Europhys. Lett.
Volume 73, Number 3, February 2006
Page(s) 415 - 421
Section Condensed matter: electronic structure, electrical, magnetic, and optical properties
Published online 21 December 2005
Europhys. Lett., 73 (3), pp. 415-421 (2006)
DOI: 10.1209/epl/i2005-10402-x

Magnetoresistance in amorphous oxide films $\chem{CoFeHfO}$

Y. Luo1, M. Eßeling1, K. Zhang2, G. Güntherodt3 and K. Samwer1

1  I. Physikalisches Institut, Universität Göttingen - Friedrich-Hund-Platz 1 D-37077 Göttingen, Germany
2  II. Physikalisches Institut, Universität Göttingen - Friedrich-Hund-Platz 1 D-37077 Göttingen, Germany
3  II. Physikalisches Institut, RWTH Aachen - Templergraben 55 D-52056 Aachen, Germany

received 15 July 2005; accepted in final form 1 December 2005
published online 21 December 2005

This paper shows results on structural and magnetotransport measurements for reactively sputtered amorphous oxide films ( ${\rm Co}_{55}{\rm Fe}_{20}{\rm Hf}_{25})_{1-{x}}{\rm O}_{x}$ with $x \leq 0.35$ and their layered structures. A strong x-dependency was found to be related to microstructural changes caused by oxygen-induced chemical phase separation. For low x, $\chem{Co\tx{-}Co(Fe)}$ pair-correlated atom clusters dominate and the films are soft ferromagnetic with excellent uniaxial magnetic anisotropy. Small coercivity $H_{c} \approx 1\tx{--}4$ Oe, enhanced magnetisation $M_{s} \approx 420$ emu/cm3 and low resistivity $\rho \approx 10^{-4}$ $\Omega$cm were measured for these films, showing a good potential for application in magnetoelectronics. For large x, however, since $\chem{Hf}$-$\chem{O}$ pair-correlated atom clusters are dominant, the films become granular-like superparamagnetic with a saturation field Hs > 20 kOe and a remanence $M_{r}/M_{s} \approx 0$. $\rho$ is extremely high presumably due to Coulomb blockade effect. A large magnetoresistance was observed, being 30% (70 kOe) at 5 K and 10% at room temperature. This effect can be attributed to spin-dependent tunnelling among lateral separated $\chem{Co}$- $\chem{Co(Fe)}$ clusters. The layered structure shows a small, but low-field magnetotunnelling effect.

75.70.-i - Magnetic properties of thin films, surfaces, and interfaces.
75.50.-y - Studies of specific magnetic materials.

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