Supporting Image
May 12, 2022 0    
Laser memory
Writing the Magnetic Alphabet

It would be rather distressing if we couldn’t alter our computer data, never able to add, delete, or change files. On the other hand, it would be quite alarming if computer memories were completely fluid, changing on a whim! Every time we save a new version of a file, we rewrite computer memories, and we take for granted that we can do it quickly, reliably, and for the long term. Magnetic materials form the heart of non-volatile computer memories, and these tiny magnetic bits are often stabilized by a phenomenon called exchange bias. The ability to control exchange bias provides a pathway for writing magnetic memories. In the image above, we use a laser to write and then read exchange bias, in the shape of the letter “N” (Go Cornhuskers!!).

But what is exchange bias? In everyday language, we associate “bias” with favoring one viewpoint over another, and its meaning in physics is not so different. Exchange bias means that the magnetism prefers one direction over another—up rather than down, say. It’s a phenomenon that occurs at the interface between a ferromagnet (the familiar materials that stick to your fridge, in which the magnetism of adjacent atoms all points in the same direction) and an antiferromagnet (materials in which the magnetism of atoms alternates between pointing up and down). Looking at Fig. 1, the difference becomes clearer: Because the magnetism of atoms in a ferromagnet points in the same direction everywhere, ferromagnets have have a net, or overall, magnetic moment—which is why playing with magnets is so much fun. In antiferromagnets, by contrast, the magnetism of one layer is “cancelled out” by the magnetism of the next layer, resulting in a zero magnetic moment overall.

Typically, in a ferromagnet, the net magnetic moment may point either up or down, but with no preference for either. Interestingly, however, when placed in intimate contact with an antiferromagnet, the ferromagnet becomes biased—that is, it prefers one direction over the other.

Here our antiferromagnetic material is chromium oxide (Cr2O3), in which the magnetism of layers alternates between pointing up and down (Fig. 2). The role of the ferromagnet is played by an alternating stack of cobalt and palladium atoms. When the two materials are put in contact at an atomic level, the result is—drumroll, please—EXCHANGE BIAS! Now, the ferromagnetic stack of cobalt and palladium orients so their magnetism preferentially points up rather than down.

As the video below shows, we can “write” the exchange bias using a tightly focused laser beam as our pencil. First, we apply an external magnetic field that opposes the existing bias. When the laser beam is applied, it heats only the tiny region directly beneath it. This very local heating, caused by the sharply focused laser beam, allows the magnetic field to alter the bias direction in the material. Away from our laser pencil, the bias stays put. This method gives us an easy way to rewrite magnetic memories using a “sharp,” non-contact writing tool (Fig. 3).

We can then “read” the exchange bias using a much weaker, polarized laser beam. The magnetic state of the material changes the polarization of the light, and by measuring  the polarization of the weak beam, we can determine the direction of the exchange bias. (Learn more about polarization here.)

Finally, we can erase the memory completely by heating the entire area in the presence of magnetic field, thereby wiping the bias clean.

By varying the combination of laser beam and magnetic field applied, we can write, read, and erase memories, much like the actions of a tiny Magna Doodle (Fig. 4).

Fig. 1 (Click to enlarge). In a ferromagnet (left), the magnetic moments of the atoms point in one direction.  In an antiferromagnet (right), the magnetic moments alternate.
Fig. 1 (Click to enlarge). In a ferromagnet (left), the magnetic moments of the atoms point in one direction. In an antiferromagnet (right), the magnetic moments alternate.
Fig. 2 (Click to enlarge). The antiferromagnet Cr2O3 with the magnetism of chromium atoms alternating between up and down.
Fig. 2 (Click to enlarge). The antiferromagnet Cr2O3 with the magnetism of chromium atoms alternating between up and down.
Fig. 3 (Click to enlarge). The letter N written as exchange bias using a focused laser.  A scanning electron micrograph of a human hair sets the scale.
Fig. 3 (Click to enlarge). The letter N written as exchange bias using a focused laser. A scanning electron micrograph of a human hair sets the scale.
Fig. 4 (Click to enlarge). By varying the combination of laser beam and magnetic field, we can create a kind of atomic-level Magna Doodle, writing, reading, and erasing data.
Fig. 4 (Click to enlarge). By varying the combination of laser beam and magnetic field, we can create a kind of atomic-level Magna Doodle, writing, reading, and erasing data.
TAGS: #ferromagnetism    #magnetic memory    #magnetism    #nanomagnetism    #polarization    
 
SHARE THIS POST:

Related Posts

11/30
Supporting Image
Supporting Image
Domains and Disks
by Shireen Adenwalla, Xiaoshan Xu

Magnets curve themselves into beautiful patterns called domains, which cannot be seen with the naked eye. Now that magnetic paint and nail polish are easily available, we can use magnets to create all kinds of magnetic patterns which we can see, photograph, erase and rewrite! Click to find out how YOU can paint with magnets!

