Techniques

AC Magnetic Susceptometer

AC magnetic susceptibility (ACMS) measurement is a useful method to probe time-dependent magnetization dynamics in materials.

How does it work?

In ACMS magnetometry, a primary coil applies a small AC magnetic field to a stationary sample, and the induced AC moment is detected with a secondary coil. The secondary coil is made up of two parts; one sample coil to detect the sample moment, and another counter-wound reference coil to null the self-inductance of the sample coil.

A lock-in method is used to detect the sample AC signal at the driving frequency. In ACMS, one measures two quantities; the real (in-phase) AC moment, X’ and the imaginary (out-of-phase) AC moment X”. The phase is relative to the driving AC field in the primary coil.

The maximum AC frequency is typically in the kHz range, and the AC amplitude is typically a few Gauss.

What does it measure?

In the low frequency limit, the real (in-phase) AC moment X’ represents the slope of the DC M(H) curve. ACMS is sensitive to small changes in M(H), not to the absolute value of M(H), therefore it is very sensitive to small magnetic shifts even when the absolute moment is large.

At higher frequencies, the AC moment of the sample does not follow the DC magnetization curve due to dynamic effects in the sample. The phase of the AC moment may lag behind the driving AC field. The imaginary (out-of-phase) AC moment X” is indicative of dissipative processes in the sample; these may be AC frequency and amplitude dependent.

What magnetic phenomena can ACMS characterize?

ACMS provides useful insight into time-dependent dynamic magnetization processes. Common examples:

Magnetic phase transitions

Typically, the susceptibility X diverges at the critical temperatures of magnetic phase transitions. ACMS allows the precise determination of critical temperatures. Furthermore, the X’ and X” versus temperature curves can be used to extract the critical exponents at the transition; this allows one to distinguish between various models of magnetic interactions (for example, SD Heisenberge, X-Y, or Ising models).

Mulder, C. A. M., A. J. Van Duyneveldt, and J. A. Mydosh. “Susceptibility of the Cu Mn spin-glass: Frequency and field dependences.” Physical Review B 23.3 (1981): 1384.Spin-glass

A spin-glass is where the magnetic spins collectively have random interactions with each other, resulting in irreversible and metastable states. The spin-glass freezing transition temperature is determined from the cusp of X’ vs temperature; this is also frequency-dependent, and is a characteristic signature of a spin-glass system. The non-zero X” signal characterizes the irreversibility of the spin-glass state below the spin-glass freezing point.

Luis, F., et al. “Resonant spin tunneling in small antiferromagnetic particles.” Physical Review B 59.18 (1999): 11837.Superparamagnetism

A ferromagnetic particle of sufficiently small size would exhibit single domain state below a blocking temperature. Above this blocking temperature, the moments are free to rotate with the collective behaviour of such particles are superparamagnetic, where the constituent moments are single ferromagnetic particles (rather than atomic moments in a normal paramagnet).

A wealth of information can be extracted from X’ and X” to characterize a superparamagnetic system. The blocking temperature of such a system depends on the measurement time scale. This is where ACMS is very useful, as by merely changing the AC frequency, different measurement time scales can be probed. Furthermore, the particle size distribution and degree of interaction between the particles can be extracted from a X’ and X” versus temperature curve.

Superconductivity

ACMS is a useful tool to characterize a superconductor. A superconductor undergoes a drastic change in magnetic susceptibility at its critical temperature, where the material has small susceptibility above its critical temperature, but becomes a perfect diamagnet below its critical temperature with very large susceptibility (X’ = -1). This allows the precise determination of critical temperature. Furthermore, one can also extract critical current density from the imaginary component of the susceptibility X”.

Singh, R., et al. “V. P: S. Awana, J. Albino Aguiar, and Md. Shahabuddin.” Phys. Rev. B 55 (1997): 1216.

Example of an AC magnetic susceptometer tool

AC magnetic susceptometer at the Istituto dei Materiali per l’Elettronica ed il Magnetismo, Parma, IT (IMEM-CNR)

Dynamic magnetization measurements

Dynamic measurements refer to the use of high frequency generators / detectors / network analyzers in the MHz to GHz frequency ranges to probe the dynamic response in devices. Examples include ferromagnetic resonance, propagating spin wave spectroscopy, spin pumping, magnonics, spin current injection, etc.

Dynamic magnetoresistance measurement setup at the Université Grenoble Alpes.

Magnetic imaging

Kerr Microscopy

Based on the same MOKE principle, Kerr microscopy is an optical technique for magnetic imaging.

Kerr microscope at the Universidad del País Vasco Bilbao.

Magnetic Force Microscopy (MFM)

Based on the scanning probe microscopy technique, a magnetized tip is scanned over the surface of interest to obtain magnetic in addition to topological contrast.

MFM at the Istituto dei Materiali per l’Elettronica ed il Magnetismo, Parma, IT (IMEM-CNR)

Example of scientific results

Achieving Giant Magnetically Induced Reorientation of Martensitic Variants in Magnetic Shape-Memory Ni–Mn–Ga Films by Microstructure Engineering (P. Ranzieri et al. Adv. Mater 27 (2015) 4760)

Vortex head-to-head domain wall in a 15nm-thick permalloy stripe of width 500nm.

