First-order Ferromagnetic Transition Research Articles

Structure, Magnetic and Magnetocaloric Properties of Attempted P Substitutions in LaFe<sub>11.5</sub>Si<sub>1.5</sub> Giant Magnetocaloric Material

Recent theoretical and experimental studies suggested that P can enter the structure of La(Fe,Si)13 alloys and lead to a significant enhancement of the isothermal entropy change, one of the two main quantities characterizing the magnetocaloric effect. Here, we report a systematic study of P for Si substitutions in La(Fe,Si)13 alloys. Eight LaFe11.5Si1.5-xPx polycrystalline bulk samples with 0 ≤ x ≤ 0.2 were prepared by arc-melting followed by heat treatment. Powder x-ray diffraction and SEM/EDX analyses show that the α-Fe secondary phase progressively increases with the increase in P substitutions and that a La-rich LaP secondary phase appears. We therefore found that P does not actually enter the main La(Fe,Si)13 phase. Magnetization and DSC measurements confirm this interpretation as the Curie temperatures of La(Fe,Si,P)13 alloys are nearly insensitive to P for Si substitutions and the latent heat of the first-order ferromagnetic transition decreases with the increase in nominal P substitutions. Our work put into questions former reports of the literature on P addition in La(Fe,Si)13 and highlights the particularly complex synthesis of these alloys.

Burst-like features observed from different techniques at the first-order magnetic transition of La(Fe,Co,Si)13

La(Fe,Si)13 alloys are prone to display burst-like phenomena at their first-order ferromagnetic transition, such as a latent heat decomposed into a succession of heat flux avalanches. For some extreme cases, these phenomena are so pronounced that they can yield recalescence/antirecalescence events. It turns out that these features are also highly sensitive to the technical specificities of the employed measurement systems. Here, we compare the thermal events at the first-order magnetic transition of LaFe11.4Si1.31Co0.29 using magnetization, two differential scanning calorimeters and a thermometry probe specially designed for the present study. These techniques are complementary to each other, and allow to make a distinction between the properties influenced by thermal coupling and those more intrinsic to the compound or to the sample. A simple thermal model is built to account for the thermal lags in the different devices, and to provide insights into the dependence of the duration of the transition on the thermal sweep rate. Looking in closer detail at the development of the transition, we found evidence of a rate independent initial regime, followed by a second one along which the same shape of temporal profile is spread over a more or less long duration depending on the sweep rate. Possible underlying mechanisms corresponding to these regimes are discussed.

Magnetization steps at the ferromagnetic transition of (Mn,Fe)2(P,Si) single crystals

Single crystals are highly desirable to study intrinsic properties of phase transitions, unfortunately due to an embrittlement at the transformation, (Mn,Fe)2(P,Si) studies were so far limited to polycrystalline samples. Here, we report the growth of (Mn,Fe)2(P,Si) single crystals which can be cycled across their first-order ferromagnetic transition. In single crystals with optimized composition, detailed magnetization measurements reveal a ferromagnetic transition whose shape is significantly different from polycrystalline samples, with unusual discontinuous magnetization steps induced by temperature or magnetic field leading to a stair-like transition. In contrast to the “virgin effect” of (Mn,Fe)2(P,Si) samples, this behavior is reproducible. This unique magnetic response of some (Mn,Fe)2(P,Si) crystals can be correlated to a microstructure very different from that of the bulk and preserving the clamping/release role played by strains and stresses at the ferro-paramagnetic transformation. On top of offering more details on how the first-order magnetic transition develops in time, space and as a function of the driving parameter, which is of importance for designing functional magnetocaloric materials, the growth of cyclable crystals opens new possibilities for future experiments.

Effect of Mn substitution in Cr1-xMnxSbSe3 van der Waals ferromagnetic material

A solid-state reaction has been used to synthesize Cr1-xMnxSbSe3 samples with a concentration of 0 ≤ x ≤ 0.1. The crystalline structure of the prepared samples was determined by X-ray diffraction (XRD) patterns in a pure single phase. The magnetic properties of the ferromagnetic samples were determined using AC-Susceptibility instruments. Mn substitutions revealed non-monotonic changes in the transition temperature. The XRD patterns of all samples Cr1-xMnxSbSe3 indicate that the crystalline structure is orthorhombic. Moreover, the Mn substitutions did not appreciably change the lattice parameters in the prepared samples. The ac-susceptibility of all samples revealed a sharp first-order ferromagnetic transition at the Curie temperature Tc. The real part of AC susceptibility for all samples is normalized on temperature for various Mn concentrations. XRD and AC-susceptibility analyses agree with the other published work. The activation energy was obtained using the ac-susceptibility measurements at various frequencies.

