Magnetic properties of Co3O4 nanoparticles

doi:10.1016/S0304-8853(02)00050-1

Journal of Magnetism and Magnetic Materials

Volume 246, Issues 1–2, April 2002, Pages 184-190

https://doi.org/10.1016/S0304-8853(02)00050-1 Get rights and content

Abstract

Magnetization and magnetic relaxation measurements of 200 Å size Co₃O₄ particles are reported. Zero-field-cooled (ZFC) susceptibility exhibits a sharp peak characterizing a phase transition at T_t=∼25 K. Above T_t, the reversal magnetizations vary linearly with H and the magnetizations scale with H/(T+θ) as would occur for an antiferromagnetic material above its Néel point. Below T_t, both ZFC and field-cooled hysteresis loops exhibit a large coercivity (∼600 Oe at 5 K) and the irreversibility in loops are found up to H=35 kOe. Exchange fields of several hundreds Oe are observed at low temperatures. Dependencies of the coercivity and the exchange field on both the temperature and the cooling magnetic field are studied. Magnetization relaxes according to the equation M(t)=M(t₀)[1−S(T)ln((t+t₀)/τ₀)]. Temperature dependence of the magnetic viscosity S(T) is thus determined for 1.8⩽T⩽40 K. S is temperature independent for T<10 K probably due to magnetic quantum tunneling, whereas it increases linearly with T in a limited range 8⩽T⩽20 K due to thermal effects. Around T_t, S exhibits a very sharp maximum which indicates a phase transition.

Introduction

The study of magnetic properties of nano-sized particles is of great importance from basic as well as applications points of view [1], [2]. Néel has originally pulled the attention to the properties of antiferromagnetic nanoparticles (AFN). Néel suggested that AFN are likely to possess induced permanent magnetic moments, which depends on the lack of an internal structural perfection and/or surface spin unbalance [3]. Also, AFN could exhibit superparamagnetic relaxation of their spin lattices. Since then, the main theme of the previous work on AFN has been devoted for the origin of particle moment.

Another interesting small particle effect, exchange anisotropy, was discovered in 1956 by Miklejohn and Bean [4]. This effect is observed in partially oxidized ferromagnetic (FM) Co-particles. Despite the large body of experimental work reporting this effect in various thin film and particle systems [5], [6], [7], the mechanism is still elusive. The unusual properties of FC Co–CoO particles are explained in terms of exchange coupling between the spins of FM Co and antiferromagnetic (AF) CoO at the interface between them. On cooling a specimen through the AF Néel temperature in the presence of an applied magnetic field, the FM layer displays a unidirectional anisotropy results in a shift of the hysteresis loop from zero on the field axis by an amount H_e called the exchange field. Indeed, with few exceptions on magnetic hysteresis [8], [9], [10], the hysteresis behavior as well as the exchange anisotropy of AFN has not been investigated. Very recently, we have observed the exchange anisotropy in NiO AFN [11], [12], [13] and horse spleen ferritin [14], most probably due to exchange coupling of the frozen moments associated with uncompensated surface spins and the AF core. Giant loop shift, more than 10 kOe, is reported for NiO 314 Å particles at 5 K [11], [13].

A third interesting property of fine particles is their low-temperature time dependence of magnetization (magnetic relaxation) [2]. At low temperatures and for a thermally activated decay mode, the relaxation rate is proportional to the temperature [15], [16]. This phenomenon has received, however, much attention after the prediction of magnetic quantum tunneling at very low temperatures [17], [18]. In particular, the subsequent observation of this phenomenon in various systems has been recently reported [19], [20], [21], [22], [23], [24], [25], [26], [27]. AFN has been believed to be the best candidate for easier observation of such quantum tunneling [28].

As a part of our study of AFN, we have done DC magnetization and magnetic relaxation measurements on 200 Å size Co₃O₄ particles.

Section snippets

Experimental

The cobalt hydroxide precursor was chemically precipitated by mixing a cobalt nitrate Co(NO₃)₂ 6H₂O aqueous solution and an aqueous sodium hydroxide (NaOH) solution whose pH was ∼12 at 296 K. The resulting pink gel was washed several times by distilled water until free of nitrate ions. The gel color turned brown during this process. The precipitate present at this moment is centrifuged and the hydrolyzing process of the solvent is done at 383 K for several hours. During the hydrolyzing process

Results and discussion

X-ray diffraction (XRD) pattern of the prepared particles indicates the formation of the single spinel phase without any observable traces of neither Co hydroxides nor Co monoxide. The lattice constant is found to be 8.082 Å. Particle size was calculated from XRD line broadening after correcting for instrumental broadening and is found to be 190 Å. The specific surface area obtained from BET measurement was 46.8 m²/g. Assuming spherical shape of the particles and using the density of bulk Co₃O₄,

Conclusion

The magnetic properties of 200 Å size Co₃O₄ nanoparticles are reported. Above T_t=∼25 K, magnetizations are perfect linear and scale with H/(T+θ). Below T_t, the FC hysteresis loops exhibit simultaneous coercivity and loop shift of few hundred Oresteds. Both of the coercivity and the loop shift decrease with increasing temperature and diminish at T_t. The magnetic viscosity varies nonlinearly with temperature. Around T_t, S exhibits a very sharp maximum characterizing the phase transition. The

Acknowledgements

The author is indebted to Prof. Dr. Ami Berkowitz at CMRR, UCSD for his valuable discussions.

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¹: Department of Physics, Faculty of Science, Assiut University, Assiut 71516, Egypt.

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Magnetic properties of Co3O4 nanoparticles

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgements

Phys. Rev.

Phys. Rev.

J. Appl. Phys.

J. Appl. Phys.

J. Appl. Phys.

Nanostructured Mater.

J. Phys. Soc. Jpn.

J. Magn. Magn. Mater.

J. Magn. Magn. Mater.

Magnetic properties of Co₃O₄ nanoparticles