Ferromagnetic Kondo Lattice Research Articles

Three-dimensional critical behavior and magnetocaloric effect in Yb14MnSb11

Systematic investigations were performed on the nature of the magnetic properties of Yb14MnSb11, which is the first ferromagnetic Kondo lattice compound in the underscreened limit and one of the highest thermoelectrically efficient materials with p-type conduction. The Rhode-Wohlfarth ratio (RWR) is estimated to be 1.01, which demonstrates the localized nature of the ferromagnetism in Yb14MnSb11, consistent with the predicted Ruderman–Kittel–Kasuya–Yosida (RKKY) like exchange interaction. Detailed analyses on critical behavior indicate a second-order ferromagnetic phase transition with critical exponents β = 0.307(2), γ = 0.923(9), and δ = 4.003(4), demonstrating a three-dimensional critical behavior with long-range interaction. Furthermore, the magnetocaloric effect and relative cooling power are estimated to be 0.772 J kg−1 K−1 and 16.43 J kg−1 at 5 T, respectively. The magnetic transition temperature of TC = 51.9(3) K is strictly estimated by evaluating the magnetic change of specific heat [ΔCp(T,H)]. The rare d-electron ferromagnetic Kondo lattice, small magnetocaloric effect, and high-temperature large thermoelectric efficiency making Yb14MnSb11 to be a system of considerable interest.

Rocksalt-type PrO epitaxial thin film as a weak ferromagnetic Kondo lattice

A unique Kondo lattice praseodymium monoxide PrO was synthesized in the form of an epitaxial thin film. The rocksalt-type PrO thin films with a $4{f}^{2}5{d}^{1}$ electronic configuration showed metallic conduction with a local resistivity minimum at 7 K as a result of the Kondo effect. In contrast to the other paramagnetic praseodymium monochalcogenides, PrO showed weak ferromagnetism below 28 K, followed by a transition to a more ferromagneticlike phase around 5 K. Emergence of the weak ferromagnetism probably originated from the enhanced $4f\text{\ensuremath{-}}4f$ exchange interaction by the short Pr-Pr distance. For the ferromagneticlike phase below 5 K, magnetic hysteresis of the anomalous Hall effect was significantly larger than that of magnetization, probably due to competing magnetic ordering.

The Study of Electronic Structure and Magnetic Properties of Ferromagnetic Kondo-Lattice Cepd2p2 From the First-Principles

The Study of Electronic Structure and Magnetic Properties of Ferromagnetic Kondo-Lattice Cepd2p2 From the First-Principles

Ce-Site Dilution in the Ferromagnetic Kondo Lattice CeRh6Ge4 Supported by the National Natural Science Foundation of China (Grant Nos. 12034017 and 11974306), the National Key R&D Program of China (Grant Nos. 2017YFA0303100 and 2016YFA0300202), and the Key R&D Program of Zhejiang Province, China (Grant No. 2021C01002).

The heavy fermion ferromagnet CeRh6Ge4 is the first example of a clean stoichiometric system where the ferromagnetic transition can be continuously suppressed by hydrostatic pressure to a quantum critical point. In order to reveal the outcome when the magnetic lattice of CeRh6Ge4 is diluted with non-magnetic atoms, this study reports comprehensive measurements of the physical properties of both single crystal and polycrystalline samples of La x Ce1–x Rh6Ge4. With increasing x, the Curie temperature decreases, and no transition is observed for x > 0.25, while the system evolves from exhibiting coherent Kondo lattice behaviors at low x to the Kondo impurity scenario at large x. Moreover, non-Fermi liquid behavior is observed over a wide doping range, which agrees well with the disordered Kondo model for 0.52 ≤ x ≤ 0.66, while strange metal behavior is revealed in the vicinity of x c = 0.26.

