Magnetic and electrical properties of NdNiSn

Magnetization and resistivity measurements have been carried out on the equiatomic ternary compound NdNiSn in the temperature range 2–200 K. The compound crystallizes in the orthorhombic CeNiSn-type structure with space group Pna2₁. Magnetic susceptibility shows a distinct feature at T_N=3 K (Néel temperature), typical of a phase transition from an antiferromagnetic to paramagnetic state. In the paramagnetic regime, the magnetic susceptibility obeys Curie–Weiss behavior yielding an effective magnetic moment μ_eff=3.32μ_B at lower temperatures, and 3.88 μ_B at higher temperatures. The reduction in the magnetic moment at lower temperatures is attributed to a crystalline electric field (CEF) effect, while the slight excess of magnetic moment at high temperatures compared to that of the free Nd³⁺ ion (3.62 μ_B) indicates that only a very small magnetic moment, at most 0.3 μ_B, is induced at the Ni sites. The electrical resistivity exhibits metallic behavior and no anomaly is observed at the respective Néel temperature. Analysis of the resistivity data in terms of crystalline electric fields including s–d electron scattering reveals that the ground magnetic state for the Nd³⁺ ions is a doublet of J=±5/2 states, with a first exited doublet of J=±7/2 states having an energy splitting of 56 K, with the next exited multiplet 139 K above the ground levels. These results are in fairly good agreement with those reported in the literature based on magnetic susceptibility and heat capacity measurements.     

Investigation of the dependency of the upper critical magnetic field on the content x in Y1−xYbx/2Gdx/2Ba2Cu3O7−y superconducting structures

The Y_1−xYb_x/2Gd_x/2Ba₂Cu₃O_7−y superconducting samples for x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 were prepared by using the solid-state reaction technique. Resistivity measurements of the samples were performed in QD–PPMS system under different magnetic fields up to 5 T in zero fields cooling regime. Using the resistivity data, the upper critical magnetic field H_c2(0) at T = 0 K for 50% of R_n was calculated by the extrapolation H_c2(T) to the temperature T = 0 K. The coherence length in T = 0 K were calculated from H_c2(0) and the effects of x in the composition on both the coherence length and the upper critical magnetic field were examined. The results showed that H_c2(0) varied from 84.05 to 122.26 T with the content x. The upper critical magnetic field in the temperature T = 0 K slightly decreased with increasing the content x. Using the content x, the upper critical magnetic field can be controlled and this can be used in the superconductivity applications.     

The isostructural phase transition and frustrated magnetic ordering in TbPd0.9Ni0.1Al studied by neutron diffraction

By using powder neutron diffraction we provide a detailed structural and magnetic investigation of the pseudo-ternary compound TbPd_0.9Ni_0.1Al with hexagonal ZrNiAl-type crystal structure and disorder of palladium and nickel atoms on two crystallographic sites. We provide experimental evidence for the occurrence of an isostructural phase transition in TbPd_0.9Ni_0.1Al at 157 K, which is affected by structural disorder compared to that observed in TbPdAl. In TbPd_0.9Ni_0.1Al the step of the c/a ratio at the first-order phase transition appears about four times smaller than in TbPdAl. TbPd_0.9Ni_0.1Al undergoes two successive magnetic phase transitions at T_N1=41 K and T_N2≈21.5 K. Two third of non-frustrated ordered Tb moments form commensurate antiferromagnetic chains () and coexist with one third of frustrated ordered Tb moments, which change from a commensurate structure () between T_N2 and T_N1 to an incommensurate structure () below T_N2. Due to the strong magneto-crystalline anisotropy all ordered Tb moments stay perpendicular to the ab-plane and reach at 1.5 K saturation values of 8.6 (2) μ_B (non-frustrated) and 7.2 (2) μ_B (frustrated). The symmetry of the magnetic structure of TbPd_0.9Ni_0.1Al is lower than that of TbNiAl.     

Coexistence of localized magnetism and itinerant ferromagnetism in a Gd5Y2Pd3 single crystal

Experimental results of the single-crystal X-ray diffraction, XPS, ac-magnetic susceptibility (ac-χ), dc-magnetization M(T), and electrical resistivity (ρ) measurements for the hexagonal Th₇Fe₃-type Gd₅Y₂Pd₃ single crystal are presented. Anomalies in (ac-χ), (T) and M(T)-curves have allowed to establish that Gd₅Y₂Pd₃ undergoes a long-range ferromagnetic-type ordering at T_C = 263 K, followed by a spin-reorientation below 190 K. The magnetization data indicate that there is an excess of the magnetic moment for the Gd³⁺ ions. The observed XPS, magnetic and electrical resistivity behaviour points to the coexistence of localized magnetism from the magnetic Gd³⁺ ions and itinerant ferromagnetism from 4d- and 5d-electron bands. We discuss the magnetic behaviour of the Gd_7−xY_xPd₃ solid solutions in terms of three competitive mechanisms: RKKY-interaction, magnetic frustration and spin-fluctuation.     

