wanda-seldon's recent activity

  1. Comment on A layperson's introduction to Spintronics Memory in ~science

    wanda-seldon
    Link Parent
    Thank you. This is my worst performing post so far however, I had hoped the new default sorting would have changed things a bit. So I don't think these posts will be making a return (they take...

    Thank you. This is my worst performing post so far however, I had hoped the new default sorting would have changed things a bit. So I don't think these posts will be making a return (they take quite some time to write up). I will try again if/when Tildes has grown a bit more.

    3 votes

  2. Comment on A layperson's introduction to Spintronics Memory in ~science

    wanda-seldon
    Link Parent
    Yes you are correct. It is currently being studied in several labs, with working prototypes.

    Yes you are correct. It is currently being studied in several labs, with working prototypes.

    1 vote

  3. Comment on A layperson's introduction to Spintronics Memory in ~science

    wanda-seldon
    Link Parent
    Hard drives work by magnetising parts of a disk/platter. That's different from this type of memory. This memory can be scaled down to the nanometre range. Each nanoscale ferromagnet would be able...

    This is the principle used in hard disk drives correct?

    Hard drives work by magnetising parts of a disk/platter. That's different from this type of memory. This memory can be scaled down to the nanometre range. Each nanoscale ferromagnet would be able to store one bit.

    If it is, what makes the boundary limitation on the domain wall? What hinders a current from influencing nearby "points" which could be used to read/write data? Do we just look what the predominant spin direction is at a given point?

    I'm not sure I understand what you mean here. Each bit is stored on its own ferromagnet which is magnetised either up or down.

    3 votes

  4. A layperson's introduction to Spintronics Memory

    ~science Text 1513 words
    Introduction I want to give an introduction on several physics topics at a level understandable to laypeople (high school level physics background). Making physics accessible to laypeople is a...

    Introduction

    I want to give an introduction on several physics topics at a level understandable to laypeople (high school level physics background). Making physics accessible to laypeople is a much discussed topic at universities. It can be very hard to translate the professional terms into a language understandable by people outside the field. So I will take this opportunity to challenge myself to (hopefully) create an understandable introduction to interesting topics in modern physics. To this end, I will take liberties in explaining things, and not always go for full scientific accuracy, while hopefully still getting the core concepts across. If a more in-depth explanation is wanted, please ask in the comments and I will do my best to answer.

    Previous topics

    Bookmarkable meta post with links to all previous topics

    Today's topic

    Today's topic is spintronics storage devices for computers. I will try to explain how we can use an electron's spin to read and write data and why this is more efficient than current technologies.

    What do we need to have a storage device?

    In order to be able to save a bit (a 0 or a 1 in computer science speak), we need to be able to represent both the 0 and the 1 in some physical way. We could for example flip a light switch and say light on is 1, light off is 0. We will also need to be able to read the information we stored, in this case we can simply look at the lamp to see if we're storing a 0 or a 1. We would also like for this information to be stored even when power is cut, so that next time we power the hardware back on, we will still be able to read the data. Lastly, we want to be able to change between 0 and 1 freely; no one wants to go back to the CD days for storage.

    Now for some basic concepts.

    What is spin?

    Spin arises from quantum mechanics. However, for the purpose of explaining spin storage devices we can think of an electron's spin as a bar magnet. Each electron can be thought of as a freely rotating bar magnet that will align itself with the fields from nearby magnets. Think of it as a compass (the electron) aligning itself to a fridge magnet when it's held near the compass.

    Why are some metals magnetic?

    Why can we make permanent magnets out of iron, but not copper? In all metals, we have spins that are free to rotate. This means that we can turn a metal into a magnet by holding it near another magnet, it will "copy" the other magnet's magnetisation - its spins will rotate in the direction of the field. But as soon as we remove the magnet, our metal will stop being magnetic. This is because the spins are freely rotating, the spins will align to the magnet's magnetisation when they feel it, but nothing is holding them in place as soon as it's removed. We call this property paramagnetism.

