Magnetic relaxation phenomena in Fe nanoparticles composited with activated carbon   Satyendra Prakash Pal DEPARTMENT OF PHYSICAL SCIENCES INDIAN INSTITUTE OF SCIENCE EDUCATION AND RESEARCH MOHALI, INDIA

Outline Introduction and motivation Experimental: Sample preparation, TEM imaging, XRD spectra, and magnetic properties Results and discussion Conclusions

Introduction and motivation Nanomagnetic materials and nanocomposites: unique properties and applications in advanced technology, environmental control and biomedical applications . Magnetic spin-spin interactions govern their magnetic behavior. Magnetic dipole-dipole and inter-particle exchange interaction are two main interactions. After dilution with non-magnetic matrices, magnetic spin interactions weaken due to spatial separation. We have synthesized different nanocomposites of Fe nanoparticles with activated carbon to alter the magnetic spin-spin interaction and hence study the dilution effect on the static and dynamic magnetic properties of the Fe nanoparticle system. Introduction and motivation

Experimental: samples preparation Fe nanoparticles have been synthesized by employing a novel, physical, top-down approach of electro explosion of wires (EEW). In the EEW technique, a wire is exploded on a plate of the same material by passing a current density ~ 1010A/m2; in a time ~ 10-6s. Flow of current through the wire-plate leads to a series of processes culminating in explosion of the parent material. Nanocomposites of activated carbon and Fe nanoparticles were obtained by mechanical mixing of activated carbon and Fe nanoparticles, with different weight ratios by grinding together in a mortar and pestle. (1) Equal amounts of both of them, by weight, denoted as (1:1) (2) 33% of Fe NPs and 66% carbon, (1:2).

Results and discussion TEM images Nanoparticles are almost spherical in shape. Most probable size of 7.5nm The diffused rings in the electron diffraction pattern can be indexed as reflection from disrupted (110), (311) and (440) lattice planes.

The particle sizes are 13nm and 13 The particle sizes are 13nm and 13.8nm for (1:1) composite and (1:2) composite, respectively. The particle size increases with increment of activated carbon weight . It seems like the interconnected pores of the activated carbon provides the van der Waal interactions between the nanoparticles, to form the clusters.

XRD spectra Most intense peak at 2θ = 44.80 Weak XRD peaks at 2θ=65.00, 82.40, due to the nonequilibrium nature of the synthesis process the planes of the Fe nanoparticles gets reoriented Peaks position of Fe nanoparticles matches with those from bulk Fe in bcc phase The peak at 2θ=44.80 shows the presence of Fe in each composites.

Magnetization measurements The nanoparticles, generated by the composite preparation conditions described so far, can be attracted by a permanent ferrite magnet. On withdrawal of the magnetic field, the particles revert to their original arrangement. On observation, lack of any remanent magnetization is clear as the particles do not cluster, and can be easily disbursed in a liquid through ultrasonic activation. Hence in all probability, the particles are superparamagnetic. However, in order to clearly ascertain this, we present a series of magnetization measurements. Magnetization measurements, for the pure nanoparticles, and different nanocomposites , were performed using SQUID.

Fe Nanoparticles ZFC FC@200Oe TB =225K Magnetization starts decreasing after 225K, i.e., the particles start random flipping of spins aided by thermal energy, to overcome the anisotropic energy barrier. The presence of a small hysteresis in the M-H data of Fe nanoparticles indicates the presence of an energy barrier and inherent magnetization dynamics.

Nanocomposites (a) (b) Composites do not show any blocking temperature at all right up to the room temperature. The pure nanoparticles were ‘leaky’ (lost their magnetization) while the isolated form was not. (c) (d) The MS value of the composites, as compare to pure Fe nanoparticles, decreases as dilution with carbon is achieved. The increase of coercivity in the composite may arise due to complex interactions, which can create strong pinning centres for the core moments during demagnetization.

M-H curve measurements data @ 300 K and Langevin function fitting parameters Sample MS MR HC Langevin Function Fitting (emu/g) (emu/g) (Oe) d (nm) Ms (emu/g) Pure Fe NPs 87.38 2.42 139.99 7.87 99.34 Composite (1:1) 24.77 3.20 342.53 14.90 26.13 Composite (1:2) 18.57 1.10 208.49 15.02 19.77 Langevin equation : M/MS= Coth(α)-1/α Where, α= µH/KBT, µ= magnetic moment of the particle µ= MS d3π/6 For nanoparticles and composites, the particle sizes obtained by Langevin function fitting are close to the ones estimated employing TEM.

Magnetic Relaxation Magnetic relaxation curves were obtained at various temperatures, 5K, 50K, and 100K after ramp up of the sample temperature to 330K followed by cool down to the pre-assigned temperatures, in the presence of an applied magnetic field (20kOe). The magnetic field was then reduced to zero, and then magnetic relaxation data was collected . The decay behavior is ascribed to a dilute ensemble of superspins with random spatial distribution, anisotropy, and spin sizes

Fe Nano Table1. Magnetic relaxation curves fitting parameters Temp. M0 A1 τ1 A2 τ2 (K) (emu/g) (s) (s) 5 11.03 0.28 638 0.70 9760 50 4.00 0.42 630 0.65 9303 100 2.27 0.21 744 0.26 7491 Due to dilute ensemble of superspins with random spatial distribution, anisotropy and spin sizes. Relaxation time decreases, i.e., relaxation rates increase with temperature increment. Thermal energy available to cross over the energy barrier increases and hence relaxation rate increases.

Nanocomposites Table1. Magnetic relaxation curves fitting parameters Temp. M0 A1 τ1 A2 τ2 (K) (emu/g) (s) (s) (1:1) comp 100 2.13 0.12 660 0.13 6727 (1:2) comp 100 1.71 0.09 688 0.12 6535 For both composites@100K: As compared to Fe nanoparticles, the relaxation is faster in the presence of the activated carbon particles. Magnetic spins are completely isolated, and can be expected as a result of increased spin – spin separation due to the presence of activated carbon particles in the intervening space.

Conclusions TEM analysis shows spherical particles with size = 7.5nm. The peak at 2θ=44.80 shows the presence of Fe in each composites. Temperature dependent magnetization measurement of the pure Fe nanoparticles gives TB~225K. Whereas composites do not show any blocking temperature at all, right up to room temperature. For nanoparticles and composites, the particle sizes obtained by Langevin function fitting are close to the ones estimated employing TEM. Magnetic relaxation can be ascribed due to dilute ensemble of superspins with random spatial distribution, anisotropy and spin sizes. Increased spin–spin separation due to the presence of activated carbon particles in the intervening space, gives faster magnetic decay.

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