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Green’s Function Theory in MagnetismThe Green’s function method is a very general method in quantum field theoiy. It can be used for various problems in many areas such as condensed matter, nuclear physics and elementaryparticle physics [115]. The general formulation is rather complicated, abstract, not suitable for a quick application. For our purpose, we present in this chapter a simplified version which, from the beginning, aims at an application of the method to systems of interacting spins. This formulation does not need a high level of knowledge in quantum field theoiy. A basic level in quantum mechanics is enough to understand and to apply the method. As seen below, the Green's function method is an alternative technique to study spin waves, in addition to the theory of magnons presented in Chapter 3. We have seen that the theory of free magnons is exact at veiy low temperatures. However, at higher temperatures we have to take into account magnonmagnon interactions. For example, we have to treat terms of higher orders in the HolsteinPrimakoff expansion [see Eqs. (3.32)(3.34)] to modify the harmonic magnon spectrum given by Eqs. (3.39) and (3.40). This is a laborious task [91]. An alternative and, by far, simpler way to take into account some correlations is the Green’s function method which can treat the whole temperature range going from Physics of Magnetic Thin Films: Theory and Simulation Hung T. Diep Copyright © 2021 Jenny Stanford Publishing Pte. Ltd. ISBN 9789814877428 (Hardcover), 9781003121107 (eBook) www.jennystanford.com lowtemperature phase up to the transition temperature T_{c}. This is possible because Green's function includes even at the lowest level some spinspin correlations. We can therefore follow the evolution of the magnon spectrum with increasing temperature. However, since all correlations are not hierarchically included, its validity is still an open question. Note that there is at present no other better method to calculate the spin wave spectrum at finite temperatures. Numerous applications of the Green's function method in surface magnetism are shown in Part II of this book. Green’s Function MethodDefinitionLet Л(Г) and B(t') be two operators in the Heisenberg representation at times t and t', respectively. We define the retarded Green's function by
and the advanced Green’s function by where
[Л(С), being the commutation relation and < • • ■ > denoting the thermal average. In spite of the complicated notation in its definition given above, Green's function is just a thermal average of a commutation relation between two operators. Green's function is connected, as will be seen below, to the physical properties of the system. So, depending on what we want to study, we choose the operators Л(Г) and B[f'). For instance, to study an electron gas, we can choose Л(Г) = o,^{+} (t) and B(f') = a,(f') where a,^{+}(f) and are creation and annihilation operators of electron state /, to study phonon excitations we can choose Л (t) and B(t') as phonon creation and annihilation operators, and for a spin system we can choose Л (t) = and B(t) = S“(t') where S,^{+}(t) and are spin operators (cf. Chapter 3). Green's function contains information on the system, in particular on the elementaiyexcitation spectrum. In what follows, we present the formulation of the method as it is applied to a spin system so that the reader can associate mathematical tools presented here to physical meanings of a phenomenon that has been seen in Chapter 3. FormulationWe consider the following functions:
The time Fourier transformation of Fba (t'  t) is written as
We show below that where p = [квТ)~^{г}. Demonstration: We use essentially the circular permutation properties of operators in Tr[ • • ] and Л(Г) = e' ^{Ht}Ae~^{,Ht}:
We have thus The Fourier transformation of F_{AB}[t — t^{7}) is Using this formula for Green’s function we obtain
where we have used the following expression:
Integrating on (t — t') and using the formula we arrive at
In the same manner, we obtain
The difference between the retarded and advanced Green's functions is the sign in front of ie in the denominator. We write from now on these functions without superscripts r and a, but we distinguish them by the sign in their arguments. Combining these functions we write We shall use this relation to calculate the spinspin correlation in the following sections. Ferromagnetism by the Green’s Function MethodWe consider the Heisenberg spins with a ferromagnetic interaction between nearest neighbors. The Hamiltonian is given by
where the spin operators obey the commutation relations
For a ferromagnet, we define the following Green's function:
We set t' = 0 hereafter to simplify the writing. Equation of MotionThe equation of motion for is written as