    However, iron (and some other metals) will retain a nearby magnet's magnetisation even when the magnet is removed. This is because in these materials, called ferromagnets, it costs energy for the spins to rotate away from the material's magnetisation. They are pinned into place.

    What happens if we expose half of our ferromagnet to a magnetisation pointing in one way (let's call it up), and the other half to a magnet whose magnetisation is pointing the other way (which we call down)? The ferromagnet would copy both magnetisation directions and create a boundary region - a so-called domain wall - in the centre. The spins in this domain wall will slowly rotate over the thickness of the wall so that at one end they're pointing up and at the other end they're pointing down.

    How can we use spin to store data?

    What if, instead of a light bulb, we used a bar magnet as our storage medium. We could magnetise our bar magnet in one direction to store a 1 and magnetise it in the other direction to store a 0. To read what we have stored, we simply check the bar magnet's magnetisation.

    Let's work out this idea. We want to be able to efficiently change the magnetisation of a bar magnet and we want to be able to read the bar magnet's magnetisation. We will use a ferromagnet because it will retain our data indefinitely (its magnetisation will not change unless we force it to). We know it costs energy to flip the spins inside a ferromagnet, so we will want to use a very tiny ferromagnet - it will have less spins which means it will cost us less energy to change the magnetisation (i.e. flip the spins).

    Magnetoresistance

    A-ha, now we're getting into the fancy-titled paragraphs. What do you, dear reader, think would happen when we send a current (e.g. a bunch of electrons) through a magnet? What would happen to the current's electrons (also called itinerant electrons, to distinguish them from the non-moving electrons of the metal)? At the boundary of the magnet, where the current enters, only the electrons who (through random chance) have a spin that's aligned to the magnet's magnetisation will pass through. We call this effect magnetoresistance, as in effect part of our current will feel a resistance - they cannot pass through to the magnet. So to rephrase, the current inside the magnet will be "magnetised" - all of the spins of the itinerant electrons are pointing the same way.

    Current induced domain wall motion

    So now we know what happens to a current that's inside a magnet. What happens when this current meets a domain wall - the region where the magnetisation changes direction? The itinerant electrons' spins will start rotating along with the magnetisation, but the static electrons of the ferromagnet will also start rotating in the opposite way due to the magnetisation they feel from the current (more experienced readers will recognise this as conservation of angular momentum). So the spins inside the current will slowly rotate until they are pointing the opposite direction and can continue passage from the up-magnetised part of the ferromagnet into the down-magnetised part. But the spins that belong to the ferromagnet itself will be rotating in the opposite manner, slowly rotating from down to up as the current passes through. This means the boundary region between up and down magnetisation, the domain wall, will move along with the current.

    So in short, by sending a current through a magnet that's magnetised in opposite directions at each end, we can force our preferred magnetisation to expand in the current's direction. By reversing the direction of the current we can then magnetise the other way again.

    So we can say magnetising up (pushing current through (let's say) from left to right) can be our 1 and magnetising down (pushing current through from right to left) can be our 0. This would allow us to store data permanently as even when we remove the current our magnet will remember its magnetisation. If we make a really tiny ferromagnet we will only need a really tiny current to flip it's magnetisation too. So we can scale this process down to get to really good efficiencies. In the lab these types of devices are down to nanometre scale and require extremely little current to be operated.

    Reading the data

    OK, so now we know how to write data. But how do we read it? The key effect here will be magnetoresistance, as explained earlier in the post.

    Let's look at this picture. The red dotted line shows our write currents, the big bar is our ferromagnet. The arrows pointing up and down at the sides are our magnetisation direction, the double-pointed arrow in the centre shows the region where we flip the magnetisation by sending through a current.

