Magnetic Phase Separation Research Articles

Magnetic phase separation and cluster-spin-glass behavior in LaMn1−xFexO3+y

The crystal structure and some magnetic properties were investigated for polycrystalline samples of LaMn1−xFexO3+y, prepared by solid-state reaction in air. All samples show the ORT-2 orthorhombic structure that suppresses the Jahn–Teller distortion and favors a ferromagnetic (FM) superexchange interaction between Mn3+–O–Mn3+. An evolution from a fairly strong FM phase, for x=0.0, to an antiferromagnetic (AFM) phase, for x=1.0, was observed. For intermediate Fe contents, a magnetic mixed-phase state occurs, with a gradual decrease (increase) in the FM (AFM) phase as x increases. A clear cluster-spin-glass (CG) behavior is observed in our samples for x≤0.1, where FM coupling dominates. In this case, the Vogel–Fulcher law describes very well the CG dynamics, giving reasonable values for the fitted parameters.

Incommensurate magnetic order and phase separation in the two-dimensional Hubbard model with nearest- and next-nearest-neighbor hopping

We consider the ground-state magnetic phase diagram of the two-dimensional Hubbard model with nearest- and next-nearest-neighbor hopping in terms of electronic density and interaction strength. We treat commensurate ferromagnetic and antiferromagnetic as well as incommensurate (spiral) magnetic phases. The first-order magnetic transitions with changing chemical potential, resulting in a phase separation (PS) in terms of density, are found between ferromagnetic, antiferromagnetic, and spiral magnetic phases. We argue that PS has a dramatic influence on the phase diagram in the vicinity of half-filling. The results imply possible interpretation of the unusual behavior of magnetic properties of single-layer cuprates in terms of PS between collinear and spiral magnetic phases. The relevance of our results to the magnetic properties of the ruthenates is also discussed.

Effects of aqueous extracts of Irvingia gabonensis seeds on the hormonal parameters of male guinea pigs

Objective To investigate the effects of the aqueous extracts of Irvingia gabonensis (Irvingaceae) seeds on the hormonal parameters of male guinea pigs. Methods The hormonal effects of Irvingia gabonensis on hormonal parameters of male guinea pigs were investigated and compared with that of proviron using enzyme immuno assay method, which was done by reaction of antibody with serum testosterone and testosterone label, magnetic solid phase separation and colour development step. The phytochemical screening of Irvingia gabonensis seeds was also carried out using standard procedures. Results The aqueous extract of the Irvingia gabonensis seeds (50-400 mg/kg) caused a statistically significance increase (P<0.05 ANOVA) of testosterone in male guinea pigs, from (2.70±0.26) ng/mL to (3.10±0.42) ng/mL on the 7th day and to (3.30±0.48) ng/mL on the 28th day of the administration of the extracts. The highest increase was (3.30±0.48) ng/mL, being obtained after 28 days of treatment. These effects were similar to that of proviron, which was (2.80±0.28) ng/mL and (3.00±0.41) ng/mL on the 7th and 28th day of treatment respectively. The phytochemical screening of Irvingia gabonensis seeds revealed the presence of flavonoids, tannin, alkaloids, carbohydrate, volatile oils, terpenoids, saponins and cardiac glycosides. Conclusions This study supports the claims on the use of the seeds of Irvingia gabonensis by traditional medicine practitioners as a fertility agent. However, further studies need to be done to isolate and characterize the active principle(s) responsible for this activity in this plant.

Absence of magnetic phase separation in MnSi under pressure

We report muon spin spectroscopy data (muSR) obtained under hydrostatic pressure on a large single crystal of the itinerant helimagnet MnSi, and recorded down to 0.235 K and up to 15.1 kbar. Up to the critical pressure p_c = 14.9 (2) kbar, where the magnetic order is suppressed, the muSR data unambiguously demonstrate that the ground state of the system is magnetic with no indication of any phase separation.

