Electron spin resonance study of the local properties of the microscopic phase separation in La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> single-crystal films

Electron spin resonance is studied in films of La_1?x Ca _x MnO₃ manganites with compositions in the vicinity of the calcium content x = 0.5, in which the phase separation is most clearly pronounced. It is found that the La_0.5Ca_0.5MnO₃ manganite undergoes different types of phase separation: (i) at temperatures above the Curie point, ferromagnetic regions exist in the paramagnetic phase; and (ii) at temperatures below the Néel point, antiferromagnetic microregions coexist with ferromagnetic microregions. Two types of ferromagnetic domains with different magnetization orientations are revealed In the temperature range between the Curie and Néel points.     

Double-exchange model: phase separation versus canted spins

We study the competition between different possible ground states of the double-exchange model with strong ferromagnetic exchange interaction between itinerant electrons and local spins. Both for classical and quantum treatment of the local spins the homogeneous canted state is shown to be unstable against a phase separation. The conditions for the phase separation into the mixture of the antiferromagnetic and ferromagnetic/canted states are given. We also discuss another possible realization of the phase-separated state: ferromagnetic polarons embedded into an antiferromagnetic surrounding. The general picture of a percolated state, which emerges from these considerations, is discussed and compared with results of recent experiments on doped manganaties. Received 17 March 1999     

Effects of bond alternation and the Dzyaloshinskii-Moriya interaction on the ground-state phase diagram of the Ising model

The bond-alternative ferromagnetic Ising model with the Dzyaloshinskii-Moriya (DM) interaction is investigated using an infinite time-evolving block decimation (iTEBD) algorithm, and two interesting phase diagrams are proposed. Without bond alternation, there are only two well-known phases (a ferromagnetic phase and an x-y spiral (chiral Luttinger liquid) phase). Once the bond alternation is taken into account, two additional interesting phases (a z-chiral and a z-stripe phases) are induced. Nonzero chiral order is observed in all phases except the ferromagnetic phase. Two kinds of chiral phase, with or without string order, are identified. With the y-axis DM interaction, the phase diagram becomes relatively simple, and consists of only two different phases (a ferromagnetic phase and a y-chiral phase). First-order metamagnetic phase transitions from the ferromagnetic phase into the other phases are observed in both cases.     

Exchange-induced phase separation in Ni–Cu films

Magneto-structural properties of films of diluted ferromagnetic alloys Ni_xCu_1−x in the concentration range 0.7<x<1.0$0.7<x<1.0$ are studied experimentally. Films deposited by magnetron sputtering show partial phase separation, as evidenced by structural analysis and ferromagnetic resonance measurements. The phase diagram of the Ni_xCu_1−x bulk system is obtained using numerical theoretical analysis of the electronic structure, taking into account the interatomic exchange interactions. The results confirm the experimentally found partial phase separation, explain it as magnetic in origin, and indicate an additional metastable region connected with the ferromagnetic transition in the system.     

Colossal magnetoresistance without phase separation: disorder-induced spin glass state and nanometer scale orbital-charge correlation in half doped manganites

The magnetic and electrical properties of high-quality single crystals of A-site disordered (solid solution) Ln0.5Ba0.5MnO3 are investigated near the phase boundary between the spin-glass insulator and colossal-magnetoresistive ferromagnetic metal, locating near Ln=Sm. The temperature dependence of the ac susceptibility and the x-ray diffuse scattering of Eu0.5Ba0.5MnO3 are analyzed in detail. The uniformity of the random potential perturbation in Ln0.5Ba0.5MnO3 crystals with a small bandwidth yields, rather than the phase separation, an homogeneous short ranged charge or orbital order which gives rise to a nearly atomic spin-glass state. Remarkably, this microscopically disordered "charge-exchange-glass" state alone is able to bring forth the colossal magnetoresistance.     

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     

Magnetically induced phase separation in the Co–Cr binary system

Direct evidence of phase separation in the Co-rich corner of the Co–Cr binary system, the transformation of high-temperature FCC α-Co into a ferromagnetic α_f phase and a paramagnetic α_p phase, has been experimentally obtained by using diffusion couple technique, scanning electron microscopy with energy dispersion analysis of X-ray (SEM-EDX), analytical transmission electron microscopy, optical microscopy and X-ray diffraction. Thermodynamic calculation, on the basis of the presently obtained phase equilibrium data, has verified that this phase separation arises from magnetic ordering, and that a similar phase separation appears in the HCP phase below 469°C which is transformed into a ferromagnetic ε_f phase and a paramagnetic ε_p phase. It is therefore concluded that magnetically induced phase separation must be responsible for the microscopic composition modulation of Cr in the CoCr thin film magnetic recording media.     

