Optical study of the antiferromagnetic phase transition in (Cd,Mn)S at highest Mn concentration

Journal of Crystal Growth 101 (1990) 911—915 North-Holland

911

OPTICAL STUDY OF THE ANTIFERROMAGNETIC PHASE TRANSITION IN (Cd,Mn)S AT HIGHEST Mn CONCENTRATION W. HEIMBRODT Bereich Haibleiteroptik. Sektion Physik der Hu,nboldt-Universität zu Berlin, Invalidenstrasse 110, DDR-1 040 Berlin, German Dem. Rep.

C. BENECKE Instirut für Festkorperphysik der Technischen Universitilt Berlin, Hardenbergstrasse 36, D-1000 Berlin 12, Germany

0. GOEDE Bereich Haibleiteroptik, Sektion Physik der Humboldt-Universität zu Berlin, Invalidenstrasse 110, DDR-1 040 Berlin, German Dem. Rep.

and H.-E. GUMLICH Inst itut für Fesrkorperphysik der Technischen Uniuersitàt Berlin, Hardenbergstrasse 36, D-1000 Berlin 12. Germany

Luminescence emission and excitation measurements are used to study the antiferromagnetic phase transition in (Cd,Mn)S single crystals and thin films in the case of highest Mn concentrations 0.8 .XMn 1. The spin-freezing temperature TN(xMfl) is determined 2~EPR linewidth. The samples having rocksalt or zincblende/wurtzite structure show from the temperature dependence of the Mn luminescence band near 1.5 and 1.8 eV, respectively. Below TN(xM,~) a significant shift of the corresponding Mn2~d—d excitation bands to higher energies is found, which is ascribed to the different spin-ordering induced energy relaxation in ground and excited states of the Mn2 + ions. The superexchange interaction parameters for the various excited Mn2 + states are obtained in a mean-field approximation. The measured luminescence decay parameters indicate the energy transfer between the Mn2~ ions to be also influenced by the antiferromagnetic spin ordering.

1. Introduction The continuous interest in the (Cd,Mn) and (Zn,Mn) chalcogemdes is based on their outstanding magneto-optical properties caused by a strong s, p—d exchange interaction between electron/hole band states and Mn2 + 3d electron states (see, e.g., refs. [1,2] for a recent review). Furthermore, these materials are characterized by an antiferromagnetic correlation between the Mn2 ± spins due to a d—p—d superexchange interaction, which leads to a transition from the paramagnetic into a spinordered phase at a Mn concentration dependent spin-freezing temperature [3,4]. In the present paper, the effects of the spinordering on the internal optical Mn2~d—d transitions were studied in the case of (Cd,Mn)S at the 0022-0248/90/$03.50 © 1990

—

highest Mn concentrations XMn 0.8. The optical investigations were carried out on bulk crystals with rocksalt (RS) and on MnS thin films with both rocksalt and zincblende/ wurtzite (ZB/W) structure. The results obtained concerning the spin-ordering induced energy relaxation of the various Mn2 + 3d5 states in both octahedral and tetrahedral coordination and the change of the energy transfer rates between Mn2 + ions are relevant also to the other (Cd,Mn) and (Zn,Mn) chalcogenides.

2. Experimental The investigated MnS and (Cd,Mn)S (XMfl 0.8) bulk crystals with RS structure were grown by

Elsevier Science Publishers B.V. (North-Holland)

912

W. Heimbrodt et al.

/ Optical study of antiferromagneric phase transition in

transport technique at about 950°C. The polycrystalline MnS thin films with thicknesses between 1 and 2 jsm were prepared on glass or quartz substrates by vacuum evaporation. Controlled by the substrate temperature 7~,MnS thin films with ZB/W (1~ 50—100°C) and RS (7~ 300°C) structure were obtained. The crystallo12

f

01

I -____________________

a2

0.1

TN(xMfl) =x~~TN(1).

-

0

The spin-freezing (Néel) temperatures TN(xMfl) of MnS and (Cd,Mn)S samples with various Mn concentrations xMfl were determined by measureEPR linewidth (see also ref. [5]). As shown in fig. ments of the temperature dependence of the Mn2 ± 1, the peak-to-peak width z~Bstrongly increases if the critical temperature TN(xMfl) for the transition from the paramagnetic into the (disordered) antiferromagnetic phase is approached from above. Simultaneously, a steep decrease of the peak-topeak height zlI is found. The resulting spin-freezing temperature decreases significantly with decreasing XMn (see fig. 2). As suggested in ref. [5J, at least at sufficiently high Mn concentrations, the results can well be approximated by (1)

4

.__

0.2

3. Results and discussion 2 ± d—d

I

I

I

graphicdiffractometry. X-ray structure of the Thesamples Mn concentrations was proved by of fluorescence measurements using an electron-beam the (Cd,Mn)S samples were determined by X-ray microprobe equipment. For the EPR measure ments, a conventional 9 GHz homodyne spectrometer was applied. The optical studies were performed using a grating monochromator, photon-counting or lock-in technique, and halogen lamp or excimer-laser pumped dye laser for excitation.

