Interlayer Exchange Coupling Energy Research Articles

TaFeB spacer for soft magnetic composite free layer in CoFeB/MgO/CoFeB-based magnetic tunnel junction

CoFeB/MgO/CoFeB-based magnetic tunnel junctions (MTJs) with a soft magnetic composite free layer have been developed for magnetic sensor applications. Tunnel magnetoresistance (TMR) ratios in the sensor-type MTJs have reached a ceiling due to a trade-off between the TMR ratio and interlayer exchange coupling (IEC) depending on the spacer thickness of the composite free layer. In this study, we developed a paramagnetic amorphous TaFeB-alloy spacer to replace the conventional Ta spacer and solve this trade-off. The TaFeB film showed a wider thickness window for a sufficient IEC, resulting in IEC energy values of 0.18–0.19 erg/cm2 at a thickness of 1.0 nm. In addition, we confirmed that the TaFeB film had an ability to function as a boron sink comparable to that of pure Ta. These characteristics allowed us to thicken the TaFeB spacer up to 1.0 nm in the sensor-type MTJs and attain an enhanced TMR ratio of up to 234%, which is the highest compared with cases using the conventional Ta spacer reported to date. These findings demonstrate that TaFeB alloy is a promising material for breaking the ceiling of sensor-type MTJs and increasing sensitivity.

Sputter Gas Damage in Nanolayered Pt/Co/Ir-based Synthetic Antiferromagnets for Top-Pinned Magnetic Tunnel Junctions

We examine the role of sputter damage on limiting the perpendicular magnetic anisotropy and interlayer exchange coupling in Pt/Co/Ir-based synthetic antiferromagnets, a key building block used in the microelectronics community for standalone and embedded magnetic random access memories. By replacing the more conventional Ar process gas with Kr or Xe, we reduce the energy of backscattered sputter gas ions bombarding the substrate from in excess of 100 eV to a few 10 s of eV. Reducing this kinetic bombardment is critical to avoid intermixing of this nanolayered functional heterostructure, with the constituent Pt, Co, and Ir layer thicknesses each below 1 nm. Our approach leads to a simultaneous enhancement in the perpendicular magnetic anisotropy (>2 × 106 J/m3) and interlayer exchange coupling energy (>3.5 mJ/m2). This advantageous improvement in perpendicular anisotropy and interlayer exchange coupling is evident on a 5 nm-thick Pt buffer layer and additionally on an MgO/CoFeB/Mo underlayers complex, showing that this method can improve the performance of both top- and bottom-pinned reference layers in perpendicular magnetic tunnel junction devices. These experimental findings may help advance the processing of conventional and synthetic antiferromagnetic devices for emerging spintronic memory technologies.

Temperature-dependent magnetic transitions in CoCrPt-Ru-CoCrPt synthetic ferrimagnets

The magnetic orientations and switching fields of a CoCrPt-Ru-CoCrPt synthetic ferrimagnet with perpendicular magnetic anisotropy have been studied in the temperature range from 2 K to 300 K. It was found that two sets of magnetic transitions occur in the CoCrPt-Ru-CoCrPt ferrimagnet across this temperature range. The first set exhibits three magnetic transitions in the 50 K–370 K range, whereas the second involves only two transitions in the 2 K and 50 K range. The observed magnetic hysteresis curves of the synthetic ferrimagnet are explained using the energy diagram technique framework pioneered by Koplak et al. [1] which accurately describes the competition between interlayer exchange coupling energy, Zeeman energy, and anisotropy energy in the system. In this work we expand the framework to include synthetic ferrimagnets (SFMs) comprising higher perpendicular magnetic anisotropy materials and large (4X) interlayer exchange coupling energies which are promising for the development of ultrafast (ps) magnetic switching free layers in MTJ structures. Furthermore, we apply the analysis to predict SFM magnetic hysteresis curves in a temperature regime that includes temperature extrema that a synthetic ferrimagnet would be expected to reliably operate at, were it to be utilized as a free layer in a memory or sensor spintronic device.

Very strong antiferromagnetic interlayer exchange coupling with iridium spacer layer for perpendicular magnetic tunnel junctions

We systematically studied the interlayer exchange coupling (IEC) in a perpendicular synthetic antiferromagnetically coupled structure having an Ir spacer layer for perpendicular magnetic tunnel junctions (p-MTJs). We found a broader peak in IEC energy density (Jex) versus spacer thickness (tIr) compared with the case of using a Ru spacer. The highest IEC energy density was 2.6 erg/cm2 at a tIr of about 5 nm. The p-MTJ nanopillars had a high magnetoresistance ratio (131%) as well as a high spin-transfer torque (STT) switching efficiency (about 2). An Ir spacer can be used to make a stable reference layer for STT magnetoresistive random access memory.

