Phason interpretation of the specific heat anomaly in TiBe2−xCux

Data of the specific heat C for TiBe_2.06,TiBe_1.95Cu_0.05 and TiBe_1.79Cu_0.21 are analyzed under the assumption that the anomaly which peaks at T?2K is due to thermal phasons, the excitations of a spin density wave. Consistent fits are obtained. A careful analysis of the anomaly below the peak is found important for the determination of $\text{C}\text{T}$ at T = 0 and gives evidence for the existence, in TiBe_2.06, of a phason gap.     

A neutron small angle scattering study of the onset of ferromagnetism in TiBe2-xCux alloys

The onset of ferromagnetic order in TiBe_2-xCu_x alloys, with x = 0.5, 0.4, 0.3, 0.2 and 0.15, has been examined using the technique of neutron small-angle scattering. The Curie temperature, T_c, for these alloys has been determined from the temperature dependence of the magnetic critical scattering. A linear extrapolation of T_c versus copper concentration yields a critical concentration for ferromagnetic order of x_c = 0.05 ± 0.02. For the alloys with x = 0.4, 0.5 the lineshape of the magnetic critical scattering, at and above T_c, is well explained by the Ornstein-Zernicke form of the spin correlation function. For the lower concentration alloys the exact form of the spin correlation function is still unclear.     

Enhanced paramagnetism of TiBe2 and ferromagnetic transitions in TiBe2-xCux

The magnetic behavior of TiBe₂ is found to be similar to that of strongly enhanced paramagnets like Pd and Ni₃Ga. The susceptibility curves _χ(H, T = 0) and _χ(H, T = 0) both go through a smooth maximum, at 55 kOe and 10 K, respectively, which might be due to the electron-electron interaction. For TiBe_2-xCu_x compounds the transition from paramagnetism to ferromagnetism is analyzed, starting from Arrott plots of the magnetization. A critical concentration x_cr = 0.155 ± 0.005 is obtained. The Stoner-Edwards-Wohlfarth model of itinerant-electron magnetism is well followed near x_cr. The low temperature behavior of the specific heat of TiBe₂ is tentatively ascribed to the effect of the electron-phonon interaction on the electronic specific heat coefficient γ.     

Itinerant ferromagnetism in the C-15 Laves phase - TiBe2−xCux

By substituting copper for part of the beryllium in the C-15 Laves phase, TiBe₂, the phase is converted into an itinerant electron ferromagnet. The stable C-15 phase forms over the general composition range, TiBe_2?xCu_x where 0 ? x <.50 and the lattice parameter expands from an a_o=6.4532 Å for TiBe₂ to an a_o=6.554 Å for the composition TiBe_1.5Cu_.5. The Curie temperature increases with increasing copper content. Since none of the constituent elements have a local moment in their metallic state, this material joins the other two truly itinerant ferromagnets, Sc₃In and ZrZn₂.     

Raman scattering from magnetic excitations in Cd1−xMnxTe semiconductor alloys

We have observed antisymmetric Raman scattering from low frequency magnetic excitations in Cd_1?xMn_xTe alloys with 0.1 ≤ × ≤ 0.7. Crystals with x = 0.65 and 0.7 exhibit a magnon line in the antiferromagnetic phase, centered at 10.5 and 12.0 cm^-1 respectively, at 5 K. The frequency and the intensity of these lines decrease markedly with increasing temperature, finally disappearing in the paramagnetic phase. Crystals with 0.4 ≤ × ≤ 0.6 exhibit a similar feature at 5 K over the frequency range 4.5 – 5.5 cm^-1. These are attributed to spin waves originating from antiferromagnetically ordered clusters within their spin-glass phase. All the alloys show a distinct inelastic wing in the paramagnetic phase, which may be related to the magnetic excitations within residual clusters of coupled Mn⁺⁺ spins.     

Preparation and magnetic properties of LaMnO3 + δ (0 ≤ δ ≤ 0.154)

The structural, electrical, and magnetic properties of ceramic perovskite manganites LaMnO_{3 + δ} (δ = 0–0.154) are investigated. It is found that, in a weak magnetic field (B = 2 G), the LaMnO_{3 + δ} manganite with δ = 0.065 at temperatures below the Curie temperature T _C of the paramagnet-ferromagnet phase transition has a mixed (spin glass + ferromagnet) phase. In LaMnO_{3 + δ} manganites with the parameter δ = 0.100–0.154, this phase transforms into a frustrated ferromagnetic phase. A similar transformation was observed previously in La_1?x Ca_xMnO₃ compounds at calcium contents in the range 0 ≤ x ≤ 0.3. This similarity is explained by the fact that, in both materials, the Mn⁴⁺ concentration and, accordingly, the hole concentration c change equally in the concentration range from ～0.13 to 0.34 with an increase in x or δ. However, the magnetic irreversibility, the concentration dependences of the Curie temperature T _C(c) and the magnetic susceptibility X(c), and the critical behavior of the temperature dependence of the susceptibility X(T) in the vicinity of the Curie temperature T _C differ substantially for these two materials. The observed differences are associated with the distortion of the cubic perovskite structure, the decrease in the degree of lattice disorder, and a more uniform distribution of holes in the LaMnO_{3 + δ} manganites as compared to the La_{1 ? x} Ca_xMnO₃ compounds.     