Magnetism in PrRhSn studied on a single crystal

We have grown a single crystal of PrRhSn, analyzed the structure by X-ray diffraction methods and measured the specific heat, ac susceptibility, magnetization, and electrical resistivity as functions of temperature and magnetic field. Also we have calculated the electronic structure of this compound by ab initio methods. Ferromagnetism below T_C = 2.9 K characterized by strong uniaxial anisotropy (an anisotropy field 65 T) has been confirmed. Pronounced crystal field (CF) effects were observed on the temperature dependences of the magnetic susceptibility, electrical resistivity and specific heat. The calculations revealed that besides the stable Pr magnetic moment owing to the localized 4f-electrons a small magnetic moment of at most 0.2μ_B is induced at the Rh site (and 0.1μ_B at the Sn site) due to the polarized Rh 4d-electron states (Sn 5p-states) hybridizing with the Pr 5d-electron states, i.e. the Rh and Sn moments play some role in the total balance of the magnetic moments in this compound. This result is in agreement with the experimentally determined saturated magnetic moment of PrRhSn, which is about 0.3μ_B larger than the ordered moment value expected for the Pr³⁺ free ion.     

Magnetic properties of a new iron lead vanadate Pb2FeV3O11

Temperature dependence of dc/ac magnetization and electron paramagnetic resonance (EPR) spectra of Pb₂FeV₃O₁₁ iron lead vanadate has been investigated. The dc magnetic measurements have shown the presence of antiferromagnetic interactions with Curie-Weiss temperature, T_CW = −15.2 K, in the high temperatures range while the field cooled (FC) magnetization revealed a maximum at T_N = 2.5 K which coincides with a long range magnetic ordering. Temperature dependence of χ′ has shown a maximum at the same temperature. EPR spectrum of Pb₂FeV₃O₁₁ at room temperature is dominated by nearly symmetrical, very intense and broad resonance line centered at g_eff ∼ 2.0 that could be attributed to the correlated system of iron ions. The temperature dependence of magnetic resonance parameters (amplitude, g-factor, linewidth, integrated intensity) has been determined in the 4-300 K range and it suggests the existence of short range correlated spin system up to high temperatures. The temperature dependence of the amplitude of the resonance line has shown a pronounced maximum at 12.5 K that indicates on the existence of two subsystems of weakly and strongly coupled iron pairs. Comparison of dc magnetic susceptibility and EPR integrated intensity points to the presence of correlated spin agglomerates that play an important role in determination of the magnetic response of Pb₂FeV₃O₁₁.     

Magnetic properties of the 1/1 approximant Ag42In42Gd16 to the icosahedral quasicrystal Ag-In-Gd

The structural, magnetic, and ¹⁵⁵Gd Mössbauer spectral properties of the 1/1 approximant Ag₄₂In₄₂Gd₁₆ to an icosahedral quasicrystal Ag-In-Gd are reported. Based on dc magnetic susceptibility measurements, it is shown that the studied compound develops no long-range magnetic order in the temperature range 1.8-300 K. The dc zero-field-cooled and field-cooled susceptibility data indicate that the 1/1 approximant Ag₄₂In₄₂Gd₁₆ is a spin glasss with freezing temperature T_f = 3.6(1) K. This is further confirmed by the analysis of the frequency dependence of T_f using the Vogel-Fulcher law and the dynamic scaling behavior near T_f. It is argued that the spin freezing process is a true equilibrium phase transition rather than a nonequilibrium phenomenon. The large frustration parameter of the studied compound indicates that it belongs to a category of strongly geometrically frustrated magnets. The ¹⁵⁵Gd Mössbauer spectra of the 1/1 approximant Ag₄₂In₄₂Gd₁₆ confirm that the Gd spins are frozen at 1.5 K and are fluctuating at 4.6 K. The Debye temperature of the 1/1 approximant Ag₄₂In₄₂Gd₁₆ is 200(1) K.     