Electrical measurements

Various magneto-transport phenomena require electrical measurements to be performed in under controlled magnetic fields and temperatures. Industrially-important magneto-transport phenomena include:

• Giant magneto-resistance (GMR)
• Tunneling magneto-resistance (GMR)
• Anisotropic magneto-resistance (AMR)
• Hall effect (HE)
• Anomalous Hall effect (AHE)

PPMS has options to conduct these experiments. Furthermore, some of our facilities have in-house custom-made setups to conduct specialist electrical measurements.

PPMS dedicated for electrical measurements at the Université de Lorraine

Custom-made sample holder for electrical measurements at the Université de Lorraine

Dynamic magnetoresistance measurement setup at the Université Grenoble Alpes.

MOKE magnetometry

MOKE (Magneto-optical Kerr effect) magnetometry measures the polarization rotation of a polarized laser light after it has been reflected from a reflective magnetic sample; this Kerr rotation is due to magneto-optical interaction. Two different geometries are possible; polar (moment component perpendicular to the sample surface) and longitudinal (moment component parallel to the sample surface and the plane of incidence). Temperature and field control depends on the apparatus. Due to this being an optical method, MOKE magnetometry has surface sensitivity.

MOKE magnetometer at the Université Grenoble Alpes.

Example of scientific results

Magnetic anisotropy measurement of Ni/Co/Ni multilayer film as function of the growth of the top Ni layer. The change of the shape of the MOKE hysteresis loop shows the magnetic moment increasingly attain in-plane direction as the layer grows. (Université de Lorraine)

Magneto-calorimetry

The magneto-caloric effect is one where change in magnetic field induces temperature change. This effect is used in magnetic refrigeration applications.

Magneto-calorimeter at the Institute of Non-Ferrous Metals, Gliwice, Poland.

Thermomagnetic analyzer at the Istituto dei Materiali per l’Elettronica ed il Magnetismo, Parma, IT (IMEM-CNR)

Example of scientific results

Magnetic properties and Curie temperatures of disordered Heusler compounds: Co1+xFe2-xSi (J. E. Fisher et al. Phys. Rev. B 94, (2016) 024418)

Magnetometers

Alternating Gradient Magnetometer

AGFM at the Istituto dei Materiali per l’Elettronica ed il Magnetismo, Parma, IT (IMEM-CNR)

Example of scientific result:

Tunable spin-wave frequency gap in anisotropy-graded FePt films obtained by ion irradiation (S. Tacchi et al. Phys. Rev. B 94 (2016), 024432)

Permeagraph

Hysteresisgraph at the Istituto di Struttura della Materia, Roma, IT (ISM-CNR)

Example of scientific result:

Demagnetization curve (in red) at room temperature of a strontium ferrite permanent magnet. (Istituto di Struttura della Materia, Roma, IT (ISM-CNR))

SQUID

SQUID magnetometers are extremely sensitive and capable of detecting very weak magnetic signals of the order of E-8 emu. Quantum Design MPMS SQUIDS have superconducting 7 T magnets, with temperature range 1.8 – 400 K. The newer MPMS models are capable of operating in VSM mode, AC susceptibility (1 – 1000 Hz), and with temperatures extendable with the oven option (400 – 1000 K) or with He-3 cryogen system (0.5 K).

Quantum Design MPMS XL SQUID magnetometer at KU Leuven.

Example of scientific result:

FC/ZFC magnetization curves at fixed magnetic field to study the magnetic order as a function of temperature and, for nanoparticle systems, the effect of nanoparticle size and size distribution; b) Hysteresis loops to study the effect of magnetic field on the magnetization and to measure the coercive field. (Istituto di Struttura della Materia, Roma, IT (ISM-CNR)

MPMS XL with pressure cell option at the Istituto dei Materiali per l’Elettronica ed il Magnetismo, Parma, IT (IMEM-CNR)

Example of scientific result:

Strong magneto-volume effects and hysteresis reduction in the In-doped (NiCo)2MnGa Heusler alloys (J. Kamarad et al. J Alloys Comp. 685 (2016) 142)

Ultra-high magnetic fields

One of our partners – High Field Magnet Laboratory (HFML) at Nijmegen – house various extremely high continuous field magnets. There are 6 extremely high field magnets available:

• Cell 1: 30 tesla Bitter Magnet (Ø50mm room temperature bore)
• Cell 2: 37.5 tesla Bitter Magnet (Ø32mm room temperature bore)
• Cell 3: 33 tesla Bitter Magnet (Ø32mm room temperature bore)
• Cell 4: 38 tesla Hybrid Magnet (Ø32mm room temperature bore)
• Cell 5: 33 tesla Bitter Magnet (Ø32mm room temperature bore)
• Cell 6: 45 tesla hybrid Magnet (under construction)

One of the ultra high magnetic field magnets at Radboud University.

Inside one of the magnets at Radboud University.