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SEM image of the patterned magnetic medium with 60 nm diameter and 100 nm pitch (distance from center to center of dots).
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Switching Field distribution (SFD) is one of the critical issues for writing in bit patterned media (BPM) for high areal densities. It is believed that the magnetostatic interaction is one of the several factors that contribute to the SFD. With the antiferromagnetically coupled (AFC) structure, the magnetostatic interaction can be tailored to under...
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... the other hand, the AFC effect may not be observed when the films are patterned, as there have been several observations of increase in coercivity by a huge amount (10 15 times) after patterning [13]. Therefore, it is essential to study the SFD of the patterned samples. Fig. 2 shows scanning electron microscopy (SEM) image of the final nanomagnetic dots with 60 nm diameter dots with 40 nm spacing between dots after ion milling. Fig. 3 shows magnetic force microscopy (MFM) images for the sample deposited at a pressure of 1 Pa. It can be seen that the magnetizations of dots with different anisotropy fields ...
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We report an experimental study on the dynamic thermomagnetic (TM) reversal mechanisms at around Curie temperature (Tc) for isolated 60 nm pitch single-domain [Co/Pd] islands heated by a 1.5 mu m spot size laser pulse under an applied magnetic reversal field (Hr). Magnetic force microscopy (MFM) observations with high resolution MFM tips clearly sh...
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... This phenomenon was described in a more generalized theory and is referred to as Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction 129,130 . The RKKY coupling constant JRKKY shows an alternating sign behavior between ferromagnetic (positive) and antiferromagnetic (negative) coupling between the magnetic layers and a strength both depending on the thickness of the spacer layer 131 . ...
The need for low-power, wideband, fast, nonvolatile logic and memory devices paved the way for profound research in chiral and linear spin structures in thin films and 2D materials. Skyrmions are promising for nonvolatile logic and memory due to their nanoscale topological protection against external magnetic fields or thermal perturbations and their ability to be manipulated using spin-polarized current. In this thesis, we use micromagnetic models to computationally explore chiral and linear spin textures for their stability and efficient transfer as functions of intrinsic magnetic film parameters, geometries, and extrinsic driving conditions. We also grew yttrium iron garnet films and characterized their properties for magnetic insulator-based efficient spintronics. We first investigated spin waves in nanoscale magnon filters. Spin waves in magnonic crystals with width-modulated geometries transfer signals selectively, filter spectra, and have logic functionalities. We study the effects of Gilbert damping, uniaxial magnetic anisotropy, geometry, and excitation frequency on the temporal and spectral profiles of spin wave modes under external DC and RF magnetic fields. The transmission spectra showed that both spectral filtering and nonlinear frequency conversion can be achieved. Second, we developed numerical and analytical models of synthetic antiferromagnetically coupled (SAF) skyrmions in magnetic heterostructures. SAF skyrmions are promising because they might be driven with a lower current than their ferromagnetic counterparts and without the topological Hall effect, unlike ferromagnetic skyrmions. SAF skyrmions have been found to require 1-3 orders of magnitude lower current and experience no topological Hall effect compared with their ferromagnetic skyrmion counterparts, where these effects are detrimental. Current density reductions could lead up to 6 orders of magnitude lower power consumption and improvements in device reliability. We found that the intrinsic material properties (Ms, uniaxial anisotropy, and Dzyaloshinskii–Moriya interaction) determine SAF skyrmions' stability and equilibrium radii. We showed that eliminating the topological Hall effect in SAF skyrmions increases their velocity and stability against the external current drive, temperature, film polycrystallinity, and edge roughness. We modeled SAF skyrmion logic gates and compared their performance to ferromagnetic skyrmion logic. The higher velocity and lower switching times suggest that SAF skyrmion logic gates might enable efficient, fast, and in-memory logic applications. Third, we grew yttrium iron garnet (YIG) on gadolinium gallium garnet substrates in pulsed laser deposition (PLD) to investigate the role of growth conditions on the films' structural and magnetic properties. The PLD growth parameters such as growth temperature, target-substrate distance, laser pulse rate, and energy can greatly affect their epitaxial quality and crystallinity. Structural, magnetic, and optical characterization showed that YIG unit cells have compressive strain and slight iron deficiency. Lower-than-bulk Ms values of YIG films are attributed to the slight iron deficiency of the films. The optimization of the growth process is necessary to obtain YIG films with high epitaxial and magnetic quality. Our models and experiments pave the way for efficient, fast, non-volatile, and nanoscale spintronic device implementations to complement conventional microelectronics in the future.
