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The brain’s connectome continually rewires throughout the life of an organism. In this study, we sought to elucidate the operational principles of such rewiring by analyzing the functional connectomes in mouse primary motor cortex (M1) during a 14-session (day) lever-press task learning in response to an auditory cue. Specifically, we employed Calcium imaging recordings of L2/3 and L5 of M1 in awake mice to reconstruct and analyze functional connectomes across learning sessions. Our results show that functional connectomes in L2/3 and L5 follow a similar learning-induced rewiring trajectory. More specifically, the connectomes rewire in a biphasic manner, where functional connectivity increases over the first few learning sessions, and then, it is gradually pruned to return to a homeostatic level of network density. We demonstrated that the increase of network connectivity in L2/3 connectomes, but not in L5, generates neuronal co-firing activity that correlates with higher motor performance (shorter cue-to-reward time), while motor performance remains relatively stable throughout the pruning phase. The results show a biphasic rewiring principle that involves the maximization of reward / performance and maintenance of network density. Finally, we demonstrated that the connectome rewiring in L2/3 is clustered around a core set of movement-associated neurons that form a highly interconnected hub in the connectomes, and that the activity of these core neurons stably encodes movement throughout learning.Significance StatementConnectomes in the motor cortex rewire during motor skill learning, but the operational principle behind this rewiring is yet to be determined. Here, we characterized the rewiring dynamics of functional connectomes in L2/3 and L5 of M1 in mice engaging in a lever-press learning, using two-photon fluorescence microscopy data. We identified a universal biphasic rewiring trajectory across animals and layers in the motor cortex that reflects two objectives: an exploratory phase that increases functional connectivity and optimizes motor performance, and a pruning phase that brings connectivity back to a homeostatic level while maintaining motor performance. We found further that connectome rewiring during motor skill learning concentrates around a core set of highly interconnected neurons in L2/3 that reliably encode movements.

We report on temperature-dependent broadband ferromagnetic resonance studies of magnetite (Fe3O4) film of thickness 30 nm grown on magnesium gallate [MgGa2O4(001)] single crystal substrate by pulsed laser deposition. The isotropic point, i.e., the temperature at which magnetocrystalline anisotropy vanishes, is found to be approximately 120 K, which is 10 K lower than the bulk value. The Verwey transition is close to 110 K, again about 10 K lower than the bulk crystal value. Below the Verwey transition temperature, the uniaxial anisotropy of the monoclinic phase increases gradually and asymptotically approaches a constant value indicating a smooth structural transition rather than a sudden change. Resonance linewidth and effective damping increase abruptly below 110 K. The effective damping constant is determined to be ~0.01 and is independent of temperature from 300 K to 120 K.

Objective. Neural decoding is an important tool in neural engineering and neural data analysis. Of various machine learning algorithms adopted for neural decoding, the recently introduced deep learning is promising to excel. Therefore, we sought to apply deep learning to decode movement trajectories from the activity of motor cortical neurons. Approach. In this paper, we assessed the performance of deep learning methods in three different decoding schemes, concurrent, time-delay, and spatiotemporal. In the concurrent decoding scheme where the input to the network is the neural activity coincidental to the movement, deep learning networks including artificial neural network (ANN) and long-short term memory (LSTM) were applied to decode movement and compared with traditional machine learning algorithms. Both ANN and LSTM were further evaluated in the time-delay decoding scheme in which temporal delays are allowed between neural signals and movements. Lastly, in the spatiotemporal decoding scheme, we trained convolutional neural network (CNN) to extract movement information from images representing the spatial arrangement of neurons, their activity, and connectomes (i.e. the relative strengths of connectivity between neurons) and combined CNN and ANN to develop a hybrid spatiotemporal network. To reveal the input features of the CNN in the hybrid network that deep learning discovered for movement decoding, we performed a sensitivity analysis and identified specific regions in the spatial domain. Main results. Deep learning networks (ANN and LSTM) outperformed traditional machine learning algorithms in the concurrent decoding scheme. The results of ANN and LSTM in the time-delay decoding scheme showed that including neural data from time points preceding movement enabled decoders to perform more robustly when the temporal relationship between the neural activity and movement dynamically changes over time. In the spatiotemporal decoding scheme, the hybrid spatiotemporal network containing the concurrent ANN decoder outperformed single-network concurrent decoders. Significance. Taken together, our study demonstrates that deep learning could become a robust and effective method for the neural decoding of behavior.

