Exploring correlation between local structure, magnetic and transport properties of multifunctional cr- based chalcospinels

Abstract

Present study Motivated from above background, we attempted to explore the correlations in crystal structure – magnetic –electronic properties of ACr2Se4 spinels, with A = Cd, Cu and Zn. As slight lattice distortions and cation valence/site disorder play an important role in determining the properties of this class of materials, substitutions of different elements at various tetrahedral (Cd/Cu/Zn) site, octahedral Cr-site and anion sites were carried out. All samples were prepared by solid state reaction method. The crystal structure of the prepared compositions was thoroughly studied using experimental probes like powder X-ray diffraction (XRD), Extended X-ray Absorption Fine Structure spectroscopy (EXAFS), and Raman spectroscopy. Magnetic properties were measured using a SQUID magnetometer. Transport measurements were carried out using standard four-probe resistivity technique. The influence of Cr-Cr and Cr-Se bond distances in governing the magnetic ground state of the materials was clearly established. Tweaking the physical properties with suitable substitutions gives rise to emergent properties that are technologically relevant, like magneto-resistance, spin-phonon coupling and possible half-metallic ground state. These possibilities were explored for the studied samples. In particular, the exact compositions studies here are: i. CdCr2Se4 a) Cd1-xMxCr2Se4 with M = Sb, In, Sn and 0  x ≤ 0.1 b) CdCr2Se3.8S0.2 and CdCr2Se3.6S0.4 ii. CuCr2Se4 a) Cu1-xMxCr2Se4 with M = Mn, Fe, Co, Ni and x = 0.1 b) CuCr2-xTixSe4 with 0  x ≤ 1 iii. Cd0.9Cu0.1Cr2Se4, Zn0.9Cu0.1Cr2Se4, and Zn0.1Cu0.9Cr2Se4 Chapter wise organization of the thesis The organization of this thesis is as follows: Chapter 1: The first chapter provides a brief review of existing literature related to various Cr – based spinel chalcogenides. This includes studies on the crystal structure, magnetic, vibrational and electronic -transport properties relevant to spinel chalcogenides. This chapter also defines the objective of the thesis.Chapter 2: This chapter contains details about the method of sample synthesis and a brief description of various characterization techniques employed in the present study. Experimental probes such as X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX), extended X-ray absorption spectroscopy (EXAFS), soft X-ray absorption spectroscopy (XAS), Raman spectroscopy, magnetic properties measurement, four-probe resistivity measurement and heat capacity measurement are described here. A brief mention of the working principles of various instruments like SQUID – based magnetic properties measurement system (MPMS) and physical property measurement system (PPMS), vibrating sample magnetometer (VSM), EXAFS spectrometer and Raman spectrometer has also been made.Chapter 3: This chapter summarizes the investigation carried out on the A-site substituted chalcospinels, viz. Cd1-xMxCr2Se4 (M = Sb, In, Sn). CdCr2Se4 orders magnetically below TC ∼130K. Partial substitution of Cd2+ by a small percentage ( 10%) of non-magnetic In3+, Sb3+, or Sn4+ results in same cubic symmetry in spite of the differences between the cationic radii and ionic charge. However, manifestation of local lattice distortion and charge-imbalance effect is seen on the structural and magnetic properties of the studied compositions. While Sb and In substituted samples reflect expected trends in its lattice parameters and magnetic properties, the Sn-substituted compositions show interesting deviations and correlation in lattice constant and TC. With increasing Sn content, the TC (~ 130 K for CdCr2Se4) drops down to 116 K (for 5% Sn) and slowly regains the value of 124 K for 10% of Sn. Overall the magnetic properties of all the studied compositions display the manifestation of competition between Cr-Se-Cr ferromagnetic and Cr-Cr antiferromagnetic interactions. EXAFS measured at Cr K-edge shows an enhancement in the thermal displacement parameters associated with Cr-Se bonds, that signal presence of local disorder while maintaining the overall cubic structure. Also, an apparent change in the feature related to Cr-Cr interaction occurs with changing concentration of substituted ions. Certain features in magnetic properties hint towards a low-spin transition of Cr2+. XANES recorded at the Cr K-edge provides final evidence of the presence of Cr mixed valence (Cr3+/Cr2+) state.Chapter 4: This chapter presents evidence of local lattice distortion and its effect on the spin-phonon coupling in CdCr2Se4-xSx and Cd1-xSnxCr2Se4 for (0 x  0.1). Our earlier attempt at understanding crystal structure–magnetic properties correlation in leads to the inference that, both, local lattice distortion and formation of mixed valence of chromium ions influence the Cr-Cr and Cr-Se-Cr interactions. In this work, we attempt to isolate the role of local lattice distortions in controlling the magnetic superexchange interactions. Slight substitution of isovalent S in place of Se indicates that S-doping leads to the decrease in the unit cell parameters without affecting the valence state of Cr ions. From EXAFS analysis, we obtain two different bond distances for Cr-Se and Cr-S, indicating structural distortion of the Cr-Se octahedra at a local level within the overall cubic unit cell. The interesting fact is that despite