Magnetic Materials and Dynamics

Key personnel: Prof Rob Hicken, Dr Feodor Ogrin, Dr Volodymyr Kruglyak

The principle of operation of the time resolved MOKE experiment. In this case an optically triggered photoconductive switch is used to generate a pulsed current and associated magnetic field. A side track allows current auto-correlation measurements to be performed.

Today most research into magnetic materials is driven by the needs of the data storage industry where there is an insatiable demand for devices such as disk drives that are able to record and retrieve larger quantities of data on ever shorter timescales. Advances in nano-fabrication have led to the development of new materials with tunable magnetic properties and to the miniaturization of the components of the disk drive system. In Exeter we investigate the basic physical properties of nanoscale magnetic materials and are particularly interested in dynamic magnetic phenomena that may occur on femtosecond through to geological timescales. These include precessional switching of the macroscopic magnetisation, spin wave excitations, and thermally activated reversal of the magnetization.

The time resolved MOKE apparatus is shown. A pulsed field is generated by means of an optically triggered phtoconductive switch

Picosecond precessional magnetization dynamics are fundamental to the operation of microwave frequency devices and high bit rate data storage systems.  We use time resolved optical techniques to explore these dynamics.  The sample magnetization is 'pumped' with an optically triggered magnetic field pulse that has arise time in the range 10 to 100 ps.  The response is  'probed' with a Magneto Optical Kerr effect (MOKE) measurement made with a 100 fs laser pulse.  The technique is non-resonant and well suited to the study of non-linear effects and relaxation processes.  We study Ferromagnetic Resonance and large angle magnetization switching in thin film elements and the measurements may be used to characterize the magnetic parameters of the sample including the Gilbert damping constant.

Paul Keatley aligns the the probe station optics

The quadrant bridge polarimeter

The principle of operation of the time resolved MOKE measurements is shown in the schematic diagram above. An ultrafast Titanium-sapphire laser supplies pulses of light, less than 100 fs in duration at a repetition rate of 80 MHz. Each pulse is divided into two parts and the first part is used to "pump" the sample. The second is time delayed before being used to "probe" the sample at a later time. Every pair of pulses pump and probe the same process. By averaging for just 1 second, 80 million events are collected, making for an excellent signal to noise ratio. The time delay can be varied by reflecting the probe beam off a mirror mounted on a translation stage, so that the sample response can be mapped out as a function of delay time. The MOKE measurement senses the instantaneous magnetic state of the sample. The sample magnetization causes the plane of polarization of the probe beam to be only slightly rotated, but an optical "bridge" detector consisting of a polarizer and two photodiodes allows the rotations of down to 1 microdegree to be resolved. The schematic shows how a pulsed magnetic field may be generated by an optically triggered Au/GaAs photoconductive switch and delivered to the sample by a coplanar strip transmission line.

The probe station is shown. High frequency probes deliver electrical waveforms to the sample structure

The XPEEM end station of Diamond beamline I06, with regenerative laser amplifier that may be phase locked to the synchrotron

Max, Paul and Andreas at the XPEEM end station of Diamond beamline I06

Time resolved MOKE measurements may also be performed using commercial pulse generators or microwave synthesizers to excite the sample. In this case measurements are performed on a microwave probe station (see photo) on which high frequency probes deliver the exciting waveform to a coplanar waveguide structure into which the sample structures are integrated. A more advanced version of the optical bridge detector employs 2 quadrant photodiodes (see photo) and allows all 3 components of the magnetization vector to be detected simultaneously.

Synchroton radiation sources also have a pulsed time structure that is ideally suited to time resolved measurements. Specifically we are using soft X-rays to measure X-ray magnetic circular dichroism (XMCD) in transition metal ferromagnets. XMCD has the advantage that is it element specific and can resolve both the orbital and spin contributions to the magnetic moment. Secondary electrons generated by X-ray absorption may be collected with an electron microscope column and used to generate magnetic images of the sample. Dynamic measurements are performed by synchronizing either an exciting electrical waveform or an ultrafast laser with the X-ray pulses.