Eu Moments Research Articles

Interplay of magnetism and band topology in Eu[formula omitted]Ca[formula omitted]Mg[formula omitted]Bi[formula omitted] (x=0, 0.5, 0.67) from first principles study

Recent discovery of the time reversal symmetry breaking magnetic Weyl semimetals (WSMs) has created a huge surge of activities in the field of quantum topological materials. In this work, we have studied systematically various magnetic orders, electronic structure and the interplay between the magnetic order and band topology in one such material, EuMg2Bi2 (EMB) and its Ca doped variant using first principle methods within the framework of density functional theory (DFT). Our thorough investigation reveals the existence of various magnetic order driven topological phases (for example, topological insulator in the A-type antiferromagnetic (A-AFM) phase with Eu moments aligned along the crystallographic a or b direction, Dirac semimetal in the A-AFM phase when Eu moments are parallel to c direction and Weyl semimetal in the ferromagnetic (FM) phase with Eu moments pointing along the c direction) in this material. These phases are found to be energetically very close and hence are expected to be tunable from one to the other by an external handle such as magnetic field or chemical substitution. Most importantly, we observe the existence of a single pair of Weyl points (WPs) connecting valence and conduction band very close to the Fermi level (FL) along Γ-A direction in the FM state of EuMg2Bi2 with Eu moments aligned along the c direction making it an ideal Weyl semimetal in its FM state exactly similar to EuCd2As2. On doping 50% and 67% Ca at Eu sites, we observe the single pair of WPs to move even closer to the FL which is highly desirable for application purposes. Further, we observe that the separation between the WPs in the pair decreases in doped compounds compared to that in the parent compound which has direct consequence on anomalous Hall conductivity (AHC). Our first principles calculation of AHC shows high peak values exactly at these WPs and the peak height decreases when we dope the system with Ca. Therefore, Ca doping can be a good external handle to tune AHC in this system.

Magnetism and Transport Properties of EuCdBi2 with Bi Square Net

We report a possible coexistence of nontrivial topology and antiferromagnetism in the newly discovered compounds EuCdBi2, with magnetic Eu layer locating above and below Bi square net. The X-ray diffraction on single crystals and powder indicats that this 112-type material crystalizes in space group of I4/mmm, the same as SrMnBi2 and EuMnBi2. Our combined measurements of magnetization, electrical transport and specific heat consistently reveal antiferromagnetic (AFM) transition of Eu2+ moments at TN = 20 K. The Eu moments are not saturated under a field of 7 T at 1.8 K. The anisotropic susceptibility suggests the Eu moments lie in the ab plane, and a metamagnetic (MM) transition is observed near 1 T below TN. Large positive magnetoresistance (MR) present for both H ‖ ab and H ‖ c, which are considered to contain part contributions from Dirac bands. Hall measurements show the electron-hole compensation effect is prominent above 100 K, with a crossover of Hall resistance from negative to positive values at ∼150 K. The fitted mobility of electrons is as high as 3250 cm2 V−1 S−1 at 1.8 K. Interestingly, the rapid increase of carrier density and suppression of mobility appear at around TN, indicating non-negligible interaction between Eu moments and electron/hole bands. EuCdBi2 may provide a new platform to investigate the interplay of topological bands and antiferromagnetic order.

Low-field magnetic anomalies in single crystals of the A-type square-lattice antiferromagnet EuGa4

