Single Crystal Growth of Multiferroic Double Perovskites: Yb₂CoMnO₆ and Lu₂CoMnO₆

Abstract

We report on the growth of multiferroic Yb₂CoMnO₆ and Lu₂CoMnO₆ single crystals which were synthesized by the flux method with Bi₂O₃. Yb₂CoMnO₆ and Lu₂CoMnO₆ crystallize in a double-perovskite structure with a monoclinic P2₁/n space group. Bulk magnetization measurements of both specimens revealed strong magnetic anisotropy and metamagnetic transitions. We observed a dielectric anomaly perpendicular to the c axis. The strongly coupled magnetic and dielectric states resulted in the variation of both the dielectric constant and the magnetization by applying magnetic fields, offering an efficient approach to accomplish intrinsically coupled functionality in multiferroics.

1. Introduction

A multiferroic is a material that simultaneously exhibits ferroelectricity and magnetism [1,2,3,4]. Strong interplay between the electric and magnetic order parameters in multiferroics provides opportunities for novel device applications, such as magnetoelectric data storage and sensors [5,6,7,8,9,10]. In particular, in type-II multiferroics, ferroelectricity originates from the lattice relaxation via exchange strictions in a particular spin order with broken spatial inversion symmetry, which naturally leads to strong controllability of the dielectric properties via external magnetic fields [11,12,13].

Recently, a new type-II multiferroic was discovered: a double-perovskite structure of Lu₂CoMnO₆ [14,15,16,17]. From the polycrystalline work, the ferroelectricity was predicted to be along the crystallographic c axis originating from the symmetric exchange striction of the up-up-down-down (↑↑↓↓) spin arrangement with alternating charge valences [15,17,18]. However, previous studies on single crystals revealed ferroelectricity perpendicular to the c axis, which can be explained by the net polarization induced by the uniform oxygen displacements perpendicular to the c axis on neighboring ↑↑↓↓ spin chains when the symmetric exchange striction is activated [16,17,18]. Lu₂CoMnO₆ belongs to the double-perovskite RE₂CoMnO₆ series (RE = La, …, Lu). These materials crystallize in a monoclinic perovskite structure (space group P2₁/n) with alternating Co²⁺ and Mn⁴⁺ ions in a corner-shared oxygen octahedra [19]. In these compounds, additional antiferromagnetic clusters can arise from another valence state of Co³⁺-Mn³⁺ and antisites of ionic disorders and/or antiphase boundaries leading to Co²⁺-Co²⁺ or Mn⁴⁺-Mn⁴⁺ pairs [20]. As the size of rare earth ions decreases, the magnetic transition temperature decreases from 204 K for La₂CoMnO₆ [21] to 48 K for Lu₂CoMnO₆ [22].

We successfully grew single crystals of multiferroic Yb₂CoMnO₆ and Lu₂CoMnO₆ using the flux method with Bi₂O₃ flux. X-ray diffraction (XRD) confirmed the double-perovskite structure with a monoclinic P2₁/n space group. The up-up-down-down (↑↑↓↓) spin order arose below T_c = 52 and 48 K, respectively, leading to a dielectric anomaly perpendicular to the c axis because of cooperative displacements of oxygen ions.

2. Experimental Methods

We synthesized single crystals of Yb₂CoMnO₆ (YCMO) and Lu₂CoMnO₆ (LCMO) by utilizing a conventional flux method with Bi₂O₃ flux in air [22]. Before the growth, the polycrystalline specimens were prepared by the solid-state reaction method. High purity powders of Yb₂O₃ (Lu₂O₃), MnO₂ and Co₃O₄ were mixed and ground in a mortar, followed by calcining at 1000 °C for 12 h in a box furnace. The calcined compound was finely re-ground and sintered at 1100 °C for 24 h. The same sintering procedure after regrinding was done at 1200 °C for 48 h. A mixture of pre-sintered YCMO (LCMO) polycrystalline powder and Bi₂O₃ flux with a ratio of 1:12 ratio was heated to 1300 °C in a Pt crucible, cooled slowly to 985 °C, and then cooled in a furnace after the power was turned off. The grown single crystals are shown in Figure 1.

