E-iron oxide powder, composition including the same, magnetic recording medium, and magnetic recording and reproducing device

Abstract

The ε-iron oxide powder has an average particle size in a range of 5.0 to 16.0 nm and an uneven distribution of an M atom in a surface layer portion, in which the M atom is one or more kinds of atoms selected from the group consisting of an aluminum atom and an yttrium atom, and a content of the M atom with respect to 100 atom % of iron atoms is in a range of 4.0 to 9.5 atom %.

Claims

1. An ε-iron oxide powder having an average particle size in a range of 5.0 to 16.0 nm, and an uneven distribution of an M atom in a surface layer portion, wherein the M atom is one or more kinds of atoms selected from the group consisting of an aluminum atom and an yttrium atom, a content of the M atom with respect to 100 atom % of iron atoms is in a range of 4.0 to 9.5 atom %, and the uneven distribution of an M atom in a surface layer portion indicates that a ratio of surface layer portion content (with respect to bulk iron atom)/bulk content is equal to or greater than 0.90, wherein the surface layer portion content (with respect to bulk iron atom) is a content of the M atom in a solution obtained by partially dissolving the ε-iron oxide powder with acid with respect to 100 atom % of iron atom in a solution obtained by totally dissolving the ε-iron oxide powder with acid, and the bulk content is a content of the M atom in a solution obtained by totally dissolving the ε-iron oxide powder with acid with respect to 100 atom % of iron atom in a solution obtained by totally dissolving the ε-iron oxide powder with acid.

2. The ε-iron oxide powder according to claim 1, wherein the content of the M atom is in a range of 4.3 to 8.0 atom %.

3. The ε-iron oxide powder according to claim 1, wherein the average particle size is in a range of 6.0 to 16.0 nm.

4. The ε-iron oxide powder according to claim 1, further comprising: one or more kinds of atoms selected from the group consisting of a gallium atom, a cobalt atom, and a titanium atom.

5. The ε-iron oxide powder according to claim 1, wherein at least an aluminum atom is included as the M atom.

6. The ε-iron oxide powder according to claim 1, wherein at least an yttrium atom is included as the M atom.

7. The ε-iron oxide powder according to claim 1, which is a ferromagnetic powder for magnetic recording.

8. A magnetic recording medium comprising: a non-magnetic support; and a magnetic layer including a ferromagnetic powder, wherein the ferromagnetic powder is the ε-iron oxide powder according to claim 1.

9. The magnetic recording medium according to claim 8, further comprising: a nitrogen-containing polymer in the magnetic layer.

10. The magnetic recording medium according to claim 8, further comprising: a non-magnetic layer including a non-magnetic powder between the non-magnetic support and the magnetic layer.

11. The magnetic recording medium according to claim 8, further comprising: a back coating layer including a non-magnetic powder on a surface of the non-magnetic support opposite to a surface provided with the magnetic layer.

12. The magnetic recording medium according to claim 8, wherein the content of the M atom of the ε-iron oxide powder is in a range of 4.3 to 8.0 atom %.

13. The magnetic recording medium according to claim 8, wherein the average particle size of the ε-iron oxide powder is in a range of 6.0 to 16.0 nm.

14. The magnetic recording medium according to claim 8, wherein the ε-iron oxide powder further comprises one or more kinds of atoms selected from the group consisting of a gallium atom, a cobalt atom, and a titanium atom.

15. The magnetic recording medium according to claim 8, wherein the ε-iron oxide powder includes at least an aluminum atom as the M atom.

16. The magnetic recording medium according to claim 8, wherein the ε-iron oxide powder includes at least an yttrium atom is included as the M atom.

17. A magnetic recording and reproducing device comprising: the magnetic recording medium according to claim 8; and a magnetic head.

18. A composition comprising: the ε-iron oxide powder according to claim 1.

19. The composition according to claim 18, further comprising: a binding agent.

20. The ε-iron oxide powder according to claim 1, wherein the average particle size of the ε-iron oxide powder ranges from 5.7 nm to 15.3 nm and the content of the M atom with respect to 100 atom % of iron atoms is in a range of 4.2 atom % to 9.4 atom %.

