Oxygen isotope effect in the Ruddlesden-Popper phases

The oxygen isotope effect in ceramic samples of bilayer manganites (La_1−z Pr _z )_1.2Sr_1.8Mn₂ ^16–18O₇(z = 0, 0.3) has been investigated. The real and imaginary parts of the magnetic susceptibility have been measured and a significant (more than 20 K) decrease in the ferromagnetic transition temperature upon oxygen isotope substitution ¹⁶O—¹⁸O is found. At temperatures T > 170 K, a number of additional ferromagnetic transitions are observed, whose critical temperatures also shift as a result of the isotope substitution. The obtained results are compared from the data for other manganite systems where giant isotope effect is observed. Original Russian Text ? A.N. Taldenkov, N.A. Babushkina, A.V. Inyushkin, R. Suryanarayanan, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 1, pp. 124–127.     

层状钙钛矿La1.3Sr1.7Mn2-xCuxO7的磁性及电特性

通过固相反应烧结法成功制备了层状钙钛矿La_1.3Sr_1.7Mn_2-xCu_xO₇多晶,主要研究了其磁电特性.结果表明,样品为Sr₃Ti₂O₇型钙钛矿结构.随着温度的降低,其磁性经历了一个很复杂的转变过程.当x=0时,在T^*=231K出现二维短程铁磁有序,在< 关键词： 层状钙钛矿 磁性 电特性     

层状钙钛矿La1.3Sr1.7Mn2-xCuxO7的磁性及电特性

通过固相反应烧结法成功制备了层状钙钛矿La_1.3Sr_1.7Mn_2-xCu_xO₇多晶，主要研究了其磁电特性.结果表明，样品为Sr₃Ti₂O₇型钙钛矿结构.随着温度的降低，其磁性经历了一个很复杂的转变过程.当x=0时，在T^*=231K出现二维短程铁磁有序，在<     

Weak ferromagnetism in copper metaborate CuB2O4

CuB₂O₄ single crystals have been grown and their magnetic and resonance properties have been investigated for the first time. The temperature dependence of the susceptibility was found to contain features at T=21 and 10 K. The CuB₂O₄ single crystal transformed at T=21 K to a weakly ferromagnetic state. The sharp drop in susceptibility at T<10 K is caused by a transition of the magnetic system of CuB₂O₄ to an antiferromagnetic state. The effective magnetic moment of the Cu₂₊ ion, determined from the high-temperature part of the magnetic susceptibility, is 1.77 μ _B. The room-temperature g factors are, respectively, 2.170 and 2.133 for magnetic field parallel and perpendicular to the c axis of the crystal. The antiferromagnetic resonance parameters in the weakly ferromagnetic and antiferromagnetic phases were measured. Fiz. Tverd. Tela (St. Petersburg) 41, 1267–1271 (July 1999)     

Investigations of the sublattice magnetizations M(T) in antiferromagnets with fourth-order exchange interactions: EuSrTe

We present a neutron scattering study of the temperature and composition dependence of the MnO-type superstructure reflection intensities in the diamagnetically diluted antiferromagnetic compounds Eu_xSr_1-xTe. In these materials antiferromagnetic biquadratic and ferromagnetic three-spin interactions have been identified recently. These fourth-order non-Heisenberg interactions are able to create their own order parameter which is believed to govern the order of the transverse moment components and which, hence, is directed perpendicular to the common Heisenberg order parameter. The observed MnO-type diffraction intensities originate in the sublattice magnetizations, , of both order parameters. Due to the different composition dependencies for biquadratic interaction processes and three-spin interaction processes , the ferromagnetic three-spin interactions dominate for x > x c =0.85, while for x <0.85 the antiferromagnetic biquadratic interactions dominate. Associated with this sign change in the fourth-order interaction sum the transverse order parameter changes from the antiferromagnetic MnO type for x <0.85 to ferromagnetic for x >0.85. This is noticed as a sudden decrease of the low-temperature MnO scattering intensities at x c =0.85. Although susceptibility measurements reveal clearly a ferromagnetic component for x >0.85 no ferromagnetic Bragg intensities were observed in standard neutron scattering spectra using EuTe powder samples. We explain this by the competition of antiferromagnetic biquadratic and ferromagnetic three-spin interactions whereby a disturbed ferromagnetic superstructure may be generated which gives rise also to weak MnO-type diffraction lines. It is found that the resulting obeys a T² law until a temperature as large as 0.75T_N irrespective of the nature of the transverse order parameter. The T² law must, hence, be common to both types of order parameter showing that the fourth-order interactions re-define the spin dynamics of both completely. From the linear composition dependence of the normalized T² coefficient the existence of three-spin interactions is again confirmed. Received 23 July 1998 and Received in final form 12 October 1998     

