Study of rare earth local moment magnetism and strongly correlated phenomena in various crystal structures

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Kong, Tai
Major Professor
Paul C. Canfield
Committee Member
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Physics and Astronomy
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Benefiting from unique properties of 4f electrons, rare earth based compounds are known for offering a versatile playground for condensed matter physics research as well as industrial applications. This thesis focuses on three specific examples that further explore the rare earth local moment magnetism and strongly correlated phenomena in various crystal structures.

The first two Chapters of the thesis will be dedicated to general introductions to rare earth physics and to quasicrystals that are related to the forthcoming Chapters. Chapter 3 describes the high-temperature solution growth technique for single crystal synthesis as well as the measurement techniques that were used during the course of this thesis research.

We then look at how local magnetic moments behave in a quasicrystalline lattice. Chapter 4 presents the discovery and characterization of i-R-Cd (R = Y, Gd-Tm), icosahedral quasicrystals, six of which belong to the world’s first family of magnetic, binary quasicrystals. We first show how these quasicrystals were discovered via high temperature solution growth utilized as an exploratory tool. We then present a detailed characterization of i-R-Cd (R = Y, Gd-Tm) by means of x-ray diffraction, temperature dependent dc and ac magnetization, temperature-dependent resistance and temperature-dependent specific heat measurements. i-Y-Cd is weakly diamagnetic and manifests a temperature-independent susceptibility. i-Gd-Cd can be characterized as a spin-glass below 4.6 K via a dc magnetization cusp, a third order non-linear magnetic susceptibility peak, a frequency-dependent freezing temperature and a broad maximum in the specific heat. i-R-Cd (R = Ho-Tm) is similar to i-Gd-Cd in terms of features observed in thermodynamic measurements. i-Tb-Cd and i-Dy-Cd do not show a clear cusp in their zero-field-cooled dc magnetization data, but instead show a more rounded, broad local maximum. The resistivity for i-R-Cd is of order 300 Ã ÂµΩ cm and weakly temperature-dependent. The characteristic freezing temperatures for i-R-Cd (R = Gd-Tm) deviate from the de Gennes scaling, in a manner consistent with crystal electric field splitting induced local moment anisotropy.

Chapter 5 focuses on the search for a hexagonal system that exhibits a strong planar magnetization with a 6-state-clock, in-plane magnetic anisotropy. In Chapter 5, we look at a specific system, RMg2Cu9. Single crystals of RMg2Cu9 (R=Y, Ce-Nd, Gd-Dy, Yb) were grown using a high-temperature solution growth technique and were characterized by measurements of room-temperature x-ray diffraction, temperature-dependent specific heat and temperature-, field-dependent resistivity and anisotropic magnetization. YMg2Cu9 is a non-local-moment-bearing metal with an electronic specific heat coefficient, γ ∼ 15 mJ/mol K2. Yb is divalent and basically non-moment-bearing in YbMg2Cu9. Ce is trivalent in CeMg2Cu9 with two magnetic transitions being observed at 2.1 K and 1.5 K. PrMg2Cu9 does not exhibit any magnetic phase transition down to 0.5 K. The other members being studied (R=Nd, Gd-Dy) all exhibit antiferromagnetic transitions at low-temperatures ranging from 3.2 K for NdMg2Cu9 to 11.9 K for TbMg2Cu9. Whereas GdMg2Cu9 is isotropic in its paramagnetic state due to zero angular momentum (L=0), all the other local-moment-bearing members manifest an anisotropic, planar magnetization in their paramagnetic states. To further study this planar anisotropy, detailed angular-dependent magnetization was carried out on magnetically diluted (Y0.99Tb0.01)Mg2Cu9 and (Y0.99Dy0.01)Mg2Cu9. Despite the strong, planar magnetization anisotropy, the in-plane magnetic anisotropy is weak and field-dependent. A set of crystal electric field parameters are proposed to explain the observed magnetic anisotropy.

The topic of Chapter 6 switches to strongly correlated phenomena. We study the evolution of the Kondo effect in heavy fermion compounds, Yb(Fe1−xCox)2Zn20 (0 < x < 1), by means of temperature-dependent electric resistivity and specific heat. The ground state of YbFe2Zn20 can be well described by a Kondo model with degeneracy N = 8 and a TK ∼ 30 K. In the presence of a very similar total CEF splitting with YbFe2Zn20, the ground state of YbCo2Zn20 is close to a Kondo state with degeneracy N = 2 and a much lower TK ∼ 2 K. Upon Co substitution, the coherence temperature of YbFe2Zn20 is suppressed, accompanied by an emerging Schottky-like feature in specific heat associated with the thermal depopulation of CEF levels upon cooling. For 0.4 < x < 0.9, the ground state remains roughly the same which can be qualitatively understood by the Kondo effect in the presence of CEF splitting. There is no clear indication of Kondo coherence observable in resistivity within this substitution range down to 500 mK. The coherence re-appears at around x > 0.9 and the coherence temperature increases with higher Co concentration levels.

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Fri Jan 01 00:00:00 UTC 2016