We report on broadband and in-plane angle dependent ferromagnetic resonance measurements of MnN (30 nm)/CoFeB ( t CoFeB ) exchange bias bilayer systems with CoFeB thicknesses ranging from 5 to 20 nm. We find that the strong unidirectional anisotropy in this system is accompanied by a strong uniaxial in-plane anisotropy and a small interfacial perpendicular anisotropy ( K ⊥ , i = 0.043 ± 0.001 erg / cm 2 ). The strength of the interfacial exchange bias coupling in this system Δ σ eb = 0.193 ± 0.005 erg / cm 2 is only 2.5 times larger than the interfacial uniaxial coupling strength Δ σ u = 0.076 ± 0.003 erg / cm 2 . The relaxation in this system also shows a strong unidirectional and uniaxial in-plane anisotropy. While the observed thickness dependence of the uniaxial anisotropy of the relaxation is consistent with an interfacial two-magnon scattering contribution, we find that this is not the case for the unidirectional relaxation.

Spin transfer torque random access memory (STT-RAM) based on magnetic tunnel junction (MTJ) is a popular type of memory because of its non-volatility, small size, and nanosecond access time. The temperature effect on the magnetic properties and the initial angle between the free and pinned layer magnetizations in the MTJ cell are significant for the fast switching process in STT-RAM. In this study, the switching energy was investigated with the temperature effect on the magnetic properties at various initial angles. The results show that the saturation magnetization was decreased by increments of the initial temperature, which also resulted in the reduction of the intrinsic critical current density. Consequently, switching energy can be reduced by the increase of both the initial temperature and the initial angle. Although the initial temperature increment in the MTJ cell is interesting for the fast switching process in STT-RAM, the thermal stability factor greater than 40 and the Curie temperature were both considered for the analysis of the switching energy with magnetic properties, for stabilizing the operation of STT-RAM.

Two-dimensional (2D) van der Waals (vdWs) materials have gathered a lot of attention recently. However, the majority of these materials have Curie temperatures that are well below room temperature, making it challenging to incorporate them into device applications. In this work, we synthesized a room-temperature vdW magnetic crystal Fe$_5$GeTe$_2$ with a Curie temperature T$_c = 332$ K, and studied its magnetic properties by vibrating sample magnetometry (VSM) and broadband ferromagnetic resonance (FMR) spectroscopy. The experiments were performed with external magnetic fields applied along the c-axis (H$\parallel$c) and the ab-plane (H$\parallel$ab), with temperatures ranging from 300 K to 10 K. We have found a sizable Land\'e g-factor difference between the H$\parallel$c and H$\parallel$ab cases. In both cases, the Land\'e g-factor values deviated from g = 2. This indicates contribution of orbital angular momentum to the magnetic moment. The FMR measurements reveal that Fe$_5$GeTe$_2$ has a damping constant comparable to Permalloy. With reducing temperature, the linewidth was broadened. Together with the VSM data, our measurements indicate that Fe$_5$GeTe$_2$ transitions from ferromagnetic to ferrimagnetic at lower temperatures. Our experiments highlight key information regarding the magnetic state and spin scattering processes in Fe$_5$GeTe$_2$, which promote the understanding of magnetism in Fe$_5$GeTe$_2$, leading to implementations of Fe$_5$GeTe$_2$ based room-temperature spintronic devices.

We report on the experimental investigation of the perpendicular magnetic anisotropy in $\mathrm{Pt}/{[\mathrm{Co}/\mathrm{Ni}]}_{\mathrm{N}}/\mathrm{Ir}$ multilayers. Broadband ferromagnetic resonance (FMR) and polar angle dependent FMR measurements reveal the presence of a strong fourth-order contribution to the perpendicular anisotropy. By numerically solving the Smit-Beljers relation, we are able to fit the polar angle dependence of the resonance field and extract both the second and fourth-order anisotropy constants. While this model provides a qualitative description for all samples, we note that, for samples near or in the easy cone state of the phase diagram, systematic deviations emerge in the polar angle dependent data. The exchange stiffness of the multilayers is found to be lower than the weighted average of the exchange stiffness reported for Co and Ni in the literature. We find that the effective Gilbert damping scales with the inverse multilayer thickness, indicating a significant interfacial contribution to the damping process.

