Session J13: Focus Session: Magnetic Nanostructures - Thin Films, Surfaces and Interfaces

A Symmetry Microscope for Disordered Materials

We use position-resolved coherent speckle patterns in concert with rotational x-ray autocorrelation analysis to uncover ``hidden'' ordering at specific length scales in labyrinthine magnetic domains. Central to this measurement is understanding how finite-size effects influence the statistics of rotational cross-correlations. To this end, we present numerical simulations of domain configurations whose speckle patterns display rotational ordering similar to experimental candidates, and examine how the real-space structure of ordered and disordered domains differs. In a scanning measurement, we observe what appears to be an island of ordering in an otherwise disordered perpendicular magnetic thin film.   

New Magnetic State In Thin-Film Antiferromagnets and Uncompensated Magnetization

We observe a new magnetic state, for which the origin (M(H=0)=0 point) is outside the major hysteresis loop, which means that zero magnetization at zero applied field cannot be reached isothermally. We observe this in antiferromagnet-only, (110)-FeF$_{2}$ grown on MgF$_{2}$, samples. The surface is responsible for the macroscopically broken time-reversal symmetry, uncompensated magnetization (UM) in a nominally compensated antiferromagnet, and, ultimately, for the new magnetic state. We argue that it is an equilibrium state. Magnetometry and polarized neutron reflectivity (PNR) measurements indicate that the UM is present in this AF. We report rather unusual properties of this UM, including the ``intrinsic exchange bias.'' While its manifestation in a shift of the hysteresis loop is similar to that of the ``classical'' exchange bias observed in bilayers, here, it is observed in a single layer material. We discuss the implications of the origin of the UM on the exchange bias mechanism.   

Existence of Ferromagnetism in Stacked Bilayers of Pd/C60

We report on an experimental and theoretical study of the magnetic properties of multilayer structures fabricated by alternating layers of sputter-deposited Pd and thermally-sublimated C$_{60}$. Auger Electron Spectroscopy and SEM techniques have been used to characterize samples for which magnetic measurements in a commercially available SQUID have been made. The magnetization measurement reveals ferromagnetism in the Pd/C$_{60}$ system, which has Curie temperature T$_{C} \quad \sim $ 450K, modified Bloch coefficient P $\sim $ 2.7 and a temperature-independent coercive field of 50 Oe. The observed ferromagnetism is surprising since both C$_{60}$ and Pd are non-ferromagnetic in the non-interacting limit. Density functional theory (DFT) calculations show that while the C$_{60}$ molecules are nonmagnetic unless polymerized, Pd films have a degenerate ground state that can become ferromagnetic with a weak perturbation. Though the calculated charge transfers of $\sim $0.06 e between C$_{60}$ and Pd are not the cause of ferromagnetism, DFT shows that ferromagnetism can be associated with Pd clusters or an interaction of C$_{60}$ molecules with sharp edges of the Pd thin film.   

Spin-Wave Transmission in Ferromagnetic Thin Films: Flexoelectric Control and Spin-Wave Drag

Spin waves in insulating ferromagnets have recently emerged as an effective low-dissipation carrier of spin currents. In this work we explore a novel form of control of spin waves by {\it flexoelectric interactions}, which couple an electric field to the spatial gradient of the magnetization. We show that not only the short-wavelength exchange spin waves, but also the long-wavelength magnetostatic spin waves in a thin-film of magnetic insulator can be effectively controlled by an electric field. In fact, the relative electric-field-induced phase shift is even larger for magnetostatic spin waves than for exchange spin waves. We further show that spin waves in an insulating ferromagnetic film can excite a spin current in an adjacent conducting or insulating film, by way of long-ranged dipole-dipole interaction between the layers, in a magnetic analogue of the electronic Coulomb drag.   

Impact of preparation conditions on the magnetocaloric properties of Gd thin films

The impact of the deposition temperature and gettering were investigated on Ta(5nm)/Gd(30nm)/Ta(5nm) thin films' magneto caloric(MCE) properties. The samples were grown by magnetron at temperatures up to 600\r{ }C, with and without gettering. Structure of the samples was investigated by X-ray diffraction and ray reflectivity. The isothermal magnetization of the samples was above and below the Curie temperature of the Gd. The entropy change associated with the second order phasewas calculated from M(H,T) using the thermodynamic Maxwell. Increasing the deposition temperature generally improves entropy peak (magnitude, FWHM, and temperature of the peak), but leads to significant oxidation. The ungettered sample grown at00\r{ }C was purely GdO (111). Gettering the chamber by sputtering Tathe walls of the chamber for 30 minutes prior to deposition this oxidation issue, and increased the relative cooling power RCP) of films grown at elevated temperatures. The RCP values of the sample set were increased by as much as 42{\%} over ungettered. Supported by AFOSR and NSF.   

