arXiv subjects

D. M. Phase

Publications and source records attributed to D. M. Phase.

At least 37 records · Page 2Linked to original sources

arXiv2019

Strain Healing of Spin-Orbit Coupling: A Cause for Enhanced Magnetic Moment in Epitaxial SrRuO3 Thin Films

Enhanced magnetic moment and coercivity in SrRuO3 thin films are significant issues for advanced technological usages and hence are researched extensively in recent times. Most of the previous reports on thin films with enhanced magnetic moment attributed the high spin state for the enhancement. Our magnetization results show high magnetic moment of 3.3 Bohr-magnetron/Ru ion in the epitaxial thin films grown on LSAT substrate against 1.2 Bohr-magnetron/Ru ion observed in bulk compound. Contrary to the expectation the Ru ions are found to be in low spin state and the orbital moment is shown to be contributing significantly in the enhancement of magnetic moment. We employed x-ray absorption spectroscopy and resonant valance band spectroscopy to probe the spin state and orbital contributions in these films. The existence of strong spin-orbit coupling responsible for the de-quenching of the 4d orbitals is confirmed by the observation of the non-statistical large branching ratio at the Ru M2,3 absorption edges. The relaxation of orbital quenching by strain engineering provides a new tool for enhancing magnetic moment. Strain disorder is shown to be an efficient mean to control the spin-orbit coupling.

cond-mat.str-el↗

arXiv2018

Interfacial spin glass mediated unusual spontaneous exchange bias effect in epitaxial self-assembled La0.7Sr0.3MnO3:NiO nanocomposite thin films

Zero field cooled spontaneous exchange bias (SEB) is observed in epitaxial La0.7Sr0.3MnO3 (LSMO):NiO self-assembled nanocomposite thin films grown on (001) SrTiO3 single crystal substrate by pulsed laser deposition. SEB is displayed by the novel asymmetry in the hysteresis loop of the composite film along with the field cooled conventional exchange bias (CEB) effect. The training effect shows that exchange bias relaxation is disorder mediated. It is revealed from DC magnetization results that such nanocomposite film divulges spin glass like behaviour, which is arising due to competing ferromagnetic and antiferromagnetic inteactions at the assorted interfaces of ferromagnetic LSMO and antiferromagnetic NiO. Also corroborated by x-ray magnetic circular dichroism (XMCD) measurements, we conclude that SEB is mainly originating due to ferromagnetic coupling of unstable interfacial antiferromagnetic spin due to NiO with the ferromagnetic LSMO at the disordered interface. These results of self-assembled thin films provide a useful input to realize and understand microscopic origin of SEB for device application.

cond-mat.mtrl-sci↗

arXiv2018

Electronic structure by X-ray absorption spectroscopy and observation offield induced unusually slowspin relaxation from magnetic properties in pyrochlore Eu$_{2-x}$Fe$_x$Ti$_2$O$_7$

X-ray absorption spectroscopy (XAS) as well as x-ray magnetic circular dichroism (XMCD) and magnetization of hybrid pyrochlore Eu2-xFexTi2O7 were investigated, where the rare earth Eu (4f) was replaced with transition metal Fe (3d) to introduce competing 4f-3d interactions. It is confirmed that the valence states of Eu and Fe ions are formally trivalent while that of Ti ions are tetravalent (3d0). The analysis yielded that the tetravalent Ti ions occupy octahedral sites with distorted Oh symmetry which is triggered by the presence of vacant 8a anionic site adjacent to TiO6 octahedra. Further study with Fe doping revealed that it essentially reduces the octahedral distortion by introducing anionic disorder (migration of 48f oxygen ions to 8a site). Analysis of O K edge XAS spectra further confirmed the Fe substitution causing the systematic change in the ligand (O2-) coordination of the Ti4+ cations. On the other hand, a new field induced transition (with Fe doping) at low temperature T* (4 K<T*< 8 K) in ac susceptibility with unusually slow spin relaxation was observed. The transition shifted towards higher temperatures both with increasing applied field and Fe concentration. However, the single ion spin freezing (Tf ~35 K) appears to be suppressed with Fe substitution. Interestingly, small amount of Fe3+ ion substitution showed significant enhancement in the dc magnetization at lower temperatures (<100 K). Analysis further indicated rise of dipolar FM exchange interaction with Fe doping.

