Magnetic properties of exchange-enhanced Pauli paramagnetic metals AeCo2P2 (Ae = Sr, Ba)

We have synthesized single crystals of AeCo2P2 (Ae = Sr, Ba) and polycrystalline powder sample of Sr1−xBaxCo2P2, and measured their magnetic properties. SrCo2P2 shows an itinerant-electron metamagnetic transition at Hc = 600 kOe and its temperature dependent magnetic susceptibility shows characteristic two anomalies at Tmax1 = 25 and Tmax2 = 115 K, suggesting it is in the vicinity of a ferromagnetic quantum critical point. While, BaCo2P2 does not show the metamagnetic transition with the applied field up to 600 kOe, and its magnetic susceptibility shows only one anomaly corresponding to Tmax1 at 33 K. In addition, the Weiss temperature of Sr1−xBaxCo2P2, decreases to negative as x increases. These results indicate the system recede from the quantum critical point by the Ba substitution.


Introduction
Since the discovery of the iron pnictide superconductor LaFeAsO 1−x F x [1], layered transition metal pnictides and chalcogenides have been extensively studied.
These iron based superconductivities are classified by the structure as 1111-type with ZrCuSiAs type structure [1], 122-type with ThCr 2 Si 2 type structure [2], 111-type with anti-PbFCl structure [3], and so on, and they are based on transition metal layers in which the transition metals are coordinated by pnictogens and chalcogens.
The same structural compounds have been discovered in cobalt systems, e.g., ACo 2 X 2 (A = Alkali metals, Alkaline earth metals, and rare earth metals; X = P, As, S, Se) [4,5,6,7,8,9,10,11,12], LaCoP nO (P n = P and As) [13,14], and Sr 2 ScO 3 CoP n [15,16]. In contrast to the iron system with antiferromagnetic interaction, these cobalt compounds have ferromagnetic interaction in their CoP n layer, thus, they have been investigated as candidates of quasitwo dimensional itinerant-electron ferromagnets. For example, two dimensional ferromagnetic behavior in LaCoP nO (P n = P and As) is observed by magnetization and NMR measurements [13,14], and highly magnetic anisotropic behavior is observed in single crystals of ACo 2 Se 2 [11]. Furthermore, an itinerant-electron metamagnetic transition is discovered in SrCo 2 P 2 , suggesting it is in the vicinity of ferromagnetic quantum critical point [17]. In the Ca substituted system Sr 1−x Ca x Co 2 P 2 , the metamagnetic transition field decreases as x increases to 0. 5 antiferromagnetic ground state appears in the region of x > 0.6 [17]. In this system, magnetic properties are closely related to two types of crystal structure. One is the collapsed tetragonal (cT) structure with making interlayer P-P bond, and the other is the uncollapsed tetragonal (ucT) one. Sr 1−x Ca x Co 2 P 2 (0 ≤ x ≤ 0.5) with the ucT structure does not show any magnetic orderings and Sr 1−x Ca x Co 2 P 2 (0.6 ≤ x ≤ 1) with cT structure shows antiferromagnetic ordering [17,18]. In this paper, taking up the magnetic properties of Sr 1−x Ba x Co 2 P 2 as a counterpart of Sr 1−x Ca x Co 2 P 2 , we synthesized single crystals of AeCo 2 P 2 (Ae = Sr, Ba) and polycrystalline samples of Sr 1−x Ba x Co 2 P 2 with the ucT structure, and measured their magnetic susceptibility and high magnetization process. We will show that temperature dependent susceptibility and magnetization curves of BaCo 2 P 2 show an anomaly which is not the itinerantelectron metamagnetic transition. and that magnetic interactions of Sr 1−x Ba x Co 2 P 2 decreases by the Ba substitution.

Experimental method
Single crystalline and polycrystalline samples of Sr 1−x Ba x Co 2 P 2 were prepared from Sr(2N), Ba(2N), Co(3N) and P(red, 5N). The single crystals with x = 0 and 1 were obtained by a tin flux method [6]. Excess tin was removed by a dilute hydrochloric acid in the case of SrCo 2 P 2 . Because BaCo 2 P 2 is easily soluble in dilute acids, excess tin was removed by centrifugation after heating to 673 K. Polycrystalline samples were synthesized as shown in Ref [17]. These samples were characterized by powder x-ray diffraction (XRD) using Cu Kα radiation. Polycrystalline samples and single crystals were placed on a glass plate and measured in Bragg-Brentano geometry.
The temperature dependent magnetizations of Sr 1−x Ba x Co 2 P 2 were measured by a Quantum Design MPMS-XL system at the Research Center for Low Temperature and Materials Sciences, Kyoto University. Magnetization curves beyond 700 kOe were measured by using an induction method with a multilayer pulsed magnet at the ultrahigh magnetic field laboratory of the Institute for Solid State Physics, the University of Tokyo.

