SUSCEPTIBILITY MAGNETIC AND HIGH MAGNETIC FIELD ESR MEASUREMENT OF SrCu 2 ( PO 4 ) 2

The magnetic susceptibility and high magnetic ESR measurement of SrCu2(PO4)2 has been performed at temperatures ranging from 4.2 K to 300 K and 4.2 K to 77 K, respectively. The magnetic susceptibility shows a broad maximum around T = 40 K. The magnetic susceptibility has been interpreted in terms of one-dimensional magnetic systems. The temperature dependence of the magnetic susceptibility indicated a good agreement with 4-spin alternating configuration model. In the ESR measurement, clear electron spin resonance (ESR) was observed. The integrated intensity for 120 and 301 GHz has a broad maximum at around 40 K, which is consistent with the susceptibility result. A quantitative description gives resonance is the first and second triplet excited states of the excitation spectrum of 4-spin alternating chain configuration. The g1, g2 and g3 values are approximately 2.21 at temperature above 40 K. The g2 dan g3 values have the dependence of temperature under 40 K.

It is very interesting to investigate 1D magnetic properties of MCu 2 (PO 4 ) 2 (M = Ba, Sr).Susceptibility magnetic of BaCu 2 (PO 4 ) 2 and SrCu 2 (PO 4 ) 2 powders have confirmed these compound to be a good 1D spin-1/2 Heisenberg antiferromagnetic (AF) chain system.The susceptibility of BaCu 2 (PO 4 ) 2 shows a broad peak around 60 K and claimed it originated from the ladder chain [13][14], while SrCu 2 (PO 4 ) 2 shows broad peak about 40 K and originated from 4-spin alternating chain [11].In the ESR measurement, the first triplet state of the excitation magnetic of ladder configuration in BaCu 2 (PO 4 ) 2 was observed [14].There seems to be a significant different in formation singlet ground state between BaCu 2 (PO 4 ) 2 and SrCu 2 (PO 4 ) 2 .Therefore, detailed studies of the mechanism for formation singlet ground state in SrCu 2 (PO 4 ) 2 are clearly needed.For this purpose, magnetic susceptibility was reexamined to confirm the correct interpretation of the existence of a spin singlet ground state with an energy gap in SrCu 2 (PO 4 ) 2 .Further, electron spin resonance (ESR) was performed on SrCu 2 (PO 4 ) 2 powder to observe the transitions between the singlet ground state and the excited triplet.

Methods
SrCu 2 (PO 4 ) 2 sample powder was prepared from appropriate mixtures of the source materials, (NH 4 ) 2 HPO 4 , SrCO 3 , and CuO in a molar ratio 2:1:2.The mixtures were heated at 900 0 C for 15 hours in an evacuated silica tube and cooled to 800 0 C. To complete the chemical reaction, the resulting product was reacted with repeated procedure until X-ray diffraction indicated the existence of a single-phase.
Magnetic properties of SrCu 2 (PO 4 ) 2 powder were studied by magnetic susceptibility and electron spin resonance (ESR) measurements.Magnetic susceptibility was measured using a Quantum Design Superconducting Quantum Interference (SQUID) device magnetometer in the 4.2 K T 300 K temperature range.Approximately 10 mg of sample were placed in a sealed quartz tube of 3 mm in diameter filled with 10% H 2 /Ar gas.In the magnetometer, magnetic susceptibility vs temperature curves were obtained by applying a magnetic field of 1 T.
ESR measurement was performed by a submillimeter wave ESR spectrometers using high pulse magnetic of 35 T, temperature control from 4 K to 300 K, and Gyrotron and Gunn Oscillator as a radiation source.The electromagnetic wave was transmitted through a sample placed at the center of the pulse magnet.The transmitted wave was detected by an InSb hot electron detector.The detected signal is amplified by a low noise amplifier.The output signal and signal from the pick up coil for field measurement were recorded by two channel digital memory oscilloscope and then fed to a computer.
We have used a pulse magnet generating high magnetic field up to 35 T. The measurement of magnetic field intensity was made by integration of the induced voltage