“Two-steps” process in the first-order transformation of giant magnetocaloric materials

The most promising materials for applications in magnetic refrigeration exhibit a large entropy change at a first-order ferromagnetic transition. Often, it has been observed that the transformation proceeds via two bursts surrounding a regime of slow transformation. Within a simplified framework, the present article reports on a thermodynamic model able to account for this “two-steps” response. This model is confronted to magnetic and calorimetric data recorded in the magnetocaloric compound LaFe11.4Si1.31Co0.29 .

Ferromagnetic and spin-glass like transition in the q-neighbor Ising model on random graphs

The q-neighbor Ising model is investigated on homogeneous random graphs with a fraction of edges associated randomly with antiferromagnetic exchange integrals and the remaining edges with ferromagnetic ones. It is a nonequilibrium model for the opinion formation in which the agents, represented by two-state spins, change their opinions according to a Metropolis-like algorithm taking into account interactions with only a randomly chosen subset of their q neighbors. Depending on the model parameters in Monte Carlo simulations, phase diagrams are observed with first-order ferromagnetic transition, both first- and second-order ferromagnetic transitions and second-order ferromagnetic and spin-glass-like transitions as the temperature and fraction of antiferromagnetic exchange integrals are varied; in the latter case, the obtained phase diagrams qualitatively resemble those for the dilute spin-glass model. Homogeneous mean-field and pair approximations are extended to take into account the effect of the antiferromagnetic exchange interactions on the ferromagnetic phase transition in the model. For a broad range of parameters, critical temperatures for the first- or second-order ferromagnetic transition predicted by the homogeneous pair approximation show quantitative agreement with those obtained from Monte Carlo simulations; significant differences occur mainly in the vicinity of the tricritical point in which the critical lines for the second-order ferromagnetic and spin-glass-like transitions meet.Graphic abstract

Crystal structure and physical properties of Yb2In and Eu2−xYbxIn alloys

While binary $R{E}_{2}\mathrm{In}$, where $RE=\mathrm{rare}\phantom{\rule{0.16em}{0ex}}\mathrm{earth}$, have been reported a few decades ago, recent investigations revealed intriguing new physical insights. For instance, the discovery of a nearly ideal first-order ferromagnetic transition in ${\mathrm{Eu}}_{2}\mathrm{In}$ calls for further exploration of structures and properties of $R{E}_{2}\mathrm{In}$, in particular for the least-documented $RE=\mathrm{Eu}$ and Yb cases. Here, we investigate ${\mathrm{Eu}}_{2\text{\ensuremath{-}}x}{\mathrm{Yb}}_{x}\mathrm{In}$ pseudobinaries with nominal values of $x=0.25$, 0.5, 0.75, 1, 1.5, 2 by powder x-ray diffraction (including as function of temperature from 100 to 375 K for ${\mathrm{Yb}}_{2}\mathrm{In}$), magnetization (5--300 K), as well as electrical resistivity (5--300 K) and calorimetric (2--150 K) measurements for ${\mathrm{Yb}}_{2}\mathrm{In}$. Compared to other RE, Yb or Eu always raise challenging questions linked to their valence states. From average atomic volume, Yb is anticipated to be divalent in ${\mathrm{Yb}}_{2}\mathrm{In}$, at least between 100 and 375 K, which is in line with the absence of $4f$ magnetism. In agreement with x-ray diffraction and magnetization data, the resistivity of ${\mathrm{Yb}}_{2}\mathrm{In}$ is rather featureless and typical of a metal. Establishing ${\mathrm{Yb}}_{2}\mathrm{In}$ as a nonmagnetic isostructural reference for ${\mathrm{Eu}}_{2}\mathrm{In}$ allows one to use its heat capacity to revisit that of the latter, and get experimental insights into the exceptional magnetocaloric effect of the compound with Eu. In particular, we show that a third of the total magnetic entropy (${S}_{\mathrm{m}}\ensuremath{\approx}35.6\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at $T=100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) is concentrated in a 3 K temperature window around the ${T}_{\mathrm{C}}$ of ${\mathrm{Eu}}_{2}\mathrm{In}$. Starting from the ferromagnetic compound ${\mathrm{Eu}}_{2}\mathrm{In}$ $[{T}_{\mathrm{C}}=55.2(5)\phantom{\rule{0.16em}{0ex}}\mathrm{K}]$, we show that Yb substitutions in ${\mathrm{Eu}}_{2\text{\ensuremath{-}}x}{\mathrm{Yb}}_{x}\mathrm{In}$ lead to a decrease in both the Curie temperature [${T}_{\mathrm{C}}=41(2)$ and 32(2) K for $x=0.25$ and 0.5] and magnetic saturation, while weakening the first-order character of the transition as $x$ increases. A significant isothermal entropy change of $5.1(4)\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for $\mathrm{\ensuremath{\Delta}}B=2\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ is found at 44 K in ${\mathrm{Eu}}_{1.75}{\mathrm{Yb}}_{0.25}\mathrm{In}$, demonstrating that the giant magnetocaloric effect of ${\mathrm{Eu}}_{2}\mathrm{In}$ can be tuned to lower temperatures by Yb substitutions.

