Hard (magnetic) disk (I)

One 、 background

I've sorted it out before Disk related information , But there is no logic , Did not explain disk clearly , Even today, I still don't have a deep understanding of disk , As a result, I often confuse the concept between operating system and disk , Cause some mistakes , therefore , I decided to clean up the disk concepts from scratch .

The following disks = Hard disk .

Two 、 A little hard disk history

The first hard disk in the world （Hard Disk Drives,HDD） By IBM stay 1956 Invented in , Its model is IBM 350 disk storage, It is carried in IBM 305 RAMAC In the computer . This hard disk is considered the rudiment of modern hard disk , It weighs up to 600 kg , The volume reaches the sum of the two refrigerators .

attach ：

later , stay 1980 year ,IBM Invented the world's first capacity breakthrough GB The hard disk of IBM3380.

2007 year , Hitachi （2003 Hitachi acquired IBM Hard disk segment of ） stay CES A piece with a capacity of TB Level of hard disk .

2021 year , The capacity of the hard disk has reached 20TB.

On capacity , The capacity of hard disk is getting larger and larger , The size is also reduced from the original Big Mac to today's standard size 3.5 And 2.5 Inch .

The above is from wiki.

3、 ... and 、 The basic principle

Electromagnetic phenomena are 1819 By Hans · Christian · Auster found , At that time, he found that the compass could not accurately indicate the direction when it was close to the electrified wire . When the current stops , The compass returns to normal .

The magnetic field produced by the wire conductor will affect the magnetic material around it , When the current direction or voltage polarity changes , The magnetic poles also change . ——— Electromagnetism

 

Faraday in 1831 Another electromagnetic effect was discovered in . He found that if a conductor passes through a moving magnetic field , Will produce an electric current . The direction of the current changes with the direction of the magnetic pole .   —— Faraday electromagnetic induction

According to electromagnetic phenomena , Wind the coil around a paramagnetic （ Paramagnetism can be magnetized in a magnetic field , So as to have magnetism .） On , This paramagnetic will be magnetized , It is magnetic , The direction of the magnetic field is consistent with that of the coil . This is the principle of electromagnets , and U The strongest point of the A-type electromagnet is U Type opening .

And according to electromagnetic induction , After the research of later generations , As long as the magnetic flux of the coil changes , The coil will produce an induced electromotive force , The faster the magnetic flux changes , The stronger the induced electromotive force .

The figure above is a schematic diagram of traditional magnetic storage , among 4 For the head （ It's just one. U Type electromagnet , ha-ha ）,1 and 2 Is the magnetized magnetic medium area , The direction of the magnetic field intensity in these two regions is different ,3 At their junction .

The disks or tapes that make up the storage media form some form of media , Above , A layer of magnetized material is stored . The substance is usually some impurity iron oxide . Each magnetic particle on the storage medium has its own magnetic field . When the medium is not magnetized , The poles of these magnetic fields are usually disordered . Since the magnetic field of each particle indicates a random direction , So there is a mutual offset , Thus, there is no obvious magnetic pole . 

4 For the head , In fact, it is an electromagnet , When there is current flowing through the coil , Its U The external magnetic field of the mouth part will affect the magnetic medium of the contact surface , Magnetize the magnetic medium material in this region . When writing , By applying current in different directions to the magnetic head , Thus, magnetic fields in different directions can be left on the magnetic medium , Write information .

among 1 and 2 It is two magnetized regions with opposite magnetic field directions , When the head is in 1 In or in 2 In the time , The magnetic flux does not change , But from 1 Switch to 2 When , Start passing 3 It's about , Because the direction of the magnetic field is reversed , The magnetic flux will change dramatically , Thus, the coil generates an induced electromotive force , There is a weak current in the coil , This current can be amplified by a peripheral circuit , To read out the previously stored information .

You can see , When the magnetic head moves from one magnetized region to another magnetized region with opposite magnetic field direction , The magnetic flux will flip , This produces a peak voltage , Only in such a region can peak voltage be generated , In other regions, though, the magnetic flux also changes , But the change is small , The voltage signal is very weak , These signals are considered noise , Noise can be filtered through peripheral circuits , At the same time, the peak voltage signal is amplified , Restore the written voltage signal .

Iron oxide is a kind of magnetic material , Magnetic materials can be magnetized by a magnetic field , When the magnetic field is removed , It remains magnetic . In the process of being magnetized , Its magnetism will vary with the intensity of the applied magnetic field H Increase and gradually increase , But it will reach a magnetization M after , The magnetization will not change as the applied magnetic field continues to increase , This magnetization is the saturation magnetization . Then remove the applied magnetic field , It can continue to maintain its magnetism , The magnetization of the material at this time is the residual magnetization , The saturation magnetization must be reached before the self magnetism can be maintained .

In the process of increasing the applied magnetic field , Magnetic materials are not immediately magnetic at first , When the applied magnetic field reaches a certain magnetization Hc after , Magnetic materials begin to have magnetization , This Hc It is called the coercivity of magnetic materials . When the magnetic material is magnetized , The material retains its residual magnetization , If you want to remove the remanence , A magnetic field in the opposite direction must be applied , The magnetic field strength of the magnetic field must be greater than Hc The residual magnetization in this direction of the magnetic material can be removed .

