Spin-reorientation

Scientific awareness of a low-temperature transition in magnetite began in 1929 with the observation of a A-type anomaly in the specific heat at about 120 K. The anomaly was typical of an order-disorder transition, but it was well below the magnetic-ordering temperature Tc = 850 In 1931, Okamura observed an abrupt semiconductor-semiconductor transition near 120 K. The transition exhibits no thermal hysteresis, but the transition temperature is sensitive to the oxygen stoichiometry. More recent specific-heat measurements show the presence of two resolvable specific-heat peaks at the transition temperature the lower-temperature peak near 110 K appears to be due to a spin reorientation. 

Figure 8.5 (a) Zeeman-shifted energy levels, (b) Cross section for magnetic spin reorientation. 

F — ferromagnetic A — antiferromagnetic P — paramagnetic imp — impurity phase SR — spin reorientation Tq — Curie temperature Tn — Ndel temperature a and c — tetragonal lattice parameters, z — coordinate of B with c as its unit. 

Fig. 34. Temperature dependence of the hyperfine field components along the tetragonal b and c axes, (W, r)b and (Whf)c, of a GdNi2B2C sample, reflecting the temperature dependence of the corresponding components of the Gd magnetic moment. The lines leading to the ordering temperature Tn = 20 K and the spin reorientation temperature Tr = 14 K are guides for the eye (after Tomala et al. 1998).

Ivanova, T.I., Pastushenkov, Yu.G., Skokov, K.P., Telegina, I.V., Tskhadadze, I.A. (1998) Spin-reorientation transitions and magnetic anisotropy in TbFen-xCoxTi compounds, J.Alloys Comp. 280 20-25. 

The intrinsic magnetic properties of the Er2Fei4B intermetallic compound have been previously investigated [1], According to the literature data [1,2] the magnetic ordering temperature of this compound is Tc = 554 K. Er2Fei4B exhibits one successive spin reorientation transition (SRT) at about 325 - 327 K. 

The aim of this work is to study the effect of hydrogenation on the magnetic phase transitions (Curie and spin-reorientation transition temperatures) in the Er2Fei4B compound using the magnetization measurements with continuous control of the hydrogen content in the examined sample. 

Figure 3. The dependence of spin-reorientation transition temperature on the hydrogen pressure for Er2Fei4BHx.

The spin-reorientation transition in Er2Fei4B compound can be attributed to the competing of the uniaxial Fe sub lattice and the planar rare-earth sublattice anisotropy, with the former being dominant at higher temperatures and the latter being dominant at lower ones. 

Absorption and fluorescence do not require any spin reorientation. However, intersystem crossing and phosphorescence require a spin reorientation. Therefore, absorption and fluorescence are much faster than phosphorescence. Absorption occurs within a time equal to 1(T15 s, and the fluorescence lifetime goes from 10-9 to 10-12 s. Phosphorescence is a long transition that can last from milliseconds to seconds, minutes, or even hours. 

A further difference between rhodium(II) and cobalt(II) complexes becomes apparent from a study of the monomeric complexes. Whereas virtually all cobalt(II) complexes are high spin d1 species, with the exception of [Co(CN)5]3 and related species, the rhodium(II) complexes are all low spin complexes. Thus, because of the lack of spin reorientation required in forming a low spin rhodium(III) complex, they are excellent reducing agents. The stability of the rhodium-rhodium bond in the dimers prevents their facile oxidation. 

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