57Fe MÖSSBAUER MEASUREMENTS IN HIGH MAGNETIC FIELDS : A STUDY OF MAGNETIC PHASE TRANSITIONS IN FeI2

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57Fe MÖSSBAUER MEASUREMENTS IN HIGH MAGNETIC FIELDS : A STUDY OF MAGNETIC

PHASE TRANSITIONS IN FeI2

G. Calis, J. Trooster

To cite this version:

G. Calis, J. Trooster. 57Fe MÖSSBAUER MEASUREMENTS IN HIGH MAGNETIC FIELDS : A

STUDY OF MAGNETIC PHASE TRANSITIONS IN FeI2. Journal de Physique Colloques, 1980, 41

(C1), pp.C1-165-C1-166. �10.1051/jphyscol:1980144�. �jpa-00219713�

JOURNAL DE PHYSIQUE Colloque C1, supplkment au no 1 , Tome 41, janvier 1980, page C1-165

3 I

Fe

&SBAUER

MEASUREMENTS IN HIGH M4GNETIC FIELDS

:

A STUDY OF MAGNETIC PHASE TRANSITIONS IN FeI2

G.H. Calis and J . M . Trooster

Department of MoZermZar Spectroscopy, University of Nymegen, The NetherZands.

1. Introduction

A t low temperature Fe12 undergoes several magnetic phase transitions when subjected t o a magnetic f i e l d (1,2), see Fig. 1. Using the

f a c i l i t i e s of the magnet laboratory of the

University of Nymegen, Mtissbauer measurements were carried out on a single crystal of Fe12 i n mag- netic f i e l d s up t o 12.5 Tesla. The experimental setup d i f f e r s from that used elsewhere (3) and i s therefore described i n some d e t a i l .

2. H i & m e t i c f i e l d and low temperature MCssbzdei- speckromzter

The spectrometer makes use of a Bitter- magnet with a maximum f i e l d of 15T. A bore diameter of 60 mn allows the insertion of a He- cryostat. For the Mijssbauer measurements vibra-

Fig. 1 Magnetic Phase Diagram of Fe12. Be* i s parallel t o the crystallographic c-axis. Open circles represent Mksbauer measurements discussed i n t h i s paper.

tions caused by the flow of cooling water are the main problem ( 3 ) . Air f i l l e d tubes and rubber supports were used t o i s o l a t e the cryostat mechanically from the magget. Without magnetic f i e l d , but with water flowing, no l i n e broadening could be observed. With the magnetic f i e l d , Bext, on, inductive coupling of the vibrating magnet with the t a i l of the cryostat and with the pick-up coil of the Mgssbauer transducer, r e s u l t s i n considerable l i n e broadening: f o r Be* = 10T the minimum observed linewidth i s 0.6 m / s .

Source, absorber and detector are placed close together inside the magnet. A small end- window proportional counter was used (4). ^{A l -} though the efficiency of this counter i s l e s s than that of conventional types and reduces further with increasing Bext, the count r a t e during the measurements in the 14 keV window was 20,000 c/s with a 25 mC source.

A s the source i s inside the magnet i t s spec- trwm consists of four lines. This complicates the measured spectrum but when the net magnetic f i e l d i n the absorber i s (anti-) p a r a l l e l t o Bext, o d y one variable (source s p l i t t i n g ) has t o be added in least-squares f i t t i n g of the spectra. Moreover, with the source, 5 7 ~ o ( ~ ) , at liquid He-temperature the net f i e l d a t the source nuclei i s small: -3T for Bext = 15T (5).

The transducer sensitivity depends strongly on Bext, therefore the velocity was measured slmul- taneously with the Mtissbauer spectra, using a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980144

C1-166 JOURNAL DE PHYSIQUE

Michelson interferometer.

3. High field measurements in. Fe12

Mijssbauer measurements on single crystals in magnetic fields up to 6.5T and temperatures down to 1.5K have shown that phases I, I1 and I11 (see Fig. 1) are characterized by two spin lattices, aligned (anti-) parallel to the c-axis, with different ratios, N+/NS, of the number of Fe ions aligned parallel and antiparallel to Bext(2). For the present measurements a single crystal of Fe12 mounted with the c-axis parallel to Bext was used.

A sample of the spectra is shown in Fig. 2.

Source splitting proved to be negligible for Be*

<9.5T, but had to be included for larger fields.

All three spectra show weak lines at

-5

and +6 Imn/s, probably due to a small ~ e impurity. ~ + The spectrum of Fig. 2a corresponds with phase 111.

The fit shown gives N+/NG=2.5 which is smaller than reported by deGraaf and Trooster (2). This

may be due to unresolved lines of Fe3+. In phase IV, Fig. 2b, the spectrum shows again two absorber hyperfine patterns without aMT = 0 transitions and N+/NS=3 .O. Thus, as predicted (2), phase IV is also characterized by two sublattices with spin (anti-) parallel to the c-axis. The ratio N+/NI is larger than in phase 111, but the predicted ratio of 5 cannot be confirmed. In phase V, Fig. 2c, only one absorber hyperfine spiitting is found in agreement with the paramgnetic nature of this phase. At the phase transition 111 -+ IV the field dependence of the hyperfine field of the two sublattices, which is proportional to sublattice magnetization, is analogous to that found for the transition

I

-t 111 (2): The mag- netization of the antiparallel lattice increases, accompanied by a decrease in magnetization of the parallel lattice. SMlarly the field depen- dence of Bhf at the transition IV -+ V is compatible with a first order transition as found for the transition 111 -t V. (6)

Acknowledgement: Invaluable technical assis- tance was given by Ad Swolfs and by the staff of the magnet laboratory. This research was funded in part by the Dutch Foundation for Chemical Research (SON) and the Dutch Organization for Fundamental Research ( ZWO)

.

1. A. R. Fert, J. Gelard and P. Caryara, Solid St. Corn. 13 (1973) 1219.

2: H. d e ~ r a a f ~ d J.. M. Trooster, Solid St. C m . 16 (1975) 1387.

'3. ^Teillet, D. Joly, F. Varret and J. Chappert, Nucl. Instr. Methods 155 (1978) 97.

4. l@rwell, type PK 498.-

5. B. B. Schwartz and R. B. Frankel, Mijssbauer Methodology

1

(1971). 21, Ed.

.

J. Gruverman, Plenum Press, New York.

6. J. M. Trooster and W. deValk, Hyperfine Int.

11

(1978) 457.

Fig. 2 Mijssbauer spectra of Fe12.