Preparation and characterization of FeAg granular alloys

ARTICLES PRESENTED AT THE XV CBECIMAT, NATAL - RN, NOVEMBER/2002

Preparation and characterization of FeAg granular alloys

J. M. SoaresI, ^*; J. H. de Araújo^I; F. A. O. Cabral^I; J. A. P. da Costa^I; J. M. Sasaki^II

^IDepartamento de Física Teórica e Experimental, Centro de Ciências Exatas e da Terra, Universidade Federal do Rio Grande do Norte, 59072-970 Natal - RN

^IIDepartamento de Física, Universidade Federal do Ceará, 60455-970 Fortaleza - CE

ABSTRACT

Fe₁₀Ag₉₀ granular alloys have been fabricated, using a sol-gel method, for a range of nitric acid concentrations in the start solution. The samples have been characterized by X-ray diffraction and magnetization measurements. The average Fe particle sizes derived from X-ray diffraction are in the range 24-29 nm, indicating a large variation. The coercivity obtained from the hysteresis curve is two orders of magnitude larger that of pure iron in all the samples. Moreover, the hysteresis curves do not saturate, even in fields of up to 1 T. These observations indicate that the samples contain both superparamagnetic and blocked particles. A comparison between the coercive field and the average particle diameter, determined by the Sherrer's formula, is displayed in all acid concentrations.

Keywords: sol-gel, granular alloy, ferromagnetism

1. Introduction

Granular magnetic materials composed of magnetic nanoparticles embedded in a nonmagnetic matrix show interesting physical properties, such as superparamagnetism (SPM), giant magnetoresistance and giant magnetoimpedance^1-3. Moreover, they have important technological applications in magnetic recording, in optical devices and in sensors^4-7. Such materials have been produced as films by sputtering, as ribbons by melt-spinning, and in powder form by sol-gel methods^8,9 and mechanical alloying¹⁰.

The magnetic grain size of these materials varies from a few Å to hundreds of Å, with the grains randomly distributed in a nonmagnetic metallic matrix. When these alloys are prepared by melt-spinning or by sputtering methods, the mean grain size is very small (a few Å) displaying a predominantly SPM behavior. However, granular alloys produced by other techniques such as sol-gel and mechanical alloying contain both SPM and blocked (BL) particles^10,11,14,15, displaying a more complex magnetic behavior.

Magnetization and Mössbauer Spectroscopy measurements have been used to study magnetic properties of the granular alloys FeCu and FeAg in powder form^8,10. The average sizes of magnetic particles were estimated by the coercive field.

In this work a series of Fe₁₀Ag₉₀ granular alloys has been produced by a sol-gel method and characterized by X-ray diffraction, Mössbauer spectroscopy and magnetization. We have observed that in all samples there is a size distribution of both SPM and BL particles with a strong influence of changes in acid concentration.

2. Experimental

Fe₁₀Ag₉₀ granular alloys were produced by a sol-gel method^8,9. The start solution was prepared from an aqueous solution of the nitrates with five different nitric acid concentrations: 0.25, 0.50, 0.75, 1.00 and 1.25 ml, respectively, in 300 ml of mixture. The powder obtained was reduced in a hydrogen atmosphere for 45 min at a temperature of 400 ºC.

The crystalline structure of the sample was investigated by conventional X-ray diffraction using a Rigaku diffractometer with Mo-K_a radiation. The magnetization and hysteresis curves were measured in a vibrating sample magnetometer with a maximum magnetic field of 1 T. The Mössbauer spectrum was obtained in a conventional constant acceleration spectrometer with a ⁵⁷Co source in a rhodium matrix at room temperature.

3. Results and Discussion

Figure 1 shows the X-ray diffractometry patterns of Fe₁₀Ag₉₀ granular alloy reduced at temperature of 400 ºC. It is shows that the iron in the Fe₁₀Ag₉₀ is b.c.c. structure and the silver is 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, D_m= (0.9l)/[d(2q)cosq], where l is the Mo wavelength, d(2q) is the Fe(211) peak width, and q the scattering angle¹⁶. Figure 2 shows details of the spectral regions around the Ag (222) and Fe (211) Bragg peaks. Applying Scherrer's formula, we obtain the average Fe particle sizes D_m from the (211) line width for different nitric acid concentrations V_ac, as shown in Table 1. We observe that D_m increases with V_ac up to a concentration of 0.75 ml followed by a decrease.

Figure 3 shows the hysteresis curves at room temperature obtained for Fe₁₀Ag₉₀ granular alloy samples with different nitric acid concentrations. None of the samples reach saturation in fields up to 1 T, showing the presence of SPM particles. The coercive field, determined from the hysteresis curves, ranges from 354 to 430 Oe, which is much higher than that of pure iron, indicating fine single-domain particles.

Figure 4 shows the Mössbauer spectrum of the Fe₁₀Ag₉₀ (0.5 ml) sample. We can observe that the spectrum consists of two components, a singlet, characteristic of an SPM phase, and a sextet corresponding to the blocked particles in the ferromagnetic a-Fe. Observe that we were able to see the presence of SPM particles, even though the measurement time window of the Mössbauer effect is 10^-8 s.

Figures 5a, 5b, 5c, and ^5d show the change of the maximum field magnetization M_Hmax, the remanent magnetization M_r, the coercive field H_c, and D_m with acid concentration. M_Hmax increases linearly with acid concentration up to V_ac = 1.00 ml, then decreases. This shows that the acid concentration in the start solution significantly affects the maximum field magnetization and consequently the atomic dispersion in the sample. In Fig. 5b, we can see that the remanent magnetization increases linearly with V_ac, despite a small dip at V_ac = 0.75 ml. Figure 5c shows that H_c increases linearly with acid concentration, up to a maximum value (H_c = 434 Oe) at V_ac = 0.75 ml, then decreases linearly. In ^{Fig. 5d}, we can observe that D_m increases non-linearly with V_ac up to V_ac = 0.75 ml, followed by a non-linear decrease. There is thus not a perfect correlation between H_c and D_m, as there is in a uniform particle system. Therefore, there appears to be a distribution of Fe particles sizes whose width depends on the acid concentration. Since the H_c value depends directly on the number of blocked particles in the distribution, the coercive field ought to have a strong dependence on the distribution width, not just on the average particle sizes. However to obtain a correlation between the particles size distribution width and the coercive field more experimental and computational work is necessary. The distribution width varies between 1.14 and 1.67¹⁷.

The experimental results obtained from the hysteresis curve and Mössbauer spectroscopy measurements both indicate that the magnetization of the Fe₁₀Ag₉₀ samples is composed of two contributions: one from the SPM particles and other from the BL particles, as observed in other systems reported in the literature^11,14,15.

4. Conclusion

Samples of Fe₁₀Ag₉₀ granular alloys were produced in various concentrations of nitric acid. Magnetic measurements show that in all samples there was a size distribution of both SPM and BL particles. The magnetic properties of the samples are strongly influenced by changes in acid concentration.

Acknowledgment

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

Received: October 03, 2003; Revised: March 21, 2004

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Publication Dates

• Publication in this collection
  13 Aug 2004
• Date of issue
  Sept 2004

History

• Reviewed
  21 Mar 2004
• Received
  03 Oct 2003