Short-range Ferromagnetic Clusters Research Articles

Magnetic phase diagram of Ge1–x–y(SnxMny)Te multiferroic semiconductors: Coexistence of ferromagnetic and cluster glass ordering

Noncentrosymmetric IV-VI semiconductors doped with magnetic ions offer exciting possibilities such as carrier mediated ferromagnetism, ferroelectric-controlled spin texture and integration of semiconducting features with ferromagnetic order. Also, coupling between more than one ferroic orders and giant Rashba spin splitting can be studied to develop materials for spintronic applications. In that context, we report the structural and magnetic results of polar α–GeTe doped with Sn and Mn from x = 0.185–0.841 and y = 0.02–0.086, respectively. The magnetic results of Ge1–x–y(SnxMny)Te (GSMT) crystals identify Mn-clustering effect with scaling parameter, R = 0.033 for x ≈ 0.2 and y = 0.06. The excessive Sn ions are assumed to drive the inception of short range ferromagnetic clusters. For the crystals revealing glassy magnetic behavior, the irreversibility temperature, Tirr shifts at high dc magnetic field which is well described by de Almeida-Thouless equation, δTirr ∝ HΦ/2 yielding Φ values of 1.55 and 1.7 that validates the formation of Mn clusters. The spin relaxation time, τ0 ∼ 10–9 s, activation energy, Ea/kB ∼ 5 TF where TF is freezing temperature and Vogel-Fulcher temperature, T0 ∼ TF also signify intracluster interactions. Also, the Mn-hole magnetic exchange constant, Jpd drops from 0.24 eV for the samples with smaller Sn content of x ≈ 0.2 to 0.16 eV for Sn rich alloy with x ≈ 0.8. Consequently, we present a magnetic phase diagram for GSMT bulk crystals as a function of Mn ions.

Percolative transport and metamagnetic transition in phase separated La0.55Ca0.45Mn1-xAlxO3-δ

The Al3+ doping effects on the ferromagnetic metallic state of La0.55Ca0.45MnO3 have been studied by preparing the La0.55Ca0.45Mn1-xAlxO3-δ (x = 0, 0.05, 0.1, 0.15) ceramics. Various measurements such as ac susceptibility, relaxation, magnetization and resistivity were performed to study the magnetic and magnetotransport properties. The magnetic disorder produced by the Al3+ dopants and the oxygen vacancies suppress the long range ferromagnetic ordering and induce a short range charge ordered antiferromagnetic clusters, leading to a phase separated behavior in the Al3+-doped samples. A Griffith-like behavior is observed due to the occurrence of short range ferromagnetic clusters in the paramagnetic phase. The 15% Al3+ doping sample exhibits a spin-glass state. The 10% Al3+ doping system is blocked in a metastable state upon cooling at zero field, and a sharp metamagnetic transition is induced by applying a high magnetic field, in which the antiferromagnetic phase is transformed into ferromagnetic phase and the ferromagnetic clusters grow in size. The ferromagnetic domains grows rapidly when applying a high magnetic field. Once the conducting ferromagnetic domains are interconnected throughout the sample, a percolative insulating-to-metallic state transition would occur, accompanied by a sharp drop in resistivity.

Evidence of ferromagnetic clusters in magnetic Weyl semimetal Co3Sn2S2

Cobalt based sulfides of compositional formula Co3A2S2 (A = Sn and In) are endowed with frustrated kagome lattice structure and a plethora of novel phenomena due to the topological band structure. We report on the detailed exploration of magnetic properties of single crystals of ferromagnetic Weyl semimetal Co3Sn2S2. A non-linear behaviour observed in the inverse susceptibility above the critical temperature TC in the paramagnetic state corroborates to the presence of short-range ferromagnetic clusters above TC in Co3Sn2S2. The upward deviation indicates the antiferromagnetic interactions between the nearest neighbouring magnetic clusters. The slow spin dynamics behaviour above TC in isothermal remanent magnetization provide another evidence of ferromagnetic clusters. The magnetic hysteresis loops represent the magnetization reversal which, in turn, also confirm the short range magnetic correlations. Further, non-linear isothermal magnetization curves and zero spontaneous magnetization derived from corresponding Arrott plots above TC prove the short range ferromagnetic clusters in Co3Sn2S2. Our experimental results emphasize an intuitive understanding of the complex nature of magnetism present in Co-based shandite systems.

