Session R23: Magnetic Phenomena in Bulk Nickelates and Other Oxides

Spatial Electronic Textures in Rare Earth Nickelate Thin Films

Rare earth nickelates (RENiO₃) exhibit metal-insulator transitions accompanied by the appearance of charge and spin order, at times concurrently. The description of the ground state of these correlated systems is hindered by the existence of multiple competing orders which are generally susceptible to multiple perturbations (strain, defects, disorder), leading to a highly inhomogeneous electronic fabric. Very recently, electron doping has been successfully realized in rare earth RENiO₃ via control of the oxygen stoichiometry. In this talk, I will present recent Raman, photoemission electron microscopy, nano- (resonant) scattering and spectroscopy measurements on both pristine and electron doped RENiO₃ thin films. Our results cast new light on the granular nature of the electronic and magnetic texture of RENiO₃.   

Origin of Magnetic Order in the Rare-Earth Nickelate Perovskites Investigated by 17O-NMR

Because of their poorly understood high-temperature metal–insulator transition and their unusual low-temperature magnetic order, the rare-earth nickelate perovskites RENiO₃ have received a lot of attention in recent years. The antiferromagnetic order with a propagation vector q = (1/4, 1/4, 1/4) (w.r.t. the pseudocubic unit cell) breaks the crystalline inversion symmetry. The microscopic origin of this non-trivial magnetic order is still unclear. Our temperature dependent ¹⁷O-NMR relaxation measurements on isotopically enriched SmNiO₃ reveal the appearance of additional magnetic fluctuations below the metal–insulator transition, which then freeze out at the magnetic phase transition. We argue that this indicates that fluctuations with q = (1/4, 1/4, 1/4) are “switched on” only in the insulating phase, growing stronger upon cooling, and finally causing the magnetic phase transition.   

Spin stripe order in a square planar trilayer nickelate

Highly orbitally polarized trilayer nickelates represent a potential platform for searching for high-T_C superconductivity. La₄Ni₃O₈ undergoes a semiconductor-insulator transition on cooling at T_SI ~ 105 K, which is accompanied by formation of charge stripes. Here we report polarized and unpolarized neutron diffraction measurements which show that the magnetic ground state consists of antiferromagnetic spin stripes that are commensurate with the charge stripes. The magnetic stripes are modeled as occurring within uncorrelated trilayers. Comparison of the charge and spin stripe order parameters reveals that, in contrast to single-layer nickelates and most transition metal oxides, the orders both appear at T_SI, suggesting a strong coupling between the two.   

Probing Lattice Distortions Associated with Resistive Switching in La2NiO4

Tuning materials properties by various stimuli remains at the forefront of research because it can generate novel phenomena and enhance functionality of future electronic devices. Of particular interest are transition metal oxides (TMO) whose transport properties can be tuned by an applied electric field. For example, a reversible resistive switching induced by an electric bias was previously demonstrated in 5d [1] and 4d [2] TMOs with potential applications in resistive and magnetic random access memories (Re-RAM and MRAM). The switching was tentatively attributed to electric-field driven lattice distortions. Here we study the resistive switching in single crystals of 3d TMO La₂NiO₄ and use an ultra-high precision capacitive displacement gauge to probe the distortions. We observe a clear correlation between the displacement gauge signal and sample resistance. Our results provide unequivocal evidence that the resistive switching is related to structural distortions induced by electric fields. [1] C. Wang et al. Phys. Rev. B 92, 115136 (2015); H. Seinige et al. ibid 94, 214434 (2016). [2] S. Shen et al. http://meetings.aps.org/link/BAPS.2017.MAR.E43.11   

Magnetic and Structural Properties of LaNiO3 Single Crystals

RNiO₃ (R = rare earth) perovskites form a series of compounds for which the magnetic and electronic properties have been studied for decades. All compounds possess an antiferromagnetic, insulating state, with the lone exception of LaNiO₃ (LNO) which has traditionally been described as a Pauli paramagnetic metal. Previous bulk experimental investigations have been relegated to work on polycrystalline materials; however, recently, successful growth of single crystals of LNO was reported [1,2]. The single crystals obtained provide an opportunity for deeper investigations into the physics of LNO, and indeed the traditional view of LNO has been challenged by a report of an antiferromagnetic metallic ground state [2]. Here, we discuss the magnetic and structural properties of single crystal LNO prepared in our laboratory.

[1] J. Zhang et al., Cryst. Growth Des., 17, 2730 (2017).
[2] Z. W. Li et al., arXiv:1705.02589 [cond-mat.str-el]   

Neutron Scattering Study of the Magnetic Phase Transition in Rare-Earth Titanates

The Mott-insulating rare-earth (R) titanates (RTiO₃) are known to exhibit a ferromagnetic-antiferromagnetic transition as the Ti-O-Ti bond angle increases in the GdFeO₃-type distorted perovskite structure. Evidence for a similar transition at a critical doping x_c~ 0.3 has been found in the solid-solution system Y_1-xLa_xTiO₃. Although neutron scattering has been used to study the magnetic response of single crystals of the parent compounds YTiO₃ and LaTiO₃, such studies have not been reported for the solid solution. Here we report on our neutron scattering work on the magnetic order and excitations in doped system Y_1-xLa_xTiO₃.   