Magnetic and electrical properties of (U, Th)T2Sn2 (T Co, Ni, Cu) intermetallics

We report preliminary results of our structural, magnetic and electrical studies on the new ternary stannides (U, Th)T₂Sn₂ (T Co, Ni, Cu). All the phases crystallize in a tetragonal symmetry with the CaBe₂Ge₂-type crystal structure. While UCu₂Sn₂ and UNi₂Sn₂ are antiferromagnets with T_N values of 108 K and 35 K respectively, UCo₂Sn₂ exhibits a more complex magnetic behaviour with two subsequent maxima in the temperature variation of the magnetic susceptibility and a strong field dependence of the magnetization. The thorium intermetallics do not exhibit any magnetic ordering except for ThCo₂Sn₂ that orders antiferromagnetically at 65 K. Both a complex magnetic behaviour of UCo₂Sn₂ and antiferromagnetic properties of its thorium counterpart suggest that as well as uranium the cobalt atoms in this stannide also carry magnetic moments. For all the uranium compounds the electrical resistivity in the paramagnetic region rises monotonically with increasing temperature, but it exhibits a more metallic behaviour for the thorium-based materials.     

A neutron diffraction study of the hexagonal pseudo-ternary compounds ErMn6−xFexSn6 (x = 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 3.0, 4.0)

The hexagonal ErMn_6−xFe_xSn₆ solid solution (0.2 < x < 4) has been studied by magnetisation measurements and neutron diffraction. The ordering temperature of the T = (Mn,Fe) sublattice almost continuously increases from T = 386 K for x = 0.2 to T = 498 K for x = 4. The T sublattice orders in the successive magnetic structures helimagnetic H1, antiferromagnetic AF2, helimagnetic H2 and antiferromagnetic AF1 with increasing iron content. While structures AF2 and H1 were already observed in ternary Mn compounds and AF1 in ternary iron compounds, the structure H2 is of a new kind characterized by an AF slab around the Er(1a) site. At low temperature, a change of the easy direction of the Er moment from easy plane to easy axis is observed. The iron-rich compounds display a ferromagnetic order of the Er sublattice. A new kind of magnetic structure characterized by a sine-wave modulated arrangement with a propagating vector Q = (0, 0, q_z) is also observed. The evolution of the magnetic properties (enhancement of the AF character of the (Mn,Fe) sublattice and magnetocrystalline anisotropy of erbium) is discussed.     

Antiferromagnetic ordering below 1.7 K in PrIr2Ge2

We present our investigations on magnetic and transport properties of polycrystalline PrIr₂Ge₂ which forms in CaBe₂Ge₂-type primitive tetragonal structure (space group P4/nmm). The ac magnetic susceptibility data exhibit two well pronounced peaks at 2.08 K and 0.76 K due to the onset of magnetic order. The specific heat also exhibits a sharp λ-type anomaly at 1.7 K confirming the onset of bulk antiferromagnetic order. The temperature dependence of magnetic part of entropy suggests a quasi-triplet ground state in this compound. The onset of magnetic order is also confirmed by the electrical resistivity data.     

Magnetoresistance of the La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single crystal

The dependence of the resistance ρ of the La_0.7Ca_0.3MnO₃ single crystal on the temperature (in a range of 77 < T < 410 K) and magnetic field H is studied. The dependence of the magnetoresistance Δρ/ρ of the ferromagnetic phase on the field is shown to be determined by the competition of two mechanisms. In low magnetic fields, the magnetoresistance is positive Δρ/ρ > 0 and is determined by changes in the resistance with changing magnetization orientation with respect to the crystallographic axes; in high magnetic fields, the magnetoresistance is negative Δρ/ρ < 0, since it is the suppression of spin fluctuations in the magnetic field that plays the principal role. The phase transition from the ferromagnetic to paramagnetic state is a first-order transition close to the second-order one. In the transition range, the magnetoresistance is determined by the resistivity in the zero field ρ(T) and by the shift of the transition temperature T _C(H) in the magnetic field. In the paramagnetic state, the resistivity ρ(T) has an activation character; similarly to the magnetoresistance of other lanthanum manganites, the magnetoresistance of this single crystal is controlled by a change in the activation energy in the magnetic field.     

A new ternary compound YbZn2As2 with mixed valency of Yb

A new compound with chemical formula YbZn₂As₂ and ‘anti’-La₂O₃-type crystal structure (space group P3m1) has been synthesized for the first time. The trigonal lattice constants of the compound are a=0.4157 and c=0.6954 nm. In the temperature range 77–500 K, the magnetic susceptibility of YbZn₂As₂ follows the Curie–Weiss law, indicating antiferromagnetic interactions of the Yb ions and yielding a Curie temperature θ=−52.8 K and an effective magnetic moment μ_eff=2.35 μ_B (the Bohr magneton) per Yb ion. This means that a part of the Yb ions has valency 3+, instead of 2+ for all the Yb ions, as would be expected from their formal oxidation number, i.e. YbZn₂As₂ is a compound with mixed valency of Yb. YbZn₂As₂ exhibits p-type conductivity with a room temperature electrical resistivity of 0.15 Ω cm which decreases when lowering temperature and reaches a practically constant value of 0.08 Ω cm below 20 K.     

