55Mn nuclear magnetic resonance experiments are reported on a series of fully strained epitaxial La(2/3)Ca(1/3)MnO3 thin films on SrTiO3. We have found evidence of multiple phase segregation into ferromagnetic metallic and nonmetallic regions as well as regions that are nonferromagnetic and insulating. These insulating regions are mainly located close to interfaces and may have a significant impact on the performance of spin-tunnel devices. As a result of phase segregation, the ferromagnetic coupling within the metallic regions is depressed. This accounts for the reduction of the Curie temperature and conductivity in nanometric thin films.

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, Co 2 Cr 1−x Fe x Al(CCFA(x)) and Co 2 FeSi 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 Co 2 FeSi 0.5 Al 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 L2 1 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 CoFe 2 film deposited on a MgO (001) single crystal substrate, wherein the spinel structure of CoFe 2 O 4 (CFO) and an epitaxial relationship of MgO(001)[100]/CoFe 2 (001)]110]/CFO(001)[100] were induced. A SFD consisting of CoFe 2 /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 CoFe 2 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.

We have studied the thickness dependence of the magnetotransport properties of La 2/3 Ca 1/3 MnO 3 thin films epitaxially grown on SrTiO 3 , LaAlO 3 , and NdGaO 3 single-crystalline substrates. When thickness decreases, a global disruption of the magnetoelectronic properties occurs, namely the resistivity and the low-temperature magnetoresistance increase while the metal-to-insulator transition temperature (T P ) is lowered. We state that the electronic properties of these films, especially close to the film/substrate interface, differ from those of the bulk material. This is confirmed by nuclear-magnetic-resonance measurements which provide evidence that these films have an inhomogeneous magnetoelectronic nanostructure with distinguishable regions containing localized charges. These regions are scattered within the films, with a higher density close to interfaces in the case of La 2/3 Ca 1/3 MnO 3 films on SrTiO 3 but more homogeneously distributed for films grown on NdGaO 3 . Since our manganite films have a virtually unrelaxed crystal structure, the thickness dependence of T P can neither be related to the strain states nor to dimensional effects. Alternatively, we show that the coexistence of different electronic phases leads to a modification of the carrier density in the metallic regions and, presumably, to an enhancement of the disorder in the Mn-O bond length and Mn-O-Mn angles. We will argue that the conjunction of both factors promotes a decrease of the double exchange transfer integral and, consequently, accounts for the reduction of the Curie temperature for the thinnest films. The possible mechanisms responsible for this phase separation are discussed in terms of the microstructure of the interfaces between the manganite and the insulating perovskite.

We have investigated the structure and magnetization of Co2(Cr1−xFex)Al (0 ⩽ x ⩽ 1) and Co2FeSi full-Heusler alloy films deposited on thermally oxidized Si (SiO2) and MgO (001) single crystal substrates by ultra-high vacuum sputtering at various temperatures. The films were also post-annealed after deposition at room temperature (RT). Magnetic tunnel junctions with a full-Huesler alloy electrode were fabricated with a stacking structure of Co2YZ (20 nm)/Al (1.2 nm)-oxide/Co75Fe25 (3 nm)/IrMn (15 nm)/Ta (60 nm) and microfabricated using electron beam lithography and Ar ion etching with a 102 µm2 junction area, where Co2YZ stands for Co2(Cr1−xFex)Al or Co2FeSi. The tunnel barriers were formed by the deposition of 1.2 nm Al, followed by plasma oxidization in the chamber. The x-ray diffraction revealed the A2 or B2 structure depending on heat treatment conditions and the substrate, but not L21 structure for the Co2(Cr1−xFex)Al (0 ⩽ x ⩽ 1) films. The L21 structure, however, was obtained for the Co2FeSi films when deposited on a MgO (001) substrate at elevated temperatures above 473 K. The maximum tunnelling magnetoresistance (TMR) was obtained with 52% at RT and 83% at 5 K for a junction using a Co2(Cr0.4Fe0.6)Al electrode. While the junction using a Co2FeSi electrode with the L21 structure exhibited the TMR of 41% at RT and 60% at 5 K, which may be improved by using a buffer layer for reducing the lattice misfit between the Co2FeSi and MgO (001) substrate.

Spin-dependent coherent tunneling has been experimentally observed in high-quality sputtered-deposited Co 2 FeAl/ MgO/ CoFe epitaxial magnetic tunneling junctions ͑MTJs͒. Consequently, the microfabricated MTJs manifest a very large tunnel magnetoresistance ͑TMR͒ at room temperature and an unexpectedly TMR oscillation as a function of MgO barrier thickness. First-principles electronic band calculations confirm the pronounced coherent tunneling effect and are in good agreement with the experimental data. The present work demonstrates the importance of coherent tunneling for large TMR with Heusler alloys

We report on the structural and magnetic characterization of ͑110͒ and ͑001͒ La 2/3 Ca 1/3 MnO 3 ͑LCMO͒ epitaxial thin films simultaneously grown on ͑110͒ and ͑001͒SrTiO 3 substrates, with thicknesses t varying between 8 and 150 nm. It is found that while the in-plane interplanar distances of the ͑001͒ films are strongly clamped to those of the substrate and the films remain strained up to well above t Ϸ 100 nm, the ͑110͒ films relax much earlier. Accurate determination of the in-plane and out-of-plane interplanar distances has allowed concluding that for t Ͼ 20 nm, unit cell volume expansion does not change substantially for ͑001͒ films whereas it relaxes towards bulk value for ͑110͒ ones. However, in all cases, an abnormal unit cell expansion is observed for t Ͻ 20 nm. It is observed that the magnetic properties ͑Curie temperature and saturation magnetization͒ of the ͑110͒ films are significantly improved compared to those of ͑001͒ films. These observations, combined with 55 Mn-nuclear magnetic resonance data and x-ray photoemission spectroscopy, signal that the depression of the magnetic properties of the more strained ͑001͒LCMO films is not caused by an elastic deformation of the perovskite lattice but rather due to the electronic and chemical phase separation caused by the substrateinduced strain. On the contrary, the thickness dependence of the magnetic properties of the less strained ͑110͒LCMO films are simply described by the elastic deformation of the manganite lattice. We will argue that the different behavior of ͑001͒ and ͑110͒LCMO films is a consequence of the dissimilar electronic structure of these interfaces.

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