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Submitted on 1 Jan 1971
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MÖSSBAUER STUDIES OF AMORPHOUS MAGNETS
A. Boyle, G. Kalvius, D. Gruen, J. Clifton, R. Mcbeth
To cite this version:
A. Boyle, G. Kalvius, D. Gruen, J. Clifton, R. Mcbeth. MÖSSBAUER STUDIES OF AMORPHOUS MAGNETS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-224-C1-225.
�10.1051/jphyscol:1971171�. �jpa-00214498�
A. J. F. BOYLE (**), G. M. KALVIUS, D. M. GRUEN, J. R. CLIFTON (***) and R. L. McBETH
Argonne National Laboratory, Argonne, Illinois 60439
R6sum6. - La dependance de la tempkrature du champ magnktique hyperh (Hhp) d'un kchantillon magnktique amorphe a ete ktudiee avec le but d'examiner la nettete de la transition magnktique. L'echantillon utilisk ktait FeC12, dkpos6 d'un rayon moleculaire sur une surface froide (10 OK). Les spectres Mossbauer etaient enregistres sans chauffe- ment de l'kchantillon. Les spectres observes sont assez diffkrents compares avec les proprietbs connues de FeClz crystallin.
On observe une distribution considkrable pour les valeurs de Hhf, mais il est pourtant possible de destiller une valeur moyenne assez bien definie pour HW et de l'ktudier en fonction de la tempkrature. A 8 OK les valeurs des paramktres hyper- fins sont : Hhf = 85 kOe et e2qQ = 5,3 mm/s. En suivant Hhf(T) nous trouvons une transition magnetique bien defmie 22 OK avec une largeur qui n'exdde pas 0,5 OK. La transformation de l'kchantillon vers un solide crystallin &cause de l'echauffement a des plus hautes temperatures, est complexe.
Abstract. - The temperature dependence of the hyperfine magnetic field (Hhf) of an amorphous magnet has been investigated with the objective of examining the sharpness of the magnetic transition. The sample used was FeC12 depo- sited from a molecular beam onto a cold (- 10 OK) surface. Mossbauer spectra were taken without warming the sample.
The observed spectra differ significantly from the known results for crystalline FeC12. A considerable spread in Hhf is noticeable, yet it was possible to extract a rather well defined average Hhn as a function of temperature. At 8 OK the values of the hf parameters are Hhf = 85 kOe and e2qQ = 5.3 mm/s. Following Hhf(T) we find a well defined magnetic transition at 22 OK, broadened not more than 0.5 OK. The transformation of the sample into a crystalline solid upon warming to higher temperatures is complex.
I. Introduction. - It is well established that amor- phous (amph) solids can exhibit ferromagnetism. We report here an examination of the sharpness of the magnetic transition in such a system by determining, the hyperfine (hf) field a t the nucleus using nuclear gamma resonance (NGR), that is the Mossbauer effect.
Since a considerable spread of the hf field is to be anticipated in an amph solid, other resonance techni- ques like NMR are precluded. Our results (see below) confirm this spread of hf fields. The system chosen was FeCl,. Crystalline (crys) FeCI, is antiferromagnetic below 24 K [I]. Its NGR spectrum [2] reveals the unusually small saturation hf field of 4 kOe. This is explained by the fact that the orbital and spin contri- butions to the hf field are of opposite sign and nearly cancel each other. The magnitude of the orbital field contribution will be very sensitive to the crystalline electric field, therefore, the hf field is expected to have a quite different value in a non-crystalline sample of FeCl,.
To produce an amph layer we have condensed FeCI, from a molecular beam onto a substrate at cryogenic temperatures. It is known from infrared spectroscopy on other metal chlorides that this tech- nique can produce a condensate of molecules, which will not crystallize as long as the sample is kept at low temperatures [3, 41.
11. Experimental. - About 100 mg of FeC1, enri- ched in 57Fe were heated in the molecular oven to 7500K. The beam of FeCl, molecules was directed onto an ultra-pure A1 foil, about 75 pm thick. The
(*) Based on work performed under the auspices of the U. S.
Atomic Energy Commission.
(**) Permanent address : Physics Department, University of Western Australia, Nedlands W. A. 6009.
