Comment on “Nature and entropy content of the ordering transitions in RCo₂”

In their analysis of our perturbed angular correlation (PAC) study of the magnetic phase transitions of RCo₂ [Phys. Rev. B 68, 014409 (2003)], Herrero-Albillos [Phys. Rev. B 73, 134410 (2006)] come to the conclusion that it is very difficult for PAC spectroscopy to distinguish a first-order from a second-order phase transition. The statement is incorrect and does not resolve the conflict between the conclusion drawn from the PAC data and the differential scanning calorimetry data of Herrero- Albillos on the order of the magnetic phase transitions of PrCo₂ and NdCo₂. In this Comment we show that measurements of hyperfine interactions by PAC and other microscopic techniques are a very powerful tool for the investigation of phase transitions which may provide details on the transition not accessible to macroscopic methods. We explain why the PAC data leave no alternative to the conclusion that the spontaneous magnetization of PrCo₂ and NdCo₂ undergoes a discontinuous, first-order phase transition at T_(C)

Perturbed angular correlation study of the magnetic phase transitions in the rare-earth cobalt Laves phases RCo₂

The order and other properties of the magnetic phase transitions in the rare-earth (R)-cobalt Laves phases RCo2 have been studied for R=Gd, Tb, Dy, Ho, Er, Sm, Nd, and Pr by measuring the temperature dependence of the magnetic hyperfine interaction of the nuclear probe ^111Cdon the cubic R sites using the perturbed angular correlation technique. Both for heavy and light R constituents the transitions change from second order (Gd, Tb, Sm) to first order (Dy, Ho, Er, Nd, Pr) at order temperatures of 150-200 K. For heavy R constituents, the order deduced from the hyperfine interaction is in agreement with previous investigations. The observation of first order transitions in NdCo₂ and PrCo₂, however, is unexpected. In earlier studies the transitions in these compounds are usually classified as second order transitions. Both in the heavy and the light RCo₂ the discontinuous jump of the hyperfine interaction at the first order transitions increases with decreasing order temperature. This trend implies that the Co magnetization at the transition increases with decreasing T_C which can be related to the temperature dependence of the coefficient of the M^(4) term of the free energy in the Wohlfarth-Rhodes-Shimizu theory of itinerant electron magnetism. All compounds investigated presented a spread of the order temperature of similar to1-2 K which results in a coexistence of the paramagnetic and the magnetically ordered phase near the transition and causes a critical increase of the relative linewidth of the hyperfine frequency diverging as δ ∞ [(1-T/T_(C)]^ε with ε =-1.0(1)

Spectrum of magnetic hyperfine fields at ^111 C probe nuclei in the pseudobinary rare-earth Laves-phase compounds R_(1-x)Y_xCo₂

The spectrum of the magnetic hyperfine fields at the closed-shell probe nucleus ^111 Cd on the rare earth (R) site of the pseudobinary Laves-phase compounds R_81-x)Y_xCo₂ has been investigated by perturbed angular correlation (PAC) spectroscopy at 10 K for the rare earth R=Tb and Ho at various Y concentrations x ≤ 0.8 and for R=Gd, Dy, Er at the concentration x=0.3. Up to four components with different magnetic interaction frequencies ν^(i) _(M) could be resolved from the PAC spectra. The relative intensities of these components are in fair agreement with those of a binomial distribution of Y atoms on the four nearest neighbor (NN) R sites of the probe nucleus. For all R constituents, one finds a strictly linear relation between the number n_(R) of NN R atoms and the magnetic hyperfine frequencies: ν^(i) _(M) =ν_(M)(4Y)+∆ ν(M) Xn_(R).The frequency ν_(M)(4Y)=35(2) MHz is independent of the R constituent and of the Y concentration up to x ≤ 0.6. These properties identify ν_(M)(4Y) as the contribution of the Co 3d moments to the hyperfine interaction at the ^111 Cdsite. The frequency steps ∆ ν_(M)[≤ 0.1 ν_(M)(4Y)] reflect the spin polarization directly induced by the 4f spins at the probe nucleus. From Gd to Er, the spin polarization decreases much stronger than expected from the linear variation of the 4f spin in the heavy R series. An indirect 4f contribution caused by a dependence of the Co 3d moment on the number of R neighbors can be excluded. The relation ν_(M)^(i) = ν_(M)(4Y) + ∆ν_(M) X n_(R) then implies that the contributions of the 3d and 4f spins to the magnetic hyperfine field in RCo₂ have the same relative sign