The body-centered-tetragonal antiferromagnet ${\mathrm{EuGa}}_{4}$ was recently identified as a Weyl nodal-line semimetal that exhibits the topological Hall effect below its reported antiferromagnetic (AFM) ordering temperature ${T}_{\mathrm{N}}=15--16.5$ K which we find to be ${T}_{\mathrm{N}}=16.4(2)$ K. The ${\mathrm{Eu}}^{+2}$ ions are located at the corners and body centers of the unit cells. ${\mathrm{EuGa}}_{4}$ exhibits A-type antiferromagnetic order below ${T}_{\mathrm{N}}$, where the ${\mathrm{Eu}}^{2+}$ spin-7/2 moments are ferromagnetically aligned in the $ab$ plane with the Eu moments in adjacent Eu planes along the $c$ axis aligned antiferromagnetically. Low-field magnetization versus field $M({H}_{ab})$ data at $T=2$ K with the field aligned in the $ab$ plane are reported that exhibit anomalous positive curvature up to a critical field ${H}_{\mathrm{c}1}$ at which a second-order transition occurs with ${H}_{c1}\ensuremath{\approx}0.85$ kOe for $\mathbf{H}\ensuremath{\parallel}[1,1,0]$ and $\ensuremath{\approx}4.8$ kOe for $\mathbf{H}\ensuremath{\parallel}[1,0,0]$. For larger fields, the linear behavior ${M}_{ab}=\ensuremath{\chi}({T}_{\mathrm{N}}){H}_{ab}$ is followed until the critical field ${H}_{\mathrm{c}}$ is reached at which all moments become aligned with the applied field. A theory is formulated for $T=0$ K that fits the observed $M({H}_{ab})$ behavior at $T=2$ K well, where domains of $\mathrm{A}$-type AFM order with fourfold rotational symmetry occur in the AFM state in zero field. The moments in the four domains reorient to become almost perpendicular to ${\mathbf{H}}_{ab}$ at ${H}_{c1}$, followed by increasing canting of all moments toward the field with increasing field up to ${H}_{\mathrm{c}}$ which is reported to be 71 kOe. A first-order transition in $M({H}_{ab})$ at ${H}_{ab}={H}_{\mathrm{c}1}$ is predicted by the theory for $T=0$ K when ${\mathbf{H}}_{ab}$ is at a small angle from the [1,0,0] direction.

Superconductivity in a Magnetic Rashba Semimetal EuAuBi

We report the observation of superconductivity with multiple magnetic ordering and Rashba-type spin-orbit coupling in a layered polar semimetal EuAuBi. Magnetic transition is observed at 4 K, followed by a superconducting transition at 2.2 K, which is sensitive to the crystal surface conditions. The upper critical field Hc2 of 9.8 T for the out-of-plane field is three times higher than that for the in-plane field, which can be associated with the two-dimensional structure or the surface state. On the basis of first-principles calculations, it is found that the characteristic Hc2 possibly reflects the anisotropic modification of the Fermi surface by the effective combination of Rashba-type spin splitting and Zeeman spin splitting enhanced by Eu moments.

Magnetic field induced ab -plane rotation of the Eu magnetic moments in trigonal EuMg2Bi2 and EuMg2Sb2 single crystals below their Néel temperatures

The thermodynamic and electronic-transport properties of trigonal ${\mathrm{EuMg}}_{2}{\mathrm{Bi}}_{2}$ in $ab$-plane magnetic fields ${\mathbf{H}}_{x}$ and the A-type antiferromagnetic structure have recently been reported. The Eu magnetic moments with spin $S=7/2$ remain locked in the $ab$ plane up to and above the $ab$-plane critical field ${\mathbf{H}}_{x}^{\mathrm{c}}=27.5$ kOe at which the Eu moments become parallel to ${\mathbf{H}}_{x}$. Here additional measurements at low fields are reported that reveal a new spin-reorientation transition at a field ${H}_{c1}\ensuremath{\approx}465$ Oe where the Eu moments remain in the $ab$ plane but become perpendicular to ${H}_{x}$. At higher fields, the moments cant toward the field resulting in $M\ensuremath{\propto}{H}_{x}$ up to ${\mathbf{H}}_{x}^{\mathrm{c}}$. Similar results are reported from measurements of the magnetic properties of ${\mathrm{EuMg}}_{2}{\mathrm{Sb}}_{2}$ single crystals, where ${H}_{\mathrm{c}1}\ensuremath{\approx}220$ Oe is found. Theory is formulated that models the low-field magnetic behavior of both materials, and the associated anisotropies are calculated. The $ab$-plane trigonal anisotropy in ${\mathrm{EuMg}}_{2}{\mathrm{Sb}}_{2}$ is found to be significantly smaller than in ${\mathrm{EuMg}}_{2}{\mathrm{Bi}}_{2}$.

