Session F54: Emergent Properties of Complex Oxides Bulk, Thin Films, and Heterostructures I

Current-controlled chiral orbital currents in a colossal magnetoresistance material

Colossal magnetoresistance (CMR) is an extraordinary enhancement of the electric conductivity in the presence of a magnetic field. It is conventionally associated with a field-induced spin polarization that drastically reduces spin scattering and electric resistance.   Ferrimagnetic Mn₃Si₂Te₆ is an intriguing exception to this rule: it exhibits a 7-order-of-magnitude reduction in ab-plane resistivity that occurs only when a magnetic polarization is avoided [1]. Here we report an exotic quantum state that is driven by ab-plane chiral orbital currents (COC) flowing along edges of MnTe₆ octahedra [2]. The c-axis orbital moments of ab-plane COC couple to the ferrimagnetic Mn spins to drastically increase the ab-plane conductivity (CMR) when an external magnetic field is aligned along the magnetic hard c axis. The COC-driven CMR is thus highly susceptible to external currents. When a small DC current is applied beyond a critical threshold, the coupling of the COC to MnTe₆ octahedra induces a time-dependent, bistable switching that mimics a first-order "melting transition”, which is a hallmark of the COC state. The demonstrated current-control of COC-enabled CMR offers a new paradigm for quantum technologies.

    

Rare-Earth Mediated Cross-Over in Dimensionality of a Spin Density Wave in a Ruddlesden-Popper Nickelate

Nickelates allow for the investigation of charge and spin density waves (CDWs and SDWs), which are quantum states recognized to be of fundamental importance in modern condensed matter physics . An intriguing metal-metal transition occurs at temperature T_MMT in R₄Ni₃O₁₀ (R=trivalent rare earth). Recently, the metal-metal transition was argued to arise from the simultaneous formation of a strongly coupled quasi-2D SDW and 3D CDW based on analysis of incommensurate superlattice peaks observed in single-crystal neutron and X-ray diffraction of La₄Ni₃O₁₀ below T_MMT ≈ 148 K [1]. In this work, we investigated the effect of replacing non-magnetic La³⁺ cations with Pr³⁺ (4f²). Neutron and resonant x-ray scattering evidenced an ordered, induced SDW on the Pr³⁺ cations well below T_MMT which leads to a 2D-3D magnetic cross-over. We developed models of stacked SDW layers for the intermediate-temperature (Ni moment only) state on warming and cooling as well as the ground-state which contained SDWs on both the Ni and Pr³⁺ sites. The modelling determined that there was a reorientation of the spin direction in the ground-state as well as the phase shifts between layers that occurred due to exchange interactions along c. A Hamiltonian including the exchange interactions and single ion anisotropy was developed which explains observed thermal hysteresis.

[1] J. Zhang et al., Nat. Commun. 11, 6003 (2020).   

Magnetic structure of Colossal Magnetoresistance Mn3(Si1-xGex)2Te6 under c-axis field

Recently discovered colossal magnetoresistance (CMR) Mn₃Si₂Te₆  [1–3] has shown stark contrast with conventional CMR manganites and pyrochlore materials. The resistivity in Mn₃Si₂Te₆ drops by orders of magnitude leading to an insulator-metal transition with an applied magnetic field just above 4 T. The CMR occurs only when the field is applied along the hard axis (c-axis), while such effect is absent with the field applied in the basal plane, where magnetization is fully saturated. Using single crystal neutron diffraction, we have conducted systematic study of the ground state spin structures of the parent and Ge-doped Mn₃Si₂Te₆ and evolution of the corresponding spin configurations with magnetic field up to 14 Tesla along the c-axis. The modification of  the spin order and the relevant mechanism responsible for the CMR will be discussed  [4].

[1]        Y. Ni et al., Phys. Rev. B 103, L161105 (2021).

[2]        J. Seo et al., Nature 599, 576 (2021).

[3]        Y. Zhang et al., Nature (2022). https://doi.org/10.1038/s41586-022-05262-3

[4]        F. Ye et al., to be published in Phys. Rev. B (2022).   

