Ferromagnetic spiral

Fig. 42. Structural and magnetic phase diagram tor the Co Ni MnGe system. Tu is the temperature of the first-order structural transition, the Neel temperature, Tc the Curie temperature. Structures SS, simple spiral CS, cycloidal spiral FS ferromagnetic spiral (Zach et al. 1984, Niziol et al. 1982).

The magnetic term in the heat capacity is what would be expected from the ferromagnetic spiral (i.e. the cone) structure of erbium (Kaplan 1961) and has been supported by the discovery of a linear spin-wave dispersion law along the c-axis in neutron scattering experiments (Nicklow et al. 1971). 

Therefore, the cubic phase is metallic below as well as above T, and the room-temperature Mossbauer spectra can be treated as an averaged charge state. At 4.2 K, in the AFM phase below Tn, Mossbauer parameters can distinguish Fe(III) from Fe(IV) ions, but the isomer shifts at the Fe(III) remain anomalously low [240]. The ferromagnetic-spiral spin configuration with q [111] found [48, 49] below Tn = 134 K in SrFeOs is probably retained throughout the cubic phase. 

The c-axis spiral has been successfully described in a quasi-linear mean field model taking into account crystalline electric fields and the RKKY interaction and supposing the presence of the ferromagnetic sheets (Amici and Thalmeier, 1998). Furthermore, in a small temperature range above Tn an a-axis modu-... 

Amici and Thalmeier (1998) used the quasi one-dimensional model mentioned in Section 4.9.1. In their approach the presence of ferromagnetically ordered Flo layers with the magnetic moments oriented perpendicular to the tetragonal c-axis is adopted and the competition of the RKKY interaction along the c-axis with the crystalline electric field is analyzed in order to determine the transition between the commensurate antiferromagnetic structure and the incommensurate c spiral shown in Figure 39. 

The parameters rp and 7J2 are linear combinations of the magnetic modes converted on the representation T5. The fact that below TN the magnetic subsystem of copper metaborate forms an easy-plane weak ferromagnet, twisted below 7] in a spiral, permits to compose rp as a combination of the ferromagnetic modes (1) and (3), and tj2 as a combination of the antiferromagnetic modes (2) and (4). Accordingly II = (Hn, Hn) = (Hx, -Hy). It is necessary to note that in the thermodynamic potential given by Eq. (5) the order parameter responsible for the transition at 7] is not chosen in an explicit form as it was done in our previous paper [8],... 

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.

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