Critical properties of the paramagnetic-to-ferromagnetic transition in nanocrystalline Gd diluted with Fe

Nanocrystalline Gd_0.946Fe_0.054 of average grain size 68 nm was prepared by melt-spinning. The magnetic behavior in the vicinity of the paramagnetic to ferromagnetic transition was investigated via dc magnetization and ac susceptibility measurements. The transition temperature and effective critical exponents for the order parameter and zero-field susceptibility were determined using Arrott-Noakes and Kouvel-Fisher analyses. The values obtained were T_C=291.71±0.07 K, β_eff=0.385±0.009, and γ_eff=1.24±0.03, respectively. Correction to scaling analysis indicated that the asymptotic exponents were both smaller than the effective ones within the reduced-temperature range investigated, contrary to the behavior seen in monocrystalline Gd. This behavior can be explained in terms of a crossover from 3D short-range Heisenberg universality class to the 3D Ising universality class due to increased anisotropy induced by the high magnetic fields used in the measurements and also possibly due to strain.     

Critical behaviour of Ho 2 Fe 17 − x Mn x —magnetisation and Mössbauer spectroscopy

The magnetic properties of Ho₂Fe_17???xMn_x compounds (x = 0–2) of ferromagnetic ordering temperatures up to T_C ~344 K have been investigated by DC magnetization and Mössbauer effect measurements. The nature of the magnetic phase transitions and the critical behaviour around T_C has been investigated by analysis of the magnetisation data and the critical exponents β, γ and δ determined. The critical exponents are found to be similar to the theoretical values of the mean-field model for which β?=?0.5 and γ?=?1.0, indicating the existence of a long-range ferromagnetic interactions. The isothermal entropy changes ΔS around T_C have been determined as a function of temperature in different magnetic fields.     

Pressure dependence of temperature variation of AC susceptibility for Fe50Ni37Mn13 and comparison to Fe68.1Ni31.9

Fe₅₀Ni₃₇Mn₁₃, which is a ferromagnetic alloy with FCC crystal structure, has been reported to show the Invar effect below the Curie temperature; however, this alloy shows a typical anti-Invar effect above the Curie temperature. In this paper, we discuss the pressure dependence of the temperature variation of the alternative current (AC) susceptibility at a frequency of 1 kHz for Fe₅₀Ni₃₇Mn₁₃ at various pressures up to 7.5 GPa above 77 K; we then compare the results with those for Fe_68.1Ni_31.9, which were obtained in a previous study. Fe₅₀Ni₃₇Mn₁₃ was in a ferromagnetic state throughout the entire pressure range measured. T_C decreases in inverse proportion to the increasing pressure; dT_C/dp and dln T_C/dp for Fe₅₀Ni₃₇Mn₁₃ are −26 K GPa⁻¹ and −0.07 GPa⁻¹, respectively. Further, the temperature variation of the shape of the χ′-T curve for different pressures indicates continuous combining of magnetic interactions occurs at high pressures. These results are similar to those obtained for Fe_68.1Ni_31.9.     

Field-induced structural transition and the related magnetic entropy change in Ni43Mn43Co3Sn11 alloy

Magnetic properties and magnetic entropy change ΔS were investigated in Heusler alloy Ni₄₃Mn₄₃Co₃Sn₁₁. With decreasing temperature this alloy undergoes a martensitic structural transition at T_M=188 K. The incorporation of Co atoms enhances ferromagnetic exchange for parent phases. Austenitic phase with cubic structure shows strong ferromagnetic behaviors with Curie temperature T_C^A at 346 K, while martensitic phase shows weak ferromagnetic properties. An external magnetic field can shift T_M to a lower temperature at a rate of 4.4 K/T, and a field-induced structural transition from martensitic to austenitic state takes place at temperatures near but below T_M. As a result, a great magnetic entropy change with positive sign appears. The size of ΔS reaches 33 J/kg K under 5 T magnetic field. More important is that the ΔS displays a table-like peak under 5 T, which is favorable for Ericsson-type refrigerators.     

