Optical,electrical and magnetic properties of nanostructured Mn3O4 synthesized through a facile chemical route

Nanostructured Mn₃O₄ sample with an average crystallite size of ∼15 nm is synthesized via the reduction of potassium permanganate using hydrazine. The average particle size obtained from the Transmission Electron Microscopy analysis is in good agreement with the average crystallite size estimated from X-ray diffraction analysis. The presence of Mn⁴⁺ ions at the octahedral sites is inferred from the results of Raman, UV–visible absorption and X-ray photoelectron spectroscopy analyzes. DC electrical conductivity of the sample in the temperature range 313–423 K, is about five orders of magnitude larger than that reported for single crystalline Mn₃O₄ sample. The dominant conduction mechanism is identified to be of the polaronic hopping of holes between cations in the octahedral sites. The zero field cooled and field cooled magnetization of the sample is studied in the range 20–300 K. The Curie temperature for the sample is about 45 K, below which the sample is ferrimagnetic. A blocking temperature of 35 K is observed in the field cooled curve. It is observed that the sample shows hysteresis at temperatures below the Curie temperature with no saturation, even at an applied field (20 kOe). The presence of an ordered core and disordered surface of spin arrangements is observed from the magnetization studies. Above the Curie temperature, the sample shows linear dependence of magnetization on applied field with no hysteresis characteristic of paramagnetic phase.     

Magnetic properties of rare-earth transition metal aluminides R6T4Al43 with Ho6Mo4Al43-type structure

The magnetic properties of 53 aluminium-rich intermetallic compounds R₆T₄Al₄₃ with R=rare-earth elements and T=Ti, V, Nb, Ta, Cr, Mo, W were investigated using polycrystalline samples and a SQUID magnetometer in the temperature range from 2 to 300 K with magnetic flux densities up to 5.5 T. The yttrium and lutetium compounds are Pauli paramagnetic, indicating that the transition metal atoms do not carry magnetic moments. The samarium compounds show van Vleck behavior and antiferromagnetic order with Néel temperatures of less than 12 K. Of these Sm₆Ti₄Al₄₃ becomes metamagnetic. The ytterbium compounds show a mixed or intermediate valent behavior and no magnetic order down to 2 K. All other compounds obey the Curie–Weiss law above 30 K. Their effective magnetic moments correspond to the theoretical moments of the rare-earth ions. They show ferromagnetic or metamagnetic behavior with ordering temperatures all below 20 K. The magnetization curves of most compounds (recorded up to 5.5 T) reach about 50% of the theoretical magnetization already at 0.5 T. The gadolinium compounds are exceptional in that they reach at 0.5 T only about 10% of their theoretical magnetization. The crystal structures of the isotypic compounds Yb₆V₄Al₄₃ and Yb₆Ta₄Al₄₃ were refined from single-crystal X-ray data.     

57Fe Mössbauer spectroscopy and magnetization study of YFexAl12−x (4.4⩽x⩽5)

YFe_xAl_12−x in the composition range 4.4⩽x⩽5 was prepared by induction melting followed by annealing in vacuum at 1270 K. Magnetization data below 150 K show complex magnetic behaviour dependent on applied field, composition and temperature. The transition temperature T_c, corresponding to the main maximum of the magnetization vs. temperature curves and below which magnetic interactions are observed for a significant fraction of the Fe atoms in the Mössbauer spectra, decreases from 180 K for x=4 down to 100 K for 4.2⩽x⩽4.7 and rises again up to 160 K for x=5. The analysis of the spectra obtained at 5 K is consistent with full occupation of the 8f sites by Fe atoms and sharing of the 8j sites by Fe and Al as deduced from the Rietveld analysis of X-ray powder diffraction data. The Mössbauer spectra further show a dependence of magnetic hyperfine fields and isomer shifts on the crystallographic site and on the number of the Fe nearest neighbours similar to that observed in UFe_xAl_12−x (4⩽x⩽6) and RFe_xAl_12−x (R=Y, Lu, x=4, 4.2). The magnetic properties of the UFe_xAl_12−x and YFe_xAl_12−x series are compared and the magnetic interactions between the different Fe sublattices are discussed.     

