Investigation Of Magnetic Phase Transitions Research Articles

Investigation of magnetic phase transitions in Ni0.5Cu0.25Zn0.25Fe2-xLaxO4 nanoferrites using magnetic and in-situ ultrasonic measurements

In this study, Ni0.5Cu0.25Zn0.25Fe2-xLaxO4 (x = 0.000, 0.025, 0.050, 0.075 and 0.100) spinel ferrites were prepared using sonochemical reactor. X-ray diffraction spectra were used to authorize/confirm the cubic spinel ferrite single-phase crystalline nature of the sample. The morphology of the prepared Ni0.5Cu0.25Zn0.25Fe2-xLaxO4 spinel ferrites was investigated by scanning electron microscope. It is observed that all the particles have devised spherical-like morphology. The ultrasonic non-destructive technique is also a versatile/flexible method/tool to explore/measure the Curie temperature (TC) in the spinel ferrites. In-situ ultrasonic velocity measurements in the range of temperature from 300 to 900 K were carried out. The detected anomalous characteristic/behavior in the ultrasonic velocity was used to explore the TC of the prepared sample. TC of Ni0.5Cu0.25Zn0.25Fe2-xLaxO4 is 735, 705,690, 675 and 655 K for the different composition x = 0.000, 0.025, 0.050, 0.075 and 0.100 respectively. The value of TC decreases with increasing La3+ doping concentration. Because La3+ ions are strongly paramagnetic, their presence reduces TC, which increases the paramagnetic zone. The vibrating sample magnetometry and susceptibility measurements have been used to investigate the influence of La3+ substitution on magnetic characteristics for further validation.

The investigation of magnetic phase transitions and magnetocaloric properties in high-pressure annealed MnNiFeGe alloy

ABSTRACT The MnNiFeGe alloy is prepared by high-pressure annealing method. It is found that the martensitic transformation temperature after high-pressure annealing of this alloy decreases with the increasing thermal hysteresis. Besides, another magnetic phase transition above the martensitic transformation is observed, which is due to the existence of a large amount of retained austenite induced by high-pressure annealing. As a result, this alloy shows two positive magnetic entropy change peaks, corresponding to the ferromagnetic phase transition of the austenite and the magnetic martensitic transformation, respectively. The magnetic phase transitions, the variable-temperature crystal-structure evolution, and the magnetocaloric properties of this alloy are investigated in detail.

Investigation of magnetic phase transition and magnetocaloric effect of (Ni,Co)-Mn-Al melt-spun ribbons

Magnetic phase transition, magnetocaloric effect and critical parameters of Ni50-xCoxMn50-yAly (x = 5 and 10; y = 17, 18 and 19) rapidly quenched ribbons have been studied. X-ray diffraction patterns exhibit a coexistence of the L21 and 10M crystalline phases of the ribbons. Magnetization measurements show that all the samples behave as soft magnetic materials with a low coercive force less than 60 Oe. The shape of thermomagnetization curves considerably depends on Co and Al concentrations. The Curie temperature (TC) of the alloy ribbons strongly increases with increasing the Co concentration and slightly decreases with increasing the Al concentration. The Ni45Co5Mn31Al19 and Ni40Co10Mn33Al17 ribbons reveal both the positive and negative magnetocaloric effects. Under magnetic field change (ΔGH) of 13.5 kOe, the maximum magnetic entropy change (|ΔSm|max) of the Ni45Co5Mn31Al19 ribbon is about 2 and -1 J·kg−1·K−1 for negative and positive magnetocaloric effects, respectively. Basing on Arrott - Noakes and Kouvel - Fisher methods, critical parameters of the Ni45Co5Mn31Al19 ribbon were determined to be TC ≈ 290 K, β ≈ 0.58, γ ≈ 0.92 and δ ≈ 2.59. The obtained values of the critical exponents indicate that the magnetic order of the alloy ribbon is close to the mean-field model.

The investigation of magnetic phase transition in cobaltite perovskites by high-resolution neutron powder diffraction under 14 T magnetic field

We carried out neutron powder diffraction (NPD) experiments on the layered perovskite cobaltite PrBaCo2O5.5+x (x = 0.24) at Super High Resolution Neutron Powder Diffractometer (SuperHRPD) with a newly installed magnet. At low temperatures, the antiferromagnetic (AFM) reflections of the NPD profile are suppressed whilst ferromagnetic (FM) reflections are enhanced under 14 T magnetic field. The magnetic-field-induced AFM-FM transition reveals the ground-state energy of FM is close to that of AFM.

