- Email: [email protected]
Search for irreversibility line in a conventional hard superconductor
Mm
Physica C 185-189 (1991) 2265-2266 North-Holland
SEARCH FOR IRREVERSIBILITY LINE IN A CONVENTIONAL HARD SUPERCONDUCTOR
RAVI KUMAR~+, A.K. GROVER P. CHADDAH"
, S. RAMAKRISHNAN+, P.L. PAULOSE+ S.K. MALIK+AND
+Tata Institute of Fundamental Research, Bombay 400005, India sDepartment of Physics, Panjab University, Chandigarh 160014, India S.S.P. Division, Bhabha Atomic Research Centre, Bombay 400085, India Four procedures have been looked into to determine Tr(H) values in niobium. The DC magnetic data specify a lower limit, whereas caution needs to be exercised in assigning peak temperature in x~(T) with T~(H). The absence of DPE in x~(T) implies non-existence of reversible region below T~(H).
The advent of high temperature superconductors (HTSC) has given rise to the beliefx that there exists a small region below the T~(H) line where the magnetization response in hard type-II materials is path independent, and the hysteretic behaviour sets in below an irreversibility (Tr,H~) line. Many interesting theoretical ideas2, such as, flux-lattice melting, flux depinning, vortex-liquid to glass transition, etc., which pertain to the nature of (T~,H~) line, are curreutly being discussed in connection with the physics of HTSC. Some of these concepts may not be relevant for conventional low temperature type-II superconductors. In view of this, we have attempted to search for (T~,H~) line via DC and AC magnetic techniques in two specimens o£ elemental niobium (To = 9.3 K). Our results focus attention onto the relative efficacy of different procedures usually employed to determine (T~,H~) line in HTSC. The niobum specimens chosen are disc (thickness = 0.09ram and d i a . = 3.59mm) and niobium powder, which have been shown 3 to display thermo-magnetic history effects similar to behaviour reported in HTSC. The disc specimen is such that no flux escapes from it on field cooling (MFc(H,T) = 0 , H > 20e), whereas the powder syecimen displays partially reversible behaviour.
0.005
Nb P o w d e r
0 d
-0.005
~
F
-0.010
m m A
&
3 kOe • FC 3 kOe o z , ' c 100 Oe
~
,~,.,,o,~
&
"
n
o
><- -o.ot5
u
FC
I00 Oe
~ ZFC 20 Oe " FC 20 0e
~
0 OQ
2 2 ..... -0.020
. . . .
4
L , , , -
. . . . . . . . . .
, . . . .
,,,
. . . . . . . . . .
5
6 7 8 9 10 11 12 TEMPERATURE (K) Fig.1 ZZFc and ~ c vs T at H = 20, 100, and 3000 Oe in Nb powder. The arrows identify Tr(H) values by (ZZFc - ~rc ) -~ 0 criterion.
. . . . . i
~ -IX)
:, .-o°°° ~.-
. , -500
0-F
-o~
ao
.4
~
Nb Powder 6.2 K
1.0 I 0.0
o
The different procedures employed to determine Tr(H) values are, (i) the merger of MZFc(T) and MFc(T) data, (ii) the vanishing of hysteresis in isothermal DC M-H curves, (iii) the appearence of differential paramagnetic effect (DPE) 4,s in the in-phase AC susceptibility X~t(T) and (iv) the peak in X~t(T). Figures 1 to 5 provide a glimpse into the results obtained.
C
......
a,
o ~
l"*" i Hc/r~
..,:.::"
1500 2500 3500 4500 Field (Oe) Fig.2 M-H curves in Nb powder specimen at 6.2K. The inset shows forward and reverse curves near He2 value.
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reservcd.
500
~_o,,j . f
2266
K Kumar et at
/ Search for irreversibili~y line
o 0
POWDER
NIOBIUM
. . . .
-~--
-~
Nb Powder o5
0
t/;
21Hz,1Oe r,m.s.
n n
/ C
, la|
IO
o o
o o
o
-,5!
o
o T (HI AX
-20
3[
"~ - 4
//e--H-O Oe ffJj
H- I00 Oe
25 | 0 OOr
1.5
:~0
2 5
:3.0
-6 I III
4
I
3.5
4O
Nb Disc
1
-05
o
Tc(H )
•
Tp(H)
(b)
Q -10
I:~
2
°
H-IO00e
..x
m o
-15
o
e
-2.0
~ o
'
'
4
' ......
