Free Layer Magnetization Research Articles

Highly-scalable disruptive reading and restoring scheme for Gb-scale SPRAM and beyond

We propose a disruptive reading and restoration scheme for a high density spin-transfer-torque random access memory (SPRAM). The proposed scheme uses the feature that – with a desired error rate and a tunnel magneto resistance (TMR) device, which is the memory device of the SPRAM – does not switch its magnetization of free layer in a specific period of large current pulse. The restoration operation is performed to secure the storing data. As a result, by keeping good scalability of spin-transfer-torque writing toward Gb-scale and beyond, high-speed reading with read-disturbance-free operation can be achieved. This operation also enables the SPRAM to accept the DDRx-SDRAM compatible operation. In addition, we also proposed a 4- F 2 cell structure with a vertical transistor and prospected the reliability of a tunnel barrier of the TMR devices for a Gb-scale SPRAM.

Annular Spin-Transfer Memory Element

An annular magnetic memory that uses a spin-polarized current to switch the magnetization direction or helicity of a magnetic region is proposed. The device has magnetic materials in the shape of a ring (1 to 5 nm in thickness, 20 to 250 nm in mean radius and 8 to 100 nm in width), comprising a reference magnetic layer with a fixed magnetic helicity and a free magnetic layer with a changeable magnetic helicity. These are separated by a thin non-magnetic layer. Information is written using a current flowing perpendicular to the layers, inducing a spin-transfer torque that alters the magnetic state of the free layer. The resistance, which depends on the magnetic state of the device, is used to read out the stored information. This device offers several important advantages compared to conventional spin-transfer magnetic random access memory (MRAM) devices. First, the ring geometry offers stable magnetization states, which are, nonetheless, easily altered with short current pulses. Second, the ring geometry naturally solves a major challenge of spin-transfer devices: writing requires relatively high currents and a low impedance circuit, whereas readout demands a larger impedance and magnetoresistance. The annular device accommodates these conflicting requirements by performing reading and writing operations at separate read and write contacts placed at different locations on the ring.

Influence of Spin Torque on the Noise of TMR Heads in the GHz Range

We investigated noise spectra in the GHz range of tunneling magnetoresistive (TMR) read heads with resistance area product (RA) values of 4.0 and 1.7 Ω·μm2 in order to understand the influence of spin torque on the noise in these TMR heads. Strong noise spectral peaks were observed at 4.2 and 6.5 GHz in TMR heads with respective RA values of 4.0 and 1.7 Ω·μm2. Each peak intensity was enhanced with increasing external magnetic field depending on its polarity when bias current flowed from a free layer to reference one. These results mean that the spin torque influences the thermally excited magnetization fluctuation noise (thermal mag-noise) from the free layers. The peak became sharper and its intensity was enhanced with increasing current density up to 2.7 × 1011 A/m2 depending on its direction when the magnetization of free and reference layers were nearly anti-parallel to each other. These results demonstrate that the influence of spin torque on the thermal mag-noise becomes stronger as the current density increases.

Nonlinear magnetization dynamics excited by the spin-transfer torque effect

The macrospin model based on Landau-Lifshitz-Gilbert equation is used to study the current-induced magnetization dynamics in magnetic spin valves. We find that the DC spin-polarized current could either switch the magnetization of free layer or excite the steady-state precessional motion via the so-called spin-transfer torque effect. The AC current could drive the chaotic oscillations. The route to chaotic oscillation depending on the strength of current is demonstrated through a series of period doubling bifurcations.

Ultrafast switching in magnetic tunnel junction based orthogonal spin transfer devices

Orthogonal spin-transfer magnetic random access memory (OST-MRAM) uses a spin-polarizing layer magnetized perpendicularly to a free layer to achieve large spin-transfer torques and ultrafast energy efficient switching. We have fabricated and studied OST-MRAM devices that incorporate a perpendicularly magnetized spin-polarizing layer and a magnetic tunnel junction, which consists of an in-plane magnetized free layer and synthetic antiferromagnetic reference layer. Reliable switching is observed at room temperature with 0.7 V amplitude pulses of 500 ps duration. The switching is bipolar, occurring for positive and negative polarity pulses, consistent with a precessional reversal mechanism, and requires an energy of less than 450 fJ.

