Regarding the magnetic properties of FH, the main results point to a cluster-like structure with Fe³⁺ ions antiferromagnetically ordered within the core and a small uncompensated magnetic momento (probably at the particle surface). But the actual physical units from which the observed cluster or superparamagnetic (SPM) behavior is originated have not been univocally defined. For antiferromagnetic (AFM) particles of few-nanometer diameter, a small uncompensated magnetic moment is expected to arise from defects within the particle and/or from the unpaired surface moments of the particle. The net magnetic moment of the particle reverses among different spatial orientations with a characteristic time τ that depends on temperature and particle volume.

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University of Zaragoza

Spinel Oxides (a.k.a Ferrites)

Ferrites are most common in nature. From the daily iron oxide Fe3O4 (magnetite) formed at the surface of many iron-containning objects (fences, cars, doors, barbecue grills, etc), we witness the effect of the atmosphere's oxygen acting on the surface of metal. These materials are quite simple from the structural point of view. However, their magnetic properties can greatly vary from one element to another, since the microscopic (atomic) structure is composed of two or more magnetic sub-units (sublattices).

The ionic distribution in this kind of structure may be represented by [M_dFe_1-d]^A[M_1-dFe_1+d]^BO₄, where d is the inversion parameter and d=0 and 1 stand for the inverse and normal cases, respectively. Although most spinel ferrites are cubic, there are some exceptions like CuFe₂O₄ that can have tetragonal unit cell symmetry if the sample is slowly cooled from high temperatures.

 Further References:

 For a more advanced view, go to:

· Amethyst Galleries

· The Institut Laue-Langevin (ILL) Site

· The Institute of Rock Magnetism Page on Magnetsm

· Physical Review FOCUS

 The spinel ferrite structure MFe₂O₄ can be described as a cubic close-packed arrangement of oxygen ions, with M²⁺ and Fe³⁺ ions at two different crystallographic sites. These sites have tetrahedral and octahedral oxygen coordination (A- and B-sites respectively), so the resulting local symmetries of both sites are different.

Magnetite  (Fe3O4)

Ferrihydrite (Fe₅HO₈ · 4H₂O)

)

Ferrihydrite (FH) phase is found in many biological systems (e.g., iron-reducing bacteria, Aedes aegypti mosquito), as well as in hydrometallurgical operations (as an undesired precipitate). Along with the interest of the basic physical mechanisms governing structure and surface formation, FH has also significance because it constitutes the core of ferritin, a protein that plants and animals use to sequester and store iron, providing a fully biocompatible material to carry iron particles in potential drug delivery applications.

This text on FH was extracted from

“Large magnetic anisotropy in ferrihydrite nanoparticles synthesized from reverse micelles”, E L Duarte, R Itri, E Lima Jr, M S Baptista, T S Berquó and G F Goya, Nanotechnology 17 5549-5555   doi:10.1088/0957-484/17/22/004

Electron Microscopy and Mossbauer spectroscopy have been employed quite successfully for identifying the properties of both kind of structures regarding their local structural and magnetic properties. The current landscape is that some aggregates with ordered structure, with small regions of ca. 2-3 nm, display definite lattice fringes and narrow distribution of hyperfine parameters.

Ferrihydrite is a poorly crystalline Fe³⁺ oxyhydroxide whose detailed crystal structure has remained elusive up to now, as reflected by the identification of the known phases as '2-line' and '6-line' on the basis of their two or six broad X-ray diffraction (XRD) peaks, as shown in the figure on the rigth. It has been proposed that the 2-line type consists of extremely small crystalline domains, whereas the 6-line has larger crystal domains of hexagonal structure with unit-cell with parameters a = 2.96 Å, c = 9.40 Å. More recently, Jansen et al.  have refined their XRD data by considering a space group P-31c phase plus a defective phase with P3 space group.

