ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ, 2012, том 86, № 2, с. 323-326
ФИЗИЧЕСКАЯ ХИМИЯ НАНОКЛАСТЕРОВ ^^^^^^^^ И НАНОМАТЕРИАЛОВ
STUDY ON Bi—Fe3O4 NANOCOMPOSITE PREPARED VIA MECHANOCHEMICAL PROCESSING
© 2012 Ahmad Hasanpour*, Morteza Mozafari**, Mohammad-Reza Azani***,
*Physics Department, Payame Noor University (PNU), Tehran, Iran **Physics Department, Razi University, Kermanshan, Iran ***Departamento de Química Inorgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain
E-mail: firstname.lastname@example.org Received February 01, 2011
Abstract — In this work Bi—Fe3O4 nanocomposite was synthesized by room temperature milling of Bi2O3 and Fe powders using a planetary ball mill in air. The synthesis reaction proceeds with increase in milling time and is finished by about 4 h. The XRD pattern of the as-milled powder shows that the main phases are Bi and Fe3O4 without any extra phases. The average crystallite sizes of the constituents have been determined by Scherer's formula and they were 22 and 18 nm for Bi and Fe3O4 respectively. This was also confirmed by Transmission Electron Microscopy (TEM). Magnetic hysteresis loops at room temperature were recorded using a vibrating sample magnetometer (VSM). A tow-probe method was used to measure resistivity variation of the nanocomposite as a function of magnetic filed and temperature. We have observed a room temperature magneto resistance (p0 — p#)/p0 as large as 17% in a magnetic field of 1 T.
Keywords: giant magneto resistance, bismuth nano composite, mechanochemical processing.
Giant magneto resistance (GMR) effect has stimulated great interests of physicists and material scientists, due to its application in magnetic information storage, sensors and magneto electronics [1, 2]. Magnetic nanostructures, such as multilayer [3—5] and granular solids [6, 7] have attracted a great deal of attention, because of the realization of negative giant magneto resistance (GMR) due to spin-dependent scattering at inter face between the particles and matrix [8—11]. In the last decade, different measurements have revealed that a negative isotropic magneto resistance is a common feature of a wide class of nano-structured magnetic materials. Very similar effects have emerged in various magnetic systems, both diluted and concentrated, such as Curie—Weiss paramagnets, core—shell nano-particulate materials, bulk frustrated Ferro magnets, whose common feature is the loss of magnetic coherence on the nanometer scale . Most reported GMR values, particularly those in spin-valve devices, are much smaller, in the range of 10% at room temperature [12, 13].
Bismuth is a semimetal with highly anisotropic Fermi surface, high mobilities, long carrier mean free path and small effective mass. A large magneto-resistance has been observed in single-crystal bismuth thin films, which were fabricated by electro deposition from aqueous solution, up to 300% at room temperature and in a field of 5 T [14, 15]. The production of nano composite ultra fine powders by simultaneous
evaporation, nucleation, and growth is an attractive way to incorporate nanoinclusions into a material . Quite recently, we have developed mechanochemical processing method for producing Bi—Fe3O4 nanocomposite and investigated it's magnetic and magneto resistance properties at room temperature .
The raw materials were powders of Bi2O3 and Fe both from E. Merck Co. with minimum purity of 99%. The powders were weighed in 1 : 2 mole ratio with 10% Fe deficiency to compensate for iron wear in the course of milling. A total of 30 g of the powders together with different sizes of hardened-steel balls, was loaded into a hardened-steel vial (500 ml) in an air, and milled for up to 4 h in a high-energy planetary mill (FRITSCH "Pulverisett6"). The number and size of the balls were chosen so that a powder to ball weight ratio of1 : 10 is obtained. In order to maintain an equal powder to ball mass ratio in all experiments, the vial was cleaned and reloaded with 30 g of new raw materials. Phase formation of the as milled composite and cold pressed samples were studied by a diffractometer, (BRUKER, D8 model), using Cu^ radiation (X = = 1.5405 A). Crystallite size was calculated by the Scherer's formula:
d = 0.9 V B cos 0,
Hi i mini ifHMWli
Fig. 1. XRD pattern of the as milled powder.
where B is the broadening of the diffraction peak due to purely crystallite size, measured at half its maximum intensity, d is mean diameter of crystallite size and 9 is the Bragg angle at maximum intensity. The broadening due to the X-ray machine was accounted by using a standard powder, which has a particle size greater than 1000 nm . Then B was calculated by Warren's formula:
J)2 _ n2 n2
in which Bm and Bs are measured breadths, at half maximum, of the diffraction lines from the sample and
the standard, respectively. Morphology of the powders was investigated by TEM, (Philips, CM20FEG model). For resistivity measurements, the as milled powders were shaped in a cylindrical die of 10mm in diameter and about 5mm in height under a pressure of 10 ton/cm2. Magnetic hysteresis loops at room temperature were recorded using a vibrating sample magnetometer. A tow-probe method was used to measure resistivity variation of the nanocomposite as a function of magnetic filed and temperature . We have observed a room temperature magneto resistance (p0 — p#)/p0 as large as 17% in a low magnetic field.
Fig. 2. The SEM image of the as milled powder.
RESULTS AND DISCUSSION
The XRD pattern of the as-milled powder (Fig. 1), shows that a nanocomposite of only two phases, bismuth (Bi, marked by asterisks) and magnetite (Fe3O4) has been formed. The mean particle sizes of the Bi and Fe3O4 were 22 and 18 nm, respectively; using Scherer's formula. Figure 2 shows the SEM image of nano composite morphology as can be seen the particles are highly agglomerated.
Figure 3 is the TEM picture of the nanocomposite As can be seen average size of the particles is about 50 nm, which is greater than the mean crystallite sizes of the Bi and Fe3O4, estimated by Scherer's formula.
STUDY ON Bi—Fe3O4 NANOCOMPOSITE PREPARED
Fig. 3. The TEM image of the as milled powder.
Fig. 4 Magnetization curve of the as milled composite.
p, Q cm
Fig. 5. "Variation of composite resistivity as a function of magnetic field.
This shows that each particle is composed of few crystallites. Figure 4 shows the magnetization curve of the as-milled composite.
A saturation magnetization of 22 A m2/Kg was obtained, which is related to the magnetic portion of the nanocomposite. Using Fig. 4 and calculations based on the weight percent of each phase, a saturation magnetization of 80 emu/g is achieved for magnetic phase (Fe3O4). This of course is smaller than the value of 90 emu/g, related to bulk Fe3O4. This is due to the fact that as the particle size is inclined, the value of magnetization decreases . Figure 5 shows the variation of magneto resistance (p0 — pH)/p0 as a function of magnetic filed. As can be seen this nano composite shows a very high magneto resistance at room temperature which is comparable with multilayer magnetic films and is much more than other granular nano composites. This property will be due to unusual electrical property of semimetal bismuth.
This study shows that Bi—Fe3O4 nanocomposite can be achieved simply by mechanochemical processing. It shows a giant magneto resistance property at room temperature which is comparable with multilayer magnetic films and is much better than other granular nanocomposites.
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