Three-dimensional Magnetic Ordering Research Articles

Calorimetric study of the green phases R 2BaCuO 5 (R = Gd, Dy, Ho, Er, Lu, Y)

Adiabatic heat-capacity measurements between 5 and 60 K have been performed on a series of green phases R 2BaCuO 5. Independent peaks are generally observed for the three-dimensional magnetic ordering of the Cu and the magnetic rare earth ions. Contrarily, the compounds with nonmagnetic rare earth ions show rounded anomalies characteristic of low-dimensional magnetic interactions.

Molecular based magnetic materials

A short resume of the principal approaches which are currently followed in order to synthesize molecular based magnetic materials is provided, stressing the interest in this class of compounds. Particular attention is devoted to systems in which stable organic radicals are coupled to metal ions, showing how many different kinds of low-dimensional materials can be obtained in this way and how in some cases three-dimensional magnetic order is obtained. Further it is shown that unusual properties arise in this new class of materials.

Three-dimensional magnetic structures and rare-earth magnetic ordering in Nd2CuO4 and Pr2CuO4.

The three-dimensional (3D) magnetism present in ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$ and ${\mathrm{Pr}}_{2}$${\mathrm{CuO}}_{4}$ has been examined in neutron-scattering experiments. Antiferromagnetic long-range order of the ${\mathrm{Cu}}^{2+}$ moments develops below ${\mathit{T}}_{\mathit{N}}$=255 K in both compounds. In ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$ a series of magnetic phase transitions in which the ${\mathrm{Cu}}^{2+}$ spins reorient has also been observed below ${\mathit{T}}_{\mathit{N}}$, while in ${\mathrm{Pr}}_{2}$${\mathrm{CuO}}_{4}$ the 3D ${\mathrm{Cu}}^{2+}$ spin structure is stable. No changes in the crystalline structure of ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$ associated with the spin-reorientation transitions were found. Induced ordering of the rare-earth magnetic moments has been observed in both compounds. The ${\mathrm{Nd}}^{3+}$ (${\mathrm{Pr}}^{3+}$) magnetic moments align parallel (antiparallel) to ${\mathrm{Cu}}^{2+}$ moments which are nearest neighbors along the c axis, and the ordered moments of the rare-earth ions were determined to be \ensuremath{\sim}1.3${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ for ${\mathrm{Nd}}^{3+}$ at 0.4 K and \ensuremath{\sim}0.08${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ for ${\mathrm{Pr}}^{3+}$ at 8 K. The temperature-dependent ${\mathrm{Pr}}^{3+}$ moment is shown to be proportional to the ${\mathrm{Cu}}^{2+}$ moment times the bulk susceptibility. In this paper we also discuss the role of the XY anisotropy in the 3D ordering.

Antiferromagnetic spin correlations in (Nd,Pr)2-xCexCuO4.

Neutron-scattering measurements have been performed on (Nd,Pr${)}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ (0.0x0.125) single crystals. All samples exhibit three-dimensional (3D) long-range magnetic order; the temperature dependence of the 2D magnetic correlation lengths in all is similar in character to that observed in pure and lightly doped ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ albeit with renormalized spin stiffness constants ${\mathrm{\ensuremath{\rho}}}_{\mathit{s}}$. Thus, in the nonsuperconducting doping regime the hole- and electron-doped systems exhibit analogous magnetic behavior except for the drastically different dopant-concentration scales for the destruction of magnetic order.

Two- and three-dimensional magnetic order of the rare-earth ions in RBa2Cu4O8.

Neutron scattering has been used to investigate the magnetic ordering of the rare-earth ions in the ${\mathrm{ErBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ and ${\mathrm{DyBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ materials. For the Er system a simple three-dimensional (3D) antiferromagnetic structure is observed, in which the magnetic unit cell is twice the chemical unit cell along the a axis, with a N\'eel temperature ${\mathit{T}}_{\mathit{N}}$\ensuremath{\approxeq}0.49 K. In the vicinity of the N\'eel temperature, however, scattering characteristic of two-dimensional behavior, indicative of the strong anisotropy of the magnetic interactions, is observed. The magnetic anisotropy arises naturally from the crystal structure, as the c-axis spacing of the magnetic ions is \ensuremath{\sim}3 times the a-b spacing. 2D behavior is also observed in the Dy material near its N\'eel temperature of \ensuremath{\sim}0.9 K. However, the scattering in the Dy system does not cross over to the expected 3D Bragg peaks at low temperatures, but rather the 2D line shape persists to the lowest temperatures measured. We explain this result as originating from a geometric cancellation of interactions between c-axis layers for the specific Dy a-b spin configuration, which together with the weak intrinsic c-axis interaction renders the net interaction along the c axis much smaller than all other known ``2D magnets.'' Hence the 2D and 3D ordering temperatures appear to be quite different for the Dy material. We anticipate that a similar cancellation of interactions is likely to occur in other R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ (R being a rare-earth element) systems.

