Magnetic Memory Elements Research Articles

Quantitative Magneto-Mechanical Detection and Control of the Barkhausen Effect

Quantitative characterization of intrinsic and artificial defects in ferromagnetic structures is critical to future magnetic storage based on vortices or domain walls moving through nanostructured devices. Using torsional magnetometry, we observe finite size modifications to the Barkhausen effect in the limiting case of a single vortex core interacting with individual pointlike pinning sites in a magnetic thin film. The Barkhausen effect in this limit becomes a quantitative two-dimensional nanoscale probe of local energetics in the film. Tailoring the pinning potential using single-point focused ion beam implantation demonstrates control of the effect and points the way to integrated magneto-mechanical devices incorporating quantum pinning effects.

Oriented single crystals of Ni–Mn–Ga with very low switching field

Ni–Mn–Ga single crystals were grown from an oriented seed using a modified Bridgman method. The grown crystals can be oriented very easily for final preparation of magnetic shape memory (MSM) elements with all faces parallel to {100}. The twinning stress depends on the orientation of the twin habit plane. For type I twin boundaries with traces perpendicular to the sample edge, the equivalent stress to move the twin boundary was 0.9MPa. For type II twin boundaries with traces slightly inclined to those of type I twin boundaries, the equivalent stress to move the twin boundary was 0.01MPa. The presented method of growing and processing Ni–Mn–Ga single crystals is more efficient than existing methods and produces high-quality MSM elements.

Thermal relaxation rates of magnetic nanoparticles in the presence of magnetic fields and spin-transfer effects

We have measured the relaxation time of a thermally unstable ferromagnetic nanoparticle incorporated into a magnetic tunnel junction (MTJ) as a function of applied magnetic field, voltage $V$ (\ensuremath{-}0.38 V $V$ $+$ 0.26 V), and temperature (283 K $T$ 363 K). By analyzing the results within the framework of a modified N\'eel-Brown formalism, we determine the effective attempt time of the nanoparticle and also the bias dependences of the in-plane and out-of-plane spin-transfer torques. There is a significant linear modification of the effective temperature with voltage due to the in-plane torque and a significant contribution of a ``field-like'' torque that is quadratic with voltage. The methods presented here do not require complicated models for device heating or calibration procedures but instead directly measure how temperature, field, and voltage influence the energy landscape and thermal fluctuations of a two-state system. These results should have significant implications for designs of future nanometer-scale magnetic random access memory elements and provide a straightforward methodology to determine these parameters in other MTJ device structures.

Spin-dependent transport for armchair-edge graphene nanoribbons between ferromagneticleads

We theoretically investigate the spin-dependent transport for the system of anarmchair-edge graphene nanoribbon (AGNR) between two ferromagnetic (FM) leads witharbitrary polarization directions at low temperatures, where a magnetic insulator isdeposited on the AGNR to induce an exchange splitting between spin-up and -downcarriers. By using the standard nonequilibrium Green’s function (NGF) technique, it isdemonstrated that the spin-resolved transport property for the system depends sensitivelyon both the width of AGNR and the polarization strength of FM leads. The tunnelingmagnetoresistance (TMR) around zero bias voltage possesses a pronounced plateaustructure for a system with semiconducting 7-AGNR or metallic 8-AGNR in the absence ofexchange splitting, but this plateau structure for the 8-AGNR system is remarkablybroader than that for the 7-AGNR one. Interestingly, an increase of the exchange splittingΔ suppresses the amplitude of the structure for the 7-AGNR system. However, the TMR ismuch enhanced for the 8-AGNR system under a bias amplitude comparable to the splittingstrength. Further, the current-induced spin-transfer torque (STT) for the 7-AGNR systemis systematically larger than that for the 8-AGNR one. The findings here suggest the designof GNR-based spintronic devices by using a metallic AGNR, but it is more favorable tofabricate a current-controlled magnetic memory element by using a semiconducting AGNR.

Magnetic crossover effect in Nickel nanowire arrays

A crossover effect in the magnetic reversal mechanism within arrays of Nickel nanowires whose diameter varies from 15 to 100 nm is observed around 50 nm. Hysteresis loops and FMR measurements confirm that nanowire diameter controls effectively the nanowire easy axis as well as the magnetization reversal mechanism. This might be very interesting for spintronic devices based on current-induced domain motion such as non-volatile magnetic memory elements (MRAM) and low Ohmic loss devices.

Feedback control of noise in spin valves by the spin-transfer torque

The miniaturization of magnetic read heads and random access memory elements makes them vulnerable to thermal fluctuations. We demonstrate how current-induced spin-transfer torques can be used to suppress the effects of thermal fluctuations. This enhances the fidelity of perpendicular magnetic spin valves. The simplest realization is a dc to stabilize the free magnetic layers. The power can be significantly reduced without losing fidelity by simple control schemes, in which the stabilizing current-induced spin-transfer torque is controlled by the instantaneous resistance.

