Journal of Magnet ism and Magnetic Materials 116 (1992) 55-57 North-Holland

Change of magnetic short range order at metal-insulator phase transition *

F. Meze i a, p. Canf ie ld b and O. Sch~irpf c

"Hahn-Meitner-lnstitut, If. 390128, 1000 Berlin 39, Germany MSC, Los Alamos National Lab., Los Alamos, NM 87545, USA " Institut Laue- Langet'in, 156X, 38042 Grenoble Cedex,

France

The wavenumber dependent susceptibility in VsO 9 shows that the jump of susceptibility at the metal-insulator transition is not due to antiferromagnetic correlations expected in a Mort insulator but to the onset of ferromagnetic correlations in the metallic phase with a small, intinerant type V moment of 0.6tz B in both phases.

Members of the series gnO2n 1 including the extrema V203 and VO 2 display archetypical metal- insulator phase transitions with several or- ders of magnitude change of the resistance be- tween the high temperature metallic and the low temperature insulating phases [1]. The metallic phase is paramagnetic in each case with a Curie-Weiss susceptibility consistent with the as- sumption of full atomic magnetic moments on the V atoms. The insulator phase is (with the excep- tion of VO 2) antiferromagnetic at T = 0, and in most members of the series paramagnetic just below the metal- insulator transition with a sus- ceptibility much reduced compared to that of the metallic phase. These observations have been considered to provide evidence for the Mott char- acter of the transition, viz. that the reduced sus- ceptibility in the insulating phase is due to the strong antiferromagnetic coupling between neigh- bouring V moments. Actually the supposition of spin dimerization, i.e. the formation of pairs of oppositely oriented nearest neighbour V mo- ments has been given tentative support by crystal-

Correspondence to." Dr. F. Mezei, Hahn-Meitner-Inst i tut , Pf. 390128, 1000 Berlin 39, Germany. * Paper presented at the International Conference on Mag-

netism (ICM '91), Edinburgh, Scotland. 2 -6 September 1991.

lographic evidence [2]. The progress of neutron polarizatio n analysis techniques at the Institut Laue-Langevin made now possible to check these asumptions and we have found them blatantly incorrect. This finding basically puts in question previous considerations about the nature of the transition in this series.

We have applied the so called "3 directional polarization analysis" method [3], which allows us to single out the magnetic scattering contribu- tions above a high background of nuclear scatter- ing in paramagnets, antiferromagnets, etc. The magnetic cross section, proportional to the gener- alized susceptibility x(q), reveals the nature of short range correlations. The bulk of the data presented here was obtained using the D7 spec- trometer, completed by a set of data taken on INl l at small momentum transfers q not accessi- ble on D7. The results obtained in a V50, ~ pow- der sample show that;

a) There is a marked ferromagnetic short range order in the metallic phase with a correlation length of about 6 ,& (assuming an Ornstein- Zernicke type correlation function). Complemen- tary inelastic scans indicate that the relaxation time of the ferromagnetic correlations is about 10 12 s at 170 K.

b) The localized V moment (determined from the approximately fiat part of the cross section

0304-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

56 F. Mezei et al. / Short range order at metal-insulator phase transition

40

oa :zL

>'2 = 0

t~

V5 09

0 I I I I I

0 100 200 300

Temperature [ K ]

Fig. 1. Temperature dependence of the magnetic susceptibil- ity around the metal-insulator transition at around 130 K in our VsO,~ powder sample, compared with the behaviour of

more ideal single crystals.

curve above 0.6 A 1) is (0.61 _+ 0.07)#B, within error identical in both phases, compared to the 2.3p. B free atomic value expected for the relevant valence state.

c) The jump in the susceptibility at the transi- tion is due to the practical disappearance of the ferromagnetic correlations and not to the ex- pected onset of antiferromagnetic correlations in the insulator phase. Note that this does not nec- essarily rule out the existence of some antiferro-

03

0.2

V s O 9

e metall ic • insulator

0.1

~]~. @ w m ~ @ m,~immwm o o ~,a , *=~=.

I 2 q [ 1 / A ]

Fig. 2. Wavenumber dependence of the magnetic neutron scattering cross section in a y sO 9 powder sample in the metallic phase at 170 K and in the insulating phase at 70 K. The error bars not indicated are comparable to the size of the

data points.

0.08

"Z" 006 e~

-~ 0.04

0.02

0.00

V 0 5 9

{

B m [ ]

L i

1 2

q l l/A] Fig. 3. The higher q data from fig. 1 corrected for the magnetic form factor of V. The error bars indicated are

representative for all data points.

magnetic correlations in the insulator (or even in the metallic) phase, because it could be masked by a remainder of the ferromagnetic short range order. In fact the smaller susceptibility jump in our powder samples compared to more ideal sin- gle crystals (fig. 1) suggests that a fraction of the sample volume remains metallic below the transi- tion.

The magnetic correlations are best repre- sented by the cross section corrected for the atomic form factor as shown in fig. 3 for the larger q data. It appears that there is further maximum at about q = 2 ~ - 1 in both phases. This q value is not inconsistent with that ex- pected for nearest neighbour V pairs, i.e. 2.2

i. Nevertheless, this observation is at the limit of statistical significance.

These results contradict all expectations con- cerning the magnetic behaviour at metal - insula- tor phase transitions, and therefore they call for new theoretical ideas for the understanding of the transition in the V,,O2, , ~ series. The reduced value of the V magnetic moments in both phases suggests that we have to do with a strongly corre- lated electron system and not with a Mott insula- tor in the low temperature phase. The nature of magnetism should be itinerant in the ferromag- netically short range ordered metallic phase. This could also apply to the insulator phase, in partic- ular since there is no essential change in the magnetic moments at the transition. The primary

F. Mezei et al. / Short range order at metal-insulator phase transition 57

magne t i c effect of the change of e lec t ron ic s truc- ture at the t rans i t ion is the r educ t ion of the f e r r omagne t i c coupl ing. The a p p a r e n t C u r i e - Weiss behav iou r in the meta l l ic phase roughly c o r r e s p o n d i n g to f ree a tomic m o m e n t s is pure ly acc identa l , and it is in rea l i ty the resul t of s t rong co r re l a t ion effects. This might ac tual ly be an indi- ca t ion of weak i t ine ran t magne t i sm.

References

[1] S. Kachi, K. Kosuge and H. Okinaka, J. Solid State Chem. 6 (1973) 258. N.F. Mott, Metal-Insulator Transitions (Taylor and Fran- cis, London, 1974).

[2] M. Marezio, P.D. Dernier, D.B. McWhan and S. Kachi, J. Solid State Chem. 11 (1974) 301.

[3] See F. Mezei, in: Neutron Spin Echo, ed. F. Mezei (Springer Verlag, Heidelberg, 1980) p. 3.