spinw.genmagstr method

generates magnetic structure

Syntax

genmagstr(obj,Name,Value)

Description

genmagstr(obj,Name,Value) is a Swiss knife for generating magnetic structures. It allows the definition of magnetic structures using several different ways, depending on the mode parameter. The generated magnetic structure is stored in the [obj.mag_str] field. The magetic structure is stored as Fourier components with arbitrary number of wave vectors in the spinw object. However spin waves can be only calculated if the magnetic structure has a single propagation vector (plus a k=0 ferromagnetic component perpendicular to the incommensurate magnetization), we simply call it single-k magnetic structure. Thus genmagstr enables to input magnetic structures that comply with this restriction by defining a magnetic structure by the moment directions (S) in the crystallographic cell, a propagation vector (km) and a vector that defines the normal of the rotation of the spin spiral (n). The function converts these values into Fourier components to store. To solve spin wave dispersion of magnetic structures with multiple propagation vectors, a magnetic supercell has to be defined where the propagation vector can be approximated to zero.

Examples

The example creates the multi-k magnetic structure of USb given by the FQ Fourier components and plots the magnetic structure:

USb = spinw
USb.genlattice('lat_const',[6.203 6.203 6.203],'spgr','F m -3 m')
USb.addatom('r',[0 0 0],'S',1)
FQ = cat(3,[0;0;1+1i],[0;1+1i;0],[1+1i;0;0])

Output

FQ(:,:,1) =
0000 + 0.0000i
0000 + 0.0000i
0000 + 1.0000i
FQ(:,:,2) =
0000 + 0.0000i
0000 + 1.0000i
0000 + 0.0000i
FQ(:,:,3) =
0000 + 1.0000i
0000 + 0.0000i
0000 + 0.0000i

k  = [0 0 1;0 1 0;1 0 0];
USb.genmagstr('mode','fourier','S',FQ,'nExt',[1 1 1],'k',k)
plot(USb,'range',[1 1 1])

Input Arguments

obj: spinw object.

Name-Value Pair Arguments

'mode'

Mode that determines how the magnetic structure is generated:

'random' (reads -) generates random zero-k magnetic structure.
'direct' (reads S, n, k) direct input of the magnetic structure using the parameters of the single-k magnetic structure.
'tile' (reads S, n, k) Simply extends the existing or input structure (S) into a magnetic supercell by replicating it. If no structure is stored in the spinw object a random structure is generated automatically. If defined, S is used as starting structure for extension overwriting the stored structure. If the original structure is already extended with other size, only the moments in the crystallographic cell wil be replicated. Magnetic ordering wavevector k will be set to zero. To generate structure with non-zero k, use the 'helical' or 'direct' option.
'helical' (reads S, n, k, r0) generates helical structure in a single cell or in a supercell. In contrary to the 'extend' option the magnetic structure is not generated by replication but by rotation of the moments using the following formula:
$\mathbf{S}^{gen}_i(\mathbf{r}) = R(2 \pi \mathbf{k_m} \cdot \mathbf{r})\cdot \mathbf{S}_i$
where $S_i$ has either a single moment or as many moments as the number of magnetic atoms in the crystallographic cell. In the first case $r$ denotes the atomic positions, while for the second case $r$ denotes the position of the origin of the cell.
'rotate' (reads S, n, k) uniform rotation of all magnetic moments with a phi angle around the given n vector. If phi=0, all moments are rotated so, that the first moment is parallel to n vector in case of collinear structure or in case of planar structure n defines the normal of the plane of the magnetic moments.
'func' (reads func, x0) function that defines the parameters of the single-k magnetic structure: moment vectors, propagation vector and normal vector from arbitrary parameters in the following form:
```
[S, k, n] = @(x)func(S0, x)
```
where S is matrix with dimensions of $[3\times n_{magExt}]$ . k is the propagation vector in a 3-element row vector. n is the normal vector of the spin rotation plane also 3-element row vector. The default value for func is @gm_spherical3d. For planar magnetic structure use @gm_planar. Only func and x have to be defined for this mode.
'fourier' (reads S, n, k, r0) same as 'helical', except the S option is taken as the Fourier components, thus if it contains real numbers, it will generate a sinusoidally modulated structure instead of a spiral.

'phi'

Angle of rotation of the magnetic moments in radian. Default value is 0.

'phid'

Angle of rotation of the magnetic moments in °. Default value is 0.

'nExt'

Size of the magnetic supercell in multiples of the crystallographic cell, dimensions are $[1\times 3]$ . Default value is stored in obj. If nExt is a single number, then the size of the extended unit cell is automatically determined from the first magnetic ordering wavevector. E.g. if nExt = 0.01, then the number of unit cells is determined so, that in the extended unit cell, the magnetic ordering wave vector is [0 0 0], within the given 0.01 rlu tolerance.

'k'

Magnetic ordering wavevector in rlu, dimensions are $[n_K\times 3]$ . Default value is defined in obj.

'n'

Normal vector to the spin rotation plane for single-k magnetic structures, stored in a 3-element row vector. Default value [0 0 1]. The coordinate system of the vector is determined by unit.

'S'

Vector values of the spins (expectation value), dimensions are $[3\times n_{spin} n_K]$ . Every column defines the three $(S_x, S_y, S_z)$ components of the spin (magnetic moment) in the $xyz$ Descartes coodinate system or in lu. Default value is stored in obj.

'unit'

Determines the coordinate system for S and n vectors using a string:

'xyz' Cartesian coordinate system, fixed to the lattice. Default value.
'lu' Lattice coordinate system (not necessarily Cartesian. The three coordinate vectors are parallel to the lattice vectors but normalized to unity.

'epsilon'

The smalles value of incommensurability that is tolerated without warning in lattice units. Default is $10^{-5}$ .

'func'

Function handle that produces the magnetic moments, ordering wave vector and normal vector from the x parameters in the following form:

[M, k, n] = @(x)func(M0,x)

where M is a matrix with dimensions of $[3\times n_{magExt}]$ , k is the propagation vector, n is the normal vector of the spin rotation plane. The default function is gm_spherical3d. For planar magnetic structure use gm_planar.

'x0'

Input parameters for func function, row vector with $n_X$ number of elements.

'norm'

Set the length of the generated magnetic moments to be equal to the spin of the magnetic atoms. Default is true.

'r0'

If true and only a single spin direction is given, the spin phases are determined by atomic position times k-vector, while if it is false, the first spin will have 0 phase. Default is true.

Output Arguments

The [obj.mag_str] field will contain the new magnetic structure.