Results and Discussion
Result of Magnetic Susceptibility Measurement.Figure 1 shows the temperature dependence of magnetic susceptibility of SrCu 2 (PO 4 ) 2 measured at 1 T. Solid circles, dashed curve, open circles, and solid curve represent the measured susceptibility , the susceptibility to impurities , and the theoretical susceptibility , respectively.The susceptibility shows a broad maximum at around 40 K, a characteristic of a low dimensional antiferromagnets, and decreases rapidly with decreasing temperature.
In order to analyze the susceptibility quantitatively, the Curie term is eliminated in .It is assumed that the spins to impurities of Cu 2+ ions are isolated spins.Thus the data below 10 K were fitted to Curie-Weiss susceptibility , where is the curie constant and is the Weiss temperature.The magnetic susceptibility is fitted numerically by [16]: ( where is the Boltzmann constant and .The coefficients are given by , , , and .The B-F is a function of , so that is estimated to be 0.75.The , and g parameters were estimated as 40 K, 30 K, and g = 2.24, respectively. The theoretical curve agrees well with qualitatively.This agreement indicates that the magnetic properties of SrCu 2 (PO 4 ) 2 can be explained approximately by the spin system with 4-spin alternating-chain model.The 4-spin alternating-chain model configuration is shown in Figure 2. Below 40 K we observe three resonance lines P 1 , P 2 , P 3 , while above 40 K, one resonance line P 1 is observed.The resonance P 1 at H = 9.7 T increases its intensity as the temperature is increased up to 40 K, while its linewidth decreases continuously.When the temperature is increased further, the intensity of the resonance P 1 decreases.As the temperature is decreased, one resonance line P 2 shifts towards lower field and another one P 3 shifts towards higher field.
Figure 4 and 5 show the integrated intensity of P 1 , P 2 , P 3 absorption lines as a function of the temperature for 120 GHz and 301 GHz.For the ESR intensity, we used the 4-spin (S=1/2) alternating-exchange-interaction chains model, which gives the equation [15]: , where k B and H represent Boltzmann s constant and the applied magnetic field, respectively.Since is defined by Hase [15], we can determine the values of dan , so as to make the temperatures at maximum I 1 (T) and the calculated result coincide with each other.Qualitatively, the calculated result agrees well with the experimental result.The temperatures dependence of the integrated intensity indicated that we observe transitions in the singlet ground state, first and second triplet excited states.The ESR intensity I 1 (T) has a broad peak about 40 K. Below 40 K, the intensity I 1 start to decrease rapidly for all frequencies as temperature is decreased, which is consistent with the susceptibility result.A quantitative description gives resonance is the first and second triplet excited states of the excitation spectrum of 4-spin alternating chain configuration.The results of the experiments are all in good agreement with theory.
Fig. 6 shows the temperature dependence of g-value of SrCu 2 (PO 4 ) 2 .The g 1 , g 2 and g 3 values are g-value for the absorption lines P 1 , P 2 and P 3 , respectively.The g 1 , g 2 and g 3 values are approximately 2.21 at temperature above 40 K.The g 2 value increases and the g 3 value decreases at temperature under 40 K.

Conclusion
Magnetic susceptibility and high magnetic field ESR measurements of SrCu 2 (PO 4 ) 2 have been performed at temperatures from 4.2 K to 300 K and 4.2 K to 77 K, respectively.The temperature dependence of the magnetic susceptibility indicated a good agreement with 4-spin alternating configuration model.In the ESR measurement, clear electron spin resonance (ESR) was observed.The integrated intensity for 120 and 301 GHz has a broad maximum at around 40 K, which is consistent with the susceptibility result.A quantitative description gives resonance is the singlet ground state, first and second triplet excited states of the excitation spectrum of 4-spin alternating chain configuration.The g 1 , g 2 and g 3 values are approximately 2.21 at temperature above 40 K.The g 2 dan g 3 values have temperature dependence below 40 K.

Figure 1 .
Figure 1.The temperature dependence of magnetic susceptibility of SrCu 2 (PO 4 ) 2 measured 1.0 T from the pick up coil.The pulse magnet system has the field intensity pattern similar to a half sinusoidal one with the width of 2.5 ms.SrCu 2 (PO 4 ) 2 and DPPH powder were set in a sample holder, which placed at the center of the pulse magnet.DPPH powder was used as a standard sample with g = 2.0023.The temperature was measured by a FeAu 0.07 at.%-Ag

Figure 4 .
Figure 4. Temperature dependence of the integrated intensity at 120 GHz.Closed, open circles and close square denote the measured intensity for I 1 , I 3 and I 2 .The solid curve represents the calculated result.

Figure 5 .
Figure 5. Temperature dependence of the integrated intensity at 301 GHz.Closed, open circles and close square denote the measured intensity for I 1 , I 3 and I 2 .The solid curve represents the calculated result.

Figure 6 .
Figure 6.Temperature dependence of g-values.Opened circles, closed circles and opened squares stand for the g-values at X-band 11) , 120 GHz and 301 GHz, respectively.The solid line in Fig. 4 and 5 is a fit to the experiment with the parameters as follows: E 02 = -2605 GHz, E 01 = 416 GHz, E 13 = -1354 GHz, E 12 = 729 GHz, E 11 = -521 GHz, and E 21 = 1146 GHz and the corresponding magnetic field H = 9.7 T for the frequency of 301 GHz and H = 3.5 T for frequency of 120 GHz.