Description

EXAMPLES

(1) Hereinafter, the invention will be described with reference to examples. However, the invention is not limited to aspects shown in the examples. “Parts” and “%” in the following description are based on mass, unless otherwise noted. In addition, steps and evaluations described below are performed in an environment of an atmosphere temperature of 23° C.±1° C., unless otherwise noted. “eq” described below indicates equivalent and a unit not convertible into SI unit.

Example 1

(2) Producing of ε-Iron Oxide Powder

(3) 3.6 g of ammonia aqueous solution having a concentration of 25% was added to a material obtained by dissolving 8.3 g of iron (III) nitrate nonahydrate, 1.01 g of gallium (III) nitrate octahydrate, 189 mg of cobalt (III) nitrate hexahydrate, 152 mg of titanium (III) sulfate, and 1.0 g of polyvinyl pyrrolidone (PVP) in 92.3 g of pure water, while stirring by using a magnetic stirrer, in an atmosphere under the conditions of an atmosphere temperature of 25° C., and the mixture was stirred for 2 hours still under the temperature condition of the atmosphere temperature of 25° C. A citric acid aqueous solution obtained by dissolving 0.85 g of citric acid in 9.15 g of pure water was added to the obtained solution and stirred for 1 hour. The powder precipitated after the stirring was collected by centrifugal separation, washed with pure water, and dried in a heating furnace at a furnace inner temperature of 80° C.

(4) 800 g of pure water was added to the dried powder and the powder was dispersed in water again, to obtain a dispersion liquid. The obtained dispersion liquid was heated to a liquid temperature of 50° C., and 40 g of ammonia aqueous solution having a concentration of 25% was added dropwise while stirring. The stirring was performed for 1 hour while holding the liquid temperature of 50° C., and 13.3 mL of tetraethoxysilane (TEOS) was added dropwise and stirred for 24 hours. 51 g of ammonium sulfate was added to the obtained reaction solution, the precipitated powder was collected by centrifugal separation, washed with pure water, and dried in a heating furnace at a furnace inner temperature of 80° C., and a precursor of ε-iron oxide was obtained.

(5) The heating furnace at a furnace inner temperature of 1028° C. (firing temperature) was filled with the obtained powder of precursor in the atmosphere and subjected to heat treatment for 4 hours.

(6) The heat-treated powder was put into sodium hydroxide (NaOH) aqueous solution having a concentration of 4 mol/L, the liquid temperature was held at 70° C., stirring was performed for 24 hours, and accordingly, a silicon acid compound was removed from the heat-treated powder. This powder was collected by a centrifugal separation process and washed with pure water, and a slurry was obtained.

(7) A powder concentration of the slurry obtained was adjusted with pure ware to be 8%, 500 g of this slurry was stirred at a liquid temperature of 85° C. at rotation rate of 300 revolutions per minute (rpm), 6.3 g of an aluminum chloride aqueous solution having a concentration of 1.0% was added and stirred for 1 hour, a sodium hydroxide aqueous solution was added dropwise so that pH becomes 8.6, and stirring was continued for 1 hour to adhere a hydroxide of Al on the particles of the powder. This slurry was washed with water, the powder was collected by decantation, and the collected powder was dried.

(8) Regarding the obtained powder, an X-ray diffraction analysis was performed. The X-ray diffraction analysis was performed by scanning CuKα ray under the condition of a voltage of 45 kV and intensity of 40 mA and measuring an X-ray diffraction pattern under the following conditions. It was confirmed that the produced ferromagnetic powder does not have a crystal structure of an a phase and a γ phase and has a crystal structure of a single phase which is an ε phase (ε-iron oxide type crystal structure) from the peak of the XRD pattern obtained by the X-ray diffraction analysis. That is, it was confirmed that the ε-iron oxide powder was produced.

