Successive Magnetic Transitions Research Articles

Magnetic properties and magnetocaloric effects in Er1 − xGdxCoAl intermetallic compounds**Project supported by the National Natural Science Foundation of China (Grant Nos. 51322605 and 51271192).

The magnetism and magnetocaloric effect in Er1 − xGdxCoAl (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1) were investigated. The Er1 − xGdxCoAl compounds were synthesized by arc melting. With the increasing Gd content, the Néel temperature (TN) linearly increases from 14 K to 102 K, while the magnetic entropy change (−ΔSM) tends to decrease nonmonotonously. Under the field change from 0 T to 5 T, the −ΔSM of the compounds with x = 0.2–1 are stable around 10 J/kg ·K, then a cooling platform between 20 K and 100 K can be formed by combining these compounds. For x = 0.6, 0.8, 1.0, the compounds undergo two successive magnetic transitions, one antiferromagnetism to ferromagnetism and the other ferromagnetism to paramagnetism, with increasing temperature. The two continuous magnetic transitions in this series are advantageous to broaden the temperature span of half-peak width (δT) in the −ΔSM−T curve and improve the refrigeration capacity.

Low-temperature properties of theS=12spin systemYb3Ru4Al12with a distorted kagome lattice structure

We have synthesized single crystals of ternary intermetallic ${\mathrm{Yb}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Al}}_{12}$ with a distorted kagome lattice structure, and investigated the low-temperature resistivity, specific heat, magnetization, and magnetic phase transitions. ${\mathrm{Yb}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Al}}_{12}$ is the first $4f$ system that has a ${\mathrm{Gd}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Al}}_{12}$-type crystal structure where antiferromagnetic interaction acts on the spin. The crystal electric field (CEF) ground state of this compound is determined as a well isolated twofold degenerate state that is subjected to a strong easy-plane-type magnetic anisotropy. In the present study, the spin system of ${\mathrm{Yb}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Al}}_{12}$ is regarded as an AFM $XY$ model of $S=1/2$. This compound undergoes successive magnetic phase transitions at 1.5 and 1.6 K, and the resistivity exhibits ${T}^{2}$ behavior below 1 K. The ratio of the coefficient of the ${T}^{2}$ term in the resistivity $A$, and that of the electronic specific heat coefficient ${\ensuremath{\gamma}}_{0}$, deviates from the Kadowaki-Woods (KW) law. The successive phase transitions and low-temperature properties of ${\mathrm{Yb}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Al}}_{12}$ where geometrical frustration and heavy fermion behavior occur are discussed.

Large entropy change accompanying two successive magnetic phase transitions in TbMn2Si2 for magnetic refrigeration

Structural and magnetic properties in TbMn2Si2 are studied by variable temperature X-ray diffraction, magnetization, electrical resistivity, and heat capacity measurements. TbMn2Si2 undergoes two successive magnetic transitions at around Tc1 = 50 K and Tc2 = 64 K. Tc1 remains almost constant with increasing magnetic field, but Tc2 shifts significantly to higher temperature. Thus, there are two partially overlapping peaks in the temperature dependence of magnetic entropy change, i.e., −ΔSM (T). The different responses of Tc1 and Tc2 to external magnetic field, and the overlapping of −ΔSM (T) around Tc1 and Tc2 induce a large refrigerant capacity (RC) within a large temperature range. The large reversible magnetocaloric effect (−ΔSMpeak ∼ 16 J/kg K for a field change of 0–5 T) and RC (=396 J/kg) indicate that TbMn2Si2 could be a promising candidate for low temperature magnetic refrigeration.

Pressure-induced successive magnetic and structural phase transitions in Fe1.087Te

The crystal and magnetic structures of Fe1.087Te have been studied by neutron powder diffraction in the temperature range from 5 to 170 K at pressures in the range from ≈0.8 to ≈7 GPa. The p, T-phase diagram contains three phases with monoclinic, orthorhombic and tetragonal symmetry. The monoclinic and orthorhombic phases are both antiferromagnetically ordered and stable at temperatures below ≈69 K while the non-magnetic tetragonal phase is stable above this temperature. The monoclinic phase is stable for p ≲ 1.2 GPa while the orthorhombic phase is stable for 1.2 ≲ p ≲ 1.7 GPa and the tetragonal phase becomes stable at higher pressures at the lowest measured temperatures. The magnetic ordering is antiferromagnetic bicollinear and commensurate in the monoclinic phase but incommensurate and pressure dependent in the orthorhombic phase. The pressure-induced collapse of magnetic order at ≈1.7 GPa is accompanied by abrupt changes in volume and compressibility, suggestive of a spin state change of the Fe2+ ions in the FeTe layers.

