Antiferromagnet Ce Research Articles

Rocksalt CeO epitaxial thin film as a heavy-fermion system transiting from p -type metal to partially compensated n -type metal by 4f delocalization

Rocksalt CeO (001) epitaxial thin films were synthesized and their electronic properties were investigated. A simple $4{f}^{1}{5d}^{1}$ electronic configuration with $4f--5d$ hybridization in CeO was confirmed by x-ray photoemission and absorption spectroscopy. While $5d$ conduction holes governed the metallic conduction at high temperatures, a partially compensated $n$-type conduction appeared below \ensuremath{\sim}10 K with a rapid decrease in resistivity corresponding to a Fermi liquid state. The hole mobility was as high as $646\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{--1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{--1}$ at 2 K in contrast to the two decades lower electron mobility, reflecting the large and small dispersion of $5d$ and $4f$ bands, respectively. At 0.8 K, a resistivity minimum was observed as a manifestation of the Kondo effect, indicating partial $4f$ localization. These results represented that the significant $4f--5d$ hybridization induced a Kondo coherent state in CeO with a short Ce-Ce interionic distance unlike the other antiferromagnetic Ce chalcogenides.

Unconventional magnetic ferroelectricity in the quadruple perovskite NaMn7O12

By means of magnetic, specific heat and pyroelectric measurements, we report on magnetic ferroelectricity in the quadruple perovskite \namno, characterized by a canted antiferromagnetic (AFM) CE structure. Surprisingly, ferroelectricity is concomitant to a dramatic broadening of the magnetic hysteresis loop, well below the AFM ordering temperature. This unconventional behavior shows that the formation of ferroelectric domains is induced by the symmetric exchange interaction in the local scale, e.g. at magnetic domain boundaries or defects. The value of electric polarization, $P = 0.027 \mu$C cm$^{-2}$, measured in polycrystalline samples is comparatively large as compared to other magnetic multiferroics, suggesting that the above scenario is promising indeed for the rational design of practical multiferroic materials.

Magnetic Field Driven Electronic Singularities through Metamagnetic Phenomena: Case of the Heavy Fermion Antiferromagnet Ce(Ru0.92Rh0.08)2Si2

Thermoelectric power measurements on the heavy fermion antiferromagnet Ce(Ru0.92Rh0.08)2Si2 under magnetic field (H) show clearly that the antiferromagnetic state below the critical field Hc ~ 2.8T can be fully decoupled from the pseudo-metamagnetic crossover at Hm ~ 5.8T which occurs when the magnetization reaches a critical value. By contrast to the weak field variation of the Sommerfeld coefficient of the specific heat in the field window from Hc to Hm, two electronic singularities with opposite signs are detected in the thermoelectric power. The interplay between the magnetic instability and the topological change of the Fermi surface is discussed and we argue similarities to other field instabilities in various heavy fermion compounds.

Electrical Resistivity of Ce2Ir3Ge5under High Pressure

We measured electrical resistivity on antiferromagnet Ce 2 Ir 3 Ge 5 under high pressure up to 8 GPa. It is shown that the Néel temperature (=9.5 K) at ambient pressure decreased by increased pressure. The Néel temperature seems to be zero at a critical pressure P c ≃2.0 GPa suddenly. The variation by higher pressure above 2.0 GPa does not seem prominence up to 8 GPa.

Ferromagnetic tendency at the surface of CE-type charge-ordered manganites

Most previous investigations have shown that the surface of a ferromagnetic material may have antiferromagnetic tendencies. However, experimentally, the opposite effect has been recently observed-ferromagnetism appears in some nanosized manganites with a composition such that the antiferromagnetic charge-ordered CE state is observed in the bulk. A possible origin is the development of ferromagnetic correlations at the surface of these small systems. To clarify these puzzling experimental observations, we have studied the two-orbital double-exchange model near half doping, n = 0.5, using open boundary conditions to simulate the surface of either bulk or nanosized manganites. Considering the enhancement of surface charge density due to a possible AO termination (A = trivalent/divalent ion composite, O = oxygen), an unexpected surface phase-separated state emerges when the model is studied using Monte Carlo techniques on small clusters. This tendency suppresses the CE charge ordering and produces a weak ferromagnetic signal that could explain the experimental observations.

