Session A23: Bulk Manganites and Cobaltites

Identification and control of domain wall order in spinel ferrimagnets

Spinel antiferromagnets have long been at the center of research into strong spin-lattice coupling and orbital effects. Among other properties, these materials frequently demonstrate concomitant magnetic and structural phase transitions, heightened magneto-elastic or dielectric response functions, and low-temperature multiferroism. There is very little agreement on the microscopic picture to be associated with these effects, but recent work has shown that mesoscale inhomogeneity can play a key role in raising the susceptibilities of complex materials to external perturbations.

In this talk, I will discuss recent work at the University of Illinois which establishes the importance of mesoscale heterogeneity in determining bulk magnetic properties of spinel ferrimagnets Mn₃O₄ and MnV₂O₄. This will first include a review of Raman and x-ray scattering results which reveal the existence of low-temperature magnetostructural transitions and magnetic force microscopy data which show the existence of stripe-like magnetic domains, before turning to our more recent neutron scattering and muon spin rotation (μSR) measurements. Our μSR work associates stripe-like domains in Mn₃O₄ with a real space separation into magnetically ordered and disordered volumes, and further shows that the ordering fraction can be grown with the application of moderate-sized fields. With small angle neutron scattering, we observe Bragg signatures of domain wall order in the bulk of both materials, with wall separations of ~200nm. Lastly, I will present data that demonstrates how domain wall motion can be used to drive these systems out of their low-temperature ferrimagnetic states, with an H_c which is highly sensitive to the applied field direction. I will discuss these results in the context of “colossal” control of magnetic properties, and implications for materials beyond the specific case of spinel antiferromagnets.   

Direct Visualization of Incommensurate Charge-Lattice Order in Correlated Oxides

The strong coupling between charge, spin, and lattice generates complex correlated phases and, in many cases, ordered patterns that break the spatial symmetries of the crystal. Charge-order stripes are particularly noteworthy because they alter electronic transport, compete with unconventional superconductivity, or spawn novel magnetic orders. As demonstrated by various real space probes, mapping striped electronic modulations locally is a powerful approach for visualizing stripe formation, observing defects and excitations, and discerning broken symmetries. Direct, atomic-scale measurements of lattice degrees of freedom, however, are lacking. Here, we use cryogenic scanning transmission electron microscopy (STEM) to map picometer-scale lattice displacements associated with charge-ordering in manganites. We directly visualize temperature-dependent nanosocale inhomogeneity and the interplay between commensurate and incommensurate modulations. In particular, the real space structure of incommensurate charge order suggests a strong tendency to lattice-locked modulations interspersed with phase defects. These cryogenic STEM measurements reveal a novel methodology for exploring lattice coupling in other correlated systems such layered nickelates and transition-metal dichalcogenides.   

Emissivity and Reflectivity of Orthorhombic and Cubic ErMnO3: Small Polarons and Bipolarons

We report on far- and mid-infrared emission and reflectivity from 40 cm^-1 to 13000 cm^-1 of ambient orthorhombic O-ErMnO₃ to sample dissociation at ~1800 K. While at 300 K the number of phonons are in close agreement with the predicted for D_2h¹⁶(Z=4), it gradually decreases by progressive broadening, merging, and band softening into the orbital disordered phase in which the orthorhombic O' lower temperature cooperative coexists with orthorhombic O. Reik’s small polaron[1] fits of the optical conductivities in the Jahn-Teller cooperative phase and in the orbital disordered phase below 1250 K single out the octahedral symmetric and antisymmetric as main phonons in the electron-phonon interactions. At higher than the order-disorder transition at T_cubic ~1329 K ± 10 K three infrared active F_1u modes coincide with the expected for perovskite Pm-3m (Z=1). A distinctive band peaking at ~6000 cm^-1 is associated to small bipolarons (two electrons self-trapped in a bonding connecting orbital mediated by the longitudinal mode distortion). At ~1700 K its optical conductivity only involves the cubic highest longitudinal optical mode. An overdamped Drude term by itinerant carriers coexisting with localized bipolarons is found at the highest temperatures.[1] H. G. Reik, Z. Phys. 203,346 (1967).   

Mapping out the Fermi surface of CMR manganites

The rare-earth manganites have been the subject of intensive research for many years. Despite this attention, many questions still remain, especially regarding the low-temperature ferromagnetic phase that exhibits colossal magnetoresistance (CMR). By applying pulsed magnetic fields of up to 100 T, we have been able to observe two series of magnetic quantum oscillations in the resistivity of high-quality La_1-xSr_xMnO₃ crystals in the CMR phase. The oscillations exhibit frequencies less than 1 kT and their temperature dependence yields effective masses around 2m_{e .} These observations suggest that the large “woolsack” Fermi surface sections predicted by bandstructure calculations fragment into smaller pockets at low temperatures, in a manner analogous to the behavior of the Fermi surface of the hole-doped cuprates.   

