Post-deposition control of ferroelastic stripe domains and internal electric field by thermal treatment

The dependence of the formation of ferroelastic stripe domain patterns on the thermal history is investigated by detailed piezoresponse force microscopy and X-ray diffraction experiments after and during annealing of tensile strained tetragonal Pb(Ti,Zr)O-3 epitaxial thin films on DyScO3 substrates. In particular, the ferroelastic pattern is reversibly interchanged between a cross-hatched and a stripe domain pattern if the films are cooled at different rates after annealing above the formation temperature of a-domains. Different types of 180 degrees and non-180 degrees patterns can be created, depending on the thermal treatment. The changes in the 180 degrees domain structure and lattice parameters are attributed to a change of oxygen vacancy concentration, which results in a modification of the internal electric field and unit cell size, causing also a shift of T-C. Thermal treatment is done on rhombohedral La:BiFeO3 thin films as well. It is observed that also in these films, appropriate heat treatment modifies the domain pattern and films with a stripe domain pattern can be created, confirming the general validity of the developed model. (C) 2015 AIP Publishing LLC.

Highly piezoelectric AlN thin films grown on amorphous, insulating substrate

Alvaro Artieda, Paul Muralt, Silviu Cosmin Sandu

AlN thin films were grown by reactive sputtering on amorphous SiO2 thin films. Film texture, x-ray rocking curve width, mechanical stress, and the clamped piezoelectric constant d33,f were studied as a function of rf bias power and substrate roughness. A high d33,f of 5.0 pm/V was achieved at low substrate roughness and low mechanical AlN film stress. Increasing substrate roughness and stress leads to a deterioration of d33,f, which is correlated with a higher density of opposite polarity grains detected by piezoresponse force microscopy. Extrapolating to 100% uniform polarity, a d33,f of 6.1 pm/V is derived as highest possible value, probably corresponding to the d33,f=e33/cof perfect single crystalline material. Growth mechanisms are proposed and underlined by high resolution transmission electron microscopy to explain the observed phenomena.

2010

Growth and Magnetism of Nanostructures Investigated by STM, MOKE, and XMCD

Alberto Cavallin

This thesis presents our studies on nanostructures growth and magnetism, mainly based on scanning tunneling microscopy (STM), magneto-optical Kerr effect (MOKE), and x-ray magnetic circular dichroism (XMCD) measurements. In most of these experiments, nanostructures were grown by molecular beam epitaxy on single crystal substrates under ultra high vacuum conditions. Our combined morphological and magnetic investigations aim at an atomic-level description of the nanostructures magnetic properties. In particular, we are searching for strategies to design the smallest ferromagnet with stable magnetization at room temperature to be used in devices such as magnetic random access memories, storage media, and sensors. A critical factor in pursuing this aim is the determination and control of the magnetization reversal mechanism. Different reversal mechanisms may occur in the nanostructures we considered: either all the spins quasi-coherently rotate or a domain wall is nucleated and propagated along the particle. We prepared monolayer-high Co islands on Pt(111) with compact and ramified shape by adjusting the deposition temperatures and coverages for a sequence of deposition and annealing steps. Combining field-dependent magnetization curves and temperature dependent zero-field susceptibility measurements we identify a size- and shape-dependent transition from quasi-coherent to domain wall propagation reversal regimes. In a second study we investigated a system which according to a recent publication presents many of the characteristics desired in bit patterned magnetic media for ultra-high density storage devices. In particular, it was claimed that Co nanoclusters, growing in registry with the GdAu2/Au(111) moiré periodic substrate, and featuring a density of 52 Tera/in2, present out-of-plane remanent magnetization at T = 300 K. At odds with this claim, no perpendicular magnetic anisotropy was found in our combined MOKE and STM study, which shows an in-plane remanent signal appearing only for coalesced islands. A third study is devoted to FeRh, an alloy material which, thanks to its peculiar magnetic phase diagram, has been proposed coupled to FePt for advanced magnetic recording schemes. We present the first observation of low temperature stabilization of the ferromagnetic (FM) phase in FeRh chemically ordered crystals. Nanocrystals measuring 3.3 nm in diameter have B2 structure with alternating atomic Fe and Rh layers. XMCD demonstrates FM alignment of the Fe and Rh magnetic moments of 3 and 1 μB, respectively. In sharp contrast to FeRh bulk and thin films, which show antiferromagnetic iii Abstract (AFM) order below 300 K, the low-temperature FM 3.3 nm nanocrystals are the hallmark of a size-dependent AFM-FM transition temperature. Finally we present our results towards creation by chemical vapor deposition of a “mille-feuille” structure composed of graphene and magnetic nanoclusters. To realize this objective we optimized the cluster size, composition, and annealing temperature to obtain a compromise between graphene syntesis and cluster coalescence. We demonstrate by STM that annealing to T = 720 K in ethylene stabilizes Ir-seeded clusters against diffusion and coalescence, and by Auger Electron Spectroscopy (AES) that Ni-coated clusters can be covered with up to half of the Carbon contained in a graphene layer.

EPFL2013

Growth and Magnetism of Nanostructures Investigated by STM, MOKE, and XMCD

Alberto Cavallin

EPFL2013