The magnetic structures of RMgSn compounds (R = Ce, Pr, Nd, Tb)

The synthesis of the new compounds RMgSn (R = La-Nd, Sm, Gd-Tm, Lu and Y) has been recently reported. The compounds formed by La and Ce crystallise in the TiNiSi structure type (oP12, Pnma), while from Nd they adopt the CeScSi-type (tI12, I4/mmm); PrMgSn is dimorphic: its high-temperature form (HT) is TiNiSi-type while the low-temperature one (LT) is CeScSi-type.In this paper we now report the results of a neutron diffraction investigation which has been performed in order to refine the crystal as well as the magnetic structures for the RMgSn compounds with R = Ce, Pr, Nd and Tb. All these compounds see at low temperature the establishment of long range magnetic ordering with a predominantly antiferromagnetic interaction; only PrMgSn-HT orders ferromagnetically. These results agree with those from magnetic measurements recently reported.The magnetic structure of CeMgSn is of the amplitude-modulated type, the value of the magnetic propagation vector refined at 2 K is τ = [0, 0.1886(4), 0.3384(8)]. The PrMgSn-HT phase below T = 52 K adopts first a purely ferromagnetic structure, then at about T = 15 K a second magnetic coupling leads to a spin-canted magnetic structure. Both PrMgSn-LT and NdMgSn have the same antiferromagnetic commensurate magnetic structure. The TbMgSn compound below T_N = 35 K orders antiferromagnetically with an equal moment cycloidal structure; however a second magnetic transition at a temperature corresponding to T_N2 = 65 K is likely also present.     

TbRhSn and DyRhSn – Detailed magnetic and 119Sn Mössbauer spectroscopic studies

The results of magnetic studies and Mössbauer spectroscopic investigations are reported for the stannides TbRhSn and DyRhSn crystallizing in the hexagonal ZrNiAl-type structure. The polycrystalline samples of these ternary intermetallics were synthesized by arc melting from metallic precursors. Detailed ¹¹⁹Sn Mössbauer spectroscopic studies are used to investigate the hyperfine interactions and their temperature evolutions at places occupied by the diamagnetic tin nuclei. Magnetic properties of DyRhSn and TbRhSn were studied by AC/DC magnetometry in a wide temperature range. The results show that both compounds are magnetically ordered at low temperatures. DyRhSn is a non-collinear antiferromagnet with the Néel temperature T_N = 7.5 K, whereas TbRhSn undergoes a transition from a paramagnetic to an antiferromagnetic state at T_N = 20.2 K. An additional transition at T_SR = 10.3 K is detected for TbRhSn which corresponds to some changes in the magnetic moments ordering. The role of the magnetostriction effect in the evolution of the hyperfine parameters and its influence on the observed TbRhSn Mössbauer spectra is discussed. Triangular-like antiferromagnetic arrangements with rare-earth magnetic moments lying in the hexagonal plane are proposed for both compounds at very low temperatures.     

Grain size dependent magnetization, electrical resistivity and magnetoresistance in mechanically milled La0.67Sr0.33MnO3

We have studied magnetization, ac susceptibility, resistivity and magnetoresistance in mechanically milled La_0.67Sr_0.33MnO₃. The material with grain size micron to nanometer scale has stabilized in rhombohedral crystal structure with space group R3C. We have found various grain size effects, e.g., decrease of ferromagnetic moment, increase of surface spin disorder, and appearance of insulator/semiconductor type resistivity. In addition to these conventional features, we have identified a magnetic anomaly at 45 K in bulk sample. Ferromagnetic to paramagnetic transition temperature (T_C) is above room temperature for all samples. The samples are typical soft ferromagnet that transformed from multi-domain state to single domain state in nanocrystalline samples. The remarkable observation is that low temperature freezing of ferromagnetic domains/clusters does not follow the conventional spin glass features. Experimental results clearly showed the enhancement of high field magnetoresistance in nanocrystalline samples below 200 K, whereas low field magnetoresistance gradually decreases above 200 K and almost absent at 300 K. We have discussed few more magnetic and electrical changes, highly relevant to the progress of nanomaterial research in ferromagnetic manganites.     