Magnetic phase transitions and large magnetic entropy change with a wide temperature span in HoZn

CsCl-type HoZn undergoes two successive magnetic phase transitions: (i) paramagnetic to ferromagnetic (FM) at TC∼72K and (ii) a spin reorientation (SR) at TSR∼26K. Magnetization and modified Arrott plots indicate that HoZn undergoes a second-order magnetic phase transition around TC. The obtained critical exponents have some small deviations from the mean-field theory, indicating a short range or a local magnetic interaction which is properly related to the coexistence of FM and SR transitions at low temperature. Two successive magnetic transitions in HoZn induce one broad pronounced peak together with a shoulder in the temperature dependence of the magnetic entropy change −ΔSM(T) curves, resulting in a wide temperature range with a large relative cooling power (RCP). For a field change of 0–7T, the maximum value of −ΔSM is 15.2J/kgK around TC with a large RCP value of 1124J/kg. The large reversible magnetocaloric effect (MCE) and RC indicate that HoZn is a good candidate for active magnetic refrigeration.

Successive magnetic phase transitions inα−RuCl3: XY-like frustrated magnet on the honeycomb lattice

The layered compound $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ is composed of a honeycomb lattice of magnetic ${\mathrm{Ru}}^{3+}$ ions with the $4{d}^{5}$ electronic state. We have investigated the magnetic properties of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ via magnetization and specific heat measurements using single crystals. It was observed that $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ undergoes a structural phase transition at ${T}_{t}\phantom{\rule{0.16em}{0ex}}\ensuremath{\simeq}\phantom{\rule{0.16em}{0ex}}150$ K accompanied by fairly large hysteresis. This structural phase transition is expected to be similar to that observed in closely related ${\mathrm{CrCl}}_{3}$. The magnetizations and magnetic susceptibilities are strongly anisotropic, which mainly arise from the anisotropic $g$ factors, i.e., ${g}_{ab}\phantom{\rule{0.16em}{0ex}}\ensuremath{\simeq}\phantom{\rule{0.16em}{0ex}}2.5$ and ${g}_{c}\phantom{\rule{0.16em}{0ex}}\ensuremath{\simeq}\phantom{\rule{0.16em}{0ex}}0.4$ for magnetic fields parallel and perpendicular to the $ab$ plane, respectively. These $g$ factors and the obtained entropy indicate that the effective spin of ${\mathrm{Ru}}^{3+}$ is one-half, which results from the low-spin state. Specific heat data show that magnetic ordering occurs in four steps at zero magnetic field. The successive magnetic phase transitions should be ascribed to the competition among exchange interactions. The magnetic phase diagram for $H\phantom{\rule{0.16em}{0ex}}\ensuremath{\parallel}\phantom{\rule{0.16em}{0ex}}ab$ is obtained. We discuss the strongly anisotropic $g$ factors in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ and deduce that the exchange interaction is strongly XY-like. $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ is magnetically described as a three-dimensionally coupled XY-like frustrated magnet on a honeycomb lattice.

Characterization of Ferromagnetic Order in CePd2P2

We examine magnetic ordering in CePd2P2 by neutron-scattering experiment and confirmed a ferromagnetic order at low temperatures. From the neutron-scattering data, the ordered moment is evaluated to be ~ 1.3(3) μB per Ce ion at 3 K and is considered to be parallel to the c-axis. In addition, no evidence of the change in the magnetic structure was observed below TC within the experimental accuracy. In DC magnetization measurement with an as-grown powder sample and a magnetically aligned sample, a clear magnetic anisotropy in CePd2P2 was observed in both ferromagnetic and paramagnetic phases. To obtain further insight to the large Curie temperature TC of CePd2P2, we have examined the magnetic properties of GdPd2P2 as a reference material. In DC magnetization measurement, successive magnetic transitions at TI = 10.6 K and TII = 7.4 K were observed in GdPd2P2. By comparing the transition temperatures of the Ce and Gd compounds, we argue the origin of the large Curie temperature of CePd2P2.

Classical Heisenberg antiferromagnet on a triangular lattice in the presence of single-ion anisotropy

The Heisenberg antiferromagnet on a triangular lattice is one of the prototype examples of frustrated systems and has been studied for several decades. In the presence of magnetic fields, the system exhibits a phase diagram consisting of exotic phases. At low temperatures successive magnetic phase transitions occur as the magnetic field is increased. We analyze the ground states of the system to obtain the critical fields at zero temperature. It is also shown through Monte Carlo simulations that the easy-axis single-ion anisotropy in classical Heisenberg antiferromagnet results in the expansion of the uud-phase region.

