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Structural and magnetic properties of Fe/Pt core-shell nanostructure prepared by a sequential reduction process are reported. Transmission electron microscopy shows nearly spherical particles fitting a lognormal size distribution with Do=3.0 nm and distribution width λD=0.31. In x-ray diffraction, Bragg lines only from the Pt shell are clearly identified with line-widths yielding crystallite size=3.1 nm. Measurements of magnetization M vs. T (2K-350K) in magnetic fields up to 90 kOe show a blocking temperature TB=13K below which hysteresis loops are observed with coercivity HC increasing with decreasing T reaching HC=750Oe at 2 K. Temperature dependence of the ac susceptibilities at frequencies fm=10 Hz-5 kHz is measured to determine the change in TB with fm using the Vogel-Fulcher law. This analysis shows the presence of significant interparticle interaction, the Neel-Brown relaxation frequency fo=5.3×1010Hz and anisotropy constant Ka=3.6×106 ergs/ cm3. A fit of the M vs. H data up to H=90 kOe for T>TB to the modified Langevin function taking particle size distribution into account yields magnetic moment per particle consistent with the proposed core-shell structure; Fe core of 2.2 nm diameter and Pt shell of 0.4 nm thickness. [ABSTRACT FROM AUTHOR]

The optoelectronic and piezoelectric properties of AlN:Er thin films have been of great recent interest for potential device applications. In this work, the focus is on the electronic state of Er in AlN:Er thin films prepared by reactive magnetron sputtering on (001) p-type Si substrate. X-ray diffraction shows that Er doping expands the lattice and the AlN:Er film has preferential c-plane orientation. To determine whether Er in AlN:Er is present as Er metal, Er2O3, or Er3+ substituting for Al3+, detailed measurements and analysis of the temperature dependence (2 K-300 K) of the magnetization M at a fixed magnetic field H along with the M vs. H data at 2K up to H=90 kOe are presented. The presence of Er2O3 and Er metal is ruled out since their characteristic magnetic transitions are not observed in the AlN:Er sample. Instead, the observed M vs. T and M vs. H variations are consistent with Er present as Er3+ substituting for Al3+ in AlN:Er at a concentration x=1.08% in agreement with x=0.94%60.20% determined using x-ray photoelectron spectroscopy (XPS). The larger size of Er3+ vs. Al3+explains the observed lattice expansion of AlN:Er. [ABSTRACT FROM AUTHOR]

Photoluminescence spectra of YVO4:Eu3+ nanoparticles are presented, with and without the attachment of organic molecules that are proposed for linking to biomolecules. YVO4:Eu3+ nanoparticles with 5% dopant concentration were synthesized via wet chemical synthesis. X-ray diffraction and transmission electron microscopy show the expected wakefieldite structure of tetragonal particles with an average size of 17 nm. Fourier-transform infrared spectroscopy determines that metal-carboxylate coordination is successful in replacing native metal-hydroxyl bonds with three organic linkers, namely, benzoic acid, 3-nitro 4-chloro-benzoic acid, and 3,4-dihydroxybenzoic acid, in separate treatments. UV-excitation photoluminescence spectra show that the position and intensity of the dominant 5D0 - 7F2 electric-dipole transition at 619 nm are unaffected by the benzoic acid and 3-nitro 4-chloro-benzoic acid treatments. Attachment of 3,4-dihydroxybenzoic acid produces an order-of-magnitude quenching in the photoluminescence, due to the presence of high-frequency vibrational modes in the linker. Ratios of the dominant electric- and magnetic-dipole transitions confirm infrared measurements, which indicate that the bulk crystal of the nanoparticle is unchanged by all three treatments. [ABSTRACT FROM AUTHOR]

Bulk NiO is a well-known antiferromagnet with Neel temperature TN(∞) = 524 K and an optical band-gap Eg = 4.3 eV. With decrease in particle size D from 40 nm to 4 nm of NiO, systematic changes of TN and Eg are observed and discussed here. From magnetic measurements, the changes in TN with D are found to fit finite-size scaling equation TN(D) = TN(∞) [1 - (ξo/D)λ] with λ = 3.2 ± 0.5 and ξo = 3.2 ± 0.2 nm, in good agreement with the predictions for a Heisenberg system. The observed blue shifts of Eg with decrease in D reaching Eg = 5.12 eV for D ∼ 4 nm are likely due to quantum confinement and non-stoichiometry. [ABSTRACT FROM AUTHOR]