Site	Bhf (T)	IS (mm/s)	QS(mm/s)	G (mm/s)	I (%)
A	47.9(1)	0.48(1)	-0.06(2)	0.66(3)	53(6)
B	44.2(1)	0.43(1)	-0.04(2)	0.76(4)	47(5)

Hyperfine parameters of four ferrihydrite samples with increasing crystallite sizes from 2.8 to 4.0 nm

Magnetite Fe3O4 has recently attracted attention because bulk Fe3O4 has a high Curie temperature (TC ~ 850 K) and nearly full spin polarization at room temperature, both properties of great potential for applications in giant magnetoelectronic and spin-valve devices based on magnetite films. In spite of the fact that magnetite is perhaps the oldest magnetic material known (see here and here), some aspects on the basic mechanisms related to the Verwey transition are still being discussed, especially in nanosized systems.  An additional issue to be solved for potential applications is that the magnetic properties of magnetite-based nanostructured systems (particles or films) strongly depend on the synthesis route, as well as on the nonmagnetic matrix (or substrate) chosen. Moreover, for a given synthesis technique the final magnetic properties seem to depend heavily on tiny changes in the local structure like antiphase boundaries (APB), oxygen deficiency and local ionic disorder.

Above the Verwey temperature, T_V ~120 K, magnetite has a cubic spinel structure (space group Fd3m), with lattice parameter a₀ ~ 8.397 Å and the O atoms arranged in an face-centered cubic (fcc) lattice. The lattice can accommodate Fe³⁺ on the tetrahedral site (A) and Fe³⁺ and Fe²⁺ on the octahedral site (B) in antiparallel arrangement, yielding ferrimagnetic order below T_C. Bulk magnetite has cubic magnetic anisotropy, with the <1 1 1> and <1 0 0> directions being the easy and hard axes of magnetization, respectively. At room temperature (RT), the first-order magnetocrystalline anisotropy constant has a negative value (K₁ = -1.35x10⁵ erg/cm³) that changes sign at low temperature, passing through an isotropic point at a temperature few degrees above the Verwey transition. On cooling below T_V, the change from cubic to triclinic structure yields a change to uniaxial anisotropy with <0 0 1> easy axis. Since the magnetic hardness of single-domain particles results from the concurrent effects of shape and crystal anisotropies, there is an obvious need for separating these effects to study each of these alone.

The saturation magnetization values found in nanostructured materials are usually smaller than the corresponding bulk phases, provided that no changes in ionic configurations occur. Accordingly, experimental values for M_S in magnetite nanoparticles have been reported to span the 30-50 emu/g range, lower than the bulk magnetite value ~90 emu/g.  Many studies have been reported on the origin of the observed reduction in magnetization in fine magnetic particles. The first studies on the decrease in magnetization performed in g-Fe₂O₃ by Coey showed that this reduction is due to the existence of noncollinear spins at the surface, making the same mechanism appealing for Fe₃O₄.

The Mössbauer spectra of Fe₃O₄ particles with different sizes, at 296 show the gradual passage from bulk-like to SPM behavior, as the average particle size decreases. For sample with <d> = 150, the RT spectrum displays two magnetic components of hyperfine fields B_hyp = 49.5 T and 46.0 T, corresponding to Fe³⁺ ions at sites A and (Fe²⁺ Fe³⁺) ions at site B, respectively with nearly null quadrupole splitting QS and intensity ratio B:A ~ 1:1. These well known features of MS are related to the electron transfer process (electron hopping) between Fe²⁺ and Fe³⁺ ions on the octahedral B site taking place for T > 115–120 K (Verwey transition). The absence of SPM signal (doublet) in the MS shows that these particles have a multidomain structure, in agreement with the theoretical estimation for maximum linear dimensions of single-domain particles of ~128 nm.

As particle size decreases, both hyperfine fields merge into a single sextet (sample with <d> = 50 nm), and for sample with <d> =  10 nm the thermal relaxation effects are clearly observed. Sample with <d> 5 nm displays only a central doublet corresponding to a full superparamagnetic state. In this figure, the sample with <d> = 50 nm showed a small component from goethite (B_hyp = 37.7 T) amounting a ~3% of the total spectral area.

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