FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003 COMPLETE SPECIFICATION [See section 10, Rule 13] RARE EARTH SINTERED MAGNET, METHOD FOR PRODUCING RARE EARTH SINTERED MAGNET, ROTOR, AND ROTARY MACHINE MITSUBISHI ELECTRIC CORPORATION, A CORPORATION ORGANISED AND EXISTING UNDER THE LAWS OF JAPAN, WHOSE ADDRESS IS 7-3, MARUNOUCHI 2-CHOME, CHIYODA-KU, TOKYO 100-8310, JAPAN THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED. 2 DESCRIPTION Field [0001] The present disclosure relates to a rare earth 5 sintered magnet which is a permanent magnet obtained by sintering a material containing a rare earth element, a method for producing a rare earth sintered magnet, a rotor, and a rotary machine. 10 Background [0002] Known R-T-B-based permanent magnets have a tetragonal R2T14B intermetallic compound as a main phase. R is a rare earth element, T is a transition metal element such as Fe (iron) or Fe having its portion replaced with 15 cobalt (Co), and B is boron. R-T-B-based permanent magnets are used for various high value-added components including industrial motors. In particular, Nd-Fe-B-based sintered magnets where R is neodymium (Nd) have excellent magnetic properties, and are thus used for various components. In 20 addition, because industrial motors are often used in a high temperature environment exceeding 100°C, attempts have been made to improve coercive force and heat resistance by adding heavy rare earth elements such as dysprosium (Dy) to Nd-T-B-based permanent magnets. 25 [0003] In recent years, the production of Nd-Fe-B-based sintered magnets has been expanded, and the consumption of Nd and Dy has been increased. Unfortunately, Nd and Dy are expensive and also have a procurement risk due to high distribution unevenness. In view of this, a possible 30 measure for reducing the consumption of Nd and Dy is to use other rare earth elements as R, such as cerium (Ce), lanthanum (La), samarium (Sm), scandium (Sc), gadolinium (Gd), yttrium (Y), and lutetium (Lu). With these elements 3 substituted for all or a part of Nd, unfortunately, magnetic properties are known to be significantly degraded. In the case of using these elements for producing Nd-Fe-Bbased sintered magnets, therefore, attempts have been 5 conventionally made to develop technology that allows for preventing the magnetic properties from degrading with temperature rise. [0004] Patent Literature 1 discloses a permanent magnet having a tetragonal R2Fe14B crystal structure and having the 10 composition formula (Nd1-x-yLaxSmy)2Fe14B, in which x is in the range of 0.01 to 0.16 and y is in the range of 0.01 to 0.16. According to Patent Literature 1, the addition of La and Sm in the above composition range to the Nd-Fe-B-based permanent magnet prevents the magnetic properties from 15 degrading with temperature rise. [0005] Patent Literature 2 discloses a rare earth sintered magnet expressed by the composition formula (R1x+R2y)T100-x-y-zQz and including crystal grains having Nd2Fe14B-type crystal structures as main phases, in which x 20 is in the range of 8 at% to 18 at%, y is in the range of 0.1 at% to 3.5 at%, z is in the range of 3 at% to 20 at%, and at least a part of the grain boundary phase has a higher concentration of R2 than the main phase crystal grains. R1 is at least one element selected from the group 25 consisting of all rare earth elements except La, Y, and Sc, and R2 is at least one element selected from the group consisting of La, Y, and Sc. T is at least one element selected from the group consisting of all transition elements, and Q is at least one element selected from the 30 group consisting of B and carbon (C). According to Patent Literature 2, diffusion of elements such as Y throughout the grain boundary phase allows rare earth elements essential for the main phase such as Nd and Pr to be 4 efficiently used without being consumed in the grain boundary phase. As a result, it is possible to provide a rare earth sintered magnet which maintains high magnetization of the main phase and exhibits excellent 5 magnetic properties. Citation List Patent Literature [0006] Patent Literature 1: PCT Patent Application Laid10 open No. 2019/111328 Patent Literature 2: Japanese Patent Application Laid-open No. 2002-190404 Summary 15 Technical Problem [0007] The permanent