... As can be seen, the system has four different configurations of six layers. If the configuration of the system is such that consecutive magnetic layers are antiferromagnetically coupled, then this configuration is called the ground state or single-phase configuration [311][312][313][314]. However, if the magnetization of the top three layers is rotated by 180 • , then this configuration is called a soliton state or two antiphase domain state. ...
Digital data, generated by corporate and individual users, is growing day by day due to a vast range of digital applications. Magnetic hard disk drives (HDDs) currently fulfill the demand for storage space, required by this data growth. Although flash memory devices are replacing HDDs in applications like mobile phones, laptops, and desktops, HDDs cover the majority of digital data stored in the cloud and servers. Since the capacity growth of HDDs is slowing down, it is essential to look for a potential alternative. One such alternative is domain wall (DW) memory, where magnetic domains in the form of two-dimensional or three-dimensional wires are used to store the information. DW memory (DWM) devices should satisfy the four basic operations, such as writing (nucleating domains or inserting DWs in memory element), storing (stabilizing DWs), shifting (moving DWs), and reading (reading magnetization direction). An external magnetic field or spin-transfer torque can be used to write the information. Spin–orbit torque or electric field may be used for shifting the DWs. The information can be read using tunneling magnetoresistance. The domains may be stored along the tracks using artificial pinning potentials. The absence of moving parts makes the DWM consume less power as compared to HDDs, and be more robust. The potential to stack many layers to store information in three dimensions makes them potentially a large storage capacity device. In addition to memory, DW devices also offer a route for making synaptic devices for neuromorphic computing.
Despite these potential advantages of DWM, significant advances in research are needed before DWM could become commercially viable. One of the major challenges associated with DWM is DW dynamics. Many problems, such as controlled DW motion, the stability of domains, reducing the dimensions of the DW devices are still to be addressed. Artificial pinning sites fabricated through either geometrical or non-geometrical methods have been proposed for controlling DW motion. This review paper presents a survey of the investigations carried out so far and the future perspective of such devices.
... As can be seen, the system has four different configurations of six layers. If the configuration of the system is such that consecutive magnetic layers are antiferromagnetically coupled, then this configuration is called the ground state or single-phase configuration [311][312][313][314]. However, if the magnetization of the top three layers is rotated by 180 • , then this configuration is called a soliton state or two antiphase domain state. ...
... Alloys with ferromagnetic and ferrimagnetic nanograins are interesting because they show a different magnetization process due to the presence of diverse types of nanograins and a variety of magnetic interactions such as dipolar, isotropic, super-exchange, anisotropic, and magneto-elastic interactions [10]. Magnetostatic interactions between nanograins are an issue key in the high recording densities, where such interactions lead to a broader switching field distribution, because of which the intrinsic properties of the materials differ from particle to particle [11]. For using nanomaterials in the magnetic recording, high magnetization is necessary and avoid the superparamagnetic apparitions for reduction and elimination of the magnetic interactions. ...
Alloys of Sm-Fe-Ti were synthesized by mechanical milling for 5 h; the hysteresis loop for nanocrystalline Sm-Fe-Ti alloys showed bistable behavior, which is partly repressed because of the presence of an effective field. The magnetic properties of remanence for nanocrystalline Sm-Fe-Ti alloys were measured to study the interactions between nanograins. Henkel et al. confirmed the structural disorder of the nanocrystalline Sm-Fe-Ti alloys obtained after 5 h of mechanical milling, while for nanocrystalline Sm-Fe-Ti alloys, obtained by mechanical milling for 5 h and annealing, the predominant effects were due to the mean field. Mössbauer spectroscopy was used to study magnetic interactions in the nanocrystalline Sm-Fe-Ti alloys.
... 40,41) Similarly, several research studies have shown the fabrication of AFC media for perpendicular magnetic recording and reported they are effective in reducing switching field distribution. [42][43][44] More recently, MAS of granular AFC media has been experimentally evaluated. 45) Because AFC media can reduce the distribution of FMR frequency, the use of AFC media for MAMR is proposed. ...