    Now we jam a third, permanently magnetised, bit of metal (let's call it the read terminal) on top of the centre of our bar. We send a current from this read connector to one of the ends of the ferromagnet. If the ferromagnet's magnetisation is aligned to that of the read terminal we will experience a low (magneto)resistance, but if the ferromagnet is magnetised in the opposite direction we will experience a high resistance. By measuring the difference in resistance we can determine if we have a 0 or a 1 stored. We just need to be careful not to send too big of a current, else that would influence our ferromagnet's magnetisation. But small currents means better efficiency, so this is not a problem at all.

    Conclusion

    This concludes the post, we have seen how to use spins and magnets to both write and read data and we understand why this is efficient.

    Feedback

    As usual, please let me know where I missed the mark. Also let me know if things are not clear to you, I will try to explain further in the comments!

    27 votes

  5. Comment on Genetic Algorithms in ~comp

    wanda-seldon
    Link
    I have added this post to the layperson's introduction to meta-post, with @Soptik's approval.

    I have added this post to the layperson's introduction to meta-post, with @Soptik's approval.

    2 votes

  6. Comment on Fermi problem game thread in ~misc

    wanda-seldon
    Link Parent
    A water molecule consists of 3 = 10^0 atoms and weighs 18 = 10^1 atomic masses. A hydrogen atom weighs 10^-27 kg. So a water molecule weighs 10^1 times this; 10^-26 kg. A human consists mainly of...
    • A water molecule consists of 3 = 10^0 atoms and weighs 18 = 10^1 atomic masses.
    • A hydrogen atom weighs 10^-27 kg. So a water molecule weighs 10^1 times this; 10^-26 kg.
    • A human consists mainly of water and weighs 10^2 kg.
    • There are then 10^2/10^-26 = 10^(2+26) = 10^28 water molecules in a human body.
    • There are 10^0 * 10^28 = 10^28 atoms in an average human body.
    2 votes

  7. Comment on Fermi problem game thread in ~misc

    wanda-seldon
    Link Parent
    I came up with the following: There are 10^6 people in Amsterdam, half of which are women. The average number of children per woman is 2. Women are able to have children over a period of ~30 years...

    I came up with the following:

    • There are 10^6 people in Amsterdam, half of which are women.
    • The average number of children per woman is 2.
    • Women are able to have children over a period of ~30 years (15 to 45).
    • This means the probability of any of those women giving birth in a specific year is 2/30=1/15 = ~ 0.1 = 10^-1 (very close to your guess @gpl!).
    • Assuming half of the women in Amsterdam are between 15 and 45, we get 10^6 / 4 =10^5.
    • 10^(5-1) = 10^4 births in Amsterdam per year, so 10^2 births per day.
    • A midwife will supervise in 10^0 births per working day.

    So you would need around 10^(2-0) = 100 midwives in Amsterdam.

    1 vote

  8. Comment on Fermi problem game thread in ~misc

    wanda-seldon
    Link Parent
    Ah yeah, that should definitely be 10^9. Gonna fix that. I rounded down the number of keystrokes per hour because the values quoted in keystrokes per minute or words per minute are usually not...

    Ah yeah, that should definitely be 10^9. Gonna fix that. I rounded down the number of keystrokes per hour because the values quoted in keystrokes per minute or words per minute are usually not sustainable over long periods of time.

    Edit: Actually, I'll change it to 10^10 as 7 billion is closer to that then to 10^9.

    2 votes

  9. Comment on Fermi problem game thread in ~misc

    wanda-seldon
    Link
    How many midwives are there in Amsterdam?

    How many midwives are there in Amsterdam?

    1 vote

  10. Comment on Fermi problem game thread in ~misc

    wanda-seldon
    (edited )
    Link Parent
    This method is very common in physics, where we call these 'back of the envelope' calculations. Rounding to the nearest power of 10 for all values (coincidentally this is why I switched from...

