Simple rules for the understanding of Heusler compounds

Spintronics is a multidisciplinary field involving physics, chemistry, and engineering, and is a new research area for solid-state scientists. A variety of new materials must be found to satisfy different demands. The search for ferromagnetic semiconductors and stable half-metallic ferromagnets with Curie temperatures higher than room temperature remains a priority for solid-state chemistry. A general understanding of structure-property relationships is a necessary prerequisite for the design of new materials. In this Review, the most important developments in the field of spintronics are described from the point of view of materials science.

Spintronics: a challenge for materials science and solid-state chemistry.

Magnetic tunnel junctions (MTJs) with a stacking structure of Co2MnSi∕Al–O∕Co2MnSi were fabricated using magnetron sputtering system. Fabricated MTJ exhibited an extremely large tunneling magnetoresistance (TMR) ratio of 570% at low temperature, which is the highest TMR ratio reported to date for an amorphous Al–O tunneling barrier. The observed dependence of tunneling conductance on bias voltage clearly reveals the half-metallic energy gap of Co2MnSi. The origins of large temperature dependence of TMR ratio were discussed on the basis of the present results.

Giant tunneling magnetoresistance in Co2MnSi∕Al–O∕Co2MnSi magnetic tunnel junctions

In this paper, recent developments in magnetic tunnel junctions (MTJs) are reported with their potential impacts on integrated circuits. MTJs consist of two metal ferromagnets separated by a thin insulator and exhibit two resistances, low (Rp) or high (Rap) depending on the relative direction of ferromagnet magnetizations, parallel (P) or antiparallel (AP), respectively. Tunnel magnetoresistance (TMR) ratios, defined as (Rap $Rp)/Rp as high as 361%, have been obtained in MTJs with Co40Fe40B20 fixed and free layers made by sputtering with an industry-standard exchange-bias structure and post deposition annealing at Ta = 400 degC. The corresponding output voltage swing DeltaV is over 500 mV, which is five times greater than that of the conventional amorphous Al-O-barrier MTJs. The highest TMR ratio obtained so far is 500% in a pseudospin-valve MTJ annealed at Ta = 475 degC, showing a high potential of the current material system. In addition to this high-output voltage swing, current-induced magnetization switching (CIMS) takes place at the critical current densities (JCO) on the order of 106 A/cm2 in these MgO-barrier MTJs. Furthermore, high antiferromagnetic coupling between the two CoFeB layers in a synthetic ferrimagnetic free layer has been shown to result in a high thermal-stability factor with a reduced JCO compared to single free-layer MTJs. The high TMR ratio enabled by the MgO-barrier MTJs, together with the demonstration of CIMS at a low JCO, allows development of not only scalable magnetoresistive random-access memory with feature sizes below 90 nm but also new memory-in-logic CMOS circuits that can overcome a number of bottlenecks in the current integrated-circuit architecture

/pdf/magnetic-tunnel-junctions-for-spintronic-memories-and-beyond-2tm3rrxd78.pdf

Magnetic Tunnel Junctions for Spintronic Memories and Beyond

Giant tunneling magnetoresistance in Co_2MnSi/Al-O/Co_2MnSi magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) with a stacking structure of epitaxial Co2MnSi/Al–O barrier/poly-crystalline Co75Fe25 were fabricated using an ultrahigh vacuum sputtering system. The epitaxial Co2MnSi bottom electrode exhibited highly ordered L21 structure and very smooth surface morphology. Observed magnetoresistance (MR) ratios of 70% at room temperature (RT) and 159% at 2 K are the highest values to date for MTJs using a Heusler alloy electrode. A high spin-polarization of 0.89 at 2 K for Co2MnSi obtained from Julliere's model coincided with the half-metallic band structure that was predicted by theoretical calculations.