To meet the ever-increasing demand for data storage, future magnetic recording devices will need to be made three-dimensional by implementing multilayer recording. In this article, we present methods of detecting and manipulating the magnetization direction of a specific layer selectively in a vertically stacked multilayer magnetic system, which enable layer-selective read and write operations in three-dimensional magnetic recording devices. The principle behind the methods is ferromagnetic resonance excitation in a microwave magnetic field. By designing each magnetic recording layer to have a different ferromagnetic resonance frequency, magnetization excitation can be induced individually in each layer by tuning the frequency of an applied microwave magnetic field, and this selective magnetization excitation can be utilized for the layer-selective operations. Regarding media for three-dimensional recording, when layers of a perpendicular magnetic material are vertically stacked, dipolar interaction between multiple recording layers arises and is expected to cause problems, such as degradation of thermal stability and switching field distribution. To solve these problems, we propose the use of an antiferromagnetically coupled structure consisting of hard and soft magnetic layers. Because the stray fields from these two layers cancel each other, antiferromagnetically coupled media can reduce the dipolar interaction.
... Hereafter, they are respectively referred to as the hard layer and the soft layer. These two layers have nearly the same magnetic moment and are antiferromagnetically coupled (AFC) by insertion of a Ru layer between them [23,24], allowing the static stray fields from the two magnets to cancel each other. The purpose of employing the AFC structure is to suppress the influence of its static stray fields on the magnetization dynamics of the STO, which helps to clearly detect the dynamic coupling through dipolar interaction. ...
Technology for detecting the magnetization direction of nanoscale magnetic material is crucial for realizing high-density magnetic recording devices. Conventionally, a magnetoresistive device is used that changes its resistivity in accordance with the direction of the stray field from an objective magnet. However, when several magnets are near such a device, the superposition of stray fields from all the magnets acts on the sensor, preventing selective recognition of their individual magnetization directions. Here we introduce a novel readout method for detecting the magnetization direction of a nanoscale magnet by use of a spin-torque oscillator (STO). The principles behind this method are dynamic dipolar coupling between an STO and a nanoscale magnet, and detection of ferromagnetic resonance (FMR) of this coupled system from the STO signal. Because the STO couples with a specific magnet by tuning the STO oscillation frequency to match its FMR frequency, this readout method can selectively determine the magnetization direction of the magnet.
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.
The development of high spatial resolution and element sensitive magnetic characterization techniques to quantitatively measure magnetic parameters of individual nanoparticles (NPs) and deeply understand and tune their magnetic properties is a hot topic in nanomagnetism. Magnetic force microscopy (MFM), thanks to its high lateral resolution, appears as a promising technique for the magnetic characterization of single nano-sized materials although it is still limited by some drawbacks, especially by the presence of electrostatic artifacts. Recently, these limitations have been overcome by the development of a particular MFM based technique called controlled magnetization - MFM (CM-MFM) allowing, in principle, a quantifiable correlation between the measured magnetic signal and the magnetization of the object under investigation. Here we propose an experimental procedure, based on the use of CM-MFM technique, to measure the magnetization curve of single magnetic NPs individuating their saturation magnetization, magnetic field, and coercivity. We measured, for the first time, the magnetization curves of individual Fe3O4 nanoparticles with diameters in the range of 18-32 nm by using a MFM instrument. Results are in very good agreement with the quantitative data obtained by SQUID analysis on a macroscopic sample, showing the high potential of the technique in the field of nanomagnetometry.
Magnetic force microscopy (MFM) is scanning probe technique which enables the analysis of magnetic properties of the samples at the nanoscale using a microfabricated tip coated with a magnetic layer. In this chapter, we describe MFM and give an overview of its applications, ranging from well established to advanced new applications.
With the emergence of nanotechnology, the feature size of devices is getting smaller and smaller and the demand for higher resolution imaging tools is growing. For the visualization of magnetic domains of nanostructures and thin films, magnetic force microscopy is the feasible and most utilized technique. The resolution of MFM is about 20—30 nm in the current instruments and efforts to improve the resolution in the sub-10 nm range are in progress. In this chapter, an overview of key research findings of several researchers and recent advances in the direction of improving the MFM resolution is provided.
