Structural and magnetic properties of the layered perovskite system Sr_2−xPr_xCoO₄ (x = 0.7, 0.9, 1.1)

Highlights

• Sr_2-xPr_xCoO₄ (0.7≤ x ≤ 1.1) is a phase separated system.

• Below T_C, it contains FM phase within AFM and PM matrices.

• Below T_f, Sr_1.1Pr_0.9CoO₄ consists of a spin glass phase.

• With increasing x, the Jahn-Teller distortion weakens the FM interaction.

• Below T_C, IRM relaxes in an identical way for all three samples (0.7≤ x ≤ 1.1).

Abstract

We report the detailed structural and temperature dependent magnetic properties of well characterized, polycrystalline samples of Sr_2−xPr_xCoO₄ (x = 0.7, 0.9, 1.1), in the temperature range 5 K–300 K. In Sr_2−xPr_xCoO₄ (0.7 $\le x\le$ 1.1), short range or long range ferromagnetic (FM) order appears below $T_{\text{C}}$ $~$ 150 K. In general the low temperature magnetic state is a mixture of FM, paramagnetic (PM) and antiferromagnetic (AFM) matrices. The spin glass phase (SG) is observed in Sr_1.1Pr_0.9CoO₄ below $T_f$ = 15 K due to comparable strengths of FM and AFM interactions. With increasing x, the Jahn-Teller distortion weakens the FM interaction and AFM interaction starts dominating. Isothermal remanent magnetization (IRM) as a function of time (t) shows that the relaxation mechanism is associated with the interfaces between different types of magnetic matrices.

Introduction

In the layered oxide compounds with K₂NiF₄-type structure, the 2D confinement of O–Co–O network reduces the $e_g$ electron bandwidth. Such reduction in the $e_g$ electron bandwidth introduces strong electron correlations that can change the coupling between different microscopic degrees of freedom such as charge, spin and orbital. As a result we observe interesting electronic and magnetic properties in these materials [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]]. Bulk or single crystalline film of Sr₂CoO₄, a member of the K₂NiF₄-type 2D layered perovskite is a metallic ferromagnet below $T_{\text{C}}$ = 255 K [13,14]. The unit cell of Sr₂CoO₄ in the body-centered tetragonal lattice consists of tetragonally distorted CoO₆ octahedra at the corners and the body-center positions. Two dimensional O–Co–O network is separated by insulating double layers of SrO [14]. Both, the theoretical as well as the experimental studies on Sr₂CoO₄ suggest that the magnetism in this compound can be well described by the spin only moment ($t_{2g}^4{e}_g^1$, $S=\frac{3}{2}$) of the Co⁴⁺ ions in the intermediate-spin (IS) state. The strong Co⁴⁺ 3d-O-2p ($pd\sigma$) hybridization gives rise to FM metallicity in the system [[14], [15], [16]]. Recent studies show that Sr₂CoO₄ possesses half-metallic character [15,16]. The mBJ calculations within the density functional theory suggest an AFM contribution from the O ions [16].

Substitution of Sr²⁺ ions by rare-earth (R³⁺) ions has two major effects: (a) the ionic state of the Co⁺ – ions changes from Co⁴⁺ state for charge neutrality (b) the volume of the unit cell decreases as the size of any R³⁺ ion is smaller than Sr²⁺, accompanied by a change in the extent of distortion of the CoO₆ octahedron. In Sr_2−xR_xCoO₄, Co⁺ ions can be in one of the three states depending on x (Co⁴⁺, Co³⁺, Co²⁺). The energy-splitting of the d-state electrons of Co⁺ ions due to the crystalline electric field is comparable to that due to Hund's coupling [17]. This results into multiple spin states of a single ion. Practically, Co³⁺ ions can exist in either of the three spin states: Low Spin (LS), ($t_{2g}^6{e}_g^0$, S = 0), IS ($t_{2g}^5{e}_g^1$, S = 1) and High Spin (HS) ($t_{2g}^4{e}_g^2$, S = 2). Co⁴⁺ ions can be either in LS ($t_{2g}^5{e}_g^0$, S = 0.5) or in IS ($t_{2g}^4{e}_g^1$, S = 1.5), whereas Co²⁺ ions always prefer HS ($t_{2g}^5{e}_g^2$, S = 1.5) state [18,19]. The amount of octahedral distortion determines the spin state of the Co-ions belonging to particular ionic state [20]. Therefore, the interactions between magnetic ions are very complex. In fact, almost all sorts of magnetic orderings/phases: long range FM ordering [21,23], commensurate as well as incommensurate AFM ordering [23], short-range FM clusters [9,24], Griffith's phase [25,26], SG phase [20,25], have been predicted in this system. In order to explain the existence of such magnetic phases, the proposed interactions between different Co⁺ ions are also of diverse types e.g., direct exchange between Co⁴⁺ ions, FM double exchange between Co⁴⁺– Co³⁺ ions, AFM superexchange between Co³⁺- Co³⁺ and Co⁴⁺-Co⁴⁺ ions. In the past, a considerable amount of research work has been published on the study of magnetic properties of Sr_2−xR_xCoO₄ [4,9,17,20,[22], [23], [24], [25],[27], [28], [29], [30], [31], [32], [33]]. For x $>$ 1, Sr_2−xLa_xCoO₄ shows AFM ordering, incommensurate for lower values of x and commensurate for higher values of x [23]. LaSrCoO₄ with Co³⁺ ions, is a PM compound irrespective of the temperature. For $x<$ 1, in Sr_2−xLa_xCoO₄ with Co⁴⁺ and Co³⁺ ions, short range FM ordering is observed for comparatively higher values of x whereas long range FM ordering is favored for lower values of x [9]. Substitution of Y³⁺ for Sr²⁺ in Sr₂CoO₄ initially reduces the Curie temperature and finally destroys ferromagnetism for $x\ge$ 0.67 [4,[29], [30]]. Nd-doped Sr₂CoO₄ system, Sr_2−xNd_xCoO₄ (0.4 $\le x\le$ 0.75) shows absence of long range FM ordering [31]. However, a high temperature FM state with controversial PM ground state has been reported for Sr_1.05Nd_0.95CoO₄ [32]. For Sr_2−xPr_xCoO₄, the pure Co³⁺ compound SrPrCoO₄ is PM down to the lowest temperature [20]. The Curie-Weiss fit in the PM region suggests that the overall interaction between magnetic ions is AFM for x $<$ 1, whereas the same is FM for $x>$ 1 [20]. Thermomagnetic irreversibility as evident from the difference of the zero field cooled (ZFC) and field cooled (FC) magnetization has been observed even up to x = 1.5 [24]. The higher temperature Griffith's phase along with low temperature short-range FM phase have been predicted for x = 0.4 [28]. A. Hassen et al. claimed the existence of multiple magnetic phases in Sr_2−xPr_xCoO₄ (0.6 $\le x\le$ 0.8) namely, PM state, Griffith's phase, short range ordered FM state and spin glass phase with the lowering of temperature [25].

For the Sr_2−xPr_xCoO₄ system, the structural and magnetic properties are highly correlated and a convincing picture about the magnetic phase is still missing. Both types of interactions are present in any member of Sr_2−xPr_xCoO₄, only the dominance of one over the other depending on the value of x determines the magnetic state at a particular temperature. The situation is more controversial in the neighborhood of x = 1 where the FM and AFM interactions are comparable. We plan to study the detailed structural and magnetic properties of Sr_2−xPr_xCoO₄ (0.7 $\le x\le$ 1.1). This will help us to have a better understanding about the system.