Kramers Doublet Ground State Research Articles

Electrostatic Potential Determined Magnetic Dynamics Observed in Two Mononuclear β-Diketone Dysprosium(III) Single-Molecule Magnets.

Two β-diketone mononuclear Dy(III) compounds, formulated as Dy(BTFA)3(H2O)2 (1) and Dy(BTFA)3(bpy) (2) (BTFA = 3-benzoyl-1,1,1-trifluoroacetone, bpy = 2,2'-bipyridine), were prepared. Compound 1 can be identified to transform to 2 in the attendance of bpy coligand, when the local geometry symmetry of eight-coordinated Dy(III) ion changes from a dodecahedron (D2d) in 1 to a square antiprism (D4d) in 2. Fine-tuning structure aroused by auxiliary ligand has dramatical impact on magnetic properties of compounds 1 and 2. Magnetic investigations demonstrate that both 1 and 2 display dynamic magnetic relaxation of single-molecule magnets (SMMs) behavior with different effective barriers (ΔE/kB) of 93.09 K for 1 under zero direct-current (DC) field as well as 296.50 K for 1 and 151.01 K for 2 under 1200 Oe DC field, respectively. As noticed, compound 1 possesses higher effective barrier than 2, despite 1 exhibiting a lower geometrical symmetry of the Dy(III) ion. Ab initio studies reveal that the Kramers doublet ground state is predominantly axial with the gz tensors of two compounds matching the Ising-limit factor of 20 anticipated for the pure MJ = ±15/2 state. Electrostatic analysis confirms the uniaxial anisotropy directions, highlighting that the proper electrostatic distribution of the coordination sphere around Ln(III) center is the critical factor to improve the magnetic anisotropy and determine the dynamic behaviors of SMMs.

Crystalline Electric-Field Randomness in the Triangular Lattice Spin-Liquid YbMgGaO_{4}.

We apply moderate-high-energy inelastic neutron scattering (INS) measurements to investigate Yb^{3+} crystalline electric field (CEF) levels in the triangular spin-liquid candidate YbMgGaO_{4}. Three CEF excitations from the ground-state Kramers doublet are centered at the energies ℏω=39, 61, and 97meV in agreement with the effective spin-1/2 g factors and experimental heat capacity, but reveal sizable broadening. We argue that this broadening originates from the site mixing between Mg^{2+} and Ga^{3+} giving rise to a distribution of Yb-O distances and orientations and, thus, of CEF parameters that account for the peculiar energy profile of the CEF excitations. The CEF randomness gives rise to a distribution of the effective spin-1/2 g factors and explains the unprecedented broadening of low-energy magnetic excitations in the fully polarized ferromagnetic phase of YbMgGaO_{4}, although a distribution of magnetic couplings due to the Mg/Ga disorder may be important as well.

Magnetic, resonance, and optical properties ofCu3Sm(SeO3)2O2Cl: A rare-earth francisite compound

In this combined experimental and theoretical paper, we study the properties of ${\mathrm{Cu}}_{3}\mathrm{Sm}{({\mathrm{SeO}}_{3})}_{2}{\mathrm{O}}_{2}\mathrm{Cl}$ belonging to the francisite family of compounds, which are novel frustrated layered compounds. ${\mathrm{Cu}}_{3}\mathrm{Sm}{({\mathrm{SeO}}_{3})}_{2}{\mathrm{O}}_{2}\mathrm{Cl}$ is synthesized through a solid state reaction. Characterizations through measurements of magnetization, specific heat, X-band electron spin resonance, and rare-earth optical spectroscopy, establish that the compound orders antiferromagnetically at ${T}_{\mathrm{N}}=35\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and undergoes a spin-reorientation phase transition at ${T}_{\mathrm{C}}=8.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ due to the interplay of anisotropies in transition metal and rare-earth subsystems. The ground state Kramers doublet of Sm is found to split only at $T<{T}_{\mathrm{C}}$ when the exchange field is aligned along the nonzero component of its $g$ factor. Prior to full saturation with ${M}_{\mathrm{s}}\ensuremath{\le}3\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$, which is reached under application of moderate magnetic field, the magnetization is found to exhibit complicated hysteresis behavior related to field-induced metamagnetic phase transitions. The first principles calculations reveal the hierarchy of exchange interaction parameters in the kagome-type magnetic subsystem of ${\mathrm{Cu}}_{3}\mathrm{Sm}{({\mathrm{SeO}}_{3})}_{2}{\mathrm{O}}_{2}\mathrm{Cl}$.