Synthesis, crystal structure, and magnetism of Fe 4(OH) 3(PO 4) 3 and V 4O(OH) 2(PO 4) 3: Chains of M2O 9 dimers connected by hydroxyl groups

The new compounds Fe 4(OH) 3(PO 4) 3 and V 4O(OH) 2(PO 4) 3 are prepared hydrothermally by reaction of Fe 2O 3 and V 2O 3 with aqueous H 3PO 4 solutions. X-ray powder diffraction shows that the two compounds have the same monoclinic structure. Fe 4(OH) 3(PO 4) 3 crystallizes with unit cell parameters a = 19.555(2), b = 7.376(1), c = 7.429(1) Å, and β = 102.26(1)° in the space group C2 c with Z = 4. Dimers of face-sharing (nonequivalent) FeO 6 octahedra are interconnected by OH groups to form a double-chain of dimers extended along the c axis. Double chains are linked to one another by PO 4 tetrahedra that share corners with the octahedra. The Mössbauer spectrum of Fe 4(OH) 3(PO 4) 3 at 293 K exhibits two narrow width quadrupole doublets of equal intensity, with nearly identical isomer shifts and different quadrupole splittings: δ 1 = 0.407 mm/sec, δ 2 = 0.403 mm/sec, ΔE 1 = 0.657 mm/sec, ΔE 2 = 0.294 mm/sec. Low-temperature spectra indicate magnetic hyperfine splitting and three-dimensional magnetic order among the dimers at 86.7 ± 0.2 K. Below the ordering temperature, the spectrum consists of two equal intensity sextets with saturation hyperfine fields (4.2 K) of 52.9 and 49.1 T, respectively. The temperature dependence of the magnetic susceptibility confirms that ordering occurs at 87 ± 0.5 K with an abrupt rise to a high-moment ferromagnetic state. Above the ordering temperature, the susceptibility data exhibits a broad maximum at about 125 K characteristic of antiferromagnetically coupled dimers. The moment vs temperature behavior of V 4O(OH) 2(PO 4) 3 also shows antiferromagnetic exchange that is likely pairwise in nature, analogous to the iron compound, in view of the broad maximum in the susceptibility data at ∼150 K.

Three-dimensional magnetic ordering of K 2FeF 5· H 2O:Mössbauer spectroscopy investigation in the helium-3 temperature range

Mössbauer spectroscopy investigations were performed on the quasi-one-dimensional complex K 2FeF 5·H 2O between 0.3 and 4.2 K. The results indicate three-dimensional order below 1 K in agreement with recent susceptibility data. The saturation hyperfme field is about 520 kOe, typical of an ionic ferric compound. Measurements were performed in different magnetic fields in both the ordered and para-magnetic states. They support the antiferromagnetic character of the magnetic order, confirming a low field spin-flop transition ( H sf ≈ 1 T) and suggest that the b -axis is the hard magnetic direction.

Magnetic correlations in La2NiO

We report neutron-scattering experiments on single crystals of the layered perovskite ${\mathrm{La}}_{2}\mathrm{Ni}{\mathrm{O}}_{4+\ensuremath{\delta}}$. Three-dimensional magnetic order sets in at ${T}_{N}=70$ K for $\ensuremath{\delta}\ensuremath{\cong}0.05$. Strong two-dimensional magnetic correlations exist for $T$ between ${T}_{N}$ and the tetragonal-to-orthorhombic transition temperature, ${T}_{s}\ensuremath{\approx}240$ K. As $T$ is increased through ${T}_{s}$, the 2D correlation length decreases from more than 50 to 10 \AA{}. The excitation spectra for the 3D ordered state are characterized by large effective in-plane spin-wave velocities which, like ${T}_{N}$, depend strongly on $\ensuremath{\delta}$. For $\ensuremath{\delta}=0\ifmmode\pm\else\textpm\fi{}0.001$, $c\ensuremath{\gtrsim}0.2$ eV-\AA{}, while for $\ensuremath{\delta}=0.05$, $c\ensuremath{\approx}0.1$ eV-\AA{}.

