Andrew D. Kent

Current-induced torques in magnetic materials

The magnetization of a magnetic material can be reversed by using electric currents that transport spin angular momentum. In the reciprocal process a changing magnetization orientation produces currents that transport spin angular momentum. Understanding how these processes occur reveals the intricate connection between magnetization and spin transport, and can transform technologies that generate, store or process information via the magnetization direction. Here we explain how currents can generate torques that affect the magnetic orientation and the reciprocal effect in a wide variety of magnetic materials and structures. We also discuss recent state-of-the-art demonstrations of current-induced torque devices that show great promise for enhancing the functionality of semiconductor devices.

Spin-transfer-induced precessional magnetization reversal

A magnetoelectronic device is proposed in which a spin-current pulse produces a rapid reversal of the magnetization of a thin film nanomagnet. A spin-transfer torque induces the reversal and the switching speed is determined by the precession frequency of the magnetization in a thin film element’s demagnetization field. Micromagnetic simulations show that this switching occurs above a threshold pulse current and can be faster than 50 ps. In contrast to present spin-transfer devices, the switching does not require an initial fluctuation or deviation of magnetic layers from collinear alignment and is far more energy efficient. This device operates at room temperature and can be realized with present-day magnetic nanostructure technology.

A new spin on magnetic memories

Solid-state memory devices with all-electrical read and write operations might lead to faster, cheaper information storage.

Negative Domain Wall Contribution to the Resistivity of Microfabricated Fe Wires

The effect of domain walls on electron transport has been investigated in microfabricated Fe wires (0.65 to 20 mm linewidths) with controlled stripe domains. Magnetoresistance (MR) measurements as a function of domain wall density, temperature, and the angle of the applied field are used to determine the low field MR contributions due to conventional sources in ferromagnetic materials and that due to the erasure of domain walls. A negative domain wall contribution to the resistivity is found. This result is discussed in light of a recent theoretical study of the effect of domain walls on quantum transport. [S0031-9007(98)06392-3]

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.

A vertical piezoelectric inertial slider

We have developed a linear translation device using piezoelectric‐induced slip‐stick motion. Reproducible single steps of about 30 A, as well as continuous stepping with an overall translation speed of 0.25 mm/s, are routinely realized. The notable feature of this device is that this performance is achieved in the vertical orientation with the translator moving against gravity. This remarkable result is made possible using cycloidal functions instead of sawtooth signals to activate the motion. We have realized a very simple translator which can be used in any orientation with a displacement onset voltage of 15 V. The instrument was successfully tested in the temperature range from 1.6 to 300 K. Since no mechanical connections are required, this design is well suited for many applications, including scanning tunneling microscopy.

Ultrafast switching in magnetic tunnel junction based orthogonal spin transfer devices

Orthogonal spin-transfer magnetic random access memory (OST-MRAM) uses a spin-polarizing layer magnetized perpendicularly to a free layer to achieve large spin-transfer torques and ultrafast energy efficient switching. We have fabricated and studied OST-MRAM devices that incorporate a perpendicularly magnetized spin-polarizing layer and a magnetic tunnel junction, which consists of an in-plane magnetized free layer and synthetic antiferromagnetic reference layer. Reliable switching is observed at room temperature with 0.7 V amplitude pulses of 500 ps duration. The switching is bipolar, occurring for positive and negative polarity pulses, consistent with a precessional reversal mechanism, and requires an energy of less than 450 fJ.

Reactive magnetron sputtering of thin‐film superconductor YBa2Cu3O7−x

Superconducting thin films of YBa2Cu3O7−x have been successfully made by co‐sputtering from the three targets in an Ar and O2 mixture atmosphere. After high‐temperature annealfilms are superconducting with an onset temperature at 90 K, a full transitiontemperature as high as 88 K, and a critical current density in excess of 1×105 A/cm2 at 78 K and 2×106 A/cm2 at 4.2 K. Highly oriented thin films have been obtained on SrTiO3 {100} substrates.

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.

Magnetotransport and magnetic domain structure in compressively strained colossal magnetoresistance films

We have studied the magnetoresistance (MR) of compressively strained La0.7Sr0.3MnO3 (LSMO) films in various magnetic states in order to understand the role of magnetic domain structure on magnetotransport. In thin films of LSMO on (100) LaAlO3, the perpendicular magnetic anisotropy results in perpendicularly magnetized domains with fine scale ∼200 nm domain subdivision, which we image directly at room temperature using magnetic force microscopy. The main MR effects can be understood in terms of bulk colossal MR and anisotropic MR. We also find evidence for a small domain wall contribution to the MR, which is an order of magnitude larger than expected from a double exchange model.