LOW TEMPERATURE PARAMAGNETIC PROPERTIES OF CU2+ AND TI3+ IN OCTAHEDRAL ENVIRONMENTS

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LOW TEMPERATURE PARAMAGNETIC PROPERTIES OF CU2+ AND TI3+ IN

OCTAHEDRAL ENVIRONMENTS

N. Rumin, D. Walsh, K. Lee

To cite this version:

N. Rumin, D. Walsh, K. Lee. LOW TEMPERATURE PARAMAGNETIC PROPERTIES OF CU2+

AND TI3+ IN OCTAHEDRAL ENVIRONMENTS. Journal de Physique Colloques, 1971, 32 (C1),

pp.C1-946-C1-947. �10.1051/jphyscol:19711339�. �jpa-00214373�

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JOURNAL DE PHYSIQUE

Colloque

C

I, supplkment au no 2-3, Tome 32, Fe'vrier-Mars 1971, page C I - 946

LOW TEMPERATURE PARAMAGNETIC PROPERTIES OF cu2+ ^AND ^{TI^+} ^IN OCTAHEDRAL ENVIRONMENTS

N. RUMIN and D. WALSH

Eaton Electronics Research Laboratory, McGill University, Montreal, P. Q.

K. P. LEE

CRAM, UniversitC de Laval, QuCbec, P. Q.

R4sumB.

-

Des expkriences RPE et des mesures de relaxation spin-reseau en dessous de

4.2 OK

mettent en evidence un effet Jahn-Teller. Dans le cas du CuzC dans LazMg3(NO3)12.

24 H z 0

on observe un processus direct plus rapide par cinq ordres de grandeur que pour le mCme ion dans K2Zn(S04)~.

6

HzO. Pour leTi3+ dans l'alum methyl ammonium les valeurs observkes du facteur

g,

gll

= 1.37 -f 0.01,

gl

= 1.61 & 0.01,

contredisent le modele du champ cristallin sta- tique. Une relaxation rapide Raman est observke pour

T > 1.7 OK,

mais quand

T < 1.7 OK

la relaxation devient indk- pendante de la temperature.

Abstract. - E.

P. R.

and spin-lattice relaxation rates measured below

4.2 OK

indicate the presence of a Jahn-Teller effect. For Cuz+ in Lanthanum Magnesium nitrate a Direct process is observed, the rate of which is five orders of magni- tude faster than for the same ion in a Tutton salt. For Ti'+ in Methyl Ammonium alum the

g

values,gl

= 1.37 & 0.01,

gll

⁼1.61

+

0.01,

are evidence against a static crystal field model.

A fast

relaxation rate, controlled

by

a Raman process, is also observed here above

1.7 OK

but below the rate tends towards a temperature-independent one.

I. Cu2+ in lanthanum magnesium double nitrate (LMN).

-

For an E orbital state coupled to E modes of vibration, the vibronic ground state consists of an E doublet above which lies an A singlet with energy separation given by the tunneling splitting 6. For Cu2"

in LMN the electron spin S

=

4 and 6

&

gPH. It was shown by Lee and Walsh [I], and in more detail by Williams et al. [2], that the spin-lattice relaxation time in the liquid He temperature range between a Kramers conjugate pair, corresponding to one of the three possible distortions of the magnetic complex, was shortened by a factor of between lo4 and lo5 due to at two-step direct process consisting of a reorientation without spin flip followed by another direct process which flips the spin. The latter step involves a non-conjugate pair and hence spin-phonon coupling is strong. We assumed that the effects of random strain in the crystal were considerably less than gPH. Since the rate for the spinless step was established to be fast, a phonon bottleneck was pre- dicted [I].

An alternative relaxation process for the reorien- tation without spin flip is a non-resonant two phonon process. Since we expect a phonon frequency distri- bution in the range of 0 to kT/h in the vicinity of the magnetic ion due to the non-linear scattering property of the Jahn-Teller complexe [3], the Raman process can satisfy the conditions hv,,, 4 k T and

hv, - hvJ

=

6 ,

where v, _and _{v j} are the frequencies of the two pho- nons. Consequently the rate is proportional to T2 in first order. Our experimental results varied with microwave power, pulse duration, and concentration

;

but, generally, the recovery signal following pulse saturation showed two decay constants, their respec- tive temperature dependence being approximately

T-' and T - 2 . Further investigation has failed to confirm definitely the phonon bottleneck interpreta- tion.

On the other hand Williams et al. assumed that

strain was - based on work by Culvahouse et al.

[4].

The corresponding splitting is 5 x lo-' cm-', putting a large band of phonons on speaking terms with the Zeeman energy, which had some experi- mental support. Their data, although it contained considerable scatter, had in general a T - ' dependence for the relaxation time below 8 OK. Recent experi- mental data on Ti3' (reported below) suggests that is - which is more in line with the strain in strain MgO, and thus the corresponding splitting in LMN is - ⁵ x l o d 2 cm-'. The two theories are slightly different because the two sets of measurements differ somewhat. In view of the large scatter in the measure- ments more accurate data are essential to resolve this difference.

