⁵⁷ Fe NMR study of spin-modulated magnetic structure in BiFeO ₃

Abstract

Zero-field 57 Fe NMR spectrum has been observed in the 57 Fe-enriched ceramics of BiFeO3. The analysis of NMR lineshape confirms the existence in this ferroelectricantiferromagnetic compound of a long-period cycloidal magnetic structure and enables the first reconstruction of the actual spatial spin distribution along the cycloid which is found to be essentially anharmonic.

Long-wavelength magnetic and magnetoelectric excitations in the ferroelectric antiferromagnet BiFeO3

“…The ψ ±3 mode is separated from the ψ ±1 modes by 5.3 cm −1 in both polarizations h 1 and h 2 , which is compatible with the ∼ 7.5 − 8 cm −1 estimate from Raman experiments 23,24 . The ψ ±3 mode softens with elevated temperature following the same dependence as the more prominent ψ ±1 modes and becomes unobservable between 150 and 200 K, most likely due to the reduction in cycloid anharmonicity observed in nuclear magnetic resonance studies 26 . We now consider resonance 3 that happens at 18.4 cm −1 (Fig 1(a)) and disappears at approximately 150 K, close to the temperature at which a magnetic phase transition occurs, possibly associated with a spin reorientation.…”

“…At low temperatures (T ≤ 77 K), the cycloid was found to develop strong anharmonicity due to uniaxial magnetic anistropy with an easy c axis 14,26 . The anisotropy causes the spins to orient preferentially within a small angle with the c axis for most of the cycloid period, as was deduced from asymmetric shapes of nuclear magnetic resonance lines 26 . The bulk BFO crystal used in this work was grown using Bi 2 O 3 flux.…”

Phase transitions in multiferroic BiFeO3crystals, thin-layers, and ceramics: enduring potential for a single phase, room-temperature magnetoelectric ‘holy grail’

“…It is noteworthy to mention that before these measurements spatially modulated structures were only observed in magnetic metals: so, the observation of a spin cycloid in magnetic dielectric BiFeO 3 was rather surprising. Additional experimental evidence of a spin cycloid was provided by nuclear magnetic measurements (NMR) in BiFeO 3 ceramic doped with 57 Fe [26]. Instead of a single peak corresponding to a homogeneous structure, the NMR line was more complicated, exhibiting two maxima corresponding to the spin orientations perpendicular and parallel to [111] c (see Fig.3b).…”

“…The NMR lineshape was asymmetrical at low temperature (T=4.2 K) and became increasingly symmetric at higher temperatures. This behavior was explained in terms of anharmonic cycloid: over most of the cycloid's period, spins are at a small angle (θ) with respect to [111] c , as can be seen from the stronger intensity of the high frequency peak, and with increasing temperature the anharmonicity of the spin profile decreases resulting in nearly linear dependence of θ on coordinate along spin cycloid direction at room temperatures [26]. However in a recent report on a high resolution neutron powder diffraction study [27] it was stated that the character of the modulated cycloidal ordering of the Fe 3+ magnetic moments remains the same from 4 K up to the Neel temperature of 640 K. Thus the temperature behavior of the spin cycloid remains an intriguing question.…”

“…However in a recent report on a high resolution neutron powder diffraction study [27] it was stated that the character of the modulated cycloidal ordering of the Fe 3+ magnetic moments remains the same from 4 K up to the Neel temperature of 640 K. Thus the temperature behavior of the spin cycloid remains an intriguing question. [9]; (b) Nuclear magnetic resonance measurements [26].…”