Large Switching Field Research Articles

2D MoS2 under switching field conditions: Study of high-frequency noise velocity fluctuations

The transient high-frequency noise response of two-dimensional MoS2 under abrupt large signal switching field conditions is studied by means of an ensemble Monte Carlo simulator. Low-to-high and high-to-low transitions are analyzed at low (77 K) and room temperature, considering several underlying substrates. The incorporation of stochastic individual scattering events allows capturing the transient collective phonon–electron coupling, which is shown to be responsible for the appearance of an oscillatory behavior in the average velocity and energy at low temperature in the case of MoS2 on SiO2, hBN, and Al2O3. Activation and deactivation of surface polar phonon emissions in the low-to-high field switching process yield the appearance of a relevant peak in the power spectral density of velocity fluctuations in the THz range. The results show the important influence of the substrate type in the noise behavior of MoS2 at very high frequencies, which is critical for the design of future FET devices based on 2D TMD technology.

Influence of Elastic and Optical Properties on AgFeO2 and AgCrO2 Delafossite to be Applied in High-Frequency Applications

Nanometric AgFeO2 and AgCrO2 delafossite were easily prepared by the flash auto-combustion method. The two main bands estimated from FTIR (Fourier-transform infrared) analysis were the tetrahedral A-site (573 cm−1 for AgFeO2, 630 cm−1 for AgCrO2) and the octahedral B-site (484 cm−1 for AgFeO2, 595 cm−1 for AgCrO2). This study is mainly focused on the elastic properties evaluated from the FTIR analysis and showed that AgCrO2 delafossite is more elastic than AgFeO2 delafossite. The elastic properties can be explained by studying the longitudinal and transverse velocities. Owing to the optical properties results, AgCrO2 delafossite is a promising material to be applied in optical devices. However, AgFeO2 delafossite is a promising material in magnetic applications because it showed a large switching field distribution by 9-fold more than that of AgCrO2 delafossite. Moreover, high-frequency applications were calculated from the magnetic analysis and showed that both samples could be applied in ultra-high microwave applications.

Improved coercivity and squareness in bulk hot-deformed Nd–Fe–B magnets by two-step eutectic grain boundary diffusion process

We introduce a novel method to develop high coercivity with improved squareness in bulk hot-deformed Nd–Fe–B magnets with a thickness of 5.6 mm. A low-melting-point Tb20Dy10Nd40Cu30 eutectic diffusion source was infiltrated into the thick magnets through grain boundaries as the 1st step, followed by the 2nd step eutectic diffusion using Nd80Cu20. The 1st step led to the formation of rare earth (RE)-rich intergranular phase as well as heavy rare earth (HRE)-rich shell surrounding platelet-shaped Nd2Fe14B grains. A large concentration gradient toward the center of the thick magnet caused a large switching field distribution, which was the main limitation of this method after the first step and in the conventional infiltration process. The second step Nd-Cu diffusion makes the HRE-shell more uniform throughout the entire volume, which improved the squareness factor from 0.83 to 0.91 with a slight coercivity increase from 2.38 T to 2.43 T. Magneto-optical Kerr effect microscopy showed unlike the conventional infiltration processed magnet, strong pinning of the magnetic domain wall motion at the side grain boundaries occur even in the near center of the two step diffusion processed magnet.

Tailoring the magnetic properties of galfenol film grown on single-crystal diamond

The thin film properties are often compromised but can be tuned by optimizing the growth parameters. Galfenol (FeGa), a magnetostrictive material can be used in MEMS technology particularly due to its large magnetostriction coefficient, low saturation and switching fields. Hence the fabrication of high quality FeGa thin film is important. Recently, the growth of FeGa thin film on semiconductor substrate offer a promising platform for micromechanical sensor. Here, we report the successful growth and high magnetic properties of FeGa thin film on single crystal diamond substrate. In order to obtain FeGa thin film with high soft magnetic properties, the variations of the growth parameters including working pressure, Ar flow rate, sputtering power and growth duration are investigated. It is shown that the microstructure, surface morphology, and magnetic properties of the FeGa thin film can be tailored by the growth parameters. Crystallized FeGa thin films with surface roughness less than 1 nm, low coercivity (Hc) of 26.2 Oe, and low saturation magnetization field (Hs) of 450 Oe and high remanence ratio (Mr/Ms) of 0.9 are obtained. The integration of FeGa thin film on diamond promotes the development of device based on magneto-strictive film integrated with semiconductors.