    This method is very common in physics, where we call these 'back of the envelope' calculations. Rounding to the nearest power of 10 for all values (coincidentally this is why I switched from astrophysics to material physics) is common in this scenario, because you do not know all data with a greater accuracy than that anyway. So let's try this.

    I define a keystroke as taking place on a computer keyboard (so no smartphones etc).

    An average person does 10^2 keystrokes per minute, or 10^3 keystrokes per hour.
The average person spends 10^0 hours per day typing. (Much more in the West probably, and much less in Africa, I feel it's definitely closer to 1 than to 10 hours).
There are 10^10 people in the world.

    This leads to 10^(3+0+10) = 10^13 or 1000000000000 keystrokes per day.

    5 votes

  11. Comment on What is your favorite Wikipedia page? in ~talk

    wanda-seldon
    Link
    The Wikipedia page for Lev Landau (one of the most influential physicists of the previous century) has this gem:

    The Wikipedia page for Lev Landau (one of the most influential physicists of the previous century) has this gem:

    In 1937, Landau married Kora T. Drobanzeva from Kharkiv.[26] Their son Igor was born in 1946. Landau believed in "free love" rather than monogamy and encouraged his wife and his students to practise "free love". However, his wife was not enthusiastic.[17]

    3 votes

  12. Comment on Intel tried to bribe reseachers to downplay the severity of MDS vulnerability in ~tech

    wanda-seldon
    Link Parent
    Hoi, I am Dutch. This article is indeed in Dutch, you can tell from the lack of German characters like ü and ö and the inclusion of the Dutch digraph ij. The source you linked (which I'd say is...
    • Exemplary

    Hoi, I am Dutch.

    This article is indeed in Dutch, you can tell from the lack of German characters like ü and ö and the inclusion of the Dutch digraph ij.

    The source you linked (which I'd say is quite trusted) doesn't mention much about bribes.

    De Amsterdamse universiteit krijgt ook als enige partij een beloning: 100.000 dollar (89.000 euro), Intels maximale beloning voor ontdekkers van kritische lekken.
    Er zit wel een bijsmaakje aan de premie. Volgens de VU probeerde Intel de ernst van het lek te bagatelliseren door 40.000 dollar beloning officieel uit te keren en daarnaast nog eens 80.000 dollar ‘los’.

    This says that intel wanted to give the VU 40k dollars officially as a reward for finding the big and another 80k seperately. This offer was refused and they ended up getting 100k on the books, the max amount available in the program.

    7 votes

  13. Comment on <deleted topic> in ~talk

    wanda-seldon
    Link Parent
    I'm still around. I might post some more in the future and see if my threads are more popular then. Currently real life is very busy though so it might be a while.

    I'm still around. I might post some more in the future and see if my threads are more popular then. Currently real life is very busy though so it might be a while.

    6 votes

  14. Comment on <deleted topic> in ~science

    wanda-seldon
    Link Parent
    Done! And I have to agree it's a very interesting article.

    Done! And I have to agree it's a very interesting article.

    3 votes

  15. Comment on A layperson's introduction to the nature of light and matter, part 1 in ~science

    wanda-seldon
    Link Parent
    Hi, sorry for the very late reply, I came down with the flu. So pilot wave theory is an example of a hidden-variable theory. Hidden-variable theories share the core idea that the randomness in...
    • Exemplary

    Hi, sorry for the very late reply, I came down with the flu.

    So pilot wave theory is an example of a hidden-variable theory. Hidden-variable theories share the core idea that the randomness in quantum mechanics is not really randomness but instead a deterministic hidden variable that we cannot measure.

    There are two classes of hidden-variable theory; local hidden-variable and non-local hidden-variable. Local hidden-variable theories have the added requirement of local realism. Simply put this means that these theories require distant events (i.e. separated by some distance) cannot communicate instantaneously. As this class of theory is very sensible, they used to be very popular amongst physicists. So what happened? Bell wrote Bell's theorem which simply states "No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics.". This theorem has been proven.

