Magnetic Transition Changes Research Articles

Magnetic tricritical point and nematicity in FeSe under pressure

Magnetism induced by external pressure ($p$) was studied in a FeSe crystal sample by means of muon-spin rotation. The magnetic transition changes from second-order to first-order for pressures exceeding the critical value $p_{{\rm c}}\simeq2.4-2.5$ GPa. The magnetic ordering temperature ($T_{{\rm N}}$) and the value of the magnetic moment per Fe site ($m_{{\rm Fe}}$) increase continuously with increasing pressure, reaching $T_{{\rm N}}\simeq50$~K and $m_{{\rm Fe}}\simeq0.25$ $\mu_{{\rm B}}$ at $p\simeq2.6$ GPa, respectively. No pronounced features at both $T_{{\rm N}}(p)$ and $m_{{\rm Fe}}(p)$ are detected at $p\simeq p_{{\rm c}}$, thus suggesting that the stripe-type magnetic order in FeSe remains unchanged above and below the critical pressure $p_{{\rm c}}$. A phenomenological model for the $(p,T)$ phase diagram of FeSe reveals that these observations are consistent with a scenario where the nematic transitions of FeSe at low and high pressures are driven by different mechanisms.

Effect of synthesis route on the structural, magnetic and magnetocaloric properties of La0.78Dy0.02Ca0.2MnO3 manganite: A comparison between sol-gel, high-energy ball-milling and solid state process

La0.78Dy0.02Ca0.2MnO3 compound has been synthesized using different methods: solid state reaction, sol-gel route and ball-milling process. The Rietveld refinement of X-ray powder diffraction shows that all our samples crystallize in the orthorhombic system with Pnma space group. Morphological properties investigated using Scanning Electron Microscopy (SEM), indicated various microstructure ranging from micrometric grain size (solid-state and sol-gel techniques) to an ultrafine grains one (ball-milling method). Based on Banerjee criterion, Landau theory and universal curve we successfully confirmed that the nature of the magnetic transition changes from first order for the sample elaborated by sol-gel route to second order transition for those synthesized by both solid-state route and ball-milling methods. Therefore, it can be deduced that the process of elaboration presents an intensive impact on the magnetic behavior. The compound elaborated by Ball-milling undergoes the larger magnetocaloric effect (MCE). It exhibits a large relative cooling power (RCP) of 346.70 J/Kg under 5 T. Thus it has the appropriate properties as suitable candidate for magnetic refrigeration application.

Effect of partial magnetic order on resistivity and thermopower of Ho(Co1−x Al x )2 alloys

The electrical resistivity ρ and the thermopower S of Ho(Co1−xAlx)2 alloys (x = 0 to 0.2) were measured at temperatures from 2 K to 300 K in magnetic fields up to 10 T. While ρ(T) and S(T) of pure HoCo2 reveal abrupt changes at the magnetic ordering temperature TC, indicating a firstorder transition, the temperature variations of both properties across the ordering temperature in Ho(Co1−xAlx)2 alloys clearly show that the type of magnetic transition changes under substitution of Al for Co from first order to second order around x ≈ 0.06. ρ(T) of the Al-substituted samples (x = 0.15 and 0.2) has a very unusual variation at temperatures below TC: in contrast to the expected and usually-observed decrease of ρ(T), the resistivity of Ho(Co1−xAlx)2 alloys (x = 0.15 and 0.2) increases at temperatures below TC. Moreover, the magnetoresistance of these alloys is positive around TC; this is also in a sharp contrast to the usually observed negative magnetoresistance. We show that the anomalous resistivity and magnetoresistance in these alloys are related to the metamagnetic instability of the Co 3d subsystem and to the fluctuating local magnetic susceptibility due to atomic substitution. Due to this fluctuating local magnetic susceptibility, the uniform exchange field of 4f moments induces a non-uniform polarization of the 3d system, with static spatial fluctuations of the local magnetization. This static magnetic disorder gives an additional contribution (ρm) to the electrical resistance in the alloys at temperatures below TC. The degree of this disorder changes with decreasing temperature, resulting in specific variation of ρm with temperature.