(9) PANalytical X'Pert Pro diffractometer, PIXcel detector

(10) Soller slit of incident beam and diffraction beam: 0.017 radians

(11) Fixed angle of dispersion slit: ¼ degrees

(12) Mask: 10 mm

(13) Scattering prevention slit: ¼ degrees

(14) Measurement mode: continuous

(15) Measurement time per 1 stage: 3 seconds

(16) Measurement speed: 0.017 degrees per second

(17) Measurement step: 0.05 degrees

(18) A part of the produced ε-iron oxide powder was used in the manufacturing of the magnetic recording medium and another part thereof was used in evaluation of the powder performed by methods which will be described later.

(19) Regarding each ferromagnetic powder produced by the method which will be described later, the X-ray diffraction analysis was performed in the same manner as in Example 1, the produced ferromagnetic powder does not have a crystal structure of an a phase and a γ phase and has a crystal structure of a single phase which is an ε phase (ε-iron oxide type crystal structure). That is, ε-iron oxide powder was confirmed.

(20) Manufacturing of Magnetic Recording Medium (Magnetic Tape)

(21) (1) List of Magnetic layer forming composition

(22) Magnetic liquid

(23) Ferromagnetic powder (see Table 1): 100.0 parts

(24) SO.sub.3Na group-containing polyurethane resin: 14.0 parts (Weight-average molecular weight: 70,000, SO.sub.3Na group: 0.4 meq/g)

(25) Cyclohexanone: 150.0 parts

(26) Methyl ethyl ketone: 150.0 parts

(27) Oleic acid: 2.0 parts

(28) Abrasive solution

(29) Abrasive solution A

(30) Alumina abrasive (average particle size: 100 nm): 3.0 parts

(31) SO.sub.3Na group-containing polyurethane resin: 0.3 parts (Weight-average molecular weight: 70,000, SO.sub.3Na group: 0.3 meq/g)

(32) Cyclohexanone: 26.7 parts

(33) Abrasive solution B

(34) Diamond abrasive (average particle size: 100 nm): 1.0 part

(35) SO.sub.3Na group-containing polyurethane resin: 0.1 parts (Weight-average molecular weight: 70,000, SO.sub.3Na group: 0.3 meq/g)

(36) Cyclohexanone: 26.7 parts

(37) Silica sol

(38) Colloidal silica (average particle size: 100 nm): 0.2 parts

(39) Methyl ethyl ketone: 1.4 parts

(40) Other components

(41) Stearic acid: 2.0 parts

(42) Butyl stearate: 6.0 parts

(43) Polyisocyanate (CORONATE manufactured by Tosoh Corporation): 2.5 parts

(44) Finishing Additive Solvent

(45) Cyclohexanone: 200.0 parts

(46) Methyl ethyl ketone: 200.0 parts

(47) (2) List of Non-Magnetic Layer Forming Composition

(48) Non-magnetic inorganic powder: (α-iron oxide): 100.0 parts

(49) Average particle size: 10 nm

(50) Average aspect ratio: 1.9

(51) BET (Brunauer-Emmett-Teller) specific surface area: 75 m.sup.2/g

(52) Carbon black (average particle size: 20 nm): 25.0 parts

(53) SO.sub.3Na group-containing polyurethane resin: 18.0 parts (Weight-average molecular weight: 70,000, SO.sub.3Na group: 0.2 meq/g)

(54) Stearic acid: 1.0 part

(55) Cyclohexanone: 300.0 parts

(56) Methyl ethyl ketone: 300.0 parts

(57) (3) List of Back Coating Layer Forming Composition

(58) Non-magnetic inorganic powder

(59) α-iron oxide: 80.0 parts

(60) Average particle size: 0.15 μm

(61) Average aspect ratio: 7

(62) BET specific surface area: 52 m.sup.2/g

(63) Carbon black (average particle size: 20 nm): 20.0 parts

(64) Vinyl chloride copolymer: 13.0 parts

(65) Sulfonic acid group-containing polyurethane resin: 6.0 parts

(66) Phenylphosphonic acid: 3.0 parts

(67) Cyclohexanone: 155.0 parts

(68) Methyl ethyl ketone: 155.0 parts

(69) Stearic acid: 3.0 parts

(70) Butyl stearate: 3.0 parts

(71) Polyisocyanate: 5.0 parts

(72) Cyclohexanone: 200.0 parts

(73) (4) Manufacturing of Magnetic Tape

(74) Various components of the magnetic liquid were dispersed by using a batch type vertical sand mill for 24 hours to prepare a magnetic liquid. As dispersion beads, zirconia beads having a particle diameter of 0.5 mm were used.

