Transition Metal Synthetic Ferrimagnets: Tunable Media for All-Optical Switching Driven by Nanoscale Spin Current

All-optical switching of magnetization has great potential for use in future ultrafast and energy efficient nanoscale magnetic storage devices. So far, research has been almost exclusively focused on rare-earth based materials, which limits device tunability and scalability. Here, we show that a perpendicularly magnetized synthetic ferrimagnet composed of two distinct transition metal ferromagnetic layers, Ni₃Pt and Co, can exhibit helicity independent magnetization switching. Switching occurs between two equivalent remanent states with antiparallel alignment of the Ni₃Pt and Co magnetic moments and is observable over a broad temperature range. Time-resolved measurements indicate that the switching is driven by a spin-polarized current passing through the subnanometer Ir interlayer. The magnetic properties of this model system may be tuned continuously via subnanoscale changes in the constituent layer thicknesses as well as growth conditions, allowing the underlying mechanisms to be elucidated and paving the way to a new class of data storage devices.

Applications of Filled Single-Walled Carbon Nanotubes: Progress, Challenges, and Perspectives

Single-walled carbon nanotubes (SWCNTs), which possess electrical and thermal conductivity, mechanical strength, and flexibility, and are ultra-light weight, are an outstanding material for applications in nanoelectronics, photovoltaics, thermoelectric power generation, light emission, electrochemical energy storage, catalysis, sensors, spintronics, magnetic recording, and biomedicine. Applications of SWCNTs require nanotube samples with precisely controlled and customized electronic properties. The filling of SWCNTs is a promising approach in the fine-tuning of their electronic properties because a large variety of substances with appropriate physical and chemical properties can be introduced inside SWCNTs. The encapsulation of electron donor or acceptor substances inside SWCNTs opens the way for the Fermi-level engineering of SWCNTs for specific applications. This paper reviews the recent progress in applications of filled SWCNTs and highlights challenges that exist in the field.

Free Layer Thickness Dependence of the Stability in Co2(Mn0.6Fe0.4)Ge Heusler Based CPP-GMR Read Sensor for Areal Density of 1 Tb/in2

Current-perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) read sensors based on Heusler alloys are promising candidates for ultrahigh areal densities of magnetic data storage technology. In particular, the thickness of reader structures is one of the key factors for the development of practical CPP-GMR sensors. In this research, we studied the dependence of the free layer thickness on the stability of the Co₂(Mn_0.6Fe_0.4)Ge Heusler-based CPP-GMR read head for an areal density of 1 Tb/in², aiming to determine the appropriate layer thickness. The evaluations were done through simulations based on micromagnetic modelling. The reader stability indicators, including the magnetoresistance (MR) ratio, readback signal, dibit response asymmetry parameter, and power spectral density profile, were characterized and discussed. Our analysis demonstrates that the reader with a free layer thickness of 3 nm indicates the best stability performance for this particular head. A reasonably large MR ratio of 26% was obtained by the reader having this suitable layer thickness. The findings can be utilized to improve the design of the CPP-GMR reader for use in ultrahigh magnetic recording densities.

Magnetic-Pole Flip by Millimeter Wave

In the era of Big Data and the Internet of Things, data archiving is a key technology. From this viewpoint, magnetic recordings are drawing attention because they guarantee long-term data storage. To archive an enormous amount of data, further increase of the recording density is necessary. Herein a new magnetic recording methodology, "focused-millimeter-wave-assisted magnetic recording (F-MIMR)," is proposed. To test this methodology, magnetic films based on epsilon iron oxide nanoparticles are prepared and a focused-millimeter-wave generator is constructed using terahertz (THz) light. Irradiating the focused millimeter wave to epsilon iron oxide instantly switches its magnetic pole direction. The spin dynamics of F-MIMR are also calculated using the stochastic Landau-Lifshitz-Gilbert model considering all of the spins in an epsilon iron oxide nanoparticle. In F-MIMR, the heat-up effect of the recording media is expected to be suppressed. Thus, F-MIMR can be applied to high-density magnetic recordings.

A New Highly Anisotropic Rh-Based Heusler Compound for Magnetic Recording

The development of high-density magnetic recording media is limited by superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat-assisted magnetic recording was developed, rapidly heating the media to the Curie temperature T_c before writing, followed by rapid cooling. Requirements are a suitable T_c , coupled with anisotropic thermal conductivity and hard magnetic properties. Here, Rh₂ CoSb is introduced as a new hard magnet with potential for thin-film magnetic recording. A magnetocrystalline anisotropy of 3.6 MJ m^-3 is combined with a saturation magnetization of μ₀ M_s  = 0.52 T at 2 K (2.2 MJ m^-3 and 0.44 T at room temperature). The magnetic hardness parameter of 3.7 at room temperature is the highest observed for any rare-earth-free hard magnet. The anisotropy is related to an unquenched orbital moment of 0.42 μ_B on Co, which is hybridized with neighboring Rh atoms with a large spin-orbit interaction. Moreover, the pronounced temperature dependence of the anisotropy that follows from its T_c of 450 K, together with a thermal conductivity of 20 W m^-1 K^-1 , make Rh₂ CoSb a candidate for the development of heat-assisted writing with a recording density in excess of 10 Tb in.^-2 .

Single-Shot Multi-Level All-Optical Magnetization Switching Mediated by Spin Transport

All-optical ultrafast magnetization switching in magnetic material thin film without the assistance of an applied external magnetic field is explored for future ultrafast and energy-efficient magnetic storage and memories. It is shown that femtosecond (fs) light pulses induce magnetization reversal in a large variety of magnetic materials. However, so far, only GdFeCo-based ferrimagnetic thin films exhibit magnetization switching via a single optical pulse. Here, the single-pulse switching of Co/Pt multilayers within a magnetic spin-valve structure ([Co/Pt]/Cu/GdFeCo) is demonstrated and four possible magnetic configurations of the spin valve can be accessed using a sequence of single fs light pulses. The experimental study reveals that the magnetization final state of the ferromagnetic [Co/Pt] layer is determined by spin-polarized currents generated by the light pulse interactions with the GdFeCo layer. This work provides an approach to deterministically switch ferromagnetic layers and a pathway to engineering materials for opto-magnetic multi-bit recording.

Nanoscale Confinement of All-Optical Magnetic Switching in TbFeCo--Competition with Nanoscale Heterogeneity

Single femtosecond optical laser pulses, of sufficient intensity, are demonstrated to reverse magnetization in a process known as all-optical switching. Gold two-wire antennas are placed on the all-optical switching film TbFeCo. These structures are resonant with the optical field, and they create a field enhancement in the near-field which confines the area where optical switching can occur. The magnetic switching that occurs around and below the antenna is imaged using resonant X-ray holography and magnetic circular dichroism. The results not only show the feasibility of controllable switching with antenna assistance but also demonstrate the highly inhomogeneous nature of the switching process, which is attributed to the process depending on the material's heterogeneity.