D. R. Hines

Nonmagnetic semiconductors as read-head sensors for ultra-high-density magnetic recording

A mesoscopic nonmagnetic magnetoresistive read-head sensor based on the recently reported extraordinary magnetoresistance (EMR) effect has been fabricated from a narrow-gap Si-doped InSb quantum well. The sensor has a conservatively estimated areal-density of 116 Gb/in.2 with a 300 K EMR of 6% and a current sensitivity of 147 Ω/T at a relevant field of 0.05 T and a bias of 0.27 T. Because this sensor is not subject to magnetic noise, which limits conventional sensors to areal densities of order 100 Gb/in.2, it opens a pathway to ultra-high-density recording at areal densities of order 1 Tb/in.2.

Extraordinary magnetoresistance in externally shunted van der Pauw plates

We show that extraordinary magnetoresistance (EMR) exhibited by a composite van der Pauw (vdP) disk consisting of a semiconductor with an internal shunt can also be obtained from an electrically equivalent, externally shunted structure that is amenable to fabrication in the mesoscopic sizes required for important magnetic sensor applications. As an example, we use bilinear conformal mapping to transform the composite vdP disk into an externally shunted rectangular plate and calculate its EMR by solving Laplace’s equation with appropriate boundary conditions using no adjustable parameters. The calculations are in good agreement with measurements of InSb plates with Au shunts. Room-temperature EMR values as high as 550% at 0.05 T are obtained.

Nanoscopic magnetic field sensor based on extraordinary magnetoresistance

The design, fabrication, and performance of a nanoscopic magnetic field sensor based on the newly discovered phenomenon of extraordinary magnetoresistance (EMR) are reported. It is shown that a sensor with an active volume of 35u2009nmu2009length×30u2009nmu2009width×20u2009nm height yields room temperature EMR values as high as 35% at an applied field of 0.05 T. The mesoscopic physics implications of these new results are discussed.

Random stacking of a commensurate guest layer in an ordered host: NiAl layer-double-hydroxides

Abstract In the solid solution layer double hydroxides with the general composition [Ni1−xAlx(OH)2] [ ( CO 3 ) x 2 ·y H 2 O ], 0 ≤ x ≤ 0.4 the Ni2+Al3+ ions decorate a positively charged triangular lattice that is sandwiched between two hydroxyl layers to form brucite-like host layers. Guest layers of co-intercalated water molecules and carbonate anions oriented with their three-fold axes perpendicular to the basal plane occupy the galleries. For x = 0.30, the carbonate ions form a ( 13 × 13 R13.9° in-plane superlattice structure relative to the host layers which themselves exhibit an ABC stacking sequence that is only slightly (20%) disordered. Despite their commensurate structure, the guest layers yield Warren line shape X-ray reflections characteristic of a random stacking arrangement. This unusual result is attributed to the high (26-fold) degeneracy of the ground state structure of the guest layer.

Enhanced room-temperature piezoconductance of metal–semiconductor hybrid structures

Metal–semiconductor hybrids (MSHs) are found to exhibit enhanced room-temperature piezoconductance in the presence of uniaxial tensile strain. The magnitude of the enhanced piezoconductance is more than five times greater than that of the homogeneous semiconductor alone and is strongly dependent on both the location and properties of the metal–semiconductor interface. MSHs may be useful in determining the electrical properties of low-resistance metal contacts on semiconductors.

Extraordinary magnetoresistance of a semiconductor-metal composite van der Pauw disk

We have calculated the extraordinary magnetoresistance of an InSb van der Pauw disk with a centrally embedded cylindrical metallic inhomogeneity by solving the Maxwell equations with corresponding boundary value conditions. The room temperature MR of such a device can exceed 100% at 500 G. Our calculations incorporate no adjustable parameters and are in excellent agreement with our recent experimental results provided the finite size of the contact electrodes is included.

