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The electrical resistance of single-crystalline Yb3Si5 was measured under high pressure up to 60 GPa generated in an diamond anvil cell with the main aim to search for pressure-induced long-range magnetic ordering. Though no magnetic transitions were found, a clear crossover behavior in the pressure dependence of the characteristic temperature T* was observed. The results are discussed in terms of complex influence of pressure on the Kondo temperature, specific to Yb-based heavy fermion systems.

We present resistivity and AC calorimetry measurements of single crystalline CeNiGe 2 under high pressure and low temperature in a diamond anvil cell. CeNiGe 2 is a highly anisotropic, antiferromagnetic Kondo system at ambient pressure, located close to the boundary with non-magnetic systems. It has two ordering temperatures, T N 1 = 3.9 K and T N 2 = 3.2 K , which are suppressed under pressure. Between 0.7 and 1.4 GPa only a single transition was identified, with T N reaching 1.9 K by 1.4 GPa. At 1.9 GPa two transitions could again be seen by AC calorimetry at 2.0 and 1.3 K, accompanied by a qualitative change in the behaviour of the resistivity. The Sommerfeld coefficient showed a maximum of 755 mJ mol - 1 K - 2 at 1.4 GPa, up from 448 mJ mol - 1 K - 2 at close to ambient pressure. The residual resistivity was found to decrease sharply above 1.4 GPa after an initial increase with pressure.

We have measured electrical resistivity of single crystalline α -manganese ( α -Mn) under high pressure. α -Mn shows an antiferromagnetic ordering at T N = 95 K and has four inequivalent crystal sites with different magnetic moments. With increasing pressure, T N shifts toward lower temperature. At higher pressure, the shape of anomaly at T N changes to a kink-like one, which may indicate different magnetic ordering from that at lower pressure. Such a difference between lower and higher pressure regions is considered to be caused by the four inequivalent magnetic moments in α -Mn. At ∼ 1.9 GPa , the boundary of the ordered phases, we observe a sudden decrease of the residual resistivity and a peak of the A -coefficient of T 2 -term of the resistivity.

CePdSb 3 crystallizes complex orthorhombic structure that has two Wyckoff positions of Ce. The recent studies on the single crystal CePdSb 3 revealed an antiferromagnetic ordering at T N =3.1 K. We have measured the temperature dependence of the electrical resistivity up to 4.1 GPa and, down to 60 mK. With applying pressure, another anomaly in ρ ( T ) curve appeared at the temperature lower than T N appeared above 1.0 GPa, suggesting the existence at second magnetic ordering caused by the complex structure that has two Ce sites.

Electrical and magnetic properties of highly correlated electron systems are investigated under multi-extreme conditions: high pressure, low temperature and high magnetic fields. The instability of the electronic state under multi-extreme conditions is systematically discussed on the basis of the data obtained. It is suggested that one kind of interaction is enhanced but another is suppressed under multi-extreme conditions, which gives rise to a lot of interesting electronic transitions.

High pressure studies on the thermal expansion and electrical resistance of the heavy fermion U-compound have been carried out to examine the electronic state under volume contraction. Clear anomalies around 17 K were found in the temperature dependence of the thermal expansion coefficient and electrical resistivity due to antiferromagnetic ordering. A T 2 dependence in electrical resistivity and T -linear term in the thermal expansion coefficient were obtained at low temperature. These results are analyzed on the basis of thermodynamic relations and discussed by using volume-dependent Gruneisen parameter in comparison with the results for other concentrated Kondo compounds.

The electrical resistivity of single crystalline ferromagnetic UGe 2 has been measured under high pressure up to 2 GPa. It is found that the Curie temperature T C decreases with increasing pressure and the ferromagnetism disappears at high pressure. The temperature dependent electrical resistivity is analyzed on the basis of spin fluctuation theory. The T 2 term was observed at low temperature and the coefficient shows a divergence near the critical pressure. A T 5/3 dependence in the ρ ( T ) curve was observed below and above T C and its pressure effect is discussed briefly.

Electrical resistivity of CePtSi2 (TN=1. 7 K), CePtGa (TN=3. 5 K) and Y1-x UxPd3 (x=0. 198) have been measured at high pressure. It is found that the antiferromagnetic ordering of the two Ce compounds is suppressed by pressure, i. e., Neel temperature TN becomes zero at a critical pressure Pc to show a non fermi liquid behavior above Pc. By pressurizing more above Pc, the electronic states show a crossover from non Fermi liquid (NFL) to Fermi liquid (FL) states or Kondo state. On the other hand for U-compound which shows NFL properties at ambient pressure a crossover from NFL to FL is also observed by applying pressure. These facts indicate that the NFL state becomes unstable at high pressure. The electronic states of these compounds show a general trends of magnetic ordering → NFL → FL as pressure increases.

