Magnetics Business & Technology - Winter 2014 - (Page 8)

FEATURE ARTICLE Magnetocaloric Measurements: From Energy Efficient Refrigeration to A Tool for the Study of Phase Transitions By Victorino Franco | Sevilla University with Brad C. Dodrill & Cosmin Radu | Lake Shore Cryotronics In the past 20 years, there has been a surge in research on the magnetocaloric response of materials, due mainly to the possibility of applying this effect for magnetic refrigeration close to room temperature. But in addition to the studies of magnetocaloric materials for increasing the energy efficiency of temperature control systems, the magnetocaloric effect can be used to gain fundamental insight into the characteristics of magnetic phase transitions. This article will discuss magnetometry measurements of relevant magnetocaloric materials and the resultant analysis and procedures proposed to characterize the phase transition using purely magnetic measurements. The magnetocaloric effect (MCE) is the reversible change in temperature of a magnetic material with the application or removal of a magnetic field. Magnetic refrigeration based on the MCE is more efficient than the process based on compression/expansion of gasses, and since no refrigerant gasses are used it is environmentally friendly in that there are no concerns about ozone depletion or greenhouse effect.1-4 To characterize a magnetocaloric material (MCM) one needs to measure the adiabatic temperature change, ΔTad, when the material is adiabatically magnetized/demagnetized, or one can measure the magnetic entropy change, ΔSM: (1) where μo is the magnetic permeability of vacuum and Hmax is the maximum applied field. Another important magnitude of a magnetic refrigerant material is its refrigerant capacity, RC, which is the amount of heat that can be transferred between the cold and hot reservoirs, at temperatures Tcold and Thot, respectively, and is expressed as: (2) From equation (1) it is clear that the largest MCE occurs at temperatures where the magnetization is changing most abruptly. For practical magnetic refrigeration this needs to occur close to room temperature, hence MCMs with phase transitions (abrupt changes in M) near room temperature are desirable. Therefore, there is an intrinsic connection between the characterization of MCMs and the study of their phase transitions, which might be of first-order type (FOPT) or second-order type (SOPT). And while the largest magnetocaloric response is associated to FOPT materials, it is also at the expense of larger thermal and magnetic hysteresis and usually smaller narrower peak than for SOPT MCMs, which usually relates to a lower RC. Magnetocaloric Effect Measurements Direct Measurements - In principle, the simplest method for characterizing a MCM is to measure ΔTad(H) by adiabatically isolating a sample and measuring its temperature change with a sensor as the magnetic field is varied. This technique is appropriate when the thermal mass of the sample is much larger than the thermal mass of the addenda (i.e., sample holder plus temperature sensor). However, as it requires a good adiabatic isolation, it has to be performed in dedicated experimental setups. Standard versions of this technique would not be appropriate for measuring small samples, like thin films, small amount of powders, nanostructured materials, etc. For these types of samples other methods are needed. Indirect Measurements - Indirect measurements are much simpler to perform and only rely on experimental devices that 8 Magnetics Business & Technology * Winter 2014 are readily available commercially, such as magnetometers and calorimeters. Magnetometry is the most common technique that is used to measure the temperature- and field-dependent magnetization curves to calculate ΔSM using equation (1). One reason being that magnetometry, unlike calorimetry, is a contactless measurement, thus it can easily measure materials in any form (e.g., powders, solids, thin films, etc.). Magnetometry measurements are most commonly performed using either vibrating sample magnetometers (VSM) or superconducting quantum interference device magnetometers (SQUID). Magnetic Measurement Techniques Magnetometry techniques can be broadly classified into two categories: inductive and force based. In this article we will focus on two most commonly employed inductive techniques: vibrating sample magnetometry (VSM) and superconducting quantum interference device magnetometry (SQUID). Vibrating Sample Magnetometry (VSM) - In this method, originally developed by Foner,5 a magnetic material is vibrated within a uniform magnetic field, H, inducing an electric current in suitably placed sensing coils. The resulting voltage induced in the sensing coils is proportional to the magnetic moment of the sample. The magnetic field may be generated by an electromagnet, or a superconducting magnet. Variable temperatures from cryogenic to high temperatures (<4 K to 1,273 K) may be achieved using cryostats and furnace assemblies, respectively. Commercial VSM systems are available that provide measurements to field strengths of ~3 T (30,000 Oe) using conventional electromagnets,6,7 as well as systems employing superconducting magnets to produce fields to 16 T.8,9 When used with electromagnets, one can make very small step changes in field (i.e., ~1 mOe) and the measurement is very fast. When used with superconducting magnets, higher field strengths are possible; however, this limits the field setting resolution, and the measurement speed is inherently slower due to the speed at which the magnetic field can be varied in superconducting magnets. The ultimate noise floor of commercially available VSMs is 10-7 emu. This is sufficient sensitivity for many magnetic materials. SQUID Magnetometry - Quantum mechanical effects in conjunction with superconducting detection coil circuitry are used in SQUID-based magnetometers to measure the magnetic properties of materials. Theoretically, SQUIDs are capable of achieving sensitivities of 10-12 emu, but practically, they are limited to sensitivities of 10-8 emu, because the SQUID also picks up environmental noise. As in a VSM, SQUIDs may be used to perform measurements from low to high temperatures (<2 K to 1,000 K), and to field strengths of 7 T employing superconducting magnets.8,9 Like the superconducting magnet based VSM systems, the measurement is inherently slow due to the speed at which the magnetic field can be varied in superconducting magnets.

Table of Contents for the Digital Edition of Magnetics Business & Technology - Winter 2014

Editor's Choice
Magnetocaloric Measurements: From Energy Efficient Refrigeration to a Tool for the Study of Phase Transitions
Exceeding the Performance of Si-Fe with High End Magnetic Materials and Technologies
Magnetics, Materials & Assemblies
Research & Development
2015 Magnetics Magazine Resource Guide
Magnetics 2015 Conference Preview
Industry News
Marketplace/Advertising Index
Spontaneous Thoughts: The Last Large Gathering

Magnetics Business & Technology - Winter 2014