Magnetization reversal in Pt/Co(0.5 nm)/Pt nano-platelets patterned by focused ion beamlithography

Arrays of ultrathin Pt/Co(0.5 nm)/Pt nano-platelets with lateral sizes ranging from 30 nm to1 µm have been patterned by focused ion beam (FIB) lithography under a weakGa + ion fluence. From polar magneto-optical Kerr microscopy it is demonstrated thatnano-platelets are ferromagnetic with perpendicular anisotropy down to a size of50 nm. The irradiation process creates a magnetically soft ring at the nano-plateletperiphery in which domain nucleation is initiated at a low field. The magnetizationreversal in nano-platelets can be interpreted using a confined droplet model. All ofthe results prove that ultimate FIB patterning is suitable for preparing discretemagnetic recording media or small magnetic memory elements and nano-devices.

(Invited) MgO Tunnel Barriers in Magnetic Nanostructures and Devices

In recent years, there has been a great deal of success in the development of MgO based tunnel barriers for use in magnetic devices, such as nanoscale magnetic random access memory (MRAM) elements and magnetic field sensors. Much of the focus has been on creating tunnel barriers that have ever-increasing tunneling magnetoresistance values, which today approach as much as 500%, and lower resistance-area products, which are as low as a few Ohm-micrometers squared. These improvements afford the potential for future scaling of MRAM devices into the deep nanometer regime and solid-state CMOS compatible sensors with much improved sensitivity. This paper gives an overview of the fundamental device concepts of these MgO-based structures. Contribution of NIST, not subject to copyright

Magnetic cellular automata and the formation of glassy and magnetic structures from a chain of magnetic particles

We show that magnetic materials made of chains of small magnetic particles display many unusual properties. This is associated mainly with a variety of stable different magnetic structures which can arise there. In particular, there arises a magnetic glass, which may be characterized by a whole set of hysteresis loops and by a large variety of Barkhausen jumps arising in the returned branches of the hysteresis loops. We consider in detail a simple example of such a system---a chain of magnetic nanoparticles. To describe such a single chain first we use numerical micromagnetic simulations. On the basis of these simulations, with the use of a perturbation theory, we derive an analytical model which is an anisotropic Heisenberg model. This is a Heisenberg model with an additional anisotropy term arising due to the shape of the particles. Such a term also arises naturally in some classical magnetic materials such as ${\mathrm{Mn}}_{2}\mathrm{Ni}$ chains. We describe all possible stable states of the system as well as transitions between the states induced by magnetic field. Each of these transitions is arising $a\phantom{\rule{0.3em}{0ex}}la$ the spin flop transition. It may be displayed and detected in experiments as a Barkhausen jump in a hysteresis loop. The series of described spin flop transitions will lead to the formation of different types of returned branches in hysteresis loops. We present exact analytical and numerical results describing the energy spectrum and the magnetization of such systems. The results may be used in the design of nanomaterials as well as for magnetic random access memory and magnetic quantum cellular automata elements.

Effect of Resistance-Area-Product and Thermal Environment on Writing of Magneto-Thermal MRAM

Blocking temperature written magnetic random access memory element test structures of various sizes and tunnel barrier resistance area products were fabricated in order to study the dependence of writing efficiency and tunnel junction integrity on the thermal environment of the memory element and tunnel junction resistance area product. The test structures were programmed using a CPP writing mode, where the device is heated by passing a small current through the tunnel junction. The device is then field cooled to set the direction of an IrMn/NiFeCo storage layer. Quasistatic write current was measured as a function of resistance area product and for underlayers with differing thermal conductivities. Linear fits to the size dependent write current data suggest that properly designed submicron bits can be written quasistatically at <100 mu A. Write current for a fixed thermal environment was found to depend inversely on resistance product, but too large a resistance area product causes the tunnel barriers to fail before the memory element can be heated above the blocking temperature of the storage layer. In addition, if the thermal conductivity between the magnetic tunnel junction and substrate is too small, the magnetic tunnel junction will fail before the blocking temperature is reached, even at very low resistance area product values. Proper device design should thus optimize cell thermal resistance and tunnel junction resistance for both reliability and minimum power consumption

Transitions Between Vortex and Transverse Walls in NiFe Nano-Structures

This paper investigates the possible transitions between transverse and vortex type domain walls in magnetic domain wall trap memory elements, induced by a rotating magnetic field. A phase relation is shown to exist relating the final domain wall configuration to the field strength and field rotation frequency. Thus, it is possible to controllably switch from a transverse domain wall to either a clockwise or counterclockwise one

Nonlocal Magnetization Dynamics in Ferromagnetic Heterostructures

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Planar Hall effect in magnetite (100) films

Giant planar Hall effect (GPHE) has been observed in epitaxial magnetite (100) films grown on MgO substrates. The effect is manifested as jumps in the transverse resistivity when the film is subjected to a swept, in-plane magnetic field. The jumps are two orders of magnitude higher than previously observed in metallic ferromagnets. Recently, the same effect has been reported for other materials, but unlike our results, they present GPHE at low temperature only. The magnitude of the GPHE observed at room temperature has potential applications such as magnetic sensors and nonvolatile memory elements.