cond-mat.mtrl-sci↗

arXiv2018

In situ soft x-ray absorption spectroscopic study of Fe/MgO interfaces

Interfacial reaction between iron and MgO has been studied in situ during deposition of iron on MgO surface, using soft x-ray absorption spectroscopy. High sensitivity of the technique which is capable of detecting even a sub monolayer of a given phase, combined with in situ measurements as a function of iron layer thickness, allowed one to make a quantitative estimate of interfacial phases. Two different substrates namely, MgO (001) single crystal, and a polycrystalline MgO film on Si substrate have been used in order to elucidate the role of the state of MgO surface in controlling the interface structure. It is found that at the interface of iron and MgO film, about two monolayers of Fe3O4 is formed. Fe3O4 being the oxide of iron with the highest heat of formation, the reaction appears to be controlled thermodynamically. On the other hand, on the interface with MgO (001) surface, FeO is formed, suggesting that the reaction is limited by the availability of oxygen atoms. Magnetic behaviour of the FeO layer gets modified significantly due to proximity effect of the bulk ferromagnetic iron layer.

cond-mat.mtrl-sci↗

arXiv2018

Density and Microstructure of Amorphous Carbon Thin Films

In this work, we studied amorphous carbon ($a$-C) thin films deposited using direct current (dc) and high power impulse magnetron sputtering (HiPIMS) techniques. The microstructure and electronic properties reveal subtle differences in $a$-C thin films deposited by two techniques. While, films deposited with dcMS have a smooth texture typically found in $a$-C thin films, those deposited with HiPIMS consist of dense hillocks surrounded by a porous microstructure. The density of $a$-C thin films is a decisive parameter to judge their quality. Often, x-ray reflectivity (XRR) has been used to measure the density of carbon thin films. From the present work, we find that determination of density of carbon thin films, specially those with a thickness of few tens of nm, may not be accurate with XRR due to a poor scattering contrast between the film and substrate. By utilizing neutron reflectivity (NR) in the time of flight mode, a technique not commonly used for carbon thin films, we could accurately measure differences in the densities of $a$-C thin films deposited using dcMS and HiPIMS.

cond-mat.mtrl-sci↗

arXiv2016

Giant Rashba effect at the topological surface of PrGe revealing antiferromagnetic spintronics

Rashba spin-orbit splitting in the magnetic materials opens up a new perspective in the field of spintronics. Here, we report a giant Rashba-type spin-orbit effect on PrGe [010] surface in the paramagnetic phase with Rashba coefficient α_R=5 eVÅ. Significant changes in the electronic band structure has been observed across the phase transitions from paramagnetic to antiferromagnetic (44 K) and from antiferromagnetic to the ferromagnetic ground state (41.5 K). We find that Pr 4f states in PrGe is strongly hybridized with the Pr 5d and Ge 4s-4p states near the Fermi level. The behavior of Rashba effect is found to be different in the k_x and the k_y directions showing electron-like and the hole-like bands, respectively. The possible origin of Rashba effect in the paramagnetic phase is related to the anti-parallel spin polarization present in this system. First-principles density functional calculations of Pr terminated surface with the anti-parallel spins shows a fair agreement with the experimental results. We find that the anti-parallel spins are strongly coupled to the lattice such that the PrGe system behaves like weak ferromagnetic system. Analysis of the energy dispersion curves at different magnetic phases showed that there is a competition between the Dzyaloshinsky-Moriya interaction and the exchange interaction which gives rise to the magnetic ordering in PrGe. Supporting evidences of the presence of Dzyaloshinsky-Moriya interaction are observed as anisotropic magnetoresistance with respect to field direction and first-order type hysteresis in the X-ray diffraction measurements. A giant negative magnetoresistance of 43% in the antiferromagnetic phase and tunable Rashba parameter with temperature across the magnetic transitions makes this material a suitable candidate for technological application in the antiferromagnetic spintronic devices.

