Abstract
Several lanthanide elements and their compounds of heavy rare earths (RE) have attracted
much attention, especially in magnetism for their exotic spin structures. Indeed,
thanks to the implementation and improvement of adapted experimental tools, a variety
of new and original magnetic states have been discovered. Among others, there is
magnetic frustration that generates magnetic states with spin correlations. This work
focuses on systems with conditions prone to magnetic frustration: RE2TX3 (RE = Ce,
Pr, Sm, Gd, T = Ag, X = Si); Pr2Pd2X (X = In, Sn), RE2Cu2In (RE = Er, Tm),
and RE3Os4Al12 (RE = Ce, Gd). The study is done with the view to investigate how
a frustrated lattice impacts the spin correlation effects. The properties of the systems
RE2AgSi3 and RE3Os4Al12 (RE = Ce, Gd) are studied for the first time in the literature.
We also investigated in more depth the compounds Pr2Pd2In, Pr2Pd2Sn, Er2Cu2In,
and Tm2Cu2In. The RE2AgSi3 compounds crystallize in the α-ThSi2-type of tetragonal
structure. Gd2AgSi3, Pr2AgSi3, and Sm2AgSi3 all present long-range magnetic ordering
at low temperatures while Ce2AgSi3 is a short-range magnetic ordered material. In
particular, Gd2AgSi3 has two antiferromagnetic (AFM) phase transitions (TN1 = 9 K,
and TN2 = 17 K) consistently found and analyzed in dc-susceptibility χ(T), specific heat
CP(T), and electrical resistivity ρ(T). A long-range ferromagnetic (FM) ordering is observed
at TC ≈ 15 K in Pr2AgSi3, and a spin glass behavior with freezing temperature
Tf ≈ 13 K is also observed in the compound. Spin glass behavior was confirmed using
ac-magnetic susceptibility, time aging, and memory effect measurements in both zero
field cooled (ZFC) and field cooled (FC) modes. An asymmetric response during the
v
heating and cooling of the Pr2AgSi3 system is observed in the magnetic relaxation measurement
with a positive heating cycle in both ZFC and FC modes. Sm2AgSi3 however
shows large magnetic anisotropy confirmed by the large coercive field obtained in the
hysteresis loops. In Ce2AgSi3, magnetic feature such as strong curvature is observed in
the field magnetization. This behavior suggests the presence of spin correlations or lowlying
phase transition. A short-range order-like transition is observed in the specific heat
data at TC ≈ 4 K. The low value of the Sommerfeld coefficient (γ = 22 mJ/moleCe.K2)
suggests a metallic density of states at the Fermi energy. The compounds Pr2Pd2X (X =
In, Sn), RE2Cu2In (RE = Er, Tm) have also been studied. They all form in the Mo2B2Fe
Shastry-Sutherland lattice (SSL) which is an intrinsically frustrated system due to the
triangular arrangement of the RE atoms. An AFM behavior is observed in Pr2Pd2In
and Pr2Pd2Sn at TN ≈ 5 K and TN ≈ 2.5 K, respectively. These AFM orders are unstable
in applied magnetic fields in both compounds. Furthermore, the field-dependence
magnetization shows a metamagnetic behavior in both compounds with the critical field
of 1.5 T at 2 K for Pr2Pd2In and the critical field of 1.3 T at 2 K for Pr2Pd2Sn. The electronic
specific heat Sommerfeld coefficient values of Pr2Pd2In (γ = 235 mJ/molePr.K2)
and Pr2Pd2Sn (γ = 311 mJ/molePr.K2) estimated from the CP(T) data indicated that
the compounds belong to the heavy-fermion family. RE2Cu2In (RE = Er, Tm) are FM
systems below 40 K for Er2Cu2In and below 32.5 K for Tm2Cu2In. The ac-susceptibility
measurements reveal an evident spin glass transition for these two compounds at a static
spin freezing temperature of Tf = 40 K for Er2Cu2In and Tf = 31.1 K for Tm2Cu2In.
The shift of the freezing temperature as a function of frequency has been analyzed based
on the power-law divergence and Vogel-Fulcher law. The Mydosh parameter was estimated
to be 0.014 and 0.0008 for Er2Cu2In, and Tm2Cu2In respectively. These values
lie typically in the range of a canonical spin glass system. The RE family of RE3T4X12
type is of particular interest among intermetallics because the structure contains layers
as well as triangular and distorted Kagome lattice features. Therefore, we also studied
the RE3Os4Al12 (R = Ce, Gd). They crystallize in the hexagonal Gd3Ru4Al12-structure
type with space group P63/mmc. The temperature (T) dependent dc-magnetic susceptibility
(χ) reveals that the compound Gd3Os4Al12 undergoes a ferro- to AFM ordering
below 30 K. Ce3Os4Al12 presents a short-range order-like transition below 6 K observed
from the temperature (T) dependent dc-magnetic susceptibility χ(T). The obtained
Sommerfeld coefficient value of 100 mJ/mole.K2 reflects an enhancement of about 20
times over that of a normal metal. The metallic behavior is identified from the electrical
resistivity of both compounds with a characteristic of electron-phonon scattering in the
paramagnetic (PM) region. It has been found that, with a careful selection of RE elements,
we can have magnetically ordered ground states in frustrated lattices. Moreover,
intuitively, one may expect from first principles that the magnetic ordering in a frustrated
lattice is essentially FM. However, we also found that with a careful selection of
the magnetic species in the frustrated lattice, we can anomalously see magnetic ground
states such as ferromagnetism and antiferromagnetism, or even double phase transitions
and as an energy solution of the frustrated lattice: a spin glass state.