Three-dimensional Magnetic Ordering Research Articles

The exchange interactions and magnetic behaviour of Cu(L-alanine)2: specific heat measurements

We report specific heat measurements in the temperature range 0.05 K<or=T<or=1.95 K for the copper-amino acid complex Cu(L-alanine)2. The data display a peak at 0.068 K, and a broad maximum at 0.55 K. Above 0.2 K they reflect a one-dimensional antiferromagnetic spin-chain behaviour. The temperature dependence of the specific heat is compared to that calculated for Heisenberg finite spin chains in order to evaluate the magnitude and characteristics of the exchange interaction. We obtain an antiferromagnetic exchange coupling parameter J0/k=(-0.57+or-0.01) K between nearest-neighbour copper ions. The peak observed at 0.068 K indicates a transition to a phase with three-dimensional magnetic order. The effect of this phase transition on the specific heat measured at higher temperatures is analysed. The experimental results, together with existing magnetic susceptibility and EPR data, are discussed considering the crystal structure and the superexchange paths connecting the copper ions in Cu(L-alanine)2. The nature of the spin chains and the possibility of competing exchange interactions between first and second copper neighbours along the chains are analysed in order to model the magnetic behaviour of Cu(L-alanine)2.

Heat capacity study of La 2NiO 4 and Pr 2NiO 4

A calorimetric study of La 2NiO 4 and Pr 2NiO 4 has been carried out with adiabatic and ac calorimetry. Thermodynamic functions of both compounds have been calculated at rounded temperatures. Heat capacity anomalies corresponding to structural transitions from the low-temperature tetragonal P4 2/ncm phase to the higher-temperature orthorhombic Bmab phase have been found at 56K for La 2NiO 4 and at 121.2K for Pr 2NiO 4. The three-dimensional magnetic ordering of the Ni ions produces a small peak in the heat capacity at 328K annd 324K, respectively, for both compounds in agreement with the expected low-dimensional behavior. No long-range order of Pr 3+ has been detected down to 2K. A broad excess heat capacity in Pr 2NiO 4 over the estimated lattice contribution was found to be due to the electronic energy levels of Pr 3+. An energy gap of around 50K between the ground and the first-excited level with equal degeneracies has been deduced.

Phase transitions and crystal-field levels in Nd 2NiO 4

The heat capacity of Nd 2NiO 4 has been measured between 2 and 350K with adiabatic and ac calorimetry. Thermodynamic functions have been calculated at rounded temperature intervals. The three-dimensional magnetic ordering of the Ni ions produces a small peak in the heat capacity at 325K, indicating strong two-dimensional magnetic behavior. A sharp first-order peak has been found at 138K corresponding to the structural transition from the low-temperature tetragonal P4 2/ncm phase to the higher-temperature orthorhombic Bmab phase. No three-dimensional ordering of the Nd sublattice has been detected. The excess heat capacity, after subtracting the estimated lattice contribution, is in agreement with the crystal-field energy levels of the Nd 3+ ions reported from inelastic neutron diffraction experiments. A broad shoulder below 10K can be adscribed to an additional splitting of 17K in the lowest crystal-field doublet of Nd 3+. It is due to the interaction with the internal magnetic field created by a canting of the Ni 2+ magnetic moments.

Hyperfine-enhanced nuclear polarization in NdGaO3.

The three-dimensional magnetic ordering of the ${\mathrm{Nd}}^{3+}$ ions in ${\mathrm{NdGaO}}_{3}$ has been studied by means of neutron powder diffraction. The transition to the antiferromagnetic state takes place at ${\mathit{T}}_{\mathit{N}}$\ensuremath{\approxeq}1 K. Below this temperature, the ${\mathrm{Nd}}^{3+}$ magnetic moments are oriented along the c axis in a ${\mathit{c}}_{\mathit{z}}$ mode. The value of the saturated magnetic moment (\ensuremath{\approxeq}1.1${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$) is drastically reduced below the free ion value (3.27${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$) due to crystal-field effects. The main contribution to the Nd-Nd magnetic interaction appears to be superexchange, the classical dipole-dipole interaction being too small to account for the observed N\'eel temperature. Below 300 mK, we observe an additional enhancement of the magnetic reflections. The analysis of the Q dependence of the neutron powder-diffraction intensities allowed us to interpret this increase as due to the polarization of the nuclear moments of the $^{143}\mathrm{Nd}$ and $^{145}\mathrm{Nd}$ isotopes in the hyperfine field created by the ${\mathrm{Nd}}^{3+}$ electronic moments. The particular conditions which lead to the observation of this phenomenon in ${\mathrm{NdGaO}}_{3}$ are briefly discussed.

