Arley Cleveland Marley

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.

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.

MnxPt1−x: A new exchange bias material for Permalloy

We report exchange biasing of Permalloy in as-grown MnxPt1−x/Permalloy structures grown by coevaporation in ultra-high vacuum. Polycrystalline and epitaxial structures with x between 0.35 and 0.75 were studied. X-ray diffraction confirms the presence of chemical ordering in the MnxPt1−x films (grown at 200 °C) in as-grown polycrystalline and epitaxial structures. An interfacial coupling energy of 0.032 erg/cm2 is found for structures where the MnxPt1−x (x=0.56) is grown at 200 °C onto epitaxial Permalloy (001). Polycrystalline structures have a smaller (0.027 erg/cm2) interfacial coupling energy. No exchange bias is observed for films with x≲0.4. The blocking temperature, for a sample with x=0.56, is >500 K.

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.

Low-field magnetoresistance in magnetic tunnel junctions prepared by contact masks and lithography: 25% magnetoresistance at 295 K in mega-ohm micron-sized junctions (abstract)

Interest in magnetic tunnel junctions (MTJ) has been increased by the recent observation of large room-temperature magnetoresistance (MR) in structures comprised of sandwiches of two ferromagnetic (FM) layers with different coercivities separated by a thin Al2O3 tunnel barrier. Using an ultrahigh vacuum dc magnetron sputtering system, a variety of MTJ structures have been explored. Junctions were fabricated directly using computer-controlled placement of successive contact masks, and by patterning using optical lithography techniques and processes, compatible with MR head structures. Structures were prepared in which one of the FM layers is exchange biased with an antiferromagnet MnFe layer. Thus, the junctions exhibit two well-defined magnetic states in which the FM layers are either parallel or antiparallel to one another. The tunnel barrier was prepared by in situ plasma oxidation of thin Al layers sputter deposited at room temperature. Using FM layers comprised of Co or permalloy (Ni81Fe19), MR values...

Electronic noise in magnetic tunnel junctions

We have studied bias and magnetic field dependence of voltage noise in metallic magnetic tunnel junctions with areal dimensions on the order of 1 μm. We generally observe noise with Gaussian amplitude distribution and pure 1/f power spectra at low frequencies. The 1/f noise scales with bias voltage as V2. Two kinds of deviations from this low frequency behavior have been observed. One is at fixed magnetic field when the junction bias reaches above a critical value, the other occurs at a fixed bias when the external magnetic field brings the sample to certain magnetic configurations. In both cases the noise spectra become dominated by Lorentzian noise and in both cases we have observed two level fluctuators in the time domain. We attribute the bias dependent noise to charge traps in the tunnel barrier. The field dependent noise is associated with the switching of the magnetization direction of portions of the top electrode, which we believe to be reversible.

Voltage dependence of the magnetoresistance and the tunneling current in magnetic tunnel junctions (abstract)

Magnetic tunnel junctions (MTJ), structures consisting of two ferromagnetic layers separated by a thin barrier of Al2O3, have recently been shown to exhibit large magnetoresistance (MR) at room temperature. The possible use of these structures in devices requires a thorough characterization and understanding of the transport properties of MTJs, which has motivated the present work. The junctions studied here were fabricated via dc sputtering on silicon substrates at room temperature with a series of contact masks to define the junction area (80×80 μm2), and plasma oxidation to produce the insulating barrier. A long standing problem of MTJs is the sensitivity of their MR to bias voltage. For example, in a recent work1 it was reported that the peak MR of 11% exhibited by a CoFe/Al2O3/Co junction was decreased by half at a bias voltage of <200 mV. In this work, we will show a significant improvement of the MR voltage dependence, with peak MR values of 20%–25% near zero bias decreasing to half these values fo...