Magnetic susceptibility. Basic principles and features on the PPMS.

Transcription

1 Magnetic susceptibility. Basic principles and features on the PPMS. Experimental methods: DC Magnetometry AC Magnetometry Torque Magnetometry Heat Capacity 1. DC Magnetometry. Basic principles. DC magnetic measurements determine the equilibrium value of the magnetization in a sample. The sample is magnetized by a constant magnetic field and the magnetic moment of the sample is measured, producing a DC magnetization curve M(H). Inductive measurements are performed by moving the sample relative to a set of pickup coils (oneshot extraction, for example). In conventional inductive magnetometers one measure the voltage induced by the moving magnetic moment of the sample in copper coils. The Faraday s Law is used in this case to extract useful information (see Appendix I).

2 DC extraction signal

3 2. AC Magnetometry. Basic principles. In AC measurements, a small AC drive magnetic field is superimposed on the DC field, causing a time-dependent moment in the sample. The field of the time-dependent moment induces a current in the pickup coils, allowing measurements without sample motion. Sample Space AC Drive Coil (blue) Calibration Coils (dash) Thermometer Detection Coils (red) AC Drive Compensation Coils Electrical Connections Case 1: Low AC frequency As long as the AC field is small, the induced AC moment is: MAC = (dm/dh) HAC sin(ω t) HAC is the amplitude of the driving field, ω is the driving frequency χ = dm/dh is the slope of the M(H) curve Case 2: Higher frequencies At high frequencies the AC moment of the sample doesn t follow along the DC magnetization curve due to the dynamic effects on the sample. That is why the AC

4 susceptibility is often known as the dynamic susceptibility. The magnetization of the sample may lag behind the drive field. The AC method yields two quantities: a) the magnitude of the susceptibility χ b) the phase shift ϕ The magnetic susceptibility is defined as follows: χ(q, ω)=χ' + χ'' or χ(q, ω)= χ exp(iϕ) χ' - real part, or dynamical susceptibility χ'' - imaginary part has the character of absorption coefficient ϕ = arctan(χ''/χ') describes a phase lag of M(t) behind the external field H exp(iωt)

5 3. AC vs DC Magnetometry. Examples. In the limit of low frequencies an AC measurement is rather similar to a DC extraction, the real component χ' is just the slope of the magnetization curve M(H). χ'' or the imaginary part is missing in the DC method. Example #1 Ferromagnetic metal Pr 0.75 La 0.25 Ni measured on PPMS (E21/TUM). Since the sample is conductive eddy(whirlpool) currents contribute to the dissipative part χ''.(see Appendix II) a) Pr 0.75 La 0.25 Ni measured by the DC method Magnetization (emu) 2 1 DC magn 1/M (emu -1 ) experiment Curie-Weiss T (K) T (K) χ = T, Θ = 19.5 K T Θ T c =20.5K in pure PrNi

6 b) Pr 0.75 La 0.25 Ni measured by the AC method Magnetization (emu) DC magn M' Moment M'' T (K) Phase (degree), M' (arb. units) M' Phase T (K) - In a ferromagnetic metal both χ' and the phase ϕ reveal well-defined maxima close to the critical temperature T c. - χ'' or the dissipative part is huge in the phase transition region.

7 Example #2 AF insulator Ca 2 Y 2 Cu 5 O 10 measured on PPMS (E21/TUM). DC: H0 = 10 koe AC: f = 1 khz, HAC = 10 OeThe Curie Law: χ(t)=c/t, C = NN A g 2 2 µ BS( S + 1) 3k B C = emu K/mole S = 1/2 (g = 2.24) χ (emu/mole Cu) experiment Curie law T (K) Since the sample isn t conductive there (emu/mole Cu) χ DC magnetization Re{X} AC measurements Im{X} AC measurements is no contribution due to eddy currents. DC = AC and χ'' T (K)

8 4. Torque Magnetometry The torque magnetometer is based on the principle that a magnetic sample in a (magnetic) field experiences a force. The torque exerted on a sample is proportional to its magnetisation. Furthermore the torque is proportional to the derivative of the anisotropy energy towards the angle of rotation. The PPMS torque magnetometer incorporates a torque-level chip mounted on a rotator platform for performing angular-dependent magnetic moment measurements. It utilizes a piezoresistive technique to measure the torsion of the lever created by the applied magnetic field on the sample moment: τ = m B Some specifications of the PPMS Torque magnetometer: - Moment sensitivity emu - Maximum Torque Nm - Maximum sample size mm 3 - Maximum sample weight 10 mg

9 4. Specific heat option of the PPMS The PPMS (Quantum Design) heat capacity option measures the heat capacity at constant pressure: C p dq = dt p It controls the heat added and removed from a sample while monitoring the resulting change in temperature. The picture below illustrates the thermal relaxation method employed by the PPMS specific heat option.

10 The simplest model to analyse the thermal relaxation data assumes that the sample and the sample platform are in good thermal contact with each other and are at the same temperature. Within this model the temperature of the platform as a function of time obeys the equation: dq Ctotal = = Kw( T Tb ) + P( t) dt where C total is the total heat capacity of the sample and platform K w is the thermal conductance of the supporting wires T b is the temperature of the thermal bath P(t) is the power applied by the heater The solution of this equation is given by exponential functions with a characteristic time-constant equal to C total /K w

11 PPMS. General description. The Quantum Design PPMS (Physical Property Measurement System) represents an open architecture, variable temperature-field system, designed to perform a variety of automated measurements. Sample environment controls include fields up to ± 9 Tesla and temperature range of K. Available PPMS applications: - DC and AC Magnetometry - Heat Capacity - Electro-Transport - Thermal Transport

12 Appendix I. Faraday's Law Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc. Faraday's law is a fundamental relationship which comes from Maxwell's equations. It serves as a succinct summary of the ways a voltage (or emf) may be generated by a changing magnetic environment. The induced emf in a coil is equal to the negative of the rate of change of magnetic flux times the number of turns in the coil. It involves the interaction of charge with magnetic field.

13 Appendix II. Eddy(Whirlpool) currents. Frequency Hz

14 Appendix III. AC susceptibility of a weakly interacting spin system

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16 Literature: R.W.White Quantum theory of magnetism (1970) McGraw-Hill Inc. Quantum Desing PPMS Brochure (2003)/