ICOD: 1 December 2009 UNCLASSIFIED/fP8R err1e1Rt U9E OIILI Intelligence Acquisition Threat Support Metallic Spintronics UNCLASSIFIED/,FOII: OPFilltftk Wil OPHa\t UNCLASSIFIED,l/FOR OFFICIO! Metallic Spintronics Prepared by: Administrative Note COPYRIGHT WARNING: Further dissemlnatlon of the photographs in this publication is not authorized. This product is one in a series of advanced technology reports produced in FY 2009 under the Defense Intelligence Agency, J<b)(3):10 USC 424 I Advanced Aerospace Weapon System Applications (AAWSA)rogram. Comments or questions pertaining to this document should be addressed to b 3 :10 USC 424 b 6 AAWSA Program Manager, Defense Intelligence Agency, ATTN: b 3 :10 use 42 Bldg 6000, Washington, UNCLASSIFIED/ (FOR OFFICICP UNCLASSIFIED//&OA O&&I&l,.k WiE 8Pll.lf Figure 1. In a Magnetic Multilayer, Several Atomic Layers of Magnetic Material Alternate With Layers of Nonmagnetic Material ...................................... 1 Figure 2. Resistance of a Magnetic Multilayer R Versus Magnetic Field ................... 2 Figure 3. Differential Resistance dV/dI of a Mechanical Point Contact as a Function Figure 6. Torques on a Magnetic Moment in a Magnetic Field and Subject to an Figure 9. Scanning Transmission X-ray Microscopy Images ................................. 10 Figure 12. Schematic of Point Contact to Sample Geometry ................................. 15 UNCLASSIFIED//POfl 8PPl@ltlil: lt!!IE OHi! UNCLASSIFIED//EOR AEEJCJOP !PSF ON!~ Metallic Spintronics 1. Introduction The rapid pace of progress in the computer industry over the past 40 years has been based on the miniaturization of chips and other computer components. Further miniaturization, however, faces serious challenges-for example, increasingly high power dlsslpation. To continue on pace, the industry must go beyond incremental improvements and embrace radically new technologies. A promising nanoscale technology known as spintronics (a neologism for "spin- based electronicsu) has emerged. Spintronics refers to the role an electron spin plays in solid-state physics. Spintronics researchers aim to develop a revolutionary new class of electronic devices based on the spin of electrons in addition to the charge. In spintronic devices, information is carried not by the electron's charge, as in conventional microchips, but by the electron's intrinsic spin. Changing the spin of an electron is faster and requires less power than moving it. Therefore, if a reliable way could be found to control and manipulate spins, spintronic devices could offer higher data processing speeds, lower electricity consumption, and many other advantages over conventional chips, perhaps including the ability to carry out radically new quantum computations. Spintronics in ferromagnetic systems is built on a complementary set of phenomena in which the magnetic configuration of the system influences its transport properties and vice versa. Giant magnetoresistance (GMR) (Reference 1, 2) and spin-transfer-torque {STT) (Reference 3-5) phenomena exemplify such interconnections in multilayers composed of ferromagnetic (F) and nonmagnetic (N) layers. The physics and applications of metal lie spfntronics are discussed in this report from the perspective of these two phenomena. GMR, research on which was awarded the Nobel Prize in Physics in 2007, refers to a large change in resistance of magnetic multilayers when the relative orientation of magnetic moments in their constituent ferromagnetic layers is altered by an applied magnetic field. The inverse effect, STT, in which a large electrical current density j can perturb the magnetic state of a multilayer, has also been predicted (Reference 3, 4) and observed in experiments on current-induced reversal and precession of magnetization (Reference 5-9) and magnetic domain wall motion (Reference 10, UNCLASSIFIE:D/,'P8N 8PPleIAt 09 Gilli UNCLASSIFIED;/P8fl.. 8PPl81AL ltDI!! fJHLY Spintronics is a broad research field with ( currently) three major subfields: ( 1) materials research that is attempting to create new materials that are both magnetic and semiconductors, (2) research on novel magnetotransport effects in ferromagnetic metals, and (3) research on techniques that can be used to manipulate individual electron spins. The first subfield is targeting magnetic semiconductors because devices based on such materials would be the easiest to integrate with the present semiconductor device technology and processing capabilities. However, despite extensive research, most semiconductor spintronic devices are still theoretical concepts awaiting experimental demonstrations. This report focuses on spintronics research in metallic systems within the scope of the second and third subfields. The second subfield has experienced an unprecedented period of new discovery over the past 20 years, including the discovery of GM R, and has already spawned major technological change in the information storage industry with the use of GMR sensors and read heads. The third subfield is vital for spintronic devices, as virtually any processing of information in such devices is associated with transport and manipulation of spins. New and efficient methods for manipulating spins that stimulate active research programs In spintronics at a large number of academic institutions and a half-dozen industrial research labs around the world are highly desirable. The prize to be gained is active control and manipulation of spin distributions (magnetic moments) for new and improved functionality