Session U17: Focus Session: Femtoscale Multiferroics

Ultrafast Imaging of Real Space Response Functions

Understanding the dynamics of spin and charge excitations are critical for the study of correlated materials, such as cuprates. Inelastic X-ray scattering can reveal extensive information related to ultrafast dynamical details about the spin and charge structure factors. To obtain a theoretical understanding, we performed small-cluster exact diagonalization calculations utilizing single-band and three-band Hubbard models with both periodic and open boundary conditions. We demonstrate the ability to track long time behavior; and show that this method can be utilized to study the response dynamics of various materials, such as correlated and chemical systems as well as biological molecules.   

Time resolved terahertz and second harmonic investigations in multiferroic \textit{R}MnO$_3$ and \textit{R}Mn$_2$O$_5$

The dynamical aspects of magnetoelectric interactions has been a very active area of research in multiferroic materials. Through linear far infrared and terahertz spectroscopies it has been shown that electric dipolo active excitations, called electromagnons, exist in some multiferroic materials and complex magnets. An unexplored area of investigation has been the non-linear response of these excitations to strong electromagnetic fields. We investigate the time resolved response of electromagnons in multiferroic materials using high-electric-field terahertz spectroscopy and second harmonic generation at infrared frequencies. We will report results on the well characterized multiferroics TbMnO$_3$, TbMn$_2$O$_5$ and YMn$_2$O$_5$.   

Transient magnetic states in the multiferroic frustrated spin chain compound Ca$_{3}$CoMnO$_{6}$

We report the discovery of transient magnetic states in a frustrated Ising spin chain system Ca$_{3}$CoMnO$_{6}$ that are observed only within a certain range of magnetic field ($B)$ sweep rates. Spin chains are composed of alternating Co$^{2+}$ and Mn$^{4+}$ spins along the $c$-axis and arranged in a triangular lattice in the \textit{ab}-plane. At zero field, the spins order in a $\uparrow \uparrow \downarrow \downarrow $ configuration that allows for ferroelectric polarization ($P)$. Previous work shows that when DC field is applied along the $c$-axis, a $\uparrow \uparrow \uparrow \downarrow $ spin structure with a 1/2 magnetization ($M)$ plateau is stabilized around $B$ $\sim$ 15 T and $P$ disappears. However, when applying $B$ with various sweep rates using a 60 T shaped-pulse magnet we find transient features in the $M$, $P$, and magnetostriction ($\Delta L$/$L)$. We found one step at 4 T with sweep rate of 75 T/s and another step at 6 T when further increasing the rate to 960 T/s, both below the $M=$1/2 plateau. We attribute this time dependence to the magnetic frustration from both interchain and intrachain exchange interactions between Ising-like Co$^{2+}$ spins which can leads to the creation of magnetic microphases. Thus the evolution of $M$ with external parameters is not a straightforward canting or rotation of spins, but could be a progression through many different ordered microphases that are close in energy. This strongly suggests that an ANNNI-like model is appropriate to describe this system.   

Femtoscale magnetically induced lattice distortions in multiferroic TbMnO$_3$

Magnetoelectric multiferroics, as exemplified by TbMnO$_3$, exhibit both magnetic and ferroelectric long range order. Whilst the magnetic order is mostly understood, the origin of the ferroelectricity has proved more elusive. Competing models ascribe the ferroelectricity to either charge transfer\footnote{H. Katsura, N. Nagaosa, A. V. Balatsky, \textit{Phys. Rev. Lett.} \textbf{95} 057205 (2005).} or ionic displacements.\footnote{I. A. Sergienko, E. Dagotto, \textit{Phys. Rev. B} \textbf{73} 094434 (2006).} I will review how a new experimental technique, exploiting the interference between charge and magnetic X-ray scattering, enabled our resolution of femtometric ionic displacements\footnote{H. C. Walker \textit{et al., Science} \textbf{333} 1273 (2011).} in TbMnO$_3$. In so doing, I will demonstrate not only that our data provide decisive support for microscopic models attributing $\mathrm{P}$ to ionic displacements, but also the importance of including both symmetric and antisymmetric magnetic interactions in any such models.   

