Unusual low-temperature magnetic behaviour of some cubic crystals

Palma-Vittorelli, Palma,

M. B.

Physica

M. U.

26

922-930

Drewes, G. W. J. Koerts, W. 1960

UNUSUAL

LOW-TEMPERATURE MAGNETIC OF SOME CUBIC CRYSTALS

by M. B.

PALMA-VITTORELLI,

Istituto

di Fisica

Kamerlingh Communication

No. 323~

Ormes

from

the

M. U. PALMA

dell’UniversitA,

G. W. J. DREWES

BEHAVIOUR

Palermo,

Italia.

and W. KOERTS

Laboratorium, Kamerlingh

Leiden,

Onnes

Nederland.

Laboratorium,

Leiden,

Eederland

Synopsis Electron exhibit

spin resonance

results

a low-temperature

on hexammine

transition

similar

nickel

to that

halides observed

are reported, in some

which

antiferro-

magnetics. Static susceptibility transition.

data,

These results are rather the current

knowledge

given concerning

also reported,

striking

do not evidence

and they

cannot

on antiferromagnetism.

such a low-temperature

easily be interpreted

Some

remarks

and

in terms

suggestions

of are

th-ir interpretation.

1. Introduction. In the last few years, work on electron spin resonance in undiluted crystals has yielded a considerable amount of information concerning exchange effects and related cooperative phenomena, such as antiferromagnetism 1)2)3)4). R ecently, preliminary electron spin resonance experiments, at room- and liquid air temperatures, on undiluted single crystals of nickel hexammine halides, have shown 5) that in these crystals a strictly cubic symmetry is preserved in the ammonia octahedra surrounding the nickel ions, and that this lack of distortion and the measured g-values suggest the presence of six strongly directional bonds in the Ni(NHs)$+ complexes. This called for an investigation on possible cooperative effects due to the expected super-exchange through the ammonia molecules. We wish to report here the results of that investigation, accomplished by low-temperature electron spin resonance and static magnetic susceptibility measurements. These measurements have revealed a striking behaviour which possibly widens the field of research on antiferromagnetics. 2. Crystal structure and samfiles. The single crystals used for the present of the work have been prepared and kindly supplied by Dr. T. Garofano Istituto di Fisica dell’Universit8 di Palermo. Their structure 6), shown in -

922

-

LOW-TEMPERATURE

MAGNETIC

BEHAVIOUR

fig. 1 (not to scale) is f.c.c. (CaFs-like).

OF SOME

CUBIC

CRYSTALS

923

There are 4 molecules in the elementa-

ry cell. The nickel ions are at the corners and the face-centres, surrounded each by an octahedron of six ammonia molecules, whose vertices lie along the and equivalent directions. The approximate dimensions of the cubic elementary cell are: 10.9rf for the iodide; 10.3A for the bromide; 10.1 A for the chloride 6).

CRYSTAL

STRUCTURE

OF

I Fig.

1. Crystal

structures

of the hexammine

nickel halides

(not to scale).

It is of interest to compare the above values with an estimate of the extension of the charge distribution of the ammonia octahedron, which may be done on the basis of recent work of Kamimura e.u.7). This points out that sizable superexchange effects between next-to-nearest neighbours Nisf may be expected through the ammonia molecules. The single crystals have a macroscopic octahedral shape and a deep blue colour. The purity of the crystals is the following: magnetic impurities < 5 ’ 10-S; other impurities < 1O-3; water content < 1O-4 (spectroscopically determined). Because of a possible escape of ammonia, a proper thermal annealing treatment is hardly found, so that the concentration of internal imperfections such as dislocations, etc. might be rather large. The best grown crystals have been carefully selected and used for the present work. 3. Experimental results on electron spin resonance. 3.1 Par am agn e t ic r e s o n an c e. Electron spin resonance experiments have been performed on single crystals of the mentioned salts, at 8 mm, 3 cm, 21 cm wavelengths, at room temperature and at temperatures in the regions of liquid nitrogen,

924

M. B. PALMA-VITTORELLI,

M. U. PALMA,

G. W.

J.

