The order of magnetic phase transition in La(Fe1−xCox)11.4Si1.6 compounds

Magnetic transitions in La(Fe_1−xCo_x)_11.4Si_1.6 compounds with x=0–0.08, have been studied by DC magnetic measurements and Mössbauer spectroscopy. The temperature dependence of the Landau coefficients has been derived by fitting the magnetization, M(μ₀H), using the Landau expansion of the magnetic free energy. For x0.02 there is a strongly first-order magnetic phase transition between ferromagnetic and paramagnetic (F–P) states in zero external field and a metamagnetic transition from paramagnetic to ferromagnetic (P–F) above T_c. Increasing the cobalt content drives the F–P transition towards second order and eliminates the metamagnetic transition.     

Magnetic phase transition between ferromagnetic and antiferromagnetic states in Ce(Fe1−xAlx)2

The cubic Laves phase compound Ce(Fe_0.95Al_0.05)₂ exhibits a first order magnetic phase transition from antiferromagnetic to ferromagnetic state at ≈ 100 K with increasing temperature. This note gives the results of the Mössbauer effect of ⁵⁷Fe and the magnetic phase diagram in the external field vs. temperature plane.     

Monte Carlo study of the magnetic properties of Fe0.9−qMn0.1Alq-disordered alloys

Within the framework of a random site-diluted Ising model with nearest-neighbor interactions, and using the Metropolis algorithm for equilibration and energy minimization, we have computed the ensemble and configurational averages for magnetization per site, magnetic susceptibility and specific heat of Fe_0.9−qMn_0.1Al_q-disordered alloys with 0.1q0.55. In the model, atoms have been randomly distributed on a body-centered cubic lattice in order to simulate the disorder and structure as that obtained in arc-melted Fe_0.9−qMn_0.1Al_q alloys treated at high temperatures during long periods of time and followed by fast quenching. Competitive interactions coming from Fe–Fe ferromagnetic bonds and Fe–Mn and Mn–Mn antiferromagnetic couplings, as well as the Al dilutor effect, have been taken into account in our study. Results allow us to conclude that, in agreement with previous Mössbauer data of the average hyperfine field, for which a comparison is also carried out, the Fe_0.9−qMn_0.1Al_q-disordered alloys are well characterized by a critical concentration at room temperature at around 40 at% Al, for which the system undergoes a transition from a ferromagnetic state to a paramagnetic one. The finite size scaling analysis to obtain the critical Al concentration in the thermodynamic limit, as well as the critical exponents, is also presented and discussed.     

Antiferromagnetism and the Kondo behaviour in Ce(Pt1−xPdx)Ga

We present the results of lattice parameters at room temperature, the static magnetic susceptibility and the magnetic resistivity between 1.8 and 300 K, and the low-temperature specific-heat measurements for the series Ce(Pt_1−xPd_x)Ga, (x=0.0, 0.2, 0.5, 0.8 and 1.0). Two maxima in the temperature dependence of the magnetic resistive curve for each sample are observed, one above 100 K, and another at around 4 K, which due to an interplay between crystal-field effect and the Kondo effect. As determined from the peak values of the temperature dependence of the specific heat data C(T), all samples exhibit antiferromagnetic ordering from 1.3 K for CePdGa to 3.4 K for CePtGa. The large reduction of entropy for each sample below T_N is associated with the Kondo effect.     

Effects of the Mn/Co ratio on the magnetic transition and magnetocaloric properties of Mn1+xCo1-xGe alloys

We have investigated the magnetic transition and magnetocaloric effects of Mn 1+x Co 1 x Ge alloys by tuning the ratio of Mn/Co.With increasing Mn content,a series of first-order magnetostructural transitions from ferromagnetic to paramagnetic states with large changes of magnetization are observed at room temperature.Further increasing the content of Mn (x=0.11) gives rise to a single second-order magnetic transition.Interestingly,large low-field magnetic entropy changes with almost zero magnetic hysteresis are observed in these alloys.The effects of Mn/Co ratio on magnetic transition and magnetocaloric effects are discussed in this paper.     

Magnetic behavior of 3d dopants in superconducting REBa2Cu3−xFexO7+δ (RE=Dy, Er)

Mössbauer studies on ⁵⁷Fe-doped superconducting REBa₂Cu₃O_7+δ (RE=Er, Dy) were made as a function of temperature for x=0.15 and 0.30. The magnetic behavior of the 3d dopants, which mainly occupy Cu(1) sites, undergoes antiferromagnetic ordering which is coexistent with superconductivity at low temperature. The dimensionality of the magnetic interaction changes from 2D to 3D when the rare earth changes from Er to Dy. the line-widths of the Mössbauer subspectra are characteristic of magnetic fluctuation behavior in the vicinity of a phase transition. Combining these results with those of Fe-doped Y-123 (pseudo 1D) and Gd (3D), the magnitude of the rare earth moments appears to be strongly correlated with the dimensionality of the magnetic interaction of Fe dopants in these compounds. However, the Mössbauer spectrum for ¹⁵⁵Gd in GdBa₂Cu_2.85Fe_0.15O_7+δ (T_{N(Fe) 14 K}) shows no magnetic order at 4.9 K.     