Spin reorientation inBa0.65Na0.35Fe2As2studied by single-crystal neutron diffraction

We have studied the magnetic ordering in Na doped BaFe$_2$As$_2$ by unpolarized and polarized neutron diffraction using single crystals. Unlike previously studied FeAs-based compounds that magnetically order, Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ exhibits two successive magnetic transitions: For x=0.35 upon cooling magnetic order occurs at $\sim$70\ K with in-plane magnetic moments being arranged as in pure or Ni, Co and K-doped BaFe$_2$As$_2$ samples. At a temperature of $\sim$46\ K a second phase transition occurs, which the single-crystal neutron diffraction experiments can unambiguously identify as a spin reorientation. At low temperatures, the ordered magnetic moments in Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$ point along the $c$ direction. Magnetic correlations in these materials cannot be considered as Ising like, and spin-orbit coupling must be included in a quantitative theory.

Magnetic phase diagram of pseudo-ternary solid solution [formula omitted

A systematic study of the solid solution URu1−xPdxGeby means of X-ray diffraction, dc-magnetization, specific heat and electrical resistivity measurements is presented. The investigated alloys (0.0≤x≤0.9) crystallize in the orthorhombic TiNiSi-type structure with space group Pnma. The variation of the unit cell volume with Pd content x follows approximately Vegard׳s law, through the lattice parameters a, b and c exhibit a little anomaly around x=0.35, 0.7 and 0.8, respectively. It is found that the magnetic ground state of URu1−xPdxGestrongly depends on Pd content. The compositions with x≤0.32 are nonmagnetic down to 0.4K, while these in the concentration range 0.35≤x≤0.8 order antiferromagnetically. The Néel temperature TN is found to follow a relation TN∝(x−0.32)2/3 for the alloys x<0.45 and to attain a maximum value of 32K at x=0.8. The composition x=0.9 alike UPdGe manifests a complex magnetic order, probably undergoes two successive magnetic phase transitions: ferromagnetic-like at TC=33K and unknown ground state characterized by an irreversible magnetization below TIR. The observed development of magnetism has been discussed in terms of changes in the degree of 5f electron localization as well as due to the competition between the Kondo and Ruderman–Kittel–Kausuya–Yosida interactions. Remarkably, in the compound x=0.32 located at the nonmagnetic-magnetic border, we found a non-Fermi liquid behaviour reflected by the power-law temperature dependencies of magnetic susceptibility χ(T)∝T−0.48, specific heat Cp(T)∝−T and electrical resistivity ρ(T)∝T3/2. We compared the results with theoretical predictions for systems with spin fluctuations nearby antiferromagnetic quantum critical point.

ChemInform Abstract: Structural Properties and Singular Phase Transitions of Metallic Pr0.50Sr0.50CoO3 Cobaltite.

AbstractTwo successive magnetic transitions of Pr0.50Sr0.50CoO3 contrast with the single ferromagnetic transition in other Ln0.50Sr0.50CoO3 compounds.

The Successive Component-separated Magnetic–Transitions on Pseudoternary Compounds Ho1-xGdxRh2Si2

Magnetic measurements on pseudoternary compounds Ho1-xGdxRh2Si2, which substitute Gd having no quadrupole for Ho, were performed. They exhibit a successive component-separated magnetic transition; the c- and ab-components of magnetic moments independently order at different temperatures TN1 and TN2, respectively. The partial ordered state, a frustration appears for TN1 >T >TN2: for the phase II in the magnetic phase diagrams. In the ordered phase, step-like metamagnetic processes appear for TN2 >T; two-step ones appear along the [001] and [100] directions, and a one-step one appears along the [110] direction. The B-T magnetic phase diagrams were constructed. There are six, four and three ordered phases in the B001-T, B100-T and B110-T phase diagram, respectively. Two diagrams of the basal plane directions, B100-T and B110-T, resemble each other. Some interesting or peculiar phase boundaries appear. The Gd composition x dependence of transition temperatures is determined. The transition temperatures TN1 and TN2 increase with increasing x. The x-dependency of TN1 is well scaled by the de Gennes factor: (g-1)2J(J+1) whereas the transition of TN2 is not scaled. Some magnetic features declare that quadrupole interactions play an important role in this compound system.

