Magnetic-field-controlled phase separation in manganites: Electron magnetic resonance study

The electron magnetic resonance spectra of Sm_1?x Sr _x MnO₃ (x = 0.30, 0.40, 0.45) manganites have been studied. At temperatures that are higher than the Curie point by several tens of kelvins, samples with x = 0.40 and 0.45 exhibit a ferromagnetic resonance (FMR) spectrum imposed on their usual EPR spectrum. The FMR spectrum appears as the applied magnetic field H exceeds a certain critical field H _c , which decreases upon cooling and becomes zero at T = T _C . These results agree with the magnetic-measurement data and indicate the magnetic-field-induced nucleation and growth of ferromagnetic domains in a paramagnetic matrix. In the initial growth stage, the volume of the ferromagnetic domains is proportional to (H ? H _c )^β, where β = 4.0 ± 0.3, and it changes in phase with magnetic field modulation up to a frequency of 100 kHz. In the same field and temperature ranges, hysteretic phenomena and narrow unstable spectral lines are detected; these lines indicate a dynamic character of the phase separation. The results obtained are interpreted in terms of the competition of different types of magnetic and charge ordering.     

Non-linear spin fluctuations and phase transitions in itinerant electron magnets: CMR manganites

We present a review analyzing the effects of coupling of transverse magnons with longitudinal spin fluctuations in isotropic itinerant ferro- and antiferromagnets. It is shown that this coupling essentially changes the spectrum of longitudinal fluctuations. At low-temperatures their spectrum is dominated by the linear Landau relaxation, is purely quasielastic and described by a broad central peak of a paramagnon type. On approaching the critical temperature non-linear magnetic relaxation due to mode–mode couplings can dominate and lead to a rapid increase of the central peak and to a new mechanism of magnetic phase transitions governed by non-linear spin fluctuations. The formalism is applied to the CMR manganites where the observed quasielastic fluctuations can be viewed as non-linear spin-lattice fluctuations strongly affected by magnons.     

钙钛矿锰氧化物中的极化子研究

钙钛矿锰氧化物(以下简称锰氧化物,如La_1-xSr_xMnO₃等,x为掺杂浓度)因其优异的电、磁性质受到人们广泛的关注,但是对于其材料内部载流子性质的认识至今仍没有统一定论.本文基于锰氧化物内Mn—O链的特点,建立一维紧束缚模型,对锰氧化物载流子的性质展开研究.发现在掺杂浓度x=0.5时,系统处于铁磁态,自旋能级完全劈裂,价带和导带之间存在带隙,所有电子态呈现扩展行为.进一步掺杂,将出现局域电子态,同时伴随着晶格的局域畸变,形成所谓的极化子.伴随着极化子的出现,带隙中出现极化子深能级.极化子携带的电荷量越多,形成的晶格缺陷越深,局域能级也越深.当极化子的电荷量继续增加时,极化子解离,载流子倾向于形成能量更低的正反"孤子"对.     

Room-temperature phase separation in weakly doped lanthanum manganites

The properties of single crystals of weakly doped lanthanum manganites La_1?xA_xMnO₃ (A = Ca, Ce, Sr; x = 0, 0.07?0.1) have been studied in the temperature range from 77 to 400 K. It is established that these lanthanum manganites exhibit (in addition to the well-known characteristic features observed in the region of the temperature of magnetic ordering) changes in the electrical and magnetic properties in the region of room temperature (T ≈ 270–300 K), which is about two times the Curie temperature (T ≈ 120–140 K) and is far from the temperature of structural transitions in the samples studied. The results are explained in terms of phase separation related to the formation of magnetic clusters in the nonconducting medium. The phase separation is caused by a gain in the exchange energy and by the development of elastic stresses in the crystal lattice and proceeds via combination of small-radius magnetic polarons into a large-size magnetic cluster containing several charge carriers. The short-range order in the cluster appears and the phase separation begins at a temperature T_ps, which is close to T_C ≈ 300 K, typical of doped conducting manganites. The results of magnetic measurements show that, as the temperature decreases from 300 to 190 K, the size of superparamagnetic droplets increases from about 8 to 15 Å.     

