Magnetoelastic Transition Research Articles

Magnetocaloric effect of nonstoichiometric La1−xFe11.4+xSi1.6 alloys with first-order and second-order magnetic transitions

Variation from a first-order magnetoelastic transition (FOMT) to a second-order magnetic transition (SOMT) could be controlled by finely adjusting the Fe/La ratio in the non-stoichiometric La1−xFe11.4+xSi1.6 (x = 0, 0.05, 0.10, 0.15 and 0.20) alloys. Particularly, a nearly stoichiometric ratio of NaZn13-type phase was obtained with an addition of excess 0.15 at% Fe. When x value increased from 0 to 0.20, the Curie temperature (TC) increased from 198.6 to 216.6 K due to lattice shrinkage and the magnetic entropy change (−ΔSM) decreased from 18.4 to 8.0 J/(kg·K) (at 0−2 T) and from 22.5 to 13.8 J/(kg·K) (at 0−5 T), which may be ascribed to the change from the itinerant-electron metamagnetism (IEM) to second order magnetic transition. Thus their effective refrigeration capacities declined from 347.6 to 245.9 J kg−1 under 0–5 T, which values were comparable to the Gd5Si2Ge1.9Fe0.1. The Fe/La ratio in NaZn13-phase was possibly dominant to induce the transition from first to second order among La1−xFe11.4+xSi1.6 alloys. The thermal hysteresis decreased by 87% to 0.3 K and magnetic hysteresis losses reduced by 90% to 1.4 J kg−1 under 0–5 T. Near the transition border, the alloys with x = 0.15 produced a giant magnetocaloric effect of about 11.8 J/(kg·K) under 0–2 T at TC = 205.6 K. The magnetic hysteresis was 3.01 J kg−1 and the thermal hysteresis was 1.9 K. The material near the border of transition could be suitable for future magnetic refrigeration application.

Magnetic phase transition and magnetocaloric effect in Mn–Fe–Ni–Ge ribbons

Two Mn–Fe–Ni–Ge ribbon samples are prepared. Due to the inherent antiferromagnetic (AFM) interaction and its instability, the Fe-substitution induces the magnetic-field-induced AFM-ferromagnetic (FM) conversion and then, the competition between the established FM coupling and the intrinsic AFM interaction triggers the first-order AFM–FM magnetoelastic transition in TiNiSi-type state for these two ribbons. When the AFM–FM conversion completes, the coupled magnetostructural transformation from the FM TiNiSi-type to paramagnetic Ni2In-type state with large magnetocaloric effect near room temperature is achieved in these two ribbons.

Large reversible magnetostrictive effect in the Gd1−xSmxMn2Ge2 (x = 0.37, 0.34) alloys at room temperature

The metamagnetic transition and magnetic-field-induced strain are investigated in the Gd66Sm34 and Gd63Sm37 alloys. The thermal- or magnetic-field-induced magnetoelastic transitions are observed in these alloys at room temperature, which are accompanied by large lattice distortion and minimal hysteresis. Large reversible magnetostriction of 900 and 350ppm can be obtained in the Gd63Sm37 alloy at 290K with the magnetic fields of 1T and 0.5T, respectively. This excellent magnetostrictive performance is attributed to the lattice change during the magnetic-field-induced magnetoelastic transition.

Tuning the giant magnetoelastic transition in Ba3BiIr2O9 and Ba3BiRu2O9

We have experimentally investigated the effects of pressure on the magnetoelastic transitions associated with the opening of spin-gaps in Ba3BiIr2O9 and Ba3BiRu2O9. For both compounds, reducing the unit cell volume by either external physical and internal chemical pressure was found to reduce the temperature T* of the transition and, to a lesser extent, the magnitude of the associated negative thermal volume expansion. The results yield the latent heat associated with the transitions, −3.34(3) × 102 J mol−1 for Ba3BiIr2O9 and −7.1(5) × 102 J mol−1 for Ba3BiRu2O9. The transition in Ba3BiRu2O9 is significantly more robust than in Ba3BiIr2O9, requiring an order of magnitude higher pressures to achieve the same reduction in T*. The differing responses of the two compounds points to differences between the 4d and 5d metals and hence to the importance of spin-orbit coupling, which is expected to be much stronger in the Ir compound.

