Table Of ContentAntiferromagnetism with divalent Eu in EuNi As
5 3
W. B. Jiang,1 M. Smidman,1,∗ W. Xie,1 J. Y. Liu,2 J. M. Lee,3 J. M. Chen,3 S. C. Ho,3
H. Ishii,3 K. D. Tsuei,3 C. Y. Guo,1 Y. J. Zhang,1 Hanoh Lee,1 and H. Q. Yuan1,4,†
1Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou, 310058, China
2Department of Chemistry, Zhejiang University, Hangzhou, 310058, China
3National Synchrotron Radiation Research Center, Hsinchu, 30076, Taiwan
4Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China
(Dated: January 20, 2017)
7 WehavesuccessfullysynthesizedsinglecrystalsofEuNi5As3usingafluxmethodandwepresenta
1 comprehensivestudyofthephysicalpropertiesusingmagneticsusceptibility,specificheat,electrical
0 resistivity,thermoelectricpowerandx-rayabsorptionspectroscopy(XAS)measurements. EuNi5As3
2 undergoes two close antiferromagnetic transitions at respective temperatures of TN1 = 7.2 K and
TN2 = 6.4 K, which are associated with the Eu2+ moments. Both transitions are suppressed upon
n
applying a field and we map the temperature-field phase diagrams for fields applied parallel and
a
perpendicular to the easy a axis. XAS measurements reveal that the Eu is strongly divalent, with
J
very little temperature dependence, indicating the localized Eu2+ nature of EuNi5As3, with a lack
9
of evidencefor heavy fermion behavior.
1
PACSnumbers: 75.30.Kz,75.50.Ee,78.70.Dm
]
l
e
-
I. INTRODUCTION sistivity, magnetization, specific heat and partial fluo-
r
t rescenceyieldX-rayabsorptionspectroscopy(PFY-XAS)
s
t. In recent decades, Kondo related physics has been measurements. Polycrystalline samples of EuNi5As3
a were also obtained to verify the crystal structure and
intensively investigated in rare-earth element inter-
m to measure the thermoelectric power S(T). Our results
metallics. In the Kondo lattice, the on-site Kondo in-
- teraction screens the magnetic moment of localized f provide evidence for two antiferromagnetic (AFM) tran-
d
sitionsatT = 7.2KandT = 6.4K.ThetwoAFM
n electrons, leading to a nonmagnetically ordered heavy N1 N2
transitionsaresuppressedbyappliedmagneticfieldsand
o Fermion state. Besides the Kondo interaction, another
we map the field-temperature phase diagrams for H k a
c competing interaction, the Ruderman-Kittel-Kasuya-
[ Yosida(RKKY)interactionmediatedbythe surrounding andH ⊥a. AdivalentEuvalenceisdeducedfromPFY-
1 conduction electrons conversely favors long-range mag- XAS measurements and the weak temperature depen-
dence ofthe Euvalence configurationconfirms the Eu2+
v netic order. The competition between the Kondo and
AFM ordering in EuNi As .
6 RKKY interactions in heavy fermion systems may re- 5 3
2 sult in various ground states, such as, magnetic order,
3
superconductivity, heavy fermion and intermediate va-
5
lence states1–5. Such phenomena related to the Kondo II. EXPERIMENTAL DETAILS
0
effect have often been observed in Ce, Yb and U-based
.
1 compounds, but Eu based heavy fermion materials have Single crystals of EuNi As were synthesized using a
0 5 3
not been commonly reported. In Eu based compounds, NiAsself-fluxmethod. EuAsandNiAswerefirstsynthe-
7
the Eu ion typically adopts one of two electronic config- sizedasdescribedelsewhere9. Subsequently,EuAs,NiAs
1
: urations: divalent, magnetic Eu2+ (4f7, J=7/2, L=0) or andNiwerecombinedintheratioEuNi5As3:NiAsof1:3.