Peculiarities of the magnetic state in the system La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>3−γ</Subscript> (0 ≤ γ ≤ 0.25)

The results of experimental investigation of the chemical phase composition, crystal structure, and magnetic properties of the manganite La_0.70Sr_0.30MnO_3?γ (0 ≤ γ ≤ 0.25) with perovskite structure depending on the concentration of oxygen vacancies are presented. It is found that the mean grain size of the stoichiometric solid solution of La_0.70Sr_0.30MnO₃ amounts approximately to 10 μm, while the grain size for anion-deficient solid solutions of La_0.70Sr_0.30MnO_3?γ is approximately 5 μm. It is found that samples with 0 ≤ γ ≤ 0.13 have a rhombohedral unit cell (with space group \(R\bar 3c\), Z = 2), while samples with γ ≥ 0.20 have a tetragonal unit cell (space group I4/mcm, Z = 2). It is proved experimentally that the magnetic phase state of the manganite La_0.70Sr_0.30MnO_3?γ changes upon a decrease in the oxygen content. It is shown that anion-deficient solid solutions of La_0.70Sr_0.30MnO_3?γ experience a number of successive magnetic phase transformations in the ground state from a ferromagnet (0 ≤ γ ≤ 0.05) to a charge-disordered antiferromagnet (γ = 0.25) via an inhomogeneous magnetic state similar to a cluster spin glass (0.13 ≤ γ ≤ 0.20). The mean size of ferromagnetic clusters (r ≈ 50 nm) in the spin glass state is estimated. It is shown that oxygen vacancies make a substantial contribution to the formation of magnetic properties of manganites. The generalized magnetic characteristics are presented in the form of concentration dependences of the spontaneous magnetic moment, coercive force, and the critical temperature of the magnetic transition. The most probable mechanism of formation of the magnetic phase state in Sr-substituted anion-deficient manganites is considered. It is assumed that in the absence of orbital ordering, a decrease in the magnetic ion coordination number leads to sign reversal in indirect superexchange interactions Mn³⁺-O-Mn³⁺.     

The specific heat of two cubic laves compounds: TiBe2 and ThMg2

The low temperature specific heats of two isomorphous Laves compounds, TiBe₂ and ThMg₂, have been measured. The shape of the transition for the itinerant antiferromagnet TiBe₂ is characteristic of magnetic ordering; the entropy of this transition is quite small, approximately 107 mJ/mole-K, a further indication of itinerancy. The γ, proportional to the density of states, is 42 mJ/mole-K² for TiBe₂, and the Debye temperature, θ_D, is 650 K. These numbers are compared to those for the itinerant ferromagnet, ZrZn₂. The results for ThMg₂ are γ=9.2 mJ/ mole-K² and θ_D = 217 K. Results for both compounds are discussed in regards to the theory of Enz and Matthias.     

Critical nuclear relaxation in cobalt metal

Nuclear magnetic resonance of cobalt metal was investigated in the paramagnetic and ferromagnetic states and in the critical region below T_c. The Knight shift and spin lattice relaxation times were measured in the paramagnetic phase in the solid and liquid states from 1578 K to 1825 K. The resonant frequency, spin-lattice and spin-spin relaxation times were measured in the ferromagnetic phase from room temperature to 1385 K. The main part of (T₁T)^-1 results from fluctuating orbital moments in both phases except near T_c where this process forms the background for critical spin relaxation. The critical exponents for T^-1₁ and for the magnetization in the ferromagnetic state were found to be n' = 0.96 ± 0.07 and β = 0.308 ± 0.012, respectively.     

Crystal structure and magnetic susceptibility of (CuInSe<Subscript>2</Subscript>)<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>(2MnSe)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The crystal structure of samples in the (CuInSe₂)_{1 ? x} (2MnSe) _x system at room temperature and their magnetic susceptibility in the temperature range 77–1000 K are investigated. It is established that compositions with concentrations 0≤ x ≤ 0.2 form solid solutions with a tetragonal structure, space group I \(\bar 4\)2d (122). The specific magnetic susceptibility χ of samples with 0.1 ≤ x ≤ 0.4 at 77 K lies in the range 9 × 10^?4?1.6 × 10^t-3cm³/g. The temperature dependence of the inverse magnetic susceptibility of the sample with x = 0.4 suggests the presence of a component with antiferromagnetic ordering and a reliably measured Néel temperature that is characteristic of MnSe. The dependences χ = f(T) of the compositions with x = 0.1, 0.2, 0.3, and 0.4 indicate the occurrence of magnetic phase transitions with a change in the spin state.     