Microwave-magnetic-field-induced magnetization excitation and assisted switching of antiferromagnetically coupled magnetic bilayer with perpendicular magnetization

Antiferromagnetically coupled (AFC) magnetic bilayer is a candidate media structure for high-density magnetic recording. Because the stray fields from the two magnetic layers of the AFC bilayer cancel each other out, switching field distribution originating from the stray fields from the adjacent data bits can be suppressed. Furthermore, in microwave-assisted magnetic recording (MAMR), which utilizes ferromagnetic resonance (FMR) excitation in a microwave field to reverse a high-anisotropy magnetic material, AFC media can suppress the distribution in FMR frequency originating from the stray fields and improve MAMR performance. In this study, we fabricate an AFC magnetic bilayer consisting of two Co/Pt multilayers with perpendicular magnetization. We use anomalous-Hall-effect-FMR in combination with a circularly polarized microwave field and carry out layer-selective analysis of FMR excitation of the two magnetic layers. We then investigate the switching behavior of an AFC bilayer nanodot in a microwave magnetic field. The switching field decreases with increasing microwave field frequency and increases abruptly at the critical frequency, and a large switching field reduction by applying a microwave field is demonstrated. This switching behavior is similar to that of a single-layer perpendicular magnetic nanodot, showing that the AFC structure does not hinder the microwave assist effect.

Ion Irradiation for Planar Patterning of Magnetic Materials

Kr+ ion dose dependence of the magnetic properties of MnGa films and the fabrication of planar-patterned MnGa films by the local ion irradiation technique were reviewed. The magnetization and perpendicular anisotropy of the MnGa vanished at an ion dose of 1 × 1014 ions/cm2 due to the phase change of the MnGa from ferromagnetic L10 to paramagnetic A1 phase. The average switching field Hsw of the planar-patterned MnGa increased with decreasing the bit size, implying low bit edge damage in the patterned MnGa, whereas a rather large switching field distribution (SFD) of 25% was confirmed for a bit size of ~40 nm. Time resolved magneto-optical Kerr effect measurements revealed that as-prepared MnGa exhibits an effective anisotropy field Hkeff = 20 kOe, its distribution ΔHkeff = 200 Oe, and Gilbert damping α = 0.008. The ion-irradiated MnGa films exhibited larger Hkeff = 22–23 kOe than that of the MnGa before the ion dose. Thus, ion irradiation does not decrease the perpendicular anisotropy, which suggests a small bit edge in the patterned MnGa. ΔHkeff increased from 0.2 kOe to 3 kOe, whereas the length of disorder in the film ξ decreased from 10 nm to 3 nm by ion irradiation.

Magnetic domain wall engineering in a nanoscale permalloy junction

Nanoscale magnetic junctions provide a useful approach to act as building blocks for magnetoresistive random access memories (MRAM), where one of the key issues is to control the magnetic domain configuration. Here, we study the domain structure and the magnetic switching in the Permalloy (Fe20Ni80) nanoscale magnetic junctions with different thicknesses by using micromagnetic simulations. It is found that both the 90-° and 45-° domain walls can be formed between the junctions and the wire arms depending on the thickness of the device. The magnetic switching fields show distinct thickness dependencies with a broad peak varying from 7 nm to 22 nm depending on the junction sizes, and the large magnetic switching fields favor the stability of the MRAM operation.

Subnanosecond microwave-assisted magnetization switching in a circularly polarized microwave magnetic field

We study microwave-assisted magnetization switching (MAS) of a perpendicularly magnetized nanomagnet with a diameter of 50 nm in a circularly polarized microwave magnetic field. The MAS effect appears when the rotation direction of the microwave field matches that of the ferromagnetic resonance excitation, and a large switching field decrease from 7.1 kOe to 1.5 kOe is demonstrated. In comparison with a linearly polarized microwave magnetic field, the circularly polarized microwave field induces the same MAS effect at half the microwave field amplitude, thereby showing its efficiency. We also examine MAS in the subnanosecond region and show that the magnetization switching can be induced by a microwave field with the duration of 0.2 ns.

