experimental apparatus and characterization...

Chapter 2

Experimental Apparatus and Characterization

Techniques

In this chapter we discuss the experimental apparatus that was

implemented during this thesis work. We start with a review of cryogen

free low temperature-high field magnet system. Then we describe in

detail the design of the set up for rf penetration depth measurement. We

also describe the specifications of some other standard characterization

tools. Sample preparation techniques and basic characteristics of the

samples used in the study are presented.

Chapter 2 Experimental Apparatus and Characterization Techniques 32

2.1 Introduction

In this chapter we discuss the experimental apparatuses that are used and

some of them that are designed and implemented during this thesis work. The

working principle of the low temperature high magnetic field facility installed

during the course of this thesis is discussed. The main characterization

techniques include magneto-resistance, the relative rf penetration depth, and

magnetization as a function of temperature and magnetic field. They have been

described in detail. Sample preparation techniques have been briefly reviewed

and basic characterization e.g., X-ray diffraction, scanning electron microscopy

and energy dispersive X -ray spectroscopy results have also been discussed.

2.2 The cryogen free low temperature - high magnetic field

system

Temperature and magnetic field are the most crucial variables for the

entire spectrum of pure scientific research. Particularly in material science and

condensed matter physics, all characterization techniques are primarily centered

on these facilities. Studies over the widest range of temperature and magnetic

field therefore hold the key to discovering new science and functional materials.

While measurements above the room temperature can be performed by

controlled heating, most low temperature experiments are carried out using a

variety of gases, prominently nitrogen and helium, in their liquid form. The

standard method is first to liquefy the gas by mechanical means, then to transfer

it to the measurement set up, and then to vaporize it over a small volume. The

latent heat of vaporization during this process is obtained from the liquid itself

leading to further lowering of temperature of the liquid phase of the gas. The

time honored procedure involves liquid helium (2He 4), which remains a gas at


atmospheric pressure down to 4.2 K. By evaporating helium at low pressure,

temperature down to 1.5 K can be easily achieved. For generating high

magnetic field too, liquid helium is indispensable. Almost all high field

magnets routinely used in research laboratories and industry are made up of

niobium based superconductors (e.g. Nb-Ti, Nb3Sn). These materials need

cooling down to 4.2 K to achieve superconducting state and to sustain high

currents in solenoidal geometry for up to 18 T field. Therefore, the production,

supply and storage of liquid helium are the most critical infrastructure

requirement for a quality condensed matter program.

Current situation however has been discouraging on two counts. For

over ten years now American Physical Society has been predicting severe

shortage of helium gas by early 21st century. A recent study indicates that

helium reserves are fast depleting and this has resulted in large scale shortage of

helium [1] and has led to postponement of projects world over. Till the new

production plants comes up the prices are expected to be almost prohibitive.

The cost of liquefier (with reasonable throughput and longevity) and its

maintenance have skyrocketed in the recent past making it next to impossible

for small groups to conduct low temperature measurements. Fortunately, with

the advent of high wattage closed cycle refrigerators, the dependence on costly

liquid helium to carry out quality low temperature experiments is a thing of the

past. In the following, we describe here the principle and successful operation

of an entirely liquid cryogen-free low temperature high magnetic facility in our

laboratory.

2.2.1 Principle of closed cycle cryocoolers

The first patent for industrial production of liquid nitrogen by

liquefaction of atmospheric gas was filed by Carl Linde (dated July 9, 1895).

The invention was based on Joule-Thomson effect which states that any real gas


undergoes a change in temperature while traversing from a high pressure region

to low pressure region under isoenthalpic condition. The change in

temperature, whether increase or decrease, is determined by change in internal

energy of the gas when the average separation between the molecules of the gas

increases under expansion [2]. Almost 65 years later, Kohler and Jonkers at the

Philips Lab. were the first to implement a closed cycle based air liquefier,

formally known as the Philips-Sterling cycle. The cooling was achieved by free

expansion of gas rather than the throttling process of J-T effect. This was based

on compression of gas followed by transfer through a regenerator onto a space

where the gas is expanded and cooled before returning to uncompressed state.

The regenerator acts as a heat exchanger as well as a thermal seal between the

warm and cold ends. The 'Kohler' design involved out of phase movement of

two pistons with helium gas as the working substance [3]. The Gifford

McMahon (GM) cycle [4, 5] on which most of the present day sub-4 K ----cryogen-free refrigerating systems are based, is a straight forward modification

of the Philips-Sterling method.

Single stage GM cryocoolers ( ~ 30 K), have been widely used as cryo

pumps and as radiation shield in MRI machines for decades. Shown in Fig 2.1

is the schematic diagram of the GM cycle. The main parts of the GM apparatus

are, a) compressor CP, b) displacer piston D, c) regenerator R, d) intake valve

V" and e) exhaust valve VE. The working material could be pure helium gas.

The displacer is cased inside a cylinder and its basic function is to displace a

volume of gas through the thermal regenerator R. The regenerator maintains a

large thermal gradient between the cold and hot ends of the cylinder. It is made

up of materials with large molar heat capacity. The displacer is tight fitted to

the cylinder through sliding seals that prevent gas flow through the radial space

between the displacer and the cylinder. Effectively, the cold (C) and warm (W)

volumes can be varied by the movement of the displacer but the total volume

remains constant throughout the cycle. The opening and closing of the intake


and exhaust valves are synchronized with the position of the displacer in the:

cylinder through a rotary drive mechanism. The four distinct steps of the GM

cycles as depicted in Fig 2.1 are;

i) With displacer at the cold end, intake valve VI is opened and the

compressed helium gas fills the volume W.

ii) The displacer is moved to warm end with VI open. In order to keep

the pressure constant gas slowly flows into the cold volume C

through the regenerator R.

iii) The intake valve is then closed and the exhaust valve V E is opened

forcing the compressed gas at the chamber C to undergo expansion

and consequent cooling.

iv) The dis placer is moved towards the cold end to drive the remaining

gas in the volume C. The exhaust valve is closed and the cycle is

repeated.