A uniaxial tensile stress apparatus for temperature-dependent magnetotransport and optical studies of thin films

A mechanical apparatus for the application of variable uniaxial tensile stress to thin films grown on bulk material has been designed for use in measuring the electrical and/or optical properties of a thin layer over a temperature range 4.2u2009K<T<300u2009K, and in magnetic fields up to 7 T. The induced strain is measured with a resolution of 0.0015% by monitoring the position of a laser beam reflected off the surface of the strained sample. The use of the apparatus is demonstrated on n-type InSb layers grown on GaAs where the uniaxial tensile stress is applied in the [001] direction. At 300 K and strains of up to 0.05%, an increase in the conductivity of approximately 3.5% is observed, most of which (∼2.5%) is the result of an increase in the carrier concentration. The remaining 1% is due to an increase in the carrier mobility. Using band-structure k⋅p theory and the deformation potential parameters obtained from optical spectroscopy measurements under uniaxial compression, these observations are shown to be well described by a reduction in both the fundamental band gap and the carrier effective mass with increasing tensile strain.A mechanical apparatus for the application of variable uniaxial tensile stress to thin films grown on bulk material has been designed for use in measuring the electrical and/or optical properties of a thin layer over a temperature range 4.2u2009K<T<300u2009K, and in magnetic fields up to 7 T. The induced strain is measured with a resolution of 0.0015% by monitoring the position of a laser beam reflected off the surface of the strained sample. The use of the apparatus is demonstrated on n-type InSb layers grown on GaAs where the uniaxial tensile stress is applied in the [001] direction. At 300 K and strains of up to 0.05%, an increase in the conductivity of approximately 3.5% is observed, most of which (∼2.5%) is the result of an increase in the carrier concentration. The remaining 1% is due to an increase in the carrier mobility. Using band-structure k⋅p theory and the deformation potential parameters obtained from optical spectroscopy measurements under uniaxial compression, these observations are shown to be we...

Layer rigidity in layer double hydroxides containing a fixed host-layer

Abstract We report here the first layer rigidity study which has been performed on layer double hydroxides (LDHs) having a fixed-host layer. The LDH materials studied have the chemical formula [(CO3)0.195(1−x)Cl0.39x]:[Zn0.61Al0.39(OH)2], 0≤x≤1. An inter-layer rigidity parameter of p=4.84±0.06 was determined for these compounds from a fit of the standard version of the discrete finite layer rigidity model to the composition-dependence of the normalized basal spacing measured using X-ray diffraction. This result validates the previous results obtained from the more complex, variable-host layer LDH materials.

Unique composition-dependent basal expansion of CO3–Cl (H2O) layer double hydroxides

Abstract Layer double hydroxides of the form [(CO 3 ) 0.195(1− x )Cl 0.39 x (H 2 O) y ]: [Zn 0.61 Al 0.39 (OH) 2 ], 0≤ x ≤1, y =0.4+0.2 x have been synthesized. Dehydration studies indicate that water in the CO 3 end-member ( x =0) is passive while sterically active in the Cl end-member ( x =1). This results from a Cl hydration shell whose water molecules tilt around their C 2 v axes when y >0.51 yielding an anomalously expanded basal spacing of the Cl end-member. The dehydrated system exhibits a unique basal response where the basal spacing is larger for the smaller CO 3 ion.

Anharmonic effects in the basal expansion of layered solids: The layered perovskites

Abstract The composition-dependent basal-spacing of the mixed-alkali layered perovskites Cs x Rb 1− x Ca 2 Nb 3 O 10 , has been measured over the range, 0 ≤ x ≤ 1 using high resolution X-ray diffraction and is compared with previous results on other layered solids. The perovskite results are incompatible with both the flexible layer model, which has been successfully applied to graphite, dichalcogenides and clays and also with the harmonic rigid layer model but are in excellent agreement with the anharmonic Leonard-Jones model. The crucial role of anharmonicity is experimentally confirmed for the first time.