Session Y23: Magnetic Phenomena in Bulk Oxides II

Phases under magnetic field and domain wall physics in pyrochlore iridates

Rare earth pyrochlore iridates (R₂Ir₂O₇, R=Pr, Y, etc.) incorporate both strong spin-orbit coupling and correlations, and consequently can have distinct physical properties and novel topological phases. We theoretically study the effects of magnetic field and metal-insulator transition in these materials, with a combination of ab initio calculation and Hartree-Fock method. Various possible magnetic phases are explored. Metallic magnetic domain wall states are found under a substantial range of correlation strength U. Finally, we will compare results with recent experiments.   

Strain dependence of the electrical resistivity in BaIrO3: A probe for magneto-elastic coupling.

We report a strong hysteretic response in electrical resistivity upon application of anisotropic biaxial strain in ferromagnetic BaIrO₃. As T is lowered from room temperature through the magnetic ordering temperature (T_C ~ 180K ), we find that the hysteretic behavior is frozen out. This temperature dependence indicates the presence of a strong magneto-elastic coupling manifested as lattice stiffening due to the ferromagnetic ordering. We discuss this relationship in connection to the variance of the Ir-O-Ir bond angle within the family of iridates.   

Direct observation of magnetic states in an actinide ferromagnet UMn2Ge2

We present studies of the magnetic domain structure of UMn₂Ge₂ single crystals using a home-built low temperature magnetic force microscope. The material has two distinct magnetic ordering temperatures, originating from the Mn and U moments. At room temperature, where the Mn moments dominate, flower-like domain patterns are observed. After exposing the sample to a one-tesla magnetic field near 40K, the evolution of the magnetic domains is imaged through zero field warming up to 200 K. Near the ordering temperature of the uranium moments a clear change in the domain wall motion is observed and can be explained by a 'Slingshot' mechanism. The anti-ferromagnetic coupling between Mn and U moments, responsible for the rapid domain wall motion, is further verified by local magnetic roughness measurement. The domain size analysis of the flower-like pattern reveals that the domain structure is consistent with a model of branching domains. In addition, an in-plane anisotropy is identified to be responsible for the highly self-similar flower-like pattern. Finally, there is also evidence of nanometer scale sub-domain structure inside each branching domain, comparable with what has been reported on uniaxial magnetic material Nd₂Fe₁₄B.   

Local Moment Paramagnetism in Insulating CrAl3Bi2O9

CrAl₃Bi₂O₉ forms in a mullite-type structure characterized by linear chains of edge-sharing, Cr- and Al-centered octahedra. Measurements of dc magnetic susceptibility χ carried out on high quality single crystals of this compound reveal a fluctuating moment of 2.60 μ_B/Cr -- reduced from the 3.87 μ_B/Cr expected for Cr³⁺ -- while χ deviates from the Curie-Weiss law below temperature T ~ 50 K, remaining enhanced down at least to T = 3 K. The Weiss temperature Θ_W = -23 K suggests that the prevailing local moment interactions are antiferromagnetic in nature. Measurements of magnetization vs. field H below T = 10 K reveal deviations from H/T scaling, suggesting that the additional contribution to χ is not Brillouin-like. UV/Vis spectroscopy reveals a rapid increase in absorption corresponding to transitions across an insulating gap of ~ 3 eV, slightly larger than the DFT direct gap of Al₄Bi₂O₉.   

Superconductivity in SrTiO3 doped with different rare earth ions

Recently, the superconducting dome of electron-doped SrTiO₃ has attracted renewed interest. For example, it becomes superconducting even at extremely low carrier densities (< 10¹⁸ cm^-3) and it has been suggested that its superconductivity cannot be explained by BCS theory. The n-type dopants used in the literature include La, Nb, and oxygen vacancies. Here we compare the superconducting transitions of SrTiO₃ films grown by molecular beam epitaxy and doped with different trivalent rare earth ions, such as La or Sm. The La-doped samples show transitions similar to those of bulk SrTiO₃ reported in the literature. The Sm-doped films show metallic resistivity at high temperatures at carrier densities in the range of 0.5 to 1 10²⁰ cm^-3. At low temperatures, around 100 – 200 mK, onset of superconductivity is observed but characteristics differ sharply from those of La-doped films. We discuss the relevance of the findings for the mechanisms of superconductivity on doped SrTiO₃.
  

