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.
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.
Through the analysis of the magnetic properties and of the nuclear magnetic resonance response of La2/3Ca1/3MnO3 ceramics with different grain sizes, we have found that poorly conducting regions, some ferromagnetic and some weakly magnetic, are located at the surface of the grains. We state that these regions constitute the tunnel barrier responsible for the low-field magnetoresistance usually observed in powders of half-metallic oxides. In addition, the spin disorder accompanying the coexistence of phases with different magnetoelectronic character could contribute to the large high-field magnetoresistance also typical of such ceramic samples. From a more general perspective, these findings can be of relevance to understand the microscopic origin of phase separation in manganites.
95, 97 Mo NMR experiments have been performed on a series of Sr 2 FeMoO 6 and electron-doped Sr 2-x La x FeMoO 6 ceramics. Detailed analysis of the NMR spectra from pristine Sr 2 FeMoO 6 conclusively shows that the Mo hyperfine field is mainly due to atomic Mo magnetic moments. No contribution of transferred hyperfine field has been observed, confirming the absence of s-electrons in the conduction band. Upon La doping, the NMR frequency (hyperfine field) gradually increases proving that the concentration of spin polarized electrons at Mo ion is enhanced by the La substitution.A simple linear correlation between magnetic moment at Mo sites and the Curie temperature of the system has been found. Implications for understanding the electronic structure and the ferromagnetic coupling in these systems are underlined. PACS: 76.60.Lz , 75.20.Hr
Granular Co 10 Cu 90 alloys displaying giant magnetoresistance have been obtained by melt spinning followed by an appropriate heat treatment in the range 0-700°C. Their structural and magnetic properties have been studied on a microscopic scale using 59 Co NMR technique and thermoremanent magnetization measurements. The study reveals that in the as-quenched samples Co is involved in two main structural components: small, irregular, strained Co particles ͑60% of the entire Co population͒ and a composition modulated CoCu alloy. A high modulation amplitude of the concentration profile in the alloy subdivides the latter in two parts with distinctly different properties. One part consists of ferromagnetic alloy ͑average Cu concentration of about 20%͒ with a blocking temperature of about 35 K ͑involving 6% of the entire Co population in a sample͒. The other part, containing the remaining 34% of the entire Co population, is a paramagnetic alloy with a blocking temperature below 4.2 K. The ferromagnetic alloy is magnetically soft-its transverse susceptibility is lower by a factor of 7 than the transverse susceptibility of the quenched-in Co particles. The latter population has a blocking temperature of about 150-200 K. During the heat treatment, each of the two main structural components undergoes respective decomposition processes: both of them display two temperature regimes. One process consists in dissolving the quenched-in Co particles after annealing at around 400°C, followed at higher temperatures by a nucleation and growth of the more regular in shape Co particles. The other process resembles a spinodal decomposition of the quenched-in CoCu alloy, resulting in sharpening of the concentration profile and eventually leading to Co cluster formation in samples annealed above 450°C. Both processes end at about T an ϭ700°C, in formation of large, pure Co clusters that are ferromagnetic at least up to 400 K.
Single-crystalline NiMnSb(111) films with negligibly low defect levels have been grown epitaxially on GaAs(111)B using molecular beam epitaxy and characterized by nuclear magnetic resonance. In a film with only 1% deviation from stoichiometry, 1.1% of all Mn atoms is involved in planar defects, ∼0.5% of all Sb sites is occupied by AsSb substitutional atoms, and ∼0.2% of all Sb atoms has a modified environment. Both the average concentration of defects and the interface orientation are compatible with maintaining a half-metallic band structure at the ferromagnet/semiconductor interface, making these films a good candidate for spin injection into a semiconductor.
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