https://iopscience.iop.org/article/10.1143/JJAP.44.L1100/pdf

Huge Spin-Polarization of L21-Ordered Co2MnSi Epitaxial Heusler Alloy Film

We fabricated B2-ordered Co2MnAl and L21-ordered Co2MnSi Heusler alloy films by optimizing various fabrication conditions (substrate, composition of sputtering target, substrate and post-annealing temperature, etc) and applied these films to bottom electrodes of magnetic tunnel junctions (MTJs). We used Al-oxide insulating tunnel barriers for our MTJs and varied oxidation times of Al films to control qualities of the Al-oxide insulating layer and Heusler-alloy/Al-oxide interface. Observed tunnel magnetoresistance (TMR) ratios were extremely sensitive to the structure and surface morphology of the prepared Heusler alloy films. Epitaxially grown Heusler alloy films showed good structural quality, very flat surfaces and enhanced TMR ratios. The behaviour of the TMR ratios towards oxidation time for the preparation of the Al-oxide barriers and the measurement temperature dependence of the TMR ratios were quite different between the MTJs with Co2MnAl and Co2MnSi electrodes. The obtained TMR ratio of 83% at 2 K in the MTJ with epitaxially grown B2-ordered Co2MnAl was large among the MTJs with an amorphous Al-oxide tunnel barrier. This result suggests that B2-ordered Co2MnAl is a highly spin-polarized material, as predicted by our theoretical calculation. Moreover, we observed a very large TMR ratio of 159% at 2 K in the MTJ with a high-quality epitaxially grown L21-ordered Co2MnSi electrode. This TMR ratio is the highest value to date in MTJs using an amorphous Al-oxide tunnel barrier. Spin-polarization of the Co2MnSi bottom electrode obtained from Julliere's formula was about 0.89. This value is also the largest achieved to date for a Heusler material and is much larger than those of conventional ferromagnetic materials such as Co–Fe. This large spin-polarization is attributed to a half-metallic band structure, as predicted by theoretical calculations.

http://iopscience.iop.org/article/10.1088/0022-3727/39/5/S09/pdf

Large tunnel magnetoresistance in magnetic tunnel junctions using Co2MnX (X = Al, Si) Heusler alloys

Magnetic tunnel junctions were fabricated with epitaxially grown Co2MnAl bottom electrodes combined with an Al–O tunnel barrier using a magnetron sputtering system. The epitaxial Co2MnAl electrode had very low surface roughness of 0.2nm and a highly ordered B2 structure. Magnetic tunnel junctions (MTJs) with a stacking structure of epitaxial-Co2MnAl∕Al–O∕CoFe∕IrMn exhibited large tunnel magnetoresistance (TMR) ratios of 65% at room temperature and 83% at 10K. The TMR ratios were larger than those of a MTJ with a Co2MnAl polycrystalline electrode.

Magnetic tunnel junctions using B2-ordered Co2MnAl Heusler alloy epitaxial electrode

Magnetic tunnel junctions with a stacking structure of Cr/Co-Mn-Al/Al-O/Co-Fe/Ir-Mn were fabricated using a UHV magnetron-sputtering machine. Co-Mn-Al films showed B2 structure, which involves partial disorder between Mn and Al sites. Tunnel magnetoresistance ratios at room temperature were 27% and 40% before and after annealing at 250°C, respectively. Those values are much higher than the ones obtained in previous experiments using half Heusler alloys.

https://iopscience.iop.org/article/10.1143/JJAP.43.L984/pdf

Large Magnetoresistance in Magnetic Tunnel Junctions Using Co-Mn-Al Full Heusler Alloy

Thin films of the full-Heusler compound, Co2MnAl, were grown on Cr-buffered MgO(001) substrates at different growth temperature and post-annealing temperature by magnetron sputtering. Atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements show that Cr-buffer layer can largely improve the surface morphology and structural quality of Co2MnAl film. The (001)-oriented epitaxial growth and B2 structure were confirmed by XRD measurements for all prepared samples. The smallest surface roughness (~2 A) and an identical Ms value to the bulk value were obtained in the sample which was deposited at ambient temperature and post-annealed at 300°C.

https://iopscience.iop.org/article/10.1143/JJAP.44.6535/pdf

J. Nakata

Papers

Huge Spin-Polarization of L21-Ordered Co2MnSi Epitaxial Heusler Alloy Film

Large tunnel magnetoresistance in magnetic tunnel junctions using Co2MnX (X = Al, Si) Heusler alloys

Magnetic tunnel junctions using B2-ordered Co2MnAl Heusler alloy epitaxial electrode

Large Magnetoresistance in Magnetic Tunnel Junctions Using Co-Mn-Al Full Heusler Alloy

Fabrication of Co2MnAl Heusler Alloy Epitaxial Film Using Cr Buffer Layer