THE MÖSSBAUER STUDY OF ULTRASOUND STIMULATED BY RADIOFREQUENCY FIELD IN FERRODIELECTRICS

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THE MÖSSBAUER STUDY OF ULTRASOUND STIMULATED BY RADIOFREQUENCY FIELD IN

FERRODIELECTRICS

Yu. Baldokhin, V. Goldanskii, E. Makarov, A. Mitin, V. Povitskii

To cite this version:

Yu. Baldokhin, V. Goldanskii, E. Makarov, A. Mitin, V. Povitskii. THE MÖSSBAUER STUDY OF ULTRASOUND STIMULATED BY RADIOFREQUENCY FIELD IN FERRODIELECTRICS.

Journal de Physique Colloques, 1972, 33 (C6), pp.C6-145-C6-149. �10.1051/jphyscol:1972633�. �jpa-

00215151�

JOURNAL DE

Colloque C6, suppliment au no 11-12, Tome 33, Novembre-Dgcembre 1972, page 145

THE MO SSBAUER STUDY OF ULTRASOUND STIMULATED BY RADIOFREQUENCY FIELD IN FERRODIELECTRIC S

Yu. V. BALDOKHIN (**), V. I. GOLDANSKII (**), E. F. MAKAROV (**), A. V. MITIN (*), V. A. POVITSKII (**)

(*) Kasan Physico-Technical Institute Ac. Sci. USSR, Kasan, USSR (**) Institute of Chemical Physics Ac. Sci. USSR, Moscow, USSR

Rksumk. - La production d'ultrasons dans des ferrodiklectriques de structure a domaines multiples est observbe avec un pompage a radiofrkquence. On propose un mkcanisme de pro- duction du changement d'aimantation dans les parois de Bloch ^& 180°, qui exciterait les vibrations ultrasonores de la plaque ferromagnktique. On ktudie expkrimentalement les spectres Mossbauer des YIG polycristallins soumis a un champ de radiofrkquence. L'analyse spectrale a montrk que le comportement sous excitation par un champ a radiofrequence confirme I'hypothkse de la production des ultrasons par les parois de Bloch a 180°.

Abstract. - Ultrasonic generation in ferrodielectrics with multidomain structure is considered at rf field pumping. The mechanism inducing the magnetization change in 1800 Bloch walls and leading to the longitudinal ultrasonic vibrations of the ferromagnetic plate is assumed. Mossbauer spectra of the polycrystalline YIG are investigated experimentally, under applied rf field. The spectra analysis showed that the rf field excitation behavior confirms the hypothesis of the ultra- sound generated by 1800 Bloch walls.

1. Introduction. - Some time ago Hack and Hammermesh [I] suggested the first theory of the influence of a radiofrequency (rf) field on the spon- taneous emission distributions. Recently, one of the authors of the present work investigated independently the problem of

y-magnetic resonance

[2]

for Mossbauer absorption and showed a real possibility for the observation of this effect in ferromagnets.

These works stimulated the interest in the investigations of the influence of a rf field on Mossbauer effect.

Some of these [4]-[9] continued the investigations in the spirit of [I], [2]. However, the experiments have shown that in a number of cases there is a stronger influence of rf field on Mossbauer spectrum than it follows from the theory of y-magnetic reso- nance. The effect has been found to have a magneto- strictive character [lo]-[14]. As a result, the ultrasonic modulation of y-radiation occurs. But until recently serious difficulties were raised by invoking magneto- striction as an explanation since the displacements of nuclei are quite small at applied amplitudes of rf field. Though recently, Pfeiffer et al. [14] made an assumption that ultrasonic vibrations along the propagation direction of y-radiation are caused by strong scattering (of the order of a few percent) of the shear vibrations to the longitudinal ones.

However, apart from this assumption no other proofs of this strong scattering effect have been presented up to now.

(1)

In a recent work [3] it has been confirmed that the Mossbauer spectra in the presence of rf field is described ^by the theory of y-magnetic resonance.

In this paper we consider the influence of rf field on Mossbauer absorption spectrum experimentally and theoretically in yttrium iron garnet (YIG).

For the explanation of the experimental results the new model of the ultrasonic excitation in Bloch walls is suggested.

2. Experimental results. - We undertook the experimental investigation of rf field effect in ferro- dielectrics by means of Mossbauer spectroscopy.

We dealt with polycrystalline samples of YIG, its Mossbauer spectra and magnetoelastic properties being well known.

The magnetic field with amplitude up to 5 Oe oscillating at the megacycle frequency was applied in the plane of the YIG sample. The sample was prepared by the precipitation of YIG powder on plexiglas substrate from suspension of the synthetic glue. We used isotropic samples and sample which had been precipitated in the external magnetic field of the order of 1 kOe. In this case the formation of fibres with orientations along the magnetic field lines takes place.

