1. Field of the Invention The present invention relates to a magnetic element capable of reading information with a high recording density and a thin-film magnetic head using this magnetic element as a reproduction element. 2. Related Background Art Reading operations in a conventional thin-film magnetic head having a magnetoresistive effect element as a reproduction element will be described below. A leakage magnetic flux from a recording medium causes a change in magnetization direction of a free magnetic layer of a magnetoresistive effect element. A change in electrical resistance of the magnetoresistive effect element brought about by the change in the magnetization direction of the free magnetic layer is read by passing a sensing current through the magnetoresistive effect element. With increasing recording density in recent years, there is a demand for reducing the thickness of the free magnetic layer of the magnetoresistive effect element in order to increase the rate of change in electrical resistance. However, when the thickness of the free magnetic layer is reduced, the effect of a domain structure therein will become more significant. This is due to the fact that a free magnetic layer is generally divided into a single magnetic domain, but will no longer form a single magnetic domain if the thickness of the free magnetic layer is reduced to a critical value. In the single magnetic domain, the magnetization of the free magnetic layer and the magnetization of the fixed magnetic layer are aligned in a vertical direction to each other and maintained unaltered. When a pair of hard magnets are provided on both sides of a magnetoresistive effect element with the thickness direction of the free magnetic layer interposing therebetween, the magnetization of the free magnetic layer is constrained to the Y direction as illustrated in FIG. 9 and remains aligned in that direction. As illustrated in FIG. 9, it is possible to align the magnetization of the free magnetic layer in the Y direction and maintain it in that direction by such a structure that the hard magnets, such as Permalloy, generate magnetic fluxes in the X direction (the direction of track width) when a recording current is passed. However, there is a case where the above-mentioned constraint fails, for example, at the time of manufacturing, resulting in the magnetization of the free magnetic layer being aligned in the X direction, as shown in FIG. 9. In this case, the external magnetic field from a recording medium will cause the magnetization of the free magnetic layer to rotate. However, a pair of magnetic domain walls exist in a stable position at both ends of the magnetic domain having the magnetization aligned in the Y direction. As a result, the magnetization of the free magnetic layer in the vicinity of both ends of the magnetic domain remains unchanged when an external magnetic field is applied to rotate the magnetization of the free magnetic layer. The magnetization of the free magnetic layer rotates in accordance with the position where it is influenced by the external magnetic field. It is therefore impossible to effectively perform magnetic-flux conduction by such a magnetic domain structure as described above. To make the magnetization of the free magnetic layer hard to rotate, there are reported methods such as increasing the film thickness of a magnetoresistive effect element and increasing the saturation flux density of the fixed magnetic layer in a magnetic circuit including the magnetoresistive effect element and an electrode connected to the magnetoresistive effect element (refer to Japanese Patent Application Laid-Open No. Hei 8-171842). However, simply increasing the film thickness of the magnetoresistive effect element leads to an increase in the distance from a reproduction position to the hard magnet, resulting in a poor sensitivity and difficulty in manufacturing. When increasing the saturation flux density of the fixed magnetic layer, the saturation flux density is required to be increased to a larger value than is necessary, resulting in an increase in the saturation magnetic field of the magnetic domain that causes a problem of an increase in the recording current. Moreover, when using a thin film for forming the hard magnet in order to enhance the permeability, it is required to select a substrate having a small magnetic-flux density and to reduce the film thickness of the hard magnet in order to avoid the occurrence of an eddy current. It is therefore difficult to ensure a sufficient magnetoresistive effect and an excellent resistance to noise. On the other hand, a method of aligning the magnetization of the free magnetic layer in