Magnetoresistive effects ͑ ͑ ͑R͑0͒-R͑H͒͒ ͒ ͒͞R͑H͒ exceeding 500% are found at room temperature in a field of 7 mT in nanocontacts between Fe 3 O 4 crystallites. The shape of the I͑V͒ curve depends on field and the magnitude of the magnetoresistance is correlated with the resistance, the largest effects occurring when R . 100 kV. The explanation proposed involves hopping transport of spin-polarized electrons through a narrow domain wall pinned at the nanocontact; spin pressure on the domain wall pushes it out into the electrode, leading to the nonlinearity of the I͑V͒ characteristic. Application of current-induced wall motion in a simple fast-switching magnetic memory element is proposed. DOI: 10.1103/PhysRevLett.87.026601 PACS numbers: 72.25. -b, 73.63.Rt, 75.70. -i Conventional electronics ignores the spin of the electron. If the budding science of spin electronics is to bear fruit, devices have to be designed around effects which are large at room temperature. Half-metallic oxides are potential sources of fully spin-polarized electrons, and when they are used to inject electrons across thin tunnel barriers [1,2], grain boundaries [3], or interparticle contacts [4] large magnetoresistance effects are observed, at liquid helium temperatures. For reasons that remain uncertain, the spin polarization falls rapidly with increasing temperature and the magnetoresistance at room temperature in all-oxide structures is usually no more than 1%. In this Letter, we show that the conductivity of a nanocontact between two crystallites of magnetite (Fe 3 O 4 ) can increase sixfold at room temperature in an applied field of 7 mT (70 Oe). This large low-field effect is attributed to spin-polarized electrons hopping across a very narrow domain wall at the contact. Spin pressure on the domain wall at the nanocontact leads to nonlinearity of the I͑V ͒ characteristic, and the possibility of fast switching in a bistable two-or threeterminal device via current-induced wall motion.Magnetite, the ferrimagnetic inverse spinel Fe 3 O 4 , is the half-metallic oxide with the highest known Curie temperature (860 K). Carriers are small polarons in a minority spin 3d # ͑t 2g ͒ band [5] which hop among the B sites; the majority spin band is full [6]. Tiny magnetite crystals grown by chemical vapor transport are used for our experiments [7]. Two of them are glued in a simple piezoelectric device with vibration isolation that allows electrical contact to be made or broken in a controlled manner [8], following a method of Costa-Krämer et al. [9]. All measurements are carried out at room temperature (290 K) in ambient air. A magnetic field of up to 14 mT (140 Oe) can be applied in any direction in a plane by means of two pairs of Helmholtz coils.When the contact is broken rapidly in a relay (ഠ100 ms), the conductance falls in an irregular way. A histogram based on 500 such breaks shows a single weak peak near the quantum of conductance G 0 2e 2 ͞h ͑12.9 kV͒ 21 . There is much less structure than was found previously for contacts between crystal...

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A classification of half-metals is illustrated with examples of ferromagnetic and ferrimagnetic oxides. I : V characteristics and magnetoresistance of contacts between oxides with localized electrons (Fe3O4) and delocalized electrons ((La0.7Sr0.3)MnO3) are discussed. The magnetoresistance is large when the conductance of the contact is less than the quantum of conductance G0 = 2e2/h. Broad distributions of point contacts are investigated in pressed powder compacts, illustrated by powder magnetoresistance measurements of CrO2, (La0.7Sr0.3)MnO3 and Fe3O4. When exchange coupling exists across an atomic scale contact, a narrow domain wall of width comparable to the contact size may be pinned there. Some consequences are thermal magnetic mode fluctuations, nonlinear I : V characteristics due to electron pressure on the wall and current-induced switching in a bistable `peanut' device with two closely spaced nanoconstrictions.

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