The YIG sample served as the absorber for a Fe5' Mossbauer experiment in usual transmission geometry. Before the rf field was applied YIG absorber had yielded a well understood Mossbauer absorp- tion pattern which indicated two Zeeman sextets arising from the Fe57 nuclei in the a- and d-sublat- tices where they were subjected to two different effective magnetic fields Heff (Fig. la) [I 51-[18].

After rf field had been switched on, the absorption

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972633

C6-146 YU. V. BALDOKHIN, V. I. GOLDANSKTI, E. F. MAKAROV, A. V. MITIN, V. A. POVITSKIL

FIG. 1. - Mossbauer spectra of YIG, T

25 OC.

a) H l = O ; H , = O .

b) H I

5 Oe, (v

30 MHz), Ho

0.

c)

H I

5 Oe, ^(v

30 MHz), Ho

1 kOe.

lines broadened and came closer and produced a pattern similar to the one that would have been obtained if the YIG sample had been heated to above IOOOC with great inhomogeneity of tempe- rature (Fig. lb).

This is not a resonant phenomenon because the effect is observed for the different values of frequency from 15 to 80 MHz. Actually the observed effect is partly connected with the heating of the samples because of hysteretic losses due to the domain wall motion. Special experiments have proved that the temperature dependence H e f f ( T ) for YIG [I91 is not different (within the limits of 5 10 OC) from the temperature of the sample determined indepen- dently. Therefore the line shifts of the Mossbauer spectrum under the rf field influence are mainly connected with heating of the samples.

Nevertheless it has been that broadening of the Zeeman lines in the spectrum cannot be explained by the temperature inhomogeneity throughout the sample.

So the direct influence of rf field is manifested in broadening of every line of the Mossbauer spectrum.

As a result of the investigation the field anisotropy influence was established. Under the same strength of the rf field HI the maximum broadening of the lines occurs when H , is directed along the fibres.

If H~ is perpendicular to the fibres, the line broadening is negligible.

We also investigated the rf field influence in the presence of constant magnetic field in the plane of the sample. It was found that the effect of rf field decreased with the increase Ho and under Ho -- 1 kOe it disappeared (Fig. lc).

Thus the nonresonant effect of rf fields on Moss- bauer spectra which cannot be explained by simple heating of the sample.

3. Ultrasonic influence on Mossbauer spectrum. - Let us point out the main properties of the ultra- sonic influence on the Mossbauer spectrum. The effect manifests in the sideband appearance that are located at ^ID,, + nw,, n ⁼ 1, 2, ... [20], where we, is Mossbauer line resonant frequency and w, is the ultrasonic frequency, for the absorber or source vibrating as whole. The intensities of the sidebands are proportional to Jn2(uZ/iZ) and

~xP[- (4 AI2] I n [ ( ~ z l A ) ~ ]

in the case of coherend and incoherent vibrations,

respectively. Here Jn and I, are usual and modified

Bessel functions of the n-th order, u, is the vibrations

amplitude in y-radiation direction. The effect of the

sidebands becomes appreciable for the ultrasonic

displacements of the order of the wavelength of y-radiation (for a Fe57 nucleus

2 = 4 2

1.4 x lo-' cm).

This estimation shows the high ultrasonic sensitivity of Mossbauer spectra.

In ferromagnets rf field stimulated the ultrasonic vibrations with the help of the magnetostriction.

According to the theory only shear elastic vibrations are generated in single-domain ferromagnets for the directions of a constant magnetic field which are parallel or perpendicular to the plate plane. This fact is confirmed by our experimental results. However, the ultrasonic behaviour in multidomain structures requires a special consideration.

4. Ultrasonic generation by Bloch walls. - Let us consider a 1800 Bloch wall. In this case the Bloch wall is perpendicular to the plane plate (see Fig. 2).

As may be seen from formula (2) the longitudinal vibration excitation effect depends on the magnitude of My component. In order to determine My, ,we consider the a set of equations for the magnetization components, deduced by Janak [21] for the excitation which is parallel to 1800 Bloch wall :

aM: Y

---- at + r0 ^{M! =} o,(t) sech - _d . ^M, + ^{2 K o M} MI

In the case when rf field is directed perpendicularly to the Bloch wall, a similar set of equations may be deduced with the help of the Janak's method 1211 :

I where To is the damping parameter

by: ^~e thickness. K is the restoring force constant, d is Bloch wall Let us write the equations for the My components :

E;ie

2 - a2M! + 2 r 0 2 aM" + ~r,2 + ~ K W & I M! = FIG. 2. - The magnetization in Bloch wall. The planes

at2 at

xOz are parellel to the absorber plane and the Bloch wall plane, respectively. The axis Oz is parallel to the easy magne-

tization axis. = sech 2 ^{. M,} [ q ^{+ T o o,(t)] (50)}

a2M;

