Low Switching Field Research Articles

Magnetoresistance and switching properties of magnetic tunnel junctions using amorphous CoSiB free layer

We have investigated the tunneling magnetoresistance (TMR) effect and switching characteristics for magnetic tunnel junctions (MTJs) with amorphous ferromagnetic Co 75Si 15B 10 free layer compared to MTJs with Co 75Fe 25, Ni 80Fe 20 and Co 70.5Fe 4.5Si 15B 10 free layers. Since amorphous CoSiB had a low saturation magnetization ( M s=470 emu/cm 3) and a modest anisotropy constant ( K u=1500 erg/cm 3), the MTJ with this free layer showed better switching characteristics, especially CoSiB-based MTJ exhibited much lower switching field ( H sw=3 Oe) and interlayer coupling field ( H int=10 Oe) than MTJs with the other free layers, but TMR ratio showed relatively low value (23%). In addition, the bias voltage dependence of TMR ratio in CoSiB-based double MTJ was improved compared to that of single MTJ. It was expected that CoSiB was a potential candidate material as the free layer for use in high-density magnetic random access memory (MRAM) with an MTJ element.

Characteristics of magnetic tunnel junctions comprising ferromagnetic amorphous NiFeSiB layers

Magnetic tunnel junctions (MTJs), which consisted of amorphous ferromagnetic Ni 16Fe 62Si 8B 14 free layers, were investigated. NiFeSiB has a lower saturation magnetization ( M s: 800 emu/cm 3) than Co 90Fe 10 and a higher anisotropy constant ( K u: 2700 erg/cm 3) than Ni 80Fe 20. By increasing the free layer thickness, the tunnel magnetoresistance (TMR) ratio of up to 41% was achieved and it exhibited a much lower switching field ( H sw) than the conventionally used CoFe free layer MTJ. Furthermore, by inserting a thin CoFe layer (1 nm) at the tunnel barrier/NiFeSiB interface, the TMR ratio and switching squareness were enhanced.

Interfacial mixing in double-barrier magnetic tunnel junctions with amorphous NiFeSiB layers

Double-barrier magnetic tunnel junctions (DMTJs) comprising Ta 45/Ru 9.5/IrMn 10/CoFe 7/AlO x /free layer (CoFe 4/NiFeSiB 2/CoFe 4, CoFe 10, or NiFeSiB 10)/AlO x /CoFe 7/IrMn 10/Ru 60 (nm) have been examined with an emphasis given on understanding the interfacial mixing effects. The DMTJ, consisted of NiFeSiB, shows low switching field and low bias voltage dependence because the amorphous NiFeSiB has lower M S (=800 emu/cm 3) and offers smoother interfaces than polycrystalline CoFe. An interesting feature observed in the CoFe/NiFeSiB/CoFe sandwich free layered DMTJ is the presence of a wavy MR transfer curve at high-resistance region. Because the polycrystalline CoFe usually grows into a columnar structure, diamagnetic CoSi, paramagnetic FeSi, and/or diamagnetic CoB might have been formed during the sputter-deposition process. By employing electron energy loss spectrometry (EELS) and Auger electron spectroscopy (AES), we were able to confirm that Si and B atoms were arranged evenly in the top and bottom portions of AlO x /CoFe interfaces. This means that the interfacial mixing resulted in a distorted magnetization reversal process.

Micromagnetics of Percolated Perpendicular Medium for 1 Tb/in$^2$and Beyond

A fully exchange coupled perpendicular medium with densely distributed nonmagnetic pinning sites has been proposed. In the novel microstructure configuration, the dense domain wall pinning sites enable the perpendicular medium to obtain narrow transitions and small transition noise. Using the micromagnetic modeling, the macromagnetic properties and recording performances of the novel medium have been investigated. The results show that the medium transition noise can be optimized with a moderate exchange coupling constant. The dynamic recording processes are studied with different damping constants and recording field amplitudes. Our study indicates that the switching time of the percolated medium is smaller than 1 ns even with low switching field and small damping constant.

Thermal robustness in synthetic antiferromagnetic free layer for magnetic random access memory applications

We have investigated the magnetic switching properties of synthetic antiferromagnetic (SyAF) structures consisting of Co90Fe10∕Ru∕Co90Re10 structure with a Ru capping layer annealed at 300°C. After annealing, the coercivity Hc of the structures with a Co90Fe10 thickness below 5nm increased. On the other hand, the Hc was maintained below 70Oe in structures with a Co90Fe10 thickness above 6nm after annealing. Thus, a low switching field for a free layer can be achieved by using the SyAF structures with a Ru capping layer, even after annealing at 300°C, and the SyAF free layer is considered to be effective for downsized memory cells in magnetic random access memories.

