Ferromagnetic Curie Temperature Research Articles

“Treasure maps” for magnetic high-entropy-alloys from theory and experiment

The critical temperature and saturation magnetization for four- and five-component FCC transition metal alloys are predicted using a formalism that combines density functional theory and a magnetic mean-field model. Our theoretical results are in excellent agreement with experimental data presented in both this work and in the literature. The generality and power of this approach allow us to computationally design alloys with well-defined magnetic properties. Among other alloys, the method is applied to CoCrFeNiPd alloys, which have attracted attention recently for potential magnetic applications. The computational framework is able to predict the experimentally measured TC and to explore the dominant mechanisms for alloying trends with Pd. A wide range of ferromagnetic properties and Curie temperatures near room temperature in hitherto unexplored alloys is predicted in which Pd is replaced in varying degrees by, e.g., Ag, Au, and Cu.

Room temperature ferromagnetism in Mn-doped NiO nanoparticles

Mn-doped NiO nanoparticles of the series Ni1−xMnxO (x=0.00, 0.02, 0.04 and 0.06) are successfully synthesized using a low temperature hydrothermal method. Samples up to 6% Mn-doping are single phase in nature as observed from powder x-ray diffraction (XRD) studies. Rietveld refinement of the XRD data shows that all the single phase samples crystallize in the NaCl like fcc structure with space group Fm-3m. Unit cell volume decreases with increasing Mn-doping. Pure NiO nanoparticles show weak ferromagnetism, may be due to nanosize nature. Introduction of Mn within NiO lattice improves the magnetic properties significantly. Room temperature ferromagnetism is found in all the doped samples whereas the magnetization is highest for 2% Mn-doping and then decreases with further doping. The ZFC and FC branches in the temperature dependent magnetization separate well above 350K indicating transition temperature well above room temperature for 2% Mn-doped NiO Nanoparticle. The ferromagnetic Curie temperature is found to be 653K for the same sample as measured by temperature dependent magnetization study using vibrating sample magnetometer (VSM) in high vacuum.

Transport and magnetic properties of cobalt disulfide prepared by solid state hybrid microwave heating and hot pressing

In this study, single phase cobalt disulfide [Formula: see text] was synthesized by temperature-controlled solid state hybrid microwave heating. The structure, composition and morphology of the obtained samples were studied using X-ray diffraction (XRD), scanning electron spectroscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and high resolution transmission electron microscopy (HRTEM), respectively. The loose [Formula: see text] polycrystalline precursor was then hot pressed to dense bulk sample. The subsequent transport and magnetic properties measurements reveal the ferromagnetic Curie temperature at the magnetic transition near 128 K. These results suggest that the magnetic transition in [Formula: see text] is susceptible to the preparation conditions and the microstructure of the samples.

Zero-Field Dissipationless Chiral Edge Transport and the Nature of Dissipation in the Quantum Anomalous Hall State.

The quantum anomalous Hall (QAH) effect is predicted to possess, at a zero magnetic field, chiral edge channels that conduct a spin polarized current without dissipation. While edge channels have been observed in previous experimental studies of the QAH effect, their dissipationless nature at a zero magnetic field has not been convincingly demonstrated. By a comprehensive experimental study of the gate and temperature dependences of local and nonlocal magnetoresistance, we unambiguously establish the dissipationless edge transport. By studying the onset of dissipation, we also identify the origin of dissipative channels and clarify the surprising observation that the critical temperature of the QAH effect is 2 orders of magnitude smaller than the Curie temperature of ferromagnetism.

Impact of bismuth incorporation into (Ga,Mn)As thin films on their structural and magnetic properties

AbstractStructural and magnetic properties of thin films of the (Ga,Mn)(Bi,As) quaternary diluted magnetic semiconductor grown by the low‐temperature molecular‐beam epitaxy technique on GaAs substrates have been investigated. High‐resolution X‐ray diffraction has been applied to characterize the structural quality and misfit strain in the films. Ferromagnetic Curie temperature and magneto‐crystalline anisotropy of the films have been examined by using SQUID magnetometry and low‐temperature magneto‐transport measurements. Post‐growth annealing treatment of the films has been shown to reduce the strain in the films and to enhance their Curie temperature. Significant increase in the magnitude of magneto‐transport effects caused by incorporation of a small amount of Bi into the films is interpreted as a result of enhanced spin‐orbit coupling in the (Ga,Mn)(Bi,As) films. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evolution of Griffith phase in hole doped double perovskite La2−xSrxCoMnO6 (x = 0.0, 0.5, and 1.0)