Effects of Bi3+ doping on the charge ordering and high-field phase diagram of La0.5-xBixCa0.5MnO3

La0.5-xBixCa0.5MnO3 (x = 0, 0.05, 0.1 and 0.2) compounds have been prepared to study the effects of Bi3+ substitution on the structure, magnetic and magnetotransport properties. Experimental results show that Bi3+ dopants have induced a distinct lattice distortion and phase separated state in which the long range ferromagnetic order turns into short range ferromagnetic clusters, and the charge ordering/antiferromagnetism and ferromagnetic clusters coexist in the system. Bi3+ doping induced disorders lead to the presence of a Griffith-like phase. The hybridization between Bi3+-6s orbitals and O2–-2p orbitals produces a localization of O2–-2p electrons and lattice distortion, which plays an important role in favoring charge ordering state and multiple magnetic ordering. The charge ordering antiferromagnetic collapses into ferromagnetic phase through a metamagnetic transition with applying a sufficiently high magnetic field. A T-HC phase diagram was determined based on the magnetization and electrical transport measurements, in which the phase boundary is figured out.

Observation of exchange-bias in the inverse Heusler alloy Mn1.5Ru1.5Si

Recently, Mn-Ru-Ga based inverse Heusler alloys have been found to be potential candidates for half-metallic fully compensated ferrimagnetic materials. In this work, we have studied another Heusler based alloy, Mn1:5Ru1:5Si, intending to design a compensated ferrimagnet. The alloy Mn1:5Ru1:5Si crystallizes in an inverse Heusler (XA) structure with a small amount of anti-site disorder. Our magnetic study indicates that the alloy orders magnetically below about TN = 50 K, which is characterized by a sharp drop in magnetization. Nevertheless, the isothermal magnetization curve shows a tendency of saturation and finite coercivity at 5 K, which rules out a pure antiferromagnetic state. Interestingly, Mn1:5Ru1:5Si shows vanishingly small magnetic moment, which also rules out possibility of a pure antiferromagnetic state. The alloy shows a significant amount of exchange-bias at low temperature. The alloy fails to obey a Curie-Weiss law between 300 and 50 K, indicating short-range correlations prior to the long-range order at TN. The exchange bias possible arises from the interfacial coupling of the short range ferromagnetic clusters with the long range antiferromagnetic state. The observation of exchange bias for such a system along with a low net magnetization value, has immense potential for magneto-electric applications and switching devices. The resistivity versus temperature data of the sample indicates a metallic character, albeit with a rather large value resistivity. The resistivity data show an upturn below 25 K that follows a logarithmic behaviour. Such upturn may be due to weak localization effect due to the presence of antisite disorder and short range magnetic clusters.

Analysis of the Griffiths–like phase observed in binary ε-Fe2N nitride

The structure and magnetic properties of binary ε-Fe2N nitride are investigated systematically and a Griffiths-like phase (GP) behavior is observed at low magnetic field. On cooling, the inverse magnetic susceptibility (1/χ) curve reveals that the GP behavior appears at 260 K. By the electron spin resonance spectra measurement and the density functional theory calculation, we suggest that the short-range ferromagnetic clusters and the local disorder within the crystallographic structure, which are stabilized and enhanced from the completion of the intra-layer Fe–Fe and inter-layer Fe–N–Fe interactions, contribute to the GP behavior.