Spin currents can exert spin-transfer torques on magnetic systems even in the limit of vanishingly small net magnetization, as recently shown for antiferromagnets. Here, we experimentally show that a spin-transfer torque is operative in a macroscopic ensemble of weakly interacting, randomly magnetized Co nanomagnets. We employ element- and time-resolved X-ray ferromagnetic resonance (XFMR) spectroscopy to directly detect subnanosecond dynamics of the Co nanomagnets, excited into precession with cone angle ≳0.003° by an oscillating spin current. XFMR measurements reveal that as the net moment of the ensemble decreases, the strength of the spin-transfer torque increases relative to those of magnetic field torques. Our findings point to spin-transfer torque as an effective way to manipulate the state of nanomagnet ensembles at subnanosecond time scales.

To develop low-moment, low-damping metallic ferromagnets for power-efficient spintronic devices, it is crucial to understand how magnetic relaxation is impacted by the addition of nonmagnetic elements. Here, we compare magnetic relaxation in epitaxial Fe films alloyed with light nonmagnetic elements of V and Al. FeV alloys exhibit lower intrinsic damping compared to pure Fe, reduced by nearly a factor of 2, whereas damping in FeAl alloys increases with Al content. Our experimental and computational results indicate that reducing the density of states at the Fermi level, rather than the average atomic number, has a more significant impact in lowering damping in Fe alloyed with light elements. Moreover, FeV is confirmed to exhibit an intrinsic Gilbert damping parameter of $\simeq$0.001, among the lowest ever reported for ferromagnetic metals., 28 pages, 9 figures

We examine the substructures of magnetic domain walls (DWs) in [Pt/(Co/Ni)$_M$/Ir]$_N$ multi-layers using a combination of micromagnetic theory and Lorentz transmission electron microscopy (LTEM). Thermal stability calculations of Q=$\pm$1 substructures (2-$\pi$ vertical Bloch lines (VBLs) and DW skyrmions) were performed using a geodesic nudged elastic band (GNEB) model, which supports their metastability at room temperature. Experimental variation in strength of the interfacial Dzyaloshinskii-Moriya interaction (DMI) and film thickness reveals conditions under which these substructures are present and enables the formation of a magnetic phase diagram. Reduced thickness is found to favor Q=$\pm$1 substructures likely due to the suppression of hybrid DWs. The results from this study provide an important framework for examining 1-D DW substructures in chiral magnetic materials.

A systematic study of the film-thickness dependence of soft magnetic properties and magnetization dynamics of Fe73Co25Al2 alloy thin films has been carried out. These films have columnar structures grown along the film normal, with the average width of the columns of about 30 nm. The saturation magnetization $M_{s}$ is about 1550 emu/cc, which remains nearly unchanged with film thickness over a range from 10 to 120 nm. The coercivity $H_{c}$ is found to increase with film thickness from 15 to 64 Oe over a range of 11 to about 50 nm, and then slightly decrease with further increase of thickness. The effective damping parameter $\alpha _{\mathrm {eff}}$ measured by ferromagnetic resonance over a frequency range from 12 to 66 GHz is found to decrease with film thickness, becoming 0.0004 and 0.0006, for the films deposited onto fused silica and MgO (100) substrate, respectively.

Magnetic damping impacts essential dynamics for spintronic device applications, but its fundamental mechanisms in various materials – including simple ferromagnetic metals – have yet to be understood. Here, we experimentally correlate damping with structural and transport properties of epitaxial thin films of Fe. At room temperature, the effective Gilbert damping parameter is independent of whether these films are coherently strained or partially relaxed. However, at low temperature, we find that coherently strained Fe films with higher crystalline quality and conductivity exhibit higher damping. The enhancement of low-temperature damping is greater than that from classical eddy current loss. Our observation of such conductivity-like damping, possibly governed by the intraband scattering mechanism [1], provides fundamental insight into the role of crystallinity in damping in ferromagnetic metals. [1] K. Gilmore, Y. U. Idzerda, M. D. Stiles, Phys. Rev. Lett. 99, 027204 (2007); M. A. W. Schoen et al. Nat. Phys. 12, 839 (2016).

