An infrared study of antiferromagnetic medium and high entropy rocksalt structure oxides and post-annealing

The infrared reflectance of a set of antiferromagnetic entropy-stabilized rocksalt structure oxides is measured in order to study the effect of a random array of cations, spin-phonon coupling and anharmonicity on phonon mode behaviour. Samples were prepared by conventional solid state reaction from quaternary or quinary equimolar mixtures of CoO,ZnO,CuO,NiO and MgO. They were characterized by magnetometry, x-ray diffraction and reflectance spectroscopy. It is found that the Néel temperature (TN) depends on the concentration of magnetic ions. The far-infrared reflectance spectra consist of one strong restrahlen band and a weaker low frequency band, which is unexpected in a rocksalt structure. The resonance frequency of the main transverse optical (ωTO) mode varies between 355 < ωTO < 386 cm-1 and is controlled by both the average reduced mass of a cation-anion pair and the average effective charge. Unlike the parent compounds CoO and NiO, no splitting of ωTO due to spin-phonon coupling appeared in the reflectance spectra below TN. This is possibly due to the high scattering rate of the associated transverse optical mode, which is connected to static disorder.

J. Appl. Phys. 130 184103 (2021)

An infrared transmission study of Ge:Mn thick films prepared by ion implantation and post-annealing

Ge:Mn thick films (t≈ 3μ m) with low average Mn concentration (≤ 0.3%) were prepared by ion implantation at 77 K followed by either conventional or flashlamp annealing. The films were characterized by x-ray diffraction, secondary ion mass spectrometry, magnetometry, and infrared transmission (100–6500 cm-1). Post-annealing at a high enough temperature recrystallizes the amorphous Ge:Mn films without significant migration of Mn to the surface, while solid phase epitaxy does not occur, resulting in polycrystalline films. Annealing causes an estimated 50%–80% of the implanted Mn to migrate to Mn-rich clusters or form Mn5Ge3, while the remainder enters the Ge lattice substitutionally creating free holes. Evidence for free holes comes from the structure in the mid-infrared absorption coefficient that is similar to previous observations in p-type Ge. The data suggest that the maximum solubility of Mn in the Ge crystalline lattice has an upper limit of ≤ 0:08%.

J. Appl. Phys. 127 103902 (2020)

Wide range dielectric and infrared spectroscopy of (Nb+In) co-doped rutile ceramics

The dielectric response of ceramics of co-doped rutile Ti1-x(Nb0.5In0.5)xO2 has been measured via a combination of impedance, high-frequency co-axial, THz transmission and infrared reflectivity spectrscopies spanning 15 decades of freqency between 0.1 Hz and 240 THz. It is argued that the colossal dielectric permittivity reported by Hu et al. [Nat. Mater. 12, 821 (2013)] has the same explanation as in the original colossal permittivity material (CCTO)., namely, a combination of the internal barrier layer capacitor (IBLC) and the surface barrier layer capacitor (SBLC) effects. The IBLC effect is caused by a microstructure consisting of insulating grain boundaries (thickness ~1 nm and conductivity ~10-6 S/cm) surrounding interior regions of bulk conductivity approximately 10-1 S/cm. The SBLC is the result of a depletion layer adjacent to the contacts approximately 100 nm thick with conductivity of 5 X 10-7 S/cm. the SBLC and IBLC effects give rise to dielectric relaxation rates measured in low-frequency and radiofrequency regions, respectively. The temperature dependence of both relaxation rates measured down to 20 K is thermally activated. No separate absorption process has been observed that could be linked to giant defect dipoles. Infrared spectroscopy has revealed four weak defect-related modes and frequencies close to the Raman mode frequencies of rutile, activated in the infrared through a symmetry-breaking process. Co-doping produces a significant loss in the partial spectral weight between 75 and 350 cm-1 associated with the lowest transverse optical mode, which softens upon cooling. The loss in its spectral weight corresponds to the gain in the near-infrared spectral weight, previously assigned to small polaron absorption.