Microscopic observation of coherent 2D metallic behavior on the Pd-terminated surface of metallic delafossite PdCrO2

The extraordinarily strong in-plane conductivity of metallic delafossites such as PdCoO₂ and PdCrO₂ and their possible origins have been subjects of considerable attention. We performed Scanning Tunneling Microscopy/Spectroscopy (STM/STS) study on the cleaved surfaces of single crystal PdCrO₂. The CrO₂-terminated surface shows a 0.5 eV gap, which is consistent with Mott insulating state of CrO₂ layer. The Pd-terminated surface has metallic density of states and shows hexagon-shaped phase domains of Pd atoms displaced from their bulk-like Oxygen(O) on-top sites to O-O bridge sites. On the Pd-terminated surface, we observed incommensurate quasiparticle interferences with long coherence lengths due to Pd 5s band in the empty states, whereas strongly localized 4d electronic states on individual Pd atoms in the filled states. The extraordinarily long coherence length of the empty-state quasiparticle interferences that extends beyond multiple phase boundaries on Pd-terminated surface may be attributed to the coupling to the underlying CrO₂ layer and is consistent with the unusually high in-plane conductivity in this class of materials.   

Temperature dependence of the elastic properties of LaCrGe3 studied by resonant ultrasound spectroscopy

LaCrGe₃ is an itinerant ferromagnet with T_C ~ 85 K, fragile magnetic order and a rich and complex temperature – pressure – magnetic field phase diagram [1,2]. We will report a study of the temperature – dependent elastic properties of LaCrGe₃, through its ferromagnetic transition, using resonant ultrasound spectroscopy. The overall behavior will be discussed together with other thermodynamic and transport measurements. Experimental elastic constants will be compared with the results of recent first principles band structure calculations [3].
[1] V. Taufour, et al., Phys. Rev. Lett. 117 (2016) 037207.
[2] U.S. Kaluarachchi, et al., Nat. Comm. 8 (2017) 546.
[2] M. C. Nguen, et al., preprint (2017).
  

Magnetocaloric Effect in Quintenary and Sextenary Heusler Alloys

Measurements on alloys based on the shape-memory compound Ni₂MnGa have revealed large entropy changes, ΔS, in magnetic fields, H. Significant magnetocaloric effects are generally observed in materials containing expensive rare-earth elements like Gd or toxic elements like As. Such materials are ill-suited for commercial magnetic refrigeration applications. In contrast, alloys based on Ni₂MnGa contain relatively inexpensive and non-toxic elements; a large magnetocaloric effect in such materials would be of particular interest. In this talk, we present heat capacity and magnetization measurements on polycrystalline samples of several quintenary and sextenary Heusler alloys. We determined ΔS using heat capacity measurements performed in applied magnetic fields. Isothermal magnetization measurements as a function of H were performed on selected samples at temperatures in the vicinity of the magnetic phase transition. Using a combination of magnetization and heat capacity data, we estimated the adiabatic temperature change, ΔT_a, that results from applying a magnetic field of H = 5 T. The largest value we observed was ΔT_a ~ 3.5 K, but further work may reveal larger effects as chemical compositions are optimized.   

Mn-Induced Structural and Physical Property Modifications in Sr4(Ru1-xMnx)3O10

Triple-layered Sr₄Ru₃O₁₀ is a unique ferromagnetic system with the central RuO₆ layer behaving differently from two outer layers both crystallographically and magnetically. We report that the partial substitution of Ru by smaller Mn gives rise to modifications in crystal structure, and electronic and magnetic properties of Sr₄(Ru_1-xMn_x)₃O₁₀. Through the single crystal x-ray diffraction refinement, we find that (Ru/Mn)O₆ octahedral rotation is reduced with increasing x and becomes undetectable at x ≥ 0.23, making the system changing from the orthorhombic to tetragonal structure. The magnetization measurement indicates the ferromagnetic transition temperature T_C decreases from 105 K (x = 0) to 70 K for x = 0.41, with the reduced average magnetic moment as well. What is remarkable is that Mn doping induces magnetic anisotropy change from the easy c axis in x = 0 to the easy ab plane in x = 0.41. Such change also removes the ab-plane metamagnetic transition observed in x = 0. Furthermore, the electrical resistivity increases with increasing x showing semiconducting behavior for x = 0.41. We discuss our results in the context of strong spin-charge-lattice-orbital interaction.   

Phonon and Magneto-electric ordering in Gallium Ferrite multiferroic

Multiferroic materials with transition temperatures around room temperature are of current interest due to their technological importance and novel applications. Herein we report phonons, magnetic and ferroelectric ordering in perovskite GaFeO3 using magnetization, dielectric and Raman scattering measurements. Single phase GFO ceramics, prepared by solid state reaction shows orthorhombic phase (space group C_2v⁹) studied under xrd pattern. Temperature dependent magnetization behaviors studied in between 5-395 K at both field-cooled and zero-field-cooled conditions identify Néel transition (T_N) at around 225 K. Evidence of spin-glass like magnetic ordering is observed at low temperature. Phonon frequencies and thier line-widths exhibit anomalies across T_N. The changes in phonon frequencies observed in Raman scattering are explained based on spin-phonon coupling in the low temperature ferrimagnetic phase. Examination of the phonon behavior indicates that three phonon anharmonicity process is dominant over other higher order processes. The temperature and frequency dependent dielectric measurements indicated a dielectric anomaly at around 590 K suggesting that the system exhibits ferroelectric relaxor behavior. The detailed experimental results and their analysis will be presented.   

