Session W15: Magnetic Cooperative Phenomena

Ultrafast time-resolved magneto-optical Kerr study of coherent spin waves in La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films

We report on ultrafast time-resolved optical measurements on La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) thin films. The differential polarization shows coherent spin wave excitations in LCMO. We determine the magnetic anisotropy and spin stiffness constants from the magnetic-field dependence of the frequency of the spin waves. The gyromagnetic factor, g $\sim $ 2, is consistent with the value determined for the 3d electrons of Mn ions. Values of the magnetic bulk anisotropies for all the LCMO samples are in good agreement with those from other techniques [1-2]. Moreover, we observed a high-order spin wave mode in LCMO and its frequency shows strong sample dependence. The results are explained by a possible existence of nanoscale ferromagnetic clusters in LCMO. 1. J. O'Donnel, M. S. Rzhowski, J. N. Eckstein, and I. Bozovic, Appl. Phys. Lett. 72, 1775 (1998). 2. C. M. Xiong, J. R. Sun, and B. G. Shen, Solid State Commun. 134, 465 (2005).   

Spin band gap and emerging low energy excitations in a doped cobaltate perovskite

We investigated spin dynamics in half doped cobaltate La$_{1.5}$Sr$_{0.5}$CoO$_{4}$, using inelastic neutron scattering. At high temperatures we observe a broad band of continuum antiferromagnetic dynamic spin correlations, extending to energies above 20 meV. As temperature decreases below spin ordering transition (T$_{SO}$ = 30K), a single coherent magnon mode emerges at low energies (bandwith ~ 15 meV), separated by an energy band gap from a broad continuum scattering residing at high energies $20 \le E \le 30$ meV. This points to an emerging 2D-XY behavior of spins, consistent with the strong planar anisotropy of this material.   

Multi phonon resonance Raman scattering, spin phonon coupling and lattice dynamics of antiferromagnetic NdFeO$_{3}$

The Raman-active phonons in the orthorhombic NdFeO$_{3}$ single crystals were studied by means of polarized Raman scattering and lattice dynamics computations (LDC). The A$_{g}$-symmetry zone-center phonons were distinguished from the B$_{1g}$ eigenmodes by performing polarized Raman scattering experiments using two parallel polarization configurations, X$\prime $(ZZ)X$\prime $ and Z(X$\prime $X$\prime )$Z. Observed phonon spectra at 100, 300, 347 cm$^{-1}$ showed anomalous temperature dependence in the range of the magnetic spin reorientation temperature (100 --200K) indicating strong spin -phonon coupling. The anomalously shaped observed phonon between 500 to 1500 cm$^{{\-}1}_{, }$observed in the A$_{g}$-symmetry X$\prime $(ZZ)X$\prime $ and B$_{1g}$ spectrum was attributed to a multi-phonon scattering caused either by multiple combination of B$_{1g}$ or by A$_{1g }$phonons. With the help of lattice dynamics calculations, we were able to assign most of the observed Raman-active modes, including B$_{2g}$ and B$_{3g}$-symmetry phonons. The LDC results indicated that among the sixteen force constants employed, the force constant corresponding to the stretching vibration between the central Fe cation and the axial oxygen atom in a FeO$_{6}$ octahedron unit had the largest value.   

Dynamic Spin Correlations in a Pyrochlore Slab

We use neutron scattering to study the spin dynamics of Ba$_2$Sn$_2$ZnGa$_3$Cr$_7$O$_{22}$, an $S=3/2$ strongly frustrated, disorder free, quasi-two-dimensional spin system based on a kagom\'{e} sandwich. Low temperature spin freezing, with timescale dependent onset and magnitude, is observed. Relaxation rate distribution displays $\omega/T$ scaling, characteristic of quantum critical regime, and anomalously broadens below 11 K. Wavevector dependence of the excitation spectrum supports the notion that the dynamics is driven by the interaction of spin simplexes, which carry no dipole moment.   