The nature of magnetic ordering in the inverted spinel Co2SnO4 = [Co2+][Co2+Sn4+]O4 is investigated by measuring the temperature dependence of its magnetization (M) and ac-susceptibilities (χ′ and χ″) in different magnetic fields (H) using a superconducting quantum interference device magnetometer. The polycrystalline sample with a grain size of 3-5 μm was prepared by the solid-state route and characterized by x-ray diffraction and scanning electron microscopy. Above the ferrimagnetic Néel temperature TN = 41 K, χdc = M/H fits well with the Néel' expression for the two-sublattice model of ferrimagnetism. The ferrimagnetic ordering is suggested to result from different magnetic moments of Co2+ on the A sites (3.87 μB) and B sites (4.16 μB) and antiferromagnetic molecular field constants with magnitudes NAA = 13.6, NBB = 36.8, and NAB = 33.2. The temperature variations of χ′ and χ″ under various dc-bias fields suggest the existence of a spin-glass (SG) phase for T < TSG = 39.1 K in which only the transverse spin components are frozen below TSG. This co-existence of longitudinal ferrimagnetic order below TN = 41 K and transverse SG state below TSG = 39.1 K is suggested to result from the presence of non-magnetic Sn4+ ions on the B sites. [ABSTRACT FROM AUTHOR]

Magnetic properties of bulk MgMnO3, a cubic defect spinel with the structure 4MgMnO3=3[Mg2+][Mg+2 1/3Mn4+ 4/3□1/3]O4, are reported. For T > 150 K, magnetization M vs T data fits the Curie-Weiss law with μ ≃ 4μB expected for Mn4+ and θ ≃ -40 K indicating antiferromagnetic Mn4+-Mn4+coupling. For T < 150 K, a blocking temperature TB ≃ 15 K is observed below which coercivity increases with decreasing T. The electron magnetic resonance spectra yield a single broad line with g ≃ 1.9 whose linewidth increases with decreasing T leading to a peak at TB. As predicted in spinels with magnetic ions only on the B-sites, these observations suggest a magnetically frustrated state with short-range order only. [ABSTRACT FROM AUTHOR]

The coexistence of the magnetic and the electrical properties in lanthanum (La)-modified bismuth ferrite (Bi1-xLaxFeO3, x=0.05, 0.1, 0.15, and 0.2) ceramics was studied and compared with those of bismuth ferrite (BiFeO3). The presence of a small secondary phase of BiFeO3 (arises due to excess Bi2O3) was removed on La substitution at the Bi site, as observed in x-ray diffraction (XRD). The effect of La substitution on dielectric constant, loss tangent, and remnant polarization of the samples was studied in a wide range of temperature (77–400 K) and frequency (1 kHz–1 MHz). The variation of magnetization, coercive field, and exchange bias with temperature (2–300 K) and La concentration were investigated. These changes in the magnetic parameters with La doping along with those of the electron magnetic resonance parameters measured at 300 K and 9.28 GHz are understood in terms of increase in the magnetic anisotropy and magnetization. These results also show that stabilization of crystal structure and nonuniformity in spin cycloid structure by La substitution enhances the multiferroic properties of BiFeO3. [ABSTRACT FROM AUTHOR]

