TITLE MAGNETIC CORE AND ADHESIVE RESIN COMPOSITION FOR MAGNETIC CORE TECHNICAL FIELD The present invention relates to a magnetic core comprising a core material made of a laminate of an amorphous alloy ribbon and a coated material arranged on an outer surface of the core material, a process for producing the magnetic core, and an adhesive resin composition used for producing the magnetic core. BACKGROUND ART An amorphous alloy is a non-crystalline solid produced by rapidly cooling a starting material, such as iron, boron or silicon, from its molten state, and such an amorphous alloy can be usually obtained as a ribbon having a thickness of usually about 0.01 to 0.1 mm. The amorphous alloy has a random structure with no regularity of atomic configuration and has excellent properties as a soft magnetic material, such as high magnetic permeability, small coercivity force, and usability even in a high-frequency region because of a wide region of applicable frequencies. In case of the amorphous alloy, further, the energy loss (hysteresis loss) that is produced when the magnetic flux passes through is small and the thickness of a ribbon of the amorphous alloy is smaller than that of a silicon steel belt, so that the core loss (sum of hysteresis loss and eddy current loss) can be reduced. On this account, magnetic cores produced by laminating the amorphous alloy ribbons are used as, for example, transformers in many electric and electronic instruments making the best use of their excellent properties, and they greatly contribute to not only magnetic properties but also miniaturization or weight reduction of parts. Moreover, transformers using amorphous ribbon core that is one of magnetic cores have been recently paid much attention from the viewpoint of energy saving. Magnetic cores using amorphous alloy ribbons are generally produced in the following manner. First, an amorphous alloy ribbon is rolled or superposed one upon another to form a magnetic core precursor in a given shape. Then, in order to allow the resulting magnetic core to exhibit specific magnetic properties, the precursor is subjected to a prescribed annealing heat treatment. Although the conditions of the annealing heat treatment vary depending upon the desired magnetic properties, this treatment is generally carried out under the conditions of a temperature of about 330 to 440°C and an annealing time of 0.1 to 100 hours in an inert atmosphere. Through the annealing heat treatment, the amorphous alloy ribbon becomes an extremely brittle ribbon. In the production of the magnetic core, therefore, it is an important problem to retain the shape of the magnetic core having been subjected to the annealing heat treatment and to prevent breakage thereof. It is known that in the case where the shape of the magnetic core having been subjected to the annealing heat treatment is intended to be retained, if an excess stress is applied to the amorphous alloy ribbon for forming the magnetic core, the hysteresis loss that is one constituent of the core loss is increased and exerts an influence on the properties of the magnetic core, and the influence becomes large particularly in the low-frequency region. For retaining the shape of the magnetic core, therefore, it is necessary to take measures sufficiently considering change of properties of the magnetic core caused by the method for retaining the shape of the magnetic core. In the case where the magnetic core produced as above is used as a part of a transformer by coiling a conductor wire around the magnetic core, in order to secure insulation between the magnetic core and the conductor wire, or in order to prevent breakage of the amorphous alloy ribbon that is made brittle by the annealing heat treatment and to impart excellent shape retention properties to the magnetic core, there has been heretofore generally used a method of introducing the magnetic core having been subjected to the annealing heat treatment into a case made of a resin or the like or a method of coating the outer surface of the magnetic core having been subjected to the annealing heat treatment with a thermosetting resin such as an epoxy resin by electrostatic powder coating. In the method of using a case, however, the case needs to be made larger than the magnetic core from the viewpoint of workability, and the magnetic core with the case becomes bulky as compared with the original magnetic core. Such a magnetic core is unfavorable as an electronic part that needs to be miniaturized. Moreover, a mold to produce a case is necessary, and from the viewpoint of cost, this method is only applicable to a magnetic core of certain shape which is consumed in large amounts. Considering preparation of molds for producing many kinds o.f magnetic cores of different shapes in small amounts, this method is of no practical use. On the other hand, in the coating method using a thermosetting resin such as an epoxy resin, problems of the above method using