Non-equilibrium cation influence on the Néel temperature in ZnFe2O4

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Abstract

We present results on the thermal dependence of the magnetization above ambient conditions of ZnFe2O4 nanoferrites having different degrees of inversion (c  0.05–0.40) and grain sizes (D  6–50 nm). The disordered ferrites present a net magnetization in the 400–510 K range that mainly depends on the degree of inversion. The reciprocal magnetization deviate from different temperature values of the linear behavior showed at high temperatures. Our results show that, contrary to similar nanoferrite systems, a simple scaling law cannot be applied to explain the Néel temperature TN dependency with the grain size. These results can be interpreted in terms of the lack of correlation between size and inversion that occurs in ZnFe2O4 and the strong TN dependence on the superexchange JAB interaction strength. A model assuming a random distribution of superexchange interactions can reproduce the Curie–Weiss temperature and the saturation magnetization values of the disordered ferrites.

Introduction

The antiferromagnetic (AF) ordering that occurs in normal ZnFe2O4 at the Néel temperature TN = 10 K has been attributed to Zn2+ and O2− defects rather than to its intrinsic behavior [1]. In any case, the AF configuration is driven by JBB superexchange interactions between Fe3+ ions at octahedral B sites since the totality of diamagnetic Zn2+ ions are preferentially located at tetrahedral A sites. Numerous works have reported that nanosized ZnFe2O4 presents a non-equilibrium distribution of cations where Zn2+ and Fe3+ occupy both A and B sites [2], [3], [4]. This gives rise to JAB interactions that changes its long range ordering to a ferrimagnetic one with TN  460–490 K [3], [5]. A TN of about 120 K was reported for ZnFe2O4 nanoparticles of 20 nm, although in this case it is difficult to distinguish whether it actually corresponds to the Néel temperature or to the blocking temperature of the particle moments [6]. Studies related to the magnetism of disordered ZnFe2O4 have been mainly focused on its low temperature behavior and, to our knowledge, only one work has reported on its magnetization above ambient conditions [5]. However, the exploration of the magnetic behavior of disordered ferrites above room temperature can provide useful information for some ferrite applications. For instance, potential hyperthermia applications might require the control of TN that guarantees a ferrimagnetic response a few tens of degrees above room temperature [7].

We present here an investigation about the temperature dependence above room temperature of the magnetization of ZnFe2O4 nanoferrites having different degrees of inversion and sizes. Our results and others from the literature are compared with theoretical predictions by assuming a random distribution of superexchange interactions.

Section snippets

Experimental

Nanocrystalline ZnFe2O4 (2ZF) was obtained by a hydrothermal procedure. Samples 2ZF4H and 2ZF10H were obtained after high energy ball milling (HEBM) 2ZF for 4 and 10 h [4]. Nanostructured ferrite (BZF10H) was obtained by HEBM bulk ZnFe2O4 for 10 h. The milling was carried out in a horizontal miller with a stainless steel vial and ball (mass to powder ratio of 10:1). Sample 2ZF10H-T was obtained after annealing 2ZF10H under oxygen flow from ambient up to 673 K (during 60 min), 773 K (60 min) and 873 K

Results and discussion

The X-ray diffraction studies showed that all the samples consist of ZnFe2O4 cubic spinel. The grain sizes D are shown in Table 1 [4], [8]. The degree of inversion, c, defined as fraction of iron ions at site A, was estimated from EXAFS data [4], [8] (Table 1).

Fig. 1 shows the hysteresis loops taken at room temperature. We observe that no matter HEBM is causing an increment or a reduction of the grain size, it always produces an enhancement of the inversion and, consequently, an increase of the

Acknowledgments

Financial support by CONICET (PIP 6524, PIP 6075); ANPCyT Argentina (PICT 38337) is acknowledged. SJS is member of Carrera del Investigador, CONICET, Argentina.

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