You can see the magnetism after being magnetized , Able to remain magnetic , And its magnetism is kept because of the existence of coercivity , Will not be easily disturbed by the external magnetic field . meanwhile , Because of the existence of coercivity , Magnetic materials are not easily magnetized .

Because of this characteristic of magnetic materials , It can be used as a material for recording information .

So what kind of magnetic material can be used as a magnetic storage medium ？

• High saturation magnetization , The greater the saturation magnetization , The less likely magnetic materials are to have residual magnetization , Then the magnetic material is less likely to have residual magnetization due to environmental impact , So in the long process of use , The area where the magnetic material is not magnetized will not obtain the residual magnetization due to the environment , Thus causing information error .
• Coercivity Hc It should be higher , The higher the coercivity , The stronger the ability of magnetic materials to maintain magnetization , It will not be degaussed due to external influence , Thus losing information .
• Rectangle ratio （ Residual magnetization is higher than saturation magnetization ） Be big , In this case , When a magnetic material is magnetized , It won't be too wasteful to add magnetic field , If the rectangle is smaller , The intensity of the applied magnetic field needs to be much higher than the remanence to reach the saturation magnetization , This is a waste of energy ; conversely , If the rectangle ratio is large , The intensity of the applied magnetic field only needs a little more than the remanence intensity to reach the saturation magnetization , Energy efficiency is higher .
• Besides , Its insulation 、 Heat resistance 、 Magnetic heat resistance and magnetic compression resistance 、 mechanical properties 、 Chemical properties should be relatively good .

The coercivity mentioned above is an important standard to measure the magnetization of magnetic materials , According to the coercivity , Magnetic materials can be divided into soft magnetic and permanent magnetic , Obviously, iron oxide is a kind of permanent magnetic material , So it can be used as a magnetic storage medium .

# So how do you write information ？

Recording principle of digital signals ：

in other words , Or use the magnetization direction to record the digital signal 1 and 0, Or use magnetization inversion to record 1 and 0.

however , Specifically 1 or 0 Whether to use magnetization direction or magnetization reversal for recording is decided by the equipment manufacturer （ In fact, it is related to the specific coding method , But the coding method is also determined by the manufacturer ）.

The function of encoding here is to convert the data into a form that can be written to the disk and read out .

Let's first look at the simplest coding method ：

Return to zero system （RZ,return zero）

   The so-called zeroing system is actually , High level represents 1, Low level represents 0, Two digital signals are always separated by a zero level .

As shown in the figure above , After writing magnetic media in this way , There is always an unmagnetized region between two magnetized regions . When writing , According to the level signal , Determine the magnetization direction , The current always returns to zero when a data is written . When I read it out , head （ Suppose from left to right , The right direction of the image is positive ） Moving from left to right , When an area that has never been magnetized enters 1 Area No （ The magnetization direction is positive ） When , The magnetic flux of the coil of the magnetic head increases instantaneously , Thus, a forward pulse voltage is sent , The peripheral circuit can output high level according to the forward pulse voltage ; When the magnetic head moves from 1 When the No. 1 region leaves and enters an unmagnetized region , The magnetic flux continues to decrease , So as to send out a negative pulse voltage , The peripheral circuit returns the level signal to zero according to the negative pulse . The rear head enters 2 Area No （ The magnetization direction is negative ）, The magnetic flux of the coil of the magnetic head decreases instantaneously , Thus, a negative pulse voltage is sent , The peripheral circuit outputs a low level according to the negative pulse ; Then the head leaves 2 Area No , The magnetic flux of the coil of the magnetic head increases instantaneously , So as to send out a positive pulse voltage , The peripheral circuit returns the level signal to zero according to the forward pulse . hinder 3、4、5 Areas are similar .

You can see , Data encoded in this way , After it is written to disk , It can be read out easily .

And this zeroing system （RZ） It is also the encoding method adopted by the early disk , Used to record data to disk .

Non return to zero system （NRZ,non return zero）

  The so-called non return to zero system is the high-level representation 1, Low level represents 0, There is no zero level between the two digital signals .

As shown in the figure above , After writing magnetic media in this way , There is no unmagnetized region between the two magnetized regions . When writing , According to the level signal , Determine the magnetization direction . When I read it out , head （ Suppose from left to right , The right direction of the image is positive ） Moving from left to right , When an area that has never been magnetized enters 1 Area No （ The magnetization direction is negative ）, The magnetic flux of the coil of the magnetic head decreases instantaneously , The coil sends a negative pulse , Peripheral circuit output low level , from 1 Get into 2 When , The magnetic flux is reversed , The magnetic flux of the coil of the magnetic head increases instantaneously , The coil sends a positive pulse , Peripheral circuit output high level , from 2 Get into 3 When , The magnetic flux is reversed again , The magnetic flux of the coil of the magnetic head decreases instantaneously , The coil sends a negative pulse , Peripheral circuit output high level . Others are similar . When from 5 When entering an unmagnetized region , The magnetic flux of the coil of the magnetic head increases instantaneously , The coil sends a positive pulse , The peripheral circuit will output high level .

