Highly spin-polarized materials and devices for spintronics∗

Abstract

The performance of spintronics depends on the spin polarization of the current. In this study half-metallic Co-based full-Heusler alloys and a spin filtering device (SFD) using a ferromagnetic barrier have been investigated as highly spin-polarized current sources. The multilayers were prepared by magnetron sputtering in an ultrahigh vacuum and microfabricated using photolithography and Ar ion etching. We investigated two systems of Co-based full-Heusler alloys, Co₂Cr_{1 - x} Fe _x Al (CCFA(x)) and Co₂FeSi_{1 - x} Al _x (CFSA(x)) and revealed the structure and magnetic and transport properties. We demonstrated giant tunnel magnetoresistance (TMR) of up to 220% at room temperature and 390% at 5 K for the magnetic tunnel junctions (MTJs) using Co₂FeSi_0.5Al_0.5 (CFSA(0.5)) Heusler alloy electrodes. The 390% TMR corresponds to 0.81 spin polarization for CFSA(0.5) at 5 K. We also investigated the crystalline structure and local structure around Co atoms by x-ray diffraction (XRD) and nuclear magnetic resonance (NMR) analyses, respectively, for CFSA films sputtered on a Cr-buffered MgO (001) substrate followed by post-annealing at various temperatures in an ultrahigh vacuum. The disordered structures in CFSA films were clarified by NMR measurements and the relationship between TMR and the disordered structure was discussed. We clarified that the TMR of the MTJs with CFSA(0.5) electrodes depends on the structure, and is significantly higher for L2₁ than B2 in the crystalline structure. The second part of this paper is devoted to a SFD using a ferromagnetic barrier. The Co ferrite is investigated as a ferromagnetic barrier because of its high Curie temperature and high resistivity. We demonstrate the strong spin filtering effect through an ultrathin insulating ferrimagnetic Co-ferrite barrier at a low temperature. The barrier was prepared by the surface plasma oxidization of a CoFe₂ film deposited on a MgO (001) single crystal substrate, wherein the spinel structure of CoFe₂O₄ (CFO) and an epitaxial relationship of MgO(001)[100]/CoFe₂ (001)]110]/CFO(001)[100] were induced. A SFD consisting of CoFe₂ /CFO/Ta on a MgO (001) substrate exhibits the inverse TMR of - 124% at 10 K when the configuration of the magnetizations of CFO and CoFe₂ changes from parallel to antiparallel. The inverse TMR suggests the negative spin polarization of CFO, which is consistent with the band structure of CFO obtained by first principle calculation. The - 124% TMR corresponds to the spin filtering efficiency of 77% by the CFO barrier.

Keywords: Co-ferrites; Heusler alloys; magnetic tunnel junctions; nuclear magnetic resonance; spin filters; spin polarization.

Figure 1

Schematic representation of the DOS for a half metal.

Figure 2

L2₁ crystalline structure of full-Heusler alloys.

Figure 3

Spin-resolved DOS for Co₂MnZ compound with Z = Al, Si, Ge and Sn [13].

Figure 4

Schematic illustration of origin of gap in minority bands in full-Heusler alloys. Here, d1, d2 and d3 denote the d_xy, d_yz and d_zx orbitals, respectively, and d4 and d5 the d_3z2-r2 and d_x2-y2 orbitals, respectively [13].

Figure 5

(a) Calculated total spin moments for full Heusler alloys. The dashed line represents the Slater–Pauling behavior. (b) Schematic band structure for explaining the Z_t - 12 rule for total magnetization [13].

Figure 6

(a) Total DOS for defective (solid line) and ideal (dashed line) Co₂MnSi with (a) Mn antisite and (b) Co antisite defects in Co₂MnSi [14].

Figure 7

(a) XRD patterns obtained with Cu-Kα source for as-deposited Co₂(Cr_{1 − x}Fe_x)Al films fabricated at RT on thermally oxidized Si substrates. The inset shows the (200) reflection intensity relative to that of the (220) peak. (b) Lattice constant as a function of x in Co₂(Cr_{1 − x}Fe_x)Al films. The triangles represent the lattice constants for the bulk materials with x= 0 and 1 and the L2₁ structure.

Figure 8

X-ray reflectivity for 100 nm thick Co_{2 (}Cr_0.6Fe_0.4) Al film deposited at RT and at elevated temperatures.

Figure 9

(a) Temperature dependence of magnetization for Co₂Cr_{1 − x}Fe_xAl films with x= 1.0, 0.6, 0.4 and 0 from the top, and (b) Fe concentration dependence of the magnetic moment per formula unit estimated from the magnetization measurements at 5 K. The dotted line represents the theoretical values in the case of the L2₁ structure.