in electronic/spintronic devices. The confluence of intense basic science and industry interest in ferromagnetic metal spintronics has not occurred on this scale in physics in a long time. The report is arranged as follows: Section 2 is dedicated to magnetotransport effects in magnetic systems where magnetic configuration can influence the system's transport properties. It discusses GMR in magnetic multilayers and related phenomena, highlights basic physical principles responsible for GMR, and describes technological applications of the effect. Section 3 focuses on the reverse connection between the system's magnetic configuration and its transport properties-the so-called STT effect. The physical origin and potential applications are discussed. Section 4 discusses other new directions in metallic spintronics, with a particular focus on spintronics with antiferromagnetic materials. Section 5 summarizes, with an eye to the future, the development of spintronic technologies and their aerospace applications. UNCLASSIFIED//P8R: BPPH!Ill.ih ~81! OHl:ltf UNCLASSIFIED/JFOA OFF16l 1.k W&& &Hlk>S 2. Giant Magnetoresistance 2.1 GMR BASICS This section discusses the phenomenon of giant magnetoresistance (GMR). Excellent reviews of GMR are available elsewhere (Reference 12-22). The focus on physical concepts important for the sections to follow are discussed. GMR in magnetic multilayers refers to a dramatic reduction in the resistance of the multilayers when subjected to an external magnetic field. GMR's size is usually defined as the resistance change in magnetic field relative to its peak value. The effect can be distinguished from the ordinary magnetoresistance (MR) coming from the direct action of the magnetic field on the electron trajectories via the Lorentz force (Reference 23), and from the anisotropic MR, which comes from dependence of the resistivity on the relative orientation of magnetic moment to the current (Reference 24). To prepare the magnetic multilayers, where several atomic layers of one (ferromagnetic) material alternate by layers of another (nonmagnetic) material (see Figure l)r a wide variety of deposition methods have been used, such as electrochemical deposition techniques (Reference 25, 26) and various vacuum deposition techniques (Reference 27, 28). The latter shares mainly between two methods using either sputter deposition or molecular beam epitaxy (MBE) systems. Sputter deposition involves knocking off the atoms of the material of interest from a target by particle bombardment, followed by the deposition of high- energetic atoms ( ~2-30 electronvolts [ eV]) onto the substrate. A principal advantage of sputter deposition is the ease with which many different materials can be deposited at relatively high deposition rates. In contrast, deposition rates in MBE systems are usually much lower than for sputtering systems, but much lower energies (~0.1 eV) of the evaporated material Perpendicular ,, to the Plane Figure 1, In a Magnetic Multilayer, Several Atomic Layers of Magnetic Material (shown in grey) Alternate With Layers of Nonmagnetic Material (shown in white). GMR occurs in one of two dlfferent geometries: (1) when the current flows in the plane (CIP geometry) of the layers or (2) when the current flows perpendicular (CPP geometry) to the layers. make this technique favorable for growth of highly oriented single-crystalline films. The original observation of GMR (Reference 1) was made on MBE grown iron-chromium (Fe/Cr) multilayers with nearly perfect crystallinity. Subsequently, by using sputtered samples that are grown much more rapidly than MBE samples, it was possible not only to reproduce these results but also to observe oscillations in the magnetoresistance as the thickness of the nonmagnetic spacer layers was varied (Reference 29). Subsequent UNCLASSIFIED;; ret\ SPPH!IAb 1116& OPU.V UNCLASSIFIED//FOR: OFFIGl1\L t.Hil! 8HI: studies (Reference 30} on sputtered cobalt-copper (Co/Cu) multilayers revealed magnetoresistances at room temperatures 3 to 4 times larger than those for iron- chromium and 13 times greater than those for the permalloy films that were used as magnetoresistive sensors in magnetic reading heads at that time. The much higher numbers observed in magnetic multilayers predetermined the fate of GMR in magnetic recording technology. The current understanding is that GMR observed in magnetic multilayers arises from the dependence of the resistivity on their internal magnetic configuration and the role of the external magnetic field to change this configuration. Figure 2b illustrates GMR in the simple limit where the electron mean-free-path is much longer than the layer thicknesses. The electrical transport properties of the system are described in terms of the so- called two-current model (Reference 31), based on the suggestion by Mott (Reference 32) that, at temperatures lower than the Curie temperature, the spin-up and spin-down electrons will be almost independent and carry current in parallel. Electrons are much more strongly scattered by a magnetic layer if they and the local magnetization spin in opposite rather than the same direction (R > r). For simplicity, the figure is drawn with scattering only at interfaces; however, there is also scattering within the layers. At zero magnetic field, where the magnetizations of adjacent magnetic layers are aligned antiparallel-for example, because of exchange coupling between the layers (Reference 29)-the spin-down electrons are weakly scattered in layer Magnetic Field