Degree Type

Dissertation

Date of Award

2014

Degree Name

Doctor of Philosophy

Department

Materials Science and Engineering

First Advisor

Vitalij K. Pecharsky

Second Advisor

Karl A. Gschneidner, Jr.

Abstract

Rare earth-based intermetallic compounds continue to draw considerable attention due to their fundamental importance in understanding structure-property relationships and potential for practical applications based on a variety of phenomena. The focus of this project is to employ two rare earth intermetallic systems: RFe4Ge2 and R117M52+xX112+y ternary intermetallic systems as model candidates to uncover the underlying electronic, atomic and microscopic interactions that result in a strong coupling between the crystallographic and magnetic sub-lattices.

A systematic investigation of the structure, magnetic and thermal properties of DyFe4Ge2 has been performed. Magnetization of DyFe4Ge2 measured as a function of temperature in 1 kOe magnetic field indicates antiferromagnetic (AFM) ordering at TN=62 K followed by two spin reorientation transitions at Tf1 = 52 K and Tf2 = 32 K and another anomaly at 15 K (Tf3). Three transitions (Tf1, Tf2, and TN) are further confirmed by heat-capacity measurement in a zero magnetic field. The two low temperature magnetic transitions are broadened and gradually vanish when the applied magnetic field exceeds 30 kOe, and the AFM transition shifts toward low temperature with increasing magnetic field. Reentrant magnetic glassy state is observed below the freezing point Tf3 = 15 K. Two field-induced metamagnetic phase transitions are observed between 2 and 50 K in fields below 140 kOe. The temperature-magnetic field phase diagram has been constructed. The first principles electronic structure calculations show that the paramagnetic tetragonal structure of DyFe4Ge2 is stable at high temperature. The calculations with collinear Dy spins confirm ferrimagnetic orthorhombic DyFe4Ge2 as the ground state structure.

The structural, magnetic, heat capacity, electrical resistivity and magnetoresistance properties of compound HoFe4Ge2 have been thoroughly investigated. The temperature dependencies of the magnetization and heat capacity show three magnetic transitions at TN = 51 K, Tf1 = 42 K, and Tf2 = 15 K. The high temperature transition is antiferromagnetic ordering and the two low temperature phase transitions are due to rearrangements of the magnetic structure. A kinetically arrested phase is observed below a freezing point of ~11 K. Below 35 K, the behavior of the isothermal magnetization reflects a first-order metamagnetic phase transition. Multiple phase transitions are also manifested in the electrical resistivity behavior. For a field change of 30 kOe, a large magnetoresistance of ~ 30% is observed near Tf2 (15K).

The magnetic properties of Pr117Co54.5Sn115.2 - a member of a family of materials with a giant unit cell - have been investigated by dc magnetization, ac magnetic susceptibility, specific heat, and electrical resistivity measurements. A magnetic glassy state at freezing temperature of ~ 11 K was detected from the magnetic susceptibility and specific heat data. The glassy state in Pr117Co54.5Sn115.2 is not the conventional spin glass with randomly oriented magnetic moments, but it is related to clusters of atoms that exist in the complex crystal lattice of the material. Furthermore, the glassy state coexists with short range antiferromagnetic order, leading to the development of antiferromagnetic clusters. A weak anomaly in the specific heat data centered around 11 K supports the formation of magnetic cluster glass state in Pr117Co54.5Sn115.2. Semiconductor-like resistivity with a negative temperature coefficient from 2 to 300 K is also observed in Pr117Co54.5Sn115.2.

The ternary intermetallic compound Pr117Co56.7Ge112 adopts the cubic Tb117Fe52Ge112-type related structure with the lattice parameter a = 29.330 (3) Å. The compound exhibits one prominent magnetic transition at ~ 10 K and two additional weak magnetic anomalies are observed at ~ 26 K and ~ 46 K in a 1kOe applied field. At a higher field of 10 kOe, only one broad ferromagnetic-like transition remains at 12 K. The inverse magnetic susceptibility of Pr117Co56.7Ge112 obeys the Curie-Weiss law with a positive value of the paramagnetic Curie temperature (θP = 24 K), indicating that ferromagnetic interactions are dominant. The effective magnetic moment is 3.49 μB / Pr, which is close to the theoretical effective paramagnetic moment of 3.58 μB for the Pr3+ ion.

The presence of cluster spin glass in Tb117Fe52Ge113.8(1) is evidenced via ac and dc susceptibility, magnetization, magnetic relaxation and heat capacity measurements. The results clearly show that Tb117Fe52Ge113.8(1) undergoes a spin glass phase transition at a freezing temperature of ~38 K. The good fit of frequency dependence of the freezing temperature to the critical slowing down model and Vogel-Fulcher law strongly suggest the existence of a cluster glass in the Tb117Fe52Ge113.8(1) system. The heat capacity data also show absence of long-range magnetic order and a large value of Sommerfeld coefficient is obtained. The spin glass behavior of Tb117Fe52Ge113.8(1) is understood in terms of competing interactions among the multiple non-equivalent Tb sites arising from the highly complex unit cell.

DOI

https://doi.org/10.31274/etd-180810-3761

Copyright Owner

Jing Liu

Language

en

File Format

application/pdf

File Size

140 pages

Share

COinS