Exploring the Magnetic Susceptibility of a Haldane Compound Sm $$_{2}$$ 2 BaNiO $$_{5}$$ 5 : Optical Spectroscopy of Sm $$^{3+}$$ 3 + Kramers Doublets

An optical spectroscopic study of quasi-Haldane chain nickelate Sm\(_{2}\)BaNiO\(_{5}\) is presented. A temperature-dependent splitting of the ground-state Kramers doublet of the Sm\(^{3+}\) ion due to an antiferromagnetic ordering at \(T_{N}\) = 55 K has been obtained experimentally and used to calculate the Schottky-type anomaly in magnetic susceptibility. The value of the magnetic moment of Sm\(^{3+}\) ion at zero temperature has been estimated within the model of the ground doublet. One-dimensional magnetic behavior of the nickel subsystem is emphasized.

Effective spin-12scalar chiral order on kagome lattices inNd3Sb3Mg2O14

We introduce $\mathrm{Nd_{3}Sb_{3}Mg_{2}O_{14}}$ with ideal kagome lattices of neodymium ions in ABC stacking. Thermodynamic measurements show a Curie-Weiss temperature of $\Theta_{CW}=-0.12~$K, a Nd$^{3+}$ spin-1/2 Kramers doublet ground state, and a second order phase transition at $T_N=0.56(2)~$K. Neutron scattering reveals non-coplanar scalar chiral ${\bf k} =0$ magnetic order with a correlation length exceeding 400 \AA = 55 $a$ and an ordered moment of $1.79(5)~\mu_B$. This order includes a canted ferromagnetic component perpendicular to the kagome planes favored by Dzyaloshinskii-Moriya interactions.

Role of Magnetic Exchange Interactions in the Magnetization Relaxation of {3d-4f} Single-Molecule Magnets: A Theoretical Perspective.

Combined density functional and ab initio calculations are performed on two isomorphous tetranuclear {Ni3 (III) Ln(III) } star-type complexes [Ln=Gd (1), Dy (2)] to shed light on the mechanism of magnetic exchange in 1 and the origin of the slow magnetization relaxation in complex 2. DFT calculations correctly reproduce the sign and magnitude of the J values compared to the experiments for complex 1. Acute ∢Ni-O-Gd bond angles present in 1 instigate a significant interaction between the 4fxyz orbital of the Gd(III) ion and 3d${{_{x{^{2}}- y{^{2}}}}}$ orbital of the Ni(II) ions, leading to rare and strong antiferromagnetic Ni⋅⋅⋅Gd interactions. Calculations reveal the presence of a strong next-nearest-neighbour Ni⋅⋅⋅Ni antiferromagnetic interaction in complex 1 leading to spin frustration behavior. CASSCF+RASSI-SO calculations performed on complex 2 suggest that the octahedral environment around the Dy(III) ion is neither strong enough to stabilize the mJ |±15/2〉 as the ground state nor able to achieve a large ground-state-first-excited-state gap. The ground-state Kramers doublet for the Dy(III) ion is found to be the mJ |±13/2〉 state with a significant transverse anisotropy, leading to very strong quantum tunneling of magnetization (QTM). Using the POLY_ANISO program, we have extracted the JNiDy interaction as -1.45 cm(-1) . The strong Ni⋅⋅⋅Dy and next-nearest-neighbour Ni⋅⋅⋅Ni interactions are found to quench the QTM to a certain extent, resulting in zero-field SMM behavior for complex 2. The absence of any ac signals at zero field for the structurally similar [Dy(AlMe4 )3 ] highlights the importance of both the Ni⋅⋅⋅Dy and the Ni⋅⋅⋅Ni interactions in the magnetization relaxation of complex 2. To the best of our knowledge, this is the first time that the roles of both the Ni⋅⋅⋅Dy and Ni⋅⋅⋅Ni interactions in magnetization relaxation of a {3d-4f} molecular magnet have been established.