${\mathrm{Fe}}_{69}{\mathrm{Co}}_{26}{\mathrm{Si}}_{5}$ alloy thin films of various thicknesses were deposited on MgO(100) single crystal substrates by magnetron sputtering at $230{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ substrate temperature. The thickness dependence of the dynamic magnetic properties was investigated using broadband ferromagnetic resonance (FMR). The x-ray diffraction results indicate that all films are of the bcc structure with an in-plane epitaxial alignment of ${[100]}_{\mathrm{FeCoSi}}//{[110]}_{\mathrm{MgO}}$. The exchange constant was determined from the field shift between the uniform precession FMR mode and the first order perpendicular standing spin wave resonance mode in the FMR spectra. The effective damping parameter decreases dramatically with increasing film thickness up to 16 nm due to the decrease of the spin pumping contribution and then remains relatively constant as the film thickness increases. In-plane angle dependent FMR measurements reveal that the in-plane anisotropy of these films is dominated by a fourfold magnetic anisotropy, which increases sharply with increasing film thickness up to 16 nm and then shows a slightly decreasing trend as the film thickness increases. In-plane angular dependence of the FMR linewidth shows a strong two-magnon scattering contribution.

We report the chemical partitioning behavior in a series of melt spun, partially crystallized Co(78−x)Fe2MnxB14Si2Nb4 (x = 0.5, 2.0, 4.0, and 6.0) alloys. Upon annealing at 520 °C anneal for 20 min, nanocrystallites precipitated from the amorphous matrix that have been previously reported to decrease the magnetic permeability with increasing Mn content. Transmission electron microscopy identified the crystallites within the continuous amorphous matrix as a mixture of hexagonal close packed and face centered cubic phases with an average size of 7.0 ± 3.0 nm. Atom probe tomography (APT) revealed an increase in chemical partitioning between the ferromagnetic and glass-forming elements upon annealing with the crystallite density increasing with Mn content. This implied an increase in crystallinity and a reduction of point defects (vacancies) that accompanied changes in Mn content during the chemical partitioning process, where the extent of crystallinity would influence magnetic properties.

Electrostatic discharge (ESD) has been an important issue in the manufacturing processes of hard disk drive. It can also generate electromagnetic interference (EMI) which could possibly damage magnetic recording heads. The aims of this work are to measure the EMI from ESD events and to examine the effects of EMI on the heads. The discharge current and the EMI generated by an ESD simulator were experimentally measured. Also, the EMI was applied to the heads to determine if this can cause changes of head parameters. Our results show that the discharge current waveform is consistent with the theoretical waveform of the IEC ESD standard. Additionally, we found that the EMI applied due to ESD at distances greater than 2 cm does not have any significant effect on the head parameters. Hence, further detailed experiments are proposed to evaluate the EMI effects on recording head parameters in order to improve the measurement methodologies to prevent the degradation of the heads performance and to increase the robustness of the heads.

