominates and leads to the narrowing of the Moss- bauer lines. The central collapsed line and the accom- panying sideband lines are distinctly seen in figures 3G and 3H. However, when the frequency of the driving rf field is lowered to 21 MHz, which is almost equal to the Larmor frequency, no restoration of the Mossbauer pattern is seen for the rf field intensity range studied (Fig. 4). Even for 6.0 Oe rf field (Fig. 4F) no Mossbauer

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FIG. 4. - Mossbauer spectra obtained for 12 pm permalloy foil : (A) with no rf field ; (B) to (F) with a 21 MHz rf field

of 1.3, 2.2, 3.7, 5.0 and 6.0 Oe, respectively.

lines are observed. In this case we see a complete degradation of the rf collapse effect.

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C6-110 M. KOPCEWICZ

(Figs. 5G-5J). The considerable difference between the For example, spectra of similar shape are observed for spectra obtained for invar (Fig. 5) and pernlalloy invar (Fig. 5G) and permalloy (Fig. 2F) for the rf fields (Fig. 2) at the same frequency of 64 MHz consists in of 3.8 Oe and 5.7 Oe for invar and permalloy foils, that the rf collapse effect occurs for substantialy lower respectively. Thus it is considerably easier to force fast rf field intensities for invar as compared with permalloy. relaxation of the hyperfine field in invar samples of the composition studied ; the hyperfine field having almost the same value as in the permalloy samples studied. Since the maximum rf field intensity available with the generator used is limited to about 7 Oe, the invar samples are more convenient for the study of the

-10 - -5

-10 -5 0 5 l O r n r n 1 ~ 0 5 lOrnrnl,

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FIG. 5.

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Mossbauer spectra obtained for 18 p m invar foil : FIG. 6.

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Mossbauer spectra obtained for 18 pm invar foil :

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FAST RELAXATION OF HYPERFINE FIELD FORCED BY AN EXTERNAL rf MAGNETIC FIELD C6-111

narrowing of the collapsed line for increasing rf field intensity.

The same effects as described above for 64 MHz can be followed for a 33.5 MHz rf field (Fig. 6). In this case the single collapsed line appears at lower rf field intensity (3.2 Oe) and is accompanied by rf sidebands whose intensity increases with increasing rf field. Three orders of rf sidebands are easily observed together with the central collapsed line (Fig. 61).

Considerably different effects compared with permalloy (Fig. 4) were observed for invar samples at 21 MHz (Fig. 7). In this case no complete disappearance of Mossbauer lines was observed. The shape of the spectra changes from the six line pattern (Fig. 7B), through the triangular one (Figs. 7C, 7D), to a collapsed single line accompanied by collapsed rf sidebands (Figs. 7E, 7F). The Mossbauer lines are very distinct

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FIG. 7. - Mossbauer spectra obtained for 18 pm invar foil : (A) with no rf field ; (B) to (F) with a 21 MHz rf field of 1.4,2.2,

4.3, 5.2 and 6.0 Oe, respectively.

despite the frequency of 21 MHz is almost equal to the nuclear Larmor frequency.

The results obtained for invar samples at 53 and 46 MHz are very similar to that seen in figure 5 and show the same characteristic features as described above. For all rf field frequencies studied the rf collapse effect occurs at considerably lower rf field intensities for invar than for permalloy.

Let us discuss now what effect is responsible for the narrowing of the six line hfs spectra to a single-line when the external rf magnetic field is applied. Undoubtedly the change of the shape of the Mossbauer spectra from the resolved six line pattern, through unresolved partly narrowed hfs spectra, triangular shape, broadened single line to the narrow single line is caused by the relaxation of the hyperfine field in the materials studied. Since, as can be seen from the results presented above, this relaxation time depends strongly on the external rf field intensity (at fixed frequency

larger than nuclear Larmor frequency of the nuclear magnetic moment of iron in the material studied) the successjve shapes of the spectra observed for increasing rf field intensity are due to the relaxation times longer, comparable to and shorter than the nuclear Larmor precession period. Even if the frequency of the rf driving field which forces the relaxation is markedly larger than the nuclear Larmor frequency, the effects observed depend on the switching time of internal magnetization. When this time is long, the magnetization reversal does not follow the reversal of external rf field. In this case the shapes of the spectra only weakly depend on the frequency of the rf field, and depend strongly on the frequency of the magnetization reversal. Since the switching time of magnetization reversal depends on the external field intensity [12], the shape of the spectra depend on the rf field intensity even when the frequency of the external rf field is fixed. This effect is clearly seen in figures 1, 2, 5 and 6. For low rf field intensities the switching time is too long, and hence the forced relaxation of internal magnetization is too slow and does not follow the rf driving field frequency. Then the Mossbauer spectrum has the shape typical for incomplete relaxation and consists of poorly resolved or unresolved hfs lines (Figs. 1B-ID, 2B-2E, 5B-5E, 6B-6E). Increasing of the rf field intensity makes the switching time shorter, and for high rf field intensities the forced reversal of internal magnetization is fast enough to follow the rf field frequency. In this case in the period of nuclear Larmor precession the hyperfine field will reverse its direction several times, and hence the local field at the Mossbauer nucleus will be averaged to zero. The Mossbauer spectrum is then collapsed to a single line (Figs. lE, 2F, 2G, 5G-5J, 6G-6J).

