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Methods

Many aspects of interest in solid state physics require close control of external parameters, most commonly temperature and an external magnetic field. To achieve this we use two different magneto-cryostats that allow measurements at a broad temperature range from 0.01 K up to and above room temperature at 300 K. At the same time external magnetic fields up to 14 T - the earth's magnetic field has an intensity of approximately 65 µT - can be applied to closely control the ground state of the samples we are interested in.

Both cryostats we use are commercially available, a Quantum Design PPMS and an Oxford Instruments Kelvinox top-loading dilution refrigerator. The probes have been customized in-house to accommodate the required electronics for our measurements.

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  • The critical slowing down of the polarization dynamics can be observed in multiferroic materials both as function of temperature ..
  • .. and magnetic field. From this results we can learn something about the critical dynamics exponent of the material.

Dielectric spectroscopy

The signature method of our group is dielectric spectroscopy, which is the measurement of the complex permittivity for different frequencies.

Experimentally we focus on the lower end of the frequencies spectrum, from 1 mHz to 20 GHz. This frequency range is also widely used in today's communication technology, for example wireless LAN (2.4 GHz and 5 GHz), mobile phones (ranging from 700 MHz up to 2.6 GHz), and computers that typically operate within the low GHz regime. Therefore this frequency range is sometimes also called the radio frequencies (RF).

Within this frequency range we use commercially available materials, such as coaxial cables as well as jacks, connectors and resistors, that are industry standard for RF applications in our self-build measurement setups that enable us to access a broad temperature and magnetic field range for our experiments. This broad range of control parameters allows us to observe, for example, the critical slowing down in multiferroic MnWO4 both with temperature at zero magnetic field or with magnetic field at 12 K.

Polarization

Additionally to the dynamics of the permittivity we also measure the polarization P of our samples directly. This can be done either as function of temperature and is then called a pyro-current measurement as the change in polarization of the sample at a ferroelectric phase transition generates a measurable current. Similar measurements can also be done with changing magnetic field which are analogously called magneto-current measurements. Both types of measurements are typically done with constant electric field of the order of several hundred V/mm.

Much more common are measurements of P(E), so-called hysteresis loop measurements. Here the polarization is measured with changing electric field E. From this measurements we can evaluate both the switchable polarization as well as the maximum speed where the polarization can be switched for a given applied electric field strength. On the right is an example for the switchable polarization in MnWO4 and its dependence on both electric field and frequency.

     

  • D. Niermann, C. P. Grams, P. Becker, L. Bohatý, H. Schenck and J. Hemberger

    Critical Slowing Down near the Multiferroic Phase Transition in MnWO4

    Phys. Rev. Lett., 114, 037204, (2015)

  • D. Niermann, C. P. Grams, M. Schalenbach, P. Becker, L. Bohatý, J. Stein, M. Braden and J. Hemberger

    Domain dynamics in the multiferroic phase of MnWO4

    Phys. Rev. B, 89, 13, 134412, (2014)

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