ULTRASONIC PHONON VELOCITIES IN Cdl-xMnxTe BETWEEN 1.5 AND 96 K : ANOMALIES NEAR THE MAGNETIC TRANSITION

HAL Id: jpa-00221271

https://hal.archives-ouvertes.fr/jpa-00221271

Submitted on 1 Jan 1981

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

ULTRASONIC PHONON VELOCITIES IN

Cdl-xMnxTe BETWEEN 1.5 AND 96 K : ANOMALIES

NEAR THE MAGNETIC TRANSITION

A. Wu, R. Sladek

To cite this version:

JOURNAL DE PHYSIQUE

CoZZoque C6, suppZ6ment au n022, Tome 42, dgeembre 1982 page C6-646

ULTRASONIC PHONON VELOCITIES IN Cdl-,Mn,Te

BETWEEN 1.5 AND

96

K

:

ANOMALIES

NEAR T H E MAGNETIC TRANSITION

A.Y. Wu and R.J. Sladek

Department of Physics, Purdue University, West Lafayette, I N 47907, U.S.A.

Abstract.- Transit times of 30 MHz ultrasonic waves have been measured from 1.5 K t o 96 K for CdlmxMnxTe samples w i t h 0.05 r

x

< 0.65. Below 60 K the v e l o c i t i e s f o r higher concentrations exhibit anomalous features which depend on x and are stronger f o r shear modes than f o r longitudinal modes. The shear mode anomalies are f l a t t e n i n g of the temperature dependence f o r

x

= 0.20 and 0.35 a t low temperatures and a wide minimum centered around 22 K f o r samples with

x

r 0.55. The location of each minimum indicates t h a t i t i s connected w i t h a t r a n s i t i o n t o the spin glass o r antiferromagnetic s t a t e . The depth of the minimum i s much greater than the e l a s t i c constant depression observed i n

t r a n s i t i o n metal alloy spin glasses (TMASG). This may be due t o electrons ( o r holes) in large o r b i t s around shallow-level impurities affecting the Mn magnetic moments.

Ultrasonic velocity measurements have proved useful f o r investigating mag- n e t i c phase t r a n s i t i o n s including those t o the antiferromagnetic s t a t e in various insulating compounds1 and t o the spin glass s t a t e in t r a n s i t i o n metal alloys. 2

In t h i s paper we present the f i r s t ultrasonic velocity measurements on Cdl-xMnxTe, a so-called semimagnetic-semiconductor which i s paramagnetic a t a l l tem- peratures f o r

x

< 0.17, becomes a spin glass below Tsg when 0.17 <

x

< 0.62 and i s antiferromagnetic a t low temperatures when 0.62 <

x

< 0 . 7 1 . ~ Our r e s u l t s should contribute t o understanding phase t r a n s i t i o n s in spin-glass insulators which are due presumably to the f r u s t r a t i o n of a n t i ferromagnetic alignment of ionic

We have measured the t r a n s i t times of 30 MHz longitudinal and shear u l t r a - sonic waves propagating normal t o the (110) cleavage plane in Cdl-xMnxTe samples with

x

= 0.05, 0.20, 0.35, 0.55, 0.57, and 0.65. Laue x-ray photographs, and visual microscopic examination a f t e r etching a t room temperature, reveal many small twins with (111) bounding planes in samples with x = 0.20 and 0.35 and many small t r i l l s o r hexagonalings with cubic (111) bounding planes ( o r hexagonal (0001) bounding planes because we have seen an apparent hexagonal s t r u c t u r e

with

c/a r a t i o of

46)

i n

samples with

x

= 0.55 and 0.65. However,we believe t h a t these s t r u c t u r a l imperfec- tions a r e not the cause of the e l a s t i c moduli anomalies reported herein since e l a s t i c anomalies were also found in the

x

= 0.05 and 0.57 single crystal samples and in the

x

= 0.20 sample whose f a s t e r shear wave modulus has a value very close t o those of the f a s t shear wave modulus in the single crystal samples.

