Advanced Magnetic and Dielectric Nanomaterials

<p>Abstract: Multiferroics are a novel class of next generation multifunctional materials that exhibit simultaneous magnetic spin, electric dipole, and ferroelastic ordering. It gives an additional degree of freedom to design new devices. Magnetoelectric effect in these materials result in the manipulation of magnetic spins via applied electric field and vice versa, making them suitable for next generation applications. Single phase multiferroics show low magnetoelectric coefficient, hence there is a need to look at composite multiferroic structures with respective magnetic and ferroelectric phase. The magnetoelectric coefficient in the composite structures depends on the magnitude of strain induced by one phase to the other. This requires the need to study suitable magnetic and ferroelectric materials that can be combined to create magnetoelectric multiferroic composite structures. Also, higher surface to volume ratio at the nanoscale should enhance the interaction between the two phases. Here, in we synthesised and studied magnetic and ferroelectric structures that have potential to be used as the respective phases of multiferroic magnetoelectric composites. Magnetic materials with high magnetostriction and low coercivity are suitable candidates for the formation of multiferroic composite. The size dependent and tuneable magnetic properties of cobalt ferrite and nickel-iron composites, respectively fulfil the above-mentioned criteria. Herein, the properties of the above magnetic materials were explored at nanoscale where efficient techniques such as thermal decomposition and electrospinning were applied. Cobalt ferrite nanoparticles with varying sizes were synthesised at the nanoscale and magnetic studies were performed to study their size dependent suitability to be used as a potential magnetic material in multiferroic composite formation. The nanoparticle synthesis by thermal decomposition of metal oleate precursors displayed reaction time dependent growth. The nanoparticles sized below superparamagnetic limit showed a negligible coercivity fulfilling an essential requirement to display magnetoelectric effect. Alongside, a successful synthesis of novel cobalt iron oxide (Co0.33Fe0.67O) nanoparticles was also performed. This displayed a synthesis dependent ferrimagnetic to antiferromagnetic phase transition in Co Fe-O structure at nanoscale. A controlled oxidation of Co0.33Fe0.67O could lead to the formation of antiferromagnetic-ferrimagnetic core-shell nanostructure that can overcome the superparamagnetic limit in nanoparticles system. They are potential materials in ME-RAMs. 1-D magnetic nanostructure show a sharp shape anisotropy and hence can be used as magnetic components of composite multiferroic structures. Nickel-iron composites in FCC phase were studied at the nanoscale in the form of fibres. Electrospinning of suitable metal precursors with PVP polymer followed by the reduction of nanofibres in H2 led to the formation of Ni0.47Fe0.53 fibre mats. They were ferromagnetic and displayed high saturation magnetisation along with low coercivity fulfilling the requirement to be used in magnetoelectric applications. 1-D flexible ferroelectric composite structures were studied alongside to be used as the ferroelectric component of multiferroic composites. Polyvinylidene fluoride was doped with DIPAB at varying ratios to study the improvement in the ferroelectric properties of the composite structure in comparison to just PVDF with low dielectric constant. Electrospinning of composite polymer solution led to the formation of DIPAB doped PVDF nanofibres. They displayed improved relative dielectric constant and low loss tangent and find use in composite magnetoelectric materials formation. The ease of processability of DIPAB doped PVDF nanofibres aids in incorporating the above studied magnetic materials. The studies proved the worth of as-synthesised magnetic and ferroelectric materials at the nanoscale for the formation of magnetoelectric multiferroic composite nanomaterials. The cobalt ferrite nanoparticles doped in DIPAB-PVDF nanofibres can result in core-sheath ME composite structure. A coating of DIPAB-PVDF composite on the formed Ni0.47Fe0.53 fibres will result to the formation of 1-D magnetoelectric structures.</p>

