Disordered local moment , spin

Figure 7.3. Planar spin spiral energetics for bcc Fe, fee Co, and fee Ni directly calculated from the BGFM (filled symbols) and evaluated using the magnetic force theorem with ferromagnetic (FM-MFT, dashed lines) and disordered local moment reference states (DLM-MFT, continuous lines). Energies are measured relative to the NM energy, and the moment is fixed to the ground state moment.

In addition to substitutional randomness, the DMSs are characterized also by some degree of magnetic disorder" which we treat in terms of the disordered local moment (DLM) method" that can be naturally incorporated in the CPA the Mn atoms have collinear, but random spin-up (Mn ) and spin-down (Mn ) orientations. It is worthwhile to mention that for total energy calculations it is not necessary to consider all possible 25-1-1 projections of the atomic spin it is sufficient to carry out the calculations for only two extremal values, namely 5 . The corresponding concentrations x and a fnlfill the condition x = +x. The degree of magnetic disorder... 

Ramirez et al (1970) discussed a metal-insulator transition as the temperature rises, which is first order with no crystal distortion. The essence of the model is—in our terminology—that a lower Hubbard band (or localized states) lies just below a conduction band. Then, as electrons are excited into the conduction band, their coupling with the moments lowers the Neel temperature. Thus the disordering of the spins with consequent increase of entropy is accelerated. Ramirez et al showed that a first-order transition to a degenerate gas in the conduction band, together with disordering of the moments, is possible. The entropy comes from the random direction of the moments, and the random positions of such atoms as have lost an electron. The results of Menth et al (1969) on the conductivity of SmB6 are discussed in these terms. 

Mossbauer spectroscopy is also able to give local moment orientations, with respect to the crystalline lattice, or the correlations between moment orientations and local distortion axis orientations in a chemically disordered or amorphous material. This arises from the interplay between the structural (electric field gradient) hyperfine parameters and the magnetic hyperfine parameters. In this way, the spin flop Morin transition of hematite, for example, is easily detected and characterized (e.g., Dang et al. 1998). The noncollinear magnetic structures of nanoparticles can also be characterized. 

In crystals with order-disorder phase transitions the magnetic moments (spins) of local centers or other local low-symmetry units (e.g., dipole moments) may have random orientations in space in the higher-symmetry phase, and they become ordered along one direction at the phase transition to the lower-symmetry phase induced by their intercenter interactions. But in many cases SSB in structural phase transitions take place in the absence of such apparent polarized or otherwise... 

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