Never heard of it. If such a term ever existed, it
must have long-since been superseded by something
else. I suggest not worrying about it.
The lambda transition is the superfluid transition,
so named because of the lambda-shaped spike in the
specific heat.
By extension, on a phase diagram the boundary of
the superfluid phase is called the lambda line,
and if restricted to a lower-dimensional subspace
(e.g. constant pressure) it reduces to a lambda
point.
How does one magnetical cool below 1K?
Using a spin system.
The quantity
magnetization times applied magnetic field
enters the thermodynamics exactly the same way as
pressure times volume
would.
The basic story goes something like this. Imagine
that the spin system is coupled weakly but not too
weakly to the ordinary translational degrees of
freedom of the lattice. When you ramp up the
magnetic field, the spins have constant entropy
over the short term. But their magnetic energy
increased. That means their temperature went up.
Over a longer timescale, they equilibrate with
the lattice and you suck the entropy away using
an ordinary refrigerator. (This is called the
precooling stage.) Now you ramp down the magnetic
field. By the mirror-image of the previous
argument, the spin temperature goes down, well
below the temperature achieved during precooling.
Over a somewhat longer timescale, the temperature
of the lattice will go down also.
If you want a crude but graphic analogy, consider
the spin-system as a *sponge* for entropy. If
you squeeze a sponge, you can remove the water
from it. If you then unsqueeze it, you can
use it to soak up water from somewhere else.
sponge = spin system
squeezing force = magnetic field
water = entropy
Spins are perhaps the best pedagogical example in
all of thermodynamics, because you can visualize
the entropy and quantify it from first principles.
You can also easily visualize how the spin variable
couples to the magnetic-field variable. It also
avoids numerous booby-traps associated with
over-relying on ideal gasses as "the" pedagogical
example (e.g. the misconception that all thermal
energy is kinetic).
Kittel & Kroemer _Thermal Physics_ is far and away
the most sensible thermo book I've ever seen.
Interestingly, it uses spins-in-a-field as the
introductory example (postponing ideal gasses
until much later).