Canted antiferromagnetic structure of HoPtGe2

Neutron powder diffraction has been performed on orthorhombic HoPtGe₂ crystallizing in space group Immm. Lattice constants are a=4.321(3) Å, b=16.344(3) Å, and c=8.731(5) Å. Refined atomic positional parameters are given. A magnetic phase transition from paramagnetism into a canted antiferromagnetic structure occurs at the Néel temperature T_N=8.5 K. The ordered moments at Ho sites 4i and 4h are 8.1 μ_B and 4.0 μ_B at 4.2 K, respectively; the moments are aligned within the ab and bc plane, respectively.     

Low-temperature hysteresis in transport properties of UNi0.5Sb2

We report detailed data of the electrical resistivity and thermoelectric power (TEP) measurements carried out at temperatures 0.4–300 K on single crystals of a deficit diantimonide UNi_0.5Sb₂. Although the composition of this compound and the transition temperature to the antiferromagnetic state are almost the same as those previously reported, we found the hysteretic transition occurring in the ordered state at different temperature (T_t = 81 K) from those estimated in earlier works, i.e. at either T_t = 64 K or T_t1 = 40 K and T_t2 = 85 K. This fact observed in both kinds of applied measurements indicates clearly a high sensitivity of these temperatures [connected to some moment reorientation and the following reconstruction of the Fermi surface (FS)] in this compound, to the used technology of growing its single crystals by different groups of authors. Following the results of a similar character of the parent compound USb₂ and taking into account the recent literature reports, we suppose that the observed anomalies in the resistivity and TEP of UNi_0.5Sb₂ are also caused by the complex electronic structure. Thus changes at its FS in the ordered state (e.g. f -electron dualism leading to band renormalization involving magnons) are forcing the reorientation of the uranium magnetic moments in the successive magnetic unit cells.     

Magnetic,electronic and thermodynamic properties of the heavy fermion compound CeNiAl4

We have carried out the magnetic susceptibility, electrical resistivity, specific heat, thermoelectric power and X-ray photoemission spectroscopy (XPS) measurements for CeNiAl₄. The magnetic susceptibilities have revealed a Curie–Weiss behavior above 30 K with a large negative paramagnetic Curie temperature θ_P = ?66 K and an effective moment of 2.45μ_B. The f-occupancy and the coupling between the f level and the conduction states are derived from XPS to be about 0.83 and ～40–64 meV, respectively. The valence state of the Ce ion is close to 3+. Extrapolation of the lowest temperatures range of C/T(T²) yields the electronic specific heat coefficient γ = 0.5 J mol^?1 K^?2. In combination with the thermoelectric power and resistivity data, a heavy fermion state is confirmed in CeNiAl₄.     

Synthesis, characterization and low temperature studies of iron chalcogenide superconductors

We have synthesized tetragonal iron selenide and telluride superconductors through solid state reaction at 450 °C and 550 °C, respectively. These synthesis temperatures have been established by optimization. Electrical resistivity and magnetic susceptibility measurements (4.2–300 K) confirm superconductivity with T_C of around 8.5 K in all selenide samples of nominal composition Fe_1+δSe (δ = 0.02–0.22). However, Scanning Electron Microscopy/Energy Dispersive Spectroscopy studies clearly indicate that the actual stoichiometric ratio of Fe:Se for all the samples synthesized is around 1:1. Also all the samples exhibit identical phase transition temperatures from tetragonal to orthorhombic structure below 100 K implying identical phase composition. The partial substitution of Se by Te leads to an enhancement of T_C to 13 K. The non-superconducting telluride Fe_1.09Te exhibits a metal–insulator phase transition at 82 K. Substitution studies of this telluride system by S and Si have in addition been carried out to investigate if chemical pressure induces superconductivity.     

A new ternary arsenide Eu20Ni42As31: Preparation, crystal structure and physical properties

We have investigated the crystal structure, magnetic and electrical transport properties of the new ternary compound Eu₂₀Ni₄₂As₃₁, crystallizing in the hexagonal Sm₂₀Ni₄₂P₃₁ structure type, space group P6₃/m, a = 21.1496(7), c = 4.0734(2) Å, according to X-ray powder diffraction. From magnetic investigations it is evidenced that in this compound the europium is divalent, Eu²⁺. The compound orders antiferromagnetically at a Neèl temperature T_N = 5 K. However, in moderate magnetic fields an overall ferromagnetic-like magnetization is observed. The measurements of the electrical resistivity prove this magnetic behaviour as well as the metallic character of the compound Eu₂₀Ni₄₂As₃₁.