Candidate Quantum Spin Liquid in the Ce3+ Pyrochlore Stannate Ce2Sn2O7.

We report the low-temperature magnetic properties of Ce2Sn2O7, a rare-earth pyrochlore. Our susceptibility and magnetization measurements show that due to the thermal isolation of a Kramers doublet ground state, Ce2Sn2O7 has Ising-like magnetic moments of ∼1.18 μ_{B}. The magnetic moments are confined to the local trigonal axes, as in a spin ice, but the exchange interactions are antiferromagnetic. Below 1 K, the system enters a regime with antiferromagnetic correlations. In contrast to predictions for classical ⟨111⟩-Ising spins on the pyrochlore lattice, there is no sign of long-range ordering down to 0.02 K. Our results suggest that Ce2Sn2O7 features an antiferromagnetic liquid ground state with strong quantum fluctuations.

Ferromagnetic Transition in a Caged Compound NdOs2Zn20

Magnetic properties in a caged compound NdOs2Zn20 have been studied by the measurements of magnetization M, magnetic susceptibility M/B, and specific heat C. The measurements indicate that NdOs2Zn20 shows a structural transition at Ts = 62K and a ferromagnetic transition at TC = 0.6K. The structural transition is also observed in the La counter-part LaOs2Zn20 at Ts = 151K. The results of M (T)/B, M (B), and C(T) of NdOs2Zn20 are reproduced by a crystalline electrical field scheme with a Kramers doublet ground state.

Spin-cluster excitations in the rare-earth kagome systemNd3Ga5SiO14

Spin-cluster excitations induced by microwave radiation have been detected as a function of an applied magnetic field in the rare-earth kagome system $\mathrm{N}{\mathrm{d}}_{3}\mathrm{G}{\mathrm{a}}_{5}\mathrm{Si}{\mathrm{O}}_{14}$. The results are interpreted using a Heisenberg model which allows for anisotropic exchange interactions between neighboring spins. At a microwave frequency of 120 GHz the conventional independent-ion electron spin resonance (ESR) spectrum, due to Zeeman splitting of the ground state Kramers doublet, is not observed at low temperatures. Instead, a large number of resonances are seen that are shifted from the conventional ESR position. These resonances persist to temperatures well above 20 K with similar behavior found at 230 GHz. The observations are consistent with the excitation of spin waves in short-range antiferromagnetically correlated spin loops or clusters and are similar to the ESR spectral features seen previously in the $\mathrm{P}{\mathrm{r}}_{3}\mathrm{G}{\mathrm{a}}_{5}\mathrm{Si}{\mathrm{O}}_{14}$ kagome system. Upon increasing the temperature and/or the microwave frequency, a broad Zeeman resonance does eventually emerge at the predicted ground state doublet resonance field.

Magnetic phase transitions and magnetic entropy in the XY antiferromagnetic pyrochlores (Er1-x Y x )2Ti2O7.