We report a detailed study of the structural and ferromagnetic resonance properties of spinel nickel ferrite (NFO) films, grown on (100)-oriented cubic MgAl 2 O 4 substrates by direct liquid injection chemical vapor deposition (DLI-CVD) technique. Three different compositions of NFO films (Ni x Fe 3− x O 4 where x =1, 0.8, 0.6) deposited at optimized growth temperature of 600 °C are characterized using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Vibrating Sample Magnetometry (VSM), and broadband ferromagnetic resonance (FMR) techniques. XRD confirms the growth of epitaxial, single crystalline Ni x Fe 3− x O 4 films. The out-of-plane lattice constant ( c ) obtained for Ni 0.8 Fe 2.2 O 4 film is slightly higher than the bulk value (0.833 nm), indicating only partial strain relaxation whereas for the other two compositions ( x =1 and x =0.6) films exhibit complete relaxation. The in-plane and out-of-plane FMR linewidths measurements at 10 GHz give the lowest values of 458 Oe and 98 Oe, respectively, for Ni 0.8 Fe 2.2 O 4 film as compared to the other two compositions. A comprehensive frequency (5–40 GHz) and temperature (10–300 K) dependent FMR study of the Ni 0.8 Fe 2.2 O 4 sample for both in-lane and out-of-plane configurations reveals two magnon scattering (TMS) as the dominant in-plane relaxation mechanism. It is observed that the TMS contribution to the FMR linewidth scales with the saturation magnetization M s . In-plane angle-dependent FMR measurements performed on the same sample show that the ferromagnetic resonance field ( H res ) and the FMR linewidth (Δ H ) have a four-fold symmetry that is consistent with the crystal symmetry of the spinel. SEM measurements show formation of pyramid-like microstructures at the surface of the Ni 0.8 Fe 2.2 O 4 sample, which can explain the observed four-fold symmetry of the FMR linewidth.

Electromagnetic interference (EMI) has been a significant issue for the current perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) read heads because it can cause magnetic failure. Furthermore, the magnetic noise induced by the spin transfer torque (STT) effect has played an important role in the CPP read heads because it can affect the stability of the heads. Accordingly, this work proposed an investigation of the magnetic instabilities induced by EMI through the STT effect in a CPP-GMR read head via micromagnetic simulations. The magnetization fluctuation caused by EMI was examined, and then, magnetic noise was evaluated by using power spectral density analysis. It was found that the magnetization orientation can be fluctuated by EMI in close proximity to the head. The results also showed a multimode spectral density. The main contributions of the spectral density were found to originate at the edges of the stripe height sides due to the characteristics of the demagnetization field inside the free layer. Hence, the magnetic instabilities produced by EMI become a significant factor that essentially impacts the reliability of the CPP-GMR read heads.

We demonstrate the formation of metastable N\'eel-type skyrmion arrays in Pt/Co/Ni/Ir multi-layers at zero-field following \textit{ex situ} application of an in-plane magnetic field using Lorentz transmission electron microscopy. The resultant skyrmion texture is found to depend on both the strength and misorientation of the applied field as well as the interfacial Dzyaloshinskii-Moriya interaction. To demonstrate the importance of the applied field angle, we leverage bend contours in the specimens which coincide with transition regions between skyrmion and labyrinth patterns. Subsequent application of a perpendicular magnetic field near these regions reveals the unusual situation where skyrmions with opposite magnetic polarities are stabilized in close proximity.

The dependences of static and dynamic magnetic properties on both substrate deposition temperature (TS) and film-thickness (d) for (FeCo)-(AlSi) alloy thin films are discussed in conjunction with structure. The samples of a multi layered type (FeCo)-(AlSi) alloy thin films were fabricated at Ts by DC sputter-deposition onto (1 0 0) MgO substrates by using two targets of Fe75Co25 and Al50Si50. For the first time, the DO3 phase in the quaternary alloy films are realized for TS at around 400 ℃ with d larger than 25 nm. The order parameter (S) was estimated by taking into account crystal structure factor, absorption factor, multiplicity factor, temperature factor, and Lorentz polarization factor. It is found that S increases with d, becoming nearly 1 for d larger than 50 nm. Observations by high resolution transmission electron microscopy support this result. The coercivity (Hc) and effective damping parameter (αeff) decrease with increasing d and remain nearly constant, approximately 4 Oe and 5 × 10−3, respectively. The result that HC decreases with increasing S suggests that (FeCo)-(AlSi) thin films with the DO3 phase are an attractive candidate for future high frequency device applications.

Confirming the origin of Gilbert damping by experiment has remained a challenge for many decades, even for simple ferromagnetic metals. In this Letter, we experimentally identify Gilbert damping that increases with decreasing electronic scattering in epitaxial thin films of pure Fe. This observation of conductivity-like damping, which cannot be accounted for by classical eddy current loss, is in excellent quantitative agreement with theoretical predictions of Gilbert damping due to intraband scattering. Our results resolve the longstanding question about a fundamental damping mechanism and offer hints for engineering low-loss magnetic metals for cryogenic spintronics and quantum devices.