Effect of Fe substitution on magnetocaloric effect in La0.7Sr0.3Mn1−xFexO3 (0.05≤x≤0.20)

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La_0.7Sr_0.3Mn_1−xFe_xO₃ (x=0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T=10-400 K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (T_C) and saturation magnetization up to x=0.15 but a dramatic change occurs for x=0.2. The x=0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (−ΔS_m) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4 J kg⁻¹ K⁻¹ at 343 K (x=0.05) to 1.3 J kg⁻¹ K⁻¹ at 105 K (x=0.2), under ΔH=5 T. The La_0.7Sr_0.3Mn_0.93Fe_0.07O₃ sample shows negligible hysteresis loss, operating temperature range over 60 K around room temperature with refrigerant capacity of 225 J kg⁻¹, and magnetic entropy of 4 J kg⁻¹ K⁻¹ which will be an interesting compound for application in room temperature refrigeration.     

Magnetic and optical properties of self-organized InMnAs quantum dots

Ten layers of self-assembled InMnAs quantum dots with InGaAs barrier were grown on high resistivity (1 0 0) p-type GaAs substrates by molecular beam epitaxy (MBE). The presence of ferromagnetic structure was confirmed in the InMnAs diluted magnetic quantum dots. The ten layers of self-assembled InMnAs quantum dots were found to be semiconducting, and have ferromagnetic ordering with a Curie temperature, T_C=80 K. It is likely that the ferromagnetic exchange coupling of sample with T_C=80 K is hole mediated resulting in Mn substituting In and is due to the bound magnetic polarons co-existing in the system. PL emission spectra of InMnAs samples grown at temperature of 275, 260 and 240 °C show that the interband transition peak centered at 1.31 eV coming from the InMnAs quantum dot blueshifts because of the strong confinement effects with increasing growth temperature.     

Medium range ordering and some magnetic properties of amorphous Fe90Zr7B3 alloy

High-resolution electron microscopy (HREM) reveals in the as-quenched Fe₉₀Zr₇B₃ alloy the existence of medium range ordered (MRO) regions 1-2 nm in size. Transmission Mössbauer spectroscopy confirms that these regions are α-Fe MRO ones. Above the Curie point of the amorphous phase (T_C=(257±2)K) they behave like non-interacting superparamagnetic particles with the magnetization decreasing linearly with the temperature. For these particles the average magnetic moment of 390μ_B and the average size of 1.7 nm, in excellent agreement with HREM observations, were estimated. The maximum of the isothermal magnetic entropy change at the maximum magnetizing field induction of 2 T occurs at the Curie temperature of the amorphous phase and equals to 1.05 Jkg⁻¹ K⁻¹. The magnetic entropy changes exhibit the linear dependence on the maximum magnetizing field induction in the range 0.5-2 T below, near and above T_C. Such correlations are attributed to superparamagnetic behavior of α-Fe MRO regions.     

Effect of magnetic field on the TC and magnetic properties for the aligned MnBi compound

In the compound MnBi, a first-order transition from the paramagnetic to the ferromagnetic state can be triggered by an applied magnetic field and the Curie temperature increases nearly linearly with an increase in magnetic field by ∼2 K/T. Under a field of 10 T, T_C increases by 20 and 22 K during heating and cooling, respectively. Under certain conditions a reversible magnetic field or temperature induced transition between the paramagnetic and ferromagnetic states can occur. A magnetic and crystallographic H-T phase diagram for MnBi is given. Magnetic properties of MnBi compound aligned in a Bi matrix have been investigated. In the low temperature phase MnBi, a spin-reorientation takes place during which the magnetic moments rotate from being parallel to the c-axis towards the basal plane at ∼90 K. A measuring Dc magnetic field applied parallel to the c-axis of MnBi suppresses partly the spin-reorientation transition. Interestingly, the fabricated magnetic field increases the temperature of spin-reorientation transition T_s and the change in magnetization for MnBi. For the sample solidified under 0.5 T, the change in magnetization is ∼70% and T_s is ∼91 K.     