Mutual information in heavy-fermion systems

A key notion in heavy-fermion systems is the entanglement between conduction electrons and localized spin degrees of freedom. To study these systems from this point of view, we compute the mutual information in a ferromagnetic and antiferromagnetic Kondo lattice model in the presence of geometrical frustration. Here the interplay between the Kondo effect, the Ruderman-Kittel-Kasuya-Yosida interaction, and geometrical frustration leads to partial Kondo screened, conventional Kondo insulating, and antiferromagnetic phases. In each of these states the mutual information follows an area law, the coefficient of which shows sharp crossovers (on our finite lattices) across phase transitions. Deep in the respective phases, the area law coefficient can be understood in terms of simple direct product wave functions thereby yielding an accurate measure of the entanglement in each phase. The above-mentioned results stem from approximation-free auxiliary field quantum Monte Carlo simulations.

Ferromagnetic ordering along the hard axis in the Kondo lattice YbIr3Ge7

Ferromagnetic Kondo lattice compounds are far less common than their antiferromagnetic analogs. In this work, we report the discovery of a new ferromagnetic Kondo lattice compound, YbIr3Ge7. Like almost all ferromagnetic Kondo lattice systems, YbIr3Ge7 shows magnetic order with moments aligned orthogonal to the crystal electric field (CEF) easy axis. YbIr3Ge7 is unique in that it is the only member of this class of compounds that crystallizes in a rhombohedral structure with a trigonal point symmetry of the magnetic site, and it lacks broken inversion symmetry at the local moment site. AC magnetic susceptibility, magnetization, and specific heat measurements show that YbIr3Ge7 has a Kondo temperature TK = 14 K and a Curie temperature TC = 2.4 K. Ferromagnetic order occurs along the crystallographic [100] hard CEF axis despite the large CEF anisotropy of the ground state Kramers doublet with a saturation moment along [001] almost four times larger than the one along [100]. This implies that a mechanism which considers the anisotropy in the exchange interaction to explain the hard axis ordering is unlikely. On the other hand, the broad second-order phase transition at TC favors a fluctuation-induced mechanism.

Suppression of long-range ferromagnetic order in the CexLa1−xTiGe3 system

We report specific heat, magnetization, and resistivity measurements on single crystals of ${\mathrm{Ce}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{TiGe}}_{3}$ systems. When the Ce concentration $x$ is increased, the system changes from a single-ion Kondo system for $x=0.05$ into a ferromagnetic Kondo lattice for $x=1$, where the magnetic part of electrical resistivity reveals a single-ion scaling with $x$. The isoelectric substitution of Ce by La atoms causes a change of the relative strength of competing energy scales of Kondo and RKKY interaction and crystalline electric field (CEF). The substitutions induce the continuous evolution of the Kondo temperature ${T}_{K}$ and the linear variation of ferromagnetic ordering temperature ${T}_{c}$, which are accompanied by a change of the CEF level scheme of the Ce ions. The composition-temperature ($x\text{\ensuremath{-}}T$) phase diagram for ${\mathrm{Ce}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{TiGe}}_{3}$ is constructed by a combination of magnetization, specific heat, and resistivity measurements. The ferromagnetic ordering temperature is linearly suppressed as $x$ decreases and vanishes near the critical concentration ${x}_{c}=0.1$, but conventional quantum criticality is absent near ${x}_{c}$. The specific heat measurement for $x=0.05$ reveals the power law increase of the electronic specific heat coefficient ${C}_{m}/T\ensuremath{\propto}1/T$ with a large value of $\ensuremath{\sim}3.5$ J/mol ${\mathrm{K}}^{2}$ at $T=0.4\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The magnetic susceptibility for $x=0.05$ also shows a power law dependence $\ensuremath{\chi}(T)\ensuremath{\propto}1/T$ below 10 K.