a case, such as a problem of miniaturization and a problem of small amounts and various kinds, can be solved. In this coating method, however, the magnetic core having been subjected to annealing heat treatment and cooling is coated with the thermosetting resin, so that the core loss properties are deteriorated. Moreover, heat treatments of two times, namely, heat treatment for annealing and heat treatment for resin setting, must be carried out, and this makes the production process complicated. Some of large-sized magnetic cores, e.g., amorphous ribbon cores used for transformers, weigh more than 100 kg, and in the heavy substances, there are always involved difficulties in delicate handling or risks of partial fine breakage attributable to their own weights. Further, in order to cure the thermosetting resin applied, it is necessary to perform heat treatment again after the annealing heat treatment. This operation is troublesome, and installment of an exclusive oven is necessary. Furthermore, there are other problems. For example, it is difficult to increase viscosity of the resin, and because of the core material's own heavy weight, it is also difficult to apply the resin with changing the position of the coating surface so that the resin can be applied in an optimum state, and hence the coating operation is restricted. There has been disclosed another method wherein amorphous alloy ribbons are laminated, then the laminate is coated with a varnish containing as a main component an organic substance capable of withstanding the annealing temperature, such as a polyimide resin, and the laminate with the varnish is heated to perform curing of the resin and annealing heat treatment at the same time (Japanese Patent Laid-Open Publication No. 126615/1987). According to this method, the surface of the magnetic core after the annealing treatment is protected by the thermally cured resin and the shape of the magnetic core is retained, so that the magnetic core constituted of the annealed brittle amorphous alloy ribbons can be more easily handled than before, and the operational problem of the magnetic core can be solved. In addition, curing of the varnish can be carried out simultaneously with annealing, and the mechanical strain of the magnetic core due to curing of the varnish can be relatively reduced. Therefore, variability of the core loss can be made smaller as compared with the aforesaid method wherein the magnetic core having been subjected to annealing and cooling is coated with the thermosetting resin of high shrink properties, such as an epoxy resin. In this method, however, an organic solvent solution of a heat-resistant resin is used, so that it is difficult to form a resin layer of sufficient thickness on the outer surface of the magnetic core. Moreover, because an organic solvent solution (liquid varnish) of a heat-resistant resin is used, the varnish penetrates between layers of the amorphous alloy ribbons, and because of strain due to curing shrinkage of the resin, unnecessary stress is applied to the amorphous alloy ribbons to thereby produce a core loss. To cope with this, a means of fastening the laminate with an iron plate or the like is also disclosed. The means, however, needs extra instrument and operation. Further, the effects are not satisfactory, and the core loss cannot be reduced to the practical level. Furthermore, because of low viscosity of the organic solvent solution (varnish) of a heat-resistant resin, it is difficult to effectively coat the surface of the laminate (core) of the amorphous alloy ribbons. Therefore, insulating performance between the. laminate and a conductor wire coiled around the laminate is liable to become unstable. The varnish is prepared by dissolving a heat-resistant resin in an organic solvent, such as N,N-dimethylacetamide, N-methylpyrrolidone, dimethylimidazolidinone or dimethyl sulfoxide. Although the organic solvent exhibits excellent dissolving power for the heat-resistant resin, it has a fear of exerting an evil influence on the human body. In the production of the magnetic core using varnish, therefore, sufficient care should be taken in handling of the varnish or diffusion of the varnish into the atmosphere. For example, complicated equipment is necessary, and considerably great investment in the equipment is necessary. Because the properties of the varnish are liable to be deteriorated with time, special equipment for storing the varnish, such as refrigerating equipment, is necessary, and it is very difficult to maintain the varnish in a favorable state. In order to relax the shrinkage strain caused by curing the heat-resistant resin in the use of such a varnish as mentioned above, a method of reducing a stress by the use of a varnish of a siloxane-modified polyimde resin having a low elastic modulus has been disclosed (Japanese Patent Laid-Open Publication No. 251439/1990). Although the core loss can be improved by decreasing the elastic