Young Scientist
4000

MnO2 bilayer
La2O2 layer
magnetic momentspin-polarized electron(La,Sr)3Mn2O7
FM(Ferromagnetic Metal)

I(Nonmagnetic Insulator)

Dr. Tsuyoshi Kimura
Department of Applied Physics,
University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo
113-8656, Japan
E-mail:tkimura@ap.t.u-tokyo.ac.jp

cation of pressure. However,
the high-resistance state in theFM
2000
I

absence of magnetic fields

7Kbar

consequently gave rise to thecolossal interplane TMR effect.

0Kbar

domain wall

The most important-5

point of the finding is not justAfter I finished my Ph.D. course five years

FM0La, Sr50
Magnetic Field(kOe)

Mnthe colossal value itself of theago, I joined a group directed by Prof. Yoshinori

low-field TMR, but also the demonstration of theresults were obtained by measuring polycrystal-

Tokura at the Joint Research Center for Atom Tech-feasibility, or importance, of the use of the highlyline samples in those days, although nowadays

nology (JRCAT) in Tsukuba, Japan as a postdoc andspin-polarized electron motion across the domain

most research into the high-Tc cuprates is per-started an investigation to explore novel two-di-boundary (or the insulating barrier) in the bulk crys-formed using single-crystal samples. In order tomensional metallic systems with strongly correlatedtal.3) The manganite showed the nearly 100% spin-

obtain large single crystals, I just kept on working

electrons. We believed that such systems to bepolarization due to its strong spin-charge couplingat floating-zone machines day after day. Finally, I

capable of showing superconductivity similar to(Hund’s-rule coupling) as compared with the caseobtained a number of large single-crystal high-Tc

high-Tc cuprates. During the research, we unex-of the transition metal (e.g., 〜 40% for ferro-cuprates. The grown crystals were then distributed

pectedly discovered the “tunneling” magnetore-

magnetic Ni). The success in observing the low-overseas for collaborative research: I sometimes

sistance (TMR) effect in a newly prepared layered

field CMR encouraged not only the search for CMR

hand-delivered them myself. When I was a master’s

perovskite-type manganite crystal.

materials in the layer-type compounds, but also

student, my adviser, Prof. Koichi Kitazawa of theThe TMR is usually investigated in tailor-

the fabrication and characterization of the artifi-

University of Tokyo, gave me an opportunity tomade trilayer films composed of a ferromagnetic-cial tunneling junctions, or spin valves, with thestudy with outstanding researchers at AT&T’s Bellmetal (FM)/insulator/FM junction using transitionuse of a material with a high degree of spin polar-

Laboratories. I was unable to obtain the expected

metals as FM layers. The TMR is also currently ofization.

results experimentally at that time, but I was for-great interest from the viewpoint of its techno-

Nowadays, extensive studies of the corre-

tunate enough to develop good relationships withlogical applications to the magnetoresistance

lated-electron systems, such as high-Tc cuprates

some of the researchers there that continue to this(MR) head or magnetoresistive random-accessand magnetoresistive manganites, have shedday. Moreover, in my doctoral course, I worked

memory (MRAM). On the other hand, thelight on the importance of the “orbital” degree

mostly in Prof. Hidenori Takagi’s laboratory at theperovskite-type manganites show an extremelyof freedom on the magnetic, electronic, andInstitute for Solid State Physics, in the Universitylarge MR (so-called CMR) in a bulk (or crystalline)

structural properties.4, 5) The orbital implies theof Tokyo. In my present studies, I benefit from thoseform with application of a relatively large (Tesla-shape of an electron cloud around an atom withitinerant research experiences.

order) magnetic field. The CMR was one of thean open shell. In the correlated-electron systems;

After my stay at JRCAT, I moved to Prof.

physical science subjects that were being exten-

the anisotropic-shaped d-orbital (or f-orbital)

Tokura’s laboratory at the University ofsively investigated in those days. Hence, we tried

electrons play a central role in various proper-

Tokyo,where I struggled to find my “newto take the best of both worlds: we demonstratedties. So far, orbital-ordered states have beencheese”. The orbital physics of the correlated-that a similar but more striking TMR phenomenon

found in several compounds. LaMnO3 is amongelectron systems is only a basic science at

can be observed in the interplane transport of thethe most typical compounds with orbital-orderedpresent. However, if we can obtain materials in

spin-polarized electrons in the layered perovskites

ground states. Recently, we demonstrated thewhich the orbital state can be easily controlledof manganite, (La,Sr)3Mn2O7.1) The compound canpresence of a novel elementary excitation

by light-irradiation or electric fields (hopefully

be viewed as a virtually infinite stacking of ferro-termed “orbital wave” in the orbital-orderedat room temperature), a new technology willmagnetic-metal (MnO2 bilayer) and insulator (La/compound LaMnO3.6) The “orbital wave” can be

begin to develop that will be called “orbital elec-

Sr2O2 block layer). A striking TMR phenomenon

observed as modulations in the relative shapetronics”.

was found in the ferromagnetic state. In a field ofof orbitals in solids and its quantized object isseveral hundreds of Oe, we have observed a large

an “orbiton”. (cf. the phonon for a lattice vi-MR up to several hundreds of percent. Further in-bration in a crystal; and the magnon for a modu-2)

References
1) T. Kimura, Y. Tomioka, H. Kuwahara, A. Asamitsu, M.

vestigations revealed that the more striking

lation of the magnetic moment in a magneti-

interplane TMR phenomena can be observed bycally ordered phase). The characters of the exci-

an application of quasi-hydrostatic pressure. Thetations play a significant role in determining theinterplane TMR was extremely enhanced up to 〜

response of the system to external stimuli, such

4000% at 〜 10kbar, perhaps the largest value

as electric and magnetic fields or temperature,

Sakaki, T. Kimura, and Y. Tokura, Science 284, 1969 (1999).

ever attained for TMR. To tell the truth, we started

that is to say, most of the properties in solids.

4) T. Kimura, Y. Tomioka, A. Asamitsu, and Y. Tokura, Phys.

the high-pressure measurements to lower theWhen I was an undergraduate student at themagnitude of resistivity and hopefully to achieve

University of Tokyo, I started my research experi-

superconductivity in the two-dimensional system.

ence as a crystal-grower for high-Tc cuprates. At6) E. Saitoh, S. Okamato, K.T. Takahashi, K. Tobe, K.

The result was contrary to our expectations. The

that time, research into the high-Tc cuprates wasYamamoto, T. Kimura, S. Ishihara, S. Maekawa, and Y.

resistivity was drastically increased with the appli-most active. However, most of the experimental

Tokura, Nature 410, 180 (2001).

Tamura, and Y. Tokura, Science 274, 1698 (1996).
2) T. Kimura, A. Asamitsu, Y. Tomioka, and Y. Tokura, Phys.
Rev. Lett. 79, 3720 (1997).
3) T. Fukumura, H. Sugawara, T. Hasegawa, K. Tanaka, H.

Rev. Lett. 81, 5920 (1998).
5) T. Kimura, Y. Tomioka, R. Kumai, Y. Okimoto, and Y. Tokura,Phys. Rev. Lett. 83, 3940 (1999).

JSAP International No.4 (July 2001)

JSAPI̲Vol4.p65

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