Magnetic interaction between rare-earth moments in high-temperature superconductors RBa2Cu3O7-x.

It is shown on the basis of a simple model that a modified Ruderman-Kittel-Kasuya-Yosida interaction involving electrons at energies away from the Fermi level can stabilize the three-dimensional magnetic order of the rare-earth moments in R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$, where R=Gd and possibly Sm, Dy. The interaction favors antiferromagnetism.

A Mossbauer study of57Fe-doped CsCoBr3and RbCoCl3

57Fe Mossbauer spectra of CsCo0.99757Fe0.003Br3 and RbCo0.9957Fe0.001Cl3 between 5 K and room temperature are reported. In both compounds, the quadrupole slitting is found to be negative, and it is deduced that the Fe2+ ion has a singlet ground state with nearly zero orbital and spin angular momentum for temperatures above the three-dimensional magnetic ordering temperature TN1. The Mossbauer spectra of CsCo0.99757Fe0.003Br3 and RbCo0.9957Fe0.01Cl3 are consistent with a transition to a partially disordered antiferromagnetic phase at TNl, and a transition to a fully ordered ferrimagnetic phase at TN2. The transition temperatures TN1 are not significantly lower than those in the pure compounds, i.e. 28.3 K in CsCoBr3 and 28 K in RbCoCl3. TN2 is found to be 12 K in CsCo0.99757Fe0.003Br3, and 4-14 K in RbCo0.9957Fe0.01Cl3. Two types of relaxation process are found in the Mossbauer spectra of these two compounds at temperatures below TNl. One is attributed to electronic relaxation between the lowest states of the Fe2+ ion; the other, which is present only at TN2 and above, to solitons in the chains of magnetic ions that are excluded from the three-dimensional order between TN1 and TN2. The temperature variation of the linewidths in the Mossbauer spectra of RbCo0.9957Fe0.01Cl3 above TN1 is consistent with solitons being present in this material.

Ordering in quasi-two-dimensional planar ferromagnets: A neutron scattering study of graphite intercalation compounds.

The magnetic ordering of stage-2 ${\mathrm{CoCl}}_{2}$- and ${\mathrm{NiCl}}_{2}$-graphite intercalation compounds (GIC's) has been investigated by neutron diffraction. Both of these compounds are observed to undergo a two-step ordering process. For temperatures within the range ${T}_{l}$${T}_{u}$ purely two-dimensional (2D) spin correlations are observed, while at temperatures below ${T}_{l}$ ferromagnetically aligned planes couple weakly with an average antiferromagnetic interplanar correlation. No three-dimensional long-range magnetic order is observed even at the lowest temperatures attained. The absence of fine structure in scans along (h,k,l) with fixed h and k indicates that successive intercalate layers are translationally uncorrelated from each other. The in-plane peaks of ${\mathrm{CoCl}}_{2}$-GIC appear to have a shape characteristic of long-range spin order below ${T}_{l}$. However, the structure factor expected for a finite-sized bound vortex phase cannot at present be discounted for ${\mathrm{CoCl}}_{2}$-GIC. A similar scan at ${T}_{l}$${T}_{u}$ is likewise inconclusive concerning the existence of the bound vortex phase of coplanar spins in the intermediate temperature regime.

Low-temperature heat capacity of linear-chain magnetic compounds CsNiCl3, RbNiCl3, and CsCuCl3

Heat-capacity measurements from 0.5 to 15 K have been made on single-crystal specimens of the linear-chain magnetic compounds CsNiCl3, RbNiCl3, and CsCuCl3. In all cases anomalies are observed at the temperature corresponding to the onset of three-dimensional magnetic order. The magnitude of the anomaly in RbNiCl3 varies slightly from specimen to specimen but shows no structure corresponding to the existence of spin frustration. At low temperatures all compounds show a T3 spin-wave contribution to the specific heat associated with a simple cubic antiferromagnet, even though they have a complex spiral-spin structure.