Correlation between interlayer exchange coupling and hydrogen absorption in V-Fe layered structures

Abstract 4 nm – Fe/d V -V/4 nm – Fe trilayers were prepared at room temperature using UHV (5 × 10 − 10 mbar) magnetron sputtering. A capping layer of 5 nm Pd was used to allow a fast uptake and release of hydrogen and to avoid oxidation of the top Fe sublayers. As a substrate we have used Si(100) wafers with an oxidised surface. The cleanness and planar growth of all layers was checked in-situ , immediately after deposition by X-ray Photoelectron Spectroscopy. Planar growth was also revealed in ex-situ studies by Atomic Force Microscopy and Scanning Tunnelling Microscopy. In the case of the trilayers the bottom and the top Fe sublayers were deposited in different deposition conditions to obtain relatively high difference in their coercive fields. Interlayer exchange coupling across vanadium was studied for “as prepared”, hydrogenated and dehydrogenated samples using minor hysteresis loop measurements. We have observed the strongest antiferromagnetic (AFM) interlayer exchange coupling energy of about 0.023 mJ/m 2 for hydrogenated sample. Results from the minor hysteresis loop measurements as a function of vanadium layer thickness for “as prepared” or dehydrogenated samples show three AFM peaks near d V ~ 1.95, 2.85, and 3.9 nm without transition to pure ferromagnetic coupling. Hydrogenation of the trilayers changes reversibly the interlayer exchange coupling strength and position of the 2nd and 3rd AFM peak.

Perpendicular magnetic tunnel junctions with strong antiferromagnetic interlayer exchange coupling at first oscillation peak

We systematically developed a perpendicularly magnetized synthetic antiferromagnetically (p-SAF) coupled reference structure with strong interlayer exchange coupling (IEC) at the “first peak”, where the thickness of the Ru layer between the Co/Pt superlattice layers was 0.4–0.5 nm. A high IEC energy density (2.2 erg/cm2) and a high magnetoresistance ratio (110–150%) with an ultralow resistance-area product (2–5 Ω·µm2) were simultaneously achieved in the integrated magnetic tunnel junctions of the p-SAF structure. The use of p-SAF coupling with strong IEC is attractive because of its ability to produce highly stable and unidirectional reference layers for memory applications.

Effect of Ta Insertion in Reference Layers of MTJs With Perpendicular Anisotropy

We analyze the opportunity of inserting a Ta layer between the polarizing section and the high anisotropy section in the reference subsystem of CoFeB-based tunnel junctions with perpendicular magnetic anisotropy. Using vector network analyzer ferromagnetic resonance, polar Kerr magnetometry, and magnetization dynamics modeling, we deduce the strength of the ferromagnetic interlayer exchange coupling energy J through various thickness of tantalum that impacts on the overall performance of the tunnel junctions. J culminates at 0.44 mJ/m 2 through 3 A of Ta, but the Co/Pt properties are then suboptimal. Our methodology can be used to rationally find the performance optimum in the reference layers of perpendicularly magnetized tunnel junctions.

Effect of the synthetic antiferromagnetic polarizer layer rigidness on the spin transfer torque switching current density

We investigate the effect of the synthetic antiferromagnetic polarizer layer rigidness on the switching current density of spin transfer torque magnetic tunneling junction with micromagnetic simulations. In contrary to the general belief, surprisingly, the rigidness of the polarizer layer affects to the spin dynamics of the free layer and it causes the switching current density changes. When the rigidness of the synthetic antiferromagnetic polarizer layer is weakened by reducing the interlayer exchange coupling energy, we found that the switching current density can be reduced due to the more effective switching process.

Domain size and structure in exchange coupled [Co/Pt]/NiO/[Co/Pt] multilayers

We investigate the competing effects of interlayer exchange coupling and magnetostatic coupling in the magnetic heterostructure ([Co/Pt]/NiO/[Co/Pt]) with perpendicular magnetic anisotropy (PMA). This particular heterostructure is unique among coupled materials with PMA in directly exhibiting both ferromagnetic and antiferromagnetic coupling, oscillating between the two as a function of spacer layer thickness. By systematically tuning the coupling interactions via a wedge-shaped NiO spacer layer, we explore the energetics that dictate magnetic domain formation using high resolution magnetic force microscopy coupled with the magneto-optical Kerr effect. This technique probes the microscopic and macroscopic magnetic behavior as a continuous function of thickness and the interlayer exchange coupling, including the regions where interlayer coupling goes through zero. We see significant changes in domain structure based on the sign of coupling, and also show that magnetic domain size is directly related to the magnitude of the interlayer exchange coupling energy, which generally dominates over the magnetostatic interactions. When magnetostatic interactions become comparable to the interlayer exchange coupling, a delicate interplay between the differing energy contributions is apparent and energy scales are extracted. The results are of intense interest to the magnetic recording industry and also illustrate a relatively new avenue of undiscovered physics, primarily dealing with the delicate balance of energies in the formation of magnetic domains for coupled systems with PMA, defining limits on domain size as well as the interplay between roughness, domains and magnetic coupling.

