Magnetic properties of the granular alloy Fe10Ag90 as a function of annealing temperature

Abstract

Fe10Ag90 granular alloys have been prepared using a sol-gel process, sintered at 300 °C and annealed at temperatures between 400 °C and 700 °C. The mean size of the iron particles, obtained from X-ray diffraction, is 30.0 ± 0.7 nm. Due to the existence of a distribution of particle sizes in these samples, both blocked (BL) and superparamagnetic (SPM) particles are present simultaneously, as confirmed by magnetization measurements at room temperature. AC susceptibility measurements as a function of temperature reveal a magnetic phase transition at about 770 °C, indicating the presence of particles exhibiting bulk behavior, in the samples annealed above 550 °C. The presence of these particles has been attributed to an atomic diffusion process between the grains, forming bulk-like multiple-domain Fe particles having Curie temperatures near that of bulk alpha-Fe phase (T C = 770 °C).

superparamagnetism; granular alloy; size distribution

REGULAR ARTICLES

Magnetic properties of the granular alloy Fe₁₀Ag₉₀ as a function of annealing temperature

João Maria SoaresI, ^*; José Humberto de Araújo^II; Francisco de Assis Olímpio Cabral^II; Thomas Dumelow^III; Milton Morais Xavier Júnior^III; José Marcos Sasaki^IV

^IDepartamento de Física, Universidade Federal de Pernambuco, 50670-901 Recife - PE, Brazil

^IIDepartamento de Física Teórica e Experimental, Universidade Federal do Rio Grande do Norte, 59072-970 Natal - RN, Brazil

^IIIDepartamento de Física, Universidade do Estado do Rio Grande do Norte, 59633-010 Mossoró - RN, Brazil

^IVDepartamento de Física, Universidade Federal do Ceará, 60455-760 Fortaleza - CE, Brazil

ABSTRACT

Fe₁₀Ag₉₀ granular alloys have been prepared using a sol-gel process, sintered at 300 °C and annealed at temperatures between 400 °C and 700 °C. The mean size of the iron particles, obtained from X-ray diffraction, is 30.0 ± 0.7 nm. Due to the existence of a distribution of particle sizes in these samples, both blocked (BL) and superparamagnetic (SPM) particles are present simultaneously, as confirmed by magnetization measurements at room temperature. AC susceptibility measurements as a function of temperature reveal a magnetic phase transition at about 770 °C, indicating the presence of particles exhibiting bulk behavior, in the samples annealed above 550 °C. The presence of these particles has been attributed to an atomic diffusion process between the grains, forming bulk-like multiple-domain Fe particles having Curie temperatures near that of bulk a-Fe phase (T_C = 770 °C).

Keywords: superparamagnetism, granular alloy, size distribution

1. Introduction

The synthesis of nanometer-scale materials has been the focus of intense study in materials science and solid-state chemistry^1,2. Nano-sized particles of noble metals have attracted considerable interest in various fields of chemistry and physics because of their conspicuous physiochemical catalytic properties and their potential applications in microelectronics, optical, electronic, and magnetic devices^3-6. Much attention have been paid to granular iron solids because of their interesting physical properties, in particular, the discovery of giant magnetoresistance (GMR)^7,8 and giant magnetoimpedance (GMI)⁹, as well as their application in magnetic recording and optical devices and in sensors^10-12.

Nanocrystalline Fe particles containing a single magnetic domain exhibit very different magnetic properties from those of bulk Fe. New applications require materials that combine high magnetization and coercivity of single domain Fe particles with a highly conducting matrix. Iron can form alloys or solid solutions with almost all metals, there being few metallic elements with which Fe is immiscible. Ag is an example of such a metal. However, in recent years, it has been shown that metastable and homogeneous alloys of Fe-Ag system can be formed by using special techniques, such as thermal evaporation^13,14, liquid quenching¹⁵, ion implantation¹⁶, sputtering^17-21, mechanical alloying^22-24, gas condensation²⁵ and a sol-gel process^26-28. FeAg granular alloy, however, is metastable, and upon recrystallization at elevated temperatures, a transformation into separated phases of BCC Fe and FCC Ag occurs.

In this work a series of Fe₁₀Ag₉₀ granular alloys has been produced by a sol-gel method and characterized by X-ray diffraction and magnetization measurements. We have observed that in all samples there are particle size distributions composed of SPM and BL particles. A method for fitting the magnetization curve to a size distribution with these two components has been applied to determine their relative amounts²⁹. The changes in annealing temperature T_ann have a strong influence in the coercive field. The presence of particles exhibiting bulk behavior has been observed by AC susceptibility measurements in the samples annealed above 550 °C.

2. Experimental

Fe₁₀ Ag₉₀ granular alloys were produced by a sol-gel process^26-28 . The start solution was prepared from an aqueous solution of Fe and Ag nitrates and nitric acid. The precursor powder obtained was reduced in a hydrogen atmosphere for 45 minutes at a temperature of 400 °C. The resultant powder was pressed and sintered at 300 °C for 8 hours. After sintering, the samples were thermally treated in the temperature range of 400-700 °C.