For spin transfer torque (STT), the switching energy and thermal stability of magnetic tunnel junctions (MTJ) bits utilized in memory devices are important factors that have to be considered simultaneously. In this article, we examined the minimum energy for STT induced magnetization switching in MTJ devices for different in-plane angles of the magnetization in the free layer and the pinned layer with respect to the major axis of the elliptical cylinder of the cell. Simulations were performed by comparing the analytical solution with macrospin and full micromagnetic calculations. The results show good agreement of the switching energy calculated by using the three approaches for different initial angles of the magnetization of the free layer. Also, the low-energy location specifies the suitable value of both time and current in order to reduce the heat effect during the switching process.

We investigate the influence of lateral variations of the second-order perpendicular anisotropy in thin films on the effective anisotropies required to represent this structure using a macrospin approximation. Second-order and fourth-order effective anisotropies are required for the macrospin approximation. In the case of quasistatic calculations, the fourth-order effective anisotropy is closely linked to deviations of the average magnetization angle from the field direction and lateral variations of the magnetization direction in the structure leading to dependence on the field strength and the lateral length scale of anisotropy variations of the effective anisotropies. We find that the field and lateral length-scale dependence of the effective anisotropies extracted from simulations of the magnetization dynamics are profoundly different from those of the quasistatic simulations. This is caused by resonance localization that depends on the orientation of the external magnetic field.

It is well established that the spin-orbit interaction in heavy metal/ferromagnet heterostructures leads to a significant interfacial Dzyaloshinskii-Moriya interaction (DMI), which modifies the internal structure of magnetic domain walls (DWs) to favor N\'eel- over Bloch-type configurations. However, the impact of such a transition on the structure and stability of internal DW defects (e.g., vertical Bloch lines) needs to be explored. We present a combination of analytical and micromagnetic calculations to describe a type of topological excitation called a DW skyrmion characterized by a ${360}^{\ensuremath{\circ}}$ rotation of the internal magnetization in a Dzyaloshinskii DW. We further propose a method to identify DW skyrmions experimentally using Fresnel-mode Lorentz transmission electron microscopy; simulated images of DW skyrmions using this technique are presented based on the micromagnetic results.

We study the temperature-dependent static and dynamic magnetic properties of polycrystalline bilayers of permalloy ($\mathrm{N}{\mathrm{i}}_{80}\mathrm{F}{\mathrm{e}}_{20}$, or Py) and gadolinium (Gd) bilayers using DC magnetometry and broadband ferromagnetic resonance. Magnetometry measurements reveal that the 3-nm-thick Gd layers undergo a magnetic ordering transition below 100 K, consistent with finite size suppression of their Curie temperature. Upon cooling below this Gd ordering temperature, ferromagnetic resonance spectroscopy reveals a sharp increase in both the gyromagnetic ratio (\ensuremath{\gamma}) and effective Gilbert damping parameter (${\ensuremath{\alpha}}_{\mathrm{eff}}$) of the neighboring Py layers. The increase of \ensuremath{\gamma} is attributed to the onset of strong antiferromagnetic coupling between the Gd and Py layers as the Gd orders magnetically. We argue that the increase of ${\ensuremath{\alpha}}_{\mathrm{eff}}$, on the other hand, can be explained by spin pumping into the rare-earth layer when taking into account the increase of \ensuremath{\gamma}, the decrease of the Gd spin diffusion length as it orders, and, most significantly, the corresponding increase of the Py/Gd interfacial spin mixing conductance in the vicinity of the magnetic ordering transition. We propose that these observations constitute a qualitative confirmation of a recent theoretical prediction of spin sinking enhancement in this situation.

Interlayer exchange coupling of two ferromagnetic layers through a nonmagnetic spacer is an integral part of many magnetic sensors and devices. The authors use broadband ferromagnetic resonance to investigate the oscillatory interlayer exchange coupling between two Co-Fe layers, as a function of Ru spacer thickness and of temperature. The results show that the temperature dependence of the coupling is governed by the thermal spin-wave model---important insight for engineering magnetic and spintronic components.