Enhanced temperature-independent magnetoresistance below the metal-insulator transition temperature of epitaxial La0.2Nd0.4Ca0.4MnO3 thin films

Epitaxial La_0.2Nd_0.4Ca_0.4MnO₃ thin films have been deposited at 800°C on LaAlO₃ substrate using pulsed laser deposition technique. The structural and magnetotransport properties of the films have been studied. The sharp peak in the temperature dependence of the resistance corresponding to metal-to-insulator transition (T _p) has been observed at a temperature of T _p=82 K, 97 K and 110 K for 0 Oe, 20 kOe and 40 kOe magnetic fields, respectively. The film exhibits a large nearly temperature-independent magnetoresistance around 99% in the temperature regime below T _p. The zero field-cooled (ZFC) and field-cooled (FC) magnetization data at 50 Oe shows irreversibility between the ZFC and FC close to the ferromagnetic transition temperature T _c=250 K. The ZFC temperature data of the film displays ferromagnetic behavior for higher temperature regime T _c=250 K>T>T _p=82 K, and a decrease in magnetization with decreasing temperature up to 5 K below 82 K exhibiting a sort of antiferromagnetic behavior in the low temperature regime (T<82 K=T _p=T _N).     

Optical study of the antiferromagnetic-paramagnetic phase transition in chromium oxide Cr2O3

Abstract

Optical effects of the first and second order with respect to the order parameter (1 is antiferromagnetic vector) have been studied in Cr₂O₃ around its phase transition at T_N = 306 K from the antiferromagnetic to paramagnetic state. The magnetic linear birefringence is characterized by a rather large magnitude Δn _sp ? 10^?3 and by a large contribution of fluctuations of the order parameter to the birefringence. The study of the nonreciprocal optical rotation induced by an electric field has shown that the phase transition has a well defined first-order character. This result is also supported by the observation of a nonlinear (quadratic in the electric field) nonreciprocal rotation in a narrow temperature region ΔT = 0.15 K around T_N . The temperature variation of the order parameter l(T) below T_N is well described by a power low l where τ^β = (T_N - T)/T_N and β = 0.355. We also observed a very reproducible effect of the rotation of the optical indicatrix in opposite directions for two types of antiferromagnetic domains. The possible explanation of this effect could be related to the so-called gyrotropic birefringence, an effect related to k_il_i terms in the dielectric permeability.     

Thermal conductivity of (La0.25Pr0.75)0.7Ca0.3MnO3 under giant isotope effect conditions

The thermal conductivity of (La_0.25Pr_0.75)_0.7Ca_0.3MnO₃ manganite has been studied. The isotope substitution of ¹⁸O for ¹⁶O in this compound leads to a ferromagnetic-antiferromagnetic phase transition at low temperatures. It has been found that the thermal conductivity in the ferromagnetic state is approximately two times higher than in the antiferromagnetic state. It has been shown that the small value of thermal conductivity and its temperature dependence can be due to strong phonon scattering from crystal lattice defects, which are thought of as Jahn-Teller distortions. The parameters of this scattering can be determined within the Debye model of thermal conductivity from a comparison of samples differing in their isotope composition.     