(75) The abrasive solution was prepared by dispersing various components of the abrasive solutions A and B with a batch type ultrasonic device (20 kHz, 300 W) for 24 hours.

(76) The magnetic liquid and the abrasive solution obtained as described above were mixed with other components (silica sol, other components, and the finishing additive solvent) and subjected to treatment (ultrasonic dispersion) with a batch type ultrasonic device (20 kHz, 300 W) for 30 minutes. After that, the obtained mixture was filtered with a filter having a hole diameter of 0.5 μm, and a magnetic layer forming composition was prepared.

(77) For the non-magnetic layer forming composition, the various components were dispersed by using a batch type vertical sand mill for 24 hours. As dispersion beads, zirconia beads having a bead diameter of 0.1 mm were used. The obtained dispersion liquid was filtered with a filter having a hole diameter of 0.5 μm and a non-magnetic layer forming composition was prepared.

(78) For the back coating layer forming composition, the various components described above excluding the lubricant (stearic acid and butyl stearate), polyisocyanate, and 200.0 parts of cyclohexanone were kneaded and diluted by an open kneader. Then, the obtained mixed liquid was subjected to a dispersion process of 12 passes, with a transverse beads mill dispersing device by using zirconia beads having a particle diameter of 1 mm, by setting a bead filling percentage as 80 volume %, a circumferential speed of rotor distal end as 10 m/sec, and a retention time for 1 pass as 2 minutes. After that, the remaining components were added into the obtained dispersion liquid and stirred with a dissolver. The obtained dispersion liquid described above was filtered with a filter having a hole diameter of 1 μm and a back coating layer forming composition was prepared.

(79) After that, the non-magnetic layer forming composition was applied and dried on a biaxial stretching polyethylene naphthalate support having a thickness of 5.0 μm so that a thickness after drying is 0.1 μm, and the magnetic layer forming composition was applied so that a thickness after drying is 70 nm, a coating layer was formed. While this coating layer is wet, a homeotropic alignment process was performed by applying a magnetic field having a magnetic field strength of 0.6 T in a direction vertical to the surface of the coating layer, and the coating layer was dried. After that, the back coating layer forming composition was applied to a surface of the support on a side opposite to the surface where the non-magnetic layer and the magnetic layer are formed, so that the thickness after drying becomes 0.4 μm, and dried, and accordingly, a back coating layer was formed.

(80) Then, a surface smoothing treatment (calendar process) was performed with a calendar configured of only a metal roll, at a speed of 100 m/min, linear pressure of 294 kN/m, and a surface temperature of a calendar roll of 100° C., and the heating treatment was performed in the environment of the atmosphere temperature of 70° C. for 36 hours. After the heating treatment, the slitting was performed to have a width of ½ inches (1 inch is 0.0254 meters), and a magnetic tape was obtained.

Comparative Example 1

(81) An ε-iron oxide powder was obtained in the same manner as in the producing of the ε-iron oxide powder of Example 1, except that gallium (III) nitrate octahydrate, cobalt (III) nitrate hexahydrate, and titanium (III) sulfate were not added, and the firing temperature was changed to 975° C.

(82) A magnetic tape was obtained in the same manner as in Example 1, except that the ε-iron oxide powder obtained as described above was used as the ferromagnetic powder for forming the magnetic layer.

Example 2

(83) A magnetic tape was obtained in the same manner as in Comparative Example 1, except that the firing temperature in the preparation of the ε-iron oxide powder was changed to 983° C.