We report on the magnetic properties of hole doped La2−xSrxCoMnO6 (x = 0.0, 0.5, and 1.0) double perovskite synthesized by the low-temperature sol-gel method. X-ray photoelectron spectroscopy (XPS) analysis reveals the increase of the number of Mn3+ and Co3+ ions with Sr content. The reduction in magnetic moment with the increase in Sr content is assigned to the increase of antiferromagnetic (AFM) phases. Detailed analysis of zero field cooled dc magnetic susceptibility as a function of temperature for the samples reveals the features of Griffiths-like singularity. Second harmonic nonlinear ac magnetic susceptibility (χ2) also demonstrates the aspects of Griffiths phase (GP) singularity above the ferromagnetic Curie temperature for all the samples. The value of the exponent of the GP (λ < 1) is found to decrease with Sr substitution in the La site, and it is not related to the structural disorder originating from the Jahn–Teller active Mn3+ions. Hence, it is concluded that with increasing Sr content, the nucleation of GP singularity is supressed due to the increase in the AFM phase volume fraction.

Magnetic polarons in doped La0.7Ca0.3MnO3, La0.7Ba0.3MnO3, and La0.7Sr0.3MnO3 manganites

The characteristics of the paramagnetic state of doped manganites containing 30% Mn4+ ions are studied. It is shown that the ferromagnetic Curie temperature for the materials with chemical composition La0.7A0.3MnO3 (A = Ca and Ba) characterized by semiconductor conductivity at T > T C can be determined from the measured χ–1(T) dependence. In the conducting La0.7Sr0.3MnO3 compound, the nonlinear behavior of χ–1 (T) is interpreted as a result of summation of different contributions related to individual Mn ions, as well as to the paramagnetic and ferromagnetic polarons with enhanced magnetic moments. The order (first or second) of the magnetic transition and the crystal lattice symmetry do not affect the formation of correlated polarons in the paramagnetic range.

Effect of metamagnetism on multiferroic property in double perovskite Sm2CoMnO6

The magnetic and multiferroic properties of a B-site ordered double perovskite Sm2CoMnO6 are investigated. The electric polarization is observed below the ferromagnetic Curie temperature of 135 K. This electric polarization of Sm2CoMnO6 can be influenced by applying a magnetic field, showing an interesting magnetoelectric effect. The origins of multiferroic properties and magnetoelectric effect are discussed.

Multiferroic and dielectric properties of Bi4LaTi3FeO15 ceramics

Single phase Bi4LaTi3FeO15 ceramics with a four-layered Aurivillius structure were synthesized by successfully inserting LaFeO3 into Bi4Ti3O12 matrix, and their structural, multiferroic and dielectric properties were investigated. Typical ferroelectric behavior with remnant polarization (2Pr) of about 12μC/cm2, large dielectric constant, small dielectric loss and a weak ferromagnetic behavior are obtained simultaneously at room temperature. As a result, possible magnetoelectric coupling effect is obtained near the ferromagnetic Curie temperature. Meanwhile, the weak ferromagnetism can be enhanced at lower temperature, and thus a larger saturation magnetization of about 1.2emu/g is obtained at 5K. In addition, two sets of dielectric relaxation are observed and discussed based on defect relaxation theory.

Magnetism and magnetocaloric effect study of CaFe0.7Co0.3O3

The CaFe0.7Co0.3O3 single crystal was grown for the first time by a two-step method and its magnetism and magnetocaloric effect were investigated. This compound experiences a second-order paramagnetism-to-ferromagnetism transition in a wide temperature window between 200 and 150 K due to the presence of multiple ferromagnetic interactions. Since the spin entropy is gradually released above the ferromagnetic Curie temperature (∼177 K), no sharp λ-type anomaly is observed in specific heat. On the basis of magnetization measurements, however, a considerable entropy change is found in this perovskite oxide. More interesting, this compound exhibits a broadening working temperature, and a significant refrigerant capacity (∼355 J kg−1 at 6 T) which is comparable with those found in some giant magnetocaloric alloys with first-order magnetic transitions. The present study therefore provides an example on how to enhance the refrigerant capacity by extending the working temperature of magnetocaloric material.

Persistent ferromagnetism and topological phase transition at the interface of a superconductor and a topological insulator.

At the interface of an s-wave superconductor and a three-dimensional topological insulator, Majorana zero modes and Majorana helical states have been proposed to exist respectively around magnetic vortices and geometrical edges. Here we first show that randomly distributed magnetic impurities at such an interface will induce bound states that broaden into impurity bands inside (but near the edges of) the superconducting gap, which remains open unless the impurity concentration is too high. Next we find that an increase in the superconducting gap suppresses both the oscillation magnitude and the period of the Ruderman-Kittel-Kasuya-Yosida interaction between two magnetic impurities. Within a mean-field approximation, the ferromagnetic Curie temperature is found to be essentially independent of the superconducting gap, an intriguing phenomenon due to a compensation effect between the short-range ferromagnetic and long-range antiferromagnetic interactions. The existence of robust superconductivity and persistent ferromagnetism at the interface allows realization of a novel topological phase transition from a nonchiral to a chiral superconducting state at sufficiently low temperatures, providing a new platform for topological quantum computation.