Vortex-dynamics-mediated low-field magnetization switching in an exchange-coupled system

A magnetic vortex has attracted significant attention since it is a topologically stable magnetic structure in a soft magnetic nanodisk. Many studies have been devoted to understanding the nature of magnetic vortex in isolated systems. Here we show a new aspect of a magnetic vortex the dynamics of which strongly affects the magnetic structures of environment. We exploit a nanodot of an exchange-coupled bilayer with a soft magnetic Ni81Fe19 (permalloy; Py) having a magnetic vortex and a perpendicularly magnetized L10-FePt exhibiting a large switching field (Hsw). The vortex dynamics with azimuthal spin waves makes the excess energy accumulate in the Py, which triggers the reversed-domain nucleation in the L10-FePt at a low magnetic field. Our results shed light on the non-local mechanism of a reversed-domain nucleation, and provide with a route for efficient Hsw reduction that is needed for ultralow-power spintronic devices.

Study of [Co/Ni]N/ [Co/Pt]N-based spin valves with perpendicular magnetic anisotropy

A structure of Pt/ [Co/Ni]2/Au/ [Co/Pt]N with perpendicular magnetic anisotropy was prepared successfully by magnetron sputtering technique. The magnetic properties dependence of Au thicknesses and [Co/Pt] layer repetition number have been studied. The results show that when the [Co/Pt] layer repetition number is 4, the strongest coupling among the samples occurs when the Au layer is 3 nm for its largest Hall voltage and large switching field plateau. The Hall loop of the sample has obvious perpendicular magnetic anisotropy. The magnetoresistance of Pt/ [Co/Ni]2/Au(t)/ [Co/Pt]4 has vibration behaviour due to the Ruderman-Kittel-Kasuya-Yosida interaction with the increase of tAu. As the thickness of the Au spacer is determined to be 3 nm, the study of dependence of [Co/Pt] layer repetition number to the magnetic properties shows that the N has significant influence to the magnetoresistance of the multilayer films. When N is 5, the MR of the sample has the maximum value and the optimum sample is structured as Pt(4)/[Co(0.2)/Ni(0.4)]2/Co(0.2)/Au(3)/[Co(0.4)/Pt(2.0)]5/Pt(0.5) (unit:nm). It has obvious perpendicular magnetic anisotropy features and large switching field plateau and the integration with device can be studied furtherly.

Role of TbFe on Perpendicular Magnetic Anisotropy and Giant Magnetoresistance Effect in [Co/Ni]N-Based Spin Valves.

The exchange-coupled [Co/Ni]N/TbFe nano-magnetic films can display strong perpendicular magnetic anisotropy (PMA) which depends on the Tb:Fe component ratio, TbFe layer thickness and the repetition number N of [Co/Ni]N multilayer. Perpendicular spin valves in the nano thickness scale, consisting of a [Co/Ni]3 free and a [Co/Ni]5/TbFe reference multilayer, show high giant magnetoresistance (GMR) signal of 6.5 % and a large switching field difference over 3 kOe. However, unexpected slanting of the free layer magnetization, accompanied by a reduced GMR ratio, was found to be caused by the presence of a thick Fe-rich or even a thin but Tb-rich TbFe layer. We attribute this phenomenon to the large magnetostriction effect of TbFe which probably induces strong stress acting on the free layer and hence reduces its interfacial PMA.

5 Tdots/in2 bit patterned media fabricated by a directed self-assembly mask

FePt bit patterned media (BPM) was fabricated with a self-assembled polymer mask with a feature size of 12 nm pitch (equivalent to 5 Tdots/in2 ). A 3.5 nm FePt film with high c-axis crystal orientation was prepared for the magnetic recording layer. A solvent vapor annealing process was applied for obtaining uniform directed self-assembling of polystyrene (PS)-polydimethylsiloxane (PDMS) diblock copolymer pattern. Pattern transfer from a polymer mask to FePt layer was achieved by employing a carbon hard mask. In spite of excellent magnetic characteristics of FePt layer, the fabricated FePt BPM showed small coercivity (Hc) of 6 kOe and large switching field distribution (SFD) of 21%. These results are due to the etching damage of FePt dots. Disordering of FePt L10 phase by the etching damage reduced magnetic anisotropy energy (Ku). The damaged portion became a nucleus of the magnetization reversal and reduced Hc. Distribution of the damaged volume and the extent of the Ku reduction contributed to large SFD. This model is supported by the experimental data of magnetic field angle dependence of switching field. The result suggests the domain wall motion type of magnetization reversal mode, where the domain wall is created at the interface between the damaged portion and the internal high-Ku region.

