OPTICAL GENERATION OF MAGNONS BY DIRECT SPIN-MAGNON RELAXATION IN MnF2 : Er3+

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OPTICAL GENERATION OF MAGNONS BY

DIRECT SPIN-MAGNON RELAXATION IN MnF2 :

Er3+

G. Jongerden, A. Kil, J. Dijkhuis, A. Arts, H. de Wijn

To cite this version:

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OPTICAL GENERATION OF MAGNONS BY DIRECT SPIN-MAGNON RELAXATION IN

M n F

2

: E r

3 +

G.J. Jongerden, A.J. K i l , J . I . Dijkhuis, A.F.M. Arts and H.W. de Wijn

Fysiseh Laboratorium, Rijksuniversiteit Utveaht, P.O. Box 80.000,

3508 TA Utrecht, The Netherlands

Résume - La génération et la détection optiques des magnons monochromatiques sont démontrées en MrôVîEr à 1.5 K en utilisant la transition directe des niveaux Zeeman les plus bas de la nultiplette vq/2' u"e calculation perturbationelle est en bon rapport avec les expérimentations. Des indications pour la réabsorption des magnons sont obtenues.

Abstract - Ihe optical generation and detection of monochromatic magnons is demonstrated in MnF2:Er^1" at 1.5 K employing the direct transition between the lowest Zeeman levels of the F9/2 multiplet. A perturbation calculation yields good agreement with the experimental results. Indications of magnon reabsorption are obtained.

In this paper we report the optical generation and detection of monochromatic magnons employing optical impurity centers in a magnetic system. The magnons are resonantly generated at the excited impurity center in a direct spin-magnon 2eeman transition. The scheme potentially allows for a study of non equilibrium magnons throughout the entire Brillouin zone. We have chosen MiF2 doped with a small amount of Er ions as an example. M1F2 has the rutile crystal structure and orders antiferromagnetically along the c axis below 67 K. The interactions are mainly Heisenberg exchange with additional dipolar contributions, leading to a gap of 8.7 cm in the spin-wave spectrum.

The crystal was grown by the Czochralski pulling technique, cut perpendicular to the [001], [110] and the [110]-axes to dimensions of 12.0 x 2.4 x 2.2 mm3, and polished to 0.1 \m. The sample contains about ~ 0.01 % Er . It was immersed in pumped liquid Helium at 1.5 K. External magnetic fields up to 6 T were available. The optical excitation was performed with a 3-Watt argon laser operating at 514 nm, or selectively with an excimer-pumped dye laser with a peak power of 50 kW, a linewidth of about 0.3 c m , and a pulse length of 25 ns. The luminescence was analysed with a 0.85 m double monochromator equipped with standard photon-counting apparatus in combination with a transient recorder with a time resolution of 20 ns per channel.

The energy level scheme of Er3* in MiF2 is schematically drawn in Fig. 1. The luminescence pertinent to our experiments is from the lowest Foy2 Kramers doublet to the lowest doublet of the 1^5/2 ground-state multiplet, as shown in Fig. 2. The four intense lines observed point to a lifting by the internal magnetic fields of the Kramers degeneracy, as suggested earlier /l/.

In an external magnetic field along the c axis, the E r3 + ions on the two sublattices become nonequivalent. This is shown in Fig. 3, where the excited-state splittings of the lowest

F9/2 d°ublets of the two different Er are plotted versus the magnetic field. The observation that the lower branch splitting has a minimum value of 1.1 c m- 1, points to a perpendicular component of the internal field. This is corroborated by experiments in external fields perpendicular to the c axis.

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JOURNAL DE PHYSIQUE

Fig.

1

-

Energy level scheme of ~

r

in

~

+

MnF2 Splittings of the 4 ~ 1 5 1 2

ground state

doublet and the lowest 4 ~ g 1 2

doublet are

indicated.

spin-or

bi t

coupling

crystal field

exchange

ENERGY

(cm-'1

Fig. 2

-

Luminescence spectrum at 1.5 K

>-

L

of the lowest 4 ~ g 1 2

Kramers doublet to

V) Z

4~,512

Kramers doublet. The

the lowest

LLI

C

weak line is absent in lower-doped crys-

Z C-(

tals. Optical excitation was performed

with an argon laser operating at 514

nm.

Optical generation of magnons

in

the direct transition between this doublet will only occur

-

.

-

_..

_.,

-

.

..

-

_..

. :

. .

-

I.

. .

.

when

the Zeeman splitting exceeds the magnon energy gap.