Effect of Fe doping on structure and magnetotransport properties of perovskite manganite

Neutron powder diffraction experiments to refine the crystal and magnetic structures of a La0.7Ca0.3Mn0.5Fe0.5O3 compound were carried out at temperatures of 10, 50, 80, 100, 120, 150, 200, 250 and 300 K. The obtained results support the conclusion that there is no long-range ferromagnetic order at all these temperatures. Structural parameters and, especially, the magnetic moments are calculated as a function of temperature. Their polynomial extrapolation is performed. Magnetization and electrical resistivity measurements were also performed in the temperature range of 5–300 K in magnetic fields up to 1 T. Field magnetization is measured to confirm the formation of the spin-glass-like state. These experimental results indicate formation of a complex magnetic state in which the long-range antiferromagnetic G-type phase coexists with short-range ferromagnetic clusters. Electrical conductivity of La0.7Ca0.3Mn0.5Fe0.5O3 demonstrates an anomalous temperature behavior suggesting a switching between different states. Correlation between the critical temperatures of magnetic and electrical transitions is demonstrated. Resistivity data in various theoretical models are processed. We discuss the origin of the unconventional magnetic state, the mechanisms of the electrical conductivity, and the correlation between magnetic and transport properties in this manganite.

Magnetotransport properties and phase separation in iron substituted lanthanum-calcium manganite

Crystal and magnetic structure of iron substituted La0.7Ca0.3Mn0.5Fe0.5O3 manganite was investigated by powder neutron diffraction in the 10–300 К temperature range. It is established that the unit cell parameters at 300 K are a = 5.447(2) Å, b = 7.709(3) Å, c = 5.467(3) Å and they change vs. temperature with rates da/dT ∼ 1.3*10–4 Å*K−1, db/dT ∼ 1.7*10–4 Å*K−1 and dc/dT ∼ 1.6*10–4 Å*K−1, respectively. It was also realized the measurements of magnetization and electrical resistivity in the 5–300 K temperature range including in magnetic fields up to 1 T. Magnetic phase separation was observed. Upon the field increase the glass, Tg, and irreversibility, Tirr, temperatures decrease, while the magnetic ordering temperature, Tmo, increases. Upon the temperature increase both the residual magnetization, Mr, and coercive field, Hc, decrease. A complex frustrated magnetic state in which the long-range G-type antiferromagnetic phase coexists with the short-range ferromagnetic clusters was established. An average size of ferromagnetic clusters is Dav ∼ 20 nm. An anomalous temperature behavior of electrical conductivity for La0.7Ca0.3Mn0.5Fe0.5O3 was found which confirms a transformation of various magnetic states. Different models of conduction mechanism were considered. The origin of the unusual magnetic state, the electrical conduction mechanisms, and the relationship between magnetic and electrotransport properties in this sample were discussed.

Effect of Fe doping on structure, magnetic and electrical properties La0.7Ca0.3Mn0.5Fe0.5O3 manganite

The crystal and magnetic structures of La0.7Ca0.3Mn0.5Fe0.5O3 compound have been studied by neutron powder diffraction in the temperature range of 10–300 К. The magnetization and electrical resistivity measurements have been also performed in the temperature range of 5–300 K in magnetic fields up to 1 T. These experimental results indicate a formation of a complex magnetic state in which the long-range antiferromagnetic G-type phase coexists with the short-range ferromagnetic clusters. The electrical conductivity of La0.7Ca0.3Mn0.5Fe0.5O3 demonstrates an anomalous temperature behavior suggesting a switching between different states. The origin of the unconventional magnetic state, the mechanisms of the electrical conductivity, and correlation between magnetic and transport properties in this manganite have been discussed.

Facile Synthesis, Magnetic and Electric Characterization of Mixed Valence La0.75K0.25AMnTiO6 (A = Sr and Ba) Perovskites.