Creating and controlling Dirac fermions, Weyl fermions, and nodal lines in the magnetic antiperovskite Eu3PbO

The band topology of magnetic semimetals is of interest both from the fundamental science point of view and with respect to potential spintronics and memory applications. Unfortunately, only a handful of suitable topological semimetals with magnetic order have been discovered so far. One such family that hosts these characteristics is the antiperovskites, ${A}_{3}B\mathrm{O}$, a family of 3D Dirac semimetals. The $A={\mathrm{Eu}}^{2+}$ compounds magnetically order with multiple phases as a function of the applied magnetic field. Here, by combining band structure calculations with neutron diffraction and magnetic measurements, we establish the antiperovskite ${\mathrm{Eu}}_{3}\mathrm{Pb}\mathrm{O}$ as a new topological magnetic semimetal. This topological material exhibits a multitude of different topological phases with ordered Eu moments which can be easily controlled by an external magnetic field. The topological phase diagram of ${\mathrm{Eu}}_{3}\mathrm{Pb}\mathrm{O}$ includes an antiferromagnetic Dirac phase, as well as ferro- and ferrimagnetic phases with both Weyl points and nodal lines. For each of these phases, we determine the bulk band dispersions, the surface states, and the topological invariants by means of ab initio and tight-binding calculations. Our discovery of these topological phases introduces ${\mathrm{Eu}}_{3}\mathrm{Pb}\mathrm{O}$ as a new platform to study and manipulate the interplay of band topology, magnetism, and transport.

High-field Studies on Layered Magnetic and Polar Dirac Metals: Novel Quantum Transport Phenomena Coupled with Spin-valley Degrees of Freedom

Recently, the interplay between the Dirac/Weyl fermion and various bulk properties, such as magnetism, has attracted considerable attention, since unconventional transport and optical phenomena were discovered. However, the design principles for such materials have not been established well. Here, we propose that the layered material $A$Mn$X_2$ ($A$: alkaline and rare-earth ions, $X$: Sb, Bi) is a promising platform for systematically exploring strongly correlated Dirac metals, which consists of the alternative stack of the $X^-$ square net layer hosting a 2D Dirac fermion and the $A^{2+}$-Mn$^{2+}$-$X^{3-}$ magnetic block layer. In this article, we shall review recent high-field studies on this series of materials to demonstrate that various types of Dirac fermions are realized by designing the block layer. First, we give an overview of the Dirac fermion coupled with the magnetic order in EuMnBi$_2$ ($A$=Eu). This material exhibits large magnetoresistance by the field-induced change in the magnetic order of Eu layers, which is associated with the strong exchange interaction between the Dirac fermion and the local Eu moment. Second, we review the Dirac fermion coupled with the lattice polarization in BaMn$X_2$ ($A$=Ba). There, spin-valley coupling manifests itself owing to the Zeeman-type spin-orbit interaction, which is experimentally evidenced by the bulk quantum Hall effect observed at high fields.

Thermodynamics of the Nonrelativistic Free-Electron Fermi Gas in One, Two, and Three Dimensions from the Degenerate to the Nondegenerate Temperature Regime

The thermodynamic and electronic-transport properties of trigonal EuMg2Bi2 in ab-plane magnetic fields Hx and the A-type antiferromagnetic structure have recently been reported. At a temperature of 1.8 K < TN, the Eu magnetic moments with spin S = 7/2 remain locked in the ab plane up to and above the ab-plane critical field Hxc = 27.5 kOe at which the Eu moments become parallel to Hx. Here additional measurements at low fields are reported that reveal a new spin-reorientation transition at a field Hc1 = 465 Oe where the Eu moments remain in the ab plane but become perpendicular to Hx. At higher fields, the moments cant towards the field resulting in M proportional to Hx up to Hxc. Similar results are reported from measurements of the magnetic properties of EuMg2Sb2 single crystals, where Hc1 = 220 Oe is found. Theory is formulated that models the low-field magnetic behavior of both materials, and the associated anisotropies are calculated. The ab-plane trigonal anisotropy in EuMg2Sb2 is found to be significantly smaller than in EuMg2Bi2.