Phys. Rev. Mat. 2 075002 (2018)

Origin of the colossal permittivity of (Nb+In) co-doped rutile ceramics by wide-range dielectric spectroscopy

Searching for giant dipolar defects, suggested as the explanation of the colossal permittivity (CP) in (Nb+In) co-doped rutile, we have recently published (J.Appl.Phys. 199, 154105 (2016) and Phys. Rev. Mat. (in press) temperature-dependent dielectric spectra from sub-Hz to THz range (including 4-pt. DC conductivity) of several (Nb+In) co-doped rutile ceramics. Using the model of two leaky capacitor in series below ~1 MHz, we estimate the depletion layer thickness and conductivity. The spectra below ~10 GHz were fitted with two Cole-Cole relaxations obeying the Arrhenius law, assigned to the effect of depletion layers and grain boundaries. In this paper, using effective medium models, we have for the first time modelled the spectra as a composite of semiconducting grains and substantially less-conducting grain boundaries and near-electrode depletion layers. The CP effect has been fully explained by the combination of surface and internal barrier-layer capacitor effects, without any measureable contribution from the dipolar defects.

Phase Transitions 91 932 (2018)

Electrode effects in dielectric spectroscopy measurements on (Nb+In)co-doped TiO2

Recently, several papers reported the discovery of giant permittivity and low dielectric loss in (Nb+In) co-doped TiO2. A series of tests was performed which included the measurement of the frequency dependence of the dielectric permittivity and alternating current (ac) conductivity of co-doped (Nb+In)TiO2 as a function of electrode type, sample thickness, and temperature. The data suggest that the measurements are strongly affected by the electrodes. The consistency between four-contact van der Pauw direct current conductivity measurements and bulk conductivity values extracted from two-contact ac conductivity measurements suggest that the values of colossal permittivity are, at least in part, a result of Schottky barrier depletion widths that depend on electrode type and temperature.

J. of Applied Physics 119 154105 (2016)

Infrared Spectroscopy of Cr- and V- doped Sb2Te3: Dilute magnetic semiconductors

Temperature Dependent optical reflectance measruements on well characterized samples of nonintentionally doped, Cr-doped, and V-doped Sb2Te3 show that both the parent compound and the Cr-doped version are narrow-gapped semiconductors (Eg~ 0.25 eV) with a conventional Drude free carrier absorption. The carrier density increases slightly with decreasing temperature while the scattering rate increases quadratically with temperature, which is a sign of the importance of optical phonon scattering. Vanadium doping introduces a change in the temperature dependence of the scattering rate as well as higher electrical resistivity than Cr-doped Sb2Te3. An analysis of the literature values of the saturation magnetization of Sb2-xVxTe3 for H parallel to the c-axis suggests V is in a mixed valence state V3+/4+.

Phys. Rev. B 90 205205 (2014)

Ge:Mn Dilute magnetic semiconductors

This work aims to develop Ge:Mn dilute magnetic semiconductor and study the fundamental origin of ferromagnetism in this system. Using ion implantation at 77 K, a single crystal Ge wafer was doped with magnetic Mn ions. The implantation was done at ion energy of 4.76 MeV with a fluence of 2 x 1016 ion/cm2. X-ray diffraction (XRD) of the as-implanted sample showed that the implanted layer was amorphous. An implanted sample was annealed at 330 oC in a tube furnace to achieve a solid phase epitaxial regrowth of the implanted layer. XRD of the sample annealed at 330 oC for 33 hours showed a polycrystalline layer. The depth profile of Mn in the as-implanted sample and the post-annealed sample at 330 oC was determined using secondary ion mass spectroscopy (SIMS) and it was found that some Mn diffused to the surface during the annealing. A SQUID was used to measure the magnetic properties of all samples. At low temperature, the as-implanted sample showed a paramagnetic behaviour. A magnetic hysteresis at 5K and up to 200K was observed for the sample annealed at 330 oC .