The performance of the GM cryocooler depends to a great extent on the

effectiveness of the regenerator material. _ In conventional two stage GM

cryocoolers, usually lead (Pb) is used as regenerator and temperature down to 6

K can be achieved. However the heat capacity of lead is negligible as

compared to pressurized helium below 6 K and therefore it is impossible to

reach sub liquid helium temperatures with lead as the regenerator. Using rare

earth alloy GdxEr1_xRh, Yoshimura et al. were the first group to reach 3.3 K by

using a 3 stage GM cryocooler [6]. These compounds undergo a magnetic

ordering transition below 20 K. Here the displacer has three stages, which

means that it uses three different materials as regenerators depending on the

temperature of the stage. In the process Yoshimura et al. were able to produce

liquid helium at the rate of I 0 cc/h by condensing helium gas near the cold

head. However high price of the Rh based materials prevented large scale

commercialization of this technique.


(ii)

(iii) (iv)

v. v.

Fig 2.1: Schematic description of four stages ofGM cycle based cryocooler.

Recent development in regenerator technology has enabled the use of

Er3Ni/Er0 9 Yb0 1Ni hybrid regenerator which is cost effective and can provide

cooling power up to 1.5 W at 4.2 K [7]. Now even sub 2 K cryocoolers are

available using a variety of magnetic resonators. A typical design as described ... ~

by Numazawa et al. uses spherical Pb particles in copper mess as the first stage .,.-regenerator and a mixture of Pb, HoCu2, Gd20 2S, GdA103, and GdV04 as the

second stage regenerator [8]. Sumitomo Heavy Industry (SHI) of Japan is the

industry leader in this technology. Most of the cryogen-free high magnetic

field systems built by Janis, Cryogenics, Cryolndustries, Cryomagnet, and

Cryomech are based on SHI compressors and displacers.


2.2.2 Details of cryogen free system at JNU

The low temperature - high magnetic field facility installed at JNU can

cool down to 1.6 Kin temperature and can generate upto 8 Tesla magnetic field

without any use of liquid cryogen. With separate attachments the sample space

can be cooled to 300 mK (He3) and heated upto 1000 K (encapsulated oven).

More advanced models with hybrid magnets can produce upto 18 T field.

These models require two compressors, one for cooling the magnet and the

other for cooling the sample space.

The system that is installed at JNU is a single compressor open ended

system. It involves a SHI compressor model CS W 71 in conjunction with a two

stage displacer model SRDK 408D. The compressor is water cooled and

requires continuous supply of chilled water ( ~ 15 °C) at the rate 7 lit/min. The

3 phase power requirement is~ 9 kW. Fig 2.2 shows the sketch of the system.

The cooling powers at the first and second stage cold heads are 34 W @ 40 K

and 1 W@ 4.2 K respectively. The second stage cooling is shared between the

sample space and the Nb-Ti superconducting magnet (T c = 9 K). The first stage

is connected to an ultra pure aluminum radiation shield. The magnet is latched

to the second stage of the compressor. A condensation pot is attached to the

magnet. When low pressure helium gas from a separate close cycle reservoir is

brought in contact with condensation pot the gas liquefies and with suitable

pumping, the base temperature of 1.6 K can be achieved. It is to be emphasized

that the process is entirely liquid cryogen-free and during the run over the last

almost three years we have not spent a single rupee on consumables like liquid

nitrogen or liquid helium or helium gas. Further since the sample space is

always cooled by the helium vapor, the vibrations due to mechanical movement

of displacer hardly interfere with the measurements. Moreover, for entirely

vibration-less environment, as mandated by scanning tunneling microscope


(STM) and Andreev point contact spectroscopy (PCS) measurement, the more

advanced pulsed-tube displacer could be opted.

2.2.3 Working of the low temperature high magnetic field system

The block diagram of the cryogen-free system is shown in Fig 2. 2. The

static and dynamic pressures in the compressor are 1.7 MPa and 2.5 MPa

respectively. Compressed helium from the compressor reaches the cold head

via flexible high pressure hoses. The cold head is housed in a specially

designed cryostat chamber that shields it from outside with vacuum and layers

of radiation shield. The cryostat also includes a vertical column where the

sample is inserted using a dip-stick (variable temperature insert or VTI), a

superconducting magnet and a condensation pot. The temperature of the

superconducting magnet always remains ~ 4 K except for small variation of the

order of0.5 K during charging and discharging ofthe magnet.

The radial sample space is 30 mm and the field homogeneity in this

region is 0.09% over 10 mm. The condensation pot is connected to a separate

helium gas recycle consisting of a reservoir ( ~ 50 liter helium gas at ~ 3 psi

pressure), an air filter and a dry pump. The helium gas gets liquefied locally at

the pot and its vapor flow (and therefore the temperature) in VTI chamber is

controlled by a needle valve. To reach 1.6 K a pressure or? .JP.!J~r i.~- m~il_!_tain~d

in the VTI chamber. Controlled heating using heater of variable wattage up to

25 W near VTI base dictates the temperature of the helium vapor at the sample

space. Resistive temperature sensors are placed at various points in the cryostat

such as the first stage, shield, second stage, magnet, condensation pot, and

exchanger exhaust to continuously monitor the system parameters. By

measunng the resistance at these points temperature is measured using a

Keithley 2700 multimeter with a 10 channel scanner. --------- - ~-- - -


Temperature Scanner

Keithley - 2700

Compressor Sumitomo CSW-71

Helium reservoir

Temperature controller

Lakeshore-340

VTI pressure

Dry Pump

16 pin Connector foir sample

characterization

Air lock valve

Magnet Power Supply

SMS 120C

Sample holder

Cryostat

Fig 2.2: Block diagram of the GM cycle based cryocooler and cryostat. The primary

helium closed cycle comprises of the compressor connected to the displacer through

flexible hoses. Separate helium gas close loop consists of a low pressure helium

reservoir, a condensation pot, a needle valve and a dry pump. The position of

superconducting magnet attached to the second stage of the cold head is also shown.

