Short Notes K129

phys. stat. sol. (a) Is, K129 (1973)

Subject classification: 18.4; 22.6

Physik-Department , Technische Universitgt Miinchen

Hyperfine Splitting in V 0 Measured by Inelastic Neutron Scattering 3- 5 By

A. HEIDEMANN

Introduction The system of the Magneli-phases V n 0 2n-1' n = 2, 3 .. . has been

investigated extensively in recent years (1 to 7). Measurements of the electric con-

ductivity of V305 by Okinaka (4) showed that this substance is a semiconductor in

the temperature range between 4.2 K and room temperature in contrast to V 0 2 3'

which undergoes a metal-insulator phase transition a t approximately 150 K.

measurements by Okinaka et al. (8) with 1% Fe57 in V305 revealed a hyperfine

splitting at temperatures below 69 K. From this an internal magnetic field at the

Fe57 nucleus in V 0 of approximately 390 kOe at 4.2 K was obtained, This ex-

periment is a rather convincing evidence for the occurrence of antiferromagnetism

in V 0 a t temperatures below 69 K. But as it i s generally known, these Mijssbauer

experiments using doped Fe57 for proving magnetic order, a re not always fully re-

liable. Therefore, we tried to measure directly the hyperfine field at the vanadium

nucleus in V 0 using inelastic spin-flip-scattering of neutrons (9).

The susceptibility curve of V 0 has a maximum near 133 K. Moesbauer effect 3 5

3 5

3 5

3 5 The physical principle is a s follows:

If neutrons a re scattered spin-incoherently from nuclei, the probability for spin-

flip is equal to 2/3. Due to the conservation of angular momentum the nucleus at

which the neutron was scattered changes its magnetic quantum number by one. If

the nucleus is situated in a magnetic field, then the spin-flip i s connected with a

change of the nuclear ground state energy. This energy change is transferred to

the scattered neutron. From the energy spectrum one obtains therefore directly

the splitting of the nuclear ground state,

Experiments The experiments have been performed using the backscattering

spectrometer of the FRJ-2-reactor in Jiilich. A detailed description of the spectro-

meter is given by Alefeld (10). It i s essentially a three-axis spectrometer with

fixed analyser energy (11). The energy of the incident, monochromatic neutrons is

K130 physica status solidi'(a) 16

df@eVl-- Fig. 1. Energy spectrum of -20 15 -10 -05 U 05 10 15 20 neutrons scattered inV,O,,

I I

-621 -4E8 -372 -156 0 156 312 46.8 62

t a r et temperature d d

10 6 K, scattering probability

48 , measuring time 27 h

varied by Doppler motion of the monochromator. Using nearly perfect silicon single

crystals with a Bragg angle of 90 for the monochromator and the analyser, one ob-

tains an extremely high energy resolution of 0.35 peV (FWHM).

0

Results Polycrystalline V 0 which was kindly supplied by H. Okinaka, was 3 5 ' investigated. An X-ray Debye-Scherrer pattern showed the known spectrum of

V 0 (12) at room temperature. A chemical analysis with an accuracy of 1% revealed

that the sample was stoichiometric. Approximately 10 g of V 0 were pressed in a

vessel of pure aluminium. The sample was kept in a helium cryostat. Fig. 1 shows

the energy spectrum of neutrons scattered in V 0 at a temperature of 10 K. The 3 5 distribution of the measured points shows quite clearly the existence of three peaks,

an elastic and two inelastic ones. The drawn curve was obtained from a least squares

fit assuming a sum of three Lorentzians. Multiple scattering corrections were not

considered because the scattering probability was approximately 4%.

3 5

3 5

The line splitting 6E = 0.68 peV is connected with the internal magnetic field

I Hint by

= - 6E = (146 + 6)kOe , Hint p - I is the spin of the vanadium nucleus, p its magnetic moment.

Discussion The experiment shows that V 0 has a magnetic order at 10 K in 3 5 agreement with measurements of Okinaka et al. (8). The vaiue of the hyperfine field

Short Notes K131

at the vanadium nucleus i s (146 + - 6)kOe. The corresponding value in V 0 was

185 kOe (13, 14) . 2 3

The intensity ratio of the elastic to one of the inelastic peaks i s essentially

larger than i ts theoretical value of unity. The reasons a r e not known until now.

Coherent scattering due to the non-magnetic lattice was eliminated by choosing

a suitable scattering angle. But it i s possible that magnetic Bragg reflections in-

creased the intensity of the elastic peak. One can hope to be able to answer this

question after having measured the spectrum of coherently scattered neutrons

at temperatures below 68 K and after having investigated in detpil the temper-

ature dependence of the energy spectrum of the incoherently scattered neutrons.

These experiments a r e planned in the next future.

The author i s indebted to Prof. T. Springer from the Institut fur FestkiSrper-

forschung Julich, to Dr. B. Alefeld, Prof. S. Kachi, and Dr. K. Kosuge for

stimulating discussions. He i s very grateful to Dr. H. Okinaka for preparing the

sample.

References

(1) G. ANDERSON, Acta Chem. Scand. g, 1600 (1954).

(2) G. GROSSMANN, O.W. PROSKURENDO, and S. M. ARIYA, 2. anorg. allg.

Chem. 3 0 5 , 212 (1960).

(3) H. KOSUGE, J. Phys. Chem. Solidsz8, 1613 (1967).

(4) H. OKINAKA, J. Phys. SOC. Japan27, 1366 (1969).

(5) H. OKINAKA, J. Phys. SOC. Japan 28, 798 (1970).

(6) H. OKINAKA, J. Phys. SOC. Japan29, 245 (1970).

(7) H. OKINAKA, K. KOSUGE, and S. KACHI, Japan. J. appl. Phys. 2, 224 (1970).

(8)H. OKINAKA, H. KOSUGE, S. KACHI, M. TAKANA, andT. TAKADA, J. Phys.

SOC. Japan 32, 1148 (1972).

(9) A. HEIDEMANN, Z . Phys. 238, 208 (1970).

(10) B. ALEFELD, Kerntechnik l4, 15 (1972).

(11) M. BIRR, A. HEIDEMANN, and B. ALEFELD, Nucl. Instrum. and,Methods 95,

435 (1971).

(12) G. ANDERSON, Acta Chem. Scand. S , 1599 (1954).

(13) H. YASUOKA, H . NISHIHARA, Y. NAKAMURA, and J.P. REMEIKA, Phys.

K132 physica status solidi (a) 16

Letters A - 37, 299 (1971).

(14) A. HEIDEMANN and B. ALEFELD, V . IAEA Symp. Inelastic Neutron Scatter-

ing, Grenoble 1972, IAEA-SM- 155/G-4.

(Received February 2,1973)