Physics in Canada 70 166 (2014)

A Study of Ferromagnetic Signals in SrTiO3 Nanoparticles

It has been suggested that ferromagnetism may be a universal fature of nanoparticles related to particle size. We study this claim for the case of commercially produced SrTiO3 nanoparticles purchased from Alfa-Aesar. Both loosely-packed nanoparticle samples and pellets formed using uniaxial pressure were studied. Both loose and pressed samples were annealed in either air or vacuum of 5X10-6 Torr at 600,800 and 1000oC. Then x-ray diffraction and SQUID measurements were made on the resulting samples. It was found that annealed loose powder samples always had a linear diamagnetic magnetization versus field response, while their pressed pellet counterparts exhibit a ferromagnetic hysteresis component in addition to the linear diamagnetic signal. Williamson-Hall analysis reveals that the particle size in pressed pellet samples increases with annealing temperature but does not change significantly in loose powder samples. The main conclusion is tha the act of pressing pellets in a die produces a spurious ferromagnetic signal into SQUID measurements.

AIP Conf. Proc. 1590 Electronic,Photonic, Plasmonic, Phononic and Magnetic Properties of Nanomaterials (2014) 87.

Reflectance Spectroscopy of Ba3+xZn1+yNb2O9 Perovskites

Complex perovskites in the Ba3+xZn1+yNb2O9 family were studied via reflectance spectroscopy for photon energies between .006 and 1 eV. %at various temperatures. These materials are of interest as potential dielectric resonator materials which require large ε1 (to enable device miniaturization), large Q~ ε12 (for selectivity) and small temperature dependence of optical functions (for device stability). The dielectric functions were modeled by fitting the reflectance spectra to both Lorentz oscillator and factorized dielectric functions in order to get a sense of the uncertainty in extrapolating the measured far-infrared dielectric function to the microwave (MW) region (300 GHz). Both models suggest that for the stoichiometric composition ε1 ~ 40 while the extrapolated value of ε2 has much more uncertainty. The extrapolated value of Q~ 20000 at room temperature at MW frequency for Ba3ZnNb2O9 and Q is largest near stoichiometric composition. ε1 is only weakly composition dependent, except for the sample furthest from stoichiometric composition (Ba2.7ZnNb2O9). Comparison of the present data for Ba3ZnNb2O9 with previous work reveals that the phonon scattering rates and low frequency ε2 are much higher in the present samples yielding lower Q values, which were prepared at somewhat higher temperature than previous workers. It is possible that microstructure - which depends strongly on sample preparation temperature - is influencing ε2 in the microwave region more strongly than deviation from ideal stoichiometry Ba3ZnNb2O9.

Progress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug. 12-15 (2013) 225.

Infrared Spectroscopy and the ferromagnetic transition in Gd

The low energy electronic structure of Gd has been investigated experimentally by infrared reflectance spectroscopy, and theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital (LMTO) method in the local spin density approximation (LSDA) as well as within the LSDA+U approach. The reflectance of a Gd single crystal was measured with the electric field in the plane perpendicular to the c-axis for temperatures between 50K and slightly above the Curie Temperature (293K) in the frequency range between 100 and 12000 cm-1 (0.013 to 1.5 eV). As Gd enters the ferromagnetic state, the dissipative part of the optical conductivity exhibits interesting spectral weight transfers over the whole spectral range measured. It is shown that the {\em ab initio} calculations reproduce well the experimental spectra in the ferromagnetic state and allow one to explain the microscopic origin of the optical response of Gd in terms of interband transitions.

J. Phys. Cond. Mat. 25 (2013) 036002

Far-IR excitations in Cd2Re2O7 in the normal and superconducting states

Cd2Re2O7 is a pyrochlore superconductor with a transition temperature Tc near 1K. We report on the far-infrared optical properties of Cd2Re2O7 at temperatures above and below Tc with a particular emphasis on changes in the spectrum below Tc. Seventeen phonon modes are observed in the normal state optical conductivity spectrum of Cd2Re2O7 at low temperatures in good agreement with a factor group analysis. In the superconducting state, a softening (~1 cm-1) of the phonon modes at 35 and 65 cm-1 occurs and thermal reflectance spectra show the development of two additional strong absorption features, near 9.6 and 19.3 cm-1. The dominant presence of lattice vibrations in the optical spectrum suggests that the electron-phonon interaction plays an important role in the normal and superconducting state of Cd2Re2O7 .

