Manipulating magnetic and magnetoresistive properties by oxygen vacancy complexes in GCMO thin films.

The effect ofin situannealing is investigated in Gd0.1Ca0.9MnO3(GCMO) thin films in oxygen and vacuum atmospheres. We show that the reduction of oxygen content in GCMO lattice by vacuum annealing induced more oxygen complex vacancies in both subsurface and interface regions and larger grain domains when compared with the pristine one. Consequently, the double exchange interaction is suppressed and the metallic-ferromagnetic state below Curie temperature turned into spin-glass insulating state. In contrast, the magnetic and resistivity measurements show that the oxygen treatment increases ferromagnetic phase volume, resulting in greater magnetization (MS) and improved magnetoresistivity properties below Curie temperature by improving the double exchange interaction. The threshold field to observe the training effect is decreased in oxygen treated film. In addition, the positron annihilation spectroscopy analysis exhibits fewer open volume defects in the subsurface region for oxygen treated film when compared with the pristine sample. These results unambiguously demonstrate that the oxygen treated film with significant spin memory and greater magnetoresistance can be a potential candidate for the future memristor applications.

Tuned AFM-FM coupling by the formation of vacancy complex in Gd0.6Ca0.4MnO3thin film lattice.

The effect ofin situoxygen and vacuum annealings on the low bandwidth manganite Gd1-xCaxMnO3(GCMO) thin film withx= 0.4 was investigated. Based on the magnetic measurements, the AFM-FM coupling is suppressed by the vacuum annealing treatment via destroying the double exchange interaction and increasing the unit cell volume by converting the Mn4+to the Mn3+. Consequently, resistance increases significantly compared to pristine film. The results are explained by a model obtained from the positron annihilation studies, where the vacuum annealing increased the annihilation lifetime in A and B sites due to the formation of vacancy complexesVA,B-VO, which was not the case in the pristine sample. The positron annihilation analysis indicated that most of the open volume defects have been detected in the interface region rather than on the subsurface layer and this result is confirmed by detailed x-ray reflection analysis. On the other hand, the effect of oxygen annealing on the unit cell volume and magnetization was insignificant. This is in agreement with positron annihilation results which demonstrated that the introduction of oxygen does not change the number of cation vacancies significantly. This work demonstrates that the modification of oxygen vacancies and vacancy complexes can tune magnetic and electronic structure of the epitaxial thin films to provide new functionalities in future applications.

Energy Transfer into Molecular Vibrations and Rotations by Recoil in Inner-Shell Photoemission.

A mixture of CF_{4} and CO gases is used to study photoelectron recoil effects extending into the tender x-ray region. In CF_{4}, the vibrational envelope of the C 1s photoelectron spectrum becomes fully dominated by the recoil-induced excitations, revealing vibrational modes hidden from Franck-Condon excitations. In CO, using CF_{4} as an accurate energy calibrant, we determine the partitioning of the recoil-induced internal excitation energy between rotational and vibrational excitation. The observed rotational recoil energy is 2.88(28) times larger than the observed vibrational recoil energy, well in excess of the ratio of 2 predicted by the basic recoil model. The experiment is, however, in good agreement with the value of 2.68 if energy transfer via Coriolis coupling is included.

Mechanisms of photoinduced magnetization in Pr0.6Ca0.4MnO3 studied above and below charge-ordering transition temperature.

We report the effect of photonic field on the electronic and magnetic structure of a low bandwidth manganite [Formula: see text] [Formula: see text]MnO3 (PCMO) thin film. In particular, the present study confirmed a mechanism that was recently proposed to explain how optical excitation can bias or directly activate the metamagnetic transition associated with the colossal magnetoresistance (CMR) effect of PCMO. The transition is characterized by a shift in the dynamic equilibrium between ferromagnetic (FM) and antiferromagnetic clusters, explaining how it can be suddenly triggered by a sufficient external magnetic field. The film was always found to support some population of FM-clusters, the proportional size of which could be adjusted by the magnetic field and, especially in the vicinity of a thermomagnetic irreversibility, by optical excitation. The double exchange mechanism couples the magnetic degrees of freedom of manganites to their electronic structure, which is further coupled to the ion lattice via the Jahn-Teller mechanism. In accordance, it was found that producing optical phonons into the lattice could lower the free energy of the FM phase enough to significantly bias the CMR effect.

Analysis of electronic structure and its effect on magnetic properties in (001) and (110) oriented La0.7Sr0.3MnO3 thin films.

