Asymptotic critical behavior of Ni

Unusual critical behavior of Griffiths ferromagnet Ho2NiMnO6at magnetic phase transition

The critical behavior at the ferromagnetic to paramagnetic phase transition of a Griffiths ferromagnet, Ho2NiMnO6, is studied using modified Arrott plot, Kouvel-Fisher, and critical isotherm analysis. Here, we report a second-order phase transition and conclude from the estimated critical exponents that it is unusual and do not belong to conventional universality classes. However, they obey scaling relationships, which indicates the renormalization of interactions around the phase transition temperature. The presence of Griffiths phase in the system accounts for the unusual critical exponents observed.

Physical properties of RIr3 (R = Gd, Tb, Ho) compounds with coexisting polymorphic phases

The binary compounds GdIr₃, TbIr₃ and HoIr₃ are synthesized successfully and found to form with macroscopic co-existence of two polymorphic phases: AuBe₅ (C15b) and AuCu₃-type. The dc magnetization and heat capacity studies confirm that the C15b phase orders ferromagnetically, whereas the AuCu₃ phase remains paramagnetic down to 2 K. The frequency dependent ac-susceptibility data, time dependent magnetic relaxation behavior and magnetic memory effect studies suggest that TbIr₃ and HoIr₃ are cannonical spin-glass systems, but no glassy feature could be found in GdIr₃. The critical behavior of all three compounds has been investigated using the magnetization and heat capacity measurements around the transition temperature (T_C). The critical exponents α, β, γ and δ have been estimated using different techniques such as the Arrott-Noakes plot, Kouvel-Fisher plot and critical isotherm as well as analysis of specific heat data and study of magnetocaloric effect. The critical analysis study identifies the type of universal magnetic class in which the three compounds belong.

Investigation of the Itinerant Electron Ferromagnetism of Ni2+xMnGa1-x and Co₂VGa Heusler Alloys

Experimental investigations into the field dependence of magnetization and temperature dependences of magnetic susceptibility in Ni_2+xMnGa_1−x (x = 0.00, 0.02, 0.04) and Co₂VGa Heusler alloy ferromagnets were performed following the spin fluctuation theory of itinerant ferromagnetism, called as “Takahashi theory”. We investigated the magnetic field dependence of magnetization at the Curie temperature T_C, which is the critical temperature of the ferromagnetic–paramagnetic transition, and also at T = 5 K, which concerns the ground state of the ferromagnetic state. The field dependence of the magnetization was analyzed by means of the H vs. M⁵ dependence, and the field dependence of the ground state at 5 K was investigated by means of an Arrott plot (H/M vs. M²) according to the Takahashi theory. As for Ni_2+xMnGa_1−x, the spin fluctuation parameter in k-space (momentum space, T_A) and that in energy space (T₀) obtained at T_C and 5 K were almost the same. On the contrary, as for Co₂VGa, the H vs. M⁵ dependence was not shown at T_C. We obtained T_A and T₀ by means of an Arrott plot at 5 K. We created a generalized Rhodes–Wohlfarth plot of p_eff/p_S versus T_C/T₀ for the other ferromagnets. The plot indicated that the relationship between p_eff/p_S and T₀/T_C followed Takahashi’s theory. We also discussed the spontaneous magnetic moment at the ground state, p_S, which was obtained by an Arrott plot at 5 K and the high temperature magnetic moment, p_C, at the paramagnetic phase. As for the localized ferromagnet, the p_C/p_S was 1. As for weak ferromagnets, the p_C/p_S was larger than 1. In contrast, the p_C/p_S was smaller than 1 by many Heusler alloys. This is a unique property of Heusler ferromagnets. Half-metallic ferromagnets of Co₂VGa and Co₂MnGa were in accordance with the generalized Rhodes–Wohlfarth plot with a k_m around 1.4. The magnetic properties of the itinerant electron of these two alloys appeared in the majority bands and was confirmed by Takahashi’s theory.

Large magnetic entropy change and prediction of magnetoresistance using a magnetic field in La0.5Sm0.1Sr0.4Mn0.975In0.025O3

A detailed study of the structural, magnetic, magnetocaloric and electrical effect properties in polycrystalline manganite La_0.5Sm_0.1Sr_0.4Mn_0.975In_0.025O₃ is presented. The X-ray diffraction pattern is consistent with a rhombohedral structure with R3̄c space group. Experimental results revealed that our compound prepared via a sol-gel method exhibits a continuous (second-order) ferromagnetic (FM) to paramagnetic (PM) phase transition around the Curie temperature (T _C = 300 K). In addition, the magnetic entropy change was found to reach 5.25 J kg^-1 K^-1 under an applied magnetic field of 5 T, corresponding to a relative cooling power (RCP) of 236 J kg^-1. We have fitted the experimental data of resistivity using a typical numerical method (Gauss function). The simulation values such as maximum resistivity (ρ _max) and metal-semiconductor transition temperature (T _M-Sc), calculated from this function, showed a perfect agreement with the experimental data. The shifts of these parameters as a function of magnetic field for our sample have been interpreted. The obtained values of β and γ, determined by analyzing the Arrott plots, are found to be T _C = 298.66 ± 0.64 K, β = 0.325 ± 0.001 and γ = 1.25 ± 0.01. The critical isotherm M (T _C, μ ₀ H) gives δ = 4.81 ± 0.01. These critical exponent values are found to be consistent and comparable to those predicted by the three-dimensional Ising model with short-range interaction. Thus, the Widom scaling law is fulfilled.