Effect of carbon on magnetocaloric effect of LaFe11.6Si1.4 compounds and on the thermal stability of its hydrides

La(Fe,Si)13 alloys display a giant magnetocaloric effect when a magnetic field is applied near the Curie temperature TC. However, to use these alloys for domestic refrigeration based on magnetic cooling, it is vital to increase TC near to the room-temperature range while simultaneously maintaining a large magnetocaloric effect. With this aim, we studied the effect of interstitial carbon on the microstructure and magnetocaloric effect in LaFe11.6Si1.4Cx (x = 0–0.4). The investigation was carried out in cast samples annealed for seven days at 1323 K. The study of microstructure shows that annealing led to about 90 wt. % of 1:13 magnetocaloric phase. Magnetization data revealed that the addition of carbon leads to an increase in TC and a decrease of the thermal hysteresis width. For x > 0.2, the magnetic transition changes from first-order to second-order, with a corresponding reduction in magnetocaloric effect. A small amount of C (x up to 0.2) improves the magnetocaloric properties of the parent alloy La(Fe,Si)13, and, furthermore, the carbon addition leads to an increase in the thermal stability of hydrided LaFe11.6Si1.4Cx. The onset of hydrogen desorption increases from 460 K for the x = 0 (carbon-free alloy) to 500 K and 540 K, respectively, for x = 0.1 and x = 0.2.

Magnetic/structural diagram, chemical composition-dependent magnetocaloric effect in self-doped antipervoskite compounds Sn1−xCMn3+x (0≤x≤0.40)

The effects of Mn substitutions on the crystal structure, magnetic properties, and magnetocaloric effect (MCE) of antiperovskite Sn1−xCMn3+x (0≤x≤0.40) have been investigated detailedly. Both the Curie temperature (TC) and the magnetizations at 40kOe decrease with increasing x firstly for x≤0.10, and then increase with increasing x further. The type of magnetic transition changes from first-order to second-order around x=0.10 with increasing x. Chemical composition-dependent MCE is also studied around TC. With increasing x, the maximal magnetic entropy changes decrease and the magnetic phase transitions broaden. Accordingly, the relative cooling power (RCP) increases with increasing x, reaching the largest values of ∼0.56J/cm3 (∼75J/kg) and ∼1.66J/cm3 (∼221J/kg) with the magnetic changes of 20kOe and 48kOe, respectively. Considering the large RCP, inexpensive, and innoxious raw materials, these serial samples Sn1−xCMn3+x are suggested to be potential room-temperature magnetic refrigerant materials.

Effect of Ca on the microstructure and magnetocaloric effects in the La1−xCaxFe11.5Si1.5 compounds

The effect of Ca on the microstructure and magnetocaloric effects has been investigated in the La1−xCaxFe11.5Si1.5 (x=0, 0.1, 0.2 and 0.3) compounds. The introduction of Ca leads to the appearance of minor α-Fe and Ca-rich phases, which affects the actual compositions of the main phases for the Ca containing samples. With increasing the Ca concentration, the Curie temperature TC increases from 183 to 208K, and the maximum magnetic entropy changes |ΔS| at the respective TC with a magnetic field change from 0 to 5T are 21.3, 19.5, 16.9, and 11.2J/kgK for x=0, 0.1, 0.2, and 0.3, respectively. The nature of the magnetic transition changes from first-order to second-order with an increase in Ca concentration, which leads to a reduction of the hysteresis and a decrease of the magnetic entropy change. However, the relative cooling power for La1−xCaxFe11.5Si1.5 compounds remains comparable with or even larger than that of other magnetocaloric materials over a wide temperature range. The higher TC and the smaller hysteresis in comparison with those of the parent compound suggest that the La1−xCaxFe11.5Si1.5 compounds could be suitable candidates for magnetic refrigerants in the corresponding temperature range.