The other interfaced instruments such as magnet power supply, Keithley multimeter

with scanner, and temperature controller are also shown.


Fig 2.3: Low temperature and high magnetic field :;ystem along with 8 T magnet

power supply and other tools for measurements such as resistivity, magnetoresistance,

and r_f penetration depth at SPS, JNU.

The temperature in the VTI and on the sample holder is measured and

controlled by Cernox sensors using a Lakeshore 340 temperature controller.

The Magnet power supply is cryogenic Model SMS 120 C. The maximum field

of 8 Tesla requires current flow of I 08 Ampere. The magnet is fitted with a

persistent switch that enables very stable homogeneous field.

The fi rst cool down of the system from room temperature to the base

temperature takes about ~ 12 hours. At the base temperature, second stage,

magnet, and condensation pot maintain ~ 4 K where as first stage, shield, and

exchange exhaust remain at around ~ 37 K. At this point helium from second

closed loop is allowed to flow through the sample space via the condensation


pot at a pressure of~ 8 mbar. The He pressure can be controlled by the needle

valve, and at 8 mbar, the temperature goes down to 1.6 K in ~ 2 hour. An

advantageous feature of this system is that sample under test can be taken out of

the system without warming the system to room temperature (unlike the

Quantum Design PPMS). This is achieved by an airlock valve. Further, we

have the flexibility to design the attachments based on our requirements.

Measurement of de and ac resistivity, Hall effect, magneto-resistance needs

little modification to the sample holder as supplied by the manufacturer.

Attachments for measuring rf penetration depth and dielectric constant have

been designed and implemented at JNU. Evidently, the experimental data

quality is not compromised in spite of the mechanical movement of the piston

and due to the varying heat load during the isofield temperature scan.

2.2.4 Attachments for various measurements

Some of the attachments designed for cryogen free low temperature and

high magnetic field system are shown below in the Fig 2.4. Depicted in Fig 2.4

(a) is LC tank circuit, that is part of rf penetration depth measurement setup.

The coil and capacitor attached to sample holder are shown. Coil was wound

tightly on a teflon mantel and was put inside a G-1 0 coil holder. This was then

attached to the sample holder. Teflon mantel helps the coil in adopting uniform

shrinkage at low temperature. Fig 2.4 (b) shows the attachment for dielectric

constant measurement. It is derived from capacitance. Sample is sandwiched

between copper electrodes placed on specially designed G-1 0 plates. The

movement of these plates is controlled by springs. The attachment is then

screwed to the sample holder. The temperature sensor on the sample holder is

also visible in this figure.


"

Capacitor [C] Coi l [L]

Fig 2.4: (a) Attachment for LC tank circuit that is part of 1.[ p enetration depth

measurement setup.

Fig 2.4: (b) Attachmentfor capacitor measurement is depicted. Sample can be placed

between the copper electrodes. The tight fitting of electrodes is controlled by .springs.


2.3 Linear four probe technique for resistivity measurement

For resistivity measurement we used linear four probe technique. Four

contact points situated linearly are taken out from the sample and a known

current from a current source (Keithley 2400/Keithley 224) is made, to pass

through the outer tabs, and potential drop across inner tabs is measured through

a voltmeter (Keithley 2182 nano-voltmeter). DC voltage with resolution~ 50

nano-Volt has been achieved with the GM compressor running. Resistivity is

derived from resistance thus measured by determining the area of cross section

and the thickness of the sample. The contacts on the sample were deposited

using conducting silver paint. For magneto-resistance we applied the field in a

direction always orthogonal to the current direction.

2.4 Interfacing software

Instruments used in the experiments e.g. Lakeshore 340 temperature

controller, Keithley 224 current source, Keithley 2400 source meter, Keithley

2182 dual channel nano-voltmeter, Keithley 2000 digital multimeter, Agilent

5 3131 A frequency counter, SMS 120 C cryogenic magnet power supply, and

Stanford Research System SR530 lock in amplifier were all interfaced and the

data acquisition was controlled by a computer. The program for interfacing

these machines is written in the Labview 6.1 software. One such software for

resistance measurement for two samples simultaneously is shown in the Fig 2.5.

The first figure depicts the front panel of the program while the second figure

shows the arrangement of various Labview components.