DREWES

AND

W.

KOERl

hydrogen and helium. The equipments of the Kamerlingh Onnes Labor; torium 2) and of the Istituto di Fisica di Palermo s) were used. At ar temperature and frequency, only one resonance line has been found, whit is angular-independent (as far as both position and width are concerned within the limits of the experimental accuracy. Data concerning the results at about 300°K and 90°K are summarize in table I. The rather large variation of the line-width in this range of tempe ature is attributable to the temperature-dependence of the spin-lattit relaxation time. TABLE Electron

Spin

Resonance

I

results Full

at 3 cm wavelen&h

line width

in oersteds

(AH“)%

g-values

Crystals/

~zsjl Maximum

half

slope

power

// ,&K)

1

300

1

90

1

300

90

/

300

/

90

I300/

NiClz.bNHs

2.175 & 0.002

2.169*0.001

320:20

95=10

440520

160110

1.3

1.48

NiBrz’bNHa

2.178&0.002

2.173*0.001

340120

70>10

480*20

120~10

1.3

1.56

NiIZ.6NH3

2.191 LO.002

2.177_LO.O01

39OA.20

85&10

600-20

145~10

1.3

1.45

A comparison

of these experimental

results

with the optical

absorptic

spectra of the same crystals (which give the distance between the ds ar dy levels), obtained at the Istituto di Fisica di Palermo (unpublished) poin out that the numerical value of the spin-orbit coupling coefficient has to 1 taken about 30% lower than that of the free ion. This may be regarded as the result of a charge-transfer which caus either a decrease of the interaction between the paramagnetic electrons ar the Nis+ ion and/or an increase of the shielding of the nuclear charge of tl process is expected to gi7 Ni nucleus s)ia)r1)12) : such a charge-transfer rise to an increased exchange effect. Accordingly the experimentally determined values for the ratio 6 =


at liquid air temperature are large compared with the values expected fs d a gaussian shape, and compared with most of the experimentally termined values for other substances 13). Comparison of the calculate dipole-dipole broadening with the experimental line-width at liquid a temperature yields a value of the exchange field of the order of a few ki oersteds.

LOW-TEMPERATURE

MAGNETIC

BEHAVIOUR

OF SOME

CUBIC

CRYSTALS

925

When the temperature is lowered 3.2 Low temperature results. down to a critical value T, (which is characteristic for each of the three salts, and lies in or close to the liquid hydrogen temperature region), a sudden broadening of the absorption line is observed, together with a weakening of the peak intensity *). The line width becomes larger and larger when the temperature is lowered to 1°K. No changes in the position of the absorption maxima are detectable at or below the transition temperature. It should be noted, however, that, due to the large line width and low peak intensity, accurate measurements of the positions of the peak and of the half intensity points could hardly be made at T < T, and the errors may be considerable. In figs. 2, 3 and 4 the behaviour

of the resonance

5

line in the hydrogen and

1

ABSORBED POWER I(ARBITRARY UNITS)

Ii !i )j

i!

i! iI

bi

MAGNETIC

Fig. 2.

FIELD

(~ERSTED)

Electron spin resonance lines of NiIz’6HNa

in the hydrogen

region,

at 8 mm

wavelength. -.-

o -.-

20.5”K

... . .. + ....

---

q ---

_.._

__

A __

19.77”K 19.73”K

*) In the course of the present work, me of us (M.U.P.)

.

_.._

was informed

..

18,07”K 14.4”K

by Dr. J. Owen

in Oxford,

that in preliminary measurements he had observed the disappearance (at a critical temperature) of the electron spin resonance in one of the crystals investigated in the present work. We are indebted to Dr. 0 we n for this communication.