Effects of grain microstructure on magnetic properties in FePtAg-C media for heat assisted magnetic recording

L10 FePt has been proposed as a magnetic medium suitable for heat assisted magnetic recording, owing to its high magnetocrystalline anisotropy and moderate Curie temperature. Carbon has previously been used as a segregant creating small magnetically decoupled grains by sputter deposition at high temperature. Here, we deposit granular, segregated FePt films having a high degree of chemical order, coercive fields of 4.8 T, high remanence, and average grain size of 7.2 nm with 17% size distribution. Magnetic characterization reveals a large intrinsic switching field distribution of 15 kOe. We incorporate the results obtained by transmission electron microscopy, magnetometry, and x-ray diffraction into a micromagnetic model to better understand the origin of the intrinsic switching field distribution. We find that the dominant contributions to the observed magnetic properties are (i) a grain-size dependent distribution in anisotropy field and (ii) small angle variations in perpendicular anisotropy axis orientation. The presence of small superparamagnetic grains, contributions originating from the grain size distribution, as well as from grains with longitudinal or randomly oriented anisotropy axis are found to play a relatively small role. The results help understand the observed magnetization loops used to evaluate FePt media.

Magnetization and magnetostriction of Terfenol-D near spin reorientation boundary

Magnetization and magnetostriction behaviors of quasi-binary TbxDy1−xFe2 alloys (Terfenol-D) near spin reorientation boundary (SRB) between 〈111〉 and 〈100〉 easy axes are studied by computer simulation of magnetic domain rotations under increasing magnetic field and compressive stress along [112] growth direction. The simulation results reveal coexistence of 〈111〉 and 〈100〉 domains near SRB and show that the switching field first decreases then increases when crossing the SRB from the 〈100〉 region into the 〈111〉 region of the quasi-binary phase diagram, while the magnetostriction first increases then saturates, thus providing an optimal combination of large magnetostriction and low switching field near the SRB as desired for actuator application. This finding has general implications for other magnetostrictive quasi-binary alloys that also exhibit spin reorientation phenomenon, allowing material design for improved magnetostrictive property near SRB, in analogy to morphotropic phase boundary ferroelectrics.

Co/Ni]N-Based Synthetic Antiferromagnet with Perpendicular Anisotropy and Its Application in Pseudo Spin Valves

The dependence of magnetic properties on layer repetition number and Ru thicknesses have been studied for perpendicularly magnetized synthetic antiferromagnet (SAF) in a structure of [Co/Ni] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> /Ru/[Co/Ni] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . The optimum SAF with strong antiferromangetic coupling field and large switching field of the net magnetization have been determined and utilized as the reference layer in the pseudo spin valves. Compared with the rapid drop of GMR signal for the normal [Co/Ni]-based pseudo spin valves after annealing at low temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) of 150°C, the spin valve with SAF reference layer exhibits much stable thermal stability due to the large switching field difference between the free and reference layers which avoids the simultaneous magnetization rotation. The GMR signal of the SAF spin valve sample is 6.0% at room temperature, it decreases very gradually with the increase of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> . We attribute the slow GMR reduction observed in the SAF spin valve to the effects of domain formation and perpendicular anisotropy deterioration caused by high temperature anneals.

Thermally induced switching field distribution of a single CoPt dot in a largearray

Magnetic dot arrays with perpendicular magnetic anisotropy were fabricated by patterningCo80Pt20-alloy continuous films by means of laser interference lithography. As commonly seen inlarge dot arrays, there is a large difference in the switching field between dots. Here weinvestigate the origin of this large switching field distribution, by using the anomalous Halleffect (AHE). The high sensitivity of the AHE permits us to measure the magnetic reversalof individual dots in an array of 80 dots with a diameter of 180 nm. By taking1000 hysteresis loops we reveal the thermally induced switching field distributionSFDT of individual dotsinside the array. The SFDT of the first and last switching dots were fitted to an Arrhenius model, and a clear differencein switching volume and magnetic anisotropy was observed between dots switching at lowand high fields.

