Λ-type Peak Research Articles

Synthesis, crystal structure, and magnetic properties of Ni[formula omitted]CrBO[formula omitted]

Ni2CrBO5 has been synthesized and investigated by X-ray diffraction, dc magnetization, and specific heat measurements. The unusual cation distribution has been established: the M1 and M3 sites are occupied by Ni2+ ions, the M2 site is by Cr3＋ ions, and the M4 site is mixed, featuring both Ni2+ and Cr3＋ ions. The magnetic order onsets at TN= 140 K, which is confirmed by a λ-type peak in specific heat, followed by the dome-shaped anomaly of the magnetization at about 30 K. In the magnetically ordered state, a remarkable sequence of temperature-induced magnetization reversal sensitive to the measurement’s regime is observed. Dominant antiferromagnetic interactions are characterized by the Weiss temperature θ= −73 K. The effective magnetic moment of 5.37 μB per formula unit is close to the spin-only one. The effect of the cation distribution on the magnetic properties is discussed.

From a conventional ferromagnetism to a frustrated magnetism: An unexpected role of Fe in Nd(Al1-xFex)2 (x ≤ 0.2)

The structure and magnetic properties of Nd(Al1-xFex)2 alloys with x = 0, 0.05, 0.1 and 0.2, which adopt cubic MgCu2-type structure with space group Fd3¯m, are studied using x-ray powder diffraction, static and dynamic magnetization, and calorimetric measurements. The lattice constant decreases linearly with increasing Fe content from 8.0106(4) Å to 7.9044(4) Å. Large thermal irreversibilities between zero-field-cooled and field-cooled conditions are observed for all substituted samples. The magnetic transition temperatures exhibit a minimum for x = 0.1, and coercive fields at 2 K increase from 70 Oe for x = 0 to 6.3 kOe for x = 0.2. The absence of λ-type peak in the heat capacity data, frequency dependence of ac magnetic susceptibility, and memory effects indicate formation of a cluster-glass magnetic state when x ≥ 0.1.

The crystal-field states and the magnetism in the Kondo-lattice antiferromagnet CeRh[formula omitted]Si[formula omitted

We have theoretically analysed temperature dependence of the specific heat at temperatures below 50 K of a Kondo-lattice antiferromagnet CeRh2Si2. We have reproduced experimentally-observed λ-type peak at the magnetic-ordering temperature TN of 36.5 K as originating from the splitting of the Kramers-doublet ground state of the Ce3+ ion. We have derived a set of crystal-field parameters, which, apart of the reproduction of crystal-field excitations of 30 and 52 meV, of the temperature dependence of the specific heat and of the paramagnetic susceptibility, reproduces a value of the magnetic moment, of 1.60 μB. Our results for the temperature dependence of the specific heat and of the entropy are physically adequate and substantially improved compared to theoretical descriptions reported in a recent publication in Nature Comm. (7 (2016) 11029).A good reproduction of the physical magnetic and electronic properties indicates that 99±1% of Ce ions are in the 4f1 configuration. A large value of the entropy removed in the magnetic transition, very close to Rln2, indicates that the hybridization effects and a broadening of crystal-field states in CeRh2Si2 are very small. The Quantum Atomistic Solid-State Theory (QUASST) approach can be used for the theoretical description of magnetic and electronic properties of other Ce, rare-earth and actinides compounds.

Large magnetocaloric effect and critical behaviour analysis in Gd2Cu2In

The ternary intermetallic compound Gd2Cu2In crystallizes in the Mo2Fe2B-type structure with the space group . The compound undergoes a ferromagnetic-paramagnetic (FM-PM) phase transition at 94.1 K and large magnetocaloric effect has been observed over a wide range of temperature. The isothermal magnetic entropy change and adiabatic temperature change were observed to be 13.8 J/kg · K and 6.5 K for 7 T applied field. We have employed a modified Arrott plot (MAP) and Kouvel-Fisher (KF) procedures to estimate the critical exponents near the FM-PM phase transition. Critical exponents and are estimated through a non-linear fitting. The β value is close to the three-dimensional (3D) Ising value, whereas γ and δ values lie closer to the mean-field values. Scaled magnetic isotherms collapse into two branches below and above TC in accordance with a single scaling equation. Specific-heat measurements show a λ-type peak near 94 K indicating long-range magnetic ordering. The specific-heat exponent α was estimated by a non-linear fitting using the function . The fitting in the temperature range yields .

