Spintronics and its application

1. Spintronics and its application Y. Tzeng July 2003

2. Outline • What’s spintronics and its advantages over traditional charge-based electron devices • GMR effect basics • GMR application in hard disk drive read sensor • Magnetic tunneling junctions • Challenges and difficulties remaining in spintronics area and possible approaches of solution

3. Moore’s Law

4. Spintronics • What’s spintronics Spintronics: Short for spin-based electronics FACT: Quantum Spins of electrons http://www.physics.umd.edu/rgroups/spin/

5. Magnetoresistance (MR) Electric Current Magnetic field - S N + Lorenz force acting on the charge carriers  increase of the resistance in an applied magnetic field MR: When a metal or semiconductor in a magnetic field, its electrical resistance usually increases by small amount mrsec.wisc.edu/Edetc/IPSE/HTML%20Presentation%20folder/GMRHardDrive.PPT

6. Spintronics 1   In addition to their mass and electric charge, electrons have an intrinsic quantity of angular momentum called spin, almost as if they were tiny spinning balls. 2   Associated with the spin is a magnetic field like that of a tiny bar magnet lined up with the spin axis. 3   Scientists represent the spin with a vector. For a sphere spinning "west to east" the vector points "north" or "up.“ It points "down" for the opposite spin. 4   In a magnetic field, electrons with "spin up" and "spin down“ have different energies. 5   In an ordinary electric circuit the spins are oriented at random and have no effect on current flow. 6   Spintronic devices create spin-polarized currents and use the spin to control current flow. (D. Awschalom et al., Spintronics, Scientific American, June 2002)

7. Spintronics Logic • The magnetic orientations of electron spin, “spin up” and “spin down” can represent “1” and “0” respectively. • Unlike traditional digital logic, which is based on existence and absence of electrons. Spintronics, American Scientist, Volume 89, by Sankar Das Sarma

8. Spintronics • Its advantages over classical charge-based devices 1. Can easily manipulated by externally applied magnetic field 2. Long coherence, or relaxation time 3. Allow devices to be much smaller, consume less electricity and be more powerful in certain types of computation Spintronics, American Scientist, Volume 89, by Sankar Das Sarma

9. GMR effect basics • What’s GMR Giant Magnetoresistive Effect is an effect of very large resistance change in materials comprised of alternating very thin layers of various metallic elements. Usually two layers of ferromagnetic materials sandwiching one layer of non-magnetic material. The total resistance of this material is lowest when the magnetic orientations of the two ferromagnetic layers are aligned, is highest when the orientations are anti-aligned. ferromagnetic Non-magnetic ferromagnetic Spintronics, American Scientist, Volume 89, by Sankar Das Sarma

10. Ferrimagnetic minerals have two types of magnetic crystal lattice sites that naturally align antiparallel. The net magnetic moment within the ferrimagnet is due to either a difference in the ionic make up of different crystal sites, or a crystallographic inhomogeneity between different sites. In antiferromagnetic minerals there are again two different magnetic crystallographic sites, however the magnetic moments of the ions at different sites entirely cancels, hence leaving no net magnetic moment. A net magnetic moment can only exist within an antiferromagnet if its individual magnetic ion sites are not entirely antiparallel, this is called imperfect or canted antiferromagnetism. In practice ferrimagnets have strong magnetic properties and moderate coercivities. Imperfect antiferromagnets have weaker magnetic properties but very high coercivities. These differences can be used to detect them in natural materials (Thompson and Oldfield, 1984). Arrangment of magnetic moments in ferrimagnets ferromagnets and anti-ferromagnets (F.D. Stacey 1992). http://www.glg.ed.ac.uk/home/John.Braisby/TRANSF/magnetics.html

11. -Ferromagnetic minerals are attracted into a positive magnetic field gradient. The attraction for ferromagnetic minerals, however, is far larger than it is for paramagnetic minerals. Ferromagnetic minerals have internal magnetic dipole moments, due to the spin of electrons within the minerals. For ferromagnets, however the individual dipoles within the mineral, when aligned by an external magnetic field, will tend to remain aligned even when the field is subsequently removed due to quantum mechanical effects arising from the ferromagnetic minerals crystal lattice structure. This remanence leads to magnetic hysteresis, where the ferromagnet remembers past applied fields. http://www.glg.ed.ac.uk/home/John.Braisby/TRANSF/magnetics.html

12. Paramagneticminerals are attracted towards a positive magnetic field gradient. The attraction into a magnetic field is due to permanent magnetic dipole moments within the paramagnetic mineral which are normally randomised by thermal activity. An external magnetic field supplies the necessary energy to align the magnetic dipoles. The randomisation due to thermal activity still persists within a magnetic field, so the number of the materials magnetic dipoles aligned with the external field, and hence the force of attraction, depends on both the temperature and the strength of the applied field. When a paramagnetic mineral is removed from a magnetic field it immediately loses its magnetism. http://www.glg.ed.ac.uk/home/John.Braisby/TRANSF/magnetics.html

