Here, under the subtitle 'principle,' it describes what happens when you have a static magnetic field along the z axis, $ B_0 \hat{z}$ and microwave field parallel to the $x$ axis, $ B_1 \hat{x}$. I have two questions regarding what's happening here.
- In the animation shown on the wikipedia page, the first show the effect of $ B_0 \hat{z}$ on the spin, which just precesses around $ B_0 \hat{z}$. Then, they go into the rotating frame where $ B_1 \hat{x}$ is just a fixed magnetic field along the x axis.
I understand that if you go into the Larmor frequency rotating frame (prior to introducing the B field in the x direction), you have a spin just poiting in some diagonal direction and at this point, you can ignore $ B_0 \hat{z}$. The wikipedia page says that $ B_1 \hat{x}$ is a microwave field parallel to the x axis. Doesn't this mean the magnetic field in the x axis is some oscillating field in the form of $B_1 cos(\omega t) $? So why isn't the B field along the x axis in the animation oscillating along the negative and positive sides of the x axis instead of being a field fixed in magnitude and stationary in the rotating frame? or are they applying some B field in the x-y plane such that it's in the form of $B_1 cos(\omega t) + B_1 sin(\omega t)$, so fixed in magnitude and rotating around the z axis at the same rate as the Larmor frequency due to $ B_0 \hat{z}$ ?
- Even if you were able to somehow achieve a $ B_1 $ such that in the rotating frame with the same frequency as the Larmor frequency where you have effectively cancelled out the effect of $B_0$, how do you make sense of the wikipedia page's statement thatfollowing statement?
Assuming B1 to be parallel to the x-axis, the magnetization vector will rotate around the +x-axis in the zy-plane as long as the microwaves are applied
In the animation, the electron spin, represented by the red arrow, isn't embedded in the x-y plane, which seems to contract the statement quoted above. (I may be conflating net magnetization and spin?)