The crystallographic structures of both crystals were confirmed by a powder X-ray diffractometer (Ultima IV, Rigaku, Tokyo, Japan) using Cu-K radiation at room temperature. The detailed characterization for the structures was performed by Rietveld refinement by applying the FullProf software to the measured data. The temperature and magnetic-field dependence of magnetization for the single crystals were measured at temperatures of T = 2–300 K under applied magnetic fields up to 9 T along (H//c) and perpendicular (H⊥c) to the c axis using a VSM technique in a Physical Properties Measurement System (PPMS, Quantum Design, San Diego, CA, USA). The temperature and magnetic-field dependence of the dielectric constant and the tangential loss under various magnetic fields were measured using a LCR meter (E4980, Agilent, Santa Clara, CA, USA).

3. Results

Figure 2 shows the XRD pattern for ground single crystals of YCMO and LCMO. Resulting from the Rietveld refinement [23], the crystal structure of YCMO (LCMO) was refined as a monoclinic double-perovskite structure (P2₁/n space group) with good agreement factors, χ² = 3.18 (4.89), R_p = 7.78 (9.04)%, R_wp = 6.62 (8.23)%, and R_exp = 3.71 (3.72)%. Figure 3 shows the crystallographic structure of YCMO viewed from the c and a axes, respectively. Co²⁺ and Mn⁴⁺ ions were alternately located in corner-shared octahedral environments.

More specific refinement results are summarized in

Table 1. Unit cell parameters, positional parameters, and reliability factors for YCMO and LCMO.

	Yb2CoMnO6	Lu2CoMnO6
Structure	Monoclinic	Monoclinic
Space group	P21/n	P21/n
a (Å)	5.19438 (6)	5.17567 (9)
b (Å)	5.56824 (7)	5.56266 (9)
c (Å)	7.44012 (9)	7.43049 (13)
β (deg.)	90.39966 (40)	90.43100 (45)
V (Å3)	215.1895	213.9214
Yb/ Lu (x, y, z)	0.51951 (1)	0.51905 (3)
	0.54826 (5)	0.57526 (1)
	0.25018 (2)	0.25111 (3)
Co (x, y, z)	(0.5, 0, 0)	(0.5, 0, 0)
Mn (x, y, z)	(0, 0.5, 0)	(0, 0.5, 0)
O1 (x, y, z)	0.38995 (7)	0.37392 (18)
	0.95622 (6)	0.95756 (11)
	0.26595 (11)	0.26913 (16)
O2 (x, y, z)	0.15383 (12)	0.16546 (17)
	0.18749 (12)	0.19740 (16)
	−0.05387 (7)	−0.06343 (10)
O3 (x, y, z)	0.28909 (16)	0.29886 (26)
	0.69364 (12)	0.67911 (24)
	−0.06026 (7)	−0.051403 (16)
Rp (%)	7.78	9.04
Rwp (%)	6.62	8.23
Rexp (%)	3.71	3.72
χ2	3.18	4.89

and

Table 2. Bond lengths for YCMO and LCMO.

Bond Length (Å)	Yb2CoMnO6	Lu2CoMnO6
Yb/Lu-O1	2.228 (4)	2.141 (10)
Yb/Lu-O2	2.167 (7)	2.181 (9)
Yb/Lu-O3	2.268 (7)	2.26 (13)
Co-O1	2.077 (8)	2.121 (12)
Co-O2	2.115 (7)	2.101 (9)
Co-O3	2.075 (8)	2.100 (13)
Mn-O1	1.845 (15)	1.846 (12)
Mn-O2	1.957 (7)	1.948 (9)
Mn-O3	1.905 (8)	1.882 (15)

, including the unit cell parameters, positional parameters, reliability factors, and bond lengths. La₂CoMnO₆ exhibited an almost pseudocubic structure with a = 5.495 Å, b = 5.492 Å, and c/$\sqrt{2}$ = 5.506 Å [22]. However, as the size of the rare earth ion decreases, the lattice constants a and c/$\sqrt{2}$ also decrease linearly while b slightly increases, which promotes the monoclinic distortion. The lattice constants for YCMO (LCMO), where the largest monoclinic distortion was developed, were a = 5.194 (5.176) Å, b = 5.568 (5.563) Å, and c = 7.440 (7.434) Å with β = 90.400 (90.431)°.

Figure 4a,b show the temperature dependence of the anisotropic magnetic susceptibility for YCMO and LCMO, respectively, measured upon warming in H = 0.01 T after zero-field cooling (ZFC) and upon cooling in the same H (FC) for H//c and H⊥c. The χ values for YCMO and LCMO exhibited pronounced peaks at T_c = 52 and 48 K, respectively, which may correspond to a reentrant spin-glass behavior [24,25]. The temperature at which the ZFC and FC curves start to separate depends on the crystallographic orientations. The χ values for two different orientations exhibited strong magnetic anisotropy, which indicates that the spins were mainly aligned along the c axis. A recent neutron diffraction study on polycrystalline LCMO suggested the up-up-down-down (↑↑↓↓) spin arrangement along the c axis as the mostly probable magnetic ground state, originating from the frustrated exchange couplings [15,26].