926

M. B. PALMA-VITTORELLI,

M. U. PALMA,

G. W. J. DREWES

AND W. KOERTS

helium regions is illustrated, for Ni(NHa)eJs, at 8 mm wavelength. The line width is the same at 3 cm wavelength. The zero-field absorption at 21 cm wavelength is such that line width measurements have no real meaning in that range.

s

t

A3SOQBED

FOWER

(nR3rr!=!L\RY

“NITS,

MAGNETIC

Fig. 3. Electron

spin resonance

FIELD

(OEffiTED)

lines of NiIz.6NH3

in the helium

region,

at 8 mm

wavelength. 0

Fig. 4. Electron

1.5”K

spin resonance

A

3°K

G

line width vs temperature wavelength.

4°K

in NiIz.6NHs,

at 8 mm

Within the rather large experimental errors, at all the investigated the resonance field-to-frequency ratio is frequencies and temperatures, constant. No anisotropy was found at any temperature or external field value. For the two other salts, the transition occurs at a temperature just higher than the liquid hydrogen region. For this reason, a complete set of measurements of line widths and resonance fields in the neighbourhood of the transition was performed only for the iodide. Qualitative measurements on the bromide and on the chloride show that the general features of the magnetic behaviour are similar for the three salts investigated.

LOW-TEMPERATURE

MAGNETIC

OF SOME

BEHAVIOUR

4. Static magnetic suscefitibility

CUBIC

measurements.

CRYSTALS

In order

to clarify

927 the

nature of the above described transition, we have performed measurements of the static magnetic susceptibility, in the same temperature regions as were used for the electron spin resonance. Single crystals were also used for these The equipment used has already been described by measurements. Gijsman 14). Strikingly, the magnetic susceptibility of the three investigated salts, appears to follow a normal paramagnetic behaviour, describable by a Curie-Weiss law, down to the liquid helium temperature range ; the sign of the Weiss temperature corresponds to an antiferromagnetic transition. In agreement with the electron spin resonance results, the magnetic susceptibility appears to be isotropic and there is no indication of a presplitting in the spin multiplet.

Fig. 5. Static plotted

susceptibility

against

temperature.

results. Molar

and therefore o

The

cannot

NiI2.6NHs

reciprocal

susceptibility clearly

static

be drawn

NiBr2.6NHs

A

susceptibility

plots are very

per gram

is

close to each other

together. q NiC12.6NHs

The experimental results are summarized in fig. 5 and in table II. In the latter the last column shows the g-values calculated from the susceptibility data, which agree with the g-values obtained by electron spin resonance TABLE Static Crystal

! ’

II

susceptibility

results

g (calculated, c

1 A

in “I<

using the C-values)

NiC12.6NHa

1.18

9

2.17 1 0.02

NiBrz.bNHa

1.21

7

2.19 f

NiIz.6NHs

1.21

3

2.18 & 0.02

0.02

928

M. B. PALMA-VITTORELLI,

M. U. PALMA,

G. W.

and given in table I, well within the experimental measurements.

J. DREWES

AND

W.