11. Ti3+ in methyl ammonium aIum (M. A. alum).

-The E. P. R. of Ti3+ in M. A. Alum was reported briefly by us in 1969

[ 5 ] .

The spectrum consisted of four lines with trigonal symmetry, and the spin-lattice relaxation time appeared to be quite short. Although the ground T2 orbital state can couple to E modes and/or T2 modes strong coupling to T2 modes was assumed leading to four stable configurations of tri- gonal symmetry. The resulting vibronic ground state is a T, state with an excited A singlet, the separation of which from T2 tends to zero as the coupling strength tends to infinity. S

=

3 for Ti3+ and consequently the spin lattice relaxation could be expected to proceed in a manner similar to Cu2+ in LMN. As show11 below the situation is somewhat more complicated.

Support for the use of such a dynamic model can be found in the fact that the experimental g values,

g, =

1.61 + 0.01 and g ,

-

1.37 + 0.01, are incon- sistent with static crystal field theory which predicts either

g, =

0 or g , < g l l . On the other hand it can be shown that it is always possible to obtain g , > gll by coupling the T2 orbital state to T2 or E modes separately, but further calculations are necessary before numerical comparisons can be made.

More detailed relaxation time measurements were

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711339

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LOW TEMPERATURE PARAMAGNETIC PROPERTIES OF C U ~ + A N D TI'+

c

¹

-

947

recently performed. Several factors, including the

existence of a residual recovery (possibly associated with the He exchange gas which was used to cool the sample and cavity), restricted accurate measurements to the range 1.45-2.3 OK. Since a temperature differen- tial between the cavity wall and the He bath, depen- ding on the exchange gas pressure and the microwave power level, was observed, temperature was monito- red by a carbon resistance thermometer mounted on the cavity wall. All recorded measurements were obtained with atmospheric pressure in the exchange gas at room temperature. Recovery signals after pulse saturation were in all cases, within experimental error, simple exponentials. Figure 1 shows the results for H in the ( 111 ) and along a direction corresponding

FIG. 1.

- Temperature-dependence

of

relaxation

time for

Ti3+ in

M. A.

Alum.

to lo0 from the projection of a trigonal (111) axis on the plane. A theoretical best fit is also shown.

These results have certain similarities to those for Ti3+ in AI,O, by Kask et al. [6]. The theory of Ti3+ in A1203 has been described recently by Macfarlane et al. [7] and also by Bates and Bentley 181, who assumed strong J-T coupling to E modes only. Spin-orbit coupling plus a trigonal distortion split the T, vibro- nic ground state into three doublets, the splitting being reduced considerably by a factor proportional to the oscillator overlap integral. A similar model, except for the sign and magnitude of A, may suggest itself for Ti3+ in M. A. Alum.

There is serious difficulty, howcver, with this model since the observed relaxation time below 1.4

OK

is independent of temperature. We can estimate the direct spih-lattice relaxation time

^{2 ,}

from the value of this quantity in Titanium Alum [9] by rccognizing that all parameters for Ti3+ will be more or less iden- tical, with the exception of the trigonal splitting A.

For strong coupling to E modes, the splitting of a T, vibronic state by either a static or a dynamic tri- gonal distortion will be considerably reduced from that for a T2 orbital state. Since

7,

is proportional to A4, we have at 1.20K

^{2 ,}x

s if A = 10' cm-'

and

^{2 ,}w 5 x lo-' s. if A =

500 cm-'. The observed

absorption lines are approximately 18 gauss wide and thus the constant relaxation time of 4 x lo-' s is indicative of strain broadening. Our conclusion then for the absence of a direct process is that the corresponding

7 ,

is considerably longer than

4 x s

and therefore A is at least a few hundred wave numbers.

Thus the trigonal field in M. A. Alum, which is expec- ted to be smaller than in A120,, can not be substan- tially reduced by the Ham effect.

Unlike the Cu2+ situation the

g

values of Ti3+

in M. A. Alum are sensitive to strain. Our estimate of crystal strain is - lo-", and presumably that in LMN is comparable.

Acknowledgements. - The authors wish to thank Professor K. W. H. Stevens for some comments on the Jahn-Teller coupling of T2 ions, and Dr. Gloria Simpson for growing the alum crystals. Financial support by the National Research Council is grate- fully acknowledged.

References

[I] LEE

(K.

P.) and WALSH (D.), Phys. Letters, 1968, [5] RUMIN

(N.),

WALSH (D.) and WOONTON

( G .

A.), CAP

27A, 17. Conference Waterloo, Ontario, 1969.

121

WILLIAMS

(F. I. B.),

KRUPKA

(D.

C.) and BREEN

@. P.),

[6] KASK (N. E.), KORNIENKO

(L.

S.), MEDELSHTAMP

(T.

S.) Phys. Rev., 1969,

179,

255. and PROKHOROV

(A. M.), SOV.

Phys. JETP, [3] BATES

(C.

A.), DIXON (J. M.), FLETCHER (J.

R.)

and 1964,

5,

1677.

STEVENS