Appendix A
If a
\(a^{\prime }\times b^{\prime }\) rectangle locates at
\(z^{\prime }=0\), and the observation position vector is located at
\({\mathbf{ r} }=(x,y,z)=(r_1,r_2,r_3)\), as seen in Fig.
2.4a, the three nonzero elements
\(N^{\mathrm{tri}}_{13},\;N^{\mathrm{tri}}_{23}\; {\text {and}}\; N^{\mathrm{tri}}_{33}\) in the demagnetizing matrix of a rectangular surface are (
\(\alpha =1,3\)):
$$\begin{aligned} N^\mathrm{ rec} _{\alpha 3} ({\mathbf{ r} })= -{\frac{1}{{4\pi }}} \int \limits ^{a^{\prime }/2}_{-a^{\prime }/2} \mathrm{ d} r^{\prime }_1 \int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} r^{\prime }_2 {\frac{{r_\alpha -r^{\prime }_\alpha }}{{\ \left[(r_1-r^{\prime }_1)^2+(r_2-r^{\prime }_2)^2+r^2_3\right]^{3/2}\ }}} \end{aligned}$$
(2.79)
We just have to do two integrations for
\(N^\mathrm{ rec} _{33}\) and
\(N^\mathrm{ rec} _{13}\), and the integration of
\(N^\mathrm{ rec} _{23}\) is totally analogy to
\(N^\mathrm{ rec} _{13}\). Let’s start with
\(N^\mathrm{ rec} _{13}\):
$$\begin{aligned} N^\mathrm{ rec} _{13}&= -{\frac{1}{{4\pi }}} \int \limits ^{a^{\prime }/2}_{-a^{\prime }/2} \mathrm{ d} x^{\prime } \int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} y^{\prime } {\frac{{x-x^{\prime }}}{{\ \left[(x-x^{\prime })^2+(y-y^{\prime })^2+z^2\right]^{3/2}\ }}} \nonumber \\&= -{\frac{1}{{4\pi }}} \int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} y^{\prime } \left.{\frac{1}{{\ \left[(x-x^{\prime })^2+(y-y^{\prime })^2+z^2\right]^{1/2}\ }}} \right|^{a^{\prime }/2}_{x^{\prime }=-a^{\prime }/2} \nonumber \\&= -{\frac{1}{{4\pi }}}\sum _q (-q) \ln \left(y^{\prime }-y+ \sqrt{R^2_1+(y-y^{\prime })^2+R^2_3}\right)^{b^{\prime }/2}_{y^{\prime }=-b^{\prime }/2} \nonumber \\&= -{\frac{1}{{4\pi }}}\sum _q \sum _w qw \ln \left(R-wR_2\right) \end{aligned}$$
(2.80)
The variables are defined in Table
2.2. The integration for
\(N^\mathrm{ rec} _{33}\) can also be done:
$$\begin{aligned} N^\mathrm{ rec} _{33}&= -{\frac{1}{{4\pi }}} \int \limits ^{a^{\prime }/2}_{-a^{\prime }/2} \mathrm{ d} x^{\prime } \int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} y^{\prime } {\frac{{z}}{{\big [(x-x^{\prime })^2+(y-y^{\prime })^2+z^2\big ]^{3/2}\ }}} \nonumber \\&= -{\frac{1}{{4\pi }}} \int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} y^{\prime } {\frac{{z}}{{\big [(y-y^{\prime })^2+z^2\big ]\ }}} \left.{\frac{{x^{\prime }-x}}{{\big [(x-x^{\prime })^2+(y-y^{\prime })^2+z^2\big ]^{1/2}\ }}} \right|^{a^{\prime }/2}_{x^{\prime }=-a^{\prime }/2} \nonumber \\&= -{\frac{1}{{4\pi }}}\sum _q \left. \arctan \left( \frac{R_1}{R_3} \frac{y^{\prime }-y}{\sqrt{R^2_1+(y-y^{\prime })^2+R^2_3}} \right) \right|^{b^{\prime }/2}_{y^{\prime }=-b^{\prime }/2} \nonumber \\&= -{\frac{1}{{4\pi }}}\sum _q \sum _w \arctan \,\frac{R_1R_2}{R_3R} \end{aligned}$$
(2.81)
The
\(N^\mathrm{ rec} _{\alpha 3} ({\mathbf{ r} })\) of a rectangular surface have be listed in Table
2.2 respectively.
Appendix A
If the surface located at
\(z^{\prime }=0\) is a right-angle triangle with right-angle side lengths
\((a^{\prime },b^{\prime })\), the origin at the midpoint of the hypotenuse, and the hypotenuse defined by equation
\(y^{\prime }=-sx^{\prime }\) or
\(x^{\prime }=-vy^{\prime }\), and the observation position vector is located at
\({\mathbf{ r} }=(x,y,z)\), there are still three nonzero elements in the demagnetizing matrix of a triangular surface, as seen in Fig.
