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Paramagnetic relaxation in two double nitrates
Volume 40, number 5
PHYSICS LETTERS
14 August 1972
PARAMAGNETIC RELAXATION IN TWO DOUBLE NITRATES J. FLOKSTRA*, W. A. VERHEIJ, G. J. C. BOTS
Delft Universityof Technology,Delft, The Netherlands and L.C. Van der MAREL and L.J.M. Van de KLUNDERT
Twente Universityof Technology, Enschede, The Netherlands Received 14 June 1972 Relaxation measurements have been performed on a single crystal of Ce2Mg3(NO3)t2 • 24H20 and of La2Mna(NO3)12 • 24H20 at liquid He temperatures.
Cerium magnesium nitrate (CMN) and lanthanum manganese nitrate (LMnN) have the same crystal structure, space group R3. The two cerium-ions per unit cell occupy identical places in the lattice; the three manganese-ions are situated on different places: one ion has symmetry 3, the other two ions have only 3. The purpose of our experiments was: a) to measure the influence of the surroundings on the relaxation behaviour of CMN, b) to investigate the relaxation behaviour of LMnN at high field values. CMN. By means of a Hartshorn mutual inductance bridge dispersion and absorption curves have been obtained on a cylindrical crystal ~diameter 6.5 mm and length 9 mm) perpendicular to the triad axis. Frequencies between 3 and 1200 Hz and magnetic field strengths up to 3.8 × 105 A/m (4.8 kOe) have been used. The bath temperature varied between 1.8 and 4.2 K and the pressure of the heliumgas, providing the contact between sample and helium bath, between 10 -3 torr and saturation pressure. At saturation pressure the dispersion and absorption curves can be well described with the formulas of Casirnir and du Pr6. For 2.1 < T < 2.6 K the temperature dependency of the relaxation time r obeys the expression for the Orbach process with A/k = 32 K (saturation pressure) in good agreement * Present address: Twente University of Technology, Enschede, The Netherlands.
with values found by other authors [1,2] (r between 0.3 and 5 ms). For T < 2.1 K r shows some influence of the Raman process; for T > 2.6 K r is too fast to be detected with our equipment (r < 0.3 ms). We have found (St/~H)T > 0 in the whole temperature range; r increases with increasing pressure of the contact gas. The experimentally determined ratio of r at vacuum and r at saturation pressure is in good agreement with the ratio CL/(CH + CL), C t and CH being respectively the lattice specific heat and the specific heat of the spin system at constant field It is found that b/R = 6.0 X 10-6 K 2 in good agreement with results of others [3]. For the a/Rvalue (C L = aT 3) we obtain 1.09 X 10 -3 K -3 ,which fits very well the value given by Bailey [4]. In fig. 1 the influence of the heat conductance of the surroundings on the observed curves is clearly demonstrated. It is difficult to define a limiting value for v + 0 of the dispersion curve. Evaluation of the shape of the curves at lowest frequencies indicates that near the saturation pressure a helium film covers the crystal, playing the role of the bath. Some preliminary calculations showed that near the saturation pressure the fluctuations of the thickness of the helium film, delivered by the alternating field, may influence greatly the shape of the dispersion curve [51. LMnN. By means of a mutual inductance bridge described by Van de Klundert et al. [6] we measured x'(H) and ×"(H) at frequencies between 1.5 and 363
Volume 40A, number 5
PHYSICS LETTERS
lx o
....- . .
Q9
0.8
0.7
14 August 1972
of the absorption curve has been found and this was not the case for T < T x. From this it may be concluded that the heat transport in this salt plays an important role in the relaxation process. For T < T~, superfluid helium penetrates into the sample and the heat contact between sample and bath is improved. For the b/R-value per gramion we found 0.083 K2; Mess [3] found 0.060 K 2 and Sapp [7] 0.070 K 2. The exchange part of the specific heat is 0.064 K 2. The paramagnetic saturation in this salt will be described elsewhere [8]. Finally it is mentioned here that the ratio of the b-values of LMnN and CMN equals roughly the square ratio of their magnetic phase transition temperatures (resp. T = 0.230 K and 0.0019 K): these ratios are respectively 14 × 103 and 15 × 103 .
io log ~J 06 1.0
15
Fig. 1. Dispersion curves of CMN at low frequencies, temperature T = 2.815 K, magnetic field strength H = 3.84 × 10 s A/m (4.8 kOe) and different pressures of the contact gas: 138 torr (= saturation pressure), • 126 torr, zx99 torr, (~ 60 torr, ~ 26.5 tort, • 9 torr, • 2 torr, * 10 -3 torr.
