MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE
MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE
MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
40 Chapter 3<br />
3.2.1.1 Apparatus<br />
The Quantum Design SQUID (Superconducting Quantum Interference<br />
Device) MPMS (Magnetic Property Measurement System) is a commercially<br />
available Òturn-keyÓ magnetometer. The device runs on liquid He and can<br />
reach temperatures of 2 K to 400 K and fields up to 7 Tesla. A sample heater is<br />
available to achieve temperatures 300 K to 800 K but the sample space<br />
diameter is reduced from 8 mm to about 3 mm. The revision 2 (MPMSR 2 )<br />
improves the software, allowing simple programming for measurements<br />
lasting several days without requiring maintenance.<br />
The SQUID coils detect the longitudinal component of the magnetization<br />
as the sample is pulled through them. The coils are wound in a second-<br />
derivative configuration in which the upper and lower single turns are<br />
counter wound with respect to the two-turn center coil. This configuration<br />
strongly rejects interference from nearby magnetic sources and allows the<br />
system to function without benefit of a superconducting shield around the<br />
SQUID sensing loops. The raw data are a set of voltage readings taken as a<br />
function of position as the sample is moved upward throughout the sensing<br />
loops. The data are normally fit to a theoretical signal to calculate the<br />
magnetic moment. Several vertical scans are averaged to obtain a standard<br />
deviation.<br />
The two important data reduction algorithms are compared in Figure 3-1<br />
for a standard Yttrium Iron Garnet (YIG) sample. The Òlinear regressionÓ<br />
assumes the sample is properly centered. If the sample is not exactly centered<br />
(longitudinally), the measured moment will be noticeable different. The<br />
Òiterative regressionÓ iterativly finds the center, so if the sample is off-center<br />
the reported moment does not change. When doing temperature scans, the<br />
length of the sample rod will significantly change. The iterative regression is<br />
clearly superior for these measurements. The precision of these two<br />
algorithms are the same (Figure 3-1). The Òfull scanÓ algorithm is simply the
Change language