We succeeded in growing a single crystal of the Eu-divalent compound EuAl4 with the BaAl4-type tetragonal structure by the Al self-flux method and measured the electrical resistivity, magnetic susceptibility, magnetization, specific heat, and thermoelectric power. EuAl4 orders antiferromagnetically below TN1 = 15.4 K, with three successive antiferromagnetic transitions at TN2 = 13.2 K, TN3 = 12.2 K, and TN4 = 10.0 K. The latter two transitions are of the first-order. The corresponding magnetization curve indicates three successive metamagnetic transitions with hystereses and saturates above 16 kOe. We observed a plausible characteristic feature of the charge density wave (CDW) below TCDW = 140 K. We also studied an effect of pressure on the electronic state by measuring the electrical resistivity and thermoelectric power. TCDW is found to decrease with increasing pressure at a rate of dTCDW/dP = −54.7 K/GPa and becomes zero at about 2.5 GPa. The present antiferromagnetic ordering is, however, found to be ...

Transport and magnetic properties of EuAl4 and EuGa4

Pressure is a powerful tool for exploring novel phenomena in solid-state physics. We review the electronic states of single-crystalline EuT2X2 (T: transition metal, X: Si or Ge) and related compoun...

Unique Electronic States of Eu-based Compounds

The magnetic topological materials have attracted much attention recently for their potential realization of various novel quantum states. However, the onset of magnetization in these materials usually occurs at low temperatures, impeding further applications. Here, by means of high pressure, we have significantly increased the magnetic transition temperature in an antiferromagnetic axion insulator candidate $\mathrm{Eu}{\mathrm{In}}_{2}{\mathrm{As}}_{2}$. Both crystal and magnetic structures remain the same with pressure up to 17 GPa. The N\'eel temperature can be monotonously increased from 16 K (ambient pressure) to 65 K (14.7 GPa). This is mainly attributed to the enhancement of intralayer ferromagnetic exchange coupling by pressure. With increasing pressure up to 17 GPa, a crystalline-to-amorphous phase transition occurs, which impedes further enhancement of the N\'eel temperature. Our results show that high pressure is an effective pathway to greatly enhance the magnetic transition temperature in topological materials. It is helpful for the realization of novel quantum states at elevated temperatures.

Elevating the magnetic exchange coupling in the compressed antiferromagnetic axion insulator candidate Eu In 2 As 2

We succeeded in growing single crystals of Eu2Ni3Ge5 and EuRhSi3 by the In-flux method, and measured the electrical resistivity under pressures. Both compounds are Eu-divalent antiferromagnets with the Neel temperatures TN = 18.6 and 47.7 K, respectively. With increasing pressure, the resistivity in Eu2Ni3Ge5 indicates a clear kink at Tv = 220 K under 8.0 GPa, for example, which corresponds to a valence transition. The valence of Eu ions deviates from divalence toward trivalence. Correspondingly, antiferromagnetic ordering disappears completely. The low-temperature resistivity is, surprisingly, very similar to the resistivity of a well-known heavy fermion superconductor CeCu2Si2, possessing a two-peak-structure based on the Kondo effect. In fact, the A value of the resistivity ρ = ρ0 + AT2 in the Fermi liquid relation becomes large, A = 0.3 µΩ·cm/K2 at 8.0 GPa. The similar characteristic features are observed in another compound EuRhSi3.

Pressure-Induced Valence Transition and Heavy Fermion State in Eu2Ni3Ge5 and EuRhSi3

We report 153Eu and 69,71Ga NMR under a zero magnetic field on the antiferromagnetic state of EuGa4 with $T_{\text{N}}\approx 16$ K. We have successfully observed a 153Eu zero-field NMR signal with well-resolved nuclear quadrupole splitting in the antiferromagnetic state of EuGa4. 69,71Ga zero-field NMR spectra were also observed below $T_{\text{N}}$. The internal field and nuclear quadrupole frequency are estimated from a simulation of the spectra by the exact diagonalization of the nuclear spin Hamiltonian matrix. The asymmetrically split zero-field NMR spectra were explained by considering a configuration of the magnetic moments of Eu2+ lying in the basal $ab$-plane. The temperature dependence of the internal field, which is proportional to the sublattice magnetization, can be explained by the Brillouin function with $J=S=7/2$.