2000 Hz. The sample was cylindrical (diameter 6.5 mm and length 20 mm) and placed in liquid helium at temperatures between 2.0 and 4.2 K. The magnetic field was applied perpendicular to the triad axis and varied up to 1.75 × 106 A/m (22 kOe). The r versus H curve shows a maximum at about 2.4 × 105 A/m (3 kOe) (at 3.0 K rmax ~ 60 ms). Possibly there is a j u m p in the relaxation time as a function of temperature at the ),-point T x. For T > T x and high field values a striking broadening
364
We wish to thank Prof. Dr. B.S. Blaisse and Dr. H.C. Meijer for stimulating discussions.
References I I ] C.B.P. Finn, R. Orbach and W.P. Wolf, Proc. Phys. Soc. 77 (1961) 261. [2] O.S. Leifson and C.D. Jeffries, Phys. Rev. 122 (1961) 1781. [3] K.W. Mess, Thesis, Leiden 1969. [4] C.A. Bailey, Phil. Mag. 4 (1959) 833. [5] T.P. Valkering and J. Flokstra, unpublished results. [6] L.J.M. van de Klundert, C. de Rooy and L.C. van der Marel, Phys. Comm. Twente Univ. of Techn. 3 (1972) nr. 3. [7] R.C. Sapp and D.A. Nelson, Bull. Am. Phys. Soc. 11 (1961) 911. [8] J. Flokstra, H.C. Meijer, G.J.C. Bots, W.A. Verheij and L.C. van der Marel, Phys. Comm. Twente Univ. of Techn. 4 (1972) hr. 1.
PHYSICS LETTERS
14 August 1972
PARAMAGNETIC RELAXATION IN TWO DOUBLE NITRATES J. FLOKSTRA*, W. A. VERHEIJ, G. J. C. BOTS
Delft Universityof Technology,Delft, The Netherlands and L.C. Van der MAREL and L.J.M. Van de KLUNDERT
Twente Universityof Technology, Enschede, The Netherlands Received 14 June 1972 Relaxation measurements have been performed on a single crystal of Ce2Mg3(NO3)t2 • 24H20 and of La2Mna(NO3)12 • 24H20 at liquid He temperatures.
Cerium magnesium nitrate (CMN) and lanthanum manganese nitrate (LMnN) have the same crystal structure, space group R3. The two cerium-ions per unit cell occupy identical places in the lattice; the three manganese-ions are situated on different places: one ion has symmetry 3, the other two ions have only 3. The purpose of our experiments was: a) to measure the influence of the surroundings on the relaxation behaviour of CMN, b) to investigate the relaxation behaviour of LMnN at high field values. CMN. By means of a Hartshorn mutual inductance bridge dispersion and absorption curves have been obtained on a cylindrical crystal ~diameter 6.5 mm and length 9 mm) perpendicular to the triad axis. Frequencies between 3 and 1200 Hz and magnetic field strengths up to 3.8 × 105 A/m (4.8 kOe) have been used. The bath temperature varied between 1.8 and 4.2 K and the pressure of the heliumgas, providing the contact between sample and helium bath, between 10 -3 torr and saturation pressure. At saturation pressure the dispersion and absorption curves can be well described with the formulas of Casirnir and du Pr6. For 2.1 < T < 2.6 K the temperature dependency of the relaxation time r obeys the expression for the Orbach process with A/k = 32 K (saturation pressure) in good agreement * Present address: Twente University of Technology, Enschede, The Netherlands.