Comparative studies of the thermal conductivity of spinel oxides with orbital degrees of freedom

We studied the thermal conductivity of various transition-metal oxides with the spinel structure $({\mathrm{MnV}}_{2}{\mathrm{O}}_{4}$, ${\mathrm{FeV}}_{2}{\mathrm{O}}_{4}$, ${\mathrm{CoV}}_{2}{\mathrm{O}}_{4}$, and ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4})$ upon varying the temperature and magnetic field. We found that for the spinel oxides having ${\mathrm{V}}^{3+}$ ions (two electrons in the ${t}_{2g}$ states) at the octahedral site, the orbital ordering suppresses the thermal resistivity (inverse thermal conductivity) in contrast to purely magnetic ordering, indicating that the orbital fluctuation of the V ${t}_{2g}$ states is the main factor affecting the thermal conductivity. On the other hand, for ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$, which has ${\mathrm{Mn}}^{3+}$ ions (one electron in the ${e}_{g}$ states) at the octahedral site, the thermal resistivity is suppressed in association with the successive magnetic phase transitions with decreasing temperature and increasing magnetic field. This can be explained by the frustration of ${\mathrm{Mn}}^{3+}$ spins and the fluctuation of ${\mathrm{Mn}}^{3+}$ ${e}_{g}$ orbitals.

Magnetic phase diagram and multiferroicity ofBa3MnNb2O9: A spin-52triangular lattice antiferromagnet with weak easy-axis anisotropy

We have performed magnetic, electric, thermal, and neutron powder diffraction (NPD) experiments as well as density functional theory (DFT) calculations on ${\mathrm{Ba}}_{3}{\mathrm{MnNb}}_{2}{\mathrm{O}}_{9}$. All results suggest that ${\mathrm{Ba}}_{3}{\mathrm{MnNb}}_{2}{\mathrm{O}}_{9}$ is a spin-5/2 triangular lattice antiferromagnet (TLAF) with weak easy-axis anisotropy. At zero field, we observed a narrow two-step transition at ${T}_{\mathrm{N}1}=3.4$ K and ${T}_{\mathrm{N}2}=3.0$ K. The neutron diffraction measurement and the DFT calculation indicate a ${120}^{\ensuremath{\circ}}$ spin structure in the $ab$ plane with out-of-plane canting at low temperatures. With increasing magnetic field, the ${120}^{\ensuremath{\circ}}$ spin structure evolves into up-up-down ($uud$) and oblique phases showing successive magnetic phase transitions, which fits well to the theoretical prediction for the 2D Heisenberg TLAF with classical spins. Multiferroicity is observed when the spins are not collinear but suppressed in the $uud$ and oblique phases.

Successive magnetic transitions in TbNiAl2 studied by neutron diffraction

We report measurements of DC (AC) magnetic susceptibility and neutron diffraction on TbNiAl2 alloy. The Rietveld refinements of the x-ray and neutron diffraction data are consistent with an orthorhombic structure of the type MgCuAl2 (space group Cmcm). The results of DC (AC)- magnetic susceptibility show two successive magnetic transitions at 20 K and 11.7 K with antiferromagnetic and ferromagnetic (or ferrimagnetic) features, respectively. On the other hand, neutron diffraction patterns show that, below 20 K and down to 12 K, new reflexions appear, confirming the antiferromagnetic character of the transition observed in the macroscopic measurements. Also, at least one of these new reflexions, located at Q = 1.2 Å−1, shifts to higher angles when the temperature decreases, indicating an incommensurate magnetic structure. Below 11 K, many reflexions disappear and new reflexions increase, evidencing a new magnetic transition.

Structure, magnetism, specific heat, and dielectric properties of Eu2Ta2O7

Polycrystalline Eu2Ta2O7 containing layered perovskite slabs was prepared and analyzed. Eu2Ta2O7 crystallizes in the orthorhombic centrosymmetric Cmcm space group (with unit cell: a = 3.95156(9), b = 27.0775(6), and c = 5.68279(13) Å) isomorphous with high-temperature Sr2Ta2O7. Dielectric measurements reveal that, in contrast to Sr2Ta2O7 which is ferroelectric below 166 K, Eu2Ta2O7 remains paraelectric down to at least 0.45 K and shows no magneto-dielectric coupling. Magnetic data in the 2–400 K range indicate an antiferromagnetic phase transition with a sharp susceptibility peak at 2.71(5) K. Further analysis using specific heat measurements reveals that the second magnetic phase transition occurs at 1.10(5) K and dominates the spin entropy of the Eu2+ 4f7 ions. The possible origin of the two successive magnetic phase transitions in Eu2Ta2O7 requires further studies.