2 2    
05/12
Supporting Image
Supporting Image
Laser memory

Instead of pencil, paper, and eraser, we can use combinations of lasers and magnetic materials to write, read, and and erase information by varying the temperature and magnetic field. Here we apply our laser "pencil" to magnetic "paper" to write the letter “N” (Go Huskers!!). This technique allows us write, erase, and rewrite tiny magnetic memories like those found in your computer hard drive and other devices. Click to learn how it works!

0 0    
02/01
Supporting Image
Supporting Image
Liquid magnetism
by Robert Streubel, Scott Schrage

You may have heard that there are three main phases of matter: solids, liquids, and gases (plus plasma if you want to get fancy). Liquids can take virtually any shape and deform instantly. Solid materials possess interesting electronic and magnetic properties essential to our daily life. But how about designing rigid liquids with magnetic properties? Impossible? Not anymore. Click to learn more!

0 0    

More Funsize Research

04/20
Supporting Image
Supporting Image
Fluids and filling

You take a pristine-looking Oreo from a package of seemingly identical sandwich cookies, and you decide to open it up to eat the creme filling first. You gently twist the cookie apart without breaking the chocolate wafers, but the creme sticks to one side only. Why? Happily, the physics of fluids helped two MIT students solve this delicious mystery. Read on to find out what they learned, and how you can test their results at home.

0 0    
05/12
Supporting Image
Supporting Image
Laser memory

Instead of pencil, paper, and eraser, we can use combinations of lasers and magnetic materials to write, read, and and erase information by varying the temperature and magnetic field. Here we apply our laser "pencil" to magnetic "paper" to write the letter “N” (Go Huskers!!). This technique allows us write, erase, and rewrite tiny magnetic memories like those found in your computer hard drive and other devices. Click to learn how it works!

0 0    
04/20
Supporting Image
Supporting Image
Thermalizing nanowires

It’s a hot summer day. You desperately want something cold to drink, but unfortunately, your bottle of root beer has been sitting in a hot car all day. You put it into a bucket full of ice to cool it down. But it’s taking forever! How, you wonder, could you speed the process up? The same question is important for understanding how electronic devices work, and how we can make them work better by controlling the temperature of the electrons that power them. Read on to find out what a bottle of root beer in a cooler full of ice and a nanowire in a vat of liquid helium have in common!

0 0    
02/23
Supporting Image
Supporting Image
World's smallest diode

Diodes, also known as rectifiers, are a basic component of modern electronics. As we work to create smaller, more powerful and more energy-efficient electronic devices, reducing the size of diodes is a major objective. Recently, a research team from the University of Georgia developed the world's smallest diode using a single DNA molecule. This diode is so small that it cannot be seen by conventional microscopes.

0 0    
02/01
Supporting Image
Supporting Image
Liquid magnetism
by Robert Streubel, Scott Schrage

You may have heard that there are three main phases of matter: solids, liquids, and gases (plus plasma if you want to get fancy). Liquids can take virtually any shape and deform instantly. Solid materials possess interesting electronic and magnetic properties essential to our daily life. But how about designing rigid liquids with magnetic properties? Impossible? Not anymore. Click to learn more!

0 0    
12/08
Supporting Image
Supporting Image
Nanoscale fluid mechanics

We think we're pretty familiar with how ordinary liquids behave, but it turns out that some of the basic things we know are no longer true when we look at these liquids on short enough length scales and fast enough time scales. The liquids start to behave more like solids, pushing back when you push on them, and slipping across solid surfaces instead of being dragged along. Click to ride the tiny-but-mighty new wave of nanofluidics!

0 0    
11/05
Supporting Image
Supporting Image
Building a better computer
by Peter Dowben, Jocelyn Bosley

Scientists are working to develop electronic devices that store and process information by manipulating a property of electrons called spin—a research area aptly known as spintronics. The semiconductors we are developing will not only be faster and cheaper than those used in conventional devices, but will also have more functionality.

1 1    
11/05
Supporting Image
Supporting Image
Good vibrations

Materials that are absolutely perfect—in other words, materials that contain no defect of any kind—are usually not very interesting. Imagine being married to a saint: you would quickly be bored out of your mind! Defects and impurities can considerably change many properties of materials in ways that allow a wide range of applications.

