Phase transitions in Sr 0.61 Ba 0.39 Nb 2 O 6 :Ce 3+ : I. Susceptibility of clusters and domains

The complex dielectric susceptibility of Sr _0.61 Ba _0.39 Nb ₂ O ₆ :Ce ³⁺ (SBN61:Ce) has been measured at frequencies and temperatures before and after poling. The relaxor behaviour with large polydispersivity observed above the ferroelectric phase transition temperature, T _c = 360 and 340 K for x (Ce) = 0 and 0.0066, respectively, is perfectly modeled within the framework of Chamberlin's dynamically correlated domain approach. Below T _c the dynamic nanodomain state crosses over into a ferroelectric state with polydispersive domain wall dynamics at very low frequencies. Presumably SBN61:Ce belongs to the three-dimensional random field Ising rather than to the dipole glass universality class. Received 1 October 1999     

Synthesis and microwave properties of TI2Ba2CaCu2O8 superconducting films grown by MOCVD

We report on the synthesis, structural and electrical characterization of high quality Tl₂Ba₂Ca₁Cu₂O₈ (Tl-2212) superconducting films. The samples have been grown ex-situ on mm² LaAlO₃ (100) substrates by a combined approach of metal-organic chemical vapor deposition (MOCVD) and thallium vapor diffusion. The morphological and compositional nature of the c-axis oriented films has been investigated by SEM and X-ray analyses. Typical values of K and MA/cm² at 77 K have been measured. Microwave measurements have been performed at f = 87 GHz inserting the film in a copper cavity and at f =1.5 GHz on patterned samples using a microstrip resonator technique. A penetration depth nm is evaluated by fitting the microwave data with phenomenological equations. The minimum value of the surface resistance measured at 4.2 K is 60 and 6 m at 1.5 GHz and 87 GHz respectively. The microwave data are described in the context of a modified two fluid model. An evaluation of the temperature dependence of the scattering rate has been performed through the simultaneous measurement of the surface resistance and the penetration depth. Received 16 December 1999 and Received in final form 17 March 2000     

Charge ordering and elastic constants in Fe3-xZnx xO4

We present the temperature dependence of elastic modes of Fe_3-xZn_xO₄ with x = 0, x = 0.02 and x = 0.032. The c₄₄ shear modes show a pronounced softening which can be normalized to a common behavior for all these substances. We can explain the softening with a bilinear coupling of the elastic strain to an order parameter linked to charge ordering processes. The other elastic modes (c₁₁, c₁₂ and the bulk modulus) do not show any softening. We present a symmetry analysis for the charge ordering model. Received 27 April 1999     

Spin-wave excitations in finite chain segments of the diluted one-dimensional Heisenberg antiferromagnet CsM{n_{1 - x}}M{g_x}B{r_3}

The inelastic neutron scattering technique was employed to study the magnetic excitation spectra in the diluted one-dimensional Heisenberg antiferromagnet CsMn_1-xMg_xBr₃ (x =0, 0.05, 0.10, 0.25, 0.50). The spectral response is interpreted in terms of spin-wave excitations in finite chain segments of Mn²⁺ ions, which are found to exist as long as the chain length exceeds twice the wavelength of the spin excitation. This limit determines the crossover into the mesoscopic regime. Received 31 December 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     

Unconventional antiferromagnetic correlations of the doped Haldane gapsystem Y 2 BaNi 1 - x Zn x O 5

We make a new proposal to describe the very low temperature susceptibility of the doped Haldane gap compound Y₂BaNi_1-xZn_xO₅. We propose a new mean field model relevant for this compound. The ground state of this mean field model is unconventional because antiferromagnetism coexists with random dimers. We present new susceptibility experiments at very low temperature. We obtain a Curie-Weiss susceptibility χ( T ) ∼ C /(Θ + T ) as expected for antiferromagnetic correlations but we do not obtain a direct signature of antiferromagnetic long range order. We explain how to obtain the “impurity” susceptibility ( T ) by subtracting the Haldane gap contribution to the total susceptibility. In the temperature range [1 K, 300 K] the experimental data are well fitted by T ( T ) = C _imp 1 + T _imp / T . In the temperature range [100 mK, 1 K] the experimental data are well fitted by T ( T ) = A ln( T / T _c ), where T _c increases with x. This fit suggests the existence of a finite Néel temperature which is however too small to be probed directly in our experiments. We also obtain a maximum in the temperature dependence of the ac-susceptibility ( T ) which suggests the existence of antiferromagnetic correlations at very low temperature. Received 17 July 2001     

