DYNAMICAL ASPECTS OF THE MORIN TRANSITION IN HEMATITE AS REVEALED THROUGH A MÖSSBAUER MAGNETIC DIFFRACTION SPECTROSCOPY

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DYNAMICAL ASPECTS OF THE MORIN TRANSITION IN HEMATITE AS REVEALED

THROUGH A MÖSSBAUER MAGNETIC DIFFRACTION SPECTROSCOPY

B. Furubayashi, I. Sakamoto

To cite this version:

B. Furubayashi, I. Sakamoto. DYNAMICAL ASPECTS OF THE MORIN TRANSI- TION IN HEMATITE AS REVEALED THROUGH A MÖSSBAUER MAGNETIC DIFFRAC- TION SPECTROSCOPY. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-677-C2-679.

�10.1051/jphyscol:19792235�. �jpa-00218617�

JOURNAL DE PHYSIQUE Colloque C2, suppl6ment au n o 3, Tome 40, mars 1979, page C2-677

DYNAMICAL ASPECTS OF THE NORIN T R A N S I T I O N I N HEMATITE AS REVEALED THROUGH A MoSSBAUER MAGNETIC DIFFRACTION SPECTROSCOPY

B. Furubayashi and I. Sakamoto

Electrotechnical Laboratory, Tanas h i , Tokyo, Japan

Rbsum6.- Le but de cet article est double; premisrement, nous montrons que la diffraction magndtique

%ssbauer (MMD) peut effectivement Stre appliqude 1 la mesure d'une vitesse de relaxation magnbtique critique associde 1 une transition de phase de premier ordre (WOPT); deuxismement, nous discutons les rdsultats obtenus relatifs 1 la transition de Morin dans l'hbmatite (a-Fe203) sur la base d'une thdorie de soliton magndtique critique que nous dbveloppons. Un progres considbrable a ddj1 dtd rdalisb, dans la limite de certaines imprdcisions explrimentales.

Abstract.- The purpose of this paper is twofold; first, to show that the Sssbauer Magnetic Diffraction (MMD) can effectively be applied to measure a critical magnetic relaxation rate associated with a Magnetic First-Order Phase Transition (MFOPT); second, to discuss the results obtained with the Morin transition in hematite (a-Fe203) from the stand point of a critical magnetic soliton theory developed by ourselves. Except for some experimental uncertainties unsettled, a considerable progress has already been achieved.

I. Introduction.- The problem concerning dynamical processes occuring and entities dominating during the (M)FOPT has acquired a central importance in physics. However, lacking adequate experimental means, direct experimental access to the most rele- vant variables has been severely restricted. The elaborate pioneering work by Balko and Hoy /I/ uti- lizing a SEDM method /2/ has shown for the first time that there certainly exists a relatively slow critical relaxat ion (- second) associated with the Morin transition.

Among several distinctive features of MMD 131, the present work providesan exampleof its new ap- plication as a means to measure directly the averaged low frequency scattering function. The novel point is a; combination therewith of the SEDM. At first glance, this may seem inappropriate, because we have no general relaxation theory of dynamical MMD in such a non-equilibrium situation as encountered in the SEDM yet. But if one has at hand a thick "per- fect" single crystalline sample /4/ and restrict oneself to the second order degeneracy condition/5/

and sufficiently slow relaxation rates, it was found that the lowest order perturbation theory /6/ can be applied mutatis mutandis.

One of the most attractive features of this method is that, by specifying a definite Bragg re- flection, the obtained relaxation rates is essential- ly just a space-time Fourier component ofanelectro- nic spin fluctuation correlation function averaged

transmitting ~sssbauer component. This is quite help- ful, as critical spin fluctuations are in many cases of interest strongly anisotropic; this can be used in advantage in inferring which physical mechanism be responsible for the observed phenomena.