Chiral modulations in MnSi thin films

We present an investigation of the magnetic textures present in MnSi thin films grown on Si(111) by molecular beam epitaxy. The magnetic structure is investigated with SQUID magnetometry and polarized neutron reflectometry (PNR). For an out-of-plane magnetic field, a conical phase is formed with a reduced wavelength of $2 \pi / Q =$~13.9 nm that has both left-handed and right-handed chirality due to the presence of inversion domains in the films. We show that the epitaxially induced tensile stress in the MnSi thin films creates an easy-plane uniaxial anisotropy. The magnetoelastic coefficient is obtained from SQUID measurements in combination with transmission electron microscopy and x-ray diffraction data. The agreement between density functional calculations of the coefficient with the experimental value support the conclusion that the uniaxial anisotropy originates from the magnetoelastic coupling. For an in-plane magnetic field, theoretical calculations based on a Dzyaloshinskii model that includes an easy-plane anisotropy predict a variety of modulations to the magnetic order that are not observed in bulk MnSi crystals. Evidence for these states is found in the SQUID and PNR measurements.   

Stabilization of surface magnetism by organic-acid adsorption

Magnetically-disordered layers at the surface of nanoparticles and low dimensionality magnetic oxides significantly reduce the magnetization density with respect to bulk values. However, high crystal quality Fe$_3$O$_4$ nanoparticles capped with non-magnetic organic acid molecules display a surprisingly high magnetization, of unknown origin. Here, we present a real space structural, chemical and magnetic characterization of oleic-acid-caped Fe$_3$O$_4$ nanoparticles with nanometer resolution, demonstrating the presence of a strong magnetic surface layer. In combination with theoretical calculations, we establish the key role of the nanoparticle/organic-acid bond. Magnetization is restored in the surface layer because the bonding with the acid's O atoms partially lifts the surface reconstruction, resulting in surface O-Fe atomic configuration and distances close to the bulk values. Our findings have implications for the optimization of magnetic properties of nanoparticles and thin films.   

Surface Structure of L1$_{0}$-MnGa(111) Ultra-Thin Films Studied Using Scanning Tunneling Microscopy and First Principles Calculations

Manganese gallium alloys have drawn lots of attention recently for their many desirable properties such as high spin polarization and low damping terms. Furthermore, the magnetic properties depend sensitively on the Mn:Ga stoichiometry, ranging from ferromagnetic for MnGa, to ferrimagnetic for Mn$_{2-3}$Ga, and antiferromagnetic for Mn$_{3}$Ga, giving this material great magnetic tune-ability. In this talk, we will focus on the stoichiometric L1$_{0}$-structured MnGa(111) surface and study its atomic, electronic, and magnetic properties. Ultra-thin MnGa films ($<$ 10 nm) are grown on GaN(0001) substrates using molecular beam epitaxy, and transferred \textit{in-situ} to the analysis chamber for room-temperature scanning tunneling microcopy studies. Atomic resolution images reveal the existence of both 1$\times $1 and 2$\times $2 surface structures. We investigate these structures using periodic, spin-polarized density functional theory with the generalized gradient approximation (GGA), Vanderbilt ultra-soft pseudo-potentials, and a repeated slab geometry. The parameters are optimized to achieve the lowest-energy configuration. Results from these calculations and their comparison with the STM images will be reported.   

Surface Ferromagnetism in Pt films

It has long been recognized that Pt as well as some other noble metals are on the verge of being magnetic, with ferromagnetism in these metals readily detected in nanoparticles. Here we report the observation of surface ferromagnetism in Pt thin films. Both sputtered and e-beam evaporated Pt films of various thicknesses (from 10 $\sim $ 100 nm) have been studied. Detailed SQUID measurements indicate that the film magnetization is largely independent of its thickness. This result is consistent with the neutron scattering data, indicating the presence of a magnetic moment near the top Pt surface, and with the spin polarization measurements by point contact Andreev reflection spectroscopy, demonstrating a sizable spin polarization. Our experimental observations are supported by the first principle density functional calculations showing that certain configurations of Pt atoms on Pt (111) surface exhibit a magnetic moment of up to 1.1 $\mu _{B}$ per Pt atom. We argue that magnetism in this system is due to band narrowing, related to reduced coordination numbers of Pt atoms located at the surface.   

TiO2 as an electrostatic template for epitaxial growth of EuO on MgO(001) by reactive molecular beam epitaxy

Interfacial electrostatics play a key role in determining epitaxial quality in the heteroepitaxy of ionic rock salt materials. We investigate the initial growth modes and the role of interfacial electrostatic interactions of EuO epitaxy on MgO(001) by reactive molecular beam epitaxy. A TiO2 interfacial monolayer is employed to alleviate electrostatic interactions between the ions of the EuO and MgO to produce high quality epitaxial growth of EuO on MgO(001) with a 45 degree in plane rotation. For comparison, direct deposition of EuO on MgO, without the TiO2 layer, is discussed. A key difference of EuO epitaxy on TiO2/MgO is the ability to form EuO by substrate assisted oxidation and without the introduction of external oxygen to the interface. Such ultrathin films are shown to have bulk like magnetic properties   

