Magnetic and electronic properties of Eu4Sr4Ga16Ge30

Applied Physics Letters, 2008

Long-range ferromagnetism and low-field giant magnetocaloric effect are observed in Eu 8 Ga 16 Ge 30 with the type VIII clathrate crystal structure, a material that is better known for its thermoelectric properties. Magnetization and modified Arrott plots indicate that the system undergoes a second-order ferromagnetic-paramagnetic phase transition at ϳ13 K. The low-field giant magnetic entropy change ͑−⌬S M ϳ 11.4 J / kg K for ⌬ 0 H =3 T͒ coupled with the absence of thermal hysteresis and field hysteresis makes the system very attractive for low temperature magnetic refrigeration. The giant magnetic entropy change originates from the large magnetization ͑7.97 B per Eu ion͒ and the sharp change with temperature at the paramagnetic-ferromagnetic transition.

Journal of Applied Physics, 2015

We observe an enhanced magnetic entropy change (-S M) at cryogenic temperatures (T < 20 K) in Eu 8 Ga 16 Ge 30 clathrate (type-I) nanocrystals prepared by a ball milling method. With reduction in the crystal size to 15 nm-S M is enhanced at low temperatures, reaching the highest value (10 J/kg K) at 5 K for a field change of 5 T. For all samples investigated there is a cross-over temperature (~25 K) in-S M (T) above which-S M decreases with crystal size, opposite to that observed at low temperatures. A careful analysis of the magnetic and magnetocaloric data reveals that as the crystal size decreases the magnetic interaction between Eu 2+ ions on the Eu2 site governing the primary ferromagnetic transition at 35 K becomes gradually weaker, in effect altering the interaction between Eu 2+ ions occupying the Eu1 and Eu2 sites responsible for the secondary ferromagnetic transition at 15 K. As a result, we have observed a strong change in magnetization and the enhancement of-S M at low temperature.

Physical Review B, 2001

Structural, magnetic, electrical and thermal transport, and heat-capacity measurements are reported on single crystals of Eu 8 Ga 16 Ge 30 , Sr 8 Ga 16 Ge 30 , and Ba 8 Ga 16 Ge 30. These compounds all crystallize in a cubic type-I ice clathrate structure, and are of interest as potential thermoelectric materials. Neutron-diffraction measurements were made on a single crystal of Eu 8 Ga 16 Ge 30 that was grown using isotopically pure Eu 153. Nuclear density maps clearly show that Eu atoms at the 6d sites ͑ 1 4 , 1 2 ,0͒ can move away from the cage center to one of four nearby positions. Ferromagnetism is observed in Eu 8 Ga 16 Ge 30 for temperatures below 32 K, with the preferred direction of the Eu spins along the ͑100͒ axis. Ferromagnetism in these heavily doped semiconductors (Ϸ10 21 electrons/cm 3) is likely due to a Rudermann-Kittel-Kasuya-Yoshida-type interaction. A large ͑Ϸ10% at 8 T͒ negative magnetoresistance was measured near the Curie temperature of Eu 8 Ga 16 Ge 30. The lattice thermal conductivities of Eu 8 Ga 16 Ge 30 and Sr 8 Ga 16 Ge 30 single crystals show all of the characteristics of a structural glass. The thermal conductivity of Ba 8 Ga 16 Ge 30 is low at room temperature ͑1.3 W/m K͒, but exhibits a temperature dependence characteristic of a crystal. A magnetic field has no significant effect on the thermal conductivity of any of the crystals for temperatures between 2 and 300 K. Heat-capacity measurements indicated Einstein contributions from each of the rattlers, with characteristic temperatures of 60, 53, and 30 K for Ba, Sr, and Eu atoms respectively. No superconductivity was observed in heavily doped single crystals of Ba 8 Ga 16 Ge 30 for temperatures above 2 K, contrary to a previous report.

Physical Review Letters, 2006

Mössbauer-effect and microwave absorption experimental evidence unambiguously demonstrates the presence of slow, 450 MHz, tunneling of magnetic europium between four equivalent sites in Eu 8 Ga 16 Ge 30 , a stoichiometric clathrate. Remarkably, six of the eight europium atoms, or 11% of the constituents in this solid, tunnel between these four sites separated by 0.55 Å . The off centering of the atoms or ions in crystalline clathrates appears to be a promising route for producing Rabi oscillators in solid-state materials.