Chapter 2 Exrerimental Arrwratus and Characterization Techniques 44

J-:-'

~~~ I

w .. ;;E

f

·~~-I ~

- :1

Fig 2.5: Sofhmrej(Jr simultaneous resistance measurementfor tHD samples is sh•n\'11.

It is Hritten in Lahvinr 6. I. Cpper plate shows theji-ont panel and lo'>t·er plate .\l'ows

diagram panel.


2.5 Radio frequency penetration depth setup

2.5.1 The technique

Several techniques are available for penetration depth measurement such

as pSR, microwave surface impedance, and small angle neutron scattering

(SANS) [9-12]. The simplest yet the time honored technique is the tunnel diode

oscillator technique. Tunnel diodes are almost obsolete but for this interesting

application. This technique is based on a highly stable tunnel diode (BD-4)

based radio frequency oscillator that includes a LC tank circuit. The sample is

placed inside the inductor of the tank circuit. The LC part is attached to the low

temperature facility to access temperature down to 1.6 K and field up to 8

Tesla. The effective change in the rf penetration depth is measured as shift in

frequency of the oscillator using an ultra-stable frequency counter.

In this type of LC oscillators, the tunnel diode is biased in negative

resistance region, and acts as a power source to overcome the losses in the

Ohmic, inductive and capacitive components. The main component of a tunnel

diode is a narrow (~100 °A), heavily doped p-njunction. Shown in Fig 2.7 is a

measured 1-V characteristic of this diode that prominently features a negative

resistance region. Once the oscillations set in, an rf current at the resonant

frequency flows in the inductor of the tank circuit and a corresponding rf

magnetic field is generated. This flux penetrates the sample kept in the coil and

depending on the magnetic state of the sample, the effective area allowed for

the flux through the inductor changes. The corresponding change in the

frequency of the oscillator thus accurately reflects the magnetization state of the

sample. In case of superconductors, as the temperature goes below the

transition temperature, the flux is expelled due to the ensuing diamagnetic state.

This leads to decrease in L and an increase in resonant frequency of the


oscillator. The magnitude of the shift in frequency is proportional to sample

density and geometry.

Schawlow and Devlin [13] were amongst the first to present the

apparatus to accurately measure the change in penetration depth using oscillator

technique. We know that the self inductance L of a solenoid depends purely on

geometrical parameters (L = ~0 n nr2, n is the number of turns per unit length).

As the temperature is decreased, along with a contribution from the

magnetization state of the sample, the inductance can also change because of

shrinking coil diameter and length. Therefore a proper background subtraction

is important for quantitative determination of penetration depth. For a coil of

inductance L and radius r of the sample, let A be the cross-sectional area

occupied by the flux. The change in penetration depth with the change in the

inductance of the coil will then be;

L\L/L= L\ A 2m/ A

In terms of oscillator frequency (since F2

depth can be written as

L\ A= (A/m) (L\F/F) = GL\F

(2.1)

1 /LC), the change in penetration

(2.2)

where G is a calibration constant that depends on sample and coil geometry.

Thus, the shift in oscillator frequency L\F is proportional to the change in

penetration depth L\A.

The value of G can be determined by using a specimen with known radio

frequency skin depth. We used oxygen free high purity copper for the

calibration. The specimen geometry and its orientation inside the coil was

made similar to the actual superconducting samples under study. However, our


entire analysis is based on relative shift in penetration depth and the uncertainty

in the determination of calibration constant does not affect our conclusions. At

room temperature for the ~ 4 mm dia, ~ 1 00 tum coil, the frequency was

1506017.15 Hz which increased to 1515413. 73Hz after keeping the copper

specimen. The orientation of the specimen was such that it covered maximum

cross sectional area of the coil and similar orientation was later used for the film

samples. The skin depth for the copper specimen 8 = [2/ Jl0 cr2nF] 112 where a is

the conductivity. This gives the value of G ~56 OA/Hz. We must emphasize

that the calibration constant may depend on the density and size distribution of

the grains of a material [ 14].

In most of the other tunnel diode based oscillator circuits that report the

stability of 0.05 Hz at the resonance frequency ~ 10 MHz the whole circuit is

kept at low temperature. For instance in Prozorov's case, the circuit is kept at a

constant temperature of 4.8 K ± 1 mK, while the sample is kept on a sapphire

rod with grease that can be temperature controlled separately [15]. In our case

we have kept the circuit at room temperature while the LC part of the circuit, in

which the sample is placed, sees the variation in the temperature. The stability

for our case is of the order of 2-5 Hz at 1.6 K. As Prozorov has described in

[16, 17] it is difficult to obtain the absolute value of the penetration depth due to

uncertainties in the sample dimension.

2.5.2 Study of vortex dynamics using tunnel diode oscillator

As proposed by J. I. Gittleman, B. Rosenblum [ 18] and further refined

by M. W. Coffey, J. R. Clem [19], in radio frequency range, the dynamics of

the vortices of a superconducting type - II superconductor can be effectively

studied. When an external de field is applied to create the vortices, the

superimposed orthogonal rf field can effectively induce tilt motion of the

vortices. If the pinning is strong, this tilt motion is not propagated into the bulk


and is confined only to the surface. In the other extreme, under negligible

pinning, the oscillating vortices can permeate the entire sample. This is shown

in Fig 2.6. Thus the rf penetration depth becomes a true measure of bulk

pinning force density. At high fields one can also access the flux flow regime

using this technique. As defined in Eq. 1.2, the vortex displacement can be

written as kpu + 11(du/dt) = Jx~o where u is the instantaneous displacement

from the vortex from its equilibrium position, 11 is the viscous drag coefficient

and kp is the restoring force constant of the pinning potential also termed as

Labush Parameter or pinning force constant. On solving above equation in the

dilute limit of flux density and periodic pinning potential, the rf penetration

depth can be written as [19]

(2.4)

2 . ~ where 'Ac = B~o/11-okp defines the Campbell penetratiOn depth and 'At =

2B~ol!l-o11CO defines the flux flow penetration depth. Thus with isothermal fi.eld

dependent measurement of rf penetration depth, one can access information

about pinning force density and coefficient of viscosity in the flux flow regime.

t Hdc~ X Hac ~

~A.=O

Strong pinning

Medium pinning

~~~. = 00

Weak pinning

Fig 2.6: Shift in rf field penetration depends on the strength of flux pinning.


2.5.3 Design of the oscillator circuit

The paper by C. Boghosian, H. Meyer, and J. E. Rives in 1966 is the first

published report of tunnel diode oscillator used for measurements in low

temperature physics [20]. Stability of the oscillator's resonant frequency is the

key for this technique. There are many reports on designing the tunnel diode

based ultra stable oscillator [21-26]. Though concept of basic circuit remains

same, optimizing the circuit is non trivial. The main component of the circuit,

i.e. tunnel diode is the biggest hurdle, it is tough to find but easy to bum. We

could find tunnel diode BD-4 but we lost 2 of them during the testing of the

circuit.

50

40

-30 ~ -

20

10

0 0 0 0 0 0 0 0 0 0 0 0 6 0 6 0 0 I

9

25 50 75 100 125 150 175 200 V(mV)

Fig 2. 7: Room temperature 1-V characteristics of tunnel diode BD-4 used in the

oscillator circuit. The value of negative resistance is ~2 kQ and the maximum current

is~ 55 ;d.


Shown in Fig 2.7 is the current-voltage (1-V) characteristic of the tunnel

diode used in the circuit. From 0 to ~ 40 m V, current increases. Beyond 40

m V current decreases and the negative resistance region sets in. The tunnel

diode is biased in this region. Post a series of optimization, the circuit and the

value of the components are as given below.

