Research articlesHematite: Morin temperature of nanoparticles with different size
Introduction
The ferric oxide (Fe2O3) at ambient conditions occurs in four crystalline polymorphic forms that have distinctly different structural and magnetic properties: α-Fe2O3 (hematite), γ-Fe2O3 (maghemite), β-Fe2O3 and ε-Fe2O3 [1], [2], [3].
Nanoparticulate α-Fe2O3 represents a cheap and thermodynamically stable material with a high specific surface that is well utilized in a wide spectrum of applications – e.g. as selective gas sensors based on the change of its surface resistance [4], or as reusable catalysts for organic transformations [5]. A colloidal suspensions of functionalized nanoparticles (NPs) were even proposed as soft tissue adhesives [6]. Taking advantage of its VIS photon absorption in the range of 295–600 nm and large absorption capacity up to 15% [7], the enhanced photo-electrochemical performance makes the hematite-based materials suitable for solar water splitting [8].
Due to its thermodynamic stability, a mineral hematite as the stable iron oxide polymorph often occurs at and below the Earth's surface. It crystallizes with the symmetry of the R-3c space group, with lattice parameters a = 5.036 Å and c = 13.749 Å, and six formula units per unit cell [9]. Bulk (crystalline) α-Fe2O3 shows a weak ferromagnetic (WF) spin ordering due to the Dzyaloshinsky-Moriya [10], [11] mechanism below Néel temperature TN ≈ 950 K [12]. At Morin temperature [13] TM∼264(2) K generally accepted for bulk material [14], it undergoes a spin-reorientation transition to an antiferromagnetic (AF) spin ordering. Below TM, the antiferromagnetically arranged spins are lying along the hexagonal c-crystallographic axis as well as the main axis of the electric field gradient (Vzz component of EFG tensor), present at the Fe site. Above TM, the spins are reoriented by ∼90° so as to lie approximately in the basal plane as it was demonstrated by an early neutron diffraction study [15]. The canting of about ∼5° leaves a weak ferromagnetic moment pointing in the direction perpendicular to this plane [16], [17], [18]. The coexistence of WF and AF phases was also evidenced by Mössbauer spectroscopy of microcrystalline (5–75 nm) and bulk α-Fe2O3 at the Morin transition [19], as this method provides the precise information on the hyperfine interactions in the system.
It was observed that magnetic field, pressure, particle size and lattice strain originating in crystal defects affect the temperature of Morin transition [20]. The value of TM generally decreases with reducing the volume [14], [21], [20], [22], and also the spin-flop transition field increases with increasing crystallite size as shown in acicular hematite nanoparticles in [23]. For particles smaller than approximately 20 nm, the Morin transition may be even suppressed and disappears entirely as near-surface spins deviate strongly from the antiferromagnetic easy axis [21].
The size dependence of Morin temperature of the nanoparticles can be described by a finite-scaling theoretical model [24], [25], [22]:where TM(∞) is the Morin temperature of bulk material, ξ0 the correlation length in bulk material at temperature T away from the ordering temperature, d the particle diameter and λ a scaling parameter. Here, the scaling parameter λ is equal to 1 for MF theory [26], 1.4 for 3D Heisenberg model and 1.6 for Ising spin system [27], [28]. The empiric relation derived from the mean-field approach [14] is
In this paper, we report an investigation on nanoparticles of pure α-Fe2O3 differing in the mean size of the crystallites in order to clarify a dependence of Morin temperature on the size of particles. Both the synthesis procedure and their structural and magnetic properties are thoroughly discussed. Nanoparticles were studied by means of transmission electron microscopy (TEM), X-ray diffraction (XRD), SQUID magnetometry, and transmission Mössbauer spectrometry (MS).
Section snippets
Sample preparation
Samples of pure α-Fe2O3 phase differing in the mean size of the nanoparticles were prepared by two distinct chemical techniques. Three samples of the nanoparticles with mean diameter below 100 nm were prepared by hydrothermal method described in [25] using following amounts of reactants: 10 mmol (400 mg) of sodium hydroxide, 20 mmol (5.5 g) of oleic acid and 5 mmol (810 mg) of iron chloride. The preparation was carried out in the mixture of 15 ml of alcohol (ethanol/1-pentanol) and 20 ml of
Theoretical model for Morin transition with a size distribution of particles
To model the gradual transition of an ensemble of particles from AF to WF ordering, one has to embed a distribution of particle sizes into the starting theoretical relation (1). The particles prepared by chemical routes tend to have a log-normal distribution [33]with the mean particle diameter <d> = d0.exp(σ2/2), variance V = d02.(exp(2σ2) − exp(σ2)) and polydispersity index s = (exp(σ2) − 1)1/2. The fraction of particles that at a given temperature T already passed
Transmission electron microscopy
The experimental distributions of the equivalent diameter, minimal and maximal projections of the particles, obtained from the TEM micrographs see examples in Fig. 1, exhibit the log-normal profile according to expression (3) as can be seen in Fig. 2. The definition of the equivalent diameter and the maximal and minimal projections, used in Fig. 2 are given and elucidated in Fig. S2 in the Supplementary Information.
Preparation by hydrothermal technique led to small spherical particles with
Conclusions
Various samples of α-Fe2O3 phase differing in the mean size of nanoparticles were prepared by the sol-gel or hydrothermal routes. The distributions of nanoparticle sizes were determined from TEM; their fitting indicated log-normal type. Morin temperatures and transition widths in prepared samples were determined using the temperature dependence of magnetization after cooling in zero and non-zero magnetic field with an applied field of 100 Oe. By combination of magnetic measurements, Mössbauer
Acknowledgments
This work was supported by the Czech Science Foundation (GACR) [grant number 16-04340S]. Magnetic experiments were performed at Materials Growth & Measurement Laboratory (MGML) supported within the program of Czech Research Infrastructures (https://mgml.eu).
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