Magnetic Order and Interactions in Ferrimagnetic Mn3Si2Te6

Mn₃Si₂Te₆ is a three-dimensional material with a trigonal unit cell that is structurally-related to the van der Waals bonded ferromagnetic semiconductors CrSiTe₃ and CrGeTe₃. In Mn₃Si₂Te₆, the filling of an interlayer octahedral position leads to three frustrated magnetic exchanges that result in a ferrimagnetic spin configuration. The magnetism has been investigated using thermodynamic and transport measurements on single crystals, first principles calculations, and neutron diffraction and diffuse scattering. A net moment exists due to an imbalance in the multiplicity of two different Mn sites, and an unexpectedly large anisotropy field of ≈9T is present at 2K. The 3^rd nearest neighbor exchange is stronger than the 2^nd nearest neighbor exchange. Frustration results in a relative suppression of the Curie temperature with T_C=78K compared to a Weiss temperature of -277K. As a result, short range correlations exist well-above T_C, as demonstrated via diffuse neutron scattering. This study has revealed complex magnetic behavior in Mn₃Si₂Te₆ and highlights the variety of states that may be accessible in this family of materials. This research was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.   

Electronic structure of Ba(Zn0.875Mn0.125)2As2

The electronic structure of single-crystalline Ba(Zn0.875Mn0.125)As2, the parent compound of the recently found high temperature ferromagnetic semiconductor-(Ba,K)(Zn,Mn)2As2, was studied by high-resolution angle-resolved photoemission spectroscopy. A flat band related to the 3d states of the doped Mn atoms was observed at the binding energy of ~-1.6eV besides the previously reported feature at about 3.3eV. The spectral weight of this feature is much stronger than that obtained from density functional theory calculations. We proposed that the large spectral weight could originate from the hybridization between Mn and As orbitals. The overall band dispersions agree well with the first-principles calculations of undoped BaZn2As2 except that the total bandwidth of the valence band is larger than that calculated. The increased bandwidth after Mn doping is compatible with the proposed theoretical model.   

Magneto-orbital ordering in the divalent A-site quadruple perovskite manganites AMn7O12 (A = Sr, Cd, and Pb)

Through analysis of variable temperature neutron powder diffraction data, we present solutions for the magnetic structures of SrMn₇O₁₂, CdMn₇O₁₂, and PbMn₇O₁₂ in all long-range ordered phases. The three compounds were found to have magnetic structures analogous to that reported for CaMn₇O₁₂. They all feature a higher temperature lock-in phase with commensurate magneto-orbital coupling, and a delocked, multi-k magnetic ground state where incommensurate magneto-orbital coupling gives rise to a constant-moment magnetic helix with modulated spin helicity. CdMn₇O₁₂ represents a special case in which the orbital modulation is commensurate with the crystal lattice and involves stacking of fully and partially polarized orbital states. Our results provide a robust confirmation of the phenomenological model for magneto-orbital coupling previously presented for CaMn₇O₁₂. Furthermore, we show that the model is universal to the A²⁺ quadruple perovskite manganites synthesised to date, and that it is tunable by selection of the A-site ionic radius.   

Strong dependence of the oxygen-isotope effect on the A-site radius in half-doped A-site disordered Ba0.5Ln0.5MnO3

Oxygen-isotope effects have been studied in half-doped A-site disordered Ba_0.5Ln_0.5MnO₃ with different ionic radii of the A-site (Ba_0.5Ln_0.5). We find that the ground states of the half-doped manganites strongly depend on the A-site ionic radii. Below a critical ionic radius, the ground state is spin-glass like and the freezing temperature T_SG is independent of the oxygen-isotope mass. Above the critical ionic radius, the ground state is ferromagnetic and the Curie temperature T_C depends strongly on the oxygen-isotope mass. As the ionic radius increases, the Curie-temperature increases while the oxygen-isotope exponent decreases monotonically. The current results are difficult to understand in terms of the double-exchange model along with strong lattice polaronic effects.   

Magnetic ground state and magnon-phonon interaction in multiferroic h-YMnO3

Inelastic neutron scattering has been used to study the magneto-elastic excitations in the multiferroic manganite hexagonal YMnO₃. An avoided crossing is found between magnon and phonon modes close to the Brillouin zone boundary. Neutron polarization analysis reveals that this mode has mixed magnon-phonon character. An external magnetic field along the c-axis is observed to cause a linear field-induced splitting of one of the spin wave branches. A theoretical description is performed, using a Heisenberg model of localized spins, acoustic phonon modes and a magneto-elastic coupling via the single-ion magnetostriction. The model quantitatively reproduces the dispersion and intensities of all modes in the full Brillouin zone, describes the observed magnon-phonon hybridized modes, and quantifies the magneto-elastic coupling. The combined information, including the field-induced magnon splitting, allows us to exclude several of the earlier proposed models and point to a magnetic ground state, of the P6'₃c'm symmetry, and provides an effective dynamic model relevant for the multiferroic hexagonal manganites.   