2.4b. Here we can first do the integral for element
\(N^\mathrm{ tri} _{13}\):
$$\begin{aligned} N^\mathrm{ tri} _{13}=&\; -{\frac{1}{{4\pi }}} \int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} y^{\prime } \int \limits ^{a^{\prime }/2}_{-vy^{\prime }} \mathrm{ d} x^{\prime } {\frac{{x-x^{\prime }}}{{\ \left[(x-x^{\prime })^2+(y-y^{\prime })^2+z^2\right]^{3/2}\ }}} \nonumber \\=&\; -{\frac{1}{{4\pi }}} \int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} y^{\prime } \left\{ \frac{1}{\ \left[(a^{\prime }/2-x)^2+(y-y^{\prime })^2+z^2\right]^{1/2}\ }\right. \nonumber \\&\;- \left.{ \frac{1}{\ \left[(x+vy^{\prime })^2+(y-y^{\prime })^2+z^2\right]^{1/2}\ }} \right\} \nonumber \\=&\; \left.-{\frac{1}{{4\pi }}} \ln \left(y^{\prime }-y+ \sqrt{(a^{\prime }/2-x)^2+(y-y^{\prime })^2+z^2}\,\right) \right|^{b^{\prime }/2}_{y^{\prime }=-b^{\prime }/2} \nonumber \\&\;+{\frac{1}{{4\pi }}}\int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} y^{\prime } \frac{1}{\sqrt{1+v^2}} \frac{1}{\ \left[(y^{\prime }-c_1)^2+r^2/(1+v^2)-c^2_1\right]^{1/2}\ } \nonumber \\=&\; {\frac{1}{{4\pi }}} \sum _w w \ln \left(R_\mathrm{ I} -wR_2\right) \nonumber \\&\;\left.+{\frac{1}{{4\pi }}}{\frac{1}{\sqrt{1+v^2}}} \ln \left(y^{\prime }-c_1+ \sqrt{(y^{\prime }-c_1)^2+c^2_2}\,\right) \right|^{b^{\prime }/2}_{y^{\prime }=-b^{\prime }/2} \nonumber \\=&\; -{\frac{1}{{4\pi }}} \sum _w w \left\{ {\frac{1}{\sqrt{1+v^2}}}\ln (P-wP_2)- \ln (R_\mathrm{ I} -wR_2)\right\}
\end{aligned}$$
(2.82)
The symbols
\(c_1\),
\(c^2_2\),
\(P\),
\(P_2\),
\(R_\mathrm{ I} \), and
\(R_2\) used in Eq. (
2.82) have be defined in Tables
2.2 and
2.3 respectively. The integration for
\(N^\mathrm{ tri} _{23}\) is totally analogy to
\(N^\mathrm{ tri} _{13}\), just with a
\(x\leftrightarrow y\) symmetry, therefore the derivation of
\(N^\mathrm{ tri} _{23}\) will be omitted here.
The integral for the matrix element
\(N^\mathrm{ tri} _{33}\) is the most complicated one, which include three parts:
$$\begin{aligned} N^\mathrm{ tri} _{33}&= -{\frac{1}{{4\pi }}} \int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} y^{\prime } \int \limits ^{a^{\prime }/2}_{-vy^{\prime }} \mathrm{ d} x^{\prime } {\frac{{z}}{{\ \left[(x-x^{\prime })^2+(y-y^{\prime })^2+z^2\right]^{3/2}\ }}} \nonumber \\&= -{\frac{1}{{4\pi }}} \int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} y^{\prime } \left.{\frac{{z}}{{\ \left[(y-y^{\prime })^2+z^2\right]\ }}} {\frac{{x^{\prime }-x}}{{\ \left[(x-x^{\prime })^2+(y-y^{\prime })^2+z^2\right]^{1/2}\ }}} \right|^{a^{\prime }/2}_{x^{\prime }=-vy^{\prime }} \nonumber \\&= -{\frac{1}{{4\pi }}}\left\{ N^{(1)}_{33} + N^{(2)}_{33} + N^{(3)}_{33}\right\} \end{aligned}$$
(2.83)
In Eq. (
2.83), the derivation of the first term
\(N^{(1)}_{33}\) is actually very similar to one of the two terms in Eq. (
2.81) for rectangular surface; in the second term, the numerator of the integrand is
\(z(-vy^{\prime }-x)\), which can be disassembled into two parts
\(zv(-y^{\prime }+y)\) and
\(z(-vy-x)\), and these two part just corresponds to the
\(N^{(2)}_{33}\) and
\(N^{(3)}_{33}\) respectively.
