AFMR AND SUBTHRESHOLD TWO-MAGNON ABSORPTION IN QUASI-ONE-DIMENSIONAL ANTIFERROMAGNET CsMnCl3, 2H2O

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AFMR AND SUBTHRESHOLD TWO-MAGNON

ABSORPTION IN QUASI-ONE-DIMENSIONAL

ANTIFERROMAGNET CsMnCl3, 2H2O

A. Anders, A. Zvyagin, Yu. Pereverzev, A. Petutin, A. Stepanov

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplbment au no

8,

Tome

39,

aoiit

1978,

page

C6-739

AFMR AND

SUBTHRESHOLD TWO-MAGNDN ABSORPTION

IN

QUASI-ONE-DIMENSIONAL ANTIFERROMAGNET CsMnCl 2H 0

3' 2

A.G. Anders, A.I. Zvyagin, Yu.V. Pereverzev, A.I. Petutin and A.A. Stepanov

Physico-Technical Institute of La, Temperatures, UkrSSR Academy of Sciences, 47, Lenin prospect, Kharkov, 310164. U. S. S. R.

Resume.- On a 6tudiL le spectre d'absorption en hyperfrequence sur CoMnC13 2H20 en couvrant de larges plages de frequences, temperatures et champs magnstiques. La dependance en temperature du spectre de resonance antiferromagnetique suit l'aimantation des sous-r6seaux jbsqu'ii 0.7 TN. En

plus de la resonance antiferromagnetique en champ parallsle, on a observe des bandes d'absorption

a

deux magnons, de type somme et difference, pour des orientations differentes du champ expdrieur. La valeur du couplage d'echange inter-chaTnes est determin6e.

Abstract.- Microwave absorption in CsMnC13.2H20 is studied in a wide range of frequencies, temperatures and magnetic fields. The temperature dependence of energy gaps in the AFMR spectrum is proportional to the sublattice magnetization p to 0.7 T

.

In addition to

AFMR,

in the parallel polarized microwave and external magnetic fields $//R the suuthreshold two-magnon absorption bands of sum and difference types are observed in the external field for different orientations. The temperature and frequency dependences of these bands are studied. Energy of the weak interchain exchange interaction is estimated.

3 1 2

Low dimensional magnets possess specific ma- to [M(T~ gnetic, thermal and other properties. The specific

behaviour of the resonance properties of low dimensional magnets has been studied in sufficient de- tails for paramagnetic temperatures /l/, while be- low the magnetic ordering point these effects are yet not clear.

We have studied the resonance properties of quasi-one-dimensional antiferromagnet CsMnC1,.2H20 at 10-120 GHz in the 1.5-5 K range and at external magnetic fields up to 30 kOe. Along with AFMR lines

the suthreshold two-magnon magnetic dipolar absorption bands have been observed. The frequency vs. field dependences for

AFMR

(curves 1-8) and subthreshold two-magnon absorption (curves 9-14) bands at

1.5 K are shown in figure l.

The

AFMR

lines in the magnetic field are quite well described by the conventi.ona1 phenomeno- logical theory for a biaxial antiferromagnet with the parameters

m

= 35.5 kOe and

m

=

17.2 kOe for the upper and lower gaps, respectively and the critical spin-flop field l&- = 17.2 kOe and

K

Fig. 1 : Frequency versus field dependences of O K' The low structure of CsMnC132H20

and subthreshold two-magnon absorption lines effected only the temperature dependence of AFMR in CsMnC13.2H20 at 1.5 K. see the text;

frequencies. In the range of AFMR lines (up to T

% 3.5 Km0.7 TN) the temperature behaviour of the The subthreshold twolnagnon absorption lines energy of the upper and lower gaps is proportional in 3d magnets in the microwave range are very weak to the magnetization variation with temperature /2/. In low dimensional magnets, which have very M(T),nhilein 3d magnets it is usually proportional flat branches in the spin wave spectrum at low fre-

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quencies, some peculiar features are observed in the magnon density of states, therefore the intensity of subthreshold twolnagnon absorption bands in such crystals becomes comparable with that of

FMR

or AFMR bands

.

The subthreshold two-magnon absorption lines have so far been studied on 2d ferromagnets /3/.

In low dimensional antiferromagnets we have observed these lines for the first time in quasi- one-dimensional CsMnClS.2H20 /4/. By now, the subthreshold two-magnon absorption lines have been studied in detailes in CsMnC1,.2H20 and the follo- wing results are available.

Unlike ferromagnets, these lines are observed not only at photon frequencies equal to two fre quencies of magnons excited on one branch of the

-+

spin-wave spectrum (bands 9, 10, 1 l for ;//a, H//b,

-+

and H//c, respectively), but also at frequencies equal to the difference of magnon frequencies from two different spectrum branches ( bands 12, 13, 14

-+

for H//a, z//b, ;//c, respectively).

The subthreshold two-rnagnon absorption lines are observed mainly in the parallel polarization. Since the two-magnon absorption is accomplished by

-f

magnons with k ranging from zero to maximum for flat branches, the suthreshold two-magnon absorption lines are of asymmetric shape and occupy a suffi- ciently wide range of fields. In figure I the field and frequency ranges, where the lines are observed, are shaded. Here the line shapes are also allustra- ted. Lines 9, 11, 12, 14 of the subthreshold two- magnon absorption, and lines 10, 13 to a weak field one.

The intensity and shapes of the subthreshold two-magnon absorption are dependent on frequency and temperature. Figure 2 shows the temperature va- raition of sum type line 9 for frequencies of 32GHz (figure 2a) and 50 GHz (figure 2b). Line 9 is the most intensive due to the comparatively largenumber of thermally axcited magnons from the non-activated spin-flop spectrum branch. As frequency grows, the line intensity at first falls and then increases again at frequencies of the order of AFMR frequency of branch /4/. As temperature rises, the line intensity growssharply so that at low frequencies (lower than 30 GHz) one can trace the line up to T % T

N.

At high frequency (50 GHz) and rising temperature the line shape is appreciably affected by the de- creasing frequency of the upper branch. In addition

to the factors mentioned, the line intensity and shape are effected by damping due to magnon interaction increasing with the temperature rise.

Fig. 2 : Variation of intensity and shape of sum type line 9 of subthreshold twolnagnon absorption with temperature for different frequencies :

(a) 32 GHz, (b) 5 0 GHz. Line 10

-

weak absorption in low fields. In (a) the intensity is 5 times sca- led up as compared to (b).

The subthreshold two-magnon absorption lines originating from activation branches of the spectrum are more strong-dependent on temperature. Even at 2-2.5 K the difference type lines broaden noticeably.

The theoretical analysis of the shapes of sum type lines 9 and 1 1 of the subthreshold two- magnon absorption /5/ gives the minimum energy of

the intrachain exchange energy.

References

/I/ Hone, D.V. and Richards, P.M., Ann-Rev-Mater. Sci.

4

(1974) 337.

/ 2 / Kazanov, M.I. and Tsukernik. V.M., zh.Eksper. Teor.. Fiz. 37 (1959) 823.

_-

131 Yamazaki, H., J.Phys.Soc.Japan

37

(1974) 667 ;

42 (1977) 57.

L

/4/ Anders, A.G., Zvyagiri, A.I., Pereverzev, Yu.V., Petutin, A.I. and Stepanov, A.A., Fiz.Nizk.Temp

1 (1975) 1409.

-