Researchers Tap Antiferromagnets for Better RAM

These magnets could write data more quickly than other magnetic RAM options

2 min read
Antiferromagnetic ordering
Erik Vrielink

Strange magnetic materials known as antiferromagnets are promising candidates for a next generation of computer memory. Now scientists have discovered magnetic effects within these materials that could help them operate with greater speed and energy efficiency than current devices.

Standard magnets have particles that are magnetically all oriented in the same direction. As a result, their magnetic polarity can influence their surroundings. In contrast, antiferromagnets , which are far more abundant in nature than regular magnets, have particles that align themselves into opposing directions throughout the material. This leads antiferromagnets to have virtually no magnetic effects on their environment.

Antiferromagnets could find use in state-of-the-art magnetoresistive random-access memory (MRAM), which stores data using permanent magnets, unlike standard computer memory technologies such as dynamic and static RAM, also known as DRAM and SRAM, which are based on storing electric charge. MRAM can read and write data at speeds similar to DRAM and SRAM, but MRAM consumes less power and, like flash memory, is nonvolatile, meaning it does not require a steady power supply to retain data.

One problem of MRAM based on ferromagnets is how external magnetic fields can inadvertently tamper with their data. Moreover, neighboring ferromagnets can interfere with each other unless there is enough distance separating them, limiting MRAM's ability to scale to higher densities for lower costs.

"Antiferromagnets have a huge potential for ultrafast magnetic recording"

MRAM based on antiferromagnets could avoid such problems. In addition, the magnetic properties of antiferromagnets suggest antiferromagnet-based MRAM can write and rewrite data thousands of times faster than ferromagnet-based MRAM.

"Antiferromagnets have a huge potential for ultrafast magnetic recording," says study lead author Davide Bossini, a physicist at the University of Konstanz in Germany.

On the inside, antiferromagnets are typically split into many smaller areas called domains, in which opposing magnetic polarities are arranged in an orderly way. "Domains are very hard to manipulate," Bossini says.

Antiferromagnet domains are separated from each other by transitional areas known as domain walls. Until now, little was known about how domain walls could influence the magnetic properties of antiferromagnets, especially during the kinds of dynamics useful for ultrafast operations.

In the new study, Bossini and his colleagues investigated what happened when antiferromagnetic crystals of nickel oxide were exposed to ultrafast laser pulses lasting only femtoseconds, or quadrillionths of a second.

The scientists found that domain walls could help manipulate oscillations of magnetism known as spin waves. Specifically, domain walls could help couple different frequencies of spin waves that would normally not interact, and do so across different domains.

"Our results show that domains are not a nuisance to be avoided and overlooked, but offer additional possibilities in antiferromagnets," Bossini says.

These findings suggest that coupling the quasiparticles making up spin waves, known as magnons, could help transfer information within an antiferromagnet. This in turn could lead to computing done with magnons on ultrafast time scales with extremely limited energy dissipation. "No complicated interfaces requiring expensive and technologically challenging nanofabrication techniques are required," Bossini says.

Future research can engineer antiferromagnets with a wide variety of differently tailored domain structures to explore how they perform, Bossini says.

The scientists detailed their findings August 9 in the journal Physical Review Letters.

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The Inner Beauty of Basic Electronics

Open Circuits showcases the surprising complexity of passive components

5 min read
A photo of a high-stability film resistor with the letters "MIS" in yellow.
All photos by Eric Schlaepfer & Windell H. Oskay

Eric Schlaepfer was trying to fix a broken piece of test equipment when he came across the cause of the problem—a troubled tantalum capacitor. The component had somehow shorted out, and he wanted to know why. So he polished it down for a look inside. He never found the source of the short, but he and his collaborator, Windell H. Oskay, discovered something even better: a breathtaking hidden world inside electronics. What followed were hours and hours of polishing, cleaning, and photography that resulted in Open Circuits: The Inner Beauty of Electronic Components (No Starch Press, 2022), an excerpt of which follows. As the authors write, everything about these components is deliberately designed to meet specific technical needs, but that design leads to “accidental beauty: the emergent aesthetics of things you were never expected to see.”

From a book that spans the wide world of electronics, what we at IEEE Spectrum found surprisingly compelling were the insides of things we don’t spend much time thinking about, passive components. Transistors, LEDs, and other semiconductors may be where the action is, but the simple physics of resistors, capacitors, and inductors have their own sort of splendor.

High-Stability Film Resistor

A photo of a high-stability film resistor with the letters "MIS" in yellow.

All photos by Eric Schlaepfer & Windell H. Oskay

This high-stability film resistor, about 4 millimeters in diameter, is made in much the same way as its inexpensive carbon-film cousin, but with exacting precision. A ceramic rod is coated with a fine layer of resistive film (thin metal, metal oxide, or carbon) and then a perfectly uniform helical groove is machined into the film.

