Magnetic Memory: Data Storage and Nanomagnets Magnetic Memory: Data Storage and Nanomagnets Mark Tuominen UMass Kathy Aidala Mount Holyoke College

Data Data is information iTunes

How do we store data digitally? Everything is reduced to binary, a “1” or a “0”. We look for ways to represent 1 or 0, which means we need to find physical systems with two distinct states. We have to be able to switch the state of the system if we want to “write” data. The bit has to stay that way for long enough. We have to be able to “read” if the bit is a zero or one to use the data. What physical systems have these properties??

10 GB GB GB GB GB 2007 Data Storage. Example: Advancement of the iPod Hard drive Magnetic data storage Uses nanotechnology! Review

MAGNETISM Electrical current produces a magnetic field: "electromagnetism" B I

MAGNETISM myfridge Refrigerator magnets provide an external magnetic field, permanently; no wires, no power supply and no current needed. Permanent Magnets = FERROMAGNETS

Ferromagnet uniform magnetization anisotropy axis ("easy" axis) Electron magnetic moments ("spins") Aligned by "exchange interaction" Bistable ! Equivalent energy for "up" or "down” states Iron, nickel, cobalt and many alloys are ferromagnets

The Bistable Magnetization of a Nanomagnet A single-domain nanomagnet with a single “ easy axis ” (uniaxial anisotropy) has two stable magnetization states “ topview ” shorthand z or H MzMz MzMz MzMz H Bistable! Ideal for storing data - in principle, even one nanomagnet per bit. hysteresis curve E = K 1 sin 2    H switching field

“Writing” data to a ferromagnet ? Ferromagnet with unknown magnetic state Current N S ‘0’ S Current N ‘1’

Magnetic Data Storage A computer hard drive stores your data magnetically Disk NS direction of disk motion “Write” Head __ “Bits” of information NS “Read” Head Signal current

Scaling Down to the Nanoscale Increases the amount of data stored on a fixed amount of “real estate” ! Now ~ 100 billion bits/in 2, future target more than 1 trillion bits/in 2 25 DVDs on a disk the size of a quarter.

Nanofabrication with self-assembled “cylindrical phase” diblock copolymer films Deposition Template Remove polymer block within cylinders (expose and develop) UMass/IBM: Science 290, 2126 (2000)

Filling the Template: Making Cobalt Nanorods by Electrochemical Deposition WE REF electrolyte CE Co 2+ Co metal

Binary Representation of Data one bit“1” or “0” only 2 choices two bits00, 01, 10, 11 4 choices three bits 000, 001, 010, 011, 100, 101, 110, choices n bits has 2 n choices For example, 5 bits has 2 5 = 32 choices… more than enough to represent all the letters of the alphabet or

Binary representation of lower case letters 5-bit "Super Scientist" code: For example, k = S N S N S N N S N S OR (Coding Activity: Use attractive and repulsive forces to "read" the magnetic data!)

Ferromagnetic Nanorings as Memory "0""1" Vortex Magnetization Nanotechnology(2008); PRB (2009) Pt solid tip

AFM: Electromagnetic Forces Lift height Anything that creates a force on the tip can be “imaged” Electromagnetic force is long range, but generally weaker than the repulsive forces at the surface Image electromagnetic forces 10 – 100nm above the surface

Magnetic Force Microscopy magnetic tip Computer Hard Drive

Magnetic Force Microscopy magnetic tip Computer Hard Drive Topography

Magnetic Force Microscopy Lift height magnetic tip Computer Hard Drive Topography

Magnetic Force Microscopy Lift height magnetic tip Computer Hard Drive Topography Magnetism

Magnetic Force Microscopy dB/dz small dB/dz large, negative dB/dz large, positive Image contrast is proportional to the derivative of the magnetic field 200 nm Magnetic state MFM simulation

MFM of Ring States Symmetric Rings

MFM of Ring States Symmetric Rings vortex onion No contrast in the vortex state in a perfect ring. Cannot determine circulation (CW or CCW) Light and Dark spots indicate Tail to Tail and Head to Head domain walls.

Switching: Onion to Vortex 1 um T. Yang, APL, 98, (2011).

1 um Stronger field (40 mA = 178 Oe) Weaker field (30 mA = 133 Oe) T. Yang, APL, 98, (2011). Switching: Onion to Vortex

1 um Stronger field (40 mA = 178 Oe) Weaker field (30 mA = 133 Oe) T. Yang, APL, 98, (2011). Switching: Onion to Vortex

Improved MRAM Proposal Zhu, Proceedings of the IEEE 96(11), 1786 (2008) Trapped DWs lead to lower switching current

Proof of Principle Cobalt, 12nm thick Nanotechnology, 22 (2011)

Ferromagnetic Nanorings as Memory "0""1" Vortex Magnetization Nanotechnology(2008); PRB (2009) Aidala and Tuominen, APL (2011); Nanotech. 2011; J.A.P Manipulation of magnetization with local circular field Pt solid tip