Magnetic polarons,clusters, and their effect on the electric properties of weakly doped lanthanum manganites

The resistivity, magnetoresistance, thermopower, and magnetic susceptibility of La_1?xA_xMnO₃(A≡Ca,Sr;x=0.07–0.1) single crystals are investigated in the temperature range from 77 to 400 K. Sharp changes in the properties (the resistivity activation energy ΔEρ, its temperature coefficient γ, the thermopower activation energy ΔE _S , the magnetoresistance, and the appearance of spontaneous magnetization) of these crystals occur near a temperature of 275±25 K, which is approximately twice as high as their Curie point T_C and approximately half of the structural transition temperature. The results are explained by the phase separation: the formation of ferromagnetic clusters. The phase separation occurs through the coalescence of small-radius unsaturated magnetic polarons, in which only two or three magnetic moments of Mn are polarized, into a large-radius ferromagnetic polaron (a cluster about 10–12 Å in size) with several charge carriers. As a result, the short-range order occurs in the cluster at a temperature of about 275 K, which is close to T _C of conducting doped manganites. The results of the experimental studies of the resistivity and the magnetoresistance as functions of temperature and magnetic field and the estimates agree well with the cluster model.     

Signatures of phase separation across the disorder broadened first order ferromagnetic to antiferromagnetic transition in doped-CeFe2 alloys

Experimental evidences of phase coexistence and metastability are presented over a substantial field-temperature regime across the ferromagnetic to antiferromagnetic transition in CeFe₂ based alloys. The idea of phase separation due to the influence of quenched disorder on a first order transition is used to interpret the results. Certain thermomagnetic history effects raise additional questions concerning the kinetics of such phase transitions.     

Phase separation induced by oxygen isotope substitution in manganites of the Sm<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>MnO<Subscript>3</Subscript> system

The effect of ¹⁶O → ¹⁸O isotope substitution on the electrical and magnetic properties of the manganite system Sm_1?xSr_xMnO₃ has been studied. It is shown that oxygen isotope substitution brings about a substantial change in the phase diagram in the intermediate region 0.4<x<0.6 between the ferromagnetic metal and antiferromagnetic insulator regions and induces phase separation and transformation of the ground metallic into insulating state for x=0.475 and 0.5. The specific features of the metal-insulator transitions in the Sm-Sr system and the nature of the low-temperature phase are discussed.     

Imaging of percolative conduction paths and their breakdown in phase-separated (La1-yPry)0.7Ca0.3MnO3 with y=0.7

Local magnetization and current distribution in (La(1-y)Pr(y))0.7Ca0.3MnO3 (y=0.7) crystals are studied by a magneto-optical (MO) imaging technique. MO images below 120 K visualize inhomogeneous magnetization and conduction paths that manifest the percolative conduction originated from the mesoscopic phase separation into ferromagnetic metals and antiferromagnetic insulators. Application of large amounts of current switches the current distribution from inhomogeneous to homogeneous concomitantly with a steep increase in resistivity. These phenomena are discussed in view of current induced collapse of the phase separation through a local heating.     

Magnetic and electrical properties of the ZnGeAs<Subscript>2</Subscript>: Mn chalcopyrite

Doping of the ZnGeAs₂ semiconductor with manganese has produced compositions with spontaneous magnetization and high Curie temperatures of up to 367 K for the composition 3.5 wt% Mn. Their magnetic properties are characteristic of spin glasses at temperatures T < T _S and magnetic fields H < 11 kOe. In stronger fields, the spin glass state transforms into a phase with a spontaneous magnetization 4–5 times weaker than that to be expected under ferromagnetic ordering of all Mn ions. This is obviously a singly-connected ferromagnetic phase containing regions with frustrated bonds. The frustrated regions and the spin glass phase have inclusions of noninteracting ferromagnetic clusters, because these regions and the spin glass phase at low temperatures exhibit a strong increase in the magnetization M, with the dependence M(T) being described by the Langevin function. Measurements of the electrical resistivity ρ and the Hall effect have revealed that, for T < 30 K, the resistivity ρ of compositions with 1.5 and 3.5 wt % Mn is higher that at 30 K, which makes superexchange dominant and gives rise to the onset of the spin glass state. The nonuniform distribution of Mn ions in the spin glass phase accounts for the existence of isolated ferromagnetic clusters, their ferromagnetism being generated by carrier-mediated exchange. As the temperature increases still more, the increase in the mobility occurs faster than the decrease in the concentration, thus promoting an enhancement of the carrier-mediated exchange and growth of the ferromagnetic clusters in size, which at T = T _S come in contact. This signifies a transition from a multiply-to a singly-connected ferromagnetic phase, which contains microregions with frustrated bonds.     

Superconductivity and ferromagnetism in the Hubbard model

In the framework of the Hubbard model at U → ∞, the phase transitions related to the lower underfilled subband are studied. The self-consistent equations determining the superconducting and ferromagnetic order parameters are derived in the one-loop approximation. The phase boundaries within which ferromagnetic and superconducting phenomena can coexist are discussed.     