Tuning the phase transition in transition-metal-based magnetocaloric compounds

Neutron-diffraction experiments on the (Mn,Fe)2(P,Si)-type compounds have shown a site preference of Si atoms in the hexagonal structure. The degree of ordering of Si depends on the Si/P ratio, while it is independent of the Mn/Fe ratio. The ferromagnetic-paramagnetic magnetoelastic transition is closely related to the size of the magnetic moment on the 3f site. A preferred occupation of Si atoms on the 2c site stabilizes and decreases the magnetic moment on the 3f and 3g site, respectively, which is supported by our first-principles density functional theory calculations. This effect, together with the contribution from the Si substitution-induced changes in the interatomic distances, leads to a phase transition that is tunable in temperature and degree of first order in Mn1.25Fe0.70P1?xSix compounds. These results provide us with further insight into the relationship between the magnetoelastic phase transition and the local atomic coordination.

Pressure induced magneto-structural phase transitions in layered RMn2X2 compounds (invited)

We have studied a range of pseudo-ternaries derived from the parent compound PrMn2Ge2, substituting for each constituent element with a smaller one to contract the lattice. This enables us to observe the magneto-elastic transitions that occur as the Mn-Mn nearest neighbour distance is reduced and to assess the role of Pr on the magnetism. Here, we report on the PrMn2Ge2−xSix, Pr1−xYxMn2Ge2, and PrMn2−xFexGe2 systems. The pressure produced by chemical substitution in these pseudo-ternaries is inherently non-uniform, with local pressure variations dependent on the local atomic distribution. We find that concentrated chemical substitution on the R or X site (e.g., in Pr0.5Y0.5Mn2Ge2 and PrMn2Ge0.8Si1.2) can produce a separation into two distinct magnetic phases, canted ferromagnetic and canted antiferromagnetic, with a commensurate phase gap in the crystalline lattice. This phase gap is a consequence of the combination of phase separation and spontaneous magnetostriction, which is positive on transition to the canted ferromagnetic phase and negative on transition to the canted antiferromagnetic phase. Our results show that co-existence of canted ferromagnetic and antiferromagnetic phases depends on chemical pressure from the rare earth and metalloid sites, on local lattice strain distributions and on applied magnetic field. We demonstrate that the effects of chemical pressure bear close resemblance to those of mechanical pressure on the parent compound.

ChemInform Abstract: Key Role of Bismuth in the Magnetoelastic Transitions of Ba3BiIr2O9 and Ba3BiRu2O9 as Revealed by Chemical Doping.

AbstractPolycrystalline samples of Ba3La1‐xCexRu2O9, Ba3Ce1‐xBixRu2O9, Ba3La1‐xBixRu2O9, and Ba3La1‐xBixIr2O9 (0 ≤ x ≤ 1) are prepared by stoichiometric solid state reaction of BaCO3, La2O3, Bi2O3, CeO2, and Ru and Ir metal in air.

Key Role of Bismuth in the Magnetoelastic Transitions of Ba3BiIr2O9 and Ba3BiRu2O9 As Revealed by Chemical Doping

The key role played by bismuth in an average intermediate oxidation state in the magnetoelastic spin-gap compounds Ba3BiRu2O9 and Ba3BiIr2O9 has been confirmed by systematically replacing bismuth with La(3+) and Ce(4+). Through a combination of powder diffraction (neutron and synchrotron), X-ray absorption spectroscopy, and magnetic properties measurements, we show that Ru/Ir cations in Ba3BiRu2O9 and Ba3BiIr2O9 have oxidation states between +4 and +4.5, suggesting that Bi cations exist in an unusual average oxidation state intermediate between the conventional +3 and +5 states (which is confirmed by the Bi L3-edge spectrum of Ba3BiRu2O9). Precise measurements of lattice parameters from synchrotron diffraction are consistent with the presence of intermediate oxidation state bismuth cations throughout the doping ranges. We find that relatively small amounts of doping (∼10 at%) on the bismuth site suppress and then completely eliminate the sharp structural and magnetic transitions observed in pure Ba3BiRu2O9 and Ba3BiIr2O9, strongly suggesting that the unstable electronic state of bismuth plays a critical role in the behavior of these materials.