J. Phys. Cond. Mat. 24 (2012) 505701

Ferromagnetism on the unpolished surfaces of single crystal metal oxide substrates

It is shown that a variety of single crystal substrates (Al2O3, LaAlO3, SrTiO3, TiO2, and ZnO), purchased from commercial suppliers, contain a ferromagnetic component to the magnetization in addition to the expected linear magnetic response. This ferromagnetic contribution is only observed on the unpolished surfaces and can be eliminated by either polishing or annealing at 600oC in air, but not by annealing at 600oC in a vacuum of 5 x 10-6Torr. Particle induced x-ray emission spectra demonstrate that there is excess Fe on the unpolished surfaces of these single crystal substrates. While defect related ferromagnetic signals have been reported in some of these substrates, and while our results do not exclude this origin of ferromagnetism, we clearly show that the ferromagnetic signals observed in our samples are largely due to excess iron on the unpolished surfaces, possibly in the form of a mixture of Fe, Fe3O4, and or y-Fe2O3.

J. Appl. Phys. 110 (2011) 033906

Effect of vanadium deficiency on properties of polycrystalline LaVO3

We report the influence of v deficiency on structure, transport, and magnetic properties of polycrystalline LaVO3. Up to 10% V-deficient samples were synthesized using standard solid state chemistry route. Structural and spectroscopic studies show that it intricately modifies lattice parameters and oxidation states of V. Further temperature dependent resistivity data reveal that V deficiency induces an enhancement in the resistivity and activation energy. The paramagnetic moment of these samples depends on net V composition while all samples exhibit a paramagnetic-to-antiferromagnetic transition at about 140 K. These distinct properties are understood owing to the presence of multiple oxidations states of V, predominantly V3+, and V4+, in nonstoichiometric LaVO3.

Appl. Phys. Letters 98 (2011) 052509

A search for defect related ferromagnetism in SrTiO3

Room temperature ferromagnetic hysteresis is observed in commercial SrTiO3 substrates purchased from a variety of suppliers. It is shown that the ferromagnetic signal comes from the unpolished surfaces. Surface impurity phases cannot be detected using either x-ray diffraction or energy dispersive x-ray spectra on the unpolished surfaces. However, a possible correlation between surface disorder (x-ray diffraction peak linewidth) and ferromagnetism is observed. Ar ion bombardment (10-90 keV) can be used to produce surface layer disorder but is not found to induce ferromagnetism. Annealing of the substrates at temperatures ranging from 600 to 1100 oC is found to alter the hysteresis curves differently depending on whether the annealing is performed in air or in vacuum. Identical annealing behavior is observed if the substrates are artificially spiked with iron. This suggests that the ferromagnetic hysteresis of as purchased SrTiO3 could be due to Fe contamination of the unpolished surfaces. In addition, it is shown that no ferromagnetism is observed in samples that contain 1019-1021 cm-3 of oxygen vacancies if all the faces are polished.

J. of Appl. Phys. 108 (2010) 053908

Evidence for random networks of diodes in thin films of LaVO3 on SrTiO3 substrates

Highly conducting interfaces of transition metal oxide heterolayers could provide a promising access to transition metal oxide based electronic devices similar to those based on semiconductor heterostructures. Recently, metallic conductivity has been reported at the LaAlO3/SrTiO3 heterointerface. Here we report the observation of diodelike behavior and the formation of a random diode network in thin films of LaVO3 deposited on as-polished SrTiO3 substrates. Depending on films annealing conditions we observed linear or nonlinear I-V characteristic below 50 K.

