arXiv subjects

R. S. Liu

Publications and source records attributed to R. S. Liu.

10 recordsLinked to original sources

arXiv2013

Electric-field-induced strain-mediated magnetoelectric effect in CoFeB-MgO magnetic tunnel junctions

Magnetoelectric coupling between magnetic and electric dipoles is one of the cornerstones of modern physics towards developing the most energy-efficient magnetic data storage. Conventionally, magnetoelectric coupling is achieved in single-phase multiferroics or in magnetoelectric composite nanostructures consisting of ferromagnetic and ferroelectric/piezoelectric materials. Here, we demonstrate an electric-field-induced strain-mediated magnetoelectric effect in ultrathin CoFeB/MgO magnetic tunnel junction employing non-piezoelectric material, which is a vitally important structure for spintronic devices, by using dynamical magnetoelectric and piezoresponse force microscopy measurement techniques. We show that the applied electric-field induces strain in a few atomic layers of dielectric MgO which is transferred to magnetostrictive CoFeB layer, resulting in a magnetoelectric effect of magnitude up to 80.8 V cm-1 Oe-1 under -0.5 V. The demonstrated strain-mediated magnetoelectric effect with an electric field in magnetic tunnel junctions is a significant step towards exploring magnetoelectrically controlled spintronic devices for low-power and high density magnetic data storage applications.

cond-mat.mes-hall↗

arXiv2008

Tunneling Anisotropic Magnetoresistance in Co/AlOx/Au Tunnel Junctions

We observe spin-valve-like effects in nano-scaled thermally evaporated Co/AlOx/Au tunnel junctions. The tunneling magnetoresistance is anisotropic and depends on the relative orientation of the magnetization direction of the Co electrode with respect to the current direction. We attribute this effect to a two-step magnetization reversal and an anisotropic density of states resulting from spin-orbit interaction. The results of this study points to future applications of novel spintronics devices involving only one ferromagnetic layer.

physics.pop-ph↗

arXiv2007

Large Magnetoresistance in Co/Ni/Co Ferromagnetic Single Electron Transistors

We report on magnetotransport investigations of nano-scaled ferromagnetic Co/Ni/Co single electron transistors. As a result of reduced size, the devices exhibit single electron transistor characteristics at 4.2K. Magnetotransport measurements carried out at 1.8K reveal tunneling magnetoresistance (TMR) traces with negative coercive fields, which we interpret in terms of a switching mechanism driven by the shape anisotropy of the central wire-like Ni island. A large TMR of about 18% is observed within a finite source-drain bias regime. The TMR decreases rapidly with increasing bias, which we tentatively attribute to excitation of magnons in the central island.

cond-mat.other↗

arXiv2006

Probing Spin Accumulation in Ni/Au/Ni Single-Electron Transistors with Efficient Spin Injection and Detection Electrodes

We have investigated spin accumulation in Ni/Au/Ni single-electron transistors assembled by atomic force microscopy. The fabrication technique is unique in that unconventional hybrid devices can be realized with unprecedented control, including real-time tunable tunnel resistances. A grid of Au discs, 30 nm in diameter and 30 nm thick, is prepared on a SiO2 surface by conventional e-beam writing. Subsequently, 30 nm thick ferromagnetic Ni source, drain and side-gate electrodes are formed in similar process steps. The width and length of the source and drain electrodes were different to exhibit different coercive switching fields. Tunnel barriers of NiO are realized by sequential Ar and O2 plasma treatment. Using an atomic force microscope with specially designed software, a single non-magnetic Au nanodisc is positioned into the 25 nm gap between the source and drain electrodes. The resistance of the device is monitored in real-time while the Au disc is manipulated step-by-step with Angstrom-level precision. Transport measurements in magnetic field at 1.7 K reveal no clear spin accumulation in the device, which can be attributed to fast spin relaxation in the Au disc. From numerical simulations using the rate-equation approach of orthodox Coulomb blockade theory, we can put an upper bound of a few ns on the spin-relaxation time for electrons in the Au disc. To confirm the magnetic switching characteristics and spin injection efficiency of the Ni electrodes, we fabricated a test structure consisting of a Ni/NiO/Ni magnetic tunnel junction with asymmetric dimensions of the electrodes similar to those of the SETs. Magnetoresistance measurements on the test device exhibited clear signs of magnetic reversal and a maximum TMR of 10%, from which we deduced a spin-polarization of about 22% in the Ni electrodes.