Epitaxial thin films of half-metallic oxide La0.7Sr0.3MnO3 (LSMO) have been grown in two crystalline orientations, one with the c-axis out-of-plane, the (001) orientation, and one with the c-axis in-plane, the (110) orientation. For the (110) oriented growth, there is no polar discontinuity at the substrate-film interface and hence no dead layer formation, which improves ferromagnetic interaction in the LSMO, making it highly attractive for spintronic applications. In our experiments, with x-ray diffraction, x-ray photoelectron spectroscopy and magnetic measurements, we have demonstrated that in the (110) oriented LSMO the lattice is more relaxed, leading to less deformation of electronic density around the La atom or in the MnO6 octahedra. This improved crystal and electronic structure improves the ferromagnetic properties of the films, making the Curie temperature higher by almost 15 K, which is of potential interest for spintronics. However, substrate strain induced magnetic anisotropy causes domain formation with out-of-plane components in these films, which poses some concern for practical spintronic devices.

Evolution of structural and magnetic properties with varying oxygen content in low-bandwidth manganite Pr0.9Ca0.1MnO3 thin films.

The effects of ex situ vacuum and oxygen annealing treatments on thin films of the low-bandwidth compound Pr(1-x)Ca(x)MnO(3) (PCMO) are investigated. Structural and magnetic measurements reveal that increased ferromagnetism can be achieved by oxygen annealing treatment, which is linked to the increased Mn(4+) ion content, as observed from x-ray photoelectron spectroscopy (XPS) measurements, as well as relaxation of the substrate-induced tensile strain of the PCMO unit cell. The increased number of Mn(4+) ions and partial release of strain lead to stronger double-exchange interaction in the system. Vacuum annealing increases the ferromagnetic (FM) interaction as well; however, the increased FM ordering is not directly related to the improved double-exchange interaction, as XPS measurement reveals an indication of a slight increase in Mn(3+) ions in this case. Trapping of carriers in the oxygen vacancies and formation of magnetic polarons have been suggested as the causes of the increase in ferromagnetic ordering, and this is also supported by the large coercivity and longer spin memory in the vacuum annealed PCMO.

Persistent photoinduced magnetization in the coexisting spin-glass and ferromagnetic phases of Pr0.9Ca0.1MnO3 thin film.

The persistent photoinduced magnetization (PPM) in the low bandwidth material Pr(1-x)Ca(x)MnO3 at the low hole doping level of x = 0.1 is reported. Upon zero-field cooling under photoexcitation, significant improvement of the ferromagnetic (FM) ordering was observed in the low temperature spin-glass phase. However, upon field cooling, the FM ordering was found to be suppressed due to weakening of the double-exchange interaction. High kinetic energy x-ray photoelectron spectroscopy measurements indicated a slight increase in the Mn³âº peak under photoexcitation which clarifies the weakening of the FM interaction. The fast relaxation of the PPM is discussed in view of localization of spin polarons in sites of magnetic disorders and the results are compared with previous reports of PPM in intermediate bandwidth Pr0.9Ca0.1MnO3 samples.

Optimization of Pr(0.9)Ca(0.1)MnO(3) thin films and observation of coexisting spin-glass and ferromagnetic phases at low temperature.

Optimization of thin films of small bandwidth manganite, Pr(1-x)Ca(x)MnO3 (for x = 0.1), and their magnetic properties are investigated. Using different pulsed laser deposition (PLD) conditions, several films were deposited from the stoichiometric target material on SrTiO3 (001) substrate and their thorough structural and magnetic characterizations were carried out using x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy (XPS), SQUID magnetometry and ac susceptibility measurements. A systematic investigation shows that irrespective of the growth temperature (between 550 and 750 °C), all the as-deposited films have twin boundaries and magnetic double phases. Post-annealing in partial or full oxygen pressure removes the extra phase and the twin boundaries. Zero-field-cooled magnetization data show an antiferromagnetic to paramagnetic transition at around 100 K whereas the field-cooled magnetization data exhibit a paramagnetic to ferromagnetic transition close to 120 K. However, depending on the oxygen treatments, the saturation magnetization and Curie temperature of the films change significantly. Redistribution of oxygen vacancies due to annealing treatments leading to a change in ratio of Mn3+ and Mn4+ in the films is observed from XPS measurements. Low temperature (below 100 K) dc magnetization of these films shows metamagnetic transition, high coercivity and irreversibility magnetizations, indicating the presence of a spin-glass phase at low temperature. The frequency dependent shift in spin-glass freezing temperature from ac susceptibility measurement confirms the coexistence of spin-glass and ferromagnetic phases in these samples at low temperature.

Fragmentation patterns of doubly charged acrylonitrile molecule following carbon core ionization.

Dissociation of acrylonitrile into pairs of cations and neutral fragments following molecular core ionization was investigated using the photoelectron-photoion-photoion coincidence (PEPIPICO) technique. The fragment ion mass spectra were recorded in coincidence with the carbon 1s photoelectrons. Deuterated and (13)C-substituted samples were used for resolving fragment mass ambiguities. Slope analysis of the PEPIPICO patterns was used in determining the fragment separation sequences in case of multiparticle processes. The results show that there are several fragmentation channels producing a wide range of charged coincident fragments. The dynamics of the dominant fragmentation processes is investigated in detail.