Preparation and physical properties of the copper manganese telluride Cu 5MnTe 4

The ternary phase Cu 5MnTe 4 with a rhombohedral, but pseudo-f.c.c. structure at room temperature, was characterized by various physical measurements. Copper is present in the monovalent Cu(I;d 10) state, as indicated by the transport properties: there is essentially one hole (effective mass 1.3 m 0) per formula unit in the Te 5p valence band. In addition to p-type metallic conduction, the delocalized holes give rise to an exchange-enhanced Pauli paramagnetic term which is swamped by diamagnetism and contributions to the magnetic susceptibility χ from localized manganese(II) S = 5 2 spins. χ versus T follows a Curie-Weiss law with a negative asymptotic temperature demonstrating large antiferromagnetic exchange interactions; however, no transition to a three-dimensional magnetic ordering is found down to 10 K.

Magnetic order in a dimerized singlet-triplet system: A neutron scattering study of Cs3Cr2I9.

${\mathrm{Cs}}_{3}$${\mathrm{Cr}}_{2}$${\mathrm{I}}_{9}$ belongs to a class of antiferromagnetic dimerized singlet-triplet compounds. Its magnetic ordering has been studied by elastic neutron scattering and magnetic-susceptibility measurements. A transition to three-dimensional magnetic order is found at 6.8 K. The ordered magnetic dimer moment is K=0.91. The ordering of the iodide is interpreted by relating it to the spin dynamics in the disordered phase which is known for the analogous bromide and chloride compounds. The true soft mode which induces magnetic order is not the singlet-triplet exciton mode but an intratriplet mode. The ordered structure corresponds to a dimerized triangular antiferromagnet. The small incommensurability of the magnetic unit cell is related to the energy minima of the exciton mode in the disordered phase. The temperature dependence of the magnetic moment is interpreted in terms of a two-sublattice antiferromagnetic mean-field model. The interaction parameters are estimated as J=-0.79 meV (intradimer), ${J}_{p}$=-0.13 meV (interdimer, within the basal plane), and ${J}_{c}$=-0.019 meV (interdimer, along c).

Synthesis, structure and low temperature magnetism of the new lithium and sodium insertion compounds: LiFe(II)ClMoO 4 and NaFe(II)ClMoO 4

The recently described tetragonal layered compound Fe(III)ClMoO 4 is found to undergo reversible lithium and sodium ion insertion by stirring the solid in an acetonitrile solution of lithium or sodium iodide under argon. Reaction occurs via a topochemical redox-insertion mechanism where Fe 3+ is reduced to Fe 2+. LiFe(II)ClMoO 4 is monoclinic with a = 6.9944(6), b = 6.8712(6), c = 5.0146(5) A ̊ and β = 91.270(5)° (space group P2 I/m). NaFe(II)ClMoO 4 appears to be orthorhombic with a = 7.174(2), b = 7.185(1) and c = 5.183(1) A ̊ . The structure of LiFe(II)ClMoO 4, determined by a Rietveld refinement of neutron powder diffraction data, is essentially the same as that of the precursor and contains Li ions in distorted octahedral sites between the layers. Also, the Fe atoms are shifted to a position where they are 6-coordinated compared to the 5-coordination observed in the ferric precursor. Zero field Mössbauer spectra for the lithium compound undergo hyperfine splitting that corresponds to a three-dimensional magnetic ordering process with T Néel∼ 68.5K. Low-dimensionality antiferromagnetism (probably one-dimensional chain in nature) is evident as a, broad maximum in the D.C. susceptibility at Tχ max ∼ 73 K. This is approximately 30 kelvins less than a similar low-dimensionality maximum observed in χ m for the ferric precursor. Mössbauer spectra for the sodium insertion compound suggest cooperative three- dimensional magnetic order below ∼ 45 K.

Static and dynamic spin correlations in the random one-dimensional antiferromagnetic CsCoxMg1-xCl3

Static and dynamic spin correlations in the random quasi-one-dimensional Ising-like antiferromagnet CsCo0.83Mg0.17Cl3 have been investigated by neutron scattering. The three-dimensional magnetic ordering temperature is strongly suppressed by the presence of impurities. The one-dimensional correlation length tends to a geometric limit at low temperatures in analogy with percolation in higher-dimensional magnetic systems. The quasi-elastic scattering is well described by exact calculations of Szz(Q) for a dilute chain. In addition to the usual spin wave band of the host crystal, the spectrum contains localised modes that arise from the oscillations of spins at the ends of magnetic chains. Their frequency is approximately half that of the host band. Calculations have been made of the weighted contributions to Sxx (Q, omega ) from each randomly occurring length of chain and good agreement with experiment is obtained.

