Spin Electronics

Goals

Bill Rippard depositing magnetic multilayers.

The Spin Electronics Program is creating the foundations for the development of new magnetoelectronictechnologies that utilize the electron spin instead of its charge. It investigates the transfer of spin angular momentum from electrons to magnetic thin films to induce magnetization dynamics for applications as microwave oscillators in high-speed signal processing and switching of discrete memory elements.

Customer Needs

Wireless communications devices are ubiquitous, ranging from simple radios to more complex structures such as cell phones and wireless Internet systems. All these devices are based upon the transmission and reception of electromagnetic signals, with higher frequencies being required for high data-transmission rates. Common oscillators for wireless applications operate in the gigahertz regime but are large (several millimeters on a side) and must be added onto semiconductor chips after their manufacture, increasing component cost. Further, magnetic data storage technologies require novel methods of high-speed operation of nanoscale memory elements. Traditional magnetic recording and magnetic random access memory (MRAM) technologies are encountering problems as they seek to push dimensions below 50 nanometers and speeds above 1 gigahertz.

This project concentrates on spin-momentum transfer (SMT) from electron currents to multilayer, ferromagnetic films. SMT is a newly discovered phenomenon that appears in nanometer-scale magnetic devices. We are studying metallic devices that use SMT to induce coherent magnetic precession. The precession frequency can be tuned from 1 gigahertz to more than 40 gigahertz by changing the current amplitude, polarization angle, or magnetic field angle. Spin-polarized currents can also be used to switch small magnetic elements used in nanoscale magnetic recording and MRAM technologies. These new techniques may enable more efficient switching of sub-50-nanometer structures at speeds above 5 gigahertz with considerably less power and better selectivity.

Technical Strategy

Sketch of spin-momentum transfer with mechanical point contacts.

We are using electron-beam-lithographed point contacts and nanopillar structures to achieve the high current densities needed to induce magnetic excitations in multilayer films. For sufficiently high current densities and applied magnetic fields, there is an abrupt increase in the resistance of a pointcontact junction. The resistance step is attributed to the generation of magnons (spin waves) by the SMT effect. We have found that SMT is a generic effect occurring for a wide range of experimental conditions: for both in-plane and out-of-plane fields, for multilayers grown at the both the first and second maxima in "giant magnetoresistance" (GMR), and for ferromagnetically coupled multilayers. We have discovered that SMT occurs in a number of different and previously unexplored alloys of Co, Fe and Ni.

The origin of the SMT effect is conservation of angular momentum. When current flow is perpendicular to the plane (CPP) of a GMR "spin-valve" device, electrons are spin-polarized by the "reference" magnetic layer. Inelastic electron scattering then leads to the transfer of spin angular momentum to the "sense" magnetic layer.

Passing a DC current through magnetic nanostructures can result in oscillations ranging from a few gigahertz to more than 40 gigahertz, the same range used for wireless applications. These new devices are only 40 nanometers in diameter and compatible with standard semiconductor processing, making the new technology attractive for applications. Work is now focusing on developing tunable oscillators and on investigating the fundamental mechanisms that govern the interaction between magnetization and spin current.

Accomplishments

Random telegraph switching induced by spin momentum transfer. Variation in resistance was measured at four values of current.

Switching probability vs. pulse duration for several current pulse amplitudes. As the current is decreased, larger pulse durations are required for consistent switching.

Reciprocal of the pulse duration required for consistent reversal vs. current amplitude. The deviation from linear behavior at low currents indicates thermally activated reversal.

Tunable Coherent Spin Waves Generated by Direct Current — We have successfully excited high-frequency magnetic oscillations in a magnetic multilayer structure with a direct current. The oscillations are caused by the torque exerted on the magnetization by the electron spins in the current. The current is injected at high density through a 40 nanometer contact into the magnetic multilayer. At a certain critical value, the current induces precessional oscillations in one of the two magnetic film layers.

