Session U42: Magnetism

Low temperature physical properties of R$_{3}$Co$_{4}$Sn$_{13}$ (R = La, Ce, Pr, Nd and Gd)

We report the low temperature physical properties of the series of compounds R$_{3}$Co$_{4}$Sn$_{13}$ where R=La, Ce, Pr, Nd and Gd. They crystallize in a cubic Yb$_{3}$Rh$_{4}$Sn$_{13}$ type structure, space group Pm-3n, which has 40 atoms per unit cell. Measurements of magnetic susceptibility, electrical resistivity, and low temperature heat capacity were carried out on single crystals grown from Sn-flux. These compounds order antiferromagnetically at low temperature (T$_{N}$ $<$ 15 K) for R = Nd and Gd, while Pr$_{3}$Co$_{4}$Sn$_{13}$ and Ce$_{3}$Co$_{4}$Sn$_{13}$ are paramagnetic down to 2K. In addition Ce$_{3}$Co$_{4}$Sn$_{13}$ display heavy fermion behavior and La$_{3}$Co$_{4}$Sn$_{13}$ is a Pauli paramagnetic which superconducts at 2.3 K. The present data are compared to the magnetic properties of the isostrucutural R$_{3}$(Rh,Ir)$_{4}$Sn$_{13}$ compounds, and the validity of de Gennes scaling as a function of rare earth for these materials is discussed.   

Properties of Heusler alloy Co$_{2}$Cr$_{1-x}$Fe$_{x}$Al epitaxial thin films

We have studied properties of thin films of the new compound Heusler alloy Co$_{2}$Cr$_{1-x}$Fe$_{x}$Al. Recently, calculations have shown ordered compounds with small amounts of Fe doping to be half-metallic, and a magnetoresistance of approximately 30{\%} has been measured in bulk polycrystalline samples by others. Using physical vapor deposition, we have grown Co$_{2}$Cr$_{1-x}$Fe$_{x}$Al epitaxially on MgO. Our results show that the films are highly disordered and have a reduced magnetization as compared to the bulk and to theoretical predictions. Studies of films incorporated into current in plane spin valves show relatively large giant magnetoresistances, especially for a Heusler alloy. Recent results also indicate the presence of a large spin orbit coupling, which is unusual for a transition metal system.   

Pressure-temperature magnetic phase diagram of Au$_4$V investigated by electrical resistivity using Designer Diamond Anvils

The electrical resistivity of Au$_4$V has been measured up to a pressure of 20~GPa between room temperature and 15~K. These measurements were performed using designer diamonds, which consist of lithographically deposited tungsten micro-leads embedded within a single crystal of diamond. The electrical resistivity of Au$_4$V has a kink in its slope at 45~K under ambient pressure, which is associated with a ferromagnetic transition. Designer diamonds can be used with a diamond anvil cell to track the pressure dependence of this ferromagnetic transition, which is found to increase under the application of pressure.   

Synthesis and device applications of doped perovskite manganite nanowires

Doped perovskite manganite films such as LaCaMnO$_{3}$ and LaSrMnO$_{3}$ have been studied intensively over the past years due to their colossal magnetoresistance (CMR) properties. It is expected that these doped manganite in the form of nanowires may offer great opportunities to explore intriguing physics and also practical applications in the emerging field of spintronics. We have developed a ``nanocasting'' technique to produce high-quality single-crystalline La$_{0.67}$Ca$_{0.33}$MnO$_{3 }$and La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ nanowires [1]. This was achieved by using pulsed laser deposition to epitaxially grow the desired manganite on single-crystalline MgO nanowire templates. The core-shell structures of these novel nanowires were clearly revealed using transmission electron microscopy (TEM). Following the material characterization, systematic transport studies have been performed based on devices consisting of individual manganite nanowires. We observed the metal-insulator transition with pronounced colossal magnetoresistance and also anisotropic effects related to the high aspect ratio of these nanowires. These devices could find application for spin injection and magnetic data storage. \newline [1] Nano Letters 4, 1241 (2004).   

Epitaxial growth of half metal thin films on GaAs(100) for spin injection

We report epitaxial thin films of half metal Fe$_{3}$O$_{4}$ and Fe$_{3}$Si on the GaAs(100) buffer layer grown by \textit{in-situ} MBE method. With only one spin band at $E_{F}$, half metals are 100{\%} spin polarized and are considered as an ideal candidate for spin injection. Fe$_{3}$Si is a ferromagnet with a T$_{c}$ of 840K, and a cubic DO$_{3}$ structure almost perfectly lattice matched to GaAs (100) surface. Preliminary RHEED studies showed the attainment of (100) FeO$_{x}$ thin films epitaxially grown on (100) GaAs with an in-plane 45\r{ } rotation in matching the major crytstallographic axes. The crystallinity of FeO$_{x}$ depends significantly on oxygen partial pressure during growth, film thickness, and the surface may undergo decomposition during cooling process. The chemical composition of the FeO$_{x}$ film was determined by XPS analysis by fitting the Fe 2p spectrum with two components of Fe$^{2+}$ and Fe$^{3+}$. Low temperature magnetic and electrical transport measurements are now underway.   

