A. B. Beznosov

Charge ordering, internal structural parameters, and magnetic susceptibility of Nd2∕3Ca1∕3MnO3: driving forces of a phase transition

The temperature dependences of the long diagonals dMn–O of the MnO6 octahedron and the magnetic susceptibility χ of Nd2∕3Ca1∕3MnO3 in the temperature interval 100–290K are investigated. The functions dMn–O(T) and χ(T) are found to have anomalies in the charge-ordering range (Tco≈212K). The sharp decrease of the diagonal dMn–O2s agrees with phase-transition notions, according to which the spatial modulation of the charge density is due to the modulation of the Mn–Mn bond lengths. The most likely driving forces of the transition are the Peierls lattice instability and Jahn–Teller stability of the MnO6 octahedron at the Mn3+ ions. The hysteresis of the temperature dependence χ(T) in the paramagnetic region shows indirectly that structural phase separation occurs together with the transition. The parameters of the function χ(T) indicate that ferromagnetic clusters consisting of one, two, or three ion pairs Mn3+–Mn4+ form in the system in the temperature intervals 274K⩽T⩽290K, 224K⩽T⩽252K, and 130K⩽T⩽198K, res...

d-f-Exchange resonance in gadolinium: Theory and experiment

Abstract The single-crystal gadolinium dielectric permeability spectrum is investigated experimentally and theoretically in the frequency range 0.5 ⩽ h ω ⩽1 eV. The temperature dependence of d-f exchange resonance spectrum observed in the above range is interpreted. This is done in terms of the concept of intra-atomic light absorption and the magnetic order parameter dependence of the coherence degree of angular parts of quasi-bound 5d-electron wave functions.

Exchange interaction and magnetoresistance in La2/3Ca1/3MnO3: experiment and models

The magnetization M(T) and electrical resistivity ρ(T) of a La2/3Ca1/3MnO3 film have been studied in the temperature range 5 K⩽T⩽320 K in the magnetic field intervals 10 Oe⩽H⩽400 Oe and 0⩽H⩽50 kOe, respectively. It is found that the M(T)/M(0) value is larger than that predicted by the conventional molecular field model below the Curie point T=267 K, and that the ln ρ(T) dependence is close to linear in the temperature range 80 K<T<200 K (accordingly, ∂ ln ρ/∂T is constant in this region). A model of the electrical conductivity and magnetoresistivity of the system describing qualitatively the experimental results is proposed (the Δmτ model). The model includes a thermally activated (with characteristic energy Δ) mechanism of conductivity, dependence of the concentration and the effective mass (m) of the itinerant charge carriers on the magnetization, as well as scattering (with characteristic time τ) of the charge carriers by static breakings of the translational symmetry, thermal fluctuations of the magne...

Low-temperature phase segregation in La2∕3Ba1∕3MnO3: Manifestation of nonequilibrium thermodynamics

Thermodynamic characteristics of the perovskite-like compound La2∕3Ba1∕3MnO3, exhibiting a structural phase transformation of the martensitic type with a characteristic temperature Ts≈200K, are studied in the temperature range 2–340K. Step-like hysteretic temperature behavior of the effective heat capacity is revealed at 150–250K and attributed to the discrete kinetics and a latent heat of the martensitic transformation. The magnetic subsystem is found to exhibit a magnetic glass state below 220K and temperature hysteresis of the magnetic susceptibility clearly pronounced in the 40–100K and 180–230K regions. The Debye and Einstein temperatures, θD=230K and θE=500K, respectively, derived from the experimental Debye–Waller factors for the La∕Ba, Mn, and O sublattices, are used to refine contributions from the structural and magnetic transformations to the heat capacity and to reveal thermodynamically nonequilibrium states.

Spontaneous magnetostriction in the Gd–Y system: analysis of phase transformations

The transformations of the states of the magnetic and lattice subsystems of Gd100−xYx (x=0,5.5, 7.5, 10.2) alloys are studied at temperatures 5–370 K in magnetic fields up to 4 kOe. The temperature dependences of elastic moduli, thermal expansion, low-field magnetization, and magnetic anisotropy parameters are obtained. The linear spontaneous striction ΔL(T)/L=ξη2 is analyzed, and the effective order parameters η and magnetostrictive parameters ξ are evaluated for the four magnetic phases (canted ferromagnetic, deformed ferromagnetic helix, ferromagnetic helix, and the ferromagnetic cone, respectively): ξcf=2.2×10−5, ξdh=−5.3×10−5, ξfh=7.1×10−5, ξfc=1.4×10−3.