C3

TDBD-4

R2

C2

c

+9V

MAR SA

Inductor Ll

800mV

Out put frequency

Fig 2.8: Figure shows the design of the tunnel diode circuit with the components. The

values of the optimized components are L1 = 0.1mH, R1= 4. 7 kQ, C1 = 10 pF, R2 =

686 Q, C2 = 680 pF, C = 120 pF, R3 = 220 Q, C3 = 68 pF. The microwave amplifier

MAR 8-Afor amplification of output voltage is also shown in the circuit.

Frequency is measured by an Agilent frequency counter 5 3131 A. Since

the minimum trigger voltage of 53131A is 50 mV, we sought to amplifY the

Chapter 2 Experimental Apparatus and Characterization Techniques 5.[

oscillator signal (~ 35 mV) by using a microwave amplifier MAR-8A. The

circuit for amplifier is shown in Fig 2.9. To remove the persistent grounding

problem a printed circuit board was specially designed and the entire circuit

was kept inside a grounded metal box to shield it from external electromagnetic

interference.

IOOohm

Q--i Vee =+9V

1-----0 473nF

MAR- 8A

Fig 2.9: The design along with the values of the components for biasing the amplifier

MAR-8A.

After optimizing the oscillator circuit at room temperature it was

optimized in low temperature and high magnetic field environment. Due to

constraints in sample space, only LC tank circuit was attached to the VTI and

the rest of the circuit was kept at room temperature. This required addition of

cable of length ~ 2 meter. We found the stability to be least compromised with

semi rigid co-axial cable RG 176. Optimization of inductance of the LC tank

circuit was also carried out to improve the stability of oscillator. Initially

copper wire on top of cigarette paper was tightly wound but it was found that

while decreasing the temperature the change in the frequency was not uniform

and this could have been caused by uneven decrease in the cross sectional area

of the coil. So we decided to wrap the coil on some thin walled teflon tube. A

tube of 1 mm thickness and 3.5 mm inner diameter was chosen. Around 70

Chapter 2 Exrerilntal Armaratus and Characterization Techniques 52

turns of the 42 gauge copper wire were tightly wound (single layer) on the

Teflon tube and soaked in GE varnish to give strength to the coil. A coil holder

of G-1 0 material was designed and coil was glued permanently with GE

varnish. The coil holder is attached to the sample holder of the VTI by a set of

lmm screws. The 120 pf capacitor that forms a part of the tank circuit is

soldered to the inductor and as mentioned above, only the tank circuit is

exposed to low temperature.

The sample inside the coil can be kept in two position. The c- axis of the

sample parallel to the coil axis (that is also the direction ofHrr), and ab plane of

the sample parallel to the coil axis. The sample is placed on a notch of a

wooden cylinder that is inserted into the coil after wrapping it with teflon tape.

The circuit stability was found to be 2-5Hz at 2.34 MHz and 1.6 K for a typical

time interval of 1 0 min. It is important to note that the ac probe fidd in our

system is of the order of l~T. Fig 2.10 shows the block diagram of the

penetration depth measurement system

2.5.4 Frequency stability of tunnel diode oscillator

Fig 2.11 (a) shows the stability of the tunnel diode circuit (without

MAR-8A amplifier) at room temperature with respect to time for approximately

6 hours. The data are taken using a Tektronix oscilloscope. The figure shows

drift in frequency in the range of± 50 Hz. Fig 2.11 (b) on the other hand shows

the stability of the circuit after including the amplifier. As elucidated,, a

stability of ± 5 Hz could be achieved.

Chapter 2 Experimental Apparatus and Characterization Techniques

]~-------·

Sample inside the LC tank circuit

placed in cryogen free system

BD-4 J Frequency based counter

oscillator 1---- Agilent 53131A

Temperature controller Lakeshore

340

r---

Labview interfaced

Data acquisition Computer

53

Fig 2.10: Block diagram for the rf penetration depth measurement, only the sample

kept in the coil of LC part of tunnel diode oscillator is inside the low temperature high

magnetic field system. The temperature controller, frequency counter are connected to

Labview interfaced computer for data acquisition system.

The variation in the frequency without any sample with respect to externally

applied field was also taken. The data taken at 35 K are presented in Fig 2.12.

A variation in the frequency is ~ 40 Hz up to DC magnetic field of strength 2.5

T is observed. It is to be noted that this is negligible compared to the actual

change in signal in the presence of the sample.


-N :I: u: <I

-75~------------~~-----~------------~----------._-----~-----~ 0 100 200

Time (min) 300 400

10~--~-.--~---r--~--~.--~-.--~---.-,-,

-5 0

odb --10L---~~~~~---i~---~~--~--~.~~~--_.--~

0 20 40 60 80 100 Time (min)

54

Fig 2.11: Room temperature stability of the circuit (a) without using the amplifier and

(b) with using microwave amplifier MAR-8A.

500r---~--~.--~--T---T---T-,--~--r-.--~--.

400 -

300 f-

-N :I: -LL 200 f-<l

100 f- -

oooooooooooooo oooo

0 ~oooooooooooO -I i

0 5000 10000 15000 20000 25000

H (G)

Fig 2.12: Variation in the frequency with respect to the DC magnetic field at 30 K.


Tunnel diode oscillator is an extremely sensitive measurement technique

and resolution up to 1 o A has been reported. However it is also extremely

susceptible to external noise and interference. Craig T. Van Degrift who

perfected this technique, has discussed the possible sources of noise and about

the measures to reduce them [22]. Shielding and proper isolation is of prime

importance. To quote VanDegrift, "when using the tunnel diode oscillator in

Helium dewar and if it is not properly shielded then just running a hair drier can

alter the frequency of the system". In our case it is assumed that noise due to

running of close cycle compressor and other necessary machines such as UPS,

water chiller would be taken care of by the excellent radiation shielding of the

cryostat. Nevertheless, if the rest of circuit components could be placed in the

sample space, one would expect a substantial increase in stability.

2.6 Miscellaneous instruments

For the magnetization study of MgB2 thin film we used a MPMS - 5

system at the Technical Physics and Prototype Engineering Division, Bhabha

Atomic Research Center, Mumbai. The magnetic property measurement

system (MPMS) from Quantum Design is a Superconducting Quantum

Interference Device that can measure magnetization of the order of 10-7 emu.

