Blocking temperature
of nFe O /(1-n)ZnO nanocomposites
2
3
as determined by dc magnetization
and ferromagnetic resonance
1
1
1
K. Wardal , J. Typek , G. Zolnierkiewicz ,
N. Guskos , U. Narkiewicz
1, 2
3
1
Institute of Physics, West Pomeranian University of Technology, Al. Piastow 48, 70-311 Szczecin, Poland
2Department of Solid State Physics, Faculty of Physics, University of Athens Panepistimiopolis, 15 784 Zografos, Greece
3Institute of Chemical and Environmental Engineering,
West Pomeranian University of Technology, K. Pulaskiego 10, 70-322 Szczecin, Poland
The T temperature determined from FMR is higher than B calculated from static magnetization not only for isolated nanoparticles but also for strongly agglomerated nanoparticles
For agglomerates of equal sizes the T temperature B increases with concentration of magnetic nanoparticles
For the same concentration of magnetic nanoparticles the T temperature scales with the size of a agglomerateB
For magnetic agglomerates dispersed in a polymer the T B temperature is smaller than in case of concentrated agglomerates what indicates on the role of dipol interaction between agglomerates
The T temperature distribution, due to different sizes of B agglomerates, can be extracted from the ZF and ZFC magnetization curves
Conclusions
Blocking temperature T is one of the more important magnetic characteristics of a nanomaterial which could severly limit its applications. Above T nanoparticles are in so called superparamagnetic phase B B (narrow lines in FMR spectrum). Value of T could be determined from the maximum of the temperature dependence of ZFC magnetization. From FMR measurements T coulb be determined from temperature B B dependence of the integrated intensity Iint. The peak in integrated intensity curve is often interpreted as occuring at T , although this subject is still under debate. Blocking temperature calculated from FMR is B usually much higher than obtained from static magnetization.
Introduction
The investigated samples were synthesized using the wet chemistry method. A mixture of iron and zinc hydroxides was obtained by adding ammonia solution to solution of Zn(NO ) ·6H O and Fe(NO ) ·4H O in water. The obtained 3 4 2 3 3 2 hydroxides were filtered, dried and calcined.
The obtained samples were characterized by means of X-ray diffraction which revealed the presence of only two phases: ZnO and ZnFe O (details are written in Table 1).2 4
FMR measurements were performed on a conventional X-band (í=9.4 GHz) Bruker E 500 spectrometer in the temperature range of 4-290 K using an Oxford helium-flow cryostat.
Static magnetization measurements were perfomed on a MPMS XL7 magnetometer with maximum filed 7 T in 2-300 K temperature range.
Experimental
Fig. 3. TEM image of sample n=0.4 Fig. 2. TEM image of sample n=0.7
Fig. 7. Comparison of blocking temperatures determined from FMR (upper curve) and dc magnetization (lower curve)
measurements Fig. 9. Temperature dependence of
temperature gradient of FC and ZFC magnetization diffrence for three
investigated samples Fig. 4. FMR integrated intensity of sample
n=0.2 obtained by integration Fig. 1. SEM image of sample n=0.7
Fig. 8. The values of the blocking temperature as a function of static magnetic field in four investigated samples determined
from ZFC magnetization 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 20 40 60 80 100 n=0.7 n=0.4 n=0.2 n=0.7 (polymer)
Magnetic field [kOe]
B lo c k in g te m p e ra tu re TB [K ] 5 10 15 20 25 30 0,000 0,001 0,002 0,003 0,004 0,005 0,006 0,007 n=0.7 n=0.4 n=0.2 Temperature [K] D (m F C -m Z F C )/ D T [a rb . u n it s ] 20 30 40 50 60 70 20 30 40 50 60 FMR nanoparticles nanoparticles in polymer Concentration of Fe2O3 [wt%] B lo c k in g te m p e ra tu re T B [K ] dc magnetization, H=0.6 kOe ¨ - nanoparticles 4 -nanoparticles in polymer 0 50 100 150 200 250 300 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 Temperature [K] F M R In te g ra te d in te n s it y [a rb . u n it s ] 0 50 100 150 200 250 300 0,3 0,6 0,9 1,2 1,5 1,8 2,1 2,4 F M R In te g ra te d in te n s it y [a rb . u n it s ] Temperature [K]
Fig. 5.FMR integrated intensity of sample n=0.4 obtained by integration 0 50 100 150 200 250 300 2 3 4 5 6 7 Temperature [K] F M R In te g ra te d in te n s it y [a rb . u n it s ]
Fig. 6. FMR integrated intensity of sample n=0.7 obtained by integration 0 10 20 30 40 50 60 70 80 90 0,01 0,02 0,03 0,04 0,05 n=0.2 n=0.4 n=0.7 0.1% of n=0.7 in PEN-b-PTMO Temperature [K] M a g n e ti c s u s c e p ti b ili ty [e m u /( g *O e )]
Fig. 10. Magnetic susceptibilyty measured in 0.6 kOe field in ZFC mode
Nominal concentration (n)
ZnO phase ZnFe2O4 phase Fe2O3 phase
XRD d [nm] Raman XRD d [nm] Raman XRD d [nm] Raman 0.2 + + 8 + - -0.4 26 + 12 + - -0.7 - + 12 + - -Table 1.
Figure 4, 5 and 6 presents integrated intensity of FMR spectras of examined samples. Maximum on each figure is intereprated as blocking temperature in FMR measurements.
Ferromagetic resonance (FMR)
Blocking temperature T is one of the more important magnetic B characteristics of a nanomaterial which could severly limit its applications. Above T nanoparticles are in so called superparamagnetic B phase (narrow lines in FMR spectrum). Value of T could be determined B from the maximum of the temperature dependence of ZFC magnetization. As could be seen in Fig. 1 the largest values of T are B obtained for both n=0.7 samples. This could be explained as the result of the strongest magnetic system (highest concentration of iron) in all investigated samples. That in general T is higher in n=0.2 sample than in B n=0.4 sample could by explained by assuming existence of larger agglomerates in n=0.2 sample than in n=0.4 sample.
From FMR measurements T coulb be determined from temperature B dependence of the integrated intensity Iint. The peak in Iint(T) curve is often interpreted as occuring at T , although this subject is still under B debate. Blocking temperature calculated from FMR is usually much higher than obtained from static magnetization. In Fig. 7 comparison of T determined from both metods is shown. Polymer sample (n=0.7, B polymer) displays consistently smaller values of T compared to similar B
concentrated sample (n=0.7) what indicates on weaker magnetic interaction between nanoparticles in a polymer.
In Fig. 9 temperature dependence of temperature gradient of FC and ZFC magnetization diffrence for three investigated samples is presented. It is argued that T distribution (due to different sizes of B nanoparticles) can be extracted from ZFC and FC curves. Comparing the obtaine curves for our three samples it is evident that sample n=0.7 contains nanoparticles with a broader distribution of sizes in comparison to nanoparticles in n=0.2 and n=0.4 samples.
Conclusions
In Fig. 9 temperature dependence of temperature gradient of FC and ZFC magnetization diffrence for three investigated samples is presented. It is argued that T distribution (due to different sizes of nanoparticles) can be extracted from ZFC and FC curves. Comparing the B obtaine curves for our three samples it is evident that sample n=0.7 contains nanoparticles with a broader distribution of sizes in comparison to nanoparticles in n=0.2 and n=0.4 samples.
