Successive field-induced transitions in BiFeO3 around room temperature

Abstract

The effects of high magnetic fields applied perpendicular to the spontaneous ferroelectric polarization on single crystals of BiFeO3 were investigated through magnetization, magnetostriction, and neutron diffraction measurements. The magnetostriction measurements revealed lattice distortion of 2 × 10 −5 , during the reorientation process of the cycloidal spin order by applied magnetic fields. Furthermore, anomalous changes in magnetostriction and electric polarization at a larger field demonstrate an intermediate phase between cycloidal and canted antiferromagnetic states, where a large magnetoelectric effect was observed. Neutron diffraction measurements clarified that incommensurate spin modulation along [110] Recently, multiferroic materials have been widely investigated due to their coupling between magnetic and ferroelectric ordering. BiFeO 3 is perhaps the most extensively studied multiferroic material as it possesses robust multiferroicity at room temperature as well as various possible applications [1][2][3][4][5][6][7][8][9][10]. The effects of magnetic fields on the coupled multiple degrees of freedom behind these phenomena are not fully understood.BiFeO 3 exhibits a cycloidal magnetic order below 640 K [11]. This state is known to exhibit marginal quadratic magnetoelectric (ME) effect at low magnetic fields [12]. High magnetic fields of ∼20 T stabilize the canted antiferromagnetic (CAFM) phase as opposed to the cycloidal phase [13][14][15][16][17]. Although several groups succeeded in realizing the CAFM phase at zero field [18][19][20][21][22][23], the ME effect in this phase has not been clarified. In this study, several features were observed when a magnetic field was applied normal to the trigonal c-axis, and they indicated that a third magnetic phase emerged in bulk BiFeO 3 between the cycloidal and CAFM phases at approximately room temperature.BiFeO 3 has a crystal structure with the polar space group of R3c. A large switchable spontaneous electric polarization emerges along the c-axis of the trigonal cell [24][25][26]. Degeneracy exists in selecting the polarization direction from eight 111 directions in the cubic unit. Hence, depending on synthesis methods, BiFeO 3 crystals can contain multiple ferroelectric domains [27].Magnetic domains can be present even in single ferroelectric domain crystals. The magnetic propagation vector Q points in one of the 110 directions of the trigonal cell [11] in the cycloidal spin ordered state below ∼640 K. The three-fold rotational symmetry around the c-axis (Z direction in this paper) leads to three equivalent Q i (i = 1, 2, 3), as shown in Fig. 1(d). The spins rotated primarily in the Q i -Z plane in a magnetic domain with a given Q i .Recently, Tokunaga et al. indicated the emergence of an electric polarization perpendicular to the Z direction that was controlled by magnetic fields [17]. Theoretical calculations suggested that the cycloidal spin order in BiFeO 3 could involve electric polarization perpendicular to the Q i -Z plane as illustrated as P T in...

Tensile deformations of the magnetic chiral soliton lattice probed by Lorentz transmission electron microscopy

“…This approach has been realized in a wide range of systems, including the switching dynamics of single-domain nanoparticles by stress [5], by ultrafast acoustic pulses [6], and by tensile strain induced from a piezoelectric substrate [7][8][9]. Strain engineering techniques have been successfully employed to manipulate magnetic moments in perovskite-based multiferroics [10][11][12][13][14][15], multiferroic thin-film heterostructures [16], and in nanowires [17]. A change in the magnetic anisotropy governed by strain is clearly demonstrated in ferromagnetic-ferroelectric [18,19] and ferrite-ferroelectric heterostructures [20] due to structural or metal-insulator [21] phase transformations in the material.…”

The Magnetoelastic Distortion of Multiferroic BiFeO$_3$ in the Canted Antiferromagnetic State

“…Since |D 2 | K Z , the spins lie primarily in the XY plane with S iZ ≈ 0. The spin canting induced by the DM interaction and by magnetic fields [15] up to about 35 T is less than 2 • . Consequently, the zero-order spin state is S 1 ≈ −S 2 and (S 1 − S 2 ) ⊥ B.…”

“…Other modes do not exhibit clear changes when entering this intermediate state. Theoretical studies [69] and neutron diffraction spectroscopy [15,70] reveal that the intermediate state in magnetic field B ⊥ Z is a conical spin structure with ordering vector along the magnetic field. While earlier measurements suggested that it disappears at low T [15], our data indicate that the intermediate state exists even at low T when B Z.…”

“…Increasing epitaxial strain transforms the structure of thin films from rhombohedral to "tetragonal-like" monoclinic [10][11][12]. Recent work on thin films [13,14] reveals that epitaxial strain can rotate the AF vector S 1 − S 2 with respect to the electric polarization P. Despite great interest in controlling its magnetic properties, comparatively little is known about the effects of magnetoelastic strain on bulk BiFeO 3 [15].…”

“…Although the crystal structure of bulk ferroelectric BiFeO 3 was first assigned to the rhombohedral space group R3c [28,[38][39][40][41], high-resolution structural studies suggested that the crystal symmetry might be monoclinic Cc [42] or even lower, triclinic P 1 [43]. Additional evidence for broken symmetry comes from magnetostriction measurements: as the cycloid unwinds in a magnetic field, the contraction or expansion of the lattice depends on the direction of the applied field [15]. Based on a study of the THz absorption spectra in the high-field canted AF phase, this paper shows that magnetoelastic coupling transforms the crystal structure of BiFeO 3 from rhobomhedral to monoclinic.…”