cond-mat.mtrl-sci↗

arXiv2016

Electronic Structure of FeAl Alloy Studied by Resonant Photoemission Spectroscopy and Ab Initio Calculations

Resonant photoemission spectroscopy has been used to investigate the character of Fe 3d states in FeAl alloy. Fe 3d states have two different character, first is of itinerant nature located very close to the Fermi level, and second, is of less itinerant (relatively localized character), located beyond 2 eV below the Fermi level. These distinct states are clearly distinguishable in the resonant photoemission data. Comparison between the results obtained from experiments and first principle based electronic structure calculation show that the origin of the itinerant character of the Fe 3d states is due to the ordered B2 structure, whereas the relatively less itinerant (localized) Fe 3d states are from the disorders present in the sample. The exchange splitting of the Fe 3s core level peak confirms the presence of local moment in this system. It is found that the itinerant electrons arise due to the hybridization between Fe 3d and Al 3s-3p states. Presence of hybridization is observed as a shift in the Al 2p core-level spectra as well as in the X-ray near edge absorption spectra towards lower binding energy. Our photoemission results are thus explained by the co-existence of ordered and disordered phases in the system.

cond-mat.mtrl-sci↗

arXiv2015

Correlation of size and oxygen bonding at the interface of Si nanocrystal in Si-SiO2 nanocomposite: A Raman mapping study

Si-SiO2 multilayer nanocomposite (NCp) films, grown using pulsed laser deposition with varying Si deposition time are investigated using Raman spectroscopy/mapping for studying the variation of Si phonon frequency observed in these NCps. The lower frequency (LF) phonons (~ 495 - 510 cm-1) and higher frequency (HF) phonons (~ 515 - 519 cm-1) observed in Raman mapping data (Fig. 1A) in all samples studied are attributed to have originated from surface (Si-SiO2 interface) and core of Si nanocrystals, respectively. The consistent picture of this understanding is developed using Raman spectroscopy monitored laser heating/annealing and cooling (LHC) experiment at the site of a desired frequency chosen with the help of Raman mapping, which brings out clear difference between core and surface (interface) phonons of Si nanocrystals. In order to further support our attribution of LF being surface (interface) phonons, Raman spectra calculations for Si41 cluster with oxygen termination are performed which shows strong Si phonon frequency at 512 cm-1 corresponding to the surface Si atoms. This can be considered analogous to the observed phonon frequencies in the range 495 - 510 cm-1 originating at the Si-SiO2 interface (extended). These results along with XPS data show that nature of interface (oxygen bonding) in turn depends on the size of nanocrystals and thus LF phonons originate at the surface of smaller Si nanocrystals. The understanding developed can be extended to explain large variation observed in Si phonon frequencies of Si-SiO2 nanocomposites reported in the literature, especially lower frequencies.

cond-mat.mes-hall↗

arXiv2014

Effect of gamma irradiation on the structure and valence state of Nd in phosphate glass

Fourier transform infrared (FTIR) spectra and X-ray photoelectron spectra (XPS) of Nd doped phosphate glasses have been studied before and after gamma irradiation in order to find the behavior of chemical bonds, which decide the structural changes in the glass samples. IR absorption spectra of these glasses are found dominated mainly by the characteristics phosphate groups, water (OH) present in the glass network as well as on the composition of glass matrix. The effects of gamma irradiation are observed in the form of bond breaking and possible re-arrangement of the bonding in the glass. Energy dispersive X-ray spectroscopy (EDX) and XPS measurements show changes in the relative concentration of elements; particularly decrease in the concentration of oxygen in the glass samples after gamma irradiation, a possible source of oxygen vacancies. The decrease in the asymmetry in O 1s spectra after gamma irradiation indicates towards decrease in the concentration of bridging oxygen as a result of P-O-P bond breaking. Asymmetric profile of Nd 3d5/2peak after gamma irradiation is found to be due to conversion of Nd3+ to Nd2+ in the glass matrix.