Theory ofI−Vcharacteristics of magnetic Josephson junctions

We analyze the electrical characteristics of a circuit consisting of a free thin-film magnetic layer and source and drain electrodes that have opposite magnetization orientations along the free magnet's two hard directions. We find that when the circuit's current exceeds a critical value there is a sudden resistance increase which can be large in relative terms if the currents to source or drain are strongly spin polarized and the free magnet is thin. This behavior can be partly understood in terms of a close analogy between the magnetic circuit and a Josephson junction.

Feature of Current-Induced Microwave Oscillation in Nano-Contacts Magneto-Resistive Devices After High-Temperature Annealing

We measured microwave spectra of nano-contacts magneto-resistive (NCMR) devices with a current-perpendicular-to-plane (CPP) spin-valve (SV) structure under dc current. This CPP-SV structure has a FeCo/FeCo-AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> nano-oxide layer (NOL)/FeCo structure as reference/spacer/free layer, respectively. NCMR is a new MR effect whose origin is thought to be spin-dependent scattering due to confined domain walls at nano-contacts (NCs) inside NOL with a potentially high MR ratio. We have observed high-level oscillation with narrow line-width which has been reported in NCMR-SV devices with the MR ratio of 3%-5%. In this study, we investigated current-induced microwave oscillation in NCMR devices with high MR ratio of 7%-11% after high-temperature annealing in order to obtain larger integrated oscillation power. High-temperature annealing not only increases the MR ratio, but also causes instability in the reference layer magnetization. It also leads to an increase in the oscillation power but broadening of the line-width, compared with low MR ratio samples. It is suggested that large integrated oscillation power in NCMR device is caused by the distorted alignment between free and reference layer magnetization at ferromagnetic NCs.

A Study of Write Margin of Spin Torque Transfer Magnetic Random Access Memory Technology

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Key design parameters of 64 Mb STT-MRAM at 90-nm technology node are discussed. A design point was developed with adequate TMR for fast read operation, enough energy barrier for data retention and against read disturbs, a write voltage satisfying the long term reliability against dielectric breakdown and a write bit error rate below <emphasis emphasistype="italic"><formula formulatype="inline"><tex Notation="TeX">$10^{-9}$</tex></formula></emphasis>. A direct experimental method was developed to determine the data retention lifetime that avoids the discrepancy in the energy barrier values obtained with spin current- and field-driven switching measurements. Other parameters detrimental to write margins such as backhopping and the existence of a low breakdown population are discussed. At low bit-error regime, new phenomenon emerges, suggestive of a bifurcation of switching modes. The dependence of the bifurcated switching threshold on write pulse width, operating temperature, junction dimensions and external field were studied. These show bifurcated switching to be strongly influenced by thermal fluctuation related to the spatially inhomogeneous free layer magnetization. An external field along easy axis direction assisting switching was shown to be effective for significantly reducing the percentage of MTJs showing bifurcated switching. </para>

The critical current density in composite free layer structures for spin transfer torque switching

The critical current density for spin transfer torque switching with composite free layer structures is investigated using the macrospin Landau–Lifshitz–Gilbert equation. We consider a magnetic tunneling junction with a rigid fixed layer, and a composite free layer consisting of two coupled ferromagnetic layers in which the coupling is parallel or antiparallel. The dependence of critical current density on thickness, coupling sign and strength, spin torque efficiency, and magnetization of the composite free layer is explored. We determine that reduction in the critical current density can be achieved only through careful design of the composite free layer structures. We show the detailed spin dynamics of the composite free layer when the reduction in the critical current density is accomplished.