Effect of Cr-doping on magnetic phase separation in Bi0.5Ca0.5MnO3

The effect of Cr doping on the magnetic phase separation in Bi0.5Ca0.5Mn(1-x)CrxO3 (x = 0 − 0.05) was explored by temperature dependent Electron Spin Resonance (ESR) studies. The volume fraction of ferromagnetic (FM) domains increase at the expense of antiferromagnetic (AFM) domains and the Charge Ordering (CO) is suppressed with the increase in Cr content. With increase in Cr content the CO transition is smeared and it almost disappears at x = 0.05. The CO temperature, TCO, decreases from 317K to 278K as x increases from 0 to 0.05. The Neel temperature TN, is ~160K for the x = 0 and 0.01 samples and it decreases to ~ 120 K for sample with x = 0.05. A strong competition between the FM and AFM correlations is observed as a function of composition and temperature in the temperature range TN < T < TCO.

Effect of anisotropy in temperature dynamics of magnetic phase separation in europium hexaboride

The ESR measurements of the EuB6 single crystal samples were executed on frequency 9.25 GHz in TE102 rectangular cavity in the temperature range from 15 to 300 K. We used samples of identical form and size, but different crystallografic orientation to estimate their magnetization. At T = 30 – 40K was observed the magnetic phase separation, which, most likely, is accompanied also by charging separation. The anisotropy magnetization of more intensive magnetic phase (with anti-Kondo interaction) along the different crystallographic directions was found above a temperature of the ferromagnetic transition. We conclude that this result connect with existence the ferronic states and charging separation in the EuB6 single crystal. Estimations of angular distribution of the magnetic moment of ferrons in EuB6 are made.

The effects of aqueous extracts of Tetracarpidium conophorum seeds on the hormonal parameters of male guinea pigs

Objective To investigate the effects of the aqueous extracts of the seeds of Tetracarpidium conophorum and the effect of Proviron (12.5 mg/kg) (as standard) on the hormonal parameters of male guinea pigs, compare the effects of the seeds of Tetracarpidium conophorumand Proviron, and screen the phytochemical constituents of the seeds of Tetracarpidium conophorum. Methods The hormonal effects of the Tetracarpidium conophorum and Proviron were tested by hormonal assay, using enzyme immuno assay method. This was done by reaction of antibody with serum testosterone and testosterone label, magnetic solid phase separation and colour development step. Phytochemical screening was done using standard procedures. Results The aqueous extract of the Tetracarpidium conophorum seeds (100–400 mg/kg) caused a statistically significant increase ( P < 0.05, ANOVA) in the level of testosterone of male guinea pigs, from (2.60 ± 0.06) ng/mL to (3.40 ± 0.05) ng/mL, (3.00 ± 0.60) ng/mL and (3.30 ± 0.45) ng/mL on the 7th, 14th and 21st day of the administration of the extracts, respectively. The highest increase was obtained after the 7th day of treatment [(3.40 ± 0.05) ng/mL]. These effects were very comparable to the effects of Proviron on the testosterone of male guinea pigs, which were obtained to be (2.80 ± 0.01) ng/mL, (2.90 ± 0.16) ng/mL and (3.10 ± 0.30) ng/mL on the 7th, 14th and 21st day, respectively. These effects were dose-and time-dependent. The optimum effect on testosterone level under dose-dependent study [(4.70 ± 0.45) ng/mL] was obtained at 300 mg/kg of Tetracarpidium conophorum after 7 days treatment. Finally, the phytochemical screening of the seeds of Tetracarpidium conophorum revealed the presence of flavonoids, tannin, alkaloids, carbohydrate, volatile oils, terpenoids, saponins and cardiac glycosides. Conclusion This study supports the claims on the use of the seeds of this plant by traditional medicine practitioners as a fertility agent. However, further studies need to be done to isolate and characterize the active principle(s) responsible for this activity in this plant.

Percolative Nano-Sized Phase Separation in Mix-Valent Manganites

Combining transport and magnetic properties measurements, we find that ferromagnetic clusters are embedded in an antiferromagnetic matrix in the mix-valent R1−xCaxMnO3 (R denotes rare-earth element, x = 0.88−1) manganites at low temperature, namely, magnetic phase separation phenomenon. In contrast to the hole-doped manganites, ferromagnetic clusters in the electron-doped manganites are likely ellipsoidal-type rather than stripe-type, evidenced by modeling the low temperature electrical transport behavior. Percolative conduction transport between the ferromagnetic clusters takes place in the case of phase separation state. As the R ion content exceeds a certain level, colossal magnetoresistance is observed in a narrow range in this system. The rotation of ferromagnetic domains under magnetic field may underlie the magnetoresistance effect in the phase separation state of these electron-doped manganite systems.

