Magnetic properties of R2Pt compounds with R = Gd,Tb, Dy,Ho, Er and Tm

Magnetic properties of polycrystalline samples of R₂Pt compounds (R = Gd, Tb, Dy, Ho, Er and Tm) are presented. The Gd, Td, Dy, Ho based compounds are ferromagnetic with Curie temperatures ranging between 155 and 17 K. Er₂Pt and Tm₂Pt are antiferromagnetic with Néel temperatures of 9 and 5 K respectively. The observed properties are discussed considering indirect exchange interactions and crystal field effects acting on the rare earth ions which lies in very low symmetry sites.     

Effect of the crystalline electric field on the Néel temperatures of RCu2 compounds

Values for the Néel temperature of the RCu₂ compounds (R=Tb–Tm) have been calculated using a molecular-field model including crystalline-electric-field effect as presented by Noakes and Shenoy. The calculated results show that the unusual behavior, at the Néel temperatures, of these compounds can be explained on the basis of this model.     

Magnetic properties of RCoSi2 compounds (R=rare earth)

New ternary silicides of composition RCoSi₂ (R=rare earth and Y) have been prepared and found to crystallize in the orthorhombic CeNiSi₂-type structure. Their magnetic properties have been studied by means of susceptibility measurements between 2 and 250 K. The Ce and Y compounds show essentially temperature independent Pauli paramagnetism. The compounds with R=Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm show antiferromagnetic ordering below 20 K. The effective rare earh moments in the paramagnetic state agree well with the free ion values, and, for the heavy rare earths, the Néel temperatures vary with the De Gennes factor. There is no indication for a magnetic contribution from the Co sublattice.     

Susceptibility measurements on some R5In3 compounds

The magnetic susceptibility of some rare earth-indium compounds has been measured in the temperature range 4.2–300 K under a constant field of 1 kOe. The compounds with R = Gd, Tb and Dy are antiferromagnetic, having Néel temperatures between 4.2 and 78 K ; those with R = Ho, Er and Tm seem to be ferromagnetic with ordering temperatures probably below 4.2 K. Y₅In₃ presents a temperature independent susceptibility.     

Spin and temperature dependence of the magnetic hyperfine field of Cd in the rare earth-aluminum Lavesphase compounds RAl2

Perturbed angular correlation spectroscopy has been used to investigate the combined magnetic and electric hyperfine interaction of the probe nucleus ¹¹¹Cd in ferromagnetically ordered rare earth (R)-dialuminides RAl₂ as a function of temperature for the rare earth constituents R=Pr, Nd, Sm, Eu, Tb, Dy, Ho and Er. In compounds with two magnetically non-equivalent Al sites (R=Sm, Tb, Ho, Er), the magnetic hyperfine field was found to be strongly anisotropic. This anisotropy is much greater than the anisotropic dipolar fields, suggesting a contribution of the anisotropic 4f-electron density to magnetic hyperfine field at the closed-shell probe nucleus. The spin dependence of the magnetic hyperfine field reflects a decrease of the effective exchange parameter of the indirect coupling with increasing R atomic number. For the compounds with the R constituents R=Pr, Nd, Tb, Dy and Ho the parameters B₄, B₆ of the interaction of the crystal field interaction have been determined from the temperature dependence of the magnetic hyperfine field. The ¹¹¹Cd PAC spectrum of EuAl₂ at 9 K confirms the antiferromagnetic structure of this compound.     

Raman scattering studies of the magnetic ordering in hexagonal HoMnO3 thin films

We present the results of the temperature dependence of the Raman spectra of hexagonal HoMnO₃ thin films in the 13–300 K temperature range. The films were grown on Pt(111)//Al₂O₃ (0001) substrates using the laser ablation method. In the HoMnO₃ thin films, we initially observedseveral broad Raman peaks at ∼510, 760, 955, 1120, and 1410 cm⁻¹. These broad Raman peaks display an anomalous behavior near the magnetic transition temperature, and the intensity difference of the Raman spectra at different temperatures shows several pairs of negative and positive peaks as the temperature is lowered below the Néel temperature. Our analyses indicate that all the broad peaks are correlated with magnetic ordering, and we have assigned the origin of all the broad peaks. Purely on the basis of the Raman analysis, we have deduced the Néel temperature and the spin exchange integrals of HoMnO₃ thin films. We also investigated the effects of the growth condition on the strongest broad peak at ∼760 cm⁻¹, which is related with pure magnetic ordering. This result indicates that the oxygen defect in the HoMnO₃ sample has negligible effect on magnetic ordering. Copyright © 2009 John Wiley & Sons, Ltd.     