Magnetic and magnetocaloric properties of Ba and Ti co-doped SrRuO3

Temperature evolution of magnetic properties in Ba and Ti doped SrRuO3 has been investigated to observe the effects of larger ionic radius Ba at Sr site and isovalent nonmagnetic impurity Ti at Ru site. Ionic radius mismatch and different electronic configuration in comparison with Ru modify Sr(Ba)-O and Ru(Ti)-O bond lengths and Ru-O-Ru bond angle. The apical and basal Ru-O-Ru bond angles vary significantly with Ti doping. Ferromagnetic Curie temperature decreases from 161 K to 149 K monotonically with Ba (10%) and Ti (10%) substitutions at Sr and Ru sites. The zero field cooled (ZFC) magnetization reveals a prominent peak which shifts towards lower temperature with application of magnetic field. The substitution of tetravalent Ti with localized 3d0 orbitals for Ru with more delocalized 4d4 orbitals leads to a broad peak in ZFC magnetization. A spontaneous ZFC magnetization becomes negative below 160 K for all the compositions. The occurrence of both normal and inverse magnetocaloric effects in Ba and Ti co-doped SrRuO3 makes the system more interesting.

Multicaloric effect in multiferroic Y2CoMnO6

We have investigated multiple caloric effects in multiferroic Y2CoMnO6. The polycrystalline sample prepared by the solid state method has shown a ferromagnetic Curie temperature ~75 K with a second order phase transition; a maximum magneto entropy change of ~ 7.3 J kg−1 K−1 with a reasonable relative cooling power ~220 J kg−1 is found without thermal and magnetic hysteresis loss. An electric field driven entropy change of ~ 0.26 J m−3 K−1 was obtained using the Maxwell’s relation and estimated magnetically induced a total temperature change of ~5.45 K around the Curie temperature that confirms the multicaloric effect in the sample.

&amp;ldquo;1111&amp;rdquo; 型稀磁半导体的研究进展

We successfully fabricated “1111” type bulk form diluted ferromagnetic semiconductors (DMS) with decoupled carriers and spins doping:(La,AE)(Zn,TM)AsO (AE Ba, Sr; TM = Mn, Fe), of which the Curie temperature TC up to 40 K. We investigated the individual influence of carriers and local moments on the ferromagnetic ordering in (La1- x Sr x )(Zn0.9Mn0.1)AsO by fixing Mn at the level of 0.10, and vary the doping levels of Sr. When the Sr doping level is 10%, a ferromagnetic ordering below ~ 30 K is observed. But both TC and saturation moments are heavily suppressed when the Sr doping level is increased to x = 0.30. We investigated the spin dynamics by microscopic techniques of mSR and Neutron Scattering. The results of mSR measurements indicate that the ferromagnetic order transition takes place in entire volume, namely, these DMSs are bulk nature. The μSR measurements also show an universal linear trend between the static internal field parameter as and the ferromagnetic Curie temperature TC for 1111-type system, (Ga,Mn)As, the 111- type system Li(Zn,Mn)As and the 122-type system (Ba,K)(Zn,Mn)2As2, which suggests that the exchange interaction supporting ferromagnetic coupling in these systems has a common origin. The researches on this series of DMS will be helpful to reveal the general mechanism of ferromagnetism shared in diluted ferromagnetic semiconductors including (Ga, Mn)As.

Quantum anomalous Hall effect in magnetically doped InAs/GaSb quantum wells.

The quantum anomalous Hall effect has recently been observed experimentally in thin films of Cr-doped (Bi,Sb)(2)Te(3) at a low temperature (∼ 30 mK). In this work, we propose realizing the quantum anomalous Hall effect in more conventional diluted magnetic semiconductors with magnetically doped InAs/GaSb type-II quantum wells. Based on a four-band model, we find an enhancement of the Curie temperature of ferromagnetism due to band edge singularities in the inverted regime of InAs/GaSb quantum wells. Below the Curie temperature, the quantum anomalous Hall effect is confirmed by the direct calculation of Hall conductance. The parameter regime for the quantum anomalous Hall phase is identified based on the eight-band Kane model. The high sample quality and strong exchange coupling make magnetically doped InAs/GaSb quantum wells good candidates for realizing the quantum anomalous Hall insulator at a high temperature.

Bi3Cr2.91O11: a ferromagnetic insulator from Cr(4+)/Cr(5+) mixing.

