S. von Molnar

Properties and measurement of scanning tunneling microscope fabricated ferromagnetic particle arrays (invited)

The low temperature magnetic properties of arrays of scanning tunneling microscope (STM) fabricated ferromagnetic particles have been studied as a function of their dimension using a novel high sensitivity Hall magnetometer. Iron deposits with controlled shape and nanometer scale diameters (∼25 nm) are formed using a STM to decompose a metalorganic precursor [Fe(CO)5] in the active area of the measurement device. The hysteresis loops change significantly in going from nearly isotropic to oriented high aspect ratio (6:1 length to diameter) filamentary particles. In particles of intermediate aspect ratio and diameter the largest coercive field of 2.7 kOe is observed. This behavior as well as the characteristics of the Hall magnetometer (spin sensitivity of 10−14 emu/ Hz1/2) are described.

Giant Negative Magnetoresistance in Ferromagnetic Eu1−xGdxSe

The NaCl‐type compounds Eu1−xGdxSe form a continuous series of solid solutions between the ferromagnetic insulator EuSe and the antiferromagnetic metal GdSe. We have studied the temperature dependence of electrical resistivity ρ, in Eu1−xGdxSe single crystals and polycrystals between 2.5° and 300°K; we find, in qualitative agreement with earlier observations, for small x, an exponential increase in ρ similar to normal semiconductors but a very sharp drop in ρ below the magnetic transition temperature. The resulting peak in ρ decreases sharply with applied magnetic field strength and increasing Gd concentration. The temperature dependence of the Hall effect in single‐crystal samples of Eu0.95Gd0.05Se indicates as origin of the resistivity anomaly an unusually large effect of the magnetic order on the carrier mobility. Preliminary thermoelectric power measurements support this conclusion. The possibilities of describing strong interactions between conduction electrons and localized spins by scattering theor...

New III‐V diluted magnetic semiconductors (invited)

A new class of diluted magnetic semiconductor (DMS) based on a III‐V semiconductor is reviewed. The new DMS, (In,Mn)As, was made possible by low temperature molecular beam epitaxial growth. Magnetic measurements and x‐ray diffraction showed homogeneous incorporation of Mn in the films under certain growth conditions, and inclusion of a MnAs‐like phase if the conditions are not optimized. The films can be made either p‐ or n‐type by choosing the growth conditions and/or doping. Homogeneous n‐type (In,Mn)As layers were paramagnetic and showed negative magnetoresistance. On the other hand, remanent magnetization was observed in p‐type samples at low temperature and an anomalous Hall effect associated with it. The presence of such effects was most readily explained in terms of formation of bound magnetic polarons. A first result of anomalous Hall effect in a heterojunction is also presented.

Growth of high aspect ratio nanometer-scale magnets with chemical vapor deposition and scanning tunneling microscopy

A combination of chemical vapor deposition and scanning tunneling microscopy techniques have been used to produce nanometer-scale, iron-containing deposits with high aspect ratios from an iron pentacarbonyl precursor both on a substrate and on the tunneling tip itself. The structure and composition of the resulting nanodeposits were determined by transmission electron microscopy and high spatial resolution Auger electron spectroscopy. Either polycrystalline, relatively pure, body-centered-cubic iron or disordered carbon-rich material can be deposited, depending on the bias conditions of the tip sample junction and the precursor pressure. Two mechanisms of decomposition are inferred from the growth phenomenology.

Transport Properties of YBa2Cu3O7: Resistivity, Thermal Conductivity, Thermopower and Hall Effect

For the high-Tc superconductor YBa2Cu3O7, we present results on the resistivity, Hall effect, and thermopower for temperatures up to 350 K as well as on the thermal conductivity in the range 0.2 K ≤ T ≤ 120 K. The Hall coefficient is positive and varies approximately with T-1. The thermopower is very small and sample dependent. These results suggest transport in more than one band. The thermal conductivity indicates dominant phonon heat transport. For the residual resistivity an upper bound, ρ0 ≤ 250 μΩ cm, is deduced. Combining this with published results for the slope of the upper critical field and the penetration depth, we estimate the specific-heat coefficient, 5 ≤ γ < 9 mJ/K2 mole-Cu, and the effective carrier density, 1.1 1022 cm-3.