Uniaxial pressure effect on magnetic susceptibility of perovskite manganite La0.66Ba0.34MnO3

Magnetization of La_0.66Ba_0.34MnO₃ and its temperature behavior under a uniaxial pressure of 0.1 kbar are measured between 5 and 270 K in magnetic fields 0<H<120 Oe. The magnetization represents nearly linear dependence on an external magnetic field. Temperature dependence of the magnetic susceptibility found represents a plateau, that is considered as an evidence of the formation of a long period magnetic structure (probably a sort of helix) below the Curie point. Pressure derivative of magnetization displays a sharp minimum at 200 K, pointing to an instability of electronic structure of the compound near this temperature.     

Magnetic properties of RPdIn (R=Gd–Er) compounds

AC susceptibility and DC magnetization measurements were performed for the RPdIn (R=Gd–Er) compounds both in the paramagnetic and in the ordered state. In opposite to GdPdIn, which is a ferromagnet (T_c=102 K), the other samples show a complex ferrimagnetic behavior with the additional transition at T_t<T_c. In the high-temperature phase (for T_t<T<T_c), a ferromagnetic interaction dominates, while in the low-temperature phase (for T⩽T_t) antiferromagnetic interactions with the magnetocrystalline anisotropy, especially strong for TbPdIn, come into play. The ordering temperatures are T_c=70, 34, 25 and 12.3 K for Tb-, Dy-, Ho- and ErPdIn respectively, while transition temperatures are T_t=6, 14 and 6 K for Tb-, Dy- and HoPdIn respectively. TbPdIn reveals an additional transition at 27 K connected with the intermediate ferrimagnetic phase. The critical fields for the magnetization process of the low-temperature phase are high (52 and 150 kOe for TbPdIn and 32 kOe for DyPdIn at T=4.2 K) yet these values decrease remarkably with increasing temperature. Results of the study are compared with magnetic and neutron diffraction data hitherto available. We state that irreversibility of the zero-field cooled–field cooled magnetization is not connected with the spin-glass phase claimed elsewhere.     

Effect of Cr substitution on the superconducting and magnetic properties of RuSr2Eu1.5Ce0.5Cu2O10−δ

In this paper we investigate the properties of polycrystalline series of Ru_1?xCr_xSr₂Eu_1.5Ce_0.5Cu₂O_10?δ (0.0 ? x ? 0.40) by resistivity, XRD and dc magnetization measurements. EuRu-1222 is a reported magneto superconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity occurs in Cu–O₂ planes below T_c ? 40 K. The exact nature of Ru spins magnetic ordering is still being debated and no conclusion has been reached yet. In this work, we found the superconducting transition temperature T_c = 20 K from resistivity and dc magnetization measurements for pristine sample. DC magnetization measurements exhibited ferromagnetic like transition for all samples.     

Exchange bias effects in ferromagnetic wires

We have investigated the exchange bias effect in micron-sized ferromagnetic wires made from Co and Ni₈₀Fe₂₀ films. The wires were fabricated using optical lithography, metallization by sputtering and lift-off technique. Magnetotransport measurements were performed at temperatures ranging from 3 to 300 K. We observed marked changes in the magnetoresistance (MR) properties as the temperature is varied. At 300 K, the field at which the sharp peak occurs corresponding to the magnetization reversal of the Co wires is 167 Oe and is symmetrical about the origin. As the temperature was decreased to 3 K, we observed a shift in the peak positions of the MR characteristics for both the forward and reverse field sweeps corresponding to a loop shift of 582 Oe in the field axis. The asymmetric shift in the MR loops at low temperatures clearly indicates the exchange bias between ferromagnetic (Co) and antiferromagnetic parts (Co-oxide at the surfaces) from natural oxidation. Ni₈₀Fe₂₀ wires of the same geometry showed similar effect with a low exchange bias field. The onset of exchange biasing effect is found to be 70 and 15 K for the Co and Ni₈₀Fe₂₀ wires, respectively. A striking effect is the existence of exchange biasing effect from the sidewalls of the wires even when the wires were capped with Au film.     

A magnetic study of the CeScSi-type RZrSb compounds (R=Gd–Tm). Large coercive field at low temperature in TbZrSb and DyZrSb

X-ray single crystal refinement performed on the GdZrSb and HoZrSb compounds confirms the CeScSi-type structure for the RZrSb series. Spontaneous magnetization is detected for all the studied compounds (5 K (TmZrSb)<T_C<61 K (TbZrSb)). Both TbZrSb and DyZrSb compounds exhibit a second magnetic transition at low temperature (29 and 21 K, respectively). Large coercive field is measured for these last compounds below the second magnetic transition temperature.     