Magnetic properties of neptunium diarsenide

The magnetic properties of a collection of single crystals of NpAs₂ have been investigated in the temperature range 4.2-300 K and in applied fields up to 100 kOe. For small magnetic fields (3 kOe), NpAs₂ is ferromagnetic up to T_IC=18 K, antiferromagnetic from 28 K to T_N=52 K, then paramagnetic. Both transitions are first order. When the applied field increases T_IC is shifted towards T_N. The antiferromagnetic phase disappears for H#62;30 kOe. The ferromagnetic range is characterized by a very large anisotropy. In the paramagnetic state, NpAs₂ has an effective moment 1.88 μ_B.     

Magnetic behaviour of Cu2FeGeSe4

Measurements of magnetic susceptibility χ as a function of temperature T and of magnetisation M as a function of applied magnetic field H at a number of fixed temperatures were made on polycrystalline samples of Cu₂FeGeSe₄. The χ versus T data show that an antiferromagnetic transition occurs at 20 K and that a second transition occurs at 8 K, indicating a transition to weak ferromagnetic form. The M versus H curves indicated that at all temperatures below 70 K bound magnetic polarons (BMP) occur, in the paramagnetic, antiferromagnetic and weak ferromagnetic ranges. Below 8 K, the M versus H curves exhibited magnetic hysteresis, and this is attributed to the interaction of the BMPs with tetragonally anisotropic matrix. The B versus H curves were well fitted by a Langevin-type of equation, and the variation of the fitting parameters determined as a function of temperature. These showed that above 20 K the total BMP magnetisation fell almost linearly with increasing temperature and effectively disappeared at 70 K. The number of BMPs remained practically constant with temperature having a mean value of 6.55×10¹⁸/cm³. The analysis gave a value of 213 μ_B for the average magnetic moment of a BMP, corresponding to 42.4 Fe atoms. Using a simple spherical model, this gives the radius of a BMP as 12.0 Å.     

Dynamical transport behavior in electron-doped manganites La0.4Ca0.6(Mn1−x Ru x )O3

Measurements of the dynamical electrical transport behavior are performed on electron-doped manganites La_0.4Ca_0.6(Mn_1−x Ru _x )O₃ (x=0 and 0.02). An undoped sample possesses a robust charge-ordered antiferromagnetic ground state, and only a positive resistivity relaxation can be observed. However, a low-temperature negative relaxation behavior arises after inducing a few ferromagnetic orders to the charge-ordered matrix by tiny Ru doping. We assigned this difference to the dynamical competition between ferromagnetic metallic and charge-ordered insulating phases. Consistently, for a doped sample, the crossover from positive to negative resistivity relaxation behavior ensues around T=115 K, which is just below the ferromagnetic Curie temperature.     

Observation of 18O/16O isotope effects at the cathode of a polymer electrolyte membrane fuel cell

¹⁸O/¹⁶O isotope effects were observed at the cathode of a polymer electrolyte membrane fuel cell at 25 and 35°C. Results of experiments in which the ¹⁸O/¹⁶O isotope ratios of the oxygen gases supplied to and exhausted from the cell were measured revealed that the lighter isotope ¹⁶O reacted more preferentially to form water molecules at the cathode than the heavier one, ¹⁸O. The value of the oxygen isotope separation factor, S₁, defined as the ratio of the ¹⁸O/¹⁶O isotope ratios of the oxygen gases supplied to and exhausted from the cell, ranged from 1.0030 to 1.0139, and tended to decrease with decreasing rate of oxygen utilisation (θ) and with increasing flow rate of the feed oxygen gas (D_F). The value of another separation factor, S₂, defined as the ratio of the ¹⁸O/¹⁶O isotope ratios of the exhausted oxygen gas and oxygen having reacted to form water molecules at the cathode, ranged from 1.0049 to 1.0304. The S₂ value was much less affected by the change in θ and D_F than the S₁ value with the majority of the S₂ value being in the range of 1.0240–1.0304.     

