- Email: [email protected]
Low temperature spin state of a model compound for organic ferrimagnets as studied by single crystal cw-ESR spectroscopy
ELSEVIER
Synthetic Metals 85 (1997) 1727-1728
Low temperature spin state of a model compound for organic ferrimagnets as studied by single crystal cw-ESR spectroscopy M. Nishizawa a, D. Shiomia, K. Satob, T. Takuib, K. Itoh a, H. Sakurai c, A. IzuokaC, T. SugawaraC, H. Sawa d*, R. Kato d aDqazbnen! ofMderialSdena? wzibDqameti ofchemistry, Faxlty ofsdence, OsakaCity Universiry, Suniyoshi-k4 OsakaS58, Jqan CDeparima?t ofPwe andAppliedSciences, GraluateSchool ofArts andSciences, The Universiry ofTokyo, Megrcro-ku, Tokyo 153, Jop~ dThelnstituteforSoiidStae Physics, TheUniversiIy ofTokyo, Minato-ku, Tokyo 106, Japan
Abstract We have studied a model compound for organic ferrimagnets by single crystal cw-ELSR. The system under study is composed of two kinds of nitronylnitroxide molecules with theground states of S=1/2 and S=l. These molecules are stacked in an alternating chain in the crystal. The cw-ESR signal of the compound split into two lines below 10 K, which were reproduced by the superposition of two Lorentzian lines. The X-ray measurements at 9.6 K revealedthat the crystal structure remainedunchanged at low temperatures, indicating that the origin of the ESR line splitting is attributable to some change in the spin state: TWO distinguishable spin species appear in the crystal to give the line splitting. The magnetic interaction fields from the two species are elucidated on the basis of the angular dependence of the EBR linewidths: One has isotropic (three-dimensional) interaction field, while the other has one-dimensional field which is axially symmetric along the alternating chain. The appearance of the two kinds of spin species demonstrates that the model system violates the classical picture of ferrimagnetic states (antiparallel alignment of adjacent spins with different spin quantum numbers). This presumably results from the internal magnetic degree of freedom within the S=l molecule, i.e., finite intramolecular ferromagnetic interaction. Keywords: Electron spin resonance, X-ray emission,
diffraction
1. Introduction Fenimagnets have been attracting attention as one of the facile approach to organic ferromagnets. However, longrange fenimagnetic or&r has not been documented so far in organic molecular crystalline solids. Izuoka et al. have reported a molecular complex crystal composedof two kinds of nitronylnitroxide molecules with the ground states of S=1/2 (Z)andS=l (2)[1].
The crystal of the molecular complex (I+2) consists of alternating chains of I and 2 which seemingly favors a ferrimagnetic spin alignment in the crystal. However, the XT value has been found to decrease below 3 K, demonstrating that ferrimagnetic spin alignment has not been achieved. * Present address: Department of Physics, Faculty of Science, Chiba University, Chiba 263, Japan 0379-6779J971S17.00
0
PII SO379-6779(96)04564-X
1997 Elsevier Science S.A. All rights resmed
andscattering
We have calculated the spin state energy of-an S=1/2 and S=l alternating chain in which the intramolecular magnetic degree of freedom remains within the S=l site [2], assuming that the S=l spin is composedof two S=1/2 spins coupled by finite ferromagnetic interaction. The calculation has shown that in the repeating unit of the chain one S=1/2 spin remains free of magnetic coupling in the ground state. This peculiar result suggests that the two of three S=1/2 spins in the repeating unit no longer contribute to the bulk magnetic moment at low temperatures but recover their contribution owing to the thermal activation as the temperature is raised Although such temperature-dependent behavior of magnetic moment has been supported by the spin-spin relaxation time data measured for the single crystal of (1+2) [2], no direct evidence for the interaction-free S=1/2 spin mentioned above has been obtained. In this paper, in order to clarify the spin state of the molecular chain of (1+2), single crystal cw (continuouswave)-ESR and low temperature crystal structure of the compound are examined. The spin state of (1+2) deduced from theexperiments is comparedwith thecalculatedresult and the difference between the obtained spin state and the classical ferrimagnetic spin alignment is discussed.