For the structural analysis of bulk samples, we used a Bruker D-8 Advance

Diffractometer at Indian Institute of Technology, New Delhi and at the Inter

University Accelerator Center, New Delhi. For thin film XRD, we used the

facility at National Physical Laboratory, New Delhi. Both these diffractometers

have Cu Ka target with wave length 1.5418 ° A. The scanning electron

micrograph of the surface morphology is taken in a Leo 435VP Surface electron

microscope with EDAX attachment at AIIMS. The electron dispersive X-ray

spectroscopy (EDAX) attachment in the SEM is an additional tool to identify


the chemical composition of the samples. The EDAX attachment sensor at

AIIMS is primarily suited for heavier ions and can only identify the element

ranging between N a to U. So in the case of MgB2 it was not much useful.

2. 7 MgB2 thin film sample preparation

Thin film superconductors are required for practically all applications

involving superconducting electronics. After the discovery of

superconductivity in MgB2 at 39 K, various groups involved in the

superconductivity research world wide started working on the preparation of

MgB2 thin films [27]. So far various techniques have been developed for

preparation of MgB2 thin films that fall in two broad categories. These are

1. Ex-situ MgB2 film growth

2. In-situ MgB2 film growth

Ex-situ film growth involves the deposition of boron precursor film on a

substrate. This boron precursor film is then annealed in the magnesium vapor

at a suitable temperature. In the in situ growth process, the deposition of boron

is carried out followed by magnesium and the annealing is done usually in the

deposition chamber. Magnesium layer is deposited either by co-deposition, or

producing the magnesium vapor locally close to the boron film. The In-situ

growth process is favored for application where junctions are required on the

film. The best quality MgB2 films are prepared by in-situ hybrid physical

chemical vapor deposition (HPCVD) technique. Volatile nature of magnesium

does not favor the direct deposition of film by MgB2 target as it results in

magnesium deficiency [28]. This deficiency again requires the annealing in

magnesium vapor and in this process quality of the film is compromised. The


boron precursor film can be deposited by using various available techniques

such as rf magnetron sputtering, pulse laser deposition, molecular beam and

electron beam epitaxial growth, and hybrid physical chemical vapor deposition

techniques.

2.7.1 Role ofMg vapor in the preparation ofMgB2 phase

The ex-situ two-step process is the most commonly followed technique

for MgB2 thin film growth. The technique seems straight forward but it is

reasonably tricky as it involves maintaining the correct Mg vapor for high

quality films. The steps include,

a. Deposition of a boron film unto an appropriate substrate.

b. Sealing the boron precursor film and magnesium lump inside

a niobium or tantalum tube.

c. The Nb/Ta tube is then put inside a quartz ampoule that is

sealed under few mbar of high purity argon gas.

d. The ampoule is then heated up to 900 °C. The vapor pressure

ofMg plays a significant role for the correct phase formation.

S. I. Lee's group from Pohang, Korea was one of the first to report the

ex-situ process for thin film MgB2 [29]. In this process a precursor thin film of

boron was prepared followed by heat treatment in Mg vapor inside a Ta tube.

The heat treatment was carried out in an evacuated quartz ampoule to prevent

oxidation of the Ta tube. The sintering procedure included fast heating to 900

oc and holding at this temperature for 10 to 30 min followed by rapid

quenching to room temperature. Films of 400 nm thickness were grown.

Almost at the same time C. B. Eom et al. reported [30] MgB2 films prepared by

pulsed laser deposition at room temperature. They used 111 oriented single


crystal SrTi03 substrates and deposited from MgB2 targets. The base pressure

Of the chamber before deposition WaS 3 X 1 o-S mbar, and depositiOn took place

under 3 x 1 o-3 mbar of Argon. As prepared MgB2 films were post annealed in

presence of magnesium. The thickness of the film was ~ 500 nm.

Paranthaman et al. followed a similar route where in boron films on

Alz03 single-crystal substrates were deposited by electron-beam evaporation

technique at room temperature and at a chamber pressure of~ 1 x 1 o-6 mbar [31].

These 5000 to 6000 ° A thick shiny amorphous boron films were sandwiched

between cold-pressed MgB2 pellets, along with excess Mg turnings, and packed

inside a crimped tantalum cylinder. The sealed tantalum cylinder containing

the precursor film was put into a quartz tube which was then evacuated to ~

1 x 1 o-5 mbar, and sealed. The sealed quartz capsule was heated rapidly to 600

°C, and maintained there for 5 minutes. Then, the furnace temperature was

rapidly increased to 890 °C, and held there for 10 to 20 minute, and then cooled

to room temperature. The as formed purplish gray film had a low two probe

resistance of - 1 Q. The MgB2 phase was confirmed by XRD analysis.

Similarly, S. R. Shinde et al. used pulsed laser deposition for boron precursor

film that was sintered in Mg vapor at 900 °C [32]. The boron films were

deposited at 800 °C on SrTi03 (100) and (111) substrates. The background

pressure was below ~ 10-7 mbar. In fact, small Mg balls were found to be

sticking on the film surface after the ex situ reaction in Mg vapor. This excess

Mg was removed by evaporation or by selective etching, without affecting the

underlying film properties. S. H. Moon et al. on the other hand wrapped the

boron films together with magnesium pieces and several pieces of titanium in a

tantalum foil [33]. Titanium pieces prevented the direct contact of the boron

film to the tantalum foil. Then, this whole thing was encapsulated in a quartz

tube. To find an optimum annealing condition, Moon et al. have varied the

annealing temperature and annealing time. The thickness of thin films

increased about 70% to 80% due to the annealing. S.D. Bu et al. have reported


the preparation of MgB2 thin films by depositing boron precursor film via RF

magnetron sputtering, followed by a post deposition anneal at 850 oc in the

presence of magnesium vapor [34]. Two different epitaxial MgB2 thin films on

(000 1) Ah03 were reported. The base pressure before boron deposition was

3x10-6 mbar. Deposition was carried out at 5x10-3 mbar Argon at 500 oc using

a pure boron target. The thickness of the boron films was 230 nm. The films

were annealed in an evacuated quartz tube using a tantalum envelope. The

quartz tube was filled with - 7-10 mbar of Argon gas after evacuation to reduce

the magnesium loss. One of these films showed extremely high upper critical

field [35].