magnet described in Patent Literature 1 does not have a crystalline subphase, and there is a high possibility that La and Sm added to Nd2Fe14B are uniformly dispersed in the permanent magnet. For this 20 reason, the concentration of Nd in the main phase can be relatively lower than that of a normal Nd2Fe14B magnet, resulting in deterioration of magnetic properties at room temperature. For the rare earth sintered magnet described in Patent Literature 2, there is a possibility that 25 magnetic properties are significantly degraded as the temperature rises. In order for the rare earth sintered magnet described in Patent Literature 2 to maintain high coercive force, in addition, an element contributing to improvement of magnetic properties such as Co and nickel 30 (Ni) needs to be added. Furthermore, if Ce is added in the rare earth sintered magnet described in Patent Literature 2, Ce exists substantially uniformly in the sintered magnet, and the magnetization monotonically decreases as the amount 5 of Ce added increases. Thus, the rare earth sintered magnet described in Patent Literature 2 has room for improvement in terms of magnetic properties. For this reason, there has been a demand for permanent magnets 5 capable of maintaining magnetic properties at room temperature equivalent to those of Nd-Fe-B-based sintered magnets as well as preventing the magnetic properties from degrading with temperature rise. [0008] The present disclosure has been made in view of 10 the above, and an object thereof is to obtain a rare earth sintered magnet capable of maintaining the magnetic properties at room temperature equivalent to those of NdFe-B-based sintered magnets as well as preventing the magnetic properties from degrading with temperature rise, 15 reducing the use of Nd and Dy. Solution to Problem [0009] To solve the above problem and achieve the object, the present disclosure provides a rare earth sintered 20 magnet comprising: a main phase satisfying general formula (Nd, La, Sm)-Fe-B and including a crystal grain based on an R2Fe14B crystal structure; and a crystalline subphase based on an oxide phase represented by (Nd, La, Sm)-O. The subphase has a higher concentration of Sm than the main 25 phase. Advantageous Effects of Invention [0010] The present disclosure can achieve the effect of maintaining magnetic properties at room temperature 30 equivalent to those of Nd-Fe-B-based sintered magnets as well as preventing magnetic properties from degrading with temperature rise, reducing the use of Nd and Dy. 6 Brief Description of Drawings [0011] FIG. 1 is a diagram schematically illustrating an exemplary sintered structure of a rare earth sintered magnet according to a first embodiment. 5 FIG. 2 is a diagram illustrating atomic sites in a tetragonal Nd2Fe14B crystal structure. FIG. 3 is a flowchart illustrating an exemplary procedure of a method for producing a rare earth magnet alloy according to a second embodiment. 10 FIG. 4 is a diagram schematically illustrating the method for producing a rare earth magnet alloy according to the second embodiment. FIG. 5 is a flowchart illustrating an exemplary procedure of a method for producing a rare earth sintered 15 magnet according to the second embodiment. FIG. 6 is a cross-sectional view schematically illustrating an exemplary configuration of a rotor equipped with a rare earth sintered magnet according to a third embodiment. 20 FIG. 7 is a cross-sectional view schematically illustrating an exemplary configuration of a rotary machine equipped with a rotor according to a fourth embodiment. FIG. 8 is a composition image obtained by analyzing a cross section of a rare earth sintered magnet according to 25 Examples 1 to 5 with a field emission electron probe microanalyzer. FIG. 9 is an element map of Nd obtained by analyzing a cross section of a rare earth sintered magnet according to Examples 1 to 5 with FE-EPMA. 30 FIG. 10 is an element map of La obtained by analyzing a cross section of a rare earth sintered magnet according to Examples 1 to 5 with FE-EPMA. FIG. 11 is an element map of Sm obtained by analyzing 7 a cross section of a rare earth sintered magnet according to Examples 1 to 5 with FE-EPMA. FIG. 12 is an element map of Fe obtained by analyzing a cross section of a rare earth sintered magnet according 5 