3.1. Spin-ordering induced shift of the Mn excitation bands

(Cd,Mn)S

10

300

I (K)

Fig. (S), 1. Temperature dependence width, ~B and peak-to-peak height, of ~I the (o),peak-to-peak of the Mn2~ EPR signal for (Cd,Mn)SRS with Mn concentrations XM,, = 1(a) and 0.8 (b).

4

I

150

I

I

-

/ / /

50

/ /

/ / /

//

/

/ /

/4 __I_,,.f

0

0.2

I

0.4

I

0.6

0.8

1.0

xpln

The successive spin-ordering below these spinfreezing temperatures now leads to an energy relaxation of the various Mn2 ± 3d5 states which is expected to be maximum at xMfl = 1. Therefore, luminescence excitation measurements of oc-

Fig. 2. Spin-freezing (Néel) temperature of (Cd,Mn)S with RS (•) and[5] ZB/W (•)wurtzite-type structure, respectively, EPR measurements and of (Cd,Mn)S from (0) and (Zn,Mn)S (X) from susceptibility measurements [6,7] for various Mn concentrations. Curves calculated after eq. (1).

W. Heimbrodt et a!.

/ Optical study of antiferromagneticphase transition

‘4T ‘D’ ~‘ ‘

\

-

-

-

216

2.91

-

I

-

proximation by

.

2.75

~g,ex

( T,

XMfl)

4T2

2.~ .-

9(G)

=

_(S~)(J

(~)).

~ nfl

2

2.4.4

-

~G, N

~

‘~

-

2.52

-

.

-

\.

~.. ~

2.50

~

=

sponding to fcc-types II and III in the case of RS

-

0

nnn

—

2.06~4Tig (G)~

204

(2)

Here jg.ex and J~ denote the exchange interac2 ± ion 1 in tion parameters between the central Mn a ground or excited state and another ground state Mn2~ionj in nn and nnn position, respectively. The difference ~ yields the spin-ordering induced shifts of the various excitation peaks as a function of are the derived temperature concentration which in refand[8].Mn Assuming an antiferromagnetic spin-ordenng at T 0 K, corre-

-2.54

-

2.42

913

The d—p—d superexchange-interaction induced energy relaxation (T, xMfl) of a considered 2~ ion in the Egex ground or excited state can Mn sufficiently be described on the basis of the isotropic Heisenberg Hamiltonian in mean-field ap-

2.93

-

in (Cd,Mn)S

I

I

I

I

I

i

temperature/K

and ZB/W structure, respectively, one obtains for the total peak shift between TN (x Mn) and T 0 K:

200

=

Fig. 3. Peak positions of the Mn2 + d—d excitation bands as a function of the temperature for MnS thin films with RS ~ and ZB/W (o) structure. Ecm=1.45 eV (MnSR 5) and 1.80 eV

RS

(MnSZB/W).

Z~EzB/w=

=

—6x

Mn\ —

=

=

—,

~2

_Jex

finn

finn

4xMfl(J~S2—

55~’l

(3a

/‘

J~ss’),

(3b)

where S ~ and 5’ are the quantum 2 ± ground and spin excited (quarnumbers the Mn tet) states,for respectively. In eq. (3b), the additional assumption I JI~I~ I I I is made, which is fulfilled for the ground state and can be expected to be true also for the excited states in the case of =

tahedrallytheand tetrahedrally coordinated yielding temperature dependence of theMnS, d—d transition energies, can well be applied to study this effect. The observed luminescence bands are situated in the infrared (1.45—1.65 eV) and red (1.80—1.95 eV) region for MnSR 5 and MnSZB/W, respectively, and are excited via energy transfer by 6Aiig ground the usual transitions fromstates the of the Mn2 ± state into d—d the various excited ions. As shown in fig. 3, both for MnSRS and MnSZB/w an almost step-like shift of these excitation peaks to higher energies is found just below the indicated spin-freezing temperature TN 152 K (MnSRS) and 90 K (MnSZB/W), which clearly demonstrates the correlation with the antiferromagnetic spin-ordering in these materials. The maximum total shift between TN and T 0 K is observed for the 6A 4T 4G) transition in 15 2g( MnSRS.

(jg

=

ZB/W structure. Using K eqs.(MnSRS) (3) and the known values J~~/k 12.5 and J~/k 12.4 K (MnSZB/W) (see ref. [2] for references), the exchange interaction parameters for the excited Mn2 + states can be obtained by fitting the experimental zlE values after fig. 3 (see table 1). As demonstrated in fig. 4, also the measured temperature dependence of the excitation peak shifts in MnSRS can well be represented by the proposed model using the j~~(values given in table 1. The dashed curves in fig. 4 are calculated for (Cd,Mn)S with xMfl 0.8 and are in sufficient agreement with the measured ~ E for this material. —

=

=

—

=

W. Heimbrodt et a!. / Optical study of antiferromagnetic phase transition in (Cd,Mn)S

914

Table 1 2~3d5 states in Exchange interaction parameters of excited Mn octahedrally and tetrahedrally coordinated MnS

corresponding peak shift due to a relaxation of the excitation ion positions.