v trivalent, nonmagnetic Eu3+ (4f6, J=0, L=3). Therefore The mixtureswerethencombinedandsealedinanevac-
Xi usually either a magnetic state with localized Eu2+, a uated quartz ampoule. The ampoule was slowly heated
r non-magneticstate with valence fluctuations or trivalent to 1000◦C, and held at this temperature for 24 hours to
a Eu3+ occur in Eu systems. However, there have been a allow for homogenization before being slowly cooled to
few proposed examples of heavy fermion systems such 800◦C at a rate of 3◦C/hour, and then quickly cooled
as EuNi2P2 and EuCu2(Si1−xGex)26,7.The Eu valence to room temperature. Rectangular rod-like single crys-
of EuNi2P2 also shows a significant temperature depen- tals with a typical length of 2mm were obtained after
dence with the valence changing from +2.25at 300 K to mechanical separation from the remaining flux. Single
+2.50 at 1.4 K8, exhibiting strong valence fluctuations crystalsofSrNi As , whichwere usedasa non-magnetic
5 3
in the ground state. The reason for the rare existence analogwere obtained using the same procedure. The as-
of heavy fermion behavior in Eu based compounds re- grown single crystals are stable in air. To further clarify
mains an open question, requiring further experimental the crystalstructure, we also synthesized polycrystalline
and theoretical investigations. samplesofEuNi As usingasolidstatereactionmethod.
5 3
Inthispaper,wereportthesuccessfulsynthesisofsin- EuAs, NiAs and Ni powder were combined stoichiomet-
gle crystals of EuNi As using a self flux method and rically and sintered at 800◦C for 4 days. The resulting
5 3
study the physical properties by means of electrical re- pelletwasthenthoroughlygroundandpressedbeforebe-
2
TABLEI.Atomiccoordinatesandisotropicdisplacementpa-
rametersUeq fromtherefinementofsinglecrystalXRDdata.
Atom Site X Y Z Ueq
As 8f 0 0.3792 0.5436 0.01338
As 4c 0.5 0.6146 0.75 0.01394
Eu 4c 0.5 0.3348 0.75 0.01632
Ni 8f 0 0.1937 0.5654 0.01645
Ni 4a 0.5 0.5 0.5 0.01431
Ni 8f 0 0.5492 0.6445 0.01527
ingannealedat850◦Cforaweekinordertoimprovethe
sample homogeneity.
The crystal structure was examined using single crys-
tal x-ray diffraction on an Xcalibur, Atlas, Gemini ul-
tra diffractometer with an x-ray wavelength of λ =
0.71073˚A. Room temperature powder x-ray diffrac-
tion (XRD) data were collected using a PANalytical
X’Pert MRD diffractometer with Cu K radiation and
α1
a graphite monochromator. The chemical composition
wasalsocheckedbyenergy-dispersivex-rayspectroscopy
using a FEI SIRION-100 field emission scanning elec-
tron microscope. Resistivity ρ(T,H), magnetization
M(T,H), specific heat C(T) and thermoelectric power
measurements were performed using a Quantum Design FIG. 1. (Color online)(a) Crystal structure of EuNi5As3.
(b) The powder x-ray diffraction pattern of polycrystalline
Physical Property Measurement System. The Eu L -
III EuNi5As3. The solid red line shows the calculated results of
edge partial fluorescence yield X-Ray absorption spec- theRietveldrefinement,whiletheverticalbarsshowthethe-
troscopywas measuredat the beamline 12XU in Spring- oretical Bragg peak positions and the green line shows the
8, Japan, as described in Ref. 9 differencebetween theobserved and calculated data.
B. Magnetic order in EuNi5As3
III. RESULTS AND DISCUSSION
To characterize the magnetic properties of EuNi As ,
5 3
we performed magnetic susceptibility measurements on
A. Crystal structure
both single crystalline and polycrystalline EuNi As .