Engineering spin-orbit effects and Berry curvature by deposition of a monolayer of Eu on WSe2

Motivated by recent progress in two-dimensional (2D) spintronics, we present a monolayer of Eu deposited on $1\mathrm{H}\text{\ensuremath{-}}\mathrm{W}{\mathrm{Se}}_{2}$ as a promising platform for engineering spin-orbit effects and Berry curvature. By first-principles calculations based on density functional theory, we show that $\mathrm{Eu}/\mathrm{W}{\mathrm{Se}}_{2}$ exhibits intriguing properties such as high magnetic anisotropy, valley-dependent polarization of spin and orbital angular momenta, and Rashba textures. These originate from magnetic and spin-orbit proximity effects at the interface and the interplay between localized $4f$ magnetic moments of Eu and mobile charge carriers of Eu and $\mathrm{W}{\mathrm{Se}}_{2}$. The analysis of the magnetic properties reveal a ferromagnetic configuration with an out-of-plane easy axis of the magnetization, which favor a pronounced anomalous Hall effect in the proposed system. Thus, we promote $4f$ rare-earth metals deposited on top of a transition-metal dichalcogenides as a promising platform for 2D spintronics.

A-type antiferromagnetic order in semiconducting EuMg2Sb2 single crystals

Eu-based Zintl-phase materials EuA$_2$Pn$_2$ (A = Mg, In, Cd, Zn; Pn = Bi, Sb, As, P) have generated significant recent interest owing to the complex interplay of magnetism and band topology. Here, we investigated the electronic, magnetic, and electronic properties of the layered Zintl-phase single crystals of EuMg$_2$Sb$_2$ with the trigonal CaAl$_2$Si$_2$ crystal structure (space group $P\bar{3}m1$). Electrical resistivity measurements complemented with angle-resolved photoemission spectroscopy (ARPES) studies find an activated behavior with the intrinsic conductivity at high temperatures indicating a semiconducting electronic ground state with a narrow energy gap of 370 meV. Magnetic susceptibility and zero-field heat-capacity measurements indicate that the compound undergoes antiferromagnetic (AFM) ordering at the Neel temperature $T_{\rm N}$ = 8.0(2) K. Zero-field neutron-diffraction measurements reveal that the AFM ordering is A-type where the Eu ordered moments (Eu$^{2+}$, S= 7/2) arranged in ab-plane layers are aligned ferromagnetically in the ab plane with the Eu moments in adjacent layers aligned antiferromagnetically. We also find that Eu-moment reorientation in the trigonal AFM domains within the ab planes occurs below $T_{\rm N}$ at low fields < 0.05 T due to very small in-plane anisotropy. Although isostructural semimetallic EuMg$_2$Bi$_2$ is reported to host Dirac surface states, the observation of narrow-gap semiconducting behavior in EuMg$_2$Sb$_2$ implies a strong role of spin-orbit coupling in tuning the electronic states of these materials.

Large Thermopower Enhanced by Spin Entropy in Antiferromagnet EuMnSb2

AbstractThermoelectric materials can realize the mutual conversion between electricity and heat in a pollution‐free process, and are thus exceptionally important for applications like energy generation and thermoelectric cooling. Although thermoelectric properties are conventionally believed to be determined by the charge and lattice degrees of freedom, the electron spin degree of freedom can also make significant contribution with electron‐electron correlation involved. Here, a large magneto‐thermopower (MTEP) in the antiferromagnet EuMnSb2 is reported. At zero field, the thermopower first increases with decreasing temperature (T), exhibits a maximum at about 70 K, and then decreases rapidly when cooled toward the antiferromagnetic ordering temperature (TN) of the Eu spins. With magnetic fields applied, the thermopower shows weak field dependence below TN, whereas at T &gt; TN it is suppressed significantly, highlighting the contribution related to the spin degree of freedom. The negative MTEP above TN is closely tracked by the field‐dependent spin entropy. These observations indicate that the spin entropy of the Eu moments can serve as the most possible source of the strongly enhanced thermopower in EuMnSb2. These results pave new paths to improve the thermoelectric performance by utilizing the spin degree of freedom and search for better thermoelectric materials.

