Morin transition temperature

As shown in Fig. 7.6, the Mossbauer data show a reduction in Morin transition temperature with increasing shock severity. At temperatures below the transition, increasing shock severity causes greater retention of the higher temperature, weak ferromagnetic contribution. The measure of weak ferromagnetic (WF) fraction (the high temperature form) is a sensitive indication of shock modification. 

Arrangement of iron and oxygen atoms in the a-Fe203 structure where the circles represent Fe " " ion positions and the dashed lines indicate planes within which oxygen atoms lie. The rhombohedral unit cell is drawn with dashed lines and the arrows mark the orientation of Fe magnetic moments with respect to a coordination crystal system below the Morin transition temperature. (Adapted from Ref. 27 with permission of World Scientific Publishing Company.)... 

This region broadens considerably with decreasing particle dimensions and with increasing structural defects [91], and the coexistence of both the AF and WF phase even can extend down to 0 K. Such a case is represented in Fig. 3.11 where the relative areas (RA) and the quadrupole splitting do not change below 150 K. The Morin transition temperature can then be defined as that temperature at which the amount (RA) of AF phase is reduced to half of its initial value at low temperatures. 

The influence of the average particle size on the Morin transition temperature has been mainly investigated for synthetic samples and is represented in Fig. 3.12. From... 

Fig. 3.12 Morin transition temperature vs. inverse average particle size for differently prepared hematite samples Black square prepared from decomposition of lepidocrocite for the other symbols, see Ref, [99])...

Somewhat more pronounced effect of A1 substitution is reflected in the behavior of the Morin transition. With increasing A1 content the transition temperature Tm decreases and the transition region becomes significantly broader [97]. Moreover the Morin transition is completely suppressed at about lOat % A1 in bulk hematite [99] and even at somewhat lower concentrations (8 at %) for less crystalline hematite [100]. On the other hand, the effect of A1 on the Morin transition temperature is smaller in the case of more homogeneous A1 substitution in samples prepared from oxinates [103]. Using the aforementioned definition, the Morin transition temperature for as-such obtained hematite species decreases by 8 K per at % Al. Because the spectral implications of A1 substitution are quite similar to those of morphological effects, the separation of both effects remains a major problem and additional techniques are necessary for the characterization of natural samples. 

When a sufficiently large magnetic field is applied along the preferred axis, the so-called spin-flop reorientation occurs, i.e., a 90° rotation of the sublattice vectors, in the antiferromagnetic a-Fe203 nanosolids [165]. Both the spin-flop field, Hs.p (T — 0), and the Morin transition temperature (Tm) decrease with particle size in a way and approach to zero when the diameter is smaller than 8 nm, for spherical particles [166]. Table 14.3 features the size-dependent Hg.p and values. 

An interesting situation arises in the spectra of a-Fe203 where the angle

Morin transition). At temperatures below TM the parameter . equals +0.20 mm/s whereas, at the transition temperature, c changes sign and magnitude to a value of -0.10 mm/s [15], in agreement with Expression (5-5). 

The superparamagnetic properties of -Fe203 have also been studied via the Mossbauer effect. Nakamura et al, (32) have investigated the temperature dependence of the internal field in -Fe203 particles of approximately 50 A. diameter. At 120 °K. they obtain a spectrum which is almost identical to the bulk material, except that no Morin transition has occurred and the spins still lie in the basal plane. At room temperature the magnetic hyperfine spectrum collapses (even though the bulk... 

Both plane wave and LCCO methods can now reproduce the band gap and nature of the valence and conduction bands but only if recent improvements are used. Neither have yet managed to correctly predict both the band gap and the equilibrium structure. A further point to note is that the calculations in Table 1 refer to a-Fe203 at low temperatures. At the Morin transition the alignment of the spins with respect to the crystal axes changes. This effect can only be modelled by including spin-orbit coupling. 

A material such as hematite, that can host a WF magnetic structure, also is able to host a classic AF structure with magnetic sublattices along a different crystalline axis. A spin flop transition, known as the Morin transition in hematite, can occur where the AF axis abruptly changes from one crystal orientation to another, at a certain transition temperature. Such spin flop transitions are sensitive to sample features such as impurity chemistry and particle size and shape, as discussed below (Dang et al. 1998). 

There is a small apparent drop in the quadrupole interaction above the Morin temperature which suggests a small departure from axial symmetry and an asymmetry parameter of rj 0 03 [e(r < Tt /e T > Tm) = 1-94, not 2 00 as predicted]. The quadrupole splitting increases above the Neel temperature due to a small contraction along the trigonal axis. The chemical isomer shift does not show any irregularities at the Morin transition (Fig. 10.3). 

The system (Rh Fei x)203 has been examined for x = OT1,0-22,0-25,0-41, and 0-81 [114]. It has the homogeneous haematite structure with randomisation of the cations. The Neel temperature falls with increasing x while the Morin transition rises. For a-Fc203 the Morin transition is 257 K, but at X = OTl it is 507 K and at x = 0-22 it is 544 K. Significant collapse of the hyperfine splitting is seen below the Neel temperatures. 

There are three phase transitions in the alloys Fei cCO cS (0 < x < 0.13) within the temperature range from 27 to 800°C [1981Ter] 1) a metal - semiconductor transition 2) a spin-rotation transition and 3) a transition from an antiferromagnetic to a paramagnetic state. The spin-rotation transition (Morin transition at about 130°C) proceed in a certain temperature interval. Increasing of Co content leads to an increase of the Neel temperature (347°C Xx = 0.09 and 407°C stx = 0.13). 

It is well known, and also well documented, that iron oxides can be prepared in the form of nanoparticles. The majority of such studies [26-39] have concentrated on the size effects on the magnetic properties in different iron-oxide phases. Typically, it has been found that transition temperatures decrease with decreasing particle size. For example, in hematite the Morin transition shifts from Tm= 263 K in bulk to temperatures below 4K in particles smaller than 8-20 nm [33]. It is interesting to note that magnetic anisotropy rapidly increases for particles with diameters less than... 

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