where the sum on p is performed on the vectors connecting spin S/ to its nearest neighbors. Remark: We have used the identity [AB, С] = [А, С] В + A [£?, C] then (4.19) and (4.20) to calculate ['И, S,^{+}] (see Problem 2 in Section 4.6). We see that the righthand side of Eq. (4.22) contains Green's functions of higher order with three operators. Writing the equation of motion for these functions will generate functions of five operators. In a first approximation, we reduce higher order Green’s functions by using the socalled Tyablikov decoupling scheme [346]
We obtain on the righthand side of Eq. (4.22) Green's functions of the same order as the one initially defined in (4.21). This decoupling bears the same spirit as the meanfield theory (Chapter 2): replacing an operator in a product by its average value, i.e., by a ''cnumber.” This approximation is called sometimes "randomphase approximation” (RPA). The RPA is hierarchically higher than the meanfield theory in the sense that one operator in a threeoperator product is replaced by its average value in the RPA while in the meanfield theory one operator in a twooperator product is replaced. We notice that Sf is a constant of motion. Therefore, in a homogeneous system, we can suppose that
We obtain then Dispersion RelationThe time Fourier transformation of Eq. (4.26) gives
We use next the following spatial Fourier transformation:
where the sum on the wave vector к is performed in the first Brillouin zone of N states. We obtain
where Z is the coordination number. We get where As seen in Eq. (4.30), the singularity of the Green’s function is at hw = e_{k}. This determines the eigenenergy hw_{k} of the magnon of wave vector k:
This is the ferromagnetic magnon dispersion relation which is to be compared to Eq. (3.40) obtained by the theory of magnons:
We see that apart from the factor 2 due to the model defined by (3.30) the only difference is that < S^{z} > appears in (4.33) instead of S. As < S^{2} > depends on the temperature, the magnon spectrum varies with T. We calculate < S^{2} > in the following. Magnetization and Critical TemperatureWe write
where we have used (4.19). For S = 1/2, we have (Sf)^{2} = 1/4 (see Pauli's matrices in Chapter 1). We get then
from which we have
We now calculate < Sj~S+ >. We have
from which, for t = 0,
Using (4.17) we write
Replacing G_{k}(ш + /e) and G_{k}(a>— ie) by (4.30) and using (4.14), we obtain Equation (4.36) becomes
This is an implicit equation for < Sf >: The righthand side contains also < Sf > in w_{k}. Therefore, we have to solve this equation by iteration to get < Sf >. At low temperatures, we follow the same method as that used for (3.46). We obtain the following result:
where o, (/ = 1, 2, 3, • • •) are constants. We notice that the T^{3} term does not exist in the lowtemperature expansion (3.49) of the theory of magnons. It may be due to the Tyablikov decoupling (4.23) and (4.24), which is the only approximation used here. To improve the decoupling, the reader is referred to the references [222, 340]. At high temperatures, < Sf >*■ 0. An expansion of e^^{/kUk} yields
At T = T_{c}, < Sf >= 0. Equation (4.43) gives then
In the same manner, for spins of amplitude S Ф 1/2, using (Sf)^{2} = (S/ • S/)/3 = S(S + 1)/3 in Eq. (4.34) one obtains
The transition temperature is given by Remark: When transforming the sum on к in (4.44) into an integral, we clearly see that the integral diverges at small к for dimensions d = 1 and 2 (see the discussion in the conclusion of Chapter 3). As a consequence, T_{c} = 0 for these low dimensions, in agreement with exact results [231] (see Chapter 5). For d = 3, the values of T_{c }numerically calculated by (4.44) are
These values are much lower than those given by the meanfield theory: 3, 4 and 6, respectively. The present results are better than those of the meanfield theory because in the Green's function method, fluctuations due to spinspin correlations are partially taken into account. Antiferromagnetism by the Green’s Function MethodIn a spin system with an antiferromagnetic interaction, we have to define two Green's functions, the first one concerns the correlation between spins of the same sublattice and the second one expresses the correlation between spins belonging to the two sublattices. We consider a system of Heisenberg spins with an antiferromagnetic interaction J between nearest neighbors
where the spin operators satisfy the commutation relations (4.19) and (4.20). We define the following Green's functions:
where /, /' belong to sublattice of f spins and m belongs to sublattice of i spins. For writing simplicity, we take t = 0. The equations of motion for G//(t) and are written as, after the Tyablikov decoupling,
where we have used the fact that nearest neighbors of a / site are on m sites and vice versa, namely 1 + p is a site m of the 4 sublattice and that m + p is a site / of the t sublattice. As in the ferromagnetic case we have supposed < Sf > = < S^{z}m+ > =  < S^{z}n > = — < S^{z}+~ > = < S^{z} >, independent of the lattice site. We use the Fourier transformations
where the factor 2/N comes from the fact that each sublattice has N/2 sites. The equations (4.50) and (4.51) become
where A_{k} = ZJ < S^{z} > and B_{k} = ZJ < S^{z} > yu. The solution of (4.54) and (4.55) is
where