Generating pure spin currents is a necessary part of many spintronic devices. Now there is a new mechanism for doing this, utilizing nuclear spin waves.

The magnetization dynamics of exchange-biased IrMn/CoFe bilayers have been investigated using broadband and in-plane angle-dependent ferromagnetic resonance spectroscopy. The interface energy of the exchange bias effect in these bilayers exceeds values previously reported for metallic antiferromagnets. A strong perpendicular magnetic anisotropy and a small in-plane uniaxial anisotropy are also observed in these films. The magnetization relaxation of the bilayers has a strong unidirectional contribution, which is in part caused by two-magnon scattering. However, a detailed analysis of in-plane angle-- and thickness-dependent linewidth data strongly suggests the presence of a previously undescribed unidirectional relaxation mechanism.

We report our progress on material improvement, device design, wafer processing, integration with CMOS, and testing of STT-RAM memory chips at 54 nm node with cell sizes of 14 and 28 F2 (F=54 nm). A dual tunnel barrier MTJ structure was found to have lower and more symmetric median spin transfer torque writing switching currents, and much tighter parallel to antiparallel switching current distribution. In-plane MTJ devices write endurance data, read and write soft error rates data and simulation fits, and solutions to the long write error rate tail at fast write speeds are discussed.

Recently, there has been a growing interest in the effects of electrostatic discharge (ESD) failure on tunneling magnetoresistive (TMR) recording heads because it directly affects reliability in manufacturing of these heads. Therefore, we study the magnetic degradation in TMR junctions caused by the temperature increase using three different ESD models. A 3-D finite-element method is used for analyzing the spatial and temporal profiles of the temperature during the discharge. The results from the three models show that, although the highest temperature occurs in the MgO barrier layer, the initial magnetic modification likely arises in the IrMn antiferromagnetic layer due to its low Neel temperature. We also found that the increase in temperature is proportional to the square of the ESD voltage. The magnetic instability of the antiferromagnetic layer due to the ESD effect is the important parameter realized for development of the future TMR devices.

MRAM relevant to current induced magnetization switching (CIMS) is studied due to thermal increment caused by CIMS. In this paper, the instability of storage and the thermal increment caused by the transient current from the HBM ESD in nanopillars of MRAM are studied. We determine the voltage which can cause erroneous switching in MRAM by inducing CIMS. The finite element method is used to calculate the temperature increase caused by the discharge. Results indicate that this voltage is not sufficient to cause permanent physical or magnetic damage to MRAMs but only affects the reliability of the stored information.

The dependences of magnetic properties and effective damping parameter (αeff) on film-thickness (d) and growth temperature (Ts) for Fe69Co26Si5 alloy films are investigated. The saturation magnetization (Ms) are found to decrease from approximately 1,700 to 1,500 emu/cm3 with d for Ts = 230 °C. For 40 nm films, the magnetization remains constant over the entire range of Ts from 35 to 360 °C. The coercivity (Hc) values for Ts above 120 °C are approximately 20 Oe. Those films deposited at Ts =120 °C or higher are epitaxial with the relation of FeCoSi// MgO in the film plane. For the films deposited at Ts =230 °C, the αeff rapidly decreases, and then slightly increases with d. The minimum αeff value of 0.002 is obtained at Ts = 230 °C.The dependences of magnetic properties and effective damping parameter (αeff) on film-thickness (d) and growth temperature (Ts) for Fe69Co26Si5 alloy films are investigated. The saturation magnetization (Ms) are found to decrease from approximately 1,700 to 1,500 emu/cm3 with d for Ts = 230 °C. For 40 nm films, the magnetization remains constant over the entire range of Ts from 35 to 360 °C. The coercivity (Hc) values for Ts above 120 °C are approximately 20 Oe. Those films deposited at Ts =120 °C or higher are epitaxial with the relation of FeCoSi// MgO in the film plane. For the films deposited at Ts =230 °C, the αeff rapidly decreases, and then slightly increases with d. The minimum αeff value of 0.002 is obtained at Ts = 230 °C.