Superconducting property of Zr–Co–Al–Nb metallic glasses

The superconducting property of Zr₅₅Co_(30?x)Al₁₅Nb_x (x = 0–20 at.%) metallic glasses fabricated by rapid solidification was investigated. The Zr₅₅Co_(30–x)Al₁₅Nb_x (x = 5–20 at.%) metallic glasses with a mixture structure of amorphous and nanocrystal phases exhibited superconductivity of T_c,on = 1.8–2.6 K. The maximum T_c,on = 2.6 K was obtained for the Zr₅₅Co₁₀Al₁₅Nb₂₀ metallic glass. This was attributable to the superconducting property of nanocrystalline particles contained in the Zr₅₅Co₁₀Al₁₅Nb₂₀ alloy. The increase of Nb element in the Zr₅₅Co_(30–x)Al₁₅Nb_x alloy led to the increase of T_c,on and the decrease of glass transition temperature. The glass transition temperature was between 704 and 749 K for the Zr₅₅Co_(30–x)Al₁₅Nb_x (x = 0–20 at.%) alloys. The temperature interval of supercooled liquid state was between 51 and 68 K for the Zr₅₅Co_(30–x)Al₁₅Nb_x (x = 0–20 at.%) alloys.     

Magnetism of UCoGe and U3Co4Ge7

A series of UCoGe and U₃Co₄Ge₇ polycrystalline samples has been prepared by arc melting and studied with respect to the phase composition and crystal structure, magnetization, a.c. susceptibility, electrical-resistivity and specific-heat behavior (down to 350 mK). U₃Co₄Ge₇ has been found to exhibit a spontaneous magnetization below T_C=21 K. Clear anomalies at T_C typical for a ferromagnetic transition have been observed in a.c. susceptibility, electrical-resistivity and specific-heat data. No additional anomaly, which would indicate the second magnetic phase transition below T_C reported in the literature, has been indicated. In all our UCoGe samples a transition to superconductivity has been revealed. On the other hand, no clear evidence of any transition to ferromagnetism with zero-field cooling down to 1.8 K has been obtained. The zero-field state is most probably governed by strong ferromagnetic spin fluctuations and seems to transform to a ferromagnetic state only when applying a magnetic field ≥10 mT. Simultaneously, an increase of the superconducting transition temperature is increasing with a magnetic field up to 10 mT and starts to decrease when increasing the field above this value. Measurements on bulk samples, which are by rule textured, indicate strong magnetocrystalline anisotropy in both investigated compounds.     

First-principles study of small palladium clusters on NiAl(1 1 0) alloy surface

Theoretical calculations focused on the geometry, stability, electronic and magnetic properties of small palladium clusters Pd_n (n=1–5) adsorbed on the NiAl(1 1 0) alloy surface were carried out within the framework of density functional theory (DFT). In agreement with the experimental observations, both Ni-bridge and Al-bridge sites are preferential for the adsorption of single palladium atom, with an adsorption energy difference of 0.04 eV. Among the possible structures considered for Pd_n (n=1–5) clusters adsorbed on NiAl(1 1 0) surface, Pd atoms tend to form one-dimensional (1D) chain structure at low coverage (from Pd₁ to Pd₃) and two-dimensional (2D) structures are more stable than three-dimensional (3D) structures for Pd₄ and Pd₅. Furthermore, metal-substrate bonding prevails over metal–metal bonding for Pd cluster adsorbed on NiAl(1 1 0) surface. The density of states for Pd atoms of Pd/NiAl(1 1 0) system are strongly affected by their chemical environment. The magnetic feature emerged upon the adsorption of Pd clusters on NiAl(1 1 0) surface was due to the charge transfer between Pd atoms and the substrate. These findings may shade light on the understanding of the growth of Pd metal clusters on alloy surface and the construction of nanoscale devices.     

