Apparatus having a magnetic turnaround roller to reduce magnetostrictive knockdown

An apparatus is disclosed having means for shunting demagnetization fields to prevent "magnetostrictive knockdown" of a signal recorded on an isotropic recording medium.

1. Field of the Invention 
The present invention relates in general to magnetic recording, and in 
particular to recording apparatus having means for reducing signal 
knockdown. 
2. Background of the Invention 
Advances in the art of magnetic recording have led to the development of a 
magnetic recording medium having both cubic crystalline as well as 
acicular shape anisotropy. Such a magnetic recording medium exhibits 
magnetic isotropy as to coercity and intensity of remanent magnetization, 
i.e. the magnetic medium exhibits coercivities that are substantially the 
same along three mutually perpendicular axes. An example of the 
abovedescribed isotropic magnetic recording medium is disclosed in U.S. 
Pat. No. 4,451,535. 
Isotropic magnetic recording media are especially useful when recording 
short wavelength signals using "microgap recording" (magnetic record gap 
12.mu."; see for example U.S. Pat. No. 4,302,790) where the vertical 
component of the magnetic flux vector, i.e. the vector perpendicular to 
the plane of the magnetic recording medium, provides significant 
contribution. Isotropic recording media are responsive to both the 
vertical, as well as the longitudinal, components of the magnetic field 
from the record head and therefore provide greater signal at high 
recording density. 
Problems with signal reduction, however, have been experienced with 
isotropic recording media in the form of magnetic tape. The main source of 
the signal reduction or knockdown is apparently due to a magnetostriction 
effect. The knockdown due to magnetostriction occurs when the magnetic 
particles of the isotropic tape are subjected to a uniaxial physical 
strain. When the magnetic tape is compressed in one direction, the 
coercivity along the axis of the compression increases while the 
coercivity in the two axes perpendicular to the compression decreases. 
This unbalanced change in the anisotropy fields allows easier switching of 
the magnetic particles of the isotropic tape. If demagnetization fields 
exist when the strain is applied, signal knockdown occurs. The magnetic 
particles of the isotropic magnetic tape are subjected to a uniaxial 
physical strain sufficient to cause "magnetostrictive knockdown" when the 
tape is wrapped around a member such as the turnaround roller or post of a 
tape transport mechanism. 
The problem then, which is the basis for the present invention, is to 
prevent magnetostrictive knockdown in an isotropic magnetic recording tape 
caused by passing the tape over a turnaround roller or post. 
SUMMARY OF THE INVENTION 
The present invention resides in the realization that if demagnetization 
fields are not present when a magnetic medium is subjected to a uniaxial 
physical strain, then the magnetic particles of the magnetic medium will 
not switch and demagnetization will be avoided. In a presently preferred 
embodiment, the invention provides a neat and simple means for reducing 
demagnetization fields by employing--as part of an appropriate tape 
handling apparatus--a magnetic turnaround roller(s) or post(s) that serve 
to shunt the deleterious demagnetization fields.

DETAILED DESCRIPTION 
Referring now to FIG. 1 of the drawings, a closed-loop magnetic tape 
transport mechanism is shown having idler rollers 10 and 12, a capstan 14, 
pressure rollers 16 and 18, and a turnaround roller 20. An erase head 22, 
a write head 24, and a read head 26 are located on the inside of a loop 
formed by an isotropic magnetic tape 28. 
The isotropic magnetic tape 28, as shown in FIG. 2, is composed of a 
support layer 30 that is coated with a magnetic recording layer 32 
containing magnetic particles 34 dispersed in a binder 36. The magnetic 
particles 34 typically have a maximum acicularity (length to diameter 
ratio) of about 2.5 giving the particles 34 a stubby "football" shape. The 
recording layer 32 exhibits magnetic isotropy as to coercivity and 
intensity of remanent magnetization. As previously mentioned, an example 
of an isotropic magnetic medium is disclosed in U.S. Pat. No. 4,451,535. 
When the magnetic tape 28 is wrapped around the turnaround roller 20, the 
magnetic layer is on the inside of the circle and hence is compressed 
resulting in a physical change in the shape of the magnetic particles 34. 
Maximum compression of the tape takes place at points 29 (see FIG. 1). 
The compression of the particles 34 causes the coercivity of the particles 
to increase along the axis of the compression (axis parallel to the length 
of the tape 28) while the coercivity along the two axes perpendicular to 
the compression (i.e. the axes in the plane of the magnetic tape 28) 
decreases. The unbalanced change in the three anisotrophy fields of the 
particle 34 causes switching in the presence of demagnetization fields, 
resulting in signal knockdown. 
This "magnetostrictive knockdown" is prevented by forming the turnaround 
roller 20 out of a magnetic material, for example permalloy, or some other 
material with a permeability of at least about 3, instead of the 
nonmagnetic materials typically used for the roller such as stainless 
steel. The magnetic turnaround roller 20 does not prevent the change in 
coercivity when the magnetic particles 34 are subjected to the uniaxial 
physical strain, but does prevent the particle 34 from switching by 
shunting the demagnetization fields through the roller 20, thus preventing 
signal knockdown. Once the particles 34 are past the pressure point 29, 
they return to their normal shape and the returning demagnetization fields 
cause no knockdown. 
The invention has been described with reference to certain preferred 
embodiments thereof, but it will be understood that variations and 
modifications can be effected within the spirit and scope of the 
invention. For example, the turnaround roller 20 could be constructed by 
coating a nonmagnetic material with a magnetic layer. In addition, the 
capstan, preferably, should also be magnetic. Indeed, if possible, all 
parts of the transport (or cassette) that touch the coated surface of the 
tape should be magnetic. The coercivity of the magnetic material should be 
small. The permeability should be high, although a permeability as low as 
3 will provide some benefit.