Elastic and anelastic properties of polycryatalline α-Mn at low temperature have been studied using electromagnetic acoustic resonance (EMAR) and resonance ultrasound spectroscopy coupled with a laser Doppler interferometer (RUS/Laser). α-Mn showed low Poisson's ratio both in the paramagnetic state at 290 K and in the anti-ferromagnetic state at 4 K. The acoustic Debye temperature was 479 K, showing good consistency with the previous work. Around the magnetic transition temperature (TN=98 K), remarkable changes in Poisson's ratio and internal friction are identified, indicating strong magnetic acoustic interaction at the transition temperature.

We have measured the electrical resistance of the filled skutterudite SmRu 4 P 12 , which exhibits a metal–insulator (MI) transition at T MI = 16 K, at high pressures up to 15 GPa. With increasing pressure, the semiconductor-like resistance was suppressed. We observed metallic behavior in the resistance above 3.5 GPa, while semiconductor-like increase of the resistance was observed below 2 K. Two characteristic anomalies below T MI , a peak and a kink in the resistance curve, are observed at T 1 and T 2 .

Thermal expansion and magnetostriction of UNiGa single crystal have been measured at the range of pressures up to 2.2 GPa and magnetic fields up to 1.5 T. The results are reported with a special emphasis on anomalies accompanying the magnetic phase transitions and the effect of pressure on the critical parameters (transition temperature and magnetic field). Formation of a new antiferromagnetic phase induced by pressures above 1.8 GPa is revealed. Analysis of results enabled us to construct P-T magnetic phase diagrams for fields up to 1.5 T. The magnetoelastic effects are discussed in terms of marked variations in electronic structure accompanying transitions between different magnetic structures with various couplings of U magnetic moments along the c -axis and involvement of strong spin-orbit interaction.

The electrical resistivity of Y 1− x Y x Pd 3 ( x = 0.198), which is well known to exhibit non-Fermi-liquid (NFL) behaviors, has been measured under hydrostatic pressure up to 5.8 GPa in order to investigate the stability of NFL state at high pressure. It is found that the coefficient of T -linear term in the ϱ ( T ) curves at low temperatures decreases with increasing pressure and at 5.8 GPa, the T 2 -term characteristic of the normal Fermi liquid is apparently observed below 15 K. On the basis of the present data, the Gruneisen parameter of T K is estimated to be about 10. The electronic structure of NFL at high pressure is briefly discussed.

Electrical resistance of the nearly two-dimensional compound PrSb 2 has been observed at high pressures up to 2.2 GPa and at magnetic fields up to 5 T. It is found that the antiferromagnetic ordering temperature T N increases with pressure having a rate d T N /d P = 1.3 K GPa −1 . Metamagnetic phase transitions are found to be induced at magnetic fields of several T .

The electrical resistivity of the intermediate valence compound CeBe13 has been measured at high pressures to 2 GPa. The coefficient of T2-term in the electrical resistivity rapidly decreases with increasing pressure, associated with an enhancement of the Kondo temperature TK. Pressure dependence of the lattice constants was also measured to 12 GPa at room temperature. The volume dependence of the TK and its Gruneisen parameter will be discussed briefly.

Thiospinel CuCrZrS4 exhibits ferromagnetic properties below its Curie temperature at T C = 60 K . X-ray diffraction studies and the electrical resistance measurements were carried out on the thiospinel under high pressure up to 50 and 58 GPa, respectively, at room temperature using a diamond-anvil cell. X-ray powder diffraction studies elucidated that a new high-pressure phase existed above 15 Gpa. An anomaly in the electrical resistance was observed at around 15 GPa, which is associated with structure transition to a high-pressure phase discovered by X-ray diffraction. Temperature dependence of the electrical resistance measurements revealed that CuCrZrS4 transforms from semi conductive to metallic compound under high pressure.