Magnetic, magnetocaloric and critical behavior investigation of La0.7Ca0.1Pb0.2Mn1−x−yAlxSnyO3 (x, y = 0.0, 0.05 and 0.075) prepared by a sol–gel method

A systematic study on the magnetic, magnetocaloric and critical behavior properties of polycrystalline La_0.7Ca_0.1Pb_0.2Mn_1−x−yAl_xSn_yO₃ prepared via a sol–gel method are studied.

Co[V2]o4: a spinel approaching the itinerant electron limit

Studies of the structure, magnetization, and resistivity under pressure on stoichiometric normal spinel Co[V(2)]O(4) single crystals show (i) absence of a structural distortion, (ii) abnormal magnetic critical exponents, and (iii) metallic conductivity induced by pressures at low temperatures. All these results prove that Co[V(2)]O(4) sits on the edge of the itinerant-electron limit. Compared with similar measurements on Fe[V(2)]O(4) and other A[V(2)]O(4) studies, it is shown that a critical V-V separation for a localized-itinerant electronic phase transition exists.

Finite-size scaling in band ferromagnets with non-universal critical behavior

The ultra-high sensitivity of the ferromagnetic resonance (FMR) technique has been fully exploited to study the finite-size effects in the critical region near the ferromagnetic to paramagnetic phase transition in Cr(75-x)Fe(25+x) (x = 0, 5) thin films of high structural and magnetic quality. Conclusive experimental evidence is provided for the validity of finite-size scaling. Irrespective of the film thickness and composition, the critical exponents β, γ and ν for spontaneous magnetization, initial magnetic susceptibility and spin-spin correlation length retain their bulk values so that no dimensionality crossover occurs within the film thickness range covered in the FMR experiments. The present results indicate that (i) like Cr(75-x)Fe(25+x), the previously studied Fe, Co, Ni, and CoNi(3) thin films behave as itinerant-electron (band) ferromagnets in which the isotropic long-range interactions between spins decay as J(r)∼r(-(d+σ)) (σ>0), and (ii) the lattice dimensionality d, spin dimensionality m, and range of spin-spin interactions (via the material-specific parameter σ) decide the (non-universal) values of the critical exponents.

Magnetic properties of the field-induced ferromagnetic state in MnSi

We present results of dc magnetization measurements focusing on the magnetic properties of the field-induced ferromagnetic state in MnSi. The temperature dependence of saturation magnetization in this ferromagnetic state exhibits the signatures of both spin wave excitations and itinerant electron ferromagnetism. The Arrott plots obtained from the isothermal field dependence of magnetization, however, are found to be distinctly nonlinear and hence cannot be explained within a simple framework of itinerant electron magnetism.

Critical behavior of the ferromagnetic perovskite BaRuO3

A thorough study has been made of the physical properties under high pressure of the perovskite BaRuO3 synthesized under pressure; it includes the critical behavior in the vicinity of Tc to 1 GPa and the temperature dependences of resistivity and ac magnetic susceptibility up to 8 GPa. The ferromagnetism in BaRuO3 is suppressed at 8 GPa. Critical fluctuations in the vicinity of Tc have been found in BaRuO3 and they are enhanced under pressure. These observations are in sharp contrast to SrRuO3 where mean-field behavior is found at Tc.

Critical phenomena and the quantum critical point of ferromagnetic Zr(1-x)NbxZn2

We present a study of the magnetic properties of Zr(1-x)NbxZn2, using an Arrott plot analysis of the magnetization. The Curie temperature Tc is suppressed to zero temperature for Nb concentration xc = 0.083+/-0.002, while the spontaneous moment vanishes linearly with Tc as predicted by the Stoner theory. The initial susceptibility chi displays critical behavior for x <or= xc , with a critical exponent which smoothly crosses over from the mean field to the quantum critical value. For high temperatures and x <or= xc, and for low temperatures and x >or= xc we find that chi(-1) = chi0(-1) + aT(4/3), where chi0(-1) vanishes as x-->xc. The resulting magnetic phase diagram shows that the quantum critical behavior extends over the widest range of temperatures for x=xc, and demonstrates how a finite transition temperature ferromagnet is transformed into a paramagnet, via a quantum critical point.