Infrared study in high magnetic fields of the crystal-field excitations in PrMnO3

Pr³⁺ ion crystal field (CF) excitations in PrMnO₃ single crystals have been studied by infrared transmission, in the 1800–8000 cm⁻¹ range, as a function of temperature and applied magnetic field up to 13 T. No noticeable frequency shifts which might occur below T_N∼100 K, as a result of the antiferromagnetic transition, are observed in the Pr³⁺ CF levels. A set of CF parameters that fit the experimental levels as well as the low temperature Pr³⁺ magnetic moment in PrMnO₃ has been determined.     

Magnetism of Rare-Earth Compounds with Non-Magnetic Crystal-Field Ground Levels

Among rare-earth compounds, there are many materials having non-magnetic crystal-field （CF） ground levels. To understand their magnetic behaviour at low temperatures, we study the effects of the CF levels and the Heisenberg-like coupling on the magnetic process of such a crystalline with mean-field and CF theory. It is found that the material can be magnetically ordered if the Heisenberg exchange is sufficiently strong. Additionally we obtain a condition for initial magnetic ordering, and derive a formula for estimating the Curie temperature if the ordering occurs.     

Reduced Magneto-Resistivity of a Rare-Earth Crystalline and the Degeneracy Removals of Its Crystal-Field Levels

A theoretical approach is generalized and employed to calculate the magneto-resistivity of a rare-earth crystalline （CeAh） with degenerate ground crystal-field （CF） level in the presence of external fields. The calculated results show that when a magnetic field is applied in the c-direction, the magneto-resistivity may be reduced by more than 90% in certain cases in comparison with the pure CF contribution at the same temperature, demonstrating the strong effects of the degeneracy removals of the CF levels on the magnetic resistivity.     

Crystal field investigation on the magnetic properties of Yb in Yb(CF3SO3)3·9H2O

Ytterbium tri-fluoromethanesulfonate (YbTFMS) single crystals are prepared from the slow evaporation of the aqueous solution of YbTFMS and the principal magnetic susceptibility perpendicular to the c-axis of the hexagonal crystal (χ_⊥) is measured from 300 K down to 13 K. Principal magnetic anisotropy Δχ(=χ_∥−χ_⊥) is measured from 300 K down to 80 K which provides principal magnetic susceptibility parallel to the c-axis (χ_∥) down to 80 K. Very good theoretical simulation of the observed magnetic properties of YbTFMS has been obtained using one electron crystal field (CF) analysis having C_3h site symmetry. No signature of ordering effect in the observed magnetic data is noticed down to the lowest temperature (13 K) attained, indicating the inter-ionic interaction to be of predominantly dipolar type. The calculated g-values are found to be g_∥=2.67 and g_⊥=2.51, respectively. CF analysis provides the electronic specific heat which gives two Schottky anomalies in its thermal variation down to ∼13 K. The temperature dependences of quadrupole splitting and hyperfine heat capacity are studied from the necessary information obtained from the CF analysis.     

A magnetic study of the ThCr2Si2-type RPd2Ge2 (R=Pr, Nd) compounds. Magnetic structure of PrPd2Ge2 from powder neutron diffraction

The magnetic properties of the PrPd₂Ge₂ and NdPd₂Ge₂ compounds have been investigated by magnetic measurements, specific heat measurements and neutron diffraction experiments. The PrPd₂Ge₂ compound orders antiferromagnetically below T_N=5.0(2) with an original modulated magnetic structure characterized by a magnetic cell three times larger than the chemical one by tripling of the c parameter. The palladium atom is non magnetic and the Pr moments are parallel to the c-axis with a value of ≈2.0 μ_B at 2 K. The specific heat measurements clearly detect a low temperature transition for the NdPd₂Ge₂ compound, interpreted as a Nd sublattice antiferromagnetic ordering below 1.3(2) K.     