~
6
,
8
'
IO 2.5
TEMPERATURE t K )
10
Fig.3 ~ and X~ vs T in H = 0 and 100 Oe at (21Hz, 1 0 e rms) in Nb powder. ,--(0
| , ,_..;.:
T,O
T.5
~
:
~
~
~
9.6
10.0
R m O ~
Fig.4 X~ and X~ vs T at (21Hz, 1 0 e rms) in Nb disc for different H (applied parallel to disc plane). The data at different H are not normalized to each other. The H = 3 kOe data in Fig. 1 shows that Z F C and FC curves nearly merge at 6.2 K, howe-~':c, the inset of Fig. 2 indicates that the difference between forward and reverse hysteresis curves persists far beyond 3.5 kOe at 6.2 K. No D P E or any other peak like structure can be seen in the data of Fig. 3 as well as in other data recorded by varying the frequency and the amplitude of A C field. W e believe that the failure to observe D P E implies absence of genuinely reversible region near T~(H) line. The situation here is such that at a given H, though XZFC "-* XFC as T approaches a quasi-irreversibility temperature, the width of the iso. thermal hysteresis remains significantly larger. The latter width vanishes as H~H~2 (or T ~ Tc(H)).
15
2.0
2.5
3.0
3.5
4.0
Log (H) Fig.5 Log-log plots of (I-T fro(I-I)) vs H in Nb powder and disc. In the case of Nb disc, M z F c values approach M F c (~-- 0) values only at T = Tc(H). The isothermal forward and reverse M-H curves also meet only at Hc2(T). As anticipated, no DPE like structure is present in x~(T) data of Fig. 4. The peak like structure in x~(T) curves of Fig. 4 are believed to be a consequence of changes in normal state electrodynamics 6 and peak temperatures in x~(T) do not identify T~(H) values. The characteristic temperatures (T*) which can be noted from the data collected are T~(H) and Tr(H) (from X Z F C - X F C -~ 0 criterion) in Nb powder and Tc(tt) and Tp(EI) [peak temperature in x~(T)] in Nb disc. Figure 5 shows that in Nh powder, Tc(H) and T,.(tt) values between H = 0.1 and 1.0 kOe can be fitted to a power law, (1 - T * / T ~ ( O ) ) cx H q, with q = 2/3 and 11/20, respectively. In Nb disc, T¢(H) do not appear to fit to the power law, whereas Tp(H) values do with q = 4/5. REFERENCES
1. A.P. Malozemoff, MRS Bulletin 15 (1990) 50 and references therein. 2. E.H. Brandt, Int. J. Mod. Phys. B (1991) to appear and reierences therein. 3. A.K. Grover et al, Physica C162-164 (1989) 335; Pramana J.Phys. 33 (1989) 297. 4. R.A. tiein and R.L. Falge Jr, Phys. Rev. 123 (1961) 407. 5. A.F. Khoder, M. Couach and J.L. Jorda, Phys.
Rev. B42 (1990) 8714. 6. R.A.Hein, Phys. Rev. B33 (1986) 7539.
Physica C 185-189 (1991) 2265-2266 North-Holland
SEARCH FOR IRREVERSIBILITY LINE IN A CONVENTIONAL HARD SUPERCONDUCTOR
RAVI KUMAR~+, A.K. GROVER P. CHADDAH"
, S. RAMAKRISHNAN+, P.L. PAULOSE+ S.K. MALIK+AND
+Tata Institute of Fundamental Research, Bombay 400005, India sDepartment of Physics, Panjab University, Chandigarh 160014, India S.S.P. Division, Bhabha Atomic Research Centre, Bombay 400085, India Four procedures have been looked into to determine Tr(H) values in niobium. The DC magnetic data specify a lower limit, whereas caution needs to be exercised in assigning peak temperature in x~(T) with T~(H). The absence of DPE in x~(T) implies non-existence of reversible region below T~(H).
The advent of high temperature superconductors (HTSC) has given rise to the beliefx that there exists a small region below the T~(H) line where the magnetization response in hard type-II materials is path independent, and the hysteretic behaviour sets in below an irreversibility (Tr,H~) line. Many interesting theoretical ideas2, such as, flux-lattice melting, flux depinning, vortex-liquid to glass transition, etc., which pertain to the nature of (T~,H~) line, are curreutly being discussed in connection with the physics of HTSC. Some of these concepts may not be relevant for conventional low temperature type-II superconductors. In view of this, we have attempted to search for (T~,H~) line via DC and AC magnetic techniques in two specimens o£ elemental niobium (To = 9.3 K). Our results focus attention onto the relative efficacy of different procedures usually employed to determine (T~,H~) line in HTSC. The niobum specimens chosen are disc (thickness = 0.09ram and d i a . = 3.59mm) and niobium powder, which have been shown 3 to display thermo-magnetic history effects similar to behaviour reported in HTSC. The disc specimen is such that no flux escapes from it on field cooling (MFc(H,T) = 0 , H > 20e), whereas the powder syecimen displays partially reversible behaviour.
0.005
Nb P o w d e r
0 d
-0.005
~
F
-0.010
m m A
&
3 kOe • FC 3 kOe o z , ' c 100 Oe
~
,~,.,,o,~
&
"
n
o
><- -o.ot5
u
FC
I00 Oe
~ ZFC 20 Oe " FC 20 0e
~
0 OQ
2 2 ..... -0.020
. . . .
4
L , , , -
. . . . . . . . . .