Investigation of magnetic phase transition on the layered inorganic-organic hybrid perovskites (C6H5CH2CH2NH3)2MnCl4 by single-crystal neutron diffraction

In previous work, the inorganic-organic hybrid perovskite system (C6H5CH2CH2NH3)2MnCl4 exhibit well-isolated 2D Heisenberg antiferromagnetc properties, including a canted antiferromagnet below TN = 44.3 K. This study looked at the antiferromagnetic spin structure of the titled compound using single-crystal neutron diffraction. At 43 K, a new forbidden (−1 0 0) peak was observed and this peak showed a clear temperature dependence. Additionally, non-integer forbidden peaks were found. It was assumed that the magnetic cell was equal to the chemical cell for Mn-PEA system (k = 0). No magnetic peaks were observed along the c-axis. From this data it can be assumed that the spin is parallel to the c-axis. This result coincides with previous magnetic measurement studies. According to Turov, in the case of an antiferromagnetic structure with weak ferromagnetism, the magnetic cell should be equal to the chemical cell. Our neutron diffraction measurement and former study strongly support the hypothesis of Turov.

Magnetocaloric effect of LaFe11.35Co0.6Si1.05 alloy

The aim of the present paper was to study the large magnetocaloric effect observed in LaFe11.35Co0.6Si1.05 alloy. X-ray diffraction (XRD) result reveals a coexistence of two crystalline phases: a dominant La(Fe,Si)13-type and a minor α-Fe(Co,Si). It is confirmed by the Mössbauer spectroscopy and microstructural observations accompanied by an energy-dispersive spectroscopy (EDS) analysis. The value of the magnetic entropy changes (|SM|) in the vicinity of the Curie temperature (TC = 268 K) was calculated using thermomagnetic Maxwell relation, and it equals to 21.4 J·kg−1·K−1 under the change in an external magnetic field of μ0ΔH = 3T. The investigation of magnetic phase transition was carried out using the Landau theory, an analysis of the field dependences of the magnetic entropy change and universal scaling curve, revealing the second order of phase transition in the studied material.

Investigation of magnetic phase transitions in dilute copper ferrites

Mossbauer spectra of copper ferrites CuGaxAl2xFe2 − 3xO4 (x = 0.3, 0.4, 0.5) have been measured at temperatures in the range from 90 to 295 K. The degree of disturbance of long-range magnetic order has been determined as a function of the temperature. The Curie temperature has been found for the composition with x = 0.5, and the dependence of the Curie temperature on the concentration of the nonmagnetic impurity has been constructed.

Investigation of magnetic phase transitions in diluted ferrites: Nickel ferrite

The Mossbauer spectra of nickel ferrite NiGaxAlxFe2 − 2xO4 were measured with significant dilution by nonmagnetic ions (with x = 0.5, 0.7, and 0.8). In the temperature range of 90–295 K, the ratio of the near- and long-range forces was experimentally found and the Curie temperatures were determined.

Investigation of Magnetic Phase Transitions in HfFe6Ge6‐Type Gd1‐xErxMn6Sn6 Intermetallics.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Broadband ultrasonics applied to phase transition characterisation in chromium antiferromagnetic alloy