In this work, we report the synthesis of sol-gel-derived lead iron niobate [Pb1.10(Fe0.5Nb0.5)O3] (PFN) powders and sintered ceramics. The PFN powders were calcined at (Ta), 973, 1073, 1173, 1273, and 1373 K for 3 h in air. As envisaged from x-ray-diffraction analyses, PFN powder calcined at 1173 K was crystallized into pure monoclinic perovskite phase whereas powders calcined at all other temperatures had varied amounts of retained pyrochlore (Pb3Nb4O13) phase coexisted with dominant monoclinic perovskite phase. The PFN pellet (prepared using the phase pure powder calcined at 1173 K) sintered at 1373 K for 4 h in air also had a minute quantity of retained pyrochlore phase coexisting with desired perovskite phase. From the temperature dependence of measured capacitance and loss tangent at different frequencies, the ferroelectric to paraelectric phase-transition temperature of PFN ceramics was observed at Tc≈370 K. The diffused nature of this transition and high dielectric constant of PFN is related to the cation disorder at the B site of A(BI+3BII+5)O3 lattice. For PFN powders, calcined at different temperatures, the temperature dependence of the magnetic susceptibility (χ) was measured in a temperature range of 2–360 K, whereas the magnetic hysteresis loops and electron magnetic resonance (EMR) spectra were measured at room temperature. Room-temperature ferromagnetism is observed in all the calcined powder samples and it was found that the magnetization increases with the increase in calcination temperature (Ta). The symmetric EMR line shape with g≈2.01 observed in all calcined samples was identified to be due to Fe3+ ions. It is suggested that the observed weak ferromagnetism, which increases with an increase in Ta, may be due to canting of the Fe3+ spins. These observations suggest PFN to be a very attractive single-phase ferroelectric/ferromagnetic material for room-temperature applications. [ABSTRACT FROM AUTHOR]

Magnetic properties of nanorods of NiO, with and without oleic acid coating, are reported, and the temperature variations of the magnetization and electron magnetic resonance spectra were measured. The differences in blocking temperatures are due to stronger interparticle interaction present in the uncoated particles.

By reacting manganese nitrate and sodium hydroxide at pH [nearly equal to] 12 and heating the product at 400 [degrees] C for 3 hrs, [Mn.sub.5][O.sub.8] with crystallite size [nearly equal to] 14 nm and the [Cd.sub.2][Mn.sub.5][O.sub.8] structure is produced, as confirmed by x-ray diffraction. Magnetic state of [Mn.sub.5][O.sub.8] is investigated by measuring the temperature variations of the magnetic susceptibility [Chi] and the electron spin resonance spectra. The [Chi] versus T variation is characteristic of antiferromagnetic ordering with Neel temperature [T.sub.N] [nearly equal to] 128 K. For T [is greater than] [T.sub.N], [Chi] follows the Curie-Weiss law, [Chi] = C/(T - [Theta]), with C = 14.26 [cm.sup.13]-K/mole, and [Theta] = -164 K. In ESR, only a single line is observed whose linewidth [Delta]H fits the variation [Delta]H(Oe) = 1570 + A/[(T - [T.sub.N]).sup.[Gamma] with [Gamma] = 1.26, characteristic of a uniaxial antiferromagnet. The magnitude of C is consistent with the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] valence state of [Mn.sub.5][O.sub.8]. Index Terms--Antiferromagnetism, electron spin resonance, mixed valences, synthesis.

Electron spin resonance investigations at 9.24 GHz in Si-doped two-line ferrihydrite (Si/FeOOH [multiplied by] n[H.sub.2]O) nanoparticles of size [nearly equals to] 4 nm are reported for x = Si/(Si + Fe) = 0, 0.02, 0.05, 0.10 and 0.15. For all samples, a single broad resonance near g [nearly equals to] 2 is observed; however with increase in x, both the resonance field H, and linewidth [Delta]H increase but integrated intensity I of the line decreases, similar to the observed decrease in magnetization with x. These observations suggest that the observed resonance is due to uncompensated [Fe.sup.3+] surface spins. Temperature variation (300 K to 80 K) of H, and [Delta]H for x = 0.05 shows a downward shift ([Delta][H.sub.r]) of the line with a rapid increase in [Delta]H so that [Delta][H.sub.r] [varies] [([Delta]H).sup.3] predicted for randomly oriented nanoparticles is observed. Index Terms--Doped ferrihydrites, electron spin resonance, magnetometry, nanoparticles.