Vibrational modes of rare‐earth formates

In this work we investigate the room‐temperature Raman spectra of several rare‐earth [RE(HCOO)₃; RE = La, Pr, Nd, Gd, Tb and Y] formates. Polarized measurements were carried out for La(HCOO)₃ yielding the observation of most of the Raman‐active phonons predicted by group theory analysis. The respective assignment is given based on the formate vibrations and correlations with previous results in other RE(HCOO)₃ systems. The wavenumber ‘softening’ observed when RE goes from Y to La is accounted for the lattice expansion. Copyright © 2009 John Wiley & Sons, Ltd.     

Mössbauer effect study of γ-FeMn alloys

The disordered antiferromagnetic γ-FeMn alloys in the range of Mn concentration from 0 up to 50 at.% Mn were investigated by means of X-ray diffraction analysis (XRD), differential scanning calorimeter (DSC) and Mössbauer spectroscopy (ME) at room temperature and at 80 K. The lattice parameter is constant for Mn contents below 27 at.% and increases linearly for higher Mn concentration. The RT Mössbauer spectra for the alloys with less than 20 at.% Mn were fitted with quadrupole splitting (QS) distribution, and above 20 at.% Mn they were fitted with magnetic hyperfine field (HF) distribution. The averageQS decreases, while the HF maintain a constant value by increasing the Mn concentration. At 80 K, the HF increases linearly up to 30 at.% Mn and then maintains a constant value at about 38 kOe. The peak of the specific heat (c _p) at the Néel temperature changes from broad to sharp as the Mn content increases. The Néel temperature increases with Mn concentration.     

Spectroscopy of La0.5Sr1.5MnO4 orbital ordering: a cluster many-body calculation

Orbital ordering (OO) in La_0.5Sr_1.5MnO₄ has been studied using soft X-ray resonant diffraction (SXRD) at the Mn L_2,3 edges in combination with many-body cluster calculations. The SXRD intensity is modelled in second quantization using a small planar cluster consisting of a central active Mn site with first-neighbour shells comprising O and Mn sites. The effective Hamiltonian includes Slater-Koster parameters and charge transfer and electron correlation energies obtained from previous measurements on manganites. The energy dependence of the SXRD OO peak is calculated using the Jahn-Teller distortions of the oxygen octahedra and in-plane spin correlations as adjustable parameters. These contributions are clearly distinguished above the Néel temperature with a good spectroscopic agreement. The results also suggest a significant charge separation between the Mn sites.     

Magnetic ordering in Co<Subscript><Emphasis Type="Italic">c</Emphasis></Subscript>Mg<Subscript>1−<Emphasis Type="Italic">c</Emphasis></Subscript>O solid solutions

The influence of dilution by diamagnetic ions on the magnetic ordering in single-crystal Co _c Mg_1−c O solid solutions was studied by Raman spectroscopy and magneto-optical microscopy in a wide range of temperatures (6 < T < 200 K). Far infrared absorption measurements of antiferromagnetic resonance (AFMR) were also performed for pure CoO. It was found that the domain structure and the contribution from Brillouin zone center magnons to Raman scattering and AFMR disappear well below the Néel temperature, whose value was determined from neutron diffraction and magnetic susceptibility measurements. The text was submitted by the authors in English.     

Spin-phonon coupling in multiferroic manganites RMnO3: comparison of pure (R = Eu, Gd, Tb) and substituted (R = Eu1-xYx) compounds