Single crystal growth and pressure effect of an antiferromagnet [formula omitted

We succeeded in growing single crystals of an antiferromagnet Ce 2 CuGe 6 with the non-centrosymmetric orthorhombic crystal structure by the Bi-flux method. The magnetic properties were studied by measuring the magnetic susceptibility, specific heat and electrical resistivity at ambient pressure and under high pressure. The antiferromagnetic easy-axis is found to be the [0 1 0] direction ( b-axis), while the other directions correspond to hard-axes in magnetization. Furthermore we studied an effect of pressure on the electronic state by measuring the electrical resistivity under pressures up to 8 GPa. The Néel temperature T N with T N = 15.4 K at ambient pressure increases slightly with increasing pressure, and then decreases rapidly above 5 GPa. The Néel temperature is found to be zero around 7.2 GPa. The heavy fermion state is formed in this pressure region.

Electronic phase separation in manganite-insulator interfaces

By using a realist microscopic model, we study the electric and magnetic properties of the interface between a half metallic manganite and an insulator. We find that the lack of carriers at the interface debilitates the double exchange mechanism, weakening the ferromagnetic coupling between the Mn ions. In this situation the ferromagnetic order of the Mn spins near the interface is unstable against antiferromagnetic CE correlations, and a separation between ferromagnetic/metallic and antiferromagnetic/insulator phases at the interfaces can occur. We obtain that the insertion of extra layers of undoped manganite at the interface introduces extra carriers which reinforce the double exchange mechanism and suppress antiferromagnetic instabilities.

Pressure effect on magnetic transitions in heavy-fermion antiferromagnet Ce 2RhIn 8

Detailed magnetization measurements under pressure have been performed on tetragonal heavy-fermion antiferromagnet Ce 2RhIn 8 single crystals. From the results of field and temperature dependences of the magnetic transitions, we construct the magnetic field–temperature phase diagrams of Ce 2RhIn 8 at several pressures up to 6 kbar, in magnetic fields up to 55 kOe between 1.8 and 3 K. The antiferromagnetic long-range order of Ce 2RhIn 8 is suppressed by pressure.

Magnetic field dependence of resistivity for Ce 0.8La 0.2(Ru 0.85Rh 0.15) 2Si 2

We measured resistivity of heavy fermion antiferromagnetic Ce 0.8La 0.2(Ru 0.85Rh 0.15) 2Si 2 in magnetic field up to 4 T. The resistivity for Ce 0.8La 0.2(Ru 0.85Rh 0.15) 2Si 2 shows a downward bend at T N, indicating an antiferromagnetic order with a local moment character. For 1 T⩽ B<1.75 T, it shows a jump at T N as in the case of Ce(Ru 0.85Rh 0.15) 2Si 2, where the downward bend was no longer observed. This result indicates that instead of the local moment character, a gap is opened on the fermi surface by magnetic field. For B>1.75 T, the resistivity is proportional to T 2 without any anomalies, suggesting that the developed gap closes again for B>1.75 T, and the Fermi liquid state is induced by magnetic field.

Electronic States in a Semimetal Ce3Sn7

We carried out the de Haas–van Alphen (dHvA) measurement and the pressure experiment of the electrical resistivity for an antiferromagnet Ce 3 Sn 7 with the orthorhombic crystal structure. The detected dHvA branches are many in number and their dHvA frequencies are extremely small in magnitude, indicating that Ce 3 Sn 7 is a semimetal. This is consistent with the result of the energy band calculations under the assumption that the 4 f electrons of two Ce atoms in the 2( a ) site are localized and the 4 f electron of one Ce atom in the 4( i ) site is itinerant. In the pressure experiment, the Neel temperature of T N = 5.3 K is found to slightly increase with increasing pressure, becomes maximum around 1 GPa, then decreases steeply with further increasing pressure and most likely becomes zero between 3 and 4 GPa. The antiferromagnetic state is changed into the non-magnetic state under pressure.

