Optimization of Pr(0.9)Ca(0.1)MnO(3) thin films and observation of coexisting spin-glass and ferromagnetic phases at low temperature

Optimization of thin films of small bandwidth manganite, Pr(1-x)Ca(x)MnO3 (for x = 0.1), and their magnetic properties are investigated. Using different pulsed laser deposition (PLD) conditions, several films were deposited from the stoichiometric target material on SrTiO3 (001) substrate and their thorough structural and magnetic characterizations were carried out using x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy (XPS), SQUID magnetometry and ac susceptibility measurements. A systematic investigation shows that irrespective of the growth temperature (between 550 and 750?°C), all the as-deposited films have twin boundaries and magnetic double phases. Post-annealing in partial or full oxygen pressure removes the extra phase and the twin boundaries. Zero-field-cooled magnetization data show an antiferromagnetic to paramagnetic transition at around 100 K whereas the field-cooled magnetization data exhibit a paramagnetic to ferromagnetic transition close to 120 K. However, depending on the oxygen treatments, the saturation magnetization and Curie temperature of the films change significantly. Redistribution of oxygen vacancies due to annealing treatments leading to a change in ratio of Mn3+ and Mn4+ in the films is observed from XPS measurements. Low temperature (below 100 K) dc magnetization of these films shows metamagnetic transition, high coercivity and irreversibility magnetizations, indicating the presence of a spin-glass phase at low temperature. The frequency dependent shift in spin-glass freezing temperature from ac susceptibility measurement confirms the coexistence of spin-glass and ferromagnetic phases in these samples at low temperature.     

Dynamical mean-field theory of a simplified double-exchange model

Simplified double-exchange model including transfer of the itinerant electrons with spin parallel to the localized spin in the same site and the indirect interaction J of kinetic type between localized spins is comprihensively investigated. The model is exactly solved in infinite dimensions. The exact equations describing the main ordered phases (ferromagnetic and antiferromagnetic) are obtained for the Bethe lattice with (z is the coordination number) in analytical form. The exact expression for the generalized paramagnetic susceptibility of the localized-spin subsystem is also obtained in analytical form. It is shown that temperature dependence of the uniform and the staggered susceptibilities has deviation from Curie-Weiss law. Dependence of Curie and Néel temperatures on itinerant-electron concentration is discussed to study instability conditions of the paramagnetic phase. Anomalous temperature behaviour of the chemical potential, the thermopower and the specific heat is investigated near the Curie point. It is found for J=0 that the system is unstable towards temperature phase separation between ferromagnetic and paramagnetic states. A phase separation connected with antiferromagnetic and the paramagnetic phases can occur only at . Zero-temperature phase diagram including the phase separation between ferromagnetic and antiferromagnetic states is given. Received 28 May 1999 and Received in final form 14 July 1999     

Sr掺杂Eu1-xSrxMnO3体系的核磁共振和磁结构研究

对Eu_1-xSr_xMnO₃ (x=0.4, 0.5, 0.6, 0.7) 体系的磁结构进行了系统的研究. 通过核磁共振实验, 磁化测量, 并结合电输运测量结果表明, Sr的掺入使得 EuMnO₃反铁磁母相中出现铁磁相. 铁磁相和反铁磁相的竞争导致样品在低温下的自旋玻璃行为.分析认为, Eu_0.4Sr_0.6MnO₃和Eu_0.3Sr_0.7MnO₃的磁结构在低温下呈现更加复杂的特征, 主要源于铁磁团簇的形成以及无序相的存在.     

Reentrant spin-glass transition in a dilute magnet

We perform a large-scale Monte Carlo simulation of a dilute classical Heisenberg model with ferromagnetic nearest neighbor and antiferromagnetic next-nearest neighbor interactions. We found that the model reproduces a reentrant spin-glass transition. That is, as the temperature is decreased, the magnetization increases rapidly below a certain temperature, reaches a maximum value, and then disappears at some lower temperature. The low temperature phase was suggested to be a spin-glass phase that is characterized by ferromagnetic clusters.     

An investigation of phase separation by magnetic force microscopy in La0.45Sr0.55MnO3-δ(δ≈0.01)

We have found phase separation in La_0.45Sr_0.55MnO_3-δ (LSMO) by means of electron spin resonance, magnetic force microscopy (MFM) and magnetic measurements. Ferromagnetic and antiferromagnetic phases can coexist at low temperatures, and ferromagnetic and paramagnetic phases coexist when the temperature lies between the Néel and Curie temperatures. The size and shape of the ferromagnetic phases (the minority phases) was first observed directly from MFM images. It is suggested that the phase separation in LSMO is not the charge segregation type, but an electroneutral type due perhaps to the nonuniform distribution of oxygen vacancies.     

