https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1222&context=ameslab_manuscripts

Current progress and future challenges in rare-earth-free permanent magnets

Owing to their reduced dimensions, the magnetic properties of ultrathin magnetic films and multilayers, e.g. magnetic anisotropies and exchange coupling, often depend strongly on the surface and interface structure. In addition, chemical composition, crystallinity, grain sizes and their distribution govern the magnetic behaviour. All these structural properties can be modified by light-ion irradiation in an energy range of 5–150 keV due to the energy loss of the ions in the solid along their trajectory. Consequently the magnetic properties can be tailored by ion irradiation. Similar effects can also be observed using Ga+ ion irradiation, which is the common ion source in focused ion beam lithography.Examples of ion-induced modifications of magnetic anisotropies and exchange coupling are presented. This review is limited to radiation-induced structural changes giving rise to a modification of magnetic parameters. Ion implantation is discussed only in special cases.Due to the local nature of the interaction, magnetic patterning without affecting the surface topography becomes feasible, which may be of interest in applications. The main patterning technique is homogeneous ion irradiation through masks. Focused ion beam and ion projection lithography are usually only relevant for larger ion masses. The creation of magnetic feature sizes below 50 nm is shown. In contrast to topographic nanostructures the surrounding area of these nanostructures can be left ferromagnetic, leading to new phenomena at their mutual interface.Most of the material systems discussed here are important for technological applications. The main areas are magnetic data storage applications, such as hard magnetic media with a large perpendicular magnetic anisotropy or patterned media with an improved signal to noise ratio and magnetic sensor elements. It will be shown that light-ion irradiation has many advantages in the design of new material properties and in the fabrication technology of actual devices.

https://iopscience.iop.org/article/10.1088/0022-3727/37/16/R01/pdf

Tailoring magnetism by light-ion irradiation

The ordering and disordering of solid solutions under irradiation

The Technical Committee of the IEEE Magnetics Society has selected seven research topics to develop their roadmaps, where major developments should be listed alongside expected timelines: 1) hard disk drives; 2) magnetic random access memories; 3) domain-wall devices; 4) permanent magnets; 5) sensors and actuators; 6) magnetic materials; and 7) organic devices. Among them, magnetic materials for spintronic devices have been surveyed as the first exercise. In this roadmap exercise, we have targeted magnetic tunnel and spin-valve junctions as spintronic devices. These can be used, for example, as a cell for a magnetic random access memory and a spin-torque oscillator in their vertical form as well as a spin transistor and a spin Hall device in their lateral form. In these devices, the critical role of magnetic materials is to inject spin-polarized electrons efficiently into a nonmagnet. We have accordingly identified two key properties to be achieved by developing new magnetic materials for future spintronic devices: 1) half-metallicity at room temperature (RT) and 2) perpendicular anisotropy in nanoscale devices at RT. For the first property, five major magnetic materials are selected for their evaluation for future magnetic/spintronic device applications: 1) Heusler alloys; 2) ferrites; 3) rutiles; 4) perovskites; and 5) dilute magnetic semiconductors. These alloys have been reported or predicted to be half-metallic ferromagnets at RT. They possess a bandgap at the Fermi level $E_{F}$ only for its minority spins, achieving 100% spin polarization at $E_{F}$ . We have also evaluated $\text{L1}_{0}$ alloys and $D0_{22}$ –Mn alloys for the development of a perpendicularly anisotropic ferromagnet with large spin polarization. We have listed several key milestones for each material on their functionality improvements, property achievements, device implementations, and interdisciplinary applications within 35 years time scale. The individual analyses and the projections are discussed in the following sections.

Roadmap for Emerging Materials for Spintronic Device Applications

The Technical Committee of the IEEE Magnetics Society has selected 7 research topics to develop their roadmaps, where major developments should be listed alongside expected timelines; (i) hard disk drives, (ii) magnetic random access memories, (iii) domain-wall devices, (iv) permanent magnets, (v) sensors and actuators, (vi) magnetic materials and (vii) organic devices. Among them, magnetic materials for spintronic devices have been surveyed as the first exercise. In this roadmap exercise, we have targeted magnetic tunnel and spin-valve junctions as spintronic devices. These can be used for example as a cell for a magnetic random access memory and spin-torque oscillator in their vertical form as well as a spin transistor and a spin Hall device in their lateral form. In these devices, the critical role of magnetic materials is to inject spin-polarised electrons efficiently into a non-magnet. We have accordingly identified 2 key properties to be achieved by developing new magnetic materials for future spintronic devices: (1) Half-metallicity at room temperature (RT); (2) Perpendicular anisotropy in nano-scale devices at RT. For the first property, 5 major magnetic materials are selected for their evaluation for future magnetic/spintronic device applications: Heusler alloys, ferrites, rutiles, perovskites and dilute magnetic semiconductors. These alloys have been reported or predicted to be half-metallic ferromagnets at RT. They possess a bandgap at the Fermi level EF only for its minority spins, achieving 100% spin polarisation at EF. We have also evaluated L10-alloys and D022-Mn-alloys for the development of a perpendicularly anisotropic ferromagnet with large spin polarisation. We have listed several key milestones for each material on their functionality improvements, property achievements, device implementations and interdisciplinary applications within 35 years time scale.

Spherical Fe–Ni (50–50) single crystals were ordered by neutron irradiation at 295°C in the presence of a magnetic field. Magnetization curves and magnetic anisotropy energy measurements demonstrate the very important directional effect of the irradiation magnetic field on the bulk anisotropy of the single crystal. When the magnetic field is applied along a fourfold axis, the bulk magnetic anisotropy is tetragonal with a very large value for K3. When the magnetic field is applied along a three fold axis, the bulk magnetic anisotropy is cubic and the threefold axis becomes an easy axis of magnetization; the magnetization measured along the easy direction reaches saturation with a certain law of approach, while the magnetization measured along the [100] direction reaches saturation immediately. The model proposed by Neel, in which three types of quadratic ordered domains are assumed to be strongly coupled, fits well the experimental results. An electron microscopy image shows the different type of ordered d...

Magnetization Curves of Fe–Ni (50–50) Single Crystals Ordered by Neutron Irradiation with an Applied Magnetic Field

The magnetic anisotropy of a FeNi 50/50 single crystal changes considerably when the sample is ordered by irradiation in a magnetic field, showing either cubic or quadratic symmetry in the ordered state, according to the direction of the field. The experimental values of the anisotropy constants are interpreted in terms of the coupled-crystallite model proposed by Neel and confirmed by X-ray measurements. The order-disorder transformation diagrams for FeNi- and FeNi 3 -type order are studied in the composition range of 37 to 70 percent of nickel. The simultaneous existence of the two types of order is remarked. In the composition range from 50 to 75 percent of nickel, a detailed study of the observed magnetecrystalline symmetry shows up the different contributions of FeNi, FeNi 2 , and directional order. The Curie temperature in the ordered state is higher than that in the disordered state, the difference between the two values reaches 300° for same composition.

J. Pauleve

Papers

Magnetization Curves of Fe–Ni (50–50) Single Crystals Ordered by Neutron Irradiation with an Applied Magnetic Field

Transformations ordre-désorde et propriétés magnétiques des alliages fer-nickel

J. Pauleve

Papers

Magnetization Curves of Fe–Ni (50–50) Single Crystals Ordered by Neutron Irradiation with an Applied Magnetic Field

Transformations ordre-d&#233;sorde et propri&#233;t&#233;s magn&#233;tiques des alliages fer-nickel

Transformations ordre-désorde et propriétés magnétiques des alliages fer-nickel