We present the results of experimental determination of the heat capacity of the pyrochlore Er2Ti2O7 as a function of temperature (0.35-300 K) and magnetic field (up to 9 T), and for magnetically diluted solid solutions of the general formula (Er1-x Y x )2Ti2O7 (x≤0.471). On either doping or increase of magnetic field, or both, the Néel temperature first shifts to lower temperature until a critical point above which there is no well-defined transition but a Schottky-like anomaly associated with the splitting of the ground state Kramers doublet. By taking into account details of the lattice contribution to the heat capacity, we accurately isolate the magnetic contribution to the heat capacity and hence to the entropy. For pure Er2Ti2O7 and for (Er1-x Y x )2Ti2O7, the magnetic entropy as a function of temperature evolves with two plateaus: the first at [Formula: see text], and the other at [Formula: see text]. When a very high magnetic field is applied, the first plateau is washed out. The influence of dilution at low values is similar to the increase of magnetic field, as we show by examination of the critical temperature versus critical field curve in reduced terms.

Study of crystal-field excitations and infrared active phonons in the multiferroic hexagonal DyMnO3

In hexagonal DyMnO3, Dy3+ crystal-field excitations are studied as a function of temperature and applied magnetic field. They are complemented with the measurements of infrared active phonon frequency shifts under applied magnetic field at T = 4.2 K. Between TN = 68 K and T = 10 K, the absence of Dy3+ Kramers doublet splittings at either the C3 or the C3v site symmetries indicates that the Mn3+ magnetic order effective exchange field has no component parallel to the c-axis at either site. Below T = 10 K, the ground state Kramers doublet splits under the Dy3+ internal effective field as well as the applied magnetic field. Also, relatively strong infrared active phonon energy shifts are observed in magneto-infrared reflectance measurements at T = 4.2 K, allowing the calculation of the induced electric polarization changes as a function of the applied magnetic field. Such changes are associated with a large magnetoelectric effect in DyMnO3, arising from a charge transfer between Dy3+ and apical oxygen ions.

Ground-state properties of the mixed-valence cobaltites Nd0.7Sr0.3CoO3, Nd0.7Ca0.3CoO3 and Pr0.7Ca0.3CoO3

The electric, magnetic, and thermal properties of three perovskite cobaltites with the same 30% hole doping and ferromagnetic ground state were investigated down to very low temperatures. With decreasing size of large cations, the ferromagnetic Curie temperature and spontaneous moments of cobalt are gradually suppressed: TC = 130 K, 55 K and 25 K and m = 0.68 μB, 0.34 μB and 0.23 μB for Nd0.7Sr0.3CoO3, Pr0.7Ca0.3CoO3 and Nd0.7Ca0.3CoO3, respectively. The moment reduction with respect to the moment of the conventional ferromagnet La0.7Sr0.3CoO3 (TC = 230 K, m = 1.71 μB) in the so-called low spin/intermediate spin (IS/LS) state for Co3+/Co4+ was originally interpreted using a phase-separation scenario. Based on the present results, mainly the analysis of the Schottky peak originating from Zeeman splitting of the ground-state Kramers doublet of Nd3+, we find, however, that the ferromagnetic phase in Nd0.7Ca0.3CoO3 and likely also in Pr0.7Ca0.3CoO3 is uniformly distributed over the whole sample volume, despite the severe drop of moments. The ground state of these compounds is identified with the LS/LS-related phase derived theoretically by Sboychakov et al (2009 Phys. Rev. B 80 024423). The ground state of Nd0.7Sr0.3CoO3 with an intermediate cobalt moment is inhomogeneous due to competition between the LS/LS and IS/LS phases. In the theoretical part of the study, the crystal field split levels for 4f3 (Nd3+), 4f2 (Pr3+) and 4f1 (Ce3+ or Pr4+) are calculated and their magnetic characteristics are presented.

Single-crystal EPR spectroscopy of a Co(II) single-chain magnet

An electron paramagnetic resonance (EPR) study of a single crystal of CoII-based single-chain magnets (SCM) is presented. Discrete resonant absorptions are associated to the presence of magnetic domains within the chains of finite lengths determined by a competition between intra-chain exchange interactions and thermally excited single spin fluctuations. The results are interpreted as a transition from single spin dynamics at high temperature (T∼20K), associated to the Kramers doublet ground state of the individual CoII ions, to archetypical SCM dynamics at low temperatures, where intra-chain correlations form long magnetic domains, whose average length is imposed by the concentration of defects in the sample.