X-ray powder diffraction study of PbHPO 4 was carried out under hydrostatic conditions at room temperature. We could not detect any difference in the diffraction patterns except peak shift with increasing pressures up to 6 GPa. By further compression, a new pressure-induced phase appears at about 6.5 GPa. We have determined that the structure of this high-pressure phase has a possible space group of Pmmm, and which is stable up to 17 GPa. During decompression process, this phase is converted back to the original phase II at 0.7 GPa.

CeSb 2 is characterized as an anisotropic ferromagnet with T C ∼ 16 K , having layered orthorhombic structure. We have measured the electrical resistance of single crystalline CeSb 2 at high pressure and low temperature. The T C is decreased by applying pressure. At 2 GPa above which ferromagnetic transition was not clearly observed, the coefficient of quadratic dependence in ρ ( T ) shows a peak and the residual resistivity drops sharply.

The metal to insulator transition is observed at T MI = 62 K in filled skutterudite PrRu4P12. We have measured the electrical resistance of single crystalline at high pressure up to 15 GPa. With increasing pressure, the semiconductor-like resistance was suppressed. We observed metallic behavior in the resistance above 11 GPa and below 50 K, while an anomaly was still observed at around 60 K.

X-ray diffraction, Raman spectroscopy, and electrical conductivity measurements of molybdenum disulfide MoS(2) are performed at pressures up to 81 GPa in diamond anvil cells. Above 20 GPa, we find discontinuous changes in Raman spectra and x-ray diffraction patterns which provide evidence for isostructural phase transition from 2H(c) to 2H(a) modification through layer sliding previously predicted theoretically. This first-order transition, which is completed around 40 GPa, is characterized by a collapse in the c-lattice parameter and volume and also by changes in interlayer bonding. After the phase transition completion, MoS(2) becomes metallic. The reversibility of the phase transition is identified from all these techniques.

An x-ray diffraction study reveals the valence-ordering structure in an intermediate-valence magnet YbPd with a CsCl structure. The valence of the Yb ions forms an incommensurate structure, characterized by the wave vector ($\ifmmode\pm\else\textpm\fi{}$0.07 $\ifmmode\pm\else\textpm\fi{}$0.07 1/2) below 130 K. At 105 K, the incommensurate structure turns into a commensurate structure, characterized by the wave vector (0 0 1/2). Based on the resonant x-ray diffraction spectra of the superlattice reflections, the valences of the Yb ions below 105 K are found to be 3$+$ and 2.6$+$. The origin of the long-wavelength modulation is discussed with the aid of an Ising model having next-nearest-neighbor interaction.

The pressure-temperature phase diagram of an intermediate-valence compound YbPd was revealed via simultaneous ac-calorimetry and electrical resistivity measurements. Two successive structural phase transition temperatures, $T_1$ = 125 K and $T_2$ = 110 K, are found to decrease with increasing pressure. The magnetic ordering transition at $T_{\rm N} ~sim$ 1.9 K decreases monotonically up to 1.7 GPa and disappears discontinuously at $P_{\rm c} sim$ 1.9 GPa, where the structural phase transition at $T_2$ is also suppressed. At $P_{\rm c}$, enhancement of the effective mass and residual resistivity are observed. Another probable magnetic phase transition, however, is found to take place at $T_{\rm ML} sim$0.3 K and smoothly varies even when crossing $P_{\rm c}$. Structural phase transitions at ambient pressure were also studied by a single crystal X-ray diffraction measurement. Below $T_2$, a tetragonal lattice distortion with doubling of the unit cell along the c-axis is observed, and thus there are two inequivalent Yb-sites below $P_{\rm c}$. These results indicate that one Yb-site, having larger valence configuration and thus smaller Kondo temperature, orders at $T_{\rm N}$. The other phase transition at $T_{\rm ML}$ seems to couple to the crystal structure for $T_2 < T < T_1$. YbPd is a unique system exhibiting magnetic orderings in metallic charge-ordered Yb systems., 7 pages, 5 figures, to be published in J. Phys. Soc. Jpn

Effects of high pressure exceeding 10 GPa on spin-driven ferroelectricity were investigated for a multiferroic, triangular-lattice antiferromagnet (TLA), CuCrO${}_{2}$. For this purpose, we developed a system which enables us to measure ferroelectric polarization under a pressure of 10 GPa by using a diamond anvil cell. We found that the magnetic transition temperature accompanying the ferroelectric one in CuCrO${}_{2}$ was remarkably enhanced by applying pressure. The result is simply explained by considering the pressure-induced enhancement of inter- and/or intralayer magnetic interaction due to the compression of the lattice. In addition, the coercive electric field for the polarization reversal was also increased with increasing pressure, while the amplitude of the ferroelectric polarization was steeply suppressed at around 8 GPa. A possible origin of the observed pressure effects on the ferroelectric property in the multiferroic TLA is discussed in terms of a ferroelectric-antiferroelectric transition and structural domain rearrangement by uniaxial stress.

