Calorimetric method for the determination of Curie temperatures of magnetic nanoparticles in dispersion

Abstract

Mn(x)Zn(1-x)Fe(2)O(4)-based magnetic fluids with x = 0.1-0.9 are synthesized by coprecipitation. The samples are heated in a radio frequency (rf) magnetic field using an rf generator at different powers, and the temperature is measured as function of time using an optical thermometer. The heating effect of the dispersed magnetic nanoparticles is proportional to the imaginary part of the dynamic magnetic susceptibility of the ferrofluid, a quantity that depends on the temperature through the magnetization of the ferrite nanoparticles and the Néel or Brownian relaxation times, respectively. We propose an extrapolation method to actuate the Curie temperatures of the dispersed magnetic nanoparticles. By means of appropriate fitting functions for (dT/dt) versus T for both the heating and the cooling process, we deduce the Curie temperature of the samples under investigation. For Mn(x)Zn(1-x)Fe(2)O(4)-based magnetic nanoparticles the Curie temperatures decrease with increasing Zn content. They turn out to be lower than the literature values for bulk Mn(x)Zn(1-x)Fe(2)O(4), a phenomenon which is generally observed for phase transitions of nanocrystalline materials.

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“…The mean crystallite size of nanoparticles for each sample is slightly smaller than the average diameter obtained from TEM measurements (Table S1, Supporting Information) and may be explained by the existence of amorphous surface layers . The lattice parameter ( a ) of the cubic unit cell for each sample (Table S1, Supporting Information) is very close to the magnetite ( a = 8.380 Å) .…”

“…The mean crystallite size of nanoparticles for each sample is slightly smaller than the average diameter obtained from TEM measurements ( Table S1 , Supporting Information) and may be explained by the existence of amorphous surface layers. 52 The lattice parameter ( a ) of the cubic unit cell for each sample ( Table S1 , Supporting Information) is very close to the magnetite ( a = 8.380 Å). 31 We note a slight decrease in lattice constant for SHS and SSH samples compared to SH and SS samples, respectively, which can be attributed to the lattice mismatch between the seed and the outermost layer but that also may be due to lattice distortion or internal stress.…”

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“…Under the ideal condition of non-interacting, monodisperse particles, the fitting functions for (dT/dt) versus T for both the heating and the cooling process is often referred as to the Tc. [76] As a result, by using a linear extrapolation for the cooling and heating branches, we expect to have Tc as the intersection point of temperature change over time with temperature (Figure 4.12c), and yields 394 K, 378 K, 368 K, and 360 K for samples S1, S2, S3, and S4, respectively. The decrease of Tc with the decrease in particle size in the present case is attributed to the fluctuations of the electron spins become more prominent in the smaller size of particles.…”