These oscillations, detected as a voltage change due to the GMR effect, range in frequency from 6 to 40 gigahertz, with spectral widths as small as 2 megahertz. The small widths imply that the oscillations have lower damping and greater coherence than most known magnetic excitations. The frequency of the oscillations increases with an externally applied magnetic field. For in-plane fields, frequencies as high as 40 gigahertz were obtained. An increase in current causes the oscillation frequency to decrease, with a tunable range of 1 to 5 gigahertz.
Telegraph Switching Induced by Spin-Momentum Transfer — We demonstrated that a highdensity spin-polarized direct current passing through a small magnetic element induces classic two-state random telegraph switching of the magnetization via the SMT effect. The magnetization undergoes large, 40 to 90 degree angular rotations between two states at rates up to 2 gigahertz for a wide range of currents and applied magnetic fields. The switching involves the collective motion of a large number of spins. Such a collective, dynamic source for random telegraph noise is substantially different from that those found in superconducting devices and semiconductor circuitry, where isolated atomic-level defects are usually responsible for two-state fluctuations. The dependence of the switching rates on field and current indicate that device heating alone cannot explain the dynamics. The switching rate also approaches a stochastic regime at sufficiently large currents, where the dynamics are governed by both precession and thermal perturbations.

Previous studies have discovered that SMT can cause both irreversible switching and continuous precession in the sense layer of a GMR device. Both of these effects have potential for practical applications in data storage and telecommunications. However, random telegraph noise is a sign that not all SMT-based phenomena are advantageous. Such a noise source could pose a serious problem for the data storage industry as CPP technology is implemented in future sub-150-nanometer size read heads in disk drives. For example, because the noise persists even in large applied fields of 0.5 to 1 tesla, conventional biasing schemes may not stabilize these sensors.
Spin-Transfer Switching of Magnetic Devices Using Pulsed Currents — SMT utilizes angular-momentum conservation and spin-dependent transport to manipulate the magnetic state of a device. As a result of SMT, an electrical current can reverse the magnetization orientation. These effects have potential applications in writing data in magnetic random-access memory (MRAM) and magnetic hard disk drives. In collaboration with Hitachi Global Storage Technologies, San Jose, California, we have demonstrated magnetization reversal in 100-nanometer-sized magnetic thin-film devices with ultrashort spin pulses. The devices have two magnetic layers separated by a nonmagnetic spacer layer. One of the magnetic layers, the "fixed" layer, is deposited on an antiferromagnet, which pins the magnetization in one direction. The magnetization of the other layer, the "free" layer, is free to rotate. The devices have two stable states: parallel and antiparallel alignment of the magnetizations of the free and fixed layers. The state can be read by using the GMR effect, which causes the antiparallel orientation to have a higher resistance than the parallel orientation. When a current pulse is applied to the device, the electron spins are polarized by the fixed layer, and they transport this angular momentum to the free layer, thereby applying a torque to the free layer and potentially causing reversal of the free-layer magnetization.

We studied rapid reversal due to current pulses whose durations ranged between 100 picoseconds and 100 nanoseconds. The pulse duration required for consistent reversal decreased with increasing current amplitude; for the highest currents applied, reversal occurred in less than 300 picoseconds. This is the shortest reversal time reported yet for SMTbased switching. Such a result is promising for applying SMT as a method of writing magnetic data in MRAM and magnetic hard disk drives, which will be required to operate at gigahertz rates in the near future. By studying the reversal probability over a range of pulse durations and pulse amplitudes, we determined the crossover between thermally activated reversal and a fully dynamic reversal mechanism. In addition, conditions exist for finite switching probability less than unity over a large range of pulse duration, even in the dynamic reversal regime. Such behavior indicates the presence of large room-temperature thermal fluctuations in these magnetic nanostructures, which may represent a fundamental limitation to nanoscale magnetic data storage.
Frequency Modulation and Phase Locking in Spin-Transfer Microwave Oscillators — Initial work on SMT focused only on the continuouswave output of the new devices. However, such pure tones transmit no information. Instead, the outputs must be modulated, for instance in amplitude (AM) or frequency (FM), in order to transmit data. We are now able to modulate the current passed through the device. As the current is modulated at frequencies much less than the natural oscillation frequency set by the device, sideband lobes appear on both sides of the original carrier frequency. The details of the outputs can be understood in terms of standard communications theory.

The devices can also be "injection-locked" to an external drive signal close to their natural oscillation frequencies. In this scenario, the devices are forced to oscillate at the same frequency as the injected signal. Over this locking region the relative phases of the two signals can be controlled by the amplitude of the current passed through the device. We are investigating the locking and phase-shifting architectures for possible directional microwireless communications and synchronization schemes.

Output of the device with DC current injection only (single peak at 9.8 GHz); device output with additional modulation included at 50 megahertz and amplitude of 400 microamperes (peak at 9.8 GHz with satellites at 9.75 and 9.85 GHz).

Output frequency with no external modulation (squares); device output frequency with presence of an external drive at 9.8 gigahertz (circles).