Now let us write the expression for the magneto- ^- + ² r, ^---- aM; + ^[IT: + oh] M: = elastic energy. Bearing in mind YIG, we restrict at2 at

the following expression :

= 2 Kw, ~ , ( t ) Mo . (5b) Now let us show that the perpendicular excitation is considerably less effective than the parallel exci- MXMZ M Mz tation. Setting for YIG 2 K -- lo-,, v,=4 x lo8 Hz, +

(

^{+ +} ) (I) r 0 j 2 n = lo7 Hz, v = lo7 Hz, we obtain :

M,2 M,2 M,2

where B,, B, are magnetoelastic constants, M i are magnetization components, M, is the saturation magnetization,

au, auj

e . . =

+ --

rJ

ax, axi

is the deformation tensor.

With the help of (1) the motion equation of the longitudinal vibrations plate may be obtained :

If we take into account the fact that the intensity of the sidebands is proportional to (My)2 since the amplitude of the displacements of nuclei is propor- tional to My, then the perpendicular excitation is considerably less effective than the parallel one.

Let us consider in more details the parallel exci-

tation case. Setting H,(t) = H, cos o t ,

C6-148 YU. V. BALDOKHIN, V. I. GOLDANSKII, E. F. MAKAROV, A. V. MITIN, V. A. POVITSKII let us write the solution of (5) :

M , = o, M , sech Y - Re x d

Taking into account that sin 8 = sech y/d and also that the effect of the wall motion will be determined by the factor d l 9 where 9 is the thickness of the domains, eq. (2) becomes

Here we eliminate the term 13~u,/az~ since the excitation does not depend on the coordinate z (homogeneous excitation) and introduce the term which describes ultrasonic damping. The bracket shows the averaging over nuclei in Bloch wall. It can shown that

Let us write the solution of (8) :

Assuming o,

o % T o , ^LY ( 9 ) is simplified :

U,

= - _{p 9 o :} Bz

cos (@t - y) ^{. (10)}

It can be seen in this case the ultrasonic phase lags 3 n/2 behind the applied rf field (B, > 0 for YIG).

The last circumstance may be tested experimentally with the help of Mossbauer delayed-coincidence technique using the pulse rf field.

To estimate the displacement amplitude u, we determine the domain thickness :

9 = Jm ( 1 1 ~ )

where L is the plate thickness, o , is the surface energy density of a Bloch wall. Taking for YIG B2 = 7.4 x 106erg/cm3,p = 5.2g/cm3,v = 5 x 107Hz, H I = 5 Oe, we find u,

2.6 x l o p 9 cm.

Now let us calculate the relative intensity of the sideband of the first order for incoherent vibrations

Thus the ultrasonic vibrations generated by Bloch walls may considerably change the Mossbauer spec- trum.

5. Discussion of the experimental results and conclusion. - Let us make an attempt to explain our experimental results on the basis of possible theoretical models.

The domain-wall passage model [23], [24] is ruled out since the amplitude rf field (5 Oe) is small in respect to the magnetization field in YIG (140 Oe).

Let us estimate the possibilities of the explanation of the experiment from the point of view of y-magne- tic resonance. The dependence on the directions respective to Bloch walls (parallel or perpendicular) and also the dependence on the magnitude of the applied constant magnetic field may be explained with the help of this mechanism. However, the field strength at 5 Oe under the enhancement coefficient lo3 evidently is insufficient to produce noticeable changes in Mossbauer spectra.

Now let us suppose the possibility of the ultrasonic Bloch wall generation. In this case the character of the rf field influence on the Mossbauer spectrum possesses a number special properties. Firstly, it has a nonresonant character. Secondly, since Bloch walls have greater relative volume in thinner plates, rf field effect will be stronger with the decrease of the thickness of the plate. Thirdly, the excitation of rf field parallel to the plate has considerably stronger effect than that perpendicular to this plate. Fourthly, the application of additional constant magnetic field will favour the disappearance of Bloch walls.

The rf field will excite only shear waves. Their influence on Mossbauer effect may be neglected.

The first, third and fourth properties are confirmed experimentally in our paper. The second property was noted recently [14] for the absorbers of pure iron.

It should be also noted that the ultrasonic vibra- tions have been subjected to the ultrasonic scattering over microscopic inhomogeneities in polycrystalline samples of YIG. This leads to the incoherent vibra- tion and its considerable damping. The last circum- stance explains the absence well-resolved sidebands in Mossbauer spectra. Only the broadening of the principal spectrum lines is noticeable.

Thus, the hypothesis of the ultrasonic vibration

excitation by rf field in Bloch walls has real founda-

tions. The Mossbauer study of this phenomenon is

promising due to its high sensitivity to the ultrasonic

displacements of nuclei.

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