Second-order Magnetic Phase Transition Research Articles

The magnetocaloric effect and heat capacity of ferrimagnetic nanosystems: High-dispersity magnetite

The magnetocaloric effects of aqueous and ethanolic high-dispersity magnetite suspensions and the magnetite magnetic liquid were determined calorimetrically over the temperature range 15–80°C. The temperature dependence of the magnetocaloric effect of suspensions was evidence of the thermal oxidation of magnetite to maghemite. The temperature dependences of the magnetocaloric effect of the magnetic liquid passed extrema related to the second-order magnetic phase transition.

Magnetocaloric effect near a second-order magnetic phase transition

The temperature and magnetic field dependences of the magnetocaloric effect (adiabatic temperature change Δ T) were measured in single crystalline and polycrystalline Gd near the Curie point ( T C) using direct method with continuously changing magnetic field ( H) at different rates and the more conventional stepping of the magnetic field. The peculiarities in Δ T( H) behavior are discussed. The Curie temperature of Gd was obtained by means of Belov–Goryaga (Arrott) plots on the basis of magnetization and magnetocaloric data, and from AC susceptibility measurements. The differences in the obtained values and possible reasons of the differences are discussed.

A [formula omitted] magnetic study of frustrated [formula omitted] and [formula omitted

Results are reported of a μ SR study on FeSc 2 S 4 and MnSc 2 S 4 , two highly frustrated spinels with the Fe compound possessing in addition orbital frustration. In FeSc 2 S 4 we see inhomogeneous magnetism. Below 100 K two μ SR signals are present, one typical for a strongly spin correlated paramagnet, the other for normal paramagnetism of free spins. No transition into an ordered magnetic state occurs down to 25 mK. The strongly correlated fraction increases with lower temperature but never reaches 100%. At 100 K its spin fluctuations are on the order of 1 GHz, at 2.5 K the fluctuations have slowed down to 200 MHz, but spin freezing does not occur. The spin fluctuation rates in the weakly correlated state are above 10 GHz at all temperatures. In MnSc 2 S 4 magnetism is homogenous. A transition into a long-range magnetically ordered state at T N = 2.1 K is confirmed, yet this state is characterized by a high degree of local spin disorder. The coexistence of an additional spin frozen state is excluded. No second magnetic transition occurs down to 13 mK. Above T N we see normal paramagnetic behavior with the typical features for an approach to a second-order magnetic phase transition.

Quantum phase transitions and the H–T phase diagram of a multisublattice Van Vleck antiferromagnet

The magnetic ordering of an easy-plane Van Vleck magnet with a large anisotropy of the easy-plane type and an antiferromagnetic exchange interaction between spins is considered in the case when the ground state of the integer-spin ions is a singlet and the antiferromagnetic phase is realized only in an external magnetic field. It is found that the phase transformation of the singlet (paramagnetic) phase to the antiferromagnetic phase is a quantum phase transition belonging to the magnetic phase transitions of the displacive type. It is shown that the Landau theory of phase transitions is applicable to such a transformation, with the order parameter being the polarization of the singlet state. Nonstandard manifestations of the paraprocess converting the second-order magnetic phase transition of the displacive type to a first-order transition are noted. The phase diagram is constructed; it differs from those of classical antiferromagnets in that the antiferromagnetic phase on it is surrounded only by the paramagnetic phase.

Thickness dependence of second-order magnetic phase transitions in films

Using a simple Landau model, we discuss the different possibilities of generating near-surface magnetic effects at a second-order transition for films. For experimental comparison, we define an order parameter averaged on the film thickness. Varying the sample size d and/or surface coupling γ, one can decrease or increase substantially the surface critical temperature T s and the saturation magnetization M s . In the case of γ > 0 , both M s and T s decrease from the bulk values as the film thickness is reduced. These theoretical results are, in particular, in nice agreement with the experimental data on superconducting MgB 2 thin films. By contrast, for γ > 0 , an enhancement of both quantities is expected. This extraordinary transition has been rarely observed experimentally and usually the situation is far from been clear. We analyze a new experiment on NiFe 2O 4 ultra-thin films, where a very strong enhancement of M s is observed.

