Electrical conductivity of magnetite at low temperatures

Electrical conductivity of magnetite at low temperatures

Solid State Communications, Vol. 9, pp. 275—278, 1971. Pergamon Press. Printed in Great Britain ELECTRICAL CONDUCTIVITY OF MAGNETITE AT LOW TEMPER...

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Solid State Communications,

Vol. 9, pp. 275—278, 1971.

Pergamon Press.

Printed in Great Britain

ELECTRICAL CONDUCTIVITY OF MAGNETITE AT LOW TEMPERATURES J.R. Drabble, T.D. Whyte and R.M. Hooper Dept. of Physics, University of Exeter, Exeter, England

(Received 8 October 1970 by C.W. McCombie)

Measurements of the electrical conductivity as a function of temperature down to liquid helium temperature are reported for four single crystal specimens of magnetite, Fe 304, differing in impurity content and in stoichiometry. Such variations have little effect on the low temperature electrical properties.

WE REPORT here measurements of the electrical conducti~vityof several single crystal specimens of magnetite, Fe304, down to liquid helium ternperature. Previous measurements have concentrated on the transition of some three orders of magnitude in the electrical conductivity which occurs in this material at about 120 K and the lowest temperature to which investigations have been made is about 40 K. 1.2

applying indium—tin solder and tests showed that these contacts had a very low resistance compared with the specimen resistance at room temperature. In the conductivity range 102_ 1O~F~ cm~, corresponding approximately to the temperature range from room temperature to 85 K, measurements were made using standard potentiometric techniques. Below this temperature, the electrical conductivity decreased rapidly with decreading temperature and measurements down to helium temperature were made the high resistivity apparatus described by with Drabble and Whyte 4which,

Measurements were made on four typical specimens. Three of3these were grown Specimen A was by cutthe arc-transfer process. from a boule which was grown using spectro-

under the cx~nditionsused, has been shown to

graphically pure material. Specimen B was prepared in the same way except that commercial grade impure material was used. Specimen D was made using high purity material but the conditions of growth were varied so as to alter the stoichiometry of the final crystal. A further specimen C was obtained from an external source and was reported to be non-stoichiometric.

have a conductivity limit of 5 x 10

cm

—t

Although there are detailed differences in the results for the four samples, the general behaviour is very similar, particularly at low temperatures. The usual way of plotting the logarithm of the conductivity versus the inverse temperature did not seem to indicate any significant features. In Figs. 1—4 we have chosen to present the results in the form of graphs of log a versus as first suggested by Mott5 in connection with impurity conduction. The significant features of these results are summarised below.

Specimens, with typical linear dimensions of 4mm, were cut from a boule using a diamond saw and the surfaces were lightly sandblasted. Electrical contacts to two opposing faces were applied by electrodepositing a 1~mfilm of nicl~el from a sulphate bath at a current density of 10 mA cm2. Connections to these were made by

For specimen A (Fig. 1), grown using material

275

276

MAGNETITE AT LOW TEMPERATURES

Vol. 9, No.4

T(K) Q (ne,

0 ~ —

0

C,

88~ ~ —

.

r,t.

I

2

T(K) C,

I

I

______________________________________ LI I I I

2

.~

~

*

0 8p.ci~sn A —2

Sp.ciLen B

a,

‘0

—4 -4 S.

42

‘Se

~-6

.2

-8 42’

—8

.

0

-10•

5S V

-10

5

0~

‘5

*0\ —12 —12

5

~0 —14

5’

~0 0

02

03

04

0.5

-~

0.6

C

—14



so S~OO0

0.7 0.2

1

T~ (K)

0.3

0.4

0.5

—i

T



0.6

I 0.7

(K)

FIG. 1. Graphs of log conductivity as a function

of T’ for four specimens of Fe~O 4.

