On the Behavior of CoF2 in a High Magnetic Field Normal to the c Axis

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Abstract
A magnetization curve of the AF single crystal CoF2 (tetragonal, TN=37. 7°K) in the magnetic field applied perpendicular to the c axis along x ∥ [100], as measured earlier,1 displays an anomaly, consisting in the sudden drop of dm/dH at the field Hx=Hc∼160 kOe. The anomaly of this kind is anticipated in a great number of easy axis AF with a nonzero Dzyaloshinsky field HD and is connected with a rotation of sublattice moments around the field, which ceases at Hx=Hc= (2HAHE−HD2)/ (HA2+HD2)½. For CoF2 the experimental mx vs Hx curve lies close to the calculated one with HE=240 kOe, HA=75 kOe, HD=115 kOe. At the same field one of the AFMR frequencies turns to zero: ω10=ω0 (I−Hx2/Hc2)½, where ω0= γ(2HAHE−HD2)½, while immediately after Hc ω10 increases according to ω10=ω0(Hx/Hc−I)½. The g‐factor anisotropy and magnetoelastic interaction do not influence the vanishing of ω10 at Hc (changing only the expression for Hc) and so should not prevent observation of AFMR near Hc at microwave frequencies. The ``linear'' magnetostriction of a shear type2 (reversed piezomagnetic effect) for CoF2 in the field H̄ ∥ [100] must follow the law uyZ=λHx (I−Hx2/H2)½ and should vanish at Hx>Hc. The maximum value uyZ∼10−4 is expected at Hx=Hc/2½.

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