Jiyu Fan

Investigation of critical behavior in Pr0.55Sr0.45MnO3 by using the field dependence of magnetic entropy change

One method of calculating critical exponents based on the field dependence on magnetic entropy change was applied to study critical behavior in Pr0.55Sr0.45MnO3. By using the obtained critical exponents, the modified Arrott plot [A. Arrott and J. E. Noakes, Phys. Rev. Lett. 19, 786 (1967)] is consistent with that by using conventional method. The calculated critical exponents not only obey the scaling theory, but also inosculate the deduced results from the Kouvel–Fisher method [J. S. Kouvel and M. E. Fisher, Phys. Rev. 136, A1626 (1964)]. It eliminates the drawback due to utilization of multistep nonlinear fitting in a conventional manner. Therefore, this means can be applied to investigate critical behavior.

Effect of Ru doping in La 0.5 Sr 0.5 Mn O 3 and La 0.45 Sr 0.55 Mn O 3

We investigated the structural, magnetic, and transport properties of polycrystalline doping systems La0.5Sr0.5Mn1−xRuxO3 0 x 0.15 with ferromagnetic matrix and La0.45Sr0.55Mn1−yRuyO3 0 y 0.5 with antiferromagnetic matrix. X-ray photoelectron spectroscopy shows that Ru ions exist mainly in the form of Ru4+ while there also exist a small quantity of Ru5+. For the low doping samples 0.05 x y 0.15 0.2 , the ferromagnetism is enhanced and Curie temperature TC rises with increasing Ru doping level, but TC is much higher than the temperature TIM corresponding to insulating-metallic transition. For the high doping samples 0.3 y 0.5 , oppositely the ferromagnetism is suppressed and the insulating property is enhanced with further increasing Ru doping level. These results show that there is an exchange interaction between Mn3+ and Ru4+ Ru5+ . The electron spin resonance spectra clearly and directly inspect that the interaction between Mn3+ and Ru4+ Ru5+ is ferromagnetic. Furthermore, the electron spin resonance spectra also show that a Ru-Ru antiferromagnetic interaction takes place in the high doping region y=0.4,0.5 . We explained the disagreement between TC and TIM.

Evolution of the intrinsic electronic phase separation in La 0.6 Er 0.1 Sr 0.3 MnO 3 perovskite

Magnetic and electronic transport properties of perovskite manganite La0.6Er0.1Sr0.3MnO3 have been thoroughly examined through the measurements of magnetization, electron paramagnetic resonance(EPR), and resistivity. It was found that the substitution of Er3+ for La3+ ions introduced the chemical disorder and additional strain in this sample. An extra resonance signal occurred in EPR spectra at high temperatures well above TC gives a strong evidence of electronic phase separation(EPS). The analysis of resistivity enable us to identify the polaronic transport mechanism in the paramagnetic region. At low temperature, a new ferromagnetic interaction generates in the microdomains of Er3+-disorder causing the second increase of magnetization. However, the new ferromagnetic interaction does not improve but decreases electronic transport due to the enhancement of interface resistance among neighboring domains. In view of a really wide temperature region for the EPS existence, this sample provides an ideal platform to uncover the evolution law of different magnetic structures in perovskite manganites.

Griffiths phase and magnetic polaronic behavior in B-site disordering manganites

We report a detailed study of magnetic and transport properties in perovskite manganites Nd0.55Sr0.45Mn1−xGaxO3 (0.00⩽x⩽0.20). The Ga substitution induces the B-site disorder together with a distinct suppression of the metal-insulator transition and ferromagnetic-paramagnetic transition. Due to an appearance of inhomogeneous magnetic clusters, the Griffiths-like phase is observed in 0.08⩽x⩽0.10 compounds. Meanwhile, with the increase of Ga concentration, the carriers self-trap into magnetic polarons. Because the oxygen sublattice distortion causes the variation of the phonon density of state, an additional Raman vibrative mode appears in the system.

Critical properties of the 3D-Heisenberg ferromagnet \chem{CdCr_{2}Se_{4}}

The critical properties of the ferromagnet CdCr2Se4 around the paramagnetic-ferromagnetic phase transition have been investigated. It is found that the 3D-Heisenberg model is the best one to describe the critical phenomena around the critical point. Critical exponents β=0.337±0.03 and γ=1.296±0.109 at TC=130.48±0.34 are obtained. In addition, the critical exponent δ=4.761±0.129 is determined separately from the isothermal magnetization at TC. These critical exponents fulfill the Widom scaling relation δ=1+γ/β. Based on these critical exponents, the magnetization-field-temperature (M-H-T) data around TC collapses into two curves obeying the single scaling equation M(H, e)=eβf±(H/eβ+γ). Although the 3D-Heisenberg model is the most satisfactory model to describe this system, critical exponents for CdCr2Se4 are slightly smaller than the theoretical exponents (β=0.36, γ=1.39 and δ=4.8). This indicates that the exchange interaction J(r) decays slower than r− 5 in this system, which can be attributed to the spin-lattice coupling.