Magnetoresistance and magnetization switching characteristics of magnetic tunnel junctions with amorphous CoFeSiB single and synthetic antiferromagnet free layers

To obtain low switching field (Hsw) we introduced amorphous ferromagnetic Co70.5Fe4.5Si15B10 single and synthetic antiferromagnet (SAF) free layers in magnetic tunnel junctions (MTJs). The switching characteristics for MTJs with structures Si∕SiO2∕Ta 45/Ru 9.5/IrMn 10/CoFe 7∕AlOx∕CoFeSiB 7 or CoFeSiB (t)∕Ru 1.0/CoFeSiB (7−t)∕Ru 60 (in nanometer) were investigated and compared to MTJs with Co75Fe25 and Ni80Fe20 free layers. CoFeSiB showed a lower saturation magnetization of 560emu∕cm3 and a higher anisotropy constant of 2800erg∕cm3 than CoFe and NiFe, respectively. An exchange coupling energy (Jex) of −0.003erg∕cm2 was observed by inserting a 1.0nm Ru layer in between CoFeSiB layers. In the CoFeSiB single and SAF free layer MTJs, it was found that the size dependence of the Hsw originated from the lower Jex experimentally and by micromagnetic simulation based on the Landau-Lisfschitz-Gilbert equation. The CoFeSiB SAF structures showed lower Hsw than that of NiFe, CoFe, and CoFeSiB single structures. The CoFeSiB SAF structures were proved to be beneficial for the switching characteristics such as reducing the coercivity and increasing the sensitivity in micrometer- to submicrometer-sized elements.

A Novel Magnetic Tunneling Junction Shaped Cell with Large Write Operation Margin for High-Density Magnetoresistive Random Access Memory

Magnetoresistive random access memory (MRAM) is one of several candidates for low-power and high-density system-on-chip (SoC) memory. In this paper, we propose a novel magnetic tunneling junction (MTJ) shape, called the “sandglass” (or “hourglass”), that has both an excellent asteroid curve for preventing write disturbance and a minimum memory of 8F2, where F is the feature size. A unique magnetic switching mechanism that works under decreasing magnetic switching field was compared with that of conventional MTJs simulated using the Landau–Lifshitz–Gilbert (LLG) simulator. We fabricated both a sandglass MTJ of 260×420 nm2 and a conventional elliptic MTJ of 200×400 nm2. The excellent asteroid curve of the sandglass MTJ is confirmed which exhibits both a larger write operation margin and a 50% lower switching field than those of the conventional elliptic MTJ. The sandglass MTJ cell is a promising candidate for realizing high-density MRAM.

비정질 CoFeSiB 단일 및 합성형 반강자성 자유층을 갖는 자기터널접합의 자기저항 효과

To obtain low switching field () we introduced amorphous ferromagnetic single and synthetic antiferromagnet (SAF) free layers in magnetic tunnel junctions (MTJs). The switching characteristics for MTJs with structures 45/Ru 9.5/IrMn 10/CoFe 7/AlOx/CoFeSiB 7 or CoFeSiB (t)/Ru 1.0/CoFeSiB (7-t)/Ru 60 (in nm) were investigated and compared to MTJs with and free layers. CoFeSiB showed a lower saturation magnetization of and a higher anisotropy constant of than CoFe and NiFe, respectively. An exchange coupling energy () of was observed by inserting a 1.0 nm Ru layer in between CoFeSiB layers. In the CoFeSiB single and SAF free layer MTJs, it was frond that the size dependence of the originated from the lower experimentally and by micromagnetic simulation based on the Landau-Lisfschitz-Gilbert equation. The CoFeSiB SAF structures showed lower than that of NiFe, CoFe and CoFeSiB single structures. The CoFeSiB SAF structures were proved to be beneficial far the switching characteristics such as reducing the coercivity and increasing the sensitivity in micrometer to submicrometer-sized elements.

Novel spin lattice in Cu3TeO6: an antiferromagnetic order and domain dynamics

We report on the magnetic properties of an insulating cubic compoundCu3TeO6 studied by ac and dc susceptibility, torque magnetometry and neutron powder diffraction.A novel three-dimensional magnetic lattice composed of almost planar regular hexagons ofCu2+ S = 1/2 spins ispresent in Cu3TeO6. The magnetic susceptibility in the paramagnetic state obeys the Curie–Weiss law in the 200–330 K regimewith ΘCW = −148 Kand at TN = 61 K system undergoes an antiferromagnetic phase transition. AboveTN the susceptibility isisotropic. Below TN a largeanisotropy develops in fields H≥500 Oe. Torque measurements reveal the presence of antiferromagnetic domains belowTN. In a rather low magnetic field ( Oe) switching of domains is observed. The dynamics related to movementof domain walls is very slow at low temperatures (of the order of102 s) and interferes with all torque measurements. The presence of domains is a consequenceof the symmetry of the underlying magnetic lattice. Neutron powder diffraction revealsthat antiferromagnetic long-range order is associated with the wavevector . The dominant component of the magnetic moment is along one of the space diagonals of the cubic unit cell, but it is not possible to resolve whether thestructure is collinear or canted.