FIG. 2. Graphs of log conductivity as a function

of T~ for four specimens of Fe 1O4~ containing less than 15 ppm impurities, the results above 45 K are very similar to those reported in reference 2. There is a sharp transition of about 3 orders of magnitude in the conductivity at a temperature close to 119 K. Below this the log a vs. T~ plot shows a linear behaviour over 9 orders of magnitude, then turns 1 over flat minimum of lO’~l’ at 5.3and K. shows At stilla lower temperatures, the cm conductivity appears to be increasing slightly with decreasing temperature down to the lowest temperature of 4.2 K. The T

dependence of log a is usually

interpreted as evidence of impurity controlled conduction and it was for this reason that specimen B was grown using commercial grade

mild steel which contained about 1 per cent of manganese. In all other respects, specimen B was prepared under the same conditions as A. The principal difference between the two specimens was that the sharp transition shown by Alowest is completely absent specimen B. At the temperatures theintwo specimens have very similar electrical properties and in particular B shows a minimum value of conductivity of 2 x 10~Y-’ cm~at about 5.1 K. However, in the intermediate range, the behaviour of B can no longer be described by a simple relation between log a and T - but seems to show two separate regions. The low temperature region is similar to that of A.

Vol. 9, No.4

MAGNETITE AT LOW TEMPERATURES

277

T(I) ~

2

T(X)

9

~I

_________________

420% ~0

2 00 5%

o

0

-2

Sp.ci~.n C

—2

Spscia.n D

1~ S —



S

S

—~ I

a,

g

~ —10

‘,

a, 5

—12

..

.

S

: —10

‘0 ‘a,

‘0

5

5

—12 0 42

-14.

0.3

0.14

0.15

0*6

T~ (K)~

—14.

0.3



0.4

0.5

0.60.7

T~ (K)~

FIG.. Graphs of log conductivity as a function of T ‘ for four specimens of Fe 304.

FIG.4. Graphs of log conductivity as a function of T for four specimens of Fe 3 04.

The two remaining specimens C and D were measured to investigate possible effects associated with non-stoichiometry. The electrical behaviour of these two specimens is almost identical over the whole temperature range, the only difference being that the sharp transition occurs at 108 K for specimen C and at6 102 specimen D. suchK afor lowering of the According to Verwey, transition temperature is associated with an

temperature behaviour again is better described by two lines on the log a vs T - plot.

increasing degree of non-stoichiometry. The magnitude of the transition is somewhat reduced compared with that for specimen A. The low temperature behaviour is very similar to the other specimens but the minimum value of 3 x 10’5Q’ cm~occurs at a somewhat lower temperature of 4.8 K. The intermediate

Measurements made in different crystallographic directions on the individual specimens showed no significant differences from those described above. The two in specimens A to and D were analysed chemically an attempt assess the non-stoichiometry but no differences were found within the limits of the accuracy of the analysis. In summary, although the impurity concentration and stoichiometry in the four specimens varies over a wide range as evidenced by the behaviour of the transition, such changes appear to have

278

MAGNETITE AT LOW TEMPERATURES

very little effect on the electrical conductivity of low temperatures. One of the many interesting features is that this conductivity either saturates or possibly shows a minimum. An extension of these measurements to temperatures below 4.2 K would clearly be of considerable interest,

Vol. 9, No.4

Acknowledgements — We are grateful to Professor M. Blackman, F.R.S. for the loan of specimen C. One of us (R.M.H.) is grateful to the Science Research Council for an award which enabled the above work to be carried out.

REFERENCES 1.

CALHOUN B.A.,Phys. Rev. 94, 1577 (1954).

2.

MILES P.A., WESTPHAL W.B. and VON HIPPEL A., Rev, mod. Phys. 29, 279 (1957).

3.

DRABBLE J.R. and PALMER A.W., J. appi. Phys. 37, 1778 (1966).

4.

DRABBLE J.R. and WHYTE T.D., J. Phys. E., 3, 515 (1970).

5.

MOTT N.F., Phil. Mag. W, 835 (1969).

6.

VERWEY, E.J.W. and HAAYMAN P.W., Physica 8, 979 (1941).

Messungen der elektrischen leiffahigkeit als Funktion der Temperatur bis zur Temperatur flussigen Heliums sind fur vier Einzel Kristalle von Magnetit (Fe 304) beschrieben worden, die in Reinheitsgrad und Stoichiometrie verschieden sind. Soiche Abanderungen haben nur wenig Wirkung auf die elektrishen Eigenschaften bis Tieftemperaturen.