the sizable lattice distortions, neither the Tc nor the magnetic moment seems to be much affected by S-substitution. Apart from EXAFS analysis, room temperature Raman spectra also provide evidence for local structure distortion. Temperature dependent Raman spectra further reveal anomalous variation in the peak position of vibrational modes across the magnetic transition temperature, suggesting an existence of spin-phonon coupling in the system. As discussed in Chapter 3, Sn-substituted compositions show influence of both, charge and lattice distortion on its magnetic properties. The slight increase in lattice constants causes a considerable difference in the overlap of d-orbitals. This result in wider separation of bands, triggering a low-spin transition of Cr2+ ions that are formed due to charge imbalance created by Sn4+ substitution at Cd2+ site. Hence a detailed investigation into the changing local crystal structure and its effect on spin-phonon coupling in Cd1-xSnxCr2Se4 for (0 x  0.1) were carried out using Cr- Kedge EXAFS and Raman spectroscopy technique. EXAFS revealed that considerable change in the Cr-Cr bond distance as compared to Cr-Se. From the Raman data analysis, it is clear that the lattice is involved in modulating the lifetime of phonon, but its coupling with spin is not sufficiently strong to modulate the frequency.Chapter 5: Electronic structure calculations present in literature, indicate the possibility of inducing half-metallic ground state in CuCr2Se4 spinel system, by a careful choice of chemical substituent. From chapter 3 and chapter 4, our study on CdCr2Se4 tells us that substitutions at A-site provide an effective way to control the magneto-structural properties of these materials than substitutions at Se-site. Hence, with an intention to change the 3d density of states at Fermi level we carried out the investigation of Cu1-xMxCr2Se4 (M= Mn, Fe, Co, Ni and x = 0.1) and CuCr2-xTixSe4 for its structure, magnetic and transport properties. In this chapter, we discuss the results of Cu1-xMxCr2Se4 (M= Mn, Fe, Co, Ni and x = 0.1). The next chapter elaborates on properties of CuCr2-xTixSe4. With the 10% magnetic ion substitution at Cu site, the TC in CuCr2Se4 is reduced from 430 K to 390 K, but the saturation magnetic moment does not change much. But fascinatingly, 10% of Mn substitution causes a sufficient - reduction of TC from 430 K to 350 K and significant increase of the saturation magnetic moment of 5.5 μB/f.u. XANES confirms the monovalent state of Cu and divalent state of Mn, Fe, Co and Ni. Introducing a small amount of Fe2+ /Co2+/Ni2+ at Cu+ site leads to a formation of the mixed valence of Cr ions (Cr3+/ Cr4+) at octahedral sites and a strong FM order originates due to the double exchange mechanism. Electrical measurement of all these samples shows the metallic-type conductivity with strong electron correlations present in the low temperature region.Chapter 6: As stated above, this chapter is dedicated to the study of CuCr2-xTixSe4 spinels. We carried out the investigation of magnetic, electrical and crystal structure properties of polycrystalline Cu[Cr2-xTix]Se4 spinel with x = 0, 0.3, 0.4, 0.5, 0.75 and 1.0 respectively. The gradual increase of Ti substitution at Cr-site in CuCr2Se4 compound causes a progressive increase in lattice parameters. The predominant ferromagnetic state appears in the range of 0 < x < 0.75. However, a strong reentrant spin glass type of state observed in CuCrTiSe4 sample at 14 K. The electrical transport behavior is also quite interesting, with all composition from CuCr2Se4 up to CuCr1.5Ti0.5Se4 showing metallic conductivity, whereas CuCr1.6Ti0.4Se4 shows the semiconducting rise with fall in temperature. Further increase in Ti concentration changes the electrical conductivity back to metal – like. Frompreliminary studies, it appears that CuCr1.6Ti0.4Se4 might indeed qualify to be a halfmetal. Chapter 7: This chapter deals with the unusual magnetic behavior and local structure of Cd0.9Cu0.1Cr2Se4, Zn0.9Cu0.1Cr2Se4 and Cu0.9Zn0.1Cr2Se4 systems. Introducing a small amount of Cu at Cd /Zn site causes multiple valences of Cr ions (Cr3+/ Cr4+) at octahedral sites in the otherwise stable Cr3+ compositions of CdCr2Se4 and ZnCr2Se4. This is confirmed from Cr K-edge XANES measurement. The magnetic ground state thus becomes quite rich and competing magnetic interactions results, as evidenced from inverse dc susceptibility curve above TC. Electronic transport behaviour of both A0.9Cu0.1Cr2Se4 (A = Cd/Zn) compounds follows Mott variable range hopping process (semiconducting nature). Cd0.9Cu0.1Cr2Se4 shows ferromagnetic nature whereas Zn0.9Cu0.1Cr2Se4 compound displays complex AFM and FM behaviour. In the case of higher Cu+ substitution at Zn site, i.e. Cu0.9Zn0.1Cr2Se4 the complex magnetic AFM-FM nature changes completely to strong FM behaviour and electronic conductance changes from semiconducting to metallic nature. This complexity in magnetism is discussed in this chapter. Cr K-edge EXAFS of Cd0.1Cu0.9Cr2Se4 and Zn0.1Cu0.9Cr2Se4 shows considerable changes at its first nearest-neighbor shell (CrSe6 octahedra) in terms of (Cr-Se) and second nearest neighbor (Cr-Cr) bond length. Chapter 8: This is the last chapter of the present thesis. It concludes the present study by summarizing all the obtained results and also describes the possible future scope of the research work that can be undertaken in this area.