Influence of magnetic agglomerates
on EPR spectra of free radicals
embedded in organic matrix
1,2 2 2 2 3
N. Guskos , G. Zolnierkiewicz , J. Typek , A. Guskos , and D. Petridis
1
Solid State Section, Department of Physics, University of Athens, Panepistimiopolis, 15 784 Zografos, Athens, Greece
2
Institute of Physics, West Pomeranian University of Technology, Al. Piastow 48, 70-311 Szczecin, Poland
3
NCSR „Demokritos“, Aghia Paraskevi, Attikis, Athens, Greece
Abstract
Two samples containing an extended free radical network derived from the condensation of cyanuric chloride with p-phenylenediamine have been prepared. Both samples (designated as sample 1 and 2) contained free radicals. Additionally, sample 2 contained a very small amount of magnetic centres, part of it in form of agglomerates. The magnetic resonance spectra (EPR/FMR) of these samples were measured in the 4–290 K temperature range. At room temperature the EPR spectrum of sample 2 is the sum of two lines attributed to two different magnetic centres: a
narrow line at H =3373.46(3) Gs (g =2.0031(1)) with linewidth ÄH =8.63(2) Gs (due to free radicals) and a broad line centred at H =3053(5) r eff pp r
Gs (g =2.240(1)) with linewidth ÄH =1010(5) Gs (arising from magnetic iron oxide agglomerates). The spectrum of sample 1 constituted a eff pp
single resonance line centred at H =3371.64(3) Gs (g =2.0043(1)) with linewidth ÄH =7.02(2) Gs. The integrated intensities decreased with r eff pp
decreasing temperature in both spectra in the high temperature range. This type of behaviour is similar to what is observed for magnetic nanoparticles in nonmagnetic matrixes. The resonance field of the broad line shifts to low magnetic fields upon lowering temperature with the
-3
gradient ÄH /ÄT=2.3(1) Gs/K, while the narrow line shifts towards high magnetic fields with ÄH /ÄT =2.7(1)x10 Gs/K for sample 1 and r r
-3
ÄH /ÄT =3.6(1)x10 Gs/K for sample 2. The linewidth of the broader line increases with decreasing temperature while for the narrow line this r
change was only minor (especially at higher temperatures) in both samples. The magnetic iron oxide clusters produce an internal magnetic field which could act on free radicals and its strength depends essentially on relative concentration of clusters. The resonance field at different temperatures is essentially dependent on the existence of magnetic clusters with a stronger process observed for sample 2. This could suggest that the reorientation processes are more intense for sample without magnetic clusters.
Sample designation I4K/IRT ÄH4K [Gs] ÄHRT [Gs] ÈCW [K] ÄHr/ÄT [Gs/K] Source 1 16 2.30(2) 5.34(2) 0.40(3) 0.027(3) This work 2 34 10.51(3) 13.83(3) 0.74(3) 0.037(3) This work 3 45 7.42(2) 13.30(3) 1.06(3) 0.035(3) [10] 4 80 8.23(2) 14.07(3) 1.23(3) 0.075(3) [9] 3360 3380 3400 -40000 -30000 -20000 -10000 0 10000 20000 30000 40000 3000 3500 4000 -1500 -1000 -500 0 500 1000 1500 d c "/ d H [A rb . u n it s ] Magnetic field H [Gs] 20 K 4 K d c "/ d H [A rb . u n it s ] Magnetic field H [Gs] 4 K 130 K 3360 3370 3380 3390 -20000 -10000 0 10000 20000 2500 3000 3500 4000 0 d c "/ d H [A rb . u n it s ] Magnetic field H [Gs] 290 K 90 K d c "/ d H [A rb . u n it s ] Magnetic field H [Gs] 90 K 290 K 3365 3370 3375 3380 -30000 -20000 -10000 0 10000 20000 30000 d c "/ d H [a rb . u n it s ] Magnetic field H [Gs] 4 K 8 K 15 K 25 K 40 K 55 K 70 K 85 K 110 K 140 K 170 K 220 K 230-260 K G0 0 50 100 150 200 250 3369 3370 3371 3372 3373 G 0 G 8 R e so n a n ce fie ld H r [G s] T e m p e ra tu re T [K ] 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 2 4 6 8 1 0 1 2 G 8 G 0 Li ne w id th D H p p [G s] T e m p e ra tu re T [K ] 0 50 100 150 200 250 300 0 2000000 4000000 6000000 8000000 10000000 12000000 14000000 G 8 G 0 In te g ra te d in te n si ty I inte g r Tem perature T [G s] 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 0 .0 0 0 0 0 0 .0 0 0 0 1 0 .0 0 0 0 2 0 .0 0 0 0 3 0 .0 0 0 0 4 0 .0 0 0 0 5 G 8 G 0 1 /I T e m p e ra tu re T [K ] 160 180 200 220 240 260 280 2800 2900 3000 3100 R e so n a n ce fie ld Hr [G s] Tem perature T [K] 1 6 0 1 8 0 2 0 0 2 2 0 2 4 0 2 6 0 2 8 0 9 0 0 1 0 0 0 1 1 0 0 1 2 0 0 1 3 0 0 L in e w id th D H p p [G s] T e m p e ra tu re T [K ] 1 6 0 1 8 0 2 0 0 2 2 0 2 4 0 2 6 0 2 8 0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 A m pl itu de A pp T e m p e ra tu re T [K ]
Table 1. Values of different parameters calculated from EPR spectra: I /I – ratio of the integrated intensities at 4 K and RT; 4K RT ÄH , ÄH - linewidths at 4 K and room RT; È – Curie-Weiss 4K RT CW temperature; ÄH /ÄT – temperature gradient of resonance field at r low temperature. Sample 1 is without additional magnetic centres, samples 2 to 4 contain an increasing amount of additional magnetic centres.
Figure 1 The
p.- methylaminophenol sulfate (a) and the 8-Hydroxy-1,3,6-pyrenetrisulfonic
acid,trisodium salt (b).
Figure 2 Temperature dependence of the EPR spectra of (a) extended free radical network derived from condensation of cyanuric chloride with p-phenylenediamine, and (b) 8-Hydroxy-1,3,6-pyrenetrisulfonic acid, trisodium salt. The insets show the spectra in extended magnetic field range.
Figure 3 Temperature dependence of the parameters of EPR spectra for both samples, a) resonance field (H ), b) peak-to-peak linewidth (ÄH ),r pp c) integrated intensity (Iintegr) and d) reverse of integrated intensity.
Figure 4 Temperature dependence of the parameters of FMR spectra for sample 2: a) resonance field (H ), b) peak-to-peak linewidth (ÄH ), r pp c) signal amplitude (A )pp a) b) a) a) b) a) b) c) d) a) b) c)