Double symmetry breaking in TmFe4Ge2 compared to RFe4Ge2 (R=Y, Lu, Er, Ho, Dy) magnetic behaviour

TmFe4Ge2 undergoes a double magneto-elastic first order transition at TN,Tc where the high temperature (HT) tetragonal phase disproportionate into two distinct orthorhombic low temperature (LT) phases with commensurate and incommensurate magnetic wave vectors respectively:P42/mnm(HT)TN,Tc→Cmmmq1=(0,12,0)+Pnnm(q2=(0,qy,0),qy≈2/11(LT)Neutron diffraction shows the relative portions of the LT Cmmm and Pnnm competing phases change linearly with T. The amount of the majority HT phase Pnnm (54% at 30K) decreases linearly to 30% down to 10K in favour of the Cmmm phase that dominates the range 26–1.5K. The Tm moments point along the c-axis in both phases while the Fe moments have canted arrangements. The μTm=3.54(3)μB/atom at 1.5K is strongly reduced below the Tm3+ free ion value gJJ=7μB for the q1 phase. The q2 phase corresponds to a 3D canted sinusoidal arrangement. The results are summarised on a phase diagram and compared to the findings in RFe4Ge2 (R=Y, Lu, Er, Ho, Dy) that are reviewed. The multitude of transition paths occurring in those systems arise from the competing magnetoelastic mechanisms involving the R-crystal field anisotropy, the exchange interactions R–R, R–Fe, Fe–Fe of the two sublattices and their coupling to the lattice strain. The geometrical frustration emerging from the compact tetrahedral Fe arrangement with antiferromagnetic interactions leads to 2D and 3D canted, incommensurate and non-magnetic states. The Cmmm transition is triggered by dominating R–R and R–Fe interactions becoming stronger at LT while the Pnnm phase is promoted by Fe–Fe and R–Fe interactions that prevail at HT. Included is also the magnetic structure of the ferromagnetic impurity phase Fe3Ge.

Magnetoelastic phase transitions in the LuFe4Ge2 and YFe4Si2 compounds: A neutron diffraction study

The magnetic structure of the tetragonal antiferromagnetic LuFe4Ge2 and YFe4Si2 compounds has been studied by high resolution neutron and X-ray diffraction above and below the magneto-elastic transition at TN, Tc=32K and 76K, respectively. Below TN,Tc the tetragonal high temperature (HT) phase transforms into the low temperature (LT) orthorhombic magnetic phase: P42/mnm (HT) → Pnnm (q=0) (LT) with a 2D canted moment arrangement of the Fe moments within the (001) plane with the magnetic space group P21n'21n'21m'(Sh58399). The Fe magnetic moment value is highly reduced in both compounds due to the presence of geometrical and exchange frustration relating to the underlying crystal structure comprising columns of Fe tetrahedra expanding along the c direction with antiferromagnetic interactions. The magnetic refinements were perturbed by the presence of partly unidentified impurity phases mainly in YFe4Si2.

From first-order magneto-elastic to magneto-structural transition in (Mn,Fe)1.95P0.50Si0.50 compounds

We report on structural, magnetic, and magnetocaloric properties of MnxFe1.95−xP0.50Si0.50 (x ≥ 1.10) compounds. With increasing the Mn:Fe ratio, a first-order magneto-elastic transition gradually changes into a first-order magneto-structural transition via a second-order magnetic transition. The study also shows that thermal hysteresis can be tuned by varying the Mn:Fe ratio. Small thermal hysteresis (less than 1 K) can be obtained while maintaining a giant magnetocaloric effect. This achievement paves the way for real refrigeration applications using magnetic refrigerants.

Magnetoelastic Quantum Fluctuations and Phase Transitions in the Iron Superconductors

We examine the relevance of magnetoelastic coupling to describe the complex magnetic and structural behavior of the different classes of the iron superconductors. We model the system as a two-dimensional metal whose magnetic excitations interact with the distortions of the underlying square lattice. Going beyond the mean field, we find that quantum fluctuation effects can explain two unusual features of these materials that have attracted considerable attention: first, why iron telluride orders magnetically at a non-nesting wave vector (π/2,π/2) and not at the nesting wave vector (π,0) as in the iron arsenides, even though the nominal band structures of both these systems are similar, and second, why the (π,0) magnetic transition in the iron arsenides is often preceded by an orthorhombic structural transition. These are robust properties of the model, independent of microscopic details, and they emphasize the importance of the magnetoelastic interaction.

