Exchange interactions in antiferromagnetic salts of iridium

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Exchange interactions in antiferromagnetic salts of iridium

J. Owen

To cite this version:

J. Owen. Exchange interactions in antiferromagnetic salts of iridium. J. Phys. Radium, 1959, 20

(2-3), pp.138-140. �10.1051/jphysrad:01959002002-3013800�. �jpa-00236003�

138

EXCHANGE INTERACTIONS IN ANTIFERROMAGNETIC SALTS OF IRIDIUM

By ^J. OWEN,

Clarendon Laboratory, ^Oxford.

Résumé. 2014 On donne

résumé des travaux récents à Oxford

la

directe _{par réso-}

nance

paramagnétique ^du superéchange ^{entre les ions} paramagnétiques ^{dans des} cristaux. Les résultats pour les voisins les plus proches ^des paires dions Ir dans K2IrCl6 ^et (NH4)2IrCl6 ^sont comparés

^les susceptibilités ^{et les} propriétés antiferromagnétiques. On discute enfin brièvement le cas du couplage ^entre trois ions Ir.

Abstract.

A summary ^is given ^{of recent work at Oxford}

^direct measurement by para-

magnetic

^of superexchange ^between paramagnetic ^{ions in} crystals. The results for nearest neighbour pairs ^{of Ir ions in} K2IrCl6 and(NH4)2IrCl6

compared ^{with the} susceptibility

and antiferromagnetic properties of these salts. The behaviour of groups of three coupled ^{Ir ions}

is also discussed briefly.

LE

JOURNAL

PIIYSIQUE

20, ^FEVRIER 1959,

During ^the past ^few years the magnetic pro-

perties of chloroiridate salts have been extensively investigated[ 1-5]. ^The present report ^summarises

some of the results (Table 1) ^for K2IrCl6 ^and ( N ^H 4) 2IrC16, particularly ^{the recent} experiments ^of Griffiths, Owen, ^{Park and} Partridge [2] ^{and of} Cooke, ^Mc Kim, Lazenby, ^{Owen and Wolf} [3].

These salts have face centred cubic structure and show antiferromagnetic ^{behaviour in the} liquid

helium temperature range. The magnetic ^carriers

are the ^Ir4+ ions, which have configuration 5d5,

and only ^{one of these} d-electrons is unpaired giving

an effective electronic spin s =1 /2.

Electron spin ^resonance measurements [1] ^on magnetically ^dilute crystals, ⁱⁿ which diama-

gnetic ^{Pt ions are} substituted for Ir ions, ^{show that}

there is approximately ⁵ % ^{transfer of the} unpaired spin ^{from Ir d-orbits to} p7t-orbits ^on each of the six Cl ions in the octahedral [IrCl6]2- complex.

Fie. 1.

Section in (1, 0, 0) plane of face centred cubic Ir,

Pt salt showing

pair ^{of Ir ions. The}

^{axis is} along

the 1, 0, ^{0 direction} perpendicular ^{to the} plane ^{of the}

paper.

This transfer provides ^a basis for superexchange coupling ^{[6, 7] between} neighbouring ^{Ir ions via} intervening ^{CI ions.} The mechanism has been

investigated theoretically by ^Judd [4]. ^Because

of the types ^{of orbit} involved, ^the exchange ^inter-

action between nearest neighbours (nn) ^is expected

to be much greater ^{than that between} next nearest

neighbours (nnn). The situation is therefore dif- ferent from that in the well known face centred cubic antiferromagnetic ^{MnO discussed} by ^Ander-

son [7] ^{and others.}

The nn interaction has been studied by ^measu- ring ^{the electron} spin ^resonance spectrum ^from pairs ^{of Ir ions} occupying ⁿⁿ lattice sites ( fig. 1)

in semidilute crystal ^{where Ir : Pt 1 : 20.} The interaction between the two spins s’, s" ^{can be for-} mally ^written [8]

where S is the total spin with values 1 or 0. The

isotropic exchange Jnn splits ^{the levels into a} triplet (S =1) ^at Jnn /4 ^{and a} singlet (S = 0)

at

3Jnn /4. The smaller anisotropic exchange parameters ^{D and E remove the} degeneracy ^{of the} tiiplet ^{levels so that} they ^{are shifted to}