Results and discussions
Single crystals and polycrystalline sample of SrCo 2 P 2 were obtained as described in previous work [4] and characterized by using XRD patterns. In the case of BaCo 2 P 2 , plate-like tetragonal crystals were obtained. The inset of Fig. 1 shows the photograph of a typical crystal of BaCo 2 P 2 . Figure 1 shows XRD patterns of a single crystal and a polycrystalline sample of BaCo 2 P 2 . All diffraction peaks are attributed to the ThCr 2 Si 2 type structure with space group of I4/mmm. XRD patterns of BaCo 2 P 2 were refined by the Rietveld method, using a computer program rietan-fp [19]. The lattice constants a = 3.8040 (2) ▽: Néel temperature. The shaded region represents collapsed tetragonal (cT) region and the other does uncollapsed tetragonal (ucT) region. AF stands for the antiferromagnetic state.
Lattice parameters of Sr 1−x Ba x Co 2 P 2 (x = 0, 0.5) were determined from a refinement of the diffraction patterns by using the Le Bail method, using rietan-fp. These lattice parameters are shown in Fig. 2 with those of Sr 1−x Ca x Co 2 P 2 in Ref. [17]. As the ionic radius of alkaline earth cation Ae 2+ decreases in the order of Ba 2+ > Sr 2+ > Ca 2+ , the lattice parameter c corresponding to the inter-CoP-plane distance monotonically decreases. For the substituted system, the lattice parameter a is almost unchanged, and c decreases as the average radius of Sr 2+ and the substituted Ba 2+ or Ca 2+ decreases in the ucT region. In this region, the inter-CoP-plane distance is deemed to be controlled by the cation substitutions and BaCo 2 P 2 expects to have the weakest interlayer interaction in this system. The structural change from the ucT to cT structure occurs at the Ca content x = 0.5 with making of interlayer P-P bonds. Therefore, a increases rapidly and c decreases around x = 0.5, and magnetic ordering occurs in the cT region. Figure 3 shows the temperature dependence of magnetic susceptibility χ(T ) and its reciprocal of AeCo 2 P 2 (Ae = Sr, Ba). As shown in Fig. 3(a), χ(T ) of SrCo 2 P 2 shows double maxima at 25 and 115 K and Curie-Weiss-like temperature dependence. The susceptibility with H ∥ a is larger than that with H ∥ a, suggesting the easy axis is the c direction. The Weiss temperature θ CW and the effective Bohr magneton number p eff are −95. ? for H ∥ c from Curie-Weiss fit at between 200 to 300 K. In the case of itinerant magnetism with ferromagnetic interaction, the Curie-Weiss like temperature dependence originates in the temperature dependence of the amplitude of the local spin fluctuation, and the Weiss temperature is an indicator of the distance from the quantum critical point [20,21,22]. When the system approaches to the quantum critical point, the negative Weiss temperature increases to 0. Figure 3(b) shows χ(T ) of the polycrystalline powder sample and the single crystal of BaCo 2 P 2 . For both field directions, χ(T ) of single crystal shows maximum at 33 K and Curie-Weiss-like temperature dependence at high temperature region. An upturn at low temperature seems to be ascribed to a magnetic impurity. For the powder sample, χ(T ) shows larger upturn at low temperature and therefore does not show the maximum behavior. Thus, to estimate the intrinsic magnetic susceptibility, we calculated susceptibility as χ(T ) = ∆M (T )/∆H, where ∆M (T ) is difference between magnetization at 60 and 70 kOe, ∆H = 70 − 60 kOe. It is revealed that the calculated susceptibility also shows the maximum behavior, suggesting this maximum behavior is intrinsic in BaCo 2 P 2 . From Curie-Weiss fit at between 200 to 300 K, θ CW and p eff are −300 K and 1.67 for H ∥ a, and −400 K and 1.81 for H ∥ c. The Weiss temperature is smaller than that of SrCo 2 P 2 , suggesting the system has weaker ferromagnetic interaction and recede from the quantum critical point. Figure 4 shows χ(T ) and magnetization process at 4.2 K for Sr 1−x Ba x Co 2 P 2 . The maximum behavior was not observed at x = 0.1, 0.5 because of probably upturn at low temperature