Nonlocal magnetization dynamics in ferromagnetic heterostructures

Two complementary effects modify the GHz magnetization dynamics of nanoscale heterostructures of ferromagnetic and normal materials relative to those of the isolated magnetic constituents: On the one hand, a time-dependent ferromagnetic magnetization pumps a spin angular-momentum flow into adjacent materials and, on the other hand, spin angular momentum is transferred between ferromagnets by an applied bias, causing mutual torques on the magnetizations. These phenomena are manifestly nonlocal: they are governed by the entire spin-coherent region that is limited in size by spin-flip relaxation processes. We review recent progress in understanding the magnetization dynamics in ferromagnetic heterostructures from first principles, focusing on the role of spin pumping in layered structures. The main body of the theory is semiclassical and based on a mean-field Stoner or spin-density--functional picture, but quantum-size effects and the role of electron-electron correlations are also discussed. A growing number of experiments support the theoretical predictions. The formalism should be useful to understand the physics and to engineer the characteristics of small devices such as magnetic random-access memory elements.

Mutual phase-locking of microwave spin torque nano-oscillators

The spin torque effect that occurs in nanometre-scale magnetic multilayer devices can be used to generate steady-state microwave signals in response to a d.c. electrical current. This establishes a new functionality for magneto-electronic structures that are more commonly used as magnetic field sensors and magnetic memory elements. The microwave power emitted from a single spin torque nano-oscillator (STNO) is at present typically less than 1 nW. To achieve a more useful power level (on the order of microwatts), a device could consist of an array of phase coherent STNOs, in a manner analogous to arrays of Josephson junctions and larger semiconductor oscillators. Here we show that two STNOs in close proximity mutually phase-lock-that is, they synchronize, which is a general tendency of interacting nonlinear oscillator systems. The phase-locked state is distinct, characterized by a sudden narrowing of signal linewidth and an increase in power due to the coherence of the individual oscillators. Arrays of phase-locked STNOs could be used as nanometre-scale reference oscillators. Furthermore, phase control of array elements (phased array) could lead to nanometre-scale directional transmitters and receivers for wireless communications.

Nonlocal Magnetization Dynamics in Ferromagnetic Hybrid Nanostructures

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Bond- versus site-centred ordering and possible ferroelectricity in manganites

Transition metal oxides with a perovskite-type structure constitute a large group of compounds with interesting properties. Among them are materials such as the prototypical ferroelectric system BaTiO(3), colossal magnetoresistance manganites and the high-T(c) superconductors. Hundreds of these compounds are magnetic, and hundreds of others are ferroelectric, but these properties very seldom coexist. Compounds with an interdependence of magnetism and ferroelectricity could be very useful: they would open up a plethora of new applications, such as switching of magnetic memory elements by electric fields. Here, we report on a possible way to avoid this incompatibility, and show that in charge-ordered and orbitally ordered perovskites it is possible to make use of the coupling between magnetic and charge ordering to obtain ferroelectric magnets. In particular, in manganites that are less than half doped there is a type of charge ordering that is intermediate between site-centred and bond-centred. Such a state breaks inversion symmetry and is predicted to be magnetic and ferroelectric.

Energy barriers in magnetic random access memory elements

Minimum energy paths and energy barriers are calculated for the free data layer in elliptical magnetic random access memory elements using a recently developed method that combines the nudged elastic band method with finite-element micro-magnetics. The method calculates the magnetic states along the most probable reversal paths for applied fields below the zero temperature switching field. The reversal mode in the minimum energy path depends on the strength of the external field. With increasing easy axis field, the reversal mode becomes more inhomogeneous than at lower fields.

On the shape optimization of magnetic random access memory element design

In designing the shape of magnetic random access memory elements, a sufficient difference in switching current threshold between a full-select element and a half-select memory element is critical. In this article, we present a systematic micromagnetic study of the margin of switching threshold for two specific shapes: eye shaped and ellipse. It is found that at small magnetic thickness, the eye-shaped element exhibits a higher switching threshold margin than the ellipse. However, for relatively thick storage layers, the opposite becomes true. The switching threshold is also a strong function of the initial magnetization state for the ellipse.

Magnetic random access memories, Brown's paradox and hysterons

A novel design for magnetic random access memory elements, called hysterons, is presented. It uses the interaction between two regions of magnetization. In one the process of reversal is almost uniform rotation of a magnetically hard system. In the other it is the reversible motion of a vortex that makes it a magnetically soft system. The magnetostatic interaction between the two regions is anisotropic and hysteretic. The magnetic softness permits using thicker elements that are more stable against temperature fluctations while having low switching fields.