153Eu and 69,71Ga Zero-Field NMR Study of Antiferromagnetic State in EuGa4

We report lattice parameters as a function of temperature, magnetization curves, and magnetoresistance of polycrystalline samples of Eu(Rh1−xIrx)2Si2. The lattice parameters a and c for x = 0.3, 0.4, and 0.5 jump at around the valence transition temperature Tv. For these samples, the high-temperature (larger a, c) and low-temperature (smaller a, c) phases, the volume fraction of which changes with temperature, coexist at around Tv, which means that the valence transition is of the first order. The magnetization curves of x = 0.3, 0.4, and 0.5 at T = 4.2 K exhibit a metamagnetic transition with a large hysteresis, implying a first-order field-induced valence transition. The transition field Bv is proportional to Tv, Bv = αTv with α = 0.36 T/K. For x = 0.3, a large negative magnetoresistance associated with the field-induced valence transition is observed. The origin of the negative magnetoresistance is discussed.

Large magnetoresistance and volume expansion associated with valence transition in Eu(Rh1-xIrx)2Si2

We report the effects of a magnetic field on a pressure-induced valence transition in EuRh2Si2 by using magnetization measurements under high pressure. The M-H curves at 2 K under various pressures up to 1.18 GPa exhibit no anomalies associated with a field-induced valence transition. On the other hand, the M-H curves under a pressure of ∼ 1 GPa, where a temperature-induced valence transition occurs at around 30 K, show a clear metamagnetic transition with a large hysteresis at around 30 K. This behavior corresponds to a field-induced valence transition. The M-T curves in magnetic fields indicate that with increasing magnetic field, the temperature-induced valence transition shifts toward lower temperature and that its thermal hysteresis becomes larger. Cooling the sample under a pressure of ∼ 1 GPa in a magnetic field of 7 T collapses the valence transition and stabilizes the magnetic high-temperature phase down to 2 K. Besides, with decreasing magnetic field, three sharp valence transitions toward the nonmagnetic Eu3+ direction are observed: 4.65, 2.65 and 1.93 T.

Valence transition induced by pressure and magnetic field in antiferromagnet EuRh2Si2

Valence transition in polycrystalline Eu(Rh1–xCox)2Si2 studied by hard x-ray photoemission spectroscopy

Considering the unique properties of EuRh 2 Si 2 from the viewpoint of the Eu valence, we have examined its physical properties under external pressure. At ambient pressure, EuRh 2 Si 2 is an antiferromagnet with a Neel temperature T N of 25 K, and the Eu ion is in the divalent state. The application of pressure up to 0.84 GPa slightly shifts T N toward higher values. Under pressures higher than 1.00 GPa, an abrupt first-order valence transition emerges simultaneously with the disappearance of antiferromagnetism. For P =1.17 GPa, the valence change associated with valence transition is roughly estimated to be ∼0.19 from the thermal expansion anomaly. The valence transition temperature T v increases rapidly with increasing pressure. The temperature–pressure phase diagram of EuRh 2 Si 2 is very similar to those of the other systems showing pressure-induced valence transition.

Pressure-Induced Valence Transition in Antiferromagnet EuRh2Si2

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Large magnetoresistance and volume expansion associated with valence transition in Eu(Rh1-xIrx)2Si2

Valence transition induced by pressure and magnetic field in antiferromagnet EuRh2Si2

Valence transition in polycrystalline Eu(Rh1–xCox)2Si2 studied by hard x-ray photoemission spectroscopy

Pressure-Induced Valence Transition in Antiferromagnet EuRh2Si2