| Since the discovery of unconventional superconductivity in cuprates in 1986, great efforts have been made in searching for new unconventional superconductors and unveiling their micro-mechanisms. It is well known that the parent compounds of cuprates, Fe-based superconductors, as well as heavy fermion superconductors, are antifferomagnets. Superconductivity occurs after the long range magnetic orders being suppressed by doping or pressure. It is generally believed that unconventional superconductivity originates from the quantum fluctuations.As is widely accepted, it is an effective method to search for unconventional superconductors in 3d magnetically ordered compounds by tuning the physical parameters (i.e. carrier concentration, pressure). As a matter of fact, superconductivity was discovered in Cu(3d9), Fe(3d6), Co(3d7),Ni(3d8), Mn(3d5), Cr(3d4) based materials in the past 30 years. However, the superconducting mechanism is far from consensus. In this dissertation, we have systematically studied the magnetism, superconductivity and magnetocaloric effect in several T1-based 3d transition metal (Fe, Co, Ni and Cu) chalcogenides and several achievements have been made.This dissertation is divided into five chapters, with the main results as following:In chapter 1, we briefly review the Fe-based superconductors and its implication on exploring novel unconventional superconductors. In the mean time, we address some physical issues related to this dissertation, including critical behavior and magnetocaloric effect in ferromagnets.In chapter 2, we systemically investigate the crystal structure, magnetic structure, magnetic susceptibility and resistivity for TlCo2-xNixSe2 (0≤x≤2) and TlCo2-xNixS2 (0≤x≤2) high quality single crystals grown by self-flux method. For TlCo2-xNixSe2 (0≤x≤2), it is found that TlCo2Se2(x=0) sample is an incommensurate (121°) helical antiferromagnet (AFM) along c axis. This magnetic structure becomes commensurate ordered AFM at around x=0.2. With increasing the Ni content, the Neel temperatures (TN) first increase and then decrease, finally approach 0 K at around x=1.7. We firstly observe bulk superconductivity in this system as the Ni content increases further. The superconducting transition temperatures (TC) increase monotonously and reach maximum of 3.7 K. The phase diagram of TlCo2-xNixSe2 in which an evolution from an incommensurate AFM to superconductor was obtained from these results. For TlCo2-xNixS2 (0≤x≤2), the phase diagram is more complicated. TICo2S2 (x=0) is an itinerant ferromagnet, a little substitution of Ni for Co drives the system to exhibit metamagnetic and intriguingly critical behaviors (discussed in the next chapter). Upon doping, the system firstly evolves from itinerant ferromagnetism to metamagnetism, then to antiferromagnetism and finally to superconductivity. The phase diagrams of magnetism and superconductivity in these two systems open a new window to understand the Ni-based superconductivity, which we will discuss in the main text.Chapter 3. As mentioned above, TICo2S2 is an itinerant ferromagnet. A little Ni substitution for Co leads the system to be a metamagnet. We have investigated the critical behavior of TICo2S2 and TlCo1.9Ni0.1S2 single crystals by isothermal magnetization M(H) measurements. The obtained critical exponents of TICo2S2 could basically satisfy the scaling equations and are in accordance with the prediction of tricritical mean-field theory. TlCo1.9Ni0.1S2 shows metamagnetic behavior that a weak magnetic field could tune the system to ferromagnetism, further confirming the presence of a tricritical point. These results indicate that TICo2S2 provides an excellent platform to study the rare tricritical phenomenon.Chapter 4. Based on our discovery of superconductivity at Tc= 3.7 K involving heavy electron behavior in TlNi2Se2, we have successfully synthesized TlNi2-xCuxSe2 (0≤x≤0.69) single crystals and studied on their electrical transport and magnetic properties. It is shown that the maximal concentration is x=0.69 which may relate to the valence of Cu (+1) in this system. Meanwhile, we have found that TC decreases monotonously as x and finally reaches 0 K at around x=0.69. All the TlNi2-xCuxSe2 (0≤x≤0.69) samples are Pauli paramagnets.Chapter 5. At the same time, we successfully synthesized quasi one-dimensional TlFe3Te3 single crystals. It is found that a paramagnetic-ferromagnetic transition occurs at 220 K in this compound with a low-level thermal and magnetic field hysteresis. We further studied its magnetocaloric effect. The large magnetocaloric effect is elucidated that when H is applied along the c axis (i.e. the isothermal magnetic changes reach 5.9 and 7.0 J/kg K for △H=0-1 T and △H= 0-2T, respectively). The magnetic entropy changes are higher than that in the most of other materials with the same temperature and magnetic field range. Therefore, our findings should inspire the exploration of new high performance magnetic refrigerants. |
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