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Temperature dependence and angular variation of ESR line widths in two-dimensional manganese(II) bromide salts
Volume 44A, number 7
TEMPERATURE
PHYSICS LETTERS
DEPENDENCE
AND ANGULAR
IN TWO-DIMENSIONAL
13 August 1973
VARIATION
MANGANESE(
OF ESR LINE WIDTHS
II) BROMIDE
SALTS
R.D. WILLETT and M. EXTINE Chemical Physics Program, Washington State University, Pullman, Wash. 99163,
USA
Received 14 June 1973 Angular and temperature dependence of the ESR line widths of (RNH&MnBr, change coupling and the two-dimensional nature of the systems.
Recently, it has been shown that (RNH,),MnCl, salts crystallize in layer structures [l] which behave as two-dimensional magnetic systems [2-41. We have recently synthesized some of the corresponding bromide salts and have ascertained that they have analogous structures. In this communication we wish to report initial investigations of their ESR spectra. Seehra and Castner [5] have shown that the temperature dependence of the ESR line widths can be represented by AH(T) = AHotaT+
blT- TNI-’
(1)
where AH0 is the line width at T = 0°K. The linear term arises from the coupling of the spin system with the phonon system through the DzialoshinskyMoriya (D-M) antisymmetric exchange interaction [6] . The last term arises from critical fluctuations near the Ne’el temperature [7]. The exponent, y, is a critical point exponent [8]. Calculated values of this parameter depend on the dimensionality of the lattice, as well as on the specific model employed for representing the spin-spin interaction. It is observed to have a value greater than 2 in (CH,NH,),MnCl, [4]. For strongly exchange coupled systems, the angular variation in line widths is expected to show a (3~0~~0 - 1) dependence [9]. On the other hand, two-dimensional magnetic systems, in which spin diffusion controlled processes are the principle mechanism for relaxation, show a (3~0~~0 - 1)2 angular dependence [e.g. lo]. This has been verified by Boesch et al. [2]. The compounds were synthesized by dissolving stoichiometric amounts of MnBr2 and the appropriate amine hydrobromide in 95% alcohol solution and evaporating until precipitation occurred. It was
salts give evidence for ex-
very difficult to obtain single crystals. Powder diffraction patterns confirmed that the salts were isomorphous to the corresponding chlorides. Satisfactory samples have eben obtained at the present time for only the ethylammonium salt (EA2MnBr4) and the propylammonium salt (PA,MnBr,). ESR measurements were made on both a Varian E-3 and a Varian E-9 spectrometer equipped with a variable temperature device to control the temperature. The line shapes are nearly Lorentzian, and all values reported here are peak-to-peak magnetic field separation of the derivative curves. For single crystal studies, the angle reported is that between the normal to the layer and the applied field. The temperature dependence of the ESR line width for PA2MnBr4 is shown in fig. 1. Slight variations in line widths were observed for repeated runs, presum-
150 -
AH
PAeMnBr,
50
^ 0
I 200
I 100
I 300
T
Fig. 1. Temperature dependence of the ESR line width of a polycrystalline sample of (C3H2NH&MnBr4.
503
Volume
44A, number
7
PHYSICS
AH = 159+47z +llz'
I
so
I 30
60
I 0
LETTERS
13 August
1973
the angular dependence of the bromide is eiso ditterent from the chloride, which shows a ( 3cosZH I)? dependence. At 77°K. the bromide salt is represented quite well by simple a ( 3cos2B 1) dependence. with I). At -” ‘95°K ~ /w given by LW,, = 114 f lh(3co\O however. neither a ( 3cos20 I ) nor ( .icos28 1j2 dependence alone fits the observed data. but :I combined expression is required. LLH,,, =. 150 + 47( 3cos2U 1) + 1 I( 3cos20 1)?.This indicates that the exchange interaction provides the principle relaxation process at 77”K, while the temperature dependent spin diffusion mechanism provides additirjnal pathways for relaxation at room temperature The presence of the (3cos*t) l)* dependence term confirms the two-dimensional nature of the magnetic interactions,
e
Fig. 2. Angular variation of the ESR line width of a single crystal of (CZHsNHs)aMnBr4 at 77°K and 295°K. The quantity z is defined by z = 3~0s’~ 1.