EuTiO3: The low-energy electronic structure of Eu2+ from the specific heat

We present the low-energy electronic structure originating from the S7/28 ground multiplet of the Eu2+ ions derived from theoretical description of the temperature dependence of the specific heat at low temperatures, including the λ-type peak at TN, both at zero field and in external magnetic fields. The derived energy states are the physical reality - they can be probed by different techniques and on this basis results of different theoretical models can be compared. Our atomic-like approach consistently explains zero-temperature properties and thermodynamics. We separate the derived molecular field at T = 0 K of 2.73 T for contributions coming from antiferro- and ferromagnetic interactions and derived exchange constants J1 = −16.6 mK (AF) and J2 = +35.3 mK (F).

La substitution effect on the magnetic phase transition of SmTi2Al20

Powder X-ray and specific heat measurements were performed on (SmxLa1-xTi2Al20 (x = 0.2-1) single crystals grown by the Al self-flux method. All samples crystallize in the CeCr2Al20-type structure with Fd¯3m symmetry and the lattice parameter changes almost linearly with respect to the Sm concentration. From a λ-type peak appearing in the specific heat, it was found that the antiferromagnetic phase transition occurring at 6.5 K for x = 1 remains in the large Sm-concentration range. The transition temperature decreases almost linearly depending on the Sm concentration and goes to zero at the finite Sm concentration value of ∼ 0.1, indicating the possible existence of a quantum critical point. The robustness of the phase boundary against the magnetic field remains even in the La-substituted system. This behavior is in marked contrast to the field-sensitive behavior observed in Ce-based heavy fermion compounds.

Magnetic Phase Transition in Antiferromagnetic SrMnO3 Perovskites

We study thermal properties of SrMnO3 starting from analysis of the low-energy electronic structure of the Mn4+ ion in oxygen surroundings. As a start point of the studies, simulations of a SrMnO3 unit cell were performed with using the density functional theory (DFT). The DFT calculations have revealed ionic character of the compound. In the next stage of calculations the Mn4+ ion was treated as an atomic-like strongly-correlated 3d3 system with the 28-fold degenerated ground term 4F. This degeneracy is lifted under the combined action of the dominant octahedral crystal field and the on-site spin-orbit coupling. This study was concentrated on the explanation of the temperature dependence of the specific heat showing a pronounced λ-type peak at TN. We have calculated the d-electron contribution to the specific heat both in the antiferromagnetic and paramagnetic states. We have found that the entropy of the magnetic contribution is close Rln4 - a value expected from the 4A2g ground subterm of the Mn4+ ions. e.g. 11.5 J/Kmol.

Superconductivity in the splat-cooled UMo alloys*

We have investigated the superconductivity in splat-cooled UMo alloys by low-temperature resistivity and specific-heat measurements down to 0.4 K. The γ-U materials, such as U-Mo15 (with 15 at.% Mo doping), exhibit a conventional BCS superconductivity with Tc = 2.1 K and upper critical field exceeding 5 T, much higher than that for α-U materials. The alloys with <10 at.% Mo doping consist of a mixed γ + α-U phase. The superconducting transition in the U-Mo6 revealed by a smooth decrease below 1.5 K and a sharp drop at 0.6 K in the resistivity indicating that γ-U grains are embedded in the α-U matrix. The superconductivity transition was revealed by λ-type peak at Tc in the C(T) curve only for U-Mo15, while only one broad peak at Tc in the C(T) curves were observed for other UMo splats. With applying the magnetic fields, the resistivity jumps and specific-heat peaks move to lower temperatures.

Anomalously Field-Insensitive Correlated Electron Behaviors in SmTa2Al20

The low-temperature properties of single crystals of SmTa2Al20 have been studied by electrical resistivity, magnetization, and specific heat measurements. A λ-type peak structure, indicating a phase transition, was found in the specific heat at \(T_{\text{x}} = 2.0\) K. From an analysis of the nuclear specific heat contribution, the ordered moment is estimated to be 0.22 µB/Sm, which is largely suppressed from 0.84 µB/Sm for a free Sm3+ ion. The Kondo-like \(-{\log T}\) dependence of resistivity and the suppressed effective magnetic moment of 0.09 µB/Sm above \(T_{\text{x}}\) indicate strong \(c\)–\(f\) hybridization. The largely enhanced specific heat divided by temperature of about 3 J/(mol K2) below 0.4 K suggests the formation of extremely heavy quasiparticles in the ordered state. A notable feature commonly observed in all of these behaviors is robustness against magnetic fields. We discuss the possible relevance of multipole moments in the quartet ground state to the robustness against magnetic fields.