Figure 10

TMRs at RT and 5 K as a function of x for junctions with Co₂Cr_{1 − x}Fe_xAl electrode.

Figure 11

TMR curves at RT and 5 K for junction with Co₂ (Cr_0.4 Fe_0.6 )Al electrode.

Figure 12

(a) XRD patterns and (b) x-ray reflectivity of Co₂FeAl films deposited on SiO₂ substrates at various temperatures. (c) AFM image of Co₂FeAl deposited on SiO₂ substrates at 773 K.

Figure 13

TMR and resistance-area (RA) product at RT as a function of substrate temperature for junctions with Co₂FeAl electrode deposited on SiO₂ substrates.

Figure 14

⁵⁹Co NMR spectra for Co₂FeAl films with A2 and B2 structures on SiO₂ substrates.

Figure 15

(a) In-plane (upper) and out-of-plane (downward) XRDs for 20-nm-thick Co₂FeAl films deposited on MgO substrates at various temperatures. (b) Intensity of (200) relative to that of (400). The broken line indicates the theoretical intensity.

Figure 16

(a) X-ray reflectivity for Co₂FeAl films deposited at various temperatures and (b) AFM image for the film deposited at 773 K on MgO (001) substrates.

Figure 17

(a) TMR as a function of the substrate temperature for junctions deposited on SiO₂ and MgO (001) substrates with a Co₂FeAl electrode. (b) Temperature dependence of TMR for junctions deposited at RT on SiO₂ and MgO (001) substrates with a Co₂FeAl electrode.

Figure 18

In-plane (200) rocking curves for Co₂FeSi and Co₂FeAl thin films deposited on a MgO(100) substrate at 773 K.

Figure 19

TMR at RT as functions of the temperatures of substrate heating (T_s) and annealing (T_a) after deposition at RT for MgO(001)/Co₂FeSi/AlO_x /CoFe/IrMn/Ta junctions.

Figure 20

TMR curves at RT (a) and 5 K (b) for the MTJ with a Co₂FeSi electrode. (c) Temperature dependence of TMR and RA.

Figure 21

XRD patterns for (a) out-of-plane and (b) f scans for (111).

Figure 22

Surface roughness measured by AFM for CFSA films with various annealing temperatures.

Figure 23

TMR at RT as a function of post-annealing temperature for junctions with CFSA electrodes with different MgO layer thicknesses.

Figure 24

TMR curves at RT and 5 K for junctions with CFSA electrodes with post-annealing at (a) 430 °C and (b) 500 °C.

Figure 25

Cross-sectional TEM with low and high magnifications of the multilayer prepared under the same fabrication condition as the MTJ with 220% TMR.

Figure 26

(a) Local structure around Co atom in L2₁-Co₂FeAl. (b) ⁵⁹Co NMR spectrum in bulk L2₁-Co₂FeAl.

Figure 27

(a) NMR spectra for CFSA films on Cr-buffered MgO(001) for T_a = 400 ° C and 500 °C. (b) Ordered L2₁ structure of Co₂FeSi_0.5Al_0.5.

Figure 28

Concept of spin-filtering device using an insulating ferromagnetic barrier.

Figure 29

Schematic of bias voltage dependence of TMR in a spin filter.

Figure 30

Calculated DOS for Co ferrite with inverse spinel structure.

Figure 31

(a) In-plane XRD pattern for surface-oxidized CoFe₂ films on MgO (001) single crystal substrates (oxidation time is 900 s). The inset shows φ scan on Co-ferrite 440 plane. (b) Electron diffraction pattern and (c) cross-sectional TEM image for MgO(001)/CoFe₂ /Co-ferrite.

Figure 32

Hysteresis loops at RT for nonoxidized and oxidized CoFe₂ thin films.

Figure 33

Tunneling magnetoresistance at (a) RT and (b) 5 K for spin filter consisting of CoFe₂ /CoFe₂-O_x.(600 s)/Ta (5 nm) on a MgO (001) single crystal substrate. The bias voltage is 5 mV.

Figure 34

(a) Current density (J) – voltage (V) curve at 1 kOe (parallel state) at 10 K. The circles represent experimental data and solid line is curve fitted using Simmons equations. (b) Major R–H curve at 5 K and 5 mV and (c) minor R–H curve at 10 K and 1 mV for SFD consisting of CoFe₂ (20 nm)-O_x. (900 s)/Ta (10 nm) on a MgO (001) single-crystal substrate.