Analysis of magnetic and structural phase transition behaviors of La 1− xSr xCrO 3 for preparation of phase diagram

The phase transition behavior of perovskite-type compounds, La 1− x Sr x CrO 3, was investigated by differential scanning calorimetry (DSC), dilatometry, dc magnetic susceptibility measurement and X-ray diffraction analysis. Both second-order magnetic phase transition from antiferromagnetic to paramagnetic and first-order structural phase transition from orthorhombic to rhombohedral were observed in the DSC or dilatometric curve of every specimen. The temperatures of both these magnetic and structural phase transitions decreased linearly with an increase in Sr content. The structural phase transition temperature of La 1− x Sr x CrO 3 with x less than 0.11 is higher than the magnetic phase transition temperature; however, a larger decrease in structural phase transition temperature than in magnetic phase transition temperature was observed with an increase in Sr content, resulting in a structural phase transition temperature lower than the magnetic phase transition temperature for La 1− x Sr x CrO 3 with x of more than 0.12. It was also observed that the heat of absorption of the structural phase transition decreased with an increase in x. In the dependence of dc magnetic susceptibility on temperature, variations by not only magnetic but also structural phase transitions were observed. It was also revealed that thermal expansion coefficient is affected not only by structural phase transition but also magnetic phase transition. Magnetic and structural phase diagram of La 1− x Sr x CrO 3, suggesting the existence of two Sr contents and temperatures at which triple phases coexist, was proposed.

Temperature-Induced Magnetic Phase Transitions in Crystals with Competing Single-Ion and Interionic Magnetic Anisotropies

Temperature-induced phase transitions in a uniaxial ferromagnetic system of spins S = 1 with competing one-particle and two-particle anisotropies are studied. It is shown that, in the case where easy-plane single-ion anisotropy dominates over easy-axis two-particle anisotropy, the transition from the paramagnetic state to a ferromagnetic state with magnetization perpendicular to the anisotropy axis is a second-order displacive magnetic phase transition. In the opposite case, where two-particle anisotropy dominates over single-particle anisotropy, the transition to a ferromagnetic state with magnetization perpendicular to the anisotropy axis is also continuous but of the order-disorder type. In a system with competing second-order one-and two-particle anisotropies, the orientational first-order phase transition can occur to a state with the magnetization directed along or perpendicular to the anisotropy axis.

Thermal Diffusivity of La1-xSrxMnO3(x < 0.3)

Perovskite manganites are interesting because of their colossal magnetoresistance. In this work high resolution thermal diffusivity measurements of La1−x Sr x MnO3 (0 ≤ x ≤ 0.3) single crystals in the temperature range from 250 to 400 K are presented. A photopyroelectric device in the standard back configuration has been used. The thermal diffusivity through second-order magnetic phase transitions, as well as through first- and second-order structural phase transitions has been measured. The critical parameters of the sample with x = 0.3 at the ferromagnetic-to-paramagnetic transition have been obtained, and are close to the values predicted by the Ising model.

Interplay between Tm 3+ and Cr 5+ magnetic sublattices in TmCrO 4

The compound TmCrO 4 has been studied with bulk magnetization measurements, 169 Tm Mössbauer spectroscopy and muon spin relaxation (μSR). The Cr sublattice in TmCrO 4, which orders with a second-order magnetic phase transition at T C =18.75 K , gradually induces magnetic order in the non-magnetic crystal field ground state of the Tm sublattice.

Magnetic Phase Transitions in Nanosystems: Role of Size Effects, Intercluster Interactions and Defects

Shear stress under high pressure loaded nanoclusters of α, γ-ferric oxides and metallic europium showed first- and second-order magnetic phase transitions. For nanoclusters of α, γ-ferric oxides and metallic europium 57Fe and 151Eu, Mossbauer spectroscopy revealed a considerable change in the Curie (Neel) points. The thermodynamic model predicts first-to-second-order (Eu) and second-to-first-order (Fe oxides) changes in the character of magnetic phase transitions and a decrease or increase of critical Curie (Neel) points. The model defines the critical cluster size and concentration of defects in nanosystems responsible for the change in magnetic properties. For nanoclusters of Fe oxides and metallic Eu, the critical (and maximum) concentration of defects corresponds to a cluster size 20–50nm.

Magnetic entropy change in the Ge-rich alloys Gd-Si-Ge

We have studied the temperature and field dependence of the magnetocaloric effect in the Ge-rich alloys Gd/sub 5/(Si/sub x/Ge/sub 1-x/)/sub 4/, where x=0.0825, 0.15, and 0.1875. The low field susceptibility shows a two-step magnetic ordering at the Curie temperature T/sub C/ and the Neel temperature T/sub N/. The T/sub C/ of the different compositions covers the temperature range 67 K-108 K while T/sub N/ remains constant at around 133 K. The magnetic entropy change /spl Delta/S/sub M/(T) as a function of applied field up to 7 Tesla is characterized by a second-order magnetic phase transition around T/sub N/ and a first-order transition around T/sub C/. The maximum magnetic entropy change around T/sub N/ is only about 3 J/spl middot/kg/sup -1//spl middot/K/sup -1/, but it is much larger around T/sub C/. It rapidly reaches a saturation plateau of about 40-50 J/spl middot/kg/sup -1//spl middot/K/sup -1/, which widens with increasing field. The width of the plateau at an applied field of 7 T is about 20 K. These results indicate that the Ge-rich alloys are potentially good candidates for MR applications at low temperature.