The crystalline structure of the samples was investigated by conventional X-ray powder diffraction using a Rigaku diffractometer operated with a Mo-K_a radiation tube.

The magnetization and hysteresis curves were measured in a vibrating sample magnetometer (VSM) with a maximum magnetic field of 1 T. AC susceptibility was measured in a mutual inductance bridge as described previously³⁰.

3. Results and Discussion

Figure 1 shows the X-ray patterns of powdered Fe₁₀Ag₉₀ granular alloy reduced at a temperature of 400 °C, sintered at 300 °C and annealed at T_ann = 700 °C. It is seen that the iron in Fe₁₀Ag₉₀ has a b.c.c. structure and the silver has an f.c.c. structure; there are also some low intensity reflections from Fe₃O₄. Assuming that the Fe in the sample is indeed distributed as small particles, the average particle diameter should be related to the Fe Bragg peak widths by Scherrer's formula. Figure 2 shows part of the diffraction pattern shown in Figure 1 in the interval of 34.4-35.6 (2q) for samples annealed at 400, 450, 500, 550, 600 and 700 °C. The Bragg peaks for Ag(222) and Fe(211) were fitted with a pseudo-Voigt function in order to determine the line width. Applying Scherrer's formula, we obtain the average Fe particle sizes D_m from the (211) line width for different annealing temperatures, as shown in Table 1. D_m increases from 28.6 nm (powder) to 30 nm (sintered sample). However, D_m is only slightly affected by T_ann, staying at around 30 nm for all thermal treatment temperatures.

Figure 3 shows the hysteresis curves at room temperature obtained for Fe₁₀Ag₉₀ granular alloy samples annealed at different temperatures. The coercive field, determined from the hysteresis curves, ranges from 365 to 295 Oe, and is two orders of magnitude greater than that of bulk iron, showing the presence of blocked single-domain particles. These curves do not saturate in fields of up to 1 T, indicating the additional contribution of SPM particles. The figure insert shows the maximum field magnetization M_Hmax as a function of the annealing temperature, and it can be seen that M_Hmax decreases linearly with increasing T_ann, up to T_ann = 500 °C. However, there is an inflexion at T_ann = 550 °C, after which M_Hmax once again decreases linearly with T_ann, but more rapidly. The inflexion at T_ann = 550 °C suggests the formation of a different type of particle at this temperature. These particles appear to be larger than the blocked particles, and have multi-domain properties similar to bulk Fe. We attribute their presence to an atomic diffusion process between the grains. Above the inflexion, the proportion and size of large particles increases, and M_Hmax therefore reduces quickly with T_ann. This explanation is supported by AC susceptibility measurements as a function of temperature, as shown in Figure 4. It can be seen that at T_ann = 550 °C, there is a phase transition at 766 °C, near to the Curie temperature T_C of pure bulk iron, T_C = 770 °C. The transition temperature and the susceptibility drop at this temperature both increase with T_ann. This type of behavior has been observed in other systems³¹.

Figure 5a shows the remanence M_r as a function of annealing temperature. M_r decreases with increasing T_ann, despite a small peak at 550 °C, as seen in hysteresis curves. Figure 5b shows the coercive field H_c vs. T_ann. One can see that H_c decreases signicantly with increasing of T_ann, reaching a steady value of 295 Oe at T_ann = 600 °C. This saturation in coercivity is probably due to the fact that for T_ann = 600 °C there is a large number of bulk-like particles, as observed through the AC susceptibility curves (seen Figure 4). Another important observation may be made by comparing the coercive field with the average Fe grain size, displayed in Figure 5c, as a function of T_ann. We see that while H_c change significantly with T_ann, D_m stays almost constant at around 30 ± 0.7 nm. Thus, there is not a correlation between H_c and D_m, as there is in a non-interage uniform particle system. However, in these samples there is a distribution of particle size. Both BL and SPM particles are present simultaneously, as confirmed by Mössbauer effect and magnetization measurements at room temperature²⁸, even though the critical diameter of Fe spherical particle is 16 nm³², i.e. much less than D_m. Thus, heat treatment mainly modifies the distribution of particle sizes, and consequently the coercive field, which is a sum of contributions of all Fe particles in samples.

4. Conclusions

Samples of Fe₁₀Ag₉₀ granular alloys were thermally treated at various temperatures. Magnetic measurements show the existence of a distribution of particle sizes in these samples, which are composed of both SPM and BL particles. The magnetic properties of the samples are strongly influenced by changes in annealing temperature. For the samples annealed above of 550 °C appears a multidomain phase similar the a-Fe phase, that was attributed to a process of atomic diffusion between the grains of Fe.

Acknowledgments

This work was partially supported by the Brazilian agencies CNPq, FINEP and CAPES.

Received: November 1, 2004; Revised: May 25, 2005

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Publication Dates

• Publication in this collection
  10 Oct 2005
• Date of issue
  Sept 2005

History

• Accepted
  25 May 2005
• Received
  01 Nov 2004