There is a problem , When encountering continuous 1 Or continuous 0 When , When reading , It will output continuous high level and continuous low level , Then the peripheral circuit can not determine how many 1 perhaps 0 了 , This must be added to the clock , So that the circuit can judge how many 1 still 0.

For clock signals , Or add a clock signal to the peripheral circuit , How about improving this coding method , Add a clock signal to the code . that , Obviously, improving the coding method is a better choice .

For the return to zero system , It will never have a continuous high level or a continuous low level , Because it always sets the level signal to zero , such 0 The level just acts as a clock signal .

Of course , There are other ways of encoding , Such as NRZ1（ Non return to zero reversal , encounter 1 Just flip it .）、 Manchester code 、 Differential Manchester code 、4/5bit code 、8/10bit code 、 Phase modulation 、 frequency modulation 、 Improved FM will not be introduced one by one .

These encoding methods will be determined when the disk is manufactured , So the encoding method is determined by the disk manufacturer .

After introducing the coding method , We have obtained the form in which data can be written to disk , That is, a series of high and low level signals , So how should this series of high and low level signals be converted into the magnetized region in the magnetic medium ？

After so many years of development , There are two recording methods ： Horizontal magnetic recording mode and vertical magnetic recording mode .

Horizontal magnetic recording mode （Longitudinal Magnetic Record）

The horizontal magnetic recording mode adopts the combination of a ring magnetic head and a recording medium with longitudinal magnetic anisotropy , The remanence direction in the recording medium is parallel to the medium plane .

Sketch Map ：

  The gap of the ring head is perpendicular to the remanence direction .

The disadvantages of horizontal magnetic recording ：

As shown in the figure ： There is a transition zone between two magnetized regions due to mutual repulsion ,  The size and width of the transition zone directly affect the intensity of the pulse signal when reading , If the transition zone is too wide , This will cause the pulse signal to be unrecognized , Thus the data cannot be read out . therefore , For a long time , Disk manufacturing researchers are considering how to reduce this transition zone , To improve the horizontal magnetic recording density . The main factors affecting the transition width are the thickness and the coercivity of the magnetic medium , Researchers reduce the thickness of the magnetic medium layer and improve the coercivity of the magnetic medium , To ensure that the density is increased at the same time , Ensure that the width of the transition zone does not affect the readout of subsequent signals .

however , No matter how you reduce the thickness or increase the coercivity , There is no legal disk capacity breakthrough TB level , So researchers turned to vertical magnetic recording .

Vertical magnetic recording mode （Perpendicular Magnetic Record）

Vertical magnetic recording usually adopts the combination of a vertical magnetic head and a recording medium with vertical magnetic anisotropy , The remanence direction in the recording medium is perpendicular to the medium plane .

  Sketch Map ：

As shown in the figure , The vertical magnetic recording method has two layers of magnetic media , The magnetic medium in the upper layer has vertical magnetic anisotropy , The remanence direction is vertical , When the upper layer is magnetized , The lower magnetic medium is magnetized by the upper remanence , The direction of remanence that forms a horseshoe , The magnetization directions between the upper and lower layers together form a coupled horseshoe shaped magnetization direction , So as to form a stable state .

The vertical magnetic recording method solves the influence of the transition region very well , Thus, the density of magnetic recording can be improved , In the end in 2007 In, the capacity of the hard disk has broken through TB level , Now? 2020 In, the capacity of hard disk has broken through 20TB 了 .

Although the method of perpendicular magnetic recording solves the problem of transition region , But there are special requirements for vertically anisotropic materials , The properties of perpendicular magnetic anisotropy must satisfy a certain quantitative relationship before it can be used as the upper magnetic medium material of perpendicular magnetic recording , But the researchers finally found the material .

In other ways , The development of magnetic head has also greatly affected the capacity of disk , For a long time in the early days , Magnetic head adopts electromagnetic induction magnetic head , The sensitivity of this magnetic head cannot be satisfied in both reading and writing , If you improve your writing ability , Will reduce its reading sensitivity ; If you increase its reading sensitivity , It will reduce its writing ability , So it soon reached its limit . The most representative electromagnetic induction heads are giant magnetic heads and thin film magnetic heads .

later , Researchers try to use magnetoresistance effect to make magnetic heads （ namely MR head ）, This kind of magnetic head has high reading sensitivity , But this kind of head cannot be used for writing , So the researchers combined the thin film head with MR Magnetic heads are combined to make AMR head , Thin film heads are used for writing ,MR The magnetic head is used for reading , This makes the head excellent in both writing and reading , however AMR The magnetic head soon reached the bottleneck . therefore GMR head （ Also belong to MR head ） emerge as the times require , Today's hard disks are basically used GMR Magnetic head .

Today, ,PMR Has developed into CMR（ Traditional magnetic recording ） and SMR（ Laminated magnetic recording ） Two ways , Both of these methods can realize large capacity hard disk , however SMR Hard disk drives last longer than CMR The hard disk is shorter .

Reference resources ： Fundamentals of magnetism and magnetic materials Tight Pengxiaoling Huang,