the X direction has been reported (refer to, e.g., Japanese Patent Application Laid-Open No. 2001-77018). A thin-film magnetic head in the related art will be described below with reference to the drawings. FIGS. 7 to 10 are conceptual diagrams illustrating the structure of the thin-film magnetic head in the related art. FIG. 7 is a partial perspective view illustrating the main part of a thin-film magnetic head in a related art, and FIG. 8 is a partial plan view of the main part of the thin-film magnetic head in FIG. 7, illustrating the relationship between elements thereof and a recording medium. FIG. 9 is a sectional view of the main part of the thin-film magnetic head in FIG. 8, taken along line XIX-XIX shown in FIG. 8. The thin-film magnetic head in the related art includes a lower-shield layer 2 as a lower shield, a magnetoresistive effect element 3 formed on the lower-shield layer 2, an upper-shield layer 4 as an upper shield formed on the magnetoresistive effect element 3, a spin valve layer 5 having a fixed magnetization, interposed between the lower-shield layer 2 and the upper-shield layer 4 and having two magnetic layers with a magnetization fixed to each other in a direction perpendicular to a recording-medium-facing surface (an X direction), and a recording current path layer 6 including a nonmagnetic material layer interposed between the lower-shield layer 2 and the upper-shield layer 4, and formed so as to pass between the lower-shield layer 2 and the upper-shield layer 4. The magnetoresistive effect element 3 has a multilayer structure including a nonmagnetic layer 8, an intermediate layer 9, a magnetization-fixing layer 10, and a magnetic layer 11. A pair of magnetic domain walls 12 are present at both ends of the magnetic layer 11. The spin valve layer 5 is formed to include the nonmagnetic layer 8, the intermediate layer 9, the magnetization-fixing layer 10, and the magnetic layer 11. It is possible to align the magnetization of the magnetic layer 11 in the X direction in accordance with the positions where the magnetic domain walls 12 are present by arranging the magnetization of the intermediate layer 9 to be stable in one direction and arranging the magnetization of the magnetization-fixing layer 10 to be stable in one direction. When the magnetization of the intermediate layer 9 and the magnetization of the magnetization-fixing layer 10 are aligned in one direction, the magnetization of the magnetic layer 11 is aligned in a direction orthogonal to the magnetization of the intermediate layer 9 and the magnetization of the magnetization-fixing layer 10. The magnetic layer 11 is magnetized to a fixed direction (an X direction), as illustrated in FIG. 9. The recording current path layer 6 has a nonmagnetic layer 13 interposed between the upper-shield layer 4 and the lower-shield layer 2 and formed so as to pass between the upper-shield layer 4 and the lower-shield layer 2, and a current perpendicular to the plane of a recording medium flows through the upper-shield layer 4, the nonmagnetic layer 13, and the lower-shield layer 2 in a direction orthogonal to the magnetoresistive effect element 3, as illustrated in FIG. 7. FIG. 10 is a sectional view of the main part of the thin-film magnetic head in FIG. 8, taken along line XIX-XIX shown in FIG. 8. As illustrated in FIG. 10, the upper-shield layer 4 includes a magnetic pole piece 14 that faces a recording medium with the spin valve layer 5 interposed between them. The magnetization of the magnetic layer 11 is aligned in the X direction and hard to rotate by the presence of the magnetic pole piece 14 and the magnetic layer 11. A leakage magnetic field from the recording medium due to a recording current does not cause a change in magnetization direction in the magnetization-fixing layer 10, so the magnetization of the magnetic layer 11 in the X direction is unlikely to be influenced by the leakage magnetic field. In the thin-film magnetic head having the spin valve layer 5 having the above-described structure, it is possible to suppress a rotation of magnetization of the magnetic layer 11 in a magnetic field from the recording medium, thereby making the magnetization of the magnetic layer 11 hard to rotate. It is also possible to rotate the magnetization of the free magnetic layer to cause a magnetoresistive effect. A leakage magnetic flux in a vertical direction to a surface of a recording medium is required to be detected, in addition to an external leakage magnetic field in an in-plane direction of the recording medium. The above-described thin-film magnetic head does not detect a leakage magnetic field from a recording medium in a vertical direction.