DFT+DMFT study of oxygen vacancies on LaTiO3

Oxygen vacancies are a very common defect in perovskite oxides, where they play a key role in a variety of emergent phenomena, such as superconductivity or the two-dimensional electron gas. Since the treatment of electronic correlations is crucial for a correct description of these systems, one needs to make use of beyond-DFT (Density Functional Theory) techniques. In this work, we use DFT+DMFT (Dynamical Mean-Field Theory) to investigate the effect of oxygen vacancies on LaTiO₃, a prototypical Mott insulator close to the metal-insulator transition. In particular, we focus on the correlation effects on the electronic states created by the oxygen defect, and the interplay between the release of carriers and the Mott insulating state.   

Effects of Electron-Phonon Coupling on the Magnetic Order in Rare-Earth Nickelates

It is no coincidence that rare-earth nickel perovskites (RNiO$_3$) have recently been gathering a lot of attention. Their exciting and extensive array of novel properties, such as the metal-to-insulator transition (MIT), bond disproportionation and an unusual antiferromagnetic ground state, with the four-site spin order, characterized by the wavevector $\mathbf{q} = (\frac{1}{4},\frac{1}{4},\frac{1}{4})$ (with many of these properties tunable in thin film RNiO$_3$ and heterostructures), makes them highly desirable as a new generation ``quantum material'', for possible applications in electronics, energy storage, and even quantum computing. Given that a breathing-mode lattice distortion mirrors the MIT, its impact on bond disproportionation has been well-studied. However, fewer studies have considered the influence of distortion on the magnetic order. We construct an effective two-orbital Hubbard Hamiltonian and couple it to the distortion (which we treat semiclassically), then find the phase diagram using the Hartree-Fock approximation. We discuss how the different terms in the Hamiltonian affect the phase diagram, specifically the magnetic order stabilized in the various regions of parameter space.   

Multiple Transitions in Spin Glass Compound RbNiPO4

Synthesis and characterisation of a new compound RbNiPO₄ was performed. Two transitions were observed in the magnetisation measurements in the low temperature regime around 20 K and 10 K. Magnetisation measurements also showed a hysterisis in the M vs. H at 2.1 K. This hysterisis showed a decrease with increasing temperatures, signifying ferromagnetic ordering at low temperatures. Curie - Weiss fit of the inverse susceptibility showed dominant antiferromagnetic interactions around 16 K. This suggests the ground state to be either canted antiferromagnetic or spin glass ordering. Spin glass/freezing state could be the reason for the splitting in zero- field cooling (ZFC) and field cooling (FC) magnetisation at around 10 K and 20 K. Further measurements such as heat capacity and AC magnetic susceptibility have to be done to find more information about the nature of these transitions.   

Multiple phase transitions in the vanadium phosphorous oxide

The vanadium phosphates system (V-P-O) has shown distinct structural stacking and variable valence states of vanadium, providing rich 3d magnetism in this system. V-P-O single crystal was successfully synthesized and characterized by X-Ray diffraction, resistivity, magnetization, heat capacity and neutron scattering measurements. It is a magnetic semiconductor with a band gap of 0.24 eV. Two phase transitions occur at 130 K and 615 K, revealed by transport, susceptibility, and neutron diffraction measurements, suggesting the existence of strongly correlated electrons. In this talk, we will discuss the nature of the phase transitions as well as the light shed by the first principles calculations.   

NMR Studies of the Vanadium Phosphorous Oxide Single Crystal

Strongly correlated-electron system, vanadium phosphorous oxide (V_xPO_y), have shown a variety of crystal structures and distinct magnetic properties, depending on the different stoichiometric ratio x and y and the valence states on V sites. A recently synthesized member of the VxPOy family was observed to undergo two phase transitions, through transport, magnetic susceptibility and neutron scattering measurements. The low-temperature phase (T<130 K) is a canted antiferromagnet, while 13P NMR spectroscopy and relaxation in the intermediate phase (130 K<T<615 K) is consistent with paramagnetism. The transition at 615 K is of unknown origin, but may be associated with charge or orbital ordering in the system.   

Complex spin orbital orders in vanadates

The delicate interplay among spin, orbital, charge, lattice degrees of freedom and geometrical frustration can lead to emergent ground states in transition metal oxides, such as complex spin/orbital orders, valence bond solid state, spin/orbital glass, spin/orbital liquids, etc., being of great interest in condensed matter physics. Vanadates have presented rich spin orbital phases. However, many questions are still under debate, for example, the mechanism of spin orders driven by orbital order, orbital fluctuation, metallic V-V bonding, the critical V-V distance that sets the boundary between localized and itinerant electronic behaviors, etc. In the talk, I will present our recent studies on two vanadium systems that both show complex spin/orbital orders, shedding light on roles of localized orbital order and itinerant V-V bonding leading towards a complex spin order.