No abstract

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...

By exploiting the simplicity of a novel transport measurement on a ferromagnetic striped domain structure in a thin film of cobalt, we report the first direct observation of ferromagnetic domain wall scattering and what we believe to be the first clear indication of giant magnetoresistive effects in a homogeneous magnetic system. ͑ ͑ ͑The colossal MR effect, while seen in homogeneous materials, is believed to originate from a charge ordering phenomenon [see, for example, Y. Tokura et al., J. Appl. Phys. 79, 5288 (1996), and references therein], and is therefore distinct from GMR, which is an effect arising from spin dependent scattering.͒ ͒ ͒ A new model is proposed to describe these observations which highlights the crucial role played by electron spin precession in determining the electrical transport properties of magnetic interfaces. [S0031-9007(96) 00854-X]

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...

Magnetotransport measurements were made on patterned, ͑110͒ oriented CrO 2 thin films grown by the high-pressure, thermal decomposition of CrO 3 onto rutile substrates. The low-temperature Hall effect exhibits a sign reversal from positive to negative as the magnetic field is increased above 1 T, which may be interpreted within a simple two-band model as indicating the presence of highly mobile ( h ϭ0.25 m 2 /V s) holes as well as a much larger number of less mobile electrons (nϭ0.4 electrons/Cr͒. Between 50 and 100 K, the field at which the sign reversal occurs rapidly increases and a contribution from the anomalous Hall effect becomes significant, while the large, positive transverse magnetoresistance ͑MR͒ observed at low temperatures changes over to a predominantly negative MR. These changes correlate with a thermally activated dependence in the resistivity of the form T 2 e Ϫ⌬/T with ⌬Ϸ80 K, reflecting the lack of temperature dependence in the resistivity at low temperatures and a T 2 behavior above 100 K. The high mobilities at low temperature which result in the observed positive MR reflect the suppression of spin-flip scattering expected for a half-metallic system. However, the changes in magnetotransport above the temperature ⌬ must be due to the onset of spin-flip scattering, even though k B ⌬ is much less than the expected energy gap in the minority spin density of states. The significance of ⌬ is discussed in terms of recent models for another half-metallic system, the perovskite manganites, and the possible formation of ''shadow bands.''

The topographic and magnetic surface structure of a natural single crystal of magnetite (Fe(3)0(4)), a common mineral, has been studied from the submicrometer scale down to the atomic scale with a scanning tunneling microscope having nonmagnetic tungsten as well as ferromagnetic iron probe tips. Several different (001) crystal planes were imaged to atomic resolution with both kinds of tips. A selective imaging of the octahedrally coordinated Fe B-sites in the Fe-O planes, and even a selective imaging of the different magnetic ions Fe(2+) and Fe(3+), has been achieved, demonstrating for the first time that magnetic imaging can be realized at the atomic level.