with values found by other authors [1,2] (r between 0.3 and 5 ms). For T < 2.1 K r shows some influence of the Raman process; for T > 2.6 K r is too fast to be detected with our equipment (r < 0.3 ms). We have found (St/~H)T > 0 in the whole temperature range; r increases with increasing pressure of the contact gas. The experimentally determined ratio of r at vacuum and r at saturation pressure is in good agreement with the ratio CL/(CH + CL), C t and CH being respectively the lattice specific heat and the specific heat of the spin system at constant field It is found that b/R = 6.0 X 10-6 K 2 in good agreement with results of others [3]. For the a/Rvalue (C L = aT 3) we obtain 1.09 X 10 -3 K -3 ,which fits very well the value given by Bailey [4]. In fig. 1 the influence of the heat conductance of the surroundings on the observed curves is clearly demonstrated. It is difficult to define a limiting value for v + 0 of the dispersion curve. Evaluation of the shape of the curves at lowest frequencies indicates that near the saturation pressure a helium film covers the crystal, playing the role of the bath. Some preliminary calculations showed that near the saturation pressure the fluctuations of the thickness of the helium film, delivered by the alternating field, may influence greatly the shape of the dispersion curve [51. LMnN. By means of a mutual inductance bridge described by Van de Klundert et al. [6] we measured x'(H) and ×"(H) at frequencies between 1.5 and 363
Volume 40A, number 5
PHYSICS LETTERS
lx o
....- . .
Q9
0.8
0.7
14 August 1972
of the absorption curve has been found and this was not the case for T < T x. From this it may be concluded that the heat transport in this salt plays an important role in the relaxation process. For T < T~, superfluid helium penetrates into the sample and the heat contact between sample and bath is improved. For the b/R-value per gramion we found 0.083 K2; Mess [3] found 0.060 K 2 and Sapp [7] 0.070 K 2. The exchange part of the specific heat is 0.064 K 2. The paramagnetic saturation in this salt will be described elsewhere [8]. Finally it is mentioned here that the ratio of the b-values of LMnN and CMN equals roughly the square ratio of their magnetic phase transition temperatures (resp. T = 0.230 K and 0.0019 K): these ratios are respectively 14 × 103 and 15 × 103 .
io log ~J 06 1.0
15
Fig. 1. Dispersion curves of CMN at low frequencies, temperature T = 2.815 K, magnetic field strength H = 3.84 × 10 s A/m (4.8 kOe) and different pressures of the contact gas: 138 torr (= saturation pressure), • 126 torr, zx99 torr, (~ 60 torr, ~ 26.5 tort, • 9 torr, • 2 torr, * 10 -3 torr.
2000 Hz. The sample was cylindrical (diameter 6.5 mm and length 20 mm) and placed in liquid helium at temperatures between 2.0 and 4.2 K. The magnetic field was applied perpendicular to the triad axis and varied up to 1.75 × 106 A/m (22 kOe). The r versus H curve shows a maximum at about 2.4 × 105 A/m (3 kOe) (at 3.0 K rmax ~ 60 ms). Possibly there is a j u m p in the relaxation time as a function of temperature at the ),-point T x. For T > T x and high field values a striking broadening
364
We wish to thank Prof. Dr. B.S. Blaisse and Dr. H.C. Meijer for stimulating discussions.
References I I ] C.B.P. Finn, R. Orbach and W.P. Wolf, Proc. Phys. Soc. 77 (1961) 261. [2] O.S. Leifson and C.D. Jeffries, Phys. Rev. 122 (1961) 1781. [3] K.W. Mess, Thesis, Leiden 1969. [4] C.A. Bailey, Phil. Mag. 4 (1959) 833. [5] T.P. Valkering and J. Flokstra, unpublished results. [6] L.J.M. van de Klundert, C. de Rooy and L.C. van der Marel, Phys. Comm. Twente Univ. of Techn. 3 (1972) nr. 3. [7] R.C. Sapp and D.A. Nelson, Bull. Am. Phys. Soc. 11 (1961) 911. [8] J. Flokstra, H.C. Meijer, G.J.C. Bots, W.A. Verheij and L.C. van der Marel, Phys. Comm. Twente Univ. of Techn. 4 (1972) hr. 1.