Similarly A. Plecenik et al. deposited boron thin films by thermal

evaporation on unheated sapphire substrates and, subsequently enclosed it in a

niobium tube together with magnesium tunings [36]. Boron films were

deposited on a 1x1 cm2 Al20 3 (0001) substrate by electron-beam evaporation at

room temperature with a thickness of about 300 nm. For post annealing under

magnesium vapor, the as deposited boron films were wrapped in tantalum foil

with magnesium lumps followed by encapsulation in a quartz tube containing

high purity Argon gas. The base pressure inside the quartz tube was maintained

below 5x 10·5 mbar and the Argon pressure was set to 60 mbar. The

encapsulated samples were annealed at 800 oc with the annealing time varied

from 1 minute to 30 minute.

Kenji Ueda and Michio Naito have reported [37] the in-situ synthesis of

as grown superconducting thin films of MgB2 using molecular beam epitaxy

(MBE). They obtained as grown superconducting films in a limited growth

temperature range of 150 to 320 oc_ At such low temperatures, films are poorly

crystallized but showed a sharp superconducting transition Tc at 36 K. The use

of a higher growth temperature to improve the crystallinity failed because the

magnesium in the films was completely lost. Films were grown on various

substrates SrTi03 (00 1 ), r- Al20 3, c- Ab03, and Si (111 ). In specially designed


MBE chamber base pressure was ~ 1-2x 1 o-9 mbar. The flux ratio of

magnesium to boron varied from 1.3 to 1 0 times higher compared to the

nominal flux ratio. The growth rate was 2-3 ° A/s, and the film thickness was

typically around 1 00 nm.

X. X. Xi's group at Penn State University, USA [38] has reported a new

method for fabrication of high quality MgB2 film by in-situ hybrid physical

chemical vapor deposition technique. In their experiment they placed a

substrate on a plate, temperature of which can be raised separately. The

magnesium vapors are created by heating the magnesium chips placed around

the substrate. Further, pure nitrogen is used followed by hydrogen, and the

magnesium chips are heated to 700 °C. Small amount of diborane (B2H6) is

then added to the hydrogen flow, and a stoichiometric MgB2 deposits on the

substrate. Switching off the diborane supply terminates the film deposition.

The high hydrogen gas pressure helps in eliminating any chance of oxygen

contamination of the film, and restricts any evaporation of magnesium from the

film. Transition temperature as high as 39 K, and RRR value ·~ 80 has been

reported for such films [39].

R Vaglio et al. [ 40] has reported the in-situ preparation of MgB2 film by

modified DC magnetron sputtering system for in-situ annealing in magnesium

vapor atmosphere. They first deposited the boron precursor film on Ah03

substrate followed by in-situ annealing in magnesium vapor at 830 °C for 10

minute. The argon pressure was 9x 1 o-3 mbar during deposition while the

chamber base pressure before establishing the plasma was ~ 1 o-9 mbar. Their

best film showed the transition temperature ~ 35 K. Some other reports for the

growth ofMgB2 film by in-situ technique are listed in the reference [41, 42].

Here in this study we have tried to grow MgB2 films by following the

ex-situ technique. For the second step of annealing of precursor boron film, we

used stainless steel (ss) tube instead of the niobium or tantalum tube. In the ss

tube we kept boron precursor film and magnesium powder for annealing [43].


We have been partially successful in making the MgB2 film which showed

onset of Tc at~ 38 K, and some of them showed complete transition at~ 22 K

though the quality of the films needs further optimization.

2.7.2 RF sputtering system for thin film deposition

We have tried ex-situ RF sputtering method to grow MgB2 thin films. A

sputtering system consists of an evacuated chamber, a target holder (cathode),

and a substrate holder (anode). At the base pressure typically ~ 10-6 mbar an

inert gas typically Argon is introduced in the chamber. The electric field inside

the sputtering chamber between the electrodes is established due to the

application of rf power. This field accelerates the available ambient electrons,

and these electrons collide with Argon atoms thus producing Ar + ions as well as

more electrons and characteristic purple/blue plasma appears. These charge

particles are then accelerated by the electric field; the electrons move to the

anode and the Ar + ions move towards the cathode. When an ion approaches the

target, in the optimum conditions, this impact can start a series of collisions

between atoms of the target leading to the ejection of some target particles.

This process is known as sputtering. Particles ejected in this process are useful

and when they strike the substrate they get condensed and deposited on the

surface of the substrate. A rf sputtering system allows the deposition of non

conductive materials at a practical rate [ 44]. Fig 2.13 shows the schematic

diagram of the sputtering chamber and the associated rf power supply of the

system used for thin films sample preparation at JNU. The target of diameter 1

inch can be fitted into the target electrode while substrate electrode diameter is

~ 1 0 em. For our MgB2 preparation the optimized distance between the target

and substrate was kept at 4 em. The rf generator is operated at 13.56 MHz.

The material to be sputtered is made into a target and mounted onto a circular

copper backing plate. For precursor boron film, the target consists of a circular


disk of hot pressed 99.999% purity boron which is of 1 inch diameter and

approximately 1/8 inch thick. There is a shutter which separates the target and

the substrate and prevents contamination.

Hi vacuum valve

Turbo pump

Throttle valve

RF Power

Source shutter

Substrate holder

Comb valve

Rot pump

Fig 2.13: Schematic diagram of the rf sputtering system. Sf and S2 are the two

target holders. Shutter prevents the deposition of film on target during roughing of

target. Combination valves help in roughing the sputtering chamber before

connecting it to high vacuum.