Chemical Pressure Induced Phase Evolution in Trilayer Square-Planar Nickelates (La1–xYx)4Ni3O8

Low-valent nickelates featured by quasi-two-dimensional (quasi-2D) square planar Ni-O layers have drawn extensive attention because of their exotic electronic and magnetic properties, including superconductivity in the infinite layer (Nd,Sr)NiO₂  and quintuple-layer Nd₆Ni₅O₁₂. In our previous studies on trilayer quasi-2D nickelates, we have shown that La₄Ni₃O₈ is a charge- and spin-stripe-ordered insulator below 105 K while Pr₄Ni₃O₈ has a metallic ground state, moreover, we have identified the approximate location of a quantum phase transition at x ≈ 0.45 in (Pr_1–xLa_x)₄Ni₃O₈. However, it is unclear whether the evolution of phase transition is dominant by Pr-O hybridization or by simple chemical pressure effect. To understand the role of size effect in the transition, we carried out the study on (La_1–xY_x)₄Ni₃O₈ solid solution, in which large La³⁺ (r = 103.2 pm) is partially substituted by much smaller rare earth cations, Y³⁺ (r = 90 pm), and no hybridization is involved. Bulk single crystals of (La_1–xY_x)₄Ni₃O₈ (x = 0.1, 0.2 and 0.25) were synthesized via high oxygen pressure (pO₂) floating zone growth followed by topotactic reduction. Characterizations combining crystallography, thermodynamics, electrical transport, magnetic and synchrotron X-ray single crystal diffraction were performed on (La_1–xY_x)₄Ni₃O₈ materials. The results revealed that chemical pressure-stimulated size effect can lead to the evolution from insolating to metallic phase in (La_1–xY_x)₄Ni₃O₈.   

Study of Magnetic Properties of NdMn1-xCoxO3

It is a well known fact that ferromagnetism (FM) gets induced in LaMnO₃ on Co doping at Mn site; yet, there is no general agreement on exchange mechanism responsible for it . We investigated the magnetic properties of Co doped NdMnO₃ to study the effect of Co doping on it; Nd being magnetic as compared to La.

    We synthesized polycrystalline NdMn_1-xCo_xO₃ (x = 0.0, 0.3, 0.7) samples using ceramic method. Rietveld refined X- ray diffraction pattern revealed the single phase orthorhombic crystal structure of all the samples with space group Pbnm. Temperature dependent magnetic measurements performed in field cooled (FC) and zero field cooled (ZFC) mode at 0.05T  exhibit the transition from FM to paramagnetic (PM) phase at Curie temperature T_c which  increases with  increasing Co concentration. The ZFC magnetization display maximum (T_m) and thereby starts decreasing resulting in the cusp. The sharp peak in the ZFC curve and huge difference between the ZFC and FC measurements at lower temperatures are suggestive of antiferromagnetic (AFM) coupling. It is  seen that there is a decrease in peak magnetization in x = 0.7 sample as compared to x = 0.0 & 0.3 samples. The hysteresis at 10 K suggest the FM interactions at small fields and at higher fields a typical AFM character is seen which is more prominent in x = 0.7 sample. The magnetic properties are very well explained in terms of the mixed valence states of Mn and Co confirmed using X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) studies.

    

From strongly frustrated magnet to antiferromagnet: Spin-orbit-coupled trimer iridates Ba3n+1Ir3nO9n+1 (n = 1, 2, and ∞)*

Quantum spin liquids are among the most intensively sought states of matter.  Our work revealed an anomalously strong frustration in an unlikely place, the trimer lattice Ba₄Ir₃O₁₀ [1]. These findings suggest an important role of Ir₃O₁₂ trimers in producing spin entanglement and point to a new pathway to the search of new phases of matter in trimer lattices [1-3]. We have recently synthesized and studied single crystals of a series of such trimer lattices Ba_3n+1Ir_3nO_9n+1 (n = 1, 2, ∞), i.e., Ba₄Ir₃O₁₀ (n=1), Ba₇Ir₆O₁₉ (n=2), and 9R-BaIrO₃ (n=∞), where n represents the number of the trimer planes per unit cell.  This new series of trimer iridates hosts Ir⁴⁺(5d⁵) ions, which support the spin-orbit-coupled Mott state. These iridates adopt a similar crystal structure with different trimer connectivity and yet cover the extreme ends of a magnetic spectrum: the delicate quantum liquid (n=1) on one end and an antiferromagnet with T_N = 183 K with a possible charge density wave (n=∞) on the other [4]. Situated between the two is Ba₇Ir₆O₁₉ (n=2), whose ground state is the focus of this talk.     