Soliton-induced magnetic phase transition in three-dimensional systems of weakly coupled magnetic chains with local anisotropy

Thermodynamical properties of weakly coupled magnetic chains, consitituting three-dimensional classical ferromagnets with special exchange and local anisotropies, are considered. The transfer-operator method and pseudospin approach are used. It is shown that the phase transition connected with the disappearance of three-dimensional magnetic ordering is induced here by static kink solitons obtained for one-dimensional systems.

Heat capacity of the two-dimensional ferromagnet bis(monomethylammonium)tetrachlorochromate(II) from 10 to 302 K

The heat capacity Cp of the S= 2 layer ferromagnet (CH3NH3)2CrCl4 has been measured from 10 to 302 K. Although the salt at room temperature has an orthorhombic structure in which, by comparison with similar compounds, some disorder might be expected, there is no sign of any phase transition in the temperature range covered by the Cp measurements. Between ca. 100 and 210 K, however, there is a broad anomaly in the heat capacity. On the basis of a comparison of the heat capacity of the chromium compound with that of the corresponding manganese salt, it is suggested that this anomaly results from the development in the chromium compound of magnetic disorder, since the “extra” entropy so gained is approximately R ln 5, the expected magnetic entropy. On this interpretation, the magnetic entropy is acquired in two stages, the first of these being the breakdown of three-dimensional magnetic order at Tc(41 K). Values of the maximum in the magnetic heat capacity, and the temperature at which it occurs, are compared with those of S=½ and S= layer Heisenberg ferromagnets. There are features in the thermodynamic properties which present problems that cannot be satisfactorily resolved with the structural information at present available.

Neutron powder diffraction on α-Tl 4CrI 6 and β-Tl 4CrI 6

α-Tl 4CrI 6 ( a = 9.132(1), c = 9.667(1) Å, Z = 2, P4 mnc at 293 K) adopts a distorted Tl 4HgBr 6 structure. In α-Tl 4CrI 6 there occurs a random distribution of Jahn-Teller distorted octahedra which are elongated perpendicular to the c axis. Between 77 and 4.2 K a phase transition occurs. In β-Tl 4CrI 6 ( a = 12.941(3), b = 12.596(3), c = 9.602(2) Å, Z = 4, Cccm at 4.2 K) the directions of elongation of the octahedra are ordered. The structure is very much related to that of α-Tl 4CrI 6. A three-dimensional magnetic ordering takes place at 2.7(2) K. The magnetic space group at 1.2 K is C I 22′2′. The magnetic moments (3.48(6) μ B) are parallel to (0 0 1) and have an angle of 41(9)° with the a axis. Four magnetic sublattices are present, forming two independent magnetic lattices which have no interaction due to the antiparallel ordering.

Neutron powder diffraction on RbCrl 3 and magnetic measurements on RbCrl 3 and CsCrl 3

β-RbCrI 3 ( a = 13.772(3), b = 8.000(2), c = 7.069(2) Å β = 95.85(1)°, Z = 4, C2 m at 293 K) and γ-RbCrI 3 ( a = 13.586(2), b = 7.923(2), c = 14.094(3) Å, β = 96.88(1)°, Z = 8, C2 at 1.2 K) are isostructural to β-RbCrCl 3 and γ-RbCrCl 3 and are both Jahn-Teller distorted BaNiO 3 structures. In both compounds elongated octahedra occur. γ-RbCrI 3 most probably has a magnetic spiral structure at 4.2 and 1.2 K. Theoretically, a spiral propagating along the b axis is expected. A model with k = 9/19 b∗ yielded the best result. However, no good fit was obtained possibly because of a misfit in k and canting of the magnetic moments due to anisotropy. χ vs T single-crystal measurements on β-CsCrI 3 are in accordance with its magnetic structure. The three-dimensional magnetic ordering temperature T c is estimated as 27(1) K. From the χ vs T curves of γ-RbCrI 3, T c could not be determined. From fits to χ vs T powder data J k of CsCrI 3 and RbCrI 3 are estimated to be −14(2) and −11(1) K, respectively.