Magnetic correlations in Cr–Fe–Mn alloy

Small angle neutron scattering (SANS) measurement has been performed on the reentrant ferromagnet Cr₆₀Fe₂₀Mn₂₀ alloy. An increase in the SANS intensity is observed below 100 K coinciding with an increase in the magnetization. It exhibits a maximum at 40 K and shifts to lower temperatures with increase in Q. The magnetic part of the SANS intensity fits to a Lorentzian at all temperatures. The temperature dependence of the inverse correlation length exhibits a minimum at 40 K. In the antiferromagnetic spin-glass phase at 15 K ferromagnetic correlations of ~40 Å is observed.     

Magnetic phase diagrams of Fe–Mn–Al alloy on the Bethe lattice

The magnetic behaviors of the Fe–Mn–Al alloy are simulated on the Bethe lattice by using a trimodal random bilinear exchange interaction(J) distribution in the Blume–Capel(BC) model. Ferromagnetic(J 0) or antiferromagnetic(J 0)bonds or dilution of the bonds(J = 0) are assumed between the atoms with some probabilities. It is found that the secondor the first-order phase boundaries separate the ferromagnetic(F), antiferromagnetic(AF), paramagnetic(P), or spin-glass(SG) phases from the possible other one. In addition to the tricritical points, the special points at which the second- and the first-order and the spin-glass phase lines meet are also found. Very rich phase diagrams in agreement with the literature are obtained.     

Ordering and phase transitions in ising systems with competing short range and dipolar interactions

As a simple model of order-disorder ferroelectrics or dipolar magnets we consider a simple cubic Ising-system with nearest neighbor exchangeJ and dipolar interaction of strengthµ ²/a ³. ForJa ³/µ ²Ja ³/µ ²<0.1270 the ground state consists of ferromagnetic rows (in spin direction) arranged antiferromagnetically in the plane perpendicular to it. AtJa ³/µ ²=0.1279 the structure changes to a layered antiferromagnetic structure with a twocomponent order parameter, while forJa ³/µ ²>0.16429 the ferromagnetic phase becomes stable (with domain arrangements depending on the shape of the sample). For all critical values ofJa ³/µ ² where the bulk energies of two phases become equal also the interface energy between these phases is found to be zero. The ordering at nonzero temperature is studied by means of mean-field approximations (MFA) and Monte Carlo (MC) calculations. It turns out that forJa ³/µ ² of order unity the MFA overestimates ordering temperatures by about a factor of two, and predicts multicritical points (between the disordered and two ordered phases) at nonzero temperature, including two biaxial Lifshitz points which the MC work suggests to occur atT=0. In contrast to MFA the layered antiferromagnetic structure is found to be stable only at extremely lowT, because a metastable spin-glass phase (with random arrangement of ferromagnetic rows in the spin direction) has only slightly higher energy. The MFA also yields two regimes of helical phases which are “locked in” to the antiferromagnetic phases at uniaxial Lifshitz points occurring at the Brillouin zone boundary. In the MC-work various methods of treating the long-range interaction are investigated. While all kinds of truncations as well as compensating field methods are rather unsatisfactory in our case, Ewald summation techniques yield satisfactory results. Nevertheless strong fluctuations as well as strong finite size effects prevent us from making accurate exponent estimates, but arguments are given that there is no regime of broad visibility of Landaulike critical behavior. Finally the extension of our results to other lattices as well as experimental applications are briefly discussed.     

Finite size effects for the dilute coupling Derrida model

There is a certain correspondence between the problems of information theory and spin-glass physics. The Derrida model occupies an important place here, as it is optimal in a certain sense. We have considered finite size corrections in the paramagnetic, spin-glass and ferromagnetic phases. At T = 0 the finite size corrections in the ferromagnetic phase have an important information theoretic meaning. We found singular corrections in the paramagnetic region and finite size corrections in the spin-glass region, which are responsible for the zero-modes. The connection between information theory and spin-glass physics and the physical meaning of finite size corrections are discussed in detail.     

Transition from orbital liquid to Jahn-Teller insulator in orthorhombic perovskites RTiO3

Following the same strategy used for RVO3, thermal conductivity measurements have been made on a series of single-crystal perovskites RTiO3 (R=La,Nd,...,Yb). Results reveal explicitly a transition from an orbital liquid to an orbitally ordered phase at a magnetic transition temperature, which is common for both the antiferromagnetic and ferromagnetic phases in the phase diagram of RTiO3. This spin/orbital transition is consistent with the mode softening at T_{N} in antiferromagnetic LaTiO3 and is supported by an anomalous critical behavior at T_{c} in ferromagnetic YTiO3.     