Magnetization Studied as a Function of Temperature and Magnetic Field for Ferromagnetic Transition in DMNaFe

Magnetization has been calculated as a function of temperature in the ferromagnetic phase of (CH3)2NH2Na0.5Fe0.5(HCOO)3 denoted by DMNaFe as one of the metal formate framework by using molecular field theory. Calculated M(T) is compared with the magnetization measured as a function of temperature (H = 10 Oe) in field-cooling and zero-field-cooling regimes from the literature, and a power-law analysis of the experimental data was performed for DMNaFe. Magnetization measured as a function of the magnetic field, as reported in the literature, has also been analyzed by the power-law formula. The magnetization indicates a weak first-order (or nearly second-order) ferromagnetic transition in DMNaFe.

First-order ferromagnetic transitions of lanthanide local moments in divalent compounds: An itinerant electron positive feedback mechanism and Fermi surface topological change

Around discontinuous (first-order) magnetic phase transitions the strong caloric response of materials to the application of small fields is widely studied for the development of solid-state refrigeration. Typically strong magnetostructural coupling drives such transitions and the attendant substantial hysteresis dramatically reduces the cooling performance. In this context we describe a purely electronic mechanism which pilots a first-order paramagnetic-ferromagnetic transition in divalent lanthanide compounds and which explains the giant non-hysteretic magnetocaloric effect recently discovered in a Eu$_2$In compound. There is positive feedback between the magnetism of itinerant valence electrons and the ferromagnetic ordering of local $f$-electron moments, which appears as a topological change to the Fermi surface. The origin of this electronic mechanism stems directly from Eu's divalency, which explains the absence of a similar discontinuous transition in Gd$_2$In.

Structural and magnetic phase diagrams of MnFe0.6Ni0.4(Si,Ge) alloys and their giant magnetocaloric effect probed by heat capacity measurements

The structural and magnetic phase diagrams of MnFe0.6Ni0.4Si1−xGex alloys (0 ≤ x ≤ 1) are explored using powder X-ray diffraction, magnetic and calorimetric measurements. At room temperature, the crystal structure evolves from the orthorhombic TiNiSi-type to the hexagonal Ni2In-type with the increase in Ge. A magneto-structural coupling leading to a first-order ferromagnetic transition is found at 204 K for x ~ 0.5. Differential scanning calorimetry reveals a recalescence-like event at the first cooling across the structural transition. In-field specific heat measurements are used to quantify the giant magnetocaloric effect and show that the latent heat of the magneto-structural transition is less than in closely related MnMX alloys, which leads to sizable transition sensitivity to the magnetic field and finite adiabatic temperature change.

Exchange intervalley scattering and magnetic phase diagram of transition metal dichalcogenide monolayers

We analyze magnetic phases of monolayers of transition metal dichalcogenides that are two-valley materials with electron-electron interactions. The exchange intervalley scattering makes two-valley systems less stable to the spin fluctuations but more stable to the valley fluctuations. We predict a first-order ferromagnetic phase transition governed by the nonanalytic and negative cubic term in the free energy that results in a large spontaneous spin magnetization. Finite spin-orbit interaction leads to the out-of-plane Ising order of the ferromagnetic phase. Our theoretical prediction is consistent with the recent experiment on the electron-doped monolayers of ${\mathrm{MoS}}_{2}$ reported by Roch et al. [Nat. Nanotechnol. 14, 432 (2019)]. The proposed first-order phase transition can also be tested by measuring the linear magnetic field dependence of the spin susceptibility in the paramagnetic phase which is a direct consequence of the nonanalyticity of the free energy.