cond-mat.other↗

arXiv2004

Effect of Sr-for-Ba isovalent substitution on the local structure, hole distribution and magnetic irreversibility of Cu(Ba,Sr)2YbCu2O6.95(2)

The effect of Sr(II)-for-Ba(II) isovalent substitution on the magnetic irreversibility field (Hirr) of Cu(Ba1-ySry)2YbCu2O6.95(2) (Cu-1212) sample series (y = 0 ~ 0.4) is studied to reveal guiding rules for tailoring the intrinsic Hirr characteristics of high-Tc superconductors. It has been assumed that substitution of the larger alkaline earth cation, Ba(II), by the smaller, Sr(II), might improve the Hirr characteristics as a consequence of the decrease in the thickness of nonsuperconductive blocking block (BB). However, results of the present work show that Sr substitution rather depresses the Hirr characteristics of the Cu-1212-phase superconductors even though the thickness of BB decreases. Both the amount of excess oxygen and the overall positive charge are confirmed to remain constant upon the Sr substitution by wet-chemical and XANES analyses, respectively. However, from neutron diffraction data analysis it is found that Sr substitution breaks the conductive CuO chains in BB by shifting part of the excess oxygen atoms from the characteristic b-axis lattice site to the a-axis site. This is believed to decrease the concentration of mobile holes in the BB, as supported by the results of TEP measurements. The lower Hirr(T) lines of the Sr-substituted samples may thus be attributed to the lower concentration of mobile holes in BB.

cond-mat.supr-con↗

arXiv2003

The valence state of iron in the Sr2Fe(Mo,W,Ta)O6.0 double-perovskite system: An Fe K-edge and L2,3-edge XANES study

Here we employ both Fe K and L2,3 edge x-ray absorption near-edge structure (XANES) spectroscopy techniques to clarify that iron in the B-site ordered double-perovskite halfmetal, Sr2FeMoO6.0, possesses a mixed-valence state, FeII/III, and accordingly molybdenum a mixed MoV/VI valence state. A reliable interpretation of the spectral features has been made possible by using a series of samples of the Sr2Fe(Mo,W/Ta)O6.0 system. Replacing MoV/VI gradually with WVI causes increasing amount of Fe to adopt the FeII state, whereas TaV substitution shifts the valence of iron towards FeIII. As the valence of Fe increases from II to III in the Sr2Fe(Mo,W/Ta)O6.0 system, the absorption energy at the Fe K-edge gradually shifts towards the higher energy side. Similarly, in the L2,3-edge XANES spectra intermediate spectral features are revealed for the Sr2FeMoO6.0 sample in comparison with those for samples heavily substituted with either W or Ta.

cond-mat.str-el↗

arXiv2003

Iron valence in double-perovskite (Ba,Sr,Ca)2FeMoO6: Isovalent substitution effect

In the Fe-Mo based B-site ordered double-perovskite, A2FeMoO6.0, with iron in the mixed-valence II/III state, the valence value of Fe is not precisely fixed at 2.5 but may be fine-tuned by means of applying chemical pressure at the A-cation site. This is shown through a systematic 57Fe Mossbauer spectroscopy study using a series of A2FeMoO6.0 [A = (Ba,Sr) or (Sr,Ca)] samples with high degree of Fe/Mo order, the same stoichiometric oxygen content and also almost the same grain size. The isomer shift values and other hyperfine parameters obtained from the Mossbauer spectra confirm that Fe remains in the mixed-valence state within the whole range of A constituents. However, upon increasing the average cation size at the A site the precise valence of Fe is found to decrease such that within the A = (Ba,Sr) regime the valence of Fe is closer to II, while within the A = (Sr,Ca) regime it is closer to the actual mixed-valence II/III state. As the valence of Fe approaches II, the difference in charges between Fe and Mo increases, and parallel with this the degree of Fe/Mo order increases. Additionally, for the less-ordered samples an increased tendency of clustering of the anti-site Fe atoms is deduced from the Mossbauer data.

