Session R8: Correlated Electron Magnetism

Origins of ferromagnetism and antiferromagnetism in Gd$_{5}$Ge$_{4}$

The origin of ferromagnetism appearing as a result of a magnetic-field-induced first-order phase transition in Gd$_{5}$Ge$_{4}$ is explored by calculating the total energy, local exchange splitting, density of states, and magnetic moments. The calculations were performed using density functional approaches including the on-site Coulomb interaction parameter. The total energy as a function of shear distortion along the $a$ axis for two different orthorhombic structures is in agreement with experiment, indicating a first-order magnetostructural transition in Gd$_{5}$Ge$_{4}$. The rearrangement of Gd 5$d$ and Ge 4$p$ densities of states, the substantial differences in atom-projected band energies, the exchange splitting, and the magnetic moments calculated with ferromagnetic spin arrangements in the orthorhombic Sm$_{5}$Ge$_{4}$-type and Gd$_{5}$Si$_{4}$-type structures of Gd$_{5}$Ge$_{4}$ help to clarify the differences in the magnetic states of these two structures. Our calculations indicate that the Sm$_{5}$Ge$_{4}$-type structure of Gd$_{5}$Ge$_{4}$ is the structural ground state and that it is antiferromagnetic.   

Unconventional metallic magnetism in LaCr$_{\mathrm{1-x}}$V$_{\mathrm{x}}$Sb$_{3}$ system

Unconventional, low temperature ground states can often be found in the vicinity of a magnetic phase transition that has been continuously tuned to 0 K. As part of our search for exotic superconductivity, we have studied the LaCrSb$_{3}$ system. Whereas magnetization measurements of LaCrSb$_{3}$ under pressure reveal no change of $T_{\mathrm{C}}$ up to $\approx $ 5 GPa, the ferromagnetic transition is gradually suppressed with increasing V substitution. Single crystals of the LaCr$_{\mathrm{1-x}}$V$_{\mathrm{x}}$Sb$_{3}$ series have been characterized by measurements of, temperature dependent magnetic susceptibility, magnetization, electrical resistivity, and specific heat. Determinations of magnetic anisotropies as well as ferromagnetic ordering temperatures have been made. Below $T_{\mathrm{C}}$, spin reorientation has been observed within \textit{bc} plane. A $T$ -- $x$ phase diagram has been assembled to shed light on the magnetism in this system.   

Optical investigations on spin density wave instability in SrMnBi$_{2}$

We investigated the electronic response of layered transition metal pnictide SrMnBi2 using infrared spectroscopy. SrMnBi2 has a crystal structure similar with that of Fe-based superconductors and shows antiferromagnetic order at high temperature 290 K. We observe that the onset of antiferromagnetic order induces a partial gap formation. Upon entering the antiferromagnetic state, the Drude response is drastically suppressed and the spectral weight is transferred to higher energies. Our results suggest that the antiferromagnetism in SrMnBi2 may be associated with spin-density-wave instability of itinerant carriers. We will discuss possible origins of the density-wave-instability based on the first-principles-calculation results.   

Resonant Ultrasound studies of double perovskites A$_{2}$FeReO$_{6}$ (A=Ba, Ca)

The elastic response as a function of temperature (50-380) K and magnetic field (0-2) T has been studied using Resonant Ultrasound Spectroscopy (RUS) for the polycrystalline double perovskites A$_{2}$FeReO$_{6}$ (A= Ba, Ca). An elastic softening over a wide temperature range is observed below the Curie temperature (T$_{c}$ $\sim$ 305K) of Ba$_{2}$FeReO$_{6}$, which is suppressed upon the application of a magnetic field. For Ca$_{2}$FeReO$_{6}$, both the longitudinal and shear modulus show a step-like softening starting around 140K, indicative of a structural transition. A large change in the magnetoelastic coupling constant is observed at this temperature, suggesting that this transition is strongly coupled to the magnetic properties of this material.   