Curie temperature and frustrated phase separation in manganites

There is an increasing experimental evidence that phase separation between insulating and metallic phase plays an important role in the physics of manganites. On general grounds one can argue that the electronic density in the metallic and insulating region will be generally different. This implies that phase separation of the ordinary “Maxwell construction” type is frustrated by the long-range Coulomb interaction. We present a generalization of Maxwell construction to this situation. The system is assumed to separate in islands of one phases hosted by the other. The size of the islands is determined minimizing a free energy that takes into account both surface energy and Coulomb effects. We discuss the peculiarities of this kind of phase separation and the consequences for the manganites. In particular, we present an explanation for the non-monotonous behavior of the Curie temperature as a function of doping.     

Inversion of the electron spin resonance signal from a P1 center in a synthetic diamond crystal

We have studied the electron spin resonance (ESR) of a 0.59 carat synthetic diamond single crystal at room temperature. The crystal was grown on a “split-sphere” apparatus in the Fe-Ni-C system by the temperature gradient method. After high-temperature/high-pressure treatment of the diamond, it was observed that as the microwave power supplied to the sample increased from 70 μW to 70 mW in an H₁₀₂ cavity, the ESR signal from the P1 center (a nitrogen atom substituting for carbon at a lattice point of the diamond crystal: C-form nitrogen) is inverted. In the original diamond (before high-temperature/high-pressure treatment), no inversion of the ESR signal was observed. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 9–12, January–February, 2006.     

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.     

Magnetic phase diagrams of manganites in the electron doping region

The paper reviews the physical properties of the R_1?xA_xMnO₃ manganites (R=La, Pr, Nd, Sm, etc., A=Ca, Sr, Ba) in the region of their electron doping where the divalent atom concentration x in the compound lies in the interval 0.5<x<1.0. Experimental magnetic phase diagrams of the most well-studied compounds and the results of theoretical calculations of these diagrams made in the tight-binding approximation within the degenerate double-exchange model for T=0 are presented. The experimental section of the review deals primarily with neutron diffraction studies of the magnetic and crystal structures of the manganites, and the theoretical part, with the relation between their magnetic and orbital structures. The review describes, in considerable detail, the method of calculation of the energy spectrum ?(k) and of the total carrier energy for all possible magnetic and orbital configurations of the system corresponding to the translation symmetry of the lattice. The theoretical analysis is carried out separately for two models of the crystal structure, with two and four manganese atoms in the unit cell. All equilibrium magnetic and orbital configurations of the four-sublattice manganite model were determined by minimizing the total energy of the system with respect to the directions of the local manganese magnetic moments and orbital states of the e_g electrons. It is shown that, by using the effective Hamiltonian of the degenerate double-exchange model for the e_g electrons, which takes into account the e_g level splitting, and the Heisenberg Hamiltonian of the localized t_2g electrons, one can describe the diversity of the magnetic phases, the sequence of their alternation with increasing x, and the correlation between the spin and orbital degrees of freedom, which are observed in most electron-doped manganites.     

Conduction electron spin resonance in magnesium

We report on the observation of conduction electron spin resonance (CESR) in samples of specially purified magnesium. Measurements were made over the temperature range 4.2–300 K and at two frequencies, 9.27 and 21 GHz. It is found that in contrast with previous data, there is no upturn in the low temperature linewidth and little variation between samples.     

Mechanisms of flexural-strain-modulated electron spin resonance

Strain-modulated electron spin resonance (SMESR) spectra of V²⁺ and Mn²⁺ centers in MgO are reported and the results are compared with those obtained for MgO : Cr³⁺. The SMESR line intensities show a characteristic angular dependence, proving that modulation of the off-diagonal elements of the $\text{G}$-tensor is the predominant mechanism when a flexural mode of vibration is applied.For V²⁺ a small additional contribution due to modulation of the diagonal elements of $\text{G}$ is present.     

Acoustic detection of electron spin resonance

The ESR-signal of DPPH was recorded by detecting the modulation of the absorbed microwave power with a gas-coupled microphone. This photo-acoustic detection scheme is compared with conventional ESR-detection. Applications of the acoustical detection method to other modulation spectroscopic techniques, particularly NMR, are discussed.     