A magnetic and Mössbauer study of melt-spun Nd60Fe30Al10

Nd₆₀Fe₃₀Al₁₀ alloys were rapidly quenched by the melt-spinning technique with different wheel surface speeds ranging from 5 to 30 m/s. The microstructure and the magnetic properties were strongly dependent on the quenching rate. A high quenching rate led to an amorphous structure with a low coercivity at room temperature, while a mixture of amorphous and crystalline phases was found after melt-spinning at 5 m/s, which exhibited hard magnetic properties at room temperature. For both the ribbons melt-spun at 5 and 30 m/s respectively, coercivity increased with decreasing temperature and reached a maximum at around 50 K. Maximum magnetization at 10 T increased dramatically at low temperature. Our magnetic study has shown that the presence of crystalline Nd was responsible for the increase of magnetization and the decrease of coercivity, as Nd became magnetically ordered at low temperatures. The Mössbauer study has shown that the magnetic microstructures of melt-spun ribbons were not uniform, as the spectra needed to be fitted by magnetic and non-magnetic components.     

Giant magnetic refrigerant capacity in Ho3Al2 compound

Magnetic properties and magnetocaloric effects (MCEs) of the intermetallic Ho₃Al₂ compound are investigated by magnetization and heat capacity measurements. Two successive magnetic transitions, a spin-reorientation (SR) transition at T_SR=31 K followed by a ferromagnetic (FM) to paramagnetic (PM) transition at T_C=40 K, are observed. Both magnetic transitions contribute to the MCE and result in a large magnetic entropy change (ΔS_M) in a wide temperature range. The maximum values of ?ΔS_M and adiabatic temperature change (ΔT_ad) reach 18.7 J/kg K and 4.8 K for the field changes of 0–5 T, respectively. In particular, a giant value of refrigerant capacity (RC) is estimated to be 704 J/kg for a field change of 5 T, which is much higher than those of many potential refrigerant materials with similar transition temperatures.     

High coercivity and superparamagnetic behavior of nanocrystalline iron particles in alumina matrix

Single-domain nanoscale magnetic iron particles have been embedded uniformly in an amorphous matrix of alumina using a pulsed laser deposition technique. Structural characterization by transmission electron microscopy (TEM) reveals the presence of a crystalline iron and an amorphous alumina phase. Fine particle magnetism have been investigated by carrying out field and temperature dependence of magnetization measurements using superconducting quantum interference device magnetometer. The particle size of Fe in Al₂O₃ matrices prepared by changing the deposition time of Fe, have been found to be 9, 7 and 5 nm from TEM studies. At 10 K, the coercivities of these samples are found be 450, 350 and 150 Oe, respectively. At 300 K, the coercivity of Fe–Al₂O₃ sample decreases from 100 to 50 Oe as the particle size decreases from 9 to 7 nm and finally the sample turns superparamagnetic when the Fe particle size becomes around 5 nm. Based on the calculated value of blocking temperature, T_B, (481 K), magnetic anisotropy K (4.8×10⁵ erg/cm³) for Fe, and the Boltzmann constant k_B (1.38×10⁻¹⁶ erg/K) from T_B=KV/25k_B, the mean radius of Fe particles is found to be 9.3 nm. in one of the samples. This is in good agreement with the particle size measured using TEM studies.     

High-frequency magnetoresonance absorption in amorphous magnetic microwires

The Fe₆₉Si₁₆B₁₀C₅, Co₇₅Si₁₀B₁₅, Co₆₈Mn₇Si₁₀B₁₅ amorphous microwires have been studied by the magnetoresonance absorption technique in the X (9.5 GHz), K (20–27 GHz) and Q (30–37 GHz) frequency bands. The specimens under study were metal threads of about 5 μm in diameter coated with dielectric Pyrex layer with thickness 5 μm. The dependences of magnetic resonance spectra on frequency and wire orientation have been measured. The analysis of the resonance signal parameters has revealed that well-known classical equations for FMR in a cylindrical-shaped sample could not be applied for these microwires. It is shown that due to the skin depth effect the model of hollow cylindrical tube has to be applied to explain the experimental results in the frequency range measured. The values of saturation magnetization, g-factor and anisotropy field have been estimated from the frequency dependence of the field for resonance.     