Interchain interactions and three-dimensional magnetic ordering in Mn(II)Cu(II) chain compounds; crystal structure and metamagnetic properties of MnCu(pbaOH)(H 2O) 3·2H 2O, with pbaOH = 2-hydroxo-1,3-propylenebis (oxamato)

Single crystals of the novel phase MnCu(pbaOH)(H 2O) 3·2H 2O ( A) have been obtained; pbaOH stands for 2-hydroxy-1,3-propylenebis(oxamato). A crystallizes in the monoclinic system, space group P2 1/ c,with a = 21.212(4) , b = 5.033(1), c = 14.246(2) A ̊ , β = 100.72(2)° and Z = 4. The structure consits of oxamato-bridged Mn(II)Cu(II) bimetallic chains running along the a-axis direction. The shortest metal-metal separations between adjacent chains related by, respectively, the unit cell translation along the b-axis direction and the glide translation along the c-axis direction involve metal ions of the same type, in contrast with what was found in the previously described phase MnCu(pbaOH)(H 2O) 3 ( B). According to our interpretation of the three-dimensional (3-D) magnetic effects in bimetallic chain compounds, the topology of the spin carriers in A should favor a 3-D antiferromagnetic ordering, while B exhibits a 3-D ferromagnetic ordering. The study of the temperature dependence of the magnetic susceptibility as well as the temperature and field dependences of the magnetization have confirmed this prediction. The antiferromagnetic ordering occurs at 2.4 K. A magnetic field of 0.9 kOe is sufficient to overcome the weak interchain interactions, so that the compound may be described as a metamagnet built from ferrimagnetic chains.

Calorimetric study of the structural and magnetic ordering in R2NiO4 (R = La, Pr and Nd)

Abstract

New possibilities offered by high resolution Fourier transform spectroscopy in studying magnetic phase transitions

High resolution optical absorption spectra of an intrinsic R3+ ion or Er3+ probe were measured in cuprates R2Cu2O5 and R2BaCuO5 related to high Tc superconductors of the 123 type and in low-dimensional nickelates R2BaNiO5. We have registered for the first time phase transitions, some of them not detectable by other methods, in R2BaCuO5 with R=Y, Lu, Nd, Dy, Yb, and Lu, in R2BaNiO5, with R=Nd, Sm, Eu, Dy, Ho, Tm, and Lu. Coexistence of two different magnetic phases in Y2BaCuO5 and Lu2BaCuO5 in a broad temperature range was demonstrated. Fine splitting of the low temperature magnetic phase transition, possibly caused by the interaction of critical fluctuations, has been observed in Dy2BaCuO5. Low-dimensional correlations at temperatures higher than the temperature of three-dimensional (3-D) magnetic ordering have been analyzed in quasi-2D compounds R2Cu2O5 and quasi-1-D R2BaCuO5. We developed the method of the Er3+ probe in a series of isostructural f–d compounds to select the right magnetic structure of an ordered d subsystem from several structures that fitted neutron scattering data equally well.

Magnetic Ordering in One-Dimensional System CoNb2O6with Competing Interchain Interactions

We investigated the formation of three-dimensional (3D) magnetic ordering in a quasi-1D ferromagnetic chain system CoNb 2 O 6 by neutron scattering. With decreasing temperature below T 1 ∼3 K in the helical magnetic phase, anomalous broadening of magnetic satellite reflections was observed in the directions perpendicular to the 1D magnetic chain, with a rapid change in the propagation wave number of helical magnetic ordering. Furthermore, in the antiferromagnetic phase below T 2 ∼1.9 K, CoNb 2 O 6 exhibits a pronounced 2D magnetic character. These features result from the competition between interchain exchange interaction and Co 2+ single-ion anisotropy in this compound.

Neutron-diffraction study of the magnetic ordering in the insulating regime of the perovskites RNiO3 (R=Pr and Nd).