Microstructural origin of switching field distribution in patterned Co∕Pd multilayer nanodots

We have identified an important microstructural origin of the switching field distribution (SFD) in patterned Co∕Pd multilayer nanodots. In this study, we patterned a marked array of 115nm Co∕Pd nanodots on 50-nm-thick Si3N4 substrate. We identified the dots with unusually small and large (&amp;gt;2 standard deviations of the mean) switching fields with magnetic force microscopy, followed by microstructural characterization of the same dots with transmission electron microscopy (TEM). From electron diffraction, we found that most nanodots with small switching fields have strong (200) spots, whereas those with large switching fields lack these spots. While bright-field TEM images reveal an average grain size of 7nm, dark-field images of the (200) spots reveal on average, a single grain of &amp;gt;10nm in lateral dimensions. Since we observed a direct correlation between strong (200) reflections and small reversal fields, we conclude that the largest grain in each nanodot, with an in-plane [001] is the likely cause for premature switching, which in turn defines the SFD of this array.

Pinning effect and thermal stability study in L1 FePt-pinned spin valves

L 1 0 -ordered FePt alloy is introduced into the synthetic antiferromagnetically coupled spin valves as an exchange pinning layer. Large switching field of 1824Oe for the reference layer and a high giant magnetoresistance (GMR) ratio of 7.0% are observed in such FePt-pinned spin valves, which are comparable to those in MnIr-pinned samples with similar structures. However, the GMR ratio in FePt-pinned spin valve exhibits a poor thermal stability. The rough surface of the FePt layer causes the atoms to interdiffuse easily at lower temperature, resulting in some FePt formation in the CoFe reference layer. This magnetic isotropic FePt alloy with (111) texture could degrade the uniaxial anisotropy of the reference layer, leading to the reduction of GMR ratio.

Influence of Pinned-Layer Dispersion on Magnetic Tunnel Junction Switching Distributions

The influence of pinning field dispersion (PFD) on the coercivity (Hc) and offset field (Ho) of IrMn/NiFeCo/AlOx/NiFeCo magnetic tunnel junctions (MTJ) has been studied by repeated in situ measurement of the resistance as a function of magnetic field, R(H), hysteresis loops of the MTJs after resetting the IrMn/NiFeCo pinned layer. Magneto-thermal magnetic random access memory, MT-MRAM, cells were used to perform this task. Here, the pinned layer is reset by pulsing a current through the tunnel barrier of the MT-MRAM MTJ in the presence of an external magnetic field. This permits rapid field cooling of the pinned layer through the blocking temperature of the IrMn. Repeated measurements of an R(H) loop after a pinned layer reset cycle show only minor variation. R(H) loops measured for different pinned layer reset cycles however, can show large changes in Hc , Ho, and R(H) loop shape. The effect is believed to be due to the random magnetostatic field distribution resulting from the frozen-in PFD of the IrMn/NiFeCo pinning layer. In order to test this conclusion, the evolution of the R(H) loop is studied as a function of reset pulse amplitude. Because of the intrinsic distribution of blocking temperatures of the IrMn layer, PFD should decrease as reset pulse amplitude increases. Results show that Hc increases, while the R(H) loops become more square as the reset pulse amplitude is increased. Ho shows a more complex dependence, which is a competition between ferromagnetic magnetic-roughness-induced Neel coupling and anti-ferromagnetic stray field coupling. A micromagnetic model for studying the dependence of the R(H) loops on the alignment of the IrMn crystallites was developed, and simulated results are in good agreement with the measurements. This work provides a simple explanation for large switching field distributions that can result in arrays of seemingly identical MRAM bits

The Effect of Structural Defects on Magnetic Switching in Thin Ferromagnetic Patterned Films

We have utilized micromagnetic simulations to study the effect of structural defects on the magnetic switching behavior of thin elliptical permalloy films. The nonmagnetic void was found to tend to pin the adjacent magnetic moment, which changes the local equilibrium magnetization configurations and alters the switching behavior of magnetization, leading to a large variation in switching fields. In particular, for the case of voids close to the edge, the curling effect of voids is significant and induces the occurrence of the multiple-stage magnetization reversal, leading to significantly large switching fields