Inhomogeneous magnetic states in the Nd(Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript>x</Subscript>)O<Subscript>3</Subscript> system

The crystal structure and magnetic properties of the Nd(Mn_?xCr_x)O₃ system (x≤0.85) have been studied. Substitution of chromium for manganese was shown to induce a transition from the antiferromagnetic to ferromagnetic state (x≈0.2) and a decrease in the critical temperature followed, conversely, by an increase in the Néel temperature and decay of spontaneous magnetization. At low temperatures, the magnetization was found to behave anomalously as a result of magnetic interaction between the ferromagnetic and antiferromagnetic phases. The formation of the ferromagnetic phase is attributed to destruction of cooperative static orbital ordering, while the coexistence of different magnetic phases is most probably due to internal chemical inhomogeneity of the solid solutions.     

Low temperature study of micrometric powder of melted Fe50Mn10Al40 alloy

Melted Fe₅₀Mn₁₀Al₄₀ alloy powder with particle size less than 40 μm was characterized at room temperature by XRD, SEM and XPS; and at low temperatures by Mössbauer spectrometry, ac susceptibility, and magnetization analysis. The results show that the sample is BCC ferromagnetic but with a big contribution of paramagnetic sites, and presents super-paramagnetic and re-entrant spin-glass phases with critical temperatures of 265 and 35 K, respectively. The presence of the different phases detected is due to the disordered character of the sample and the competitive magnetic interactions. The obtained values of the saturation magnetization and the coercive field as a function of temperature present a behavior which indicates a ferromagnetic phase. However, the behavior of the FC curve and that of the coercive field as a function of temperature suggest that the dipolar magnetic interaction between particles contributes to the internal magnetic field in the same way as was reported for nanoparticulate powders.     

Two superconducting phases in the d=3 Hubbard model

The phase diagram of the d=3 Hubbard model is calculated as a function of temperature and electron density 〈n_i〉, in the full range of densities between 0 and 2 electrons per site, using renormalization-group theory. An antiferromagnetic phase occurs at lower temperatures, at and near the half-filling density of 〈n_i〉= 1. The antiferromagnetic phase is unstable to hole or electron doping of at most 15%, yielding to two distinct“τ" phases: for large coupling U/t, one such phase occurs between 30–35% hole or electron doping, and for small to intermediate coupling U/t another such phase occurs between 10–18% doping. Both τ phases are distinguished by non-zero hole or electron hopping expectation values at all length scales. Under further doping, the τ phases yield to hole- or electron-rich disordered phases. We have calculated the specific heat over the entire phase diagram. The low-temperature specific heat of the weak-coupling τ phase shows an exponential decay, indicating a gap in the excitation spectrum, and a cusp singularity at the phase boundary. The strong-coupling τ phase, on the other hand, has a critical exponent α≈-1, and an additional peak in the specific heat above the transition temperature possibly indicating pair formation. In the limit of large Coulomb repulsion, the phase diagram of the tJ model is recovered.     

Magnetic-breakdown oscillations of the thermoelectric field in layered conductors

We consider the quasi-two-dimensional pseudo-spin-1/2 Kitaev–Heisenberg model proposed for A₂IrO₃ (A = Li, Na) compounds. The spin-wave excitation spectrum, the sublattice magnetization, and the transition temperatures are calculated in the random phase approximation for four different ordered phases observed in the parameter space of the model: antiferromagnetic, stripe, ferromagnetic, and zigzag phases. The Néel temperature and temperature dependence of the sublattice magnetization are compared with the experimental data on Na₂IrO₃.     

V替代Mn对La_(0.45)Ca_(0.55)MnO_3电荷有序相及自旋玻璃态的影响

用固相反应法制备了La0.45Ca0.55Mn1-xVxO3(x=0.00,0.10)多晶样品.通过X射线衍射谱、质量磁化强度-温度曲线、电子自旋共振谱,研究了V5+替代Mn3+/Mn4+对La0.45Ca0.55MnO3电荷有序相和自旋玻璃态的影响.实验结果表明,当x=0.10时,不仅母体的电荷有序相基本破坏,而且母体在40K左右出现的自旋玻璃态也被融化.电荷有序相被破坏的主要原因是用V5+替代Mn3+/Mn4+后,增加了Mn3+与Mn4+的比例,使铁磁双交换作用优于反铁磁超交换作用;自旋玻璃态的融化是由于V替代Mn后破坏了反铁磁背景下有少量铁磁成分的自旋玻璃态的形成条件.     

