A tape-type magnetic recording medium, includes a substrate; and a magnetic layer provided on the substrate. The magnetic recording medium is configured such that (1) (w.sub.maxw.sub.min)/w.sub.min400 [ppm] where w.sub.max and w.sub.min are respectively maximum and minimum of average values of width of the magnetic recording medium measured under four environments whose temperature and relative humidity are (10 C., 10%), (10 C., 80%), (29 C., 80%), and (45 C., 10%), respectively, and the magnetic layer has a squareness ratio of 65% or more in the vertical direction, and (2) 4.4T.sub.B/(T.sub.AT.sub.B) where T.sub.A is an average thickness of the magnetic recording medium and T.sub.B is an average thickness of the substrate.

Provided is a magnetic tape cartridge of a single reel type in which a magnetic tape is wound around a reel, in which the magnetic tape includes a non-magnetic support, and a magnetic layer including a ferromagnetic powder and a binding agent on the non-magnetic support, a tape thickness is equal to or smaller than 5.2 m, a tape width difference (BA) between a tape width A at a position of 10 m1 m from a tape outer end and a tape width B at a position of 50 m1 m from a tape inner end is 2.4 m to 12.0 m, and the tape width A and the tape width B are values measured 100 days from the date of magnetic tape cartridge manufacture.

Provided is a magnetic tape cartridge of a single reel type in which a magnetic tape is wound around a reel, in which the magnetic tape includes a non-magnetic support, and a magnetic layer including a ferromagnetic powder and a binding agent on the non-magnetic support, a tape thickness is equal to or smaller than 5.2 m, a tape width difference (BA) between a tape width A at a position of 10 m1 m from a tape outer end and a tape width B at a position of 50 m1 m from a tape inner end is 2.4 m to 12.0 m, and the tape width A and the tape width B are values measured 100 days from the date of magnetic tape cartridge manufacture.

A tape-type magnetic recording medium, includes a substrate; and a magnetic layer provided on the substrate. In a case where w.sub.max and w.sub.min are respectively maximum and minimum of average values of width of the magnetic recording medium measured under four environments whose temperature and relative humidity are (10 C., 10%), (10 C., 80%), (29 C., 80%), and (45 C., 10%), respectively, w.sub.max and w.sub.min satisfy a relation of (w.sub.maxw.sub.min)/w.sub.min400 [ppm]. The substrate contains polyester, and the magnetic layer has a squareness ratio of 65% or more in the vertical direction.

The magnetic recording medium includes a non-magnetic support; and a magnetic layer including a ferromagnetic powder and a binding agent on the non-magnetic support, in which a difference (S.sub.0.5S.sub.13.5) between a spacing S.sub.0.5 measured by optical interferometry regarding a surface of the magnetic layer under a pressing force of 0.5 atm after n-hexane cleaning and a spacing S.sub.13.5 measured by optical interferometry regarding the surface of the magnetic layer under a pressing force of 13.5 atm after n-hexane cleaning is equal to or smaller than 3.0 nm, and a magnetic recording and reproducing device including this magnetic recording medium.

A biaxially orientated polyester film has a P.sub.L/V.sub.L ratio of 0.3 to 1.2, P.sub.L and V.sub.L representing the average diameter of the convex portions and the average diameter of the concave portions, respectively, that are defined on the basis of a slice level with a height of 0 nm (reference plane) in a roughness curve determined for at least one surface using a three dimensional surface roughness meter, and the convex portions accounting for an area ratio of 30% to 51% of the reference plane. The biaxially oriented polyester film is excellent in travelling property, slitting property, and dimensional stability, and serves, when used for magnetic recording medium production, to provide a high density magnetic recording medium having a smooth magnetic layer, suffering little dimensional change due to variations in environmental parameters such as temperature and humidity or due to storage, and having good electromagnetic conversion characteristics with little dropout.

A magnetic recording medium includes a substrate, a seed layer, an under layer, and a perpendicular recording layer having a granular structure. (Ms.Math.??.sup.1.5(1?Rs).sup.0.33), Ms, and ? satisfy (Ms.Math.?.Math.?.sup.1.5(1?Rs).sup.0.33)?0.1 [?.Math.emu.Math.(mm).sup.?1.5], Ms?450 [emu/cc], and ??1.2. In the above formulas, Ms indicates a saturated magnetization amount, ? indicates the gradient of a M-H loop around a coercive force Hc, ? indicates the thickness of the perpendicular recording layer, and Rs indicates a squareness ratio.

Provided is a magnetic tape cartridge of a single reel type in which a magnetic tape is wound around a reel, in which the magnetic tape includes a non-magnetic support, and a magnetic layer including a ferromagnetic powder and a binding agent on the non-magnetic support, a tape thickness is equal to or smaller than 5.2 a tape width difference (BA) between a tape width A at a position of 10 m1 m from a tape outer end and a tape width B at a position of 50 m1 m from a tape inner end is 2.4 m to 12.0 m, and the tape width A and the tape width B are values measured 100 days from the date of magnetic tape cartridge manufacture.

Provided is a magnetic recording medium with a tape shape capable of correcting a change in width of the magnetic recording medium by adjusting a running tension of the magnetic recording medium in a case where the magnetic recording medium is saved or caused to run in a high-temperature environment. The magnetic recording medium is a magnetic recording medium with a tape shape, an amount of change in average width of the magnetic recording medium before and after the magnetic recording medium is left still for 40 hours in an environment at a temperature of 50? C. and a relative humidity of 40% RH in a state where a tension of 0.55 N is applied in a longitudinal direction is 170 ppm or less, and an average tension responsiveness with respect to a tension in the longitudinal direction in the environment at a temperature of 50? C. and a relative humidity of 40% RH is 700 ppm/N or more.

To provide a resin film, of which dimensional stability required of an ultra-high density recording medium can be controlled easily by drive tension, and which has processability at high temperature in a processing step of the resin film into a magnetic recording medium. A resin film having a Young's modulus in the film longitudinal direction of 1 GPa or more and a film thickness of 1 m or more, wherein the product of the Young's modulus in the longitudinal direction and the thickness is 5 GPa.Math.m or more and 20 GPa.Math.m or less and wherein a dimensional change in the film longitudinal direction is 2% or more and +2% or less when the film is heated at a rate of 5 C./min under a load of 2 kg/mm.sup.2 applied in the longitudinal direction and the temperature has reached 110 C., the resin film satisfying at least either of the following (1) or (2): (1) the Young's modulus in the film longitudinal direction is 6 GPa or less and the film thickness is 4.5 m or less; and (2) the Young's modulus in the film longitudinal direction is 4 GPa or less and the film thickness is 6 m or less.