A remarkable feature of technical magnetization of a nonuniformly strained iron borate (FeBO 3 ) crystal is longperiod spatial modulation of the magnetic order parameter for this weak ferromagnet observed in external magnetic field Н 1 ≤ Н ≤ Н 2 [1]. Investigations performed in [1] have demonstrated that the modulated magnetic structure (MMS) of nonuniformly strained FeBO 3 arises when the vector Н is oriented near a certain reference crystal direction of the easy magnetization (basal) crystal plane. During magnetization in this direction, rotation of the ferromagnetism vector m (or of the antiferromagnetism vector l ⊥ m by definition) in the basal plane occurs somewhat specifically: after the termination of displacements of the domain boundaries, the single domain structure is first formed in the crystal, and the local vectors m appear oriented along the easy magnetization direction (the uniform low-field phase); then at Н = Н 1 , the magnetic structure of the crystal changes from the uniform to spatially modulated structure stable for Н 1 ≤ Н ≤ Н 2 (the sinusoidal phase); then at Н = Н 2 , the crystal passes once again to the uniform magnetic state with m || Н (the uniform high-field phase). The sinusoidal phase has the form of a static transverse spin wave linearly polarized in the basal plane in which the azimuth of the local vector m oscillates about the average magnetization direction. Since the MMS is not observed in FeBO 3 without mechanical stresses, the specific feature of magnetization of the examined crystal established in [1] was attributed to the occurrence in its basal plane of additional spatially nonuniform (random) magnetic anisotropy caused by local deformations of the crystal lattice at mechanical stresses. According to models of crystal transition from the uniform to modulated magnetic state suggested in [1], the spatial orientation of the vector m (l) in the sinusoidal magnetic phase is determined by competition between the orienting action of the exchange forces on the local sublattice magnetic moments and the disorienting action of the nonuniform magnetic anisotropy forces.Experimental investigations performed in [1] demonstrated that the MMS was observed at temperatures Т < T s ≈ 140 K; in the examined temperature interval (90 ≤ Т ≤ 140 K), the spatial MMS period D remained virtually independent of Т, but significantly decreased with increasing Н. In this case, the direction along which modulation of the azimuth of the vector m (l) is observed is independent of Т and Н; the amplitude of deviation of the local vector m from the average crystal magnetization direction decreases with increasing Н and/or Т; the field strengths at which the MMS is observed ΔН = (Н 2 -Н 1 ) → 0 when Т → T s ; transitions low-field uniform phase ↔ sinusoidal phase ↔ highfield uniform phase are the first-order magnetic phase transitions. The present work analyzes the above-listed main properties and conditions of existence of the MMS in a nonuniformly strained FeBO 3 crystal using the model suggested in [1].
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