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Submitted on 1 Jan 1988
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PARAMETRIC EXCITATION OF SPIN-WAVES
THROUGH SPIN-FLOP CRITICAL FIELD
RESONANCE IN (C2H5NH3)2 CuCl4
H. Yamazaki, J. Shi, M. Mino
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplhment au no 12, Tome 49, d6cembre 1988
PARAMETRIC EXCITATION OF SPIN-WAVES THROUGH SPIN-FLOP CRITICAL
FIELD RESONANCE IN (C2H5NH3)2 CuC14
H. Yamazaki, J. Shi and M. Mino
Department of Physics, Faculty of Science, Okayama University, Tsushima, Okayarna 700, Japan
Abstract. - Parametric excitation of spin-waves in antiferromagnetic (C2H5NH3)2 CuC14 has been studied by the perpendicular-pumping technique. Nonlinear absorption which is attributed to the first-order Suhl instability is observed at twice the frequency of the spin-flop critical field resonance.
1. Introduction
1
A variety of studies of parametric excitation of spin- waves have been performed so far. Antiferromagnetic spin-waves have been studied by this method in the an- tiferromagnetic, spin-flop and paramagnetic phases by many researchers. In this paper the first observation of parametric excitation of antiferromagnetic spin-waves through spin-flop critical field resonance is reported.
When the external static field is applied along the spin-easy axis of antiferromagnets and its strength crosses just over the spin-flop critical field from the lower side, the spin direction turns suddenly from the easy axis t o the second-easy axis. At each orienta- tion the spin system has the corresponding oscillai tion frequency which has been theoretically studied by Nagamiya [I]. We have developed the theory of the critical field resonance with including the magnetic dipolar interaction, i.e. the surface demagnetizing field, the Lorentz field and magnetic dipolar anisotropy field. As previously studied [2], the antiferromagnetic resonance frequency in (C2HrjNHa), CuC14 is consid- erably affected by the dipolar interaction because the antiferromagnetic exchange interaction is comparative in order of magnitude with the dipolar one.
Katsumata and Date [3] found a resonance absorp- tion at half the frequency of the critical field resonance
in CoC126H20 instead of twice the frequency as re- ported here. They described it by nonlinear excitation arising from the dynamical magnetoelastic coupling between spins and lattice. The origin of the nonlinear absorption in this report is attributed to the first-order Suhl spin-wave instability.
2. Critical-field resonance
Antiferromagnetic resonance (AFMR) measure- ments at the spin-flop critical field were made a t a temperature of 1.4 K by using the coaxial cable appa- ratus with sweep oscillator 8620C (HP). The crystal used in this experiment is a disk of 6 mm in diam- eter and 0.3 mm thick. Figure 1 shows the angular dependence of AFMR fields in the abplane, where the
+4.872 GHz 05.200 GHz
05.006 GHz
540 4.058 GHz T=I.LK
Fig. 1. - Angular dependence of the antiferromagnetic resonance in the abplane. The critical-field resonance is observed when the static field of 320 Oe (modified field = 350 Oe) is applied along the *axis.
modified magnetic field is introduced by the relation H = (g/2) Hexperi,ent. The g-value is 2.165 which is isotropic in the abplane. The a-axis is the spin-easy axis and the baxis is the second-easy axis. Spin-flop critical field resonance is observed at the modified field of about 350 Oe applied along the a-axis and in the frequency range between 4.87 and 5.20 GHz. When the resonance frequency is higher than 5.07 GHz the AFMR field increases with tilting the direction of the field from the a-axis, while the field decreases when the frequency is lower than 5.07 GHz. The critical field resonance is well described by the theory which includes the magnetic dipolar interactions.
C8 - 1608 JOURNAL DE PHYSIQUE
3. Spin-wave instability
s
High-power resonance experiments are carried out at the frequencies of 9.86 and 10.82 GHz which are twice of that of critical field resonance. The apparatus is a standard reflection-type setup. The necessary high mi- crowave power is obtained with a traveling-wave-tube amplifier (TWTA) which delivers 9.31 W in its maxi- mum a t the cavity site. To avoid sample-heating effect input microwave to the TWTA was switched by a PIN diode with a pulse of 20 ns rise time, 1 ms duration and 25 Hz repetition. The sample was mounted on the bottom of the TElol cavity whose loaded Q was about 4,500 at the liquid helium temperatures. The re- flected power from the cavity through a circulator was detected by a diode. An onset of spin-wave instability was identified by observing the power level at which the reflected pulse first showed a nonlinear shape.
Measurements are made by perpendicular-pumping configuration. When microwave power is increased above certain threshold, absorption appears at the f r e quency twice of criticd-field resonance and at the spin- flop critical field. Figure 2 shows the absorption line shape as a function of static magnetic field for various pumping power levels a t 9.867 GHz. There are no ab- sorption below 0 dB, which is the threshold. With in- creasing microwave power, nonlinear absorption grows up. Figure 3 shows the threshold microwave field as
9.867GHz T=lXK POWER 138 dB 0 dB 82mW I 1 I I 300 4 0 0
MAGNETIC FIELD (Oe)
Fig. 2.
-
Nonlinear absorption as a function of magnetic field.Fig. 3.
-
Threshold for spin-wave instability as a function of magnetic field.a function of static magnetic field at a pumping fre- quency of 9.864 GHz. The threshold microwave field is comparable magnitude with the previous measure- ments 141 of spin-wave instability at the antiferromag- netic phase in this crystal.
The nonlinear absorption in the frequency twice of the critical field resonance is attributed to the first- order Suhl instability [5]. Absorption is observable above the threshold and-it increases quickly with in- creasing the driving power. Above the threshold, the power transfer catastrophically from lc = 0 mode to lc
and -k magnon pairs. The generation of magnon pairs arises from nonlinear character of the three-magnon interaction. Theoretical expression of the threshold for spin-wave instability through spin-flop critical field resonance is desirable.
[l] Nagamiya, T., Prog. Theor. Phys. 11 (1954) 309. [2] Chikamatsu, M., Tanaka, M. and Yamsaki, H.,
J. Phys. Soc. Jpn 50 (1981)2876.
131 Katsumata, K. and Date, M., J. Phys. Soc. Jpn
24 (1968) 751.
[4] Yamazaki, H., Chikamatsu, M. and Tanaka, M.,
J. Phys. Soc. Jpn 56 (1987) 340.