Anti-Invar properties and magnetic order in fcc Fe-Ni-C alloy

Anti-Invar effect was revealed in the fcc Fe-25.3%Ni-0.73%C (wt%) alloy, which demonstrates high values of thermal expansion coefficient (TEC) (15-21)×10⁻⁶ K⁻¹ accompanied by almost temperature-insensitive behavior in temperature range of 122-525 K. Alloying with carbon considerably expanded the low temperature range of anti-Invar behavior in fcc Fe-Ni-based alloy. The Curie temperature of the alloy T_C=195 K was determined on measurements of temperature dependences of magnetic susceptibility and saturation magnetization. The Mössbauer and small-angle neutron scattering (SANS) experiments on the fcc Fe-25.3%Ni-(0.73-0.78)%C alloys with the varying temperatures below and above the Curie point and in external magnetic field of 1.5-5 T were conducted. Low value of the Debye temperature Θ_D=180 K was estimated using the temperature dependence of the integral intensity of Mössbauer spectra for specified temperature range. The inequality B_eff=(0.7-0.9)B_ext was obtained in external field Mössbauer measurement that points to antiferromagnetically coupled Fe atoms, which have a tendency to align their spins perpendicular to B_ext. Nano length scale magnetic inhomogeneities nearby and far above T_C were revealed, which assumed that it is caused by mixed antiferromagnetically and ferromagnetically coupled Fe atom spins. The anti-Invar behavior of Fe-Ni-C alloy is explained in terms of evolution of magnetic order with changing temperature resulting from thermally varied interspin interaction and decreasing stiffness of interatomic bond.     

Determination of critical exponents of inhomogeneous Gd films

The role of inhomogeneity on the critical behavior is studied for non-epitaxial Gd films. For this purpose, the film inhomogeneity was varied experimentally by annealing otherwise identical samples at different temperatures T_AN=200, 400, and 500 °C. Vibrating sample magnetometry (VSM) was used for magnetization M vs. T measurements at different external fields H. A method based upon the linear superposition of different sample parts having different Curie temperatures T_C was used to extract the critical exponents and the intrinsic distribution of Curie temperatures. We found that this method allows extracting reliable values of the critical exponents for all annealing temperatures, which enabled us to study the effects of disorder onto the universality class of Gd films.     

Spin-glass-like behaviour and magnetic order in Mn[Cr0.5Ga1.5]S4 spinels

Magnetization and susceptibility were investigated as a function of temperature and magnetic field in polycrystalline Mn[Cr_0.5Ga_1.5]S₄ spinel. The dc susceptibility measurements at 919 Oe showed a disordered ferrimagnetic behaviour with a Curie-Weiss temperature θ_CW=−55 K and an effective magnetic moment of 5.96 μ_B close to the spin-only value of 6.52 μ_B for Cr³⁺ and Mn²⁺ ions in the 3d³ and 3d⁵ configurations, respectively. The magnetization measured at 100 Oe revealed the multiple magnetic transitions with a sharp maximum at the Néel temperature T_N=3.9 K, a minimum at the Yafet-Kittel temperature T_YK=5 K, a broad maximum at the freezing temperature T_f=7.9 K, and an inflection point at the Curie temperature T_C=48 K indicating a transition to paramagnetic phase. A large splitting between the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations at a temperature smaller than T_C suggests the presence of spin-glass-like behaviour. This behaviour is considered in a framework of competing interactions between the antiferromagnetic ordering of the A(Mn) sublattice and the ferromagnetic ordering of the B(Cr) sublattice.     

Study of critical exponents in doped La2/3Ca1/3Mn1-y Fe y O3 (y = 0, 0.03) manganite films

In this work, we investigated close to the Curie temperature T _C the critical exponents of the magnetization of doped manganite La_2/3Ca_1/3Mn_0.97Fe_0.03O₃ (LCMFO) thin films, as well as undoped La_2/3Ca_1/3MnO₃ (LCMO). Using a T _C distribution given by the intrinsic magnetic inhomogeneities in these ferromagnets enables the determination of β and δ critical exponents [corresponding to M(T) and M(H) respectively], average Curie temperature < T _C > , and the T _C distribution width, ΔT _C. Additionally, we extracted the critical exponent η = βδ from the fits of ΔT _C as a function of the external applied magnetic field. We found a value of 1.74 ± 0.09 for this exponent, close to that reported in undoped La_2/3Ca_1/3MnO₃ thin films. Even though the substitution effects of the Mn ions by Fe affect the magnetotransport and structural properties of LCMO system, these results suggest that around T _C, the magnetic phase transition, governed by the critical exponents, is similar in both magnetic systems, and belongs to the same universality class.     