13. Diamagneticminerals are repelled from a positive magnetic field gradient. The repulsion of a diamagnetic mineral from a magnetic field is caused by the generation of a magnetic dipole moment, within the atoms of the mineral, by the disruption of their electron orbits due to the presence of the magnetic field. The repulsion of a diamagnetic mineral from a magnetic field gradient is far smaller than the attraction of a paramagnetic mineral. Like a paramagnetic mineral, a diamagnetic mineral loses its internal magnetic field once it has been removed from an external magnetic field. http://www.glg.ed.ac.uk/home/John.Braisby/TRANSF/magnetics.html

14. A hysteresis loop for a ferromagnetic mineral showing the relationship between applied magnetic field and mineral magnetisation is shown in Fig 2.1.HysteresisThe magnetic properties of a ferromagnet depend not only on the present conditions affecting it, but also on the past magnetic fields to which it has been subjected. In figure 1 the magnetic field that a ferromagnetic material is being subjected to is plotted on the horizontal axis, against the magnetisation induced within the material on the vertical axis. Also plotted is the low, field magnetic behaviour. Up to a certain applied magnetic field the material exhibits no hysteresis effects and when the external field is removed the material returns to an unmagnetised state.Some important parameters on the hysteresis curve are: Saturation Magnetisation, which is the largest magnetisation that can be imparted to a given ferromagnetic mineral; Saturation Remanence, which is the remanent magnetisation of a material that has been magnetised in a saturating field; Coercive force, which is the magnetic field that has to be applied to reduce the saturation remanence to zero magnetisation when measurement is made in the presence of the field (Thompson R., and Oldfield F., 1984). http://www.glg.ed.ac.uk/home/John.Braisby/TRANSF/magnetics.html

15. http://www.almaden.ibm.com/st/projects/magneto/giantmr/

16. TemperatureFerromagnetism is a temperature dependent phenomenon. In fact ferromagnetism and paramagnetism are at different ends of a thermal energy / magnetic energy scale. At a low enough temperature paramagnetic behaviour can become ferromagnetic due to the lack of randomising thermal energy. Likewise at a high enough temperature ferromagnets can become paramagnetic due to thermal energy randomising the direction of individual ionic dipoles. The temperature at which ferromagnetism breaks down is called the Neel or Curie Temperature and is dependent on mineral composition. Indeed it is often used to identify minerals (Thompson and Oldfield, 1984). http://www.glg.ed.ac.uk/home/John.Braisby/TRANSF/magnetics.html

17. Magnetic Grain SizeMagnetic crystal grain size is, as with temperature, a balance between opposing energies. For ferromagnetic minerals there are four different balancing points that are dependent on the size of magnetic crystals. For very small ferromagnetic crystals the effect of thermal randomisation is stronger than for larger crystals. This leads to a phenomenon called super paramagnetism. As the name suggests these very small minerals exhibit strong paramagnetic properties, and cannot retain remanences. http://www.glg.ed.ac.uk/home/John.Braisby/TRANSF/magnetics.html

18. Super viscous materialsare in a grain size between supper paramagnetic and single domain, they can hold remanence but will lose it over a short period of time, the exact amount of time depends on the grain size. Super paramagnetic and super viscous minerals both exhibit a property called frequency dependent susceptibility. When a frequency dependent sample is subjected to a high frequency field there is a time lag between the maximum in the field strength and the reaction of the sample, this leads to super paramagnetic particles having a lower susceptibility when measured in high frequency fields (Thompson and Oldfield, 1984). http://www.glg.ed.ac.uk/home/John.Braisby/TRANSF/magnetics.html

19. The next size class up is thesingle stable domain crystal. In these grains the thermal energy is overcome by the magnetic energy of larger volume crystals and a stable magnetic moment results. When the size is increased again the crystal continues to try and minimise its magnetic energy. In large crystals this can be accomplished by the division of the magnetic moment into two or more magnetic domains. These domains will substantially reduce the overall magnetic moment of the crystal, and in many cases take it to zero. These large crystals are referred to as multidomain grains (Thompson and Oldfield, 1984). http://www.glg.ed.ac.uk/home/John.Braisby/TRANSF/magnetics.html

