Field-induced magnetic phase transitions and correlated electronic states in the hexagonal RAgGE and RPtIn series Page: 45 of 283
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known; we can enumerate a few where only two metamagnetic transitions have been observed
(DyCo2Si2 Iwata, 1990], ErGa2 [Doukour4, 1982], CeZns [Gignoux, 1987]). More recently a
number compounds have been shown to have several more complex metamagnetic transition,
and they are discussed in some detail in the next section.
2.2.3 Metamagnetic transitions in tetragonal compounds with magnetic moments
in tetragonal point symmetry
One simple case of an extremely anisotropic antiferromagnetic compound is that of TbNi2Ge2
(Bud'ko, 1999), a tetragonal compound with Tb ions in tetragonal point symmetry. At low
temperatures, the crystalline electric field (CEF) anisotropy confines the local moments along
the c ([001]) crystallographic axis (Ising-like system). Up to six metamagnetic transitions are
observed, with the critical field values Hei, having a 1/ cos 8 dependence on the angle between
the applied field H and the c-axis. It is also shown that the locally saturated magnetization val-
ues vary like Mi,sat * cos 0, and from the Mi,sat values, the net distribution of moments for each
metamagnetic state can be inferred. Both of these angular dependencies are a consequence of
the extremely axial nature of the CEF splitting of the TbNi2Ge2 ground state.
A more complex situation is encountered when the magnetic moments are allowed more
degrees of freedom , i. e. when the CEF anisotropy constrains them to an easy plane. This
is the case in the tetragonal compounds RNi2B2C (Cho, 1996b; Canfield, 1997a,b), with R =
Tb - Er, and RAgSb2 (Myers, 1999a) for R = Dy. The R ions are again in tetragonal point
symmetry and the local moments are confined to four equivalent [110] or [100] crystallographic
directions; thus the angular dependent magnetization measurements, when the field is applied
in the basal plane, reveal the four-fold anisotropy of the longitudinal magnetization that reflects
the symmetry of the unit cell. The angular dependencies of the locally saturated magnetiza-
tions Msat and the critical fields Hey could be treated by simple analysis, and plausible net
distribution of moments could be inferred for each metamagnetic phase (Canfield, 1997a,b).
As an example, Fig. 2.10a shows a subset of the M(H) T = 2 K isotherms, for various orienta-
tions of the applied field within the basal plane. Up to three metamagnetic transitions can be
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Morosan, Emilia. Field-induced magnetic phase transitions and correlated electronic states in the hexagonal RAgGE and RPtIn series, thesis or dissertation, August 1, 2005; Ames, Iowa. (https://digital.library.unt.edu/ark:/67531/metadc783252/m1/45/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.