Magnetization switching and tunneling magnetoresistance effects of synthetic antiferromagnet free layers consisting of amorphous NiFeSiB

A synthetic antiferromagnet (SAF) structure comprising of ferromagnetic amorphous Ni16Fe62Si8B14 layers has been devised and employed as a free layer of magnetic tunnel junctions (MTJs) to enhance cell switching performance. We observed −0.03erg∕cm2 of exchange coupling energy (Jex) by inserting a 0.5 nm Ru layer in between NiFeSiB layers. In Si∕SiO2∕Ta 45∕Ru 9.5∕IrMn 10∕CoFe 7∕AlOx1.5∕(single NiFeSiB 7) or [NiFeSiB(t)∕Ru 0.5∕NiFeSiB(7−t)]∕Ru 60(nm) MTJ structures, we found size dependence of the switching field originating from the lower Jex both experimentally and by simulation. The NiFeSiB SAF structure showed lower switching field than traditional CoFe and CoFeB SAF structures. This is because NiFeSiB possesses low saturation magnetization (Ms=800emu∕cm3) and high anisotropy constant (Ku=2,700erg∕cm3). These properties were proven beneficial for the switching characteristics such as reducing the coercivity (Hc) and increasing the sensitivity in micrometer to submicrometer sized elements.

A large low-field tunneling magnetoresistance of CrO2/(CrO2∕Cr2O3) powder compact with two coercivities

Two channels of the conductance G exist in cold-pressed powder compacts of CrO2/(CrO2∕Cr2O3), where two types of granules have different coercivities. One of the channels is the spin-dependent intergranular tunneling conductance, and the other one is the spin-independent higher-order inelastic hopping conductance. The conductance is mainly related with the spin-dependent tunneling channel at low temperature. A large low field tunneling magnetoresistance (TMR) of 30.4% is achieved in 950 Oe field at 2 K. The reason is the relative orientation of the magnetization that tends to be antiparallel between the two coercivities. Field dependence of the tunneling magnetoresistance shows two better-separated peaks than that of CrO2 powder compact. The results suggest that the introduction of CrO2/Cr2O3 improves low field sensitivity, TMR, and switching characteristics.

Nanomagnetic Ni-Array in Porous Silicon as a Possible Magnetic Field Sensor in the High Field Range up to 7 T

AbstractA highly doped n-type silicon wafer is anodized in an aqueous hydrofluoric acid solution to generate a porous silicon skeleton with a convenient morphology. After drying, the mesoporous sample is exposed to an electrodeposition process in which the pores are filled with the ferromagnetic metal Ni. Anodization and deposition of Ni lead to a ferromagnetic nanoscopic system which shows an interesting behaviour in the high field range (> 3 T). In addition to the expected low field switching below 500 Oe a second switching at fields of a few Tesla with a steep slope is also present. This feature with its extremely high sensitivity for changes of the external magnetic field gives rise to high magnetic field sensor applications based on a silicon technology.

Pathways of structural and magnetic transition in ferromagnetic shape-memory alloys

A fundamental question in the study of ferromagnetic shape-memory alloys is, what is the nature of the magneto-elastic coupling in these alloys and to what extent does it drive the structural transformation? This question also holds the key to developing new and optimized alloys that combine high strains at low switching field. In the present study, it is shown that the reconfiguration of the micromagnetic structure is completely enslaved to and follows the martensitic structural transformation in these alloys, using Ni--Mn--Ga and Fe--Pd systems. This is determined by developing a new high-speed electronic method to study temperature-dependent domain dynamics, called ``magnetic transition spectra.'' The sequence of structural and magnetic transitions was found to be as follows: Cooling: structural transition followed by micromagnetic reconfiguration; Heating: micromagnetic reconfiguration followed by structural transition.

Multistable switching due to magnetic vortices pinned at artificial pinning sites

Magnetic vortices form the ground state in micron and submicron ferromagnetic disks. By inserting artificial defects (antidots) into a submicron ferromagnetic disk, magnetic vortices can be pinned controllably thus enabling a different way for magnetic switching. We show that by inserting n antidots into a disk magnetization reversal takes place via n-1 jumps of the vortex core between neighboring antidots. This cannot only be used to establish stable two-state switching for n=2, but also to realize a multilevel remanent state with low switching fields.