For the two stable phases of Ni(OH)2, viz. α-Ni(OH)2 and β-Ni(OH)2, structural and magnetic properties are compared employing x-ray diffraction (XRD), scanning electron microscopy/transmission electron microscopy (TEM/SEM), and variation of the magnetization (M) with temperatures (2-350 K) and magnetic fields (up to ≈65 kOe). Both phases crystallize in the layered hexagonal structure with a=3.04 Å (3.12 Å) and c=23.6 Å (4.67 Å) for the α(β) phase with clear evidence for turbostraticity in the α-phase. From TEM/SEM, the β-phase consists of nanoplates of dimensions 30(10)×3(1) nm, whereas the α-phase has flowerlike morphology with petal thickness ≈5(1) nm. The low-field M versus T data for the zero-field-cooled/field cooled cases bifurcates at Tp=25 K (13 K) for the β(α) phase. For T>Tp, Curie-Weiss variation of M versus T is observed yielding positive θ=20.6 K (32 K) with μ=2.95 μB (3.13 μB) for the Ni2+ ions in the β(α) phase. The M versus H data at 2 K shows antiferromagnetic (ferromagnetic) like variation for the β(α) phase with dM/dH showing peaks at Hc1=28 kOe and Hc2=55 kOe for the β-phase. It is concluded that β-Ni(OH)2 is a metamagnet for T

For the two stable phases of Ni(OH)2, viz. α-Ni(OH)2 and β-Ni(OH)2, structural and magnetic properties are compared employing x-ray diffraction (XRD), scanning electron microscopy/transmission electron microscopy (TEM/SEM), and variation of the magnetization (M) with temperatures (2-350 K) and magnetic fields (up to ≈65 kOe). Both phases crystallize in the layered hexagonal structure with a=3.04 Å (3.12 Å) and c=23.6 Å (4.67 Å) for the α(β) phase with clear evidence for turbostraticity in the α-phase. From TEM/SEM, the β-phase consists of nanoplates of dimensions 30(10)×3(1) nm, whereas the α-phase has flowerlike morphology with petal thickness ≈5(1) nm. The low-field M versus T data for the zero-field-cooled/field cooled cases bifurcates at Tp=25 K (13 K) for the β(α) phase. For T>Tp, Curie-Weiss variation of M versus T is observed yielding positive θ=20.6 K (32 K) with μ=2.95 μB (3.13 μB) for the Ni2+ ions in the β(α) phase. The M versus H data at 2 K shows antiferromagnetic (ferromagnetic) like variation for the β(α) phase with dM/dH showing peaks at Hc1=28 kOe and Hc2=55 kOe for the β-phase. It is concluded that β-Ni(OH)2 is a metamagnet for T2 is a hard ferromagnet with Tc=13 K and domain structure for H<5 kOe. [ABSTRACT FROM AUTHOR]

Temperature dependence of ac susceptibilities χ′ and χ″ are reported using frequencies fm=0.1, 1, 99, 499, and 997 Hz for nanoparticles of Ni dispersed in silica (Ni/SiO2:15/85) with the mean sizes D=3.8, 11.7, 15, and 21 nm (σ≃0.2 nm), as determined by transmission electron microscopy. The blocking temperatures TB, as determined by peaks in χ″ versus T data, are fit to the Vogel–Fulcher law based on the following equation: TB=To+Ta/ln(fo/fm). Using the attempt frequency fo=1.82×1010 Hz, Ta (K)=310 (21), 954(17), 1334(14), and 1405(47) are determined for D=3.8, 11.7, 15, and 21 nm, respectively, along with To (representing the interparticle interaction)=0, 0, 6.6(0.7), and 12.5(2.5) K respectively. The magnitudes of Ta=KaV/k yield the anisotropy constant Ka increasing with decreasing D (or volume V) due to contributions from surface anisotropy. The validity of the theoretical result χ″=C∂(χ′T)/∂T with C≃π/[2 ln(fo/2πfm)] is checked and the calculated values of fo are consistent with experimental value of fo=1.82×1010 Hz. [ABSTRACT FROM AUTHOR]

Magnetic properties of oleic acid/oleylamine coated magnetite nanoparticles of average diameter D=4, 6, 8, 10, and 12 nm are reported. The samples were characterized by transmission electron microscopy and x-ray diffraction (XRD) with XRD showing increasing disorder with decreasing D. Magnetization M vs temperature T data show the blocking temperature TB decreasing with decreasing D from TB=38 K for 12 nm to TB=6.5 K for D=4 nm. The saturation magnetization Ms at 2 K also decreases from Ms=62 emu/g for 12 nm to Ms=17 emu/g for 4 nm but the coercivity at 2 K increases with a decrease in D. It is shown that Ms fits Ms=92(1-2d/D)3 with d=0.68 nm for D>4 nm and d=0.86 for D=4 nm. This equation is derived assuming a core-shell model with shell of thickness d consisting of disordered spins not contributing to Ms. [ABSTRACT FROM AUTHOR]