Applied Physics Letters, 2008

Long-range ferromagnetism and low-field giant magnetocaloric effect are observed in Eu 8 Ga 16 Ge 30 with the type VIII clathrate crystal structure, a material that is better known for its thermoelectric properties. Magnetization and modified Arrott plots indicate that the system undergoes a second-order ferromagnetic-paramagnetic phase transition at ϳ13 K. The low-field giant magnetic entropy change ͑−⌬S M ϳ 11.4 J / kg K for ⌬ 0 H =3 T͒ coupled with the absence of thermal hysteresis and field hysteresis makes the system very attractive for low temperature magnetic refrigeration. The giant magnetic entropy change originates from the large magnetization ͑7.97 B per Eu ion͒ and the sharp change with temperature at the paramagnetic-ferromagnetic transition.

Journal of Applied Physics, 2015

Large magnetocaloric effects over a wide temperature range in MnCo1−xZnxGe A phenomenological fitting curve for the magnetocaloric effect of materials with a second-order phase transition

Physica B: Condensed Matter, 2003

Based on an analysis of the geometric crystallographic relations a general classification scheme is presented for intermetallides with four-coordinated networks isomorphous with hydrate clathrates. We prepared novel europium substituted clathrates, Eu 2Àx (Sr,Ba) 6Àx M y Si 46Ày (M=Al, Ga), consistent with the standardized clathrate I-Ba 8 Al 16 Ge 30 type structure (space group Pm % 3n). Europium atoms in Ba compounds preferentially occupy the 2a position and thus form a new quaternary version of the Ba 8 Al 16 Ge 30 structure type. All clathrates studied are metals with low electrical conductivity. The negative Seebeck coefficients indicate transport processes dominated by electrons as carriers. Eu 2 Ba 6 Al 8 Si 36 and Eu 2 Ba 6 Ga 8 Si 36 exhibit long-range magnetic order below 32 and 38 K of presumably ferromagnetic type. Magnetic susceptibilities indicate an Eu 2+ ground state, in fine agreement with L III absorption edge spectra. r (P. Rogl). 0921-4526/03/$ -see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 -4 5 2 6 ( 0 2 ) 0 1 8 0 6 -9

Physical Review B, 2011

Systematic dc magnetization studies using the Banerjee criterion, Kouvel-Fisher, and magnetocaloric effect methods provide physical insights into the origin of the magnetic anomaly and the tunneling effect of europium on the ferromagnetic ordering in Eu 8 Ga 16 Ge 30 type-I clathrates. We show that Eu 8 Ga 16 Ge 30 undergoes a second-order magnetic transition (SOMT) at T C ∼ 35 K, resulting from the magnetic interaction between the Eu 2+ ions at the Eu2 sites, followed by a secondary magnetic transition at T L ∼ 10 K (indicated as a magnetic anomaly in previous studies), as a result of the magnetic interaction between the Eu 2+ ions at the Eu1 and Eu2 sites. The critical exponent β = 0.388 is close to that predicted from the three-dimensional Heisenberg model (β = 0.365), while the critical exponent γ = 0.956 is close to that predicted from the mean-field model (γ = 1). The substitution of Sr 2+ for Eu 2+ retains the SOMT but largely reduces the transition temperatures (T C ∼ 15 K and T L ∼ 5 K), with the critical exponents β = 0.521 and γ = 0.917 close to those predicted from the mean-field model (β = 0.5 and γ = 1). These results point to the important fact that the tunneling of Eu 2+ between the four equivalent sites in the tetrakaidecahedral cage tends to prevent the occurrence of a long-range ferromagnetic ordering in the type-I clathrate materials.