TEMPERRTURE

(Kl

TEMPERATURE

(

K

1

Fig. 1. Cll/CI1(95.5 K) vs T f o r s i n g l e Fig. 2. CSs/CSS(95.5 K) vs T f o r t h e c r y s t a l samples. Values f o r C11(95.5 K) slower shear waves. Values f o r

are 5.56, and 5.15 x 1011 dyn/cm2 f o r Css(95.5 K) are 9.03, 8.29, 9.40, 11.03, x = 0.05 and 0.57, r e s p e c t i v e l y . 7.94, and 10.71 x 1010 dyn/cm2 f o r

x = 0.05, 0.20, 0.35, 0.55, 0.57, and 0.65, r e s p e c t i v e l y .

The e l a s t i c s t i f f n e s s moduli o f l o n g i t u d i n a l and two shear waves were deduced from t r a n s i t t i m e data u s i n g t h e thermal e x p a n s i v i t y o f CdTe t o c o r r e c t f o r sample d e n s i t y and l e n g t h changes. For s i n g l e c r y s t a l s ,

Clang

= (Cll + C12 + 2 c 4 ~ ) / 2 , C f s = C 4 ~ and Css = (Cll

-

C12)/2, where f s means f a s t shear and ss slow shear.

From F i g . 1 i t can be seen t h a t t h e normalized Cll l o n g i t u d i n a l wave modulus obtained f o r s i n g l e c r y s t a l samples has a depression a t lowest temperatures f o r x = 0.05 and between 20 K and 50 K f o r x = 0.57, i n d i c a t i n g t h e presence o f some v o l u m e t r i c magneto-elastic coupling.

C6-648 JOURNAL DE PHYSIQUE

For the x = 0.55 sample we a l s o measured the s h e a r wave moduli C f s and Css a s a function of temperature i n t h e presence o f 780 G . A minimum was observed sim-

i l a r t o t h a t which occurred i n zero applied f i e l d . This implies t h a t i t i s not

3

r e l a t e d t o t h e "zero" field-cooled a c s u s c e p t i b i l i t y ,

xac,

but r a t h e r t o t h e x - l

anomaly observed a t 8 . 5 k ~ . ~

The gradual change i n e l a s t i c anomalies with i n c r e a s i n g Mn concentration seems t o i n d i c a t e t h a t t h e t r a n s i t i o n from s p i n g l a s s t o antiferromagnetic phase i s a smooth function of x. On t h e o t h e r hand, s i n c e we f i n d no i n d i c a t i o n of a sharp, inverted 1-type e l a s t i c anomaly even i n our

x

= 0.65 sample, which presumably has an antiferromagnetic t r a n s i t i o n , the broad minimum we observe may i n d i c a t e t h a t our samples contain both spin-glass and antiferromagnetic regions i n s t e a d o f c o n s i s t i n g of a s i n g l e homogeneous phase.

Acknowledgment.- Support f o r this work was provided by t h e National Science Foundation v i a Grant No. DMR79-08538A1. Thanks a r e due t o Professors J . K. Furdyna and W. M. Becker f o r making a v a i l a b l e t h e samples used i n t h i s i n v e s t i g a t i o n .

References

1. B. L i t h i , T. J . Moran, and R. J . P o l l i n a , J . Phys. Chem. S o l i d s ,

3,

1741 (1970). 2. See f o r example, G . F. Hawkins, R. L . Thomas, and A. M. de Graaf, J . Appl. Phys.

50(3), 1709 (1979), and r e f e r e n c e s c i t e d t h e r e i n .

-

3. R. R. Galazka, S. Nagata, and P . H. Keesom, Phys. Rev.

Bz,

3344 (1980).

4. L de Seze, J . Phys. C: S o l i d S t a t e Phys.

10,

L353 (1977). 5. G. S. Grest and E . G. Gabl, Phys. Rev. L e t t .

43,

1182 (1979).

6. S. B. Oseroff, R. Calvo, W . G i r i a t , and Z. Fisk, S o l i d S t a t e Commun.

35,

539 ( 1980).