Ultrathin magnetic multilayer films for low-field microwave notch filters

Microwave filters that use thin films of ferromagnetic metals are now being established as a valuable option compared to yttrium iron garnet based filters due to their higher frequency response. In these filters the signal propagation is inhibited over a wide frequency band, depending on the applied dc magnetic field. However, the continuous application of an applied field to achieve an operating frequency in the higher gigahertz range increases the power consumption of the device. The main contribution of this article is to provide techniques which significantly boost the operating frequency of notch filters in zero or very low applied magnetic fields. To do this, the authors fabricated high quality epitaxial Fe films which are interlayer exchange coupled through nonmagnetic Si layer of different thicknesses. The films were used in flip-chip geometry on top of a Cu-coplanar waveguide to create band-stop filters. In contrast to filters based on Fe alone, the multilayer filters can operate above 25GHz with a very small applied magnetic field. The observed upshift in frequency is attributed to the induced interlayer exchange coupling energy mediated through the nonmagnetic Si layer between the two Fe layers. These frequency shifts are in good agreement with theoretical calculations of the ferromagnetic resonance modes taking into account anisotropy, exchange, and Zeeman energies.

조절 가능한 층간교환상호작용에 관한 연구

We theoretically demonstrate that the interlayer exchange coupling (IEC) energy can be manipulated by means of an external bias voltage in a four-layer system. It is well known that the IEC energy between two ferromagnetic layers separated by nanometer thick nonmagnetic layer depends on the spin-dependence of reflectivity to the four-layer system, where the reflectivities at the interface in interface also depends on interface due to the multiple reflection of an electron-like optics. Finally, the IEC energy depends on the spin-dependent electron reflectivity not only at the interfaces of , but also at the interface of . Naturally the Schottky barrier is formed at the interface between metallic ferromagnetic layer and semiconductor, the Schottky barrier height and thickness can be tailored by an external bias voltage, which causes the change of the spin-dependent reflectivity at interface. We show that the IEC energy between two ferromagnetic layers can be controlled by an external bias voltage due ti the electron-optics nature using a simple free-electron-like one-dimensional model.

Tunable interlayer exchange coupling energy by modification of Schottky barrier potentials

We theoretically demonstrate that the interlayer exchange coupling (IEC) energy can be manipulated by means of an external bias voltage in a F 1/NM/F 2/S (F 1: ferromagnetic, NM: nonmagnetic metallic, F 2: ferromagnetic, S: semiconductor layers) four-layer system. It is well known that the IEC energy between two ferromagnetic layers separated by nanometer thick nonmagnetic layer depends on the spin-dependent electron reflectivities at the interface in F 1/NM/F 2 trilayer system. We apply such dependence to the F 1/NM/F 2/S four-layer system, where the reflectivity of NM/F 2 interface also depends on F 2/S interface due to the multiple reflection of an electron like optics. Finally, the IEC energy depends on the spin-dependent electron reflectivity not only at the interfaces of F 1/NM/F 2, but also at the interface of F 2/S. Naturally the Schottky barrier is formed at the interface between metallic ferromagnetic layer and semiconductor, the Schottky barrier height and thickness can be tailored by an external bias voltage, which causes the change of the spin-dependent reflectivity at F 2/S interface. We show that the IEC energy between two ferromagnetic layers can be controlled by an external bias voltage due to the electron-optics nature using a simple free-electron-like one-dimensional model.

Ab initiodetermination of Kerr angles inCu4Ni8CunNi9∕Cu(100)(n=2–10)

Calculated polar magneto-optic Kerr rotation and ellipticity angles are presented for the system ${\mathrm{Cu}}_{4}{\mathrm{Ni}}_{8}{\mathrm{Cu}}_{n}{\mathrm{Ni}}_{9}∕\mathrm{Cu}(100)$, $n=2,\dots{},10$. Contrary to the common belief, the Kerr signals are found to be not directly proportional to the total magnetic moment. It will be shown that in order to assign at least indirectly the size (and sign) of the Kerr angles and therefore the type of coupling to a kind of total magnetic moment, one has to consider weighted layer-resolved moments, e.g., weighted by the damping factors of the reflected waves. As to be expected the occurring oscillations in the Kerr angles with respect to the spacer thickness resemble closely those for the interlayer exchange coupling energy.

Antiferromagnetic interlayer exchange coupling in all-semiconductingEuS∕PbS∕EuStrilayers

A comprehensive experimental study on the antiferromagnetic interlayer exchange coupling in high quality epitaxial all-semiconducting $\mathrm{EuS}∕\mathrm{PbS}∕\mathrm{EuS}$ trilayers is reported. The influence of substrates, of the thickness of the nonmagnetic $\mathrm{PbS}$ spacer layer, and of temperature was investigated by means of SQUID magnetometry. In trilayers with a $\mathrm{PbS}$ thickness between 4 and $12\phantom{\rule{0.3em}{0ex}}\AA{}$ the low temperature hysteresis loops showed the signature of antiferromagnetic coupling. The value of the interlayer exchange coupling energy was determined by simulating the data based on a Stoner-Wohlfarth model. An important observation was that the interlayer exchange coupling energy varies strongly with temperature, consistent with a power-law dependence of the exchange coupling constant on the saturation magnetization of the $\mathrm{EuS}$ layers. While no theoretical description is readily available, we conjecture that the observed behavior is due to a dependence of the interlayer exchange coupling energy on the exchange splitting of the $\mathrm{EuS}$ bands.