KOERTS

errors of the susceptibility

There is some indication of small variations in the C and A values, relative to temperatures above and below the transition. The order of magnitude of the differences is just at the limit of the experimental errors, and no definite statement can be made, except that a Curie-Weiss law is also satisfied below the transition temperature. The present results are in rather good agreement with those made at higher temperatures on a powder of nickel hexammine chloride, recently published 15). Small differences may be due to chemical impurities : the good agreement between the g-values obtained by the present susceptibility measurements and electron spin resonance experiments, indicates that the present data are probably more precise. 5. Discussion of the experimental results. The main results of the present experiments are the following : 4 the data of table I on the line width and the line shape indicate that exchange effects are present. b) The large broadening of the electron spin resonance occurs very sharply at a critical temperature T,. The above mentioned broadening occurs with no detectable shift of the 4 resonance field. 4 There is no detectable deviation from cubic symmetry in the octahedron surrounding the nickel ions. At any temperature, both the static susceptibility and the resonance line are isotropic. 4 The transition temperature mentioned in b) is larger (for about a factor 6) than the Curie-Weiss constant. The transition at T, in the resonance line is not paralleled by a f) change in the behaviour of the susceptibility vustemperature curve. Result f) is the most striking one. Results a) and b) suggest that we are dealing with a cooperative phenomenon. An electron spin resonance behaviour similar to the one described above, has been found in a number of antiferromagnetics 1)s) all of which exhibit a well defined transition in the susceptibility vs temperature curve. It has also been found that in CU(NH.$&OJ.H~O 2) 16) and in the DPPH 17) an unexpected broadening of the resonance line occurs in the helium range. For the former crystals, the line width steadily increases from 4°K to about 1.5”K and, more recently, it has also been observed that the static susceptibility shows a maximum ls) (at about 3.4”K) independent of the orientation of the crystal in the static field. A possible, though not very probable, explanation of the conflicting results could be that the crystals contain a small concentration of “imperfections” which would not contribute much to the magnetic properties observed.

LOW-TEMPERATURE

MAGNETIC

These “imperfections” or be of stoichiometric

BEHAVIOUR

OF SOME

CUBIC

CRYSTALS

929

should either be within the purity figures of sect. character.

They

could have a transformation

1,

of a

ferromagnetic or antiferromagnetic character at the observed transition temperature. The resulting small changes of volume could lead to irregularly distributed strains in the lattice, disturbing the cubic symmetry of the cristalline fields acting on the nickel ions. This would lead to a removal of the degeneracy of the lowest triple level of the nickel ion and to a broadening of the resonance. However, it is striking that the transition temperature does not depend on the particular specimen. Another possibility for explaining the results could be in terms of an antiferromagnetic ordering (see for instance ref. 2) with a very small threshold field. This is consistent with result d) which gives no evidence of any anisotropy. In an f.c.c. lattice (as in our case) the antiferromagnetic ordering should be built up by 4 sub-lattices. Result e) may be accounted for, if an antiferromagnetic interaction between nearest neighbours (belonging to different sub-lattices) and a ferromagnetic interaction is)sO) between next-tonearest neighbours within the same sub-lattice is assumed. From the data of table I an estimate of the value of the exchange field can be obtained. A comparison of this value with that estimated from the transition temperature, suggests a temperature-dependence of the exchange integrals, which might mask the expected transition in the susceptibility behaviour below the transition temperature. Result f) could then be accounted for. An alternative explanation of results b) c) and f) could be connected with a prevailing effect, at the mentioned transition temperature, of the anisotropit exchange. Specific heat measurements might give a clue to the difficulties encountered. We soon hope to be able to work out an acceptable interpretation of the behaviour of the salts concerned. Acknowledgements. The congenial assistance received at the Kamerlingh Onnes Laboratorium by two of us (M.B.P.V. and M.U.P.) is gratefully acknowledged together with the support of the stay by the Comitato Regionale Ricerche Nucleari (Palermo). We wish also to thank Prof. C. J. Gort er for throughout stimulating criticism of the present work, continued during his recent visit at Palermo, and for criticism of the manuscript, Prof. M. Santangelo for friendly help, Prof. J. van den Handel for permission of using the static susceptibility apparatus, Dr. T. Garofano for kind preparation and analysis of the crystals; Mr. D. de Jong for technical help and Mr. E. Frikkee for assistance during the measurements. Received 25-5-60

$30

1) 2)

LOW-TEMPERATURE

Maxwell, Gerritsen,

L. R. and McGuire, H. J., Thesis Leiden

MAGNETIC

BEHAVIOUR

Palma-Vittorrlli,

6) Wyckoff, 7) Kamimurn,

R. n’. G., Crystal structures, S., Sugano,

Cimcnto

M. U., Nuovo Interscience

Suppl.

Cimeuto

Publishers,

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CRYSTALS

(1955).

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CUBIC

T. R., Rev. mod. Phys. %Tr(1953) 279.

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