Magnetic Phase Transition and Spin Dynamics in Magnetoelectric Effect LiMnPO$_{4}$

Elastic and inelastic neutron scattering techniques were used to study the magnetic phase transition and spin dynamics in single crystal LiMnPO$_{4}$. In this mulitiferroic, antiferromagnetism coexists with induced ferroelectricity below the Neel temperature. Elastic neutron scattering technique confirmed that LiMnPO$_{4}$ has a collinear antiferromagnetic ground state with magnetic moments oriented along the $a$-axis. The temperature dependent order parameter, calculated from the integrated intensity of the (010)~magnetic reflection, was fit to a power law equation, yielding a transition temperature $T_{N}$ = 33.7 K and a critical exponent of \textit{$\beta $} = 0.114. Above the Neel temperature, in the paramagnetic phase, unusually strong magnetic fluctuations were observed to temperatures as high as 60 K. The correlation lengths in the MnO layer and between the layers were also determined. The spin-wave dispersion curves along $a-$ and $b-$axis were measured in the antiferromagnetic state at 4.5 K and the system was shown to be quasi-2D by analyzing the dispersion curves using a 3D Heisenberg model from linear spin wave theory.   

Inelastic neutron scattering study of the Verwey transition in YBaCo$_2$O$_5$

A new class of oxides with the chemical formula $R$Ba$M_2$O$_5$ ($R$~=~rare-earth, $M$~=~transition metal) are based on the perovskite structure with a doubled unit cell and a layer of oxygen vacancies. The structure consists of pyramids of five-coordinated $M$-sites, and more importantly the $M$-site is mixed valent in the stoichiometric formula unit (with an average valence of +2.5). Here, we present results of inelastic neutron scattering experiments on polycrystalline YBaCo$_2$O$_5$, which orders antiferromagnetically at $T_{\mathrm{N}}$~=~330~K and is accompanied by a tetragonal to orthorhombic distortion. Below $T_{\mathrm{V}}$~=~220~K, orbital/charge ordering occurs as well as a change in the Co$^{2+}$ spin state from low to high spin. A dramatic change in the spin wave spectrum at $T_{\mathrm{V}}$, as well as a large damping of zone-boundary spin waves above $T_{\mathrm{V}}$ are similar to those observed in YBaFe$_2$O$_5$ and are discussed in terms of a coupling of magnetism and valence fluctuations.   

Neutron scattering study of vanadium spinel MnV$_{2}$O$_{4}$.

We report the results of elastic and inelastic neutron scattering studies carried out on single-crystals of MnV$_{2}$O$_{4}$. In this spinel system the octahedral site is occupied by the V$^{3+}$ ion, having two 3d electrons in threefold $t_{2g}$ levels, and the tetrahedral site is occupied by the Mn$^{2+}$ (3d$^{5})$ with no orbital degrees of freedom. MnV$_{2}$O$_{4}$ was found to exhibit a ferrimagnetic ordering at approximately 56 K. There is a structural phase transition from a cubic to a tetragonal phase at about 53K, reportedly associated with a spin reorientation process [1]. The inelastic measurements at low temperatures reveal the presence of an energy gap in spin-wave spectrum of 1.6 meV.~ The gap decreases with increasing temperature, and vanishes as the temperature is increased above the structural transition. Linear spin wave theory is used to evaluate the exchange interactions in this material. \newline [1] R. Plumier and M. Sougi, Solid State Commun. 64, 53 (1987)   

Spin wave dispersion in the helical spin ordered system SrFeO$_3$ and CaFeO$_3$

One of the most interesting problems in condensed matter physics is the metal-insulator (MI) transition driven by strong electron correlations. The cubic perovskites SrFeO$_3$ and CaFeO$_3$ are isoelectronic to the manganite system (t$^3_{2g}$ e$^1_{g}$) and exhibit colossal magneto resistance effects. But in contrast, the ferrates show a helical instead of a collinear spin structure. Furthermore, perfectly cubic SrFeO$_{3}$ shows no charge order and is metallic whereas pseudocubic CaFeO$_{3}$ shows a MI-transition at the charge ordering transition of 290 K. Therefore, both compounds are right at the borderline between itinerant and strongly correlated systems. We have determined the static and dynamic spin properties by neutron scattering. The extracted parameters in the spin Hamiltonian are a big step towards the understanding of the mechanisms behind the helical spin order and the charge order in the ferrates.   