Coherent magnon and acoustic phonon dynamics in rare earth doped BiFeO3 multiferroic thin films

Magnetoelectric (ME) multiferroics, with coupled electric and magnetic order parameters, exhibit novel physics and have applications in information storage, spintronics and photovoltaics. BiFeO3 is one of the few room temperature multiferroics, but suffers from weak ME coupling. Lanthanide substitution on the Bi site enhances the remnant polarization, saturation magnetization and ME coupling. We investigated the dynamics of ME coupling in the time domain via ultrafast spectroscopy. Coherent magnons and acoustic phonons are impulsively excited and probed in BiLaDyFeO3 thin films using femtosecond laser pulses. Coupling to distinguishable acoustic phonon modes in the film and substrate yields the elastic constants in conjunction with spectroscopic ellipsometry. After substitution of Bi with Dy a rapid magnetoelectric coupling to weak ferromagnetic order creates a magnon oscillation at 75GHz, indicative of a Dzyaloshinskii-Moriya interaction energy of 0.31meV. Additional substitution with non-magnetic La suppresses this mode. The behaviour under a magnetic field and correlation with magnetisation studies confirms the assignment of the magnon mode. Our optical approach allows the extraction of parameters otherwise difficult to recover experimentally.   

Effect of Ultrafast Thermal Quenching on Nd$_{0.67}$Sr$_{0.33}$MnO$_{3}$

We have successfully performed an ultrafast thermal quenching of Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ (x=0.33) from 1200$^{\circ}$C down to -196$^{\circ}$C in a fraction of a second, at ambient pressure. This allowed us to freeze and investigate the physical properties of the material that have been formed at high temperatures. Resistivity measurements showed a 27 K reduction in the metal-insulator transition (MIT) temperature of the quenched sample compared to the as-grown sample. Whereas magnetic measurements revealed $<$2 K shift in the antiferromagnet-ferromagnet (AFM-FM) transition temperature with a significant broadening in the AFM-FM transition accompanied with a decrease in the low temperature magnetization. Moreover, ultrafast quenching significantly widens the temperature range of the magnetoresistance(MR) from few degrees to over 200 K. Here we present physical interpretations of the results in accordance with X-ray and structural analysis.   

Verwey Transition in Magnetite: How fast does an insulator become a metal?

Magnetite (Fe3O4), is the first oxide where a relationship between electrical conductivity and fluctuating/localized charges was observed, with a drop in conductivity by two orders of magnitude at TV=123K. The Verwey transition is accompanied by a structural change from monoclinic to cubic symmetry. Despite decades of research and indications that charge and orbital ordering play an important role, the mechanism behind the Verwey transition is yet unclear. Recently, three-Fe-site lattice distortions called trimerons have been identified as the true microscopic face of electronic order in low temperature insulating phase. We studied the real time response of insulating magnetite to optical excitation with ultrafast soft X-ray scattering. We discover this to be a two-step process. After an initial femtosecond destruction of individual trimerons in the corresponding lattice, we observe a phase separation into residual insulating trimeron and cubic metallic phases on a 1.0 $\pm$ 0.2 picosecond timescale.   

Complex Magnetic Interactions in A-site and B-site Doped Multiferroic TbMnO$_{3}$

Multiferroic materials have been of great interest in recent years due to a number of potential applications in random access memory or spintronics devices. TbMnO$_{3}$ in particular has attracted attention since the discovery of significant magnetoelectric coupling. The possibility of ferroelectricity in rare-earth chromites has also been examined recently through x-ray diffraction and dielectric measurements. Although several studies have looked at Cr-doped LaMnO$_{3}$, the nature of the Mn--Cr interactions is still controversial and no studies have been performed where the parent compound is multiferroic. In the present work, bulk Tb$_{1-x}$A$_{x}$MnO$_{3}$ (A = Ca$^{2+}$ or Sr$^{2+}$) and TbMn$_{1-y}$Cr$_{y}$O$_{3}$ have been synthesized via solution route. The structural evolution as determined through x-ray diffraction and Raman spectroscopy is consistent with a reduction in the orthorhombic distortion. Magnetic properties distinct from the parent compound, including ferrimagnetism and ferromagnetism, have been observed due to the Mn$^{3+}$--Mn$^{4+}$ or Mn$^{3+}$--Cr$^{3+}$ interactions. These complex interactions between the Mn$^{3+}$/Mn$^{4+}$, Cr$^{3+}$, and Tb$^{3+}$ moments will be discussed in detail.   

The domain walls of antiferromagnetic TbMnO$_{3}$ thin films

In bulk TbMnO$_{3}$ below 28K, the Mn sublattice orders as an antiferromagnetic cycloidal spin structure. This breaks inversion symmetry and induces a macroscopic electrical polarization: TbMnO$_{3}$ is a multiferroic material with a strong magnetoelectric coupling. Contrary to the bulk, TbMnO$_{3}$ thin films grown on (001)-SrTiO$_{3}$ substrates show ferromagnetic-like behavior with a magnetic moment of 1.5$\mu_{\mathrm{B}}$/f.u. at 15K. However, the thickness dependence of the magnetic moments is not consistent with magnetism homogeneously distributed through the film. Additionally, epitaxial strain enables the stabilization of different symmetries and particular domain configurations at nanometric scales. Large strain gradients and/or lowering of symmetry at the boundaries of these domains allow the appearance of physical responses distinct from those of the domains. In this work we investigate the contribution of the domain walls to the magnetic moment.   

