Tikalon Blog by Dev Gualtieri

Magnetocapacitive Tunnel Junctions

August 21, 2017

The transition from vacuum tubes to transistors as computing components did not occur as quickly as you might expect. At the time of the invention of the transistor, December, 1947, the vacuum tube was forty years old. Lee de Forest (1873-1961), who earned a physics Ph.D. under J. Willard Gibbs at Yale University, invented the triode vacuum tube, which he called an Audion, in 1906. At the time of the transistor's invention, the forty years of vacuum tube development produced electronic switches that functioned at ultra-high frequencies (UHF).

Vacuum tubes required considerable power to produce the thermionic electrons needed for their operation, they were also large, since their volume needed to contain bits of metal held at a particular spacing; and, as their name implies, all this was contained in a vacuum envelope. Computer engineers in the 1950s were eager to build transistorized computers, but early transistors would barely operate above a few hundred kilohertz, and only in a limited temperature range. The first consumer transistor device, the Regency TR-1 transistor radio, introduced at the end of 1954, performed miserably, and it only needed to receive radio signals up to 1600 kHz.

Eventually, the IBM 7090 was introduced in November, 1959. The 7090 contained more than 50,000 germanium bipolar junction transistors, and it was capable of computation at 100 Kflop/sec. Current amplifiers, like bipolar transistors, and transconductance amplifiers, like vacuum tubes, are not the only electronic switches. The tunnel diode, invented in August, 1957, by Leo Esaki and colleagues at what is presently Sony, can be used as a sub-nanosecond switch. Leo Esaki and Brian Josephson were awarded the 1973 Nobel Prize in Physics for research in the quantum tunneling of electrons.

Tunnel diodes can be used as electronic switches because they exhibit negative resistance; that is, a portion of their current-voltage characteristic curve has the opposite slope expected from a resistor (see figure). Since they were also made from germanium, the workhorse semiconductor material of the time, they were proposed as the high-speed replacement of transistors in computer circuitry. This, however, didn't happen, since transistors were easier to fabricate, and silicon became the predominant semiconductor material.

A current-voltage (I-V) curve descriptive of a tunnel diode.

In the red region, the derivatives of I vs V, and V vs I are both negative, so the resistance is negative.

(Modified Wikimedia Commons image by "Chetvorno."

Magnetometers, magnetic field sensors, are useful devices for a myriad of consumer applications, The most common of these are as a wheel rotation rate sensor in automobiles, and as an electronic compass. Common magnetic sensing principles are magnetoresistance, including giant magnetoresistance (GMR), Hall effect, and fluxgate. Another magnetic effect, the magnetocapacitance (MC) effect, is found in devices based on multiferroic and spintronic materials. As reported in a recent open access article in the journal, Scientific Reports, a large magnetocapacitance effect is found in magnetic tunnel junctions formed as a sandwich of Fe/AlO_x/Fe₃O₄.[1-2]

A research team comprised of scientists from Hokkaido University (Sapporo, Hokkaido, Japan), Tohoku University (Sendai, Miyagi, Japan) and Brown University (Providence, Rhode Island), followed-up on an earlier study demonstrating that changing the electron spin orientation in a quantum magnetic tunneling junction can result in a large increase in the junction capacitance.[3] In a normal magnetocapacitance effect, there's a higher capacitance when spins in the electrodes on opposite sides of the tunnel layer are parallel to each other, and a lower capacitance when spins are antiparallel.[1]

This new research reports on the inverse of that, higher capacitance when the spins are antiparallel, an inverse tunnel magnetocapacitance (TMC) effect.[1] For a tunnel diode sandwich consisting of Fe/AlO_x/Fe₃O₄, the inverse TMC has a magnitude of up to 11.4% at room temperature.[1] This is a novel spintronic device, a device whose operation depends not just on electron charge, but also electron spin.[2] The device structure, as shown below, has a tunnel layer of 2-4 nanometers of AlO_x.[1]

Structure of the tunneling magnetocapacitance device.

(Portion of fig. 1 of ref. 1, licensed under the Creative Commons Attribution 4.0 International License.)[1

In tunnel junctions made from electrodes of the highly magnetic material, iron-cobalt-boron, the research team demonstrated that a complete spin flip from anti-parallel to parallel could increase the junction capacitance by 150 percent. A theory predicting this result also predicts that the capacitance change could be as high as 1,000 percent.[2]

Tunneling magnetoresistance (TMR) of the tunneling magnetocapacitance device.

(Portion of fig. 1 of ref. 1, licensed under the Creative Commons Attribution 4.0 International License.)[1

The experimental data were found to agree with the Debye-Fröhlich model, modified by other models. Theoretical calculations predict that this inverse TMC effect could reach 150% in magnetic tunnel junctions having positive and negative spin polarization of 65% and −42%. Such a strong effect would allow magnetic logic circuits and multi-valued memory devices.[1]

The iron oxide and iron layers have different cation orientations in their crystal structures, and this causes them to have inverse electrical properties.[2] Says Gang Xiao, a paper coauthor from the Brown University physics department,

"We used iron for one electrode and iron oxide for the other... The electrical properties of the two are mirror images of each other, which is why we observed this inverse magnetocapacitance effect... Now we see that the theories fit well with the experiment, so we can be confident in using our theoretical models to maximize these effects, either the 'normal' effect or the inverse effect that we have demonstrated here."[2]

Capacitance change in a tunneling magnetocapacitance device.

(Portion of fig. 3 of ref. 1, licensed under the Creative Commons Attribution 4.0 International License.)[1

Aside from magnetic logic circuits and multi-valued memory devices, this effect could be used for magnetic sensors.[2] This research was funded by the National Science Foundation and various Japanese research agencies.[2]

Magnetocapacitive Tunnel Junctions

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