Field-induced first-order ferromagnetic transition in (La0.8Eu0.2)4/3Sr5/3Mn2O7 single crystal

Field-induced first-order ferromagnetic transition in (La0.8Eu0.2)4/3Sr5/3Mn2O7 single crystal

Magnetocaloric effect in itinerant magnets around a metamagnetic transition

The phase diagram and magnetocaloric effect in itinerant magnets is explored within the Stoner theory, which yields a reasonable description of the metamagnetic transition observed in various compounds. We obtain the phase diagram as a function of temperature and magnetic field, identifying the region of metastability around the first-order ferromagnetic transition. The impact on the magnetocaloric properties has been verified through the calculation of the isothermal entropy change ΔS, which is computed from two alternative methods based on specific heat or magnetization data. From the direct comparison between the two methods, we observe that the second one is strongly dependent on the process, and we explain under what conditions they become equivalent by using the Clausius-Clapeyron equation. We also discuss the effect of metastable states on the curves of ΔS. The evolution of the transition from first to second order is in good agreement with the phenomenological approach based on the Landau expansion. The results can be applied to different magnetic compounds such as RCo2, MnAs1−xSbx, and La(FexSi1−x)13.

Ferromagnetic ordered phase of quantum spin ice system Yb2Ti2O7 under [001] magnetic field

Measurements of magnetization (M) and specific heat (C) under a [001] magnetic field were carried out on a single crystal of a quantum spin ice system Yb2Ti2O7 in order to investigate a feature of the transition occurred at TC ∼ 0.2 K. As a result of applying the magnetic field μ0H < 0.1 T, the C/T − T curve structure and transition temperature barely changed. On the other hand, applying the more than 0.1 T magnetic field, the C/T − T curve structure drastically change from sharp peak structure to broad peak one, and the broad peak temperature of C/T − T curves linearly increases with increasing magnetic field (H). In the magnetic field μ0H < 0.1 T, the magnetization drastically increases around TC ∼ 0.2 K with decreasing T, and a thermal hysteresis loop of the M − T curve is observed. With increasing H, the thermal hysteresis loop of the M − T curves disappears above μ0HC = 0.1 T. We can understand these results, where Yb2Ti2O7 exhibits a first-order ferromagnetic transition associated with the latent heat corresponding to the energy of μ0HC = 0.1 T. Basis of the H − T phase diagram along [001] magnetic field, the feature of the transition occurred at TC ∼ 0.2 K in quantum spin ice system Yb2Ti2O7 is discussed.

Electronic and magnetic properties of phosphorus across the first-order ferromagnetic transition of(Mn,Fe)2(P,Si,B)giant magnetocaloric materials

To study the evolution of the electronic states and magnetism of the metallic and metalloid atoms across the first-order ferromagnetic transition of (Mn,${\mathrm{Fe})}_{2}$(P,Si,B) compounds, x-ray absorption (XAS) and magnetic circular dichroism (XMCD) spectra were recorded at the $K$ edges of P, Mn, and Fe. Surprisingly, while the Fe and Mn XAS spectra do not show pronounced changes, the $K$-edge XAS of P is strongly modified across the ferromagnetic transition and is a fingerprint of electronic structure changes at the phase transition. The evolution of the $3p$ electronic states of phosphorus is linked to changes in metal-metalloid bonding. The amplitude of this effect is correlated to the discontinuity in interatomic distances at the transition and progressively disappears with boron substitution. Moreover, it is found that the P atoms carry a finite magnetic moment in the ferromagnetic state in accordance with the results of band-structure calculations.

