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Submitted on 1 Jan 1985
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INTERSUBLATTICE RELAXATION OF
HIGH-FREQUENCY MAGNONS
G. Jongerden, K. Bouwers, J. Dijkhuis, A. Arts, H. de Wijn
To cite this version:
JOURNAL
DEPHYSIQUE
Colloque C7, supplement au nOIO, Tome 46, octobre 1985 page C7-263
INTERSUBLATTICE RELAXATION O F HIGH-FREQUENCY MAGNONS
G . J . Jongerden, K. Bouwers, J.I. D i j k h u i s , A.F.M. A r t s and H.W. de Wijn
Fysisch Laboratoriwn, R i j k s u n i v e r s i t e i t Utrecht, P.O. BOX 80.000, 3508 TA Utrech, The NetherZands
Abstract
-
Intersublattice relaxation of o p t i c a l l y generated high-frequency magnons is measured a t 1.5 K i n fiF2. 'lhe spectral width of these magnons is determined from the magnetic-field dependence of the magnon relaxation time.In t h i s paper we report on the i n t e r s u b l a t t i c e relaxation of high-frequency magnons i n antiferromagnetic MF2 a t 1.5 K i n external magnetic f i e l d s up to 4 T. The experimental technique i s sublattice-selective pulsed o p t i c a l excitation v i a appropiately polarized t r a n s i t i o n s of the 6 ~ 1 - 4 ~ l g exciton-gnon absorption band. The f a s t relaxation of the magnons compared to tEe excitons r e s u l t s i n a macroscopic magnetization, h s e time derivative i s detected with a pickup coil. A similar scheme, but i n zero f i e l d , was used by l b l z r i c h t e r e t a l . i n a study of the i n t e r s u b l a t t i c e relaxation of excitons i n mF2 /I/.
Here, the r i s e of the induction signal turned out to be limited by the shape of the exciting l a s e r p l l s e , pointing to a v i r t u a l l y instantaneous magnon relaxation. The present experiments, conducted i n a magnetic f i e l d along the c axis, show t h a t magnon relaxation becomes r a t e limiting, and thus accessible to experimental determination.
The experiments were carried out a t 1.5 K i n a MF2 c r y s t a l of dimensions 10.0 x 1.9 x 2.0 mm3, with the c axis along the long sides. The o p t i c a l excitation of the o l exciton-gnon sideband was performed with an excimer-pumped dye l a s e r having a pulse length of 25 ns, a spectral width of 0.3 cm-l, and a peakpower of -100 kW. 'Ihe l a s e r beam, focused to about 0.3 mm diameter, propagated along the c axis, and was polarized along the [I101 o r [IT01 axes. A sixty-turn pickup c o i l with a response time of 90 ns was wrapped around the c r y s t a l such a s to be sensitive to longitudinal magnetization changes. The signal voltage induced over the c o i l was fed v i a a lowimpedance cable to an amplifier with a response time of -3 ns, and displayed on an oscilloscope.
In Fig. 1 single-sbt traces of the induction signals a r e presented for zero f i e l d , 2 T, and 4 T. The maxiuauu signal voltage from the c o i l was -0.5 mV, which corresponds to an induced magnetization of -0.1 G, or a concentration of -7 x 10'' excited h2'. In zero f i e l d the f a s t i n i t i a l rise of the signal v i r t u a l l y r e f l e c t s the s h a r of the l a s e r p l s e . This implies a lower l i m i t of the magnon relaxation r a t e of 2 x 10' s-
.
The subsequent decay essentially follows the instrumental profile. Upon increasing the magnetic f i e l d , the r i s e of the signal slows down significantly, thus serving a s a d i r e c t measure of the magnon relaxation time. The signal i n f a c t cpnsists of t m components, viz., the resonant signal with the risetime slowed down by the raagnon relaxation, Eut of course decaying with the instrumental time constant, and a signal, also present i n the case of nonresonant o p t i c a l excitation, exhibiting an o s c i l l a t i n g behavior with a period of -lo-' s. The mnresonant component increases l i n e a r l y with the applied magnetic f i e l d , and i s hardly significant below-1 T. The nonresonant signal i s presumably caused by mechanical sample vibrations. After subtraction of the nonresonantC7-264 JOURNAL DE PHYSIQUE
contribution a sum of tw, e q n e n t i a l s was f i t t e d t o the data. The magnon relaxation r a t e , corresponding to the shorter time constant, is shown versus the magnetic f i e l d i n Fig. 2. A sharp drop of the r a t e i s seen to occur within 0.5 T, followed by a long t a i l extending to a t l e a s t 4 T. In f i e l d s perpendicular t o the c a x i s , no e f f e c t s were observed.
Fig. 1
- Magnetic induction signals in &F
2 a t 1.5 K following p l s e d e x c i t a t i o n of the a1excitonlnagnon absorption band for zero f i e l d , 2 T, and 4 T.
MAGNETIC F I E L D
(T
1
Fig. 2 - Inverse r i s e time of the induction signals versus a magnetic f i e l d along the c axis. Signals were corrected for the nonresonant background (see t r a c e a t 4 T i n Fig. 1).
i n a short time. In a p a r a l l e l f i e l d , however, a s soon a s the s p l i t t i n g of the magnon branches exceeds the s p e c t r a l width of the magnon packets, i n t e r s u b l a t t i c e relaxation i s f r u s t r a t e d since the momentum conservation cannot be f u l f i l l e d . We therefore interpret the width of the magnetic-field dependence of the relaxation r a t e i n Fig. 2 a s a measure for the width of the magnon packet, t o find a halfwidth of -0.4 cm-l a t half maximum. This is close t o the homgeneous linewidth of E2 /2/. Decay of the magnon packets a t low temperatures by magnon-phgnon relaxation is evidently very slow, consistent with the theorem of Lax e t a l .
/ 4 / a s well a s the S character of the MI*+ ground s t a t e .
Acknowledgments
The authors thank C.R. de Kok and J.J. van der Linden for technical assistance. Financial support of the Dutch Foundation Janivo is g r a t e f u l l y acknowledged.
REFERENCES
/ I / Holzrichter, J.F., Macfarlane, R.M. and Schawlow, A.L., Phys. Rev. Lett.