MnSRS

3.2. Spin-ordering effects on luminescence decay

MnSZB/w

Excited state

J,~~/k (K)

Excited state

J,?,~/k(K)

4Tig (4G)

—11.0 —0.7 —13.3

4T 4G) 4T2 (4D) 2(

+6.4 —0.5

4T

4G)

4Aig 25 (4G) (

times In all investigated samples the observed emission bands are excited via radiationless energy

transfer between the Mn2~ions and do not correspond to the 4Tsig 6Ai,sg transition in unperFor MnSRS the obtained ~ values of the 4Tig and 4Aig Mn2~states are nearly the same as for the 6A 1g ground state. The observed spin-ordering induced shifts of the corresponding bands, therefore, mainly result from the difference between ~ 4T2g( 4G) and S’. In the case of the excited states in MnSRS and 4T in MnSZB/W, the exchange interaction nearly2(~D) vanishes, and L~E approximately represents the whole spin-ordering induced energy relaxation of the ground state. The positive J~ value for the 4T 2(~G)state in MnSZB/W mdicates a ferromagnetic spin-coupling, i.e.; the energetically favoured state is characterized by2 +a parallel orientation between the excited ion and spin its nn neighbours, but the spin flipMn into this state occurs slowly compared to the excitation transition, It should be mentioned that also for the Mn2~ 4T 6A 1 1 luminescence band a spin-ordering related shift of the same order of magnitude is found in the case of (Cd,Mn)Te and (Zn,Mn)Te at XMn = 0.7 [9], which, however, may differ from the —~

e4C

I

30~

I

I

I

I

and the decay properties of these luminescence bands are essentially determined by the capture of the mobile excitation energy by centres of energy radiationless transitions and, therefore, by the 2~ions [2]. Especially transfer ratetransfer between by the the Mnexchange interaction the energy mechanism, dominating at the present highest Mn concentrations, should be influenced by spinordering processes in the Mn sublattice. The luminescence decay was measured over several decades and found to be not dependence exactly cxponential. The obtained temperature of the decay parameters r~, defined as decay times to one tenth of the initial intensity, is shown in fig. 5 for (Cd,Mn)SRS with different Mn concentrations. In all samples a significant increase of is found below TN(xMfl), which indicates a correlation to the antiferromagnetic spin-ordering. Obviously, the energy transfer rate between Teif

I

~

‘

MnS

S

Cd02 Mn05S

________

~

turbed Mn2~ions (see ref. [2] for further discussion and references). Both the quantum efficiency

~½~~-~

100

150

1(K) 2~ Fig. 4. Spin-ordering excitation peaks in MnSft induced shift, ~E, of the various Mn 5 as a function of the temperature below TN = 152 K (after fig. 3). Curves calculated after ref. [8] for XMn =1 ( ) and XMn = 0.8 (— — —).

50

700

150 T(K)

200

Fig. 5. Temperature dependence of the effective luminescence decay time, ;~, for (Cd,Mn)SRS bulk crystals with XMn = and 0.8. Eem = 1.6 eV.

W. Heimbrodt eta!.

/ Optical study

ofantiferromagnetic phase transition in (Cd,Mn)S

neighbouring Mn2 ± ions is smaller in the spinordered than in the paramagnetic phase. Usually, the temperature dependence of the

energy transfer is ascribed to a variation of the overlap integral between the 6A 1 ig ~Ti,Ig emission and absorption band. The spin-ordering in6A~ ground and 4T 2± states lead to an increase 115 of duced relaxation of could the excited Mn the Franck—Condon shift in the spin-ordered phase and, by this way, to the observed decrease of the energy transfer rates. Also the established difference between the d—p mixing in the paramagnetic and antiferromagnetic phase may contribute to the measured variation of the energy transfer as suggested in ref. [8]. A quantitative discussion of the spin-ordering effects on the energy transfer rates in (Cd,Mn)S is difficult, however, because the electron—phonon interaction induced part of the temperature dependence of ;~ can hardly be separated. *-‘

915

References [1] J.K. Furdyna, J. AppI. Phys. 64 (1988) R29. [2] 0. Goede and W. Heimbrodt, Phys. Status Solidi (b) 146 (1988) 11.

[31K.C. 8.

Hass and

H.

Ehrenreich, J. Crystal Growth

86 (1988)

[4] B.E. Larson, K.C. Hass, H. Ehrenreich and A.E. Carlsson,

Solid State Commun. 56 (1985) 347. [5] 0. Goede, D. Backs, W. Heimbrodt and M. Kanis, Phys. Status Solidi (b) 151 (1989) 311.

[6] A. Twardowski, C.J.M. Denissen, W.J.M. de Jonge, A.T.A.M. de Waele, M. Demianiuk and R. Triboulet, Solid State Commun. 59 (1986) 199. [7] M. Swagten, A. Twardowski, W.J.M. de Jonge, M. Dcmianiuk and J.K. Furdyna, Solid State Commun. 66 (1988) 791. [8] W. Heimbrodt, C. Benecke, 0. Goede and H.-E. Gumlich, Phys. Status Sohdi (b) 154 (1989) 405. [9] E. Muller and W. Gebhardt, Phys. Status Solidi (b) 137 (1986) 259.