5 3
Figure. 2 displaysthe magnetic susceptibility χ(T)inan
applied field of 0.1 T for Hka and H⊥a, where correc-
Rod-like single crystals were selected for single crys-
tions for the demagnetization have been made based on
talx-raydiffractionmeasurementsatroomtemperature,
the sample geometry. For both field directions, the data
to clarify the crystal structure. The results of the re-
followsthe Curie-Weisslawabove10K.The inversesus-
finement of the crystal structure from the single crystal
ceptibility 1/χ(T)and the fit to the Curie-Weiss law are
diffractionreflections are displayedin Table I. EuNi As
5 3
shown in the inset of Fig. 2(a). The derived effective
crystallizes in the LaCo P -type orthorhombic structure
5 3 moments for the two field directions are µa = 8.64µ
with the space group Cmcm(No.63), isostructural to eff B
EuNi5P310,11. The reliability factors R and S obtained and µbecff = 8.12µB, which are both close to the ex-
fromthe structuralrefinementare8.59%and1.2respec- pected 7.94µ for a free Eu2+(J=7/2) ion. In addition,
B
tively, confirming the accuracy of the refinement. The the obtained Curie-Weiss temperatures θ are -7.85 K
P
refined cell parameters a, b, c are 3.7381, 12.1977 and and -5.45 K for Hka and H⊥a, respectively, indicat-
11.8239˚Arespectively,whichareconsistentwiththepre- ing AFM interactions between the Eu2+ moments. The
vious report11. As shown in Fig. 1, along the [100] di- lowtemperaturemagneticsusceptibilityχ(T)isshownin
rection, the Eu atoms form one dimensional-like chains Fig. 2(b). Inthe case ofHka, χ(T)showsanAFM tran-
and are separatedby a Ni-As structure. In addition, the sition around 7.0 K with pronounced drop at lower tem-
powder XRD patterns for polycrystalline EuNi As , as peratures corresponding to another transition at 6.2 K.
5 3
showninFig.1(b), canbewellfitted usingsimilarstruc- WhenH isappliedperpendiculartotheaaxis,χ(T)also
tural parametersto those derivedfrom the single crystal showstransitionsat7.0Kand6.3K,whileatlowertem-
XRD refinement. peratures χ(T) remains almost constant. A decrease at
3
2.5 (a) 250 (a)2 K)6
0.1 T HH / /ZaaFC mu)40 K)200 J/mol 4 TN2 TN1
)ol2.0 ol/e30 mol 150 C/T (2
Oe m1.5 Oe m20 C (J/100 00 T 5(K) 10
mu/ 1/ (10 50 EuNi5As3
( e1.0 SrNi5As3
0 0
0 100 200 300
0.5 T (K) 0 20 40 60 80 100
T (K)
6 1.2
(b)
0.0
0 100 200 300 1.0
2 )
T (K) K
ol 4 0.8 n8
2.5 (b) H//a ZFC m RI
HH//aa Z FFCC TN1 T (J/ 0.6 S/
/e
2.0 H a FC C2 0.4
)ol Polycrystal TN2
m
Oe 1.5 0.2
u/ 0 0.0
m 0 5 10 15 20
( e1.0 T (K)
0.5 FIG. 3. (Color online) (a) Temperature dependence of the
specific heat C(T) for single crystalline EuNi5As3 and the
isostructural compound SrNi5As3 in zero field. The inset
0.0
2 4 6 8 10 showsthelowtemperatureregiondisplayingtwoAFMtransi-
tions. (b) Magnetic specific heat Ce(T)/T of EuNi5As3 after
T (K)
subtracting the lattice contribution. The magnetic entropy
∆S(T) is also shown. The dashed line marks the position
FIG. 2. (Color online) (a) Magnetic susceptibility as a func- where theentropy is Rln8.
tion of temperatureof single crystals of EuNi5As3 with zero-
field cooling. The inset shows 1/χ against temperature and
the corresponding Curie-Weiss fitting. (b) Low temperature
transitions at T = 7.2 K and T = 6.4 K, con-
magnetic susceptibility for single crystals with zero-field and N1 N2
field cooling, along with measurements of the polycrystalline sistent with the values from χ(T). At low temperatures,
sample. C(T)/T also shows a pronounced plateau around 2 K.
