Phase transitions in

{Mn}_{1 - x} {Zn}_{x} F_{2}

x = 0.125

, were investigated using thermal-expansion, magnetostriction, and ultrasonic-attenuation measurements. The phase diagram was determined for 4.2

K \leq T \leq T_{N}

and

0 \leq H \leq 140

kOe, where

T_{N} = 58.4

K is the Néel temperature. The magnetic field

\vec{H}

was parallel to [001]. The random field

H_{R}

induced by

H

influences the shape of the

λ

anomaly at the paramagnetic-antiferromagnetic transition, and depresses the transition temperature

T_{c}^{∥}

. Some evidence for an irreversible behavior near

T_{c}^{∥}

is found for

H \neq 0

. The paramagnetic—spin-flop boundary

T_{c}^{⊥} (H)

exhibits a large "bulge" towards high

T

. At 4.2 K, the spin-flop field

H_{sf}

is 78 kOe, which is lower than that predicted by mean-field theory. The discrepancy is explained by considering the effect of the random fields. As

T

increases from 4.2 K,

H_{sf}

first increases but then decreases as

T

approaches the bicritical temperature

T_{b} = 55

K. It is possible that the latter portion of the spin-flop line is actually one of two second-order lines which surround an intermediate phase. The Néel temperature is in reasonable agreement with mean-field theory.

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