Correlated Insulator Collapse due to Quantum Avalanche via In-Gap Ladder States

We propose a microscopic mechanism to resolve the long-standing puzzle of the insulator-to-metal transition in correlated electronic systems, most notably the large discrepancy between the experimental and predicted switching fields in charge-density-wave (CDW) materials and Mott insulators, driven far-from-equilibrium by a DC electric field. By introducing a generic model of electrons coupled to an inelastic medium of phonons, we demonstrate that an electron avalanche can occur in the bulk limit of such insulators at an arbitrarily small electric field. The quantum avalanche arises by the generation of a ladder of in-gap states, created by a multi-phonon emission process. Hot-phonons in the avalanche trigger a premature and partial collapse of the correlated gap. The details of the phonon spectrum dictate the existence of two-stage versus single-stage switching events which we associate with CDW and Mott resistive phase transitions, respectively. The behavior of electron and phonon temperatures, as well as the temperature dependence of the threshold fields, demonstrates how a crossover between the thermal and quantum switching scenarios emerges within a unified framework of the quantum avalanche.   

Magnetic resonance study of rare-earth titanates

Rare-earth titanates are model systems for investigations of the interplay among spin, orbital, and lattice degrees of freedom. Ground-state magnetic order, through subtle lattice distortions, is easily manipulated by the size of the rare-earth ion. Peculiar deviations from the Curie-Weiss law and anomalous magnetic behavior have been detected well above the ferromagnetic ordering temperature in the Mott-insulating parent compound YTiO₃. We utilize magnetic resonance (NMR, ESR) techniques to explore these discrepancies. All measurements are conducted on single-crystalline samples of (Y, La, Ca)TiO₃ [1] in wide ranges of charge (Ca) doping and isovalent (La) substitution. NMR confirms the existence of anomalies in the paramagnetic phase of these compounds. X-band ESR measurements detect a small splitting of the lower t_2g orbital which scales with the sample T_c and could explain the unusual properties. We also directly measure the crystalline anisotropies of YTO₃ in the magnetically ordered state using ferromagnetic resonance.

[1] S. Hameed et al. 10.1103/PhysRevMaterials.5.125003   

Anisotropic magnetic behavior of the Shastry-Sutherland lattice material BaNd2PtO5

The Shastry-Sutherland model is one of the quintessential models of two-dimensional quantum magnetism, however magnetic materials that can be mapped onto this model are rare. This talk will cover the physical properties and magnetic ground state of the Shastry-Sutherland lattice material BaNd₂PtO₅.  Magnetization and specific heat measurements on single crystals revealed an antiferromagnetic transition at T = 1.9K.  The magnetic response to applied fields is very anisotropic, including anisotropy within the basal plane of the tetragonal lattice.  Single crystal neutron diffraction revealed a propagation vector of (½ ½ ½) for the magnetic ground state as compared to the (½ ½ 0) observed in a related Shastry-Sutherland host lattice material BaNd₂ZnO₅. The ordered moment is mostly in the plane with nearest neighbors forming ferromagnetic dimers, however unlike BaNd₂ZnO₅ there is also a finite out of plane component.

    