μSR investigation of the magnetic ordering in EuAs3

We have investigated the magnetic ordering and the incommensurate-commensurate phase transition in EuAs₃ by zero-field (ZF) and longitudinal-field μSR. In the commensurate phase, stable at temperatures below T_L=10.3 K, the ZF muon signal exhibits oscillations corresponding to four muon precession frequencies the lowest of which behaves anomalously. The muon signal shows no oscillation but exponential decay in the incommensurate phase stable in temperature range from T_L≈10.3 K up to T_N≈11 K. The temperature dependence of the fitted relaxation rate shows divergence-like behaviour at the ordering temperature T_N≈11 K and also at the lock-in transition T_L≈10.3 K. The results are in qualitative agreement with those previously obtained by neutron and X-ray magnetic scattering investigations except for the anomalous temperature dependence of the lowest frequency in the commensurate phase. We propose a model for this anomalous behaviour.     

Effect of Al doping on the magnetic and electrical properties of layered perovskite La1.3Sr1.7Mn2−xAlxO7

The effect of Al substitution for Mn site in layered manganese oxides La_1.3Sr_1.7Mn_2−xAl_xO₇ on the magnetic and electrical properties has been investigated. It is interesting that all the samples undergo a similar and complex transition with lowing temperature; they transform from the two-dimensional short-range ferromagnetic order at T*, then enter the three-dimensional long-range ferromagnetic state at T_C, at last they display the canted antiferromagnetic state below T_N. T*, T_C and T_N are all reduced with Al content. Resistivity increases sharply with increasing Al concentration, and the metal-insulator transition disappears when x reaches 10%. Additionally, magnetoresistance (MR) effect is weakened. Al substitution dilutes the magnetic active Mn-O-Mn network and weakens the double exchange interaction, and further suppresses FM ordering and metallic conduction. Owing to the anisotropic interaction in the layered perovskite, the magnetic and electrical properties are more sensitive to Al doping level than those in ABO₃-type perovskite.     

Magnetism of ErNi2B2C investigated with a two-ion model for rare-earth antiferromagnets

A two-ion model for rare-earth antiferromagnets was employed in the present work to study the magnetic and thermodynamic properties of ErNi₂B₂C, and derive an analytic formula for the Néel temperature of the compound with the quantum perturbation theory. To apply the theory, four new base functions had to be composed due to the special forms of its crystal-field (CF) wavefunctions. Our theoretical analysis suggested that the magnetic behavior of the compound be governed by these low-lying CF levels in the temperature region at least up to 100 K.     

Magnetic,transport, thermal properties and orbital state for spinel MnTi2O4

We systematically investigated the various properties of MnTi₂O₄. The compounds are found to be conducted by small polarons with different activation energies across the temperature T₁ and deviate from the Curie–Weiss law for the magnetic susceptibility at T₁. Also at T₁ do the specific heat undergo a crossover. As indicated by the X-ray diffraction peaks, the series of phenomena near the characteristic temperature T₁ are attributed to the happening of the micro-structural distortion, and the distortion implies the short-range orbital ordering state below T₁. This short-range oribtal ordering state is supported by the little upturn of the thermal conductance at T₁. As the temperature decreases, the systems enter a long-ranged collinear ferrimagnetic state at a lower temperature T_N. Moreover, the compounds decreases in thermal-electric power with the decrease of temperature and undergoes a p-type to n-type transition at the temperature T_pn. Through the quantitative theoretical analysis, both the decrease of the thermal power and the p-type to n-type transition are cooperatively induced by spin–orbit (SO) coupling and the Jahn–Teller (JT) effect.     

Heat capacity and magnetic properties of pyrochlore Hg2Os2O7

Hg₂Os₂O₇, which has the cubic pyrochlore structure, remains metallic down to the liquid helium temperature unlike its isostructural counterpart Cd₂Os₂O₇, which shows metal-insulator transition at 226 K. Magnetization and heat capacity data for Hg₂Os₂O₇ are presented. The magnetic anomaly at T_N=88 K shares many characteristics in common with the metal-insulator transition in Cd₂Os₂O₇, though Hg₂Os₂O₇ remains metallic below T_N. The heat capacity C_p shows no or very little change in the magnetic entropy around T_N, supporting the view that there is no long-range ordering of localized spins. The measured value of electronic heat-capacity coefficient γ=21 mJ K⁻²mol⁻¹ is comparable to the value obtained from band-structure calculation on Cd₂Os₂O₇, suggesting that mass-enhancement is small in Hg₂Os₂O₇. There is a pronounced peak in C_p/T³ at 13.1 K, which corresponds to a peak in the phonon density of states at 40 cm⁻¹.     