    So that leaves us with non-local hidden variable theories, which includes the modern version of Pilot wave theory, called De Broglie-Bohm theory. This is a valid interpretation of modern quantum mechanics (along with the many-worlds interpretation, Copenhagen interpretation, modal interpretation and objective-collapse interpretation) that has plenty of weirdness of its own. The wavefunctions given by this theorem have hidden variables that can depend on the state of the entire universe. Furthermore, the pilot waves of this theory are by themselves sufficient to explain the behaviour of particles. So once again you end up describing particles as waves, just pilot waves. However, there are definitely physicists that argue for this view (and for any of the other views that I mentioned) but in the end they all produce the same results.

    3 votes

  16. Comment on A layperson's introduction to the nature of light and matter, part 1 in ~science

    wanda-seldon
    Link Parent
    We do know, we just can't create an analogy to more everyday things. Next time I will elaborate on the model we currently have.

    We do know, we just can't create an analogy to more everyday things. Next time I will elaborate on the model we currently have.

    2 votes

  17. Comment on A layperson's introduction to the nature of light and matter, part 1 in ~science

    wanda-seldon
    Link Parent
    I will elaborate on this in my next post. I'll @ you when it's up :)

    I will elaborate on this in my next post. I'll @ you when it's up :)

    2 votes

  18. Comment on A layperson's introduction to the nature of light and matter, part 1 in ~science

    wanda-seldon
    Link Parent
    It can go from its base state (state 1) to a higher energy state (state 2) by absorbing a photon that would put in a third even higher energy state (state 3) and then reemit a photon that has an...

    It can go from its base state (state 1) to a higher energy state (state 2) by absorbing a photon that would put in a third even higher energy state (state 3) and then reemit a photon that has an energy equal to the difference between the second and third state. This way it ends up at state 2 even though the photon didn't have the energy to fit the transition from 1 to 2.

    2 votes

  19. Comment on A layperson's introduction to the nature of light and matter, part 1 in ~science

    wanda-seldon
    Link Parent
    If the photon does not fit the electron's needs it will not interact with the electron. So it will not be absorbed.

    If the photon does not fit the electron's needs it will not interact with the electron. So it will not be absorbed.

    3 votes

  20. Comment on A layperson's introduction to the nature of light and matter, part 1 in ~science

    wanda-seldon
    (edited )
    Link Parent
    In essence you are correct. Light that is too low frequency to allow the electron to escape will instead only bring the electron into a higher energy level within the metal. If it would then...
    • Exemplary

    In essence you are correct. Light that is too low frequency to allow the electron to escape will instead only bring the electron into a higher energy level within the metal. If it would then absorb a second photon it could escape (or maybe it needs a third, fourth, etc). However, there are several rules that make this very tricky to achieve.

    The energy of the photon must be exactly the energy difference between the energy level the electron is in and another allowed energy state. Remember, this is quantum mechanics so the electron can only have certain discrete energies. If the photon's energy doesn't match this energy difference nothing happens.

    Say we manage to do this, then we have to get lucky and absorb a second photon before the electron falls back into its original state (and re-emits a photon). The average amount of time an electron spends in the higher energy state is called the lifetime, and it's usually very short in these cases.

    If it needs more than two photons worth in energy to escape it becomes even trickier. Say it absorbed one photon. Then it gets lucky and comes into contact with another photon that has the same energy. Just because the photons have the right frequency to help the electron go from its original state to the state it's currently in, doesn't mean the same energy can bring it into an even higher state. Energy levels are rarely evenly spaced.

    So I did lie a little bit, you could - if you have the right set of frequencies in your light that can help the photon raise its energy in steps - see a few photons being emitted but it would be very rare compared to what happens when you have the frequency that can punch out electrons in a single step. It's the difference between winning the lottery once and winning it twice within a short time span (the lifetime of the electron in the higher energy state).

    Does that make sense? :)

    1 vote