| Budget Amount *help |
¥15,100,000 (Direct Cost: ¥15,100,000)
Fiscal Year 2002: ¥1,300,000 (Direct Cost: ¥1,300,000)
Fiscal Year 2001: ¥13,800,000 (Direct Cost: ¥13,800,000)
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| Research Abstract |
One of the crucial issue for the creation of spintronics field is the development of half-metallic ferromagnet (HMF) films at RT, which have a band gap at the Fermi level (E_F) for one spin direction and thus exhibits 100% spin polarization at E_F. Heusler alloys are promising materials for spintronics applications, because a number of which have been predicted to be HMF. There are two distinct families of the Heusler alloys, one of which have the form XYZ and crystallize in the Cl_b structure, known as half-Heusler compounds. The second family of Heusler alloys are the so-called full-Heusler alloys and tey have X_2YZ formula with the L2_1 structure. The compounds of the type Co_2MnZ (Z = Si and Ge) have been predicted to be HMF by the first principle calculation. Experimentally, however, HMF for the Heusler alloys has not been demonstrated, despite of an extensive study. We grew Co_2Cr_<1-x>Fe_xAl (x = 0, 0.4) films at RT using a Co_2Cr_<1-x>Fe_xAl target on a thermally oxidized Si su
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bstrate by a conventional magnetron sputtering at a base pressure below 2 × 10^<-4>Pa. The MTJ with a 10^4 μm^2 junction area using a Co_2Cr_<1-x>Fe_xAl film was also fabricated utilizing metal masks on a thermally oxidized Si substrate without a buffer layer at RT. The tunnel barrier was formed by deposition of 1.4 nm Al, followed by plasma oxidized in the chamber during 60 seconds. We analyzed the film composition using the electron probe microanalysis (EPMA). The structure and roughness of the CCFA films were investigated by X-ray diffraction and X-ray reflectivity measurement, respectively. Magnetic properties were measured by using a vibrating sample magnetometer (VSM) and a SQUID magnetometer. The magnetoresistance measurement of the junctions was performed using a fourpoint technique. The Co_2Cr_<0.6>Fe_<0.4>Al Heusler alloy film revealed the B2 structure for both x = 0 and 0.4. The film exhibited the magnetic moment of 2.04 μ_B per formula unit for x = 0.4, nearly the integer number of Bohr magnetons, suggesting a localized nature of ferromagnetism like many Heusler compounds, which is a necessary condition for half metallicity. A spin valve type tunneling junction with a Co_2(Cr, Fe)Al Heusler alloy film was fabricated using metal masks, which consists of Co_2Cr_<0.6>Fe_<0.4>Al(10 nm)/AlO_x(1.8 nm)/CoFe (3 nm)/NiFe(5 nm)/IrMn (15 nm)/Ta (5 nm), deposited on a thermally oxidized Si substrate without a buffer layer. The junction demonstrated large tunneling magnetoresistance of 16% at room temperature and 26.5% at 5 K. This is the first observation of TMR using fullHeusler alloys. Less
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