Quantized spin-wave modes in magnetic tunnel junction nanopillars

We present an experimental and theoretical study of the magnetic field dependence of the mode frequency of thermally excited spin waves in rectangular shaped nanopillars of lateral sizes 60x100, 75x150, and 105x190 nm2, patterned from MgO-based magnetic tunnel junctions. The spin wave frequencies were measured using spectrally resolved electrical noise measurements. In all spectra, several independent quantized spin wave modes have been observed and could be identified as eigenexcitations of the free layer and of the synthetic antiferromagnet of the junction. Using a theoretical approach based on the diagonalization of the dynamical matrix of a system of three coupled, spatially confined magnetic layers, we have modeled the spectra for the smallest pillar and have extracted its material parameters. The magnetization and exchange stiffness constant of the CoFeB free layer are thereby found to be substantially reduced compared to the corresponding thin film values. Moreover, we could infer that the pinning of the magnetization at the lateral boundaries must be weak. Finally, the interlayer dipolar coupling between the free layer and the synthetic antiferromagnet causes mode anticrossings with gap openings up to 2 GHz. At low fields and in the larger pillars, there is clear evidence for strong non-uniformities of the layer magnetizations. In particular, at zero field the lowest mode is not the fundamental mode, but a mode most likely localized near the layer edges.

Spin-transfer dynamics in spin valves with out-of-plane magnetized CoNi free layers

We have measured spin transfer-induced dynamics in magnetic nanocontact devices having a perpendicularly magnetized Co/Ni free layer and an in-plane magnetized CoFe fixed layer. The frequencies and powers of the excitations agree well with the predictions of the single-domain model and indicate that the excited dynamics correspond to precessional orbits with angles ranging from zero to 90 degrees as the applied current is increased at a fixed field. From measurements of the onset current as a function of applied field strength we estimate the magnitude of the spin torque asymmetry parameter lambda ~ 1.5. By combining these with spin torque ferromagnetic resonance measurements, we also estimate the spin wave radiation loss in these devices.

Current-driven magnetization dynamics in magnetic trilayers with a tilted spin polarizer

The magnetization dynamics is studied theoretically in the tilted-polarizer magnetic trilayers. Utilizing stability analysis, we obtain the phase diagrams in the plane defined by the current density and the direction of pinned-layer magnetization. With the pinned-layer magnetization oriented in a certain range, one can realize different magnetic states, such as quasi-parallel and quasi-antiparallel stable states, in-plane and out-of-plane precessions, and out-of-plane stable states by varying the current. We find that the free-layer magnetization prefers reversal for small deviation of the fixed-layer magnetization from the film plane, while precession for big deviation.

FORC studies of exchange biased NiFe in L10(111) FePt-based spin valve

We use First-Order Reversal Curves (FORC) to study the switching distribution and exchange bias in L10 (111) FePt-based spin valves with a layer structure of Ta (6)/Pt (3)/L10 (111) Fe53Pt47 (20)/Co50Fe50 (1.5)/Cu (4.5)/Co50Fe50 (2)/Ni80Fe20 (3) (in nm). We find that the switching of the free layer magnetization is strongly influence by the magnetic state of the FePt/CoFe fixed layer, as evidenced by tunable coercivity and exchange bias fields.

Current-induced dynamics in noncollinear dual spin valves

Spin-transfer torque and current induced spin dynamics in spin-valve nanopillars with the free magnetic layer located between two magnetic films of fixed magnetic moments is considered theoretically. The spin-transfer torque in the limit of diffusive spin transport is calculated as a function of magnetic configuration. It is shown that non-collinear magnetic configuration of the outermost magnetic layers has a strong influence on the spin torque and spin dynamics of the central free layer. Employing macrospin simulations we make some predictions on the free layer spin dynamics in spin valves composed of various magnetic layers. We also present a formula for critical current in non-collinear magnetic configurations, which shows that the magnitude of critical current can be several times smaller than that in typical single spin valves.

Measurement of Effective Free Layer Magnetization Orientation of TMR Sensors

Measurements of thermal magnetization fluctuation noise spectrum in multi-directional magnetic field reveal tilt of effective free layer orientation in TMR sensors. We propose a method for effective sensor stiffness field measurement and demonstrate that deviations of stiffness from nominal values are caused by the angular dependence of the stiffness field. Ferro-magnetic resonance peak measurements indicate that the free layer magnetization orientation may be tilted from the cross-track direction. This magnetization tilt is caused by reference layer, hard bias problems and shape anisotropy effects. This reduces sensor efficiency and leads to amplitude asymmetry, multi-domain configuration and sensor instability. FMR-based method for detection of potentially unstable heads based on these observations is proposed.