20. The relative "hardness" of ferromagnetic minerals can be linked to their magnetic crystal size. A sample consisting of predominantly single domain crystals will be relatively hard and requires high fields to influence its magnetic remanence. Whereas a sample containing mostly multidomain grains will be magnetically soft and it will be easy to impart a remnant magnetisation. There is also an intermediate state where most of the magnetic crystals have only two or three domains. Intermediate hardness crystals are sometimes called pseudo single domain crystals as they have many properties that are somewhat similar to single domain crystals, but lower magnetic intensities (Thompson, and Oldfield, 1984). http://www.glg.ed.ac.uk/home/John.Braisby/TRANSF/magnetics.html

21. Magnetic Parameters There are three common magnetic measurements that are referred to in this report, these are: -Susceptibility-Isothermal remanent magnetisation (IRM)-Anhysteritic remanent magnetisation (ARM)Susceptibility. The susceptibility of a sample is the ratio of the magnetic field induced within a sample to the magnetic field required to produce the magnetisation. http://www.glg.ed.ac.uk/home/John.Braisby/TRANSF/magnetics.html

22. Isothermal remanent magnetisation. The IRM of a sample is the magnetisation retained by that sample when it has been subjected to a known field at a known temperature (usually room temperature). IRM can be measured at varying fields, typically between 20mT and 3T. By increasing the field that a sample is subjected to in stages and measuring between each stage, it is possible to gain a lot of information about a sample. The saturation IRM or SIRM is the maximum remanence that a sample can acquire by IRM magnetisation. SIRM alone can be very indicative of a materials composition, for instance Magnetite will usually reach saturation at approximately 300mT where as Haematite is often still unsaturated at applied fields of 2T or 3T. However the overall shape of the IRM curve (the IRM's gained through several successive and increasing magnetisations) contains information about the magnetic properties of a sample. For instance the sample's hardness (a term that describes the relative ease or difficulty with which a sample is magnetised) and an insight into its constituent minerals. http://www.glg.ed.ac.uk/home/John.Braisby/TRANSF/magnetics.html

23. Anhysteritic remanent magnetisation. The ARM of a sample, sometimes called perfect magnetisation, is similar to the IRM in that it is a measurement of the magnetisation of a sample after it has been subjected to a known field. However the ARM field is not applied in the same way as with IRM. Instead of a steady field being applied to the sample, it is magnetised within an alternating magnetic field, with a steady (d.c.) field applied over the top of ac field, the ac field is increased to a known maximum and then slowly decreased back to zero. The aim of ARM is to drive the magnetisation of the sample backward and forward around the origin of its hysteresis loop, while magnetising it with a small steady field. ARM is generally imparted at a high alternating field measured and then demagnetised using smaller alternating fields, without the application of the steady field. The demagnetisation of the sample using known fields builds up an ARM curve, this gives similar information to the IRM curve, however ARM properties are strongly influenced by grain size, and some minerals ARM properties are very distinctive. http://www.glg.ed.ac.uk/home/John.Braisby/TRANSF/magnetics.html

24. http://www.almaden.ibm.com/st/projects/magneto/giantmr/

25. Chief source of GMR Spin-dependent scattering of conduction electrons • Electrons scatter little when the magnetic orientation of the material is aligned with the electron spin direction, thus the resistance is low • Electrons scatter most then the magnetic orientation of the material is anti-aligned with the electron spin direction, thus the resistance is high http://www.hgst.com/hdd/technolo/gmr/gmr.htm

26. http://www.hgst.com/hdd/technolo/gmr/fig8.gif

27. Animation of electrons scattering in GMR structure • See animation http://www.research.ibm.com/research/demos/gmr/index.html

28. Some important features • Why we need the middle layer To ensure the coupling between the two ferromagnetic layers is weak • Why the layers should be very thin To ensure the spin property will not change due to collision or interaction http://www.research.ibm.com/research/gmr.html

29. GMR device: Hard disk drive read sensor • How data is stored? It’s stored as digitally as tiny magnetized region, called bits, on the disk. A magnetic orientation represents “1” or “0” • How to improve data storage? Decrease each bit area, so that more data can be packed into the fixed amount disk space. However the sensitivity of read sensor must be high enough to read these tiny bits. http://domino.watson.ibm.com/Comm/bios.nsf/pages/gmr.html

30. GMR read sensor • GMR read sensor turns out to be a great success. Due to its high sensitivity of magetoresistive effect. It improves data storage to 10 gigabyes per square inch http://searchstorage.techtarget.com/sDefinition/0,,sid5_gci214472,00.htm

31. GMR read sensor • Structure of GMR read sensor Anti-ferromagnetic Exchange layer ferromagnetic Pinned layer Spacer Non ferromagnetic ferromagnetic Free layer

32. Spin-valve MR continued • In negative applied fields the resistivity is constant at its minimum value • In weak positive fields, an abrupt increase in the resistivity • With a small field of 10Oe, a GMR of 5% at room temperature was obtained • This means that the structure is capable of detect tiny magnetic fields. www.cs.man.ac.uk/Study_subweb/PhDWeb/CS710/heping.pdf

33. Primary Application: GMR Reading Head • GMR sensor in high density data storage systems • As data storage densities increase the information bits must get smaller and their magnetic flux becomes too small for inductive heads to detect. • GMR heads can detect tiny magnetic fields • GMR sensors can be made much smaller than inductive read heads • GMR sensors be easily fabricated by deposition techniques.