Chemical Doping Induced Ferro- and Antiferro-magnetic States in non-Magnetic Insulating FeGa$_{3 }$

A ferromagnetic quantum critical point (FM-QCP) in Ge-doped FeGa$_{3}$ was reported very recently (Umeo \textit{et al}. PRB \textbf{86} (14), 144421, 2012). We have simultaneously observed the FM-QCP in this system. Furthermore, we analyzed the magnetic properties of FeGa$_{3}$ in the context of a unique structural feature, where the four Fe atoms in the unit cell exist as two Fe-Fe dimers (Yin and Picket, PRB \textbf{82} (15), 155202, 2010). We propose a phenomenological model where the extrinsic electrons from the Ge doping creates a mixed valence Fe-dimer with a net effective spin. Such a model provides a novel mechanism for the (FM-QCP) and is consistent with the system's magnetic and thermal properties. In addition to Ge doping, we have investigated effects of Ru/Mn substitution on the Fe site. Ru substitution produces an unexpected ferromagnetic (FM) insulating phase that develops immediately, and it disappears above an intermediate doping level. This behavior agrees well with our model of spin creation on the transition-metal dimers via conduction electrons, and the enhanced insulating behavior in the electrical resistivity suggests the Ru acquires a 2$+$ state. Interestingly, Mn-doped FeGa$_{3}$ shows an apparent antiferromagnetic (AFM) insulating phase, where the magnetic data is consistent with the effective moment coming from Mn(3$+)$. These results provide further evidence of the important role of the Fe-Fe dimer structure in FeGa$_{3\, }$in determining its unique magnetic properties.   

Ferromagnetic Fluctuations Enhanced by Mn Doping in Sr$_2$RuO$_4$

Sr$_{2}$RuO$_{4}$ is the first experimentally established example of a spin-triplet superconductor [1]; it has attracted a great deal of interest since its discovery in 1994. Like other unconventional superconductors, the superconductivity of Sr$_{2}$RuO$_{4}$ also occurs in close proximity to magnetic instability. Its normal state is characterized by incommensurate antiferromagnetic (AFM) fluctuations associated with Fermi surface nesting. Moreover, the other ruthenate compounds related to Sr$_{2}$RuO$_{4}$ in the Ruddlesden-Popper series are all magnetic. The Sr-based members Sr$_{3}$Ru$_{2}$O$_{7}$, Sr$_{4}$Ru$_{3}$O$_{10}$ and SrRuO$_{3}$ are either metamagnetic or ferromagnetic (FM), whereas the Ca-based members Ca$_{2}$RuO$_{4}$ and Ca$_{3}$Ru$_{2}$O$_{7}$ are AFM. We have investigated the Mn doping effect in Sr$_{2}$RuO$_{4}$ using floating-zone grown single crystal samples and observed significantly enhanced FM fluctuations in the Mn-doped Sr$_{2}$RuO$_{4}$ samples. The system becomes nearly FM with only a few percent Mn doping. This finding suggests that Sr$_{2}$RuO$_{4}$ involves competing, orbital dependent magnetic fluctuations.\\[4pt] [1] A. P. Mackenzie and Y. Maeno, Rev. Mod. Phys. \textbf{75}, 657 (2003).   

Spin-state controlled electronic and magnetic structures of Sr$_{2-x}$La$_{x}$CoO$_{4}$

Sr$_{2-x}$La$_{x}$CoO$_{4}$ is an interesting group of materials, and they display abundant electronic and magnetic properties. In this work, we studied those properties, using electron-correlation corrected density functional calculations. We find that besides a charge-state variation induced by La doping, a multiple spin-state transition takes place and determines (1) a metal-insulator transition and a ferromagnetic insulating phase for $x$=0.5 [1], (2) a paramagnetic Mott insulating phase with a mixed high-spin and low-spin state for $x$=1, instead of a ferromagnetic half-metallic solution with a homogeneous intermediate-spin state [2], and (3) a charge-ordered highly insulating phase with an active spin-blockade mechanism for $x$=1.5 [3]. [1] H. Wu, Phys. Rev. B 86, 075120 (2012). [2] H. Wu, Phys. Rev. B 81, 115127 (2010). [3] H. Wu and T. Burnus, Phys. Rev. B 80, 081105(R) (2009).   