D,

D E ₂ ^{relative to their} centre of gravity. g y Measurement ^of électron spin ^resonance transitions between the Zeeman components ^{of the} triplet

at 20 °K using 1.25 and 3.3 cm wavelength give

the values of D and E in Table 1. Because of the presence of the singlet (S

0), ^the signal intensity

1 shows a temperature dependence ^{which is} approxi- mately of the form

Measurement of this intensity ^{down to 2 °K.}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphysrad:01959002002-3013800

139

gives the values of J. ^{in Table 1. The} positive sign ^indicates antiferromagnetic exchange. ^The

size of the measured interactions shows order of

magnitude agreement [2, 4] with Judd’s theory, though ^{it is difficult to make a close} comparison

because of lack of knowledge ^of overlap integrals.

The measured symmetry properties ^{are in} good agreement ^with theory, ^the x, y and z axes being

the twofold symmetry ^{axes of a nn} pair ^{which can}

be represented ^p diagrammatically by ^Ir Cl-Cl " ^Ir.

(see fig. 1). ^The principal ^{axis z is} perpendicular

Fie. 2.

Energy ^{levels of}

pair ^{of Ir ions.}

to the plane ^{of the four CI ions} participating ^{in the}

interaction.

In concentrated crystals, ^where each Ir ion has twelve nn Ir ions, ^the high temperature suscep-

tibility (90 ^{to 300} OK) ^{is found} [3] ^{to follow a}

Curie-Weiss law _x

CI(T -E- 6) after correction for diamagnetism ^and temperature independent paramagnetism. ^{The Weiss constants 0 are} given

in Table 1. Assuming ^that Jnnn « Jnn, ^{it is to be} expected ^that

where ^z

12 is the nearest neighbour ^number.

The results appear ^to be in agreement ^{with this} assumption ^{in as much as} they give Ynn (semi- dilute) N Ynn (concentrated).

The Néel temperatures Tn ^{are found} [3, 5] ^{to be}

rather low (Table 1), giving ^an unusually large a/Tn

ratio of about 10 for the both salts. Dr. P. W. Ander-

son has suggested (private communication) ^{on the}

basis of spin wavetheorythat ^this might possibly ^be

accounted for by ^a small value of Jnnn given by

The values of Tu ^and Jnn ^{in Table 1 thus} require

that fnnn Ik "-’ 0.1 -K if the results ^are fitted

to (4).

This _very small value of Jnnn ^{is not} inconsistent with the types of orbit used in the superexchange mechanism, ^{nor with the} high temperature suscep- TABLE 1

tibility results, ^{as has} already ^{been mentioned}

above. Some further evidence is provided by ^the

fact that no absorption lines from ⁿⁿⁿ pairs ^have

been seen. This suggests ^{that the} anisotropic part

of Jnnn, say Dnnn, is less than about 0.01 cm-1,

since the spectrum ^{from nnn} pairs would then fall within the line width of the strong ^{line from iso-} lated ions, ^{and so would not be seen.} However, though ^{these results} suggests ^that Jnun may be

considerably smaller that Jnn, ^{it is still not known}

whether Jnnn ~ 0.1 cm-1 as required by ^Ander-

son’s suggestion.

Preliminary measurements on semidilute _crys- tals by ^Mr. B. A. Coles show ^that there is a many line resonance spectrum ^{which is ascribed to}

groups ^of three coupled ^{Ir ions} (triplets). ^The intensity ^is weak, being ^{of order c} times smaller than that from nn pairs, ^{where c} is the fractional concentration of Ir. Several différent types ^of triplets ^are possible. ^For example, ^{if the three}

ions are all nn to each other, ^the interaction, neglecting anisotropic ^{terms, is} expected ^{to be} [9]