2. Experimental.- In brief, the employed Msssbauer diffractometer consists of a N,-cooled cryostat

(f0.3 K) assembled with a goniometer in such a way as to allow for the incorporation of SEDM apparatus.

An 100 mCi 5 7 ~ o (Rh) source was used throughout. A single crystalline enriched a- Fe203 sample 141 has a surface parallel to (111) plane. It is of high quality such that the dynamical diffraction theory for a thick "perfect" crystal /3/ applies with suf- ficient accuracy.

3. Results.- By a similar reasoning as in reference / I / , we adhered to the same sixth line (+3/2-+1/2), and measured the temperature dependence of SEDM spectra under a symmetric (555) Bragg reflection (OB=28") near the Morin point (TM=254.0 K). Examples are shown in figure I, where the experimentally as- signed relaxation rates <y(q,w)> are also given.

(w; the doublet separation of the sixth line).

To further confirm that the observed relaxa- tion is not spurious, the same sort of experiments were repeated, this time with a modified absorber motion scheme, namely with a constant velocityscan- ned slowly enough to avoid a troublesome transient irregular movement possibly originating from an ab- sorber fly-back. The observed spectra are, however,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792235

C2-678 JOURNAL DE PHYSIQUE

cative of no positive relaxation effect. Thus, al- though in our second series of experiments, the grains of the sample irradiated by the source are not necessarily the same as in the first, it cannot be excluded that the relaxation observed in the first might only be apparent one possibly resulting from the above-envisaged irregular absorber motion 171-

Channel Number

Fig. I : The temperature dependence of SEDM spectra measured near the Morin point. Solid lines indicate the best-fit theoretical line shapes with their as- signed relaxation rates (MHz).

4. Discussions.- We have developed a theory of y(q,w) suggested by a remarkable paper by Chow and Keffer 181. The most essential element of ours is the sssumption that the critical spin fluctuation (flipping) motion along the trigonal I[ I

13

much more rapid than and decoupled dynamically from that in the basal (I 1 1 ) plane. The former is domina- ted by critical magnetic double-Sine-Gordon solitons.

Secondly, we assume the latter be described by a fm dimensianal isotropic (weak-) ferromagnet, of which metastable inhomogeneous states have lately been shown to be well in accord with an instanton model 191. An important consequence of this in our context is that the instanton, a semi-macroscopic local me- tastable singular spin configuration having finite degrees of freedom, gives rise to a natural length to the instanton scale h because of its quantum fluctuation in an otherwise scale-invariant h- distribution [pC(h) a

h - 3 7

pq(h) a

A-'

This has a zero at A = 1 and anextremum at = e2 :

data /I01 and a theory Ill/. The critical soliton may thus be envisioned to nucleate whenever it can avail itself of a proper-sized surface instanton.

These ideasreally work well and give a closed analytic expression for y(q,w) even for a metastable phase by using ananalytic continuation technique, but too lengthy to be presented here. Results are shown in figure 2 together with the experimental values. It is worthy of note that in the metastable phase (in our case T>TM), <y(q,w)> has an imaginary part, i.e., the relaxation shift which is strongly temperature dependent and far exceed the real part

(relaxation width); this may be viewed as the second order Doppler (thermal) shift due to the liberated latent heat. The latent heat and the

q

Fig. 2 : The critical temperature dependence of relaxation rates <y(q,w)> according to our model as compared with experimental values.

5. Conclusions.- We have undertaken to construct a new MMD spectrometer to obtain a similar dynamical information as in the inelastic neutron scattering, but with a much superior energy resolution and a far less expense, and applied it to study a critical spin relaxation phenomena in the Morin transition, although some experimental uncertainties are unset- tled yet. This seems to be ideally suited to inves- tigate a slow critical nucleation mode characteris- tic of a FOPT, as the latter to the soft mode.

Simultaneously, we have developed a new theoretical model emphasizing the instanton-soliton aspect of the problem, and compared it with our experiments.

the latter we normalize to 1.4 pm at 20°C with a

References

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