Structural and Magnetic Properties of Nanosized Half-Doped Rare-Earth Ho0.5Ca0.5MnO3 Manganite

We investigated the structural and magnetic properties of 20 nm-sized nanoparticles of the half-doped manganite Ho0.5Ca0.5MnO3 prepared by sol-gel approach. Neutron powder diffraction patterns show Pbnm orthorhombic symmetry for 10 K < T < 290 K, with lattice parameters a, b, and c in the relationship c/√2 < a < b, indicating a cooperative Jahn–Teller effect, i.e., orbital ordering OO, from below room temperature. In contrast with the bulk samples, in the interval 250 < T < 300 K, the fingerprint of charge ordering (CO) does not manifest itself in the temperature dependence of lattice parameters. However, there are signs of CO in the temperature dependence of magnetization. Accordingly, below 100 K superlattice magnetic Bragg reflections arise, which are consistent with an antiferromagnetic phase strictly related to the bulk Mn ordering of a charge exchange-type (CE-type), but characterized by an increased fraction of ferromagnetic couplings between manganese species themselves. Our results show that in this narrow band half-doped manganite, size reduction only modifies the balance between the Anderson superexchange and Zener double exchange interactions, without destabilizing an overall very robust antiferromagnetic state.

Magnetic and Transport Studies of Strongly Correlated Perovskite Ceramics

<p>This thesis describes the results from an experimental study of the magnetic and transport properties of two strongly correlated transition metal oxides. The firstmaterial under study is the ferromagnetic half-metal double perovskite, Sr2FeMoO6, in which we have made isoelectronic (Ba2+) and electronic (La3+) substitutions onto the strontium site. Magnetoresistance measurements on Sr2-xBaxFeMoO6 revealed that the low temperature magnetoresistance is dominated by inter-grain transport while the intra-grain contribution is evident when the temperature is close to the ferromagnetic transition temperature. Transport measurements on Sr2-xLaxFeMoO6 clearly showed that the doping dependence of the thermoelectric power is surprisingly similar to the one observed in the superconducting cuprates. In addition, it was found that the electronic doping leads to an increase in the ferromagnetic transition temperature, which supports the band filling model. Substitution on the Fe site was also investigated by partially replacing Fe with the non-magnetic aluminium element (Sr2Fe1-xAlxMoO6). It was found from thermoelectric power measurement that the Fe electronic state is below3+,which is inconsistentwith theoretical models but is in good agreement with Mossbauer measurements. In addition, magnetic measurements showed that the reduction in the ferromagnetic ordering temperature could be explained in terms of a 3D percolation model. The second compound is the oxygen deficient strontium iron oxide SrFeO3-delta . The temperature dependence of the thermoelectric power was measured in this compound for the first time and shown to be reminiscent of the charge-ordering Verwey transition observed in Fe3O4. Magnetic measurements show an increase of a weak ferromagnetic signal versus the oxygen deficiency that could originate from a Dzyaloshinsky-Moriya interaction in the distorted FeO6 octahedra. Finally, we observed a large magnetoresistance near room temperature for compounds close to the orthorhombic SrFeO2.75 phase.</p>

Magnetic and Transport Studies of Strongly Correlated Perovskite Ceramics

<p>This thesis describes the results from an experimental study of the magnetic and transport properties of two strongly correlated transition metal oxides. The firstmaterial under study is the ferromagnetic half-metal double perovskite, Sr2FeMoO6, in which we have made isoelectronic (Ba2+) and electronic (La3+) substitutions onto the strontium site. Magnetoresistance measurements on Sr2-xBaxFeMoO6 revealed that the low temperature magnetoresistance is dominated by inter-grain transport while the intra-grain contribution is evident when the temperature is close to the ferromagnetic transition temperature. Transport measurements on Sr2-xLaxFeMoO6 clearly showed that the doping dependence of the thermoelectric power is surprisingly similar to the one observed in the superconducting cuprates. In addition, it was found that the electronic doping leads to an increase in the ferromagnetic transition temperature, which supports the band filling model. Substitution on the Fe site was also investigated by partially replacing Fe with the non-magnetic aluminium element (Sr2Fe1-xAlxMoO6). It was found from thermoelectric power measurement that the Fe electronic state is below3+,which is inconsistentwith theoretical models but is in good agreement with Mossbauer measurements. In addition, magnetic measurements showed that the reduction in the ferromagnetic ordering temperature could be explained in terms of a 3D percolation model. The second compound is the oxygen deficient strontium iron oxide SrFeO3-delta . The temperature dependence of the thermoelectric power was measured in this compound for the first time and shown to be reminiscent of the charge-ordering Verwey transition observed in Fe3O4. Magnetic measurements show an increase of a weak ferromagnetic signal versus the oxygen deficiency that could originate from a Dzyaloshinsky-Moriya interaction in the distorted FeO6 octahedra. Finally, we observed a large magnetoresistance near room temperature for compounds close to the orthorhombic SrFeO2.75 phase.</p>

Crystal Structure of the Intermediate Na2V2(PO4)3 Phase and Electrochemical Reaction Mechanisms in NaxV2(PO4)3 (1 ≤ x ≤ 4) System

The Na-superionic-conductor (NASICON) Na3V2(PO4)3 is an important positive electrode material for Na-ion batteries. Here, we investigate the mechanisms of phase transition in NaxV2(PO4)3 (1 ≤ x ≤ 4) upon a non-equilibrium battery cycling. Unlike the widely believed two-phase reaction in Na3V2(PO4)3 – Na1V2(PO4)3 system, we determine a new intermediate Na2V2(PO4)3 phase using operando synchrotron X-ray diffraction. Density functional theory calculations further support the existence of the Na2V2(PO4)3 phase. We propose for the first time two possible crystal structures of Na2V2(PO4)3 analyzed by Rietveld refinement. The two structure models with the space groups P21/c or P2/c for the new intermediate Na2V2(PO4)3 phase show similar unit cell parameters but different atomic arrangements, including a vanadium charge ordering. As the appearance of the intermediate Na2V2(PO4)3 phase is accompanied by symmetry reduction, Na(1) and Na(2) sites split into several positions in Na2V2(PO4)3, in which one of the splitting Na(2) position is found to be a vacancy whereas the Na(1) positions are almost fully filled. The intermediate Na2V2(PO4)3 phase reduces the lattice mismatch between Na3V2(PO4)3 and Na1V2(PO4)3 phases facilitating a fast phase transition. This work paves the way for a better understanding of great rate capabilities of Na3V2(PO4)3.

Real-space imaging of phase transitions in bridged artificial kagome spin ice

In frustrated spin systems, magnetic phase transitions underpin the formation of exotic, frustration-driven magnetic phases. Of great importance is the ability to manipulate these transitions to access specific phases, which in turn provides a means to discover and control novel phenomena. Artificial spin systems, incorporating lithographically-fabricated arrays of dipolar-coupled nanomagnets that allow real-space observation of the magnetic configurations, provide such an opportunity. In particular, the kagome spin ice is predicted to have two phase transitions, one of which is to a low temperature phase whose long-range ground state order has not been observed experimentally. To achieve this order, we change the global symmetry of the artificial kagome system, by selectively tuning the near-field nanomagnet interactions through nanoscale bridges at the vertices. By precisely tuning the interactions, we can quantify the influence of frustration on such a transition, and find that the driving force for spin and charge ordering depends on the degeneracy strength at the vertex. For the first time, we are able to observe the evolution of the magnetic configurations associated with a phase transition in real space and time.