The singularities of these functions are ±e_{k}. The magnon mode к has thus two opposite precessions. This degeneracy comes from the reversal symmetry of the two sublattices. The antiferromagnetic dispersion relation is
It is noted that
To calculate the magnetization we follow the same method used in the ferromagnetic case. We have
Replacing /_{k}(a>and f(_{k}(a>each with its corresponding Green's function by using (4.17), we obtain
Using these relations in (4.60) and (4.61), we have
For 5 = 1/2, we replace < Sf S_{(}^{+} > by 1/2— < S^{z} > in (4.64). We finally arrive at This implicit equation for < S^{z} > should be solved by numerical iteration. At low temperatures, expanding the righthand side of (4.66) when T *■ 0 [p »■ oc), one has where AS is the zeropoint spin contraction (at 7 = 0) (see Chapter 3). As seen, the temperature dependence of < S^{z} > is not the same as in the ferromagnetic case. This is a consequence of the linear dependence on к of e_{k} at small k. The Neel temperature T_{N} is calculated by letting < S^{z} >> 0 in (4.66). One has Green’s Function Method for NonCollinear MagnetsIn frustrated spin systems, the competition between different kinds of interaction can give rise to groundstate spin configurations which are not collinear. The incompatibility of some lattice geometries with antiferromagnetic interaction yields also noncollinear spin configurations as those in the antiferromagnetic triangular lattice, in the antiferromagnetic facecentered cubic lattice or hexagonal closepacked lattice. Such frustrated systems show many striking properties. The reader is referred to Ref. [85] for recent reviews on many aspects of the frustration. For noncollinear magnets, the Hamiltonian can be expressed in the local coordinates. This has been done in Eq. (3.124). We define the following Green's functions: With the use of the Tyablikov decoupling [346], the equations of motion of these functions are given by
Note that the sinus terms in Eq. (3.124) are canceled out upon summing over symmetric neighbors at each lattice site (inversion symmetiy) in the above equations. The next steps are similar to those used in the antiferromagnetic case in the previous section: Using the Fourier transformations, the solution of the resulting coupled equations gives the dispersion relation which allows us to calculate the magnetization and the transition temperature. Some applications are given as problems in Section 4.6. ConclusionWe have presented in this chapter the Green’s function method, which is used to investigate spin waves in ferromagnetic, antiferromagnetic and noncollinear magnets. The method gives the temperaturedependence of the magnon spectrum and a compact expression which allows us to calculate the magnetization up to the transition temperature. To keep the method simple and tractable, we have used the Tyablikov decoupling scheme to reduce higherorder Green's functions. There is room for improving it but we will lose the simplicity of the method. The dispersion relation can be explicitly obtained in simple cases. However, for complicated systems where we have to define several Green's functions, we obtain a system of coupled linear equations which can be numerically solved to obtain the dispersion relation which is used to compute the temperature dependence of various physical quantities. Such applications are treated in part II of this book. We note that the method, though efficient, is not accurate enough to allow us to calculate critical exponents of the phase transition. However, it can detect firstorder transitions as well as multiple phase transitions of the system at different temperatures as seen in Ref. [285] and in part II. ProblemsProblem 1. Give proofs of the formula (4.13). Problem 2. Give the demonstration of Eq. (4.22). Problem 3. Helimagnet by Green’s function method: Consider a crystal of simple cubic lattice with Heisenberg spins of amplitude 1/2. The interaction Ji between nearest neighbors is ferromagnetic. Suppose that along the у axis there exists an antiferromagnetic interaction ]_{2} between next nearest neighbors, in addition to/j. (a) Follow the method in Section 3.4, show that the ground state is helimagnetic in they direction if I/2I//1 is larger than a critical value Determine a_{c}.
Problem 4. Apply the Green’s function method to a system of Ising spins S = ±1 in one dimension, supposing a ferromagnetic interaction between nearest neighbors under an applied magnetic field. Problem 5. Apply the Green’s function method to a system of Heisenberg spins on a simple cubic lattice, supposing ferromagnetic interactions between nearest neighbors and between nextnearest neighbors. Problem 6. Magnon spectrum in Heisenberg triangular antiferro magnet: Green’s function method Calculate the magnon spectrum of a triangular lattice with spins 1/2 interacting with each other via an antiferromagnetic interaction between nearest neighbors. Estimate numerically the zeropoint spin contraction. Guide: Use the spin configuration obtained in Problem 6 of Section 3.6. 
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