Crystal field effect and geometric frustration in Er2Ti2O7 —an XY antiferromagnetic pyrochlore

Magnetic susceptibility of powder Er₂Ti₂O₇ (ErT) is measured between 300 K and 80 K. shows a Curie-Weiss (CW) type behaviour with   ErTiO_3.5 and . A crystal field (CF) analysis of our experimental data, g-values (g_∥=0.27 and g_⊥=7.8) and the positions of two CF levels (reported earlier from an inelastic neutron scattering study) provide CF parameters and CF levels of the ground ⁴I_15/2 and excited multiplets of ErT. The theoretical follows a CW-type behaviour, with . Single-ion magnetic anisotropy (χ_⊥−χ_∥) is 9500×10⁻⁶ emu/mol ErTiO_3.5 at 300 K, which increases by ∼54 times at 10 K and ErT resembles an XY planar system. It can be inferred from CF analysis that the earlier observed change of from −13 K to −22 K below 50 K is not due to the CF effect. Nuclear hyperfine (HF) levels of ¹⁶⁷ErT and ¹⁶⁶ErT are calculated and the theoretical curve of vs. T (K) for T<T_N matches the observed results. Mössbauer lines expected for ¹⁶⁶ErT are also predicted.     

Heat capacity and magnetic phase transitions of rare-earth orthoferrite HoFeO3

Heat capacity of two rare-earth orthoferrites HoFeO₃ and LuFeO₃ were measured between 1.8 and 200 K. A distinctly large and two small heat capacity anomalies were detected for HoFeO₃ under zero magnetic field around 3.3, 53 and 58 K, respectively. The low-temperature anomaly with a peak at 3.3 K is due to the ordering of Ho³⁺ ions and the estimated magnetic entropy for this transition was favorably compared with the expected (R ln 2). Application of magnetic field significantly affects the positions and the magnitudes of the anomaly at 3.3 K. Energies of low-lying levels of the lowest J-term of Ho³⁺ ion were roughly estimated through analysis of the Schottky heat capacity.     

Low temperature thermodynamical properties of ErCu2Si2

ErCu₂Si₂ crystallises in the tetragonal ThCr₂Si₂-type crystal structure. In this paper results of magnetometric, electrical transport, specific heat as well as neutron diffraction are reported. Results of electrical resistivity and specific heat measurements performed at low temperature yield existence of magnetic ordering roughly at 1.3 K. These results are in concert with neutron diffraction measurements, which reveal simple antiferromagnetic ordering between 0.47 and 1.00 K. At temperatures ranging from 1.00 up to 1.50 K an additional incommensurate magnetic structure was observed. The propagation vector k=(0;0;0.074) was proposed to describe magnetic reflections within the amplitude modulated magnetic structure. Basing on specific heat studies the crystal field levels splitting scheme and magnetic entropy were calculated.     

Magnetic properties of the quasi-one-dimensional system BaMn2V 2O8

Magnetic properties of BaMn₂V ₂O₈ are investigated by means of susceptibility, magnetization, and heat capacity measurements. Our experimental results show that BaMn₂V ₂O₈ is a one-dimensional canted antiferromagnet. The antiferromagnetic transition at a relatively high T_N of 37 K may be due to an enhancement of interchain interaction in the system, since Mn²⁺ ions have large spin moment of S=5/2. Weak ferromagnetism in BaMn₂V ₂O₈ may be due to Dzyaloshinskii-Moriya interactions arising from its noncentrosymmetric crystal structure.     

Specific heat and magnetic susceptibility of MnF2 and Mn0.98Fe0.02F2 near TN

The antiferro- to paramagnetic phase transition of the weakly anisotropic compound MnF₂ has been studied by means of heat capacity, magnetic susceptibility and thermal expansion measurements. The critical-point parameters associated with the specific heat indicate a transition according to the theoretical Ising-model. The temperature derivative of the parallel magnetic susceptibility times temperature (d(χ∥T)/dT) and the c-axis thermal expansion coefficient show a critical behaviour very similar to that of the specific heat. The influence of iron doping on the critical behaviour has been investigated by studies on Mn_0.98Fe_0.02F₂. Specific heat and magnetic susceptibility measurements show an unexpectedly sharp transition although some rounding off is noticed as compared to pure MnF₂.     