Neutron-diffraction experiments and polarization analysis techniques reveal that the metal-insulator transition in the orthorhombic perovskites ${\mathrm{PrNiO}}_{3}$ and ${\mathrm{NdNiO}}_{3}$ is accompanied by a sudden three-dimensional magnetic ordering of the Ni sites (${\mathrm{\ensuremath{\mu}}}_{\mathrm{Ni}}$\ensuremath{\approxeq}0.9${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$). The electronic localization is followed by a magnetic ground state consisting of an unusual antiferromagnetic structure, with k=(1/2,0,1/2) relative to the orthorhombic crystal cell. This propagation vector implies the symmetrical coexistence of ferro- and antiferromagnetic couplings along the three pseudocubic axes. This suggests the existence of a nonuniform orbital distribution of the single ${\mathit{e}}_{\mathit{g}}$ electron. The orbital superlattice may result from the breakdown of the degeneracy of the ${\mathrm{Ni}}^{\mathrm{III}}$(${\mathit{t}}_{2\mathit{g}}^{6}$${\mathit{e}}_{\mathit{g}}^{1}$) state due to electronic correlation. This type of spin arrangement has not been observed in other perovskite oxides. These findings are confirmed by the magnetic behavior of the single crystallographic Nd site: half of these ions become ordered and the other half remain disordered as in a simple paramagnet. This can be explained by the combination of polarization effects due to the exchange field from nickel moments and the breaking down of point symmetry at Ni site.

Disappearance of three-dimensional magnetic ordering in Gd2CuO4

The magnetic ordering of an isotope (158Gd) enriched Gd2CuO4 single crystal has been investigated by neutron diffraction in the temperature range 1.5–50 K. The three-dimensional magnetic order of the Cu sublattice is found to disappear abruptly in a narrow temperature range close to 7 K which is just above the ordering temperature of the Gd sublattice [TN(Gd)=6.4 K]. This result is thought to be intimately related to the structural distortion of the CuO planes which gives rise to weak ferromagnetism. The lack of three-dimensional order in the structural distortion imposes itself on the antiferromagnetic arrangement of the Cu sublattice through the Cu-Gd and Gd-Gd interactions and the different and incompatible wave vectors of the Cu and Gd magnetic sublattices.

Incommensurate magnetic correlations in La 2NiO 4.125: Magnetic phase diagram and two-dimensional hole-order

The magnetic phase diagram of La 2NiO 4+δ is studied by magnetization and neutron scattering measurements. Three-dimensional (3d) magnetic order with an incommensurate Ni 2+ spin modulation in the NiO 2 plane appears beyond oxygen doping δ around 0.10. The sharpness of the magnetic phase transition in this phase dramatically depends on δ and the sharpest one is observed at δ≈0.125= 1 8 . In contrast to the carrier doping with δ<0.10, which degradates the 3D commensurate magnetic order, the Néel temperature ( T N) of this phase is raised by the increase of oxygen doping. Above T N two-dimensional (2D)_incommensurate fluctuations dominate. The magnetic phase diagram and the commensurate-incommensurate phase transition of this system are discussed based on the 2D hole-order in the NiO 2 plane.

Crystal-field excitations in high-Tcsuperconducting materials

Abstract The discovery of high-Tc superconductivity in the perovskite-type copper-oxide compounds has given rise to a large amount of materials research. Of particular interest in determining the nature of the superconductivity is the effect of substituents at various sites in these compounds. It has been realized that the superconducting transition temperature is essentially unchanged upon replacing the Y and La ions by “magnetic” rare-earth (R) ions. This surprising observation is in contrast to conventional super-conductors, for which paramagnetic ions usually have a large detrimental effect on superconductivity. It is therefore important to achieve a detailed understanding of the low-energy electronic properties which define the magnetic ground state of the R ions. In particular, information on the crystalline-electric-field (CEF) interaction at the R site is highly desirable, for the following reasons: For many high-Tc compounds superconductivity and long-range three-dimensional magnetic ordering of ...

Two- and three-dimensional magnetic order of Er in superconducting ErBa 2Cu 3O 7

Neutron diffraction has been used to study the magnetic fluctuations and long-range order of the Er ions in single crystals of superconducting ErBa 2Cu 3O 7. The temperature dependence of the rod of scattering, characteristic of 2D behavior, has been measured above and below the 3D Néel temperature ( T N⋍0.62 K). This rod intensity is observed to increase as the temperature decreases until T N is reached, and then the intensity decreases rapidly below T N. The 2D magnetic correlation length, which is obtained from measurements of the width of the rod, grows continously with decreasing temperature, then reaches a resolution-limited maximum at the Néel temperature when long-range magnetic order sets in. Below T N the sublattice magnetization, which is a measure of the thermodynamic order parameter, follows Onsager's exact solution of the 2D S= 1 2 Ising model. At low T, a single type of simple 3D antiferromagnetic structure is found, characterized by a wave vector of ( 1 2 0 1 2 ).