The incongruous observation of magnetic phase separation in La0.85Sr0.15CoO3 spin glass system

Phase separation (PS) in hole-doped cobaltites (La1−xSrxCoxO3) is drawing renewed interest recently. In particular, the magnetic behavior of La0.85Sr0.15CoO3 has been subjected to a controversial debate for the past several years; while some groups show evidence for magnetic PS, others show spin glass (SG) behavior. Here, an attempt is made to resolve the controversy related to “PS versus SG” behavior in this compound. We present the results of a comprehensive investigation of the dc magnetization, ac susceptibility, and the magnetotransport properties of La0.85Sr0.15CoO3 samples. We contemplate that the magnetic PS in La0.85Sr0.15CoO3 is neither intrinsic nor inherent, but it is a consequence of the preparation conditions. It is realized that a low temperature annealed (LTA) sample shows PS whereas the high temperature annealed (HTA) sample shows SG behavior. The Brillouin-like behavior of field cooled dc magnetization and apparently no frequency dependent peak shift in ac susceptibility for the LTA sample characterize it to be of ferromagneticlike whereas a kink in field cooled dc magnetization and a considerable amount (∼3 K) of frequency dependent peak shift in the ac susceptibility for the HTA sample characterize it to be of SG state. The magnetotransport properties show that the HTA sample is more semiconducting as compared to the LTA sample. This is interpreted in terms of the presence of isolated as well as coalescing metallic ferromagnetic clusters in the case of LTA sample. The magnetoresistance (MR) at 10 K for the HTA sample exhibits a huge value (∼65%) as compared to the LTA sample, and it monotonically decreases with the rise in temperature. Such a high value of MR in the case of HTA sample is strongly believed to be due to the spin dependent part of random potential distribution. Further, the slow decay of remnant magnetization with progress of time and the existence of hysteresis at higher temperatures (up to 200 K) in the case of LTA sample as compared to the HTA sample clearly unveil different magnetic states associated with them.

Enhanced magnetic refrigeration capacity in phase separated manganites

Multiple phase transitions and magnetic phase coexistence lead to a negative magnetocaloric effect in a wide temperature range of ∼10–250 K in La0.215Pr0.41Ca0.375MnO3. A large fraction of the magnetocaloric effect originates from the low temperature phase separated state, which is composed of coexisting, magnetic field dependent charge ordered, and ferromagnetic regions. While the maximum isothermal entropy change is modest, the persistence of the field-dependent phase separated state over a ∼240 K temperature span yields a refrigeration capacity of ∼3.2 J/cm3. Materials with magnetic field dependent phase separation can therefore be used to improve regenerative magnetic refrigerators.

Structural tuning of magnetism in the orthorhombic perovskiteNd1−xYxMnO3: Evidence for magnetic phase separation

We report on the magnetic and structural properties of the perovskite solid solution ${\text{Nd}}_{1\ensuremath{-}x}{\text{Y}}_{x}{\text{MnO}}_{3}$, using magnetization and neutron powder-diffraction measurements. The orthorhombic perovskite structure $(Pbnm)$ is obtained over the composition range of $0\ensuremath{\le}x\ensuremath{\le}0.6$. The Mn-O-Mn bond angles and the N\'eel temperature ${T}_{N}$ decrease with increasing $x$ and track closely similar behavior established for the general series of manganites $R{\text{MnO}}_{3}$ ($R=\text{rare}$ earth). For lower doped samples, the antiferromagnetic $A$-type Mn order is stable, while for higher doping the diffraction data can be modeled using an incommensurate spin-density-wave (SDW) model. In both cases, we find that Mn spins are aligned along the $b$ axis. For intermediate compositions $0.4\ensuremath{\le}xl0.55$, we find a coexistence of $A$-type and SDW magnetic order. On symmetry grounds, we argue that these two magnetic phases cannot occupy the same volume but rather this coexistence region is indicative of a notable magnetic phase separation.