This feature may be due to the Zeeman splitting of the
8S multipletoftheEu2+ ionsintheinternalmagnetic
7/2
the transitions when Hka and almost constant behavior field, similar to many magnetic Eu and Gd based com-
when H ⊥a indicates that the a-axiscorrespondsto the pounds such as, EuB6, EuCu2As2 and Gd2Fe3Si512–14.
easy direction. The lack of hysteresis between zero-field The low temperature specific heat of non-magnetic
cooling(ZFC)andfield-cooling(FC)measurementsindi- SrNi As was fitted (not shown) using C/T = γ +βT2.
5 3
catesthatthesetransitionscorrespondtoAFMordering. The derived electronic specific coefficient γ of SrNi As
5 3
The magnetic susceptibility χ(T) of the polycrystalline is 16.1 mJ/mol K2, while the Debye temperature
samplealsoshowstwosimilartransitions,supportingthe θ = 338.4Kwascalculatedusingθ = p3 12π4nR/5β,
D D
single crystal results. where β = 0.451 µJ/mol K4, n = 9 is the number of
The main panel of Fig. 3 shows C(T) in the tem- atoms per formula unit and R = 8.314 J/mol K. The
perature range 0.4 K - 100 K for EuNi As and the magnetic contribution to the specific heat of EuNi As
5 3 5 3
non-magneticisostructuralcompoundSrNi As forcom- (C /T)isshowninFig.3(b),whichisobtainedfromsub-
5 3 e
parison. At high temperatures, C(T) for both com- tracting the phonon contribution estimated from fitting
pounds overlap, indicating that the lattice contribution the data of SrNi As . The entropy ∆S(T) is also shown
5 3
of EuNi As can be taken to be the same as that of in Fig. 3 (b), where ∆S(T) continuously increases but
5 3
SrNi As . Itcanbeseenintheinsetthattwosharppeaks showstwoanomaliesaroundthetwotransitions,indicat-
5 3
areclearlyobserved,whichcanbeattributedtotwoAFM ingasecond-ordernatureofthe AFMtransitionsatT
N1
4
60
(a)m) TN2 0.1T
50 c6.0 (a) EuNi5As3 0.4T
4 0.45T
m) (5.5 TN1 ol) H//a 0.48T
c 40 m 0.49T
e 3 0.5T
5 6 7 8 9 10 O 0.6T
( 30 T (K) u/ 0.7T
m 2 0.8T
e 0.83T
20 ( 0.85T
1 0.9T
1.2T
10 1.5T
0
(b)
0 H a
0 (b) ol)1.6 0.1T
m 0.5T
e 1.4 1T
-2 O 1.5T
u/ 2T
)K -4 em1.2 22..35TT
V/ ( 3T
(S -6 1.0 4T
0.8
-8
2 3 4 5 6 7 8 9 10
T (K)
-10
0 100 200 300 FIG. 5. (Color online) Temperature dependence of χ(T) un-
der various applied magnetic fields with (a) Hk a, and (b)
T (K)
H⊥ a.
FIG. 4. (Color online) (a) Temperature dependence of the
electricalresistivityρ(T)forsinglecrystalsofEuNi5As3. The power is about −10 µV/K, and with decreasing temper-
inset shows ρ(T) in the vicinity of the magnetic transitions.
ature, S linearly increases before saturating below 20 K,
(b) Temperature dependenceof the thermoelectric power for
reaching −1 µV/K2 at 2 K. In the whole temperature
polycrystalline EuNi5As3 in zero applied field.
range, S remains negative, indicating the dominance of
electron-type carriers in EuNi As .
5 3
and T . The full magnetic entropy Rln8 expected for
N2
divalent Eu is recovered around 15 K, above the AFM C. Field dependence of the magnetic state
transition temperatures, possibly due to the formation
of short range magnetic order or magnetic fluctuations,
The temperature dependence of χ(T) under various
whichmaybeseenfromthe upturnofC /T below16K.
e applied magnetic fields is shown in Fig. 5. Upon apply-
The temperature dependence of the electrical resistiv- ing magnetic fields, the two magnetic phase transitions
ity ρ(T) down to 2 K for single crystals of EuNi As T and T are suppressed to lower temperatures. For
5 3 N1 N2
is shown in Fig. 4(a) with the current along the a-axis. H k a, T becomes broader and weaker at high fields
N2
At high temperatures, ρ(T) decreases with decreasing and is suppressed considerably more rapidly than T .