Giant magnetoresistance spin valves exchange-biased by ferroelectric BiFeO$_3$ thin films

The recent demonstrations of electric-field-driven magnetization control in ferromagnet(FM)/BiFeO$_3$ bilayer systems [1,2] have attracted considerable interest because of the potential applications in spintronics. In this study, giant magnetoresistance (GMR) spin valves (Co/Cu/Py/Ta) were fabricated on SrRuO$_3$/BiFeO$_3$ films by magnetron sputtering at a base pressure of 2 $\times$ 10$^{-8}$ Torr and with an external field of 300 Oe. The presence of exchange bias between the BiFeO$_3$ layer and the ferromagnetic Co layer is established by magnetization and electronic transport data. The heterostructure was patterned in a rectangular shape with a width of about 20 $\mu$m and a length up to 100 $\mu$m. The GMR characteristics of the patterned devices were systematically studied and directly compared to that obtained from identically fabricated structures on NiO and SiO$_2$, respectively. How these results relate to the realization of reversible control of the GMR spin valve effect by an electric field will be discussed.\\[4pt] [1] Heron et al., Phys. Rev. Lett 107, 217202 (2011);\\[0pt] [2] Ratcliff et al., submitted].   

Magnetic and magnetoelectric excitations of BiFeO3

We have determined a model which describes the magnetic and magnetoelectric excitations of multiferroic BiFeO$_3$. Using the full magnetic dispersion relations which are obtained by neutron inelastic scattering measurements [1], parameters for the Heisenberg model with 1st and 2nd neighbor exchange couplings as well as Dzyaloshinskii-Moriya interaction and the single ion anisotropy are estimated. The model also shows excellent agreements with the observed peaks in THz [2] and Raman [3] spectroscopies, which leads to successful assignments of the excitation modes to these peaks. We also discuss that the mode observed at 21.5 cm$^{-1}$ is an electromagnon excitation which should be both magnetic and electric active. This can be verified by the non-reciprocal directional dichroism measurements. {\it REFERENCES: } [1] Matsuda et al., PRL {\bf 109}, 067205 (2012). [2] Talbayev et al., PRB 83, 094403 (2011). [3] Rovillain et al., PRB 79, 180411 (2009).   

Theory of spin-orbit enhanced electric-field control of magnetism in multiferroic BiFeO$_3$

We present a microscopic theory that shows the importance of spin-orbit coupling in multiferroic compounds with heavy ions. In BiFeO$_3$ (BFO) the spin-orbit coupling at the bismuth ion sites results in a special kind of magnetic anisotropy that is linear in the applied $E$-field. We show how this interaction is capable of disrupting the magnetic cycloid state of bulk BFO, leading to a remarkable level of $E$-field control of magnetism. R. de Sousa, M. Allen, and M. Cazayous, arXiv:1209.6612.   

Evolution of the magnetic structure in (Sm,Bi)FeO3 Thin Films

BiFeO3 is a multiferroic, which is ordered at room temperature. In this compound, the magnetic and ferroelectric domains are coupled and magnetic domains can be switched with an electric field [1]. It has recently been found that doping Sm onto the Bi site drives the system from rhombohedral to orthorhombic ordering [2]. Furthermore, near the phase boundary, application of an electric field can drive the material between the two structures. It is an open question as to whether the magnetic structure follows. In this talk, I share our recent neutron diffraction results on the magnetic structure of Sm doped BiFeO3 thin films. [1] T. Zhao, A. Scholl, F. Zavaliche, K. Lee, M. Barry, A. Doran, M. P. Cruz, Y. H. Chu, C. Ederer, N. A. Spaldin, R. R. Das, D. M. Kim, S. H. Baek, C. B. Eom, and R. Ramesh, Nature Materials \textbf{5}, 823 (2006). [2] Daisuke Kan, Ching-Jung Cheng, Valanoor Nagarajan, Ichiro Takeuchi \textbf{110}, 014106 (2011) [3] Daisuke Kan, Lucia Palova, Varatharajan Anbusathaiah, Ching Jung Cheng, Shigehiro Fujino, Valanoor Nagarajan, Karin M. Rabe, Ichiro Takeuchi, Adv. Funct. Mater. \textbf{20}, 1108 (2010).