LABOEND/PEMM/EE/COPPE, Federal University of Rio de Janeiro, P.O.Box 68505, CEP 21945-970 Rio de Janeiro-RJ, Brazil(Received October 4, 2000)(Accepted in revised form January 17, 2001)Keywords: Acoustic methods; Phase transformations; Nondestructive testing; Magnetic structureIntroductionUltrasonic pulse-echo techniques are being widely used in the investigation of magnetic phasetransitions in solid materials [1,2]. In general, changes in the ultrasonic pulse amplitude and velocity aremeasured, and then the ultrasonic attenuation and elastic constants are determined for fixed frequenciesgiven by the odd harmonics of the transducer coupled to the sample. It is difficult to reproduce the sameconditions of coupling between transducer and sample, therefore most analyses are limited to a fewfixed frequencies. An investigation of the ultrasonic pulse propagation close to a magnetic phasetransformation, for a continuous frequency range, is presented using wide band transducers and digitalspectral analysis. The material chosen is a single crystal of Cr-0.18at%Re, which has a magnetic phasetransformation from the paramagnetic (PM) to an antiferromagnetic (AFM) state at 318 K [3,4].Similarly to pure chromium, the ultrasonic attenuation and internal friction for this alloy have acontribution due to the coupling between the spin density wave in the AFM phase and the oscillatoryelastic strain introduced by the ultrasonic pulse. Since the sixties the ultrasonic broadband analysis isbeing widely used for many applications [5–8]. In this paper, we have shown that the ultrasonic spectralanalysis approach is highly advantageous in many aspects for the investigation of phase transitions.Experimental MethodThe Cr-0.18at%Re single crystal used was obtained from a larger ingot previously investigated byneutron scattering [9] and cut with faces parallel to the crystallographic directions [001], [110] and[110] with dimensions of 10 3 12 3 6 mm. The full half width at half maximum (FWHM) of the Braggpeak was found to be less than 0.5° after standard procedure of electropolishing with oxalic acid.The measurement was performed using the immersion in water technique with a broadbandKrautkra¨mer transducer model IAP 50.2.1, which has a good response in the frequency interval from5 MHz to 40 MHz. The generator/receptor excites the transducer with a wide band pulse, and thegenerated ultrasonic pulse propagates through the water/sample system. Part of the pulse reflects at

Investigation of the magnetic phase transitions at high pressure by neutron depolarization

We propose to use the combination of the polarized neutron facility and the anvil high-pressure cell for investigation of magnetic phase transitions in solids. The magnetic phase transition manifests itself in the rise or loss of neutron depolarization. Neutron depolarization in Fe 70Ni 30 alloy and MnAs under high pressure was studied. Phase transitions from ferromagnetic state to paramagnetic state and vice versa were detected. Ferromagnetic state of MnAs at pressure of about 80 kbar was observed at room temperature.

Neutron diffraction investigation of magnetic phase transitions to long-range antiferromagnetic ordering in the “free-electron” praseodymium halides Pr 2X5 ( X = Br, I)

Powder neutron and single-crystal X-ray diffraction experiments of the halides Pr 2X 5 ≜ ( Pr 3+) 2(X −) 5( e −) were performed in order to determine temperature dependences of the structure and to establish the nature of the magnetic phase transitions. Bulk magnetic measurements in the temperature range of 1.6 to 300 K showed remarkably high ordering temperatures. Pr 2Br 5 orders antiferromagnetically at T N = 50(1) K, corresponding to the Shubnikov space group P2 1 m′ , i.e., k = 0. Below T ≈ 25 K the ordered magnetic moments of the two inequivalent Pr 3+ ions increase abruptly from μ Pr1 = 1.2 μ B and μ Pr2 = 2.2 μ B at 30 K to the saturation values μ Pr1 = 1.9 μ B and μ Pr2 = 2.6 μ B. Below T N = 37(1) K, Pr 2I 5 shows antiferromagnetic ordering corresponding to k = [ 1 2 ,0,0] and the Shubnikov space group P a 2 1 with ordered magnetic moments at saturation: μ Pr1 = 2.1 μ B and μ Pr2 = 1.8 μ B. Measurements of average electrical conductivities of polycrystalline Pr 2 X 5 indicate semiconducting behavior.

Neutron scattering investigation of magnetic phase transitions and crystal-field splitting in UX3 (X identical to Br, Cl)

