The phenomenon that mass changes its dimension in the presence of magntic field, as first dicovered in 1842 by Joule, is called magnetostrictive effect. The traditional magnetostrictive material such as iron, nickel, ferrite etc. are of low magnetostriction, such as iron 21¡Á10^-6, nickel -46¡Á10^-6. In the early 1970s, A.E Clark and coworkers discovered that rare earth binary alloys TbFe₂, DyFe₂, SmFe₂ etc. have very large magnetostriction above room temperatures however with high anisotropy which means very large field will be needed to get suturation, and this causes great difficulty in practical applications. In solving this problem, pseodobinary alloys were studied which led to the discovery of Tb_xDy_1-xFe_2-y material possessing giant magnetostriction with low anisotropy. Grain-oriented Tb_xDy_1-xFe_2-y single crystal or multicrystal materials exhibit "jump effect" which means that the magnetostriction increases drastically in the presence of prestress. From then on, the practical application of this material became possible, therefore attracting widespread attention from industry. Since this material having magnetostriction of 1500¡«2000¡Á10^-6 is ten to even several hundred times that of the traditional magnetostrictive materials, it is called the "giant magnetostrictive material" or "super magnetostrictive material" (hereafter refer to as GMM).

    GMM can function both as actuator material and as sensor material.

    As actuator material it changes dimensions in the presence of a magnetic field, which means the magnetic energy is changed to mechanical one. Making use of this function, firstly we can make precision displacement actuators with high precision (10^-1¡«10^-3 ¦Ìm), instant response (in microseconds) and large output force. Secondly, if an alternating magnetic field is supplied, the GMM rod

will vibrate mechanically which make sonic and ultrasonic transducers possible for applications such as louderspeakers, ultrasonic cutting, etc. The most outstanding point is that these transducers are much more powerful than that of the piezoelectric ceramic and traditional magnetostrictive transducers, and whatsmore, much more reliable comparing with that of the piezoelectric ceramic ones.

    When a stress or force is applied to the GMM, there will be changes in its magnetization. That is why it can be made into sensors for force, vibration etc. Combining the above mentioned two functions of sensing and actuating by means of computer collecting sensing information from and sending command to the GMM transducer to actuate will form the smart sensing & actuating system, so GMM can be classified into smart materials.

    As a material with multiple functions, it can be used in many different fields. Till now, more than 1000 kinds of different devices have been developed, covering the fields of machine tool industry, electronic industry, oil industry, textile industry, aircraft and aerospace, agriculture as well as civil use. All these developments have promoted advances greatly in the regarding industry areas. For example, the use of the high power GMM transducer to treat the oil well reduces the viscosity of oil therefore increases the output of the oil greatly.

    With the rapid development of the science & technology, this material is becoming more and more important in various fields. We believe it will exert a profound influence to our new century industry and high technology.