Dynamics of the infinite-range ising spin glass

The Glauber dynamics of an Ising spin glass with infinite-range interactions and additional static field, h, is investigated near the freezing temperature, T_f. We obtain critical slowing down at and below the de Almeida-Thouless instability line, h_c(T), to order (1?T/T_f)³ with algebraic decay of the spin correlations ～t^?ν, where $\text{ν=}\text{1}\text{2}$ at T_f and $\text{ν≤}\text{1}\text{2}$ for T<T_f.     

Magnetic properties of the compounds N(CH3)4MnIIMIII(CN)6 ·8 H2O(MIII = Mn,Fe)

In the isostructural cyanobridged chain compounds N(CH₃)₄Mn^IIM^III(CN)₆ · 8H₂O high spin Mn(II) ions couple antiferromagnetically to low spin Mn(III) of Fe(III) ions. The Mn^II–Mn^III compound orders ferrimagnetically below T_N = 28.5 ± 1 K. The tetragonal a and b axes are easy ones for the magnetic moments. In the Mn^II–Fe^III compound antiferromagnetic order occurs below T_N = 9.3 K, with spins aligned along the tetragonal c axis. The compound undergoes a meta-magnetic transition from the antiferromagnetic to a ferrimagnetic phase. This occurs at 2 K for a field H_crit ≈ 1.2 T. The temperature dependence of H_crit, which vanishes at T_N, is followed. The tricritical temperature T¹ is ～ 5 K.     

Anomalous pulse-angle and phase dependence of Hahn’s electron spin echo and multiple-quantum echoes of the spin correlated radical pair P700+A 1 − in photosystem I

In a spin-correlated radical pair system, anomalous pulse-angle and phase dependence of electron spin echo and multiple-quantum echoes were theoretically calculated by Tanget al. (J. Chem. Phys.106, 7471 (1997)). The maximum intensity of the out of phase signal at 45 degree of spin rotation angle was experimentally verified in two-pulse echoes of the light-induced P700⁺A ₁ ^? radical pair in Photosystem I. The values,D = 1.64 G andJ = 0.00 G, fit well with the experimental ESEEM spectra. Single and double quantum echoes were observed at the value oft = τ andT = 2τ with the laser flash-t-P1_70,ζ1-τ-P2_{140, ζ2}-T pulse sequence, which led to determination of the relaxation time T₂₃ between the singlet and triplet ¦T₀〉 states. The relaxation times of the zero and single quantum transitions were determinedT ₂₃ ≈ 100 ns andT ₂ = 1000 ns, respectively. The field sweep ESE signal shape can be fitted with the hyperfine inhomogeneities of 7 G for P700⁺ and of 10 G for A ₁ ^? .     

Proton spin-lattice relaxation by critical polarization fluctuations in KH2PO4

The observed anomalous decrease in the proton spin-lattice relaxation timeT ₁ on approaching the Curie point in a rather pure KH₂PO₄ single crystal is explained by magnetic dipolar coupling to the ferroelectric mode. The isolated “non-interacting” O?H...O proton flipping time is estimated from theT ₁ data as τ=0.66·10^?12 sec for the paraelectric phase and τ=2.24·10^?12 sec for the ferroelectric phase, in good agreement with the results obtained from other methods.     

Electron hopping and optic phonons in Eu3S4

Raman scattering on single crystals of Eu₃S₄ does not show the allowed q=o phonon modes in the cubic phase and exhibits no new modes in the distorted low temperature phase (T<186 K). Above the Curie temperature T_c=3.8 K the scattering is dominated by a spin-disorder induced one-phonon density of states allowing for the observation of the zone boundary phonon breathing mode of the S^2?ions. This mode does not show any anomaly near the charge order -disorder phase transition T_t=186 K. Temperature tunable spin fluctuations associated with the temperature activated Eu²⁺→Eu³⁺ electron hopping are detected in the scattering intensity, superimposed on the usual thermal spin disorder.     

Das kritische magnetische Verhalten von EuO

Mössbauer effect and magnetization measurements were employed in order to study the static and especially the dynamic magnetic properties of the nearly Heisenberg ferromagnet EuO near its Curie temperature,T _c=69.2 °K. The critical exponent β of the spontaneous magnetization was determined to be β=0.34±0.02. It was shown that critical slowing down of spin fluctuations takes place nearT _c with spin relaxation times between 7×10^?11 sec (T=1.01T _c) and 1.5×10^?1 sec (T=1.03T _c). The experimental values of the relaxation time were found to be in satisfactory agreement with theoretically computed ones. Just belowT _c the Mössbauer spectra exhibit relaxation effects, which are characteristic for the occurence of critical super-paramagnetism. Investigations of several samples indicated quantitatively, that critical superparamagnetism has its origin in the non ideal composition of the real crystal.     