Magnetic Response of Weakly Coupled Ladders: A Model for Stripes in the Cuprates

We study the spin response of pairs of weakly coupled Hubbard ladders, comparing the results to a recent neutron scattering experiment on stripe-ordered $L_{1.875}B_{0.125}CuO_4$ (J. Tranquada et al., Nature 429 (2004) 534.) We assume the doping is segregated: for every pair of ladders, we treat one as half-filled, the other as a 3/8-filled Hubbard ladder. The magnetic response of the half-filled ladder is treated straightforwardly using a single mode approximation. In contrast, the spin response of the doped ladder is computed using the RG-equivalent, low-energy effective field theory, the SO(6) Gross-Neveu model. The combined response of the coupled ladder pairs is then calculated under a RPA approximation. Good agreement is found to the aforementioned experimental observations at all relevant energy scales. In addition, we find that the spin response of the ladder system below the transistion to full magnetic order resembles neutron scattering studies of slightly overdoped $L_{1.82}Sr_{0.18}CuO_4$.   

Orbital Paramagnetism of Strongly Confined Micron Width 2DEG Strips in the Extreme Quantum Limit

The possibility of orbital paramagnetism in a confined degenerate electron gas arising from surface corrections was pointed out by F.S. Ham over fifty years ago [1]. Several theoretical studies of such surface effects have since been published, including confinement effects in mesoscopic systems [2,3]. Experiments have also revealed the presence of size-dependent orbital paramagnetism [4]. In this work I report the results of calculations on the orbital magnetism of strongly confined micron-width strips of 2DEG systems in the extreme quantum limit. A maximum in orbital paramagnetism is predicted at achievable steady magnetic fields for electron areal densities of 10$^{10}$ cm$^{-2}$. It is suggested that such strips, when configured parallel to each other in a plane, with similar appropriately spaced plane layers, may constitute a novel paramagnetic material. 1. F.S. Ham, Phys. Rev. \underline {92} 1113 (1953) 2. B.L. Altshuler, Y. Gefen, and Y. Imry, Phys. Rev. Lett. \underline {66} 88 (1991) 3. B.L. Altshuler, Y.Gefen, Y.Imry, and G. Montambaux, Phys. Rev.B \underline {47 } 10335 (1993) 4. L.P. Levy, D.H. Reich, L. Pfeiffer, and K. West, Physica B \underline {189 204 (1993).}   

Anisotropic photo-induced magnetism of a sequentially deposited thin films of Rb$_{j}$Co$_{k}$[Fe(CN)$_{6}$]$_{l}$$\cdot$$n$H$_{2}$O

Using sequential deposition methods, we have generated two different films of Rb$_{j}$Co$_{k}$[Fe(CN)$_{6}$]$_{l}$$\cdot$$n$H$_{2}$O for magneto-optical studies. The synthesis protocol was intentionally varied in order to generate samples with different degrees of surface homogeneity. As a consequence, film \textbf{1} possessed a powder-like rough surface, while film \textbf{2} was a smooth, quasi-two-dimensional film. Upon irradiation at 5 K with an external magnetic field of 200 G perpendicular to the film surface, the magnetization of film \textbf{1} increased, whereas the magnetization of film \textbf{2} decreased. This contrasting behavior is consistent with dipolar field model describing the phenomena* and is related to the novel anisotropy of the photoinduced magnetism in film \textbf{2}, where the photoinduced magnetization increases or decreases depending on the orientation of the film with respect to the external magnetic field.\linebreak *J.-H. Park \emph{et al}., Appl. Phys. Lett. \textbf{85}, 3797 (2004).   

From itinerant ferromagnetism to insulating antiferromagnetism: A magnetic and transport study of single crystal SrRu$_{1-x}$Mn$_{x}$O$_{3}$ (0$\le $ x$<$0.60)

We report results of a magnetic and transport study of SrRu$_{1-x}$Mn$_{x}$O$_{3}$ (0$\le $ x$<$0.60), i.e., Mn doped SrRuO$_{3}$. The Mn doping drives the system from the itinerant ferromagnetic state (T$_{C}$=165 K for x=0) through a quantum critical point at x$_{c}$=0.39 to an insulating antiferromagnetic state. The onset of antiferromagnetism is abrupt with a Néel temperature increasing from 205 K for x=0.44 to 250 K for x=0.59. Accompanying this quantum phase transition is a drastic change in resistivity by as much as 8 orders of magnitude as a function of x at low temperatures. The critical composition x$_{c}$=0.39 sharply separates the two distinct ground states, namely the ferromagnetic metal from the antiferromagnetic insulator.   

