Hematite Morin temperature

Hematite is paramagnetic above 956 K (Tc). At room temperature it is weakly ferromagnetic and at 260 K (the Morin temperature, Tm), it undergoes a phase transition to an antiferromagnetic state. Particles smaller than about 8 nm display superpara-magnetic relaxation at room temperature. A plot of the dependence of the B f (Hi) of hematite on temperature is shown in Figure 6.7 the plot follows an approximate Brillouin curve. 

Fig. 6.7 Temperature dependence of the magnetic properties of hematite. Tc = Curie temperature,Tm = Morin temperature, pm = paramagnetic region, wfm = weakly ferromagnetic region afm = antiferromagnetic region. The insets show simulated Mossbauer spectra of hematite in the paramagnetic, weakly ferromagnetic and antiferromagnetic states (Murad, 1988, with permission).

Fig. 6.9 Left Relationship between the Curie (Tc) and Morin (Tm) temperature of hematite and the extent of structural substitution of Fe " by Al, Ga, Cr and In (M) (Svab Kren, 1979, with permission). Right Relationship between the Morin temperature and the inverse mean crystal dimension (MCD) for different hematites ( ) natural from Elba ( ) heating lepido-...

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). 

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... 

Substitution of Fe " " ions by Cr + ions in hematite affected the reduction of HMF and broadening of Mossbauer lines [253,255], as well as lowering of the Neel temperature [252,261]. The Morin transition was observed only for low substituted Cr-hematites—in samples with 4.3 mol% of substituted Cr or more the Morin transition no longer occurred [261]. 

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. 

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