Phase separation, percolation and giant isotope effect in manganites

Phase separation and a tendency to form inhomogeneous structures seems to be a generic property of systems with strongly correlated electrons. After shortly summarising the existing theoretical results in this direction, I concentrate on the phenomena in doped manganites. I discuss general theoretical results on the phase separation at small doping and close to the doping x=0.5. The “global” phase diagram in this region is constructed. These general results are illustrated on the example of the particular system with rich and complicated properties — (LaPr)_1−xCa_xMnO₃, in which there exist a ferromagnetic metallic (FM) phase and a charge ordered (CO) insulating one. The experimental situation in this system is discussed and the interpretation is given in the framework of the model with competition of FM and CO, and the indications of phase separation and percolative nature of this system are given. The giant isotope effect observed in this situation is shortly discussed.     

Two-phase paramagnetic-ferromagnetic state of La<Subscript>0.7</Subscript>Pb<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single-crystal lanthanum manganite

Two phases, paramagnetic and ferromagnetic, were shown by the magnetic resonance method to coexist below the temperature T _C in La_0.7Pb_0.3MnO₃ single crystals exhibiting colossal magnetoresistance. The magnetic resonance spectra were studied in the frequency range 10–78 GHz. The specific features in the behavior of the spectral parameters were observed to be the strongest at the temperatures corresponding to the maximum magnetoresistance in the crystals. The concentration ratios of the paramagnetic and ferromagnetic phases in the samples were found to be sensitive to variations in temperature and external magnetic field. This behavior suggests realization of the electronic phase separation mechanism in the system under study.     

Phase separation of the spin system in the La<Subscript>0.93</Subscript>Sr<Subscript>0.07</Subscript>MnO<Subscript>3</Subscript> crystal

The magnetic state of the manganite La_0.93Sr_0.07MnO₃ in the range 4.2–290 K was studied using elastic neutron scattering. The magnetic state of this compound was found to occupy a particular place in the La_1?xSr_xMnO₃ solid-solution system, in which the antiferromagnetic type of order (LaMnO₃, T_N=139.5 K) switches to ferromagnetic ordering (La_0.9Sr_0.1MnO₃, T_C=152 K) with increasing x. In the transition state, this compound contains large-scale spin configurations of two types. A fractional crystal volume of about 10% is occupied by regions of the ferromagnetic phase with an average linear size of 200 Å, while the remainder of the crystal is a phase with a nonuniform canted magnetic structure. Arguments are presented for the phase separation of the La_0.93Sr_0.07MnO₃ spin system being accounted for by Mn⁴⁺ ion ordering.     

Phase coherence of the electron and hole in a ferromagnetic film in proximity with a superconductor

The scattering matrix approach between the clean and dirty limits is developed for the study of tunneling spectra in a ferromagnetic film in proximity to a superconductor. The minigap and the damped oscillation from ``0" to ``π" state in tunneling conductance are attributed to the phase coherence of the electrons and the corresponding Andreev-reflected holes in the ferromagnetic film. The calculated results provide a reasonable explanation for the behavior observed in recent experiments.     

Magnetostriction domain structure in a periodic system of magnetoelastic and elastic nonmagnetic layers

The spectrum of magnetoelastic waves in a periodic structure of alternating ferromagnetic and nonmagnetic layers was studied. In the case of ferromagnetic layers with easy magnetization axes parallel to the layer surfaces, an orientational phase transition induced by an external tangential magnetic field H_e was considered. The formation of an inhomogeneous phase with a spatially modulated order parameter, which is caused by the magnetization being coupled through magnetostriction to lattice strains near the interfaces separating the magnetoelastic from elastic media, is predicted. It is shown that at a certain critical field in excess of the orientational phase transition field in the system without magnetostriction, a magnetoelastic wave propagating in a direction parallel to the in-plane magnetization vector M becomes unstable at finite values of the wave vector and condenses into a magnetostriction domain structure. A phase diagram in the (L, T, H_e) coordinates is constructed, and the regions of existence of thermodynamically equilibrium collinear, canted, and domain phases are established (L and T are the thicknesses of the ferromagnetic and nonmagnetic layers, respectively).     

Interaction of separated ferromagnetic domains in a hole-doped manganite achieved by a magnetic field

We report the change in the magnetic microstructure with the application of a magnetic field to a hole-doped manganite La0.81Sr0.19MnO3 in the mixed-phase state, in which ferromagnetic and paramagnetic phases coexist. In situ observations by electron holography have revealed that the applied magnetic field generates a "channel" of the magnetic flux in the paramagnetic phase region, thereby connecting the separated ferromagnetic domains. The magnetic flux density of this channel is estimated at 0.33 T, which is comparable with that of the ferromagnetic domains. The connection of the separated ferromagnetic domains appears to promote the conduction in the mixed-phase state as predicted for many manganites exhibiting the magnetoresistance effect.