Magnetic phase transitions in layered perovskites K2CoF4 and K2NiF4

For the first time we have calculated low-energy electronic structure both in paramagnetic and magnetic state as well as zero-temperature properties and thermodynamics of K2CoF4 and K2NiF4. We consistently described that magnetic properties of these compounds are in agreement with its insulating ground state. The orbital moment of 1.06 μ B gives 30% contribution to the total moment of K2CoF4 of 3.25 μ B at T = 0 K. From this low-energy electronic structure we have calculated the temperature dependence of the specific heat with a λ-type peak marking the magnetic phase transition. In our ionic approach, K2CoF4 and K2NiF4 are independent insulators on distortions and the magnetic order.

T-H Phase Diagram of PrPb3 in [001] and [110] Magnetic Field Directions

We have measured the magnetic field dependence of the specific heat in PrPb3 for H || [001] and [110] up to H = 8T. At H = 0T, a sharp λ-type peak due to antiferro-quadrupolar(AFQ) ordering and a small anomaly appears at TQ = 0.43K and T* = 0.25K, respectively. The field dependence of TQ shows a reentrant behavior and a high-field phase appear above H ~ 6 T in the both field directions. T* shows a large difference between H || [001] and [110], suggesting that the anomaly is related to the anisotropic feature of the quadrupolar moment in magnetic fields. From these results, we describe the T-H phase diagram PrPb3 in H || [001] and [110].

Almandine: Lattice and non-lattice heat capacity behavior and standard thermodynamic properties

The heat capacity of three synthetic polycrystalline almandine garnets (ideal formula Fe3Al2Si3O12) and one natural almandine-rich single crystal was measured. The samples were characterized by optical microscopy, electron microprobe analysis, X-ray powder diffraction, and Mossbauer spectroscopy. Measurements were performed in the temperature range 3 to 300 K using relaxation calorimetry and between 282 and 764 K using DSC methods. All garnets show a prominent λ-type heat-capacity anomaly at low temperatures resulting from a paramagnetic-antiferromagnetic phase transition. For two Fe3+-free or nearly Fe3+-free synthetic almandines, the phase transition is sharp and occurs at 9.2 K. Almandine samples that have ~3% Fe3+ show a λ-type peak that is less sharp and that occurs at 8.0 ± 0.2 K. The low- T C P data were adjusted slightly using the DSC results to improve the experimental accuracy. Integration of the low- T C P data yields calorimetric standard entropy, S∘ , values between 336.7 ± 0.8 and 337.8 ± 0.8 J/(mol·K). The smaller value is recommended as the best S∘ for end-member stoichiometric almandine, because it derives from the “best” Fe3+-free synthetic sample. The lattice (vibrational) heat capacity of almandine was calculated using the single-parameter phonon dispersion model of Komada and Westrum (1997), which allows the non-lattice heat capacity ( C ex) behavior to be modeled. An analysis shows the presence of an electronic heat-capacity contribution ( C el, Schottky anomaly) superimposed on a larger magnetic heat-capacity effect ( C mag) around 17 K. The calculated lattice entropy at 298.15 K is S vib = 303.3 J/(mol·K) and it contributes about 90% to the total standard entropy at 298 K. The non-lattice entropy is S ex = 33.4 J/(mol·K) and consists of S mag = 32.1 J/(mol·K) and S el = 1.3 J/(mol·K) contributions. The C P behavior for almandine above 298 K is given by the polynomial [in J/(mol·K)]: C P = 649.06 ( ± 4 ) - 3837.57 ( ± 122 ) · T - 0.5 - 1.44682 ( ± 0.06 ) · 10 7 · T - 2 + 1.94834 ( ± 0.09 ) · 10 9 · T - 3 which is calculated using the measured DSC data together with one published heat-content datum determined by transposed-drop calorimetry along with a new determination in this work that gives H 1181K − H 302K = 415.0 ± 3.2 kJ/mol. Using our S∘ value and the C P polynomial for almandine, we derived the enthalpy of formation, Δ H °f, from an analysis of experimental phase equilibrium results on the reactions almandine + 3rutile = 3ilmenite + sillimanite + 2quartz and 2ilmenite = 2Fe + 2rutile + O2. A Δ H °f = −5269.63 kJ/mol was obtained.