Jrn(s’ . S" + s" . s"’ + s’’’. s’), giving ^a quadruplet (S = 3/2) ^at 3Jnn/4, ^{and two doublets} (S = 1/2)

at - 3Jnn /4. ^{If there is no} exchange coupling

between two of the three ions, the interaction is

expected ^{to be of the form} Jnn(s’ .s" + s’.s"’), giving ^a quadruplet (S

3/2) ^at Jpl) /2, ^{and two}

doublets (S = 1/2) ^{at 0 and}

^{Jnn. A third} type

of triplet ^{can be built from a nn} pair ^{one of whose}

140

memhers has a nnn Tr. The interaction, including anisotropic ^{t,erms, is} expected ^{to be} of the form

where aC1.2is given by equation (1). ^The approxi-

mate energy levels, assuming J nnn «6D, ^{are four dou-}

blets at

31nn/4, lnn/4

D, Jnn/4 ⁺ D/2

± [(E /2)2 ⁺ (lnnn/4)2Jl/2. ^The inclusion of a nnn

Ir thus produces ^a slight shift in the energy levels of a nnn pair. ^{If the} spectrum ^{from such a} triple ^could be resolved and identified experimen- tally, ^it might give ^{a direction measure of} Jn,,n.

There are two main conclusions ^to be drawn from these experiments. First, ^{these chloro-}

iridate salts from an interesting ^{series of face cen-}

tered cubic antiferromagnetics ^with relatively strong ⁿⁿ interactions and much weaker nnn inter- actions. It has been possible ^{to reach a} good understanding ^{of the} high temperature suscep-

tibility, ^{and in view of the detailed} knowledge ^of

the exchange interactions which is available, ^it

seems likely ^{that the low} temperature ^behaviour

will be equally ^well understood in due course.

Second, ^and perhaps ^more important, ^the experi-

ments show that magnetic ^resonance techniques

can be ^{used to} make direct and detailed measu-

rements of exchange interactions in strongly ^cou- pled systems. The method is applicable, ⁱⁿ prin- ciple, ^to several well-known anti- and ferrimag- gnetics ^{in which the} magnetic ^{ions can be} replaced by diamagnetic ^ions.

This report represents ^{the work of a number of} authors, ^as given ^{above and in the} references, ^to

whom 1 am greatly ^{indebted for} permission ^to quote

their results. 1 would also like to acknowledge support ^{from the} General Electric Research Labo-

ratory, Schenectady.

REFERENCES

[1] ^GRIFFITHS (J. ^H. E.) ^{and OWEN} (J.), Proc. Roy. ^Soc., 1954, ^A 226, 96. ^STEVENS (K. ^W. H.), ^Proc. _Roy. Soc., 1953, ^A 219, ^542.

[2] ^GRIFFITHS (J. ^H. E.), ^OWEN (J.), ^PARK (J. G.) ^and

PARTRIDGE (M. F.), Phys. Rev., 1957, 108, ¹³⁴⁵ and 1959, accepted ^for publication ⁱⁿ Proc. Roy. ^Soc.

[3] ^COOKE (A. H.), ^MCKIM (F. R.), ^LAZENBY (R.), ^OWEN (J).

and WOLF (W. P.), 1959, acceptec ^for publication,

in Proc. Roy. ^Soc.

[4] ^JUDD (B. R.), 1959, accepted ^for publication, in ^Proc.

Roy. ^Soc.

[5] ^BAILEY (C.A.) ^{and SMITH} (P. L.), 1959, accepted ^for publication in Phys. ^Rev.

[6] ^KRAMERS (H. A.), Physica, 1934, 1, 182.

[7] ^ANDERSON (P. W.), Phys. Rev., 1950, 79, 350, 705.

[8] ^BAKER (J. M.) and BLEANEY (B.), Conference

the

Physics ^{of Low} Temperatures, 1955 (Paris, ^Institut

International du Froid).

[9] ^YVON (J.), ^HOROWITZ (J.) and ABRAGAM (A.), ^Rev.

Mod. Physics, 1953, 25, ^{165. KAMBE} (K.), ^J. Phys.

Soc., Japan, 1950, 5, ^48.

DISCUSSION

Mr. Smit.

^If such low values of exchange integrals (0.1 OK) ^are involved, ^{is it} then ^{not to be} expected ^{that an} applied magnetic ^{field will}

influence the susceptibility ?

Mr. Owen.

I would think that the _suscep-

tibility ^{below the} transition might be influenced if the magnetic ^{energy were} comparable ^with kTN,

which means a field of order 10 000 gauss for ammonium chloroiridate where TN ^{= 2.1 OK. So}

far the measurements have been made with much lower fields than this. Dr. Wolf can probably give you further details.

Mr. Wolf.

^The susceptibilities ^at very low

temperature ^were measured in fields _up to 1 000 gauss. In this range the susceptibility ^{was found}

to be independent ^{of field.}