Adarsh SANDHU (SANDHU Adarsh)

Offer Organization: 日本学術振興会, System Name: 科学研究費助成事業, Category: 萌芽研究, Fund Type: -, Overall Grant Amount: - (direct: 3300000, indirect: -)
次世代の高保磁力永久磁石、超高密度磁気記録技術及び「スピンエレクトロニクス」等の新分野の研究には、磁性体特有のキュリー温度(Tc)の測定が非常に重要である。現在、磁性体のTcの測定は通常、振動試料型磁力計(VSM)装置を使って行われている。しかし、VSMによるTcの測定の難点として、(1)試料全体の平均的な値しか得られない;(2)熱電対と試料が離れているため、試料の正確な温度測定ができずTcの値に誤差(±20%)が生じる;(3)VSM装置は複雑な仕組みで操作が困難である;(4)VSM装置は高価(〜2000万円)である、等が挙げられる。そこで、申請者はVSM装置ではなく、Tcをホール素子により測定する方法を提案した。しかし従来のホール素子の材料であるGaAs、 InSb等の半導体は、温度変化により、磁界に対する出力であるホール電圧が変化するという問題がある。そこで、本研究ではホール素子に用いる半導体材料として二次元電子ガスを有するGaN/AlGaNヘテロ構造に着目し、ホール電圧の温度変化が極めて小さいホール素子の作製を目的とする。
平成18年度の研究予定は実際に極高温で安定な動作する50〜100μm角のGaN/AlGaNホール素子の応用として、高温におけるBaフェライトの磁化曲線(キュリー温度)の測定を行った。通常のVSM測定はマクロ的であり、GaN/AlGaNホール素子を用いた測定は局所的な情報を得ることができ、測定した円形の等方性Baフェライトのキュリー温度はVSM測定と若干異なり、460℃であることは分かった。さらに、センサ部分は2μm×2μmのSHPM用ホール素子を作製し、50μm厚のガーネットサンプルを測定し、磁区幅約10μmを有する磁区構造の観察に成功した。
以上の高温におけるBaフェライトの磁化曲線の測定に成功したこと、ガーネットの磁区観察に成功したことで高温で局所的な磁界測定が可能であるを証明した。さらに約数μm角のGaN/AlGaNホール素子をSHPMに搭載し、高温で磁性体表面の熱による磁区反転・磁壁移動の観察への応用を検討した。

Offer Organization: Japan Society for the Promotion of Science, System Name: Grants-in-Aid for Scientific Research, Category: Grant-in-Aid for Scientific Research (C), Fund Type: -, Overall Grant Amount: - (direct: 3700000, indirect: -)
A fundamental understanding of the behavior of magnetic domains in external magnetic fields is important for the development of information technology such as ultra-high density magnetic recording media, high coercivity permanent magnets and multilayered ferromagnetic structures used for fabricating novel ‘spintronic'' devices. We have developed a room temperature scanning Hall probe microscopy (RT-SHPM) for the non-invasive and quantitative imaging of localized stray fields at the surfaces of ferromagnetic magnetic microstructures in the presence of pulsed external magnetic fields of 3 T Oe. However, advances in magnetic recording media have led to increasing demands for improvements in the spatial resolution of SHPM which using semiconducting GaAs/AlGaAs micro-Hall sensors is limited to 1.0 x 1.0μm due to surface depletion effects.
The use of bismuth (Bi)(a semimetal, which exhibits a large Hall coefficient and negligible surface depletion) for fabrication of submicron Hall sensors shows promise as a means of improving the spatial resolution of RT-SHPM technology. Conventional MFM systems have a spatial resolution of 〜100nm, a figure which serves as a benchmark and target for studies on spatial resolution.
In this research we succeeded in fabricating 50nm x 50nm Hall sensors using Bi thin films deposited by thermal evaporation. The electrical properties of the films were improved by post-deposition annealing in vacuum. The nano-Hall sensors were successfully used for magnetic imaging the surface of crystalline garnet thin films.
This report describes the research carried out on "Fabrication of Bismuth Thin Films Nano-Hall Sensors for Room Temperature Scanning Micro-Hall Probe Microscope Imaging of Ferromagnetic Microstructures" as part of the Grant-in-Aid for Scientific Research (c)(2)for two years between 2003 and 2004. The fabrication and electrical characteristics of the Bi nano-Hall sensors are described. Also, the use of the Bi-nanosensors for imaging garnet thin films is demonstrated.

Offer Organization: Japan Society for the Promotion of Science, System Name: Grants-in-Aid for Scientific Research, Category: Grant-in-Aid for Scientific Research (C), Fund Type: -, Overall Grant Amount: - (direct: 3300000, indirect: -)
A fundamental understanding of the behavior of magnetic domains in external magnetic fields is important for the development of information technology such as ultra-high density magnetic recording media, high coercivity permanent magnets and multilayered ferromagnetic structures used for fabricating novel 'spintronic' devices.
In this research, a unique magnetic imaging system comprising of a room temperature scanning Hall probe microscope with an integrated mini-coil capable of generating pulsed magnetic fields up to 3 Tesla (width of 3.5ms) was developed for the direct and non-invasive magnetic imaging of ferromagnetic micro-domains in the presence of extremely large external pulsed magnetic fields without adverse vibrational disturbance of the sample during measurements. The system was successfully used for magnetic imaging of the erasure process of bit patterns on the surface of 1.4MB written floppy disks and the dynamics of micro-domain structures of demagnetized strontium ferrite permanent magnets under large external pulsed magnetic fields.
The combined mini-coil/RT-SHPM system was demonstrated to be a valuable instrument for the direct, quantitative and non-invasive observation of localized stray magnetic fields at the surfaces of magnetic recording media and ferromagnetic materials in the presence large external bias fields.