Pressure- and field-induced magnetic instabilities in a heavy-fermion antiferromagnet Ce 7Ni 3

We investigated ground state properties of a heavy-fermion antiferromagnet Ce 7Ni 3 under hydrostatic pressures and magnetic fields. This compound undergoes two antiferromagnetic phase transitions at T N1 = 1.9 K and T N2 = 0.7 K. Below T N1, a spin-density-wave (SDW) develops. Upon applying rather weak pressure 0.39 GPa = P c, both T N1 and T N2 vanish, and non-Fermi liquid behavior appears in the specific heat and magnetic susceptibility. The enhancement of residual resistivity along the a axis near P c is attributed to the increased spin fluctuations along the a axis. By applying fields B along the c axis, T N1 is suppressed and vanishes at 0.3 T. Magnetoresistance, specific-heat, and magnetization measurements revealed another field-induced magnetic (FIM) phase in the region B || c > 0.7 T and T < 0.5 K. Neutron diffraction experiments indicate that the magnetic unit cell in the c-plane for the FIM phase is treble that of the chemical unit cell. Moreover, this magnetic reflection intensity remains even in the region between the FIM phase and SDW phase. This observation indicates the presence of large spin fluctuations in the c-plane associated with the magnetic frustration, which should be responsible for the magnetic instability of Ce 7Ni 3.

CEF-scheme of a semimetal [formula omitted

We measured the magnetic susceptibility and magnetization of an antiferromagnet Ce 3Sn 7 with the orthorhombic crystal structure. The experimental data are found to be well explained on the basis of the crystalline electric field (CEF) 4f-scheme under the assumption that two Ce atoms in the 2(a) site possess a magnetic moment of 0.36 μ B / Ce and one Ce atom in the 4(i) site possesses no magnetic moment as in a valence fluctuating compound CeSn 3, which was previously proposed by Bonnet et al. Furthermore, we carried out the de Haas–van Alphen experiment. The detected Fermi surfaces are many in number but are extremely small in volume, indicating that Ce 3Sn 7 is a semimetal.

Resistivity of antiferromagnetic Ce 0.8La 0.2(Ru 0.85Rh 0.15) 2Si 2 under pressure

We have measured the resistivity of antiferromagnetic Ce 0.8La 0.2(Ru 0.85Rh 0.15) 2Si 2 along the tetragonal c-axis under various hydrostatic pressures. At P=0 GPa we observe no gap-type jump in the resistivity at T N as has been observed for Ce(Ru 0.85Rh 0.15) 2Si 2 at the spin density-wave (SDW) transition temperature, but rather downward bend with decreasing temperature. At P⩾0.48 GPa we observe the gap-type jump just below T N. These facts suggest the recovery of the gap opening due to the Fermi-surface nesting by pressure from a conventional localized antiferromagnetic state.

Magnetic and Fermi Surface Properties of an Antiferromagnet Ce3Sn7

We measured the electrical resistivity, specific heat, magnetic susceptibility, high-field magnetization, thermal expansion and magnetostriction of an antiferromagnet Ce 3 Sn 7 with the orthorhombic crystal structure. The experimental data are found to be well explained on the basis of the crystalline electric field (CEF) 4 f -scheme previously proposed by Bonnet et al. in which the two Ce atoms at the 2(a) site possess a magnetic moment of 0.36 µ B /Ce and the one Ce atom at the 4(i) site possesses no magnetic moment as in a valence fluctuating compound CeSn 3 . We also constructed the antiferromagnetic phase diagram for the three principal directions. Furthermore, we carried out the de Haas–van Alphen experiment. Fermi surfaces are many in number but extremely small in volumes, indicating that Ce 3 Sn 7 is a semimetal.

Heat capacity of Ce 2RhIn 8 under hydrostatic pressure

We report measurements of the low temperature heat capacity of the heavy-fermion (HF) antiferromagnet (AFM) Ce 2RhIn 8 (0.35 K< T<7 K) under hydrostatic pressure up to p=1.65 GPa and in magnetic fields B⩽8 T. The application of pressure leads to a decrease of both the Néel temperature ( T N) and of the coefficient of the electronic specific heat C/T| T>T N e ́ el . These studies aim to shed light on the possibilities of electronically mediated superconductivity (SC) in strongly correlated electron systems (SCES) close to a magnetic quantum critical point (QCP).