Mixing effects of light and heavy rare earths in the Laves phasecompounds Nd1−xHRxCo2 (HR = Gd,Tb)

The mixing effects of light and heavy rare earths in cubic Laves phasecompounds Nd1−xHRxCo2 (HR = Gd,Tb)have been investigated by means of x-ray powder diffraction and magneticmeasurements. In Nd1−xTbxCo2, the saturationmoment MSexhibits an anomaly and could be ascribed to an abrupt jump of theCo moment at a critical concentration xc ≈ 0.33,whereas, in Nd1−xGdxCo2,MSdoes not show such an anomaly and a relatively simplemagnetic behaviour is observed at xc ≈ 0.43.The different behaviours of these two systems can be understood by consideringthe structural distortion of a cubic unit cell at low temperature. Afield-induced metamagnetic transition from weak ferrimagnetism to strongferrimagnetism is observed in both systems. This can be interpreted bythe evolution of magnetization with the help of percolation theory. Thecompensation points are observed in both systems, which are well explainedwithin the two-sublattice model. A double peak of the AC susceptibilityobserved in Nd1−xTbxCo2 nearx0 ≈ 0.19does not necessarily mean the occurrence of a first-order phasetransition. The observed second-order magnetic phase transition nearx0can be well described by Landau theory of the phase transition.

Ideal magnetocaloric effect for active magnetic regenerators

The active magnetic regenerator (AMR) uses a magnetic solid as a thermal storage medium and as a working material in a refrigeration cycle. Thermodynamically coupled to a heat transfer fluid, the regenerator produces a cooling effect and generates a temperature gradient across the AMR. The coupling between the heat transfer fluid and the magnetic refrigerant is a key aspect governing the operating characteristics of an AMR. To increase our understanding of AMR thermodynamics, we examine the entropy balance in an idealized active magnetic regenerator. A relation for the entropy generation in an AMR with varying fluid capacity ratios is derived. Subsequently, an expression describing the ideal magnetocaloric effect (MCE) as a function of temperature is developed for the case of zero entropy generation. Finally, the link between ideal MCE and refrigerant symmetry is discussed showing that an ideal reverse Brayton-type magnetic cycle cannot be achieved using materials undergoing a second-order magnetic phase transition.

Synthesis and magnetism of the intercalation compound Fe0.95PS3(MV)0.11

The intercalation compound Fe0.95PS3(MV)0.11 (MV is the methylviol ogen cation) has been prepared. Its crystal structure and magnetism have been in vestigated by x-ray diffraction (XRD), susceptibility measurements and Mssbaue r experiments. Its XRD pattern can be indexed in a monoclinic unit cell with a=0 .879nm, b=0.944?nm, c=1.070nm, β=114.76°. The distance between layers of this intercalation compound is increased by 0.33nm compared with that of FePS3. Th e data of susceptibility show that second-order magnetic phase transitions have occurred when the temperature changed from 300K down to 4.2K. A ferromagnetic ph ase transition occurred below TC=84K. An antiferromagnetic phase tran sition occurred below TN=26K. The results of Mssbauer spectra taken in the t emperatur e range of 12 to 300K indicate that there are three kinds of divalent ions with their high spin states, which imply a charge transfer from guest to the Fe—S e g antibonding orbitals of the FePS3 host lattice, and some ions have been re leased from the FePS3 host lattice and then some vacancies have been formed. T he ferromagnetism originates from a canting of the spins of Fe2+, whe reas the antiferromagnetism occurred when the canting of the spins of Fe2+ have been modified at the lowest temperatures.

Drop of magnetocaloric effect related to the change from first- to second-order magnetic phase transition in La2/3(Ca1−xSrx)1/3MnO3

From data of initial magnetization isotherms, the magnetic entropy change ΔSM of the series of ferromagnetic perovskites La2/3(Ca1−xSrx)1/3MnO3, with x=0, 0.05, 0.15, 0.25, 0.50, 0.75 and 1, has been measured near their Curie temperatures. The results go from 3.7 J kg−1 K−1 for La2/3Ca1/3MnO3 to 1.5 J kg−1 K−1 for La2/3Sr1/3MnO3. Nevertheless, the evolution of ΔSM with x is not monotonic, it shows a steep decrease between x=0.05 and x=0.15. This point corresponds to a tolerance factor t=0.92, at which the system changes from orthorhombic (Pbnm) to rhombohedral (R3̄c) structure, and the magnetic phase transition from first to second order. Provided that rhombohedral symmetry forbids static long-range cooperative Jahn–Teller distortions, present in orthorhombic samples, we interpret our results as the strong increase of lattice effects in them. The anomalous thermal expansion at the Curie temperature, due to these lattice effects, is thought to underlie the jump in ΔSM with x.