Example 3

(84) An ε-iron oxide powder was obtained in the same manner as in the producing of the ε-iron oxide powder of Example 1, except that the amount of gallium (III) nitrate octahydrate used was changed to 51 mg, cobalt (III) nitrate hexahydrate and titanium (III) sulfate were not included, and the firing temperature was changed to 991° C.

(85) A magnetic tape was obtained in the same manner as in Example 1, except that the ε-iron oxide powder obtained as described above was used as the ferromagnetic powder for forming the magnetic layer.

Example 4

(86) An ε-iron oxide powder was obtained in the same manner as in the producing of the ε-iron oxide powder of Example 1, except that the amount of gallium (III) nitrate octahydrate used was changed to 1.51 g, and the firing temperature was changed to 1045° C.

(87) A magnetic tape was obtained in the same manner as in Example 1, except that the ε-iron oxide powder obtained as described above was used as the ferromagnetic powder for forming the magnetic layer.

Comparative Example 2

(88) A magnetic tape was obtained in the same manner as in Example 4, except that the amount of gallium (III) nitrate octahydrate used was changed to 1.63 g, and the firing temperature was changed to 1052° C. in the producing of the ε-iron oxide powder.

Comparative Example 3

(89) A magnetic tape was obtained in the same manner as in Example 1, except that the amount of the aluminum chloride aqueous solution added to the slurry for adhering the hydroxide of Al was changed to 4.9 g.

Example 5

(90) A magnetic tape was obtained in the same manner as in Example 1, except that the amount of the aluminum chloride aqueous solution added to the slurry for adhering the hydroxide of Al was changed to 5.5 g.

Example 6

(91) A magnetic tape was obtained in the same manner as in Example 1, except that the amount of the aluminum chloride aqueous solution added to the slurry for adhering the hydroxide of Al was changed to 12.3 g.

Comparative Example 4

(92) A magnetic tape was obtained in the same manner as in Example 1, except that the amount of the aluminum chloride aqueous solution added to the slurry for adhering the hydroxide of Al was changed to 12.9 g.

Comparative Example 5

(93) A magnetic tape was obtained in the same manner as in Example 1, except that the aluminum chloride aqueous solution added to the slurry for adhering the hydroxide of Al was changed to 6.6 g of an yttrium chloride aqueous solution.

Example 7

(94) A magnetic tape was obtained in the same manner as in Comparative Example 5, except that the additive amount of the yttrium chloride aqueous solution was changed to 7.8 g.

Example 8

(95) A magnetic tape was obtained in the same manner as in Example 7, except that the additive amount of the yttrium chloride aqueous solution was changed to 9.4 g.

Example 9

(96) A magnetic tape was obtained in the same manner as in Example 7, except that the additive amount of the yttrium chloride aqueous solution was changed to 16.7 g.

Comparative Example 6

(97) A magnetic tape was obtained in the same manner as in Example 7, except that the additive amount of the yttrium chloride aqueous solution was changed to 17.1 g.

Example 10

(98) A magnetic tape was obtained in the same manner as in Example 1, except that an additive A was added in the producing of the magnetic liquid.

Example 11

(99) A magnetic tape was obtained in the same manner as in Example 1, except that an additive B was added in the producing of the magnetic liquid.

Example 12

(100) A magnetic tape was obtained in the same manner as in Example 8, except that an additive A was added in the producing of the magnetic liquid.

Example 13

(101) A magnetic tape was obtained in the same manner as in Example 8, except that an additive B was added in the producing of the magnetic liquid.

Comparative Example 7

(102) A magnetic tape was obtained in the same manner as in Example 1, except that 1.01 g of gallium (III) nitrate octahydrate used was changed to 0.33 g of aluminum (III) nitrate octahydrate and the adhering treatment of the hydroxide of Al was not performed in the producing of the ε-iron oxide powder.

(103) The additive A shown in Table 1 is a polyethyleneimine derivative (J-1) (polyalkyleneimine chain-containing polymer) disclosed in a paragraph 0109 of JP2015-028830A.