Analysis of heat capacity and magnetothermal properties of the La0.775Gd0.225Mn2Si2 compound

We study magnetothermal properties of the La0.775Gd0.225Mn2Si2 compound. The magnetic contribution to the heat capacity is estimated by zero-field and 0.5 T magnetic field data. According to Arrott plot, the La0.775Gd0.225Mn2Si2 has a second order transition at a Curie temperature (TC) and a first order transition at a lower temperature. The λ-type peak in the heat capacity data also confirms the second order transition at the Curie temperature. We determined the electronic, magnetic and lattice contributions to the heat capacity. In addition, Debye temperature and N(EF) are estimated by heat capacity data as a function of temperature under zero and 0.5 T magnetic field.

Magnetic and transport properties of single-crystal Yb3Cu4Ge4

The magnetic and transport properties of single-crystal Yb3Cu4Ge4 with the Gd3Cu4Ge4-type orthorhombic structure are presented. Magnetization along the b-axis at 2K saturates to 2.8μB/Yb at 3kOe, while that along the a- and c-axes at 2K are gradually increasing to the value of 1.5μB/Yb and 0.39μB/Yb at 50kOe, respectively. The electrical resistivity within the ab-plane shows a metallic behavior in contrast to a broad maximum at around 30K for that along the c-axis. Each resistivity for the principal axis suddenly decreases below 8K. The specific heat shows a λ-type sharp peak at 7.8K. The electronic specific heat coefficient is estimated to be 29.4mJ/molYbK2 by fitting the magnetic part of the specific heat below 3K. The magnetic entropy released up to TC is 68% of that of Rln 2, expected for the doublet ground state. It is revealed that Yb3Cu4Ge4 is categorized to a weak heavy-fermion system showing a ferromagnetic transition at 7.8K with uniaxial anisotropy along the b-axis.

Crystal growth, magnetic and transport properties of single-crystal YbCuGe

The magnetic and transport properties of single-crystal of YbCuGe are presented. The magnetic susceptibility which shows uniaxial anisotropic behavior along the c-axis in all temperature range has a kink at around 4.2K. The specific heat also shows a λ-type peak at 4.2K, which moves toward low temperature side by an application of the magnetic field up to 90kOe. The linear specific heat coefficient is estimated to be 5.2mJ/molK2 from the zero field data. The electrical resistivity for the current along the principal axes shows a metallic behavior with a broad maximum at around 200K. The Hall coefficient drastically changes from the negative minimum at 70K to the positive constant value below 10K. These results indicate that YbCuGe is a metallic antiferromagnet with TN=4.2K.

Structural, Thermal, Spectroscopic, Specific-Heat, and Magnetic Studies of (C5H18N3)[Fe3(HPO3)6]·3H2O: A New Organically Templated Iron(III) Phosphite with a Pillared Structure Formed by the Interpenetration of Two Subnets

A new open framework iron(III) phosphite with formula (C5H18N3)[Fe3(HPO3)6].3H2O has been prepared by hydrothermal synthesis with N-(2-aminoethyl)-1,3-propanediamine as a templating agent. The crystal structure was solved from single-crystal X-ray diffraction data in the trigonal space group R. The unit cell parameters are a= 8.803(1) A and c= 25.292(2) A with Z = 3. The complex pillared structure can be described as two interpenetrating subnets, one organic, [(C5H18N3).3H2O]3+, and one inorganic, [Fe3(HPO3)6]3-. In the inorganic subnet, the pillars are formed by FeO6 trimers linked by vertex sharing phosphite groups, while in the cationic subnet the organic molecules act like pillars. With increasing temperature, the flexibility of the structure allows contraction due to dehydration followed by thermal expansion before reaching the thermal stability limit. The Dq and Racah parameters calculated for (C5H18N3)[Fe3(HPO3)6].3H2O are Dq = 965, B = 1080, and C = 2472 cm(-1). Mössbauer spectroscopy confirms the trivalent oxidation state of iron cations and the crystallographic multiplicities of their sites. The ESR spectra show isotropic signals with a g-value of 2.00(1). Specific-heat measurements show a three-dimensional (lambda-type) peak at a critical temperature Tc = 32 K. The value of the entropy at saturation is 46 J/mol K, very near the expected value of 44.7 J/mol K for the iron(III) cations with S = 5/2. Magnetic measurements indicate a three-dimensional antiferromagnetic ordering below 32 K and a reorientation of spins below 15 K with an incomplete cancellation of spins due to triangular interactions inherent to the structure.