The integration for
\(N^{(1)}_{33}\) is just straightforward:
$$\begin{aligned} N^{(1)}_{33}&= \int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} y^{\prime } {\frac{{z}}{{\ \left[(y-y^{\prime })^2+z^2\right]\ }}} {\frac{{a^{\prime }/2-x}}{{\ \left[(a^{\prime }/2-x)^2+(y-y^{\prime })^2+z^2\right]^{1/2}\ }}} \nonumber \\&= \left.\arctan \left( \frac{a^{\prime }/2-x}{z} \frac{y^{\prime }-y}{\sqrt{(a^{\prime }/2-x)^2+(y-y^{\prime })^2+z^2}}\right) \right|^{b^{\prime }/2}_{y^{\prime }=-b^{\prime }/2} \nonumber \\&= \sum _w \arctan [(a^{\prime }/2-x)R_2/(zR_\mathrm{ I} )] \end{aligned}$$
(2.84)
In the derivation for
\(N^{(2)}_{33}\), complicated variables such as
\(c_1\),
\(c_2\) and
\(c_5=y-\mathrm{ i} z\) in Table
2.3 have to be defined, and the respective integral is:
$$\begin{aligned} N^{(2)}_{33}&= \int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} y^{\prime } {\frac{{zv}}{{\ \left[(y-y^{\prime })^2+z^2\right]\ }}} {\frac{{y^{\prime }-y}}{{\ \left[(x+vy^{\prime })^2+(y-y^{\prime })^2+z^2\right]^{1/2}\ }}} \nonumber \\&= \int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} y^{\prime }{\frac{{zv}}{2}} \left[{\frac{1}{{\ y^{\prime }-c_5\ }}}+{\frac{1}{{\ y^{\prime }-c^*_5\ }}}\right] {\frac{1}{\sqrt{1+v^2}}}{\frac{1 }{{\ \left[(y^{\prime }-c_1)^2+c^2_2\right]^{1/2}\ }}}\ \ \ \ \end{aligned}$$
(2.85)
The previous integration include two terms which are complex conjugates of one another. Now let’s make an integration variable change
\(y^{\prime }-c_1=c_2\sinh \theta \) with the two integration limits of the angle
\(\theta \) as
\(\theta _1=\sinh ^{-1}[(-b^{\prime }/2-c_1)/c_2]\) and
\(\theta _2=\sinh ^{-1}[(b^{\prime }/2-c_1)/c_2]\), the integral in Eq. (
2.85) has the form:
$$\begin{aligned} N^{(2)}_{33}\;&= \;{\frac{{zv}}{{2\sqrt{1+v^2}}}} \int \limits ^{\theta _2}_{\theta _1} \left[{\frac{{\mathrm{ d}\theta }}{{c_1-c_5+c_2\sinh \theta }}} + c.c.\right]\ \ \ \end{aligned}$$
(2.86)
Then make another variable change
\(e^\theta =u\), and define a new constant
\(\sinh \eta =(c_1-c_5)/c_2\), the integral becomes:
$$\begin{aligned} N^{(2)}_{33}\;&= \;{\frac{zv}{\sqrt{1+v^2}}} \int \limits ^{u_2}_{u_1} \left[{\frac{1}{{c_2}}}{\frac{{\mathrm{ d}\left(e^\theta \right)}}{{\left(e^\theta \right)^2+2[(c_1-c_5)/c_2]e^\theta -1}}} + c.c.\right]\ \ \ \nonumber \\&= \;{\frac{zv}{\sqrt{1+v^2}}} \int \limits ^{u_2}_{u_1} \left[\frac{1}{c_2}{\frac{\mathrm{ d}u}{(u+e^\eta )(u-e^{-\eta })}} + c.c.\right]\ \ \ \nonumber \\&= \!\! {\frac{zv}{\sqrt{1+v^2}}} \left\{ \left. {\frac{1}{{2c_2\cosh \eta }}} \ln {\frac{u-e^{-\eta }}{u+e^\eta }} \right|^{u_2}_{u\,=\,u_1} + c.c. \right\} \ \ \ \nonumber \\&= \; {\frac{zv}{\sqrt{1+v^2}}} \sum _w \Re \left\{ \frac{w}{\sqrt{(c_1-c_5)^2+c^2_2}} \ln \frac{V_+}{V_-} \right\} \nonumber \\&= \; {\frac{zv}{\sqrt{1+v^2}}} \sum _w \Re \left\{ \frac{w}{A e^{\mathrm{ i} \theta ^{\prime }/2}} \ln \frac{|V_+|e^{\mathrm{ i} \phi _+}}{|V_-|e^{\mathrm{ i} \phi _-}} \right\} \end{aligned}$$