Single pair of Weyl nodes in the spin-canted structure of EuCd2As2

Time reversal symmetry breaking Weyl semimetals are unique among Weyl materials in allowing the minimal number of Weyl points, thus offering the clearest signatures of the associated physics. Here we present neutron diffraction, density-functional theory, and transport measurement results which indicate that ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$, under ambient field, strain, and pressure, is such a material with a single pair of Weyl points. Our work reveals a magnetic structure (magnetic space group $C{2}^{\ensuremath{'}}/{m}^{\ensuremath{'}}$) with Eu moments pointing along the [210] direction in plane and canted $\ensuremath{\sim}$ ${30}^{\ensuremath{\circ}}$ out of plane. Density functional theory calculations using this structure show that the observed canting drastically alters the relevant electronic bands, relative to the in-plane order, leading to a single set of well-defined Weyl points. Furthermore, we find the canting angle can tune the distance of the Weyl points above the Fermi level, with the smallest distance at low canting angles. Finally, transport measurements of the anomalous Hall effect and longitudinal magnetoresistance exhibit properties indicative of a chiral anomaly, thus supporting the neutron scattering and DFT results suggesting ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$ is close to the ideal situation of the Weyl hydrogen atom.

Field-induced topological Hall effect in antiferromagnetic axion insulator candidate EuIn2As2

Magnetic topological materials have attracted significant attention due to their potential realization of a variety of novel quantum phenomena. ${\mathrm{EuIn}}_{2}{\mathrm{As}}_{2}$ has recently been theoretically recognized as a long-awaited intrinsic antiferromagnetic bulk axion insulator. However, experimental studies of the transport properties arising from the topological states in this material are scarce. In this paper, we perform detailed magnetoresistance (MR) and Hall measurements to study the magnetotransport properties of this material. We find that the transport is strongly influenced by the spin configuration of the Eu moments from the concomitant change in the field dependence of the MR and that of the magnetization below the N\'eel temperature. Most importantly, an anomalous Hall effect (AHE) and a large topological Hall effect (THE) are observed. We suggest that the AHE is originated from a nonvanishing net Berry curvature due to the helical spin structure and that the THE is attributed to the formation of a noncoplanar spin texture with a finite scalar spin chirality induced by the external magnetic field in ${\mathrm{EuIn}}_{2}{\mathrm{As}}_{2}$. Our studies provide a platform to understand the influence of the interplay between the topology of electronic bands and the field-induced magnetic structure on magnetoelectric transport properties. In addition, our observations give a hint to realize axion insulator states and higher-order topological insulator states through manipulating the magnetic state of ${\mathrm{EuIn}}_{2}{\mathrm{As}}_{2}$.

Possible quantum nematic phase in a colossal magnetoresistance material

EuB6 has for a long time captured the attention of the physics community, as it shows a ferromagnetic phase transition leading to a insulator the metal transition together with colossal magnetoresistance (CMR). EuB6 has a very low carrier density, which is known to drastically change the interaction between the localized Eu moments and the conduction electrons. One of early triumphs of the quantum theory in condensed matter was the presence of Fermi surface, which is intimately linked to the symmetry of the underlying crystal lattice. This symmetry can be probed by angle resolved magnetoresistance (AMRO) measurements. Here, we present angle resolved magnetoresistance (AMRO) measurements that show a that in EuB6 this symmetry is broken, possibly indicating the presence of a quantum nematic phase. We identify the region in the temperature-magnetic field phase diagram where the magnetoresistance shows two-fold oscillations instead of the expected fourfold pattern. Quantum nematic phases are analogous to classical liquid crystals. Like liquid crystals, which break the rotational symmetry of space, their quantum analogs break the point-group symmetry of the crystal due to strong electron-electron interactions, as in quantum Hall states, Sr3Ru2O7, and high temperature superconductors. This is the same region where magnetic polarons were previously observed, suggesting that they drive the nematicity in EuB6. This is also the region of the phase diagram where EuB6 shows a colossal magnetoresistance (CMR). This novel interplay between magnetic and electronic properties could thus be harnessed for spintronic applications.