The electrical resistivity, ρ, of amorphous Ce 58 Ru 42 alloy (a-CeRu) has been measured as a function of temperature ( T ) at various pressures ( P ) up to 14 GPa. At room temperature ρ decreases rapidly with increasing pressure: ∂log ρ/∂ P =-0.9 GPa -1 below 2 GPa. The log T dependence in the ρ( T ) curve due to Kondo scattering is observed up to 14 GPa. The slope of the ρ vs log T curve decreases with increasing pressure. At 4.2 K the magnetoresistance, Δρ/ρ, is positive and decreases with increasing pressure. These results suggest that the Kondo temperature, T K , decreases with pressure in a-CeRu, i.e., the Kondo effect is suppressed by applying pressure.

The magnetoresistance or magnetostriction of α-Ce, CeAl3 and amorphous Ce58Ru42 alloy (a-CeRu) has been measured at high pressure up to 2 GPa. The results show that the Kondo temperature TK is increased by an application of pressure for α-Ce and CeAl3, while the behavior is opposite for a-CeRu.

Electrical resistivity ϱ(T) of single crystalline UGe2 has been measured at high pressure up to 20 kbar and high magnetic field to 5 T. It was found that the Curie temperature Tc (K) decreases with increasing pressure and tends to disappear around 18 kbar. The magnitude of magnetoresistance Δϱ/ϱ decreases with pressure. The result is discussed briefly on the basis of the theory of weak itinerant ferromagnetism.

The heavy-fermion antiferromagnet CeNiGe${}_{2}$ was investigated under pressure by resistivity and ac calorimetry up to 4 GPa and down to 40 mK. The two magnetic transitions found in both resistivity and specific heat at 0.1 GPa at ${T}_{N1}=3.95$ and ${T}_{N2}=3.21$ K are replaced by a single one at 0.7 GPa and 2.81 K. Increasing pressure initially reduces this further, however, at about 1.7 GPa, a new transition appears, accompanied by a marked change in the pressure dependence of the ordering temperatures, the temperature dependence of the resistivity, and the residual resistivity; the latter indicates a marked change in the quantum fluctuations present. There are signs that this new transition has some first-order character. The phase diagram of CeNiGe${}_{2}$ bears little resemblance to the Doniach phase diagram widely used to classify heavy-fermion compounds.

Electrical resistance of the heavy fermion compound CeInCu 2 has been measured at high temperatures up to ∼1000 K and pressures to 8 GPa. The resistance-maximum temperature T max increases rapidly with increasing pressure, associated with an enhancement of the Kondo temperature T K . The volume dependence of the T K and the Gruneisen parameter Γ is discussed quantitatively.

The electrical resistivity ρ ( T ) of the concentrated Kondo compound URu 2 Si 2 has been measured at high pressure up to 80 kbar in order to examine the stability of the electronic state at high pressure. The pressure dependence of volume is also observed by X-ray diffraction. The Gruneisen parameters for the Neel temperature and the Kondo temperature are estimated on the basis of the results in the present work. The present analysis emphasizes the itinerant character of 5f electrons of URu 2 Si 2 .

The magnetoresistance (MR) and magnetostriction (MS) have been measured at high pressure for heavy fermion materials CeInCu 2 and CeAl 3 . It is found that the sign of MR of CeInCu 2 at 4.2 K changes from negative to positive around 1 GPa, which indicates that the system enters a coherent state. MS of CeInCu 2 and CeAl 3 is affected strongly by an application of pressure. The decrease in magnitude of MS is explained well by assuming the large enhancement of Kondo temperature T K .

Electrical resistivity ϱ(T) of the concentrated Kondo compound CePtSi2 has been measured as a function of temperature at various pressures up to 80 kbar. Two maxima are observed near 5 K (= T1) and 28 K (= T2) in the ϱ(T) curve at ambient pressure. It is found that T2 increases with increasing pressure but T1, disappears above 5 kbar and the two maxima are merged into a broad maximum above 20 kbar. The overall behavior of the ϱ(T) curves at high pressures indicates that a crossover in the electronic state of CePtSi2 is induced by applying pressure. On the basis of the data in the present work, we examine a semi-quantitative analysis to obtain the value of |JN(0)| at ambient pressure.