Ferromagnetic phase transitions in the magnetically heterogeneous solid solution system EuxLa1−xS

The ferromagnetic phase transitions of the solid solution system Eu_xLa_1-xS with Eu-concentrations x=0.85, 0.65 and 0.50 are analyzed by measurements of the initial permeability, the specific heat and the magnetization. For the sample with x=0.85 the spontaneous magnetization develops continuously between two temperatures T_c1 and T_c2. For the sample with x=0.65 a well defined magnetic ordering temperature exists. One observes strongly curved magnetization isotherms when plotting the magnetization data in form of modified Arrott plots. The M(H, T) data above the curved region show usual ferromagnetic scaling with the critical exponents β=0.5 and δ=4.7. These exponents fulfill the scaling relations with the exponents γ=2.1 and α≈-1 derived for the initial susceptibility and the magnetic specific heat. The sample with the concentration x=0.50 turns out to be not truely ferromagnetic. It is a spin glass with strong ferromagnetic short range order.     

Critical behavior and magnetic entropy change in the La0.6Sr0.4Mn0.8Fe0.1Cr0.1O3 perovskite

Critical behavior in the La_0.6Sr_0.4Mn_0.8Fe_0.1Cr_0.1O₃ ceramics was studied using magnetization methods. Results show that the paramagnetic–ferromagnetic transition is of second order. Based on the critical behavior analysis using the Banerjee criterion and the Kouvel–Fisher method, we find the critical exponents: β=0.395±0.010, γ=1.402±0.010, and δ=5.208±0.007, for which the magnetic interaction is satisfied within the three-dimensional Heisenberg model. Results indicate the presence of short-range interactions. The magnetic entropy change (−ΔS_M) reached maximum values of 1.75, 1.45, 1.15, 0.8 and 0.43 J Kg⁻¹ K⁻¹ under a magnetic field variation of 5, 4, 3, 2 and 1 T, respectively. Nevertheless, these (−ΔS_M) values are much low for any potential application at this moment. The nature of this phenomenon is discussed in relation to the characteristics of the magnetic phase transition and critical exponents.     

Magnetic properties of half-metallic semi Heusler Co1−xCuxMnSb compounds

A study of the half-metallic character of the semi Heusler alloys Co_1−xCu_xMnSb (0?x?0.9) is presented. We investigated the saturation magnetization M_S at temperatures from 5 K to room temperature and the temperature dependence of the DC magnetic susceptibility χ above Curie temperature T_C. The magnetic moments at 5 K, for most compositions are very close to the quantized value of 4 μ_B for Mn³⁺ ion, the compound with 90% Co substituted by Cu is still ferromagnetic with M_S (5 K)=3.78 μ_B/f.u. These results emphasize the role of Co atoms in maintaining the ferromagnetic order in the material. The Curie temperature is decreased from 476 K to about 300 K as the Cu content increases from 0% to 90%. Above T_C, the χ⁻¹ vs T curves follow very well the Curie–Weiss law. The effective moment μ_eff and paramagnetic Curie temperature θ are derived. A comparison between the values of M_S at 5 K and μ_eff shows a transition from localized to itinerant spin system in these compounds.     

Critical behavior studies in Ti-substituted lanthanum bismuth perovskite manganites

Perovskite manganite La_0.4Bi_0.6Mn_1−xTi_xO₃ (x = 0.05 and 0.1) synthesized using conventional solid state route method give rise to critical phenomenon in their magnetic interactions due to the substitution of non magnetic Ti ions. The critical behavior is observed near paramagnetic–ferromagnetic transition and is studied by magnetization measurements. Various techniques like Modified Arrott plot, Kouvel–Fisher method, scaling equation of state analysis and the critical magnetization isotherm were used to analyze the magnetization data on magnetic phase transition. The values of critical exponents β and γ obtained using different techniques are in good agreement. The obtained critical exponents are found to follow scaling equation with the magnetization data scaled into two different curves below and above the transition temperature, T_C. This confirms that the critical exponents and T_C are reasonably accurate. The obtained critical exponents for both the samples deviates from mean-field model and do not completely follow the static long range ferromagnetic ordering. This behavior is consistent with non magnetic nature of Ti substituted at Mn site and can be associated with Griffiths phase like phenomenon.     