Stable mean-field solution of a short-range interacting SO(3) quantum Heisenberg spin glass

We present a mean-field solution for a quantum, short-range interacting, disordered, SO(3) Heisenberg spin model, in which the Gaussian distribution of couplings is centered in an antiferromagnetic (AF) coupling J[over ]>0, and which, for weak disorder, can be treated as a perturbation of the pure AF Heisenberg system. The phase diagram contains, apart from a Néel phase at T=0, spin-glass and paramagnetic phases whose thermodynamic stability is demonstrated by an analysis of the Hessian matrix of the free-energy. The magnetic susceptibilities exhibit the typical cusp of a spin-glass transition.     

Reentrant phases in the ± J spin glass

We consider an enlarged phase space of the ±J spin glass which includes the dilute Ising model and the frustrated system. The three orthogonal axes in this space are: (i) The fraction of ferro- to antiferro-magnetic bonds, p; (ii) the ratio of the strengths of the antiferro- to ferromagnetic interacions, q; and (iii) the temperature, T. Within this phase space we observe extended regions of the low-temperature spin-glass phase which is characterized by a unique distribution of the local-order parameter. We observe reentrant phase transitions: for fixed p and q with varying T the distribution of the local order parameter shows paramagnetic, ferromagnetic and then spin-glass phases; for fixed p and T and varying q the distribution shows ferromagnetic to paramagnetic and then spin-glass phases.     

Magnets with random uniaxial anisotropy: Large-N effective potential and infrared properties

We study the magnetic properties of systems with random uniaxial anisotropy using a large-N effective potential approach for d = 4, 3.It is found that the random interactions induce a strong infrared behavior that prevents the existence of a ferromagnetic phase and massless transverse modes.The transverse susceptibility is finite for all values of the temperature and at d = 4 it has an esential singularity in the couplings. We argue that this is indicative of a mechanism of dynamical mass generation due to the infrared instabilities of the theory.For both d = 4, 3 there is a spin-glass low-temperature phase and a paramagnetic high-temperature phase, the susceptibility having a cusp across the transition.We prove that these phases are stable and that the transverse and longitudinal susceptibilities are equal.     

Electrical and magnetic features in the perovskite-type system Y(Co, Mn)O3

Electrical and magnetic properties of the perovskite-type solid solution YCo_xMn_1−xO₃ (x=0.20-0.60) have been studied at different temperatures and magnetic fields. Electrical conductivity measurements show a semi-conducting behaviour throughout the solid solution. The room-temperature conductivity increases with the Co content up to 33 at.%, and then decreases. The effective moment in the paramagnetic state shows a non-monotonic decrease, when the Co content increases. In the ordered state, the behaviour at low or null magnetic fields corresponds to a spin-glass or antiferromagnetic system, with a transition temperature, which raises with the Co content (up to 50 at.% Co), and then decreases. At high fields, all the solid solutions show a ferromagnetic behaviour, although there is a marked difference in their ferromagnetic cycles, at a threshold value of 33 at.% Co.     

High hydrostatic pressure effect on magnetic state of anion-deficient La0.70Sr0.30MnOx perovskite manganites

The magnetic state of La_0.70Sr_0.30MnO_x (x=2.85, 2.80) anion-deficient manganites is studied experimentally under hydrostatic pressure up to 5.2 GPa. These materials possessing the properties of a spin-glass state. The La_0.70Sr_0.30MnO_2.85 exhibits a coexistence of the rhombohedral (R3¯c) and tetragonal (I4/mcm) crystal structures and below T_f 50 K the spin-glass state is formed. The La_0.70Sr_0.30MnO_2.80 exhibits the tetragonal (I4/mcm) crystal structure. For this compound a phase-separated magnetic state below T_f is formed, involving coexistence of C-type antiferromagnetic (AFM) clusters within spin-glass matrix. In both compounds the crystal structure and magnetic states remain stable upon compression. Under the hydrostatic pressure up to 1 GPa, the La_0.70Sr_0.30MnO_2.85 is a spin glass with a smeared phase transition to the paramagnetic state. The freezing temperature T_f of the magnetic moments of the ferromagnetic clusters increases at a rate of 4.30 K/GPa and the magnetic ordering temperature T_MO increases at a rate of 12.90 K/GPa. The enhancement of the ferromagnetic properties of La_0.70Sr_0.30MnO_2.85 anion-deficient manganite under hydrostatic pressure is explained by the redistribution of oxygen vacancies and a decrease in the unit cell parameters.