Doping Induced Itinerant Ferromagnetism in CoAs

The magnetism in $\alpha $-CoAs is dominated by strong spin fluctuations. In this study, we explore the effects of Phosphorus doping in $\alpha $-CoAs. Phosphorus is isovalent with Arsenic, and the resulting doping introduces disorder and chemical pressure. In CoAs$_{\mathrm{1-x}}$P$_{\mathrm{x,}}$ a cross-over from the spin fluctuation-dominated regime to an itinerant ferromagnetic (IFM) state take places around x $=$ 0.04. The IFM state persists up to x $\le $ 0.27. For compositions between x $=$ 0.28 and 0.40, the magnetization data suggests a possible Stoner enhanced state. We acknowledge the support from DOD PECASE.   

Magnetic and Orbital Ordering of KCuF$_{3}$ Studied by Soft X-ray Scattering

The interplay between charge, orbital, and spin degrees of freedom plays an important role in the underlying physics of transition-metal compounds. The charge-transfer insulator KCuF$_{3}$ is an archetype of orbitally ordered materials with large exchange interaction energy. KCuF$_{3}$ has long been known to display quantum one-dimensional antiferromagnetic properties along the $c$-axis originating from the superexchange interaction between the $e_g$ orbitals of Cu$^{2+}$. Due to the large Jahn-Teller distortion in the tetragonal structure, the degeneracy of the two $e_g$ orbitals is lifted and the $e_g$ orbitals form a pattern of orbital ordering . In this talk, we will present our recent measurements of spin and orbital ordering of KCuF$_{3}$ by soft X-ray scattering to address its magnetic transition and the coupling between spin and orbital degrees of freedom.   

Controllable chirality-induced geometrical Hall effect in the frustrated strongly-correlated metal UCu$_{5}$

A current of electrons traversing a landscape of localized spins possessing non-coplanar magnetic order gains a geometrical (Berry) phase which can lead to a Hall voltage independent of the spin-orbit coupling within the material--a geometrical Hall effect. In this talk, I will present experimental data and Monte-Carlo simulation results showing that the strongly-correlated metal UCu$_{5}$ possesses an unusually large controllable geometrical Hall effect at $T$ \textless\ 1.2K due to its frustration-induced magnetic order. The magnitude of the Hall response exceeds 20{\%} of the $\nu =$1 quantum Hall effect per atomic layer, which translates into an effective magnetic field of several hundred Tesla acting on the electrons. The existence of such a large geometric Hall response in UCu$_{5}$ opens a new field of inquiry into the importance of the role of frustration in highly-correlated electron materials. \textit{B.G. Ueland et al., Nat. Commun. }\textbf{\textit{3}}\textit{, 1067 (2012).}   

Dynamical spin correlation function in a frustrated two-leg spin-ladder system

We numerically study the magnetic excitations in a frustrated two-leg spin-ladder system, in which all magnetic exchange interactions, i.e., the nearest-, next-nearest-neighbor sites in the leg direction, and the nearest-neighbor sites in the rung direction, are antiferromagnetic. This is a minimal model describing a low-dimensional quantum spin compound, BiCu$_2$PO$_6$. We calculate a dynamical spin correlation function at zero temperature by using the dynamical density-matrix renormalization-group method in possible magnetic phases, columnar dimer and rung singlet. The columnar dimer phase is characterized by multi-spinon excitations, while the rung singlet phase is dominated by a triplon excitation, which is the triplet excitation in the rung direction. Difference between these two types of excitations appears in the spectral weight, in particular, of the bonding and anti-bonding modes in the rung direction. Therefore, we can distinguish one phase from the other by distribution of the spectral weight. In addition, we examine frustration effect on the bonding mode, so-called bound triplon, with a perturbation theory from the strong coupling limit in the rung direction. Our study is expected to be useful to analyze inelastic neutron scattering data for BiCu$_2$PO$_6$.   