Eu's generalized hydrodynamics with its derived constitutive model: Comparison to Grad's method and linear stability analysis

Recently, the generalized hydrodynamic equations (GHE) and nonlinear coupled constitutive relation (NCCR) model have been successfully utilized for the practical application in stable numerical computations of the non-equilibrium flows. However, their stability property has never been studied theoretically, and the inherent connection with classical Grad's moment still remains unclear. In order to clarify these issues, Eu's method, including the modeling of the non-equilibrium distribution function and the cumulant expansion for collision terms, is revisited to derive the modified moment system. A comparison of Eu's moment method with existing Grad's is presented in detail from the perspectives of distribution function and closure theory. The original infinite system of Eu's distribution function is first truncated into a finite system with 13 moments. Then through our attempt of Taylor expanding the truncated distribution function, a connection between Eu's distribution and Maxwellian and Grad's is established. Subsequently, a truncated closure method is conducted to clarify the relation between Eu's moment and Grad's moment equations. Finally, linear stability analysis of GHE and NCCR model is performed in one-dimensional and multi-dimensional processes, which shows that the equations are unconditionally stable for all wavenumbers and frequencies in the equilibrium rest state (ui0=0) and uniform-moving state (ui0≠0). The linear stability of GHE and NCCR model assures their numerical stability in the multi-dimensional computations, which can be deemed as one of the major benefits of Eu's theory.

Strong coupling between magnetic order and band topology in the antiferromagnet EuMnSb2

We report a comprehensive magnetization and magnetotransport study on a metallic-type of ${\mathrm{EuMnSb}}_{2}$ single crystals. A large Hall signal up to room temperature is found in our metallic ${\mathrm{EuMnSb}}_{2}$, which is dominated by hole carriers with very low carrier density and high mobility. Clear Shubnikov--de Haas oscillations are observed at low temperatures (T $&lt;$ 30 K), from which a dominant tiny hole pocket with small effective mass is deduced. The interplay of carrier density and Mn and Eu moments results in complex magnetic ground states with two successive antiferromagnetic (AFM) transitions appearing at ${T}_{N1}=24 \mathrm{K}$ and ${T}_{N2}=9 \mathrm{K}$. In line with the rich magnetic phases, the transport properties, including magnetoresistance, quantum oscillation, and Berry phase, experience abrupt disturbances when crossing the magnetic phase boundaries. The Dirac state is likely quenched once the AFM orders set in, indicating a strong coupling between the AFM magnetic order and the band topology. Our results suggest that metallic-type ${\mathrm{EuMnSb}}_{2}$ is an ideal platform for the study of the interplay between magnetism, charge transport, and band topology.

Strongly anisotropic antiferromagnetic coupling in EuFe2As2 revealed by stress detwinning

Of all parent compounds of iron-based high-temperature superconductors, EuFe2As2 exhibits by far the largest magnetostructural coupling due to the sizable biquadratic interaction between Eu and Fe moments. While the coupling between Eu antiferromagnetic order and Fe structural/antiferromagnetic domains enables rapid field detwinning, this prevents simple magnetometry measurements from extracting the critical fields of the Eu metamagnetic transition. Here we measure these critical fields by combining x-ray magnetic circular dichroism spectroscopy with in-situ tunable uniaxial stress and applied magnetic field. The combination of two tuning knobs allows us to separate the stress-detwinning of structural domains from the field-induced reorientation of Eu moments. Intriguingly, we find a spin-flip transition which can only result from a strongly anisotropic interaction between Eu planes. We argue that this anisotropic exchange is a consequence of the strong anisotropy in the magnetically ordered Fe layer, which presents a new form of higher-order coupling between Eu and Fe magnetism.

