# How do I find an expectation value for an electron's magnetic moment?

Given a spin state: $|s\rangle$ = some linear combination of $|\uparrow\rangle + |\downarrow\rangle$ possibly with an imaginary component. How do you get from the definition of a magnetic momentum operator $\hat{\mu}_e = g\mu_B\hat{\sigma}$ to the expectation value of the electron spin magnetic moment?

$g$ is the gyrmoagnetic factor and is approximately 2.0023.

$\mu_B =\frac{e\hbar}{2m_o}$ is the Bohr magneton.

$\hat{\sigma}$ is the Pauli spin matrix.

I feel like this is the operation

$\langle s| \hat{\mu}_e |s\rangle$

If it is, I need an example walk-through with some arbitrary complex $|s\rangle$

Let $$|s\rangle = \alpha|\uparrow\rangle + \beta|\downarrow\rangle$$ We assume that $s$ is normalized i.e. $\langle s | s\rangle = 1 \implies |\alpha|^2+|\beta|^2 = 1$. Then the expectation value of $\hat{\mu}_e$ is: $$\langle\hat{\mu}_e\rangle = \langle s|\hat{\mu}_e|s\rangle$$ $$\implies \langle\hat{\mu}_e\rangle = |\alpha|^2\langle\uparrow| \hat{\mu}_e |\uparrow\rangle + |\beta|^2\langle\downarrow| \hat{\mu}_e |\downarrow\rangle + \alpha^{\ast}\beta\langle\uparrow| \hat{\mu}_e |\downarrow\rangle + \alpha\beta^{\ast}\langle\downarrow| \hat{\mu}_e |\uparrow\rangle$$ Now, $\hat{\mu}_e = g\mu_B\hat{\sigma}$. We use this together with $\langle \uparrow|\hat{\sigma}|\uparrow\rangle = 1$, $\langle \downarrow|\hat{\sigma}|\downarrow\rangle = -1$, and $\langle \uparrow|\hat{\sigma}|\downarrow\rangle = \langle \downarrow|\hat{\sigma}|\uparrow\rangle = 0$, to get: $$\langle\hat{\mu}_e\rangle = g\mu_B(|\alpha|^2 - |\beta|^2)$$ Note that this expression for the expectation value is consistent with interpreting $|\alpha|^2$ and $|\beta|^2$ as probabilities of finding the spin to be $\uparrow$ and $\downarrow$ respectively, as required.
• What happens if we have two $\uparrow$ spins with $\alpha$ being real and $\beta$ being imaginary? – That1guy Apr 22 '15 at 7:08
• The result is applicable for general complex coefficients. Two $\uparrow$ spin terms with different coefficients would just add up to one $\uparrow$ spin term with the (complex) sum of the coefficients. But it is only $\uparrow$ and $\downarrow$ together that describe all possible spin states. – AV23 Apr 22 '15 at 7:13
• Ah, I could just consider it $\alpha= x+ yi$ and $\beta= 0$ and it would apply to a specific observation. Yes? – That1guy Apr 22 '15 at 7:14
• Or would only having $\uparrow$ mean that the result is a vector? – That1guy Apr 22 '15 at 7:24
• You can take $\alpha = x + iy$ and $\beta = 0$. – AV23 Apr 22 '15 at 7:29