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Manipulation of the magnetization of a perpendicular ferromagnetic free layer by spin-orbit torque (SOT) is an attractive alternative to spin-transfer torque (STT) in oscillators and switches such as magnetic random-access memory (MRAM) where a high current is passed across an ultrathin tunnel barrier. A small symmetry-breaking bias field is usually needed for deterministic SOT switching but it is impractical to generate the field externally for spintronic applications. Here, we demonstrate robust zero-field SOT switching of a perpendicular CoFe free layer where the symmetry is broken by magnetic coupling to a second in-plane exchange-biased CoFe layer via a nonmagnetic Ru or Pt spacer. The preferred magnetic state of the free layer is determined by the current polarity and the sign of the interlayer exchange coupling (IEC). Our strategy offers a potentially scalable solution to realize bias-field-free switching that can lead to a generation of SOT devices, combining a high storage density and endurance with a low power consumption.

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 ...

The metal-insulator transition is mixed-valence manganites of the ͑La 0.7 Ca 0.3 ͒MnO 3 type is ascribed to a modification of the spin-dependent potential J H s-S associated with the onset of magnetic order at T C . Here J H is the on-site Hund's-rule exchange coupling of an e g electron with sϭ1/2 to the t 2g ion core with S ϭ3/2. Above T C , the e g electrons are localized by the random spin-dependent potential and conduction is by variable-range hopping. Over the whole temperature range, the resistivity varies as ln( / ϱ ) ϭ͓T 0 ͕1Ϫ(M /M S ) 2 ͖/T͔ 1/4 , where M /M S is the reduced magnetization. The temperature and field dependence of the resistivity deduced from the molecular-field theory of the magnetization reproduces the experimental data over a wide range of temperature and field. ͓S0163-1829͑97͒04513-X͔ Interest in mixed-valence manganites of the ͑La 0.7 Ca 0.3 ͒MnO 3 type has revived 1 with the observations of large negative magnetoresistive effects, 2,3 especially in suitably annealed thin films. 4 The magnetoresistance is greatest in the vicinity of the Curie point T C of ferromagnetic compositions which exhibit ''metallic'' ͑temperature-independent͒ conduction at low temperatures and thermally activated conduction above T C . These compositions have a structure which is a variant of the cubic perovskite cell where the Mn-O bond lengths are unequal and Mn-O-Mn bond angles differ from 180°. 5 Their electronic properties are related to electron hopping among the Mn ions in octahedral sites; metallic conductivity and ferromagnetism are closely related and are generally interpreted in terms of the doubleexchange mechanism. 6 A spin-polarized * conduction band of mainly 3d(e g ↑) character 7 is supposed to be responsible for the ''metallic'' character of the current transport below T C . 8 The Mn 3ϩ ion has one e g electron, whereas the Mn 3ϩ ion has none. When the concentration of the divalent A-site cation ͑Ca, for example͒ is 0.3, the occupancy of the * band is 0.7, which corresponds to the strongest ferromagnetism and the greatest magnetoresistance. Electron transfer with spin memory is an essential ingredient for an understanding of the transport properties of mixed-valence manganites, but something more is needed to account for the metal-insulator transition near the Curie point. 9 The change of conduction regime below T C appears to be brought about by the onset of ferromagnetism. As temperature decreases, the magnetization increases and the resistivity drops. Resistivity has been reported to vary like ͓1Ϫ(M /M S ) 2 ͔, as in conventional giant magnetoresistance ͑GMR͒ systems, 10 but others find an exponential dependence 11 ln( )ϳϪM/M S . Here we propose the concept of magnetic localization to relate the resistivity at any temperature or applied field to the local magnetization, evaluated in the molecular field approximation. The model involves variable range hopping and goes beyond the purely phenomenological parallel conduction model of Nunez-Reiguero and Kadin. 12 We previously observed an impr...