We report detailed investigations of a strong perpendicular magnetic anisotropy in Ni0.8Fe0.2 thin films originating from interfaces with adjacent Ru and SiO2 layers. The anisotropy is quantified using broadband ferromagnetic resonance measurements for three types of layered heterostructures, namely an asymmetric structure of SiO2/Ni0.8Fe0.2/Ru and symmetric structures of Ru/Ni0.8Fe0.2/Ru and SiO2/Ni0.8Fe0.2/SiO2. The results show a stronger perpendicular anisotropy at the Ni0.8Fe0.2/SiO2 interface than at the Ni0.8Fe0.2/Ru interface and a lower interfacial damping contribution for the Ni0.8Fe0.2/SiO2 interface compared to the Ni0.8Fe0.2/Ru interface.We report detailed investigations of a strong perpendicular magnetic anisotropy in Ni0.8Fe0.2 thin films originating from interfaces with adjacent Ru and SiO2 layers. The anisotropy is quantified using broadband ferromagnetic resonance measurements for three types of layered heterostructures, namely an asymmetric structure of SiO2/Ni0.8Fe0.2/Ru and symmetric structures of Ru/Ni0.8Fe0.2/Ru and SiO2/Ni0.8Fe0.2/SiO2. The results show a stronger perpendicular anisotropy at the Ni0.8Fe0.2/SiO2 interface than at the Ni0.8Fe0.2/Ru interface and a lower interfacial damping contribution for the Ni0.8Fe0.2/SiO2 interface compared to the Ni0.8Fe0.2/Ru interface.

For high frequency device applications, a systematic study of the soft magnetic properties and magnetization dynamics of (FeCo)-Al alloy thin films has been carried out. A low effective damping parameter αeff of 0.002 and a high saturation magnetization of about 1,800 emu/cc are obtained at y=0.2∼0.3 for (Fe1-yCoy)98Al2 alloy thin films deposited onto fused silica and MgO(100) at an ambient temperature during deposition. Those films are of the bcc structure with the orientation normal to the film plane. They possess a columnar structure, grown along the film normal. The column width is found to be about 20 nm for y=0.25. It is concluded that the (FeCo)-Al thin films with a damping parameter as low as 0.002 and high saturation magnetization of about 1,800 emu/cc have been successfully fabricated, and that they are potential for future high frequency device applications.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; The ability of the full Heusler alloy Co1.5Fe1.5Ge(001) (CFG) to be a Half-Metallic Magnetic (HMM) material is investigated. Epitaxial CFG(001) layers were grown by molecular beam epitaxy. The results obtained using electron diffraction, X-ray diffraction, and X-ray magnetic circular dichroism are consistent with the full Heusler structure. The pseudo-gap in the minority spin density of state typical in HMM is examined using spin-resolved photoemission. Interestingly, the spin polarization found to be negative at E-F in equimolar CoFe(001) is observed to shift to positive values when inserting Ge in CoFe. However, no pseudo-gap is found at the Fermi level, even if moderate magnetization and low Gilbert damping are observed as expected in HMM materials. Magneto-transport properties in MgO-based magnetic tunnel junctions using CFG electrodes are investigated via spin and symmetry resolved photoemission.

We model "soft" error rates for writing (WSER) and for reading (RSER) for perpendicular spin-torque memory devices by solving the Fokker-Planck equation for the probability distribution of the angle that the free layer magnetization makes with the normal to the plane of the film. We obtain: (1) an exact, closed form, analytical expression for the zero-temperature switching time as a function of initial angle; (2) an approximate analytical expression for the exponential decay of the WSER as a function of the time the current is applied; (3) comparison of the approximate analytical expression for the WSER to numerical solutions of the Fokker-Planck equation; (4) an approximate analytical expression for the linear increase in RSER with current applied for reading; (5) comparison of the approximate analytical formula for the RSER to the numerical solution of the Fokker-Planck equation; and (6) confirmation of the accuracy of the Fokker-Planck solutions by comparison with results of direct simulation using the single-macrospin Landau-Lifshitz-Gilbert (LLG) equations with a random fluctuating field in the short-time regime for which the latter is practical.