(104) The additive B shown in Table 1 is a polyalkyleneimine chain-containing polymer obtained by the following method.

(105) In Table 1, in the examples in which the additive A or the additive B is shown in the column of the “additive A/B”, the magnetic liquid was prepared by adding 30.0 parts of additive A or B.

(106) Synthesis Method of Additive B

(107) Synthesis of Intermediate P-1

(108) 45.0 g of propylene glycol monomethyl ether acetate (PGMEA; reaction solvent) was added to 500 mL three-neck flask under the nitrogen atmosphere. After increasing the liquid temperature to 75° C., 6.4 g of mercaptopropionic acid (MPA; thiol compound), 90.1 g of methyl methacrylate (MMA; vinyl monomer), 180.1 g of PGMEA (reaction solvent), and 0.14 g of dimethyl 2,2′-azobis (2-methylpropionate) (V-601 manufactured by Wako Pure Chemical Industries, Ltd.; polymerization initiator) were mixed with each other in advance and added dropwise for 2 hours. After the dropwise addition, 0.14 g of V-601 was added and stirred for 2 hours. In addition, the liquid temperature was increased to 90° C., and the mixture was stirred for 2 hours to obtain a PGMEA solution of the intermediate P-1 having the following structure. The mol number of MMA (vinyl monomer) used in the above is 15 mols with respect to 1 mol of MPA (thiol compound). The weight-average molecular weight of the intermediate P-1 synthesized here was 3,500.

(109) Structure of Intermediate P-1

(110) ##STR00009##

(111) Synthesis of Additive B

(112) 2.4 g of polyalkyleneimine (SP-006 manufactured by Nippon Shokubai Co., Ltd., number average molecular weight of 600) and 144.8 g of the 30% PGMEA solution of the intermediate P-1 were mixed with each other and heated to the liquid temperature of 110° C. for 3 hours, and accordingly, a polymer including a polyalkyleneimine chain and a vinyl polymer chain (polyalkyleneimine chain-containing polymer) was obtained.

(113) The above synthesis scheme is shown below. In the following synthesis scheme, a, b, and c each independently represent a polymerization molar ratio of a repeating unit, are 0 to 50, and a+b+c=100. k, l, m1, and m2 each independently represent a polymerization molar ratio of a repeating unit, k is 10 to 90, l is 0 to 80, m1 and m2 are each independently 0 to 70, and k+l+m1+m2=100. n represents a repeating unit and is 2 to 100.

(114) ##STR00010##

(115) The reaction solution after the synthesis of the polymer was heated to a liquid temperature of 70° C., 0.4 g of phthalic anhydride was added and stirred for 1 hour, and accordingly, an acid-modified polyalkyleneimine chain-containing polymer (additive B) was synthesized. By the acid modification, one of the following partial structure which is a partial structure represented by Formula 1 is introduced per molecule in the additive B. A weight-average molecular weight of the additive B synthesized was 4,300, an amine value was 0.30 mmol/g, and an acid value was 0.59 mmol/g. The introduction of each synthesis raw material to the additive B finally synthesized at a ratio calculated from the used amount was confirmed by a measurement value of .sup.1H-nuclear magnetic resonance (NMR), the weight-average molecular weight, an amine value, and an acid value.

(116) ##STR00011##

(117) Evaluation Method of ε-Iron Oxide Powder

(118) (1) Average Particle Size

(119) Regarding each ε-iron oxide powder in the examples and the comparative examples, an average particle size was obtained by the method described above using a transmission electron microscope H-9000 manufactured by Hitachi, Ltd. as the transmission electron microscope, and image analysis software KS-400 manufactured by Carl Zeiss as the image analysis software.

