# Draw a spiral on a sphere

My goal is to visualize damped magnetic precession.

Wikipedia features an image, but it doesn't quite capture one essential constraint. Namely, that the magnetization M should be normalized. So the curve shown in the following picture should lie on the sphere.

So what I want to show is:

• The vectors M and H_eff with M on the sphere.
• A spiral lying on a sphere.
• -M x H_eff being orthogonal to M and H_eff
• M x dM/dt pointing towards H_eff (This is not exactly correct, but rather an approximation).

The tangent of the spiral at the endpoint of M should be a linear combination of MxdM/dt and -MxH_eff (to be more exact: alpha MxdM/dt - MxH_eff for some positive alpha), so the picture on Wikipedia looks fine concerning this requirement.

I have found a similar picture in the following answer: http://tex.stackexchange.com/a/56617/50081

How can this be achieved with any of the modern plotting tools for LaTeX?

Edit: As Christian pointed out one could start by generating the spiral via projection of a flat spiral. This is my first try using pgfplots.

\documentclass{minimal}
\usepackage{tikz}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\xdef\w{10}

\begin{axis}[%
axis equal,
axis lines = none,
xlabel = {$x$},
ylabel = {$y$},
zlabel = {$z$},
enlargelimits = 0.5,
ticks=none,
]
opacity = 0.2,
surf,
z buffer = sort,
samples = 21,
variable = \u,
variable y = \v,
domain = 0:180,
y domain = 0:360,
]
({cos(u)*sin(v)}, {sin(u)*sin(v)}, {cos(v)});

\addplot3+[color=blue,domain=0:4*pi, samples=100, samples y=0,no marks, smooth](
{x*cos(deg(x))/sqrt(\w*\w+x*x)},
{x*-sin(deg(x))/sqrt(\w*\w+x*x)},
{\w/sqrt(\w*\w+x*x)}
);
\end{axis}
\end{tikzpicture}
\end{document}

-
Welcome to TeX.SX! We would appreciate a minimaly working example as frame to start with! -- You need the curve vector, probably. A curve within a surface with curvature. Is the solution known? – Christian Hupfer Apr 30 '14 at 12:20
It is the solution to the Landau-Lifshitz-Gilbert-PDE, but I think generating the solution would be too much of an effort, so I would be content with any spirally shape... – knedlsepp Apr 30 '14 at 12:25
Do you know some differential geometry? – Christian Hupfer Apr 30 '14 at 12:26
To some extent: yes. – knedlsepp Apr 30 '14 at 12:28
As far as I know, Matlab allows for export to tikz. – Christian Hupfer Apr 30 '14 at 12:58

Here's an attempt using Asymptote. I took literally your statement that "any spirally shape" would be okay. To compile it, save the code below in a file called (e.g.) filename.tex and then run pdflatex --shell-escape filename. (Also, make sure you have Asymptote installed.)

\documentclass[margin=10pt,convert]{standalone}
\usepackage{asypictureB}
\begin{document}
\begin{asypicture}{name=sphere_spiral}
settings.outformat = "png";
settings.render = 16;
import graph3;
size(10cm);

triple eye = (5,2,3);
currentprojection=orthographic(eye);

surface hemisphere = surface(Arc(X,-X,c=O,normal=Z,n=16), c=O, axis=X, angle1=0, angle2=180);
draw(shift(-5 eye)*hemisphere, material(white + opacity(0.5), emissivepen=0.2 white));

usepackage("amsmath");  //for \text command
draw(O -- 1.6Z, arrow=Arrow3, L=Label("$H_{\text{eff}}$", align=W, position=EndPoint));

real theta(real t) { return t/20; }
real phi(real t) { return -t + 1; }
real r(real t) { return 1; }
triple F(real t) { return polar(r(t), theta(t), phi(t)); }

path3 spiral = reverse(graph(F, 0, 4pi, operator ..));
draw(spiral, blue + dotted + linewidth(1pt));

real t = 0.1;
triple arrowpos = point(spiral, reltime(spiral, t));

t = 0.7;
arrowpos = point(spiral, reltime(spiral, t));

triple M = point(spiral, 0);
draw(O -- M, arrow=Arrow3, L=Label("$M$", position=MidPoint));
draw(shift(M) * (O -- 0.5 cross(-M, Z)), red, arrow=Arrow3, L=Label("$-M \times H_{\text{eff}}$", position=MidPoint));
triple dMdt = dir(spiral,0);
triple crossprod = cross(M, dMdt);
draw(shift(M) * (O -- 0.5 crossprod), blue, arrow=Arrow3, L=Label("$M \times \frac{dM}{dt}$", position=MidPoint));
\end{asypicture}
\end{document}


The result:

-
Thanks! Exactly what I was looking for; sadly the LaTeX integration is not quite as smooth as with pgfplots. – knedlsepp Apr 30 '14 at 18:29
There are several packages for integrating Asymptote into LaTeX; if you state what features you would imagine for "smooth integration," I can tell you whether any of these packages currently supports those features. – Charles Staats Apr 30 '14 at 22:55
I guess there's nothing one can do about it, but: (Some really minor points I noticed playing around) - You can't simply install asymptote like a LaTeX package, (whereas pgfplots just worked out-of-the-box) - I have to change typesetting from LaTeX to asymptote from within TeXShop manually - I managed to get a vector graphic by changing the outformat, but then the sphere isn't smooth anymore. (Well, that's not really about the LaTeX integration, and generating shaded vector graphics may be somewhat difficult.) Nevertheless, asymptote seems to be the way to go for this kind of visualization. – knedlsepp May 1 '14 at 8:22
@knedlsepp: Most of these points can't really be fixed, as you say. However, if you're using TeXShop, and are willing to use shell-escape to make your life simpler, you can go to preferences and then, under Engine, in the pdfTeX box, in the lower of the two text boxes, append the option  --shell-escape. Once you've done this, pictures created with the asypictureB package will run seamlessly with the default LaTeX typesetting engine. – Charles Staats May 1 '14 at 13:52