Journal of Solid State Chemistry, 2002

Single-phase samples of REAgMg (RE ‫؍‬ La, Ce, Eu, Yb) and EuAuMg were prepared by reacting the elements in sealed tantalum tubes in a high-frequency furnace. LaAgMg and CeAgMg adopt the hexagonal ZrNiAl-type structure, while EuAgMg, YbAgMg, and EuAuMg crystallize with the orthorhombic TiNiSi type. Chemical bonding was exemplarily investigated for EuAgMg and EuAuMg on the basis of TB-LMTO-ASA calculations. Magnetic susceptibility measurements indicate Pauli paramagnetism for LaAgMg and YbAgMg with roomtemperature susceptibilities of 2.4(1)؋10 ؊9 and 1.5(1)؋ 10 ؊9 m 3 /mol, respectively. CeAgMg remains paramagnetic down to 2 K. The experimental magnetic moment of 2.52(2) B /Ce above 50 K is compatible with trivalent cerium. EuAgMg and EuAuMg are paramagnetic above 50 K with experimental magnetic moments of 7.99(5) B /Eu for the silver and 7.80(5) B / Eu for the gold compound, indicating divalent europium. Ferromagnetic ordering is detected at T C ‫؍‬ 22.0(3) K (EuAgMg) and T C ‫؍‬ 36.5(5) K (EuAuMg). At 4.2 K and 5 T the saturation magnetizations are 7.1(1) and 7.3(1) B /Eu for EuAgMg and EuAuMg, respectively. According to the very small hysteresis, EuAgMg and EuAuMg may be classi5ed as soft ferromagnets. All compounds are metallic conductors. For EuAgMg and Eu-AuMg freezing of spin-disorder scattering is observed below T C . At 78 K 151 Eu MoK ssbauer spectra show isomer shifts of ؊9.00(4) and ؊8.72(8) mm/s for EuAgMg and EuAuMg, respectively. Full magnetic hyper5ne 5eld splitting is detected at 4.2 K with hyper5ne 5elds of 17.4(1) and 18.3(2) T at the europium nuclei of EuAgMg and EuAuMg. 2002 Elsevier Science (USA)

Journal of the Physical Society of Japan, 2015

We succeeded in growing a single crystal of the Eu-divalent compound EuAl 4 with the BaAl 4-type tetragonal structure by the Al self-flux method and measured the electrical resistivity, magnetic susceptibility, magnetization, specific heat, and thermoelectric power. EuAl 4 orders antiferromagnetically below T N1 = 15.4 K, with three successive antiferromagnetic transitions at T N2 = 13.2 K, T N3 = 12.2 K, and T N4 = 10.0 K. The latter two transitions are of the first-order. The corresponding magnetization curve indicates three successive metamagnetic transitions with hystereses and saturates above 16 kOe. We observed a plausible characteristic feature of the charge density wave (CDW) below T CDW = 140 K. We also studied an effect of pressure on the electronic state by measuring the electrical resistivity and thermoelectric power. T CDW is found to decrease with increasing pressure at a rate of dT CDW =dP = −54.7 K=GPa and becomes zero at about 2.5 GPa. The present antiferromagnetic ordering is, however, found to be stable at higher pressures up to 7 GPa in EuAl 4. On the other hand, the different characteristic CDW was observed in EuGa 4 , not at ambient pressure but above 1 GPa, and T CDW increases with increasing pressure. Above 6 GPa, we found that the antiferromagnetic ordering is changed into another first-order-like phase transition. Its characteristic feature is similar to that of the valence transition, and the heavy fermion state is realized at low temperatures.

Journal of Applied Physics, 2023

Clathrates have been reported to form in a variety of different structure types; however, inorganic clathrate-I materials with a low-cation concentration have yet to be investigated. Furthermore, tin-based compositions have been much less investigated as compared to silicon or germanium analogs. We report the temperature-dependent structural and thermal properties of single-crystal Eu 2 Ga 11 Sn 35 revealing the effect of structure and composition on the thermal properties of this low-cation clathrate-I material. Specifically, low-temperature heat capacity, thermal conductivity, and synchrotron single-crystal x-ray diffraction reveal a departure from Debye-like behavior, a glass-like phonon mean-free path for this crystalline material, and a relatively large Grüneisen parameter due to the dominance of low-frequency Einstein modes. Our analyses indicate thermal properties that are a direct result of the structure and composition of this clathrate-I material.

2012

We present a detailed study of the magnetic properties of unique cluster assembled solids namely Mn doped Ge46 and Ba8Ge46 clathrates using density functional theory. We find that ferromagnetic (FM) ground states may be realized in both the compounds when doped with Mn. In Mn2Ge44, ferromagnetism is driven by hybridization induced negative exchange splitting, a generic mechanism operating in many diluted magnetic semiconductors. However, for Mn-doped Ba8Ge46 clathrates incorporation of conduction electrons via Ba encapsulation results in RKKY-like magnetic interactions between the Mn ions. We show that our results are consistent with the major experimental observations for this system.