A new series of mixed valence perovskites, La0.75K0.25AMnTiO6 (A = Sr and Ba) nanocubes, have been synthesized by a mild hydrothermal route. From powder X-ray diffraction (XRD) analysis, the crystal structure of La0.75K0.25AMnTiO6 was solved as the orthorhombic symmetry (space group Pbnm) with a random A-site or B-site arrangement. The phase purity of the products was confirmed by ICP, SEM, and EDS analyses, and the oxidation states of the B-site metal atoms were determined to be +4 for Ti and +3/+4 for Mn from XPS results. The soft ferromagnetic behavior is present in both samples. The Curie point TC has been detected as high as 309 K in La0.75K0.25SrMnTiO6 and 217 K in La0.75K0.25BaMnTiO6. A Griffiths phase can be observed at the high temperature region, which is related to the magnetic inhomogeneity induced by the existence of short-range ferromagnetic clusters. The resistivity measurements indicate that the semiconducting properties of the samples can be depicted better by variable range hopping (VRH) due to a diluted double-exchange interaction of Mn3+-O-Mn4+ with Ti4+ doping.

Low temperature nucleation of Griffiths Phase in Co doped LaMnO3 nanostructures

We have reported magnetic properties of La1-xCoxMnO3 nanostructures synthesized by hydrothermal route. The crystal structure has been characterized by X-ray diffraction (XRD) technique, which shows rhombohedral perovskite structure at room temperature. Scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) have been used to analyse morphology and chemical composition of prepared nanoparticles. Magnetic hysteresis loops of all the samples exhibit ferromagnetic behaviour at 10K. Inverse susceptibility graphs as a function of temperature represent deviation from Curie Weiss law. The indication for short range ferromagnetic clusters well above Curie temperature is observed due to the Griffiths Phase (GP). It is proposed that the presence of GP arises from induced size effects of La and Co ions.

Nonlinear magnetodielectric effect in double-perovskite Gd2NiMnO6

Magnetic and dielectric properties of the double perovskite Gd2NiMnO6 were investigated. Ferromagnetic order of alternating Ni2+ and Mn4+ spins arises below K and an additional order of Gd3+ spins occurs at K. The formation of short-range ferromagnetic clusters accompanied by a Griffiths phase-like feature below K is also suggested. Under the low enough dielectric loss at low temperature regime excluding the influence of the extrinsic effect, a highly nonlinear variation of the dielectric constant was achieved in application of the magnetic fields. Our finding offers an efficient approach to accomplish intrinsically coupled functionality utilizing both magnetic and dielectric quantities.

Griffiths phase, spin-phonon coupling, and exchange bias effect in double perovskite Pr2CoMnO6

The ceramic Pr2CoMnO6 of double perovskite structure is prepared by a solid-state reaction and the magnetic properties, phonon behaviors are studied in detail. Two ferromagnetic transitions at TC1 ∼ 172 K and TC2 ∼ 140 K are observed in the temperature-dependent magnetization curves, respectively. Furthermore, a detail analysis on the magnetic susceptibility reveals that a short-range ferromagnetic clustered state exists above TC1, which can be well described as the Griffiths phase with a well-defined Griffiths temperature TG ∼ 210 K. The presence of the B-site antisite defects is considered to contribute to the observed Griffiths singularity. Temperature-dependent Raman scattering experiment reveals an obvious softening of the phonon mode involving stretching vibrations of the (Co/Mn)O6 octahedra in FM temperature regions, indicating a close correlation between magnetism and lattice in Pr2CoMnO6. On the other hand, it is found that the phonon softening extends up to TG, which further confirms the preformation of the short-range ferromagnetic clusters up to TG. Moreover, the field-cooling magnetic hysteresis loop reveals that exchange bias phenomena is present, which is supposed to origin from the exchange coupling between Co/Mn ordered ferromagnetic phases with antiferromagnetic antiphase boundaries caused by the partially Co/Mn antisite disorders. These findings give a systematic understanding on the magnetic interaction in Pr2CoMnO6 which is closely related to the lattice and atomic distribution, and add special interest for application of this material.