Partial charge transfer and absence of induced magnetization in EuS(111)/Bi2Se3 heterostructures

Heterostructures made from topological and magnetic insulators promise to form excellent platforms for new electronic and spintronic functionalities mediated by interfacial effects. We report the results of a first-principles density functional theory study of the geometric, electronic structure, and magnetic properties of EuS(111)/Bi$_2$Se$_3$ interface, including van der Waals and relativistic spin-orbit effects. In contrast to previous theoretical studies, we find no appreciable magnetic anisotropy in such a heterostructure. We also do not see additional induced magnetization at the interface or the magnetic proximity effect on the topological states. This is due to the localized nature of Eu moments, and because of a partial charge transfer of $\sim$0.5 electron from Eu to Se. The formation of the surface dipole shifts the Dirac cone about 0.4~eV below the chemical potential, and the associated electrostatic screening moves the topological state from the first to the second quintuple layer of Bi$_2$Se$_3$.

Synthesis and study of the chiral magnetic system EuIr2P2

Chiral materials, where no improper symmetry operations such as inversion are present, are systems prone to the appearance of a skyrmion lattice. Recently it has been shown theoretically that not only ferromagnets (FMs) but also antiferromagnets (AFMs) can host such kind of phases. In this work we study a new candidate for AFM skyrmions, ${\mathrm{EuIr}}_{2}{\mathrm{P}}_{2}$, by means of magnetization and specific heat measurements on poly and single crystals. X-ray diffraction confirms a trigonal chiral crystal structure, where europium ions form helices along the $c$ direction. In spite of predominantly FM interactions, ${\mathrm{Eu}}^{2+}$ ions order antiferromagnetically at ${T}_{{N}_{1}}=5$ K in what seems to be an incommensurate amplitude-modulated magnetic state where the moments are oriented mainly along the $c$ direction. A second magnetic transition takes place at ${T}_{{N}_{2}}=2.9$ K, involving the ordering of an in-plane component of the Eu moment likely resulting in an equal-moment structure. Specific heat data show a tail above ${T}_{{N}_{1}}$. Accordingly the magnetic entropy at ${T}_{{N}_{1}}$ is strongly reduced in comparison to the expected $Rln8$ value. This evidences a significant amount of frustration. A simple analysis based on a Heisenberg model indicates that the observed properties imply the presence of several relevant interactions, with competing FM and AFM ones resulting in frustration. Thus ${\mathrm{EuIr}}_{2}{\mathrm{P}}_{2}$ is a new interesting magnetic system, where chirality and frustration might result in unconventional magnetic textures.

Long-range magnetic phase transitions and negative magnetoresistance of divalent Eu in Eu2Cu5Ni5 phase

We report the synthesis and physical properties of a new ternary Eu2Cu5Ni5 phase which crystallizes in the orthorhombic space group Pmmm, No. 47, Z = 1, with lattice parameters; a = 4.015(4), b = 4.921(2) and c = 8.621(3) Å. Analysis from magnetic susceptibility χdc(T) results reveal an effective magnetic moment μeff = 7.8 μB/Eu that is comparable to the theoretical value of 7.93 μB for a divalent Eu2+ ion. The Eu2+ electronic state of 4f7 is well localized in this phase and the results preclude both the Cu and Ni sub-lattices from bearing a magnetic moment. Two long-range phase transitions pertaining to the Eu2+ moments are observed at TC1 = 72 K and TC2 = 47.2 K, followed by another anomaly at TSR = 22 K, attributed to a spin re-orientation of Eu moments. The Seebeck coefficient S(T) attains a value of − 16.7 μV/K at room temperature (RT) and indicates that the electrons are the dominant charge carriers. Evidence from the electrical transport reveals a metallic behavior with the magnetotransport showing a negative magnetoresistance below RT.