Influence of zeolite water on paramagnetic and ferromagnetic resonances in the Co2[Nb(CN)8] · 8H2O molecular magnet

The contributions of Co²⁺ and Nb⁴⁺ ions to the high-frequency dynamic magnetic susceptibility of the Co₂[Nb(CN)₈] · 8H₂O molecular magnet in the paramagnetic state at T > 12 K are separated. It is found that the ferromagnetic ordering, which leads to the reconstruction of the electron paramagnetic resonance spectrum into the ferromagnetic resonance spectrum, occurs at T < 12 K. The influence of zeolite water on the spectra of the paramagnetic and ferromagnetic resonances is found. Dehydration leads to a decrease in the time of the spin relaxation of the ferromagnetic system from 50 ps to 17 ps at T = 4 K and to the variation in the temperature dependences of the widths of the lines and g factors in the electron spin resonance spectra.     

Mictomagnetic State in an EuBaCo<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>5.5–δ</Subscript> Single Crystal

The magnetic properties of an EuBaCo_1.9O_5.36 single crystal are studied in the temperature range T = 2–300 K and the magnetic field range H ≤ 90 kOe. This binary layered cobaltite single crystal has vacancies in the cobalt and oxygen sublattices, in contrast to the stoichiometric EuBaCo₂O_5.5 composition. All cobalt ions in EuBaCo1.9O5.36 are in a trivalent state. The single crystal has an orthorhombic structure with space group Pmmm, and its unit cell parameters are a = 3.883 Å, b = 7.833 Å, and c = 7.551 Å. The field and temperature dependences of the magnetization of the single crystal demonstrate that it is ferrimagnet below T_C = 242 K. At T < 300 K, all three spin states of the Co³⁺ ions are present. The nearest-neighbor interactions give antiferromagnetic (AFM) and ferromagnetic (FM) contributions to the exchange energy. The ratio of the AFM to the FM contributions changes when temperature decreases because of a change in the spin state of the Co³⁺ ions. The single crystal exhibits signs of mictomagnetism at low temperatures in high magnetic fields. At T = 2 K and H = 90 kOe, the zero-field and nonzero-field magnetizations are strongly different because of a uniaxial magnetic anisotropy, which tends to set magnetization along the magnetic field applied in cooling throughout the crystal volume. As a result, a complex ferrimagnetic structure with a noncollinear direction of Co³⁺ spins appears. The following phenomena characteristic of mictomagnets are also observed in the EuBaCo_1.9O_5.36 single crystal: a shift in a magnetization hysteresis loop when temperature decreases, retained hysteretic phenomena and no magnetization saturation in high magnetic fields, and an orientation transition. The mictomagnetic state in EuBaCo_1.9O_5.36 is shown to be caused by the structural distortions induced by vacancies in the cobalt and oxygen sublattices and by the frustration of AFM and FM exchange interactions.     

Spin gap, phase separation and d-wave pairing as revealed by electron spin-lattice relaxation in 123 and 124 YBaCuO systems doped with Gd3+

With an original modulation technique, the Gd³⁺ electron spin-lattice relaxation has been investigated in normal and superconducting states of YBa₂Cu₃O_6+x (123) and YBa₂Cu₄O₈ (124) compounds doped with 1% Gd. In the 123 sample withx = 0.9T _c = 90 K), theT ₁ behavior within 50 <T< 200 K reveals the [1 ? tanh²(Δ/2kT)]/T dependence typical of a spin gap opening with Δ ≈ 240 K. Below 50 K, the exponential slowing down ofT ₁ is limited by the Korringa-like behaviorT ₁ T = const); the same Korringa-like law is found in the 123 sample withx = 0.59 (T _c = 56 K) within the total 4.2–200 K temperature range. This is interpreted in terms of microscopic separation of the normal and superconducting phases allowing for the electron spin cross-relaxation between them. In the 124 sample (T _c = 82 K), the Gd³⁺ relaxation rate below 60 K is found to obey a power lawT _n with an exponentn ≈ 3. Such a behavior (previously reported for nuclear spin relaxation) is indicative of the d-wave superconducting pairing. Additional paramagnetic centers characterized by relatively slow spin-lattice relaxation are found in both 123 and 124 systems. A well-pronounced change in theT ₁ temperature dependence atTT* ≈ 180–200 K is observed for these slowly relaxing centers as well as for the conventional, fast-relaxing Gd³⁺ ions, suggesting microscopic phase separation and a change in the relaxation mechanism due to electronic crossover related with the opening of the spin gap. This hypothesis is supported by some “180 K anomalies” previously reported by other authors.