Longitudinally detected electron spin resonance: Recent developments

Multiple-quantum spectroscopies are reviewed in the frame of electron paramagnetic resonance. Some properties of different nonlinear techniques are discussed for both transverse and longitudinal detection. The connections of effects recently presented with the procedure of longitudinal detection of electron paramagnetic resonance (LODESR) in presence of double transverse irradiation are stressed. Peculiarities of LODESR spectroscopy and its capabilities in facing problems related to relaxations in presence of very slow dynamic processes are evidentiated. Recent results show the vitality of the LODESR technique, that in the future could be applied to new fields, owing to experimental updating.     

NMR probe of phase segregation in electron doped mixed valence manganites

A 55Mn and 139La NMR study of La0.35Ca0.65MnO3 is reported. The zero field 55Mn spectra consist of two lines centered at 290 and 375 MHz. Their behavior under an applied magnetic field makes it possible to attribute them to regions of antiferromagnetically and ferromagnetically coupled Mn spins, respectively. This gives evidence for the existence of electronic phase segregation of microscopic ferromagnetic regions of double exchange coupled Mn spins over a charge ordered antiferromagnetic background. The behavior of these ferromagnetic regions in the applied magnetic field is related to the magnetoresistive properties of the manganites.     

Study of doping effect,phase separation and heterojunction in CMR manganites

Mixed-valence manganites have attracted considerable research focus in recent years not only because of the potential application of colossal magnetoresistance(CMR) in magnetic devices,but also because of many intriguing physical properties observed in these materials.Doping elements at A-site can alter the filling of 3d Mn band and the tolerance factor.Therefore the hole-and electron-doped CMR manganites exhibit a rich phase diagram.In addition,more theoretical and experimental results suggest that phase separation is a critical factor for the understanding of CMR phenomena.Recently,there is an increasing interest in the fabrication and investigation on manganite-based heterojunction,which demonstrated excellent rectifying property,large MR,and photovoltaic effect.     

On the mechanism of hydrogen atom and electron addition to phosphorus: An electron spin resonance study

A partir des spectres d'absorption du méthane naturel (1,1 pour cent de ¹³CH₄) enregistrés sur un spectromètre à grilles avec un parcours de 10 m, une pression de 10 torr et une résolution de 0,060 cm^-1, nous avons pu mesurer, avec une précision de pointé allant jusqu'à 0,004 cm^-1, 130 raies rotationnelles (correspondant à 150 transitions) de la bande ν₄ de la variété isotopique ¹³CH₄. Ces données ont permis de déterminer pour la première fois un jeu de treize constantes contribuant de façon significative à l'énergie jusqu'au quatrième ordre dans l'état vibrationnel v ₄=1. Pour les 150 transitions introduites dans le calcul, l'écart quadratique moyen est 0,020 cm^-1 et le plus grand écart n'excède pas 0,057 cm^-1. Les nombres d'ondes calculés sont donnés pour 233 transitions de la bande ν₄ de ¹³CH₄ allant de P 12 à R 13.     

Optical detection of electron spin resonance in CdTe

We report the optical detection of electron spin resonance in p-type CdTe at 1.7 K in optical pumping conditions. The Overhauser shift of the electronic resonance, of the order of 45 G, is related to the sign of the electron g-factor g¹. We measure $\text{g}{}^1\text{= -1.59±0.02}$. Using this g¹ value and the previous results on the Knight shift, we deduce the value of the electron wavefunction on Cd in CdTe, which is consistent with the value in CdS.     

Non-linear transport in tetrathiofulvalene-tetracyanoquinodimethane: An electron spin resonance study

We describe an ESR study of pure and irradiated TTF-TCNQ in the low temperature range (1.2 K–4.2 K) where nonlinear transport has been observed. We find no evidence of an activated term in the susceptibility; our results yield an upper limit of about 10 ppm for nonmagnetic impurities with a spin-$\text{1}\text{2}$ excited state. Since in pure samples, the activated susceptibility term is not present whereas the transport nonlinearity is clearly seen, we conclude that the latter cannot be attributed to hot electron effects. The susceptibility and dc bias studies are consistent with a nonmagnetic soliton picture. We have monitored the spin resonance in the presence of a dc bias current. Using the Curie law as an internal thermometer we are able to set an upper limit on sample heating of 0.1 K at the highest dc levels used.