The magnetocaloric effect at the first-order magneto-elastic phase transition

This paper presents a study of the magnetocaloric effect at thefirst-order magneto-elastic phase transition. The entropy changeΔs at the transition temperature is given by the sum of the magnetic andthe structural contributions. By using a thermodynamic model, it isshown that the sign and amplitude of the structural contribution toΔs are determined by the dimensionless parameterζ (zeta) whichdepends on β, the steepness of the change of exchange forces with volume, and onαp, the thermal expansion coefficient of the structural lattice. Forζ < 0 thechange Δs can be larger than the magnetic contribution alone, because the structural entropy change and themagnetic entropy change have the same sign, giving rise to the giant magnetocaloric effect. For0 < ζ < 1 the two contributions partially cancel each other and the totalentropy change is lower than the magnetic entropy change. Forζ > 1 the structural entropy dominates and a transition occurs upon heating from a lowtemperature paramagnet to a high temperature ferromagnet.

Magneto structural transition in the DySi CrB- and micro-structural changes in the FeB-type compounds by XRPD and neutron diffraction

We present the magnetic temperature phase diagrams of the CrB- and FeB-type orthorhombic phases of the DySi compound, determined from high-quality powder XRPD and neutron diffraction, as well as the sample microstructure as determined by HRTEM. Both phase diagrams comprise a HT (Tc1, Tc2−TN) and a LT range (5K–Tc1, Tc2) separated by a monoclinic phase transition at Tc1=Tc2=23.5K well below the second-order Néel transition (TN=40K). The transition paths are for CrB-type Cmcm (HT) Tc1→C2/m11 (LT), and for FeB Pnma (HT) Tc2→P21/n11 (LT). The transitions are related to non-monotonous changes of the lattice and structural parameters displaying anomalies at Tc1, Tc2 and slight volume changes. For the CrB-type the monoclinic angle decreases smoothly from Tc1 down to 5K and the maximum strain experienced by the crystal lattice in the (021) direction was found at Tc1.In the FeB-type, in addition to the magneto-elastic transition at Tc2=23.5K, minor anomalies are found at 13.5K in the temperature dependence of the monoclinic angle and the maximum strain along (011). Both temperatures mark the sequence of changes in the magnetic domain microstructure observed in FeB: below T2=23.5K the incommensurate HT magnetic phase with q3≈(01217) disproportionates into two LT phases q3≈(012111)T2 and q2≈(01216) coexisting in the form of domains with portions varying with T going from T2 down to 13.5K (q2 increasing at the cost of q3). This behaviour could be related to structural inhomogeneities below the first-order magneto-elastic transition Tc2, if one assumes a broad two phase range, where the HT (Pnma) phase coexists with (P21/n) as a metastable phase at LT in the form of domains with different magnetic behaviour.

Large Magnetostriction from Morphotropic Phase Boundary in Ferromagnets

For more than half of a century, morphotropic phase boundary (MPB) in ferroelectric materials has drawn constant interest because it can significantly enhance the piezoelectric properties. However, MPB has been studied merely in ferroelectric systems, not in another large class of ferroic systems, the ferromagnets. In this Letter, we report the existence of an MPB in a ferromagnetic system TbCo2-DyCo2. Such a magnetic MPB involves a first-order magnetoelastic transition, at which both magnetization direction and crystal structure change simultaneously. The MPB composition demonstrates a 3-6 times larger "figure of merit" of magnetostrictive response compared with that of the off-MPB compositions. The finding of MPB in ferromagnets may help to discover novel high-performance magnetostrictive and even magnetoelectric materials.