‘This work was supported GP-6593 and GP-32458.
in part by NSF grants
References ably due to preferential orientation of the plates in sample tube. The data is quite well represented by eq. ( 1) with an extrapolated value for AH, of 70 gauss. Definite evidence for critical behavior below 77°K is indicated, as well as the presence of significant DM antisymmetric exchange. If a critical temperature near 40°K is assumed (as observed for the chloride salts [4] ). a value of y - 2.0 is obtained. This is in accord with predictions for two-dimensional systems [S] A value of gave = 7.015 was obtained. The angular dependence of the line widths for single crystals of EA,MnBr, at 77°K and 295°K are reproduced in fig. 2. For comparison, the line width for EA,MnCl, varies between 15 and 40 gauss at 295°K. Two factors are immediately evident. The line widths for the bromide salt are almost an order of magnitude larger than for the chloride salt. Secondly,
504
Ill
E.R. Peterson and R.D. Willett, J. C‘hem. Phys. 56 (1972) 1879. 121H.R. Boesch, U. Schmocker, 1,. Waldner, K. Lmercon and J.E. Drumheller, Phys. Lett. 36A (1971) 461. 131 W.D. van Amstel and L.J. de Jongh, Solid State Comm. 11 t 1972) 1423. I41 R.D. Willett, K. Chang, R. C‘aputo and B.C. C&stein, (to be submitted). 151 M.S. Seehra and T.G. Castner, Physik Kond. Materic. 7 (1968) 185. J. Phys. Chem. Solids 4 (1958) 24 1: (61 1. Dzialoshinsky, T. Moriya, Phys. Rev. 120 ( 1960) 961. L71 M.S. Seehra, J. Appl. Phys. 42 (1971) 1290. and critical phenomena 181 H.E. Stanley, Phase transitions (Oxford University Press, 1971). [91 P.W. Anderson and P.R. Weiss, Rev. Mod. Phys. 25 (1953) 269. IlO1 M.J. Hennessy, C.D. McElwee and P.M. Richards, Phys. Rev. 87 (1973) 930.
TEMPERATURE
PHYSICS LETTERS
DEPENDENCE
AND ANGULAR
IN TWO-DIMENSIONAL
13 August 1973
VARIATION
MANGANESE(
OF ESR LINE WIDTHS
II) BROMIDE
SALTS
R.D. WILLETT and M. EXTINE Chemical Physics Program, Washington State University, Pullman, Wash. 99163,
USA
Received 14 June 1973 Angular and temperature dependence of the ESR line widths of (RNH&MnBr, change coupling and the two-dimensional nature of the systems.
Recently, it has been shown that (RNH,),MnCl, salts crystallize in layer structures [l] which behave as two-dimensional magnetic systems [2-41. We have recently synthesized some of the corresponding bromide salts and have ascertained that they have analogous structures. In this communication we wish to report initial investigations of their ESR spectra. Seehra and Castner [5] have shown that the temperature dependence of the ESR line widths can be represented by AH(T) = AHotaT+
blT- TNI-’
(1)
where AH0 is the line width at T = 0°K. The linear term arises from the coupling of the spin system with the phonon system through the DzialoshinskyMoriya (D-M) antisymmetric exchange interaction [6] . The last term arises from critical fluctuations near the Ne’el temperature [7]. The exponent, y, is a critical point exponent [8]. Calculated values of this parameter depend on the dimensionality of the lattice, as well as on the specific model employed for representing the spin-spin interaction. It is observed to have a value greater than 2 in (CH,NH,),MnCl, [4]. For strongly exchange coupled systems, the angular variation in line widths is expected to show a (3~0~~0 - 1) dependence [9]. On the other hand, two-dimensional magnetic systems, in which spin diffusion controlled processes are the principle mechanism for relaxation, show a (3~0~~0 - 1)2 angular dependence [e.g. lo]. This has been verified by Boesch et al. [2]. The compounds were synthesized by dissolving stoichiometric amounts of MnBr2 and the appropriate amine hydrobromide in 95% alcohol solution and evaporating until precipitation occurred. It was
salts give evidence for ex-
very difficult to obtain single crystals. Powder diffraction patterns confirmed that the salts were isomorphous to the corresponding chlorides. Satisfactory samples have eben obtained at the present time for only the ethylammonium salt (EA2MnBr4) and the propylammonium salt (PA,MnBr,). ESR measurements were made on both a Varian E-3 and a Varian E-9 spectrometer equipped with a variable temperature device to control the temperature. The line shapes are nearly Lorentzian, and all values reported here are peak-to-peak magnetic field separation of the derivative curves. For single crystal studies, the angle reported is that between the normal to the layer and the applied field. The temperature dependence of the ESR line width for PA2MnBr4 is shown in fig. 1. Slight variations in line widths were observed for repeated runs, presum-
150 -
AH
PAeMnBr,
50
^ 0
I 200
I 100
I 300
T
Fig. 1. Temperature dependence of the ESR line width of a polycrystalline sample of (C3H2NH&MnBr4.
