Effects of Vanadium doping on BaFe2As2

TL;DR

This study explores how vanadium doping affects the structural, magnetic, and electronic properties of BaFe2As2, revealing an avoided quantum critical point and spin glass behavior without inducing superconductivity.

Contribution

It demonstrates that vanadium doping in BaFe2As2 leads to an avoided quantum critical point and spin glass state, extending previous findings from Co/Ni doping to non-superconducting samples.

Findings

Vanadium doping causes strong hole-doping effects.

Antiferromagnetic and structural transitions are suppressed with doping.

Spin glass behavior emerges near the critical doping level.

Abstract

We report an investigation of the structural, magnetic and electronic properties of Ba(Fe(1-x)V(x))2As2 using x-ray, transport, magnetic susceptibility and neutron scattering measurements. The vanadium substitutions in Fe sites are possible up to 40\%. Hall effect measurements indicate strong hole-doping effect through V doping, while no superconductivity is observed in all samples down to 2K. The antiferromagnetic and structural transition temperature of BaFe2As2 is gradually suppressed to finite temperature then vanishes at x=0.245 with the emergence of spin glass behavior, suggesting an avoided quantum critical point (QCP). Our results demonstrate that the avoided QCP and spin glass state which were previously reported in the superconducting phase of Co/Ni-doped BaFe2As2 can also be realized in non-superconducting Ba(Fe(1-x)V(x))2As2.

Tables

Table 1. Table 1: Chemical composition analysis of Ba(Fe 1-x V x ) 2 As 2 .

Sample type	Nominal x	EDS x
Single crystal	0.06	0.016
Single crystal	0.1	0.031
Single crystal	0.14	0.051
Single crystal	0.20	0.081
Single crystal	0.30	0.130
Single crystal	0.40	0.183
Polycrystalline	0.26	0.198
Polycrystalline	0.29	0.221
Polycrystalline	0.30	0.245
Polycrystalline	0.40	0.315
Polycrystalline	0.50	0.397

Full text

1 Introduction

In both cuprate and Fe-based high-Tc superconductors, investigation of the interplay between the magnetism and superconductivity through chemical doping is critical to explore the superconducting mechanism[1, 2]. In the iron-pnictides phase diagram, besides superconductivity and antiferromagnetic (AFM) phase, various exotic states including incommensurate short-range AFM order, G-type AFM order, C4 magnetic phase and spin glass state were observed which reveals the rich physics controlled by the interplay between the spin, charge and orbital degrees of freedom in this system[3, 4, 5, 6, 7, 8, 9, 10, 11].

For the well-known FeAs-122 parent compound BaFe2As2, either electron doping (e.g., Co, Ni, Cu, Rh, Ir, Pt, Pd)[12, 13, 14, 15] or isoelectronic doping (e.g., Ru, P)[16, 17] in the FeAs plane could easily instigate superconductivity. On the hole doping side, although alkali metal doping in the Ba site could induce superconductivity with transition temperature as high as 38 K[18], attempts of hole doping in the FeAs plane (e.g., Mn, Cr, Mo)[7, 5, 19] show that the stripe AFM and structural transitions are suppressed to some extent but superconductivity was never observed. This contrasting behavior has not been well understood. Furthermore, the discovery of a novel C4 magnetic phase only in the hole-doped 122 materials has drawn a lot of research interests[9]. Therefore more investigations are needed for the hole doping cases in the FeAs plane.

In this paper we report the effect of V-doping on BaFe2As2 in a bulk property and neutron/x-ray diffraction study. The hall effect measurements indicate that V substitution for Fe results in hole doping. Although superconductivity is not observed in Ba(Fe1-xVx)2As2 as in other in-plane hole-doping cases, an avoided Quantum critical point with the emergence of spin glass state are found in heavily doped samples.

2 Experiment

Single crystals of Ba(Fe1-xVx)2As2 were grown by FeAs/VAs self-flux method similar to Ba(Fe1-xCox)2As2[20]. Polycrystalline samples were synthesized by heating stoichiometric Barium, FeAs and VAs in an evacuated quartz tube at 1223 K for 30 h. The products were reground and annealed at the same temperature twice to ensure the phase purity. The elemental doping composition was measured by energy dispersive xray spectroscopy (EDS, Oxford X-Max 50). X-ray diffraction (XRD) patterns of powder and single crystals were collected from a Bruker D8 Advance x-ray diffractometer using Cu Kα radiation. Resistivity measurements were performed on a Quantum Design physical property measurement system (QD PPMS-14T). Magnetization measurement was carried out in Quantum Design MPMS3. Elastic neutron scattering measurements were performed using the HB1A triple-axis spectrometer at HFIR with incident energy Ei=14.65 meV and collimation of 40’-40’-40’-80’.