Electron spin resonance inEuB6

Cavity measurements of a high frequency (60 GHz) electron spin resonance (ESR) have been carried our for a single crystal of ${\text{EuB}}_{6}$ at temperatures 4.2--50 K in magnetic field up to 7 T, which has been aligned along [001] crystallographic direction. It is found that in the case of homogeneous magnetic field in the sample the ESR spectrum of ${\text{EuB}}_{6}$ is formed by a single line at all temperatures including the ferromagnetic ordering region, whereas the gradient of magnetic field in the samples induces double peak ESR structure at low temperatures. For the quantitative description of the ESR line shape we suggested an analytical approach applicable for the cavity measurements of a metal with an arbitrary value of magnetic permeability including strongly magnetic case and obtained full set of the ESR parameters, namely oscillating magnetization ${M}_{0}$, $g$ factor, and linewidth. Our analysis has explained the visible resonance line shift and revealed the coincidence of the oscillating magnetization defining the resonance amplitude with the static magnetization. The anomalous growth of the linewidth below Curie temperature ${T}_{C}\ensuremath{\approx}15\text{ }\text{K}$ is observed. We argue that ESR in ${\text{EuB}}_{6}$ is not considerably affected by either interaction with the spin polarons or the magnetic phase separation and reflects merely the oscillation of the ${\text{Eu}}^{2+}$ localized magnetic moments, which can be well understood within mean-field approximation.

Electronic transport and magnetic properties of the perovskites La 0.8Sr 0.2Co 1− xFe xO 3; x ≤ 0.3

Transport and magnetic properties of LaCoO 3-based compounds, doped with 20% Sr and 2.5, 5, 10, 15, 20 and 30% Fe, were investigated by means of magnetization, resistivity and magnetoresistance measurements as well as by 57Fe Mössbauer spectroscopy. While the temperature dependence of the dc and ac magnetic susceptibilities reveals the presence of magnetic phase separation accompanied by spin-glass and cluster-glass behavior, the electrical resistivity and magnetoresistance characteristics indicate that the mesoscopic structure of the present compounds is rather well described as consisting of ferromagnetic, metallic grains embedded in an insulating matrix. The effect of the partial Co → Fe substitution on the bulk magnetic and transport properties, as well as on the local state of Co and Fe ions is discussed.

Magnetic relaxation and phase separation in manganite Eu 0.55Sr 0.45MnO 3

We present a study of the magnetic relaxation behavior of the phase-separated manganite Eu 0.55Sr 0.45MnO 3. A local maximum of the magnetic viscosity S is observed around the magnetic freezing temperature and near the Curie temperature, respectively. The former is ascribed to relaxation of the proportion of coexisting ferromagnetic and non-ferromagnetic phases from a magnetic frozen matrix. A quantitative relation established between S and the initial normalized magnetization before relaxation, m , shows that S keeps constant at low m values, reaches a broad maximum around m ∼ 0.35 , then decreases rapidly, and finally remains at nearly zero for m > 0.7 . The latter local maximum of S , much smaller than the former in magnitude, is possibly associated with both the relaxation of the proportion of coexisting phases and the reorientation of magnetic moments in ferromagnetic domains. Thermoelectric and magnetic relaxation results further indicate the existence of a definite temperature T thermo between which and the metal-insulator transition T p magnetic inhomogeneity is significant. Based on these results, a temperature and field dependent scenario of phase separation in Eu 0.55Sr 0.45MnO 3 is proposed.

