Novel SQUID magnetometer to study magnetic dynamics in exotic systems

LTL Quantum Physics Seminar (Nanotalo). Speaker: Dr. Daniel Margineda (Cardiff School of Physics and Astronomy, UK).

Magnetic fluctuations play an important role in the magnetic ground state of several systems that have revealed exotic behaviour. For instance, magnetic frustration takes place when the local magnetic moments cannot simultaneously satisfy all the pairwise exchange inter- actions. It gives rise to a large landscape of energetically equivalent states with a macroscopic degeneracy, separated by low energy barriers which can be crossed by thermal or quantum processes. Under certain conditions, these frustrated systems can lead to the formation of a frozen ground state, such as spin ice or stay in a fluid-like states of matter, highly correlated, where the magnetic moments still fluctuate strongly down to zero Kelvin, called spin liquids. Other examples are systems where magnetic transitions are driven to zero Kelvin by a tuning parameter such as magnetic field, hydrostatic or chemical (doping) pressure in a quantum critical point (QCP). The uncertainty principle permits that quantum fluctuations may survive in a region of temperatures where thermal energy is lower than the quantum energy that characterises the fluctuations. Around this quantum criticality, non-Fermi liquid behaviours or novel phases such as unconventional superconductivity that seems to be driven by magnetic fluctuations or weak antifferomagnetism as spin density wave (SDW) can emerge. These magnetic fluctuations may be investigated with different techniques at different time scales. At low frequencies, AC susceptibility can measure the magnetic dynamics from few Hz to tens of KHz and at high frequencies neutron scattering is able to detect magnetic fluctuations from 10 7 to 10 12 Hz. The existing time window has forced to successfully investigated these phenomena with indirect techniques such as μSR or NMR, but the frequency dependence of the fluctuations in spin liquids or spin ice make relevant to cover the time window between the bulk susceptibility and the local μSR probe. Although experimental setups have achieved to measure susceptibility from hundred of KHz to several hours, there are not commercial equipment that can measure beyond tens of KHz and below 2 K.

I present a novel SQUID susceptometer in a liquid helium free dilution refrigerator able to work at temperatures of tens of mK and MHz resolution. To expand the frequency bandwidth, combined with the difficulties of working at milliKelvin temperatures is a challenging achievement that will allow to contrast the magnetic dynamics detected by μSR in those systems. I also present muon spin relaxation investigations of the ground states of some interesting systems. CeRhIn 5 is a heavy fermion where superconductivity and antiferromagnetism are claimed to coexist at ambient pressure. Nb 1−y Fe 2+y is a family of ferromagnetic metallic alloys where the paramagnetic to ferromagnetic transition is avoided around criticality by the formation of a weakly magnetic around state that is claimed to be SDW. Future works are orientated to improve the magnetometer capabilities and fabricated our own SQUIDs to exploit their properties as phase qubits entangled with a resonating cantilever.