Pressure and field dependence of the magnetic transition in GdBa2Cu3O7-x

The influence of hydrostatic pressure and of magnetic field strenght is presented for the low temperature antiferromagnetic ordering temperature (T_N=2.3 K) of GdBa₂Cu₃O_7-x. Data are presented for both superconducting and normal samples, the superconducting sample having a sharp 95 K transition and the oxygen-depleted normal sample being a semiconductor. For both systems the Néel temperatures, extrapolated to zero measuring field, are identical: T_N = (2.33±0.03) K. The effect of pressure is to raise the transition temperature slightly for both samples, dT_N/dP=+0.03 K/kbar for the superconducting sample and +0.04 K/kbar for the normal sample. The temperature dependence of the heat capacity made in several fixed external magnetic fields and the isothermal magnetization for T<T_N provide a measure of the antiferromagnetic-paramagnetic phase boundary, which shows T_N approaching T=0 K at about 2.5 T.     

Oxygen isotope effect in chromium-doped Pr<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> manganites

The effect of oxygen isotope substitution on the properties of Pr_0.5Ca_0.5Mn_{1 ? x} Cr _x O₃ manganites (x = 0, 0.02, 0.05) have been studied. The introduction of chromium favors (i) the decomposition of a charge-ordered state and (ii) the appearance of a ferromagnetic metallic phase in Pr_0.5Ca_0.5Mn_{1 ? x} Cr _x ^16–18O₃. The isotope substitution ¹⁶O → ¹⁸O leads to a decrease in the content of the ferromagnetic phase, an increase in the charge-ordering transition temperature (T _CO), and a decrease in the ferromagnetic transition temperature (T _FM). The isotope mass exponent is evaluated.     

Spin susceptibility of neutron-irradiation-disordered YBa2Cu3O6.9 in the normal and superconducting states

The spin-spin relaxation rate ⁶³ T ₂ ⁻¹ of ⁶³Cu nuclei in CuO₂ layers is measured in the normal and superconducting states of the compound YBa₂Cu₃O_6.9 (T _c ^onset =94 K) subjected to radiation-induced disordering by a fast-neutron flux Φ to T _c ^onset =68 K (Φ=7×10¹⁸ cm⁻²) and T _c ^onset <4 K (Φ=12×10¹⁸ cm⁻²). It is found that as the structural disorder increases, the contribution of the indirect spin-spin interaction ⁶³ T _2G ⁻¹ , which is related to the value of the spin susceptibility at the boundary of the Brillouin zone of the copper planes χ_s(q={π/a; π/a}), decreases slightly at the transition to the superconducting state for the initial sample and remains unchanged for the weakly disordered sample. This behavior of the short-wavelength contribution to the spin susceptibility attests to the stability of the x ²−y ² symmetry of the energy gap against structural disorder, in accordance with proposed theoretical models of Cooper pairing for high-T _c cuprates. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 172–177 (10 February 1998)     

Optical Tomograms of Multiple-Photon-Added Gaussian States via the Intermediate State Representation Theory

The oxygen isotope effect in PrBaMn₂^16-18 O_5.97 manganite with an ordered cation arrangement is studied. The field dependences of magnetic susceptibility and magnetization are measured in the temperature range 100–270 K and magnetic fields up to 32 T. A significant increase in the temperature of the spin-reorientation antiferromagnet–ferromagnet phase transition is detected in samples enriched in heavy oxygen ¹⁸O (negative isotope effect). The transition temperature and the isotope effect depend strongly on the magnetic field. An H–T phase diagram is plotted for samples with various isotope compositions. An analysis of the experimental results demonstrates that the detected negative isotope effect and the giant positive isotope effect revealed earlier in doped manganites have the same nature. The mechanisms of appearance of isotope effects are discussed in terms of the double exchange model under a polaron narrowing of the free carrier band.     

