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Giant Magneto Resistance (GMR) 2007 10/30 論文紹介 by 白鳥昂太郎 (ちなみに去年は10/31発表でした) Paper The Nobel Prize in Physics 2007 Albert Fert (アルベール・フェール) Unité Mixte de Physique CNRS/THALES, Université Paris-Sud, Orsay, France (国立科学研究センター(CNRS)の科学主 任 (とタレスグループ合同科学主任 )) Peter Grünberg (ペーター・グリューンベルク) Forschungszentrum Jülich, Germany (独ユーリヒ固体物理研究所教授) This year's physics prize is awarded for the technology that is used to read data on hard disks. It is thanks to this technology that it has been possible to miniaturize hard disks so radically in recent years. Sensitive read-out heads are needed to be able to read data from the compact hard disks used in laptops and some music players, for instance. Contents • Backgrounds – Ferromagnetic metals – Magneto Resistance – Giant Magneto Resistance • HDD • Summary Backgrounds Ferromagnetic metals (強磁性体) S↓ S↑ W/o Magnetic field S↓ S↑ W/ Magnetic field Parallel : Energy → decreased Anti-parallel : Energy → increased The characteristic of Ferromagnetic metals are determined from the behavior of the valence electrons under the Fermi surface. ⇒The magnetization occurs from the polarization of those electrons. Ex. Fe, Co, Ni : 3d and 4s electrons (Transition metals:遷移金属) Magneto Resistance • Resistance : The scattering of conduction electrons by the vibration of lattice ⇒The “electrical” resistance has no difference between spin-up and spin-down electrons. • Resistance of transition metals – The conductivity of transition metals are mainly determined by 4s electrons. – 4s electrons can be scattered into 3d states. ⇒ Resistance of transition metals (For Cu, all 3d states are occupied and it shows high conductivity) ⇒ Scattering process (resistance) is related to the spin-orbital transition. • By the magnetic field, the scattering process of ferromagnetic metals can be changed. ⇒ Spin-up electrons feel the different resistance from Spin-down ones. (The electronics considering the spin direction of electrons is called “spintronics”.) • Magnet resistance : DR/R ~a few %. Giant Magneto Resistance Fe | Cr | Fe layers Cr (non-ferromagnetic metals) sandwiched by Fe (ferromagnetic metals) I↑:FM-3d → NFM-3d: R大,large NFM-3d → FM-3d: R小,small I↓:FM-3d → NFM-3d: R小, small NFM-3d → FM-3d: R大,large ⇒ Larger resistance B↑ B↑ I↑:FM-3d → NFM-3d: R大, large NFM-3d → FM-3d: R大, large I↓:FM-3d → NFM-3d: R小, small NFM-3d → FM-3d: R小, small ⇒ Smaller resistance Current : I FM→NFM→FM Giant Magneto Resistance Fe | Cr | Fe layers Cr (non-ferromagnetic metals) sandwiched by Fe (ferromagnetic metals) B↑ B↑ Experimental result DR/R ~1.0 ⇒ 0.5 (50 % changed) GMR @ Half-metal CrO2 : Only one electron state for conductivity at the Fermi level →By the magnetic field, one spin direction state dose not shows the conductivity. The multilayer material shows strong GMR effects. TMR : Tunneling magnetoresistance The insulating material has only a few atomic layers thick. ⇒The probability of the tunneling through the insulator is large. The resistance can be changed by the external magnetic field as similar the GMR effects. DR/R ~500 % increased 大 野 研 究 室 (池田准教授研究室） 東北大学電気通信研究所 ナノスピン実験施設 GMR research • Discovery of GMR – The begging of Spintronics – Sensitive magnetic probe • Technological background – Development of nano-technology – Growth of lattice (Epitaxial growth) • The greatest contribution ⇒ HDD HDD Requirements for HDD • Compact size and fast reading – The size of magnetized area should be small. ⇒ It is difficult to read the direction of magnetization (binary) because of its weakness. – Writing :○⇔Reading :× • The sensitive head is needed to achieve the compact HDD. ⇒GMR head Property of HDD • ハードディスクドライブ – 出典: フリー百科事典『ウィキペディア（Wikipedia）』 • Present capacity of HDD – ~200 GBit/inch2 (25 GB/inch2) inch2= 500円 coin size *First HDD ~4 MB @ Refrigerator size Future growth • GMR head ⇒ TMR head • MRAN – Magnetic working Random Access Memory – MARN combines RAM with HDD. Summary • GMR effects was discovered by A. Fert and P. Grünberg independently. • GMR shows large magnetoresistance. – GMR ~50 % > MR ~ a few % • The greatest contribution of GMR is the sensitive reading head for HDD. • The TMR effect is based on the idea of GMR. – More sensitive reading head is being developed.