N1
temperature, indicating metallic behavior. The resid- When 0.5 T is applied the transition corresponding to
ual resistivity at 7 K is ρ = 6.01 µΩ cm with a resid- T is no longerobservedand insteadof a dropinχ(T),
0 N2
ual resistivity ratio of RRR ≈ 8.8. Upon further de- there is an upturn. This change of behavior may cor-
creasing the temperature, there is a clear drop in ρ(T) respond to the emergence of a new magnetic phase and
of EuNi As at 6.7 K due to a magnetic transition, as when0.6Tisapplied,twotransitionscanclearlybeseen.
5 3
seeninthemagneticsusceptibilityχ(T)andspecificheat Upon further increasing the field, these two transitions
C(T). While only a single transition can be clearly re- aregraduallysuppressedtolowertemperaturebeforedis-
solvedinρ(T),whichislikelyT ,thereis alsoaweaker appearing at around 0.9 T. In the case of H ⊥ a, both
N2
anomaly at a slightly higher temperature of around 7 K T and T are continuously suppressed and eventu-
N1 N2
which may correspond to T . Figure. 4(b) shows the ally disappear near 3 T. Therefore the magnetic order
N1
temperature dependence of the thermoelectric power S is suppressed significantly more rapidly for H k a and
for polycrystalline EuNi As in the temperature range this anisotropy is likely due to the a axis being the easy
5 3
2 K-300 K. At room temperature, the thermoelectric direction.
5
8 8 6
(a) (b)
2 K 2 K EuNi5As3
M (/f.u.)B46 46 IDneccrereaassiningg f ifeieldld H//a () /f.u.MB46 H a dM/dH 2 )K4 012TTT
2 02 B2 B1 2 J/mol 34TT
0 0.0 0. 5 1.0 0 (T 6T
0 1 2 3 4 5 0 1 2 3 4 5 C/ H a
0H (T) 0H (T) 2
8 8
(c) (d)
2K 4K
6K 8K
/f.u.)B6 )/f.u.B6 112050KKK
M (4 ( M4 00 2 4 6 8 10 12
2K 4K T (K)
2 6K 8K 2
H a
10K 15K
H//a 20K
0 0 FIG. 7. (Color online) C/T as a function of temperature
0 1 2 3 4 5 0 1 2 3 4 5 downto0.4Kundervariousmagneticfieldswhichareapplied
0H (T) 0H (T) perpendicular tothe a axis.
FIG. 6. (Color online) (a) Field dependence of the magneti-
zationM(H)withH kaat2K.Theinsetshowstheenlarged
sis between the FW and FC measurements. Figures 6
low field region measured upon both increasing and decreas-
(c) and (d) show the isothermal magnetization measure-
ing the field. The arrows indicate the metamagnetic tran-
ments at various temperatures for two different field ori-
sitions (b) M(H) and the corresponding derivative dM/dH
entations. With increasing temperature, all of the meta-
with H⊥a at 2 K. M(H) at various temperatures is shown
magnetic transitions are shifted to lower magnetic fields
for (c) H ka and (d) H⊥a.
and broaden upon approaching T . At high tempera-
N2
tures above T , M(H) displays an S-shape. Further-
N1
more, the saturated magnetic moments are 7.38µ and
Figure. 6 (a) shows the field dependence of M(H) at B
7.29µ for Hka and H⊥a, respectively, veryclose to the
2 K when H is parallel to the a axis. Below 0.44 T, B
expectedEu2+moment,furtherindicatingthattheAFM
M(H) increaseslinearly with magnetic field and there is
state arises from the ordering of the Eu2+ moments. At
no hysteresis, which is consistent with an AFM ground
15KwithH ka,M(H)issignificantlyreducedcompared
state in EuNi As . Upon further increasing the field,
5 3
to 10 K with a less pronounced S-shape, while there is
M(H) undergoes two sharp jumps at B = 0.47 T and
2
a much smaller change between 15 K and 20 K. This is
B = 0.81Trespectively,consistentwiththepresenceof
1
consistent with the emergence of magnetic fluctuations
two metamagnetic transitions. Hysteresis between field-
or short range order upon approaching T from higher
warming (FW) and field-cooling (FC) can be clearly ob- N1
temperatures, as suggested from specific heat measure-
served, indicating the first-order nature of these transi-
ments.