Magnetic structure and spin-wave dispersion in spinel FeMn2O4

Spinel FeMn₂O₄ is known to exhibit a cation distribution of (Mn²⁺)_A(Mn³⁺Fe³⁺)_BO₄, with Mn²⁺ mainly occupying the tetrahedral (A) sites forming a diamond lattice, and Fe³⁺ and Mn³⁺ sharing the octahedral (B) sites to form a pyrochlore lattice. Powder neutron diffraction reveals a cubic-tetragonal structural transition at ~595 K due to the Jahn-Teller distortion of MnO₆ octahedra. A collinear ferrimagnetic order with antiparallel moments between A and B sites is found below ~373 K, and non-collinear ferrimagnetic order appears below ~50 K. Spin wave dispersions in both the collinear and non-collinear ferrimagnetic ordered regions have been determined via inelastic neutron scattering on a large single crystal. At 8 K we observe a spin gap of approximately 5 meV arising from single-ion anisotropy at the orbitally active Mn³⁺ site. We argue that the orbital degree of freedom of the B-site Mn³⁺ ion and its coupling to the spin and lattice degrees of freedom play a key role in the structural and magnetic properties in FeMn₂O₄.   

Evidence of a new magnetic anomaly below the ferromagnetic Curie temperature in highly ordered La2CoMnO6

Bulk double perovskite La₂CoMnO₆ (LCMO) is an insulating ferromagnet with Curie temperature (T_C) of 225 K. The monoclinic P2₁ /n phase of LCMO was synthesized using sol-gel technique. Temperature dependence of magnetization showed two distinct transitions at 226 and 218 K corresponding to Mn⁴⁺–O–Co²⁺ ferromagnetic (FM) superexchange (T_C ~226 K) and Co²⁺–O–Co²⁺ antiferromagnetic (AFM) interactions, respectively. A relatively high value of T_C and µ_eff ~ 7.51 µ_B confirm the highly ordered state of Co and Mn ions. Interestingly, AC measurements indicate the presence of two additional transitions: a frequency dependent peak at 135 K which signifies a spin-glass-like state and an anomaly at ~40 K due to low-temperature Mn⁴⁺–O–Mn⁴⁺ AFM interactions. A careful examination of the temperature and field dependence of magnetic entropy change (ΔS_M), calculated via the magnetocaloric effect, supports the above findings. Specifically, our results indicate the presence of a low-field AFM phase (218 K) just below the ferromagnetic transition (226 K) that is suppressed at a critical field, which has not been previously reported.   

Pressure-Induced Magnetic and Electronic Transition in SrCoO3 Perovskite

Perovskite cobaltites (La,Sr)CoO₃ have been intensively studied for their complicated magnetic properties, which arise from the multiple oxidation and spin states of cobalt. Even the end members, LaCoO₃ and SrCoO₃, exhibit great complexity. For example, both temperature and strain can induce spin transition in LaCoO₃, and the spin state of magnetic Co³⁺ has been a matter of debate. Recently, SrCoO₃ has attracted considerable attention. At ambient pressure, SrCoO₃ is a ferromagnetic metal (T_c ~ 305 K) crystalizing in cubic perovskite (Pm-3m symmetry) with lattice parameter a ~ 3.829 Å [1]. The magnetic Co⁴⁺ in SrCoO₃ has been suggested to be in the intermediate-spin (IS) state (S = 3/2). Also, experiments have shown that SrCoO₃ remains in Pm-3m symmetry under compression up to 60 GPa [2]. Using first-principles calculations, we investigate the magnetic and electronic states of cubic perovskite SrCoO₃ . Our calculations indicate that SrCoO₃ undergoes a magnetic and electronic transition induced by pressure.

[1] Y. Long et al., J. Phys.: Condens. Matter 23, 245601 (2011).
[2] J.-Y. Yang et al., Phys. Rev. B 92, 195147 (2015).   

Disentangling Spin-Orbital Interactions - Extraction of far reaching exchange interactions in Cobalt Oxide

Cobalt oxide (CoO) is a classical example where the magnetic superexchange is sufficiently strong such that a significant molecular field induced mixing between single-ion spin-orbit manifolds occurs. This makes the extraction of all multi-neighbor exchange interactions and comparison with first principles theory nearly intractable. We disentangle these interactions via neutron spectroscopic measurements on the diluted structural analog Co_0.03Mg_0.97O, thereby switching off the magnetic-order-induced molecular field that is the origin of this strong admixture. By considering the prevalent `dimer' response, we extract 7 exchange constants out to the fourth coordination shell with a dominant antiferromagnetic next nearest neighbor 180^o superexchange interaction, in agreement with Kanamori's original predictions. Other interactions appear in ferromagnetic/antiferromagnetic pairs, consistent with the Goodenough-Kanamori-Anderson rules and account for the Neel and Curie-Weiss temperatures obtained from thermodynamic measurements.