to Examples 1 to 5 with FE-EPMA. FIG. 13 is an element map of B obtained by analyzing a cross section of a rare earth sintered magnet according to Examples 1 to 5 with FE-EPMA. FIG. 14 is a diagram illustrating the core-shell 10 structure of Nd in the main phase of the composition image in FIG. 8 by comparison between the composition image in FIG. 8 and the element map of Nd in FIG. 9. FIG. 15 is a diagram illustrating the core-shell structure of Nd in the main phase of the element map of Nd 15 in FIG. 9 by comparison between the composition image in FIG. 8 and the element map of Nd in FIG. 9. Description of Embodiments [0012] A rare earth sintered magnet, a method for 20 producing a rare earth sintered magnet, a rotor, and a rotary machine according to embodiments of the present disclosure will be hereinafter described in detail with reference to the drawings. [0013] First Embodiment. 25 FIG. 1 is a diagram schematically illustrating an exemplary sintered structure of a rare earth sintered magnet according to the first embodiment. The permanent magnet according to the first embodiment is a rare earth sintered magnet 1 including a main phase 10 and a 30 crystalline subphase 20. The main phase 10, which satisfies general formula (Nd, La, Sm)-Fe-B, is a crystal grain based on a tetragonal R2Fe14B crystal structure. The subphase 20 is based on an oxide phase represented by (Nd, 8 La, Sm)-O. [0014] The main phase 10 has a tetragonal R2Fe14B crystal structure where R is Nd, La, and Sm. That is, the main phase 10 has the composition formula (Nd, La, Sm)2Fe14B. 5 The result of calculation of magnetic interaction energy using a molecular orbital method shows that a composition having La and Sm added to Nd forms a practical rare earth sintered magnet 1. This the reason why R of the rare earth sintered magnet 1 having a tetragonal R2Fe14B crystal 10 structure is rare earth elements consisting of Nd, La, and Sm. Note that adding too much La and Sm causes a decrease in the amount of Nd that is an element having a high magnetic anisotropy constant and a high saturation magnetic polarization. As a result, the magnetic properties will 15 degrade. For this reason, the composition ratio of Nd, La, and Sm is preferably Nd>(La+Sm). [0015] The average grain size of the crystal grain of the main phase 10 is preferably 100 μm or less. As illustrated in FIG. 1, the crystal grain of the main phase 20 10 include a core portion 11 and a shell portion 12 provided on the outer periphery of the core portion 11. The shell portion 12 may cover a part of the outer periphery of the core portion 11. In one example, the shell portion 12 is provided in a region in contact with 25 the subphase 20. [0016] In the first embodiment, the concentration of Nd in the shell portion 12 is equal to or higher than the concentration of Nd in the core portion 11. The concentration of Nd in the shell portion 12 is desirably in 30 the range of one to five times the concentration of Nd in the core portion 11. Nd in the core portion 11 is low in concentration and is partly replaced by La and Sm, thereby making it possible to reduce the material cost as compared 9 with general Nd2Fe14B magnets. In addition, since the shell portion 12 having a higher Nd concentration than the core portion 11 is provided at the peripheral edge of the crystal grains, it is possible to improve magnetic 5 anisotropy and prevent magnetization reversal. As described above, the crystal grains of the main phase 10 having a core-shell structure make it possible to prevent degradation of magnetic properties as well as reducing use of Nd. 10 [0017] The crystalline subphase 20 is present between the main phases 10. As described above, the crystalline subphase 20 is represented by (Nd, La, Sm)-O, where (Nd, La, Sm) means that a part of Nd is replaced by La and Sm. [0018] In the rare earth sintered magnet 1 according to 15 the first embodiment, La is segregated in the subphase 20 and coats at least a part of the surface of the main phase 10. Sm is dispersed in the main phase 10 and the subphase 20 with a difference in concentration of Sm between the main phase 10 and the subphase 20. Specifically, the 20 concentration of Sm in the subphase 20 is higher than the concentration of Sm in the main phase 10. The relational expression 1