Spectroscopic and magnetic susceptibility analyses of the FJ and D4 energy levels of Tb(4f) in TbAlO3

Detailed analyses of spectroscopic and temperature-dependent magnetic susceptibility data are reported for the crystal-field split energy levels of the ⁷F_J and ⁵D₄ of Tb³⁺ in stoichiometric single crystals of ortho-aluminate TbAlO₃. The spectroscopic data include absorption spectra obtained between 2940 and 480 nm from 8 to 300 K. High resolution fluorescence spectra are reported, representing transitions from ⁵D₄ to ⁷F_6,5,4, at a sample temperature of 85 K. Using crystal-field modeling techniques recently adapted for low symmetry systems, we have assigned all 58 experimental Stark levels within the ⁷F_J and ⁵D₄ manifolds, with a fitting standard deviation of 4.5 cm⁻¹ (3.8 cm⁻¹ rms error). As a further test, the theoretical Stark levels and calculated wavefunctions were used to determine the temperature dependence of the magnetic susceptibility along the c-axis of the TbAlO₃ crystal. Good agreement is obtained between the calculated susceptibility and temperature-dependent magnetic data reported earlier, including a prediction of a 0.2 cm⁻¹ splitting of the ground-state quasi-doublet. The susceptibility calculation also confirms the predicted ordering of states within the ⁷F₆ multiplet manifold.     

Low temperature specific heats of erbium dihydrine. Magnetic transition and crystal field splitting effects

The specific heat of erbium dihydride, ErH_1.99, was measured in the temperature range from 1.8–230 K. A λ-type specific heat anomaly, most probably due to an antiferrómagnetic transition has been found at T_N = 2.13 ± 0.03 K. The influence of the crystal field of cubic symmetry on the splitting of the ground state ⁴I_{$\text{15}\text{2}$} of the Er³⁺ ion was analysed and the conclusion was drawn, that the doublet Γ₇ is the lowest lying crystal-field level.     

Specific heat and magnetic susceptibility of single-crystalline ZnCr2−xAlxSe4 (x=0.15, 0.23)

A correlation between the second critical field H_c2 of the helix to paramagnetic transition and the magnetic specific heat C-peak was found in ZnCr_2−xAl_xSe₄ spinel single crystals with x=0.15, 0.23. The specific heat peak is anomalously sharp for all finite magnetic fields used here and this points to a first order magneto-structural transition (from cubic to tetragonal symmetry). The C(T)-peak is increasingly suppressed as the external field increases. Approaching the Neel temperature T_N, a broad ac-magnetic susceptibility peak is observed for zero dc-magnetic field. That peak does not show an energy loss and thus points towards a return to a second order type of transition. The magnetic contribution to the specific heat displays a sharp peak at T_N and is maximal at the spin fluctuation temperature T_sf=34 K. T_sf is related to the maximum of the magnetic susceptibility at T_m=40 K (at 50 kOe) in the spin fluctuation region, as evidenced by the entropy exceeding 90% of the entropy calculated classically for the complete alignment of the Cr spins, (2−x)R ln(2S+1). The X-ray photoelectron spectroscopy (XPS) data indicate that Al-substitution does not affect Cr³⁺ 3d³ electronic configuration.     

Magnetic properties of PrRh2Si2: A neutron diffraction study

The magnetic properties of polycrystalline PrRh₂Si₂ sample have been investigated by neutron diffraction measurements. Antiferromagnetic transition with an anomalously high ordering temperature (T_N∼68 K) is clearly observed in magnetic susceptibility, specific heat, electrical resistivity and neutron diffraction measurements. Neutron diffraction study shows that Pr³⁺ ions carry an ordered moment of 2.99(7)μ_B/Pr³⁺ and align along the crystallographic±c-directions for the ions located at the (0,0,0) and positions. The magnetoresistance at 2 K and 10 T is rather large (∼35%).