Instead of coating the resistor with an epoxy, it’s hermetically sealed in a lustrous little glass envelope. This makes the resistor more robust, ideal for specialized cases such as precision reference instrumentation, where long-term stability of the resistor is critical. The glass envelope provides better isolation against moisture and other environmental changes than standard coatings like epoxy.

15-Turn Trimmer Potentiometer

A photo of a blue chip
A photo of a blue chip on a circuit board.

It takes 15 rotations of an adjustment screw to move a 15-turn trimmer potentiometer from one end of its resistive range to the other. Circuits that need to be adjusted with fine resolution control use this type of trimmer pot instead of the single-turn variety.

The resistive element in this trimmer is a strip of cermet—a composite of ceramic and metal—silk-screened on a white ceramic substrate. Screen-printed metal links each end of the strip to the connecting wires. It’s a flattened, linear version of the horseshoe-shaped resistive element in single-turn trimmers.

Turning the adjustment screw moves a plastic slider along a track. The wiper is a spring finger, a spring-loaded metal contact, attached to the slider. It makes contact between a metal strip and the selected point on the strip of resistive film.

Ceramic Disc Capacitor

A cutaway of a Ceramic Disc Capacitor
A photo of a Ceramic Disc Capacitor

Capacitors are fundamental electronic components that store energy in the form of static electricity. They’re used in countless ways, including for bulk energy storage, to smooth out electronic signals, and as computer memory cells. The simplest capacitor consists of two parallel metal plates with a gap between them, but capacitors can take many forms so long as there are two conductive surfaces, called electrodes, separated by an insulator.

A ceramic disc capacitor is a low-cost capacitor that is frequently found in appliances and toys. Its insulator is a ceramic disc, and its two parallel plates are extremely thin metal coatings that are evaporated or sputtered onto the disc’s outer surfaces. Connecting wires are attached using solder, and the whole assembly is dipped into a porous coating material that dries hard and protects the capacitor from damage.

Film Capacitor

An image of a cut away of a capacitor
A photo of a green capacitor.

Film capacitors are frequently found in high-quality audio equipment, such as headphone amplifiers, record players, graphic equalizers, and radio tuners. Their key feature is that the dielectric material is a plastic film, such as polyester or polypropylene.

The metal electrodes of this film capacitor are vacuum-deposited on the surfaces of long strips of plastic film. After the leads are attached, the films are rolled up and dipped into an epoxy that binds the assembly together. Then the completed assembly is dipped in a tough outer coating and marked with its value.

Other types of film capacitors are made by stacking flat layers of metallized plastic film, rather than rolling up layers of film.

Dipped Tantalum Capacitor

A photo of a cutaway of a Dipped Tantalum Capacitor

At the core of this capacitor is a porous pellet of tantalum metal. The pellet is made from tantalum powder and sintered, or compressed at a high temperature, into a dense, spongelike solid.

Just like a kitchen sponge, the resulting pellet has a high surface area per unit volume. The pellet is then anodized, creating an insulating oxide layer with an equally high surface area. This process packs a lot of capacitance into a compact device, using spongelike geometry rather than the stacked or rolled layers that most other capacitors use.

The device’s positive terminal, or anode, is connected directly to the tantalum metal. The negative terminal, or cathode, is formed by a thin layer of conductive manganese dioxide coating the pellet.

Axial Inductor

An image of a cutaway of a Axial Inductor
A photo of a collection of cut wires

Inductors are fundamental electronic components that store energy in the form of a magnetic field. They’re used, for example, in some types of power supplies to convert between voltages by alternately storing and releasing energy. This energy-efficient design helps maximize the battery life of cellphones and other portable electronics.

Inductors typically consist of a coil of insulated wire wrapped around a core of magnetic material like iron or ferrite, a ceramic filled with iron oxide. Current flowing around the core produces a magnetic field that acts as a sort of flywheel for current, smoothing out changes in the current as it flows through the inductor.

This axial inductor has a number of turns of varnished copper wire wrapped around a ferrite form and soldered to copper leads on its two ends. It has several layers of protection: a clear varnish over the windings, a light-green coating around the solder joints, and a striking green outer coating to protect the whole component and provide a surface for the colorful stripes that indicate its inductance value.

Power Supply Transformer

A photo of a collection of cut wires
A photo of a yellow element on a circuit board.

This transformer has multiple sets of windings and is used in a power supply to create multiple output AC voltages from a single AC input such as a wall outlet.

The small wires nearer the center are “high impedance” turns of magnet wire. These windings carry a higher voltage but a lower current. They’re protected by several layers of tape, a copper-foil electrostatic shield, and more tape.

The outer “low impedance” windings are made with thicker insulated wire and fewer turns. They handle a lower voltage but a higher current.

All of the windings are wrapped around a black plastic bobbin. Two pieces of ferrite ceramic are bonded together to form the magnetic core at the heart of the transformer.

This article appears in the February 2023 print issue.