cond-mat.mtrl-sci↗

arXiv2014

Valence Band Onset and Valence Plasmons of SnO2 and In2-xSnx O3 Thin Films

Valence band onset (Ev), valence band tail (VBT) and valence plasmons (VPs) have been studied as a function of sputtering of SnO2 and In2-xSnxO3 (ITO) thin films, using ultraviolet photoemission spectroscopy (UPS). Decrease in Ev with respect to the Fermi level and increase in the density of energy levels of VBT have been observed after 5 minutes of sputtering using Ar+ ions (500V). Bulk and surface components of VPs of Sn, SnO and SnO2 in sputtered SnO2 thin films have been observed in UPS spectra. Similarly, bulk and surface components of VPs of In, Sn and oxygen deficient ITO in sputtered ITO thin films have been observed in UPS spectra. Possible roles of Ev and increase in the density of energy levels of VBT are discussed in the mechanisms of current transport through heterojunctions of SnO2 with semiconductors.

cond-mat.mtrl-sci↗

arXiv2013

Studies on temperature dependent semiconductor to metal transitions in ZnO thin films sparsely doped with Al

For a detailed study on the semiconductor to metal transition (SMT) in ZnO thin films doped with Al in the concentration range from 0.02 to 2%, we grew these films on (0001) sapphire substrates using sequential pulsed laser deposition. It was found that the Al concentration in the films increased monotonically with the ratio of ablation durations of the Alumina and ZnO targets used during the deposition. Using X-ray photo electron spectroscopy it was found that while most of the Al atoms occupy the Zn sites in the ZnO lattice, a small fraction of the Al also gets into the grain boundaries present in the films. The observed SMT temperature decreased from ~ 270 to ~ 50 K with increase in the Al concentration from 0.02 to 0.25 %. In the Al concentration range of ~ 0.5 to 2 % these doped ZnO films showed metallic behavior at all the temperatures without undergoing any SMT. A theoretical model based on thermal activation of electrons and electron scatterings due to the grain boundaries, ionic impurities and phonons has been developed to explain the observed concentration and temperature dependent SMT.

cond-mat.mtrl-sci↗

arXiv2009

Effect of 200 MeV Ag15+ ion irradiation on structural and electrical transport properties of Fe3O4 thin films

Thin films of Fe3O4 have been deposited on single crystal MgO (100) and Si (100) substrates using pulsed laser deposition. Films grown on MgO substrate are epitaxial with c-axis orientation whereas, films on Si substrate are highly <111> oriented. Film thicknesses are 150 nm. These films have been irradiated with 200 MeV Ag ions. We study the effect of the irradiation on structural and electrical transport properties of these films. The fluence value of irradiation has been varied in the range of 5 x 1010 ions/cm2 to1 x 1012 ions/cm2. We compare the irradiation induced modifications on various physical properties between the c-axis oriented epitaxial film and non epitaxial but <111> oriented film. The pristine film on Si substrate shows Verwey transition (TV) close to 125 K, which is higher than generally observed in single crystals (121 K). After the irradiation with the 5 x 1010 ions/cm2 fluence value, TV shifts to 122 K, closer to the single crystal value. However, with the higher fluence (1 x 1012 ions/cm2) irradiation, TV again shifts to 125 K.

cond-mat.mtrl-sci↗

arXiv2009

Structural, electrical and magnetic properties of Mo1-xFexO2 (x=0-0.05) thin films grown by pulsed laser ablation

We report the growth of undoped and Fe (2 and 5 at. percentage) doped molybdenum oxide thin films on c-plane of sapphire substrate using pulsed laser ablation. X-ray diffraction results show that the films are oriented in (100) direction and have monoclinic structure based on MoO2 phase as also supported by Raman spectroscopy. The x-ray photoelectron spectroscopy reveals chemical state of Fe is +2, which favors the substitutional occupancy of Fe ion in the MoO2 matrix. The room temperature resistivity of all the films are very low (approx.100 microohm-cm). The Fe doped samples show ferromagnetic behavior at room temperature.