tions. Howeveruponincreasingthetemperaturetowards
T , the hysteresis becomes less pronounced , which in- Specific heat measurements under applied magnetic
N2
dicates that the transitions become more weakly first- fields with H ⊥ a are shown in Fig. 7. Upon increasing
order with increasing temperature. From comparisons the applied field, the anomalies at TN1 and TN2 become
with χ(T)in field, it canbe seen that B corresponds to less pronounced and are gradually suppressed to lower
2
the lower transition observed below 0.5 T and therefore temperatures,withnotransitionbeingobserveddownto
this suggests that at B the magnetic state which ap- 0.4 K at 3 T. In addition, the low temperature plateau
2
pears below T is suppressed and there is a transition likely due to the Zeeman splitting of the ground state
N2
to the state onsetting at T . Above the second tran- multiplet is shifted to higher temperatures with increas-
N1
sition at B , the magnetization appears saturated and ing field.
1
changes little with increasing field, indicating that this Figures 8 (a) and (b) show ρ(T) measured in various
corresponds to a transition from the antiferromagneti- fields with Hka and H⊥a respectively. For H⊥a, below
cally ordered phase to a spin polarized state. 1.1 T the AFM transition is slowly suppressed to lower
Figure. 6 (b) shows the field dependence of M(H) at temperature with increasing field before the transition
2 K when H is perpendicular to the a axis. At low fields broadens and is rapidly suppressed in higher fields. No
below2.4T,M(H)showssub-linearbehaviorbeforedis- transitions are observed down to 2 K at 3 T and the
playing two anomalies at 2.4 T and 2.8 T, clearly seen resistivity begins to show ∼ T2 behavior, as expected
as two peaks in dM/dH. Unlike the first order transi- for a Fermi liquid. In contrast for Hka, below 0.49 T
tions observed for H ka, there is no observable hystere- a sharp drop due to a magnetic transition is observed,
6
9
(a) (a) H//a 1.8K 3K
5K 6K
7
7K 8K
8 10K
)
m )m
c 6 c7
( (
H//a 6
5
0 T 0.1T 0.3T
0.49T 0.5T 0.55T
0.6T 0.7T 0.8T
0.9T 1.5T 2.5T 5
0 1 2 3 4
4
2 3 4 5 6 7 8 0H (T)
T (K) (b) H a 1.8K 3K
5.0 4K 5K
6K 7K
7 (b) 8K 10K
)m4.5
c
m)6 ( 4.0
c
( H a
3.5
5 0T 0.1T 0.3T
0.5T 0.7T 0.9T 0 1 2 3 4
1.1T 1.5T 2T
2.1T 2.3T 2.5T 0H (T)
3T
4
FIG. 9. (Color online) Magnetic field dependenceof ρ(H) at
2 3 4 5 6 7 8
different temperatures for (a) Hka and (b) H⊥a.
T (K)
FIG. 8. (Color online) Temperature dependence of electrical to the metamagnetic transition seen in M(H). With
resistivity at various magnetic fields applied (a) parallel to increasing temperature, this jump in ρ(H) decreases to
the a axis, and (b) perpendicular to the a axis. The curves lowerfields, reaching0.9 T at6 K. At 7 K near T , the
N1
are vertically shifted for clarity.