3D-Heisenberg ferromagnetic characteristics in CuCr2Se4

The critical properties of the spinel selenide CuCr2Se4 around TC have been investigated by the modified Arrott plot, Kouvel–Fisher method, and critical isotherm analysis. Reliable critical exponents β = 0.372±0.007, γ = 1.277±0.044, and δ = 4.749±0.016 with TC = 430 K are obtained. Based on these critical exponents, the magnetization-field-temperature (M-H-T) data around TC collapses into two independent curves obeying the single scaling equation M(H,ɛ)=ɛβf±(H/ɛβ+γ). Moreover, the critical exponents are confirmed by the field dependence of the magnetic entropy change relation ΔSM|T=TC∝Hn with n = 1 + (β-1)/(β+γ). The obtained critical exponents are in good agreement with the prediction of the 3D-Heisenberg model except that γ is smaller than the theoretical value. The smallness of γ may be due to the delocalization of holes produced by Se1-, which gives evidence for the model of the ferrimagnetic hybridization between localized electrons of the Cr3+ ions and delocalized holes of the Se 4p band proposed f...

Critical behavior and long-range ferromagnetic order in perovskite manganite Nd0.55Sr0.45MnO3

The critical behavior of perovskite manganite Nd0.55Sr0.45MnO3 has been investigated based on the static magnetization measurement around the paramagnetic-ferromagnetic transition temperature 273 K. A large critical exponent and a small one have been obtained by calculating the magnetic-field dependence of the magnetic-entropy change and the Widom scaling relation. These critical exponents not only obey the scaling hypothesis, but also corroborate the results obtained from the Kouvel-Fisher method. In comparison with the values given by standard models, these obtained exponents are very close to those expected from the mean-field model ( and ) and its magnetic-coupling type belongs to nearly long-range interaction. We suggest that the A-site spin disorder and localized magnetic phase competition are the main reasons for the actual critical exponents to show a slight deviation from the theoretical model.

Critical phenomenon of the near room temperature skyrmion material FeGe.

The cubic B20 compound FeGe, which exhibits a near room temperature skyrmion phase, is of great importance not only for fundamental physics such as nonlinear magnetic ordering and solitons but also for future application of skyrmion states in spintronics. In this work, the critical behavior of the cubic FeGe is investigated by means of bulk dc-magnetization. We obtain the critical exponents (β = 0.336 ± 0.004, γ = 1.352 ± 0.003 and β = 5.276 ± 0.001), where the self-consistency and reliability are verified by the Widom scaling law and scaling equations. The magnetic exchange distance is found to decay as r−4.9, which is close to the theoretical prediction of 3D-Heisenberg model (r−5). The critical behavior of FeGe indicates a short-range magnetic interaction. Meanwhile, the critical exponents also imply an anisotropic magnetic coupling in this system.

Anisotropy compensation and magnetostrictive properties in Tbx Dy1−x(Fe0.9Mn0.1)1.93 Laves compounds: Experimental and theoretical analysis

The structure, magnetic transitions, and magnetostriction of Tbx Dy1−x(Fe0.9Mn0.1)1.93 polycrystalline compounds have been investigated. The Mn substitution for Fe changes the composition for the anisotropy compensation to the Dy-rich side compared with the Tbx Dy1−xFe2 system, which was confirmed by the analysis of detailed scanned XRD, the temperature dependence of a.c. initial susceptibility χac(T), and the calculation of the first anisotropy constant K1. The spin configuration diagram accompanied with different crystal structures for TbxDy1−x(Fe0.9Mn0.1)1.93 was constructed. The large tetragonal distortion λ100 due to the Mn substitution for Fe could be ascribed to the change in d band structure. The largest ratio between magnetostriction and the absolute values of the first anisotropy constant λa/|K1| appears at the composition with x=0.26, which could make it potential material for magnetostrictive application. Based on our experimental results, a phenomenological approach was also proposed to theor...

Critical behavior and the universal curve for magnetocaloric effect in textured Mn5Ge3−xAlx ribbons

Mn5Ge3−xAlx ribbons with x=0,0.5, and 1 were prepared by single roller melt-spinning method. Textured structures were observed in these ribbons as a result of fast cooling, with highly ordered columnar grains perpendicular to the surface. The magnetocaloric effect and critical behavior were investigated by dc magnetization measurement. Curie temperatures decrease from 297 K to 283 K as the Al content increases, and the peak entropy changes also decrease from 4.92 J/kg K to 3.0 J/kg K under an external magnetic field change of 3 T. Critical exponents were obtained by modified Arrott plots technique, Kouvel-Fisher method, and critical isothermal analysis. Similar values of exponents of β∼0.37, γ∼1, and δ∼3.8 were obtained for all of the compositions. Moreover, rescaled entropy data for all compositions collapse into a universal curve, revealing universal behavior for the magnetocaloric effect in this series of compounds.