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Register a new userHyperfine Fields in an Ag/Fe Multilayer Film Investigated with 8Li beta-Detected Nuclear Magnetic Resonance
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Low energy $beta$-detected nuclear magnetic resonance ($beta$-NMR) was used to investigate the spatial dependence of the hyperfine magnetic fields induced by Fe in the nonmagnetic Ag of an Au(40 AA)/Ag(200 AA)/Fe(140 AA) (001) magnetic multilayer (MML) grown on GaAs. The resonance lineshape in the Ag layer shows dramatic broadening compared to intrinsic Ag. This broadening is attributed to large induced magnetic fields in this layer by the magnetic Fe layer. We find that the induced hyperfine field in the Ag follows a power law decay away from the Ag/Fe interface with power $-1.93(8)$, and a field extrapolated to $0.23(5)$ T at the interface.
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In low-dimensional metallic systems, lattice distortion is usually coupled to a density-wave-like electronic instability due to Fermi surface nesting (FSN) and strong electron-phonon coupling. However, the ordering of other electronic degrees of freedom can also occur simultaneously with the lattice distortion thus challenges the aforementioned prevailing scenario. Recently, a hidden electronic reconstruction beyond FSN was revealed in a layered metallic compound BaTi2As2O below the structural transition temperature Ts ~ 200 K. The nature of this hidden electronic instability is under strong debate. Here, by measuring the local orbital polarization through 75As nuclear magnetic resonance experiment, we observe a p-d bond order between Ti and As atoms in BaTi2As2O single crystal. Below Ts, the bond order breaks both rotational and translational symmetry of the lattice. Meanwhile, the spin-lattice relaxation measurement indicates a substantial loss of density of states and an enhanced spin fluctuation in the bond-order state. Further first-principles calculations suggest that the mechanism of the bond order is due to the coupling of lattice and nematic instabilities. Our results strongly support a bond-order driven electronic reconstruction in BaTi2As2O and shed light on the mechanism of superconductivity in this family.
The drive to improve the sensitivity of nuclear magnetic resonance (NMR) to smaller and smaller sample volumes has led to the development of a variety of techniques distinct from conventional inductive detection. In this chapter, we focus on the technique of force-detected NMR as one of the most successful in yielding sensitivity improvements. We review the rationale for the technique, its basic principles, and give a brief history of its most important results. We then cover in greater detail its application in the first demonstration of three-dimensional (3D) nuclear magnetic resonance imaging (MRI) with nanometer-scale resolution. Next we present recent developments and likely paths for improvement. Finally, the technique and its potential are discussed in the context of competing and complementary technologies.
We demonstrate that zero-field $beta$-detected nuclear quadrupole resonance and spin relaxation of low energy $^8$Li can be used as a sensitive local probe of structural phase transitions near a surface. We find that the transition near the surface of a SrTiO$_3$ single crystal occurs at $T_c sim 150$ K, i.e., 45 K higher than $T^{bulk}_c$, and that the tetragonal domains formed below $T_c$ are randomly oriented.
The nature of the magnetic transition of the half-filled triangular antiferromagnet Ag$_{2}$NiO$_2$ with $T_{rm N}$=56K was studied with positive muon-spin-rotation and relaxation ($mu^+$SR) spectroscopy. Zero field $mu^+$SR measurements indicate the existence of a static internal magnetic field at temperatures below $T_{rm N}$. Two components with slightly different precession frequencies and wide internal-field distributions suggest the formation of an incommensurate antiferromagnetic order below 56 K. This implies that the antifrerromagnetic interaction is predominant in the NiO$_2$ plane in contrast to the case of the related compound NaNiO$_2$. An additional transition was found at $sim$22 K by both $mu^+$SR and susceptibility measurements. It was also clarified that the transition at $sim$260 K observed in the susceptibility of Ag$_{2}$NiO$_{2}$ is induced by a purely structural transition.
We have studied the magnetization depth profiles in a [57Fe(dFe)/Cr(dCr)]x30 multilayer with ultrathin Fe layers and nominal thickness of the chromium spacers dCr 2.0 nm using nuclear resonance scattering of synchrotron radiation. The presence of a broad pure-magnetic half-order (1/2) Bragg reflection has been detected at zero external field. The joint fit of the reflectivity curves and Mossbauer spectra of reflectivity measured near the critical angle and at the magnetic peak reveals that the magnetic structure of the multilayer is formed by two spirals, one in the odd and another one in the even iron layers, with the opposite signs of rotation. The double-spiral structure starts from the surface with the almost antiferromagnetic alignment of the adjacent Fe layers. The rotation of the two spirals leads to nearly ferromagnetic alignment of the two magnetic subsystems at some depth, where the sudden turn of the magnetic vectors by ~180 deg (spin-flop) appears, and both spirals start to rotate in opposite directions. The observation of this unusual double-spiral magnetic structure suggests that the unique properties of giant magneto-resistance devices can be further tailored using ultrathin magnetic layers.
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