New materials for semiconductor spin-electronics

The success of an all-electronic semiconducting spintronic device rests on three elements, i.e., spin injection, spin manipulation, and spin detection. This paper focuses on various materials and their properties necessary to achieve these goals. Most of the focus will be on spin injection with lesser emphasis on manipulation and detection.

Magnetism of nanometer-scale iron particles arrays (invited)

The magnetization behavior in arrays of small ferromagnetic iron particles is investigated. Arrays were fabricated by a combination of chemical vapor deposition and scanning tunneling microscopy. This method allows a variety of particle arrays to be grown differing in particle height, diameter, or arrangement. Moreover, the arrays can be grown directly onto different materials such as Au or permalloy. Magnetic measurements were conducted by Hall magnetometery up to 100 K and compared to switching field measurements by means of magnetic force microscopy at room temperature. The magnetization reversal mechanisms were studied from magnetization curves measured for an arbitrary angle ϑ of the applied field with respect to the long axis of the particles. By analyzing the reversible rotation, the particles’ magnetic core diameter and the shape anisotropy could be determined. A phenomenological model based on thermally activated magnetization reversal was introduced and compared to experimental switching field d...

IMAGING AND MAGNETOMETRY OF SWITCHING IN NANOMETER-SCALE IRON PARTICLES

The reversal mechanisms in arrays of nanometer‐scale (<40 nm diameter) iron particles are studied by low‐temperature Hall magnetometry and room‐temperature magnetic force microscopy. Rotation of the net array magnetization at low temperatures (20 K) occurs by both reversible and irreversible modes, the latter revealed by Barkhausen jumps. Spatially resolved measurements at room temperature show the particles to be single domain with remanence and coercivity indicating they are not superparamagnetic. Individual particles are observed to switch irreversibly over a small field range (<10 Oe) between preferred magnetic directions parallel to the growth direction of the particles. Scaling of the arrays offers the possibility of magnetic storage at the 45 Gbit/in.2 level, nearly 50 times greater than current technology.

Epitaxy of III–V diluted magnetic semiconductor materials

A new class of III–V based diluted magnetic semiconductors, specifically In1−xMnxAs (x≲0.2) and InAs/InMnAs multilayer structures, has been prepared by molecular beam epitaxy. The x‐ray diffraction measurements reveal that the incorporation of Mn can be predominantly either homogeneous (200 °C) or inhomogeneous (300 °C), depending on the growth temperature. Semiconducting properties of the films have been examined by optical absorption and Hall effect measurements, and it has been found the films of homogeneous alloy are n‐type and have a band gap which decreases with increasing Mn composition. Magnetization measurements indicate that the homogeneous alloy exhibits paramagnetic behavior, whereas ferromagnetic behavior dominates for the inhomogeneous case. The growth of GaMnAs has also been examined.

Magnetic, transport, and thermal properties of ferromagnetic EuB6

Magnetic measurements on Al‐flux grown EuB6 crystals show that this material orders ferromagnetically with a transition temperature Tc=13.7 K. The effective moment derived from paramagnetic susceptibility measurements gives μeff=7.76 μB, and the saturation magnetization extrapolated to 0 K is within 10% of the theoretical value of 7 μB expected for Eu+2. The magnetic order, however, cannot be that of a simple colinear ferromagnet because the magnetic specific heat in zero applied magnetic field shows a broad maximum centered about 9 K rather than the expected λ‐like anomaly at 13.7 K. Finally, transport measurements suggest that EuB6 is an intrinsic semimental.