Magnetic and Physical Properties of RCuGe

We report magnetic, transport and thermodynamic measurements for AlB$_{2}$-type CeCuGe, NdCuGe and GdCuGe. In NdCuGe we observe a peak at 3K in specific heat and susceptibility corresponding to an AF transition. The magnon contribution to the specific heat is fitted very well and supports a CEF scheme, for which we found a first excited state 66 K from the ground state. GdCuGe exibits an AF transition at 18 K, with a peak in susceptibility and a kink in resistivity. For CeCuGe, ferromagnetic with $T_{c}$=10 K, the specific-heat peak is broadened and shifted to higher temperatures as the applied magnetic field increased. At high temperatures the magnetization obeys a Curie-Weiss law and the estimated magnetic moment agrees with the rare-earth free ion moment. To better understand the magnetism of these materials we also report local probe NMR measurements. Lineshape, and Knight shift measurements, show that coupling machanism is dominated by $J$ coupling, and from relaxation measurements we obtain a measure of the spin dynamics. This work was supported by the Robert A. Welch Foundation (grant A-1526).   

Magnetism in GdFe2Zn20

The magnetic structure formation and the origin of the exchange couplings in GdFe2Zn20 have been studied using the standard band-structure methods at T=0K and the ab-initio spin dynamics in the paramagnetic case. At low temperatures the relative importance of the different channels of the magnetic coupling have been analyzed. At high temperatures the soft spin deviation modes responsible for the observed high temperature of the magnetic phase transition [1] have been identified. Several strong magnetic short range order parameters have been found in this material. \newline [1] S. Jia, S.L. Bud'ko, G.D. Samolyuk, P.C. Canfield. cond-mat/0606615   

Microemulsion phases in one and two dimensional magnetic models with long-range interactions

Spivak and Kivelson\footnote{B. Spivak and S. A.Kivelson, Physical Review B, 70 155114 (2004)} have proposed that the first order phase transition between the Wigner crystal and Fermi liquid phases of the interacting electron gas in two dimensions is pre-empted by a series of microemulsion phases characterized by phase separation on the mesoscopic scale, which may be responsible for the anomalous conductivity. We have studied analogous classical magnetic models in one and two dimensions. In particular, we present an exact analytical solution of a one dimensional classical ferromagnetic Ising spin chain frustrated by a long range antiferromagnetic interaction, which clearly exhibits such phase separation in which the mesoscale varies continuously with applied magnetic field. We describe these phases in the 1D model and consider extensions to stripe and bubble phases in two dimensions.\footnote{K. Ng and D. Vanderbilt, Physical Review B, 52 2177 (1995)}   

The spin glass transition of the three dimensional Heisenberg spin glass.

We present our results[1] about the study of the critical properties of the three dimensions Edwards-Anderson model with Heisenberg spins, by means of Monte Carlo simulation and Finite Size Scaling analysis. A mixture of Heath Bath and Overrelaxation algorithms allowed us to thermalize lattices of size up to L=32. We have found a finite temperature transition where both the spin glass and the chiral glass orderings develop. The presence of logarithmic corrections suggests that the phase transition is of Kosterlitz-Thouless type, although we may not exclude a lower critical dimension barely smaller than three. \newline \newline [1] I. Campos, M. Cotallo-Aban, V. Martin-Mayor, S. Perez-Gaviro and A. Tarancon, Phys. Rev. Lett. in press (cond-mat/0605327).   

Impurity induced order and moment distribution in quantum antiferromagnets

The local response to a uniform field around vacancies in antiferromagnetic lattice models is analyzed by using a combination of spin wave calculations, an effective continuous theory, and quantum Monte Carlo simulations. The impurity breaks the sublattice symmetry and causes a tilting of the order parameter towards the field over a large range. This corresponds to an induced antiferromagnetic order {\it parallel} to the applied field, which is understood in quantitative detail. The relevance for NMR and susceptibility measurements will be discussed.