Figure 2, (a) Resistance of a magnetic multilayer R versus magnetic field. (b) Origin of GMR in terms of spin-dependent electron scattering: Fl and F2 are ferromagnetic layers with a nonmagnetic layer in between. At zero magnetic field, the magnetizations in Fl and F2 are aligned antiparallel (center panel) and can be switched to parallel orientation by an applied field. (c) The equivalent resistance circuits corresponding to the three magnetic configurations shown in (b). See text for details. Fl but strongly scattered in F2. In contrast, the spin-up electrons are weakly scattered in layer F2 but strongly scattered in Fl. As a result, two channels are equivalent, leading to a total resistance in this "antiferromagnetic" configuration RAF= (R+r)/2 (see the corresponding resistance circuit in Figure 2c). When the magnetizations of the two Flayers are set into parallel configuration by an applied magnetic field, the spin-up electrons are weakly scattered in both layers and form a low-resistivity channel, whereas the spin-down electrons are strongly scattered in all the layers and form a high-resistivity channel. The reversal of magnetic field just interchanges the roles of spin-up and spin-down channels. The current's shunting by the low-resistivity channel produces a low total resistance RF;;; 2Rr/(R+r} in this "ferromagnetic" configuration. The size of the GMR is defined as (RAF-RF}/RAF = (R- UNCLASSIFIED/ f FOR OFEJCJ0P PSF Al!! Y UNCLASSIFIED//P81l 8PPM!I.AL ~81!! 8Hl!V 5 1. The other definition l!R/R = (RAF-RF)/RF::;; (R-r) /4Rr (unbounded from above) is also in use. Figure 2a shows a magnetoresistance curve typical for magnetic multilayers. The resistance is constant at a minimum value RF above a saturation field Bs (parallel Fs) and rises to a maximum value RAF as the applied magnetic field B approaches zero {antiparallel Fs). GMR occurs in two different geometries (see Figure 1): namely when the current flows in the plane of the layers, or CIP geometry, or when current flows perpendicular to the layers, or CPP geometry. Most of experiments on GMR are carried out in the CIP geometry because measuring the fairly large resistance of a thin film is quite easy (film length is typically orders of magnitude larger than its thickness). Experiments in the CPP geometry are more difficult (Reference 33) and require special techniques for precision measurements of very small resistances ~10 n resulting from the "short and wide" geometry of a 1-mm "wide" and 1-m "long" sample. In order to increase the resistances to easily observable values, microfabrication techniques can be used to reduce the sample's cross-sectional area (Reference 34-36). Finally, a simple and inexpensive point-contact technique (Reference 37) may also be suitable for this purpose. The samples with a reduced cross-sectional area will be of interest for spin- transfer-torque experiments presented in Section 3, 2.2 GMR APPLICATIONS GMR is currently used in magnetic field sensors, including those in read heads for computer hard drives, in galvanic isolators, and in nonvolatile random access memory devices. Reading information stored on magnetic hard disk drives in computers was the first large-scale commercial application of GMR. The information is stored by magnetizing small regions (magnetic domains) of a magnetic recording disk in different directions. The stray magnetic fields from these domains are detected by a GMR sensing element called spin valve. The simplest type of spin valve consists of two ferromagnetic layers separated by a thin, nonmagnetic spacer. The spin-valve resistance is smallest when the magnetizations of the two ferromagnetic layers are parallel and largest when the magnetizations are antiparallel. The antiparallel alignment is achieved by making the two layers respond differently to an external magnetic field; an antiferromagnet in contact with one of the layers is used to effectively "pin" the magnetization in this layer through an effect called "exchange bias. ' The exceptional responsiveness of spin valves to magnetic fields has enabled very high areal packing densities in hard drives. Other sensor applications using GMR elements include monitoring of a ferrous gear rotation in machinery operation (Reference 38) via detection of a changing magnetic flux when a gear tooth passes near the sensor, monitoring of electrical current via detection of the current-induced Oersted magnetic field, and transferring high- frequency signals between isolated circuits (Reference 39) via magnetic fields generated by a high-frequency inductor in one circuit and replicated in another circuit by a GMR sensor. UNCLASSIFIED/ FAP 055JQI IL ,tiii 8t1Wf UNCLASSIFIED//F&R &FFlll/11! ~81 8Hl:!V 3. Spin-Transfer-Torque This section focuses on the spin-transfer-torque (STT) phenomenon, which refers to a novel method to control and manipulate magnetic moments in nanostructures by spin currents-one of the forefront and most exciting areas in magnetism research today. 3.1 STT BASICS The previous section showed that the magnetic state of a ferromagnet can affect its electrical transport properties; for instance, the relative orientation of the magnetic moments in magnetic multilayers underlies the phenomenon of GMR (Reference 1, 2). The inverse effect, in which a large electrical current density can perturb the magnetic state of a multilayer, has also been predicted (Reference 3, 4). Here the current transfers vector spin between the magnetic layers and induces precession and/or reversal of the