Advanced Magnetic and Dielectric Nanomaterials

<p>Abstract: Multiferroics are a novel class of next generation multifunctional materials that exhibit simultaneous magnetic spin, electric dipole, and ferroelastic ordering. It gives an additional degree of freedom to design new devices. Magnetoelectric effect in these materials result in the manipulation of magnetic spins via applied electric field and vice versa, making them suitable for next generation applications. Single phase multiferroics show low magnetoelectric coefficient, hence there is a need to look at composite multiferroic structures with respective magnetic and ferroelectric phase. The magnetoelectric coefficient in the composite structures depends on the magnitude of strain induced by one phase to the other. This requires the need to study suitable magnetic and ferroelectric materials that can be combined to create magnetoelectric multiferroic composite structures. Also, higher surface to volume ratio at the nanoscale should enhance the interaction between the two phases. Here, in we synthesised and studied magnetic and ferroelectric structures that have potential to be used as the respective phases of multiferroic magnetoelectric composites. Magnetic materials with high magnetostriction and low coercivity are suitable candidates for the formation of multiferroic composite. The size dependent and tuneable magnetic properties of cobalt ferrite and nickel-iron composites, respectively fulfil the above-mentioned criteria. Herein, the properties of the above magnetic materials were explored at nanoscale where efficient techniques such as thermal decomposition and electrospinning were applied. Cobalt ferrite nanoparticles with varying sizes were synthesised at the nanoscale and magnetic studies were performed to study their size dependent suitability to be used as a potential magnetic material in multiferroic composite formation. The nanoparticle synthesis by thermal decomposition of metal oleate precursors displayed reaction time dependent growth. The nanoparticles sized below superparamagnetic limit showed a negligible coercivity fulfilling an essential requirement to display magnetoelectric effect. Alongside, a successful synthesis of novel cobalt iron oxide (Co0.33Fe0.67O) nanoparticles was also performed. This displayed a synthesis dependent ferrimagnetic to antiferromagnetic phase transition in Co Fe-O structure at nanoscale. A controlled oxidation of Co0.33Fe0.67O could lead to the formation of antiferromagnetic-ferrimagnetic core-shell nanostructure that can overcome the superparamagnetic limit in nanoparticles system. They are potential materials in ME-RAMs. 1-D magnetic nanostructure show a sharp shape anisotropy and hence can be used as magnetic components of composite multiferroic structures. Nickel-iron composites in FCC phase were studied at the nanoscale in the form of fibres. Electrospinning of suitable metal precursors with PVP polymer followed by the reduction of nanofibres in H2 led to the formation of Ni0.47Fe0.53 fibre mats. They were ferromagnetic and displayed high saturation magnetisation along with low coercivity fulfilling the requirement to be used in magnetoelectric applications. 1-D flexible ferroelectric composite structures were studied alongside to be used as the ferroelectric component of multiferroic composites. Polyvinylidene fluoride was doped with DIPAB at varying ratios to study the improvement in the ferroelectric properties of the composite structure in comparison to just PVDF with low dielectric constant. Electrospinning of composite polymer solution led to the formation of DIPAB doped PVDF nanofibres. They displayed improved relative dielectric constant and low loss tangent and find use in composite magnetoelectric materials formation. The ease of processability of DIPAB doped PVDF nanofibres aids in incorporating the above studied magnetic materials. The studies proved the worth of as-synthesised magnetic and ferroelectric materials at the nanoscale for the formation of magnetoelectric multiferroic composite nanomaterials. The cobalt ferrite nanoparticles doped in DIPAB-PVDF nanofibres can result in core-sheath ME composite structure. A coating of DIPAB-PVDF composite on the formed Ni0.47Fe0.53 fibres will result to the formation of 1-D magnetoelectric structures.</p>

Overcoming the Trilemma Issues of Ultrahigh Density Perpendicular Magnetic Recording Media by L10-Fe(Co)Pt Materials

L10-ordered FePt and CoPt (collectively called L10- Fe ( Co ) Pt in this review) have become potential materials for future ultrahigh density perpendicular magnetic recording (PMR) media due to their high magnetocrystalline anisotropy, rendering small grains with high thermal stability. However, PMR media using such high anisotropy faces the well-known trilemma issues among thermal stability, signal-to-noise ratio (SNR), and writability. This paper will provide an overview of the impact of L10- Fe ( Co ) Pt on overcoming the superparamagnetic limit and balancing the trilemma issues for ultrahigh density PMR media. Here the research and development of L10- Fe ( Co ) Pt materials will be presented, from the perspectives of enhancing thermal stability, SNR and writability. Furthermore, we will provide some combined approaches to tackle the challenges in balancing the trilemma issues, focusing on materials engineering.