cond-mat.mtrl-sci↗

arXiv2009

Electrical and magnetic transport properties of Fe3O4 thin films on GaAs (100) substrate

Thin films of magnetite (Fe3O4) are grown on single crystal GaAs (100) substrate by pulsed laser deposition. X ray diffraction (XRD) result shows the (111) preferred orientation of the Fe3O4 film and x-ray photoelectron spectroscopy confirm the presence of single phase Fe3O4 in the film. The electrical transport property of the film shows the characteristic Verwey transition at 122 K and below 110 K, the transport follows variable range hopping type conduction mechanism. The film shows room temperature magnetization hysteresis loop suggesting the ferrimagnetic behavior of the film with saturation magnetization value close to 470 emu/cc.

cond-mat.mtrl-sci↗

arXiv2008

Electronic structure of Fe (0-5 at. %) doped MoO2 thin films studied by resonant photoemission spectroscopy

The electronic structure of pulsed laser deposited Mo1-xFexO2 (x=0, 0.02 and 0.05) thin films has been investigated using resonant photoemission spectroscopy near Mo-4p absorption edge. In all the samples a broad Fano- like resonance peak at ~46 eV is observed in the whole area of the valence band which indicates the contribution of the Mo-4d states in the entire valence band region. The doping of Fe in these films leads to decrease in Mo 4d states contributing to electronic states at lower binding energy region. In addition to this, we also observe a shoulder at 4.9 eV in the valence band spectra of doped samples. It is proposed that the origin of this shoulder is due to the Fe hybridised states.

cond-mat.mtrl-sci↗

arXiv2008

Effect of strain on the electronic structure of La0.7Ca0.3MnO3

The effect of substrate strain on the electronic valence band structure of La0.7Ca0.3MnO3 thin films has been investigated. For this purpose La0.7Ca0.3MnO3 thin films have been simultaneously grown on SrTiO3 and LaAlO3 substrates using pulsed laser deposition technique. These samples were characterized by x-ray diffraction, x-ray reflectivity, core level x-ray photoelectron spectroscopy, and scanning electron microscopy measurements. Our experiments confirmed that all these samples have same chemical composition but different strain configuration. The electronic structure of these samples is probed through valence band spectroscopy measurements on Indus-1 synchrotron x-ray source. We observe that strain has large effect on the valence band electronic structure of La0.7Ca0.3MnO3. The results are explained on the basis of strain induced orbital occupancy and change in the crystal field splitting due to Mn-O bond length.

cond-mat.str-el↗

arXiv2008

Origin of resistivity minima at low temperature in ferromagnetic metallic manganites

The resistivity and magnetoresistance measurements were carried out on thin film of La0.7Ca0.3MnO3 to investigate the possible origin of low temperature resistivity minimum observed in these samples. We observed large hysteresis in the magnetoresistance at low temperature; 5K and the sample current I has large effect on resistivity minima temperature. The observation of hysteresis at low temperatures suggests the presence of inhomogeneity at low temperatures. These in-homogeneities consist of regions of different resistive phases. It appears that the high resistive phase prevents the tunneling of charge carriers between two low resistive regions and thus giving rise to the resistivity minimum in these samples.

cond-mat.str-el↗

arXiv2008

Dispersion of Resonant Raman Peaks of CO and OH in SnO2, Mo1-x FexO2 Thin Films and SiO2 bulk glass

Resonance Raman (RR) peaks of and stretching modes and their higher harmonics have been observed superimposed on photoluminescence (PL) spectrum of thin films. Commercial fluorine doped thin films deposited by sputtering on glass and thin films deposited on Si by laser ablation have been studied. The dispersions of CO and OH stretching RR modes are ~ 600 cm-1/eV and 800 cm-1 respectively. The dispersion of the third harmonic of CO stretching mode is ~ 2000 cm-1/eV. Similar dispersion of RR peak of stretching modes and higher harmonics superimposed on PL spectra has been observed in Mo1-xFexO2 thin films and SiO2 bulk glass. Large dispersion of RR peaks seems to be a common property of oxides with impurities of and .

cond-mat.mtrl-sci↗