Identification of first- and second-order magnetic phase transitions in ferromagnetic perovskites

A criterion used for the determination of first- and second-order magnetic phase transitions from purely magnetic methods is applied to manganese perovskites of formula La2/3(Ca1−xSrx)1/3MnO3. A crossover from first- to second-order character at a tolerance factor t=0.92 is found, which also brings about several variations in other physical properties. At t=0.92 a change from orthorhombic to rhombohedral symmetry also takes place. The impossibility of establishing static cooperative Jahn–Teller distortions in the rhombohedral symmetry is suggested as being responsible for the observed behaviour.

The Properties of Voronoi Tessellations for 2D Ising Fluid Near the Second-Order Magnetic Phase Transition

The topological and geometrical properties of Voronoi cells generated for 2D fluid of hard disks with Ising-like spins near the second-order phase transition from the paramagnetic to ferromagnetic phase are described for different disk densities. The comparison with Voronoi cells generated for the random hard disk system is given.

Universality of a two-dimensional Ising ferromagnetic fluid near the second-order magnetic phase transition.

The critical behavior of a two-dimensional (2D) system of hard disks, each disk bearing an Ising spin and interacting via Ising-like interactions, is studied near the second-order phase transition from the paramagnetic-to-ferromagnetic fluid phase by Monte Carlo simulations combined with a finite-size scaling analysis and a single histogram technique. The critical line is located and values of critical exponents along this line are determined. The continuous variation of the Binder's reduced fourth-order cumulant at the critical point and of the critical exponent nu with disk density is observed and a linear relation between these quantities is found. Ratios of critical exponents gamma/nu and beta/nu are found to be the same as those in the 2D Ising lattice model at all fluid densities. A statistical analysis of Voronoi diagrams generated for typical particle configurations along the critical line is performed. The variation of parameters characterizing Voronoi diagrams at the critical point with density is compared to the variation of critical exponents with density.

Magnetism in rare earth Co2 compounds under high pressures

We have studied electrical resistivity anomalies connected with magnetism in RECo2 (RE=Nd, Tb, Er, Ho) compounds in pressures up to 8 GPa. At ambient pressure the former two compounds exhibit a second order magnetic phase transition (SOMPT) at TC, whereas a first order magnetic phase transition (FOMPT) is observed in the latter two. Although TC decreases with pressure in all four compounds, the TC(P) dependence for NdCo2 and TbCo2 differs considerably from this for HoCo2 and ErCo2. For the latter two, Tc vs P data deviate dramatically from the initial linear dependence above a critical pressure Pc to become almost pressure independent at higher pressures. We propose that this is reflecting the loss of Co metamagnetism that is also indicated by the vanishing resistivity drop at Tc for P>4 GPa and by the change from a FOMPT to a SOMPT. A scenario is discussed assuming that for P>Pc the projected Co 3d density of states at EF decreases. Hence the Co moment collapses because the RE-Co-RE exchange channel becomes ineffective to induce the itinerant 3d electron metamagnetism. The localized RE moments, however, order at a “residual” TC due to the persisting Ruderman–Kittel–Kasuya–Yosida-type exchange interaction. For NdCo2 and TbCo2 the Tc values decrease with pressure by an exponential law but remain rather high in the highest applied pressures. Results of first-principles electronic structure calculations using the full-potential linearized augmented plane wave method are presented for HoCo2 and NdCo2 compounds, as well.

Critical behavior of electrical resistivity in amorphous Fe-Zr alloys

Electrical resistivity (ρ) of the amorphous (a-)Fe100−c Zr c (c=8.5, 9.5 and 10) alloys has been measured in the temperature range 77 to 300 K, which embraces the second-order magnetic phase transition at the Curie temperature point T c. Analysis of the resistivity data particularly in the critical region reveals that these systems have a much wider range of critical region compared to other crystalline ferromagnetic materials. The value of T c and specific heat critical exponent, α has the same values as those determined from our earlier magnetic measurements. The value of α for all the present investigated alloys are in close agreement with the values predicted for three-dimensional (3D) Heisenberg ferromagnet systems, which gives contradiction to the earlier results on similar alloys. It is observed from the analysis that the presence of quenched disorder does not have any influence on critical behavior.