P. C. Hammel

Scaling of spin Hall angle in 3d, 4d, and 5d metals from Y3Fe5O12/metal spin pumping.

We have investigated spin pumping from Y3Fe5O12 thin films into Cu, Ag, Ta, W, Pt, and Au with varying spin-orbit coupling strengths. From measurements of Gilbert damping enhancement and inverse spin Hall signals spanning 3 orders of magnitude, we determine the spin Hall angles and interfacial spin mixing conductances for the six metals. The spin Hall angles largely vary as Z(4) (Z: atomic number), corroborating the role of spin-orbit coupling. Amongst the four 5d metals, the variation of the spin Hall angle is dominated by the sensitivity of the d-orbital moment to the d-electron count, confirming theoretical predictions.

Observation of ferromagnetic resonance in a microscopic sample using magnetic resonance force microscopy

We report the observation of a ferromagnetic resonance signal arising from a microscopic (∼20 μm×40 μm) particle of thin (3μm) yttrium iron garnet film using magnetic resonance force microscopy (MRFM). The large signal intensity in the resonance spectra suggests that MRFM could become a powerful microscopic ferromagnetic resonance technique with a micron or sub‐micron resolution. We also observe a very strong nonresonance signal which occurs in the field regime where the sample magnetization readily reorients in response to the modulation of the magnetic field. This signal will be the main noise source in applications where a magnet is mounted on the cantilever.

Superconductivity and magnetism in a new class of heavy-fermion materials

We report a new family of Ce-based heavy-fermion compounds whose electronic specific heat coefficients range from about 400 to over 700mJ/mol-Ce K2. Crystals in this family form as CenTmIn3n-2m, where T = Rh or Ir, n = 1 or 2, and m = 1, with a tetragonal structure that can be viewed as m-layers of CeIn3 units stacked sequentially along the c-axis with intervening m-layers of TIn2. Ambient and high-pressure studies show that the quasi-2D layers of CeIn3 produce unconventional superconducting and magnetic ground states. This family should enable new understanding of the relationship between magnetism and superconductivity in heavy-fermion materials and more generally of why heavyfermion superconductivity prefers to develop in one structure type and not another.

Anomalous NMR magnetic shifts inCeCoIn5

We report ^{115}In and ^{59}Co Nuclear Magnetic Resonance (NMR) measurements in the heavy fermion superconductor CeCoIn_5 above and below T_c. The hyperfine couplings of the In and Co are anisotropic and exhibit dramatic changes below 50K due to changes in the crystal field level populations of the Ce ions. Below T_c the spin susceptibility is suppressed, indicating singlet pairing.

Magnetic Field Independence of the Spin Gap in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}

We report, for magnetic fields of 0, 8.8, and 14.8thinspthinspT, measurements of the temperature dependent {sup 63}Cu NMR spin lattice relaxation rate for near optimally doped YBa{sub 2}Cu {sub 3}O{sub 7{minus}{delta}} , near and above T{sub c} . In sharp contrast with previous work we find no magnetic field dependence. We discuss experimental issues arising in measurements of this required precision and implications of the experiment regarding issues including the spin gap or pseudogap. {copyright} {ital 1998} {ital The American Physical Society}

Ferromagnetic Resonance Force Microscopy on Microscopic Cobalt Single Layer Films

We report mechanical detection of ferromagnetic resonance (FMR) signals from microscopic Co single layer thin films using a magnetic resonance force microscope (MRFM). Variations in the magnetic anisotropy field and the inhomogeneity of were clearly observed in the FMR spectra of microscopic Co thin films 500 and 1000 A thick and ~ 40 × 200 µm^2 in lateral extent. This demonstrates the important potential that MRFM detection of FMR holds for microscopic characterization of spatial distribution of magnetic properties in magnetic layered materials and devices.

63Cu NMR and hole depletion in the normal state of yttrium-rich Y1-xPrxBa2Cu3O7

The {sup 63}Cu Knight shift {ital K} and spin-lattice relaxation rate 1/{ital T}{sub 1} have been measured in the superconducting cuprate system Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Ba{sub 2}Cu{sub 3}O{sub 7}, 0.05{le}{ital x}{le}0.20. With Pr doping {ital K} decreases and develops a temperature dependence at both plane and chain sites. This resembles the behavior of the Cu and Y Knight shifts as well as the bulk susceptibility in oxygen-deficient YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}}. The orbital contribution to {ital K} and the anisotropy of the Cu hyperfine coupling remain essentially unchanged over the entire Pr concentration range. No appreciable direct effect of Pr magnetism on the conduction-band susceptibility was found. Instead, analysis of the bulk susceptibility and NMR data indicate that pair breaking and hole depletion both take part in the suppression of the superconducting transition temperature {ital T}{sub {ital c}}. The temperature dependence of 1/{ital T}{sub 1} for magnetic field parallel to the {ital c} axis is also similar to that for the oxygen-deficient compound. This agreement leads to a consistent picture of the role of antiferromagnetic fluctuations in these materials. An analysis of the data in the framework of the phenomenological theory of Millis, Monien, and Pines is given.

Imaging mechanisms of force detected FMR microscopy

We demonstrate spatial resolution of ferromagnetic resonance in a microscopic sample of YIG using ferromagnetic resonance force microscopy (FMRFM). Measurements were performed on a small single crystal YIG film grown on a GGG substrate, roughly rectangular in shape 20 µm×~150 µm and 3 µm thick. The perpendicular and parallel force geometries of FMRFM, in conjunction with an external bias field both parallel and perpendicular to the film, were used to scan the sample. This enabled the detection of strong signals, even at atmospheric pressure and room temperature. The fundamental and higher-order magnetostatic modes were observed to have 26–29 Gauss separation. The intensity of these modes exhibited spatial variation as the magnetic tip was scanned over the sample, and this behavior is qualitatively explained by DE theory. An improved fabrication method for magnet on cantilever was employed, which yielded a spatial resolution of 15 µm. These results demonstrate the potential of FMRFM for investigating the spatial dependence of ferromagnetic resonance, and for studying the anisotropy fields and exchange coupling effects within multilayer films and small magnetic systems.

Cuprous oxide manometer for high-pressure magnetic resonance experiments

We present a manometer designed to measure pressures of 1–20 kbar in temperatures between 4–300 K in cylinder‐piston type chambers, with an accuracy of ∼100 bar. The manometer is based on pressure‐dependent zero‐field 63Cu nuclear quadrupole resonance frequency corresponding to ‖±3/2〉↔‖±1/2〉 transition in Cu2O. The nuclear quadrupole resonance frequency νQ varies linearly with pressure and its temperature dependence is adequately explained by a model of lattice vibrational modes in O—Cu—O bonds. This manometer is particularly convenient for zero or high‐field magnetic resonance experiments.

sup 139 La NMR study of phase separation in single-crystal La sub 2 CuO sub 4+. delta

We report a study of single-crystal La{sub 2}CuO{sub 4+{delta}}, a 38-K superconductor produced by high-pressure oxygenation. We employ {sup 139}La NMR to probe the behavior of copper spins in the CuO{sub 2} planes. At low temperatures we observe two signals, one originating from regions of the crystal rich in oxygen and a second having no excess oxygen ({delta}{approximately}0). Upon warming through 265{plus minus}5 K, the volume fraction of the crystal poor in oxygen goes to zero. The magnetic shift of the peak intensity of the line originating in the oxygen-rich portion of the crystal does not change in the vicinity of 265 K. These observations provide direct microscopic evidence for phase separation in the crystal.