The electrical resistance of the heavy fermion compound CeInCu2 has been measured at high magnetic field and hydrostatic pressure. It is found that the high pressure induces positive magnetoresistance, which is characteristic of the Kondo coherent state, associated with the enhancement of the Kondo temperature. The result is discussed briefly on the basis of the theoretical model presented by Kawakami and Okiji.

The electrical resistivity p(T) of the heavy fermion compound CeAl3 has been measured under high pressure up to 1.5 GPa in order to clarify the volume dependence of the electronic state in the heavy fermion. It is found that the temperature showing a maximum in the p(T) curve increases with increasing pressure. Compressibility was also measured using the X-ray diffraction technique at high pressure. On the basis of the data, a quantitative analysis of the volume dependent energy scale is examined using a simple theoretical model.

Pressure phase diagram of the antiferromagnet UIn 3 was constructed from the electrical resistivity measurement under high pressures up to 9 GPa. A Neel temperature increases monotonically with inc...

The electrical resistivity of the antiferromagnetic dense Kondo compound UCu5In has been studied on single-crystalline specimens under hydrostatic pressure up to 2.2 GPa. Upon applying pressure, ρa(T) hardly alters but the other two resistivity components, i.e. ρb(T) and ρc(T), show some change in their magnitudes, in both the ordered and paramagnetic regions. The Neel temperature TN=25 K is almost independent of pressure, which indicates that the magnetic moments in UCu5In are rather well localized.

A cryogenic loading system for introducing H(2) and D(2) into the diamond anvil cell has been designed and constructed. The integration of pressure loading mechanism, ruby fluorescence spectrometer, and microscope camera allows for in situ tuning and calibrating the pressure. The performance of the system has been demonstrated by successful synthesis of hydride and deuteride of transition metal and rare earth metal. Our cryogenic methodology features facile start-over of loading and in situ electrical resistance measurement of as-synthesized sample.

We report the discovery of two consecutive, pressure driven magnetic instablities in Yb2Pd2Sn. They emerge in a non-Fermi liquid environment at the initial and the final point of a dome-like, single magnetic phase at pressures pc1 = 1 GPa and pc2 = 4 GPa. This singular behavior of Yb compounds is supposed to result from mutually competing, pressure modi ed energy scales, which in case of Yb2Pd2Sn cause a sign change of the pressure dependence of the Kondo temperature TK and magnetic ordering temperature TN.

We report the pressure dependence of electrical resistance $R$($T$) of a frustrated pyrochlore oxide Nd${}_{2}$Ir${}_{2}$O${}_{7}$, which shows a second-order metal-insulator transition (MIT) at 36 K. This MIT is monotonically suppressed by the application of pressure. The insulating phase disappears at around 10 GPa. Then, in the pressure-induced metallic state above 10 GPa, a new pressure-induced phase transition emerges at around 3 K, which is characterized by a resistance drop. The new phase transition is due to a ferromagnetic (FM) ordering, which is suggested to be an ordered spin ice (two-in two-out) state of the Nd moment via the RKKY interaction. We succeeded in observing the phase competition between the MIT and the FM ordering by the application of high pressure.

The electrical resistivity ρ(T) of CePtSi2 has been measured up to 20 kbar. It is found that two maxima exist in ρ(T) at ambient pressure but merge into a broad peak above 10 kbar. The thermal expansion coefficient α(T) is also measured at ambient pressure. A large enhancement in α T is observed at low temperature.

The magnetoresistance of the heavy fermion compound CeInCu 2 has been measured at both ambient and hydrostatic pressures up to 2 GPa. It is found that the high pressure induces positive magnetoresistance, which is characteristic of the Kondo coherent state, associated with the enhancement of the Kondo temperature. The result is discussed briefly on the basis of the theoretical model by Kawakami and Okiji.

The temperature dependence of electrical resistivity p ( T ) of the heavy-fermion compound CeInCu 2 has been measured under hydrostatic pressure up to 8 GPa. It is found that the p ( T ) curves indicate a pressure-induced valence transition, and the resistivity-maximum temperature T max increases rapidly by applying pressure. p ( T ) at low temperatures has a T 2 dependence and the temperature range showing the T 2 term is expanded by pressure. The coefficient A of the T 2 term in p ( T ) decreases rapidly with increasing pressure. Since T max and A are proportional to T K and T K −2 respectively, where T K is the Kondo temperature, the large increase in the value of T max and decrease in A indicates the large enhancement of T K at high pressures.