2. 7.3 Deposition process

The chamber is first evacuated by rotary pump before being opened to

the high vacuum turbo molecular pump. The pressure drops to below 1 o-3 mbar

(the pressure which has to be attained before the high vacuum valve can be

opened) in around 20 minutes. The time taken to reach the base pressure

(usually 5x 1 o-6 mbar here) is approximately 2.5 hours. Once the chamber is

evacuated to the base pressure, argon gas is filled in to chamber pressure of 1 o-2

mbar. Argon being a noble gas does not react with either the target or substrate.

The rf supply is then switched on and stabilized to the required power ~ 50 W

over a d.c. bias of 315 V. During this time the substrate is shielded by a shutter.

Once conditioning is complete, the shutter is opened to begin the deposition

process. We deposited the boron film on polished side of r-cut 0001 Al20 3

substrate. The substrate and target distance smaller than 4 em was deliberately

avoided, as it may initiate the back sputtering from substrate itself. These

conditions were first optimized on glass and silicon substrates. When the

brownish boron film started appearing on the glass substrate, the substrate was

then changed to Ab03. Under these conditions a series of boron films of

thickness ~ 200-300 nm were deposited. The thickness of the film was

monitored by the thickness monitor.

The as prepared boron films were annealed in the Mg vapor. We had to

undertake a lengthy optimization process for the proper MgB2 phase formation

on the precursor films. We took different amount of Mg and annealed the

boron precursor film in different ways. We got MgB2 thin films of reasonable

quality only when we annealed the boron precursor film in argon flow with the

film and Mg powder crimped in stainless steel tube. Only a Tc of 22 K has been

achieved so far and further optimization is under way.


2.8 Sample used during the present study

We have made measurements in the following set of samples.

a. A set of MgB2 films were prepared at Applied Superconductivity

Center, University of Wisconsin, Madison, by J. Giencke, C. B. Eom

using ex-situ growth process [34]. In the process boron precursor

films were grown by RF sputtering. These boron precursor films

were annealed in Mg atmosphere at ~ 900 °C. As prepared MgB2

films were used for irradiation as well as penetration depth

measurement.

b. Polycrystalline 'MgB2 was prepared at INFM, Italy by V. Braccini.

The samples were prepared by taking high purity magnesium and

boron powder in stoichiometric ratio and ground thoroughly and

palletized. These pellets were kept in out-gassed tantalum crucible

and arc welded in argon atmosphere. These were further sealed in

quartz ampoule under vacuum and heat treated at ~ 900 °C for few

hours. This has resulted in a good quality of sample with Tc = 39.2

K, resistivity at Tc = 3.5 11-n-cm and residual resistivity ratio = 14

[45].

c. Polycrystalline MgB2 samples were prepared by Ben Senkowich at

Applied Superconductivity Center, University of Wisconsin Madison

[ 46]. Commercial MgB2 powder was thoroughly ball milled. This

milled powder was pressed to form pellets which were then welded in

evacuated SS tube and was exposed to hot isostatic pressure (HIP)

processing at 1000 °C at~ 30 Kbar. The very dense and hard sample

thus processed had a Tc of38.6 K.

d. BizSrzCazCu30 10 in the form of oriented tape were prepared at

Applied Superconductivity Center, University of Wisconsin

Chapter 2 Experimental Apparatus and Characterization Techniques 6.5

Madison, by J. Jiang. Standard powder in tube process was followed.

Bi 2223 powder was filled in Ag tube which was then heat treated at

~830 oc in presence of mixture ofN2 and 0 for 2-12 hours [47].

e. Polycrystalline NbSe2 samples were prepared at the School of

Physical Sciences, JNU, New Delhi by I. Naik using solid state:

reaction technique. High purity niobium and selenium were taken in

stoichiometric ratio and ground and palletized and were heat treated

2.9 Structural analysis

Shown in Fig 2.14 is the recorded XRD pattern ofpolycrystalline MgB2

sample (INFM). No impurity is observed and all the peaks are identified.

:i 300 r::::: ::::s 0 (.) -

200

100

0 10 20

-0 0 ....... '-"

30 40

2e (degrees)

Fig 2.14: XRD pattern ofpolycrystallineMgB2.

-0 ....... -N ....... '-" 0 -0 N

'-" 0

50 60 70

Fig 2.15 (a) shows the SEM image of polycrystalline MgB 2 (IN FM) that

suggests well connected grains. Sub micron sized grains arc equally distribu1ed

in the sample. Fig 2.15 (b) shows the SEM image of unin·adiated thin film.

The grains of size less than I ~tm are uniformly distributed in the image.

Development of the minor cracks seen in the film can be attributed to the strain

developed during the film deposition.

EHT-15 00 kV Wllo 10 llll Hag, 6 25 K X Wm - Photo No ·4 Detector' SE!

Fig 2.15: SE\4 image oj(a) pol\'l.:n;stalline INF\f.V!gB:: and (h) tllin(llm o(J1gB:

Fig 2.16 shows the EDAX spectrum of polycrystalline NbSe2. High quality of

the sample is confirmed as we do not find much impurity phase in the sample.

Chaoter 2 ExQerimental AQoaratus and Characterization TechniQues

Se

Nb

l Nb • ••

1 2 3 ~ 5 8 7 6 9 Ftl ScalE! 11J'Xl6 lis CLM!or: o D[JJ

Fig 2.16: EDAX spectrum of NbSe2 polycrystalline sample.

1000

900

- 800 tn .....

700 t:: ::::s 0 600 (.) -~ 500 tn t:: Q) 400 ..... t::

300 ,........ co

200 0 0 ._

100

0 10 20

0

s

0

N S 0 0 ._

30

,........ ('(') N' ..- ..-..-..-._

40

28 (degrees) Fig 2.17: XRD pattern for Bi2Sr2Ca2Cu3010 sample.

50

s~ Sc

10 11 12

60

67

13

70

Fig 2.17 shows the XRD pattern of Bi2Sr2Ca2Cu30 10 which matches with

the standard formula Bil.84Pb0.34Sr191 Ca203Cu3060x. Presence of Pb has been

confirmed by EDAX spectra.


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experimental apparatus and characterization...

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