Appearance of Griffiths phase in La0.62Er0.05Ba0.33Fe0.2Mn0.8O3 manganite

Structural and magnetic properties of La0.62Er0.05Ba0.33Fe0.2Mn0.8O3 (LEBFMO) compound have been investigated. Rietveld refinement of powder X-ray diffraction data reveals that the sample crystallizes in the rhombohedral (R3¯c) structure. The dc and ac magnetic studies demonstrate aspects of Griffiths phase-like behavior above the Curie temperature. The observed anomalous paramagnetic behavior is attributed to the magnetic heterogeneity which is caused by the segregation of short-range ferromagnetic clusters within the paramagnetic matrix. A large deviation is usually observed between zero fields cooled (ZFC) and field cooled (FC) M(T) at a low temperature below the blocking temperature. At 50K a spin-glass state is seen to arise from a ferromagnetic state. This is caused by the competition between the ferromagnetic and antiferromagnetic interaction. From isothermal magnetization versus magnetic applied field, we deduced the magnetic entropy change ΔSM. The large RCP and ΔSM found in Fe-substituted sample are found to be interesting for magnetic refrigeration below room temperature.

Exchange Bias Effect Concerned with Tunneling Magnetoresistance in ${\hbox{Sm}}_{0.35} {\hbox{Pr}}_{0.15}{\hbox{Sr}}_{0.5}{\hbox{MnO}}_3$ Phase Separated Manganites

In this paper, we report the exchange bias (EB) effect along with tunneling magnetoresistance (MR) in polycrystalline Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.35</sub> Pr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.15</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> manganite. Analogous to the shift in the magnetic hysteresis loop along the field (H)-axis, a shift is also observed in the MR-H curve when the sample is cooled in a static dc magnetic field at 5 K. The values of exchange fields (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> ), coercivity (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ), remanence asymmetry (M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> ) and magnetic coercivity (M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ) are found to strongly depend on cooling magnetic field and temperature. Magnetic training effect (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> ) is supportive experimental evidence which describes the decrease of exchange bias field when sample is successively field-cycled at a particular temperature. We have also observed T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> in the shift of the MR-H curve which has been interpreted by the spin relaxation model. We have observed the time dependent resistivity below the spin freezing temperature. The unusual MR behavior is interpreted in terms of the intragranular interface effect between short-range ferromagnetic clusters and spin-glass matrix giving rise to the EB effect, which is of special interest for potential application in multifunctional spintronic devices.

The influence of disorder on the appearance of Griffiths phase and magnetoresistive properties in (La1−xNdx)2/3(Ca1−ySry)1/3MnO3 oxides

Abstract The effect of quenched disorder on the magnetic and transport properties of (La 1− x Nd x ) 2/3 (Ca 1− y Sr y ) 1/3 MnO 3 polycrystalline samples with ( J 1 ( x =0, y =0); J 2 ( x =0.05, y =0.04); J 3 ( x =0.25, y =0.20); J 4 ( x =0.3, y =0.24); J 5 ( x =0.98, y =0.8) is analyzed within the context of percolative transport and the existence of the Griffiths phase. Our results demonstrate that moderate-small chemical disorder affects the inverse dc susceptibility χ − 1 (T) at higher temperature revealing the presence of the Griffiths phase above the Curie temperature. We attribute the observed anomalous paramagnetic behavior to the magnetic heterogeneity caused by the segregation of short-range ferromagnetic clusters within the paramagnetic matrix. The electrical properties show the presence of a metal-insulator transition at T MI which decreases with increasing disorder σ 2 . In the low-temperature ferromagnetic metallic region, the ρ data follow an empirical relation, ρ F M = ρ 0 + ρ 2 T 2 + ρ 4.5 T 4.5 reflecting that the conductive mechanism mainly arises from electron–electron, electron–phonon and electron–magnon scattering changed versus disorder effect. In the high-temperature paramagnetic insulating region, the ρ data for all samples follow the adiabatic small-polaron-hopping model. To understand the dependence of disorder with transport mechanism, we used the phenomenological equation for conductivity under a percolation approach, which is dependent on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions. Also we demonstrate that magnetoresistance (MR) increase with increasing disorder σ 2 . It is very interesting to note that the MR at room temperature is enhanced, which is encouraging for potential applications.