metamagnetic transition disappears and ρ(H) displays
a negative magnetoresistance. For Hka at 1.8 K, ρ(H)
displays a peak at around 0.9 T. Both transitions can
alongwithaweakeranomalyataslightlyhighertemper- be attributed to the field-induced metamagnetic transi-
ature, which becomes more pronounced with increasing tions seen in the magnetization measurements. Similar
field. In the vicinity of 0.5 T, two transitions can still to the H⊥a case,the mainpeak is shifted to lowerfields
be observed but there is now a smaller anomaly at the with increasing temperature. In the paramagnetic state,
lower transition. At higher fields only one transition is aclearnegativemagnetoresistanceisobservedduetothe
clearly seen, which is suppressed to below 2 K upon the reduction of spin disorder scattering as a result of the
application of 0.9 T. From a comparison with zero field alignment of the spins along the applied magnetic field.
specific heat measurements and the magnetic suscepti-
bility,thestrongertransitionatlowfieldscorrespondsto
TN2, while the weaker anomaly at higher temperatures D. Eu valence
islikelyT . When0.5Tisapplied,thelowertransition
N1
agrees well with the possible new field-induced magnetic
The value of the effective Eu moment obtained from
phase suggestedto emerge in this field regionfrom mag-
fittingthemagneticsusceptibility,indicatesthelocalized
netic susceptibility measurements [Fig. 5(a)], after the
nature of the Eu in EuNi As . To further investigate
5 3
disappearance of T .
N2 the Euvalence ofEuNi As , we performedEuL edge
5 3 III
The magnetic field dependence of the electrical resis- PFY-XASmeasurements. InFig.10,EuL spectraare
III
tivity ρ(H) at different temperatures is shown in Fig. 9 shownatthreetemperatures,alongwiththespectrumof
(a)(Hka)andFig.9(b)(H⊥a). ForH⊥a,at1.8Kρ(H) EuCoO for comparison. The EuNi As measurements
3 5 3
has a pronouncedjump at2.4 T, whichalso corresponds allshowaprominentpeak ataround6974.7eV,whichis
7
Eu3+:EuCoO3 8 (a) (T): TN1 TN2 T*
units) 3 311E050u00KCKKoO3Eu2+:EuNi5As3 67 TN1 M((HT)):: TTNN11 TTNN22 T*
arb. ) 5 TN2
sity ( 2 T (K 4
n
e 3
nt
I 1
2
1
0 H//a
6960 6965 6970 6975 6980 6985 6990 0
0.0 0.2 0.4 0.6 0.8 1.0
Photon Energy (eV)
0H (T)
8
FIG. 10. (Color online) Eu LIII PFY-XAS spectra of (b) (T): TN1 TN2
EuNi5As3 at three different temperatures. The dotted line 7 TN1 M(H): TN1 TN2
showstheEureferencespectrumforEuCoO3(Eu3+)forcom- 6 C(T): TN1 TN2
parison. (T): TN2
5 TN2
)
K
ascribed to the 2p3/2 → 5d transition in Eu2+, agreeing T (4
verywellwiththepeakpositionaround6975eVobserved 3
indivalentEuF 16. IftherewereasignificantEu3+ com-
2
2
ponent, another peak is expected at higher energies, as
shownfor example by the spectrumof EuCoO315, where 1
thereisapeakaround6982.6eV.Sincesuchaprominent H a
0
peak is not clearly observed in EuNi5As3, these results 0.0 0.5 1.0 1.5 2.0 2.5 3.0
indicate that the Eu does not have significantly mixed
0H (T)
valence character, but that the valence is very close to
+2 at all temperatures down to at least 10 K. This sit-
uation is very similar to other divalent magnetically or- FIG.11. (Coloronline)Thetemperature-fieldphasediagram
dered Eu-based compounds, such as, EuT Sb (T=Fe, of EuNi5As3 for magnetic fields applied (a) parallel to the a
4 12
Ru, Os)16,17. axisand(b)perpendiculartotheaaxis. ThetransitionsTN1
and TN2 are represented by red filled and blue open symbols
respectively, while different symbols denote values obtained
by different techniques. T∗ denotes the possible antiferro-
IV. DISCUSSION AND SUMMARY magnetic transition induced upon applying magnetic fields
parallel to thea axis.