Exchange-assisted nonlinear bit shift reduction at high linear densities in synthetic ferrimagnetic media

Synthetic ferrimagnetic media comprised of two magnetic layers tend to exhibit larger nonlinear transition shift (NLTS) compared to single magnetic layer conventional media. NLTS is reduced by increasing the antiferromagnetic exchange field (Hex) and especially by employing a lower layer with high magnetization and thickness product (Mst). Due to the lower switching field of the stabilization layer, the position of the transition during writing is farther downstream than that of the upper layer. We suggest that, at high densities, the pinning of the lower layer transition below the nearest neighboring upper layer transition leads to a reduction of demagnetizing fields and therefore NLTS.

High Thermal Stable MRAM With a Synthetic Ferrimagnetic Pinned Layer

Magnetic random access memory (MRAM) with a synthetic ferrimagnetic (SF) pinned layer has been investigated experimentally and theoretically. The SF pinned layer offers the higher thermal stability due to its even higher anisotropy and larger total thickness. A smaller aspect ratio cell with single domain state, less switching field dependence on the cell size, and lower switching field can be achieved in the SF structure than that in the conventional structure because of the thinner effective thickness. The experimental results show that high heat tolerance can also be achieved in the SF MRAM structure.

Interfacial composition and microstructure of Fe/sub 3/O/sub 4/ magnetic tunnel junctions

Magnetic tunnel junctions with an Fe/sub 3/O/sub 4/ top electrode have been fabricated. High resolution transmission electron microscopy (HRTEM) shows that in as-deposited state, Fe and FeO phases exist at the interface. These two phases are believed to have formed as a result of the unstable plasma condition at the start of the Fe/sub 3/O/sub 4/ deposition. A relatively low magnetoresistance ratio (MR) (3.5%) and low switching field (H/sub C2/) (40 Oe) is observed which is associated with the fact that the FeO phase magnetically isolates the Fe phase from the Fe/sub 3/O/sub 4/ phase at the interface. After annealing at 150/spl deg/C for 5 h, the MR as well as the switching field (H/sub C2/) increases by a factor of two. HRTEM shows that the FeO phase at the interface has transformed into Fe and Fe/sub 3/O/sub 4/ in the annealed sample, resulting in the increased MR (7.0%) and higher switching field (H/sub C2/) (300 Oe). However, annealing does not completely remove Fe at the interface, thereby limiting the MR.

Controlled switching of ferromagnetic wire junctions by domain wall injection

The switching of submicrometer ferromagnetic wire junctions is investigated by controlled injection of domain walls (DWs). A three-terminal continuous Ni/sub 80/Fe/sub 20/ structure is described, consisting of two input wires and one output wire. Separate structures were fabricated by focused ion beam (FIB) milling, to inject either zero, one, or two DWs to the junction inputs. Introduction of DWs to the junction was performed using one or two DW injection pads with low switching fields. Hysteresis loops measured by magnetooptical Kerr effect (MOKE) magnetometry on the device output wires showed the coercivity of the output is strongly dependent on the number of DWs incident at the junction. By injecting either one or two DWs into the junction, it is possible to switch the output wire at two distinct field values, each markedly lower than the nucleation field of the junction. Results presented are relevant to the future development of spintronics DW logic systems, and for fundamental studies into DW resistance and interaction between DWs and wire morphology.

Domain switching characteristics of the near stoichiometric LiNbO3 doped with MgO

MgO-doped stoichiometric LiNbO3 crystal has been produced using the Czochralski method and vapor transport equilibration technique. The domain reversal characteristics of the near stoichiometric single crystals with different Mg doping levels have been investigated. The switching field required for 180° ferroelectric domain reversal was compared with the near stoichiometric and congruent crystals. The near stoichiometric crystal doped with MgO shows lower switching field than the near stoichiometric and the congruent crystals. The switching field for the domain reversal in the near stoichiometric crystal doped with 2.0 mol % MgO was 1.8±0.2 KV/mm. This is about one third of the switching field for the near stoichiometric crystal, and one tenth of the switching field for the congruent crystal.

Switchable Holographic Polymer-Dispersed Liquid Crystal Reflection Gratings Based on Thiol−Ene Photopolymerization

Holographic reflection gratings in polymer-dispersed liquid crystals (H-PDLCs) were formed by thiol−ene photopolymerization. Using UV laser light and a single prism, electrically switchable reflection gratings in blue, green, yellow, and red colors were fabricated. Results indicate that thiol−ene polymers function as better hosts for H-PDLC than multifunctional acrylate as matrixes. These differences are the result of a much different temporal structure development caused by fundamental differences in the polymerization propagation mechanism: a step-growth addition mechanism for the thiol−ene system compared to a chain-growth addition mechanism in multifunctional acrylates. Morphology studies by TEM support these conclusions, as striking differences in droplet shape and uniformity are observed. Discrete nematic droplets with a nearly spherical shape were seen. Thiol−ene polymers offer lower switching fields, higher diffraction efficiencies, better optical properties, and higher thermal stabilities. The r...