The nature of magnetism in 10% Cu-doped ZrO2, TiO2, MgO, SiO2, and Al2O3 is investigated using superconducting quantum interference device magnetometry and electron magnetic resonance (EMR). The doping was done by the incipient wetness impregnation technique. X-ray diffraction studies showed the presence of some CuO notably in TiO2, SiO2, and Al2O3. However, EMR yielded typical Cu2+ spectra in all the samples, resulting from some doping. Magnetic field (H) and temperature (T) dependences of the magnetization (M) show only paramagnetism in these oxides in contrast to ferromagnetism with Tc=380 K observed in Cu/ZnO. Concentrations x of Cu2+ doped into the oxides are determined from the M vs T and M vs H data, showing Cu/ZrO2 with the largest x and hence largest magnetization. [ABSTRACT FROM AUTHOR]

The variations of the blocking temperature TB with measuring frequency fm and applied field H are reported for Ni nanoparticles (NPs) embedded in SiO2 matrix with the nominal composition Ni/SiO2 (15/85). Transmission electron microscopy of the sol-gel prepared NPs yielded a log-normal distribution with diameter D=3.8(0.2) nm whereas x-ray diffraction data yielded D≃3.3 nm. The TB versus fm variation for fm=0.1–1000 Hz fits the equation TB=Ta/ln(f0/fm) with f0=2.6×109 Hz and Ta=270 K whereas TB versus H data follows the equation TB(H)=33(1-H/H0)m with H0=3200 Oe and m=2(3/2) for lower (higher) H. These results are compared with the predictions of the Néel-Brown model of magnetic relaxation in noninteracting NPs. [ABSTRACT FROM AUTHOR]

Magnetic studies in bulk 57% CuxZn1-xO/43% CuO composite prepared chemically with x=0.10 (0.03) are reported. Temperature dependence of the magnetic susceptibility shows clear signature of the Néel temperature TN≃230 K for pure CuO. Hysteresis loop studies in the zero-field-cooled sample show room-temperature ferromagnetism with coercivity Hc≃50 Oe at 300 K with strong temperature dependence for T<300 K. A negative exchange bias He at 5 K is observed when the sample is cooled from RT in H=20 kOe, and its temperature dependence yields He→0 at TN=230 K of CuO. These results and the temperature dependence of remanence Mr provide strong evidence that the CuxZn1-xO phase with x≃0.10 is a ferromagnet with Tc≃400 K. An electron magnetic resonance signal due to Cu2+ substituting for Zn2+ in CuxZn1-xO is also observed. [ABSTRACT FROM AUTHOR]

Experimental investigations of the magnetic dead layer in 7.6 nm thick film of La0.7Sr0.3MnO3 (LSMO) are reported. The dc magnetization (M) measurements for a sample cooled to T = 5 K in applied field H = 0 reveal the presence of negative remanent magnetization (NRM) in the M vs. H (magnetic field) measurements as well as in the M vs. T measurements in H = 50 Oe and 100 Oe. The M vs. T data in ZFC (zero-field-cooled) and FC (field-cooled) protocols are used to determine the blocking temperature TB in different H. Isothermal hysteresis loops at different T are used to determine the temperature dependence of saturation magnetization (MS), remanence (MR) and coercivity HC. The MS vs. T data are fit to the Bloch law, MS(T) = M0(1 – BT3/2), showing a good fit for T < 100 K and yielding the nearest-neighbor exchange constant J/kB ≅ 18 K. The variations of TB vs. H and HC vs. T are well described by the model often used for randomly oriented magnetic nanoparticles with magnetic domain diameter ≈ 9 nm present in the dead-layer of thickness d =1.4 nm. Finally, the data available from literature on the thickness (D) variation of Curie temperature (TC) and MS of LSMO films grown under 200, 150, and 0.38 mTorr pressures of O2 are analyzed in terms of the finite-size scaling, with MS vs. D data fit to MS(D) = MS(b)(1-d/D) yielding the dead layer thickness d = 1.1 nm, 1.4 nm and 2.4 nm respectively. Brief discussion on the significance of these results is presented. [ABSTRACT FROM AUTHOR]