Magnetization measurements of Fe1−xCoxSi

Magnetization measurements are made in pulsed magnetic fields up to 200 kOe at 4.2 K and up to 100 kOe at various temperatures on the alloys Fe_1−xCo_xSi. In the paramagnetic region of x<0.05 or x0.7, we expected a metamagnet transition at high field but no remarkable change of magnetization is observed.     

Magnetic properties and magnetocaloric effect in NdxLa1-xFe11.5Al1.5 compounds

Effects of Nd-doping on the magnetic properties and magnetocaloric effects （MCEs） of NdxLa1-xFe11.5Al1.5 have been investigated. Substitution of Nd leads to a weakening of the antiferromagnetic （AFM） coupling and an enhancement of the ferromagnetic （FM） coupling. This in turn results in a complex magnetic behaviour for Nd0.2La0.8Fe11.5Al1.5 characterized by the occurrence of two phase transitions at ～188 K （PM AFM） and ～159 K （AFM-FM）. As a result, a table-like MCE （9 J/kg.K） is found in a wide temperature range （160-185 K） for a field change of 0-5T around the transition temperature, as evidenced by both the magnetic and calorimetric measurements. Based on the analysis of low-temperature heat capacity, it is found that the AFM-FM phase transition modifies the electron density significantly, and the major contribution to the entropy change comes from the electronic entropy change.     

Zero field μ spin relaxation in Cd1−xMnxTe diluted magnetic semiconductors

Zero field μSR measurements were carried out on samples of the typical diluted magnetic semiconductor Cd_1−xMn_xTe as a function of composition in the range 0.27x0.65, at temperatures in both the “spin glass’ regions of the magnetic phase diagram. The results show the onset of complex diffusion-trapping behaviour at temperatures T60 K for all concentrations. Below 50 K the exponential relaxation found for the main signal is consistent with the interactions of the muon spin with rapidly fluctuating and rather large local hyperfine fields in these concentrated random diluted magnetic systems. In spite of the loss of signal near and below the transition temperature, the present results show that rapid spin fluctuations persist below T_g.     

NMR study of coexistence of ferro- and antiferromagnetism in (Zr1−xNbx)Fe2

The magnetic phase diagram has been investigated in the C14 type (Zr_1−xNb_x)Fe₂ with x0.7 from ⁹³Nb NMR and magnetization measurements. In the compound with x = 0.825 a first order-like transition has been found to occur around 25 K from a canted state with the ferromagnetic moment in the basal plane to a ferromagnetic state with decreasing temperature.     

Ferromagnetic Zr1−xMnxCo2 laves phase compounds

Structure and magnetic properties of the Zr_1−xMn_xCo_2+δ alloys were studied for 0 x <0.7, δ=0, 0.45. The cubic C15 Laves phase structure shows Mn solubility up to x≈0.4. The other Laves phase with the hexagonal C36 structure found for x0.5 apparently has a small region of Mn solubility in the vicinity of Zr_0.4Mn_0.6Co₂. Though the parent Mn-free compounds are known to be paramagnetic, the Mn-substituted alloys show ferromagnetic behavior with the Curie temperatures up to 625 K and the room-temperature saturation magnetization of about 100 emu/g. The onset of ferromagnetism with the Mn substitution for Zr may be caused by polarization of itinerant 3d electrons, like it was earlier supposed for the off-stoichiometric ZrCo_2+δ. The universal composition dependencies of the intrinsic magnetic properties for different δ can be obtained, if plotted against the amount of zirconium atoms missing in its sublattice. The room-temperature anisotropy with the noticeable anisotropy field of 24 kOe and the 1 1 0 easy magnetization direction laying in a basal plane was found in the hexagonal Zr_0.5Mn_0.5Co₂.     

Magnetic properties of the Cr3S4−xTex

It was found that a transition from the antiferromagnetic state to the ferromagnetic state took place at about x = 1.6 and that a monoclinic phase exists between x = 3.0 and 4.0 and a monoclinic structure became a hexagonal structure at about x = 3.5.     

The magnetic phase transitions and magnetocaloric effect in MnNi1−xCoxGe alloys

The magnetic phase transitions and the magnetocaloric effects in MnNi_1−xCo_xGe (x=0.38 and 0.40) alloys were investigated. The substitution of Co for Ni in the MnNiGe antiferromagnet results in the metamagnetic transitions from antiferromagnetic to ferromagnetic state, which associates with very small thermal and magnetic hystereses. Positive and negative values of magnetic entropy changes are exhibited around the metamagnetic transition temperature and Curie temperature, respectively. The relatively large refrigerant capacity in low magnetic field along with the good reversibility suggest that MnNi_1−xCo_xGe (x=0.38 and 0.40) alloys are potential candidates for magnetic refrigeration.     

