Labyrinthine Island Growth during Pd/Ru(0001) Heteroepitaxy

Using low energy electron microscopy we observe that Pd deposited on Ru only attaches to small sections of the atomic step edges surrounding Pd islands. This causes a novel epitaxial growth mode in which islands advance in a snakelike motion, giving rise to labyrinthine patterns. Based on density functional theory together with scanning tunneling microscopy and low energy electron microscopy we propose that this growth mode is caused by a surface alloy forming around growing islands. This alloy gradually reduces step attachment rates, resulting in an instability that favors adatom attachment at fast advancing step sections

Nanoscale periodicity in stripe-forming systems at high temperature: Au/W(110)

We observe using low-energy electron microscopy the self-assembly of monolayer-thick stripes of Au on W(110) near the transition temperature between stripes and the non-patterned (homogeneous) phase. We demonstrate that the amplitude of this Au stripe phase decreases with increasing temperature and vanishes at the order-disorder transition (ODT). The wavelength varies much more slowly with temperature and coverage than theories of stress-domain patterns with sharp phase boundaries would predict, and maintains a finite value of about 100 nm at the ODT. We argue that such nanometer-scale stripes should often appear near the ODT.Comment: 5 page

Detecting Electronic States at Stacking Faults in Magnetic Thin Films by Tunneling Spectroscopy

Co islands grown on Cu(111) with a stacking fault at the interface present a conductance in the empty electronic states larger than the Co islands that follow the stacking sequence of the Cu substrate. Electrons can be more easily injected into these faulted interfaces, providing a way to enhance transmission in future spintronic devices. The electronic states associated to the stacking fault are visualized by tunneling spectroscopy and its origin is identified by band structure calculations.Comment: 4 pages, 4 figures; to be published in Phys. Rev. Lett (2000

Reversible temperature-driven domain transition in bistable Fe magnetic nanostrips grown on Ru(0001)

© 2015 American Physical Society. High-aspect-ratio Fe nanostrips are studied with real-space micromagnetic imaging methods. We experimentally demonstrate reversible switching from essentially homogeneous single-domain states at room temperature to multidomain diamond states at elevated temperature. This temperature-dependent magnetic bistability can be understood and modeled by accounting for the temperature dependence of the magnetocrystalline, shape, and magnetoelastic anisotropies. These results show how the transition temperature between two magnetic domain states can be tailored by controlling epitaxial strain and particle geometry, which may generate new opportunities for magnetic memory and logic device design.Peer Reviewe

Unprecedented tuning of the in-plane easy axis in (100) magnetite films grown by IR-PLD

Conference paper presented at the IEEE International Magnetics Conference, held in Beijing (China) on May 11-15th, 2015.Magnetite (Fe3O4) is attracting much interest in the last years due to its robust ferrimagnetism down to nanometer thickness, good electrical conductivity and presumed half-metal character. In particular, Fe3O4 films are studied as ideal cases for the design of improved bulk magnets [1] and have been tentatively used in spin-valves and spin-LEDs. Fe3O4 presents a low-temperature metal-insulator transition, the Verwey transition (TV) which has also been proposed for spintronic applications. An open question is to what extent the preparation of Fe3O4 films can affect their detailed magnetic properties, such as the magnetic anisotropy axis. This information is required to efficiently apply Fe3O4 in technological multiphase magnets and spintronic applications [1]. Most of studies dealing with bulk and Fe3O4 thin film systems show room temperature (RT) in-plane magnetic easy axis. By contrast, we show in this work the preparation of pure stoichiometric Fe3O4 thin films with RT easy axes along the in-plane directions [2], i.e. rotated by 45º respect to previous studies. Fe3O4 films have been grown by ablation from a sintered hematite target using a nanosecond infrared (IR) laser at 1064 nm and a substrate temperature of 750 K [3]. Single crystal substrates of SrTiO3, MgAl2O4 and MgO have been used. The films were characterized using XRD, AFM, Raman and Mössbauer spectroscopies, vectorial magneto-optical Kerr effect microscopy (v-MOKE) and SQUID magnetometry. All films consisted of stoichiometric Fe3O4 and presented a Verwey transition at TV=115-118 K. RT in-plane hysteresis loops were measured by vectorial-MOKE as a function of the direction of the applied magnetic field in the 0º-360º range with an angular step of 5º. For all epitaxial films under study, the highest coercivity and remanence are found at 0º, 90º, 180º and 270º (i.e. directions), thus orthogonal to each other, while the lowest coercivity values are found between them [Figures 1(a) and 1(b), respectively]. This results in a well-defined four-fold symmetry indicative of biaxial magnetic anisotropy [2]. In order to verify this result, ferromagnetic resonance (FMR) experiments have been carried out at 9.4 GHz frequency. The angular dependence of the in-plane resonance field at RT for the Fe3O4 layers proves that the easy axes are indeed the in-plane directions (Fig. 2). Furthermore, spin-polarized low-energy electron microscopy (SPLEEM) has allowed imaging the individual magnetic domains at the surface of the films [2]. The magnetic domains present magnetization vectors along the in-plane ¿100¿ directions, while the domain walls are aligned with the in-plane ¿110¿ directions. The most probable cause for the observed magnetization easy-axis direction is the orientation of the anti-phase domain boundaries (APBs). It is known that depending on the orientation of the APBs, they can couple both ferromagnetically or antiferromagnetically the magnetite grains that lie across the boundary. We thus propose that the particular distribution and orientation of APBs that our growth conditions promote are responsible for the observed easy-axis directions of our films. Consequently, all angular studies here shown in addition to SPLEEM experiments demonstrate easy-axis orientation along in-plane directions, i.e., differing from that of bulk magnetite or films prepared by other techniques, and thus demonstrating the possibility of tuning the easy axis orientation in Fe3O4 films

Epitaxial growth of ultrathin palladium films on Re{0001}

Ultrathin bimetallic layers create unusual magnetic and surface chemical effects through the modification of electronic structure brought on by low dimensionality, polymorphism, reduced screening, and epitaxial strain. Previous studies have related valence and core-level shifts to surface reactivity through the d-band model of Hammer and Nørskov, and in heteroepitaxial films this band position is determined by competing effects of coordination, strain, and hybridization of substrate and overlayer states. In this study we employ the epitaxially matched Pd on Re{0001} system to grow films with no lateral strain. We use a recent advancement in low-energy electron diffraction to expand the data range sufficiently for a reliable determination of the growth sequence and out-of-plane surface relaxation as a function of film thickness. The results are supported by scanning tunneling microscopy and X-ray photoemission spectroscopy, which show that the growth is layer-by-layer with significant core-level shifts due to changes in film structure, morphology, and bonding

Mirror nuclei emission and isospin transport at intermediate energies

Isospin effects are studied in reactions induced by 40Ca projectiles at E/A=25 MeV on 40Ca, 48Ca and 46Ti targets. The N/Z of projectile-like, target-like and mid-velocity sources are probed by measuring isotopic (7Li/6Li and 9Be/7Be) and isobaric (7Li/7Be) yield ratios, for semi-peripheral events. The presence of isospin diffusion and drift phenomena is observed. It seems indeed that the interaction time between projectile and target does not allow a complete charge equilibration between quasi-projectile and quasi-target sources