First-order ferromagnetic transition in single-crystalline (Mn,Fe)2(P,Si)

(Mn,Fe)2(P,Si) single crystals have been grown by flux method. Single crystal X-ray diffraction demonstrates that Mn0.83Fe1.17P0.72Si0.28 crystallizes in a hexagonal Fe2P crystal structure (space group P6¯2m) at both 100 and 280 K, in the ferromagnetic and paramagnetic states, respectively. Magnetization measurements show that the crystals display a first-order ferromagnetic phase transition at their Curie temperature (TC). The preferred magnetization direction is along the c axis. A weak magnetic anisotropy of K1 = 0.28 × 106 J/m3 and K2 = 0.22 × 106 J/m3 is found at 5 K. A series of discontinuous magnetization jumps is observed far below TC by increasing the field at constant temperature. These magnetization jumps are irreversible, occur spontaneously at a constant temperature and magnetic field, but can be restored by cycling across the first-order phase transition.

First-Order Ferromagnetic Transition of Quantum Spin Ice System Yb2Ti2O7

The neutron diffraction and measurements of specific heat and magnetization were carried out on single crystals of a quantum spin ice system Yb 2 Ti 2 O 7. The thermal hysteresis loops of temperature dependence of the magnetization and neutron diffraction intensity indicate a first-order ferromagnetic transition, where the used single crystals are confirmed to have a sharp peak structure in the specific heat (C)–temperature (T) curve at T C ∼ 0.2 K. The magnetic structure analysis of neutron diffraction results revealed that the Yb 3+ moments have a ferromagnetic collinear structure along [001] directions with ordered moment value 1.1 ± 0.1 μB at 0.03 K. The mechanism of ferromagnetic transition of Yb 2 Ti 2 O 7 will be discussed.

Ferromagnetic critical behavior inU(Co1−xFex)Al(0≤x≤0.02)studied byCo59nuclear quadrupole resonance measurements

In order to investigate physical properties around a ferromagnetic (FM) quantum transition point and a tricritical point (TCP) in the itinerant-electron metamagnetic compound UCoAl, we have performed the $^{59}$Co nuclear quadrupole resonance (NQR) measurement for the Fe-substituted U(Co$_{1-x}$Fe$_x$)Al ($x$ = 0, 0.5, 1, and 2%) in zero external magnetic field. The Fe concentration dependence of $^{59}$Co-NQR spectra at low temperatures indicates that the first-order FM transition occurs at least above $x$ = 1%. The magnetic fluctuations along the $c$ axis detected by the nuclear spin-spin relaxation rate $1/T_2$ exhibit an anomaly at $T_\mathrm{max}$ $\sim$ 20 K and enhance with increasing $x$. These results are in good agreement with theoretical predictions and indicate the presence of prominent critical fluctuations at the TCP in this system.

Moment evolution across the ferromagnetic phase transition of giant magnetocaloric(Mn,Fe)2(P,Si,B)compounds

A strong electronic reconstruction resulting in a quenching of the Fe magnetic moments has recently been predicted to be at the origin of the giant magnetocaloric effect displayed by ${\mathrm{Fe}}_{2}\mathrm{P}$-based materials. To verify this scenario, x-ray magnetic circular dichroism experiments have been carried out at the L edges of Mn and Fe for two typical compositions of the ${(\mathrm{Mn},\mathrm{Fe})}_{2}(\mathrm{P},\mathrm{Si},\mathrm{B})$ system. The dichroic absorption spectra of Mn and Fe have been measured in the vicinity of the first-order ferromagnetic transition. The experimental spectra are compared with first-principles calculations and charge-transfer multiplet simulations in order to derive the magnetic moments. Even though signatures of a metamagnetic behavior are observed either as a function of the temperature or the magnetic field, the similarity of the Mn and Fe moment evolution suggests that the quenching of the Fe moment is weaker than previously predicted.

First-order magnetic transition inYb2Ti2O7

The very nature of the ground state of the pyrochlore compound ${\mathrm{Yb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ is much debated, because experimental results demonstrate evidence for either a disordered ground state or a long-range ordered ground state. Indeed, the delicate balance of exchange interactions and anisotropy is believed to lead to competing states, such as a quantum spin liquid state or a ferromagnetic state which may originate from an Anderson-Higgs transition. We present a detailed magnetization study demonstrating a first-order ferromagnetic transition at 245 and 150 mK in a powder and a single-crystal sample, respectively. Its first-order character is preserved up to applied fields of $\ensuremath{\sim}200$ Oe. The transition stabilizes a ferromagnetic component and involves slow dynamics in the magnetization. Residual fluctuations are also evidenced, the presence of which might explain some of the discrepancies between previously published data for ${\mathrm{Yb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$.