The T −H phase diagram constructed from measure-
ments ofthe electricalresistivity,magneticsusceptibility
andspecificheatofEuNi5As3isshowninFig.11forfields ThebehaviorofEuNi5As3atambientpressureappears
applied parallel and perpendicular to the a axis. The to be similar to other Eu based magnetically ordered
phase boundaries deduced from different measurements materials, with a stable Eu2+ configuration and a lack
are all consistent. As well as the two zero field magnetic of quantum criticality. This is strikingly different from
transitions at T and T , the possible new field in- many Ce and Yb-based Kondo systems, which are well
N1 N2
duced AFM phase above 0.5 T is denoted by T∗, which understood on the basis of the Doniach model1. These
occurs after the disappearance of T for H k a. The systems can often be continuously tuned to a quantum
N2
temperature evolution of the metamagnetic transitions critical point using pressure, doping or magnetic fields,
in M(H) is also shown in the phase diagram, obtained where pronouncednon-Fermiliquid behavioris observed
fromthe maximumofthe derivativeandthe closeagree- and there is generally a gradual change of the valence
ment with the χ(T) measurements indicate that these upon increasing the hybridization2–4.
correspond to the suppression of the two AFM phases. For Eu systems, the atomic size of the non-magnetic
In ρ(T) at zero field and with H⊥a, only the position trivalent Eu3+ is smaller than that of magnetic diva-
of the transition corresponding to T can be clearly lent Eu2+. Applying pressure may destabilize the mag-
N2
resolved from the derivative. For H k a the most pro- netic Eu2+ leading to an abrupt change from Eu2+ to
nounced transition below 0.5 T corresponds to T , but Eu3−δ18. There are several examples where upon in-
N2
T can still be resolvedonce a field is applied, up to its creasing the pressure, the magnetic phase suddenly dis-
N1
suppression at around 0.9 T. appears at a first-order transition P , above which the
c
8
system is non-magnetic with a mixed Eu valence. These are all absent at about 0.9 T, while for H⊥a, they are
features have often been seen in Eu intermetallics, such relativelymorerobustinfieldthanalongthechaindirec-
as EuNi (Si Ge ) 19 and EuRh Si 20,21. tion and are suppressed at about 3 T. Meanwhile both
2 x 1−x 2 2 2
The antiferromagnetic transition temperatures of magneticsusceptibility andPFY-XASmeasurementsin-
EuNi As are 7.2 K and 6.4 K, slightly smaller than dicate that the Eu are strongly divalent with an almost
5 3
AFM transition temperature of 7.5 K in the isostruc- temperature independent valence and there is a lack of
tural compound EuNi P , which has smaller lattice pa- evidence for heavy fermion behavior. To determine the
5 3
rametersandthereforecorrespondstoapositivechemical magnetic structure in the ordered phases, neutron scat-
pressure22,23. This indicates that the magnetic phase of tering measurements are desirable. Furthermore, it may
EuNi As is likely to be quite robust against pressure. be possible to tune EuNi As or EuNi P towards a va-
5 3 5 3 5 3
The antiferromagnetismin EuNi As should be deep in- lence transition by doping or hydrostatic pressure.
5 3
side the AFM region,farawayfromthe criticalline near
P . Therefore, tuning EuNi P with pressure may allow
c 5 3
for the system to either reach a mixed valence state, or
possibly even display heavy fermion behavior.
ACKNOWLEDGMENTS
To summarize, we have successfully synthesized single
crystalline and polycrystalline EuNi As and performed
5 3
a detailed investigation of its crystal structure, physi- We thank Z. Hu for valuable discussions. This
cal properties and Eu valence. From our measurements, work was supported by the National Natural Science
EuNi As is anAFM compoundwith T = 7.2K and Foundation of China (No. U1632275), National Key
5 3 N1
asubsequentAFMtransitionatT = 6.4K.TheAFM Research and Development Program of China (No.
N2
state is sensitive to an applied magnetic field and shows 2016YFA0300202),and the Science Challenge Projectof
an anisotropic response. For Hka , the AFM transitions China.
∗ [email protected] tureof a new europium nickelphosphidephase, EuNi5P3,
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