Temperature-dependence of the hyperfine interaction in the cubic phase of BaHfO₃

The temperature dependence of the hyperfine interaction in the paraelectric phase of BaHfO₃ in the temperature range from liquid-nitrogen temperature to 1350 K has been studied. The experimental results show an Abragam-Pound mechanism for the attenuation of the angular correlation. In this cubic structure the attenuation would be consistent with a relaxation mechanism that involves O-ion hopping between vacant sites in the lattice. Using the point-charge model for the estimation of the electric field gradient produced by an O vacancy in the coordination sphere of the probe ion, the process is characterized by a correlation time τ_(c) = 12 ± 1 ps at RT

Perturbed Angular Correlation Studies in SrTi0₃ Single Crystals

The quadrupole hyperfine interactions of ^111Cd/^111 probe nuclei in single-crystalline SrTiO₃ perovskite samples were investigated using Perturbed Angular Correlation spectroscopy. Three electric field gradients were detected and their fractions and hyperfine parameters were measured in the temperature range from 26 to 700 K. The fraction f₀ having a vanishing quadrupole frequency, as expected for ^111Cd on substitutional sites in a cubic lattice, starts to develop around 300 K and reaches 100% at 700 K. Two well-defined EFG's having closely lying quadrupole frequencies and asymmetry parameters of ω_(Q1) = 49.1(3) Mrad/s, η_(1) = 0.10(2) and ω_(Q2) = 51 8(3) Mrad/s, η_(2)= 0.12(2), at room temperature, were identified in the temperature range from 250 to 530 K, and their (100) orientation in the lattice was determined. They are associated with electronic defects at the probe atoms

Ligand exchange in gold-coated FePt nanoparticles

In this work, we present the magnetic properties of gold-coated FePt nanoparticles and the study of stable aqueous dispersions of FePt@Au and FePt synthesized after ligand exchange with mercaptoundecanoic acid. The particle size determined from transmission electron microscopy (TEM) micrographs goes from 4 nm for the uncoated nanoparticles to a maximum of 10 nm for the gold-coated ones indicating that the thickness of the shell ranges from 1 to 3 nm. The magnetic characterization consists in hysteresis cycles at 10 and 300 K. The results show that, at low field and room temperature, the magnetic behavior of uncoated and coated nanoparticles are surprisingly quite similar. Because the gold-coated nanoparticles keep the magnetic properties of FePt and the presence of gold improves the functionalization of nanoparticles, the system is suitable for biological application. Mercaptoundecanoic ligand transfer was used to render water stable nanoparticles in a wide pH range. Transmission electron microscopy and dynamic light scattering (DLS) results show the nanoparticles slightly agglomerate after ligand exchange. Fourier transform infrared spectroscopy results suggest that thiol binds to the gold atoms of the surface

Vacancy motion in rare-earth-deficient R_(1-x)Ni_2 Laves phases observed by perturbed angular correlation spectroscopy

Rare-earth-deficient R_(1-x)Ni_2 Laves phases, which reportedly crystallize in a C15 superstructure with ordered R vacancies, have been investigated by perturbed angular correlation (PAC) measurements of electric quadrupole interactions at the site of the probe nucleus ^111Cd. Although ^111Cd resides on the cubic R site, a strong axially symmetric quadrupole interaction (QI) with frequencies ν(q)approximate to265-275 MHz has been found in the paramagnetic phases of R_(1-x)Ni_2 with R=Pr,Nd,Sm,Gd. This interaction is not observed for the heavy R constituents R=Tb,Dy,Ho,Er. The fraction of probe nuclei subject to the QI in R_(1-x)Ni_2, R=Pr,Nd,Sm,Gd, decreases from 100% at low temperatures to zero at T>300 K and 500 K for R=Sm,Gd and R=Pr,Nd, respectively. At T=100 K the QI is static within the PAC time window, but at T=200 K fluctuations with correlation times τ_(C) 500 K nuclear spin relaxation related to vacancy hopping is observed in nearly all R_(1-x)N_i2. Auxiliary ^111Cd PAC measurements have been carried in Sc_0.95Ni_2, ScNi_2, ScNi_0.97, Gd_2Ni_(17), GdNi_5, GdNi_3, and GdNi

Structural and magnetic characterization of oleic acid and oleylamine-capped gold nanoparticles