Appl. Phys. Letters 96 (2010) 042110

Kramers-Kronig-constrained variational dielectric fitting and the reflectance of a thin film on a substrate

A test was made of the ability of Kramers-Kronig constrained variational dielectric fitting to extract the optical conductivity of a thin film from reflectance data containing structure due to both thin film and substrate. The reflectance of a series of well-characterized thin films of SrRuxMg1-xO3 and SrRuxO3 with a variety of thicknesses (56-300 nm) and dc resistivities (250-2200 &mu&Omega cm) was measured. The low frequency values of the extracted optical conductivities agree with the dc measurements, however, removal of features due to the substrate improves with increasing film thickness.

Applied Optics 47, (2008) 4205-4211

Infrared active phonons in SrRuxO3 and SrRuxMg1-xO3 thin films

Infrared reflectance measurements (100-5000 cm-1) were performed on well characterized thin films of SrRuxMg1-xO3 and SrRuxO3 fabricated on [001] oriented SrTiO3 single crystal substrates using pulsed laser deposition. In the films, which were less than 300 nm thick and some of which were insulating, the reflectance data show structure due to both the films and the substrate. Kramers-Kronig constrained variational dielectric fitting was employed to extract the real optical conductivities. Only a small number of infrared active modes appear in the reflectance spectra compared with the number allowed by orthorhombic symmetry in shell model calculations. The infrared signature for replacement of Ru by Mg in SrRuO3 is a mode near 655 cm-1.

J. Phys. D: Appl. Phys. 41, (2008) 135007.

Metal-insulator transition in Mg-doped SrRuO3thin films prepared by laser ablation

Thin films of magnesium doped SrRuO3(Mg-SRO) have been successfully prepared by laser ablation on SrTiO3 (100) substrates. The stoichiometry, structural, electrical and magnetic properties of the films depend on the substrate temperature (Ts) and oxygen pressure (PO2)during deposition. All Mg-SRO films are ferromagnetic, but the transport properties and remanent moments vary with (Ts) and (PO2). Metallic films are produced for Ts >= 700° C whereas insulating films can be produced with Ts = 500° C with PO2>=27Pa. A correlation between lattice parameter, stoichiometry and transport properties has been discovered: the larger lattice parameter correlates with the decreased Ru content and more insulating samples. Insulating samples prepared at Ts = 500° C and PO2>=27Pa have stoichiometry close to the SrMg0.15Ru0.85O3 and are ferromagnetic Anderson insulators with transport properties consistent with variable range hopping.

J. Phys. D: Appl. Phys. 39, (2006) 6-13.

Absence of the 90 K structural transition in CuV2S4crystals grown by chemical vapour transport using TeC14

Various physical properties (magnetization, specific heat, optical reflectance, electrical resistivity) of CuV2S4crystals grown by chemical vapour transport using TeC14as the transporting agent have been measured. The data show slight differences compared to samples grown using different techniques. These differences include the absence of a sharp drop in magnetization and the absence of a peak in the heat capacity near 90 K. These differences suggest that the cubic-tetragonal phase transition near 90 K does not occur in these particular crystals. The reflectance of the same crystals has been studied from (70-20000 cm-1) for temperatures between 40 and 300 K and the data are consistent with those for a disordered metal. A high frequency absorption, perhaps an interband transition, has been observed in addition to absorption due to strongly scattered free carriers.

J. Phys. Cond.Matt., 17, 4813-23, 2005.

Metal-insulator transition in SrMgxRu1-xO3 system

SrMgxRu1-xO3 ceramics for 0 <~ x <~ 0.15 have been prepared by conventional solid-state reaction. The samples have been studied by x-ray diffraction, dc resistivity, magnetic susceptibility, and optical reflectance. Mg substitution produces little change in the lattice parameters and does not destroy the ferromagnetic transition but renders the material insulating.

Phys. Rev. B, 65, 224407, 2002.