Magnetic and electronic phase transitions and magnetoresistance effect in the Pr0.5−xLaxSr0.5MnO3 (, 0.15) system

The electronic and magnetic phase transitions of Pr_0.5−xLa_xSr_0.5MnO₃ with x=0.10 and 0.15 were investigated. The M(T) and ρ(T) curves for these samples clearly show transitions from antiferromagnetic insulator to ferromagnetic semiconductor, ferromagnetic metal and finally to paramagnetic semiconductor as the temperature is increased from 5 to 300 K. Especially, two obvious protrudent peaks in the magnetoresistance curves MR(T) for these samples were clearly observed in the relative low magnetic field, 1 T. One peak appears at around the antiferromagnet-ferromagnet transition temperature T_N (150 K) with MR≈−23%, another occurs at around the ferromagnet-paramagnet transition temperature T_C(275 K ) with MR≈−8.2%. In addition, when the magnetic field was increased, the temperature corresponding to the MR peak at T_N shifts to lower temperature while the temperature corresponding to the MR peak at T_C is fixed.     

Crystal structure, superconductivity and magnetic properties of the superconducting ferromagnets Gd1.4−xDyxCe0.6Sr2RuCu2O10 (x=0–0.6)

The structural, electrical and magnetic properties of the superconducting ferromagnets, Gd_1.4−xDy_xCe_0.6Sr₂RuCu₂O₁₀ (x=0–0.6) are systematically investigated as a function of Dy doping and temperature. These compounds are characterized by high temperature superconductivity (T_c ranging from 20 to 40 K depending upon the Dy content) co-existing with weak ferromagnetism with two magnetic transitions (T_M2 ranging from 95 to 106 K and T_M1 around 120 K). Doping with Dy gives no significant structural changes except for a minor change in the c/a ratio. However the superconducting transition temperature is significantly suppressed and magnetic ordering temperature enhanced on Dy doping. These effects are described and discussed.     

Magnetic properties and magnetic entropy changes of La1-xPrxFe11.5Si1.5 compounds with 0≤x≤ 0.5

Magnetic properties and magnetic entropy changes in LaFe$_{11.5}$Si$_{1.5}$ have been investigated by partially substituting Pr by La. It is found that La$_{1 - x}$Pr$_{x}$Fe$_{11.5}$Si$_{1.5}$ compounds remain cubic NaZn$_{13}$-type structures even when the Pr content is increased to 0.5, i.e. $x = 0.5$. Substitution of Pr for La leads to a reduction in both the crystal constant and the Curie temperature. A stepwise magnetic behaviour in the isothermal magnetization curves is observed, indicating that the characteristic of the itinerant electron metamagnetic (IEM) transition above $T_{\rm C}$ becomes more prominent with the Pr content increasing. As a result, the magnetic entropy change is remarkably enhanced from 23.0 to 29.4\,J/kg$\cdot$K as the field changes from 0 to 5\,T, with the value of $x$ increasing from 0 to 0.5. It is more attractive that the magnetic entropy changes for all samples are shaped into high plateaus in a wide range of temperature, which is highly favourable for Ericsson-type magnetic refrigeration.     

Magnetic doping in Zn7−xMxSb2O12 spinels (M=Ni and Co)

In this article we present a concise report on our studies on the magnetic behavior and structural arrangements of the inverse spinel Zn_7−xM_xSb₂O₁₂ system (M=Ni, Co). Studies on the temperature dependence of the magnetization (M) of several samples in this system showed the occurrence of a spin-glass-like state in temperatures around 10 K. The capability of this system to hold magnetic ions in either octahedral and/or tetrahedral positions is responsible for the occurrence of competing ferromagnetic and antiferromagnetic interactions. This condition is likely to cause the appearance of the observed spin-glass-like behavior.     

Spin-freezing processes around the tricritical point in Mn1−xTixSb

Transitions related to the spin-glass phase in Mn_1−xTi_xSb are revisited from the nonlinear ac susceptibility. Near the tricritical point (x≈0.8), the transition between ferromagnetic and reentrant spin-glass states is sharp. At x0.8, an anomally is found slightly above the temperature of usual spin-glass freezing.     

Magnetic susceptibility, electrical resistivity and thermal expansion coefficient of NiAs-type V1−xCrxSe

V_1−xCr_xSe(0.05x0.83) shows a temperature dependence of the magnetic susceptibility χ which is similar to that of CrSe. At small x, the magnetic transition temperature T_t(x) and the Weiss constant θ_p(x) decrease with decreasing x, while the effective number of Bohr magnetons per Cr (P_eff) significantly increases.     

Positive linear magnetoresistance in Fex–C1−x composites

A large positive magnetoresistance (MR) has been found in micro-sized Fe_x–C_1−x composites. At a magnetic field of 5 T, the Fe_0.2–C_0.8 composite has the largest MR, 53.8% and 190% at room temperature and at 5 K, respectively. The magnetic field dependence of the MR can be described approximately as MR∝Bⁿ, and the value of exponent n is determined by the Fe weight concentration and temperature, ranging from 1/4 to 6/4. It appears that Fe_x–C_1−x has a linear field dependence of the positive MR at different temperatures. The possible mechanism for the positive MR is discussed.