Neutron scattering experiments were performed in the temperature range from 1.5 to 293 K on polycrystalline samples of the non-metallic 5f3 electron systems UBr3 and UCl3. The hexagonal crystal structure P63/m was refined at T=293 and 10 K. UBr3 orders at TN=5.4(1) K with a second-order transition to a 0+-configuration of magnetic moments of magnitude mu U(3.9K)=2.08(2) mu B oriented parallel to the c axis. At Tt=3.0(5) K occurs a second transition to an ordered antiferromagnetic ground state, corresponding to the lattice constants am= square root 3a, bm=a, cm=2a. This structure is anticentered in the basal plane. The magnetic moments of magnitude mu U(1.6K)=0.82(4) mu B are oriented parallel to the b axis. In both magnetic phases the antitranslation (0, 0, 1/2) suggests predominantly antiferromagnetic exchange within the chains of uranium ions along the c axis. The situation is similar for UCl3: the first transition at TN=6.5(1) K with an ordered magnetic moment mu U(4.0K) of 1.8(1) mu B, and the second transition at Tt=3.8(5) K with a magnetic moment mu U(1.6K) of 0.8(1) mu B. By means of inelastic neutron scattering the authors have measured the crystal-field splitting of UBr3 and UCl3 and determined the crystal-field parameters. The assumption of anisotropic magnetic interactions reasonably predicts the value of the zero-field magnetisation and provides an explanation for the magnetic phase transitions.

Photopyroelectric detection of magnetic phase transitions. Application to ferromagnetic and itinerant electron antiferromagnetic materials

It is demonstrated for the first time theoretically and experimentally that the photopyroelectric (PPE) method with front excitation and rear detection, is suitable for the investigation of magnetic phase transitions. Accurate plots of the PPE voltage amplitude for Ni 100− x Cu x ( x = 28, 30, 33%) alloys were obtained in the critical region with a signal-to-noise ( S N ) ratio better than 10 2. By increasing the modulation frequency of the radiation, the discontinuity of the PPE signal for the same specific heat anomaly is enhanced, as the thermally thin sample becomes thermally thick. The Curie temperatures determined by this method are close to those obtained by magnetic measurements. The itinerant electron antiferromagnetism-to-Pauli paramagnetism phase transition was also detected in chromium and the alloy γ-(Fe 55Mn 45) 80Rh 20, and the possible phase transition of α-titanium was investigated by this method. No anomaly in the PPE signal for α-Ti was found up to a S N ratio of 10 2.

Neutron scattering investigation of magnetic phase transitions in CoCl2-graphite intercalation compounds

The stage 2 CoCl2-graphite intercalation compound exhibits two magnetic phase transitions at TCl (=8.0 K) and TCu (=9.1 K). We have performed neutron scattering experiments on this compound which revealed at T < TCl a spin structure in which the 2D ferromagnetic CoCl2-sheets are stacked antiferromagnetically along the c-axis with weak 3D correlation. The intermediate phase between TCl and TCu is purely 2D in character with ferromagnetic in-plane order.

Neutron scattering investigation of magnetic phase transitions in CoCl2-graphite intercalation compounds

The stage 2 CoCl 2 -graphite intercalation compound exhibits two magnetic phase transitions at T Cl (=8.0 K) and T Cu (=9.1 K). We have performed neutron scattering experiments on this compound which revealed at T < T Cl a spin structure in which the 2D ferromagnetic CoCl 2 -sheets are stacked antiferromagnetically along the c -axis with weak 3D correlation. The intermediate phase between T Cl and T Cu is purely 2D in character with ferromagnetic in-plane order.

Investigation of magnetic phase transitions in HoPO 4 and HoAsO 4

The optical absorption spectrum of Ho 3+ in HoPO 4 and HoAsO 4 and the specific heat of HoAsO 4 has been studied to obtain information on magnetic and hyperfine interactions in these substances. Both substances are Ising systems which order antiferromagnetically in a simple two-sublattice structure with magnetic moment direction parallel to the optical axis at T N = 1.39K and 0.557 K, respectively. The interactions to nearest (nn), next nearest (nnn), and third nearest neighbours (tnn) were estimated. The most striking result is that the exchange interaction to the nearest neighbours was found to be ferromagnetic however cancelled by the third nearest neighbour's exchange interaction in HoAsO 4 and even overcompensated in HoPO 4.

Mössbauer investigation of magnetic phase transitions in FeI2

Mössbauer spectra were measured on single crystals of FeI 2 at temperatures between 1.5 and 40°K in magnetic fields up to 6.5 T. The measurements show that the recently discovered magnetic phase transitions on FeI 2 are characterized by changes in the ratio of spins parallel and antiparallel to the c -axis. The five magnetic phases have the following ratio of parallel to anti-parallel spins: 6 : 6, 8 : 4, 9 : 3, 10 : 2 and 12 : 0.