Optical properties of highly-reduced SrTiO3-x

The reflectance of highly reduced SrTiO3-x has been studied in samples where the carrier concentration-or oxygen-vacancy density - varies from about 10(18) to 10(21)cm(-3). Several phonon modes are strongly influenced by oxygen-vacancy density. Oxygen reduction induces a mid-infrared band whose oscillator strength scales with carrier density. In highly reduced samples, a weakly localized plasmon mode develops.

Phys. Rev. B, 59, 12842, 1999.

Far-infrared optical properties of antiferromagnetic SmTiO3

The polarized reflectance of nominally stoichiometric SmTiO3 has been measured in the far-infrared at temperatures above and below the antiferromagnetic Neel temperature T-N of 50 K. The appearance of a feature near 120 cm(-1) when the electric field vector is polarized along the b axis correlates with the onset of magnetic ordering in the Ti and Sm sublattices, suggesting it is likely due to a magnetic transition. Its large linewidth and relatively high-energy scale point to a two-magnon process. Because the RTiO3 family of compounds (where R is a rare earth or Y) is susceptible to doping by vacancies on the R site, the nominally stoichiometric sample examined has a small, but finite background conductivity of the order 20 Omega(-1)cm(-1) at low frequencies. This background conductivity, like that along the semiconducting c axis in underdoped cuprate superconductors appears to be of an incoherent nature with a magnitude below Mott's minimum metallic conductivity. At temperatures below T-N there is evidence that the continuum interacts strongly with a nearby allowed phonon mode causing considerable asymmetry. The correlation between the onset of magnetic ordering and the appearance of asymmetry in the allowed phonon mode suggests that this continuum, to which the mode appears to be strongly coupled, may be of magnetic origin.

Phys. Rev. B, 59, 6938-42, 1999.

Raman scattering in the RTiO3 family of Mott-Hubbard insulators

Raman-scattering measurements have been carried out for crystals of the RTiO3 R=La,Ce,Pr,Nd,Sm,Gd) system whose members are Mott-Hubbard insulators. RTiO3 hasan orthorhombically distorted perovskite unit cell. The distortion increases systematically from LaTiO3 to GdTiO3 and is accompanied by changes in electronic structure (decreasing W/U ratio). As a consequence of the changing electronic properties, he Raman spectrum shows an interesting evolution of both the phonon features and the electronic continuum. Most notable are (1) a redistribution in the spectral shape of the electronic background, (2) a systematic change in line shape, and a dramatic increase in the center frequency of one of the phonon modes from 187 cm-1 in LaTiO3 to 385 cm-1 in GdTiO3, and (3) the observation of resonance effects in the most insulating members of the series. The appearance of a free-carrier component in the electronic-scattering background, which seems to be related to systematic self-energy effects of the phonon near 300 cm-1, is unexpected. It is likely the result of increased doping due to a greater facility for rare-earth vacancies to form in large R3+ionic radius members of the series. A systematic increase in the continuum scattering rate is also observed and indicates that the free carriers are not scattering off rare-earth vacancies but rather that the scattering mechanism originates from changes in electronic structure.

Phys. Rev. B, 55, 1442-8, 1997.

Technique for measuring the reflectance of irregular, submillimeter-sized samples.

Details are given of a technique for measuring the reflectance at near-normal incidence of small, irregular, submillimeter-sized samples from the far IR (40 cm-1) to the visible (40000 cm-1) between 10 and 300 K by using a modified Michelson interferometer or grating spectrometer. The sample and a reference mirror are mounted on nonreflecting cones. At the focus the size of the beam is larger than either the sample or the reference, so that the entire area of the sample is utilized. The positions are interchanged by a 90 degree rotation by using preset mechanical stops. The scattering caused by geometrical effects is corrected for by the in situ evaporation of gold or aluminum onto the sample. The effect of diffraction is estimated from Mie theory by assuming the sample and reference to be spheres. For frequencies above ~40 cm-1 and sample diameters of ~1 mm with a detector field of view of 30 degrees, the calculations show that the ratio of the backscattered intensities gives a good approximation of the specular reflectance.

Appl. Opt., 32, 2976-83, 1993.