In Fig.

3

the gap energy,

Rw,,

is

given versus the magnetic field 121, and shown to intersect the Zeeman splittings of the

tw,

Er* at

3.6

and

4.7

T.

In Fig.

4 the spontaneous relaxation time for the transition between

these 2eeman levels for the

tm

Er* is shown versus the external magnetic field along the

c

axis. The excitation was selective into the upper Z e e m level, wtiile the detection was

accomplished

by

mnitoring the time evolution of the luminescent intensity of the lower level

to the ground state. The measured time constant was corrected for the radiative lifetime of

-

200

ps.

A sudden speed-up of the relaxation by a h s t

an

order of magnitude is observed at

3.6

and 4.7

T

for the t w different Er3, as expected from the intersection points in Fig. 3.

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Fig. 3

-

Doublet s p l i t - t i n g s of t h e lowest 4 ~ g 1 2 l e v e l s of t h e two magneti-

8 -

tally nonequivalent Er 3+ v e r s u s t h e e x t e r n a l magne- t i c f i e l d along t h e c a x i s . . - Also i n s e r t e d i s t h e spin-

7 -

wave energy gap v e r s u s t h e

-

f i e l d . Beyond t h e inter-- (3 s e c t i o n p o i n t s spin-magnon [L

w

4 -

r e l a x a t i o n i s expected t o

w

become o p e r a t i v e .

0

1

2

3

L,

5

6 MAGNETIC

FIELD ( T )

In the case of an external f i e l d p a r a l l e l t o the [I101 axis similar e f f e c t s a r e observed. We a t t r i b u t e these e f f e c t s to d i r e c t spin-magnon relaxation additional to normal spin-phonon relaxation. For Kramers doublets d i r e c t spin-phonon relaxation is forbidden, yet relaxation can take place due to the admixing of other levels by a magnetic f i e l d . The f u l l curve i n Fig. 4 represents a f i t to t h e data p i n t s i n the regime of spin-phonon relaxation only, according to /3/

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JOURNAL

DE

PHYSIQUE

1

I I I 1 1 1 1 1 1 I

-

_-

-

_-

-

- -

:

-

4 -

_-

-

_-

-

o

up sublattice

-

down

sublattice

-

0

J

1 I I I

l l l l l

I

0.5

1

5

10 MAGNEZIC FIELD ( T )

Fig. 4

-

Spontaneous d i r e c t r e l a x a t i o n time between t h e Zeeman l e v e l s of t h e lowest

4 ~ , , 2 d o u b l e t v e r s u s t h e magnetic f i e l d along t h e c a x i s . The arrows i n d i c a t e d correspond t o t h e i n t e r s e c t i o n p o i n t s acccrding t o Fig. 3 .

where z is the munber of neighbors %=5/2, N is the number of magnetic u n i t c e l l s , a and at a r e the ET* spin-deviation operators,

pg

and

&t

a r e magnon creation and annihilation operators, respectively,

%

i s a Bogoliubov coefficient and

y g

i s the geometrical sum

Y+

= ;I Z g exp

(iZ.8)

over neighbors displaced by

if.

Applying the Golden h l e , taking ut;2 '=

3.4 and

yft

= 1, w then find for the spin-magnon relaxation r a t e

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where g~ = 413. F& i s the magnetic quantum number of the lowest 4~912 doublet, taken 912. This yields zl

JI

= 0.24 em-'. The calculated spin-magnon relaxation r a t e i n t h i s case exceeds the spin-phonon relaxation r a t e i f w suppasses

wo

by 0.3 %, which is i n accord with the observations.

The present scheme of magnon generation and detection also has the p t e n t i a l of being u t i l i z e d i n studying the dynamics of nonequilibrium magnons. This is manifested by the slowing down of t h e mgnetic part of the relaxation by up to an order of magnitude t h a t has been achieved upon increasing the excited-state concentration. In t h i s respect the scheme i s very similar to the case of phonon bsttleneck./3/, indicative of reabsorption of the magnons by excited ErN spins to take place.

Acknowledgments

The authors thank C.R. de Kok for invaluable technical assistance. Financial support of the k t c h Foundation Janivo i s gratefully acknowledged.

REFERENCES

/1/ Wilson, B.A., Yen, W.M., Hegarty, J. and Imbusch, G.F., Phys. Rev.

B

E

(1979) 4238. /2/ Johnson, F.M. and Nethercot, A.H., Jr., F'hys. Rev.

114

(1959) 705.