Magnetic dynamics with spin-transfer torques near the Curie temperature

We use atomistic stochastic Landau-Lifshitz-Slonczewski simulations to study the interaction between large thermal fluctuations and spin transfer torques in the magnetic layers of spin valves. At temperatures near the Curie temperature $T_{\rm C}$, spin currents measurably change the size of the magnetization (i.e. there is a {\it longitudinal} spin transfer effect). The change in magnetization of the free magnetic layer in a spin valve modifies the temperature dependence of the applied field-applied current phase diagram for temperatures near $T_{\rm C}$. These atomistic simulations can be accurately described by a Landau-Lifshitz-Bloch + Slonczewski equation, which is a thermally averaged mean field theory. Both the simulation and the mean field theory show that a longitudinal spin transfer effect can be a substantial fraction of the magnetization close to $T_{\rm C}$.

Sensitivity of spin-torque diodes for frequency-tunable resonant microwave detection

We calculate the efficiency with which magnetic tunnel junctions can be used as resonant detectors of incident microwave radiation via the spin-torque diode effect. The expression we derive is in good agreement with the sensitivities we measure for MgO-based magnetic tunnel junctions with an extended (unpatterned) magnetic pinned layer. However, the measured sensitivities are reduced below our estimate for a second set of devices in which the pinned layer is a patterned synthetic antiferromagnet (SAF). We suggest that this reduction may be due to an undesirable coupling between the magnetic free layer and one of the magnetic layers within the etched SAF. Our calculations suggest that optimized tunnel junctions should achieve sensitivities for resonant detection exceeding 10 000 mV/mW.

Magnetization dynamics in spin torque nano-oscillators: Vortex state versus uniform state

Current-driven magnetization dynamics in spin torque nano-oscillators (STNOs) is intensely investigated because of its high potential for high-frequency (HF) applications. We experimentally study current-driven HF excitations of STNOs for two fundamental magnetization states of the free layer, namely, vortex state and uniform in-plane magnetization. Our ability to switch between the two states in a given STNO enables a direct comparison of the critical currents, agility, power, and linewidth of the HF output signals. We find that the vortex state has some superior properties, in particular, it maximizes the emitted HF power and shows a wider frequency tuning range at a fixed magnetic field.

100 ps precessional spin-transfer switching of a planar magnetic random access memory cell with perpendicular spin polarizer

Ultrafast spin-transfer precessional switching between two stable states of a magnetic random access memory device is demonstrated in structures comprising a perpendicularly magnetized polarizing layer (PL⊥), an in-plane magnetized free layer (FL), and an in-plane magnetized analyzing layer (AL) in a PL⊥/spacer/FL/spacer/AL stack. Back and forth switching can be achieved with sub-ns current pulses of the same polarity. The spin-torque influence from the analyzer leads to an asymmetric dependence of the switching properties as a function of the current sign and initial state.

Time-resolved x-ray imaging of magnetization dynamics in spin-transfer torque devices

Time-resolved x-ray imaging techniques have recently demonstrated the capability to probe the magnetic switching of nanoscale devices. This technique has enabled, for example, the direct observation of the nonuniform intermediate states assumed by the magnetic free layer during reversal by a spin-polarized current. These experiments have shown an interesting size-dependent behavior associated with the motion of vortices to mediate the magnetization reversal which cannot be explained by the macrospin picture of spin-torque switching. In this paper we present both experimental and analytical results which show the origin of the complex switching behavior. We use time-resolved x-ray microscopy to further study the switching behavior of samples with $45\ifmmode^\circ\else\textdegree\fi{}$ angle between the free and polarizing magnetic layers. A model is developed in terms of a linearized Landau-Lifshitz-Gilbert equation showing that the initial dynamics is dominated by the balance between the Oersted field and thermal fluctuations. The spin torque amplifies this dynamics, leading to a strong sensitivity to sample size, angle, and temperature. The model is in good agreement with current and previous experimental observations.