Magnetic phase transition induced by electrostatic gating in two-dimensional square metal-organic frameworks

We investigate theoretically magnetism and magnetic phase transitions induced by electrostatic gating of two-dimensional square metal-organic framework compounds. We find that electrostatic gating can induce phase transitions between homogeneous ferromagnetic and various spin-textured antiferromagnetic states. Electronic structure and Wannier function analysis can reveal hybridizations between transition-metal $d$ orbitals and conjugated $\ensuremath{\pi}$ orbitals in the organic framework. Mn-containing compounds exhibit a strong $d\ensuremath{-}\ensuremath{\pi}$ hybridization that leads to partially occupied spin-minority bands, in contrast to compounds containing transition-metal ions other than Mn, for which electronic structure around the Fermi energy is only slightly spin split due to weak $d\ensuremath{-}\ensuremath{\pi}$ hybridization and the magnetic interaction is of the Ruderman-Kittel-Kasuya-Yosida type. We use a ferromagnetic Kondo lattice model to understand the phase transition in Mn-containing compounds in terms of carrier density and illuminate the complexity and the potential to control two-dimensional magnetization.

CePd2Al8 – A ferromagnetic Kondo lattice with new type of crystal structure

The novel cerium aluminide CePd2Al8 was investigated by means of single crystal X-ray diffraction and low-temperature magnetic, electrical transport and heat capacity measurements. The compound is found to crystallize with a monoclinic structure of its own type with a single crystallographic position for Ce atoms in the unit cell. Due to magnetic moments carried on Ce3+ ions, it exhibits Curie-Weiss paramagnetic behavior and long-range ferromagnetic ordering below TC = 9.5 K. In the entire temperature range studied, the physical properties of CePd2Al8 are influenced by Kondo interactions with an energy scale close to TC.

Cascade of Magnetic-Field-Induced Lifshitz Transitions in the Ferromagnetic Kondo Lattice Material YbNi_{4}P_{2}.

A ferromagnetic quantum critical point is thought not to exist in two- and three-dimensional metallic systems yet is realized in the Kondo lattice compound YbNi_{4}(P,As)_{2}, possibly due to its one-dimensionality. It is crucial to investigate the dimensionality of the Fermi surface of YbNi_{4}P_{2} experimentally, but common probes such as angle-resolved photoemission spectroscopy and quantum oscillation measurements are lacking. Here, we study the magnetic-field dependence of transport and thermodynamic properties of YbNi_{4}P_{2}. The Kondo effect is continuously suppressed, and additionally we identify nine Lifshitz transitions between 0.4 and 18T. We analyze the transport coefficients in detail and identify the type of Lifshitz transitions as neck or void type to gain information on the Fermi surface of YbNi_{4}P_{2}. The large number of Lifshitz transitions observed within this small energy window is unprecedented and results from the particular flat renormalized band structure with strong 4f-electron character shaped by the Kondo lattice effect.

Insight into the temperature dependent properties of the ferromagnetic Kondo lattice YbNiSn

Analyzing temperature dependent photoemission (PE) data of the ferromagnetic Kondo-lattice (KL) system YbNiSn in the light of the Periodic Anderson model (PAM) we show that the KL behavior is not limited to temperatures below a temperature T_K, defined empirically from resistivity and specificic heat measurements. As characteristic for weakly hybridized Ce and Yb systems, the PE spectra reveal a 4f-derived Fermi level peak, which reflects contributions from the Kondo resonance and its crystal electric field (CEF) satellites. In YbNiSn this peak has an unusual temperature dependence: With decreasing temperature a steady linear increase of intensity is observed which extends over a large interval ranging from 100 K down to 1 K without showing any peculiarities in the region of T_K ~ TC= 5.6 K. In the light of the single-impurity Anderson model (SIAM) this intensity variation reflects a linear increase of 4f occupancy with decreasing temperature, indicating an onset of Kondo screening at temperatures above 100 K. Within the PAM this phenomenon could be described by a non-Fermi liquid like T- linear damping of the self-energy which accounts phenomenologically for the feedback from the closely spaced CEF-states.

Ferromagnetic Kondo lattice model with Rashba coupling: effects of antiferromagnetic coupling between localized spins.

