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Dr. Alagiriswamy A A, (M.Sc, PhD, PDF) Asst. Professor (Sr. Grade), Dept. of Physics, SRM-University, Kattankulathur campus, Chennai Lecture 3 UNIT II Mar. 09/10
Outline • about magnetic bubbles and applications • Giant magnetoresistance (GMR) • colossal magnetoresistance (CMR)
Each bit requires two domains to allow for error identification If two domains are magnetized in same direction, the bit is a 0; opposite directions makes the bit a 1 Direction of magnetization must change at the start of each new bit. Magnetic data is written by running a current through a loop of wire near the disk Review of Magnetic Storage
This animation shows how magnetic domains propagate through a pattern of T- and I-shaped guide pieces in magnetic bubble memory bubbles
Includes all types of information including voice, images, symbols, text, video and other time varying patterns, etc. Storage: Media must be non-volatile and readily accessible for adding, retrieving and sharing information Information Storage Defined
Magnetic Domains a high degree of magnetization in ferromagnetic materials within individual domains, but that in the absence of external magnetic fields those domains are randomly oriented conceptual illustration Even iron piece is not magnetic at room temperature
a tiny movable magnetized cylindrical volume in a thin magnetic material that along with other like volumes can be used to represent a bit of information
SPINTRONICS:Colossal Magnetoresistance Giant Magnetoresistance
Electrons moving through a current-carrying wire are moving charges If a magnetic field is present in the wire (not in the direction of current flow), the conduction electrons will experience a magnetic force perpendicular to direction of current This force pushes electrons off track, increasing resistance Magnetoresistance
Magnetoresistance is a much larger effect than induction Magnetoresistance detects magnetic field, not just the change in magnetic field, so it is less sensitive to changes in tape/disk speed and other variables Equipment needed to detect magnetoresistance simpler than coils for inductance Magnetoresistance REPLACED induction in mid-1990s Comparison
Direction of force on conduction electrons Magnetic field pointing into page (screen) Direction of velocity v of electrons Direction of qv of (negative) electrons Current-Carrying Wire
Spintronics new branch of electronics in which electron spin, in addition to charge, is manipulated to yield a desired outcome. Magnetoresistance a change in electrical resistance due to the presence of a magnetic field. Colossal effect of an exceptional or astonishing degree.
Magnetoresistance, or MR. Consider an electric current running in a material like iron. Placed in a strong magnetic field, its resistance drops or increases by several percent, depending on orientation. Giant Magnetoresistance GMR. 1988, thinly layered materials were found that increased or decreased their resistivity by 20 percent or more in relatively weak magnetic fields -- hence "giant" Magnetoresistance, or GMR. The basic effect depends on the alignment of electron spins at the interface of different kinds of magnetic materials. Colossal Magnetoresistance CMR. 1993, materials were found that could increase or decrease resistance not by a few percent but by orders of magnitude. Hence "colossal" Magnetoresistance
Colossal Magnetoresistance predominantly discovered in manganese-based perovskite oxides. * General formula of manganese oxides RE(1-x)M(x)MnO3 RE=rare earth, M=Ca, Sr, Ba, Pb. * CMR effect arises because of strong mutual coupling of following degrees of freedoms - Spin - Charge - Lattice BUT HOW ???????? Phase diagram of La1-xSrxMnO3
Conduction is by hopping electrons between Mn3+ and Mn4+ sites, magnetic moments must be parallel ! ferromagnetic state is needed Colossal MR Found in mixed valence Mn oxides, I.e. La1-xSrxMnO3 At Tc transition from metal to insulator maximum of resistivity At T>Tc increase of thermally exited carriers decrease of resistivity Applied magnetic field increase ferromagnetic ordering decrease resistivity Large fields (several Tesla) are needed Top:Magnetization against temperature for La0.75Ca0.25MnO3 for various field values Middle: resisitivity against temperature Bottom: magnetoresistance against temperature
First Evidence of GMR Resistivity versus applied field for Fe/Cr multilayers Relative resistance change as a function of the external magnetic field for Fe/Cr/Fe and 250A thick Fe film Discovered in 1988 in antiferromagnetically coupled magnetic multilayers by Baibich et al and on Fe/Cr superlattices In Fe/Cr multilayers the low field antiparallel configuration was induced by antiferromagnetic coupling between Fe layers across Cr Does not depend on the angle between the current and magnetization spin-orbit coupling and resulting anisotropy play a minor role MR=79% at T=4.2K and 20% at room temperature
Alternate layers of ferromagnetic material will naturally align with opposite magnetization All electrons coming in will scatter since they’ll have opposite spin from magnetization in some region Magnetic Superlattices Ferromagnetic material with magnetization in direction of turquoise arrow Non-ferromagnetic material spacer Warning: not to relatively Scale