Cold-pressed powders of the half-metallic ferromagnet CrO 2 are dielectric granular metals. Hysteretic magnetoresistance with maxima at the coercive field arises from interparticle contacts. Dilution with insulating antiferromagnetic Cr 2 O 3 powder reduces the conductivity by 3 orders of magnitude, but enhances the magnetoresistance ratio which reaches 50% at 5K. The negative magnetoresistance is due to tunneling between contiguous ferromagnetic particles along a critical path with a spin-dependent Coulomb gap. [S0031-9007(98)05996-1] PACS numbers: 72.15. Gd, 73.40.Gk, 75.50.Cc, 81.20.Ev Negative magnetoresistance has been widely investigated in ferromagnetic metals and heterostructures. Effects intrinsic to a material are distinguished from extrinsic effects which depend on the direction of magnetization in adjacent ferromagnetic regions. Examples of the former include the anisotropic magnetoresistance of permalloy [1] or the colossal magnetoresistance of nonstoichiometric EuO [2] and mixed-valence manganites [3]. Examples of the latter are the giant magnetoresistance of multilayers [4] and granular metals [5,6] or the behavior of spin-dependent tunnel junctions [7], where resistivity is greatest at the coercive or switching field and decreases as the sample reaches technical saturation. Recent experiments on epitaxial manganite films with a single grain boundary have allowed the high-field, colossal magnetoresistance to be separated from the low-field effect due to heterogeneous magnetization distribution in adjacent grains [8,9]. A characteristic but unexplained feature of the low-field magnetoresistance in manganite ceramics [10], polycrystalline films [11,12], and tunnel junctions [13,14] is its rapid decay with increasing temperature.Here we report a new type of extrinsic magnetoresistance. It is studied in pressed powders of CrO 2 , where it arises from contacts between particles. Chromium dioxide is an ideal material for spin-polarized electron tunneling, as it is a half-metallic ferromagnet where complete spin polarization of the conduction electrons is maintained up to the surface [15]. There are two 3d electrons in spinsplit t 2g subbands, one localized and the other in a halffilled band [16]. The two electrons are strongly coupled by the on-site exchange interaction J H ഠ 1 eV. The intrinsic metallic nature of the oxide is illustrated by the resistivity of an oriented film grown on TiO 2 , shown in Fig. 1(a). It follows Matthiessen's rule with a residual resistivity of 0.1 mV m ͑10 mV cm͒ and a room-temperature value about 30 times greater. The slope dr͞dT remains positive above the Curie temperature ͑T C 396 K͒ [17]. The films exhibit only a small linear intrinsic magnetoresistance effect, ͑1͞m 0 r͒dr͞dH ϳ1%͞T at room temperature.Our samples were made from a commercial CrO 2 powder used for magnetic recording. The powder is composed of acicular single-domain particles with an average length of 300 nm and an aspect ratio of about 8:1. Coercivity is 59 mT (590 G) at room temperature, rising up to ...

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We report an unusual magnetic behavior in powders prepared by spark-eroding ͑in liquid Ar͒ alloy electrodes containing approximately equal weights of Fe and Nd 2 Fe 14 B in an effort to prepare composite permanent magnets. Magnetization exhibits reproducible thermal hysteresis, peaking in all applied fields near 520°C when warming, but increasing monotonically when cooling to room temperature from 700°C and above. Mössbauer spectroscopy was used to show that the behavior is due to the metastability of Fe 1Ϫx O produced in the powders by partial oxidation in Ar gas flow. This compound, paramagnetic at room temperature, decomposes only slowly below 570°C into ferromagnetic Fe and ferrimagnetic Fe 3 O 4 . The reverse reaction occurs readily at higher temperatures.

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