503
Volume
44A, number
7
PHYSICS
AH = 159+47z +llz'
I
so
I 30
60
I 0
LETTERS
13 August
1973
the angular dependence of the bromide is eiso ditterent from the chloride, which shows a ( 3cosZH I)? dependence. At 77°K. the bromide salt is represented quite well by simple a ( 3cos2B 1) dependence. with I). At -” ‘95°K ~ /w given by LW,, = 114 f lh(3co\O however. neither a ( 3cos20 I ) nor ( .icos28 1j2 dependence alone fits the observed data. but :I combined expression is required. LLH,,, =. 150 + 47( 3cos2U 1) + 1 I( 3cos20 1)?.This indicates that the exchange interaction provides the principle relaxation process at 77”K, while the temperature dependent spin diffusion mechanism provides additirjnal pathways for relaxation at room temperature The presence of the (3cos*t) l)* dependence term confirms the two-dimensional nature of the magnetic interactions,
e
Fig. 2. Angular variation of the ESR line width of a single crystal of (CZHsNHs)aMnBr4 at 77°K and 295°K. The quantity z is defined by z = 3~0s’~ 1.
‘This work was supported GP-6593 and GP-32458.
in part by NSF grants
References ably due to preferential orientation of the plates in sample tube. The data is quite well represented by eq. ( 1) with an extrapolated value for AH, of 70 gauss. Definite evidence for critical behavior below 77°K is indicated, as well as the presence of significant DM antisymmetric exchange. If a critical temperature near 40°K is assumed (as observed for the chloride salts [4] ). a value of y - 2.0 is obtained. This is in accord with predictions for two-dimensional systems [S] A value of gave = 7.015 was obtained. The angular dependence of the line widths for single crystals of EA,MnBr, at 77°K and 295°K are reproduced in fig. 2. For comparison, the line width for EA,MnCl, varies between 15 and 40 gauss at 295°K. Two factors are immediately evident. The line widths for the bromide salt are almost an order of magnitude larger than for the chloride salt. Secondly,
504
Ill
E.R. Peterson and R.D. Willett, J. C‘hem. Phys. 56 (1972) 1879. 121H.R. Boesch, U. Schmocker, 1,. Waldner, K. Lmercon and J.E. Drumheller, Phys. Lett. 36A (1971) 461. 131 W.D. van Amstel and L.J. de Jongh, Solid State Comm. 11 t 1972) 1423. I41 R.D. Willett, K. Chang, R. C‘aputo and B.C. C&stein, (to be submitted). 151 M.S. Seehra and T.G. Castner, Physik Kond. Materic. 7 (1968) 185. J. Phys. Chem. Solids 4 (1958) 24 1: (61 1. Dzialoshinsky, T. Moriya, Phys. Rev. 120 ( 1960) 961. L71 M.S. Seehra, J. Appl. Phys. 42 (1971) 1290. and critical phenomena 181 H.E. Stanley, Phase transitions (Oxford University Press, 1971). [91 P.W. Anderson and P.R. Weiss, Rev. Mod. Phys. 25 (1953) 269. IlO1 M.J. Hennessy, C.D. McElwee and P.M. Richards, Phys. Rev. 87 (1973) 930.