Unusual Critical Behavior in La1.2Sr1.8Mn2O7 Single Crystal

In this paper, we present a detailed analysis on the critical behavior of La
_1.2
Sr
_1.8
Mn
₂
O
₇
single crystal via isothermal magnetization measured at different temperatures around the paramagnetic–ferromagnetic phase transition at
$T_{C} = 85$
K. Using the Landau–Lifshitz coefficients, the Arrott plots (
$H/M = a(T) + b(T)M^{2}$
) of sample have been analyzed. It showed that a(T) changed from positive to negative values at different temperatures in the field ranges of
$H = 0$
–10, 10–30, and 30–50 kOe, indicating that the critical behavior could not be described with a single model under different applied fields. Through the modified Arrott plots method, the Kouvel–Fisher method, and the critical isotherm analysis, we determined the values of the critical exponents for La
_1.2
Sr
_1.8
Mn
₂
O
₇
around its magnetic phase transition over different magnetic field ranges. The critical exponent
$\beta $
value is found to be 0.501, 0.417, and 0.371 under field ranges of
$H = 0$
–10, 10–30, and 30–50 kOe, respectively. This means that the
$\beta $
value depends strongly on the strength of the applied field, shifting from the value approaching that of the mean field model (
$\beta = 0.5$
) to the 3-D-Heisenbeg model (
$\beta = 0.365$
). Meanwhile, its
$\gamma $
value is quite stable (
$\gamma =0.973$
–1.074), almost independent on the choice of field fitting range. In addition, using the reduced temperature
$\varepsilon = (T-T_{C}$
)/
$T_{C}$
and the obtained critical exponents, almost
$M(H, T$
) data measured near
$T_{C}$
obey the scaling equation
$M(H, \varepsilon) = \varepsilon ^{\boldsymbol {\beta }}f_{\pm }(H/\varepsilon ^{\boldsymbol {\beta +\gamma }}$
), where
$f_{+}$
and
$f_{-}$
are regular analytic functions corresponding to data at
$T > T_{C}$
and
$T < T_{C}$
, respectively.