Enhanced grain surface effect on magnetic properties of nanometric La0.7Ca0.3MnO3 manganite: Evidence of surface spin freezing of manganite nanoparticles

We have investigated the effect of nanometric grain size on magnetic properties of La0.7Ca0.3MnO3 nanoparticles having average particle size (Φ) of ∼17 nm. Temperature dependence of field-cooled (FC) and zero-FC (ZFC) dc magnetization indicate the existence of two different types of relaxation processes: a relatively high temperature regime where there is a broad maximum of the ZFC curve at T=Tmax (&amp;gt;40 K) and another is a relatively low temperature regime that is characterized by a sharp maximum at T=TS (≈40 K). We believe that the broad maximum at Tmax is associated with the blocking of core particle moments, whereas the sharp maximum at TS is related to the freezing of surface spins. Waiting time (tw) dependence of ZFC relaxation measurements at T=50 K show weak dependence of relaxation rate [S(t)] on tw and dM/d ln(t) following a logarithmic variation in time. These features strongly support superparamagnetic (SPM) blocking of core particle moments at Tmax. At T=20 K, S(t) attains a maximum at tw=1000 s that establishes freezing phenomenon occurring at TS. The combination of topological disorder and magnetic phase separation may result in a magnetically disordered state at the grain surface. This, in turn, results in magnetically uncoupled assembly of nanoparticles that eventually exhibits SPM blocking. The glassy behavior at TS has been attributed to the competing magnetic interactions stabilizing a spin-glass-like frozen state at the surface region of the nanometric grains.

Magnetic phase separation in europium hexaboride and its relation to the Kondo interaction

Europium hexaboride single crystals have been investigated using the electron paramagnetic resonance at a frequency of 9.25 GHz in the temperature range 10–300 K. The magnetic phase separation of the spin system of europium ions Eu2+ is observed. The cause of the separation is the formation of the polarons of two types associated with the Kondo and anti-Kondo couplings of charge carriers of the valence and conduction bands, respectively, with the localized magnetic moments of Eu2+.

Signature of magnetic phase separation in the ground state ofPr1−xCaxMnO3

Neutron scattering has been used to investigate the evolution of long- and short-range charge-ordered (CO), ferromagnetic, and antiferromagnetic (AF) correlations in single crystals of Pr(1-x)Ca(x)MnO(3). The existence and population of spin clusters as reflected by short-range correlations are found to drastically depend on the doping and temperature. Concentrated spin clusters coexist with long-range canted AF order in a wide temperature range in the x=0.3 while clusters do not appear in the x=0.4 crystal. In contrast, both CO and AF order parameters in the x=0.35 crystal show a precipitous decrease below similar to 35 K where spin clusters form. These results provide direct evidence of magnetic phase separation and demonstrate a critical doping x(c) (close to x=0.35) that divides the inhomogeneous from homogeneous CO ground state.

Magnetic properties of electron doped Sm0.1Ca0.9−yBayMnO3 (y=0.02, 0.06) manganites: Pressure effects on competitive ferromagnetic and antiferromagnetic interactions

Magnetic properties of polycrystalline Sm0.1Ca0.9−yBayMnO3 (y=0.02, 0.06) samples have been investigated in the temperature range of 2–320 K, magnetic fields of up to 90 kOe, and under hydrostatic pressures of up to 12 kbar. It was found that the volume fraction of ferromagnetic (FM) phase at 5 K is about 27% for Sm0.1Ca0.88Ba0.02MnO3 and only ∼1% for Sm0.1Ca0.84Ba0.06MnO3. The compound Sm0.1Ca0.88Ba0.02MnO3 exhibits magnetic phase separation below TN≈TC≈99 K, which is consistent with a model of FM clusters embedded in antiferromagnetic (AFM) G-type matrix. For this sample, magnetization and ac susceptibility exhibit a cluster glasslike behavior below TC, while the resistivity exhibits pronounced thermal hysteresis around TC, indicating upon a plausible first order magnetic phase transition. For Sm0.1Ca0.84Ba0.06MnO3 two peaks observed in the temperature dependence of the ac susceptibility at low temperatures may be related to magnetic transitions, associated with the C- and G-AFM structures. Both, a significant hysteresis of magnetization observed at T&amp;lt;100 K and an increase in magnetoresistance at low temperatures are apparently an indicative of magnetic field induced structural transformation from monoclinic to orthorhombic phase. This transformation occurs concurrently with magnetic transition from C-type AFM to G-type AFM structure. It was found that an applied pressure enhances TC of Sm0.1Ca0.88Ba0.02MnO3 with a pressure coefficient dTC/dP≈0.6 K/kbar. The spontaneous FM moment of Sm0.1Ca0.88Ba0.02MnO3 decreases by 15% under pressure of about 12 kbar, while under the same pressure it only slightly increases for Sm0.1Ca0.84Ba0.06MnO3. For both samples, the paramagnetic Curie temperature decreases under pressure indicating upon an increase in AFM interactions. Stronger suppression of the FM phase for the sample with y=0.06, in comparison with that one for 0.02 sample, is attributed to the effect of the chemical disorder, playing a more significant role than the effect related to the changes in ⟨rA⟩.