Motivated by emergent phenomena at oxide surfaces and interfaces involving transition metal oxides, we examine the ferromagnetic Kondo lattice model (FKLM) in the presence of a Rashba spin-orbit coupling (RSOC) at zero temperature using numerical techniques. We find that the main effect of the RSOC is the destruction of the ferromagnetic state present in the FKLM at low electron fillings, with the consequent suppression of conductivity. In addition, near half-filling the RSOC leads to a departure of the antiferromagnetic state of the FKLM with a consequent reduction to the intrinsic tendency to electronic phase separation. A similar behavior is observed at very low electron density in the presence of an antiferromagnetic coupling between localized spins.

Magnetic and transport signatures of Rashba spin-orbit coupling on the ferromagnetic Kondo lattice model in two dimensions

Motivated by emergent phenomena at oxide surfaces and interfaces, particularly those involving transition metal oxides with perovskite crystal structure such as ${\mathrm{LaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$, we examine the ferromagnetic Kondo lattice model (FKLM) in the presence of a Rashba spin-orbit coupling (RSOC). Using numerical techniques, under the assumption that the electrons on localized orbitals may be treated as classical continuum spins, we compute various charge, spin, and transport properties on square clusters at zero temperature. We find that the main effect of the RSOC is the destruction of the ferromagnetic state present in the FKLM at low electron fillings, with the consequent suppression of conductivity. In addition, near half filling the RSOC leads to a departure of the antiferromagnetic state of the FKLM with a consequent reduction to the intrinsic tendency to electronic phase separation. The interplay between phase separation on one side, and magnetic and transport properties on the other, is carefully analyzed as a function of the RSOC/hopping ratio.

Low-energy effective theories of the two-thirds filled Hubbard model on the triangular necklace lattice

Motivated by ${\mathrm{Mo}}_{3}{\mathrm{S}}_{7}$(dmit)${}_{3}$, we investigate the Hubbard model on the triangular necklace lattice at two-thirds filling. We show, using second-order perturbation theory, that in the molecular limit, the ground state and the low-energy excitations of this model are identical to those of the spin-one Heisenberg chain. The latter model is known to be in the symmetry-protected topological Haldane phase. Away from this limit we show, on the basis of symmetry arguments and density matrix renormalization group (DMRG) calculations, that the low-energy physics of the Hubbard model on the triangular necklace lattice at two-thirds filling is captured by the ferromagnetic Hubbard-Kondo lattice chain at half-filling. This is consistent with and strengthens previous claims that both the half-filled ferromagnetic Kondo lattice model and the two-thirds filled Hubbard model on the triangular necklace lattice are also in the Haldane phase. A connection between Hund's rules and Nagaoka's theorem is also discussed.

Ferromagnetism in the Kondo-lattice compound CePd2P2

We report physical properties of CePd2P2 crystallizing in the tetragonal ThCr2Si2-type structure (space group I4/mmm). Dc-magnetic susceptibility, magnetization, specific heat, electrical resistivity and magnetoresistance measurements establish a ferromagnetic ordering below the Curie temperature TC = 28.4 ± 0.2 K. Critical analysis of isothermal and isofield magnetization yields critical exponents of β = 0.405 ± 0.005, γ = 1.11 ± 0.05 and δ = 3.74 ± 0.04. The ordered state is characterized by saturation moment Ms ∼ 0.98μB and magnon energy gap Δ/kB ∼ 25–35 K. The studied properties reflect a competing influence of the Kondo and crystalline electric field (CEF) interactions. The strength of the Kondo effect is assigned by a low-temperature Kondo scale TK ∼ 19 ± 10 K and a high-temperature Kondo scale 117 ± 10 K. A model of the inelastic scattering of the conduction electrons with an exchanged CEF energy ΔCEF was applied to the magnetic resistivity. An average value ΔCEF = 260 ± 30 K is consistent in the relationships with TK and . We argue that the CePd2P2 compound appears to be a new ferromagnetic Kondo-lattice among the Ce-based intermetallics.