For rare-earth manganite RMnO₃ compounds spin-phonon coupling manifests itself as a phonon softening in the temperature range of the magnetically ordered phases. Within this class of materials, a continuous tuning of the lattice and thus also of the magnetic properties of multiferroic manganites is achieved by Y doping in substituted Eu_{1- x} Y _x MnO₃. We compare the impact on spin-phonon coupling within this partial-substitution approach in a series of Eu_{1- x} Y _x MnO₃ samples 0 ≤ x ≤ 0.5) with the effect of a complete exchange of the rare earth ions R³⁺ in a series of pure RMnO₃ compounds (R = Eu, Gd, Tb). For this purpose we employ polarized Raman scattering in the 10–300 K temperature range. The low-temperature results show phonon softening in all investigated compounds. For decreasing R³⁺ radius, i.e. an increasing orthorhombic distortion and magnetic frustration, we observe in both systems a weakening of the spin-phonon coupling. For known sublattice magnetization within the MnO₂-plane, quantitative results for the spin-phonon coupling constant are derived for both cases within a molecular field approximation. Our results show, that the spin-phonon coupling strength in the magnetically ordered phases of the various investigated manganites does not correlate with the magnetization pattern. Instead, the pure RMnO₃ compounds and the substituted Eu_{1- x} Y _x MnO₃ fit excellently within a common scheme, in which the weakening of the spin-phonon coupling reflects the degree of tilting of the MnO₆ octahedra due to the orthorhombic distortion of the crystal lattice.     

Antiferromagnetic spin structure in the heavy-fermion compound YbNiAl

Neutron diffraction experiments have been performed in order to study the magnetic order of the heavyfermion compound YbNiAl. Below the Néel temperature at T _N = 2.9K a sinusoidally modulated structure is found with an amplitude of the modulation of μ = (1.9±0.2)μ_B at T = 200 mK. The magnetic propagation vector is k _m = (τ00) with τ = (0.779±0.001), and the magnetic Yb-moments are directed perpendicular to the hexagonal c-axis. The results obtained on YbNiAl are discussed in comparison to RNiAl compounds with other rare earth atoms R.     

The temperature dependence of the angular variation of,and the critical point exponent for the EPR linewidth in the two-dimensional canted antiferromagnetic (C2H5NH3)2MnBr4: Evidence for a structural phase transition

The angular variation of the EPR linewidths in single crystals of (C₂H₅NH₃)₂MnBr₄ has been measured as a function of temperature. The angular dependence is well characterized by δH(θ) = a + b(3 cos²θ ? 1) + c(3 cos²θ ? 1)². The temperature dependence of the expansion coefficients is reported, and the effect of critical point fluctuations near the Néel temperature as well as a linear temperature dependence at high temperature are observed. A sharp decrease in linewidths at 160°K is attributed to a structural phase transition. The Néel temperature is determined to be 46°K (± 1°) from linewidth measurements of a powder sample. The linewidths diverge exponentially near the Néel temperature with a critical point exponent of 1.5.     

Crystallographic distortion in CrAs0.50Sb0.50

X-ray diffraction has shown that CrAs_0.50Sb_0.50 undergoes at the Néel temperature a crystallographic distortion (NiAs → MnP) accompanied by a discontinuous volume change. Neutron diffraction revealed in this compound a helimagnetic structure very similar to the ‘double spiral’ in CrAs discovered recently. The pressure dependence of the Néel temperature is estimated in the framework of the Bean Rodbell theory and compared with the CrAs value.     

Raman scattering studies of (GaP)n/(InP)n (n = 1, 1.7, 2) short‐period superlattice structures

We report Raman scattering from (GaP)_n/(InP)_n (n = 1, 1.7, 2) short‐period superlattice (SPS) structures to study the effect of lateral composition modulation (LCM) on the behavior of optical phonons. Cross‐sectional transmission electron microscope images revealed that LCM was formed with complex pattern in the n = 1.7 and n = 2 samples grown at 490 °C. Interestingly, both the InP‐ and the GaP‐like longitudinal optical (LO) phonon energies increased systematically as the number of monolayers was increased from n = 1 to n = 2. A significant broadening of the phonon line shapes was also observed for the n = 1.7 and n = 2 samples. In contrast, for samples grown at 425 °C, both the increase of the LO phonon energies and the broadening of the phonon line shapes were observed only when n = 1.7. Our results demonstrate that the optical phonons in the (GaP)_n/(InP)_n SPS structures are significantly affected in the occurrence of LCM related to the growth temperature and the number of monolayers.Copyright © 2009 John Wiley & Sons, Ltd.     

The pressure response of Raman active phonon modes of Tm3Al5O12

Abstract

We present the Raman spectrum of Tm₃Al₅O₁₂ single crystal and its pressure dependence for hydrostatic pressure up to 11GPa and room temperature. Tm₃Al₅O₁₂ belongs to the crystal family of rare earth garnets (Re₃A1₂(AlO₄)₃, Re: Gd, Tb, Dy, Er,…), which crystallize in the body-centered cubic lattice and contains eight molecular units in the conventional unit cell, Group theory predicts 25 Raman active modes for these compounds, while experimentally are observed 15 modes. As crystal volume decreases all Raman peaks exhibit pressure coefficients varying from 0.7 to 5.6cm^?1/ GPa. A large part of the vibrational spectra of these compounds could be explained taking into account the vibrational properties of molecular subunits, namely AlO₄.     