Physical Review B, 2003

The magnetic ordering in the Eu 4 Ga 8 Ge 16 clathrate compound was studied using magnetic susceptibility, Mo ¨ssbauer spectroscopy, and heat-capacity measurements. Antiferromagnetic ordering is observed at 8 K. Mo ¨ssbauer spectroscopy shows the Eu atoms to exist as Eu 2ϩ with strong room-temperature absorption at Ϫ11.8 mm/s vs EuF 3 . The temperature dependence of the Lamb-Mo ¨ssbauer factor can be fitted to an Einstein model with an effective Einstein temperature of 80 K. In combination with heat-capacity measurements, this result suggests that the Eu atoms behave like isolated oscillators that are effectively decoupled from the lattice. The magnetic susceptibility data are interpreted using the Curie-Weiss law for ordering with ʈ ,eff ϭ8.00 Ϯ0.03 B /Eu 2ϩ ion and ϭ4.43Ϯ0.08 K. The positive Weiss constant is rationalized by a nearest-neighbor ferromagnetic exchange interaction within an overall antiferromagnetically ordered system. The spin-flop phase diagram is established and used to extract the next-nearest-neighbor interaction energy. The location of the tri-critical point in H,T space is determined. The magnetic heat capacity C m is extracted by modeling the lattice contribution using a weighted Debye-Einstein model with fitted Debye temperature ⌰ D ϭ314Ϯ4 K and using a fixed Einstein temperature ⌰ E ϭ80 K. Integration of C m /T gave 15.1 J/K mol Eu 2ϩ in fair accordance with expectations based on configurational considerations.

Physical Review B

Understanding the interplay between magnetic and electronic degrees of freedom is of profound recent interest in different Eu-based magnetic topological materials. In this paper, we studied the magnetic and electronic properties of the layered Zintl-phase compound EuAl 2 Ge 2 crystallizing in the trigonal CaAl 2 Si 2-type structure. We report zero-field neutron diffraction, temperature T-and magnetic-field H-dependent magnetic susceptibility χ (T, H), isothermal magnetization M(T, H), heat capacity C p (T, H), and electrical resistivity ρ(T, H) measurements, together with T-dependent angle-resolved photoemission spectroscopy (ARPES) measurements complemented with first-principles calculations. EuAl 2 Ge 2 undergoes second-order A-type antiferromagnetic (AFM) ordering below T N = 27.5(5) K, with the Eu moments (Eu 2+ , S = 7/2) aligned ferromagnetically in the ab plane while these layers are stacked antiferromagnetically along the c axis. The critical fields at which all moments become parallel to the field are 37.5(5) and 52.5(5) kOe for H ab and H c, respectively. The H = 0 magnetic structure consists of trigonal AFM domains associated with ab-plane magnetic anisotropy and a field-induced reorientation of the Eu spins in the domains is also evident at T = 2 K below the critical field H c1 = 2.5(1) kOe. The ρ(T) measurements reveal metallic behavior transforming into a slight resistivity increase on cooling towards T N. A pronounced loss of spin-disorder scattering is observed below T N. The ARPES results show that EuAl 2 Ge 2 is metallic both above and below T N , and the Fermi surface is anisotropic with two hole pockets at the zone center and one small electron pocket at each M point. In the AFM phase, we directly observe folded bands in ARPES due to the doubling of the magnetic unit cell along the c axis with an enhancement of quasiparticle weight due to the complex change in the coupling between the magnetic moments and itinerant electrons on cooling below T N. The observed electronic structure is well reproduced by first-principles calculations, which also predict the presence of nontrivial electronic states near the Fermi level in the AFM phase with Z 2 topological numbers 1; (000).

Journal of Alloys and Compounds, 2014

The compounds CeGa 2 , Ce 0.82 Mg 0.50 Ga 1.68 , Eu(Mg/Li) x Ga 2-x (0≤x≤0.4) and Eu(Mg/Li) x Ga 4-x (0≤x≤0.73) were synthesized and characterized by X-ray diffraction. CeGa 2 and Ce 0.82 Mg 0.50 Ga 1.68 crystallize in the AlB 2 structure type with P6/mmm space group. The structure is composed of infinite arrays of planar hexagonal Ga atoms stacked along the [001] direction and the Ce atoms are sandwiched between them. Eu(Mg/Li) x Ga 2-x (0≤x≤0.4) crystallize in the KHg 2 structure type with Imma space group and consists of the hexagonal rings of M (Li/Mg+Ga) atoms connected along the [100] plane forming a puckered network with voids occupied by two Eu atoms. The Eu(Mg/Li) x Ga 4-x (0≤x≤0.73) compounds crystallize in the BaAl 4 structure type having I4/mmm space group. In Eu(Mg/Li) x Ga 4-x , the Eu atom occupies the edge and the body centered position of the unit cell while the Ga and M(Li/Mg+Ga) atoms forms a 3D polyanionic framework. Magnetic susceptibility of CeGa 2 , EuGa 2 , EuGa 4 and their corresponding Li/Mg analogs are discussed.