M. Nishizawa
1728
etal. /SyntheticMetals 8.5 (1997 1727-l 728
2. Experimental
4. Conclusion
The single crystal cw-ESR spectra were recorded in the X band on a Bruker ESR spectrometer ESP 300. The crystal structure determination at low temperatures was performed using a MAC Science Weissenberg-type imaging plate system equipped with a DAIKIN closed-cycle helium refrigerator.
The splitting of FSR signal was observed for the nitronylnitroxide-based molecular crystal, (I +2). The X-ray measurements showed that the crystal structure remained unchanged at low temperature. The origin of the ESR line splitting is attributable to some change in the spin state: Two kinds of spin species appear in the crystal on lowering the temperature. The angular dependence of the ESR linewidths revealed the presence of the magnetic interaction fields from the two spin species: One has isotropic (threedimensional) interaction field, while the other has axially symmetric (one-dimensional) interaction field along the stacking chain. A schematic picture of the ground state of the alternating chain composedoftheS=1/2 andS=l molecules is represented in Fig. 2(a), which is based on the theoretical calculation [2] and the experimental results of the present ESR study. The appearance of the two kinds of spin species in the (1+2) stacking chain demonstrates that the model system as shown in Fig. 2(a) violates the classical picture of ferrimagnetic spin alignment (Fig. 2(b)).
3. Results
and discussion
The cw-ESR spectrum exhibited a single Lorentzian line at room temperature. Below 10 K, the spectrum split into two lines. The resulting spectrum was well reproduced by the superposition of two Lorentzian lines A (strong) and B (weak). Two mechanisms are proposed for the splitting of ESR spectrum on lowering the temperature: ( i ) The “sitesplitting” due to symmetry reduction associated with structural change and ( ii ) spin-state change due to magnetic interaction. The crystal structure analyses at 9.6 K, 30Kand 298K showed that the structure remained unchanged on lowering the temperature. Only a shrinkage of the unit cell was observed at low temperatures. Thus, the splitting of cw-ESR spectrum below 10 Kis attributable to some change in the spin state.
Fig. 1. Angular dependence of ESR linewidths in the a*c plane at 3 K. A and B denote the Lorentzian lines obtained from the deconvolution of the observed spectrum. We analyzed the angular dependence of the peak-to-peak linewidths AHpp of the ESR signals from spin species A and B. The angular dependence of the linewidths in the a*c plane at 3 K is shown in Fig. 1. The component A shows that the angular dependence of AHpp is proportional to cos’0 (0 is the angle between the c-axis and the magnetic field), indicating an isotropic (three-dimensional) magnetic structure. The signal intensity of A was enhanced with decreasing temperature. From these results, the component A is regarded as free of exchange interaction between the neighboring spins, indicating the free S=1/2 spin appearing in thegroundstate as calculated[2]. On the other hand, the component B shows a maximum of AHpp in the magnetic field direction parallel to the stacking chain axis (//the c-axis) andminima at the two magic angles (54.7” and 125.3”). This is characteristic of axially symmetric (one-dimensional) magnetic structure along the chain. The intensity of the B signal decreased on lowering the temperature. The component B corresponds to the thermally activated spins which are coupled by the intermolecular antiferromagnetic exchange interaction as predicted in the calculation [2].
Fig. 2. (a) Schematic representation of the ground state of the (S=lR)-(.%I) alternating chain. The open circles are spin-1R sites. The solid lines represent the intramolecular ferromagnetic interactions. The ovals show the intermolecular singlet (S=O) spin-pairs. (b) Classical picture of the ferrimagnetic spin alignment. This presumably results from the internal magnetic degree of freedom within theS=l molecules, i.e., the finite intramolecular ferromagnetic interxtion comparable to thermal activstion energy. Acknowledgment This’ work has been supported by Grants-in-Aid for Scientific Research on Priority Area “Molecular Magnetism” (Area No. 228104 242 103, 04 242 104 and 04 242 105) from the Ministry of Education, Culture and Science, Japan. The authors (D. S. and K. S.) acknowledge the Ministry of Education, Culture and Science, Japan for Grants-in-Aid for Encouragement of Young Scientists (No. 07740553 and 07740468). References [l]A.
Izuoka, M. Fukuda R. Kumai, M. Itakura, S. Hikami, T. Sugawaa, J. Am. Chetn. Sot., 116 (1994)2609. [2] D. Shiomi, M. Nishimwa, K. Sato, T. Takui, K Itoh, H. Sakurai, A. Izuoka, T. Sugawxa., Mol. Ctyst. Liq. Ctysf., 278 (1996)989.
