Structural And Physical Properties Of Doped Double Perovskite La₂Nimno₆

Double perovskite manganites with a general formula A2BB’O6or AA’bb’o6(where A and A’ are alkaline-earth and/or rare-earth metals and B and B’are manganese and other transition metals) have attracted much attention due to their unique structural and magnetic properties, and potential applications in the spintronic devices. In this dissertation, we mostly focused on the investigation of La2NiMnO6compound, a typical double perovskite material, and its related compounds. According to the basic law in the material science that structure determines performance, we have tried to tune the magnetic and transport properties of La2NiMnO6by adjusting the Ni/Mn ordering degree. Furthermore, we have investigated the inherent relationship between the microstructure and the physical properties. Details are as follows:In chapter one:beginning from the basic structure and properties, we introduced some basic concepts related to the perovskite oxides with colossal magnetoresistance (CMR) effect. Based on those concepts and the related theory, the recent developments and studys on the basic structure and properties of La2NiMnO6, a foretype of double perovskite, have been reviewed and discussed.In chapter two:the magnetic, transport, and magnetoresistance properties of the polycrystalline La2NiMnO6were investigated. The magnetic data show that the antiphase boundaries composed by the Ni/Mn antisite disorders were existed. The temperature dependent resistivity exhibits the semiconducting behaviors which can be well fitted by the3-D variable range hopping model. The field dependent magnetoresistance curve, which showes a similar characteristic to that in tunneling magnetoresistance manganites, can be divided into high-and low-field regions. Based on those results, we proposed that the nature of the magnetoresistance in La2NiMnO6is the spin dependent scattering when the electron hopping across the antiphase boundary.In chapter three:by doping the La with Ca or Sr, the hole-doping effect on the structure, local valence, and magnetic properties of La2NiMnO6compound has been studied. It is found that as the doping content increasing, the average valence of A-site decreases, which is compensated by the valence increasing of part Ni2+ions to Ni3+ions. Meanwhile, the antiferromagetic antiphase boundary composed by the Ni/Mn antisite disorder is increased. The exchange bias effect is appeared, which can be attributed to the coupling between the antiferromagnetic antiphase boundary and the Ni/Mn ordered ferromagnetism. Furthermore, the change of the structural and magnetic properties dependent on the A-site average ionic radii in La1.8RE0.2NiMnO6has been discussed. During the discussion, the doping content is kept unchang by using two-valence Ca, Sr, and Ba, respectively, as a dopant.In chapter four:the magnetic properties of Zn doped polycrystalline La1.8Sr0.2Ni1-xZnxMn06samples (0≤x≤0.5) with a double perovskite structure have been investigated. It is found that the ferromagnetic transition temperature (Tc) decreases with the increase of Zn doping content, while the value of saturated magnetic moments (Ms) increases at first, and after reaching a maximum at Zn doping content x-0.2, and it decreases as the Zn doping continue to be increased. Usually, the substitution of nonmagnetic Zn ions for the magnetic Ni ions can be related in Ni/Mn ordered and disordered sublattices. As a result, two opposite influences on magnetic properties are expected, i.e. both the ferromagnetic interaction between Ni and Mn ions in Ni/Mn ordered region and the antiferromagnetic interaction between Ni-O-Ni ions in antisite disordered region are weakened simultaneously. It is suggested that the nonmagnetic ion substitution breaks the Ni-O-Mn ferromagnetic superexchange chains in the Ni/Mn ordered region and results in the decrease of Tc, and the competition of the two opposite influences causes the change of Ms. The weakening of exchange bias (EB) effect further confirms the existence of the above-mentioned two opposite influences.In chapter five:the structure and low-temperature magnetic properties of Lai.8Sro.2Ni1-xMn1+xO6(|x|≤0.2) compounds have been invesgated. X-Ray diffraction shows that the samples with different Ni/Mn ratio are single phase with a rhombohedral structure and the lattice parameters monotonously increase with Ni content increasing (Mn content decreasing correspondingly). The Raman spectra and magnetic measurement data reveal that both the Ni/Mn ordering degree and ferromagnetic transition temperature reach the maximum when the proportion of Ni/Mn is1:1. As Ni/Mn ratio is less than one, i.e. the proportion biases to Mn side, the ferromagnetic transition temperature decreases. At the same time, an extra magnetic transition emerges at the lower temperature, which may be attributed to the double exchange interaction between Mn3+and Mn4+. When Ni/Mn ratio is greater than or equal to1:1, the sample exhibits exchange bias effect, which may come from the coupling between Ni/Mn ordered ferromagnetic domains and Ni-O-Ni antiferromagnetic spins.In chapter six, the crystal structure and magnetic properties of polycrystalline Y2NiMnO6have been investigated. Rietveld refinement based on the X-Ray diffraction data reveal that Ni/Mn are highly ordered on the B-site of the ideal perovskite ABO3unit cell, with small amount of Ni/Mn random distribution phase coexisting. Magnetometer measurement shows that the short-range FM ordered state begins to develop below T*～130K and a standard PM-FM transition appears at Tc-85K, the long-range FM ordering begins to form below Tc. The crystal structural and magnetic results can be interpreted as the scenario that short-range magnetic ordering is induced by Ni/Mn antisite disorder against long-range ferromagnetic ordering of the Ni/Mn sublattice. Heat capacity and magnetic property measurements under an external electric field confirm the robust ferromagnetic state of polycrystalline Y2NiMnO6sample.