The tunnel magnetoresistance (TMR) effect in magnetic tunnel junctions (MTJs)1,2 is the key to developing magnetoresistive random-access-memory (MRAM), magnetic sensors and novel programmable logic devices3,4,5. Conventional MTJs with an amorphous aluminium oxide tunnel barrier, which have been extensively studied for device applications, exhibit a magnetoresistance ratio up to 70% at room temperature6. This low magnetoresistance seriously limits the feasibility of spintronics devices. Here, we report a giant MR ratio up to 180% at room temperature in single-crystal Fe/MgO/Fe MTJs. The origin of this enormous TMR effect is coherent spin-polarized tunnelling, where the symmetry of electron wave functions plays an important role. Moreover, we observed that their tunnel magnetoresistance oscillates as a function of tunnel barrier thickness, indicating that coherency of wave functions is conserved across the tunnel barrier. The coherent TMR effect is a key to making spintronic devices with novel quantum-mechanical functions, and to developing gigabit-scale MRAM.

Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions

The magnetic properties of Fe‐Si‐B‐M (M: additives) alloys prepared by annealing amorphous alloys made by the single roller method over their crystallization temperature have been investigated for development of new Fe‐based soft magnetic alloys. Excellent soft magnetic properties were obtained by adding the two elements Cu and Nb to Fe‐Si‐B alloys. It was found that these new alloys, called ‘‘FINEMET,’’ have an ultrafine grain structure composed of bcc Fe solid solution. They are suitable for many kinds of magnetic components such as saturable reactors, choke coils, and transformers, because they have superior soft magnetic properties and a high saturation flux density, and because different types of B‐H hysteresis loops are obtained by magnetic field annealing.

New Fe-based soft magnetic alloys composed of ultrafine grain structure

Magnetocaloric effect and magnetic refrigeration

A survey is given of the physical properties, composition and crystal structure of intermetallic compounds formed between rare-earth elements and 3d transition elements. Apart from binary compounds the results of pseudobinary series are also considered. The magnetic properties determined by the exchange interactions involving 4f as well as 3d electrons, are discussed together with experimental results available on magnetovolume effects and various resonance techniques such as NMR and the Mossbauer effect.

https://iopscience.iop.org/article/10.1088/0034-4885/40/10/002/pdf

Intermetallic compounds of rare-earth and 3d transition metals

An overview of the structure and magnetism of spinel ferrite nanoparticles and their synthesis in microemulsions

A magnetoresistive element has a ferromagnetic double tunnel junction having a stacked structure of a first antiferromagnetic layer/a first ferromagnetic layer/a first dielectric layer/a second ferromagnetic layer/a second dielectric layer/a third ferromagnetic layer/a second antiferromagnetic layer The second ferromagnetic layer that is a free layer consists of a Co-based alloy or a three-layered film of a Co-based alloy/a Ni—Fe alloy/a Co-based alloy A tunnel current is flowed between the first ferromagnetic layer and the third ferromagnetic layer

Magnetoresistive element and magnetic memory device

A complex new magnetic refrigerant, suitable for the ideal Ericsson cycle, has been investigated. Above ∼15 K it is necessary to use ferromagnets as a magnetic refrigerant. However, temperature variation for the magnetic entropy change in a homogeneous ferromagnet is not suitable for the Ericsson cycle. The present paper verifies, from theoretical analysis, that a complex ferromagnetic material, for instance, (ErAl2)0.312(HoAl2)0.198 (Ho0.5Dy0.5Al2)0.490, has the most suitable characteristics for the ideal Ericsson cycle, including two kinds of isomagnetic field processes. On the basis of the above consideration, a sintered layer structural complex has been prepared, composed of three kinds of RAl2.15 layers, where R’s are rare‐earth atoms. From specific heat measurements made on this complex, its entropy and entropy change have been determined. It has been concluded that the complex magnetic material is the most hopeful refrigerant for the Ericsson cycle.

New application of complex magnetic materials to the magnetic refrigerant in an Ericsson magnetic refrigerator

Thin films of CoxFe3−xO4−y (x=0.8, 1.0, 1.15, 1.25, 1.5, 1.8) were prepared under 100–500 eV Ar+‐ion bombardment during film deposition. Strong crystallization enhancement and preferred crystallographic orientation were observed as the effects of ion bombardment. Crystal orientation normal to the substrate depended on the composition x; 〈110〉‐axis orientation was obtained for x=0.8 and 1.0 and 〈111〉 orientation for x=1.15, 1.25, 1.5, and 1.8. The orientation was not affected significantly by the incident angle of the ions and did not vary with ion energy in the range 100–500 eV. The mechanism of crystal orientation is thought to have a close relationship with the preferential sputtering of oxygen by Ar+‐ion impact.

Preferred crystal orientation of cobalt ferrite thin films induced by ion bombardment during deposition

Multilayer Fe/Si films with constant Fe thickness (2.6 nm) and variable Si thickness are investigated. Negative magnetoresistance is observed and two different temperature dependences are found as a function of Si thickness. For ${t}_{\mathrm{Si}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.2$ nm, the magnetoresistance decreases with temperature decrease. For ${t}_{\mathrm{Si}}g1.5$ nm, the magnetoresistance increases (weakly) with temperature decrease. The magnetoresistance is attributed to spin-dependent scattering caused by antiferromagnetic layer coupling across a semiconducting spacer: narrow gap iron silicide for thin Si spacer layers and amorphous Si for thicker spacer layers.

Magnetoresistance associated with antiferromagnetic interlayer coupling spaced by a semiconductor in Fe/Si multilayers.

Oscillatory magnetoresistance as a function of the Fe layer thickness has been found in Fe/Cr(100) multilayers deposited on MgO(100). It is mainly attributed to an oscillatory interlayer exchange coupling between adjacent Fe layers as a function of the Fe layer thickness. Further, oscillatory variation of the saturation resistivity with Fe layer thickness was also observed. Both the oscillation periods are approximately 8 \AA{}. These two oscillations can be understood in terms of the partial confinement of the perpendicular motion to the layer plane of Fe majority spin electrons.

Koichiro Inomata

Papers

Magnetoresistive element and magnetic memory device

New application of complex magnetic materials to the magnetic refrigerant in an Ericsson magnetic refrigerator

Preferred crystal orientation of cobalt ferrite thin films induced by ion bombardment during deposition

Magnetoresistance associated with antiferromagnetic interlayer coupling spaced by a semiconductor in Fe/Si multilayers.

Two oscillatory behaviors as functions of ferromagnetic layer thickness in Fe/Cr(100) multilayers.