Optical functions of the Drude model: transformation of the spectra over wide ranges of parameters

The spectral features of the optical functions of the Drude model are investigated over wide ranges of parameters — the plasma frequency ωp of the current carriers, their transport relaxation frequency g, and the dielectric constant e∞ due to high-energy electronic transitions in the system. The conditions are determined for: a) the square-root frequency dependence of the modulus and phase of the reflection; b) linearity of the phase θ=2ω/ωpe∞+θ0; c) the existence of a plasma reflection edge. Approximate relations are obtained which permit simplifying the analysis of optical reflection and electron characteristic energy loss data both in “good” metals and in materials with a strong temperature dependence of the electrical conductivity (semiconductors, doped and nonstoichiometric oxides, granular metal films, and amorphous alloys). The parameters of the systems of conduction electrons in Al and U2Zn17 are determined.

Direct evidence of the low-temperature cluster-glass magnetic state of Nd2/3Ca1/3MnO3 perovskite

A giant exchange bias is detected in the colossal magnetoresistance of Nd2/3Ca1/3MnO3 perovskite at low temperatures and is evidence of intrinsic exchange coupling in this compound. These phenomena confirm our previous assumption that the low-temperature magnetic structure of this compound consists of small (nanosized) ferromagnetic clusters embedded in a charge-ordered antiferromagnetic matrix. The magnetic behavior of the perovskite Nd2/3Ca1/3MnO3 is consistent with a cluster-glass magnetic state and inconsistent with the classical spin-glass state observed in a variety of disordered magnetic systems. We think that the cluster-glass magnetic behavior of Nd2/3Ca1/3MnO3 originates in a self-organized phase-separated state of the compound. A Cole-Cole analysis of the dynamic susceptibility at low temperatures reveals an extremely broad distribution of relaxation times, indicating that spins are frozen on a “macroscopic” time scale. Slow relaxation of the zero-field-cooled magnetization is also observed exp...

Spectrum of dynamic magnetic susceptibility of a randomized f–d magnet with spin–lattice coupling. II. “Opening” of the spin excitation bands

The transformation of the magnetic absorption spectra of a narrow-band ferromagnetic conductor containing local f and quasilocal d magnetic moments under conditions of weak spin–lattice coupling and spatial randomization of the g factors of the quasilocal and local spin subsystems is investigated. It is shown that randomization of the g factors (e.g., as a result of the introduction of an impurity) leads mainly to the “opening” (i.e., to the appearance in the magnetic absorption spectrum) of acoustic and optical magnon bands formed in the system while not affecting the position and shape of the narrow magnetic resonance lines comprising the spectrum of the impurity-free crystal. The effects of spin–lattice coupling, besides the “opening” of the phonon and magnon bands, lead to a shift and a temperature smearing of the narrow resonance lines. The relative corrections to the effective magnon masses generated by the spin–lattice coupling in a 4f–5d metal amount to ∼2×10−4 for an acoustic mode and to ∼4×10−4 ...

Magnetic phases in La0.66Ba0.34MnO3: effects of temperature and elastic strains

The static magnetization MN and dynamic magnetic susceptibility χac of polycrystalline manganite La0.66Ba0.34MnO3 are investigated in the temperature region 5 K<T<350 K. Measurements are made on a sample with a demagnetizing factor N=5.4 in static magnetic fields H in the range 0–120 Oe and on a toroidal sample with N=0 in fields with frequencies of 120 and 1000 Hz and amplitudes from 8×10−3 to 4.1 Oe. The effect of a uniaxial pressure P=0.1 kbar on the static susceptibility χN=MN/H is investigated in the region 5 K<T<270 K. It is found that in the magnetically ordered state (below 314 K) χac is independent of the field amplitude in the interval 1.5–4.1 Oe and independent of temperature between 250 and 302 K. At a temperature Tp=250 K a second-order phase transition (discontinuity of the derivative ∂χac/∂T) is observed, and at Ti=177.5 K—a first-order phase transition (discontinuity of χac(T)). The uniaxial pressure leads to an anomalous lowering of the susceptibility in the region 160–250 K with a sharp ...

Crystal-field and Nd-Mn exchange interaction in Nd2/3Ca1/3MnO3

A study of the low field magnetization and specific heat in magnetic fields up to 9 T of Nd2/3Ca1/3MnO3 perovskite in the 2-30 K temperature range has been done. All the specific heat data show broadened Schottky-like anomaly below 20 K. We suppose that such a behavior originates from the Nd magnetic ordering caused by the splitting of the Nd3+ ions ground-state doublet (GSD) in the effective molecular field Hex of Mn spin system supplemented by an applied external magnetic field. The zero field GSD splitting is an evidence of a strong exchange coupling between Nd and Mn magnetic subsystems. The Nd-ions magnetic ordering introduces an additional contribution to the ferromagnetic moment producing anomalies of the field-cooled and zero-field-cooled magnetizations of the system below 28 K. The broadened Schottky-like anomalies found are fitted for every field by a set of three Schottky functions. Applied magnetic field extends the anomaly region and shifts it to the higher temperatures. Splitting of the higher crystal field Kramers doublets gives an additional contribution to the heat capacity under magnetic fields. The GSD g-factors g|| and g⊥ was estimated as 3.4 and 2.2, respectively, and Hex as 9 T.