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Register a new userEffects of spin density wave quantization on the electrical transport in epitaxial Cr thin films
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We present measurements of the electrical resistivity, $rho$, in epitaxial Cr films of different thicknesses grown on MgO (100) substrates, as a function of temperature, $T$. The $rho(T)$ curves display hysteretic behavior in certain temperature range, which is film thickness dependent. The hysteresis are related to the confinement of quantized incommensurate spin density waves (ISDW) in the film thickness. Our important finding is to experimentally show that the temperature $T_{mid}$ where the ISDW changes from $N$ to $N$,+,1 nodes {it decreases} as the film thickness {it increases}. Identifying $T_{mid}$ with a first order transition between ISDW states with $N$ and $N$,+,1 nodes, and using a Landau approach to the free energy of the ISDW together with Monte Carlo simulations, we show that the system at high temperatures explores all available modes for the ISDW, freezing out in one particular mode at a transition temperature that indeed decreases with film thickness, $L$. The detailed dependence of $T_{mid}(L)$ seems to depend rather strongly on the boundary conditions at the Cr film interfaces.
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We report the observation of spin-glass-like behavior and strong magnetic anisotropy in extremely smooth (~1-3 AA) roughness) epitaxial (110) and (010) SrRuO3 thin films. The easy axis of magnetization is always perpendicular to the plane of the film (unidirectional) irrespective of crystallographic orientation. An attempt has been made to understand the nature and origin of spin-glass behavior, which fits well with Heisenberg model.
We present an experimental study of the changes generated on the electrical resistance $R(T)$ of epitaxial Cr thin films by the transformation of quantized spin density wave domains as the temperature is changed. A characteristic resistance noise appears only within the same temperature region where a cooling-warming cycle in $R(T)$ displays hysteretic behavior. We propose an analysis based on an analogy with the Barkhausen noise seen in ferromagnets. There fluctuations in the magnetization $M(H)$ occur when the magnetic field $H$ is swept. By mapping $M rightarrow Psi_0$ and $H rightarrow T$, where $Psi_0$ corresponds to the order parameter of the spin density wave, we generalize the Preisach model in terms of a random distribution of {it resistive hysterons} to explain our results. These hysterons are related to distributions of quantized spin density wave domains with different sizes, local energies and number of nodes.
We report on nanoscale strain gradients in ferroelectric HoMnO3 epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane X-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.
Via spin-polarized scanning tunneling microscopy, we revealed a long-range ordered spin density wave (SDW) for the first time on a Cr (001) surface, corresponding to the well-known incommensurate SDW of bulk Cr. It displays a (~ 6.0 nm) long-period spin modulation in each (001) plane and an anti-phase behavior between adjacent planes, which are confirmed by changing the magnetization of the tip. Meanwhile, we simultaneously observed the coexisting charge density wave (CDW) with half the period of the SDW. Taking advantage of real-space measurement, we found the charge and spin modulations are in-phase, and their domain structures are highly correlated. Surprisingly, the phase of CDW in dI/dV map displays a {pi} shift around a density-of-states dip at about -22 meV, indicating an anomalous CDW gap opened below EF. These observations support that the CDW is a secondary order driven by SDW. Therefore, our work is not only the first real space characterization of incommensurate SDW, but also provide new insights on how SDW and CDW coexist.
We have explored the influence of deposition pressure and temperature on the growth of BiFeO3 thin films by pulsed laser deposition onto (001)-oriented SrTiO3 substrates. Single-phase BiFeO3 films are obtained in a region close to 10-2 mbar and 580C. In non-optimal conditions, X-ray diffraction reveals the presence of Fe oxides or of Bi2O3. We address the influence of these parasitic phases on the magnetic and electrical properties of the films and show that films with Fe2O3 systematically exhibit a ferromagnetic behaviour, while single-phase films have a low bulk-like magnetic moment. Conductive-tip atomic force microscopy mappings also indicate that Bi2O3 conductive outgrowths create shortcuts through the BiFeO3 films, thus preventing their practical use as ferroelectric elements in functional heterostructures.
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