Magnetic order of Pr ions in PrBa 2Cu 3O 6

The magnetic order of Pr ions in polycrystalline, tetragonal PrBa 2Cu 3O 6.2 has been studied by neutron diffraction measurements. The basic ordering consists of an antiferromagnetic arrangement with the propagation vector k = [ 1 2 , 1 2 , 1 2 ] . The three-dimensional magnetic ordering is imperfect due to finite correlation length ξ c = 10 A ̊ along the c-axis. The Néel temperature is determined to be T N = (12 ± 1) K, and the magnetic saturation moment of the Pr ions turns out to be (1.9 ± 0.2) μ B.

Moessbauer Spectroscopic Study of Magnetic Ordering in Oxalate-Bridged Cr(III)-Fe(II) and Fe(III)-Fe(II) Systems

Abstract Magnetic ordering phenomena in mixed-metal assemblies {NBu4[Fe(II)M(III)(ox)3]}3∞ (M=Cr (1), Fe (2)) were studied by using 57Fe Moessbauer spectroscopy, where NBu4 +=tetra(n-butyl)ammonium ion, ox2-=oxalate ion. Nuclear Zeeman splittings were observed in the Moessbauer spectra of both 1 and 2 at low temperatures (below 10 K for 1 and 41 K for 2), which indicated the occurrence of three-dimensional magnetic ordering in these complexes under zero applied field.

Sudden disappearance of three-dimensional magnetic ordering in Gd2CuO4

Sudden disappearance of three-dimensional magnetic ordering in Gd₂CuO₄

Dynamic Spin Properties of La1.98Sr0.02CuO4

Neutron scattering experiments have been performed to study the dynamic spin properties of La 1.98 Sr 0.02 CuO 4 , a crystal with just enough Sr to destroy three-dimensional long range magnetic order and induce spin-glass behavior at low temperatures. The q -integrated susceptibility \(\int \,{\rm d}^{2}q_{\rm 2D}\,{\rm Im}\,\chi _{\rm 2D}(q_{\rm 2D},\,\omega )\) follows the scaling function of ω/ T suggested by Keimer et al. Below 3 meV, however, clear deviation from the scaling function is observed at low temperatures. This probably originates from a gap in the spin-wave-like excitations due to the out-of-plane anisotropy.

Spectral and magnetic studies of nickelates Lu2BaNiO5 and Yb2BaNiO5

Abstract High resolution optical spectra, ac magnetic susceptibility and magnetization in pulse fields were measured for polycrystalline Yb2BaNiO5 and Lu2BaNiO5 isostructural to the “green phase” R2BaCuO5 of high-Tc superconducting ceramics. Lu2BaNiO5 orders magnetically at 4.5 K while in Yb2BaNiO5 three-dimensional magnetic order does not, evidently, establish down to 1.9 K. Low-dimensional magnetic correlations are present in both compounds.

New family of superconducting copper oxides: GaSr 2Ln 1− xCa xCu 2O7

We have prepared a new copper oxide, GaSr 2Ln 1− x Ca x Cu 2 O 7 (0 ≤ x ≤ 0.4), that becomes superconducting a high-pressure oxygen annealing. While the undoped parent compound, x = 0, does not show three-dimensional magnetic order above 4 K, probably due to the presennce of the thick ionic separating the CuO 2 layers, superconductivity with a high- T c (∼70 K) is possible for the doped compositions.

Heat-capacity measurement of R 2BaNiO 5 (R = Y, Ho, Er) oxides

Adiabatic heat-capacity measurements between 5 and 350 K have been performed on R 2BaNiO 5 (R = Y, Er, Ho). Three-dimensional magnetic ordering transitions are observed at 32.5 and 47.6 K for the Er and Ho compounds, respectively. Additionally there are broad anomalies in the ordered temperature region centered at 8 and 16 K, respectively and other excess heat-capacity contributions at higher temperatures due to thermal population of low-energy levels of the rare earths. In Y 2BaNiO 5 there is no evidence of any of these anomalies. The one- and three-dimensional magnetic interactions have been analyzed.