Magnetic properties of the low dimensional spin system (VO)PO:ESR and susceptibility

Experimental results on magnetic resonance (ESR) and magnetic susceptibility are given for single crystalline (VO)₂P₂O₇. The crystal growth procedure is briefly discussed. The susceptibility is interpreted numerically using a model with alternating spin chains. We determine J =51 K and . Furthermore we find a spin gap of meV from our ESR measurements. Using elastic constants no indication of a phase transition forcing the dimerization is seen below 300 K. Received: 22 December 1997 / Accepted: 17 March 1998     

On the T Bloch law in magnets with fourth-order exchange interaction

For the ferromagnets EuS and GdMg, in which fourth-order exchange interactions (i.e. biquadratic, three-spin and four-spin interactions) have been identified, the deviation of the spontaneous magnetization with respect to the T =0 value is shown to follow a T² law instead of the famous T3/2 law expected for a Heisenberg ferromagnet. Moreover, the observed T² law holds for temperatures as large as 0.8T_C and the extrapolated magnetization value for does not conform to ferromagnetic saturation. This is because the fourth-order exchange interactions generate a second order-parameter which is assumed to govern the order of the transverse moment components. These moment components have a finite expectation value for at the expense of the Heisenberg order parameter. Like the spontaneous magnetization, the critical field curves B c ( T ) of the metamagnet EuSe and the antiferromagnet EuTe also start decreasing with a T² term for . It is argued that the T² law is a consequence of the fourth-order exchange interactions. This is shown experimentally by a study of the critical field curves [0pt] pertinent to the longitudinal (Heisenberg) order-parameter in the diamagnetically diluted antiferromagnets Eu_xSr_1-xTe. In this solid solution series a particular composition of x c =0.85 exists at which the different fourth-order interaction processes compensate each other in the high temperature average. As a consequence, an Eu_xSr_1-xTe sample with x =0.85 meets the requirements of a Heisenberg antiferromagnet at least if a quantity is considered for which the high-temperature average over all fourth-order interactions is decisive. This seems to be the case for the critical field curve [0pt] which gives the phase boundary to the paramagnetic phase. In fact, a crossover from a T² to a T3/2 law is observed for [0pt] on approaching x_c. This, we believe, shows the frequently observed T² law is caused by the fourth-order interactions. Received 23 July 1998 and Received in final form 12 October 1998     

A quantitative determination of magnetic fluctuations in CuGeO3

A polarised neutron scattering investigation has been carried out on a powder sample of CuGeO₃ within the temperature range of 1.5 K to 600 K. The magnetic scattering has been separated from all other contributions by using polarised neutrons and polarisation analysis and placed onto an absolute scale. At low temperatures the long wavelength components of the paramagnetic response are suppressed consistent with the formation of Cu dimers in which the magnetic moments are correlated antiferromagnetically. This form of the scattering persists to temperatures well above the dimerisation temperature T _sp ∼ 14 K. However as the temperature is raised the intensity of the long wavelength spin fluctuations increases and above ∼150 K they are the dominant feature in the wave vector dependence of the response. At all temperatures the observed scattering extrapolates smoothly to the Q = 0 value given by the uniform susceptibility. Consequently the thermal variation of the uniform susceptibility arises from the evolution of the long wavelength magnetic fluctuations. At large wave vectors the energy dependence of the scattering revealed that the response occurs below 16 meV in agreement with the reported maximum magnetic excitation energy at the zone boundary in the ground state. However the total magnetic scattering is significantly less than that expected for a local moment system suggesting that the spectrum of thermal and quantum fluctuations overlap. Received 30 May 2000 and Received in final form 22 March 2001     