From single- to double-first-order magnetic phase transition in magnetocaloric Mn1−xCrxCoGe compounds

Substitution of some Cr for Mn atoms in MnCoGe was employed to control the magnetic and structural transitions in this alloy to coincide, leading to a single first-order magnetostructural transition from the ferromagnetic to the paramagnetic state with a giant magnetocaloric effect observed near room temperature. Further increase in the Cr content in the Mn1−xCrxCoGe alloys can induce another first-order magnetoelastic transition from the antiferromagnetic to the ferromagnetic state occurring at lower temperature. The giant magnetocaloric effect as well as the simultaneous tunability of the two magnetic transitions make these materials promising for future cooling applications.

Magnetoelastic instability in Ising-like models

The magnetoelastic instability in nanostructured ring-shaped Ising-like spin 1/2 model has been investigated by using the exact diagonalization method. It is found that there exists two critical anisotropy parameters Δ c ( 1 ) and Δ c ( 2 ) ( Δ c ( 1 ) < Δ c ( 2 ) ) in the phase diagram. As the anisotropy parameter Δ > Δ c ( 2 ) , the magnetoelastic transition from dimerized phase to uniform phase is a first order transition with an increase of the lattice spring constant. While for Δ < Δ c ( 2 ) , the transition is continuous. Another critical value Δ c ( 1 ) divides the different lattice distortion behavior as the anisotropy is strengthened.

Magneto-elastic phase transition in a linear chain within the double and super-exchange model

In this work a magneto-elastic phase transition in a linear chain was obtained due the interplay between magnetism and lattice distortion in a double and super-exchange model. We consider a linear chain consisting of classical localized spins interacting with itinerant electrons. Due to the double exchange interaction, localized spins align ferromagnetically. This ferromagnetic tendency is expected to be frustrated by the anti-ferromagnetic super-exchange interaction between neighbor localized spins. Additionally, the lattice parameter is allowed to have small changes, which contributes harmonically to the energy of the system. The phase diagram is obtained as a function of the electron density and the super-exchange interaction using a Monte Carlo minimization. At low super-exchange interaction energy phase transition between electron-full ferromagnetic distorted and electron-empty anti-ferromagnetic undistorted phases occurs. In this case all electrons and lattice distortions were found within the ferromagnetic domain. For high super-exchange interaction energy, phase transition between two site distorted periodic arrangement of independent magnetic polarons ordered anti-ferromagnetically and the electron-empty anti-ferromagnetic undistorted phase was found. For this high interaction energy, Wigner crystallization, lattice distortion and charge distribution inside two-site polarons were obtained.

Monte Carlo study of the double and super-exchange model with lattice distortion

In this work a magneto-elastic phase transition was obtained in a linear chain due to the interplay between magnetism and lattice distortion in a double and super-exchange model. It is considered a linear chain consisting of localized classical spins interacting with itinerant electrons. Due to the double exchange interaction, localized spins tend to align ferromagnetically. This ferromagnetic tendency is expected to be frustrated by anti-ferromagnetic super-exchange interactions between neighbor localized spins. Additionally, lattice parameter is allowed to have small changes, which contributes harmonically to the energy of the system. Phase diagram is obtained as a function of the electron density and the super-exchange interaction using a Monte Carlo minimization. At low super-exchange interaction energy phase transition between electron-full ferromagnetic distorted and electron-empty anti-ferromagnetic undistorted phases occurs. In this case all electrons and lattice distortions were found within the ferromagnetic domain. For high super-exchange interaction energy, phase transition between two site distorted periodic arrangement of independent magnetic polarons ordered anti-ferromagnetically and the electron-empty anti-ferromagnetic undistorted phase was found. For this high interaction energy, Wigner crystallization, lattice distortion and charge distribution inside two-site polarons were obtained.

Role of striction at magnetic and structural transitions in iron pnictides

We discuss the role of striction in the intertwined magnetic and structural phase transitions in the underdoped iron pnictides. The magnetoelastic coupling to acoustic modes is then derived and estimated in framework of the multiband spectrum for itinerant electrons with nesting features. We argue that the first-order character of the magnetoelastic phase transition originates from the lattice instabilities near the onset of spin-density wave order introducing, thus, a shear acoustic mode as another order parameter. Taking nonharmonic terms in the lattice energy into account may explain the splitting of the structural and magnetic transitions in some oxypnictides. Fluctuations of the magnetic order parameter show up in the precursory temperature dependence of the elastic moduli.