Critical magnetic scattering in geometric frastrated multiferroic LuMnO$_3$

The coexistence of competing order parameters in the class of materials referred to as the multiferroics is of great interest. The hexagonal manganites \textit{A}MnO$_3$ (\textit{A} = Y, Lu, Ho and Yb) with the \textit{P6$_3$cm} space group exhibit a ferroelectric transition, at very high temperatures, typically $\sim$ 1000 K, while the antiferromagnetic transition, \textit{T$_N$}, occurs at $\sim$ 100K. Earlier studies on YMnO$_3$ and LuMnO$_3$ using neutron scattering on single crystals showed that diffuse scattering is present around the forbidden nuclear (100) Bragg peak which corresponds to \textit{Q}=1.20 \AA$^{-1}$.Its intensity rises very sharply and drops just around \textit{T$_N$}. We performed inelastic neutron scattering measurement on a powder sample of LuMnO$_3$ form 4 to 250 K using the DCS at NIST. Strong inelastic intensity, not due to magnon excitations, is observed at \textit{Q}=1.32 and 2.50 \AA$^{-1}$. With cooling, the intensity gradually rises and reaches a peak around 100 K. Below, it drops drastically once the system orders. This kind of scattering is due to critical scattering arising from magnetic fluctuations above \textit{T$_N$}. The S(Q, $\omega$) is asymmetric suggesting that the Mn spin correlations are mosmost likely 2-dimensional in nature.   

Electron mediated magnetism in two-dimensional spin-ice

In this work we study the magnetic phase diagram of classical spins which interact with itinerant electrons on a checkerboard lattice, a lattice that constitutes a two-dimensional equivalent of the three-dimensional spin-ice pyrochlore lattice. We explore both the strong coupling and weak coupling limit and find a rich ground state phase diagram as function of interaction strength and electron doping. The strong coupling limit allows for unbiased Monte Carlo simulations of the classical spins combined with exact diagonalization of the fermionic Hamiltonian. For half filling we find a very robust coplanar orthogonal spin state, the robustness of which originates from the strong geometrical frustration of the checkerboard lattice. In the weak coupling approach this double-Q spin state is a consequence of fermi-surface nesting. The electronic spectrum is this state consists of two Dirac points in full analogy with graphene. For other special electron filling fractions such as $n=p/q=1/4,3/4,3/8,5/8$, we find collinear ``loop'' states, where the spins order in disconnected loops of fixed length $q$. Interestingly, for intermediate fillings the ground state is a mixture of loops of different size, which can be captured by an emergent electromagnetic theory with fractional charge.   

Phase diagram and chirality of the spin-1/2 J1-J2 Heisenberg model on the kagome lattice

We studied the spin-1/2 Heisenberg model on the kagome lattice with nearest (J1) and next-nearest neighbor (J2) interactions by means of the density matrix renormalization group. We set J1 as antiferromagnetic coupling (J1 \textgreater\ 0), and J2 can be either ferromagnetic (J2 \textless\ 0) or antiferromagnetic (J2 \textgreater\ 0). By analyzing the spin-spin correlation function and the bond energy, we find a valence-bond crystal phase for J2 \textless\ -0.1 and a magnetically ordered phase for J2 \textgreater\ 0.2. In the intermediate paramagnetic phase, we investigate the evolution of spin and singlet gaps, topological entanglement entropy, dimer and chirality correlations as a function of the parameter J2. In particular, we investigate the local p6 chiral order parameter proposed recently by measuring the dimer-dimer correlation functions to study the possible reflection symmetry breaking in this spin liquid candidate.   

Collinear Magnetic Order in an Isotropic Triangular Antiferromagnet: The Sn/Si(111) Surface System

The one-electron spectral function is the key quantity to extract detailed information on the complex spin pattern in a frustrated magnetic system. This is demonstrated here by a detailed comparison of theory, which combines a priori density-functional (LDA) with cluster many-body (LDA + DCA) calculations, with high-precision angle-resolved photoelectron spectroscopy (ARPES). The role model in this work is the isotropic triangular antiferromagnetic Sn/Si(111). Its geometric frustration and strong electronic correlations are shown at low temperatures to combine to an unexpected magnetic, i.e. collinear order, and not the possible spiral ($120^{\circ}$) antiferromagnetic order or a disordered spin-liquid phase.