An <sup>166</sup>Er-Mossbauer Investigation of ErNiAl<sub>4</sub>

A low temperature 166Er-Mössbauer spectroscopy investigation of ErNiAl4 reveals that the Er3+ ion’s ground state Kramers doublet has ¦Jz¦ just 8% less than the fully-stretched value of J = 15/2. The persistence of a magnetically-split spectrum to well above TN = 5.8 K is consistent with Orbach type paramagnetic relaxation that proceeds via the third excited crystal field level.

X-ray Magnetic Circular Dichroism (XMCD) Study of a Methoxide-Bridged DyIII–CrIII Cluster Obtained by Fluoride Abstraction from cis-[CrIIIF2(phen)2]+

An isostructural series of dinuclear chromium(III)-lanthanide(III) clusters is formed by fluoride abstraction of cis-[CrF2(phen)2](+) by Ln(3+) resulting in LnF3 and methoxide-bridged Cr-Ln clusters (Ln = Nd (1), Tb (2), Dy (3)) of formula [Cr(III)(phen)2(μ-MeO)2Ln(NO3)4]·xMeOH (x = 2-2.73). In contrast to fluoride, methoxide bridges in a nonlinear fashion, which facilitates chelation. For 3, X-ray magnetic circular dichroism (XMCD) provides element-specific magnetization curves that are compared to cluster magnetization and susceptibility data acquired by SQUID magnetometry. The combination of XMCD and SQUID is able to resolve very small magnetic coupling values and reveals a weak Cr(III)-Dy(III) coupling of j = -0.04(3) cm(-1). The Dy(III) ion has a ground-state Kramers doublet of mJ = ±13/2, and the first excited doublet is found to be mJ = ±11/2 at an energy of δ = 57(21) cm(-1). The Cr(III) ion exhibits a uniaxial anisotropy of DCr = -1.7(1.0) cm(-1). Further, we observe that a weak anisotropic coupling of dipolar origin is sufficient to model the data, suggesting that methoxide bridges do not play a significant role in the magnetic coupling for the present systems.

Magnetic structures of the rare-earth chain nickelates [formula omitted]BaNiO5 as studied by the optical spectroscopy of the Er probe

The rare-earth chain nickelates R2BaNiO5 (R=Nd, Sm, Eu, Gd, Dy, Ho, Er, and Tm) were studied by means of the spectroscopic Er probe. The magnetic ordering was registered and the splitting of the ground-state Kramers doublet of Er3+ as a function of temperature was measured for all the compounds studied. This information was used to analyze a possible orientation of ordered magnetic moments of the nickel ions. The magnetic structures found for R=Ho, Dy, Nd, Gd, Tm, and Er are in good agreement with literature data, those for R=Sm and Eu are predicted for the first time.

Metal-insulator transition and thePr3+/Pr4+valence shift in(Pr1−yYy)0.7Ca0.3CoO3

The magnetic, electric, and thermal properties of the ${(L{n}_{1\ensuremath{-}y}{\text{Y}}_{y})}_{0.7}{\text{Ca}}_{0.3}{\text{CoO}}_{3}$ perovskites $(Ln=\text{Pr},\text{Nd})$ were investigated down to very low temperatures. The main attention was given to a peculiar metal-insulator (M-I) transition, which is observed in the praseodymium based samples with $y=0.075$ and 0.15 at ${T}_{\text{M-I}}=64\text{ }\text{K}$ and 132 K, respectively. The study suggests that the transition, reported originally in ${\text{Pr}}_{0.5}{\text{Ca}}_{0.5}{\text{CoO}}_{3}$, is not due to a mere change in cobalt ions from the intermediate to the low-spin states but is associated also with a significant electron transfer between ${\text{Pr}}^{3+}$ and ${\text{Co}}^{3+}/{\text{Co}}^{4+}$ sites, so that the praseodymium ions occur below ${T}_{\text{M-I}}$ in a mixed ${\text{Pr}}^{3+}/{\text{Pr}}^{4+}$ valence. The presence of ${\text{Pr}}^{4+}$ ions in the insulating phase of the yttrium doped samples ${({\text{Pr}}_{1\ensuremath{-}y}{\text{Y}}_{y})}_{0.7}{\text{Ca}}_{0.3}{\text{CoO}}_{3}$ is evidenced by Schottky peak originating in Zeeman splitting of the ground-state Kramers doublet. The peak is absent in pure ${\text{Pr}}_{0.7}{\text{Ca}}_{0.3}{\text{CoO}}_{3}$ in which metallic phase, based solely on non-Kramers ${\text{Pr}}^{3+}$ ions, is retained down to the lowest temperature.