The effect of magnetic field on the electrical resistivity of CeInCu2 at high pressure up to 2 GPa and on the length of crystal axes of CeCu6 has been measured in the temperature range, 2K

The electrical resistivity of single crystalline ErNi2B2C has been measured at high pressures up to 2.2 GPa and magnetic fields up to 5 T in order to make clear the relation between antiferromagnetism and superconductivity. It is found that the superconducting transition temperature Tc is almost independent of pressure, while the upper critical field Hc2 for H⊥c decreases with increasing pressure. The Neel temperature TN increases with increasing pressure. The present results are compared with those of other borocarbides and discussed taking into account the interplay between the superconductivity and the antiferromagnetism.

Lattice parameters of high-Tc cuprate superconductors La2−xSrxCuO4 (LSCO) and YBa2Cu3O7−δ (YBCO) have been measured at high pressure for a wide variety of x and δ. It is found that the linear and volume compressibilities have peaks in the vicinity of x and δ at which the superconducting state becomes unstable. Some anomalous properties in the linear compressibility κi (i=a, b and c) of YBCO are pointed out and discussed in connection with the oxygen deficient δ.

Giant magnetostriction (GMS) of Tb0.3Dy0.7Fe1.9 single crystal has been measured at hydrostatic pressure up to 9 kbar. The electrical resistance and thermal expansion were also measured at low temperature down to 4.2 K to examine low temperature behavior. It is found that the magnitude of GMS increases with increasing pressure below 4 kbar but decreases above 6 kbar having a maximum near 5 kbar. The origin of maximum is discussed briefly the difference between hydrostatic pressure and uniaxial pressure is pointed out.

Thermal expansion of α-Mn has been measured at high pressures up to 2.2 GPa in order to examine the relation between the instability of antiferromagnetism and the magnetovolume effect. It is found that TN goes down and the magnitude of spontaneous volume magnetostriction becomes small as pressure increases. The antiferromagnetism of α-Mn is suggested as disappearing below 5 GPa.

The electrical resistivity of single crystalline TmNi 2 B 2 C has been measured at high pressures up to 2 GPa and magnetic fields up to 5 T. It is found that the superconducting transition temperature T c and the upper critical field H c2 decrease with increasing pressure, ∂T c / ∂P =−0.50 K/GPa and ∂H c2 / ∂P =−0.38 T/GPa. The present results are compared with those of other borocarbides and discussed by taking into account the interplay between the superconductivity and the antiferromagnetism.

Thermal expansion coefficients of polycrystalline CeRh2Si2 under high pressure up to 2.15 GPa have been measured in the temperature range from 5 to 300 K. It is found that the antiferromagnetic ordering temperature TNl and the magnetic order-order transition temperature TN2 decrease with increasing pressure and the antiferromagnetism disappears at high pressure. A positive spontaneous magnetostriction, of the order of 10-4, is observed below TN1, which also disappears above Pc = 1.11 GPa. The results are explained qualitatively by the competition between the Kondo and the RKKY interactions. Similarity between the present P-T phase diagram and the Doniach's one is pointed out.

We report comprehensive high pressure studies on correlation between intercalated magnetic layers and superconductivity in EuFe2(As0.81P0.19)2 single crystal through in-situ high pressure resistance, specific heat, X-ray diffraction and X-ray absorption measurements. We find that an unconfirmed magnetic order of the intercalated layers coexists with superconductivity in a narrow pressure range 0-0.5GPa, and then it converts to a ferromagnetic (FM) order at pressure above 0.5 GPa, where its superconductivity is absent. The obtained temperature-pressure phase diagram clearly demonstrates that the unconfirmed magnetic order can emerge from the superconducting state. In stark contrast, the superconductivity cannot develop from the FM state that is evolved from the unconfirmed magnetic state. High pressure X-ray absorption (XAS) measurements reveal that the pressure-induced enhancement of Eu's mean valence plays an important role in suppressing the superconductivity and tuning the transition from the unconfirmed magnetic state to a FM state. The unusual interplay among valence state of Eu ions, magnetism and superconductivity under pressure may shed new light on understanding the role of the intercalated magnetic layers in Fe-based superconductors.