Microwave Assisted Magnetization Reversal on Exchange Coupled Composite Media

To overcome superparamagnetic limit, microwave assisted magnetic recording (MAMR) is one interesting magnetic recording technology. Therefore, the effect of microwave on magnetization reversal in media should be analyzed. In this work, we propose the MAMR to decrease switching field (coercivity, Hsw) in exchange coupled composite (ECC) media by using the micromagnetic simulation based on the Landau - Lifshitz - Gilbert equation. The Hsw of single layer and ECC media without microwave field is 110.90 and 7.7 kOe, respectively. When the oscillating microwave field is added, Hsw of single layer media with microwave frequency of 2.5 - 40 GHz is lower than 110.90 kOe. Likewise, Hsw of ECC media with microwave frequency of 5 - 16 GHz is lower than 7.7 kOe and has the lowest value of 4.9 kOe at frequency of 10 GHz. The results from this work lead to solve superparamagnetic limit and increase areal density in hard disk drive.

L10-CoPt Bit Patterned Media with Tilted Easy Axis for Ultrahigh Areal Density over 2.5 Tb/in2

Ultrahigh areal density is the key target of hard disk drive technology. Hence, writing field strength from head and switching field, Hsw, of media should be improved. In this work, we propose the one of alternative method to increase data density and reduce Hsw of the media by using tilted easy axis technology for bit patterned media (BPM) at areal density beyond 2.5 Tb/in2. Moreover, transition noise and superparamagnetic limit have been eliminated owing to characteristics of BPM. The effect of exchange coupled between adjacent bits, Adot, of tilted easy axis of BPM is analyzed by micromagnetic simulation software - the object oriented micromagnetic framework based on Landau-Lifshitz-Gilbert equation. The BPM with tilted easy axis perform clearly the reduction of Hsw below perpendicular media and available writing head field. The Adot of BPM has no effect on decreasing Hsw. Anisotropy and Zeeman energy density of BPM with tilted easy axis are higher and lower than perpendicular BPM, respectively. Thereby, BPM with tilted easy axis have high potentiality to improve Hsw of media at ultrahigh data density.

Effects of Thermal and Cross-Track Variations for Longitudinal Heat-Assisted Magnetic Recording Systems

The current data recording technology is approaching its capacity limit approximately 1 Tbit/in2(terabits per square inch) known as superparamagnetic limit. Heat-assisted magnetic recording (HAMR) is one of the promising technologies that is being planned to be used as a new data recording technology to achieve the storage capacity beyond 1 Tbit/in2. In HAMR, the laser is applied to heat a magnetic medium during the writing process, which results in the unique transition characteristics if compared to a conventional system. This paper investigates the effects of thermal and cross-track variations to the transition characteristics (both transition center and transition parameter) of longitudinal HAMR systems. Experimental results indicate that the longitudinal HAMR system can withstand some amount of thermal and cross-track variations and still provides satisfactory system performance.

Experimental Set-Up for Simultaneously Measuring Changes in LD Temperature and Flying Height of HAMR-HGA

Heat-assisted magnetic recording (HAMR) is a promising technology for overcoming the superparamagnetic limit, and thereby enabling the achievement of a recording density beyond 8 Tb/in2 in a hard disk drive (HDD) [1]. The HAMR head gimbal assembly (HGA) consists of a HAMR head slider, a suspension, and a laser diode (LD) mounted on the slider. An optical near-field transducer (NFT) and a waveguide are near the write-pole in the head slider. During the writing process, light energy is delivered from the LD to the NFT through the waveguide, and the NFT forms a nano-size thermal spot on the recording medium, thereby reducing its coercivity. Development of an HAMR-HGA requires solving several thermal problems. There are two heat sources. One is the LD, which transforms electrical energy into light energy and heat energy. The heat energy increases the temperature of the LD itself, which reduces the laser power and deforms the slider. The other is the NFT, which absorbs light energy. The absorbed energy is transformed into heat energy. This increases the temperature of the NFT and causes the head to protrude. The thermal deformation and protrusion cause a change in the flying-height (FH). The head thermal protrusion problem has been solved using finite-element method (FEM) simulation [2]. We have developed a novel experimental set-up for measuring the temperature increase and FH change simultaneously.