Magnetic properties of Cr-Fe-Mn alloys

The magnetic behaviour of a Cr_80−xFe₂₀Mn_x alloy system with x=2, 7, 10, 13 and 22 has been investigated in the temperature range 2-400 K through measurements of magnetization, electrical resistivity, magnetoresistivity, specific heat and thermal expansion. The temperature vs. Mn concentration magnetic phase diagram of the system is rich in magnetic behaviour with ferromagnetic (FM), antiferromagnetic (AFM) and paramagnetic phase regions and a spin-glass (SG) region at the lowest temperatures. Phase transition temperatures amongst these different magnetic phases could be identified from well-defined anomalies of magnetic origin that are displayed by graphs of the above-mentioned physical properties as a function of temperature. The time relaxation of the thermoremanent, isothermal remanent and field-cooled magnetizations below and above the SG freezing temperature show unusual aspects. These relaxations do not follow the usual superposition principle that is expected for typical SG materials. Negative giant magetoresistance (GMR) is observed in the alloys at 4 K. The GMR initially increases sharply on increasing the Mn content in the alloy system, followed by a tendency towards a saturation negative value for concentrations of more than about 10 at% Mn. Low-temperature plots of C_p/T vs. T², where C_p is the specific heat, present anomalous behaviour for alloys with x=2, 10 and 22. For x=2 the plot shows an upturn at the lowest temperatures that changes over to a prominent downturn for x=10 and 22. This behaviour is attributed to Fe concentration fluctuations in the alloys, confirming the theoretical model of Matthews.     

Crystal structural, magnetic and electrical transport properties of CeKFeMoO6 double perovskite

The crystal structural, magnetic and electrical transport properties of double perovskite CeKFeMoO₆ have been investigated. The crystal structure of the compound is assigned to the monoclinic system with space group P2₁/n and its lattice parameters are a=0.55345(3) nm, b=0.56068(2) nm, c=0.78390(1) nm, β=89.874(2). The divergence between zero-field-cooling and field-cooling M-T curves demonstrates the anisotropic behavior. The Curie temperature measured from C_p-T curve is about 340 K. Isothermal magnetization curve shows that the saturation and spontaneous magnetization are 1.90 and 1.43 μ_B/f.u. at 300 K, respectively. The electrical behavior of the sample shows a semiconductor. The electrical transport behavior can be described by variable range hopping model. Large magnetoresistance, −0.88 and −0.18, can be observed under low magnetic field, 0.5 T, at low and room temperature, respectively.     

Magnetic and electrical properties of the half-metallic ferromagnets Co2CrAl

This paper presents the results of measurements of the magnetic and electrical properties of the ferromagnetic alloy Co₂CrAl in two structural states: (i) after severe plastic deformation and (ii) after shortterm high-temperature annealing of the deformed specimens. The experiments have been performed at temperatures in the range from 2 to 900 K in magnetic fields H ≤ 50 kOe. The ferromagnetic Curie temperature T _C and the paramagnetic Curie temperature Θ have been determined (T _C = 305 K and Θ = 326 K), as well as the spontaneous magnetic moment μ _S and the effective magnetic moment μ_eff per molecule of the alloy (μ _S = 1.62 μ_B and μ _eff ² = 8.2 μ _B ² ). It has been shown that the magnetic crystalline anisotropy energy of the alloy is on the order of ～5 × 10⁵ erg/g. The specific features of the electrical properties are associated with the presence of an energy gap in the electronic spectrum near the Fermi level E _F and with the change in the parameters of the energy gap as a function of the temperature.     

The magnetic phase transition of an amorphous FePC and its alloys containing Ni and Cr

The magnetic phase transition of the amorphous ferromagnetic alloys Fe₈₀P₁₃C₇, Fe₇₀Cr₁₀P₁₃C₇ and Fe₇₀Ni₁₀P₁₃C₇ has been investigated by magnetization and specific heat measurements. From the field dependence of the isothermal magnetization, the critical exponents and the Curie temperature (T_C) of Fe₈₀P₁₃C₇ were obtained to be β = 0.38 ± 0.02, γ = 1.30 ± 0.05, θ = 4.47 ± 0.05 and T_c = 586.55 ± 0.1°K. These exponents sufficiently satisfy the static scaling law. The specific heat of Fe₈₀P₁₃C₇ showed an approximate logarithmic singularity at T_c. The specific heat from Fe₇₀Ni₁₀P₁₃C₇ shows fairly broad peak and the peak almost smears out for Fe₇₀Cr₁₀P₁₃C₇.