Itinerant ferromagnetism to insulating spin glass in SrRu 1− xCu xO 3 (0 ≤ x ≤ 0.3)

The ferromagnetic metallic oxide, SrRuO 3 ( T C ∼ 165 K) undergoes structural, magnetic and metal–insulator transitions upon substitution of Cu at the Ru-site. For x = 0.2 in SrRu 1− x Cu x O 3, the structure becomes a tetragonal with the space group I4/ mcm and there is a signature of both ferromagnetic ( T C = 65 K) and antiferromagnetic ( T N = 32 K) ordering due to possible magnetic phase separation. The antiferromagnetism arises due to short range ordering of Cu- and Ru-moments. Jahn–Teller distortion of (Ru,Cu)–O 6 octahedra indicates that the copper ions are in 2+ oxidation state with t 2 g 6 e g 3 electronic configuration. For x ≥ 0.1, narrowing of Ru-4d bandwidth by the substitution of Cu ions results in semiconducting behavior. For x = 0.3, the ac and dc susceptibility measurements indicate a spin glass behavior. The origin of spin glass behavior has been attributed to competing ferromagnetic and antiferromagnetic interactions.

Epitaxial La0.5Sr0.5CoO3 thin films: Structure, magnetism, and transport

La 1 − x Sr x CoO 3 has received considerable attention in bulk form. This is due to interest in the fundamental magnetic properties (spin-state transitions and magnetic phase separation) as well as potential applications in ferroelectric memory and solid-oxide fuel cells. The structure and properties in thin film form are not well understood, and the influence of dimensional confinement on effects such as magnetic phase separation is unknown. Here, we report a comprehensive investigation of structure, magnetism, and transport in strained epitaxial La0.5Sr0.5CoO3 (001) films deposited on SrTiO3 (001) substrates by reactive dc magnetron sputtering. The crystalline quality, phase purity, strain state, oxygen stoichiometry, morphology, and magnetic and electronic properties of the epilayers are all probed and are found to be particularly sensitive to the total sputtering gas pressure and the ratio of reactive to inert gas (PO2/PAr). The various structure-property relationships are discussed in detail, particularly with respect to the degree of oxygenation and oxygen-induced resputtering. The films are strained and tetragonally distorted due to the 1.9% lattice mismatch with SrTiO3. Significant strain relaxation occurs at thicknesses around 200 Å, resulting in a crossover from two-dimensional-like to three-dimensional growth. Polarized neutron reflectometry was combined with x-ray reflectometry to obtain chemical and magnetic depth profiles, which are compared with cross-sectional scanning transmission electron microscopy. The results indicate a thin (∼10 Å) layer at the film/substrate interface with significantly different structural properties to the bulk of the film, as well as a strongly graded magnetic and chemical profile at the film surface due to the significant roughness. The Curie temperature was found to decrease very slowly as the thickness is reduced down to ∼50 Å, at which point a rapid decrease occurs, almost coincident with a sharp decrease in saturation magnetization. At this point, the temperature dependence of the resistivity shows a crossover from metallic to insulating, accompanied by dramatic changes in the magnetoresistance. The magnetoresistance has a negative contribution peaking around the Curie point (similar to that seen in bulk), a second negative contribution occurring at low temperature (only for the thinnest samples), as well as a large anisotropic magnetoresistance, which vanishes at the Curie point. Remarkably, the low temperature contribution in the thinnest x=0.5 films bears a striking resemblance to that seen in the insulating phase (x&amp;lt;0.17) in bulk, suggesting the formation of a nonmetallic phase at low thickness that is similar to the low doping bulk phase, i.e., magnetic phase separation near the interface with SrTiO3.