modulus, satisfactory effects cannot be obtained. Moreover, if the siloxane-modifled polyimide resin is exposed to a temperature of 330 to 440°C for a period of 0.1 to 100 hours, said conditions being those for annealing the magnetic core, a part of the resin is decomposed because of its low heat resistance, and this causes a problem of shape retention particularly in a large-sized magnetic core such as an amorphous ribbon core. It has been heretofore thought that if a resin having such heat resistance as to withstand the annealing conditions in the production of a magnetic core and if the annealing and the curing of the resin are carried out at the same time, the shape of the annealed magnetic core can be retained by the resin and the annealed magnetic core can be protected by the resin present on the surface. It has been also thought that handling of the thus treated magnetic core becomes easier and there is a possibility of reducing the core loss. In the use of the above-mentioned varnish, however, the amount of the varnish that penetrates between layers of the amorphous alloy ribbons is much larger than expected, and the stress applied to the amorphous alloy ribbons by the curing shrinkage of the resin is considerably large. Therefore, it is difficult to obtain a magnetic core having satisfactory properties. Moreover, because the varnish contains a large amount of an organic solvent, it is very difficult to form a resin film of enough thickness to effectively protect a laminate of the amorphous alloy ribbons having been made brittle by annealing, and in some cases, even a film thickness for securing insulation between the magnetic core and a conductor wire wound around the magnetic core cannot be obtained. The conventional varnish contains a large amount of an organic solvent. Therefore, taking influences of the varnish on the working atmosphere and the natural environment into consideration, great equipment becomes necessary. The conventional varnish itself has a problem of stability to heat and elapse of time and also has a problem of sagging because of low viscosity. When the varnish is used, the resin penetrates and is cured between layers of the laminate of the amorphous alloy ribbons, so that unnecessary stress is applied to the laminate of the amorphous alloy ribbons to produce core loss, and the magnetic core having such core loss becomes a problem in the practical use. DISCLOSURE OF THE INVENTION It is an object of the present invention to provide a magnetic core in which an outer surface of an amorphous alloy ribbon has been coated without handling the amorphous alloy ribbon having been made brittle by annealing to thereby impart insulating properties and shape retention properties to the magnetic core after annealing heat treatment. It is another object of the present invention to provide a magnetic core having low core loss and high insulating properties, wherein a core material made of an amorphous alloy ribbon is imparted with excellent magnetic properties. It is a further object of the present invention to provide an adhesive resin composition for a magnetic core, which is used for producing the above-mentioned magnetic core. It is a still further object of the present invention to provide a process for producing the above-mentioned magnetic core, and a magnetic core obtained by the process. It is a still further object of the present invention to provide an adhesive resin composition for a magnetic core, which can efficiently produce a magnetic core without using an organic solvent and which per se is stable to heat and elapse of time. The magnetic core according to the invention is a magnetic core comprising a core material made of a laminate of an amorphous alloy ribbon and a resin-containing coated material with which at least a part of an outer surface of the core material is coated, wherein the resin-containing coated material formed on the outer surface of the core material has a thickness of not less than 10 fim, and a resin for constituting the resin-containing coated material has a tensile strength, as measured at 30°C after annealing of 365°C and 2 hours in a nitrogen atmosphere, of not less than 30 MPa and has a loss in weight, due to thermal decomposition after annealing of 365°C and 2 hours in a nitrogen atmosphere, of not more than 2% by weight. In the present invention, the resin for constituting the resin-containing coated material is preferably one or more thermoplastic resins selected from a polyimide resin, a polyether imide resin, a polyamide-imide resin, a polyamide resin, a polysulfone resin and a polyether ketone resin. In the present invention, the resin for constituting the resin-containing coated material is preferably one or more thermoplastic resins selected from resins having, in their main chain skeletons, recurring units represented by the following formulas (1) to (10): (Figure Removed) wherein a and b are numbers satisfying the conditions of a+b=l, 0c