3 Results and discussion

Single crystals with centimeter or millimeter size are obtained through self-flux crystal growth method up to nominal doping x=40%. It is not possible to grow samples in single crystal form at a higher doping level. However polycrystalline samples of Ba(Fe1-xVx)2As2 with nominal doping up to x=50% can be synthesized with only a small amount of impurity phases (impurity phases including VAs are estimated to be lower than 5% from x-ray diffraction refinement). We characterized all samples with EDS and the results are listed in Table.1. Figure 1 plots the lattice parameters as a function of V concentration. The $c$-lattice parameter increases monotonically with increasing $x$ which is the typical feature in hole-doped iron-pnictides material[21]. Comparing with other hole-doping cases (Mn, Cr, Mo or K)[21, 22], Vanadium substitution effectively expands the $c$-axis, the parameter increases 4% at x=0.397. On the other hand, there is a slight contraction for the $a$-axis lattice parameter which decreases 0.5% at x=0.397.

Magnetization results for Ba(Fe1-xVx)2As2 (0$\leq$ x $\leq$0.183) single crystals are presented in Figure 2(a). The data were measured in zero-field-cooling (ZFC) with magnetic field of 1 T applied parallel to the ab-plane and normalized by the susceptibility at 300 K. For BaFe2As2, the susceptibility decreases linearly with decreasing temperature, then drops abruptly below the AFM transition with TN$\approx$133 K. With V-doping, the susceptibility increases with decreasing temperature and approaches a Curie-Weiss-like behavior. The AFM transition remains sharp in all doped single crystals. Figure 2(b) shows both the ZFC and FC magnetization results for polycrystalline samples at higher doping levels (0.198$\leq$ x $\leq$0.397). For x=0.198 and x=0.221, the weak AFM transitions in the magnetic susceptibility is evident from the dM/dT curves with TN$\approx$43 K and 37 K respectively (inset of Figure 2(b)). For x=0.245, No sign of magnetic transition can be inferred from the M-T curve, while the separation between ZFC and FC susceptibilities at low temperatures indicates a spin glass behavior (SG). Such behavior becomes even more apparent for x=0.315 and x=0.397 samples. Furthermore, for x=0.315 and x=0.397 there are new AFM-like kinks (marked as T*) in the M-T curves at around 20 K, indicating possible new short-range AFM fluctuations. In addition, we calculated the paramagnetic moments from the Curie-Weiss fit of susceptibility data for heavily V-doped samples (x=0.315 and x=0.397). Assuming all the moment contribution is from V, the results show that the average moment brought by one V ion is about 2.9$\mu_{B}$, which is quite close to the theoretical moment value of one free V3+ ion.

Figure 3 presents the normalized electrical transport data $\rho/\rho_{300~{}K}$. The temperature dependent resistivity for BaFe2As2 shows a sharp drop anomaly at the AFM/structural transition, while for the V-doped samples the feature becomes a sharp upturn and gradually moves to lower temperatures with increasing $x$. The temperatures of resistivity anomalies determined from the minimum of d$\rho$/dT are consistent with the TN from the M-T curves. At x=0.198 and x=0.221, the weak features of resistivity anomalies are still visible from the d$\rho$/dT curves as shown in the inset of Figure 3(b). However from x$\geq$0.245, such anomaly could not be detected anymore, indicating the absence of AFM/structural transition. Furthermore, the resistivity increases approximately logarithmically with decreasing temperature below 50 K as shown in the right inset of Figure 3(b), which is a characteristic of Kondo scattering. This behavior suggests that the Vanadium dopants act as magnetic impurities.

The Hall coefficients RH at different temperatures for Ba(Fe1-xVx)2As2 (0$\leq$ x $\leq$0.13) single crystals are presented in figure 4. RH changes suddenly from a negative value in the undoped sample to a positive one with slight V-doping at low temperatures, and becomes completely positive in all temperature region measured at x=0.051. Comparing to the Hall effect in Cr-doping[5], the sign-reversal of RH occurs at a much lower doping level indicates that V acts as an effective hole dopant. The doping dependence of RH at different temperatures is shown in the inset of Figure 4. After changing sign (x$>$0.04), The value of RH(T) initially increases then decreases at x=0.130. This feature is quite similar to the Hall effect in Ba0.6K0.4Fe2As2[23], which is explained as the result of asymmetric scattering between the electron and hole bands where the mobility is much larger for the former[20, 23].

