curie point

Curie point or Curie temperature refers to the temperature at which a material can change between ferromagnetic and paramagnetic. When the temperature is below Curie point, the material becomes ferromagnetic, and the magnetic field related to the material is difficult to change. When the temperature is higher than the Curie point temperature, the substance becomes paramagnetic, and the magnetic field of the magnet easily changes with the change of the surrounding magnetic field. At this time, the magnetic sensitivity is about 10 to the negative 6th power.

/kloc-at the end of 0/9, a famous physicist discovered a physical characteristic of magnets in his own laboratory, that is, when the magnets are heated to a certain temperature, the original magnetism will disappear. Later, people called this temperature "Curie point". On the earth, rocks are magnetized by geomagnetic field during diagenesis, obtaining weak magnetism, and the magnetic field of magnetized rocks is consistent with geomagnetic field. That is to say, no matter how the geomagnetic field changes direction, as long as its temperature is not higher than Curie point, the magnetism of rocks will not change. According to this truth, as long as the magnetism of rocks is measured, the geomagnetic direction at that time can be inferred naturally. This is what people often call fossil magnetism in geoscience research. On this basis, scientists use the principle of fossil magnetism to study the variation law of geomagnetic field in the history of earth evolution, which is called paleomagnetism theory.

In order to find new evidence of continental drift theory, scientists introduced paleomagnetism into the field of marine geology and achieved encouraging results.

After World War II, scientists used highly sensitive magnetic detectors to conduct paleomagnetic surveys on the sea surface of the mid-ocean ridge of the Atlantic Ocean. Later, people used magnetometers and other instruments to measure the paleomagnetism of the Pacific Ocean in the form of dense survey lines. The data of the two surveys surprised people to find that there is a stripe with equal magnetic lines at the bottom of the ocean, which is parallel to the north-south direction of the central axis of the mid-ocean ridge, and the magnetism is alternating. Each magnetic field line belt is about several hundred kilometers long and varies in width from several tens to several hundred kilometers. The discovery of submarine magnetic tape has become a great miracle of earth science research in this century. 1963, F.J. Vain, a young scholar at Cambridge University in England, and D.H. Matthews, his teacher, suggested that if "submarine expansion" ever happened, the lava rising from the mid-ocean ridge should retain the magnetization direction of the earth's magnetic field at that time after solidification. In other words, there should be magnetic strips with the same magnetization on the seabed on both sides of the ocean ridge. When the earth's magnetic field reverses, the polarity of the magnetic stripe should also be reversed, and the width of the magnetic stripe can be used as a measure of the time of two reversals. This bold assumption was quickly confirmed, and people found the same symmetric tapes in the Pacific Ocean, the Atlantic Ocean and the Indian Ocean. Not only that, scientists also calculated that in 76 million years, the earth experienced 17 1 inversion.

It is also found that the longest period between two reversals of the earth's magnetic field is about 3 million years, the shortest period is about 50 thousand years, and the average period between two reversals is about 420 thousand to 480 thousand years. At present, the direction of the earth's magnetic field has been preserved for 700 thousand years, so people have a hunch that a new magnetic field change may be approaching us.

The research on submarine magnetic tape is still going on, and many questions still cannot be answered satisfactorily. For example, the most basic question, why the earth's magnetic field reverses back and forth, cannot be explained clearly. Although scientists have put forward various hypotheses, the real reason is still unclear. In other words, the mystery of the internal law of the earth's magnetic field turning needs scientists to continue to explore.

Let's talk about ferromagnetic materials.

Ferromagnetic substance (1) can be magnetized to saturation under a small magnetic field. Not only the magnetic susceptibility is greater than 0, but also the value is as large as 10- 106. The relationship between magnetization m and magnetic field strength h is a nonlinear and complex function. This magnetism is called ferromagnetism.

(2) Ferromagnetic substances are ferromagnetic only when they are below Curie temperature; Above Curie temperature, due to the interference of crystal thermal motion, the directional arrangement of atomic magnetic moments is destroyed, so that ferromagnetism disappears, and then the matter becomes paramagnetic.

(3) Characteristics

First, the magnetism is very strong. Usually, the magnetic substance mainly refers to this substance.

B, hysteresis.

C spontaneous magnetization: the atomic magnetic moment in ferromagnetic materials overcomes the disorder effect of thermal motion through the action of atomic electron layers in adjacent lattice nodes. Atomic magnetic moments are spontaneously arranged in parallel and orderly regions and distributed in different small regions. This phenomenon is called spontaneous magnetization.

Unparalleled 3d Electron Layer: Iron, Nickel, Cobalt and Manganese

D, magnetic domain

Small areas of spontaneous magnetization are called magnetic domains. The interface between domains is called domain wall.

Then explain the measurement experiment.

Curie point of ferromagnetic materials

Experimental purpose: To understand the microscopic principle that ferromagnetic substances become paramagnetic, and to learn the principle and method of measuring Curie temperature with JLD-Ⅱ Curie point tester.

Experimental instruments: JLD-II Curie point tester (one host, one heating furnace and five samples) and ST 16B oscilloscope.

Experimental principle: for ferromagnetic materials, due to the existence of magnetic domains, hysteresis will occur under the action of external alternating magnetic field. Hysteresis loop is the main manifestation of hysteresis phenomenon. If the ferromagnetic substance is heated to a certain temperature, due to the intensification of thermal motion in the metal lattice, when the magnetic domain is destroyed, the ferromagnetic substance will be transformed into paramagnetic material, and the hysteresis phenomenon will disappear. The transition temperature of ferromagnetic substances is called Curie point. This Curie point tester observes and measures the transition temperature of ferromagnetic materials by observing the existence of hysteresis loop displayed on the oscilloscope. The instrument generates an alternating magnetic field by applying alternating current to a coil wound around a sample. During the heating process of the heating furnace, the Curie point was found on the oscilloscope.

Experimental steps: 1. Connect the wires of the heating furnace to the two terminals in front of the power box. Connect the ferromagnetic material sample to the power box with special wires, and put the sample into the heating furnace. Connect the connectors of the temperature sensor and the cooling fan to the sensor connector connected to the front panel of the power supply.

2. Connect the B output on the oscilloscope to the Y input, and connect the H output to the X input with special wires. Turn the "heating-cooling" switch to heating, turn on the power switch on the power box, and properly adjust the Y and X adjustments on the oscilloscope, and the hysteresis loop will be displayed on the oscilloscope.

3. Turn the "Measure-Set" switches of the two air doors on the furnace to the "Set" position (the knob direction is perpendicular to the axis direction of the heating furnace). After setting the furnace temperature, go to "Measurement". When the heating furnace works, the furnace temperature gradually rises to the set temperature.

4. When the temperature reaches the Curie point of the sample, the hysteresis loop disappears, and the digital thermometer displays the measured temperature value-Curie point.

Open two air doors on the heating furnace (the knob direction on the air doors is parallel to the axis direction of the heating furnace), and rotate the "heating-cooling" switch to cool down. After the heating furnace cools down, repeat the above process with another sample until the sample is measured.