Charge ordering transition in La$_{1.67}$Sr$_{0.33}$NiO$_4$ studied by Angle Resolved Photoemission Spectroscopy

The La$_{1.67}$Sr$_{0.33}$NiO$_{4 }$compound has attracted a lot of interested due to the report of static spin-charge stripe ordering, as well as for being isostructural to the high temperature superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$. While several theoretical and experimental studies have been reported to investigate the stripe phase in this compound, a full analysis of its electronic properties and how this evolves in the stripe phase is still missing in the literature. Here we present the first high-resolution angle resolved photoemission study of a single crystal of La$_{1.67}$Sr$_{0.33}$NiO$_{4}$. Data below and above the charge ordering temperature are presented. The evolution of the electronic structure as well as the changes observed in quasiparticle lineshapes through the stripe phase are discussed.   

Magneto-Optic Properties of Small Atomic Clusters of Ga and In with As and V

The magneto-optic properties of small, virtually (i.e., fundamental theory-based, computationally) pre-designed and vacuum pyramidal clusters of Ga-As-V and In-As-V atoms have been investigated by means of the Hartree-Fock (HF) method. The optic transition energies (OTEs) of these clusters are about 3 times smaller than those specific to small Ga-As-P and In-As-P clusters of the same structure and numbers of Ga and In atoms studied earlier. The HF analysis of the spin density distributions for In-based clusters suggests that these clusters possess noticeable magnetic properties: their total spin density distributions (SDDs) expand beyond the space occupied by the cluster's atoms and can be considered as collective features of the entire corresponding clusters, rather than individual atoms. In the case of the Ga-based clusters with V atoms the absolute values of the SDDs are an order of magnitude lesser than those specific to similar In-based clusters. Unfortunately, stabilization of such In-based clusters grown experimentally may involve chemical means, both in vacuum and in confinement.   

Surface-Kondo Effect Observed for Single Gd and Ho Atoms Embedded in Lu(0001)

By using scanning tunneling spectroscopy at low temperature, we found narrow antiresonances at the Fermi energy in the vicinity of single magnetic defect atoms (Gd and Ho) that are embedded within the surface layer of Lu(0001). The effect is explained by interaction of the magnetic defect atoms with the narrow surface-state band of Lu (0001) that crosses the Fermi energy. It can therefore be interpreted as a surface-Kondo effect.   

First-principles calculations of the magnetic interactions in Fe dimers

We present the results of first-principles calculations of the magnetic interactions between the Fe(III) ions in Fe2 dimers, and those within the larger Fe8 cluster which interact by superexchange through two oxygen ions. The magnitude and sign of interaction is found to be strongly dependent on the Fe-O-Fe angle $\theta$. We rationalize the obtained behavior analyzing the valence charge and spin densities calculated versus the angle. For the experimentally relevant range $100 \le \theta \le 105^{\circ}$, in addition to the sign and magnitude of the isotropic Heisenberg exchange interaction constant, we obtain the intramolecular global and local spin anisotropy interaction constants.   

Mott-Hubbard Type Transition and Thermodynamic Properties in Nanoscale Clusters

Thermodynamic and magnetic properties of clusters of various geometries, sizes, etc. are calculated using exact diagonalization and quantum Monte Carlo simulations. Studies of electron correlations in clusters, with respect to the interaction strength $U$, number of electrons $n$, temperature $T$ and magnetic field $h$ are fundamental for understanding the nature of ferromagnetism. Small clusters contain also important technical features necessary for monitoring the Mott-Hubbard (MH) transition in thermodynamic systems in higher dimensions. Grand canonical ensemble approach for the two site Hubbard cluster gives insight into the nature of MH transition at half filling ($n=1$) with respect to variations of $U$, $h$ and $T$. At $n=1$ the developed ``pseudo gap'' at infinitesimal temperature decreases with increasing $T$. This pseudo gap in the spectrum disappears at characteristic $T_c$ similar to MH critical temperature. A four peak structure in the density of states at finite $U$ indicates the existence of a pseudo gap at $n=1$, quarter $n=1/2$ and three $n=3/2$ fillings at relatively low $T$. The differences between the spin and charge energy gaps for finite clusters are also analyzed. A comparison of the exact results for the two atomic cluster with those calculated from quantum Monte Carlo shows reasonable agreement in a wide range of $T$ and $h$.   

High Coercivity FePt Nanoparticles Synthesized Using the Particle Gun

Chemically ordered FePt nanoparticles pose a potential solution to the superparamagnetic limit for magnetic recording media. This paper presents the synthesis and characterization of high coercivity face centered tetragonal nanoparticles with sizes in the range of 4 to 6 nanometers. As deposited samples are face centered cubic as shown by selected area diffraction. Magnetic measurements made using SQUID and vibrating sample magnetometry confirm this as the nanoparticles are soft ferromagnets. Upon heat treatment the nanoparticles transform to the chemically ordered face centered tetragonal (FCT) phase. Magnetic measurements of the annealed samples reveal a high coercivity in excess of 10 kOe for the appropriate annealing conditions commensurate with the high anisotropy FCT phase. Some unwanted agglomeration is inevitable with post synthesis annealing as indicated by bright field electron micrographs. The use of an in-situ heater stage in dynamically annealing the nanoparticles will be discussed as relates to the L10 transformation and its limits.