, . . . .
,,,
. . . . . . . . . .
5
6 7 8 9 10 11 12 TEMPERATURE (K) Fig.1 ZZFc and ~ c vs T at H = 20, 100, and 3000 Oe in Nb powder. The arrows identify Tr(H) values by (ZZFc - ~rc ) -~ 0 criterion.
. . . . . i
~ -IX)
:, .-o°°° ~.-
. , -500
0-F
-o~
ao
.4
~
Nb Powder 6.2 K
1.0 I 0.0
o
The different procedures employed to determine Tr(H) values are, (i) the merger of MZFc(T) and MFc(T) data, (ii) the vanishing of hysteresis in isothermal DC M-H curves, (iii) the appearence of differential paramagnetic effect (DPE) 4,s in the in-phase AC susceptibility X~t(T) and (iv) the peak in X~t(T). Figures 1 to 5 provide a glimpse into the results obtained.
C
......
a,
o ~
l"*" i Hc/r~
..,:.::"
1500 2500 3500 4500 Field (Oe) Fig.2 M-H curves in Nb powder specimen at 6.2K. The inset shows forward and reverse curves near He2 value.
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reservcd.
500
~_o,,j . f
2266
K Kumar et at
/ Search for irreversibili~y line
o 0
POWDER
NIOBIUM
. . . .
-~--
-~
Nb Powder o5
0
t/;
21Hz,1Oe r,m.s.
n n
/ C
, la|
IO
o o
o o
o
-,5!
o
o T (HI AX
-20
3[
"~ - 4
//e--H-O Oe ffJj
H- I00 Oe
25 | 0 OOr
1.5
:~0
2 5
:3.0
-6 I III
4
I
3.5
4O
Nb Disc
1
-05
o
Tc(H )
•
Tp(H)
(b)
Q -10
I:~
2
°
H-IO00e
..x
m o
-15
o
e
-2.0
~ o
'
'
4
' ......
~
6
,
8
'
IO 2.5
TEMPERATURE t K )
10
Fig.3 ~ and X~ vs T in H = 0 and 100 Oe at (21Hz, 1 0 e rms) in Nb powder. ,--(0
| , ,_..;.:
T,O
T.5
~
:
~
~
~
9.6
10.0
R m O ~
Fig.4 X~ and X~ vs T at (21Hz, 1 0 e rms) in Nb disc for different H (applied parallel to disc plane). The data at different H are not normalized to each other. The H = 3 kOe data in Fig. 1 shows that Z F C and FC curves nearly merge at 6.2 K, howe-~':c, the inset of Fig. 2 indicates that the difference between forward and reverse hysteresis curves persists far beyond 3.5 kOe at 6.2 K. No D P E or any other peak like structure can be seen in the data of Fig. 3 as well as in other data recorded by varying the frequency and the amplitude of A C field. W e believe that the failure to observe D P E implies absence of genuinely reversible region near T~(H) line. The situation here is such that at a given H, though XZFC "-* XFC as T approaches a quasi-irreversibility temperature, the width of the iso. thermal hysteresis remains significantly larger. The latter width vanishes as H~H~2 (or T ~ Tc(H)).
15
2.0
2.5
3.0
3.5
4.0
Log (H) Fig.5 Log-log plots of (I-T fro(I-I)) vs H in Nb powder and disc. In the case of Nb disc, M z F c values approach M F c (~-- 0) values only at T = Tc(H). The isothermal forward and reverse M-H curves also meet only at Hc2(T). As anticipated, no DPE like structure is present in x~(T) data of Fig. 4. The peak like structure in x~(T) curves of Fig. 4 are believed to be a consequence of changes in normal state electrodynamics 6 and peak temperatures in x~(T) do not identify T~(H) values. The characteristic temperatures (T*) which can be noted from the data collected are T~(H) and Tr(H) (from X Z F C - X F C -~ 0 criterion) in Nb powder and Tc(tt) and Tp(EI) [peak temperature in x~(T)] in Nb disc. Figure 5 shows that in Nh powder, Tc(H) and T,.(tt) values between H = 0.1 and 1.0 kOe can be fitted to a power law, (1 - T * / T ~ ( O ) ) cx H q, with q = 2/3 and 11/20, respectively. In Nb disc, T¢(H) do not appear to fit to the power law, whereas Tp(H) values do with q = 4/5. REFERENCES
1. A.P. Malozemoff, MRS Bulletin 15 (1990) 50 and references therein. 2. E.H. Brandt, Int. J. Mod. Phys. B (1991) to appear and reierences therein. 3. A.K. Grover et al, Physica C162-164 (1989) 335; Pramana J.Phys. 33 (1989) 297. 4. R.A. tiein and R.L. Falge Jr, Phys. Rev. 123 (1961) 407. 5. A.F. Khoder, M. Couach and J.L. Jorda, Phys.
Rev. B42 (1990) 8714. 6. R.A.Hein, Phys. Rev. B33 (1986) 7539.