## 1 INTRODUCTION. THE MAIN ANOMALIES IN THE MAGNETIC PROPERTIES OF FeRh ALLOYS

_{tr}~ 320–370 K [1‐5]. The AFM–FM phase transition in FeRh is accompanied not only by an abrupt change in the magnetization, \(\Delta M,\) but also by an increase in the size of the unit cell, \({{{{\Delta }}a} \mathord{\left/ {\vphantom {{{{\Delta }}a} a}} \right. \kern-0em} a} \approx 0.01\) [6], and significant changes in the electrical resistivity [1, 5]. In the vicinity of the AFM–FM transition, anomalously large values of magnetostriction [7, 8], magnetoresistance [9, 10], and elastocaloric [11] and magnetocaloric effects [12, 13] are observed. Studies have shown that the temperature intervals for the existence of AFM and FM states and the critical temperature (T

_{tr}) in FeRh-based alloys can be changed over a wide range by applying external pressure [6, 14], as well as by small substitutions of iron or rhodium by atoms of other d-metals [10, 15‐17]. In connection with the prospects for the practical use of FeRh-based alloys not only for magnetic cooling, but also in magnetic recording devices, in recent years, intensive studies of the dynamics of the nature of the AFM–FM transition have been carried out by various methods. The main results on the magnetic and magnetothermal properties of FeRh alloys are presented in a recently published review [5].

_{loc}are given in Section 4. The results we obtained are discussed in Section 5.

## 2 A MODEL OF THE ENERGY SPECTRUM OF DELOCALIZED 3d ELECTRONS OF Fe AND 4d ELECTRONS OF Rh

_{j}, which are the quantum numbers of such localized states. The ordering of spins is determined by their exchange interaction V

_{ex}. Negative values of the exchange energy provide the FM ordering. The ordering of the AFM type is realized if terms with positive exchange parameters dominate. This means that the AFM–FM phase transition in the model of localized electrons requires changes in the atomic structure of the matter.

## 3 CONDITIONS FOR AFM ORDERING OF 3d SPINS OF Fe IN FeRh ALLOYS

### 3.1 Estimation of Parameters J_{33}, J_{44}, and J_{34}
for a Uniform Distribution of 3d and 4d Electrons

_{33}determines the exchange energy of 3d spins in the AFM phase, which, at \(T < {{T}_{{{\text{tr}}}}} \approx 370~\,{\text{K}}\), can be estimated as [5]

### 3.2 Estimation of Parameters J_{33}, J_{44}, and J_{34}
for a Nonuniform Distribution of 3d and 4d Electrons

_{0.9875}Ni

_{0.0125})

_{0.49}Rh

_{0.51}alloy in the AFM phase. The hypothesis of the existence of local moments \({{\mu }_{{{\text{loc}}}}}\) (20) radically affects the estimate of the exchange parameters in (9) and (13). First, for a nonuniform distribution of 3d electrons, the exchange interaction is no longer determined by the magnetic moments of the iron ions, but by the local moments \({{\mu }_{{{\text{loc}}}}},\) that arise at the maxima of densities of 3d electrons. Second, the estimate of the parameter \({{J}_{{33}}}\) changes, because \({{V}_{{33}}}\) becomes dependent on \(\nu \) (41). For the composition (Fe

_{0.9875}Ni

_{0.0125})

_{0.49}Rh

_{0.51}, the transition temperature is \({{T}_{{{\text{tr}}}}} = 283~\,{\text{K}}{\text{.}}\) As a result, \({{V}_{{33}}}\) is estimated as

## 4 THE TEMPERATURE AND FIELD DEPENDENCES OF THE MAGNETIC SUSCEPTIBILITY OF FeRh ALLOYS IN THE AFM PHASE

_{0.9875}Ni

_{0.0125})

_{0.49}Rh

_{0.51}alloy (Fig. 2). The sample had the shape of a parallelepiped with linear dimensions \(2 \times 2 \times 6\) mm. The field was applied along the long side of the sample. The method of obtaining and certification of the sample was described in detail in [10].