The search for materials with ferromagnetic and semiconducting/insulating properties has intensified recently because of their potential use in spintronics. However, the number of materials is rather limited because of conflicting requirements needed for the appearance of ferromagnetic and insulating properties. Here we show that Bi3Cr2.91O11 belongs to the scarce family of ferromagnetic insulators. Bi3Cr2.91O11 was synthesized at high pressure of 6 GPa and high temperature of 1570 K. Its crystal structure and properties were studied using single crystals. It crystallizes in the KSbO3-type structure with space group Pn3 and the lattice parameter a = 9.2181(2) Å. Bi3Cr2.91O11 has almost a 1:1 mixture of Cr(4+) and Cr(5+) ions distributed in one octahedral crystallographic site. Bi3Cr2.91O11 is a rare example of oxides having chromium ions in unusual oxidation states. The presence of Cr(4+) and Cr(5+) results in ferromagnetic properties with ferromagnetic Curie temperature TC = 220 K.

Dielectric and Magnetodielectric Properties of R2NiMnO6 (R = Nd,Eu,Gd,Dy, and Y)

We have prepared polycrystalline R2NiMnO6 (R = Nd, Eu, Gd, Dy, and Y) samples by conventional solid‐state reaction and all the samples have shown monoclinic structure with P21/n space group. With the decrease in rare‐earth ionic size (&lt;rR&gt;), the &lt;Ni–O–Mn&gt; bond angle decreases, correspondingly a decrease in ferromagnetic (FM) Curie temperature is noticed. In the dielectric measurement, the dielectric anomaly shifts to high temperature with the decrease in the &lt;rR&gt; and shows no correlation with the FM Curie temperature and hints the absence of apparent magnetodielectric (MD) coupling. Appearance of multiple relaxations in the dielectric study suggests the electrical heterogeneity of the system. The dielectric/impedance analysis has revealed a close correlation between dc conductivity and the dielectrics; in fact, both dc resistivity and the grain relaxations follow the variable range hopping mechanism. The thermal activation of charge carriers at the grain boundary led to Maxwell–Wagner interfacial polarization. Finally, dielectric study under magnetic field showed no effect, it implies that not only the intrinsic MD is absent, but also the extrinsic MD due to the lack of magnetoresistance.

Enhanced magnetic properties in low doped La1-xBaxMnO3+δ (x=0, 0.1 and 0.2) nanoparticles

Low doped colossal magnetoresistance La1−xBaxMnO3+δ nanoparticles are synthesized by a sol–gel method along with the parent compound LaMnO3+δ. The Reitveld refinements on the XRD patterns show that they are crystallized in structure other than their stoichiometric bulk. The crystal structure indicates excess oxygen/cationic vacancies in the compound. The magnetic measurements reveal that the ferromagnetic Curie temperature (TC) is enhanced after oxygen annealing. The observed TC is higher than that of their stoichiometric bulk compounds and the values are comparable to the reported tensile strained epitaxial thin films. The estimated Mn4+/Mn3+ ratio is found to increase in the nanocrystalline samples compared to their bulk samples annealed under the same oxygen atmosphere. The enhancement in Curie temperature could be correlated with the excess oxygen/cationic vacancies.

Magnetic and dielectric properties of Aurivillius phase Bi6Fe2Ti3−2xNbxCoxO18 (0 ≤ x ≤ 0.4)

We investigate the structural, magnetic, and dielectric properties of Bi6Fe2Ti3−2xNbxCoxO18 (0 ≤ x ≤ 0.4). The room-temperature ferromagnetism is observed in the Nb and Co co-doped samples compared with the paramagnetic behavior in Bi6Fe2Ti3O18. The ferromagnetism in Bi6Fe2Ti3−2xNbxCoxO18 can be understood in terms of spin canting of the antiferromagnetic coupling of the Fe-based and Co-based sublattices via Dzyaloshinsky-Moriya interaction. Moreover, doping Co at Ti sites can significantly enhance the ferromagnetic Curie temperature compared with the substitution of Co for Fe in the Aurivillius compounds. The dielectric loss of Bi6Fe2Ti3−2xNbxCoxO18 (0.1 ≤ x ≤ 0.4) exhibits a relaxation process. The rather large activation energy in the 0.1 ≤ x ≤ 0.3 samples implies that the relaxation process is not due to the thermal motion of oxygen vacancies inside ceramics.

Intrinsic ferromagnetism in iron doped magnetically topological insulator Fe0.015BiSbTe3

The combination of magnetism with topological insulators makes a good platform to study fundamental physical phenomena. In this work, ferromagnetic ordering in Fe0.015BiSbTe3 is observed by magnetization and magneto-transport measurements. The ferromagnetic Curie temperature TC is about 4.5K and the magnetic easy axis is out of plane. A clear AHE at 2K reveals that the ferromagnetism is intrinsic.