  We have investigated the magnetic ground state properties of the quantum spin
trimer compound strontium hydroxy copper chloride SrCu(OH)$_3$Cl using bulk
magnetization, specific heat measurements, nuclear magnetic resonance (NMR),
and electron spin resonance (ESR) spectroscopy. SrCu(OH)$_3$Cl consists of
layers with isolated Cu$^{2+}$ triangles and hence provides an opportunity to
understand the magnetic ground state of an isolated system of \textit{S} = 1/2
arranged on an equilateral triangle. Although magnetization measurements do not
exhibit a phase transition to a long-range ordered state down to \textit{T} = 2
K, they reveal the characteristic behavior of isolated trimers with an exchange
of $J = 154$~K. The Curie-Weiss behavior changes
  around 50--80~K, as is also seen in the NMR spin-lattice relaxation rate. In
zero magnetic field, our specific heat data establish a second-order phase
transition to an antiferromagnetic ground state below \textit{T}= 1.2 K. We
have drawn a magnetic field-temperature ($H$-$T$) phase diagram based on the
specific heat measurements. The ESR data show divergence of the linewidth at
lower temperatures, which precedes the phase transition to an antiferromagnetic
long-range ordered state with unconventional critical exponents. The
temperature variation of the $g$-factor further confirms the antiferromagnetic
phase transition and reflects the underlying magneto-crystalline anisotropy of
the compound.