Magnetic susceptibility, specific heat and magnetic structure of CuNi 2(PO 4) 2

The CuNi 2(PO 4) 2 phosphate has been synthesized by the ceramic method at 800 °C in air. The crystal structure consists of a three-dimensional skeleton constructed from MO 4 (M II Cu and Ni) planar squares and M 2O 8 dimers with square pyramidal geometry, which are interconnected by (PO 4) 3− oxoanions with tetrahedral geometry. The magnetic behavior has been studied on powdered sample by using susceptibility, specific heat and neutron diffraction data. The bimetallic copper(II)–nickel(II) orthophosphate exhibits a three-dimensional magnetic ordering at, approximately, 29.8 K. However, its complex crystal structure hampers any parametrization of the J-exchange parameter. The specific heat measurements exhibit a three-dimensional magnetic ordering ( λ-type) peak at 29.5 K. The magnetic structure of this phosphate shows ferromagnetic interactions inside the Ni 2O 8 dimers, whereas the sublattice of Cu(II) ions presents antiferromagnetic couplings along the y-axis. The change of the sign in the magnetic unit-cell, due to the [1/2, 0, 1/2] propagation vector determines a purely antiferromagnetic structure.

Spin-glass behaviour in the double perovskite “Sr2FeTeO6” due to mis-site disorder and cation vacancy formation

The double perovskite of nominal composition Sr2FeTeO6 has been prepared by the freeze-drying method. Simultaneous Rietveld refinement of X-ray and high resolution D2B neutron diffraction data has shown it to have space group I12/m1 but with a significant degree of pseudo-tetragonal symmetry with a = 5.6080(3) A, b = 5.5952(3) A, c = 7.9081(5) A and β = 90°, and a far more complex cation distribution over the B- and B′-sites. Refinement of Fe and Te fractions over these sites yielded the composition (Fe0.84□0.16)B(Te0.87Fe0.13)B′. This, and the absence of any significant formation of oxygen vacancies, yields an oxidation state for Fe close to +3, confirmed by EPR measurements (g ∼ 2.00). The disorder found over the B- and B′-sites implies that each Fe3+O6 octahedron is coordinated to six species, each of which can be Te, □ or another Fe, producing a statistical distribution of exchange pathways. The presence of several signals in the room temperature 57Fe Mossbauer spectrum and the broadening of the ESR signals can be related to this statistical distribution of the coordination environments around Fe3+. Magnetic DC susceptibility measurements indicate that the most important interactions are of antiferromagnetic nature. Absence of magnetic peaks in the 4 K neutron diffraction data, divergence below 40 K of field-cooled and zero-field-cooled DC susceptibilities, the frequency dependence of χ′ and χ″ magnetic AC susceptibilities and the lack of a three-dimensional λ-type peak in the specific heat measurements are all consistent with spin-glass type behaviour, as expected from the variety of frustrated magnetic interactions arising from the observed mis-site disorder and vacancies.

Neutron diffraction, specific heat and magnetic susceptibility of Ni 3(PO 4) 2

The Ni 3(PO 4) 2 phosphate was synthesized by the ceramic method in air atmosphere. The crystal structure consists of a three-dimensional skeleton constructed from Ni 3O 14 edge-sharing octahedra, which are interconnected by (PO 4) 3− oxoanions with tetrahedral geometry. The magnetic behavior was studied on powdered sample by using susceptibility, specific heat and neutron diffraction data. The nickel(II) orthophosphate exhibits a three-dimensional magnetic ordering at approximately 17.1 K. However, its complex crystal structure hampers any parametrization of the J-exchange parameter. The specific heat measurements of Ni 3(PO 4) 2 exhibit a three-dimensional magnetic ordering ( λ-type) peak at 17.1 K. Measurements above T N suggest the presence of a small short-range order in this phase. The total magnetic entropy was found to be 28.1 KJ/mol at 50 K. The magnetic structure of the nickel(II) phosphate exhibits ferromagnetic interactions inside the Ni 3O 14 trimers which are antiferromagnetically coupled between them, giving rise to a purely antiferromagnetic structure.

A Spin Glass Model with Vibrations of Crystal Lattices

With the help of the replica method and imaginary-timefunctional-integrate technique, the spin glass model with thevibrations of crystal lattices is investigated. In the limit of thereplica symmetry and the imaginary-time static approximation, themagnetic and thermodynamic quantities have been obtained. By thenumerical calculations, we found that the magnetization of the systemhas the typical spin-glass behaviour. A peak is found in thesusceptibility–temperature curve and is shifted to lower temperaturewith increasing applied field. Due to the lattice contribution, thespecific heat increases strongly at high temperature. Due to themagnetic contribution, the anomaly in the specific heat–temperaturecurve forms a λ-type peak, which agrees with the observation ofRojo et al. [Phys. Rev. B 66 (2002) 094406].