SR evidence of low frequency spin fluctuations in the AF phase of hole-doped NiO

μSR measurements in the antiferromagnetic (AF) phase of Ni_1-xLi_xO for are reported. While in pure NiO the muon longitudinal depolarization rate is found almost temperature independent, in the Li-doped compounds broad maxima around 130 K are observed. These maxima are associated with the progressive freezing of the spin fluctuations of S =1/2 defects induced by the localization of the extra-holes. From the temperature dependence of and the stretched exponential form of the depolarization, insights on the distribution of correlation times for the fluctuating field at the muon site are derived. Received: 27 April 1998 / Received in final form and Accepted: 21 August 1998     

Jahn-Teller distortion and magnetoresistance in electron doped Sr1-xCexMnO3 (x = 0.1, 0.2, 0.3 and 0.4)

Neutron and electron diffraction, electrical transport and magnetic measurements have been carried out on a newly synthesized electron doped Sr_1-xCe _x MnO₃ (x = 0.1, 0.2, 0.3 and 0.4) system. For x=0.1, while cooling, it undergoes a first-order metal-insulator transition at 315 K which is associated with a structural transition from cubic (Pm3m) to tetragonal (I4/mcm) due to Jahn-Teller ordering () which stabilizes a chain like (C-type) antiferromagnetic ground state with . The antiferromagnetic insulator state is insensitive to an applied magnetic field of 7 T. With increase of x, while the nuclear structure at room temperature for x=0.2 and 0.3 remains tetragonal, for x=0.4 it becomes orthorhombic (Imma) where the doping electrons seem to occupy mainly the d _x2-y2 symmetry. Further, the JT distortion and the antiferromagnetic interactions decrease with doping and a small negative magnetoresistance appears for . Magnetic measurements show that the dilution of antiferromagnetic interaction results into a spin glass like behaviour at low temperature for the samples with x=0.3 and 0.4. This behaviour is in contrast with the CMR properties of calcium based electron doped systems and hole doped manganites. The stability of C-type antiferromagnetic ordering in the electron doped system with large A-site cationic size may be responsible for the absence of double exchange ferromagnetism and CMR effect. Received 10 September 1999     

Lattice damage and Al-metal precipitation in 2.5 MeV-electron-irradiated AlH3

AlH₃ powder was bombarded with energetic electrons at 20 K and at room temperature and investigated by EPR, NMR, X-ray diffractometry, and microwave dielectric-constant measurements. The EPR spectra of the irradiated powder and of a selected single crystal cuboid of mm edge show a complex asymmetric line centered at g = 2.009, with a Curie-like temperature dependence, attributed to radiation-induced color centers and/or their agglomerates. At the same time, the grains, which have become shiny black after irradiation, exhibit an increase of both the real and the imaginary part of . ²⁷Al-NMR spectra of the irradiated powder present a Knight-shifted line at 1600(50) ppm, close to the position of bulk metallic Al, and corresponding to a concentration of c(Al) . In addition, the main hydride line differs from that before irradiation, demonstrating an alteration of environmental symmetry. The irradiation induces also a change in shape and width of the ¹H-NMR line, another indication of symmetry change in the lattice. Finally, a refined X-ray single-crystal structure analysis of the irradiated cuboid indicates a change of structure from trigonal R -3 c to R -3, with a loss of mirror symmetry for the two Al sites caused by the introduction of Al-defects in the vicinity of one of them. Received: 20 October 1997 / Revised: 24 December 1997 / Accepted: 30 January 1998     