Resistivity and Thermopower of Ho(Co1-xAlx)2-Effects of Pressure and Magnetic Field

The electrical resistivity ρ and thermopower S of the cubic Laves phase compounds Ho(Co 1- x Al x ) 2 ( x =0, 0.15 and 0.20) have been measured at temperatures from 2 to 300 K in magnetic fields up to 10 T. The measurements of ρ under pressures up to 3 GPa have been performed in the temperature range between 2 K and 300 K. Both ρ( T ) and S ( T ) of the pure HoCo 2 reveal abrupt changes at the magnetic ordering temperature T C , indicating the first order transition. ρ( T ) of the Al substituted samples ( x =0.15 and 0.20) show anomalous behavior in the form of an increase below T C . The variations of ρ and S at T C clearly indicate that the type of the magnetic transition changes under substitution of Al for Co from the first order to the second order. The residual resistivity of Ho(Co 1- x Al x ) 2 shows a rapid increase with increasing x . T C increases linearly for x < 0.15, having a maximum at x ≈0.15 and decreases with further increase of x . The peculiar x dependence of T C and the anomalous behav...

Magnetic and magnetothermal properties of La1−xNdxFe11.5Al1.5 compounds

In this paper, we report the structural, magnetic, and magnetocaloric effect (MCE) of La1−xNdxFe11.5Al1.5 (x=0.1,0.2) compounds. Temperature dependence of magnetization data shows that with Nd substitution, the nature of magnetic transition changes from second order transition to multiple first order transition. This observation is confirmed by the thermodynamic analysis using the Landau theory of phase transitions. The MCE has been calculated in terms of the isothermal magnetic entropy change (ΔSM) using the magnetization isotherms obtained at temperatures close to the transition temperature. The maximum values of ΔSM are found to be 5.4 and 4.6Jkg−1K−1 of x=0.1 and x=0.2, respectively, for a field change of 50kOe, whereas the value for the undoped compound is about 3Jkg−1K−1. The refrigerant capacity has been calculated to be 544J∕kgK for x=0.1 and 470J∕kgK for x=0.2.

Magnetic, magnetothermal, and magnetotransport properties in SmMn2Si2−xGex compounds

The effect of Ge substitution for Si in SmMn2Si2−xGex compounds has been studied. The Sm ordering temperature is found to be much larger in the compound with x=2, as compared to the compounds with x=0 and 1. The increase in the intralayer Mn–Mn distance is attributed to this variation. Among these three compounds, SmMn2Ge2 is found to show reentrant ferromagnetism at low temperatures. The magnetic contribution to the heat capacity has been calculated in all the three compounds. The splitting of the ground state multiplet has been estimated by fitting the magnetic part of the heat capacity data, using the Schottky formula. The isothermal magnetic entropy change is found to remain the same for x=0 and 1 but decreases in the compound with x=2, though the nature of magnetic transition changes from second order to first order, as x is increased from 0 to 2. The electrical resistivity increases with Ge concentration. The excess resistivity in the antiferromagnetic region has been calculated.

Phase separation and competition of superconductivity and magnetism in the two-dimensional Hubbard model: From strong to weak coupling

Cooperation and competition between the antiferromagnetic, $d$-wave superconducting, and Mott-insulating states are explored for the two-dimensional Hubbard model including nearest and next-nearest-neighbor hoppings at zero temperature. Using the variational cluster approach with clusters of different shapes and sizes up to 10 sites, it is found that the doping-driven transition from a phase with microscopic coexistence of antiferromagnetism and superconductivity to a purely superconducting phase is discontinuous for strong interaction and accompanied by phase separation. At half-filling the system is in an antiferromagnetic Mott-insulating state with vanishing charge compressibility. Upon decreasing the interaction strength $U$ below a certain critical value of roughly ${U}_{c}\ensuremath{\sim}4$ (in units of the nearest-neighbor hopping), however, the filling-dependent magnetic transition changes its character and becomes continuous. Phase separation or, more carefully, the tendency towards the formation of inhomogeneous states disappears. This critical value is in contrast to previous studies, where a much larger value was obtained. Moreover, we find that the system at half-filling undergoes the Mott transition from an insulator to a state with a finite charge compressibility at essentially the same value. The weakly correlated state at half-filling exhibits superconductivity microscopically admixed to the antiferromagnetic order. This scenario suggests a close relation between phase separation and the Mott-insulator physics.