(2.87)
In the previous derivation, the difficult part is to find integration limits
\(u_1\) and
\(u_2\). Actually
\(\sinh ^{-1}x=\ln [x+\sqrt{x^2+1}]\), therefore
\(u_1\),
\(u_2\),
\(e^\eta \) and
\(e^{-\eta }\) are:
$$\begin{aligned} u_1=e^{\theta _1}=\frac{1}{c_2}\left[-\left(\frac{b^{\prime }}{2}+c_1\right)+\sqrt{\left(\frac{b^{\prime }}{2}+c_1\right)^2+c^2_2}\right] \nonumber \\ u_2=e^{\theta _2}=\frac{1}{c_2}\left[+\left(\frac{b^{\prime }}{2}-c_1\right)+\sqrt{\left(\frac{b^{\prime }}{2}-c_1\right)^2+c^2_2}\right] \nonumber \\ e^\eta =\frac{1}{c_2}\left[+(c_1-c_5)+\sqrt{(c_1-c_5)^2+c^2_2}\right] \nonumber \\ e^{-\eta }=\frac{1}{c_2}\left[-(c_1-c_5)+\sqrt{(c_1-c_5)^2+c^2_2}\right] \end{aligned}$$
(2.88)
By defining
\(P_2={\frac{b^{\prime }}{2}}+w\ \!\! c_1\) (
\(w=+1\) and
\(w=-1\) are for the two integral limits
\(u_1\) and
\(u_2\) respectively),
\(A e^{\mathrm{ i} \theta ^{\prime }/2}\),
\(V_+\) and
\(V_-\) will have the forms in Table
2.3:
$$\begin{aligned} \frac{u-e^{-\eta }}{u+e^\eta }= \frac{-wP_2+\sqrt{P^2_2+c^2_2}+(c_1-c_5)-\sqrt{(c_1-c_5)^2+c^2_2}}{-wP_2+\sqrt{P^2_2+c^2_2}+(c_1-c_5)+\sqrt{(c_1-c_5)^2+c^2_2}} =\frac{V_-}{V_+} \end{aligned}$$
(2.89)
The integration of
\(N^{(3)}_{33}\) is similar to
\(N^{(2)}_{33}\), which also includes imaginary numbers:
$$\begin{aligned} N^{(3)}_{33}&= \int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} y^{\prime } {\frac{{z}}{{\ \left[(y-y^{\prime })^2+z^2\right]\ }}} {\frac{{x+vy}}{{\ \left[(x+vy^{\prime })^2+(y-y^{\prime })^2+z^2\right]^{1/2}\ }}} \\ \nonumber&= \int \limits ^{b^{\prime }/2}_{-b^{\prime }/2} \mathrm{ d} y^{\prime } {\frac{{x+vy}}{{2\mathrm{ i} }}} \left[{\frac{{-1}}{{\ y^{\prime }-c_5\ }}} +{\frac{1}{{\ y^{\prime }-c^*_5\ }}}\right] {\frac{1}{\sqrt{1+v^2}}}{\frac{1 }{{\ \left[(y^{\prime }-c_1)^2+c^2_2\right]^{1/2}\ }}} \end{aligned}$$
(2.90)
The rest of derivations are similar to Eq. (
2.87), but the result is an imaginary part:
$$\begin{aligned} N^{(3)}_{33}\;&= \; -{\frac{{x+vy}}{{2\mathrm{ i} \sqrt{1+v^2}}}} \int \limits ^{\theta _2}_{\theta _1} \left[{\frac{{\mathrm{ d} \theta }}{{c_1-c_5+c_2\sinh \theta }}} - c.c.\right]\ \ \ \nonumber \\&= \; -{\frac{{x+vy}}{{2\mathrm{ i} \sqrt{1+v^2}}}} \int \limits ^{u_2}_{u_1} \left[{\frac{1}{{c_2}}}{\frac{{\mathrm{ d} u}}{{(u+e^\eta )(u-e^{-\eta })}}} - c.c.\right]\ \ \ \nonumber \\&= \; -{\frac{{x+vy}}{\sqrt{1+v^2}}} \sum _w \Im \left\{ \frac{w}{\sqrt{(c_1-c_5)^2+c^2_2}} \ln \frac{V_+}{V_-} \right\} \nonumber \\&= \; -{\frac{{x+vy}}{\sqrt{1+v^2}}}\sum _w \Im \left\{ \frac{w}{A e^{\mathrm{ i} \theta ^{\prime }/2}} \ln \frac{|V_+|e^{\mathrm{ i} \phi _+}}{|V_-|e^{\mathrm{ i} \phi _-}} \right\} \end{aligned}$$
(2.91)
Finally, insert the results of the three parts in Eqs. (
2.84), (
2.87) and (
2.91) into Eq. (
2.81), the most difficult matrix element of a triangular surface
\(N^\mathrm{ tri} _{33}\) can be found.