34. GMR Applications • The main technological interest of GMR lies in its ability to detect tiny magnetic fields • Primary applications: • High density data storage systems – New generation computer hard drive • Magnetic RAM • Secondary applications: • Linear position sensor • Angular measurement • Current measurement www.cs.man.ac.uk/Study_subweb/PhDWeb/CS710/heping.pdf

35. Hard Disk Drive Magnetic Medium – Fe3O4 Glass Hard Drive Disk • Substrate - glass • Magnetic medium – Fe3O4 mrsec.wisc.edu/Edetc/IPSE/HTML%20Presentation%20folder/GMRHardDrive.PPT

36. Hard Drive Information Density mrsec.wisc.edu/Edetc/IPSE/HTML%20Presentation%20folder/GMRHardDrive.PPT

37. 600 nm Magnetic Storage Media 17 Gbits/inch2 commercial Hundreds of particles per bit Single particle per bit ! 50 nm 10 nm particle uw.physics.wisc.edu/~himpsel/801.ppt

38. New Phases of Magnetic Alloy Films    http://www.almaden.ibm.com/st/projects/alloys/

39. Limits of Magnetic Recording    Currently, it takes about 2 years from a laboratory demonstration to market introduction and it is expected that areal densities of 10 Gbits/in2 will be available in products by the the turn of the century (see e.g. areal density demonstration2 at 12.1 Gbits/in2). At the present 60% annual growth rate, it will thus take less than five years until the predicted areal density limit3 of 40 Gbits/in2 will be reached. This prospect has prompted major research activities in industry and academia aiming to understand the thermal effect limit in magnetic recording.

40. http://www.almaden.ibm.com/st/projects/magneto/giantmr/

41. 5 nm GMR Reading & Writing Head mrsec.wisc.edu/Edetc/IPSE/HTML%20Presentation%20folder/GMRHardDrive.PPT

42. 1 1 0 0 0 1 S N N N N S S S How the GMR Reading Head Retrieves Data Current cannot pass through Media Magnetization Reading Current Read bit 0 GMR Current can pass through Digital Data Read bit 1 mrsec.wisc.edu/Edetc/IPSE/HTML%20Presentation%20folder/GMRHardDrive.PPT

43. GMR read sensor • See animation http://www.research.ibm.com/research/demos/gmr/index.html

44. Challenges and difficulties in Spintronics technology • It’s a developing area in a very early stage • Many basic questions remain unanswered • Devices currently applicable are metal-based, semiconductor-based devices still have a long way to go Spintronics, American Scientist, Volume 89, by Sankar Das Sarma

45. Possible approaches to develop spintronics technology • Developing for new materials with larger spin polarization or making improvements in existing devices to provide better spin filtering • Finding novel ways both to generate and to utilize spin-polarized currents—that is, to actively control spin dynamics Spintronics, American Scientist, Volume 89, by Sankar Das Sarma

46. Motivations for MRAM http://www.calit2.net/events/2001/nvm/presentations/engel_files/frame.htm

47. http://www.almaden.ibm.com/st/projects/magneto/images/mtj1.gifhttp://www.almaden.ibm.com/st/projects/magneto/images/mtj1.gif

49. MTJ MRAM • MTJ MRAM • Density of DRAM • Speed of SRAM http://www.calit2.net/events/2001/nvm/presentations/engel_files/frame.htm

50. MRAM developed by IBM and Infineon To write "1", IBM scientists simultaneously force one current to flow through the top electrode and another through the write word line; writing "0" requires the same process except that the current through the top electrode flows in the opposite direction. To read "1" and "0", the scientists force another lesser current to flow from the bottom electrode through the stack of magnetic layers and out the top electrode. The magnetic stack allows greater current flows to the top electrode for reading "1" than for reading "0". The red and green spheres represent electrons spinning in opposite directions in the magnetic layers. The very thin insulator allows electrons to quantum-mechanically tunnel through it. Information is stored in the top layer by forcing its electrons to spin in one direction or another. Information is retrieved by measuring the amount of current that travels through the tunnel insulator, which depends on the spin direction of the electrons in the top layer. http://domino.research.ibm.com/comm/bios.nsf/pages/mramvlsi.html http://domino.research.ibm.com/comm/bios.nsf/pages/mramvlsi.html/$FILE/mram.avi