Universal behavior at weak first order quantum phase transitions

First order quantum phase transitions besides their general interest are clearly relevant to the study of heavy fermions and magnetic transition metal compounds. Recent results show that in many systems belonging to these classes of materials, the magnetic transition changes from second order to first order as the quantum critical point (QCP) is approached. We are interested here in the case the change in the nature of the T = 0 transition is due to a coupling of the order parameter to soft modes or to the competition between different types of order near the QCP. For these weak first order quantum phase transitions general results are obtained. In particular, we describe the thermodynamic behavior when this transition is approached from finite temperatures. This is the discontinuous equivalent of the non-Fermi liquid trajectory close to a conventional QCP, for example, in heavy fermion systems.

Magnetic phase transition in Laves phase DyCo2 probed by XRD and XMCD under high pressure

Pressure effects of the magnetic phase transition in DyCo2 were studied by XRD and XMCD. The magnetic transition changes from 1st order to 2nd order in a range of 4–5GPa. The Co K-edge XMCD is insensitive to the pressure, which is interpreted as resulting mainly from the pressure variation of Dy 4f magnetic polarization.

Magnetic Properties of Tetragonal Ce1-xLuxB2C2(0 ≤x≤0.31) Compounds

Specific heat, dc - and ac -magnetic susceptibility and magnetization measurements were performed on rare earth intermetallic compounds Ce 1- x Lu x B 2 C 2 (0 ≤ x ≤0.31) in order to elucidate the magnetic interaction in CeB 2 C 2 . In the process of the substitution of Ce by Lu in CeB 2 C 2 , the characteristics of magnetic transition change from a first-order transition to a second-order one. Furthermore, the spin-glass phase appears at Lu concentration x =0.31. We suppose that the ferromagnetic (FM) and antiferromagnetic (AFM) interactions compete and nearly cancel out each other in CeB 2 C 2 , and hence that a biquadratic interaction might cause the first-order magnetic transition at T N . The Lu substitution breaks the competition between AFM and FM interactions and brings forth randomness in interactions acting on each Ce moment. The typical spin-glass behavior of x =0.31 originates in this random distribution of magnetic interactions.

The magnetic phase transitions in (Tb, Ho)Co 2 and (Tb, Y)Co 2 compounds

The type of the magnetic phase transitions in a series of Tb x Ho 1− x Co 2 and Tb x Y 1− x Co 2 compounds is studied by means of magnetization, electrical-resistivity and specific-heat measurements. In both systems the T C values decrease with decreasing Tb content. At about x = 0.7 the type of the magnetic transition changes from second order to first order. An explanation is offered in terms of the Inoue-Shimizu model by taking into account the strong temperature dependence of the magnetic susceptibility of the 3d itinerant electrons.

Electronic and magnetic properties of ErxY1-xCo2compounds

Ferromagnetic compounds of the series ErxY1-xCo2 (x=1, 0.8, 0.7, 0.6) have been investigated in specific-heat, thermal-expansion, magnetisation, AC susceptibility and resistivity experiments. The magnetic moment on the Co atoms decreases with decreasing Er content, as does the Curie temperature. The character of the magnetic transition changes from first order to second order around x=0.7. This change is compared with the similar one observed in other RExY1-xCO2 compounds with RE-Gd, Tb, Dy and is discussed in terms of the Inoue-Shimizu model by taking into account the strong temperature dependence of the magnetic susceptibility of the 3d electrons. The spontaneous volume magnetostriction at zero temperature, the magnetic contribution to the specific heat and the magnetic entropy below the magnetic ordering temperature are determined. The experimental results for these quantities confirm the induced character of the cobalt magnetic moment in these compounds.

Magnetic and chemical properties of mechanically activated zinc and nickel ferrites

In the course of mechanical activation zinc and nickel ferrites transform to the amorphous state and change their magnetic and chemical properties due to the effects of the cation redistribution and disordering of the oxygen sublattice. In this case these ferrites exhibit the properties of amorphous magnetics and, specifically, the temperature of their magnetic transition changes. Mechanically activated substance is nonuniform in composition. A series of its states have been observed experimentally. The solubility of zinc ferrites increases in acids with increasing the time of mechanical activation and corresponds to the degree of their transformation to the mechanically activated state. The change in the coordination number of zinc cations leads to the change of the ratio between the leaching rates of zinc and iron cations.