0 0    
11/05
Supporting Image
Supporting Image
Secrets of semiconductors

Semiconductors are materials with properties intermediate between metals and non-conducting insulators, defined by the amount of energy needed to make an electron conductive in the material. The non-conducting electrons occupy a continuum of energy states, but two of these states (the “heavy hole” and “light hole”) are nearly identical in energy. The heavy hole is easy to observe and study, but the light hole eludes most observers.

0 0    
11/05
Supporting Image
Supporting Image
Locking up electrons

Solids are generally divided into metals, which conduct electricity, and insulators, which do not. Some oxides straddle this boundary, however: a material's structure and properties suggest it should be a metal, but it sometimes behaves as an insulator. Researchers at the University of California, Santa Barbara are digging into the mechanisms of this transformation and are aiming to harness it for use in novel electronic devices.

0 0    
11/05
Supporting Image
Supporting Image
Nanomagnetism

You may know that the media used in magnetic recording technologies, such as computer hard drives, are made of millions of tiny nanomagnets. Each nanomagnet can be switched up or down to record bits of information as ones and zeros. These media are constantly subjected to magnetic fields in order to write, read, and erase information. If you have ever placed a magnet too close to your laptop or cell phone, you know that exposure to an external magnetic field can disrupt information stored this way. Did you know that it is possible for the nanomagnets to "remember" their previous state, if carefully manipulated under specific magnetic field and temperature conditions? Using a kind of memory called topological magnetic memory, scientists have found out how to imprint memory into magnetic thin films by cooling the material under the right conditions.

0 0    
11/05
Supporting Image
Supporting Image
Slow reflection

Inside solids, the properties of photons can be altered in ways that create a kind of "artificial gravity" that affects light. Researchers at the University of Pittsburgh tracked photons with a streak camera and found that whey they enter a solid-state structure, they act just like a ball being thrown in the air: they slow down as they move up, come to a momentary stop, and fall back the other way. Studying this "slow reflection" will allow us to manipulate light's behavior, including its speed and direction, with potential applications in telecommunications and quantum computing technologies.

0 0    
11/05
Supporting Image
Supporting Image
Superfluid helium droplets

In a unique state of matter called a superfluid, tiny "tornadoes" form that may play an important role in nanotechnology, superconductivity, and other applications. Just as tornadoes are invisible air currents that become visible when they suck debris into their cores, the quantum vortices in superfluids attract atoms that make the vortices visible. Quantum vortices are so small they can only be imaged using very short-wavelength x-rays, however.

0 0    
11/05
Supporting Image
Supporting Image
Magnetic anisotropy

Would you rather have data storage that is compact or reliable? Both, of course! Digital electronic devices like hard drives rely on magnetic memory to store data, encoding information as “0”s and “1”s that correspond to the direction of the magnetic moment, or spin, of atoms in individual bits of material. For magnetic memory to work, the magnetization should not change until the data is erased or rewritten. Unfortunately, some magnetic materials that are promising for high density storage have low data stability, which can be improved by squeezing or stretching the crystal structures of magnetic memory materials, enhancing a material property called magnetic anisotropy.

0 0    
11/05
Supporting Image
Supporting Image
Detecting neutron radiation

Neutron radiation detection is an important issue for the space program, satellite communications, and national defense. But since neutrons have no electric charge, they can pass through many kinds of solid objects without stopping. This makes it difficult to build devices to detect them, so we need special materials that can absorb neutrons and leave a measurable signature when they do. Researchers at the University of Nebraska-Lincoln are studying the effects of solar neutron radiation on two types of materials on the International Space Station (ISS), using detectors made of very stable compounds that contain boron-10 and lithium-6.

0 0    
11/05
Supporting Image
Supporting Image
Printable perovskites

To increase our use of solar energy, we need to create more efficient, stable, and cost-effective solar cells. What if we could use an inkjet printer to fabricate them? A new type of solar cell uses a class of materials called perovskites, which have a special crystal structure that interacts with light in a way that produces an electric voltage. We've developed a method to produce perovskite thin films using an inket printer, which in the future could pave the way to manufacture solar cells that are surprisingly simple and cheap.

0 0    
09/30
Supporting Image
Supporting Image
Computing with fool's gold?

Fool's gold is a beautiful mineral often mistaken for gold, but recent research shows that its scientific value may be great indeed. Using a liquid similar to Gatorade, it can be turned into a magnet at the flick of a switch! Read on to learn more!

0 0    

Go to Top