Magnetic Thin Films of Inorganic Nanosheets

Molecule-based magnets have been fascinating materials because of the potential applications in information storage, electronic and spintronic devices. However, such applications would require arraying the active materials on a substrate or interfacing with other components. Here, we focus on fabricating multi-functional magnetic films using inorganic nanosheets as a building block. The thin films could be prepared by the modified Langmuir-Blodgett, LB, technique or the layer-by-layer, LbL, method, which are representative wet-processings for film preparation. As the magnetic LB film, we chose semiconductive titania nanosheets and magnetic Prussian Blue. Upon band gap excitation of titania nanosheets, electron injection into Prussian Blue was achieved with scavenging interlayer water molecules, leading to photoreduction to Prussian White. As the magnetic LbL film, we chose magnetic layered double hydroxide, LDH, nanosheets and non-magnetic smectite nanosheets. In powdered LDH, a coercivity increased with expanding the interlayer spacing. On the other hand, despite the larger interlayer spacing for the LbL film, a coercivity was less than that of the comparative powdered LDH. It is indicated LDH nanosheets are integrated in an anisotropic manner in the LbL films.   

Suppression and Revival of Magnetic Correlations at Interfaces

We study a model of metal-insulator interfaces consisting of a multilayer, repulsive Hubbard Hamiltonian in which the interaction is nonzero on one set of layers and zero on another. As the interface hybridization is tuned, the magnetism and spectral functions in the correlated layers undergo an evolution in which the signatures of strong interaction are first reduced and subsequently revived due to the formation of an interfacial spin liquid phase. The penetration into the correlated layers of the suppression of magnetic order is found to be 4-6 layers.   

Effects of inhomogeneous strain on the magnetization behavior of magnetic nanostructures in BiFeO$_{3}$/CoFe$_{2}$O$_{4}$ composite

In CoFe$_{2}$O$_{4}$/BiFeO$_{3}$ (CFO/BFO) nanostructured thin films, where ferromagnetic nanopillars are embedded in a ferroelectric matrix, electric field induced rotation of the easy axis of the ferrimagnetic nanopillars has been demonstrated experimentally [Zavaliche et al. 2005]. However, for applications where the magnetic pillars would be used to store information, electric control of the magnetization has to be acheived at the scale of a single pillar, without disturbing the neighbors, and the effects of local strain on the magnetic reversal is therefore of interest. We carried out finite element simulations of the strain state of an arrangement of CFO pillars when the BFO matrix surrounding one of them is under piezoelectric strain. Because of stress relaxation at the top free surface of the thin film, the strain is highly inhomogeneous along the pillar. The position-dependent strain was imported into a micromagnetic simulation, giving a position-dependant magnetoelasticity, to predict the switching behavior of the CFO pillars and estimate the feasibility of electric control of a single magnetic bit in this system. The reversal of the pillars was found to be highly incoherent, showing that the pillars cannot be treated as a macrospin.   

Magnetic field dependencies of dielectric and ferroelectric properties in PbZr0.52Ti0.48O3/La3/8Ca5/8MnO3 multiferroic thin films

In this work, the temperature and magnetic field dependencies of dielectric and ferroelectric properties were investigated for Au/PbZr0.52Ti0.48O3/La3/8Ca5/8MnO3 (PZT/LCMO) thin films grown on (001)-oriented SrTiO3 substrates. The results indicate there exists a large magnetodielectric effect up to 30{\%} at T = 220 K, f = 1 MHz and H = 0.8 T in these multi-layer films, which is promising for practical application as compared with conventional multiferroics demanding large magnetic fields and low temperatures. From the electric polarization hysteresis loops, it is found that with increasing temperature at H = 0 T the coercive field Ec of PZT decreases at low temperature range, and then starts to increase till the Curie temperature of LCMO (Tc $\sim $ 220K) where a maximum appears. This peak is obviously suppressed and shifted to a higher temperature with increasing magnetic fields, which may be related to the depolarization field which is affected by the change of the carrier concentration in LCMO. As for the variation of remnant polarization Pr, it increases with increasing temperatures from 50 K to 300 K, but decreases with increasing magnetic fields around Tc. This magnetoelectric effect implies that the strain effect due to the magnetostriction of LCMO may also have some impact on the variation of hysteresis loops. These findings provide potential for multifunctional devices in spintronics.   

Tailored inter and intra layer exchange coupled superlattices for optimized magnetocaloric effect

We explore Magnetocaloric (MC) properties of Fe/Cr superlattices with tailored inter- and intra-layer interaction using simple 3d metals. Our multilayers are fabricated by pulsed-laser deposition with emphasis on maximizing magnetic entropy changes near room temperature. Nanostructuring\footnote{T. Mukherjee, S. Sahoo, R. Skomski, D. J. Sellmyer, and Ch. Binek, Phys. Rev. B~\textbf{79}, 144406 (2009).} allows tailoring ferromagnetic and antiferromagnetic coupling. This in concert with finite size scaling of the ferromagnetic Fe films has the potential to lead to optimized MC materials. Thermodynamic and MC properties of such Fe/Cr superlattices are studied with the help of SQUID magnetometry. Entropy changes are deduced via the Maxwell relation in single phase regions, X-ray diffraction and X-ray reflectivity are used to correlate structural data with the magnetic properties.