The temperature and magnetic-field dependences of the sublattice magnetization in the hexagonal layertype compound Cr

{Cl}_{3}

(

T_{N} = 16.8 °

K) have been deduced from the

{}^{53}{Cr}

nuclear magnetic resonance (NMR) for

0.4 \leq T \leq 8.1 °

K and

0 \leq H \leq 10

kOe. The observed zero-field data can be accounted for over the whole temperature range by a renormalized spin-wave model based on isotropic ferromagnetic (

J_{T}

) intralayer and antiferromagnetic (

J_{L}

) interlayer exchange interactions in the presence of a weak effective anisotropy field (

H_{A}

). Appropriate renormalized spin-wave dispersion relations are given for the four-sublattice antiferromagnetic (weak-field) and two-sublattice ferromagnetic (strong-field) equilibrium spin configurations. The validity of the two-dimensional approximation to these states is examined in detail for

k_{B} T > 2 | J_{L} | z_{L} S

and

| J_{L} | ≪ J_{T}

. It is shown that under these conditions the sublattice magnetizations for

J_{L} < 0

and

J_{L} > 0

are identical. The three-dimensional zero-field spin-wave fit gives

\frac{J_{T}}{k_{B}} = 5.25 °

H_{A} (0) = 650

Oe and a 0°K, zero-field

{}^{53}{Cr}

frequency

ν (0) = 63.318

Mc/sec. Parallel magnetic susceptibilities calculated with these parameters in the range

0.4 < T \leq 8.1 °

K are in quantitative agreement with experimental values based on measured splittings of the

{}^{53}{Cr}

NMR in weak fields (

H \leq 100

Oe). The interlayer constant,

\frac{J_{L}}{k_{B}} = - 0.018 °

K, used in the spin-wave calculations was obtained from single-crystal bulk magnetization measurements (

χ_{⊥} = 9.9

emu/mole for

T \leq 4 °

K), corrected for demagnetizing effects. These measurements show that the net anisotropy in the ferromagnetic state (i.e.,

H \geq 1.68

kOe) is zero, presumably because of a cancellation of dipolar and single-ion contributions. The sublattice magnetization behavior in the ferromagnetic state appears to be strongly influenced by long-range dipolar interactions, as evidenced by significantly lower values of

\frac{M (T, H)}{M (0)}

for

M ∥ c

than for

M ⊥ c

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