Synthetic Metals 85 (1997) 1727-1728
Low temperature spin state of a model compound for organic ferrimagnets as studied by single crystal cw-ESR spectroscopy M. Nishizawa a, D. Shiomia, K. Satob, T. Takuib, K. Itoh a, H. Sakurai c, A. IzuokaC, T. SugawaraC, H. Sawa d*, R. Kato d aDqazbnen! ofMderialSdena? wzibDqameti ofchemistry, Faxlty ofsdence, OsakaCity Universiry, Suniyoshi-k4 OsakaS58, Jqan CDeparima?t ofPwe andAppliedSciences, GraluateSchool ofArts andSciences, The Universiry ofTokyo, Megrcro-ku, Tokyo 153, Jop~ dThelnstituteforSoiidStae Physics, TheUniversiIy ofTokyo, Minato-ku, Tokyo 106, Japan
Abstract We have studied a model compound for organic ferrimagnets by single crystal cw-ELSR. The system under study is composed of two kinds of nitronylnitroxide molecules with theground states of S=1/2 and S=l. These molecules are stacked in an alternating chain in the crystal. The cw-ESR signal of the compound split into two lines below 10 K, which were reproduced by the superposition of two Lorentzian lines. The X-ray measurements at 9.6 K revealedthat the crystal structure remainedunchanged at low temperatures, indicating that the origin of the ESR line splitting is attributable to some change in the spin state: TWO distinguishable spin species appear in the crystal to give the line splitting. The magnetic interaction fields from the two species are elucidated on the basis of the angular dependence of the EBR linewidths: One has isotropic (three-dimensional) interaction field, while the other has one-dimensional field which is axially symmetric along the alternating chain. The appearance of the two kinds of spin species demonstrates that the model system violates the classical picture of ferrimagnetic states (antiparallel alignment of adjacent spins with different spin quantum numbers). This presumably results from the internal magnetic degree of freedom within the S=l molecule, i.e., finite intramolecular ferromagnetic interaction. Keywords: Electron spin resonance, X-ray emission,
diffraction
1. Introduction Fenimagnets have been attracting attention as one of the facile approach to organic ferromagnets. However, longrange fenimagnetic or&r has not been documented so far in organic molecular crystalline solids. Izuoka et al. have reported a molecular complex crystal composedof two kinds of nitronylnitroxide molecules with the ground states of S=1/2 (Z)andS=l (2)[1].
The crystal of the molecular complex (I+2) consists of alternating chains of I and 2 which seemingly favors a ferrimagnetic spin alignment in the crystal. However, the XT value has been found to decrease below 3 K, demonstrating that ferrimagnetic spin alignment has not been achieved. * Present address: Department of Physics, Faculty of Science, Chiba University, Chiba 263, Japan 0379-6779J971S17.00
0
PII SO379-6779(96)04564-X
1997 Elsevier Science S.A. All rights resmed
andscattering
We have calculated the spin state energy of-an S=1/2 and S=l alternating chain in which the intramolecular magnetic degree of freedom remains within the S=l site [2], assuming that the S=l spin is composedof two S=1/2 spins coupled by finite ferromagnetic interaction. The calculation has shown that in the repeating unit of the chain one S=1/2 spin remains free of magnetic coupling in the ground state. This peculiar result suggests that the two of three S=1/2 spins in the repeating unit no longer contribute to the bulk magnetic moment at low temperatures but recover their contribution owing to the thermal activation as the temperature is raised Although such temperature-dependent behavior of magnetic moment has been supported by the spin-spin relaxation time data measured for the single crystal of (1+2) [2], no direct evidence for the interaction-free S=1/2 spin mentioned above has been obtained. In this paper, in order to clarify the spin state of the molecular chain of (1+2), single crystal cw (continuouswave)-ESR and low temperature crystal structure of the compound are examined. The spin state of (1+2) deduced from theexperiments is comparedwith thecalculatedresult and the difference between the obtained spin state and the classical ferrimagnetic spin alignment is discussed.
M. Nishizawa
1728
etal. /SyntheticMetals 8.5 (1997 1727-l 728
2. Experimental
4. Conclusion
The single crystal cw-ESR spectra were recorded in the X band on a Bruker ESR spectrometer ESP 300. The crystal structure determination at low temperatures was performed using a MAC Science Weissenberg-type imaging plate system equipped with a DAIKIN closed-cycle helium refrigerator.