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C6-112 M. KOPCEWICZ

ger than the nuclear Larmor frequency, then the magnetization reversal frequency is too low to cause the complete collapse of hfs spectrum. This effect is seen in figure 3. Simultaneously, for lower &equencies the rf sideband effect increases. The resulting spectra may show complete wash-out of the Mossbauer lines (Figs. 3D-3F), and for higher field intensities

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weak lines (Figs. 3G, 3H). When the rf field frequency is comparable to the Larmor frequency, the collapse effect vanishes. In the situation when the magnetization reversal may occur or not in the Larmor precession period, the magnetization reversal is too infrequent to cancel the hyperfine field and degradation of the collapse effect is observed (Figs. 4E, 4F). However, in the case of invar the degradation of the collapse effect is not complete, and one can see the distinct lines in the spectra obtained at 21 MHz (Figs. 7E, 7F) for relatively strong rf fields. Since the induced fluctuations of hyperfine field are not random but coherent in space and time, this may lead to incomplete degradation of the rf collapse effect.

It should be emphasized that the simultaneous appearance of rf sidebands in the collapsed spectra provides an evident proof that the samples investigated were in the ferromagnetic state during the rf experiments, what is essential for the interpretation given above. Let us consider the possible mechanisms of magnetization reversal. It is known 11-41 that magnetization reversal may be caused by domain wall motion, uniform and nonuniform rotation of magnetization. The domain wall motion was proposed as a dominating mechanism in [7]. However, the data obtained in [I], 121, [3] suggests that this mechanism is responsible for relatively slow magnetization reversal, with switching time longer than 500 ns, which is much slower than that observed in the present study. The switching time relevant to the Mossbauer rf collapse effect is of the

[I] SMIIH, D. O., in Magnetism, Vol. 111, ed. by G. T. Rado\

and H. Suhl (Academic Press, N. York, London) 1963

p. 465.

[2] GYORGY, E. M., in Magnetism, Vol. 111, ed. by G. T. Rado

and H. Suhl (Academic Press, N. York, London) 1963,

D. 525.

[3]

KRYDER,

M. H., HUMPHREY, F. B., J. Appl. Phys. 40 (1969)

2469.

[4] KRYDER, M. H., HUMPHREY, F. B., J. Appl. Phys. 41 (1970)

1130.

[S] PFEIFFER, L., in Mossbauer Efect Methodology, Vol. 7,

ed. by I. J. Gruverman (Plenum Press, N. York) 1972

p. 263.

order of 10 ns. For such short switching times the rotational character of magnetization reversal is of primary importance [3]. Thus, considering the model given in [I], [2] and the data obtained in [3], and taking into account that all Mossbauer nuclei are affected by the relaxation of hyperfine field (as can be seen from the spectra obtained) is seems that the uniform rotation of magnetization is the dominating mechanism of magnetization reversal observed in the present study, both for permalloy and invar.

As mentioned in [8] the Mossbauer investigations of the rf collapse effect make it possible to evaluate the switching time as a function of the rf field intensity. It may be assumed that the switching frequency begins to follow the external rf field frequency when the single collapsed line appears in the spectra. Thus, basing on the data obtained, the switching time may be evaluated to be about 12 ns at 7.0 Oe, 16 ns at 6.0 Oe, 22 ns at 5.2 Oe for permalloy, and about 16 ns at 3.8 Oe, 19 ns at 3.4 Oe and 22 ns at 3.0 for invar.

The results obtained show that the Mossbauer effect is a very powerful tool for investigating magnetization reversal in ferromagnetic materials. Applications of the rf field may be very useful in the study of magnetic properties of alloys and ferrites. This technique may considerably contribute to the better understanding of relaxation effects in magnetic materials since it gives a unique possibility to force, simulate and control the relaxation effects in ferromagnetic materials.

Acknowledgments.

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The author is indepted to Dr. I. Sosnowska for her kind interest in this study. Thanks are due to Dr. T. Panek (Silesian Univ.) for making available the Fe-Ni samples, and to Mr. S. Fijalkowski for the construction of the rf generator and technical assistance during the measurements.

[6] ALBANESE, G., ASTI, G., NUOVO Cimento 6B (1971) 153.

[7] BALDOKHIN, J. V., MAKAROV, E. F., MITIN, A. V.,

POVICKI, V. A., Proc. 5th Intern. Con$ on Miissbauer

Spectroscopy, Bratislava 1973 (ed. by Czechoslovak

Atomic Energy Comm., Praha) 1975, p. 609.

[8] KOPCEWICZ, M., Solid State Commun. 19 (1976) 719.

[9] PFEIFFER, L., HEIMAN, N. D., WALKER, J. C., Phys. Rev. B 6

(1972) 74.

[lo] &TI, G., ALBANESE, G., BUCCI, C., Phys. Rev. 184 (1969)

260.

[ l l ] KOPCEWICZ, M., Ko-rrIcKr, A., SZEFER, M., Phys. Stat. Sol.

(b) 72 (1975) 701.