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Aldrin S. Bendal 2013-79553

Superconducting Properties of Magnesium Diboride

This summary reviewed paper about MgB2 superconductors its electrical and magnetic

properties. This material known since early 1950's, but recently discovered to be

superconductive at a remarkably high critical temperature Tc=40K by the group of Akimitsu in

2001 [1].

Magnesium Diboride (MgB2) possessed simple crystal structure, large coherence lengths, high

critical current densities and fields among the binary compound family, because of that MgB2

will be a good material for both large scale applications and electronic devices [2]. This material

could be fabricated in various forms: bulk, single crystals, thin films, tapes and wires [3- 7].

Among the rest thin films formed achieved the highest critical temperature of about 40k which

can be fabricated in many ways, such as: pulsed laser deposition (PLD), co-evaporation, and

deposition from suspension, Mg diffusion, and magnetron sputtering [8-11].

One issue about deposition of MgB2 is the substrate being used. Some substrate has lower

adhesion ability so that the film easily breaks down when subjected for experimental and

application purposes.

Figure 1 shows the effect of critical temperature on the substrate and the deposition method

used. The reports using sapphire show the highest Tc’s and sharpest transitions by Mg diffusion

method. In addition, good quality films can be prepared on SrTiO3, Si, and SS. However, the

films prepared on SS have poor adhesion on the substrate [12-14].

A recent report on PLD shows that the temperature of the film varies during pulsed laser

deposition, the variation depending on the deposition parameters: substrate temperature,

pressure in the ablation chamber, deposition rate [15]. This may be an important factor due to

Mg volatility.

The best method for MgB2 thin film fabrication has proven to be Mg diffusion method. Why the

type of substrate is not so important? Probably because the hexagonal structure of MgB2 can

accommodate substrates with different lattice parameters.

Another factor that affects the critical temperature is the substitution elements in MgB2.

The substitutions are important from several points of view. First, it may increase the critical

temperature of one compound. Secondly, it may suggest the existence of a related compound

with higher Tc. And last but not least, the doped elements which do not lower the Tc

considerably may act as pinning centers and increase the critical current density.

The critical temperature decreases at various rates for different substitutions, as can be seen in

Figs. 1. The largest reduction is given by Mn followed by Co, C, Al, Ni, Fe. The elements which do

not reduce the critical temperature of MgB2 considerable are Si and Li [16-20].

Figure 1: Critical temperature and critical temperature width for MgB2 films deposited on different substrates.

Up to date, all the substitutions alter the critical temperature of magnesium diboride with an

exception: Zn, which increases Tc slightly, with less than one degree [Moritomo], [Kazakov].

There are only two reports regarding Zn doping. Both agree with the fact that at a certain

doping level Tc increases, but disagree with the doping level for which this fact occur. This may

be due to the incorporation of a smaller amount of Zn than the doping content.

Figure 3, shows Hall effect in MgB2. There are only three reports about Hall effect in MgB2 from

1950’s-2001, the worked of kang (a) polycrystal, kang (b) oriented at c-axis and Jin (a) with no

preferential orientation [].All reports agree with the fact that the normal state Hall coefficient

RH is positive (Fig. 7), therefore the charge carriers in magnesium diboride are holes with a

density at 300K of between 1.7 ÷ 2.8 × 1023 holes/cm3, about two orders of magnitude higher

than the charge carrier density for Nb3Sn and YBCO. But the researcher disagree weather the

Hall coefficient in normal state increases or decreases in temperature [21-23].

Figure 2: Critical temperature dependence on doping content x for substitutions with Zn, Si, Li, Ni, Fe, Al, C, Co, Mn (0<x<0.2).

Critical Current in different forms:

Jc in bulk [3]

As you can see in Figure 4 (a) the critical current density vs. magnetic field in bulk form. for bulk

MgB2 samples, taken at different temperatures, 5, 10, 15, 20, 25 and 30K. In self fields bulk

MgB2 achieve moderate values of critical current density, up to 106 A/cm2. In applied magnetic

fields of 6 T Jc maintains above 104 A/cm2, while in 10 T Jc is about 102 A/cm2.

Jc in powder [4]

Figure 4 (b) shows the the critical current density vs. magnetic field in powder form. Very high

current densities can be achieved in low fields, of up to 3×106 A/cm2. However, magnetic fields

of 7 T quenches the current density to low values - 102 A/cm2, Jc(H) having a steeper

dependence in field than bulk MgB2.

Jc in film [5]:

Figure 4 (c) shows the critical density dependence in magnetic field for MgB2 thin films. One can see in Fig. 4 c that in low fields, the current density in MgB2 is higher than the current in Nb3Sn films and Nb-Ti. In larger magnetic fields Jc in MgB2 decreases faster than for Nb-Sn and Nb-Ti superconductors. However, a Jc of 104 A/cm2 can be attained in 14T for films with

Figure 3: Hall coefficient versus temperature.

oxygen and MgO incorporated. These high current densities, exceeding 1 MA/cm2, measured in films, demonstrate the potential for further improving the current carrying capabilities of wires.

Jc in wire [6]:

Figure 4 (d) illustrates current density dependence in magnetic field for MgB2 wires. Compared to bulk and powders MgB2, the wires have lower values of Jc in low fields, of about 6×105 A/cm2. However, the Jc(H) dependence becomes more gradual in field, allowing larger current density values in higher fields, Jc(5T) > 105 A/cm2.

Figure 4: (a) Critical current densities versus magnetic field for MgB2 bulk samples, (b) powder, (c) thin film, and (d) wire.

a b

c d

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