R. A. Altman

Microstructured magnetic tunnel junctions (invited)

We have used a simple self-aligned process to fabricate magnetic tunnel junctions down to submicron sizes. Optical and electron-beam lithographies were used to cover a range of areas spanning five orders of magnitude. The bottom magnetic electrodes (Co or permalloy) in our junctions were exchange biased by an antiferromagnetic layer (MnFe). The top electrodes were made of soft magnetic materials (Co or permalloy). We have consistently obtained large magnetoresistance ratios (15%–22%) at room temperature and in fields of a few tens of Oe. The shape of the field response of the magnetoresistance was varied from smooth to highly hysteretic by adjusting the shape anisotropy of one junction electrode.

Observation of large low‐field magnetoresistance in trilayer perpendicular transport devices made using doped manganate perovskites

We report on the fabrication of a new class of trilayer epitaxial thin film devices based on the doped perovskite manganates La–Ca–Mn–O and La–Sr–Mn–O. We show that large resistance changes, up to a factor of 2, can be induced by a moderate applied magnetic field below 200 Oe in these trilayers supporting current‐perpendicular‐to‐plane transport. These results show that low‐field spin‐dependent transport in manganates can be accomplished, the magnitude of which is suitable for magnetoresistive field sensors.

Shape-anisotropy-controlled magnetoresistive response in magnetic tunnel junctions

We show that shape anisotropy can be used to control the response characteristics of magnetic tunnel junctions. By varying the junction shape, the resistance versus field curve was made to vary from a nonhysteretic linear curve with a high-field sensitivity (0.3%/Oe) to a hysteretic response curve with high squareness.

Bias voltage and temperature dependence of magnetotunneling effect

We have studied systematically the magnetotunneling properties of several metallic magnetictunnel-junction systems (Ni80Fe20–insulator–Ni80Fe20,Ni80Fe20–I–Co,Co–I–Co, Ni40Fe60–I–Co). The room-temperature magnetoresistance MR value at zero-bias ranges between 16% and 27%, depending on the spin polarization of the electrodes. There seems to be a general bias dependence of MR in all of these systems. In particular, it requires a bias in the range of 0.22–0.23 V to suppress the maximum MR value by half. We have also measured the bias dependence of MR as a function of barrier parameters (thickness and oxidation time). At low temperature, a sharp cusplike feature appears near zero bias. In some cases, low-temperature MR values substantially exceed expectations from established spin-polarization.

Magnetic tunnel junctions fabricated at tenth-micron dimensions by electron beam lithography

Abstract Magnetic tunnel junctions consisting of permalloy and cobalt thin film electrodes, separated by a thin aluminum oxide tunnel barrier, have been fabricated by e-beam lithography at dimensions down to 120 nanometers. The devices are fabricated by sputter deposition and ion milling. They exhibit magnetoresistances of up to 22% at room temperature. Evidence of individual domain switching is observed. The smaller junctions have resistances in the kilohm range, which are easily measured, leading to the possibility of sensing and microelectronic applications.

The response of high-Tc SQUID magnetometers to small changes in temperature

We have investigated the response of the flux-locked output of several high-Tc SQUID magnetometers to small changes in temperature and for magnetic fields 0–30 μT. The temperature response DT≡dΦS/dT is observed to be linear in the applied magnetic field Ba and can be as large as 500 mΦ0/K where ΦS is the flux through the SQUID loop and Φ0 is the flux quantum. Our measurements can be explained using a simple model that takes into account the geometry of a given device and is based on the idea that DT is due to the temperature dependence of the superconducting penetration depth. Our results can be used to optimize device performance in applications where the noise of a device is dominated by ambient temperature fluctuations.