(120) (2) Evaluation of Surface Layer Portion Content (With Respect to Bulk Iron Atom) of M Atom, Surface Layer Portion Content (With Respect to Surface Layer Portion Iron Atom) of M Atom, Bulk Content, and Uneven Distribution of M Atom in Surface Layer Portion, and Composition Analysis of ε-Iron Oxide Powder

(121) (i) Partial Dissolving

(122) 12 mg of a sample powder was collected from each ε-iron oxide powder of the examples and the comparative examples, and a beaker containing 12 mg of this sample powder and 10 ml of hydrochloric acid having a concentration of 1 mol/L was held on a hot plate at a set temperature of 70° C. for 1 hour, to obtain a solution in which a particle surface layer portion configuring the ε-iron oxide powder was dissolved (totally dissolved). The obtained solution was filtered with a membrane filter having a hole diameter of 0.1 μm. The element analysis of the filtrate obtained as described above was performed by an ICP analysis device.

(123) (ii) Total Dissolving

(124) 12 mg of a sample powder was separately collected from each ε-iron oxide powder of the examples and the comparative examples, a beaker containing 12 mg of this sample powder and 10 ml of hydrochloric acid having a concentration of 4 mol/L was held on a hot plate at a set temperature of 80° C. for 3 hours, to obtain a solution in which the ε-iron oxide powder was dissolved (totally dissolved). The obtained solution was filtered with a membrane filter having a hole diameter of 0.1 μm. The element analysis of the filtrate obtained as described above was performed by an ICP analysis device.

(125) (iii) Surface Layer Portion Content (With Respect to Bulk Iron Atom) of M Atom, Surface Layer Portion Content (With Respect to Surface Layer Portion Iron Atom) of M Atom, Bulk Content

(126) The surface layer portion content (with respect to bulk iron atom) of M atom was calculated as “(content of M atoms in the solution obtained by the partial dissolving in the section (i)/content of iron atoms in the solution obtained by the total dissolving in the section (ii))×100”.

(127) The content (bulk content) of the atom was calculated as “(content of M atoms in the solution obtained by the total dissolving in the section (ii)/content of iron atoms in the solution obtained by the total dissolving in the section (ii))×100”.

(128) The presence or absence of the uneven distribution of an M atom in a surface layer portion was evaluated based on a ratio of the surface layer portion content (with respect to bulk iron atom) obtained described above to the bulk content.

(129) The surface layer portion content of the M atom (with respect to surface layer portion iron atom) was calculated as “(content of M atoms in the solution obtained by the partial dissolving in the section (i)/content of iron atoms in the solution obtained by the partial dissolving in the section (i))×100”.

(130) (iv) Composition Analysis of ε-Iron Oxide Powder

(131) The quantity of a substitutional atom of the iron atoms was determined by the element analysis using the ICP analysis device regarding the solution obtained by the total dissolving in the section (ii), and the composition of the ε-iron oxide powder represented by a compositional formula Ga.sub.xCo.sub.yTi.sub.zFe.sub.(2-x-y-z)O.sub.3 was specified from the quantitative result.

(132) Evaluation of Electromagnetic Conversion Characteristics

(133) A magnetic signal was recorded on each magnetic tape of the examples and the comparative examples in a tape longitudinal direction under the following conditions and reproduced with a magnetoresistive (MR) head. The reproduced signal was frequency-analyzed with a spectrum analyzer manufactured by Shibasoku Co., Ltd., and noise accumulated at 0 to 600 kfci was evaluated. The unit kfci is a unit of a linear recording density (cannot be converted into the unit SI). The electromagnetic conversion characteristics (initial stage of running) of each magnetic tape of the examples and the comparative examples was evaluated according to the following evaluation standard.

(134) Recording and Reproduction Conditions

(135) Recording: Recording track width 5 μm

(136) Recording gap 0.17 μm

(137) Head saturated magnetic flux density Bs 1.8 T

(138) Reproduction: Reproduction track width 0.4 μm

(139) Distance between shields (sh-sh distance) 0.08 μm

(140) Evaluation standard

(141) 5: Substantially no noise, a signal is excellent, no error is observed.

(142) 4: A degree of noise is small and a signal is excellent.

(143) 3: Noise is observed. Signal is excellent.

(144) 2: A degree of noise is great and a signal is unclear.

(145) 1: Noise and signal cannot be distinguished or cannot be recorded.