Pressure–Temperature–Magnetic Field Phase Diagram of Ferromagnetic Kondo Lattice CeRuPO

We report the temperature-pressure-magnetic field phase diagram made from electrical resistivity measurements for the ferromagnetic (FM) Kondo lattice CeRuPO. The ground state at zero field changes from the FM state to another state, which is suggested to be an antiferromagnetic (AFM) state, above ~0.7 GPa, and the magnetically ordered state is completely suppressed at ~2.8 GPa. In addition to the collapse of the AFM state under pressure and a magnetic field, a metamagnetic (MM) transition from a paramagnetic state to a polarized paramagnetic state appears. CeRuPO will give us a rich playground for understanding the mechanism of the MM transition under comparable FM and AFM correlations in the Kondo lattice.

Magnon self-energy in the correlated ferromagnetic Kondo lattice model: Spin-charge coupling effects on magnon excitations in manganites

Magnon self-energy due to spin-charge coupling is calculated for the correlated ferromagnetic Kondo lattice model (FKLM) including the intraorbital Coulomb repulsion term $U$ for the band electrons using a diagrammatic expansion scheme. By systematically incorporating correlation effects in the form of self-energy and vertex corrections, the expansion scheme explicitly preserves the continuous spin-rotation symmetry and hence the Goldstone mode. Due to a near cancellation of the correlation-induced quantum correction terms at intermediate coupling and optimal band filling relevant for ferromagnetic manganites, the renormalized magnon energies for the correlated FKLM are nearly independent of correlation term. Even at higher-band fillings, despite exhibiting overall non-Heisenberg behavior, magnon dispersion in the $\ensuremath{\Gamma}$-$X$ direction retains nearly Heisenberg form. Therefore, the experimentally observed doping-dependent zone-boundary magnon softening must be ascribed to spin-orbital coupling effects.

Pressure-induced tetragonal-orthorhombic phase transitions in CeRuPO

CeRuPO, a ferromagnetic Kondo lattice, undergoes two novel pressure-induced structural phase transitions. CeRuPO undergoes an isostructural transition to a collapsed tetragonal structure near 7.5 GPa. Since pressure suppresses the initial ferromagnetic ordering of Ce3+, our observation suggests a quantum critical point for CeRuPO near this pressure. The collapsed tetragonal phase transforms into an orthorhombic Cmma structure. This phase is isostructural with the low temperature phase of LaFeAsO, which is superconducting under high pressure. Since CeRuPO exhibits a similar relationship between the two phases of LaFeAsO, the collapsed tetragonal and orthorhombic phases of CeRuPO may adopt a novel magnetic ground state.

Formation of spin-polarons in the ferromagnetic Kondo lattice model away from half-filling

Even though realistic one-dimensional experiments in the field of half-metallic semiconductors are not at hand yet, we are interested in the underlying fundamental physics. In this regard we study a one-dimensional ferromagnetic Kondo lattice model, a model in which a conduction band is coupled ferromagnetically to a background of localized d moments with coupling constant JH, and investigate the T = 0 phase diagram as a function of the antiferromagnetic interaction J between the localized moments and the band-filling n, since it has been observed that doping of the compounds has led to formation of magnetic domains. We explore the spin-polaron formation by looking at the nearest-neighbour correlation functions in the spin and charge regimes for which we use the density matrix renormalization group method, which is a highly efficient method to investigate quasi-one-dimensional strongly correlated systems.

The nature of 4f electron magnetism in the diluted ferromagnetic Kondo lattice, CeIr2B2

We report on the physical properties of the series Ce1−xLaxIr2B2 (x = 0–0.9), obtained by means of magnetization, heat capacity and electrical resistivity measurements as a function of temperature (down to 0.7 K for the latter two measurements). The Curie temperature of CeIr2B2 (∼5 K) is lowered due to La substitution, as expected. However, no quantum critical point or ‘non-Fermi liquid’ behavior was observed even in the dilute limit of x = 0.9. Interestingly, ferromagnetic ordering persists even for Ce0.1La0.9Ir2B2, below 0.8 K. Among the Ce systems, CeIr2B2 is one of the compounds in which direct 4f–4f interaction does not appear to play any role in the magnetism, which is controlled by the indirect exchange interaction alone. In this compound, the Kondo effect persists in the ferromagnetic ordered state, as inferred from the entropy data.