The splitting of FSR signal was observed for the nitronylnitroxide-based molecular crystal, (I +2). The X-ray measurements showed that the crystal structure remained unchanged at low temperature. The origin of the ESR line splitting is attributable to some change in the spin state: Two kinds of spin species appear in the crystal on lowering the temperature. The angular dependence of the ESR linewidths revealed the presence of the magnetic interaction fields from the two spin species: One has isotropic (threedimensional) interaction field, while the other has axially symmetric (one-dimensional) interaction field along the stacking chain. A schematic picture of the ground state of the alternating chain composedoftheS=1/2 andS=l molecules is represented in Fig. 2(a), which is based on the theoretical calculation [2] and the experimental results of the present ESR study. The appearance of the two kinds of spin species in the (1+2) stacking chain demonstrates that the model system as shown in Fig. 2(a) violates the classical picture of ferrimagnetic spin alignment (Fig. 2(b)).
3. Results
and discussion
The cw-ESR spectrum exhibited a single Lorentzian line at room temperature. Below 10 K, the spectrum split into two lines. The resulting spectrum was well reproduced by the superposition of two Lorentzian lines A (strong) and B (weak). Two mechanisms are proposed for the splitting of ESR spectrum on lowering the temperature: ( i ) The “sitesplitting” due to symmetry reduction associated with structural change and ( ii ) spin-state change due to magnetic interaction. The crystal structure analyses at 9.6 K, 30Kand 298K showed that the structure remained unchanged on lowering the temperature. Only a shrinkage of the unit cell was observed at low temperatures. Thus, the splitting of cw-ESR spectrum below 10 Kis attributable to some change in the spin state.
Fig. 1. Angular dependence of ESR linewidths in the a*c plane at 3 K. A and B denote the Lorentzian lines obtained from the deconvolution of the observed spectrum. We analyzed the angular dependence of the peak-to-peak linewidths AHpp of the ESR signals from spin species A and B. The angular dependence of the linewidths in the a*c plane at 3 K is shown in Fig. 1. The component A shows that the angular dependence of AHpp is proportional to cos’0 (0 is the angle between the c-axis and the magnetic field), indicating an isotropic (three-dimensional) magnetic structure. The signal intensity of A was enhanced with decreasing temperature. From these results, the component A is regarded as free of exchange interaction between the neighboring spins, indicating the free S=1/2 spin appearing in thegroundstate as calculated[2]. On the other hand, the component B shows a maximum of AHpp in the magnetic field direction parallel to the stacking chain axis (//the c-axis) andminima at the two magic angles (54.7” and 125.3”). This is characteristic of axially symmetric (one-dimensional) magnetic structure along the chain. The intensity of the B signal decreased on lowering the temperature. The component B corresponds to the thermally activated spins which are coupled by the intermolecular antiferromagnetic exchange interaction as predicted in the calculation [2].
Fig. 2. (a) Schematic representation of the ground state of the (S=lR)-(.%I) alternating chain. The open circles are spin-1R sites. The solid lines represent the intramolecular ferromagnetic interactions. The ovals show the intermolecular singlet (S=O) spin-pairs. (b) Classical picture of the ferrimagnetic spin alignment. This presumably results from the internal magnetic degree of freedom within theS=l molecules, i.e., the finite intramolecular ferromagnetic interxtion comparable to thermal activstion energy. Acknowledgment This’ work has been supported by Grants-in-Aid for Scientific Research on Priority Area “Molecular Magnetism” (Area No. 228104 242 103, 04 242 104 and 04 242 105) from the Ministry of Education, Culture and Science, Japan. The authors (D. S. and K. S.) acknowledge the Ministry of Education, Culture and Science, Japan for Grants-in-Aid for Encouragement of Young Scientists (No. 07740553 and 07740468). References [l]A.
Izuoka, M. Fukuda R. Kumai, M. Itakura, S. Hikami, T. Sugawaa, J. Am. Chetn. Sot., 116 (1994)2609. [2] D. Shiomi, M. Nishimwa, K. Sato, T. Takui, K Itoh, H. Sakurai, A. Izuoka, T. Sugawxa., Mol. Ctyst. Liq. Ctysf., 278 (1996)989.