In this work the study of oleic acid and oleylamine-capped gold nanoparticles is presented. The structural characterization of the sample shows 6.7 nm gold nanoparticles with a narrow size distribution. The experimental optical absorption spectrum has a maximum at 2.35 eV. The calculated optical absorption spectrum is shifted and narrower than the experimental one, indicating that the oleic acid and oleylamine do not merely passivate the metallic nanoparticles but modify its electronic structure. These gold nanoparticles show in addition a kind of magnetic order similar to other organic passivated gold nanoparticles as thiol-capped gold nanoparticles. Although the magnetic interactions seem to be weaker than in thiol-capped ones, the magnetic behavior looks similar to that, i.e., an invariant temperature dependence of the magnetization from 5 to 300 K and a noticeable coercive field. We analyze the influence of the organic layer bonding the nanoparticles on the magnetic behavior

Structure, phase transformations, and defects of HfO₂ and ZrO₂ nanoparticles studied by ^(181)Ta and ^(111)Cd perturbed angular correlations, ^(1) H magic-angle spinning NMR, XPS, and x-ray and electron diffraction

Structure, phase transformations, grain growth, and defects of bare and alumina-coated nanoparticles of HfO₂ and ZrO₂ synthesized in a microwave-plasma process have been investigated by x-ray diffraction (XRD), transmission electron microscopy (TEM), and perturbed angular correlation (PAC) spectroscopy. The PAC technique was used to measure the electric quadrupole interactions (QIs) of the nuclear probes ^(181)Ta and ^(111)Cd in nanocrystalline HfO₂ and ZrO₂ as a function of temperature. For comparison, the QI of ^(181)Ta in the bulk oxides was determined in the same temperature range 300 K ≤ T ≤ 1550 K. The oxygen-metal ratio of the as- ynthesized particles was determined by x-ray photoelectron spectroscopy to be in the range 1.4 ≤ x ≤ 1.8. A hydrate surface layer with a hydrogen content of 5–10 wt %, consisting of chemisorbed hydroxyl groups and organic precursor fragments, was detected by ^(1) H magic-angle spinning NMR. XRD and TEM show that bare n-ZrO₂, Al₂O₃-coated n-ZrO₂, and Al₂O₃-coated n-HfO₂ are synthesized in the tetragonal or cubic modification with a particle size d < 5 nm, whereas bare n-HfO₂ is mainly monoclinic. The grain growth activation enthalpy of bare n-ZrO₂ is Q_(A)=32(5)kJ/mol. Coating with Al₂O₃ stabilizes the tetragonal over the monoclinic phase, both in hafnia and zirconia nanoparticles. While TEM micrographs of the native nanoparticles reveal a well-ordered cation sublattice, the observation of a broad QI distribution in the PAC spectra suggests a high degree of disorder of the oxygen sublattice. The gradual transformation of the disordered state and the phase evolution were studied by high-temperature QI measurements. Hafnia nanoparticles persist in the monoclinic (m) phase up to T ≤ 1400 K. In coated n-ZrO₂ /Al₂O₃, the monoclinic and tetragonal (t) phases coexist over a large temperature range, whereas uncoated, initially tetragonal or cubic (t or c) n-ZrO₂ presents a sharp m↔t transition. A “defect” component involving 30%–40% of the probe nuclei appears in the ^(181)Ta PAC spectra of all nanoparticles when these are cooled from high temperatures T ≥ 1200 K. The temperature dependence of this component can be reproduced by assuming that Ta impurities in hafnia and zirconia may trap electrons at low temperatures. The observation that the defect component appears only in nanoparticles with diameter d < 100 nm suggests that mobile electrons are available only in the surface region of the oxide particles, either from oxygen vacancies (Vo) and/or Vo- hydrogen donors at the interface of the nanoparticles and their hydrate layers. This conclusion is supported by the absence of a size effect for ^(111)Cd probes in HfO₂ and ZrO₂. The temperature dependence of the ^(181)Ta defect fraction is consistent with a Ta_(+) impurity level at E_d ~ 0.9 and 0.6 eV below the hafnia and zirconia conduction band, respectively

Structural phase transitions in BaTi0₃ studied via perturbed angular correlations

Phase transitions in the ferroelectric perovskite BaTiO₃ were studied for ^111In-implanted polycrystalline samples by measuring the electric field gradients by means of Perturbed Angular Correlation spectroscopy. The phase transitions between the orthorhombic double left right arrow rhombohedral double left right arrow tetragonal double left right arrow cubic lattices were investigated in 2 - 10 K steps, for increasing and decreasing temperatures, in order to determine their hysteresis. The transition parameters are compared with results from measurements of the spontaneous polarization, electric susceptibility and neutron scattering