Spontaneous moment

The condition for localized electrons with spontaneous atomic moments is U > w that for itinerant electrons with no spontaneous moment is Un < w. The intermediate case Un = w is of considerable theoretical interest. In the case of iron oxides, a U5 = 3 eV ensures a localized 3 d majority-spin configuration at both Fe " and Fe " ions since the cubic-field splitting Ag < Ag, is small enough to leave the ions in the high-spin state. However, localization of the minority-spin electron, particularly in the mixed Fe / Fe " state, does not necessarily follow. Similarly, a much smaller U4 will be seen to make U4 = w for Fe " in the perovskites A Fe03. 

Because of domain formation, which permits minimization of internal demagnetizing fields at the expense of a fraction of the coupling energy, it is usually necessary to align the moments of the various domains by an external field in order to determine the magnitude of the spontaneous moment. 

Low-temperature magnetic properties are listed in table 5. The values of the spontaneous moments (Ms) listed pertain to single-crystal data. Exceptions are the values given for La2Fe14B and Lu2Fe14B data obtained in high fields on aligned powders have been used for these compounds. 

Other UX2 compounds with different and less-closely packed structures are ferromagnets with higher Curie temperatures and larger 5f moments than UNi2. A typical example is UGa2, the transition temperature of which is as high as — 125 K (spontaneous moment fis = 2.71 ib/U, dv u — 400 pm). 

Pressure effect on magnetism in U and Np intermetallics. iis is the spontaneous moment 7 CN the magnetic ordering temperature X4.2K. the... 

Obtained on a polycrystalline sample with a considerable amount of preferred orientation. f Spontaneous moment per U atom (ref. [16]). e / . increases rapidly and non-linearly with increasing pressure, reaching 5.9 K at 11.3 kbar. h The value could be of the order of magnitude of 100. 

No single crystals could be manufactured due to principal difficulties (the compound is formed by a peritectoid solid-state reaction which prevents a growth from the melt). The discrepancy between the bulk spontaneous moment [(0.4-0.45))iB/U] obtained for polycrystalline samples at 4.2 K, and the microscopic moment (0.8/xB) derived from neutron diffraction (Frings and Franse 1985a) point to a high uniaxial magnetic anisotropy. This may account for the lack of saturation of the magnetic isotherms in fields up to 35 T. 

UNi2 behaves as a weak itinerant ferromagnet. The value Tc — 20-21 K was determined on polycrystals (Sechovsky et al. 1980a, Havela et al. 1980) while Tc = 27 K (Frings et al. 1986) was found on a single crystal. The most appropriate value, however, would be Tc = 24 K which corresponds to a peak in the C(T) data (Frings et al. 1986). A very low spontaneous moment jus = 0.08jaB/f.u. locked in the... 

PuPt2 is ferromagnetic with Tc = 6 K and has a small spontaneous moment of 0.2/iB/f.u. (Nellis and Brodsky 1972, Harvey et al. 1973). The susceptibility and resistivity are shown in figs. 3.14a and 3.14b, respectively. No magnetic data are known for AmPt2 and CmPt2. 

UTX compounds with Caln2 structure. CAF denotes a canted antiferromagnet, AF an antiferromagnet, F a ferromagnet rC N the temperature of magnetic ordering p.s the spontaneous moment geff 6p, and Xo the CW parameters y the linear coefficient of specific heat. 

Fig. 5. Magnetic cluster spontaneous moment, /a, in Bohr magnetons, as a function of temperature. The solid line is the curve from molecular field theory for spin 5.

Pyrochlores of the type A2Mn2 07 are known for Dy, Ho, Eu, Tm, Yb, Lu, Y, Sc, In and T1 (Subramanian et al. 1988). All exhibit ferromagnetic behavior (table 6). On the other hand, there are indications that these compounds do not possess a spontaneous moment in the absence of an applied field. 

To decide whether the magnetism of RMnjXj compounds may be discussed in terms of itinerant or localized moments, the ratio njn should be estimated. The and n, are the magnetic carrier numbers derived from the Curie constant and the spontaneous moment at zero K. The njn ratio in the range of 1.0-2.87 indicates that the magnetic properties of RMn2X2 compounds cannot be explained in terms of the itinerant model only. 

Fig. 14.92. The Curie temperatures and spontaneous moments of Y2(Fe, Co)n and Y2(Co, Ni)i7 as a function of electron concentration. Note the resemblance to the Slater-Pauling curve (fig. 14.67). (Taylor and Poldy, 1975).

Fig. 14.114. Variation as a function of electron concentration of the spontaneous moment Ms, moment Mm at 160 kOe, and critical field He of the Y(Fe,Co)3 and Y(Co, Ni), systems (Poldy, 1972).

Hi = internal magnetic field corrected for demagnetization M = bulk magnetization M, = saturation bulk magnetization Hfff = magnetic hyperfine field [Pg.260]

Fig. 16.8. Spontaneous moments for amorphous and crystalline GdFe2 as a function of temperature (Burzo, 1971 and Rhyne et al., 1974b).

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