Neutron diffraction experiments were performed on single crystals at x= 0.031, 0.051 and 0.130, the results are shown in figure 5(a)-(c). The AFM transition was characterized by measuring the temperature dependence of (0.5 0.5 1)T magnetic Bragg peak and the tetragonal to orthorhombic structural transition is identified by the intensity change of the (1 1 2)T nuclear peak (T refers to tetragonal basis)[24]. Combing the neutron scattering data and resistivity results of d$\rho$/dT, where only one transition anomaly is observed, both the structural and magnetic transitions occur at the same temperature for V-doped samples. This agrees with other hole-doped FeAs-122 cases (Cr, Mn, Mo and K)[7, 6, 19, 25], but different from the electron-doping cases in which TS and TN are well separated[26]. The doping dependence of the refined magnetic moment is presented in figure 5(d). For the electron-doping cases such as Co-doped BaFe2As2, the ordered moment is linearly suppressed with doping and finally becomes zero with 5% Co-doping[27]. For the hole-doping cases like Ba(Fe1-xCrx)2As2, the magnetic moment is independent of concentration for x$\leq$0.2 despite suppression of the transition temperature from 133 K to 56 K. Then the moment quickly deceases to about 0.3$\mu_{B}$ at x=0.335[6]. While for V-doping case, the moment evolution seems to be slightly different. The average ordered Fe/V moment first decreases in the range of 0$\leq$ x $\leq$0.051, then becomes almost doping independence from x=0.051 (0.62$\mu_{B}$) to x=0.13 (0.64$\mu_{B}$).

Based on the experimental results above, the $T$-$x$ phase diagram for V-doped BaFe2As2 is presented in figure 6. There is a gradual suppression of the AFM/structural transition temperature of BaFe2As2 from 133 K to 37 K with increasing $x$. The decreasing of AFM transition temperature actually follows a linear behavior with increasing $c$, see the inset of figure 6. Usually one would expect an doping-induced magnetic quantum critical point. However the AFM/structural transition suddenly vanishes at x=0.245 and $c$=13.54Å. On the other hand, a spin glass state is detected on the vanishing point of AFM order. These observations strongly suggest an avoided quantum critical point (QCP) by the development of competing states[28, 26]. Magnetic QCP should not be a neccessary condition for Fe-based superconductivity, it is interesting to mention that for Co- or Ni-doped BaFe2As2, near the optimal doping with maximum Tc, there were also reports about an avoided QCP[26] and clustered spin glass state[10, 11]. The spin glass in BaFe2-x(Co/Ni)xAs2 was previously interpreted as an intrinsic response to the competition between the superconductivity and antiferromagnetism[10], but similar explanation is not applicable for the SG in Ba(Fe1-xVx)2As2 since no superconductivity was observed for the V-doped samples. The most plausible explanation is that the magnetic impurity V in metallic hosts could result in the Ruderman-Kittel-Kasuya-Yosida (RKKY) effective exchange interaction between the impurity spins, which leads to a spin glass behavior[29, 30].

The AFM-like kinks (T*) observed in M-T curves at x=0.315, 0.397 closely resemble the behavior of checkerboard (G-type) AFM order in Cr- and Mn-doped BaFe2As2[6, 7]. According to calculations in Mn-doped BaFe2As2, the doped magnetic impurities would exhibit a G-type magnetic structure close to their cores in heavily doped region when the RKKY interactions between the conducting electrons and the magnetic impurities are taken into account[29]. Similarly, the T∗ in the V-doped samples most probably represents the formation of short-range local G-type AFM fluctuations. The Fe-based superconductivity may be accompanied by the spin fluctuations of the 3d electrons of Fe which are possibly suppressed by the competition of G-type spin fluctuations brought by V.

4 Conclusion

As a magnetic impurity, the doping of V into BaFe2As2 suppresses the AFM order and generate effective hole-doping effect, no superconductivity is observed which is in contrast with the K-doped BaFe2As2. This indicates the extreme sensitivity of in-plane magnetic impurity for Fe-based superconductivity. On the other hand, the RKKY spin glass behavior accompanied by possible checkerboard antiferromagnetic fluctuations emerges at higher doping levels. The phase diagram of Ba(Fe1-xVx)2As2 exhibits an avoided QCP similar as that in Ni-doped BaFe2As2.

Acknowledgements.

The work at RUC is supported by NSFC (No.11204373 and No.11227906).