A. Hirohata

Future perspectives for spintronic devices

Spintronics is one of the emerging research fields in nanotechnology and has been growing very rapidly. Studies of spintronics were started after the discovery of giant magnetoresistance in 1988, which utilized spin-polarized electron transport across a non-magnetic metallic layer. Within 10 years, this discovery had been implemented into hard disk drives, the most common storage media, followed by recognition through the award of the Nobel Prize for Physics 19 years later. We have never experienced such fast development in any scientific field. Spintronics research is now moving into second-generation spin dynamics and beyond. In this review, we first examine the historical advances in spintronics together with the background physics, and then describe major device applications.

Roadmap for Emerging Materials for Spintronic Device Applications

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 EF only for its minority spins, achieving 100% spin polarization at EF. We have also evaluated L10 alloys and D022-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.

Structural and magnetic properties of epitaxial L21-structured Co2(Cr,Fe)Al films grown on GaAs(001) substrates

We have successfully grown both L21 polycrystalline Co2CrAl and epitaxial L21-structured Co2FeAl films onto GaAs(001) substrates under an optimized condition. Both structural and magnetic analyses reveal the detailed growth mechanism of the alloys, and suggest that the Co2CrAl film contains atomically disordered phases, which decreases the magnetic moment per f.u., while the Co2FeAl film satisfies the generalized Slater–Pauling behavior. By using these films, magnetic tunnel junctions (MTJs) have been fabricated, showing 2% tunnel magnetoresistance (TMR) for the Co2CrAl MTJ at 5K and 9% for the Co2FeAl MTJ at room temperature (RT). Even though the TMR ratio still needs to be improved for future device applications, these results explicitly include that the Co2(Cr,Fe)Al full Heusler alloy is a promising compound to achieve half-metallicity at RT by controlling both disorder and surface structures in the atomic level by manipulating the Fe concentration.

Magnetic nanoscale dots on colloid crystal surfaces

We demonstrate that uniform, ordered, single-domain magnetic nanoscale dots can be fabricated on concentrated colloid surfaces. The substrate consists of compact silica nanosphere arrays grown on a glass wafer. Through the subsequent deposition and oxidation treatment of a Co film, monodisperse magnetic Co nanoscale dot arrays with controlled magnetic properties and size were obtained. We suggest that magnetic dots deposited on colloidal surfaces might open a way of developing artificially nanostructured materials for fundamental studies in nanomagnetism and for applications such as patterned magnetic recording media.

Distinctive current-induced magnetization switching in a current-perpendicular-to-plane giant-magnetoresistance nanopillar with a synthetic antiferromagnet free layer

We investigated current-induced magnetization switching (CIMS) in two types of pseudo-spin-valve nanopillars with current-perpendicular-to-plane giant magnetoresistance (CPP-GMR); Co90Fe10(10nm)∕Cu(10nm)∕Co90Fe10(2.5nm) (conventional type) and Co90Fe10(10nm)∕Cu(10nm)∕Co90Fe10(1.5nm)∕Ru(0.45nm)∕Co90Fe10(2.5nm) (synthetic antiferromagnet; SyAF type). We observed the CIMS in the both CPP-GMR structures at room temperature. In particular for the SyAF type nanopillars, the CIMS was observed only in a negative current regime. We also discovered that the applied magnetic field dependence of the CIMS shows absolutely different behavior from that of the conventional type. These peculiar CIMS behaviors with the SyAF free layer are attributed to majority electron spin transfer torque from the thick to the thin Co90Fe10 layers, enhanced by the presence of a Ru layer.

Bias voltage effect on tunnel magnetoresistance in fully epitaxial MgO double-barrier magnetic tunnel junctions

Double-barrier magnetic tunnel junctions (DMTJs), consisting of a fully epitaxial Fe(001)∕MgO(001)∕Fe(001)∕MgO(001)∕Fe(001) structure, have been deposited onto MgO (001) single-crystal substrates using molecular-beam epitaxy, and have been characterized by measuring the bias voltage effects on both tunneling magnetoresistance (TMR) and conductance. The DMTJs are found to show large TMR ratios of up to 110% and extremely small bias voltage dependence(Vhalf=1.44V under a positive bias application) compared with conventional magnetic tunnel junctions (MTJs) with a single MgO barrier at room temperature. In addition, clear asymmetry is observed in the bias voltage dependence of the TMR ratios with respect to the signs of the bias, which corresponds to the asymmetric bias dependence of the conductance, especially for a parallel magnetization configuration. Such a high Vhalf with a large TMR ratio is relevant for a high-output MTJ cell for future spintronic devices.

Magnetoresistance in tunnel junctions using Co2(Cr,Fe)Al full Heusler alloys

We grew Co2(Cr1−xFex)Al Heusler alloy films using a magnetron sputtering system on thermally oxidized Si substrates at room temperature without any buffer layers. The x-ray diffraction patterns did not show the L21 structure as expected for the bulk but revealed the B2 and A2 structures, depending on the Fe concentration x, in which the structure tends to become the A2 with increasing x. The magnetic moment and the Curie temperature monotonically increased with increasing x. Spin-valve-type tunneling junctions consisting of Co2Cr1−xFexAl (100 nm)/AlOx (1.4 nm)/CoFe (3 nm)/NiFe (5 nm)/IrMn (15 nm)/Ta (10 nm) were fabricated on thermally oxidized Si substrates without any buffer layers using metal masks. The maximum tunneling magnetoresistance at room temperature was obtained as 19% for x=0.4.

Broadband ferromagnetic resonance of Ni81Fe19 wires using a rectifying effect

The broadband ferromagnetic resonance (FMR) measurement has been carried out by using a rectifying effect in two sets of

Magnetic switching properties of magnetic tunnel junctions using a synthetic ferrimagnet free layer

{\text{Ni}}_{81}{\text{Fe}}_{19}

Magnetic characterization and switching of Co nanorings in current-perpendicular-to-plane configuration

wires. One wire is deposited on the middle strip line of the coplanar waveguide (CPW) and another is deposited between two strip lines of the CPW, measuring the FMR induced by in-plane and out-of-plane magnetic fields, respectively. The FMR frequency is defined by detecting the magnetoresistance oscillation due to the magnetization dynamics induced by a radio frequency (rf) field. The magnetic-field dependence of the resonance frequency and the rectification spectrum are analytically interpreted based on our uniform magnetization precession model. These findings reveal that two distinct rectifying signals are anticipated by a rf field and a rf current, which can easily be controlled by engineering the ferromagnetic wire shape and the external field orientation. These fundamental understandings are crucial for future rf device applications in spintronics.