(146) In addition, each magnetic tape (length of 100 m) of the examples and the comparative examples was caused to repeatedly run 600 passes under the environment of the temperature of 37° C. and relative humidity of 87% at a running speed of 3 msec in a linear tester, to bring the surface of the magnetic layer and the magnetic head into contact with each other and slide on each other. The electromagnetic conversion characteristics (after repeated running) were evaluated by the same method as described above, after the repeated running.

(147) The results of the above evaluation are shown in Table 1. In Comparative Example 1, a large number of scratches was observed on the surface of the magnetic layer after the repeated running, and accordingly, the evaluation of the electromagnetic conversion characteristics was not performed, and “-” was shown in the column of the electromagnetic conversion characteristics after repeated running of Table 1.

(148) TABLE-US-00001 TABLE 1 M atom Surface Surface Surface layer uneven layer layer portion dis- Electromagnetic portion portion content tribution conversion Compositional content content (with respect of M characteristics formula (with (with respect to surface atom in Average Initial Ga.sub.xCo.sub.yTi.sub.zFe.sub.(2-x-y-z)O.sub.3 respect to bulk iron layer surface particle stage After Ga Co Ti Bulk to bulk atom)/bulk portion layer Additive size of repeated x y z Kind content iron atom) content iron atom) portion A/B (nm) running running Example 1 0.20 0.05 0.05 Al 4.8 4.5 0.94 84 Present None 11.3 4 4 Com- 0.00 0.00 0.00 Al 4.1 3.9 0.95 80 Present None 4.7 1 — parative Example 1 Example 2 0.00 0.00 0.00 Al 4.2 3.9 0.94 81 Present None 5.7 3 3 Example 3 0.10 0.00 0.00 Al 4.4 4.2 0.96 82 Present None 9.0 4 4 Example 4 0.30 0.05 0.05 Al 5.1 5.0 0.97 83 Present None 15.3 4 4 Com- 0.32 0.05 0.05 Al 5.1 4.8 0.95 85 Present None 17.4 2 2 parative Example 2 Com- 0.20 0.05 0.05 Al 3.8 3.6 0.94 82 Present None 11.4 3 1 parative Example 3 Example 5 0.20 0.05 0.05 Al 4.2 4.1 0.96 83 Present None 11.3 3 3 Example 6 0.20 0.05 0.05 Al 9.4 8.9 0.95 85 Present None 11.3 3 4 Com- 0.20 0.05 0.05 Al 9.8 9.3 0.95 81 Present None 11.4 2 3 parative Example 4 Com- 0.20 0.05 0.05 Y 3.8 3.6 0.94 80 Present None 11.4 3 1 parative Example 5 Example 7 0.20 0.05 0.05 Y 4.4 4.2 0.96 81 Present None 11.3 3 3 Example 8 0.20 0.05 0.05 Y 5.3 5.0 0.95 82 Present None 11.4 4 4 Example 9 0.20 0.05 0.05 Y 9.4 9.0 0.95 83 Present None 11.3 3 4 Com- 0.20 0.05 0.05 Y 9.6 9.3 0.96 84 Present None 11.5 2 3 parative Example 6 Example 10 0.20 0.05 0.05 Al 4.8 4.6 0.95 82 Present Additive A 11.3 5 4 Example 11 0.20 0.05 0.05 Al 4.8 4.6 0.96 83 Present Additive B 11.3 5 5 Example 12 0.20 0.05 0.05 Y 5.3 5.0 0.95 84 Present Additive A 11.4 5 4 Example 13 0.20 0.05 0.05 Y 5.3 5.1 0.96 85 Present Additive B 11.4 5 5 Com- Al0.10 0.06 0.06 Al 4.9 0.1 0.03 4.8 None None 11.4 3 2 parative Example 7

(149) From the results shown in Table 1, the magnetic tapes of Examples 1 to 13, excellent electromagnetic conversion characteristics can be observed both in the initial stage after the running and after the repeated running in the high temperature and high humidity environment.

(150) One aspect of the invention is effective in a technical field of a magnetic recording medium for high-density recording.