Figure 4c,d display the temperature dependence of the dielectric constant for YCMO and LCMO, respectively, measured perpendicular to the c axis in a zero-magnetic field. In both cases, the dielectric anomaly started from T_c, indicating a type-II multiferroic [7,14]. The broad peak in H = 0 T was observed below T_c with a dielectric loss of less than 0.025, which is rationally small. The variation of the dielectric constant, which is defined as the peak height normalized by the value at T_c, can be estimated as ~5% and 12% for YCMO and LCMO, respectively. On the other hand, the estimated variation has been reported as small as ~3% and 2% in polycrystalline YCMO and LCMO, respectively. This difference reflects the fairly porous characteristic of polycrystalline structures [27,28].

The anisotropic isothermal magnetization at 2 K for H//c and H⊥c was measured after ZFC up to 9 T, as shown in Figure 5. In YCMO, the magnetization at 9 T in H//c was ~7 μ_B per formula unit and was not saturated owing to the magnetic moments of Yb³⁺ ions in addition to the magnetization of 6 μ_B for the Co²⁺ (S = 3/2) and Mn⁴⁺ (S = 3/2) moments in a formula unit [27]. It showed a large magnetic hysteresis with abrupt jumps at ±1.4 and 4.1 T. In LCMO, the initial curve of magnetization exhibited a metamagnetic spin-state transition from ↑↑↓↓ to ↑↑↑↑ at 2 T [29]. As the magnetic field decreased from 9 T, the magnetization showed consecutive metamagnetic transitions at 0.3, −1.3 and −2.9 T, which implies evolution from the saturation to several spin states, ↑↑↓↓, ↑↓↓↓, and ↓↓↓↓ [30], similar to the Ising spin chain magnet of Ca₃CoMnO₆ [31].

Figure 6a,b display the derivative of isothermal magnetization, dM/dH, at 2 K and the field dependence of the dielectric constant at 2 K for YCMO. The dielectric constant exhibited sharp peaks at ±1.4 T and step-like features at ±4.1 T, consistent with the metamagnetic transitions shown as peaks of dM/dH. This simultaneous tunability demonstrated the manipulation of multiple order parameters in a type-II multiferroic. Figure 6c,d show the temperature dependence of the dielectric constants and the dielectric tangential losses, measured perpendicular to the c axis under various magnetic fields (H = 0, 1, 1.2, 1.4, 1.6, 1.8, 2 and 3 T) along the c axis for LCMO. Currently, the plausible explanation for the ferroelectricity can be given by the cooperative oxygen displacements perpendicular to the c axis on neighboring ↑↑↓↓ spin chains because of the symmetric exchange strictions [31]. In LCMO, upon increasing the magnetic fields along the c axis, the broad peak was gradually suppressed and completely disappeared at 3 T where the magnetization was saturated. The dielectric loss exhibited a shoulder around the peak position of the dielectric constant and another peak below 20 K. The most distinct change occurred between 1.4 and 2 T, in accordance with the precipitous increase in the isothermal magnetization, indicating the strong interconnection between the dielectric and magnetic properties. The reduction of the peak in the dielectric constant can be explained by the metamagnetic spin-state transition from ↑↑↓↓ to ↑↑↑↑ as the fully saturated magnetic moments resulted in the cancelation of oxygen-ion shifts [14].

4. Summary

The flux method was employed for the single-crystal growth of multiferroic Yb₂CoMnO₆ and Lu₂CoMnO₆ which crystallize in a double-perovskite structure with the monoclinic P2₁/n space group. The refinement of the XRD data determines the lattice constants with large octahedral distortion as a = 5.194 (5.176) Å, b = 5.568 (5.563) Å, and c = 7.440 (7.434) Å with β = 90.400 (90.431)° for Yb₂CoMnO₆ (Lu₂CoMnO₆). The temperature and magnetic field dependences of the magnetization for both specimens exhibited a highly anisotropic nature and metamagnetic transitions. The simultaneous emergence of magnetic order and the dielectric anomaly indicates that both compounds are type-II multiferroics.