(* * *) New address : National Bureau of Standards, Washing- ton, D. C.
foil was kept at 10 OK. Deposition took place in a vacuum better than Torr and required about three hours. The absorber thus created had a thickness of -- 1 mg/cm2 of 57Fe. It was then rotated 900 into the gamma ray beam of a source of 57Co in Cu. Spec- tra were produced with an electromagnetic drive sys- tem and a 400 channel multiscaler. The NGR spec- trometer gives a full width at half maximum (FWHM) of 0.23 mm/s for the two inner resonance lines of metallic iron.
An electrical heater allowed us t o vary the sample temperature which was read by a Ge resistor below 25 OK and a Pt resistor from 20 to 400 OK. Absolute thermometer calibration should be good to + 0.5 OK.
111. Results and Discussion. - Figure 1 shows the NGR spectra of amph FeC1, at 8, 15, and 24 OK. The
9 2 .:
I I I I I 1 I I I I
-8 -6 -4 -2 0 t2 t4 t6 +8 m m / s
VELOCITY
FIG. 1. - NGR spectra of amorphous FeC12 at 8, 15, and 24 OK. The source is 57C0 in Cu at 300 OK. Note the different intensity scales. The arrow indicates zero velocity for a source
of 57Co in Cr.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971171
spectrum at 24 OK gives a pure quadrupole interaction with e2 qQ = 5.3 mm/s, which is about 2.5 times the value found in crys FeCI,. The two spectra at lower temperatures are complex and considerably wider.
They indicate the presence of a magnetic hf interaction in addition to the quadrupole splitting. A satisfactoryfit could not be obtained for the latter two cases with the usual fitting routine, because of the wide spread in the values of the hf parameters. Using a simulation program [5] with which a random angle between the magnetic hf field and the electric field gradient can be treated, and averaging over a sensible range of input parameters, the spectrum at 8 OK can be reproduced roughly with an average hf field of about 85 kOe in addition to the quadrupole interaction found at 24 OK.
At 15 OK, the magnetic hf field is reduced to 45 kOe.
In order to measure the temperature dependence of the hf field close to the critical point, thermal scanning 161 was employed. For a source of 57Co in Cr the right member of the quadrupole doublet appears very close to zero velocity (indicated by an arrow in Fig. 1).
For such a source, the countrate at v = 0, (No) will be a measure of both the intensity and the position of this peak, and those in turn are dependent on the value of the hf field. The variation of No with tempe- rature is given in figure 2. It is seen that No changes rapidly with increasing temperature above 19.5 OK until 220K is reached and then remains constant.
Thus 220K is the Curie temperature (T,) of amph FeCl,. The hf field approaches zero rapidly as T reaches T,, and we set a limit of 0.5 OK for a possible broadening of the magnetic transition.
The Curie temperature of amph FeCl, lies close to the NCel temperature of crys FeCI,. This coincidence has little significance. Preliminary data on amph FeBr, show a similar behavior for both ferrous halides, with a difference in T, not larger than 2 OK. However, the NCel temperature of crys FeBr, is 11 OK [I].
In addition we followed the NGR spectra of amph
TEMPERATURE
FIG. 2. -Relative countrate at zero velocity through an absorber of amorphous FeC12 in the temperature region between
15 and 30 OK. The source is 57C0 in Cr a t 300 OR.
FeCI, sample upon warming well above the critical temperature. A conversion of the amorphous layer into the normal crystalline material can be achieved, but requires temperatures above 300 OK and occurs via an unknown intermediate phase which is characterized by its quadrupole splitting.
This work is being continued with the investigation of other ferrous halides. Preliminary data show similar behavior for all the amorphous systems, while the various crystalline ferrous halides differ considerably in their magnetic behavior.
IV. Acknowledgments. - We wish to thank Dr G. K. Shenoy for his assistance with the data analysis, and Dr R. S. Preston for lending us his 57Co in Cr source. L. Ouwerkerk's help in the Iabora- tory is greatly appreciated.
References
[I] VAN UITERT (1,. G.), WILLIAMS (H. J.), SHERWOOD [4] GRUEN (D. M.), unpublished data.
(R. C.) and RusrN (J. J.), J . Phys., 19653 [5] GABRIEL (J. R.) and RUBY ( S . L.), Nucl. Instv. Meth.
36, 1029. 1965, 36, 23.
[2] NO (K.), ITO (A.) and FUJITA (T.), Phys. Sot. JaP. [6] DUNLAP (B. D.) and DASH (J. G.), Phys. Rev., 1967,
1964, 19, 2119. 155, 460.
[3] CARLSON (G. L.), Spectrochim. Acta, 1963, 19, 1291.