Triangular antiferromagnetic order in the honeycomb layer lattice of

ErCl₃ crystallizes in the AlCl₃-type layer structure. The crystal structure was refined in the paramagnetic state by powder neutron diffraction. The monoclinic lattice parameters at 1.5 K are a = 6.8040(3)?, b = 11.7456(5)?, c = 6.3187(3)? and . The space group is C2/m. Short-range, predominantly in-plane, magnetic ordering occurs above 350 mK up to several Kelvin. Below mK a three-dimensional antiferromagnetic order with a propagation vector of sets in. The magnetic structure of ErCl₃ was determined by powder and single-crystal neutron diffraction at temperatures down to 45 mK. The Er³⁺ ions are located on two-dimensional honeycomb layers in the a–b plane. There are two antiferromagnetically coupled triangular sublattices which form right- and left-handed helices along the c-axis. The magnetic moments are oriented in the a–b plane and amount to 3.3(1) at saturation. From the temperature dependence of the integrated neutron magnetic peak intensity a critical exponent (2) was derived for the magnetic phase transition. Received 1 December 1999 and Received in final form 21 July 2000     

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     

Penetration depth measurement in high quality thin films

The parallel plate resonator method has been used for measuring high quality (YBCO) thin films, which have low temperature residual losses comparable to those previously obtained in single crystals. The surface resistance and the real part of the conductivity show a non-monotonic behaviour with a broad peak around 45 K. The penetration depth and the real part of the conductivity vary linearly at low temperatures. The lowest penetration depth linear fitting has a slope value of to up to 20 K which is lower than previous measurements on YBCO single crystals. An interpretation of this smaller slope in terms of the generally accepted d-wave order parameter symmetry presents difficulties. Received: 22 July 1997 / Revised: 11 March 1998 / Accepted: 23 June 1998     

Double-Q magnetic structures and strong planar anisotropy in tetragonal ErRu2Ge2and ErRu2Si2

Single crystal magnetization measurements and powder neutron diffraction on tetragonal ErRu₂Ge₂ as well as anisotropy of the paramagnetic susceptibility and specific heat measurements on ErRu₂Si₂ are presented. Besides the huge crystal field contribution to the uniaxial anisotropy, which favors the basal plane, a strong in-plane anisotropy is evidenced. From these features and neutron diffraction experiments it is shown that magnetic structures of these materials are double-Q and accordingly non-colinear below their Néel temperature (5.2 and 6.0 K for Ge and Si based compounds, respectively). The magnetic structures induced during the metamagnetic processes are discussed. Received 24 December 1999     

On the 240 K anomaly in the magnetic properties of LiNiO2

In this work, results of X-band ESR spectroscopy, ac-magnetic susceptibility and X-ray powder diffraction measurements on Li_1-xNi _1+x O₂ (x = 0.02 and x = 0.07) are presented and discussed. While the susceptibility of the compound with x = 0.02 is shown to follow a Curie-Weiss law, with a Weiss temperature of the order of 30 K, the compound with x = 0.07 is found to order ferromagnetically below K. However, an additional anomaly is observed in the magnetic properties of this latter compound at around 240 K. We attribute this anomaly to the presence of macroscopic Ni-rich regions which order ferrimagnetically below this temperature. This phenomenon is different from the bulk ferromagnetism that occurs at much lower temperatures, and allows us to discard earlier suggestions proposed in the literature in which the 240 K anomaly has been considered as denoting an intrinsic phenomenon. Received 14 May 1999 and Received in final form 5 August 1999     

On the evolution of the ground state in the system U x La 1 - x S: Polarized neutron diffraction and X-ray magnetic circular dichroism study

In the U_xLa_1-xS system there is an abrupt loss of the long-range ferromagnetic ordering found in pure US at a critical concentration x _c ∼ 0.57, which is far above the percolation limit. As the magnetic ground state in such a system can be strongly affected by small variations of the 5f localization, we have investigated a set of samples with different x by polarized neutron diffraction and X-ray magnetic circular dichroism (XMCD). The neutron results are consistent with early measurements performed on pure US. Even at the lowest U content (x = 0.15, below x _c ) the shape of the induced form factor (f ( Q )) is comparable with that found for x = 1 and is well reproduced by either a U⁴⁺ or a U³⁺ state. The ratio between the orbital and the effective spin moments in the XMCD measurements confirms this result, but the evolution of the shape at the M₅ edge suggests an abrupt change in the distribution of the electrons (holes) in the 5 f density of states around x _c . Received 31 January 2001