Calorimetric and spectroscopic study of quasi-one-dimensional Haldane magnets (Y1 − x Nd x )2BaNiO5 (x = 1, 0.75, 0.50, 0.25)

The temperature dependences of the heat capacity and optical transmission spectra of quasi-one-dimensional Haldane magnets (Y1 − x Nd x )2BaNiO5 (x = 1, 0.75, 0.50, 0.25) have been studied. All the compounds studied undergoes antiferromagnetic ordering. In the ordered state, the internal magnetic field produces splitting of the ground-state Kramers doublet of Nd3+ ion. The temperature dependences of the magnitude of splitting, as determined from spectroscopic data, were used to calculate the Schottky anomaly on the heat capacity curve. Nonequivalent centers related to neodymium ions have been found in compounds with x = 0.75, 0.50, and 0.25.

Magnetic properties of RPd5Al2 (R=Y, Ce, Pr, Nd, Sm, Gd)

Abstract Polycrystalline samples of a new rare-earth series R Pd 5 Al 2 crystallizing in the tetragonal ZrNi 2 Al 5 -type structure have been prepared. Their physical properties by electrical resistivity ρ , magnetic susceptibility χ , magnetization M and specific heat C p measurements are reported. The ingots are composed of elongated grains preferentially aligned in the c direction; therefore, measurements were conducted parallel and perpendicular to the grains. Antiferromagnetic ordering appears in R =Ce, Nd, Gd, and Sm at low temperatures. CePd 5 Al 2 has two AFM transitions at 4.1 and 2.9 K and ρ ( T ) indicates a Kondo metal behavior with large anisotropy. In PrPd 5 Al 2 no magnetic transition was observed down to 0.4 K. The C p ( T ) shows a broad peak around 13 K due to the CEF effect, suggesting a non-magnetic singlet ground state. In NdPd 5 Al 2 , χ ( T ) shows anisotropy and the C p ( T ) shows a sharp peak at 1.2 K. The magnetic entropy at 3 K is very close to R ln2, indicating a Kramers doublet ground state. In SmPd 5 Al 2 , C p ( T ) shows a magnetic transition at 1.7 K. C p ( T ) for GdPd 5 Al 2 shows a peak at 6 K, followed by a broad anomaly around 3 K. Within this series, T N 's for CePd 5 Al 2 and NdPd 5 Al 2 clearly deviate from the relation predicted by de Gennes scaling, which is ascribed to the CEF effect.

Infrared study of SmMnO3 crystal field excitations

SmMnO 3 infrared transmission in the 2000–8000 cm−1 range was studied as a function of temperature. We observed several infrared-active Sm3+ crystal field transitions between the lowest H65/2 ground-state level and the H69/2, H611/2, H613/2, and H615/2 excited levels. The splitting of the ground-state Kramers doublet of the Sm3+ ion below TN∼60 K varies from 3.8 up to 5.8 cm−1 at 2 K. This reflects strengthenings of the Mn3+–Mn3+ and Mn3+–Sm3+ magnetic interactions. In order to monitor the antiferromagnetic transition, the frequency softening of the most intense Raman-active B2g stretching mode (∼607 cm−1) below TN was also measured.