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Submitted on 1 Jan 1988
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ELECTRON SPIN RESONANCE IN
CsMn1-xFexCl3.2H20
G. Gerritsma, B. Klopman
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplement au no 12, Tome 49, dkcembre 1988
ELECTRON SPIN RESONANCE IN C S M ~ ~ - ~ F ~ X C I ~ . ~ H ~ O
G. J. Gerritsma and B. B. G. Klopman
University of Twente, Faculty of Applied Physics, P.O. Box 217, 7500 AE Enschede, The Netherlands
Abstract. - Electron spin resonance linewidth studies have been made at 9.51 GHz in the temperature range 10 K
5
T ;5 100 K with x 5 0.02. The additional broadening due to the iron impurities are in part explained with a model of Gulley and Jaccarino. More detailed calculations are performed using the fluctuation dissipation theorem.
It is well known that both g-shift and linewidth of electron spin resonance spectra of antiferromagnets in the paramagnetic temperature regime may be used to study the spin dynamics of these systems. Here we report on the linewidth of the linear chain antiferro- magnet CsMnl-,Fe,Cls.2Hz0 (CMFC) with 0
5
x,<
0.02 in the temperature range 10
5
T5
100 K. This is well above the NCel temperature of these systems(TN
N 5 K).
Both pure compounds, i.e. withx
= 0(CMC) and x = 1 (CFC), are orthorhombic with space group Pcca. Therefore it is expected that the same holds for CMFC. The Fe-doping is a very fast relax- ing impurity in the linear Mn-C1 chains. The CMFC crystals were mounted in a rectangular cavity resonat-
ing in the TE102 -mode a t 9.51 GHz. Both the static and the rf component of the magnetic field are in the ab-plane of the crystal, which may be rotated around its c-axis.
In figure 1 we present the measured (symbols) ex- cess linewidth AB,,, of CMFC with x = 0.02 as a function of 8, the angle between the static magnetic field and the a-axis (the direction of the linear chains), at T = 100 K, where AB,,, is defined as the difference
between the linewidth of CMFC and that of CMC. In the case of cobalt impurities such an angle dependent
AB,,, is explained by Nagata et al. [I] on the basis of
an anisotropic line narrowing resulting from the inhi- bition of the spin diffusion in the chains, whereas the Co-Mn exchange broadening effect is isotropic.
Fig. 1. - Measured AB,,, at T = 100 K as a function of
angle between applied magnetic field and linear chain axis (symbols). The solid line is calculated.
In figure 2 the measured AB,,, is given as a func-
tion of temperature for 8 = 0 and 90 degrees. It is seen that for T
>
50 K, AB,,, is approximately in- dependent of temperature. In order t o explain AB,,,in a qualitative way we used a model of Gulley and Jaccarino for impure exchange-coupled paramagnets [2]. In this phenomenological model the exchange is treated in a molecular field approximation using a coupled-equation-of-motion approach. In the limit of strong host-impurity coupling the linewidth is given by
where X = 6sL
/
6 1 ~ , the ratio of the impurity-spin- lattice relaxation rate 6 s ~ and the impurity-t~host cross relaxation rate 6 1 ~ . The intrinsic linewidth of the host spins is denoted by 62, the susceptibility of the host and impurity spins by~ ; f
and X: respectively.- - T ( K 1
Fig. 2.
-
Measured AB,, at two different angles of the applied magnetic field with the linear chain axis (0 = 0 (e)and 90 ( 0 ) degrees) as a function of temperature.
C8
-
1480 JOURNAL D E PHYSIQUEFurthermore it is assumed in this derivation that 62 is not smaller than w,,,. Within this approxima-
tion two extremes may be discerned, i.e. X
>>
1 or X<<
1. In the first case of a very fast relaxing impu- rity AB,,, is proportional to 6 1 ~ and in the latter, so called phononbottleneck, case to 6 s ~ . Assuming that 6 1 ~ is independent of temperature and ASL is depen-dent of temperature it is clear that our result for T
>
50 K indicates that we are dealing with the first sit- uation. In order to explain the anisotropy in ABexcwe calculated the effect of impurities within the frame work of the fluctuation dissipation theorem [3]. Con- trary to [I] anisotropy results from the fact that the Ligand Hamiltonian of ~ e + + is anisotropic in its spin components. Our result for the line broadening by im- purities is given by
Awe,, = 2J;;z ( f ) 2
1
( S ( S + l))--l/) xI J I
where J' and J are the Mn-Fe and Mn-Mn exchange
energies respectively, K = 2 JS (S
+
1)/
kT, u =coth K
-
K-' and h is a function that depends on 0 and T. The function h is also dependent on the crys- tal field parameters D and E , being the trigonal and rhombic components respectively, and on 00, the angle between iron spins and chain axis. A typical example of h as a function of temperature is given in figure 3 with 00 = 40 degrees, D/
k = -21 K, E/
k = 1.4 K,J / k = -3.00 K, IJI'/ k = 0.97 K and x = 0.02. For D and E we took the values known for RbFeC13.2H20. With the same values for the parameters we caculated the solid line in figure 1. It is seen that an excellent agreement is obtained.
Fig. 3. - The function h vs. temperature for 8 = 0 (a) and
0 = 90 degrees (b), reflecting the influence of the crystal field on the excess linewidth.
Furthermore for T + m, Aw,, tends t o a con- stant value in accordance with our measurements. For
T + 0, Awe,, diverges as T - ~ / ~ , in qualitative agree- ment with out observations. However a quantitative agreement as a function of temperature is not obtained with the chosen parameters. This might be due to the fact that the parameters of Fe+<- in CMC are not known. This matter needs further investigation.
[l] Nagata, K., Nishino, T., Hirosawa, T., Komat- subara, T., J. Phys. Soc. Jpn 44 (1978) 813. [2] Gulley, J. E., Jaccarino, V., I'hys. Rev. B 6
(1972) 58.
