Magnetically sensitive particles and magnetic structure

A method of magnetic field detection, the method comprising:

• modifying an external magnetic field with a magnetic structure that is integrated with a container, wherein magnetically sensitive particles within the container move in response to the modified magnetic field to provide a discernible response to the external magnetic field;
• after the modifying, detecting the magnetically sensitive particles within the container;
• and
• outputting a signal indicative of the external magnetic field based on the detecting.

A system with particles that move in response to a magnetic field, the system comprising:

• a container;
• particles within the container, wherein the particles are in at least one of a fluid or a gel;
• and
• a magnetic structure integrated with the container, the magnetic structure configured to magnetically interact with both an external magnetic field and the particles and deliver a discernible response to the external magnetic field.