Magnetic, magnetocaloric, and magnetotransport properties of RCo1.8Mn0.2 (R=Er, Ho, Dy, and Tb) compounds

The magnetic, magnetocaloric, and magnetotransport properties of RCo_1.8Mn_0.2 (R=Er, Ho, Dy, and Tb) were studied by room temperature X-ray diffraction, magnetization, and resistivity measurements at a temperature interval of 5-400 K and magnetic fields up to 5 T. The Curie temperature of RCo₂ was found to increase significantly when 10% Mn was substituted for Co. The effective paramagnetic moments were found to be in reasonable agreement with their theoretical values. A large magnetoresistance (MR) of Δρ/ρ_o≈−13.5% for R=Ho at T≈153 K for ΔH=5 T has been observed. The maximum relative cooling capacities vary from 467 J/kg at low temperature for R=Er to 202 J/kg at the near room temperature for R=Tb.     

自旋波-声子耦合对反铁磁体红外吸收谱的影响

由于交换作用常数受到磁离子间距离变化的影响是不可忽略的,在磁有序物质中存在自旋波和声子的耦合作用。本文探讨了非金属反铁磁体中自旋波-声子耦合在红外吸收谱上可能有的表现,辐射场的电偶极矩作用在激发一个光频支声子的同时,通过自旋波-声子耦合又激发两个自旋波和一个声子,这就在晶格吸收谱的高频边缘以外形成附加吸收带。当反铁磁体的Néel点足够高,并且自施波态密度有尖锐的极大值时,附加吸收带就有超出晶格吸收边缘足够大的能量,且有明显的峯形,上述吸收峯就应在实验上观测到。把这一结果应用于NiO,解释了它的红外光谱中0.24电子伏的附加吸收峯,计算得到与实验相符合的吸收能量和吸收强度,并指出这一吸收机构只能在反铁磁体中而不能在铁磁体中发生。本文最后指出,Mizuno和Koide在企图解释同一现象的工作中,所探讨者相对于本文所考虑的跃迁机构,它只可能产生极其次要的贡献。     

Quantum Monte Carlo investigation of the 2D Heisenberg model with S=1/2

The two-dimensional (2D) Heisenberg model with anisotropic exchange (Δ = 1−J _x /J _z ) and S=1/2 is investigated by the quantum Monte Carlo method. The energy, susceptibility, specific heat, spin-spin correlation functions, and correlation radius are calculated. The sublattice magnetization (σ) and the Néel temperature of the anisotropic antiferromagnet are logarithmic functions of the exchange anisotropy: 1/σ+1+0.13(1)ln(1/Δ). Crossover of the static magnetic structural factor as a function of temperature from power-law to exponential occurs for T _c /J≈0.4. The correlation radius can be approximated by 1/ξ=2.05T ^1.0(6)/exp(1.0(4)/T). For La₂CuO₄ the sublattice magnetization is calculated as σ=0.45, the exchange is J=(1125–1305) K; for Er₂CuO₄ J∼625 K and the exchange anisotropy Δ∼0.003. The temperature dependence of the static structural magnetic factor and the correlation radius above the Néel temperature in these compounds can be explained by the formation of topological excitations (spinons). Fiz. Tverd. Tela (St. Petersburg) 41, 116–121 (January 1999)     

Raman study of orbital mediated multiphonons in RMnO3 (R = Pr,Sm,Eu,Tb,Y)

We have studied RMnO₃ manganites (R = Pr, Sm, Eu, Tb, Y) Raman excitations in the 200–2800 cm^-1 range as a function of temperature. Combinations of phonon energies are observed up to the fourth order, indicating the presence of electron-phonon coupling. In comparison to Γ-point phonon combinations, double phonon excitations appear to be blue shifted in large size rare earth ion compounds. The phonon combination intensities decrease rapidly with their increasing order, confirming other studies which conclude that the electron-phonon coupling is not as strong as supposed in the localized limit. Moreover, different intensity order dependences are observed between the phonon combination and the so-called Jahn-Teller mode. These effects are better described in the orbiton-phonon coupling scheme.