Enhanced ferromagnetism and glassy state in phase separated La0.95Sr0.05MnO3 + δ

Unusual high temperature ferromagentism is reported for La0.95Sr0.05MnO3+δ with Curie temperatures (TC1 and TC2) TC1 at 290 K and TC2 ∼ 150 K. Weak antiferromagnetism (AFM) is thought to give low moment, low temperature irreversibility, and non-saturation up to 10 T at 5 K. A short-range ferromagnetism is predicted from well-defined hysteresis at 5 K and field-cooled (FC) magnetization curves. Strong irreversibility between zero field-cooled (ZFC) and FC at 5 T is due to “hard” spins, not from ferromagnetic clusters. Field-dependent peak shift of spin freezing temperature in ZFC, memory effect, magnetic relaxation shows cluster glass like transition in the system. A metal-insulator transition at 115 K also implies a completion of percolation of short range ferromagnetic clusters, giving an onset of metallic state at 115 K. A complete magnetic phase diagram is presented showing its temperature dependent rich magnetic behavior.

Evidence of two distinct dynamical freezing processes in single-layered perovskiteLa0.7Sr1.3CoO4

The static and dynamic features of magnetization have been investigated in terms of dcmagnetization, ac susceptibility, and memory effects on single-layered perovskiteLa0.7Sr1.3CoO4. The results indicate that short-range ferromagnetic clusters coexistwith the glassy magnetic state, i.e., a cluster–glass phase betweenTf andTG and a spin-glass-likephase below Tf. The clear evidence of memory effects in the dc magnetization is also observed belowTf andTG, respectively. These two glassy states with cooperative relaxation processes mayderive from the different interaction processes among nanoscale ferromagneticclusters mediated by the matrix, which is influenced by changing the spin state ofCo3 + ions in spontaneously phase-separated cobaltite.

NonmagneticB-site impurity-induced ferromagnetic tendency in CE-type manganites

Using a two-orbital model and Monte Carlo simulations, we investigate the effect of nonmagnetic $B$-site substitution on half-doped CE-type manganites. The lattice defects induced by this substitution destabilize the CE phase, which transforms into (1) the ferromagnetic (FM) metallic competing state, (2) a regime with short-range FM clusters, or (3) a spin-glass state, depending on couplings and on the valence of the $B$-site substitution. While a $C$-type antiferromagnetic state is usually associated with an average ${e}_{g}$ charge density of less than 0.5, the nonmagnetic $B$-site substitution that lowers the ${e}_{g}$ charge density is still found to enhance the FM tendency in our simulations. The present calculations are in qualitative agreement with experiments and provide a rationalization for the complex role of nonmagnetic $B$-site substitution in modulating the phase transitions in manganites.

Particle size dependent exchange bias and cluster-glass states inLaMn0.7Fe0.3O3

The cluster-glass compound LaMn0.7Fe0.3O3 was synthesized with average particle sizes∼20,∼90,and ∼300 nm. We observed a shift of the magnetic hysteresis loop in the field axis while the samplewas cooled in an external magnetic field. The systematic shift of the hysteresis loops andthe cooling field dependence of the shift indicated the phenomenon of exchange bias.The exchange bias field was found to be strongly dependent on the particle size,where the exchange bias field decreased considerably with an increase of particlesize, and the weak effect of exchange bias was observed for particles with size∼300 nm. A cluster-glass state with short range ferromagnetic clusters embedded in thespin-glass like host has been proposed, where the average size of the ferromagnetic clusterincreases with particle size.