Broadband magnetotransport in bulk and thin films of self and Sr-doped LaMnO3

Colossal magnetoresistance (CMR) discovered nearly three decades ago in the Mn-perovskite oxide family (hole-doped LaMnO₃) triggered a flurry of activities in search of a similar phenomenon in other oxides.  However, the majority of available reports deal with CMR measured with direct current or low-frequency current (f < 1kHz) and magnetic fields often more than 1 T. While ac impedance in the frequency range 100 Hz to a few MHz range is often used to study dielectric relaxation and magnetocapacitance effect in insulating oxides, ac impedance in metallic oxides is seldom reported. Recent experimental work on the ac magnetoimpedance in the frequency range from  ~1kHz to 3 GHz done in our lab has revealed many exciting phenomena: colossal magnetoresistance at low fields (~ 40-90 % for H ≤ 1 kOe at room temperature) and a transition from negative to positive magnetoresistance. As the frequency of the microwave current increases beyond 0.9 GHz, the low-field magnetoresistance exhibits a positive peak and its position shifts up linearly or non-linearly with increasing strength of the applied dc magnetic field, depending on the hole content. We propose that this feature is most likely due to current-driven paramagnetic/ferromagnetic resonance, but detected electrically. I will present some results derived from self-doped LaMnO₃, Sr-doped LaMnO₃ bulk and thin films and in a paramagnetic  DPPH molecule[1-4]. This technique can potentially probe spin dynamics in other materials too but is rarely exploited. In the end, I will relate our approach to recent works on the inverse spin Hall effect and spin-orbit torque-induced ferromagnetic resonance observed in non-oxide thin films.

    

Ferroelectricity in oxygen-deficient ferrite perovskites

The family of oxides with composition R_xA_1-xFeO_3-δ (R: rare earth, A: alkali-earth) exhibits oxygen-deficient phases that can be realized without disrupting the underlying perovskite lattice structure. For example, in ferrite, the presence of oxygen vacancies leads to the transformation of octahedral units into tetrahedral chains, thus stabilizing Brownmillerite and Grenier phases, depending on the relative number of alternating octahedral and tetrahedral layers. Interestingly, tetrahedral chains can be twisted with two different handedness leading to a local dipole. Using DFT+U calculations and the Quantum Espresso Code, we investigated the ferroelectric behavior arising from the tetrahedral chain twist in the R_0.33A_0.67FeO_2.67 Grenier structures. We find that tetrahedral twisting involves rotation of neighboring octahedral layers, leading to a parallel/anti-parallel arrangement of dipole moments in the tetrahedral chains. We also find that the relative stability of the polar Grenier phase and the barriers required to reverse the polarity of the chains depend on the size of the rare-earth and A-cations, thus providing a guideline for the optimal design of functional materials for resistive switching devices.   

THz and Raman spectroscopy of multiferroic hexagonal ferrites (h-)(Lu,Sc)FeO3

We studied multiferroic hexagonal Lu_0.6Sc_0.4FeO₃ single crystals with non-collinear spins using the THz and Raman scattering spectroscopies. Antiferromagnetic (AFM) resonances, or magnons, were found at about 0.82 THz.  The magnon doublet splits in external magnetic fields applied along the c axis with Fe3+ g-factor of 2.5. The magnons harden with the temperature increase and disappear at the temperature above 130 K, which is consistent with the magnetic susceptibility and the AFM transition temperature known for this compound. A strong dichroism at the resonance with the AFM doublet has been observed in magnetic field using both the conventional circular polarization and the THz vector vortex beams.   

Thermal Properties of the Metallic Delafossite PdCoO2: A Combined Experimental and First-Principles Study

Metallic delafossites have attracted much interest due to record-high oxide conductivities, but relatively little attention has been paid to thermal properties. Here, we address this via wide-temperature-range experimental studies of the crystal structure, thermal expansion, and specific heat of single-crystal PdCoO₂, combined with density functional theory (DFT) calculations of the electronic and phononic densities-of-states, and thus thermal properties [1]. PdCoO₂ retains the R-3m space group from 12-1000 K, with thermal expansion in quantitative agreement with DFT-based calculations. The Co-O bond lengths additionally elucidate the stability of the low-spin state of the Co³⁺ ions, which contrasts with Co-based perovskites. 1.9-400 K measurements of specific heat provide accurate values for the Debye temperature and Sommerfeld coefficient and can be modeled through a Debye-Einstein approach, quantitatively understood based on unusual features of the phonon density-of-states. These behaviors are remarkably closely reproduced by DFT, establishing quantitative understanding of key thermal properties of the metallic delafossite PdCoO₂.

* This work was primarily supported by the US Department of Energy through the University of Minnesota Center for Quantum Materials, under DE-SC0016371.

[1] Y. Zhang, A. Saha, F. Tutt, V. Chaturvedi, B. Voigt, W. Moore, J. Garcia-Barriocanal, T. Birol, and C. Leighton, Phys. Rev. Mater. (submitted).