Vapor–solid–solid growth of Ge nanowires from GeMn solid cluster seeds

We describe the fabrication of Ge nanowires during a single co-deposition step of Ge and Mn at high temperature. In these experimental conditions, a phase separation occurs and two different phases Ge and Ge_1 ? xMn_x are formed with Ge_1 ? xMn_x in the shape of small clusters distributed randomly in the Ge matrix. Because of the high deposition temperature, a new Ge_1 ? xMn_x phase with low eutectic point is stabilized; this phase is different from the one (commonly Ge₃Mn₅) stabilized at lower temperature. During the growth process at 350 °C, the crystalline clusters remain solid but they are highly mobile and can float at the surface, serving as seeds to direct the growth of crystalline Ge nanowires from the vapor. The sketch steps of NWs formation are first the phase separation with formation of specific Ge_1 ? xMn_x critical nuclei with low eutectic point and second the growth of Ge NWs directed by the Ge_1 ? xMn_x solid cluster seeds. Ge NWs growth is forced along particular crystalline axis by the cluster seeds that lower the interfacial energy Ge/Ge_1 ? xMn_x and the energy formation of the germanium crystal stabilizes the cluster position at the tip of the NWs. The density of NWs can be tuned by varying the nominal Mn concentration since this density is related to the number of clusters with the specific Ge_1 ? xMn_x phase (with low eutectic point). The single step MBE process presented here has the main advantage to fully avoid any incorporation of unintentional impurity into Ge nanowires (apart from Mn atoms) and could be applied to several other systems. This work also provides new insights into the vapor–solid–solid growth mechanisms of Ge NWs.     

Superparamagnetic properties of C60Co3 complexes

Preparation of fullerites containing cobalt and analyses of reactions based on semiempirical quantum calculations are described. The magnetic properties of thermally treated C₆₀Co₃ samples: Curie constant (C≈3500 emu K/mol Oe) temperature and field dependencies of magnetization and nonequilibrium effects of magnetization are interpreted in terms of superparamagnetic blocking model of the compound.     

The magnetization processes,spin reorientation transitions and magnetic domain structure in DyFe10CoTi single crystal

The tetragonal ThMn₁₂-type, single crystalline DyFe₁₀CoTi sample has been investigated by torque and magnetization measurements and observation of domain structure at various temperatures between 10–300 K and in magnetic field from B=0 to 0.15 T. These examinations showed that the magnetic structure of DyFe₁₀CoTi changes from “easy axis” (c-axis) type to conical at 225 K and to “easy plane” (ab plane) type at 100 K.     

The effect of chemical pressure in rutheno-cuprates

We have studied the effect of negative chemical pressure in the RuGd_1.5(Ce_0.5?xPr_x)Sr₂Cu₂O_10?δ with Pr content of 0.0 ? x ? 0.2. This is also investigated using the bond length results obtained from the Rietveld refinement analysis. The c parameter and cell volume increase with x for 0.0 ? x ? 0.15. The width of the resistivity transition also increases with Pr concentration, indicating higher inhomogeneity and oxygen deficiency. The difference in the ionic valences of Pr^3+,4+ and Ce⁴⁺ causing different hole doping, the difference in the ionic radii, and oxygen stoichiometry affect the superconducting transition. The magnetoresistance shows a cusp around 135 K which lies between the antiferromagnetic and ferromagnetic transition temperatures, which is probably due to the presence of a spin glass region. There exist two magnetic transition temperatures for 0.0 ? x ? 0.2 which respectively change from T_M = 155 K to 144 K and from T_irr = 115 K to 70 K. The magnetization versus applied magnetic field isotherms at 77 K and 300 K show that the remanent magnetization and coercivity are lower for samples with higher Pr content.     

In situ characterization of transport properties of superconducting (Cu, C)-system thin films

Transport properties of (Cu, C)Ba₂CuO_x [(Cu, C)-1201] thin films have been characterized by in situ temperature dependence of resistivity, without breaking vacuum from the deposition to the measurement. In in situ transport properties measurements, the obtained results reveal that (Cu, C)Ba₂CuO_x films exhibit T_c > 20 K on the cased of conductivity at 290 K (σ_[290 K]) > 4 × 10² S/cm and temperature coefficient of resistivity (TCR) > 1.5 × 10^?3 K^?1, and doping level of them should be in between under-doped and optimally-doped states. Their results suggest that there would be possible to further increases of T_c, and XPS data suggest that (Cu, C)-system should have the excellent dopability in their charge reservoir and the possibility of low anisotropy.