Effect of pressure on the electrical resistivity of heavy fermion antiferromagnet Ce 2RhIn 8

Heavy-fermion antiferromagnet Ce 2RhIn 8 is structurally related to the pressure-induced superconductor CeRhIn 5. We have measured pressure dependence of the electrical resistivity up to 1.6 GPa in the temperature range 1.5– 300 K for Ce 2RhIn 8. We have found that the Néel temperature T N decreases by pressure at the rate of −0.8 KGPa −1 . This large decrease of T N is a contrast to the case of CeRhIn 5 in which T N has very weak pressure dependence.

Neutron-diffraction study of field-induced transitions in the heavy-fermion compound Ce 2RhIn 8

We present neutron diffraction measurements in high magnetic fields (0–14.5 T) and at low temperatures (2.5, 2.3, 0.77 and 0.068 K) on single crystals of the tetragonal heavy-fermion antiferromagnet Ce 2RhIn 8. For B ∥[1 1 0] the field dependence of selected magnetic and nuclear reflections reveals that the material undergoes several transitions, the temperature dependence of which suggests a complex B– T phase diagram. We present the detailed evolution of the integrated intensities of selected reflections and discuss the associated field-induced transitions.

Charge, orbital, and magnetic order inNd0.5Ca0.5MnO3

In the manganite Nd0.5Ca0.5MnO3, charge ordering occurs at much higher temperature than the antiferromagnetic order (TCO=250K,TN=160K). The magnetic behavior of the phase TN<T<TCO is puzzling: its magnetization and susceptibility are typical of an antiferromagnet while no magnetic order is detected by neutron diffraction.We have undertaken an extensive study of the cristallographic, electric and magnetic properties of Nd0.5Ca0.5MnO3 and established its phase diagram as a function of temperature and magnetic field. The charge disordered, paramagnetic phase above TCO present ferromagnetic correlations. An antiferromagnetic CE phase prevails below TN, with complete charge and orbital ordering. In the intermediate temperature range, charge ordering occurs while orbital ordering sets in progressively, with no magnetic order. Strong magnetic fields destroy the charge ordered phases in a fisrt order transition towards a ferromagnetic state.

Incommensurate Magnetic Structure of the Heavy Fermion Antiferromagnet Ce7Ni3

Magnetic structures of the heavy fermion antiferromagnet Ce 7 Ni 3 have been studied by neutron diffraction using a single crystal. It undergoes the two successive phase transitions at T N1 = 1.8 K and T N2 = 0.7 K in zero field. At T N1 ordering of a single- k type magnetic structure with an incommensurate modulation vector k IC ≃0.22 c * takes place. This structure is approximately a sinusoidally modulated structure of c -axis moments, which belongs to the Γ 2 and Γ 3 irreducible representations of the space group P 6 3 m c . Root mean squares of the moments at T = 1.4 K are 0.46±0.07, 0.70±0.03 and 0.10±0.05 µ B for the Ce1, Ce2 and Ce3 sites, respectively. Below T N2 the incommensurate modulation vector becomes temperature independent, and a commensurate structure with \({\mib k}_{\rm C} = \frac{1}{4} {\mib c}^{\ast}\) appears in addition to the incommensurate structure. The metamagnetic transition under the magnetic field of H ∥ c = 0.16 T at T = 1.5 K is a first order transition to a ferromagnetic s...

Magnetization steps from Ce 3+-pairs in Pb 1− xCe xSe

The magnetization of Pb 1− x Ce x Se single crystals ( x=0.75% and 3.5%) was measured at 20 mK. For H || 〈1 1 1〉 two magnetization steps were observed at H 1=5.7±0.2 and H 2=9.5±0.2 kOe , which are attributed to two kinds of antiferromagnetic Ce 3+-pairs. For H along 〈0 0 1〉 and 〈1 0 1〉 , the first step does not change, indicating that the associated exchange interaction is isotropic ( J=−0.26±0.01 K ). The second step, on the other hand, is split, possibly due to exchange anisotropy (with an average constant J ̄ =−0.43±0.02 K ). These two J's correspond, probably, to nearest- and next-nearest neighbor pairs. The analyses of the sizes of the steps, however, indicate that the distribution of Ce 3+ in the samples studied may be non-random.