cond-mat.str-el↗

arXiv2003

Layered-specific hole concentrations in Bi2Sr2(Y1-xCax)Cu2O8+d as probed by XANES spectroscopy and coulometric redox analysis

Induction of holes not only in the superconductive CuO2 plane but also in the Bi2O2+d charge reservoir of the Bi2Sr2(Y1-xCax)Cu2O8+d superconductor upon CaII-for-YIII substitution is evidenced by means of two independent techniques, i.e., high-resolution x-ray-absorption near-edge structure (XANES) spectroscopy measurements and coulometric redox titrations. The absolute values derived for the CuO2-plane hole concentration from the Cu L2,3-edge XANES spectra are in good agreement with those obtained from the coulometric redox analysis. The CuO2-plane hole concentration is found to increase from 0.03 to 0.14 concomitantly with the increase in the BiO1+d/2-layer hole concentration from 0.00 to 0.13 as the Ca-substitution level, x, increases from 0 to 1. The threshold CuO2-plane hole concentration for the appearance of superconductivity is determined at 0.06, while the highest Tc is obtained at the hole concentration of 0.12. In the O K-edge XANES spectrum, the increases in the CuO2-plane and BiO1+d/2-layer hole concentrations with increasing x are seen as enhancement in the relative intensities of the pre-edge peaks at ~528.3 and \~530.5 eV, respectively.

cond-mat.supr-con↗

arXiv2002

Oxygen non-stoichiometry in Ru-1212 and Ru-1222 magnetosuperconductors

Here we report the results of thermogravimetric (TG) analysis on the oxygen non-stoichiometry of RuSr2GdCu2O8 (Ru-1212) and RuSr2(Gd0.75Ce0.25)2Cu2O10(Ru-1222) samples. With TG annealings carried out in O2 and Ar atmospheres it was found that the oxygen content in Ru-1212 remains less affected upon various annealings, while for Ru-1222 wider-range oxygen-content tuning is possible. When heated in H2/Ar atmosphere the both phases release oxygen upon breaking down to mixtures of metals (Ru and Cu) and binary oxides (CeO2, Gd2O3, and SrO) in two distinct steps around 300 and 450 oC. This reductive decomposition reaction carried out in a thermobalance was utilized in precise oxygen content determination for these phases. It was found that the 100-atm O2-annealed Ru-1212 sample was nearly stoichiometric, while the similarly treated Ru-1222 sample was clearly oxygen deficient. X-ray absorption near-edge (XANES) spectroscopy was applied to estimate the valence of Ru in the samples. In spite of the fact that the Ru-1212 phase was shown to possess less oxygen-deficient RuO2 layer, the valence of Ru as probed with XANES was found to be lower in Ru-1212 than that in Ru-1222.

cond-mat.supr-con↗

arXiv2002

Oxygen Content and Valence of Ru in RuSr2(Gd0.75Ce0.25)2Cu2O10(Ru-1222) Magnetosuperconductor

The valence of Ru was analyzed for two RuSr2(Gd0.75Ce0.25)2Cu2O10 samples with different oxygen contents by Ru LIII edge x ray absorption near-edge structure (XANES) spectroscopy. For the sample as-synthesized in 1 atm O2 the DC magnetization data measured in an applied field of 5 Oe showed a clear branching of the zero-field-cooled (ZFC) and field-cooled (FC) curves at around 100 K, a cusp at 85 K and a diamagnetic transition around 30 K in the ZFC part. A further confirmation for the superconductivity at 30 K was obtained from a resistance vs. temperature measurement. Annealing the as-synthesized sample in 100-atm O2 atmosphere at 420 oC increased the diamagnetic transition temperature from 30 K to 43 K. According to a thermogravimetric analysis, the oxygen content increased accordingly by ca. 0.15 oxygen atoms per formula unit. Quantitative analysis of the XANES spectra using Sr2RuO4 (RuIV) and Sr2GdRuO6 (RuV) as reference materials revealed a valence value of +4.74 and +4.81 for Ru in the as-synthesized and the 100-atm O2-annealed sample, respectively. The obtained result suggests that the valence of Ru in Ru-1222 is affected by the change in oxygen content.

cond-mat.supr-con↗