Isotope effect and characteristic features of the phase diagram for cobaltites with spin-state transitions

The effect of ¹⁶O → ¹⁸O oxygen isotope substitution has been studied for (Pr_1−y Eu _y )_0.7Ca_0.3CoO₃ cobaltites (0.12 < y < 0.26). A pronounced isotope shift has been found for the spinstate transition temperature, which increases with the oxygen isotope mass. In contrast, the ferromagnetic transition temperature has slightly lower values for the samples with heavier oxygen. The observed phenomena and constructed phase diagram confirm the results reported previously for (Pr_{1 − y} Sm _y )_0.7Ca_0.3CoO₃ in [G. Y. Wang, X. H. Chen, T. Wu, et al., Phys. Rev. B 74, 165113 (2006)]. The measurements of the specific heat have been performed for (Pr_{1 − y} Eu _y )_0.7Ca_0.3CoO₃ with the main emphasis on the analysis of the isotope effect. The contributions to the isotope effect coming from the lattice and magnetic components of the specific heat have been separated. The mechanisms underlying the large isotope effect are discussed.     

Single crystal study of the one dimensional Ca Co O compound: five stable configurations for the Ising triangular lattice

Single crystals of the one-dimensional phase Ca₃Co₂O₆ of several mm length have been grown. The magnetic study of such a crystal confirms the previous observations on polycrystalline samples: it consists of a triangular lattice of ferromagnetic [Co₂O₆] chains ( K) antiferromagnetically coupled ( K). The dynamic of these chains array, probed by AC susceptibility, is very slow as shown from the large shift of the freezing temperature from 12 K to 16.5 K as the excitation frequency increases by three orders of magnitude (10⁰ to 10³ Hz). The origin of this effect is believed to be the result of different arrangements with close energies for the chain ferromagnetic moments on the triangular lattice. Five stable magnetic configurations have been evidenced by the magnetization as a function of applied field curves registered at 2 K. Their relative magnetizations correspond to m =1/4, 1/2, 1, 2, 3 where m =3 represents the ferromagnetic ordering of three chains on the same triangle, each chain having a m =1 magnetization. A magnetic phase diagram is finally proposed. Received 7 December 1999     

Frustrated exchange interactions formation at low temperatures and high hydrostatic pressures in La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>Mn<Subscript>O2.85</Subscript>

The magnetic and thermal properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite are investigated in wide temperature (4–350 K) range, including under hydrostatic pressure (0–1.1 GPa). Throughout the pressure range investigated, the sample is spin glass with diffused phase transition into paramagnetic state. It is established, that spin glass state is a consequence of exchange interaction frustration of the ferromagnetic clusters embeded into antiferromagnetic clusters. The magnetic moment freezing temperature T _f of ferromagnetic clusters increases under pressure, freezing temperature dependence on pressure is characterized by derivative value ∼4.5 K/GPa, while the magnetic ordering T _MO temperature dependence is characterized by derivative value ∼13 K/GPa. The volume fraction of sample having ferromagnetic state is V _fer ∼ 13% and it increases under a pressure of 1.1 GPa by ΔV _fer ≈ 6%. Intensification of ferromagnetic properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite under hydrostatic pressure is a consequence of oxygen vacancies redistribution and unit cell parameters decrease. The most likely mechanism of frustrated exchange interactions formation is discussed.     

NMRON studies of the antiferromagnetic and paramagnetic phases of54Mn-MnCl2 · 4H2O

Pulsed NMRON, CW NMRON and thermal NMR-NO methods have been utilized to study⁵⁴Mn-MnCl₂ · 4H₂O. The⁵⁴Mn spin-lattice relaxation timeT ₁ in zero applied field has been measured between 35 and 90 mK in the antiferromagnetic phase. Above 65 mK the dominant relaxation mechanism is a Raman process with the electronic magnons, but at lower temperatures a direct process takes over. NMRON has been observed for the first time in the paramagnetic phase, and a line width of 300 kHz, with both homogeneous and inhomogeneous contributions, is observed. In the antiferromagnetic phase the line width is 35 kHz, and there are also homogeneous and inhomogeneous contributions. The dependence ofT ₁ for the⁵⁴Mn spins on field and temperature was studied in the paramagnetic phase. AT ₁ minimum centred atB ₀=2.64 T was observed. The hyperfine parameter <⁵⁴ AS>/h=−513.6(3) MHz in the paramagnetic phase, and comparison with the value in the antiferromagnetic phase gives 0.013(1) for the zero point spin deviation.