A coupling of martensitic and metamagnetic transitions with collective magneto-volume and table-like magnetocaloric effects

A coupling of the first-order paramagnetic-to-induced-ferromagnetic martensitic and the second-order antiferromagnetic-to-ferromagnetic metamagnetic transitions was found in MnNi0.8Fe0.2Ge alloy. Based on the coupling, a magneto-volume effect driven by the martensitic transition and a table-like magnetocaloric effect generated by the successive magnetic phase transitions arise collectively. By using the magneto-volume effect, the internal stress in the volume-expansion martensitic transition was determined at 350 MPa. The magnetocaloric effect, with a wide working temperature range of 26 K around room temperature, shows a small hysteresis loss (5 J kg−1) and a large net refrigerant capacity (157 J kg−1).

Neutron diffraction study of magnetic structure in multiferroics under pressure

RMn2O5 (R = Y, Bi, rare-earth) is one of the prototypical multiferroic materials that exhibits a rich variety of magnetoelectric effects. Since the successive magnetic and ferroelectric phase transitions simultaneously take place, magnetic order has been thought to be a primary order parameter for the ferroelectricity in this system. We recently have found that in neutron diffraction study of 153EuMn2O5, magnetic phase transition is induced by applying hydrostatic pressure. As temperature decreases upon p = 1.4 GPa, the magnetic propagation wave vector changes from qM = (1/2, 0, 1/3) to (1/2, 0, 1/2), indicating that the period of magnetic unit cell as well as the magnetic structure change at the phase transition. We have also carried out the dielectric and polarization measurements under pressure and established magnetic and dielectric phase diagram as functions of temperature and pressure as shown in the figure. This study has revealed that the ferroelectric (FE1) – ferroelectric (FE2) phase transition concomitantly occurs at the magnetic phase transition, where the electric polarization is enhanced. To clarify the relevance between the ferroelectricity and the magnetic structure, we carried out single crystal magnetic structure analysis of 153EuMn2O5 upon ambient- and high-pressure. In the magnetic phase with qM = (1/2, 0, 1/3), cycloidal magnetic structure of manganese spins propagating along c-axis is realized. On the contrary in the magnetic phase with qM = (1/2, 0, 1/2), the spins arrange almost collinearly along c-axis. The result indicates that the presence of the cycloidal spin structure plays an important role for inducing (or reducing) the electric polarization in this compound. This study was supported by "KAKENHI"-programs of Scientific Research (B) (24340064), Scientific Research (A) (21244051), Challenging Exploratory Research (23654098) and of Scientific Research on Priority Areas "Novel States of Matter Induced by Frustration" (19052001).

Ground state and magnetic phase transitions of orthoferriteDyFeO3

Low-temperature thermal conductivity (\kappa), as well as magnetization (M) and electric polarization (P), of multiferroic orthoferrite DyFeO_3 single crystals are studied with H \parallel c. When the crystal is cooled in zero field, M, P, and \kappa all consistently exhibit irreversible magnetic-field dependencies. In particular, with 500 mK < T \le 2 K, all these properties show two transitions at the first run of increasing field but only the higher-field transition is present in the subsequent field sweepings. Moreover, the ultra-low-T (T < 500 mK) \kappa(H) shows a different irreversibility and there is only one transition when the field is swept both up and down. All the results indicate a complex low-T H-T phase diagram involving successive magnetic phase transitions of the Fe^{3+} spins. In particular, the ground state, obtained with cooling to subKelvin temperatures in zero field, is found to be an unexplored phase.

Enhanced refrigerant capacity in two-phase nanocrystalline/amorphous NdPrFe17 melt-spun ribbons

The magnetocaloric properties of NdPrFe17 melt-spun ribbons composed of nanocrystallites surrounded by an intergranular amorphous phase have been studied. The nanocomposite shows two successive second-order magnetic phase transitions (303 and 332 K), thus giving rise to a remarkable broadening (≈ 84 K) of the full-width at the half-maximum of the magnetic entropy change curve, ΔSM(T), with a consequent enhancement of the refrigerant capacity RC. For a magnetic field change of 2 T, |ΔSMpeak| = 2.1 J kg−1 K−1 and RC = 175 J kg−1. Therefore, the reversible magnetocaloric response together with the one-step preparation process makes these nanostructured Fe-rich alloy ribbons particularly attractive for room temperature magnetic refrigeration.