Cluster dynamical mean-field theory study of the deficient spinel chalcogenide GaV4S8

We present a theory of the deficient spinel compound GaV₄S₈ (GVS) using a cluster DMFT approach. GVS has been of growing interest recently because of its multiferroicity below T_C ~ 13K. At T_f ~ 70K it has an additional structural transition from a cubic to a rhombohedral structure with cooling. Across the two structural transitions GVS remains insulating with a gap of about ~ 0.3 eV, and the insulating nature of its high-temperature cubic paramagnetic phase is suspected to be of Mott type. However, there have been no systematic studies of the Mott-insulating phases of GVS, which should be important for understanding the ferroelectric and multiferroic phases. For this problem, we apply a cluster DMFT method that treats selected atomic orbitals on a cluster of four V atoms, using a molecular orbital basis set instead of an atomic one. Comparing with single-V-site DMFT calculations, we show that intra-cluster correlations are essential in reproducing the Mott-insulating nature of the high-T cubic paramagnetic phase. Correlation effects on the crystal structure of GVS will be also discussed, and the role of Hund's coupling in tuning V clustering by controlling spin-state transitions will be demonstrated.   

Revisiting magnetic properties of γ- Fe2WO6

Heda Zhang¹, Gordon Peiker², Mengze Zhu¹, Brian Phelan¹, Clarina dela Cruz³, Xianglin Ke¹ (1. Physics and astronomy, Michigan State University; 2. Skyline high school; 3. Neutron scattering division, Oak Ridge National Lab)
Fe₂WO₆ is a p-type semiconductor with a large band gap (1.68 eV), serving as a promising photoelectrode material for water oxidation. Furthermore, this material also exhibits interesting magnetic properties depending on the crystalline phase. In this talk, we revisit the magnetic properties of gamma phase Fe₂WO₆ via magnetic susceptibility, heat capacity and neutron diffraction measurements. Comparing to an earlier study, we found some inconsistencies regarding the magnetic pattern and the temperature dependence of the magnetic ordering vector . Detailed discussions will be presented.   

High Magnetic Field Investigations of the Dimerized System Ba0.1Sr2.9Cr2O

The well-studied compounds Sr₃Cr₂O₈ and Ba₃Cr₂O₈ are insulating dimerized antiferromagnets with magnetic ions, Cr⁵⁺. These spin ½ ions have a strong intradimer antiferromagnetic coupling, which leads to a singlet ground state and gapped triplet states. Spontaneous long-range ordering occurs upon applying an external magnetic field. We investigate the magnetic properties of Sr₃Cr₂O₈ by introducing chemical disorder upon replacing Sr by Ba. We performed heat capacity and ultrasound velocity measurements in high magnetic fields up to 30 T and 60 T respectively, in order to map the magnetic phase transitions. Moreover, electron spin resonance (ESR) measurements were done to investigate different transitions between the singlet ground state and gapped triplet states.   

Monte Carlo simulations of the magnetic behavior of diluted hexaferrites

The hexagonal ferrites BaFe₁₂O₁₉, SrFe₁₂O₁₉ and PbFe₁₂O₁₉ are ferrimagnets with Neel temperatures of approximately 720K. They also show interesting ferrolectric properties. By randomly substituting Ga ions for the Fe ions, the Neel temperature can be suppressed to zero. Recent experiments [1] have found the phase boundary to vary as T_N ∼ (1 - x/x_c)^2/3 over a wide x range, with x_c very close to the percolation threshold of the iron spins. We report the results of large-scale Monte Carlo simulations to explain this unusual phase boundary as well as the properties of the zero-temperature phase transition.
[1] S.E. Rowley at al., Phys. Rev. B 96, 020407(R) (2017)   

Alternating-spin-chain compound AgVOAsO4 probed by 75As NMR

⁷⁵As NMR measurements were performed on a polycrystalline sample of spin-1/2 alternating-spin chain Heisenberg antiferromagnet AgVOAsO₄. Temperature-dependent NMR shift K(T), which is a direct measure of the intrinsic spin susceptibility, agrees very well with the spin-1/2 alternating-chain model, justifying the assignment of the spin lattice. From the analysis of K(T), magnetic exchange parameters were estimated as follows: the leading exchange J/k_B = 38.4 K and alternation ratio α = J′/J = 0.69. The transferred hyperfine coupling between the 75As nucleus and V4+ spins obtained by comparing the NMR shift with the bulk susceptibility amounts to A_hf = 3.3 T/μ_B. By fitting the low temperature region of K(T) and 1/T₁ to a 3D activated behaviour, a zero-field spin gap of around 15 K was obtained. This matches very well with the results and theoretical calculations of previous paper on the same compound by Tsirlin et. al. The effect of interchain couplings on the low-temperature activated behavior of K(T) and spin-lattice relaxation rate 1/T₁ is identified.